The Formation and Reactivity of I BORON CARBIDE and Related

Home , Aluminium diboride, Boron carbide, Boron carbides, Carbide

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1970 The Formation and Reactivity of BORON CARBIDE and related materials

JONES, JAMES ALFRED http://hdl.handle.net/10026.1/1880

University of Plymouth

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I BORON CARBIDE and related materials

A Thesis presented for the Research Degree of

DOCTOR OF PHILOSOPHY

of the

COUNCIL FOR NATIONAL ACADEMIC AWARDS

London

JAMES ALFRED JONES

Department of Chemistry Plymouth Polytechnic

Plymouth, Devon- February^ 1970o F'.V

flCCH.

i'iC.

1 CLASS, T Shi LJl JoH 13' ABSTRACT

1 The formation of boron carbide, (CBC)*B^^0*^(3^0 is re• viewed with special reference to newer production methods and fabrication techniques. Its crystal structure and the nature of its bonding are discussed in relation to those of other borides and carbides. Information so far available on the sintering of this material is summarised in relation to its reactivity. Sintering into monolithic compoaentBcan only be achieved by hot pressing at pressures between 200 and 300 Kgcm'^ and at temperatures above

2000°C preferably at about 2,300^0 for the most rapid achievement of theoretical density, i.e. pore free. High purity, stoichiometric boron carbide produced by the magnesium thermal reduction of boric oxide in the presence of carbon, and of submicron average particle size, has been oxidised by heating in air. The work indicates that there is preferential oxidation of boron at temperatures below SOO^C; any oxidation of the resultant carbon is inhibited by the product ^2^^ phase. The measured acti• vation energy of 23*9 Kcals per mol supports this view. Formation of mixed systems of titanitim borides are described, including both orthorhomic and cubic monoboride together with the hexagonal diboride. These are formed when titanium alloys (alpha, beta, and mixed alpha and beta) are coated with boron carbide and heated to l400^C under vacuum or inert atmosphere.

(ii) ACKNOWLEDGMENTS

The author wishes to express his sincere thanks to

Dr« O.K. Glasson for his supervision and guidance throughout the course of this work.

He is grateful to Mre Lo Bullock, Managing Director, Bullock Diamond Products Ltd., Torpoint, Cornwall, and to Dr. A.B. Meggy, Head of the Chemistry Department, Plymouth Poly• technic, for the use of facilities and the supply of materials* His thanks are also due to Noel Pearman, Andrew Johnson and Margaret Sheppard for their technical assistance and to Miss Loraine Ash who typed the manuscript.

He is indebted to his wife, Dorothy, for her constant help and encouragement.

-V- CONTENTS

SECTION Page(s)

1o Introductory Survey and Review o» o» o o 1-77 lele General discourse oo oo o. oo o« 1 1o2» The classification of borides, carbides and related materials oo oo oo oo oo ^ 1o3o The structural properties of the boron carbides and related materials oo oo oo 8 lo^o The crystallo-chemical structure of the boron carbides oo oo oo oo oo oo 13 1o5o Boron-carbon system in detail oo oo oo 22

1o6(a}« Compounds of general boron carbide structure type oo oo oo oo oo oo 29

1o6ol(a) Boron-oxygen system oo oo oo oo oo 31

1o6o2(a) Boron-silicon system oo oo oo oo oo 31

1o6o3(a) Boron-carbon-silicon oo oo oo oo oo 33

1o6o^(a) Boron-carbon-nitrogen oo oo oo oo oo 3^ .1o6(b) Compoxmds of boron with other nonmetals having low boron content oo oo oo 3^

1o6ol(b) Boron-oxygen system oo oo oo oo oo 3^

1o6o2(b) Boron-nitrogen system oo oo oo oo oo 3^a 1o7o Systems of boron with metals oo oo oo 33

1o7(a) Lower borides oo oo oo oo oo oo 33

1o7(b) Higher borides oo oo oo oo oo oo 38

1o7ol(b) Boron-aluminium system oo oo oo oo 38

1o7o2(b) Boron-al\iminium-carbon system oo oo oo 39 1o8. Chemical thermodynamics and kinetics of the production

(iv) SECTION Page(e)

1o9<. The sintering of boron carbide and other refractory materials oo oo oo 55

lolOo The chemical reactivity of boron carbide and related compounds oo oo oo 72

Idlo The applications of boron carbide and related refractories oo oo oo oo 76 2, Experimental techniques for the production and analysis of boron carbide oo o© o© oo oo ©o ©o 7^ ^ 98 2o1o The production of boron ceurbide ©© ©o o© 78

2©2(a) The chemical analysis of boron carbide and related compounds ©© ©© ©o ©o o© 79

2©2(b) X-ray diffraction identification of phases ©© ©© ©© ©o ©© ©© ©© ©© 82

2©2(c) Electron microscopy and diffraction «© ©© 86

2o2(d) Surface area measurement by gas sorption ©© 90

2o2(e) Thermometric analysis ©© ©© ©© ©© ©© 95 3o Experimental results obtained on the prepared boron carbide etnd on related materials ©© ©© ©© ©© ©© ©© o© 99-117 3o1o The analysis of the prepared boron carbide 99

3©1(a) The chemical emalysis of B^C o© ©o o© 99

3©1(b) Phase identification of B^C by X-ray diffraction ©o ©o ©© ©© ©© ©© o© 100 3©l(c) Particle size analysis by X-ray line broadening, by BoEoT* surface sorption . ^ and by electron microscopy ©© ©© ©© o« 103

3o2o The vacuum sintering of boron carbide©* ©© 112

3o3o The hot pressing of the prepared B^C ©© ©o 113

3o^© The ball-milling of the prepared B^C «© ©© 116

(v) SECTION Page(s)

4o The oxidation of bo^o;! carbide oo oo oo 1l8 ~ 127 4o1o General discussion oo oo oo .o. oo 118 ko2. Thermometric and particle size results oo 120 5o The formation of titanium boride oo oo 128 - l40 5e1o General discussion on titcmium and its alloys oo oo oo oo oo oo oe 128

5o2o The titanixun-boron system in detail .o oo 130

5o3o Experimental technique for the production of titanium boride oo oo oo oo oo 132

5o4o Identification of titanium-boron phases oo 133

5o5o Discussion of results oo o« oo oo 138 5o6. Formation of a hard surface on titanium alloys oe oo oo oo oo oo oo oo 139 6. Concluding summary oo oo oo oo oo 1^1 - 1^6 6e1o Preparation of boron carbide oo oo oo 1^1 6o2. Sintering of boron carbide oo oo o© 1^1

6o3o The oxidation of boron carbide in air oo l43 6o^o Formation of titanium boride coatings oo 1^3

6o3o Phenomologlcal theory of boron carbide formation oo ©o oo oo oo oo oo 1^^

6o6o Proposed further work oo oo oo oo oo 1^5

REFERENCES oo oo oo oo oo oo oo oo oo viii - xvi APPENDICES oo oo oo oo oo oo oo oo oo XVii - XX

(vi) SECTION 1, INTRODUCTORY SURVEY AND REVIEW

I•I. General discourse. In spite of early historical applications, long history of investigation and use, boron and its compounds have not been extensively investigated until recent years. Studies of their inorganic chemistry and metallurgical science have been limited by the relatively low concentration of the element in the earth's crust, the specifity of its raw material sources. Also there are difficulties in conducting the appropriate investigations due to the unique character of boron which occupies an intermediate position between the metals and nonmetals in the Periodic classi• fication. Boron, Latin name borax, is already mentioned in the ancient works on chemistry, and metallurgy dating from 800 A.D. in connection with its use as a flux. In 1702, Ve Goldberg described the production of an acid from borax (sodium tetraborate) but believed this acid to be a salt. In 17^8, F. Baron proved that borax is a salt of boric acid. In 1777, this boric acid was dis• covered by G. Hoeffer in one of the lakes of Tuscany.

The high affinity of boron for numerous elements, especially for oxygen, made It impossible to isolate pure boron and its alloys. Ultimately, elementary boron was obtained, independently in l8o8, by Sir Humphry Davy and by the French chemists Gay-Lussac and Thenard.

By far the most important investigations of boron and its compounds were by Henri Molssan. In l892, Moissan developed a method for producing boron by reducing boric anhydride with mag• nesium, a method which is still in use at the present time. In l899f he obtained a compound of boron with carbon, vizo, boron carbide, and subsequently the borides of chromium, titanium and tungsten and those of silicon and boron steels.

In 1909f Moissan's research was continued by V/eintraub who discovered the semiconductor properties of boron, and by

Wedekind and Kroll who produced all the refractory borides known at present.

A number of works on boron and borides were conducted in the I930's and 19^0*s by Becker, Agte, Moers and Andriexix. These investigations developed the production of boron and borides by deposition from the dissociated gaseons halides and by the elec• trolysis of fused mediso However, the basic research on boron and borides has been stimulated over the past two decades, by the interest in heat resistant alloys containing boron and refractory boridee^ fundamental work has been done by Kiessling (Switzerland)

Kieffer, Benesovsky, Hovak, Nowotny (Austria), Glaser, Schwarzkopf,

Binder, Bliunenthal, Moskowitz, Post, Brewer (United States), and

Samsonov, Neshpor, Umanskll, (U.S.S.R.).

The compound of boron with carbon produced by Moiss^n on smelting boric anhydride with sugar charcoal in an electric arc furnace was assigned the formula B^C from analytical results. It was one of a number obtained by Podzus (1933)$ viZo, BC, B^C^,

B^C, B^C and B^C, The product called boron carbide which had been on the market some years as a metallurgical source of boron had roughly the composition BCo However, Rldgway (Chipawa Plant,

Ontario, of the Norton Company of America) patented a process in

193^ for the production of a definite crystalline compound having

(2) an average composition closely approximating the formula, B^C, for use as an abrasive« This outstanding contribution laid the foundations on which are built our present knowledge of the boron-carbon system and of the related metal borides with all their complexity and profusenesso

Studies of metal and nonmetal borides have been mainly concerned with their preparation, characterisation, structure, and physical properties; bonding has received some attention but comparatively little is known about their chemistry* A number of reviews have dealt in some detail with particular aspects such as structure, preparationg and physical properties for example, Schwartzkopf and Kieffer (1953)* Aronsson et alo

<965), Post (196^), Hoard and Hughes (1967) and Thompson and

Wood (1963)9 but only those of Samsonov and Markovskii (196O),

Greenwood et al. (1966) and the recent review of Glasson and the

Author (1969) covers aspects of their chemistry as wello

The present work carried out during the period 196**-to

1969 in the John Graymore Chemistry Laboratories of The College of Technology, Plymouth, and at the Works of the Bullock Diamond

Products Limited, Torpoint,Cornwall, concerns the production on a semi-technical scale of boron carbide of a particularly high purity and in a highly crystalline stateo Its reactivity is studied with regard to (a) its homogeneous sintering with and without additives, (b) its application to the formation of tran• sition metal refractories and (c) the resistance of these materials to oxidation, nitridation and hydrolysis« This stimu• lates from a need to produce abrasive-resistant surfaces on

(3) machine tools and similar articles for application in a large number of industrial concerns©

I©2© The classification of borideSp carbides and related materials Pata for the properties of many refractory borides, carbides, silicides, etc©, have been collated and reported by a niunber of workers in their respective fields^ namely Schwarzkopf and Kieffer

(1952), Bradshaw (1958), Hiester (1957), Lange (1960) and others© However, the most truly comprehensive collations are by Shaffer

(196^) and Samsonov (196^); in particular the latter is extremely valxiable as it covers otherwise unpublished or obscure material by

the author and other workers in the DoS^SoRo

It is difficult to define the term 'refractory compoiind' since any subdivision into refractory and non»refractory compoimds is arbitrary and presupposes the fixing of some melting-point boundary, above which, chemical compoxinds are considered to be re• fractory© Such a boundary has been repeatedly established and has gradually been shifted to regions of higher and higher temperature, from 1000*^0 in the second half of the last century, to 2000°C in the first half of this century, and often currently is assumed to be 3000°Co

Nevertheless, the expression "refractory compound* is now • gradually losing its original meaning and is becoming deeper and more fundamental, encompassing a whole complex of properties, in• cluding high hardness0 brittleness, and heat of formation, as well as specific electrical and magnetic properties as determined by the electronic structure of the corresponding compounds and the

('f) position of their components in the periodic system of the ele- mentSo Thus, a refractory compound need not always be one of high melting point, but may denote a substance possessing a com• bination of other properties^ eogo high hardness, low vapour pressure and rate of evaporation, and resistance to chemical attacko The principal tenor of the concept of 'refractiveness* is increasingly becoming the character of the chemical bond be• tween the component atoms of the compounds, which is mainly metallic or covalent with a small degree of ionic bondingo Such types of bond occur as a rule (i) in compounds of metals (mainly transition metals or metals similar to them, according to a number of criteria) with non-metals of the type boron, carbon, silicon, nitrogen, sulphur, phosphorus, etCo, not having ionization poten• tials high enough to produce ionic bonds, (ii) in compounds be• tween nonmetals and (iii) in certain intermetallic compoundso

Compounds of the first class are conveniently called *metal-like* refractories; a necessary condition for the forma• tion of metal-like refractories is the participation of incomplete

d and f electron levels, loSo a transition metal characteristiCe

As a qualitative criterion, Samsonov (1953? 196|^) proposed the use of the quantity, 1/l)n, where n is the number of electrons in the n incomplete level, signified by the principal quantum number N; another factor is the ability of atoms of the nonmetal to donate valence electrons, given by the magnitude of their ionization potentialso The electron density between the atom cores in the crystal lattice and its distribution depend on the values of n, N and I.P. nonmetal; an increase in 1/^n (acceptor capacity of the metal) produces a

(5) displacement of the electron concentration towards the metallic

atom for a constant donor, whereas an increase in ionization

potential of the nonmetal for a constant acceptor produces a

displacement towards the nonmetal with a corresponding increase

in ionic character» Thusg variations in the values of 1/^n and

I.P. nonmetal produce diversity, but for a binary system the

number of combinations are not infinitely large« This in turn

determines the continually discrete character of the variation

in the type of bond, and, correspondingly, of the physical and

chemical properties of this class of compounds in which the

chemical bonds are heterodesmic<>

Borides, where the boron atoms are isolated from each

other (M2B), have the boron valence electrons predominantly in

the free d levels of the metal atom^ In the more complex struc•

tures, having chain and network structures of boron, the nonmetal

valence electrons are mainly expended in the formation of covalent

catenated bonds and a smaller proportion is transferred to the general electron aggregate, symptomatic with metallic bonding*

Conversely, in carbides the proportion of metallic bond increases as the result of the higher ionization potential of carbon, and they possess typical metallic propertiese Silicides follow more closely the behaviour of borides, while nitrides tend to be ionic and compare more closely with oxideso

The second class of refractory compounds is formed by com• pounds of nonmetals with each other or the so-called nonmetallic refractory compounds; all of these compounds, like the metal com• pounds, are characterised by a heterodesmic character of the bond.

(6) but with predominance of the covalent bond, and they have semi• conductor properties as well as high electrical resistance at room temperature; as a rule, these compounds have a structure with layer, chain or skeletal structural groups, and either melt with decomposition or decompose before reaching the melting pointo Table 1.2.1 shows a number of currently known compounds of this classo

TABLE Io2o1o

lonisatlon Element potential, Si B N ev.

Si 8ol^ Si SI S SIP SIC X B 8o28 B Si ^ B B S B P B 0 BN X X X X S 10o42 S Si B S S P X X X P 10o^3 SIP B P X

C 11o2^ SIC B C X (diamond ) N l4o51 Si3N^ BN

In crystals of the elements situated along the diagoneil, in• dicated by the arrows, the width of the energy gaps increases in the direction of the arrows, while in the compounds formed between these elements it may be assumed that there will be an increase in the pro• portion of ionic bond with increase in the difference in ionisation potentials of the components (from Si^^B to Sl^N^, from Si^N^ to BN, etCo)o Finally, three elements of the periodic system occupy an in• termediate position with regard to the ability to form refractory metal-like and nonmetalllc compounds, despite their low melting

(7) point and high vapour pressure© These elements, beryllium, magnesium and aluminium, are capable of forming fairly re• fractory semiconductor compounds, with nonmetals, for example, beryllium, magnesium, aluminimum borides, aluminium nitride,

magnesium silicides, etCo

The third class of refractory compounds of metals with each other - intermetallic compounds, has recently been

discussed in depth by Westbrook (1966), and include beryllide, magnide and aluminide systems of other metalso

On the basis of this classification, it is possible to explain a number of the properties of refractory compounds and also the direction of their variations, such as their thermal and chemical stability, their mechanical properties such as hardness and strength, and of course their electrical and optical properties©

In the present work only data of certain compounds of the second class are considered, unless there is a specific need to compare such data with that of compounds of the other two classes.

1,3* The structural properties of the boron carbides and related materials and their application

The covalent radius of an element coupled with its ionisation potentials serve as a qualitative indication of the nature of its bonding in both catenate and alternate modes© Both para• meters must be treated with reserve© Covalent radii are average values from a number of compounds or are a specific value for the element in a particular allotropic form, e^g© carbon in graphite, or in diamond, and indicate the contribution of the element, in

(8) terms of bond length, to the covalent bond formed between two elements* The term "ionisation potential" is the energy for the removal of the outer-most electron to Infinity, from an atom in the gaseous state, nevertheless, they show the contri• bution, in terms of electron density, to the covalent bondo

The use of the covalent radius of an element to give a definitive indication of an * interstitial' compound from its ration with the host element, has been applied rigorously by

H£(gg et al (I930)o In the case of refractory compounds of the second type formed by atoms of comparable size and ionisation potential, such a classification has no relevance.

Table Io3o1o shows the accepted covalent radii for related elements and the ratio of these values to one another^

TABLE Io3o1 The covalent radii of some light elements forming refractory compounds

B Al C Si N

covalent radius

atomic O082 I0I8 0o77 loll 0o75 8 weight

B IO081I I0OOO 1c 439 Oo939 1»354 0o915

Al 2609815 O0695 I0OOO O0653 Oo9^l O0636

C 12o0111 I0O65 1o532 I0OOO 1o442 Oo97^

Si 28.086 Oo739 1.063 0.69^ I0OOO O0676

N I4o0067 I0O93 1o373 1.027 I0W0 I0OOO

(9) Table Io3«2o gives the composition of the refractory compound under discussion as represented by 'ideal' stoichio• metric formulations, and it should be noted that many of these compounds exist in a form having the same crystal structure

(Table Io3o3o) for a wide range of compositions, ioe^ the so- called homogeneity range»

TABLE Io3c2o

The compositions of some related refractory compounds

'metalloid* X content % Phase Formula v:eight % atomic weight

135ol8 g/mole 90o91 8O0O5

156082 92,31 82o80

1^3o93 42 086 25o03

A1N kOo99 5O0OO 34ol8

16^068 86067 85o40

165087 SOoOO 78068

BgSi 92o95 l4o29 30o22

•B^Si- 71o37 20e00 39o36

BjSi 60o52 25o00 46o39

BN 2^083 5O0OO 43o57

SiC 5O0OO 7O0O5

I40o22 42 06 60c 06

9( the most electropositive element

ioe. • the foremost element in the phase formulation

(10) TABLE Io3o3o

Crystal Structures of some refractory compounds

Space Structure.' Phase-- Unit Call ei8 c c/a ^oup Type bX S Reference Year

Rhombic 80881 9olOO 5.690 — — Cohn et al 19^8

II If Tetrago 1O0I61 — lit 0283 —

II ft Rhombic D^„ - Imma 12o3^ 120631 100161 — —

Rhombohed. 8o53 — — 28^17' — Stackelberg 193'f

A1N Hexagonal - C6mC ZnS 3oi04 — — 10600 Bokii 195^^

Rhombohod B^C 5o598 12o12 — 2ol65 Zhdanov 195** ^3^2 II If B^C 5 0630 12,19 2ol6 195'f ^2^3 BgSi Rhombic i^o392 18^267 9,88 — — Cline 1959

B^Si Hexagonal - R32 B^C 60 330 12o736 2a012 La Place 1961

B^Si Tetragonal 2o829 •Da if, 765 1063 Samsonov 1955

BN Hexagonal Djh - P6m^ Graphite 2o504 2 0665 Piyper 1958

SiC IV Rhombohedo C^ - R5m 17o7l8 — 9^58' — Bokii 195't 3v — If SiC VI Q\ R3m 27-759 OS 6°2lo5' Bokii 195^* 3v - — SiC Cubic F.C. T^ - F53m ZnS ^•358 — Bokii 195** a —

Si3N^ Hexagonal C^^ - H3C 7o76 - — 0o725 Narite 1959

Si3N, Hexagonal - P63/m 7o59 2.92 — 0.385 Narite 1959 The crystal structures of the refractory compounds given in Table 1.5,3. ae discussed in more detail in Section Z.^-^ and SectiOTi 2..2.. their indexing by X-ray diffraction in Appondiat IIo

Table Io3o^ gives the temperature stability range of these phases, unless a remge of values is actually given, the temperature given is the maximum of the stability range^

TABLE Io5o^o

Temperature stability ranges

Temperature Phase stability Heference Year range^ C

1660 - 1850 Serebryanskii 1961

2100 Slavinskii 1952

A1N 2230 - 1938

2480 Vuillard 1959

It 2350 1959

otSiC 2100 Samsonov 1959

ft ^iC 2630 1959

oTBN 3000 Sindeband 1950

. 1900 - 1938

1900 - 1938

1370 Matkovich 1960

It BgSi 1370 1960 ^1 C pyrographite 3652 1959

(12) The cryetallo-chemical structure of the boron carbides

To imderstemd the complex structures found in these materials

requires a detailed knowledge of the element itself. Boron has an

extra-nuclear structure of five electronso In the ground state two

of these are in the K-shell and the remaining three valence elec•

trons occupy four low-lying orbitals which are available for bonding;

the boron atom is thus electron deficiento

B.5 ED m rtm Is 2s 2p In the solid form the element does not assume the usual metallic

state encountered in such cases, but rather it crystallizes into a variety of polymorphic forms, all of which are extremely hard semi conduc torAs o

In the majority of the nonmetallic elements where catenation of like atoms or alternation of dissimilar atoms take place, viz«, two-centre bonding, the bond consists of an orbital embracing the two atomic nuclei and containing the maximum of two electrons« It is often necessary to invoke 'hybridisation* of the atomic orbitals to explain the co-ordination zsaxima and stereochemistry of these elementse However, in the case of boron, polycentred bonds are the rule both in catenation and alternation; eogo, three boron atoms in a trigonal plane are bonded together by a three-centred orbital em• bracing the nuclei and filled by only two electrons; such economy allows for ftirther bonding by each atom. The wave mechanical opera• tion (after Lipscomb 1938) takes a linear combination of the avail• able 2e and 2p atomic orbitals, viz :-

(13) 0 L.C.A.O

3-centred 'closed* molecular orbital hybrid atomic orbitals 0

Higher-centred bonds are possible involving four, five, six and

even twelve boron atoms; Sogo, in the common B^^ structural unit

the atoms are located at each vertex of an almost regular icosa-

hedron (Fig, I,4»l); each boron atom is involved in three-centre

bonding with its five immediate neighbours and in a twelve-centre

bonding with these and the remaining six atoms, requiring 26 of the

36 electrons available for the intra-icoeahedral bondingo In the simpler rhombohedral form of boron, the ten remaining electrons are required for extra-icosahedral bonding of the twelve atoms to neighbouring icosahedra (six in three-centre bonding and six in two-centre bonding). In cases where the extra-icosahedral bonding are all of a two-centre kind, donation of two electrons to give a

^12*" ^^^^^ ^® needed (e.g. B^^" CBC^'*')c

(14) Fig. Io4.1,

The icosahedron shares with the tetrahedron, the octahedron

and the dodecahedron the distinction of being one of the five regular

convex Platonic polyhedra; all are ieo«ahedral with regular polygons

as faces and possess a set of equivalent vertices which lie on the

surface of a circumscribed sphere« The icosahedron has twelve vertices

with five edges and five faces at each vertex, the thirty edges defin•

ing twenty equilateral triangleso It is a highly symmetric structure

element, endowed with considerably more symmetry than can be utilised in a three-dimensional crystallographic array, having 31 rotational

symmetry axes, 15 twofold aaes connecting centres of opposite edges,

10 threefold axes connecting centres of opposite faces, 6 fivefold axes linking opposite vertices, 13 mirror planes passing through opposite edges and is centrosymmetrico Eliminating redundancy leaves a collection of symmetry of order 120; this constitutes a non-

(13) crystallographic point group comparable to the crystallographie

point group of highest symmetry Oh - m3m of order which

describes the symmetry of the cube and the regular octahedrono

Fivefold rotation is not utilised in two or three-dimensional

periodic networks* A molecule or group possessing this symmetry

can use any other crystallographic symmetry it might possess or

it can be propagated as an antisymmetric element in a general

position by the translation symmetries; in either case, the

extent to which It simulates fivefold symmetry is a measure of

its internal rigidity while interacting with external neighbours

must induce some distortions. Since these are small In even

strong intericosahedral linkage it is possible to discuss the

structures in terms of dimensionally regular icosahedrae They

are not only complex because of the various detailed accommodations

that must be made, but are also characteristically rather open

networks, with regiilarly spaced holes large enough to contain metal

and other impurity atomsc Moreover, the structures are susceptible

to modification by inclusion in the three-dimensional frameworks

of additional boron or other atoms in positions that relieve the strains imposed by the difficulty of propagating the pentagonal structure elements* An Icosahedron provides an extremely efficient way of packing twelve spheres about a point very little different

from the packing a twelve spheres around a central sphere as in cubic close-i>acking except that the cavity in an icosahedron has a dieimeter of 0.90 that of the sphere, rather than unity* This reduction of the interior volume (by about 30%) results in a serious loss of external three-dimensional packing efficiencyo

(16) Within a B^2 icosahedron each boron atom forms five bonds

symmetrically disposed to a fivefold axis of symmetryo In three-

dimensional framework structure each boron atom characteristically

forms one additional bond directed outwards towards an atom in

another icosahedron or towards an interposed framework atom giving a co-ordination number of six for each atom; the preferred co•

ordination polyhedron for a boron atom is a pentagonal, pyramid

(Figo I,4o2,) the bond to the five atoms in the base are 60° apart and are inclined at 121^3' to the bond along the unique eixiso The length of the unique bond varies between structureso r

Figo I«4o2o

According to the results of X-ray diffraction analyses by

Zhdanov and Sevastyanov (1941), and by Clark and Hoard (1943), the original description of the boron carbide structure was based upon single crystals with a stoichiometric composition B^C corresponding to the traditional formulation of the principal compound in the boron-carbon systems This B/C ratio was satisfactorily achieved with two structureJ. elements - a B^^ icosahedron and a linear

(17) group in a primitive rhombohedral unit cello All essential

features of the structure remain intact over a wide composition range; variations in composition are accommodated by interchange of boron and carbon atoms at appropriate points in the framework and perhaps in some cases by inclusion of extra atoms in the open structureo Moreover, it appears that the only congruently melting

compound in the system is not in fact B^^^^l^/^^^^ rather the

thermally more stable B^^iCBC) where the linear group is re• placed by a linear C - B - C groupo Recent work indicates the B^C

composition to be in fact (B^^C) (CBC), nevertheless this refine• ment does not affect a discussion of the basic structureo

The icosahedron depicted (Figo in a projection along

the threefold axis is labelled to distinguish between two classes

of atoms (vertices) rhombobedral and equatorialo The rhombohedral

atoms r and their centrosymmetric mates r subtend at the centre of

a vector triplet which will be used to define the direction of the

rhombohedral axeso The vectors are of eqtial length and directed

along the icosahedral fivefold axes Guid are all inclined to each

other at the angle 63*^26^ (more precisely arctan 2)o It should be"

noted that a rhombohedral lattice defined by an angle of exactly

60° corresponds to a face-centred cubic array of lattice pointso

The remaining six atoms in the eqxiatorial positions, e and e, link

in a staggered belt around the equator of the icosahedrono The

atoms lie at the vertices of a flat triangular antiprism, the five•

fold axes traversing the vertices are only slightly inclined

(10^50') to the equatorial planoo

(18) In the boron carbide structure (primitive rhombohedral unit,

space group R3m) a 6^^ icosahedron is centred at each lattice point,

appropriately oriented with respect to the cell axes to conform

with the required symmetry 3m(D^^)o In each B^^ ^ atoms lie

almost directly along the rhombohedral axes and are bonded to the

equivalent atoms, r of the unit centred at the adjacent lattice

points0 The resulting rigid three-dimensional framework involves

exactly half of the boron atoms in direct inter*ico8ahedral bonds

each formed along quasi-fivefold axes of icosahedrao

No direct lateral connections exist between eqiiatorial

atoms in adjacent icosahedra; all such bondings must be effected

through additional interposed atoms which eo-e required to complete

the frameworko With this arrangement of icosahedra a substantial

cavity is created at the centre of the cell along the threefold

axis; the cavity is surrounded by an octahedral array of B^^ units,

three above the centre £uid three belowo The quasi-fivefold axes

traversing the equatorial atoms, e, of three of these adjacent

icosahedra meet precisely at a point on the threefold £Lxis about

1.6 S from the equatorial atoms and 1o4 S above the centre of the

cell; a corresponding intersection involving the equatorial atoms, e, in the other three icosahedra occurs 2o8 £ away, below the centre of invereiouo An atom placed at or near this intersection not only stabilizes the rhombohedral framework but also satisfies the preferred coordination geometry for all the equatorial atoms«

It is required only to form three bonds to boron at a reasonable distance of 1o6 S and at approximately tetrahedral suiglee and to link across the 2o8 8 span, if necessary through a collinear third atom at the centre.

(19) The geometric constraints specified above play a pro•

minent role in determining the possible structural modifications

of the boron carbide structure, they are satisfied by not only

a C - B - C chain but by a variety of other nonmetal atomso

A number of secondary considerations also permit varia•

tions in the structure without violating the basic framework*

For example, the framework encloses two holes per unit cell each

large enough to accommodate an extra atomo They are located on

the threefold eucis just above and below the central chain* Den•

sity and stoichiometry can be modified by partial Inclusion of

extra boron, carbon and other atoms in these holes particularly

for systems prepared under conditions far from equilibrium*

Another structural degree of freedom involves the partial sub• stitution of a limited number of icosahedral boron atoms by such divergent elements as carbon^ silicon and perhaps beryllium, the substitution is driven by electron deficiencies In the Icosahedral framework as well as geometric constraintSo

The foregoing description of the boron carbide structure has been idealised in terms of perfectly regular icosahedra; in

fact, the approximations are quite good, but a number of small deviations do occur* The essential features of the framework are shown in Fig* 1*^*3* which represents a section of the structure viewed down a threefold axis* Regular Icosahedra with I080 2 edges bonded rhombohedrally at I080 S woiild be separated by

5*22 2 at an angle of 63^26'* The rhombohedral lattice constants of boron carbide are a = 5ol67 - 0*003 2 and ot = 63*60°^ 0*05° or

(20) Figo Io4,3.

o T Oe

a = 5o58 and C = 12o00 for its hexagonal cell containing o 8 o 2 three rhombohedral unit structures^ (Figo Io4o4o) and the ob• served boron-boron distances range from 1o72 to I08O S«

The intra-icosahedral bonds are all very close to I08O S except for a slight constriction aroundo The rhombohedrally directed inter-icosahedral bonds are 1o72 S considerably longer than the I06O S equatorial bonds to the carbon atoms (covalent

(21) Figo Io4o4o

(a) Two projections of the hexagonal unit of B^2('3 0 • o e

(b) Rhombohedral unit cell of

radius carbon (Sp^) = 0o77 S, covalent radiiis of boron (equatorial)

= 0«86, the calculated boron-carbon bond becomes I063 S)e

BORON-CARBON system in detail

The various structural data so far reported must be considered carefully, as it is already quite evident that the thermal treatment accorded to a reacting mixture of specified composition is of major consequence in determining the nature of the producto The role too

(22) of possible critical impurities, for example, aluminium, requires detailed investigatiouo Thus, Glasser et al (1953) report on the products obtained by heating veurious mixtures of boron and carbon to approximately 2000^C involving sintering into compositeso Hoard and Hughes are critical of their results as the temperature proba• bly falls short of fusion for any composition in the boron-carbon system; thermodynamic equilibria were not achieved and the compo• sition range of interest, k to 28 atom % of carbon, extended at both ends beyond the range of thermodynamic stabilityo Allen (1953) recorded density and X-ray powder data for three compositions; B^C,

^17^3 ^7^t took the composition B^C to represent the unique/ choice for a true compoundo He assiuned substitutional solid solu• tion, ioe« boron for carbon in the ideal B-^2^3 ^^^unework in the phases B^^C^ and B^C. Samsonov et al (196O) describe a still evolving phase diagram for the boron-carbon system; in the compo• sition range, 0 to 28 atom % of carbon, two compoiinds are reported,

^13^2 congruently at about 2^50^C ajid ^^^^^ melting incongruently at about 2350^Co More recent work by Vuillard (1968) using more refined techniques gives the values as 2^80°C for the melting point of ^^^^^ states that B^^C is completely fused at

2360°Co The phase diagram for the boron-rich part of the system has been established by Dolloff (196O) and refined by Elliot (196I )

(Figo le^ol.}; it is reduced from ^28 atom % of carbon (Glaseer

1953) and 6-2A- atom % of carbon (Samsonov^1960) to approximately

8-20 atom % of carbon, corresponding to a mean value equivalent to S^3^2 upper limit of B^C, for the composition range of the rhombohedral boron carbide phase.

(23) Fiffure 1*5^,,

Projected phase dlagreun boron-carbon

Lxquxd

0) u Boron Carbide Carbide 2 IbOO + + <1> p< Carbide Graphite s 9 EH

\'U>0

10 ^6 *^ Atomic percent carbon

It should be noted that the rhombohedral alpha boron

which has the simple boron carbide structure is not stable at

these elevated' temperatures, and it is the complex beta form

of the element which exists, and that the eutectic.containing

only 3 atom-% of carbon melts at 2100°a only a little lower

than the 2370*^C for pure B-rhombohedral boron (Vulllard 1963)0

The possibility of another compound (presumably as a

peritectic) being interpolated between the eutectic and the

(24) B^2^2 ^compositions, is tentatively put forward by Samsonov et

al (1960) on the following basis.

A plot of electrical resistivity against composition

shows a series of maxima at ^^^2^3' ^13^2 ^13^* which the

second is by far the most prominent; a similar plot of thermal

B.M.F. shows a sharp deep minimum at ^^^^^ ^^^^^V^^^^^ by maximum

at Bf2^3 ^13^ (probably a necessary consequence of the minimum

at B^^C^). The data carry conviction for the B^^C^ composition,

but are only suggestive of possible compound formation in the

other two cases»

In their re-investigation of crystalline B^C, Scott et ai

(1964) have utilized three-dimensional coiinter-recorded X-ray data

comprising the larger part of that measurable with M^Ko^ radiation,

The density of 2e52 go/cco and the lattice constants a = 5ol67 -

0,003 X, = 65e60 - Oo05°t are typical of a B^C compositiono

Fourier difference synthesis shows that true interstitial holes are not used by extra atoms; it shows also that there is a deficit of electron density associated with the central position in the triatomic chain that amounts to rather more than one electron for occupancy of this position by a neutral carbon atomo The inte• grated electron densities for all peaks in the Fourier synthesis are suggestive of either of two theoretically significant formu• lations: I. Cj^*Bi2^" - ®' - ^^'^ (CBC)'*'(B^^C)"; in either case the icosahedron is formally assigned 38 valence shell electrons, the chain IO0 The analysis generally speaks for a transfer of negative charge from the central atom of the chain to the icosa• hedron, very roughly estimated as I08 to O08 electrons according

(25) to whether the central atom of the chain is a carbon or a boron.

The nearly insignificant anisotropy in the apparent thermal

motions of the terminal carbon atoms in the chain strongly suggests

that the chains approach structural and consequently chemical

homogeneity - that the chains are predominantly either or CBC,

with little mixing of the twoo Interpretation of the structural

data on the basis of CBC chains and B^-^C icosahedra (in which the

carbon atoms are randomly distributed among the twelve positions)

is quite straightforward and unforced as compared with the classic

alternative using chains.and B^^ icosahedra (the latter being

inconsistent with the N»M.R. results obtained by Silver and Bray

(1959) for similar materiaDo Consequently, it appears that an

ordinarily well-annealed boron carbide of B^C composition is

structurally to be formulated, at least in first approximation,

as (CBC)"*"(B^^C)~ to indicate probable charge transfer and is

considered to be energetically preferredo

Revision of the theoretical discussions of electron dis•

tribution is not required; the electron counting of a boron

carbide of B^C composition, written as ^3^*B^2^~ Longuet-

Higgins and Roberts (1955), or as the more probable (CBC)*(B^^C)" after Scott et al is 2 +2 (4) +38 =48 electrons* The formal

charge transfer is in either case from the central atom of the chain to the icosahedron, although this does not mean that the central atom of the chain is thereby limited to a pair of opposed collinear sigma bonds« Either theoretical model is compatible with the predicted and observed semi-conducting behaviour of a

B^C composition (Yamazaki, 1957); in fact, the model which

(26) minimises the charge transfer should be energetically preferred.

In contrast, ^1-5^2 ^® boron carbide composition

ordered chemical compound; the electron count is inevitably one electron short of the theoretical requirement for a closed shell or filled bond configuration. Therefore, theoretically B-^^^^ should display metallic conduction by 'positive holes*, but in fact it is shown by Samsonov et al (196O) to have a higher resistivity than B^C. This suggests the inference can be made that the. .'holes' are, non-conductive by being trapped within the icosahedra, restricting conductivity to n-type (electron conduc• tion).

Studies by Tucker and Senio (195^, 1955) of neutron irra• diated boron carbide (of B^C composition) demonstrate the extra• ordinary resistance of the three-dimensional framework to extreme maltreatment. Virtually all the B - 10 nuclei are transmuted by neutron irradiation to He - ^ and Li - 7 nuclei (15% of all atoms present) yet the more fundamental characteristics of the boron carbide structure are clearly preserved, although there is expan• sion of the a axis of 0«899^ and contraction of the C axis of o o

1.38% as well as an intensification of the anisotropic thermal effect which is six times as strong in the C^-direction as in the a^ direction together with changes in the average positions of certain of the lattice atoms and very heavy diffuse scattering.

Nevertheless, most of the damage to the residual framework is re• paired by rather mild annealing at 700 to 900^0,

(27) That the boron carbide structural type, aided perhaps by

a partial filling of interstitial holes and/or a rather free use

of framework vacancies, should accommodate a variety of composi•

tions ranging from the highly boron-rich to the moderately carbon-

rich extremes, appears rather less surprising in view of the

Tucker and Senio studies• As noted by Clark and Hoard (1953)

carbon in excess of the 20 atom-% required for B^C occurs as

graphite, and it is suggested that only quenched compositions

might retain a higher content of carbon in the boron carbide

phase. Lowell (1966) has shown that at the carbon-rich end

of the boron-carbon system over a temperature interval of l800

to 2500*^C, boron occupied one of two possible sites: (l) an

interstitial position in the centre of the hexagon formed by

the carbon atoms -or- (2) a substitutional position replacing

a carbon atom. The maximmn solubility of boron in graphite is

given as 2.30 atom % at 2350^0 (the fusion maximum of B^C) and

a tentative phase diagram (Fig. 1,5.2) shows solid solution

over only a ^ery small area. The effect of boron dissolved in

graphite on^he lattice constants of the latter, are summarised

by the following equations

a = 2,^6023 + 0o00310K„, 2 O D

C = 6,71163 - 0.0059^K-,, o 0 8 where Kg is the atomic fraction of dissolved boron.

Hence, it appears that the range of solid solution forma• tion extends principally to boron-rich compositionso

(28) Figure 1.3.2.

Phase diagram boron-carbon (carbon rich region)

Solid Solution + liquid Solid T.UOO Solution

Solid Solution

lO XC !6C* Atomic Percent Boron

Ic6(a) Compounds of General Boron Carbide Structxire type

The materials listed In Table I060I, are sometimes called

'interstitial compounds ofo^-boron*» This is grossly inaccurate as by the concept formulated by Hflgg (1929) and scrupulously ob• served by the Uppsala workers to this day, requires that the lattice constants and the volume of the host phase (in this case boron) remain virtually unaltered; the small holes - the interstices

- which geometrically interleave the crystalline arrangement of quasi-spherical atoms of the host, are filled partially or wholly

(29) TABLE I060I

CRYSTALLINE MATERIALS OF GENERAL BORON CARBIDE STRUCTURE TYPE (HOARD, 19^77

Rhombohedral Hexagonal Relative Cell Cell Volume

Material a,2 a ,2 C ,2 VA VA„ 00 N o^-RhB 5.057 58° 4' 4o908 12o567 ONE O0855

Model 5o12 63°26« 5o38 12o23 1o17 ONE

^0 3o^k 62°56» 5o37 i2o3i I0I7 I0OO B'606^ '

B.C 5c174 65^30- 5o598 12o12 1o25 1c07

^13^2 ^"^^^ 65^29' 3o630 12o19 1o28 I0O9

'^12^' 5o19 67''56« 5.80 11c90 1.32 1c13

^12^1 8 ^"^^^ 69*^31 • 5o98'f 110850 lo'fO 1o20

^12^^2 5o319 70^32' 6ol42 II0892 1o48 1o26

®2o89^^ 5o592 68°49' 6o3i9 l2o7l3 I067 lo43

(30) by the sufficiently small atoms of the interstitial componento

This is wholly at variance with the boron carbide structure,

where the effective volume of an 'interstitial* chain atom is

fully as large as that of a 'host" atomo Even more important

is that the required continuity of phase betweenoC ^^^ombohedral

boron and the boron carbide compositions is lacking; instead,

the composition range for solid solution on the equilibrium

diagram terminates at the boron-rich end in a eutectic with

^-rhombohedral boron at a temperature some 900^0 higher than

that at which the thermal instability of theo<.-rhofflbohedral

polymorph is manifest.

1,6.1. dORON-OXYGEN system

For the material described as B^ ^0 (Table Io3ol) La

Placa^and Post (1961) prefer the formulation ^12^"

than the apparent (0B0^*B^2^")on the basis of density measure-

mentso The former Is more in keeping %rith the Longuet-Higgins

criteria for electron counting and suggests that the oxygens,

while filling the 'carbide' lattice site, are not bonded to one

another but each form three bonds to icosahedral boron atomso

1.602. !0R0N-SILIC0N system

The preparation of B^Si and B^Si as definite compounds

was first reported by Moisscui and Stock (1900), Cllne (19C9)

and Adamsky (1958) have characterised the orthorhomblc B^Sl;

the compound is black, opaque and very hard with a density of

2o'4^3 g/cco and is semlconductiveo The orthorhombic structure

is thought to contain kO B^Si's in the unit cello The detailed

study of B^Si by Magnusson and Brosset (I962) not only supports

(31) the Moisson and Stock formula, but strongly suggests that this compound is representative of the numerous preparations to which other investigators (Colton I96O, 1961); Matkovich (196O) and

Rizzo et al (196O a) have assigned the formula, B^Sio A careful chemical analysis, by Rizzo and Bidwell (196O b) and phase equilibrium studies by Knarr (196O) give the empirical formula

B^Sio The compound melts incongruently at about l400°C to give

BgSi + Sio

The atomic arrangement is crystalline B^Si is of genera• lised boron-carbide type, with Si2 replacing C-B-C chains and with B.^ -Si as the composition on the average, of the icosahedral groups, silicon atoms substitute for boron only in the icosahedral r and r positions since substitution in the equatorial positions with Si-Si bond lengths of 2o34 8 would produce intoler• able distortion of the framework if used to link icosahedra to chainso The value reported for the actual Si-B crosslinks,

2o002 - O0O29 S agrees with the sum of the tetrahedral bond radius of silicon (lo17 S) and that of the equatorial boron (O085 2)o

For electron counting, it is the (B^o^^2^ ^^2 ^5^^2^ which obeys exactly the Longuet-Higgins and Roberts (1955) criteria for closed shell or filled bond configurationo

Whereas the frequently reported B^Si would be (B^^Si) Si2, the B^Si composition some halfway in between requires the use in equal proportions of B^-^Si and B-,QSi2 icosahedrao

If the supposition is made that in a B^Q^i^ icosahedron the two silicon atoms occupy non-contiguous positions (ioSo one on r and the other on r) a numerous family of statistically

(32) equivalent frameworks in which there are no Si-Si bonds except•

ing in the chains, can be constructedo However, if the propor•

tion of B^^Si^ icosahedra is greater than half, there must be

Si - Si bonds in de-stabilizing position, thus B^Si appears to

be at the silicon-rich limit for stability of the phaseo On

the other hand, a ^-y^^^z ^^6^^^ composition having a boron carbide

type structure would be two electrons short of the theoretical

counting for a filled bond configiiration, so that a more complex

orthorhombic structure is preferred for the B^Si composition

(compare the - tetragonal form of elemental boron)o

I,6.3>Srstem BORON-C&RBON-SILICON

Samsonov et at (1955,, 19^0) and Meereon et al (196I) des•

cribe two ternary compositions B^Si and B^^i^ 0^; both are

extremely hard, semiconductive materials prepared by the action

of silicon carbide or silicon on boron-carbide or boron, or by

a mixture of the three elementso Taking a hypothetical composi•

tion of(B^Q C2) Si^, (B^ SiC) , as ideal, with regard to elec•

tron counting, by analogy with B^Si and B^Si, the'boron rich'

teOcing C = B, B^Si should be orthorhombic and the silicon-

rich B^Si2 should be rhombohedrale Lipp and Rttder (1966)

describe substitution compounds of the formiilation B^^(C^ Si, B,)^

(produced by heating boric oxide, sand and graphite, in ah electric

arc furnace) having rhombohedral structures and lattice constants

(hexagonal) of a^ = 5o65 2 and = 12o35 Xo The system B^C-SiC

has been considered as having a quasi-binary eutectic (at 15 atom%

carbon) at 2300^Co The mutual solubility is less than 2% said

(33) in silicon-rich compositions the hexagonal alpha-SiC is present,

and in boron-rich compositions the cubic beta-SiC is presents l.GA. system BORON-CARBON-NITROGEN

Boron carbonitride has been reported by Samsonov et al

(1962)0 Degtyarev et al (1966) describes a preparation by the

nitriding (N^) of B^C at l800-1900°Co Its electrical proper•

ties are almost that of an insulator and it is probably a mix•

ture of boron-nitride (80%) and graphite (20%)<> It has a high

thermal shock resistanceo

Io6(b)o Compounds

Io6ol(b) BORON-OXYGEN

Boron trloxide (boric anhydride) is obtained by the de•

hydration of boric acid and by the oxidation of borldeso It

occurs in a crystalline or vitreous state depending on the method

of productiouo In the crystalline state it has a rhombohedral

structure which can be indexed as hexagonal with, a = 4o325 2

andf c = 8o3l7 2o In the vitreous state it possesses a struc•

ture with short-range order depicted in Figure I060I0, after

ZacharieOen (1932) and consisting of individual groups of net•

works of triangular complexes of /BO^^o The distance of B - 0

is 1o39 2 and 0-0, 2o40 2o The vapoxxr at 1500°C consists of

the monomer B20^o

(34) Figure Ip6,1

« boron

oxygen

6 o

A lower .bxlde of formula (BO)^ is formed when B20^ is

^heated .with the" correct proportion oif elemental boron at a

temperature between 1050^ and 1550^0; it has an amber-coloured

vitreous*form and reacts vigorously with water liberating hy•

drogen with traces of boranes and is sometimes pyrophoric. In

the vapour phase at temperatures in excess of 2000°C it eicists

as a mixture of the monomer and dimer^

I,6c2(b), BORON-NITROGEN

Boron nitride is obtained as the hexagonal or 'graphitic"

white form by direct synthesis from the elements, pyrolysis of

boron-nitrogen compounds or by the action of nitrogen or ammonia

on a variety of boron compounds under reducing conditions =.

Boron nitride differs fromg-aphite by having the layers stacked

immediately above one another but still maintaining the B-N

alternation, Figure I.6c2*

(3^a) Figure Io6o2o

a) Boron nitride b) graphite

1st layer boron o carbon 2nd layer O nitrogen • J- As carbon exists in two main aliotropic forms so does boron nitride; following the synthesis of diamond by Bundy

(1955), Wenthorf (1957) produced the cubic analogue 'borazon' by converting the graphite form at l600^C at a pressure of

60 kilobarso As formed it has the zinc blende structure of diamond, but, as with diamond, a hexagonal w\irtzite form is knowne

(3^b) I>7* Systems of boron with metals

(a) Lower borldes T

The boron frameworks of three-dlmensionally linked polybedra that typify the true higher borldes are replaced in the metal dlborldes by quasl-inflnlte networks of two dlmenslonally linked boron atoms, ^he structure type of the atypical AlB^, Is utilized by most of the betterocharacterised dlborldeso The versatility of this simple hexagonal structure (Figure lo7o1) In Its ability to accom• modate metal atoois that differ widely in metallic radius and in electronic configuration Is made evident by the listing of dlborldes in Table 1o7.1; the significant departures from exact dlborlde stoichlonetry and Ideal lattice are not Indicated although such departures are quite generalo Dlborldes of the tremsltion metals from the fourth group and increasingly from latter groups of the

Periodic Table must be formed in competition with still lower borides in which the special nature of the metal atoms plays an ever more prominent roleo The rather Involved structural chemistry of the lower borldes (and of the related slllcldes) Is discussed in detail by Aronsson et al« (1965)0

Figure Io7o1*

o, boron

6 metal

(35) The AIB2 structure type has a hexagonal unit cell contain•

ing just one metal and two boron atoms in positions wholly fixed

by the space group, P6/mmmo In geometrical terms the crystalline

arrangement is completely statified along the hexagonal axis, c,

that is, layers of metal atoms alternate layers of boron atomso

The close packing of metal atoms is typically metallic, however,

the direct superposition of these layers to give a simple hexa•

gonal lattice is quite atypical for pure metal, thus the eight-

coordinate polyhdron of each metal is a hexagonal bipyramid, and

each atom has a graphite-like with a B-B bond distance of a/3o

Each atom of metal is in contact with twelve boron atoms and 'sees*

eight others at a disteuice comparable with the metallic diameter, so that, in fact, twenty coordination is showno The metal dia• meters 2R, of the listed diborldes, varies widely yet the B-B distance varies little, from 1o71A in CrB^ to 1o9lA in GdB20 It can be concluded that the lattice spacing, a9 is largely determined as a compromise between the conflicting dimensional demands of M<»M binding within the metal-layers and the B-B binding within the boron net, with the latter favouredo It can be surmised that the

B-B bond length of 1o75A observed in TiB2 represents minimxim strain of the boron neto The structure type approaches maximum thermal stability in the diborides of titanium, niobium, tantalum and hafnium; they are significant in that they have melting points some 1000^ above those of the pure metalso

(36) TABLE 1.7,1-

Structural data for the metal diborides

Metal Distance, A Density

Boride c/a C a 2Rm B>B M-B Rm+Rb go/cc calco expto

GdB^ 1e19 3o9^ 3o31 3o6l 1o91 2o74 2o68 7o96

YB^ 1al6 3o8'f 3o30 3c60 Io90 2o70 2o68 505^*

TbB2 I0I9 3o86 3o28 3o56 I089 2o70 2o66 8o34

DyB2 I0I7 3.84 3o28 3o55 I089 2o70 2o66 8o53

HoB^ I0I7 3o82 3o27 3o53 I089 2o68 2o65 8080

ErB^ I0I6 3o79 3.28 3o51 I089 2o68 ZoSk 8089

LuB^ 1o1^ 3c 74 3«25 3o47 I087 2o6k 2o62 9o76

SCB2 I0I2 3o52 3o15 3o28 I082 2o53 2o52 3o67

PUB2 1o24 3o95 3o19 3o28 loS^f 2«68 2o52 12o85>

ZrB2 loll 3o53 3o17 3o21 1083 2o54 2c48 60IO 60I7

MgB2 1olA- 3o52 3o08 3o20 lo78 2.50 2o48 2o63 2o67

HfB2 1010 3o47 3ol4 3ol6 l„8l 2o51 2o46 11.2 IO0O5

UB2 1o27 3.99 3ol3 3ol6 I081 2o68 2A6 I2o70

TaB2 1o04 3o23 3olO 2«97 lo79 2o4l 2o36 12c2 11o7

NbB2 I0O5 3o27 3oll 2o96 lo79 2o43 2o36 6o92 6060

TiB2 I0O6 3o23 3o03 2o93 lo75 2o38 2o34 4o48 4o38

M0B2 I0OI 3o06 3o0'f 2o83 lo75 2o33 2o30 80OI

WB2 I0OI 3o05 3o02 2o85 lo74 2o32 2o31 1^.2

AIB2 I0O8 3e26 3o01 2o86 1o74 2o38 2e31 3ol6 3ol7

VB2 I0O2 3o05 2o99 2o73 lo73 2o30 2o25 5o05

CrB2 I0O3 3o07 2o97 2o60 1o7l 2o30 2ol8 5o20

MnB2 I0OI 3o04 3o01 2o58 107^^ 2o31 2o17 5c35

(37) (b) Higher boridea

(i) The system BORON-ALUMINIUM

Kohn et al (1958, I96l, 1965) in a series of studies of

the higher aluminium borides, concluded that there are three

polymorphs of AlB^^s theDC't and formso The most common

phase, cC- ^^B^2 ^® tetragonal^ pseudocubic, with lattice con•

stants a = IO0I6 S and c = l4o28 The diffraction symmetry and

the systematic absences correspond to the uniquely determinable

enantiomorphic pair of space groups P^^^ 2^ 2 and P^^ 2^ 2 with

the possibility that the observed symmetry results from a polytypic ordering of identical layers and that the correct space group should be 2^ 2o The high symmetry of the quasi-spherical icosahedral structural units favours the formation of alternative frameworks systematically related to a basic framework structureo

Such twinning, which preserves all of the essential features of icosahedral stereochemistry, owes its origins to detailed bonding requirements of relatively- small amounts of the secondary component of the boridCo The ^ ^^^12 ^® reported by Kohn et al (1958) as being intricately twinned on (IIO) and (1T0) and indexed on an orthorhombic, pseudo-tetragonal cell, a = 12,3^ 2, b = 12o63 S and c = 10el6 8, in the space group I2/m2/m2/ao Matkovich et al

(1965) suggests that carbon is required for this phase^ Kohn and

Eckart (1961) emphasise twinned space groups in their formulation of A1B^2 orthorhombic phase with a = l6o56 8, b = 17o53 S, c = IO0I6 8 in P2^ 2^ 2^) as a polytypic derivative of the alphao

From analysis it appears that the phase derives from the ^- phase

(38) by a cell twinning operation involving a rotation of l80^ around

the normal to (101) after every (101)ed layer*

The role of aluminium in driving these transformations is unknown; it probably eubotitutes for boron in generating some framework links, but the possibility of partial occupancy of some

framework holes cannot be igaoredo Further evidence is provided by Giese et al (196^) who show a reversible 'marteneitic' trans• formation of alpha to gamma (C.CoP. ^ H.C.P. )<> The gamma form is the stable one at lower temperatures^

(11) The system BO-CN-ALPMINIUM-CARBON

The ternary -.o-pound Al B^^ appears as a secondary product in the Al B^^ preparations of Kohn et al (1958) and was characterised as Al B^^ Matkovich et al (196^) formulated the phase Al B2^C^ and Wills (1963) demonstrated by X-ray diffraction that nearly regular icosahedra are dominant features of the struc• ture,, Hoard and Scott (1966) have reformulated the structure for a 62-atom cell containing 2 Al atoms, ^ i<^<^B^^^d''A k linear C-B-C chains to give an empirical composition of Al ^26^4**

The effective volume of the B^^ sub-grouping in Al B^^C^ is about k% greater than that in B^Co The structure Is derived from the boron-carbide framework by a cell-twinning operation of a mirror reflection in the twinning plane* The carbon atoms are precisely located in each rhombohedral sub-cell at the usual ter• minal sites of the three-atom chain, and bond equally to three icosahedrae Again the role of the aluminium atoms is not clear, but their presence is certainly required for stability of the phase©

(39) Giese et al (196^) report that at approximately 2000°C, Al ©26^^

transforms by loss of aluminium to a rhombohedral boron carbide,

probably B^^ ^^^9 have, decided to call the ternary phase

•beta boron carbide*, and B^C 'alpha boron carbide"o

Elektroschmelzwerk Kempton GomoboH (196^) have patented

a process for the production of a hard material formed by heating

boron carbide with aluminium to between 1400-1500*^C in an inert

atmosphereo The composition of the final product is not given

but is probably the ternary compound, Al ^26^4*

Io8, Chemical thernodynamics and kinetics of the production of BORON-CARBIDE and related materials

Although the eiztensive properties, enthalpy and free energy of elements and their compounds cannot be measured in absolute. terms, it is possible to determine the change in these properties attending a chemical reactiouo The standard f^ee energy change,

^a°, for a chemical reaction affords a direct qiiantitative measure of the extent to which the reaction may proceed, related to the equilibrium constant for the reaction, by the expression:-

y^G° = - R T In K (i)

For the general reaction between an element (a metal) and another element (a nonmetal) eogo the formation of a metal oxide

" (8) ^ 02(g); =^ "°2(8) the standard free energy change at a particular temperature is equal to the standard free energy of formation of the compound, in this case the metal oxide> If data is compiled for the re• actions of a number of elements with one element, eogo oxygen, the relative affinities of these elements for the one element can

(40) be ascertained by plotting the standard free energy change per

gram equivalent of the element as a function of temperature

(^Kelvin)o This graphical method of presentation, the Ellingham

diagram (19^S^, is extremely valuable in extractive metallurgy

and preparative chemistry in indicating the feasibility of a

reaction over a particular temperature range and also the com•

patibility of materials at high temperatures The general dia•

grams drawn by Ellingham showed a plot of standard free energy

change (invariably negative) against temperature (*^C) for the

formation of a number of oxides under standard conditions; gases

participating in the reactions being at one atmosphere pressure.

A pressure correction for the departure from nonstandard

conditions is calculated from the Van't Hoff equation (1886)

^G, = ^ HT m |^fffef3 --- (ii)

The Richardson (19>^)nomographic scale on the Ellingham

diagram allows the equilibrium gas compositions to be read

directly at any temperaturco Thus for oxide reduction by carbon and hydrogen, CO/CO and H 0 , ^ ratios are relevant, and c. c. d. \ vap o }

for nitride reduction by hydrogen the ^^/^^^ ratio is requiredo

In the case of the boridea and carbides, where the reactants and products are refractory, the vapour pressures of the compo• nents concerned are only significant at high temperatures and only slightly modify the standard free energy changCo At moderate temperatures the standard free energy change plotted as a function

(^1) of temperature (°K) is linear and has generally a positive slope

providing there is no net volume change (increase) on going from

reactants to products; this is in spite of the temperature de•

pendent of the related extensive properties, enthalpy and entropy

and but since

^G"" = AH"" . TAS"" (iii)

the two terms are almost self-balancingo At higher temperatures,

fusion and evaporation of the reactants and products give more

significant changes in the enthalpy and entropy, causing inflec•

tions in the linearity of the plots especially for evaporationo

At the temperature when A^G*"*^ is equal to zero, products are at

equilibrium with reactants under standard conditionso Hence,

above this temperature (the decomposition temperature)« the pro•

ducts are thermodynamically unstable and, vide infra, below this

temperature reaction is feasible, providing the A value is negative, ioe^ the plot has positive slopeo When two or more such plots are compared to show their relative affinities for a particular element, oxygen, the oxide products can co-exist with either of the reacting elements at the point where the plots cross, ioeo have the same value of AG°^ as f (T)o Outside this tem• perature, should the slopes differ, one product will have a more negative free energy change and hence is the more stable; in turn, the element forming this product (oxide) will reduce the less stable product of the other elemento This position is re• versed on going to a temperature the opposite side of the cross• over pointo

(^2) There are a number of disadvantages in the application

of these /\G^-^/T diagrams, principally these are:-

(i) the free energy changes refer to standard

states only, conditions never realised in

dynamic systemso

(ii) The assumption is made that the compounds

are of definite composition although in

practice for many refractories this may

not be soo

(iii) the distribution of the reactants and

products between the different phases is

not taken into account,

(iv) the formation of intermetallic compounds

and other mixed phases between products

and reactants is a possibilityo

(v) they indicate only whether a process is

thermo-dynamically possible, but do not

indicate the kinetics of the process^

Their main advantage lies in their simplicity and ready

evaluationo Reliable data for their compilation are available

from a number of sources, e^go UcS« Bureau of Standards Publi•

cations (1932, etco), Janaf Thermochemical Tables and Supplements

(1960-5), and Schick (1966) (Appendix D, although much of the data for the higher temperatures have been obtained by extrapo- lationo Diagrams for a number of oxide, boride, carbide and

chloride systems have been compiled (Figures loSo2^

lo8o3* and loSok)^ The data for oxides and chlorides are

necessary as most borides and carbides are produced by the

reduction of oxides and halides by carbon or hydrogeno Having

determined the feasibility of a process, the net enthalpy change,

/\E^J must be evaluated to determine whether the process on

going from reactants to products is exothermic or endothermic

and if gaseous reactants and products are formed, on this basis

the efficacy of a 'closed" or an 'open* system can be assessedo

Thus in the production of boron carbide by the reduction of

boric oxide with carbon according to the reaction:-

the reactionds thermodynamically feasible over the temperature

range I880 - 3100 (ioe. AG®^ is

negative) is ENDOTHERMIC (ioSo A H^^ is positive) and the opera•

ting free energy change is given by:-

A^^T = In p^Q (iv) i.eo is favoured by an open system, however if account is made of the volatility of the boric oxide so that:-

AG^ = AG% - RT In pg^^^ + RT! In (v) this may not be the case, particularly at very high temperatureso

When producing boron carbide by the magnesium thermal reduction

(4if) Figure 1-.8>1 Ellingham diagram of standard free energy change for OXIDES

Change of state Element Oxide M B T

MgO

Si02 C02

^2°3 Ti02

150 —i 1000 2000 3000K Temperature

(44a) Ellingham diagram of standard free energy change for BORIDES

B^C a o s B O

(0 •H O TiB, U o TiB

OR o

1000 2000 3000K

Temperature

(44b) Figure 1.8>3 Ellingham diagram of standard free energy change for CARBIDES

s B^C u 9* B'. o SiC

Q « •H

O H TiC o

.60. —i- r 1000 2000 3000K

Temperature Figure 1,8.4, Ellingham diagram of standard free energy change for CHLORIDES

20i ccn.i

U c: O ---v 3 . U I:

(0 SiCl, o Pi Q •H »4 O TiCl 0 U o

1000 2000 3000K

Temperature

(44d) of boric oxide in the presence of carbon according to the

equation :-

(b) 23^0^ + 6 Mg + C = B^C + 6 MgO

the reaction is thermodynamically feasible over the temperature

range 0 - 2400 °K (ioeo

AG°^ is negative) is EXOTHERMIC (icee A is negative);

and the operating free energy change is given by s-

= ^^""T - ^"^ PB^O^ - PMg ^^^^

suggesting that a 'closed' system is preferred, having in mind

the kinetics of the process^

The remaining method of production, that of the gas phase

reduction of a boron trihalide by hydrogen in the presence of

methane, carbon-tetrachloride or carbon, viz

(d) 4 BCl^ + 8 + CCl^ = B^C + 16 HCl

(e) 4 BCl^ + 6 H2 + C = B^C + 12 HCl are all thermodynamically feasible over a wide temperature range

( > 1700 °K) and are highly EXOTHERMIC, the operating free

energy change is given by :-

AGrp = A G'^T - RT In p^^^^ - RT In p^^ - RT In p^jj^

or CCl^

+ RT In pjj^^ (vii)S,

(45) as there is an overall increase in volume on going from reactante to products, the reaction is favoured by a reduction of pressureo

The formation of boron carbide from the elements - ^ rhombohedral boron and graphite is feasible ig) to a temperature of 3100°K (Figure Io8o2) and is moderately EXOTHERMIC, however the difficulty of achieving thermodynamic equilibrium when both reactants are refractory solids must beemphasizedo Formation by hot pressing (Glaeeer et al 1953; Kranz 1963) can give a wide range of composition probably having dispersed phases of boron or graphite in the only two definitive compounds ^-^^^2 and ^^2^3

(see Section Io2).

The chemical kinetics and mechanism of formation of boron carbide cannot be readily assesed except possibly for the vapour phase production from the hydrocarbon-hydrogen reduction of the halide (Pring and Fielding 1909; Powell et al 1966)0 Samsonov et al (1950, 196€>) have demonstrated that there are two consecu• tive processes in the reduction of boric oxide by an excess of carbon :-

V3vap« * 3 CO(^j = 2B(^j . 3 C02(^) (above 1640«K)

""is) * ^°2(g) = 2 CO(gj and

The newly-formed boron diffuses through the boron carbide layers progressively formed on the surface of the graphite particles,

(^6) finally giving boron carbide particles retaining the original shape of the graphitOo The coefficient of diffusion of boron in graphite is given empirically, as

28,625/T 3o02 e

(Samsonov et al 19^0; Lowell 1967)9 which indicates that the diffusion of carbon in boron is correspondingly much slowero

This is to be expected from the magnitude of the lattice energy as indicated by the relative melting points and boiling points of boron and cajrbon (boron, mopo 2450°K, boPo 3931^K; carbon mopo 4000°K, bopo ^:300*^K); although the boron atom is the much larger of the two (covalent radius, O086 2 for boron5 Oo77 2 for carbon), the former has the higher polarie^bility (first ionisation potential boron, 8o28-electron volts; carbon 11o4l-electron volts) and is more readily distortedo

Magnesium reduces boric oxide, by a similar mechanism at temperatures where both the oxide and magnesium are in a liquid or vapour state, B20^ moPo 723*^K, boPo g3l6^K; Mg moPo 922*'K, bopo 1378^K, and form boron by the reaction :-

V3(vap) * 3 Mg(g) = 2 B(g) ^MgO^^j and the newly-formed boron diffuses into the added carbon

It appears improbable that the carbon does not react with some of the boric oxide, but the net reaction is the same, since

(47) (j) Mg + CO ~ MgO + C

Effusion studies of the volatilisation of boron from boron carbide

solid solutions below their fusion temperatures 2200 to 2600*^K by

Robson and Gilles (196V) and Uildenbrand and Hall (1964) show the

preferential loss of boron to the vapour phase regardless of the

composition of the sampleo The vapour pressure of the boron is

given by :-

log P(^t„j) = 7o506 - (29»630A) (ix)

Verhaegen et al (1962) identified B^C and BC^ as well as

boron in the vapour^ by mass spectrometry, the first being a minor

constituent but the pressure ratio p(B)/p(BC2) = 15 at 2500^Ko

The Hitachi workers in Japan (1966) report that production

of boron carbide by the magnesiiim thermal reduction process is

possible at temperatures as low as G£L IOOO^K when certain

'catalysts' are present, namely, magnesium oxide and other 'inert'

materialso

Considering that the formation of boron carbide takes place

in two consecutive processes by chemical equations (f) and (g),

it appears likely that at moderate temperatures the latter process

involves boron in the solid state rather than as a vapour, as

judged from the vapour pressure given by equation (ix)o Assximing

that the magnesium or carbon (monoxide) reduction of the boric oxide is rapid, the rate-determining step for the overall reaction is the diffusion of boron into the graphite grainso Budnikov and

Ginstling (1965) have constructed a model for reactions of this type based on Ficks law of diffusiouo

(48) Figure Io8o5.

In the general type of reaction A •«- B — =^ AB

(^igure Ie8o5), the thickness of the product layer, x, con• tinuously increaseso The rate of diffusion of A through AB is very much less than the rate of reaction between A and B, so that the concentration of A at the surface dividing AB and B is zero; also the concentration of A at the surface of the grain is constant, as the external resistance of diffusion is much

(49) less than that experienced in the product ABo Applying Ficks

Law to a grain of spherical symmetry, Budnikov and Ginetling obtain an expression for the rate of growth of the product layer AB 2-

dx K dt = (x) x/^d - x/^)

X = thickness of layer AB

t = time

K = rate constant = DC

D = diffusion coefficient of A through AB

C = molar concentration of A at the surface of the grain

^ = proportionality coefficient = p n/ytu.

p ' density of AB

n = stoichiometric coefficient of the reaction expressed as the number of moles of A re• acting with one of Bo

^ = formula weight of AB

r = radius of the spherical grain

Esin and Gel'd (195^) note that dx/dt has a minimum when x/r = Oo5 ioCo X = r/2«

(50) 14

0,2 0<,4 0o6 Oo8 loO

x/r

Figure Io8o6^ Relationship between rate of increase in thickness of product ratio x/r

(51) The relationship between the rate of increase in thick• ness of product layers and the ratio x/r is shown in Figure I0806, from the graph it is seen that the rate dx/dt for thickening of the product AB continuously falls with a chajige in the magnitude x/r from 0 to 0^5 and then symmetrically increases from the minimum value to infinity with a change in x/r from 0^3 to I0

It should be noted that when x/r 0, and x/r —^ 1, the rate of thickening is not infinite, since in these cases the value dx/dt is determined by the rate of chemical reaction between A and B and not by the diffusion processo The equations derived express the relationship between the thickness of the layer of product with time© In practice, it is much more interest• ing to obtain equations for calculating the degree of conversion of the substanceo

Let the particles of reagent B forming the product AB during reaction with reagent A, have respectively at the initial moment and at a certain time from the start of the reaction, a volume V and V , a mass V and V and as surface area So o t o • t and If during time, dt, the reaction occurs over a thickness, dx, in the grains of B corresponding to a mass of dV^ , then s-

dV^/^ = S^d x/^ (xi) or ^ (V^ - V^) = e Fdx hence dV, Fdx and - rS^d x — (xii)

(52) The degree of conversion of the reagent B may be expressed as

OC = 1_ ^ , (^^..j V V o o differentiating gives

^ d = li , u±v) —i iCa*—i— Vo dt dt or 1^ = So o St dx / VZ dt ^^^^ since St = f (oC) and the explicit form of this function So varies with the geometrical shapes of the graino Thus for a sphere f(oC) = (1 2/3

OC ^ 1 . (r > x)^ ^ (^^.3 r for a solid cylinder:

f(0C) = (1 -c^)'*/^

S^. = 1 - (r^2 _ (^^^3 r

The ratio S^/^^ niay be expressed as follows

_ (xviii)

= shape coefficient (sphere ^/3, cube ^/6, etc.)

1 = least thickness of the grain

(53) Equation (xv) can be rewritten :-

doC = « dx f (OL) (xix) dt -^1 dt

to give a general expression :

doC = Xf -(XX; dt

reaction coefficient

f(pC) = shape factor for the grain

Under certain conditions the kinetic equation may indicate

a pseudo-monomolecular reaction (one of first order)o Thus, if

the solid A reacts with a liquid B to produce a solid AB^ forming

with reagent B a mixture which melts at the reaction temperature,

then the diffusion layer of the product AB continuously increases

from the side A and passes into the liquid phase from the side Bo

In this case, the thickness of the diffusion layer of product AB

may have very low values, but is constant over a long periodo

Such a condition may be the state when boron carbide is oxidised

by moist airo

Finally, it should be noted that diffusion coefficient D

from Ficks Law is temperature dependento Hevesy established that:

-E/RT D = Ae (xxi) where A = pre-exponential coefficient of diffusion at a temperature of infinitye (connected with the frequency of the atomic oscilla• tions).

(5^) E = the energy of activation of diffusion

(or the energy of opening up the lattice)o

R B the gas constant

T = temperature ^Ko

This eqxiation^ analogous to the Arrhenius equation for

the velocity of mono<-molecular reactionsg is correct for most

conditions and mechanisms of diffusion so far investigatedo

Therefore the rate constant, Kj in equation (x) should

follow a similar relationshipo It is also pertinent to state

that the equation for the variation of viscosity with temperature

during hot-pressing is^ according to Koval'chenka and Samsonov

(1961)9 governed by a similar expression (see Section Io9)o

Io9 The sintering of boron carbide and other refractory materials

General principles of the mechanism of sintering

In any discussion of sintering processes two systems have

to be distinguished, homogeneous9 consisting of a single component

or components which give continuous series of solid solutions, or

heterogeneous for multiple component systemso It is unlikely that in pure homogeneous systems any significant sintering is ever achieved; even very small fractions of another component assist in their consolidation and are often of the utmost necessityo

Nevertheless, discussion of a pure homogeneous system provides insight into the general principles of sintering mechanismo Homo• geneous systems are taken as being "binder free* and are exempli• fied by borides, carbidesg nitrides 9 silicides and single phase metal powderso

(55) In the homogeneous sintering of a powder, distinction can

be made between two overlapping stages of sinteringo The first

stage is characterised by the formation and growth of bonds, that

is, contact areas between adjacent powder particleso The growth

of these contact areas takes place during the early stages of

sintering, and is manifested by improved cohesion of the com•

pact; where the material is electrically conducting there is

rapid increase of conductivityo During the second stage, the

material is densified and the pore volume decreased^ Under fa•

vourable conditions the latter is practically eliminatedo

At present, surface-free energy is generally recognised as the driving force in both stages of sinteringo The energy required for sintering is supplied by the decrease of surface areas or by the replacement of interfaces of high energy by those of lower energy (eogo grain boundaries)o

Calculations have shown that the surface free energy is sufficient to account for sintering, provided a suitable mecha• nism is available for the transport of atoms involved in the consolidation of powder compactSo The following five mechanisms are possible in the case of homogeneous materials :*

(1) Evaporation followed by condensation

(2) Siurface diffusion

(3) Volume diffusion

(k) Viscous flow (Newtonian flow characterized by a linear relationship between strain rate and stress

(5) Plastic flow (Bingham flow characterized by the existence of a yield stress)o

(56) The first attempt to develop a quantitative theory was

by Frenkel (19^3) who assumed that with both amorphous and

crystalline powders viscous flow would occur under the variation

influence of the capillary forces associated with the curved

surfaces of the pores with timeo The viscosity may be repre•

sented by the equation

= k T / DiY ^ (i)

Where ^ = viscosity

D = coefficient of self-diffusion

T = absolute temperature

k = constant

^Q^^ = atomic volume

The mechanism of deformation of solids by viscous flow

and the role of diffusion in the deformation of crystalline solids

was evolved further by Shaler and Wulff (19^), Nabarro (19^8)

and Herring (I950)e Frenkel's postulate holds for the sintering

of glasseso

At high temperatures crystalline solids can deform at stresses below the yield pointo The rate law and stress- dependence governing their deformation agree with the laws of viscous flow9 60 that their behaviour can be described by a material constant which has the dimensions of viscosltyo How• ever , the actual mechanism of deformation is considered to be the migration of individual vacancies or atoms^ ioCo volume diffusiono The driving force for this migration is the gradient in chemical potential resulting from differences in etresso

(57) The sources and sinks of this diffusion vacancy or atom

migration are grain boxmdaries and the su^rfaces of pores, as

well as the outer surface of the solido The first demonstration

that mass flow by volume diffusion occurs during sintering, was

provided by Kuczynski (I950)o Earlier, Pines (19^6) had recog•

nized that the concentration of the lattice vacancies (Schottky

defects) would be greater under concave pore surfaces than under

a plane surface and had concluded that pores could be eliminated

by the diffusion of vacancies away from the pore in the resultant

vacancy concentration gradiento Yet Kuczynski was the first to

provide quantitative proof of this by comparing the observed time

dependence of neck growth with the time dependences predicted for viscous flow, evaporation-condensation, volume diffusion and sur•

face diffusiouo He showed that in all four cases, the sintering

time, t, to produce a neck of radius, x, should be given by the

form s-

A (T) t (ii)

Where a = particle radius

A(T) = a function of temperatiire only and 2, m 1 for viscous flow

3, m 2 for evaporation-condensation

5t m 3 for voliune diffusion

7, m k for surface diffusion

6, m k for grain houndary growth

(58) Measurements of neck growth between spheres and planes of copper and silver showed that the neck diameter increased as t^^^^ indicating that volume diffusion was the predominant mechanism operatingo With very small spheres of copper (less than 30 micron) however, a deviation from the relationship was observed at the lowest sintering temperatures used, which he interpreted as being due to the increased contribution of sur• face diffusion at these particle sizes and temperatureso

Kuczynski*s model for neck-growth by volume diffusion is shown in Figure I090I9 which shows two spheres in contact, sectioned through their centreso The capillary suction in the neck is then :-

1-1 , a.o

O = surface tension

r = radius of curvature of the neck surface in the plane of the section

\ihen a vacancy is formed imder the surface of the neck, a quan* tity of work, , is therefore done by the capillary suc• tion, where ^ ^ is the volume of the vacancyp and the thermal energy, w, required to form the vacancy is decreased by this amounto Hence :-

Ac = , c = ^ ^ 1 (iv) C C rkT

(59) concentration of vacancies in the unstressed crystal

concentration of vacancies in the neck

This expression is identical with that for the increase in the vacancy concentration around a cylindrical pore of radius ro To obtain an expression for the vacancy flux away from the neck,

Kuczynski assiuned in effect that the surface of the latter could

Diffusion path

Figure Io9o1 Kuczynski model for initial sintering by vacancies being eliminated at the surface of spheres.

(60) be considered as forming one half of the surface of a cylin•

drical pore from which there was radial diffusion to sinks on

the surfaces of the sphereo For this case, the vacancy con•

centration gradient adjacent to the surface is ^ C r In d/^ where d is the distance at which C*^ = C, ln*^/r was assumed to

be unity9 and an equation could thus be set up for the rate of increase in the volume of the neck s-

(x)^ /foY^ ^Dt (v) ^ a^ k T

a = particle radius

D s coefficient of volume diffusion

Alexander and Baluffi (1930) observed that in copper, only pores in the vicinity of grain boundaries closed rapidly, which accords vrith the theoretical deduction of Nabarro ^^^k8) and Herring (1950) that the grain boundaries can act as sources and sinks for vacancies £ind are considered necessary for rapid slnteringe Two alternative mechanisms have been suggested to account for this action of grain boundaries

(1) that they act as "pipe lines" for rapid

diffusion of vacancies to free surfaces

(2) that they act as sinks for the destruction

of vacancies as envisaged in the Nabarro-

Herring mechanism

(61) The densification of compacts by the former process ex• clusively would require that vacancies should diffuse to the outside of the compacto This, it would start from the outside and be a function of compact size, contrary to general experienceo

However, grain boundary diffusion will be expected to contribute to mass transport over a short distance during sintering, as evident in the sintering of alumina (Coble (l95@))o The second mechanism provides a means for the destruction of vacancies within the compacto Kuczynski's original model assumed that vacajicy sinks are confined to the particle surfaces and could only account for ddiisification so long as the pores remain open and inter-connectedo Kingery and Berg (1955) showed that the observed rates were too rapid to be accounted for by neck growth due to volume diffusion, and proposed a model (Figxire Io9o2o) in which a grain boundary existing between two spherical particles is considered to act as the vacancy sinko

Atoms would then flow from the vacancy sinks to the neck surface, as indicated by the arrows, and thus^ by spreading out of material at the neck, cause the particles to coalesceo Subse• quently Coble and Ellis (1958, 1963) concluded that the deforma• tion produced by plastic flow is limited to a fixed value deter• mined by the hot hardness of the material, limited as a mechanism to ^ 8k% theoretical density© further densification is ascribed to a diffusion process similar to that of heat transfero Vasilos

(62) Diffusion path Grain boundary

Figure Io9o2o Kingery and Berg model for vacancies eliminated at grain boundary at neck

and Spriggs, (1963) have applied the Coble model for the hot pressing of magnesia and find a logarithmic time law of densi* fication :-

= 1 kD In t (vi)

p = relative density t„ = time at which pores vanish, valid up to relative densities of ^90%

(63) Figure Io9o3o Mackenzie and Shuttleworth model

Homoge

The Mackenzie and Shuttleworth theory (19^9) rests on a model consisting of closed pores in a homogeneous matrix

(Figure I^9o3)o This theory is really only valid for the final stage, of sintering where closed pores first appear ( ^10% porosity)g however, the theory agrees with experimental results for sintering stages at even 359^ porosity, ioeo when the pores are inter-connectedo They obtained the following expression :-

dp dt

l-a/l.l\^ In f 1 N (vii)

(.6k) p = relative density

^ = surface tension • • •(••'••)

n = number of pores per unit volume of material fV = coefficient of viscosity

'Xc= yield stress

It can be shown that:-

3 c 1 (viii) r3-3

r^ = pore radius so that for pressureless sintering s- (4 (ix)

For pressureless sintering, the driving force is the pore pressure

S^/r^ For hot pressing there is an additional external pressure

Po It can be shown that s-

dP \ + 3 P (1 - p) (x) d t /

P :§> 2t/ rj (xa)

and Tc (xb)

(65) For instcmce, If the radius of the pores r^ is 1 micron

do"'* cmo ) and the surface tension of the material, ^ , is 2 7 1000 ergs/cm , then the material pore pressxire is 2 x 10

dynee/cm^ 20 Kg/cm^o A normal value for P is 500 Kg/cm^o

Using relationship (xa) and (xb), the hot-pressing

equation may be simplified^ as the terms contained 2 /r^ and T

can be eliminated so that the rate law of the hot-pressing pro•

cess may be written as

dif. = 3 P (1 - ) (xi) or, integrated with respect to time :-

In 1 - ^ = - 3 P . (xii) when^= when t=®

The plastic flow theory succeeded In accounting for several experimental characteristics of the hot-pressing process. The theory explains the effect of external pressure in reducing the sintering temperature necessary for densificationo It further explains the effect of pressure on the end point density at con• stant temperature, the effect of pressure on the sintering rate and the influence of particle size on the end-point densityo At the end point S O

The hot-pressing equation (xii) given by Murray et al

(195^) has been \erified by Mangsen et al (196O) hot-pressing alu• mina, by Vasilos (196O) for silica and by Jaeger et al (1962) on

(66) other ceramicso . From their studies of binder-free hafnium,

./V; zirconium and tantalum, Lersmacher and Scholz (1961) state that

{ the plastic flow theory can describe the early stages of hot-

pressing, but that deviations occur at longer times, particularly

at higher temperatureSo They conclude that the density after

long sintering times (e^go 60 minso ) does not continuously in•

crease with temperature, but reaches a maximum at a specific tem•

perature o This is related to grain growth during sintering?

grain growth is directly controlled by impurities such as the

binder metals, iron, cobalt and nickel in heterogeneous sin•

tered ceramics^

Again it was clear from the many experimental results

that the initial sintering rate is often greater than that pre•

dicted from the plastic-flow equatiouc

A completely new approach was made when Koval'chenko and

Samsonov (1961) proposed their hot-pressing equation, based upon

the mathematical framework of the rheological behaviour of a dis•

persed system; later, a correction for grain growth was introduced

Their model is a system consisting of a gaseous phase in

the form of globules dispersed in an incompressible viscous medixim,

In addition, the concentration of the dispersed phase, Q, (equiva•

lent to 1 - ), is 80 small that terms involving need not be

consideredo In such a system

V2 + 1 —— (xiil)

(67) Where Q = porosity

= the volume of the viscous medium

= the volume of the dispersed phase, ioeo the volume of the pores«

V = = total volume of the system

The following relation between the material constants and ^ ia valid for a porous fluid system

C = - Q) (1 2Q) (xiv) ^ / Q(3 - Q)

Where f\ = viscosity of the medium = Ist coefficient of / viscosity

^ = 2nd coefficient of viscosity

Since Q is small, the rate at which work (W^) must be done to deform the system, may be written s->

Wd = Sis)(f^V (XV) and equated to the external work (W^^^^) supplied for the compres- sion of the system, ioeo

W = -P d\L - "^ds Vdt Vdt

2 = (--i) a

Where P = external pressure

= surface tension

S = total pore Area

dV/V c relative volume change of the system

(68) Neglecting the surface tension term, as In the Murray theory^

gives :-

- P =C LdV \ — (xvii)

hence dV a dQ , J^JN — ^2 ^ (xviii)

and = - P o Q (3 - Q) (xix) dt ?y (1-2 Q)

Since ^ for a viscous body is constant with respect to

time, we have upon integration

Pt = IQ (3 - Q„)^6 Q^V3 TTf) ° ^-1 (xx)

Where Q = for t • ®

According to Nabarro (19^) and Herring (1950) the viscosity is a

function of the grain size, since the viscous flow of a poly-

crystalline body depends upon the self-diffusion between grain boundaries subject to tensile and compressive stress© Thus s-

Yo^^ (xxi) o

Where k =: Boltzmann^s constant

R = average grain radius

D = coefficient of self-diffusion

= atomic volume

for metals and metal carbides the grain-growth is time dependent as (Burke and Turnbull (1952) Lersmacher et al (1962) )g

(69) = R2 (1 - bt) -(xii)

= grain radius a t = 0

b B rate constant C= K V

The time-dependence of the viscosity becomes

2 (t) = + bt) kT 2. (xxii)

\fj^ (t) = ^iX * bt) ^ . (xxiv)

Where fj ~ t = 0

Equation (xix) may now be vrritten:

d2 = - P Q (3 • Q) — • (xxv) dt 'ffi^^d bt) (1 - 2Q) or integrated *rf.th respect to time s-

^ In (1 + bt) = In (3 - Q ) ^3 Q O 4 n b % -2 (3 - Q)^^ Q-"/^

= F(f) (xxvi)

The plots of F(f} versus ln(l -f bt) for tujigsten carbide hot-pressed at 2300^0 and various pressures are linearo

Ao pointed out by Scholz and Lersmacher (1964) this last equation may be simplified to 'Murray form* noting the relation

= it <1 - aQ) ' (xxvii)

(70) a = constant (varies between different authors)

ioCo does not contain any second-order term in Q as in equation (xxvi)

da o 3P o (1 -Q) Q (xxviii) dt 1 - aQ for a = 1

d2 = 3P o Q —(xxix)

dt ?iy so that equation (xxvi) becomes s-

In Q (1 - Q ) + a In 1 Q = - 3P In (1 + bt)

V^^^ ""^^ 9-'> - (XXX) for a = 1 ~ 3 P " T>X\ 15 Q = qJX + bt) = Q^(l + bt) —2R2 b

= Q^(l + bt)"^"^*^ (xxxi)

The densification process during hot-pressing is thus described by a hyperbolic rate equation based on the Nabarro-Herring mechanismo

In applying the above theory to a polycrystdline« porous body, T^^ (•'^igure Io9o^) stands for the average distance from the pore centre to the surrounding grain boundarieso ^he apparent viscosity of such a body increases with the square of grain size as (1 bt) to a final value9 the diffusional viscosity of a single crystalo This is known to be extremely smallo Therefore densification by the Nabarro-Herring mechanism can operate at useful rates only for a moderate grain sizeo

(71) This last equation has been established empirically by

Scholz and Lersmacher (196^^) in studies on the hot-pressing of

pure metallic carbideso Theyput s-

Q = Q^(l + Btr° —. (xxxii)

where B is an empirical constanto The exponent n in equation

(xxxii} is a constant of the material and is obtained from the

slope of a plot of In Q versus ln(l+ Bt), they found a linear

relationship between In Q and In t, ioCo o - k 2 (xxxiii) dt t

As pointed out by Hcholtz (I965) equation (xxxii) is not

satisfied at t s 0 where Q must equal unless an additional

constant is usedo In a plot of InQ versus In t, there should be an asymptopic approach to Q^, this was evident in McElland and Whitney's (1962) work on tin powdero

I.IO0 The chemical reactivity of boron carbide and related compounds

The paucity of information on the reactivity of these compounds and the often conflicting available data results from the general inertness of these materials, particularly to non- oxidizing reagents, and the significance of the state of sub• division and the purity of the materials xinder examinationo

When considering the suitability of these refractiories for ap• plication, the reactions of greatest importance are those of oxidation, nitidation, and their degree of compatibility with metals and other refractory materialso In materials highly re-

(72) sistant to chemical attack, eogo oxididation, the reaction is

extremely slow^ even when the process is shown to be thermodyna-

mically feasible, (see Section Io8)o ^he apparent resistance to

attack often results from the low volatity and refractoriness

of the oxidation products or from the impervious nature of the

oxide film, which inhibits further reactionq eogo wHen metal

carbides suffer oxidation^ carbon is removed as the gaseous

oxide and resistance to attack depends on the protective nature of the metal oxide layero However, when a boride or a silicide is oxidised by air, oxygen or other oxidative gas, a vitreous phase of the nonmetal oxide covers the material and inhibits attacko Thus, boron carbide shows an increase in weight when heated in dry air to temperatures below lOOO^C, but above this temperature there is progressive loss in weight as the boric oxide shows significant volatitya In moist air, loss of weight occurs at a much lower temperature due to the removal of boron as the more volatile boz^c acido It follows that a study of weight change has little value unless cogniscence is taken of the exact composition and state of the reaction productSo

Oxidative attack by acidic and basic fused media and solutions is assisted by the removal of the reaction products into solution; boron carbide and most borides react spontaneously with fused alkaline metal hydroxides, nitrates and persulphateso

The action of elemental heilogens on boron carbide (and other borides) is a highly exothermic process after initiation giving the volatile boron trihalide and carbon (or the metal halide)o

(73) Niridation by the element or ammonia gae is only significant at very high temperatures,It was reported in "New Scientist",

1963« that boron carbide forms boron nitride and graphite when hot-pressed in a nitegea atmosphere to 2000^0; metal borides form the metal nitride and boron nitrideo

The reaction of boron carbide and other borides with molten metals is controlled by the contact auigle the metal makes with the surface of the sintered compacto A zero contact angle, complete wetting of the surface, shows high reactivity between the two phases9 whereas, a large angle indicates little or no reactiouo Table I0IO0I0 gives the contact angles of some re• fractory phases with metals in the molten state (Figure I0IO0I-)

Figure X0IO0I0 Surface energy relationship between solid, liquid and vapour interfaces (after Sutton).

Vapour Liquid Liquid

Solid 0> 90* a) Wetting ©< 90° b) Won»wetting

VsL surface energy Q^lid liquid interface VsV " " sbiid vapour " '^LV " " liquid vapour "

SL SV LV cos

(7'f) TABLE lolOolo

Contact angles of some molten metals with refractory boron compounds.

Phase Metal Temperature Reference

\o Zn 5^0-620 l2lo5 - 119 Samsonov 196O

It Cu 995-1090 130 - 17 If

tt Al 600-670 117 - 118 ti

tt ft Pb 225-395 121 - 113

If ft Brass 905-950 54o5 - 30

If Fe 1780 strong reaction Hamijan 1952

11 Co 1780 90 ti

tt Cr 1820-1830 0 Janes ^ Nixdorf

CrAil I^OO-l'flO 0 1966

11 Mi 1^70 k^ It

TiB2 Cu 1100-1500 158 - 154 Eremenko 1958

It Ni 1W0 100 - 38«3 II

Cu 1^50 50 II

II Ni 1^80 11 II

ZrB^ Cu 1160-1400 123 - 36 II

(75) loll. The applications of Boron Carbide and related refractorieso

The principal applications of these materials result from

their two main properties 1) refractoriness and 2) favourable

mechanical strength and hardness© For example, boron carbide

is the hardest and most abrasion—resistant material available

in massive fonso ^r some 25 years it has been used for sand•

blasting nozzles, mortars for grinding^ as a grit in grinding

polishes and wheels, and for die, spinnerets and gaugeso A

recent application has been the production of light-»weight

armour on account of its hardness combined with high strength,

high elasticity and low densityo ^ince boron has a high nuclear

cross-sectio]^ the carbide has been used for control and shield materials and as burnable poisons in nuclear reactorso Boron carbide with a boron content of 78 wt-^, has the interesting anomalous property of a higher boron density than elemental boron itself; it is also more economical to produceo Another feature of the material is its electrical conductivity, suitable for its applications as a thermocouple, electrodes in cells and spark- erosion and for resistanceso One serious disadvantage possessed by boron carbide, which limits its application, is its poor thermal shock resistance9 one-half of the heating cycle, it shatters when cooled rapidly; composite materials containing silicon carbide show greater promiseo Whiskers of boron carbide, single-crystal fibres, show extremely high strength and modulus and have been investigated as a possible reinforcement for metal and resin matriceso

(76) The utilization of other borides has been limited by

their poor oxidation resistanceo Nevertheless^ their use in mixed systems with silicides has been proposed in aircraft com• ponents at high temperatureso Both titanium and zirconium diborides have electrical resistivities compeLrable to copper and

extensive tests have been made of these borides as electrodes

for aluminixim reduction although intergranular corrosion and

their brittleness has caused failurco Many metal borides have application as heterogeneous catalysts in organic syntheses

involving hydrogenatioua

The present work has been stimulated by the need to

produce boron carbide of a controlled quality for use as an abrasive grit by growing it to size, and for a material capable of sintering with a minimum of pressure and temperatureo Also

consideration has been given to the formation of wear-resistant

surfaces on metal bases such as titanium by the in situ formation

of their borides from boron carbideo

(77) SECTION 2 - EXPERIMENTAL TECHNIQUES FOR THE PRODUCTION AND SUBSEQUENT ANALYSIS OF BORON CARBIDEo

The section covers the semi-technical production of

boron carbide by the magnesium thermal reduction process and

the experimental techniques used in determining the phase com•

position, crystallite and aggregate sizes of the material; and

the principles underlying themo

The production of boron carbide - B^2^3

Boron carbide, B^2^3i prepared by the magnesium

thermal reduction process first described by Grey (1951) and

later developed by Samsonov et al (196O )« The starting materials

were; anhydrous boric oxide, B20^ (Borax Consolidated 20 Mule Team

Technical Grade), granular magnesium turnings (Magnesium Elektron) and carbon black (Cabot, Sterling SO fluffy)o

The boric oxide and the carbon black were mixed together

in the ratio of their stoichiometric proportions (l7o5 kg B20^ +

1o25 kg Co) by milling in a steel ballmill having "hardmctal*

spheres for 2^ hours to ensure complete blendingo, A stoichig^^

metric amount of magnesium (18 kg) was blended gradually with the

other componentSo Trials had indicated the difficulty of reacting

large charges of the mixture ('"•^VO kg) owing to the powders sift•

ing and the reaction propagating slowly when initiated by a hot•

wire fuseo It was found necessary to bind the reactants together

by mixing in a small qusmtity of kerosene, this served to bind

the components together in their correct stoichiometric propor•

tions and also provide a blanket of vapour so preventing oxida•

tion of the hot products by the airo Successive runs were com-

(78) pleted with a high degree of control, unlike the experiences of the Hitachi workers (1966) who reported explosive reactions and added a diluent to the reactants, eogo magnesia, MgO.

Temperatures in excess of l600°C were measured within the charge© When cool, the charge was removed from its iron crucible, the crust was discarded and the remainder crushed down to a coarse powdero The crushed powder was added to an excess of diluted sulphuric acid (25 volo%o 1o84 SGo H2S0^) to dissolve out the magnesium oxide and decompose magnesiiim boride and any unreacted boric oxideo It was found possible to decant off the liquor together with any unreacted carbon from the boron carbide sediment<, Successive washings were made with warm water, decanting off the bulk of the liquor and finally filtering through a filter-paper supported on cloth (Whatman Nol ) and washed with copious amounts of water until free of sulphate iono The boron carbide cake was dried in an oven at 120^0 for several hours and then milled to a fine powdero

2o2(a) The chemical analysis of boron carbide and related compounds

As indicated in Section IdOthe refractory compounds of boron resist attack by many chemical reagents, particularly in aqueous mediae Therefore, fairly drastic methods are required for their dissolution before chemical analysis for boron and other elements, Blumenthal (1951) describes methods for the analysis of many of these compoundso

(79) For the determination of boron the sample is first mixed with anhydrous sodium carbonate in the ratio of 1 part to 20 parts of the alkali, the mixture is heated to fusion in a platinum dish, previously lined with sodium carbonate to prevent attack on the metal by elemental borono A powerful exothermic reaction normally occurs through the oxidation of the boron and other components, but it is sometimes necessary to add some sodium nitrate to the melt to assist the oxidationo

On cooling the fused melt is leached with dilute hydrochloric acid and the solution made up to volumeo The borate ion in an aliquot is deteraixned either titrimetrically with sodium hydroxide by the mannitol method or by flame photometry using an atomic absorptiometcro In both techniques it is necessary to remove interfering cations by ion exchangee In the present work both methods of analysis were employedo

The titrimetric method of analysis for boron is based on the method of Kramer (1955) using a cation exchange in the hydrogen form (Permutit Zeocarb 225 meshrange 'fO-So) and de• termining the equivalence point of neutralisation with a pH meter (Pye)c Firstly the stronger hydrochloric acid is neutra• lised with sodium hydroxide to pH = 7oOO, then excess mannitol is added to release hydrogen ions by the following reaction:

H^BO^ + CgH^^O^ = fl^BO^emann + + H2O

Orthoboric mannitol hydrogen acid ion

The titration is then continued to the same pH valueo

(80) Alternatively, the eluate from the ion exchange column is analysed for boron by atomic absorption spectroscopy using a Hilger and Watt Atomspek spectrophotometero The solution is excited in an acetylene/nitrous oxide flamco Using a boron lamp the wavelength is set at 2^98 A and the attenuation of the radiation is observed for samples and for standard solutions of borono

Other elements were determined qualitatively by emission spectrography using a Hilger and Watt Large Quartz spectrograph, and quantitatively, (with the exception of carbon) by atomic absorption using the appropriate element lampo Emission spectro• graphy has application in the detection of most elements, only those light elements having high ionisation potentials, eogo nitrogen, oxygen, the chalcogens and the halogens are not de• tected when excited in the usual mannero (The solid sample is excited in the usual mannerj The solid sample is excited by arcing between copper or graphite eletrodes (Johnson-Matthey- specpure) and recording the spectra on a photographic plate

(Ilford Ordinary N30), exposures being taken at two spectral ranges; viz, above 2760A and above 2200& to record all of the most sensitive emission lines of the elements in question.

Quantitative analyses were carried out for the various elements detected by qualitative analysis, by measurement with the atomic absorption spectrophotometer (Hilger»Watt

Atomepek) of the solutions obtained from either alkaline or acid fusion (potassium pyrosulphate), comparison being made with prepared standardso

(81) 2o2(b) X-ray diffraction identification of phases

A comprehensive summary of the theory and practice of

X-ray diffraction techniques is given by Peiser et al (1960)0

A crystal consists of a regular three-dimensional array of

atoms in spaceo Points having identical surroundings in the

structure are called lattice points, and collection of such

points in space forms the crystal latticec If neighbouring

lattice points are joined together the unit cell is obtained, ioeo the smallest repeating unit of the structures Sometimes it is more convenient to choose a larger repeating unit, for ex£uttple a centred cello In general the shape of the unit cell is a parallelepiped, but, depending on the symmetry of the crystal, it can have a more regular shape, Cogo cubic or rec- tangularo The shape of the unit cell is completely described by the lengths of its three edges or axes and the angles be• tween themo Conventionally, the axes are termed x, ^, 2 or

b, c, and the angles Od, (3 9 ^ ^ CC being between the ^ and z axes, etCo

Crystals can be classified into seven classes according to their symmetry as shown in Table 2o2o1.

(82) TABLE 2o2o1

Crystal Glass Conditions limiting cell Minimum 5i5£55i22 sjmmetrj

Triclinic a = b = c Ksii ^ =^ = 90^ None

Monocllnic a = b = c ©C4 ¥ = 90^,^= 90* One 2°fold axle or 1 pps*

Orthorhonibit a = b = c p = '2^= 90° Two 2-fold axis or 2 pps*

Tetragonal a = b c o6= p = ^ = 90° One ^-fold axis

Hexagonal a = b c 06= ^ = 90°^= 120* One 6-fold axis

Rhombohedral a = b c oL= ^ = 1^ = 90° One 3-fold a^is

Cubic a = b = c oC= p^H^ = 90° Four 3-fold axis

pps perpendicular planes of symmetry

Various sets of parallel planes can be drawn through the lattice points of a crystalo Each set of plants in identified by a set of three integers, namely, the Miller indices, iii k, 1 corresponding to the three axes a, b, c, respectivelyo The index, h, is the reciprdcal of the fractional value of the Intercept made by the set of planes on the a axis5 etCo When a crystal interacts with an incident beam of X-rays, the lattice can act as a diffrac• tion grating, because the lattice's dimensions are of the same order of magnitude as the wavelength of the X-raySo The diffracted beam which emerges, in phase, from a particiilar set of lattice planes obeys Bragg's Law<,

(83) X = 2d sin e- (i) where X = wavelength of the incident X-ray beam d = interplanar spacing ©• = angle of incidence (diffraction)

When the crystals are large and have a large number of lattice planes in each set the diffracted beam appears at a sharp angleo With aggregates of small crystals broadening occurs and the extent serves as a measure of crystallite sizeo The interplanar spacing, d, is related to the unit cell dimen• sions of the crystal and to the Miller indices of the set of planeso Thus, measurement of Bragg angles leads to the deter• mination of the lattice constants^ If a single crystal of a substance is rotated in a beam of monochromatic X-radiation, the diffraction pattern forms a series of spots on a photographic plateo However, if the sample is in the form of a crystalline powder or a sintered compact, the crystals in which show random orientation, the diffracted beams lie along the surfaces of a set of coaxial coneSo The pattern can be recorded using either an X-ray diffractoraeter with electronic ionisation detector and chart recorder^ or a powder camera with photographic film which gives a series of concentric ringSo The distribution with respect to the Bragg angles and intensities of the diffracted beams is characteristic of a particular structure and can be used as a means of identifica• tion, as a fingerprinto The X-ray powder diffraction patterns of most crystalline substances in their various allotropic

forms, are recorded (AoS«ToMo Powder Diffraction File in a

card form)o The powder pattern of a mixture of crystalline

structures consists of the superimposed patterns of the in•

dividual structures., It should be noted that due to instru•

ment characteristics9 and orientation effects displayed by

the sample, the relative intensities of the lines forming

these patterns do not always correspond with those of the

AoS,T<,M« cardo'.

In the present work. X-ray powder diffraction patterns

were determined usxng a Berthold diffractometer fitted with a

gas filled proportional counter fitted to a combined discri-

minator/ratemeter and EHT supply and having a chart recorder

outputo The source of X-radiation, copper KX of wavelength

105^28, was an Hilger and Watt's constant voltage generator

fitted with a sealed tube Philips copper target having nickel

filters to remove the Kp componento The generator was operated

at a standard kO Kilo volts potential, and a filament voltage of

20 milliampso

Loose powders were examined by mixing with a cellulose acetate cement and coating a suitable amorphous (X-ray trans• parent) material such as a glass cover slidoo Sintered materials and metal samples were machined with a flat face and mounted directly on to the goniometer. Phases were identified by re•

ference to the appropriate A.SoTeMo tables or to reference samples measured in the same mannero

(85) 2o2(c) Electron microscopy and diffraction

Comprehensive accounts of the theory and practice of

electron diffraction and its application to high level magni•

fications (microscopy) are given by Zworykin et al

Kay (1965) and Hirsch et al (1965)0

^ beeun of electrons possesses wave properties similar to those of a beam of electromagnetic radiation, the wavelength being given by the de Broglie relationship:

X = h - h (i) p mv

where X = wavelength

h = Planck's constant

m = electron mass

V = velocity

p = momentum

If the accelerating potential difference is V, the energy E, of an electron is given by

E = ^ mv^ = Ve (ii) where, e = electrostatic chargeo

Combining equations (i) and (ii) and eliminating V gives:

X = h (iii) /2me V

(86) SmcJl A relatively/^correction has to be applied to equation (iii) to account for the variation in the mass of the electron with velocity, which depends on the voltageo In practice, however, the wavelength, if required, is determined by recording the diffraction pattern of a substance with known unit cell dimensions and calculating a single factor, the camera constant, LX, where L is the effective camera lengths If the same in• strument is used at the same accelerating voltage, then L\ remains constanto At an accelerating voltage of 100 kilo volts

the wavelength of an electronic beam is Oo037 Xo

The electron microscope is constructed on similar prin• ciple to the optical microscope, but its optics or lens system is comprised of a series of magnetic or electrostatic fields of varying intensity and direction through which the beam passeso The shorter wavelength of electrons compared with that of visible light enables a much greater resolution to be achieved by the electron microscope; the theoretical limit of resolution of a microscope is given by half the wavelength of the radiation

used * o02A for the electron microscope and 2000 S for the optical microscope, a gain of lO^o Figure 1 gives the general layout of a typical electron microscope^ The object to be studied, by transmission, is placed in the focal plane of the instrument and its magnified image viewed directly on a phosphor screen or recorded on to photographic film held in a cassette within the instrumento Normally, both facilities are employed, one to align and select, the other to recordo The energy of electrons ie reduced when they are scattered readily on collision

(87) with gas moleculeso HcncCg the microscope has to be operated at a sufficiently low pressure to increase their mean free path^ thus a vacuum of about 10"^mm Hg is requiredo

Diffraction patterns of the samples viewed on the elec• tron microscope can be obtained by interposing a special lens system between the objector and the projectoro . (*'igure 2^201) such diffraction is possible only when the crystals or their aggregates are very thin and/or are composed of elements of low mass number as the electrons are highly absorbed by matter. The diffraction pattern of fa single crystal consists of reflec• tions from a plane of recipro.cal lattice point, here, electrons, because of their very short wavelength differ from X-rays^ Nevertheless for a polycrystalline specimen in random orients- tion, the diffraction pattern consists of the typical concen• tric ringSo

Figure 2Q2O1O

F Filament A Accelerater C Condenser Lens G Sample Grid 0 Objective Lens D Diffraction Lens I Image C Camera P Projector Lens S Screen

(88) In the present work, direct transmission and diffrac•

tion micrographs of powdered materials and transmission micro•

graph surface replicas of sintered refractory materials were

made using a Philips E<,Mo100B electron microscope, see Van

Dorsten, Kienwdorf and Verhoeff,.M95O0!

Specimens were supported by a carbon film laid on a

copper grido The carbon film was produced initially by evapora•

tion from a pair of carbon electrodes struck by a Do Co Arc on

to a clean mica surface inside the vacuum belljar of an Edwards

Speedivac High Vacuum Coating Unit 12E6o The carbon film of

roughly 20o8 thickness was floated on to distilled water and

each copper grid coated with the filmo A copper grid consists

of a 3 mm diameter of # 200 square mesh foil, each hole has side

length of 100 /^m, the width of the sashes forming the window vary in size to give a pattern which assists in the location of a particular area of the grido

Samples for direct transmission were prepared by dispersing the powder in water or other unreactive liquid medium together with a suitable deflocculating agent, for boron carbide addition of ammonium hydroxide solution (10% volo/volo) proved to be adequate for dispersiono A portion of the solution was placed on the prepared grid and evaporated to dryness under an infra-red heat leimpo

Samples prepared as replicas were prepared by pressing a gob of melted clear polystyrene polymer or by coating with a PVA emulsion on to the sintered surface, an initial impression served

(89) to remove any adherent material from the surface, until it solidifiedo The film replica so produced was then shadowed, ioeo coated with a film of platinum in the coating unit in such a manner as to 'highlight' the surface structurco The polymer was dissolved away, using chloroform for polystyrene and dilute

HCI for PoVoA„, and the film placed directly on to a copper grido

The grids were placed in the sample holder and placed in the micro• scope through a vacuum seal at the focal plane of the lens syetemo

After selection of an area of interest at the required magnifica• tion the film was exposed for a number of seconds depending on the intensity of the imageo Magnification is determined by re• ference to the voltage applied to the focussing optics and a calibration charto On completion of a set of exposures the film camera was removed from the microscope and the film, 35mm (Kodak

Fine Grain Reversal) developed in a fine grain developer to give optimxm contrasto Prints were made from the * negative' on to

Kodak bromide paper noting the magnification factor of the en• largement from its negative, in this manner magnifications of over 70,000 times the original size were obtained.

2o2(d) Surface area measurement by gas sorption

The amount of a gas adsorbed by a substance depends, inter alia, on the specific surface, ioeo the surface area per unit masso

Therefore, gas sorption measurement provide a means of determining the average particle size of a powder of regiaar shape and simple geometryo A general account of the technique is given by Gregg and Sing (l967)0

(90) The method most widely used is that due to Brunauer,

Emmett and Teller (1938)o The B«£.T. equation states :-

P 1 o E_ 1 (i) x(po-p) XmC po XmC

where p = pressure of adsorbate vapour in equilibrium with absorbent po = saturated vapour pressure of vapour adsorbed X = amount of vapour adsorbed xm = capacity of filled monolayer c = a constant

Figure 2o2o2o

II III

/ / U O (0 f;o loO loO (d 0 3 O Q <

loO loO

Relative pressure (*p/po)

Five types of adsorption isotherm in the B.E.Tp classification

(91) Adsorption isotherms are classified into five types

as proposed by Brunauer, Deming, Deming and Teller (BoE.T.

classification. See Figure 2o2o2o Of these, only Types II and IV can be used to calculate the specific surface of the ad• sorbing solid and only Type IV for making an estimate of pore size distributiono However, Type II isotherms give best agree•

ment with the BoEoT. eqtxation over limited ranges of relative

vapoxir pressure (Gregg (l96li Po31t56)o Thus a plot of p x(po-p) versus £_ results in a straight Jine graph of slope C - 1 po XmC

and intercept 1 « Elimination of c from these two expressions XmC gives too The adsorbate can be measured either gravimetricfi0.1y or volumetrically (tensiometrically) (Gregg and Sing, 1967,

Po310)o

The specific surface, S, is related to Xm by the equation: S s: Xm o No Am (ii) M where M e molecular weight of adsorbate N s ArogadroB number Am = cross-sectional area of an adsorbate molecule in a completed monolayer

For a substance consisting of cube-shaped crystallites, the average particle size, 1, is related to 5 by :-

S = _6_ (iii) PI where P = density of the adsorbent

(92) The same relationship is valid for calculating the equivalent spherical diameter, similar relationships can be derived for plate and needle-shaped crystallite powderso The sorption balance used in the present work Is bj^'ed on the design by Gregg (l9^6, 1955o) The balance used for low tem• perature nitrogen adsorption as described by Glaeson (1956) and B.S, ^359, Part I (1969) and is shown in ^igure 2o2e3o The whole assembly is encased in glass allowing it to be evacuated and filled vrith any desired gaso Balancing of the beam is affected by applying a current to the external solenoido The instrument is calibrated by measuring the current required to observe the null-point for known loadings on the balance pano The samples for which surface areas were to be measured, were placed, in turn, into the bucket and outgassed to remove physically adisorbed vapour, this was undertaken at 200°C to remove possible moisture (Glasson, ^^Gk)o The adsorbate was nitrogen gas and the coolant, boiling liquid oxygen, so that the isotherms were measured at -l83^Co The weight of the sample was determined in vacuo, and aliquots of nitrogen gas were introduced into the systemo Simultaneous readings of sample weight and the nitrogen gas pressure were taken when equilibritun was reachedo A final reading was recorded at room temperature and one atmosphere pressure of fnttrogeno (The IBM Prograiane for computing the surface areas r;aa. that

'devised by P.O'KeiH and Denise Harris (Plynouth Polytechnic^ and is detailed in Appendix II* i

(93) Figure 2o2o3a

The gas sorption balance

Magnetic balance limb

Sample Limb Balance arm Bucket for adsorbent Frame Connection to gas handling system M Permanent magnet or solenoid N - Solenoid 2«2(e) Thermometric analysis

There are a number of analytical techniques which come

under the title of thermometric analysis of these perhaps two

of the most widely appliede One, thermogravimetric analysis

(T.G.A.) is a technique whereby the weight of a sample can

be followed over a period of time while its temperature is

being changed, usually it is increased at a constant rate;

it is particularly suited for measuring the loss of weight

on the decomposition of a solid to a solid residue and to a

gas, and also Indicates the termal stability of a compound in

both inert and in reactive atmo6pheres«

A thermobalance consists of a modified single or.double

pan analytical balance (Gregg and Wineor, 19^5)o In the pre•

sent work, a Stanton balance was used having a crucible sus•

pended into an electric furnace, via a nichrome wire support

from the weighing paUo The furnace winding was energised

through a linear variable programmer (Stanton-Redcroft) and

temperature was measured using a Pt/Pt.-13%Rh thermocouple attached to a millivolt meter. (Baird and Tatlock-Resllia).

The general set up is shown in Figure 2e2o4,

0»3g of the powder under test were accurately weighed into an alumina crucible (Thermal Syndicate Thermal Alumlno), attached to the balance and the weight notede The thermal pro• grammer was set to give a linear rise (2oVmin) from the ambient to the set temperature for the run, and the weight of the cru• cible recorded at known intervals of timeo In the present work the reacting gas was air, the relative humidity eaid the ambient

(95) Figure 2o2o4, Thermal gravimetric analysis apparatus r

(ir~i

-i-i.

-wm A - Analytical balance B - Furnace C - Controller

(96) temperature being recorded throughout the run© Samples of

boron carbide and other borides were heated between 20° and

1100°C; isotherms being studied in the temperature regions

showing pronounced weight change in the specimen© The re•

action kinetics were determined from the slope of logarithmic

weight changes plotted against the logarithmic time as dis•

cussed in Section loSo The activation energy of the oxidation

by air of boron carbide was estimated by the Arrhenius equation©

A second method of thermometric analysis is that of

differential thermal analysis, D.T»A<,, it was first applied by

Houldsworth and Cobb (l922-5). Thermocouples embedded in the

test and inert materials were connected in opposition so that

any appreciable EoM<.F<, set up during the heating resulted from

the evolution or absorption of heat in the test seunple and the

temperatures at which such changes occured were noted© In the

present work, DoT«A« equipment based on the design described by

Grimshaw, Heaton and Roberts (19^5) was employed - Figure 2o2o5o

Oo5g of the sample was packed alongside the reference material,

recrystallized ^"12^3 ^ ceramic block© The block was heated

in an electric furnace ( Griffin and George ) controlled by

a linear programmer, (Stanton-Redcroft) heated at a rate of 10

to 20°/mino The temperature of the block and the temperature

differential were recorded automatically over the temperature

range of 250 to lOOO^C in order to determine the temperature

required for the onset of oxidation of boron carbide and re• lated materials by air©

(97) Figure 2o2e3c DoToAo apparatus

Ceramic Sample Block iff

R - Reference S - Sample T - Thermocouples

n II

o o 1 1

F - Furnace P - Linear programmer M - Multipen recorder

(98) SECTION 3 - EXPERIMENTAL.RESULTS- OBTAINED ON THE PREPARED BORON CARBIDE AND ON RELATED MATERIALS

The section includes resiilts of the analysis and the subsequent'treatment of the prepared boron carbide to determine its cheinges in surface activity, crystallite emd aggregate sizes on sintering.

3.1 The analysis of the prepared boron carbide

3«l(a)* The chemical analysis showing the composition of the boron carbide with regard to its stoichiometry and to its purity is given in Table 3«1o1o

TABLE 3o1.11.

^\Sample No, 1 • 2.

Element wt. % at. % Wto % eit.%

boron, 76o2 77.6 74.3 75.6-

carbon ,20.8 19.6 .20.3 ' 18.9

free carbon 2.8 5.4 ..

• ** - free ^2^3 0.1 0.1

oxygen - .1- nitrogen - - » 0.1 silicon 0.1

magnesium 0.4 0,15 • * ' 0.05 0.05 iron * n.d. n.d. others n.do not detected * * spectrophotometrically * volumetrically

(99) 5«l(b) Phase Identification of boron carbide by X-ray diffraction

Data obtained from the dlffractometer trace'of the

prepared boron carbide (Figure 3«1»1») l6 compared with

that obtained from & commercial sample of boron carbide

(Norton Norblde) (Figure 3«1«2*} and the date given In

A.S.T.M. Card 6-0555. (Table 3-1.2.)

Figure 3.1.1. Prepared B.C

101

©I

Figure 3.1.2. Norton Norblde B^C

10^ 021 113

101 003 012

6= 8 10 12 16 18 20

(100) (101) TABLE 3c1o2

^ B^C (Figure 3o1c1o) B^C (figure 3o1o2o) A.SoT. Mo CARD 6-0555

d(X) d(8) I/I, d(8) (hkl)

VoW. 't.'f9 20 30 4o49 101

w 'f,02 30 4o02 40 4o02 003

60 3.79 65 3o79 70 3o79 o-i;? • 15 3.37 i 20 3o37 100 3c35 001 w 2,8l 5 15 2o8l 30 2o8l 110

80 2o57l 80 2o57 80 2o57 104 i> 100 2o38- 100 2o38 100 2o38 021

2e30 w 2o30 10 2o30 113

w 2o02 20 2o02 10 2o62 006

w I087 10 I087 10 I087 211

; 10 1o71 20 1c71 30 1o7l4 205 i V o Wo I063 V 0 Wo I063 10 I0637 116

: — - - - 10 I0628 107

w 1o50 15 io50 20 lo505 303

1o46 20 lo^6 i • « 30 lo463 125 :

w 1o45 20 lo^5 30 .lo446 018

w lo^l 20 lo^l 30 Io407 027

w 1«35 w 1o35 20 lo345 009

[ w lo3^ w lo3^ 20 1«3^2 113 1

lo33 w 1o33 20 1o326 223

1 VoW. 1o29 V 0 Wo 1o29 10 lo286 208

1 lo26 w .1e26 20 1o26l 306 :

VoWo I0I9 V 0 W 0 I0I9 10 I0I9I 042 : 1

- relative intensity taking (021) = 100 = ^1 • Spacing for Carbon (graphite)

(102) 3cl(c) Particle size analysis

Sedimentation methods for peirtlele size aneaysls, eogo Andreasen pipette method» Indicated that when samples of the prepared boron carbide were dispersed In water the terminal velocities given by:-

V = 2 gr^Cd^-d^) cm./sec

where V = terminal velocity g = 981 cm/sec r s Stokes diameter (cm) d^ = density of boron carbide, g/cm^ d2 » density of water, g/cm^ = viscosity of water showed, that 709^ of the sample consisted of 'particles* with radii above 30 fjjn and the remainder displaying a maximum dis• tribution at or below 1 ^Ujn radius (^igure 3ol63o)« Further dispersion (by subjecting the sample to ultrasonic vibration) altered the distribution curve, and it was evident that the larger particles were, in fact, aggregates of smaller particles• On the assumption that aggregation was due to an acidic oxide layer on the surface of each particle of boron carbide, the experiments were repeated using a dilute solution of ammonium hydroxide (1 molar) to disperse the samples; results now showed the majority of the particles to be below 1 yton in size. These methods do not function accurately below 1 ^nr) radius as

(103) the terminal velocities are extremely slow, so that such dispersions are virtually stableo For this reason, other methods of particle size analysis were investigatedo

It was noted that there was some broadening of the X*ray diffraction lines produced by the prepared boron carbide

(*'igure 3o1o1o)o The lines obtained by X-ray diffraction have a finite width due to the collective effect of X-ray beam width, the aperture of the detector and mechanical imperfection of the instrument, eogo recorder responseo The instrumental broadening can be determined readily by examining the diffractometer traces of a large perfect crystal of calcite (CaCO^)o Superimposed on the instrumental line width there may be an intrinsic line broadening arising from lattice distortions, stacking faxtlts or from the sample being composed of very small crystallites; the effect becomes significant at dimensions

<' Oo^^mo The line width is related to the crystallite size; one commonly-used relationship is s-

p = integrated intensity of the line = K X intensity of peak toCoeO" where K = constant { unity) ^ = wavelength of the radiation

^ = Bragg angle t = a linear dimension = m V m = mean crystallite v.oliuse p = line width (in terms of 2 0)

(10^) p Is estimated either from the recorder trace as the peak width at half the peak maximum after subtraction of the background radiation, or using the scaler and printout to give optimum statistical accuracyo This formula can only be used experimentally when there is insignificant instru• mental line broadeningo As it is not possible to correct by simple subtraction the contribution of the instrument broaden• ing in cases where such broadening is significant, the method due to Jones (1938) is usedo The observed integral breadths of the lines are first corrected for the fact that they are produced by the o^.p^- doublet of the copper X-radiation using the graph. Figure 3o1o^o of b/Bo against A/Bo, where Bo is the observed.line broadening, b is the corrected width for the sharp, s-line of calcite (l^°^3') A is the doublet separation measured in the same units, calculated from the formula :-

A = O0285 tan© (Copper )

The corrected vrldth B, for the broadened m-line of the

(104) spacing for boron carbide (17°31') is obtained from the same graph© p, the intrinsic broadening, is obtained from the graph p/B against b/B Figure 3o1o5o In a typical calculation of the average crystallite size of the prepared boron carbide (batch4^=l) the following results were obtainedo

(105) Figure ioHok.

Curve for correcting observed line breadth for the fact that it is formed by a od-doublet Oo9

^Bo

0.8

0,8

0 Po2 OA 0,6 0.8 1.0 A/Bo

FifTure 3*1

loO Curve for correcting line breadth for the effect of instrumental broadening

(106) Boron carbide m - line B = Oo307° from Figure 3o1o1o

Calclte s - line b = 0.2^0°

m - line A= Oo2o85 tan 17^31' = O0O9O

A/Bo = Oo293

from Figure 3o1o^o B/Bo = Oo90 B = Oo276®

s - line A= O0283 tan 1^%3' = 0.075

A/Bo = Oo313

from Figure 3o1o^o b/Bo = 0.887 b = 0.213^

b/B = 0o772 from Figure 3o1o^o p = 0.28

hence p = 0.0773^ = o0773 x 2n radians 360

0% t = 1.3^ X 360 =1,2008 0O773 X 2rr cos 17 31'

(or Oo12^m)

Line-broadening determinations were carried out on samples obtained from the Andreasen method having maximum aggre< gate size between 3 ^um and 63^0 Measurements were made on the diffraction lines for (110), (10^), and (021) spacings and are shown in Figure 3o1o6o, together with their corresponding average crystallite size values which ranged between 200 and 200080

The (104) spacing in the hexagonal indexing of the rhombohedral boron carbide lattice corresponds to the (200) spacing where the rhombohedron is regarded as a deformed cube.

(107) Thus, the line broadening for this reflection should afford a measure, o^ aY,erage crystallite sIze nearest, to. the^ ^ : - lent spherical diameter. The results gave useful confirma• tion of those found by gas sorption and electron microscopy.

Measurement by the B.E.X. method of surface sorption gave a specific surface of 20 m^g~^ and the average crystallite size (or equivalent spherical dicuneter) as 0.1^ (1000 &)^ based on the assumption that the crystallites are cube-shaped (or spherical). Study by electron microscopy showed that the largest single crystals were about lyum in size and to have regular shape being very few In number. These crystals were seen against an almost uniform background of very fine particles whose average diameters were somewhat less than 0.1^. A typical sequence of electron micrographs showing the particle size range of the boron carbide and the attendant aggregation and sintering is shown in Figure 3»1«7«

(108) Figure 3«1o6o

Graph of values of average crystallite size by line-broadening for samples fractionated by sedimentation. Oo3

^ 110 spacing

^ lO^f "

• 021 "

i...

15 (0 >> u o I 4 CD Of u Ool i >

20 ^0 60 80 100

Maximum aggregate size, jjm. diameter

(109) Figure 3.1«7* Klectron nlcrogrs^bs of the pre^arra Bu.Hoii CAxiiUiJKi

U0»000 X nagnlfIcatlon

To distinguish between the relative sizes of the

particles, a sizing graticule based on the design by

Flschmeister (196I), shown in *\gure 3o1o8o, was placed

over the image recorded on photographic paper; the size of

each particle was estimated and the number of such particles recorded^ The graticule contains sets of circles and ellipses of equal area forming a geometrical progression of the form

Ao2"o The ellipses have their major axes twice the lengths of their minor axes^ Both sets of shapes were untilized de• pending on the orientation of the crystallite on the grid. A typical estimation gave an observable particle size range of l^m to O0OI ^um with the mean ranging between 6OO and 1^000 8

(0.06 - OpI^tLm) based on sample weights After allowing for the problems of obtaining a representative sample and of dis• persing the aggregates, it was considered that this last method gives the most satisfactory way of determining particle size range in the submicron regionj particularly when used in con• junction with gas sorption.

Figure ^c"\c^o

^ ° o o O O O 98765^3 2 1

6 789

(111) Electron diffraction patterns of single crystallites gave little correlation with the observed X-ray diffraction data owing to the impossibility of orientating the crystal, with respect to the electron beam, along a known axiso The phenomenon of Kikuchi lines observed on many of these samples was ascribed to lamellar layers of boric anhydride (or hjdrtte) produced on evaporating the boron carbide dispersions»

3olLo The vacuum sintering of boron carbide A vacuum furnace was constructed, being powered by a four kilowatt radiofrequency induction heater (Delapena E with the woxmd coil external to a 1 ino diameter fused silica tube (Thermal Syndicate Purox)o The tube WPR evacuated by a single stage rotary pump (Edwards Speedivac ISI50) backing a two stage oil diffusj^ion pump (Edwards Speedivac 403A ); vacua of between 10"^ to 10"^ mm Hgo were achievedo

About kg of the prepared boron carbide were pressed into a pyrolytic graphite crucible (Le Carbonne), supported on a ceramic pedestal inside the tube furnace and heated to tempera• tures ranging from 1000-l800^Ce Temperatures were measured with a disappearing filament pyrometer (Foster) the crucible being viewed through a window at the end of the tube, correc• tion being made for the spectral emissivity of boron carbide at the measured temperature (Kingery 1959)o The maximum tem• perature achieved was limited by the tendency for the crucible and the tube walling to react; alumina proved unsatisfactory as it became porous on heating and failed completely after a

(112) few runso The general arrangement of the apparatus is shown

in Figure 3o2^^t> Samples were heated for periods from one to

five hours; some outgassing was observed when freeh boron

carbide was heated to ^ lOOO^C, but not when such samples

were reheated. The initial outgassing was probably due to a

surface oxide layer which was removed as the volatile boric

anhydride, ^2^3° Surface area and average crystallite size

measurements were made on each sample after heatingo Results

are shown in Figure 3o202o and pronounced sintering of the

powders is indicated particularly at the upper temperature

regions; however, such sintering was confined to the smaller

crystallites as the samples remained in powder form and showed

no tendency to bind© X-ray diffraction patterns ^>^l|Jjm'u ^irg^*

oîÊfei^sff^v^ê shoiiân increase in the amount of free carbon

as the samples sintero The effect of adding fine chromium

powder (1096 by weight) was to further increase the degree of

sintering, as indicated by an increase in the average crystal'

lite sizee X-ray diffraction confirmed the presence of a

mixture of chromium carbide and diboride in samples heated

above l600^Co

3<3* Hot pressing of the prepared boron carbide

Samples of the boron carbide were compressed in a gra•

phite mould set between hydraulic rams and heated to temperatures

above 2000^0 by a 36 kilowatt RoFo induction heater (Wild -

Barfield)o Figure 3o5o1o Burnt lime, CaO, was used to contain

the heat of the die set (more recent work shows that graphite wool or carbon black are better media for heat retention).

(113) Figure 3o2..1 High temperature vacuum furnace for sintering studies*

Optical pyrometer Silica sheath Crucible

RoFo Coil Vacuum gauge Diffusion pump Rotary vacuum pump o o /

Figure 3o2o2(a)

Change of surface area on heating under vacuum

lOOO^C V 1200«>c

Cr Q 16000c

I8OOOC

^000 h Change In crystallite size on iSoo'^c heating under vacuum

1600^0 3000

• 1^00°C •V 1200*^0 2000 ^ lOOOOc

100

Time 0 hours

(114b) Figure 3-2»2(b)

. 20

Change of surface area on 16 heating under vacuum

S 8

1000 1200 1400 1600 1800**C

Change in crystallite size on heating under vacuum 4000

3000

2000L

lOOOh

1000 1200 1^00 1600 l800 C

(114c) Figure 3* 3>1^ Hotpreesing set up for compaction of boron carbide©

/////

V///////

Hydraulic ram R - R.Fo Coil Pressure gauge A - Asbestos shield Ram-Travel indicator D - Graphitedie Carbon black P - Optical pyrometer Boron carbide S - Sighting tube

(115) A constant pressure of 300 Kg/cm^ was applied to

the compact; at cold, consolidation of the powder amounted

to 40^ of the theoretical density, which increased to about

7096 when the temperature reached lOOO^C although no binding

occurred, and remained so until a temperature above 2000^0

was achieved., (ail estimated temperature of 23odcwas achieved

on one run). Temperatures were measured using a disappearing

filament pyrometer (FosterIhstrumoit)* When cool, the compact

was removed from the mould; the majority of the specimens

showed good surface finish and extreme hardness. The sintered

boron carbide was sectioned on a diamond grit wheel (Bullock

Diamond Products). Sections were examined for their density

by high density media (Sym. tetrabromoethane and carbon i tetrachloride mixed in various proportions) and the phase identified by X-ray diffraction Table 3*5.1«

3^^. The ball-milling of the prepared boron carbide*

6g. samples of the powder were ball-milled under standard

condition, i.e. a constant ratio of sample weight to number and

sizes of the porcelain balls, in a porcelain mill for different

lengths of time. Changes in specific surface, S, and average

crystallite size determined by gas sorption (Section 2.2(d))

are shown in Figure 3«'**1* The specific surface, S, remained

practically constant during the first 5h.^hich it progressively

increased. While the average crystallite sizes, as determined by

gas sorption, remained constant. X-ray line broadening (section

3.1(c)) showed development of strain within the crystallites as

(116) TABLE ^.3.1

j- B^C. A .S.T.M. CARD 6-0555 . i

d(8) d(8) (hkl) i ! ^ i 1 1 4.49 30 4.49 101 15 1 1 40 4,02 40 4.02 003 1 1 70 3.79 70 3.79 012 1

i 50 3«38(caTbon) 100 3.35 001 •

25 2.8l 30 2.81 110 j

80 2.57 80 2.57 104 \

1 100 2.38 100 2.38 021 ;

i 10 2.30 10 2.30 113 ;

7 20.2 10 2.02 006 :

i 10 1.87 10 1o87 211 1

30 1.714 30 1.714 205 \

I 7 1.637 10 I0637 116 -

i 5 1.628 10 I0628 107 j

' 5 1.56

; 30 1.505 20 1.505 303 ;

! 40 1.463 30 1.463 125 : i 35 1.446 30 1.446 018 •

40 1.407 30 1.407 027 !

20 1.345 20 1.345 009 ;

20 1.342 20 1.342 113 ;

20 1.326 20 1o326 223 1 !" 7 1.286 10 1o286 208 ; I i 20 1.261 20 1c26l 306 i 7 1.191 10 1.191 042

I/I^ - relative intensity taking (021) = = 100 • ASTM 13.148 spacing for graphite

(116a) there was additional li^ broadening In excess of that due to

the particle size of the materlalo Further milling decreased

the average crystallite size and further straia developed as

Indicated In Table*3«4o1o

TABLE

Milling Average IntrlnslcdO^ reflection) Internal* Apparent* time fh, cryst. broadening caused by strain strain slze,fi. Crystoslze pjj Cryst^straln ^5*2

0 1160 OoOSO® I

10 730 Oo127° 1 Oo108° 0*0030 0*0060

^ • -2e tan (see Klug and Alexander 19^4) and

In the course of milling, the material became porous, as

Indicated by the development of some hysteresis In the ad^rption

Isotherms. As noted In Section 3«1* the original material con*

slsted of large crystals of about 1fUM and below in size and aggre<

gates of smaller crystals all below Od yU.m,. Electron micrographs

of the milled samples. Figure 3o^o2o Indicated that during the

earlier stages of milling only the larger crystals are reduced in

size and hence the average crystallite size was not decreased Si markedly* Compaction of the aggregates lead to the development

of porosity, with the type II isotherms tending towards type IV and showing some hysteresis even at very low relative pressures.

(117) Table 3o^c1. Ball milling of boron carbide

60 a) specific surface

b) average crystallite size

1000

500 J-

13 20

milling time, hours

(117a) figure 3«U.1. Th« prepared BOBQM OAABXM ball<-«llled for 10 h.

140,000 X M^nirioatiOD.

Tbe i repared BORGJi OhH^Iuc ball-Billed for 20 h.

MM ^ UOyOOO X aagniricatioD. SECTION kg THE OXIDATION OF BORON CARBIDE

The section concerns the behaviour of the prepared boron carbide in reactive atmospheres, for example^ air, at high tem• perature o

Aol. The kinetics have been studies for the oxidation of boron

carbide with (i) carbon dioxide over the temperature range 600

to 750*^0 by Davies and Phennah (1959), (ii) air and water vapour between 200 and 750°C by Litz and Mercuri (1963), (ill) pure

oxygen between 515 and 6^6°C by Harvey (1965) and (iv) in dry air between 503 and 527o5°C by Dominey (1968)0 The following reactions are proposed

I0 Dry CO^

a) B^C + 7CO2 > ^^2S *

or b) B^C + 300^ ^ ^^2^3 *

2. Dry air and oxygen

a) B^C + kO^ 2B^0, + CO^ 2 3 2 or b) B^C + 3^2 2B2O3 + CO

or c) B^C + 3O2 2B2O3 + C

3o Water vapour

a) B^C + 8H2O ¥ 2B2O3 + CO2 + 8H2

or b) B^C + 6H2O • > 2B2O3 + C + 6H2

with associated B2O3 transport via

c) B2O3 + H2O » 2HBO2 (g)

(118) In all cases, study of the kinetics of the above reactions was made using thermogravimetric analysis with, generally, some analysis of the reaction productso The percentage weight increase for complete oxidation via reactions 1(a), 2(a) and 2(b) is

^32%; via Kb) 245^; via 2(c) 174%; and the percentage weight decrease for reaction via 3(a) is 100% and via 3(b) is 88% when transport of B^O^ occurs via 3(c)o

Complete reaction is rarely achieved, being dependent on the temperature and the phases present during reactiono Never- theiess, all appear to be principally controlled by the rate of diffusion of the oxidant, e.go O2, through a viscous layer of boric anhydride, ^20^, except in reactions 3(a) and 3(b) at low temperature, 450*^C, when the product B^O^ layer is removed by water vapour as it is formedo At temperatures in the region of

650^C and above the liquid viscosity and surface tension of the

B^O^ influence the reaction rate in causing the separate aggregatejt to coalesce into a glassy compacto The volatility of B20^ is insignificant at these temperatureso

In the present work, samples of the prepared boron carbide were heated in the thermogravimetric balance described in Section

2o2(e) at 500, 6OO, 65O, 700, 750 and 800°C in an atmosphere of air and the change in weight of the sample over a period extend• ing to several hourso The results obtained are shown graphically as a direct plot and as a logarithmic plot. Figure 4o1o.1<» In this way the slope of the tangent of the graph serves to indicate the order of reaction, since

-de = kc° dt

(119) where c = concentration at any time t, and is proportional

to the change In weight,

n = order of reaction

Figure ^olol* Weight change on oxidation of B^C in air. 800, 750, 700Oc

6500c

600Oc

1000 1200 t(min)

OoOl'—^ 10 100 1000 log t(min) (120) Thus, at 500°C there appeared to be little reaction

on the basis of weight change9 a situation which was main•

tained until the temperature was raised to 600^0 when oxidation

occurred as a first order reactlono At higher temperatures,

the order of reaction changes being 2/3 at 650^0 and ^, or

terval of time, the reactions came to a standstill with less

than 50% of the material oxidised on the basis of weight changOo

Determination of the water soluble boric oxide by titrimetric

analysis, Section 2o2(a), on completion of the tharmogravimetric

analysis, indicated, that in fact more than 70% of the boron

carbide had been oxidisedo It was evident that some lose of

B^O^ had occurred due to presence of moisture in the atmosphere,

maximum humidity being recorded as 70%RH at 20^C during the

oxidation (equivalent to 1o75 volo % or 12o3 mmoHg pressure of

water vapour)©

According to Litz and Mercuri (1963)5 reaction would be

favoured at the lower isotherms 9 which is evident from the re•

sults for 650^0 when ^2^3 ^® being removed almost at a rate by

which it is being formedo

The energy of activation for the oxidation of boron car•

bide was determined by the Arrheniua method of plotting the logarithm of the rate of oxidation against reciprical tempera•

ture (°K) as in Figure ^o1o2o

(121) Figure 4.1o2

to 1.2

B •H

0) 1o1 •*-»

e bO e

B 1c 04

-a Oo9 loO lolo

2 o 10- K

The value obtained of 23o9 kilo-calories per gram molecule

of B^C is in accord with the values of 11 kilocalories per molo

obtained for reaction 3(a) and k7 kilocalories per molo for

reaction 2(a) by Litz and Mercuri (1963).

k^2o Supplementary to the above work, 5g samples of the boron

carbide were heated in a furnace at 700^0 in air, for ^,

1, 2 and k hour intervals of time to estimate the extent of

sintering during oxidationo Determination of the surface area

by gas sorption analysis. Section 2o2(d), on the cooled samples

(122) freshly oxidised, and following washing with hot water to

remove the boric acid gave the results tabulated in Table

4o2o1o

TABLE 4o2o1o

Boron carbide oxidised in air at 700^C

2 -1 Time % wt % oxidation S, m g hours increase e^ 2(a) e^ 2(c). before washing after washing

0 0 0 0 20o6 —

l6o7 lloO 9o6 4o4 35o7

22o1 l4o5 12o7 4o1 42o6

50o9 20o3 l7o8 3o4

1 41 o8 27o5 24.1 2.0 5608

2 53.8 35.4 30c9 Oo8 74.8

4 68.2 44.8 39o2 Oo3 —

The expected reduction of surface area on heating is due to

(i) preferential oxidation of the smaller crystallites, and (ii)

aggregation of the remaining crystallites and sealing of the pores

by the liquid ^20^. However, none of these mechanisms correlate

with the marked increase of the surface area of the unreacted

carbide following leaching, unless the larger crystallites under•

go splitting. Phase identification by X-ray diffraction. Section

2o2(b) showed a pronounced increase in the free carbon present in

(123) these samples, Figure 'f.S.I., which was significantly broadened.

Figure 4.2.1

Carbon

where as the peaks for the (104) and (021) spacing of the boron carbide showed no appreciable changes In half peak broadening but had modified profiles. Should splitting of the unchanged boron carbide crystallites have occurred through spalllng at the carblde-oxlde Interface when the samples were cooled then reheated samples would not have proceeded to give oxidation rates similar to those of samples which had been continuously heated.

In practice, there was no distinction between these samples.

Assuming that the boron carbide and carbon are separate phases their individual surface areas were estimated on the basis of reaction 2(c), viz,

(124) B^C + 3O2 •^tB^O^ + C

and are given in Table ^o2o2

TABLE 4o2o2o

Surface areas and average crystallite sizes produced on oxidizing boron carbide

Average Time % wt % oxi• wt fra ction Surface area (m g crystal• hours increase dation B^C C B^C+C B^C c lite A Size S

0 0 0 1 0 2O06 2O06 - -

1 l6o7 9o6 0^904 0O21 35o7 19o2 l6o4 783 3^

22o1 12o7 00873 0O28 42o6 23o8 23o8 850 31

1 4108 24o1 Oo759 .052 5608 39o6 39o6 762 35

2 53o8 30o9 Oa691 0O67 74o8 58o7 58o7 876 30

The X-ray broadening of the carbon peak gives the dimension

along the C-axis of 82^, asstunlng a uniform hexagonal crystal habit,

where the edge of the prism base is a multiple of the C-C distance

of 1<,'f22 and the prism height, a multiple of the Interplanar spacing,

3e35X| the edge dimension is:

82 rA2 3^o75 2 3»35

(125) compared with the average crystallite size from gae sorption determination of 32«5So Hysteresis was shown by the sorption isotherms of the more oxidised samples^ Figure ko2o2^ indicating that the carbon has porosity and lacks crystallinityc The method of computation of the B,E»T» isotherm by digital computer (IcB«M<.

1130) is given in the Appendix^

Adsorption of on oxidised boron carbide (In air at 700°C for 2 hours? after washing)

0,03 r*

O0O2

0,01 O0I 0o2 PO

C126) ^•3» Chemical analysis and electron microscopy were employed to indicate the composition and appearance of the boron carbide following oxidation, Sections 2o2(a) and (c), the results are tabulated in Table 4»3.1« for the chemical analysis and in

Figure ^•3»1« for the microscopy.

TABLE ^.3:1* The chemical analysis of the products of^ oxidation of boron carbide in air at 700 C

% B^C" Time of Residue after washing oxidation water oxidised soluble % B % C % B^C % free cajrbon

14.7 7.0 74.2 25.8 94.4 5.6 i 23.'9 12.5 72.9 27,1 92.7 7.3 1 40.1 23.6 71.4 28.6 90,8 9.2 2 54.2 35.0 70.9 ,29.1 90.2 9.8 k ' 67.8 50,1 68.4 31.6 87.0 • 13.0

Differential thermal analysis, D.T.A., as described in

Section 2»2(e) was used to determine the minimum temperature for the onset of oxidation of boron carbide in air. The boron carbide was diluted with the alixroina powder before placing in the sample holder and with alumina as the reference, heated between 600 and

700°C at the rate of 5°C per minute^ The leading edge of the

D.T.A. curve was registered at 637*^0; reaching a maximum at SkO^C; due to the highly exothermic oxidation of boron carbide (Smith et al. 1955) a deflection was maintained to above 700°C.

(127) Figure U.3.1. Boron carbide oxidised In eir i h. ut 700 C (•) unleached, 10,000 X magnification.

lb; boron carbide oxidiaod in air, U h. m% 700 0

unleached, 10,000 X magnification. t'lfcLjrr i4.3.1. iioron carblur oxiui»ea in air tor 1 h. *t /uo C

leacheu with Aater, 10,0^0 X mii^uii ication.

^d) boron carblae oxidised in air for 1 h. -.t 700 0

lenchfG with water ,"40 ,OuO A m-.^nir icat i JH. SECTION 3' THE FORMATION OF TITANIUM BORIDE

The section covers the formation of titanium boride on

to a substrate of titanium alloy by reaction with boron carbideo

5o1, Titanium^ one of the lightest of the transition elements,

has other desirable properties of strength and toughness as well

as resistance to corrosiono In the pure state it exists as the

alpha, hexagonal9 form which transforms to the beta^, body<»centred

cubic form at a temperature of 882*^Co Alloy additions are either

alpha- or beta-stabilizing depending on whether they raise or

lower the transformation temperature; there are three basic types of alloy according to McQuillan and McQuillan (l956)o The first, alpha alloys, contain only alpha stabilizers, or a predominance of them, namely 0, N, and many nontransitional elements, eogo

Al, Sn, Cno Some have a small amount of the beta form in the structure at room temperature and are known as *near' or 'super* alpha alloyso The second type9 alpha-beta9 contain additional elements which stabilise the beta phase to such an extent that the microstructure, at room temperature^ consists of a mixture of the alpha and beta phaseso The beta phase is stabilized by the addition of transition elements such as Fe, Cr, Mo^ Mn and

Vo These alloys are of immense value as they can have their mechanical properties controlled by precipitation hardeningo The third type^ beta, are all beta at room temperature and contain a large proportion

(128) TABLE 5c1o1c

The composition and phases of commercially available titanixim (IcMoIo Limited)

• Density IoMolo BoSo Spec^ Composition Phase Class Remarks g/cm?

130 Commercially pure Alpha I (become succes- (sively mechani-J tt It rt TA, 7,8,9, I ^•051 (cally stronger

230 TA, 21,22,23,2'f Ti - 2o5 Ciit 1 II I Ductile* medium strength f Ti - k Al-4Mn Alpha-beta 111(2) — ( — 315 Ti - 2 Al-2Mn ; Alpha-beta 111(2) low strength

317 i TA, 1^,15916917, Ti - 5 Al-2o5Sn ^ Alpha-beta llKi) medium " i 318 ; TA, 10,11,12,13,28, Ti - 6 Al-^V Alpha-beta 111(2) II II

205 ' — Ti - 15 Mo beta ] II high " Figure 5»1o1o Titanium-Boron Phase Diagram

3000 r

2600

I i2^50 / TiB^+S

2200 Tx-B + TiB / . I d. d. 1 / I; / I' ^/Ti^B+Sjf 1800

1668^5 /"^ TiB+S TiB, r 1650

1^00 TiB

^ + TiB TiB + 1000 TiB '2 I 882

oC + TiB

600*^0 —f— 20 ^0 60 80 100

B, ato %

(129a) Titanium readily forms a series of binary compounds

with the light elements, boron, carbon, nitrogen and oxygen,

which are refractory and often of extreme hardness. The

phase diagram for the titanium-boron system is shown in

Figure 3.1.1. Of the compounds, Ti^B, TiB, TiB^, Ti^B^, and

TiB^2» only TiB^ is stable at room temperature according to

Samsonov (196O); both Ti«nd TiB disproportionate to TiB^

and Ti according to Palty et al (195^), the same workers

have shown that the solubility of boron in titanium does not

exceed 0.05% between 750 and 1300*^0. and is only 0,1% at the

eutectic temperature ( l650*^C).

5.2. Moissan (l895, 1896) and Wedekind (1913) first obtained

titanium diboride by sintering compacted powders of the two

elements, Andrieux (19^8) produced TiB^ by the electrolysis

of fused mixtures of TiO^, ^2^3' OaF^ at temperatures

of about 1000*^0. Gas phase production of a compound approxi-

ma ting to TiB was carried out by Moers (1931) heating TiCI and BBr^ between 1^00 and l800^C. Other methods depend on the

reduction of the metal oxide and boric anhydride by carbon by

the following reaction :-

TiO^ + B^O^ + 5C > TiB^ + 5C0

but in the proportions 1:4:7 as against 1:1:5 demanded by

stoichiometry nevertheless the product still contains 2% free

carbon (Blumenthal 1956). Kieffer et al (1952) have developed a 'borocarbide' method of production which consists of the

reactions :-

(130) Ti + B^C + B^O^ ^ TiB2 + CO

or Ti + Ti02 + B^C ^ TiB^ + CO

carried out in a Tammann furnace at between l800-2000^Co An

alternative method developed by Samsonov (1956) was based on

the reaction :-

2 Ti02 + B^C + 3C h- 2 TiB2 + ^CO

-1 «2

conducted under a vacuum of 10 to 10 mm Hgo and at a tem•

perature of l400 to l'f50^C and contained a carbon content of

between OoOl - 0o39^ of carbon after one hour., Thermodynamic

data (Section 1o7) indicates that the reaction 2 Ti + B^C 7?,TiB2 + C

is feasible over the temperature range 0 to 4000^K and is

exothermic and that TiB^ is formed in preference to TiC.

3o3o In the present work^ various alloys of titanium (Imperial

Metal Industries (Kynoch) Ltda) were sectioned by a diamond grit wheel (Bullock Diamond Products) and polishedo Each section was coated to a thickness of about Ool mm with the prepared boron carbide powder by evaporating a suspension of the material in water to dryness under an infraheat lampo The specimens about

1 cm in diameter0 were placed in fused alumina crucibles (Thermal

Syndicate) and loaded into the high temperature vacuum furnace

(Metals Research PoColOo

(131) Figure 5.3..1 High temperature Laboratory Furnace (Metals Research, Type PCA10)

9 O

water

vacuum water pump rr-r Furnace Crucible Control Unit Flowmeter Lowering device Needle valve

Argon For vacuum and inert atmospheres (argon) a small amount of titanium hydride, TiH2 was placed in the furnace to getter any residual O^ and in the systemo Specimens were heated to temperatures between 1000° and l600°C for a period of one hour, and were cooled slowly allowing the titaniiun phases to equilibriateo The specimens were surface-etched by immersing quickly in a 1:1 mixture of HCl and HKO^ and washing under watero

The phases on the surface of the titanium metal substrate were examined by X-ray diffraction and by electron micrography of replicas«

5oô The lattice spacings^ for coated specimens heated at various temperatures, obtained from X-ray diffraction traces, are tabulated in Tables 5c4ol(a), 5oôl(b), 5oôl(c) and 5o4ol(d), alongside the published data for B^C, C (graphite)9 alpha Ti, beta Ti, TiB2, TiB, TiC, TiN, TiO, TiO^ and any alloying element in order to establish concordanceo

(133) TABLE 5c^ol(a)

X-ray diffraction d-spacings, obtained from the boron coated titanium-Commo Pure (loMoIo 130) heated to I'fOO C for two hours under argon

Boron Base Ti Bo Ti Alloy TigB cubicffortho :TiBp TiC TiN TiO TiO. B4C Coating Alloy Bo0682 calc Palty calc 5o0700j8o121 670652 80117 I'f 0551 5.0555

3ol8w 3.67 3.053m 3.22W 4,49 2o98m 2.98m 3o2'f5s 4.02 2o83w 2,62m 3.798 2,585m 2o58m ec557m 2,81 2o51s 2o56m 2,538e 2.5't38 2,518 2,57s 2A6e ZASB 2.'t89 2„3^3s 2 o 3^*68 2A07 2.38s 2o3l8 2o36m |2o3'*2m 2o310s 2o336 2,297 2„30w 2„2'fw !2o2'fV6|2 2o263w 2c 1^8 2.157m 2<,l6lm ^,1798 2,188 2c12vs 2c13s 2„l'f0s 12,12s 2,0851 2,054 2,02w 2o03ve 2,0338 1o93w |l.9lm 1o950m I 1,956m 1.85W 1,856111 I 1,863m 1,87w 1.75 !l.72ni h,726w 1o7^8m I 1,755m 1„7l4m i 1.63 1o63m ;i,6l3w 1,637 I0628 1.515 1,528w;1,5lW|l,535m 1.687 1.505 1.'f67mh .332w!l.33m | 1,3628 i,'f6lw; 1,496 I 1,475 I 1,463 Ilc327m' 1,28w 1,3628: 11,311m I 1,446 ho 2^171* 11,23m 1.255W 1,407 1,277 i 1,259 I .! L 1,223 i 1.205 I

•A.SoT.M. Card Index: weak, m - mediunij s - strong, ve - very strong *"l.ure 5.U.1.(.) l.U.l ijO, florlde co.teU, 20.0O0 X m.^nlfIcatlon,

l.k.l. 130, untreatea, 10,00C 3L «u»*nifioation. TAB L E 5o4ol(b)

X-ray diffraction d-spacings, 2, obtained from the boron coated titanium, 2o5 Cu (IoM«Io 230) heated to ^kOO C for two hours under argon

Boron Base Ti iBo Ti Alloy TlpB cuHic ortho" TiB. TiC TiN TiO Coating Alloy 5o0682!calco Copper Palty calco 5o0700 O2T 6o06l^ 6o06'f2 570551 5o0555 U-

3ol8w ^o0836 3.67 3o053m 3o22w ^065 ^0^9 2o98m 3o2^5s ^o02 2o62m 3o798 2o585ra 2o5^m 2o557m 2o8l 2o5l6 205388 205^38 2e5is 2o57s 2o^6s 2o^6s 2o^'fB : 2 0^89 2o3^3s 2o3^6s 2 0^07 2o38s 2o31s 2o33m 2o3^2m 2o310s V/4 2o297 2o30w 2o2W 2o2'fvs 2o2^4s 2o263w 2ol4s 2o157m 2ol6lm 2o1798 ;2ol88 2o12vs 2o138 2ol40s 2o128 i 2o088s 2e085s;2o05^ 2o02w 2o03vs 2o033 1o93w 1c808m 1o950m 1o956m 1o85w 10856m 10863m 1o87w 1o73 1o72m 1o726w 1o7Wm 1o755m 1o71^m I063 1o63m 1o6l3w I0637 I0628 1o515 !lo^7m 1o'f75w , 1o528w 1o5lMlo535ml I0687 1o505 ilo303m! 1o332w 1o33m 1o362si 1o46lw \ ho^96 1o475 io^63 1o278w ilo28w !lo3628 Mo311m: lo4if6 1o247w !1c23m i 11o255w; 1o277 i1o259 f iio407 I Mo223 ho205 ' i i I • AoS„ToM« Card Index w weak, m - medium, s - strong, V6 - very strong >'i»urc lb) l.k-I. ki30 Boplde coated, *?0,00O X magnification.

230, untreated, 20,000 ma^nlfIcatir.n. TABLE 3Aol(c)

X-ray diffraction d-spacings, 8, obtained from the boron coated titanium - 15 Mo (loMoI- 2©5) heated to 1^00 C for two hours under argon

Boron Base Ti Bo Ti Alloy TipB |cuDicT| ortho TiBp TiC TiN TiO TiO. BZiC Coating Alloy 5o0682 calc Mo Palty Icalco 5c0700 8o121 6o06l^ 6oOS¥2 ^7iT7 '^Tb5! 5o0555

3ol8w :3o67 3o053m 3o22w ^o03 ^0^9 2e98m I 3o2^5s ^o02 ] i2.62m 3c 79s ' 2o585m ; 2o557m 2o8l 2o5l6 ;2o51m i2o538s 2o5^3s 2o5l6 2o57s 2o568 ; 2A6e 2OHB } 2o489 2o3^3ei 2e3^6s; 2o407 2o38s \ 2o3lB 2o3l6 2o3^2w|2o310s! i 2o2W |2o23s 2o225 2o263w 2o 297 2o30w 0^ !2o1^s , 2o157m I2ol6lm 2o 188 * 2o12vs i 2o13s I 2ol40s 2o12s i 2o085i 2o05^ 2e02w i 2,03V8 • 2o033 I 1,93w . 1o950mj : 1o956m 1,85w i 10856m! ; 10863m: 1 1,87w ; 1.75 i lo726w 1o7^8mi ^ 1o755m; 1,714m : 1.63 I1„63m 1o63m I 1o6l3wi 1,637 1,628 : 1.515 1 1o^75w 1o57^m i • 1o528w| 1o5lW| n535m 1,687 1,505 ; i 1o332wi1o33m i io362si :"lo^6lw: lo^96 lo^75 1.463 !1.33m 1 j1o28w i 1o3628i I lo311ra 1.446 • 1 1»2if7wt i I1«23m i ! ii«255w 1o277 1.259 1.407 1o285m 1.223 1,205 •

• A„S.T.Mo Card Index: w « weak, m - medium, s - strong, vs - very strong 1 i

li^urc 5«U.1.(c) 205» borlde coaioa, 20,000 a nagnlfIcmtloa

l.li.I. 205» untreated, 20,0^ X magnification. TABLE 3o^ol(d)

X*ray diffraction d-spacinge, 2, obtained from the boron coated titanium 6a1 kV{*^) (loM.Io 318) heated to 1200 C for two hours under argon

Boron Base Ti B. Ti Alloy [TipB rcub~ic: f ortho TiB- TiC TiN TiO TiOp B4C Coating Alloy "5o0682i i6o06l4 calc Al/V|Palty jcalco j5o0700Itto121 6006^2 ^c117 4o0551 5o0553

3oi8w 3o67 3o053nil3o22w ^o05 'fo^9 2.98m 3o245s 4o02 2o62m 3o79s 2.585m 2c557m: 2o8i 2.57 2o5l8 2o51m 2o538e 20 5^*38 2o5l6 2o57e 2 0^66 2o^6s 2o489 2o38 I 203^36 2c3^66 2 0^07 2o38s 2o31e 2o3l6 2o3^2m 2c3106 2o^^8s 2o24w 2o23s 2o2U6 2«263w |2o297 2o30w -N3 2o1^6 2ol4s 2a157m )2ol6lmj 2.188 2o12v6 i 2o13e .^ol'fOsi 2o126 i 2 o 0851 20 05*^ 2o02w 2o03v8 2o02'fml 2o033 1o93w 1«950m :1c956mi 1n85w 10856m 10863ml lo87w 1o75 1o726wi 1o7^8m .1o755m: 1o71^m Ic63 !1c63i 1.63 m 'lo6l3w I0637 I0628 lo5l5 1o'f75w: • 1o528w lo5lW. 1o535rai I0687 1c 505 1o332wi1,33m 1e^31w 1o3626 1„^6lw' 1o^96 lo^75 lo463 1c 33m i lo238w ••1o28w 1o3626 • Ic311m 1o4if6 1c2U7w| 1o221w ^1o23m • Mo255w 1o277 1.259 1o^07 1c223 i 1o205

• AoS.T.Mo Card Index: w - weak m medium strong, V8 - very strong yifcUre S.U.Hd) I.k.I. 318, boriao coated, 10,000 X B*gniric«tioa,

318, untreated, iiO,000 X MgnifIcatlon. The X-ray diffraction intensities indicated that the final surface coatings had depths of some 100^'-'m and had the

B€une composition and phase irrespective of the composition and phase of the titanium alloy substrateo It was found that the principal phases were TiB, cubic and orthorhombic, TLB^^ and beta Ti and very little alpha Ti even when the substrate was pure alpha (IoMoI» 130, Table 5o'fol(a)o Specimens heated above

^kOO^C were uncontaminated by any carbide or free carbon phase; this is in accord with Mercuri et al (1959) who showed that the oxygen and carbon content of metal borides can be reduced by

^ o ° heating above 1^00 C under partial vacuumo A possible tetra• gonal phase of a lower boride was not discounted, however, it was impossible to resolve it from the almost identical spacings of the orthorhombic TiB (a = 6o1o2, c = ^o532, and a = 6o122 b = 3o068, c = 4o562 respectively )o

3o3« When transition metals are combined with light nonmetals

(carbon, nitrogen, hydrogen) so called interstitial phases of these atoms in the pores of the metallic lattices are formedc

The interstitial phases are formed,, according to Mgg (1931 )« under the condition that the ratio of the radius of the nonmetal to the radius of the metal atom does not exceed Oo39o

They have simple structures^. FoCoC«, MoCoPo and BoC^Co Above this ratio9 more complex structures are formed but titanium and boron give the limiting ratio of Oo59o As a result^ pure alpha titanium accommodates boron (from boron carbide) giving, ini• tially, a lower boride TiB v;hich has a tetragonal lattice when

(138) 0<,2. (Palty et al, 195^)o This boride disproportionates

peritectoidally into beta Ti and TiB at a temperature above

900°Co The monoboride is normally of the orthorhombic-P,

FeB type when near stoichiometric, TiBy (y = li Ool) (Decker

and Kasper (195^+) (Taylor and Kagle (1963)), a cubic, NaCl,

form of TiB, a = Vc2lS(Ehrlich, 19^9), a = ^o26K(Glaser, 1952),

exists when some C, N or 0 is present for stabilisation; excess

carbon causes disproportionation to TiB^ and TiC but formation of the latter is precluded by the presence of excess borons

Below 680^C TiB also disproportionates to TiB2 and presumably alpha Ti (Palty et al (195^))? ^msonov (1956)^ reporting on the saturation of annealed specimens^high purity titanium with boron,

indicates that the only phase evident at room temperature is the hexagonal TiB^o In the present work, as indicated, there is a mixture of phases, despite slow cooling under argono It is con• sidered noteworthy that the spacings for (110) of the beta titanium, B.C.C. and for (ill) of the orthorhombic TiB occur at 2o3l2 thus giving mutual stabilisation when a specimen is quenched resulting in •fre^zing-in" the metastable phases^ The surfaces of the untreated and treated metals were polished and etched metallographically; direct photomicrographs and electron- micrographs of replicas of these surfaces are shown in Figures

5o5o1o and 5o5o2o respectivelyo

5o6o Attempts to polish the coated specimens by the normal metallographic techniques proved difficult as the surface was extremely hardo Hardness tests by the usual indenter were

(139) impracticable owing to the thinness of the surface and the-

relative low hardness of the titaniunio The surface withstood

the abrasion of carborundumg SiCg and corundumg AI^O^; only with diamond was it possible to remove the surfaceo Titanium alloys show a pronounced tendency to gall, ioeo metals in

contact binding together; attempts at case-hardening titanium and its alloys by either surface heat treatment of an age-

hardened alloy, or, by a superficial layer of metal containing

0, N or C, have been of limited successo Titanium heated in

0^ or at 1000°C has a layer which exhibits spalling and gas carborization yields a carbide surface of between Z3 and 1 ^jun thick, however the presence of any hydrogen is most undesirable during the fabrication and use of titanium and its alloyso The case-hardening of titanium and its alloys by boron appears to offer the control lacking in the other techniques and is the basis of a patent application by the Author (l969)o

(lifO) SECTION 6

The section summarises the conclusions derived from the results obtained in the previous sections and suggests a theory for the phenomenalogical formation of boron carbide and a number of related compounds*

6,1. The preparation of boron carbide

The material produced by the magnesium thermal reduction of boric oxide, ^2^3' presence of carbon black, described in Section 2o1o, was of composition corresponding to the stoichio• metric formula B^C, but contained some free carbon. The unusual feature of this reaction is the fact that both reaction products

- boron carbide and magnesium oxide - are refractory compounds, in which case, each boron carbide particle is separated by mag• nesium oxide which precludes grain growth and results in the ex• tremely small crystallite sizeo Further crystallite growth can only be achieved by reheating the separated boron carbide to l800°C or grain growth by ball milling for several hours (Sameonov, 196O,

P0I86).

6o2. Sintering of boron carbide

The sintering of boron carbide, inasmuch that the crystal• lites grow in size, was achieved by heating the prepared boron carbide, under vacuum, at temperatures between 1000-l800^C« Phase studies indicated some loss of boron, particularly at the higher temperatures; there was no tendency for the particles to adhere.

Thus, the Tammann temperature, which indicates the ratio of the temperature at which particles begin to aggregate and adhere

compared with their melting point, kelvin, for boron carbide

is much greater than the 50-70% found for many ceramic oxides

and nitridesc It was reported by Jackson (1961) that for most

borides and carbides, including B^C, final consolidation of the

material is not achieved until a temperature of about 90% of

the melting point is reachedo This can be explained in terms

of the increased covaiency of these compound, compared to many

oxides, inhibiting the diffusion at the surface and at the grain

boundaries characteristic of many oxides and ionic nitrideSo

For this reason it is not possible to compact either diamond or

borazon (cubic BN) by sintering (with or without pressure) to

a pore-free state, as these represent the ultimate in three-

dimensional pure covalent bonding of the constituent atomso

Compaction of boron carbide to almost theoretical den•

sities was achieved only by hot-pressing to a temperature above

2,500°C (m.po B^C 2,450°C) and at pressures between 200 and 3OO

Kg cm~^ (limited by the working pressure of the graphite mould

sets), despite a rapid consolidation of the powder to about 30%

pore density at a temperature of lOOO^Co These results confirm

those of Hashimoto and Toibana (1969) which show the mechanism

of sintering of boron carbide to occur via a process of plastic

flow as a result of the temperature and load causing deformation

of asperities and consequent adherence according to the Murray

Model equation for pressure sintering: " ^ ' 0 " where dD ^ is the densification rate, P is the applied pressure, Cj is

(1^2) the viscosity and D is the relative densityo It should be noted that a similar result for alumina, A1-0-, where T = Sk% c. ^ m of the melting point was obtained by Mangsen et al (1960),

Compacts of boron carbide, B^C, having little porosity

6o3o The oxidation of boron carbide in air.

The high-purity, near stoichiometric boron carbide of submicron size, produced as described in 60I0, was oxidised by heating in airo Changes in phase composition, surface area, crystallite and aggregate size have been correlated with the time and temperature of the oxidations The boric oxide, ^2^3* formed, acted as a matrix for the remaining boron carbide and the newly-formed carbon and progressively retarded the rate of oxidationo The activation energy of 2^ kilocalories per mole was somewhat lower than the value of ^7 kilocalories per mole obtained by Mercuri et al, on material of larger particle size, due, it is believed, to the selective oxidation of the boron under the influence of the vitreous ^2^3 P^^^Co

60^0 The formation of an adherent coating of mixed titanium borides to a titanium alloy was achieved by heating the metal and its layer of very fine boron carbide to temperatures above lOOO^C under a inert atmosphere, or in a vacuumo Only when heated above 1^00°C was the surface free of unreacted B^C and carbons The carbon which appears to stay free of the boride

(143) phase was assumed to have been oxidised to carbon manoxide by

traces of oxygen, or water vapour in the system^ "^here was no

evidence that the titanium boride, or the base metal, suffered

oxidation during the period of heating of two hourso beta

phase titanium, rather than the expected alpha phase was pre•

sent even when the chosen base metal was pure alpha,, This

could be explained by the peritectoidal decomposition of the

lower boride of titanium initially formed to the orthorhorabic

TiB and the body centred beta titanium, these metastable phases

were frozen in on coolingo The second phase of TiB, cubic,

which was present was assumed to be stabilised by carbon in the

system and the final product identified was the stable hexagonal

diboride TiB20 The surface so produced was extremely hard com•

pared with the ductile titanium alloy and represented a method

for producing a gall-resistant surface on these alloyso

So^o The formation and stability of boron carbide and related

compounds can be considered phenomenalogically on the basis of

their crystallochemical etructureo Scott et al (196?) in their re-evaluation of. the X-ray data for ^^^^^ ^^/^^^ being in fact

(CBC)* (B^^C) indicate a possible mechanism for its formation besides its likely stoichiometry; a priori, the reaction for the

formation of boron carbide involves the diffusion of boron atoms into the graphite lattice causing breakage of the intercalated carbon bondse This results in a alternate structure as.in boron nitrideo Subsequently, aided by multlcentred bonding, many more boron atoms are accommodated into the structure as icosahedrao

It should be noted that, statistically, the formula, (CBC)*

(B^^C)', represents the highest carbon content, 20 atomic

for a structure with no two carbon atoms contiguous where the

intraicosahedral carbon is not in an equitorial position to

bond to the linear grouping C-B-Co Also significant is the

fact that BN represents the highest boron compound of the B-N

system^ suggesting that elements of a greater number of valence

electrons than carbon are able to saturate the electron defi•

ciency inherent in borono This would cast doubt on the reported

structures ^0, 'B^^S* B,P ^ (Martkovich I963) unless sup- bob Id '* o Ooy

ported by small percentages of carbon in their structureso In

borides, where the metals are themselves electron deficient,

the complexity of the boron structure is unaffected unless the

metal is present in amounts indicated by the lower borides, vizo

M^B, MB, etc-. Then the valence electrons of both the metal and

the nonraetal occupy the conduction band of the metalo These

lower borides have simple structures similar to those of the metals

in displaying maximum coordinationo

6060 The present work has indicated the pau;ity of informa•

tion available which relates the observed physical data for these compounds with their'crystallo-chemical structure, due mainly to

the difficulties in the technology involved in their production in a pure and defined state^ Further work is indicated into the mechanism of formation of these compounds and to the influence

(1^5) of contaminants on the final productso Parallel work to that on borides and carbides, is being undertaken in this Department in the field of nitrides (Glasson et al (1968, etc)o Jayaweera

(1969), I Ali (1970) and N„Gc Coles (1970) )c

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(xvi) APPENDICES

Ic Janaf thermochemical tables

(a) boron carbide, B^C (b) diboron trioxide, ^^^^ (c) carbon monoxide, CO (d) carbon dioxide, CO2 (e) magnesitim oxide, MgO (f) titanium monoboride, TiB

(g) titanium diboride, TiB2 (h) titanium carbide, TiC (i) titanium nitride, TiN

IIo IBM 1130 Computer programme for BoEoT, gas sorption data^

III. Reprints of published papers -

•Formation and Reactivity of Borides, Carbides and Silicides*

lo Review and Introduction, Joappl. Chemo 22, (l969 \ 125

lie Production and sintering of boron carbide, Joapplo Chemo 22 137 (I969)

(xvii) APPENDIX I

Reprints of

JANAF Thermochemical Tables

(xviii) •ril. more 'rfm. Lr«l. Diolr'' T. *K. -(i"-ii;«)/T 11*.n AM; LcHi K TBTRABOHOH HOKOCARBIDE (CRYSTAL) KOL. -rfT. - 55.291 w .tiOO • OUQ lNF1»000 39.070 68. 168 - - 40.9*2 67,939 * 33.669 - 1.198 .084 3100 39.01* 09,478 41,49* 86.874 - 31,340 - .077 .009 Heltlntt Data. 3 700 *0.100 70.744 *?*390 90,807 37*960 .988 - .067 1300 *0.700 71.S(89 43.720 04.91)6 17.976 7.0)0 - waa datamlned by Dolloff, VADD Tech. Rept. 60-M3, 1960 and AH' 1*00 * 1.360 75.712 - .13* was es',:oated. **.OQJ 99.004 33.039 3,073 - 1)00 74.416 ,19S *1.6U0 4*.9*2 101.197 - 31,499 11 *.1U* - .796

f • • • •

PcC' 31. lOCO; Jimo 30, 1963 CB

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leo 9.0CC 7.24'9 .5 • 1.5 kcal. Bole' 7.191 .017 BO.501 73.999 D.bll ^•le.is 10.410 ir.054 7.5bb .919 flO.*9i 71.292 38,990 )0? 11.110 12.4 72 a.Jit 7.060 no.4Cb 69.C01 10.199 ^??e.i5 I 6.:o T_ - 3C0O-K 11.650 9,182 3.724 bb.72'» 24.305 7on 11.960 •.C 376 10.083 4,405 - 8O.IC18 64.479 20.130 - 16 kcal. nole'^ ^or^ 1?.1«0 10.972 • i. . •• • . >. 17.988 9.613 - 90.04> b2.247 17.004 9C0 I.-*.390 19.4)6 11,833 6.642 - 79.^17 60.028 14.976 12.990 ;:.75: 12.660 6,089 - 7<),7TiaBlc Properilee of Selected Llg-T.- i.':o 12. '5)0 24.094 14.92B 11.916 - 83,419 51.161 R.601 Elenent Conpounda", July, 1960. nJ3 25.0)9 13. .-7: 15.617 13.217 - 80.319 48,915 7.b3b 25.•b* I3.:9j lb,277 14,910 - 93.219 46,676 6.800 14 20 13.29? 2b,918 lb.910 15.8)4 - 30*125 44.441 6,070 13,*C0 2 7.b2 7 17.916 17.168 - 90.0)3 42.219 9.427 1^ •s-. 13.913 29.39b 18,100 16.to 18*534 - 79.9*2 39.993 4.85b n.oio 21.129 18.bbt 19,890 - 79.95* 37.779 4.349 noo 13.713 29,9 3C 19.232 21.256 - fl).-41 39,469 3.876 1900 22CO U.SI3 32.521 19,724 22.b32 - 95.236 33.073 3.442 r.co II, 913 3:. 146 20.229 2*.018 - 83.143 3C.686 3.0*8 ;;oo I**:'!.-' »:.767 20.717 25,414 - 82.961 29.304 2.689 ?;.3b9 2330 U.IU 71,190 26.970 - 87.910 29.9)1 7.361 2*C0 32.94) 21.b49 14.213 28.23b - 82*631 23.965 7.060 2900 2600 1^.310 33.902 32.094 29.662 • 82.443 21.207 1.783 ;7cj i-.4:o 34.344 23,576 31*097 • 82.247 19.89b 1.526 2925 14.520 ;-.569 22.9*7 32.542 • 82.042 16.510 1.289 2900 14.603 35.390 23,357 33,997 • 91.928 14,174 1.066 3 300 14.693 3;,576 23.796 39.462 11.849 .863 - 81,605 3100 14.793 Jb.SbO 24,149 36.936 9.921 .671 3200 14.883 36.931 24.929 38.419 • 81.374 7.211 .492 yvio' •i£;§93" *»b". 9V:" • "24.-596"' V9,9'f7"" 4.9V1 ".V2V 3430 19.C70 1-.-39 :».258 *1*419 • 80.89) 2.b0b .168 25,612 J500 15.170 37.377 42,977 • 80.626 .319 .020 • 80.3 63 ib:: 1).?60 3S. Kb 29,999 44.449 •182,546 3.413 .207 3I0C 19.363 ?S,T25 26,296 45,980 •192.291 S.)79 .906 39,136 3903 19.^5; 26.631 47.920 182.322 13.731 .790

.)-*•• 1-

Drccmber 31, 1960- APPENDIX II

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Reprints of published papers:-

Formation and Reactivity of Boridee, Carbides and Silicides

I« Review and Introduction

II. Production and sintering of boron carbide

(xx) Glasson <& Jones: Forniathn and Rcaciiviiy of Boru/cs. Carbulcs and Silicides. I 125 FORMATION AND REACTIVITY OF BORIDES, CARBIDES AND SILICIDES 1. REVIEW AND INTRODUCTION

IJy IX U. GLASSON and J. A. JONES

Horides, carbides and siltcidcs arc reviewed wiih special reference to newer production methods and fabrication techniques. Crystal structures and types of bonding in binary and ternary compounds are classified and discussed. The scope and hmiiations of the Pauling-Rundle theory, molecular orbital treatment and the Ubbelohde-Sam- sonov theory are esamined critically and appropriate experimental evidence is summarised. Information so far available on ihc sintering of borides, carbides and silicides is summarised in relation to their chemical reactivity. The sintering is influenced by additives or impurities such as oxides formed by partial hy• drolysis and o.vidation. Resistance to oxidation is increased by sintering and hot pressing the rcfructories, bui since the afliniiy of (he metals is exclusively higher for oxygen, exchange reactions diminish the quality of the materials. Boridc and carbide coatings generally have poor resistance in air or oxygen, but some silicides are more suitable. The kinetics and products of oxidation of borides. carbides and silicides so far studied depend mainly on the intrinsic reactivity of the material and the available surface at which oxidation can occur.

Introduction The remaining borides and carbides arc classified generally Iticicasing industrial requirements for refractory materials as ionic or 'salt-like". They are formed by the more strongly have enhanced research on borides. carbides and silicides. electropositive metals, and are colourless, transparent, Important properties of these materials now include melting crystalline solids, non-conductors of electricity, which are point and thermal stability, hardness and briitlcncss, as well decomposed by water or dilute mineral acids. as specific eiecirical and magnetic properties. Thus, modern refractories arc not ncces.sarily of high m.p.. yet possess Methods of boridc. curbide and silicide production suitable groups of other properties such as great hardness, Larger-scale production methods low vapour tensions and evaporation rates, low reactivity Borides,^'^-^ carbides^-'^ and silicides' " •'^ can be pro• towards normally corrosive chemicals, etc. Such properties duced by direct combination of their clcmenis. Variations on arc determined by the electronic structure of the compounds, this method, governed thcrmodynamically by heats of for• arising from the position of iheir components in the periodic mation, include heating the metal hydrides or reducing the •system of ihc clcincnis.' metal oxides with boron, carbon or silicon. Alternaiivcly, " The metallic components of the refractory borides, car• carbides are obtainable by reducing the metal o.xides with bides and silicides include elements of the odd (A) subgroups carbon^-"* and a readily oxidisabie metal,e.g. Ca or Mg. or ^f Groups III to VH, Group VIII. lamhantdes, aciinides and with its carbide only,'" e.g. CaCj. For silicides, the metal ituminium.^ -^ The chemical bond in ihc lattices of these oxides arc reduced with Si or by mixtures of SiO; with C, Al ;ompounds (in addition to the s- and /^-electrons of the or Mg.^ Similarly, borides arc formed by co-reduction of netallic and non-mciallic components, respectively) is metal oxides and boric oxide at high temperatures, usually .'ormcd also by the electrons of the incomplete d- and /-levels with carbon, aluminium or a Group i or II metal,^-*'-^ e.g., of ihe transition metals. Isolated atoms of metals of the odd V>05 4-B203-h8C-2VB + 8CO. This carbon reduction is not -ubgroups of Group II. the alkaline-earth metals, do not have usually satisfactory because of considerable losses of BsO.i ny electrons in the d- and /-shells, but in compounds with by volatilisation and contamination of the product with boron, on-metals, energy states corresponding to these shells may carbon and boron carbide. Reduction of the metal oxide is ccur.^'"* The 'metal-like" refractory compounds have heiero- improved by using carbon in the presence of boron carbide esmic chemical bonding, the proportion of each type of bond or the boride of another metal, e.g. CaBt,. The boron carbide • spending on the crystal structure. Hard-cast alloys, e.g. is a good source of boron and will react with most metals or :mented carbides.^ are formed with binder metals such as their oxides, e.g. production of rare-earth metal borides Tf^hromium, cobalt and nickel. such as MBe (thermionic emitters); M20j4-3BaC-'2MB(,4- A second class of borides and carbide refractories consists 3CO. The carbon (or the additional B2O3) ensures complete ' so-called non-metallic refractory compounds, i.e., com- removal of the oxygen or carbon as carbon monoxide, e.g. lunds of B and C w-ith each other, or w'iih other non-metals 7Ti + 3B4C+B20j--7TiBi + 3CO; this contrasts with the ' Vh as N, S, P and Si. Their bond character is also hctero- reaction 8Mg+3B4C—6MgB2 + Mg^Cj, which gives a 'smic, but with covalent bonding predominating. This mi.xed product in alkaline-earth metal boridc manufacture.' ' •nfers semi-conductor properties as well as high electrical Nevertheless, the direct syntheses from the elements give sistance at room temperature. They generally have layer, the purer borides required for special applications or research; chain or skeletal structural groups or patterns, and either melt both synthesis and fabrication proceed during hot pressing.' with decomposition or decompose below the m.p. The purity of the product largely depends on that of the The three elements, Be, Mg and Al (typical elements of parent metal, since the impurities in boron are usually more Groups H and HI) are iniermcdiatc in their ability to form volatile. Contamination by the crucible material is mini• refractory metal-like and non-metallic borides and carbides. mised by using boron nitride which has been heated in hy• Thus, their borides and carbides are moderately refractory drogen to remove oxygen. However, exact sioicheiometry is semi-conductors. difliculi to attain, particularly with very volatile metals.**

J. appl. Chcm., 1969, Vol. 19, May 126 Glasson Jones: formaihn and Rcactiviiy of lioridcs, Carbides and Silicides. I

Reductions of BiOj, oxides of carbon or SiO. with meials din"erenl boridc phases exhibit about equal thermal stability are generally less saiisfaciory. as the products often contain regardless of composition. On the basis of m.p.. the strength large quantities of the metal oxides. Relatively pure silicides of the M-B bond is increasing with the atomic weight of the of definite composition arc produced by reacting the metals metals with each Group (IV, V and VI) and decreasing with with silicon in a copper menstruum. the atomic weight within each period. The generally lower stability of boron carbide, Fig. 1(a). Smallcr-scalc production methods makes it an excellent source of boron, and it reacts with most Small amounts of pure borides. carbides and silicides are metals or their oxides, alone or in the presence of B2OJ. deposited (a) by hydrogen reduction of mixed vapouriscd H, However, although energetically feasible, these reactions may C or Si compounds and appropriate nieial compounds ai a be kinetically unfavourable. The solid state reactions are heated surface, or (b) by decomposing volatile B, C or Si facilitated by fine grain sizes and pressing of well homo• compounds at the heated surfaces of appropriate metals.''' genised materials as in reactive sintering. Thus, Glaser^** Boride coatings, e.g., Ti, Zr, Nb, and V diborides and B^C, obtained borides of Ti. Zr, Nb and Ta by hot-pressing may be applied by plasma-jci methods, normally using argon mixtures of boron with the respective metal hydrides or as the carrier gas.^ A ZrBi-MoSii plasma-applied coating is carbides, and borides of V, Cr, Mo and W by hot-pressing claimed to withstand 2000° in an oxidising atmosphere.^ boron with metal or carbide powders. Later, boron carbide Finely-divided non-pyrophoric carbides have been produced was hot-pressed with metals or their hydrides or carbides. recently'*' by subjecting a volatile halidc (e.g., chlorides of B. Likewise, silicon combines with metals only at relatively high Si, Ti, Zr, Hf. V. Nb. Ta. Mo. W. Th and U) and a gaseous temperatures, and silicide formation is accelerated by using hydrocarbon (e.g. CHa. CHa. C4H,o. C,H,. C.H^ or mixtures of components in finest dispersion.Silicon is able CoHo) to the action of a hydrogen plasma jet (at 2000— as well to reduce metal oxides under analogous conditions 5000° for 10-*—10-^ sec). The powdered carbide is sintered to carbon. At high temperatures (above 1700°), SiO rather in a rotating drum at 900—1000°. Mixed carbides arc than Si02 is formed under reducing conditions. Use of obtainable from mixtures of metal halides, preferably chlo• temperatures above the m.p. of SiO. is recorruncQdcd, to rides. Some carbides, e.g. Cr^Cj, arc deposited on metal facilitate ultimate separation of silica. surfaces which have been oxidised with CO or CO2 at higher The carbides of the fourth and fifth odd (A) subgroups also temperatures.'^ have the greatest stability, e.g., SiC and TiC in Fig. 1(b). Other methods include fused-salt electrolysis of the metal This progressively decreases for carbides in the lower groups, oxides, e.g, TiO,, ZrO., VjO,. NbÔj, TaÔj, CrÔj, e.g. CaC2. and AI4.C3, and for transition-metal carbides in MoOj, WOj and UjOs. with boric oxide or borax, usually in Groups VI to VIM, e.g. WC. an electrolyte flux of an alkali or alkaline-earth halide or The relative afllniiies of metals for boron, carbon and fluoroboratc.^-^-'-'^ Elemental boron impurities in the silicon are compared in Fig. 2. The borides are of greater products are separated partly by flotation. Under appro• stability and more readily formed than the corresponding priate conditions, fused salt electrolysis of carbonates can carbides and silicides. This is ascribed to the relatively open produce free carbon 10 form carbides with metals present,"* structure of elemental boron compared with that of carbon e.g. Mo and W. Well developed crystals of silicides, e.g. of or silicon. Elemental groupings (Bjj-icosahedra) are re• Ti, Zr and Co, are formed by electrolysing fused alkali tained in boron, whereas the carbon (graphite) and silicon fluorosilicatesand the respective metal oxidesor fluorides.'''•^" (diamond) lattices are disrupted completely, thus allowing Small amounts of carbides, mainly of alkali or alkaline- ready ingress of the metal component in the formation of earth metals, arc produced by passing acetylene into solutions binary borides, e.g. AlB,o (but not AlBji);^^ ScB,2 and of the metals in liquid ammonia.'° Initial products such as YB12 have Bi2-cubo-octahedra.^^--^ On the other hand, the CaC:. C2H2, 4NH3; Na.C^, C^H,; KC^. C^H^: or LiC,. solubilities of boron and silicon in solid transition metals arc C2H2, 2NH3, all decompose to simple carbides in vacuo at usually small,except where the metals have substi- temperatures up to 3(X)°. The carbides of the heavier metals, tutionally dissolved boron (Mo and \V-B alloys) or silicon notably of even Group I B, are obtained usually by passing (Mn, Fe, Co and Ni-Si alloys) with unit cell shrinkage. acetylene into metal salt solutions.'° Slight increa.ses in cell dimensions of other transition metals indicate that boron and silicon are dissolved interstitially, un• less the increases have been caused by uptake of nitrogen or Thermodynamics of boride. carbide and silicidc formation oxygen. The latter elements are more electronegative but are The stability of borides, carbides and silicides and their usually small enough to be dissolved interstitially. The free production at various temperatures are related to their energy of the intermediate phases greatly influences the standard free energies of formation, AC°r;'-^' more negative homogeneity range of the primary solid solution. Thus, the values of AC°/' indicate stabler compounds. These are com• very great stability of ZrB; and HfBj vitiates an extended pared for some of the more important compounds on an interstitial solid solution of boron in Zr and Hf although Ellingham diagram^^ (Fig. I), showing the temperature varia• the size relationships arc not too unfavourable. Electronic tion of AC°7' per g-atom of boron (a), carbon (b) and silicon factors also govern the extent to which the diflêrcnt non- (c). Of the metal diborides, Fig. 1(a), those of the fourth and metals are accommodated inicrsiitially in solid metals, and fifth odd (A) subgroups, e.g. TiBj, have the greatest stability. these are discussed in the next sections. This progressively decreases for diborides in the lower groups, e.g. MgBj. and for transition metal diborides in Groups VI to VIII, e.g. CrBj. Disilicidcs show a similar Relalionship between bonding and crystal siruclurc of trend, Fig. 1(c). Further information^^ shows that the di• borides, carbides and silicides borides are the most stable borides of the Group IV transition Hagg^^-^-' suggested that the binary refractory borides and metals, but for metals of the sixth group, the monoborides carbides of the transition elements had simple "normal' appear more stable than the diborides. For Group V, the structures when the radius ratio, t\:r„, of the non-metal and

J. appl. Chem., 1969, Vol. 19, May Glasson d Jones: Formation and Reactivity of liorides, CarhiJes and Silicides. I 127

1000 ?000 1000 ?000 TEWPERAIURE.S [-ig. I. Varioiion of free enerijies of formation {per atom of B, C or Si) of horu/es. Fig. 2. Variation of free energies cnrhiJes (iiul siliciiles wit/i temperature (Single points for silicides arc csiimaicd of formation {per atom of metal) \alues al 29S°K from i.\H(. 298" \'aliie. assuming AS, - 0) of horidcs. carbides and siiicides with temperature metal atoms was less than 0-59 (corresponding to a nieial to the electron cloud of the compound, at least partially filling non-metal radius ratio of over 1-70). Higher non-metal the electron defect of the metal atoms. The additional forces concentrations increase the unit cell dimensions of the in• of the donor-accepior inicraciion greatly strengthen the terstitial phases, cfTectively making ihe radius ratio less interatomic bond. Therefore, heats of formation of borides, favourable for normal structures. Nevertheless, many of carbides and silicides (and nitrides) increase uniformly with these compounds are still metallic in character, but ihcir greater 'acceptor ability". l/A'/;, of the atoms of the metallic structures become more complex with decreasing si/c of the components, where A' is the principal quantum number of the metal atom. Later research' indicates that this limiting radius partially filled d- (or /-) shell and // is the number of electrons ratio rule is valid only for carbides. in this shell.'*'* Likewise, the lattice energy and the hardness Kiessling^** ascribes the deviation of borides to the ten• of the metallic compounds increase. Their electrical re• dency of B atoms to form chains, sheets or three-dimensional sistance decreases with a rise in l/A''/;,'^ whereas the work networks. Hiigg's rule seems restricted to phases not con• function of the electrons increases in this direction in the case taining directly interconnected non-metal structure elements. of thermionic emission.*° The electron density also depends Accordingly. 8 dilTerent types of boride crystal structures have on the ionisation potentials of the non-metal atoms, their been described by Schwarzkopf & Kieffer^ for boride phases electron-donor ability increasing in the direction of O. N. C. ranging in composition from M^B to MB12. Further varia• B, Si. The latter concept is repudiated by Rundle,*^ but tions are included in later reviews^'-^' covering phases from Schwarzkopf & KiefTcr^ consider that suRicicnily strong metal M4B to MB,3. and possibly MB70. In transition-metal to non-meial bonds can be formed if the lighter atoms in the silicides, the comparatively large radius of the Si atom refractory compounds assume the metallic state. The (117 A) ensures that Magg's critical ratio is exceeded. Ubbelohde-Samsonov theory provides a wider and more quantitative interpretation of physical properties and chemical General theories reactivity. Its scope in rationalising existing knowledge for Many of the refractories first investigated-^^--*-* were monoborides. carbides and silicides is illustrated now; types of ex• carbides and mononiirides (MX-type) having radius ratios perimental data which may extend its applicability arc indicated. within the range 0-41—0 59. These had rock-salt structures irrespective of whether the original metal had a cubic close- Borides packed structure or not. Rundle^^ considered that there was The relative electronegativities of boron and transition octahedral metal to non-metal bonding, and developed metals suggest that electron transfer in borides should occur Paulings basic concept^''-*' of the resonance of the 4- always from metal to boron. This accords with Pauling s covaleni C or N bonds amongst the 6 positions. Physical calculations (using his relation between bond length and properties such as hardness, high m.p. and electrical con• order"*') that in borides of the type, MB, one-third to one- ductivity were interpreted partly on the basis of resonating quarter of an electron would be transferred to the boron. bond structures and on ionic structures, involving essentially However, the actual electronic structures depend con• homopolar and heteropolar forces. siderably on the types of boron and metal lattices present. A difTercni theory, first advanced by Ubbelohdc'*^-'*-* and Nevertheless, more recent molecular orbital treatment of Umanskiy."**'"*^ has been developed by Samsonov & Nesh- intermediate and higher borides (MB. to MBj.) also in• por,*^ and reviewed recently for nitrides.'*'' Bonding dicates electron transfer to the boron lattice. The Ubbelohde- is provided by transfer of non-metal valence electrons into Samsonov theory confirms the latter findings, but contradicts

J. appl. Chem., 1969, Vol. 19, May 12! Glasson & Jones: Fonnaihn and Reaciivity of Borides, Carbides and Silickles. I

Pauling's conclusions by indicaiing cicciron transfer from are consistent with ir-bonding in the boron layer, possibly boron to metal for the lower (mcial-rich) borides. ll extends achieved by electron transfer from metal to boron.The the molecular orbital treatment by explaining the inability excess valence electrons of the transition metals account for of the even (B) subgroup metals to form higher borides. the metallic properties, and the conductivity of YB; accords Appropriate experimental evidence is summarised in relation with one free electron per metal atom."' There are in• to boride crystal structures. sufficient experimental data to extend this interpretation to consider possible effects of unfilled inner orbitals in the metal Binar>- metallic phases atoms and to account for the inability of most Group B metals to form borides. Lv^ver borUies {Isolated atoms, pairs or chains) The hexagonal close-packed metal lattice in diborides Boron has a small ionisation poicniial. Hence, in com• permits considerable meial-metal bonding which might pounds where the B atoms are isolated from one another stabilise transition-metal diborides. However, formation of (M4B to MjB)," the boron valence electrons are mainly in the crystal structures characteristic for metallic compounds does free J-lcvels of the transition-metal atom if the latter has a not necessarily arise from the transitional nature of their sufllcicntly high acceptor ability.*'-*^ This gives typical atomic components. Thus, aluminium diboride, AIB2, is not metallic phases, similar to intermetallic compounds. a metallic conductor, in contrast with the isomorphic di• Electron transfer from boron to metal is confirmed by borides of the Group lll-IV transition metals.-* This boridc. magnetic measurements on CrB, MnB, CoB, NiB.*^'"* although isomorphic, docs not form solid solutions with I-CjB*^ and CoiB,^*" and Mossbauer measurements for the transition-metal diborides."° isomer shift and hyperfine magnetic field for the two iron borides.*' The latter decreases from Fe to FcaB to FcB. more greatly than expected through distortion or expansion Higher borides {Three-dimensional boron networks) of the metal lattice alone. Other metal-rich borides have not Tetragonal teiraborides, MB4, are formed by Ca,'"' Y,*'' been studied, but Mossbauer measurements on Fc-Si and jh 68 yj i^B^io 71 rare-earth clements,^^-''^-''^ Mo and Fc-AI systems crystallographically similar to Fe-B again \V.'' Their crystal structure is a hybrid of MB^ and MB„.-^'' accord with electron transfer from the non-metal to the iron Metal hexaboridcs, MB^. (cubic lattice-type CaU^. 0*h) ure aloms.*'''*^ formed by d- and /J-transiiion metals (Y, La, Ce. Pr etc.) and In the formation of pairs (M3B2), and single chains (MB), non-transition metals (Ca, Sr, Ba).Quantitative treatment double chains (M^Bj). and extended 2- or 3-dimensional of the bonding in metal hexaborides has been based initially on networks of B atoms, as in MBj. M2B5, MB4. MB(, etc.,-' a molecular orbital calculations (for isolated B^-octahcdra) considerable proportion of the /7-electrons of the boron using the 2.v and Ip boron orbitals.This suggests that forms covaleni B-B bonds, with a smaller proportion going 2 electrons must be provided by the metal atoms to give a into the electron cloud resulting in a metallic bond. The configuration M^+CBe)^". More detailed calculations in• degree of mclaliic bonding decreases as the B/M ratio in• volving the 3i-, 3p and 3t/orbitals"" are more m accord with creases. the semiconducting properties of CaBe, SrB^ and BaBf, with The essentially metallic character of the interatomic bond small energy gaps of 0 1-0-4 cV.''' The magnetic properties is comparable with the Hume-Roihcry electron phases, the of the rare-earth hexaborides suggest tervalent metal atonis.^' nature of the crystal structure depending on the electron Their conductivity data indicate generally one conduction concentration. Increasing concentration produces a sequence electron per metal atom, with probable transfer of the other of crystal lattices, viz. body-centred cubic, base-centred two electrons to the boron lattice.The borides with the hexagonal, face-centred cubic, simple hexagonal, for similar larger cell constants, e.g. EuBt; and YbBe, have almost zero atomic radii ratios r^:r^, where X = B, Si, C, N. The face- conduction electrons per metal atom, but the properties of the centred cubic lattice, most characteristic of Groups IV and V pure metals suggest a possible decrease in the effective valency metal carbides and nitrides, corresponds to an electron from 3 to 2. The other exception, SmB^, has anomalous concentration of 5-5 to 6 electrons per atom. electrical and magnetic properties.' Hall-constant measure• ments for lanthanum hexaboridc are interpreted on the Intermediate borides {Two-dimensional boron networks) availability of one "excess' valence electron per La as a negative current carrier in sioicheiomctric LaBft.^-* This im• This group includes some of the best electrically conducting plies a theoretical lower limit of homogeneity of Lao-tTBe borides of highest melting point, and hardest of all the borides. with each La contributing all 3 valence electrons to the boron Metal diborides, MB;, represent the transition between the net. Then the phase should be an electrical insulator, but metal-rich and the boron-rich types of boride. Interpretations mechanical instability caused by the large number of metal of Hall coefficient and resistivity measurements have been vacancies restricts the obser\ed lower limit to Lao-siB^.*'*' based on transfer of boron electrons to the metal-band system,making any boron-boron bonding very weak. This Molecular orbital treatment of the bonding in the dodeca- is refuted by thermal expansion and variations in lattice borides, MBjj, is in accord with a closed-sheir con• constants suggesting considerable rigidity in the boron figuration requiring 38 electrons, giving a probable M^HB,2)^~ lattice.^*' The axial ratio, cja (the V;"-axis is in the plane of the arrangement.^"* Therefore, divalent metals should give boron lattice), increases as the size of the metal atom in• dodecaborides which are insulators or at least poor con• creases. ductors. In ZrB,2, the 4 metal valence electrons confer Molecular orbital treatment of diborides is based on metallic properties similar to those in rare-earth dode- MgBi as the closed-shell prototype of the transition-metal caborides.^** diborides.*^' This is more satisfactory in that the transfer of .Samsonov & Ncshpor have suggested that cubic hexa- the two electrons from the metal to the boron atoms (in the bon'des are formed only by metals having first and second formulation M^'(B-)2) makes the boron layers isoelectronic ionisation potentials less than about 6-7 and 12 cV.^** This with graphite.**^ Nuclear magnetic resonance measurements excludes the transition metals (6 5-9 and 12-20 eV) and

J. appl. Chem., 1969, Vol. 19, May Glasson & Jones: Formation and Reactivity of Borides, Carbides and Silicides. I 129

Be (9-3 and 18-2 cV) and accounts for the failure of B sub• range of carbon content, from pure B to BjC. cf B to BijBe group meials such as Ga, In, Tl, Gc, Sn, and Pb to form to B^Be.'"^ In the higher boron phase. Bi^Cj, one B atom borides. through inability lo transfer electrons to the boron apparently replaces a C atom in the linear triad. More laiiicc.'"' Substitution of .sodium in CaBe and ThB<, up lo recently, Hoard & Hughes"^^ indicate that the BijC.^ has a concentrations of Cao sTNaoaiBt; and Tho.23Nao.77B„*^ structure of this type Bi2(C-B-C) where a C atom has re• could require the equivalent of 1-57 and I -69 electrons lo be placed a B atom in the icosahcdron. In contrast, excess C in transferred to the boron. The relative importance of elec• B12C:, is manifested as intercrystallite graphite. tronic or structural factors may be ascertained if further alkali-metal hexaborides similar to NaB^,^® and KB^,^^ can be Ternary phases isolated. However, the ionisation-potential criierion^^ does These are formed sometimes in systems of borides with not hold for diborides, Since these are formed by metals of other borides, carbides, nitrides and silicides.' Isomorphous all types, e.g.. AgB, and AuB;."*^ complete transfer of 2 borides, e.g., simple hexagonal diborides, should form con• electrons may not be necessary, nevertheless, some metal lo tinuous series of solid solutions where the radius ratios of the boron electron transfer is siill highly probable in contrast to metal atoms arc favourable. By analogy with conditions in the reverse transfer in monoborides and metal-rich borides. carbide systems, e.g., TiC-WC. solid solutions are formed Studies of the spectrum of electrons are limited and include possibly between separately non-isomorphous borides. Thus, investigations of the A'-ray spectra of La, Ti. V. Nb. Mo and (he high-temperature modification of MoB (p) is stabilised by Cr borides.-''-'"-^** and electron paramagnetic resonance solid solution with CrB:'°-* at least 50 wt.-% MoB is soluble studies of rare-earth hexaboridcs.''^ in CrB. Likewise, small amounts-of titanium boridc arc en"cclive. and both MoB and \VB dissolve in ZrBi with an Binary iioii-metal lie phases excess of boron.The compounds MojCoBj, Mo2NiB2. These phases include non-metallic refractory compounds Mo2FeBj. M02C0B4 and MojNiB^ have properties suitable of boron with C, N. Si, P and S, or analogues with As and for cutting-tool maicrials'"'-'*^^ where the low B/M ratios possibly oxygen. They arc characterised by covalent bonds in are consistent with high degrees of metallic bonding. In the crystal lattices, and cither melt with decomposition or some ternary and multiple M^'M^Bj borides {rs\>rs\'') the decompose below the m.p.'--^ They have semiconducting smaller M' atoms probably occupy the cubic holes between properties and high electrical resistance at room temperature, the A-laycrs in the UaSij-typc structures. and become p- or /;-typc conductors when normal sites of More recent research also suggests that the cubic-F type their crystal lattices arc replaced by atoms of foreign metals. structure of the so-called monoborides. TiB, ZrB. HfB and Their crystal structures generally consist of linear, lamellar PuB may be stabilised by O, N or C impurities which could or ihrcc-dimcnsionally extended structural groups. form ternary or quaternary phases.-*'•''•*•'" ' TiBN has been described and its infra-red radiation has been studied.' Low er horides {Isolated boron atotns) A number of rare-earth MB, phases, with x varying from 3 to 4 and with simple tetragonal structures, have been prepared The most important of the MB-type compounds is boron only in the presence of carbon."'-' Hence, they are probably nitride. Its hexagonal form ("white" graphite or lampblack) borocarbides of unspecified and possibly variable B and C has unit cell dimensions very similar to those of the iso- content. Several borosilicidcs crystallise with the CrjB,! elccironic graphite.-"' In both crystal structures, the planar siructure.^^ and in the ternary phases which arc isomor• hexagonal nets of atoms are separated by half the length of phous with WjSij. the B atoms replace the Si atoms only in the c spacing. The B atoms are situated directly above the N the antiprismatic holes giving MjSiaB. e.g., Fe4.BSi2B and atoms in the adjacent layers, in contrast to the C atoms in Cou.7Si2B. There is increasing deviation from stoicheio- graphite being directly above the centres of the hexagonal metry in the series CrgSij, Fe4.8Si2B, Co4.7Si2B (WjSi.^- rings in successive layers.'"' B and N atoms alternate in the structure) and the Nowotny phases"^ ZrjSia (C). Taj-wSia rings giving the structure with B-N bond lengths of 1-45 A. (C), Moa-oSijC (MnsSij-structurc). which requires explana• The basic skeleton bonds are formed by sp^ hybrid orbitals tion in terms of fundamental electronic interactions. Ternary of the B and N atoms: the remaining electrons exist in de- non-metallic boron carbonitrides probably exist in some localised rt-orbitals extending above and below the whole crucible materials"* and electrical insulators protecting plane, but there is an energy gap of sufilcient magnitude to metal thermocouples."' make BN non-conductive, cf graphite.

Carbides Higher borides (Three-di/ncnsional boron networks) The more conventional classification of carbides,"'*-"'' as The most important compound of ihis group is boron (A) salt-like or ionic, (B) covalent, (C) interstitial and (D) carbide. B,zCj. The remainder, viz. Bi2Sij, B12P2, B12AS2. FcjC-iypc. has been defined more precisely by Samsonov* B12S and B12O;. have crystal structure analogous to boron in terms of the electronic and crystal structures of the carbide carbide and a-boron.^"^ The boron carbide unit cell is rhom- phases. The carbide-forming elements arc those with (1) bohedral with one BjjCj structural unit.The B atoms valence .v-electrons with complete or incomplete deeper at the vertices of compact, nearly regular, icosahedra are shells. (2) valence 5/7-elccirons and (3) valence sd- or sfd- linked by B~B bonds to form a three-dimensional network. electrons, i.e. with completed d- and /-shells. Classes (1) and In each unit cell. 3 C atoms arc arranged linearly (parallel to (3) have been subdivided, giving 5 groups altogether. the 3-fold axis of the rhombohedral cell) in large holes formed by the approximately close-packed large boron icosahedra. Binary carbides The crystal structure of ot-boron is approximately that of boron carbide with the C atom omitted from the large Salidike or ionic carbides interstitial holes.Since boron carbide is stable over a wide These are formed by non-transition metals having valence range oT composition.'"' this suggests a possible extended .?-clectrons (with completely occupied or built-up internal

J. appl. Chcm., 1969, Vol. 19, May 130 Glasson & Jones: Formation and Reactivity of Borides, Carbides and Silicides. / electron shells) with first ionisalion potentials from 3 to formation of gaseous cyanogen and solid covalent carbides 7 eV, i.e. carbides of alkali or alkalinc-carth metals. The such as P2C6 of low thermal stability but acid and alkali formation of strong covalent links between the carbon atoms resistant.'^^ is ascribed to the stabilisation of the .v/?-electron configuration Metaldike {or metallic) carbides of the si\-iransition meials by the .v-electrons of the alkali metals. Carbide phases with relatively large carbon contents are formed, especially for Metals with up to 5 electrons in the ^/-shell form less stable metals with the lowest first ionisation potentials. Thus, J" and more stable d^ configurations.'^^-'^'^ Metals with lithium (5-57 eV) forms only one stable carbide, Li2C2. but over 5 ty-clectrons form d^ and f/'° configurations. The sodium (509eV) and potassium (4-32 cV) form poly- statistical weights of the larger configurations increase with carbides.'"* NaCj. NaC,,. NaC.e, KC^, KC,„ (or KC, more f/-electrons. Hence, the energetic stabilities and m.p. and KCj.,)- The potassium dicarbidc, K2C2. is formed only of TiC, ZrC and HfC increase with the greater statistical with difiiculiy and Rb(4 l9eV) and Cs (3-86 eV) do not form weight of the most stable c/* configurations; accordingly, dicarbidcs but give polycarbides. MCj and MC|f,. NbC and TaC have the highest m.p. On the other hand, the The higher ionisation potentials of the alkaline-earth hardness depends primarily on the antibonding action of the meials restrict the tendency to form complex anions, and only collective i-electrons, so that TiC (m.p. 3150° and hardness carbide phases of type MC2 are given. Carbides of Be and 3000 kg mm--) is harder than NbC (3480^ 1950 kg mm^') Mg are intermediate in character between this group of and TaC (3880^ 1600 kg mm-').''' The state of the col• carbide phases and the covalcnt carbides. lective electrons also specifies the type of conductivity,the moderate thermo-eleciromotive forces'-*' and thecharacteristic Covalent-nu'iallic carbides crystal structures.'-'' These are formed by metals having outer ,v-electrons with The simpler and the l-e^C-typc complex interstitial phases first ionisation potentials of 7 to II eV. such as Cu and Zn in (conventional (C) and (D) classes) are formed by transition Groups I B and II B. Under the usual conditions, these metals having fewer or more than 5 J-electrons, respectively. carbides arc not formed because of the isolation of the In the latter case, the high statistical weight of the d^ con• stable electronic configuration of metals and carbon;^' the figurations for Fe (J".v^). Co (//'.v') and Ni Ui^s^) allows part metal atom configurations r/'".v' and f/'".v* de-stabilise by of the .v-electrons to convert to the collective state. This .v-electrons the lattices of configuration f/'" and .v/;-*. ensures completeness of the electronic configuration of the carbon aioins which are isolated in the centres of triangular Covalent carbides prisms of metal atoms in the cementite lattices. It also agrees These arc formed by elements having outer i77-cIcctrons in with the endothermicity of the formation of such carbides.' the state of the isolated atoms. Group III elements, B. Al. The isolation of complex covalently bonded groups of metal Ga, In and Tl have a characteristic s'p configuration which is atoms is indicated by A'-ray spectrum investigations showing convertible to sp^. The latter can be stabilised up to sp^ or in larger numbers of trapped electrons with increasing statistical disturbance to the .v/?-* configuration of the C atoms. Boron weight of the d^ stales.'•'•'•'•'•* produces the most stable sp configuration, giving very stable Where the d^ and d^ stable configurations can vary widely, carbide phases, particularly B12C3. Iilectron transfer from as in TiC or ZrC, there is only one carbide and it has a wide carbon to stabilise the boron configuration leads to the range of homogeneity.'^^ Larger numbers of carbide phases formation of linear chains of C atoms similar to the allcnic with narrower homogeneity ranges are given, as d^ becomes groups. The separation of two very stable configurations in the more probable configuration.'-^^ Thus. V and Nb form 2 boron carbide explains the large energy breakdown.'-*^ phases, MC and M2C, Cr forms 3 phases with high metal semiconductiviiy, decomposition on melting,' •' high chemical content. M^jCft. MTCI and M3C2. while Mn gives 5 phases, stability, great hardness, etc."- The overlap between the M4C. M.jCt,. MjC. M5C2 and M-Cj. Similar patterns are electronic configurations is greater in AUCj causing lower observed in other periods and series. The homogeneity stability and greater chemical reactivity; ALC^ is decomposed ranges progressively narrow fron-i about 20-30 at.-%C for by water to form CHj similar to BciC. Gallium carbide. Ti, Zr and Hf (^/V), to 8-15% for Nb and Ta (f/^?'), to Ga2C2. is extremely unstable (heat of formation only 6 kcal 2-4% for Mo and W {d^s'^d^'s^), to I -2% for Fe (f/V)-type mole."'), while In and Tl carbides are apparently unknown.' metals. Further comparison of the hardncss'-'-'-'^^ and Group IV elements. C, Si. Ge. Sn and Pb, have an s^p^ electrical conductivity'-'^*' of the J-transition metal carbides configuration tending to acquire the sp^ stable configuration shows an uninterrupted trend from simpler interstitial phases for the tetrahedral structure of diamond (carbon carbide). to carbides of the cementite type that previously were classi• Again, increased overlap reduces the hardness of the SiC fied separately. compared with diamond and decreases the width of the for- biden zone."** There are numerous modifications of SiC Saltdike covalent metallic carbides of the sdUtransition metals based on combinations of the bond functions o{ sp^. sp^ for This group of transition metals, which includes lanthanides Si and the 5y7^-carbon.'The statistical weight of the con• and actinides, forms several types of carbide phases: M3C, figuration is much higher for SiC than B4C. Thus, SiC MC, M2C3 and MC2.The phases MjC and MC are generally has higher chemical stability in various media, but it typical interstitial phases with isolated C atoms, but the decomposes even before melting through its inability to form M2C3 and the MC2 phases (Pu^Cj and CaC: structural completely independent electron configurations. Gc. Sn and types) contain paired C atom arrangements. Pb do not form carbides, as the configurations of sp^ and of a The M3C carbides resemble BcaC by hydrolysing only to lower order become even less energetically stable.'^* CH4 and Hj.'^" The carbides MC and M.Cj hydrolyse to Group V elements, N, P, As. Sb and Bi, have an s-p^ con• hydrocarbons, mainly acetylene, and hydrogen. More figuration convertible to sp'^ and sp^-^e, with the electron acetylene and less hydrogen arc evolved from the dicarbides, capable of transferring to acceptor partners or ensuring the MC2. suggesting that the bonds in MC and M2C3 are co- formation of electron pairs. This indirectly explains the valcnt-mctallic with a larger proportion of the covalent bonds

J. appl. Chcm., 1969, Vol. 19, May Glasson & Jones: Formation and Reactivity of Borides, Carbides and Silicides. I 131

in the MC2 carbides. The quantity of hydrogen evolved atoms are isolated. The higher silicides, like the borides, seems to be related to the number of electrons present in the contain chains, two-dimensional layers or three-dimensional f/-states. The probability of f-rd transitions increases with frameworks of Si-Si bonds. However, all of the borides have decreasing number of possible terms.''*^ The very probable metallic conductivity, whereas several transition metal sili• 4f~'5d transition in La and Ce compared with the other cides are either semiconductors (CrSij, FeSii. RcSi2) or have rare-earth metals gives dicarbides forming more hydrogen. a conductivity between that of metals and semiconductors As the carbon content increases in the series M3C, MC. (MoSi2, WSij).'"*** A theoretical analysis of the electron M2C3. MC2, the relative proportion of ionic to covalent structure of MoSi2 indicated that the ^/-state of the metal (and bonds increases, and the metallic bonds accomplished by col• its corresponding energy bands) in MoSi; has vacant sites, lective electrons are decreased. Hence, the more salt-like car• just as in metallic Mo.'=" Tungsten disilicide of similar bides of high carbon content have some semiconductor structure, also exhibits p-iype conductivity, whereas in the properties while the lower carbides have high electrical con• remaining metallic disilicides of Groups IV-VI transition ductivity, distinguishing this group from the salt-like and the metals the current carriers are electrons.''*'* Nevertheless, metallic carbides. Carbides of yttrium and scandium are vacant rZ-statcs are probably still present in metallic disilicides intermediate between metallic and salt-like covalcnt metallic with /i-type conductivity and in lower silicides. according to carbides. the correlation between resistivity, 'acceptor ability' of the metals'*^ and current carrier mobility. Ternary phases The A'-ray absorption spectra of silicides of Ti'" and l-ormation of continuous series of solid solutions by iso- V confirm that the higher silicides have fewer free electrons morphous carbides and carbide-nitride systems depends and greater conductivity, but the semiconducting CrSi2 has mainly on diflcrences in atomic dimensions being less than lower conductivity.^ Crystallochemical calculations using 15%.^ '**' However, in NbC-ZrN no solid solutions form, Pauling's method"-' give approximately similar Cr valencies although the size factor is favourable, and there is limited (5-5—5-7) for Cr3Si, Cr3Si3 and CrSij. This indicates an solubility in VC-ZrN where the radius difi"erence exceeds essentially homopolar Cr-Si bond in keeping with the semi- 15%. This suggests that the state of the collective .v-electrons conductivity properties of CrSi2.^-'''**•'" There is also may be of additional importance, and their aniibonding spectrum evidence of a heteropolar component in the M-Si action would specify the properties of cemented carbides,* bond in higher silicides'and of directed covalcnt M-M i.e. readily sintered materials where metals have alloyed with bonds in lower silicides (where the disilicide has metallic binary and ternary carbides. The alloying metal may form conductivity)."* The high-temperature form of iron disili• a separate carbide, e.g.. cementite, Fe3C. in sintered compo• cide, (x-leboite, has metallic conductivity but the low-tempera• sitions of TiC with iron or steel.In carbidc-boride ture 3-Ieboite is semiconducting, having a Si lattice analogous systems, the borides generally are more stable than the to pure Si and Ge.^ At 0-400°, the //-type conductivity of corresponding carbides; some monoborides, e.g. Ti, Ta, be• the C/j-phase is changed to /7-type by 0-1% Al impurities. come unstable and form the respective diborides and mono- Ordered solid solutions are characterised by a significantly carbides.'"* higher temperature coeflicient of resistance than for alloys of low Si content;"*" the coefllcicnt remains almost constant up Silicides to the Curie point, but it is negative in the paramagnetic state.

Binary siiicides Ternary phases The Cu-Si system exemplifies wide variations often found Thermodynamic data so far available"' suggest that the in composition and crystal structures of silicides. Phases decrease in heat of formation, i.e. stability, with increasing CujSi (P-Mn structure). CujiSi^ (7-) and Cu3Si (c-) metal group number from IV A to VIII becomes less marked conform with the Humc-Rothery rules'"*-* and the general in the sequence; nitrides, carbides, borides and silicides. In theory of metals,''*'* and are analogous to CuaSn (3-brass), the ternary systems M'-M'-X, the metal with the lower Cu.uSng (7-) and CujSn (c-). Magnesium silicidc. MgzSi. group number is almost invariably concentrated in the is analogous to Mg.Ge, MgjSn, Mg^Pb and BCiC which all phase most rich in non-metals.^' Similar trends in stability crystallise in the fluorspar structure.' This enables the first are shown in the M-X'-X" systems, e.g. the M-Si-B systems Brillouin zone to just accommodate the 8/3 electrons per are dominated by the diborides and disilicides of metals of atom in all these compounds, which arc in fact either in• lower and higher group numbers respectively. Information on sulators or semiconductors. The molten compounds are good M-Si-C. M-Si-N and M-B-N systems"' demonstrates the conductors, since the ordered crystal structure is now absent. great stability of the Group IV A and VA metal carbides Higher silicides (up to MSij and also BaSi3) are given by the and nitrides. The latter phases dominate the ternary systems other alkaline-earth and the rare-earth metals for which im• of these metals, but borides and silicides occur over much proved methods of preparation have been reported re• larger fractions of the ternary systems of the later group cently.'•*'-'•*' The silicides of Groups IV A. V A and VI A transition metals. The M-B-N and M-Si-N systems are also have widely varied crystal structures.^ There are 6 complicated by the comparatively great stability of B and Si din"erent structures for the disilicides, whereas all of the nitrides which may form 2-phase equilibria with the metal- diborides arc isomorphous and most of the monocarbidcs and rich phases, including clementar>' metals. nitrides are isomorphous with each other. The compara• tively large diameter of the Si atom precludes formation of Kinetics of boride, carbide and silieidc formation interstitial structures, yet some of the silicides are metallic in Vapour-phase deposition of borides, carbides and silicides character and are therefore classifiable as hard metals. has been extensively studied by Powell and Campbell and In difl"erent silicides of J- and /fAtransition metals, variations their co-workers.'^-'^^ The kinetics are interpreted more in Si content and atomic radii ratios produce difi"erent easily than those of the preparative methods involving com• classes of structure.In the lower MjSi phases, the Si pletely solid state reactions, where the mechanisms are more

J. appl. Chcm., 1969, Vol. 19, May 132 Glasson & J less significant for scales that grow by outward migration of position from an atmosphere containing either B, C or Si and matter.'*"* 11 is more important for scales where the diffusion the metal components both as volatilised compounds, and is from the surface towards the metal/scale interface. Apart (b) boriding, carbiding or siliciding the surface layer of an from fractional volume differences between the metals and object by heating it in an atmosphere of a volatile B. C or Si their carbides, there is s(imetimes dissolution of free C in the compound. For borides and carbides, process (a) deposits carbides, e.g. in Mo carbide.'^ Formation of mixed car• coatings at a faster rate than (b). where the rate of inier- bides, carboborides or carbonitrides often improves the difTusion of B or C and the base metal limits the boride or hardness of the coalings, e.g. boronised 0-3% carbon steel carbide formation rate. Process (a) generally yields the is harder than boronised iron, particularly at higher boronising purer deposits, since the B or C to metal ratios can be con• temperatures,'*'* Gas mixtures and temperatures must be trolled belter. As most metals form more than one boride, controlled carefully to equalise the reaction velocities for the the practicable control of relative rates of diffusion and de• deposition of each component."*** The range of mixed position is usually insufficient to avoid appreciable simulta• carbide coatings might be increased by carburising alloy- neous formation of several boride phases, causing wide varia• vapour deposits, or by diffusing superposed carbide deposits tions in the properties of the coatings. The carbides require at high temperatures. temperatures to be sufilcicntly high for the interdiffusion of the deposited carbon or metal (from the decomposed hydro• Vapour deposition of silicides carbon or metal halidc vapours) with the metal or carbon Free silicon is deposited usually by hydrogen reduction of substrates respectively. On the other hand, silicon diffuses silicon tetrachloride, but the reaction is sensitive to im• so readily into most materials at comparatively low tempera• purities.'*'^ Deposits on quanz or ceramic bases""* below tures, which makes process (b) more convenient in the in• 1000" tend to be grainier, but they are smooth and hard stances so far examined. above 1100°. The deposition rate is independent of lime and proportional to ihc SiCU concentration, within limits, Vapour deposition of borides suicide deposits rich in Si on the outside are obtained by Borides directly deposit most readily when hydrogen re• successively coating Ta and momentarily Hashing each layer duces mixed vaporised chlorides of boron and the desired to the m.p. of Si; the products have the most suitable mecha• meial component at a heated surface, i.e. process (a).'*-'^'* nical and electrical properties for electrical translating However. Nb. Ta. Mo and W borides are not deposited materials.'^'' Detailed investigation of silicide formation by suitably by reduction of the mi.xed halidcs because the free diffusion of Si coatings appears to be confined so far to Ti. metals arc deposited rapidly at temperatures below those re• Zr. Nb, Ta. Cr. Mo. W. Fe. Ni and several alloy steels;''' quired for boride formation. The impure boridc deposits coatings can be applied also to Cu. Ag. Be. Al. Hf. Th. V. either contain much free metal or are non-adherent powders, Mn. rare-earth metals. Co and Pi-group metals. Minimum and process (b) becomes preferable. Deposition often is dis• Si-deposition temperatures are about 900-!(X)0'' for H;- continuous when halidcs without a common ion or atom are rcductton methods, unless the base materials arc sufliciently used. e.g. V^CU and BBrj. Limited studies on the thermal dc- reactive for Si diffusion to cause deposition by displacement compoiiiion of metal borohydrides, e.g.. Th(BH4).i.'"** at lower temperatures,'"" e.g.. Fe, W at 800". or Mo at suggest that such a method generally would produce boride 8(K)-900''. At low deposition rates, the maximum deposition deposits having excess uncombined boron. The deposition temperature is limited by the m.p. of any cutectic or pcritectic reaction is rendered inefficient by some decomposition of the compositions formed between the base and the coating. At borohydrides at the vaporisation temperature and the in- high deposition rates, it is limited by the m.p. of Si unless nammabiliiy of some of them in dry air. this is higher than the m.p. of any eutcctics. The siticides The boronising or boriding process (b) essentially requires formed arc not usually homogeneous in composition, but deposition of free boron at a lower temperature before its comprise most of the silicidc phases that can exist in a given diffusion into the base material at a higher temperature; system at the deposition temperature.'* Silicon also has otherwise the temperature must be high enough for deposition some solid solubility in most metals. Thinner silicide de• and diffusion at comparable rates. Kinetics of boron de• posits arc usually more adherent, e.g. Mo silicide up to position by low-temperature pyrolysis of boranes and organo• 80 pm thick, but the adhesion of the thicker deposits boron compounds have been determined by Schlesinger et (>200Mm) is improved by successive short-stage depositions «/."" However, there have been no comprehensive studies of and intermittent heating in hydrogen to diffuse the coating rates of boron diffusion and formation of boride layers on into the substrate. various metals. Solid slate preparative reactions Vapour deposition of carbides In the production of borides, carbides and silicides by direct Extensive studies of carbon deposition from pyrolysis of combination of their elements (or close variations of these hydrocarbons show that thermodynamics, kinetic, transport methods), the reactions arc accelerated by using mixtures of and nucleation characteristics of the systems are closely in- components in finest dispersion.-"" Rational grain-size terrclaied.' * In process (b), the carburisaiion rate depends on composition is of special interest now because of newer the specimen temperature, the hydrocarbon content in the methods of grinding materials (vibratory grinding, fluid- surrounding gases, the rate of gas flow and the geometry of energy mills, etc.). The uniformity and degree of grinding the specimen. The factors also affect the adhesion of the influences the following parameters:''' (a) surface area and carbide layer. In general, the thinner carbide layers are the energy of the grains, (b) temperature, heat of fusion and most adherent. As found for nitrides'*^ and oxides.'^^ the solution, (c) intensity of heat exchange with surroundings, formation of non-uniform, i.e. porous or cracked, scales (d) rate of solution, sublimation, dissociation and chemical

.1. appl. Chem.. 1969. Vol. 19, May Glasson <$ Jones: Formation and Reactivity of liorides. Carbides and Silicides. I 133 reaction with other reagents, (c) thickness of the product layer of the coalings. Similar considerations apply to borides, but developing on the grains during chemical reaction, and silicides partly possess suitable combinations of the desired governing diflusion rates through the layer, (f) properties of properties. the crystalline reaction products, e.g. mechanical and thermal. The only interstitial boride and carbide oxidations that (g) clTectivcncss of reaction accelerators between solids, and have been studied in any detail arc those of TiBj and TiC. (h) the economics of the process. The kinetics are determined They illustrate factors to consider and problems to be en• mainly by (d) and (e). For mixtures with components with countered in further investigations of other transition-metal similar grain sizes, Jander & Hofi'mann"- have shown that boride and carbide oxidations. Reactions having parabolic the thickness of the product layers around the grains is pro• kinetics between 600-1000" for TiB2"''-''-* and 450-1000° portional to the square root of the calcination time. In the for TiC'"^-'"" produce scales consisting essentially of sintering of each product, Berczhnoi'^-* has established that rutile. A cubic relationship with time, found by Miinstcr'*'-' the minimum porosity is given by a mixture of fine and for the oxidation of TiBj at 900°, was not observed by coarse fractions having a grain size ratio of 0-3, and contents Samsonov & Golubeva.'"- .V-ray and metallographic ex• of 30-40% and 60-70% respectively. The successful pro• amination'*'^'''"* shows that at 700" the rutile is dispersed in duction of borides by hot pressing (reactive sintering) of vitreous BjOj. but between 800-1000" the B2O3 forms a top metals, hydrides and carbides with boron or boron carbide layer covering a rather porous rutile layer with coherent powders^"*-''** illustrates the general applicability of the above ruiile adjoining the metal boride. Platinum marker experi• parameters. Ilowever. there are only detailed kinetic data ments'"-' suggest thai the oxidc/boride interface moves away available'"^ on the average solid solution rates (1400-2400") from the oxide/gas interface, but at higher temperatures the for the 50/50 TiB2-ZrB2 composition. In the production of BiOj fiows outwards (over the markers) to form a separate carbides and silicides. most metals and their oxides react with layer; at 1100". most of the B2O., volatilises and the scale C and Si far below their m.p. (1200-2200°). The lowest consists almost entirely of rutile. This indica'.es that oxygen possible carburisation temperatures arc used to avoid any rather than Ti difi'usion is raie-detcrniining al least in the deleterious grain growth.* parabolic stage of the oxidation, cf difiusion of anion vacancies in the Ti02 (//-type conductor)'"' '"" which controls Reactivity of borides. carbides and silicides oxidation of Ti between 600-700" and gives a similar energy of activation.'"*' '"" Therefore, the oxidation mechanisni Sintering of borides. carbides and silicides would be essentially the same as for TiC and TiN, where the The chemical reactivity of borides, carbides and silicides is rutile scales are separated from the carbide and nitride by thin controlled considerably by the extent to which they have films of TiO-TiC and TiO-TiN solid solutions.'"^ The carbon been sintered during their formation and any subsequent formed in the reaction TiC-fO. = TiO>-fC, is thought to calcination. At present, there is much more information difi'usc to the scale surface where it reacts with oxygen to available on the sintering of oxides and a limited amount on form carbon oxides,^'^" cf oxidation of UC which finally nitrides.-*" Theories of sinterinu have been developed bv gives UO3 containing residual C02.^"' Some carbon may be Hiitiig.''"' Kingery.'^' Coble.'^"-'Kuczynski.'"" White'^' assimilated by the TiC lattice, especially at lower tempera• and Fedorchenko & Skorokhod.'"' Sintering is enhanced by tures, for commercial TiC is usually C-deficieni.^°^ IEfi"ects compacting the powdered materials before calcining them i/i of temperature and oxygen partial pressures on TiC oxida• vacuo to prevent possible hydrolysis and oxidation.'^-'•'**"* tion have been investigated further,^**^ and also the reactivity Hot pressing often extensively densifies materials,^ giving with watcr^""* and the related oxidations of Zr and Hf car• almost the theoretical densities for o.xidcs such as MgO. bides.^*'* TiC-Co aggregates containing 18% Co also oxidise CaO and AI2OJ.Development is limited by im• parabolic-ally in air between 600-1000°.'"" and WC oxi• purities, particularly gas-producing contaminants such as dises more readily than TiC'*' The hard metals TiC-WC-Co hydroxides and carbonates. Also, nitrogen reacts extensively oxidise linearly for high WC-contcnts (as found for TiC at with some carbides, e.g.. TiC'"**-'^" and ZrC,'"*' and pro• very low temperatures. 300-400"). Re-entrant edges in the duces carbide-nitride solid solutions. Hence, often vacuum product scales are typical of uninhibited oxidation at the hot pressing is preferred.'^*' Sintering is accelerated generally hard metal-oxide interface. In contrast, the parabolic curve by additives of low melting point.'*" but these may cause for oxidation of TiC-Co-Cr hard metal produces much serious reductions in optical and mechanical properties. thinner scales, which are tenaciously adherent and pro• However, extremely brittle borides. carbides and silicides may tective; chromiutn carbide improves the oxidation resist- be sintered with metals such as Co to give satisfactory ance.'«« cermets.* or may be used as surface coatings.'* Atmospheric oxidation of transition metal silicides has Hydrolysis and oxidation of horides, carbides and silieides been studied systematically by KiefTer and his co-workers, The resistance of borides. carbides and silicides to the mainly between 1100-1500°. The maximum oxidation re• action of water and aqueous acids and alkalis has been sistance in the systems Ti-Si,'*'" Zr-Si.''° V-Si,^" Nb-Si,''^ summarised by Shaffer & Samsonov.' The materials listed Cr-Si^'^ and Mo-Si''•* is exhibited by compositions of by Campbell el H/."^ for high-iemperaiure coatings include approximately MSi;. or higher Si contents of about 30 wt.-% .several metals such as Zr, Th. Nb and Ta which all have m.p. in the systems Ta-Si'"" and W-Si."* However, NbSi2 oxi• above 1700° and generally reasonable ductility, but their dises rapidly compared with the NbjSij phase stabilised with oxidation resistance is poor. The numerous carbides of high Cr.'" When oxidised at 1200^ TiSi: (m.p. 1540"), VSi: melting-point, e.g. TiC. ZrC. NbC. M02C. W.C. SiC, also (1670°). MoSi: (2030°) and W'Sl^ (2160") form vitreous well- have generally poor oxidation resistance, and their ductility adhering scales which arc highly protective. MoSi: is is inferior to that of the metals. The afiiniiy of the metals is especially resistant to oxidation and the scales are self- exclusively higher for oxygen than for carbon, ef Figs 1 and healing.-'" The mechanism of the low ionic dilTusion in 2. and consequently exchange reactions diminish the quality these vitreous silicate scales is not yet established. ZrSi: and

J. appl. Chem., 1969. Vol. 19, May 134 Olasson & Jones: Formation and Reactivity of Borises, Carbides and Silicides. I

Ta silicides of lower Si content than TaSi2 form scales which Procedure are vitreous but brittle, tending to flake otT during tempera• Hydrolysis and oxidation of the borides, carbides and ture nuciuations.-^"-^'" These silicides arc more oxidaiion- silicides arc followed by weight changes on vacuum^-*'-^-*' resistant than those of Cr and Nb which form porous, and thermal--'^ balances. Samples are outgassed usually at loosely adherent scales. More recently, the continuous solid 200^* /•// vacuo before determination of their surface areas by solution at 50% ReSii-MoSi: is claimed as a composition the li.E.T. procedure^"*' from nitrogen (or occasionally of extreme oxidation resistance at high temperatures,^'"' oxygen) isotherms recorded at - 183" on an electrical sorp• but poor oxidation resistance for ReSii has been reported.^'^ tion balance. The deduced average crystallite sizes (equiva• Improvements generally would seem to depend on the mutual lent spherical diameters) are compared with particle size alloying of transition metal silicides (and borides) to produce ranges determined by optical or electron microscope or sedi• ternary phases of new and independent structures, the mentation balance. Where necessary, particle size fractions stabilities being controlled by electron relations.^'"' The new of the materials arc sintered or hot pressed for further periods structures would be unlike those of the end members, but at fi.xed temperatures. isomorphic to other single silicides with the best oxidation resistances. Further improvements in protective performance Phase composition identification will depend on slight ductility possessed by some silicides, e.g., Samples are examined for phase composition and crystal- NbSi2. TaSi:, MoSi:, ^^'Si;, matching of lattice volumes, linity using an A'-ray powder camera and a Solus-Schall A'-ray eutcctic and melting temperatures, and the extent of sintering dirTractomeicr with Geiger counter and Panax rate-meter. of the oxidised overlayers. The average crystallite size of some of the phases can be determined from A'-ray line- or peak-broadening.^'*- Certain samples arc further examined by optical and electron micro• E.vptTlmciital techniques scopes (Philips FM-lOO). More detailed investigation in• Materials volves determination of pore size distribution in refractories, where the microscopic examination may be supplemented Borides, carbides and silicides usually contain impurities' advantageously by methods based on mercury penetration, introduced during their production by the initial reaciants expulsion of water from a saturated specimen and electrical and apparatus materials or surrounding gases. Purification detection of capillary penetration.^"*^ Electron-probe micro• is required before determination of physical properties such analysis often provides further information.^*'* as m.p.. hardness, electrical conductivity etc.The thermo• chemistry of interactions of metals and refractory materials Acknowledgments commonly used for containers has been reviewed recently by Kubaschewski,^^" who emphasises the lack of accurate The authors thank Dr. S. J. Gregg for his interest and en• thcrmochemical data for the nonstoicheiomciric compositions couragement in this work; Mrs. M. A. Sheppard for her in refractory systems, particularly non-stoicheiometric metal- assistance in the translation and classification of the chemical non-metal phases of high afTmity. Purer boride. carbide and literature; the University of London. Imperial Chemical silicide products are obtained by using high frequency heating Industries Ltd., and the Science Research Council for grants /// vacuo or in argon, as developed by Brewer et ol. for bo- for apparatus and a S.R.C. Research Tcchnicianship (for rides^'' and silicidcs-^^ and A\gte & Moers for carbides.^^-* M.A.S.). Agte & Moers embed the sintering carbide bars in carbide John Graymore Chemistry Laboratories. powders to protect against carburisation, oxidation and College of Technology, nitridation. Extremely high temperatures of final sintering Plymouth -. n,, . -. . n give self-purification, by evaporation of the oxides, metals see aJ-so "Special ceramics", and other impurities with vapour pressures higher than those (B.C.R.A. ), 1970. of the carbides. Additives promoting sintering, e.g. 0-2-1-5% Cr. Fe. Co, References Ni or their oxides, often form liquid phases and promote ' ShaiTcr. P. T. H.. »S: Samsonov, C. V.. "High temperature self-purification through difi'tision processes, particularly for materials". 1964. Handbooks 1 and 2 (New York: Plenum) - Schwarzkopf. P.. & KicfTcr, R., 'Rcfractorv hard metals'. 1953 carbides of Groups IV and v.^^"*-^^^ The auxiliary metal (New York: Wiley) additives can be completely vacuum-evaporated in high- ^ Samsonov. G. V., "Refractory transition metal compounds", frequency furnaces at pressures of about 01 mm Mg at 1964 (London: Academic Press) Samsonov. G. V.. Ukr. khim. Zh.. 1965. 31 (10). 1005 (N.L.L. 2000-2500^ The refractories arc hot pressed by the present translation, RTS 4233: Dec. 1967): summarised in Izv. Akad. authors using apparatus designed by Scholtz.-^*' Roeder & Nmik SSSH, 1967. 58 (10). 76 Scholiz--' and Oudemans.^^^ Pressure sintering bonds ' Schwarzkopf. P.. & KicfTcr, R.. -Cemented carbides", 1960 compacts of ceramic friction materials on to metal back- (London: Macmillan) plates, for which special types of furnace are required.^'" " Samsonov. G. V.. & Markovskii. L. Ya.. Usp. Khim.. 1956. 25. 190 (Associated Technical Services translation RJ 631) Production of finely divided and more reactive materials ' .Samsonov. G. V.. Usp. Khim., 1959. 28, 189 (U.K.A.L.A. by milling can produce considerable strain.*-'" This may be translation, report AERF. Trans. 849) increased by impurities, e.g. the presence of oxygen in (he Thompson, R.. & Wood. A. A. R., Chem. Enar. 1965. p. CF5I Greenwood, N. N.. Parish. R. V.. & Thornton, P., Q. Rev. lattice causes some carbides to lose cubic symmetry and de• chem. Soc, 1966. 20 (3), 441 form more isotropically.^^' Modern apparatus for closely '° Mellor. J. W.. 'Comprehensive treatise on inorganic and theo• controlling the milling of materials^-*^-"'' can give products retical chemistry". Vol. 5, p. 844 (London: Longmans) having narrow panicle size distributions. Changes in crystal• '' Samsonov, G. V.. Zh. leor. l-iz., 1956. 26. 216 lite and aggregate sizes during sintering and milling of the Samsonov, G. V.. & Ncshpor, V. S.. 'Subjects of powder metallurgy and strength of materials". 1959. Issue 7, p. 99 borides, carbides and silicides are determined by the methods (Kiev: Izdvo Akad. Nauk SSSR) described below. Elektroschmelzwcrk Kcmpten, H.P. 1.070, 325

J. appl. Chem., 1969, Vol. 19, May Olasson & Jones: Formation and Reactivity of Boricles, Carbides and Silicides. I 135

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In the reduction of B,Oj. viz., 2».Oj + 7C - BX + GCO. presence of comparatively low-melting additives'- and by AG'' - +397,193-215-22 7' cal mole-' B4C. making the application of pressure (hot pressing).'-* The importance of theoretical initiating temperature 1845°K. This temperature rational grain-si/e composition has been discussed also in will be reduced at lower pressures since AC'r = AC,+ Part L^ R7'ln /7co i>"d A6"'^ has a negative variation with T. In prac• In the present work, finely-divided boron carbide was pre• tice, the reactants arc heated to temperatures above 2400' in pared by reduction of B2O3 with C and Mg. Rates of sinter• an electrical furnace, the reaction being cndothermic. When ing were determined from changes in surface areas and average part of the carbon is replaced by magnesium, the reaction crystallite and aggregate sizes. These are correlated with 2B,0., + 6Mg-fC = BaC + 6MgO. has AG" - -478,333 + temperature conditions during and after boron carbide pro• 195-67 7" cal mole"' B4C. above the b.p. of Mg. HTS^K. A duction. Also, effects of additives and hot pressing were reversal of the reaction is feasible above 2445''K (AC° = 0 ai determined. 2445°K), but since AGr = AG^-RTIn p^^, this diminishes as the reaction continues, as AG° has a positive temperature Experimental variation. After the highly exothermic reaction is initiated, it Production of boron carbide is self-propagating if there is adequate thermal contact be• The materials used were magnesium technical grade (Mag• tween the magnesium fragments. nesium Elektron), diboron irioxide (Borax Consolidated) The last production method may proceed by alternative and carbon black (Cabot's). routes: (i) 4BCli + 4H2 + CH4 = B4C+ 12HCI, (ii) 4BCU + Stoicheiomctric proportions of the boron oxide and carbon 8ll, + CCl4= B4C+I6HCI, or (iii) 4BCIj + 6H> + C = B\C black were mixed thoroughly by ballmilling. The mixing + I2HCI. All are highly exothermic, and once initiated are was continued with an additional stoicheiometric excess of subsequently self-propagating; carbide formation is favoured magnesium powder. The completely mixed powder was at lower pressures. transferred to an open reaction vessel, and a volatile fuel oil was added to bind the powder and prevent sifting. The Mechanism of formation mixture was ignited by a glowing fuse wire and the reaction General information on the kinetics of vapour phase de• was allowed to continue spontaneously. The burning fuel oil position of borides and carbides has been summarised in provided a non-oxidising blanket for the reaction and Pan L^ Pring & Fielding' reduced BCI.i vapour with an ex• assisted in the propagation; temperatures reached a maxi• cess of H2 and deposited boron on a carbon surface at mum of 1600^. After complete reaction, the products were 1500—1750". At higher temperatures, 1750—1950=', carbide allowed to cool overnight. The top crusts of the product coatings (reported as BgC) were obtained which became more were discarded and the remainder were crushed to small crystalline and non-adherent as the deposition temperatures size before the magnesia was leached out with dilute sul• were increased further to 2200°. Powell ct a/.* also obtained phuric acid. The boron carbide powder was filtered, washed adherent deposits superficially resembling B4C on Mo and C free of sulphate ion with hot water and finally dried at 120° bases from 1:1 BCIj and M2 mixtures containing about 2 for 2 h. The yield (allowing for the crust) was better than vol.-% toluene vapour at specimen temperatures of 800— 90% for batches of about 3 kg. 1200°. There have been no comprehensive studies of rates of boron diffusion and formation of boride layers on carbon Sinlcring of boron carbide or various metals. Empirically, the cocfTicient of diffusion of Since boron carbide is oxidised in air at higher tempera• boron in graphite. D = 3-02 cxp (-28,525/7'), which in• tures, the samples were sintered /// vacuo. The furnace con• dicates that the diffusion of carbon in boron is correspondingly sisted of a fused silica tube connected to a high-vacuum much slower.' -^ Although B (0-9 A) is larger than C (0-7 A). system, a single-stage rotary pump and a 3-stage oil diffusion the former has the higher polarisability (first ionisation pump. Pressures were monitored using a Penning gauge, potentials: B, 8-28 cV; C, 11 41 eV). and vacuum conditions of better than 2x10"' torr were In the reduction of B^Oj by an excess of C. Samsonov ct possible during the heating cycle. The sample was in a al.^-^ have demonstrated that there arc two consecutive pyrolyiic graphite crucible inductively heated by a 4 kW processes: B2OJ + 3CO = 2B + 3C02 above 1640°K. and radiofrequency heater operating at 450 kHz. The tempera• 4B + C = B4C. The newly formed B diffuses through the ture was measured by a disappearing filament pyrometer. boron carbide layers progressively formed on the surface of Slight outgassing during the first minutes of heating became the graphite particles, finally giving boron carbide particles almost negligible when samples were reheated, particularly retaining the original shape of the graphite, i.e. there is no at the higher temperatures. penetration of liquid B2O3 through the carbide layers. The The samples were calcined (A) for fi.xed limes at different equilibrium temperature for boron carbide formation ac• temperatures, (B) for various times at each of a number of cording to the total reaction 2B>03 + 7C= B4C + 6CO is fixed temperatures. The cooled samples were removed and about 1400* at 1 aim pressure of CO, allowing for fusion and ouigasscd at 200° in vacuo (where necessary) before deter• evaporation of B^Oj.'*' Volatilisation of boron from boron mination of their surface areas by the B.E.T. procedure'"* carbide at higher temperatures. 2300—2500°, leaves carbon from nitrogen isotherms recorded at - 183° on an electrical as fine filaments and graphite inclusions in the carbide grains. sorption balance.''''*" This substantially reduces the abrasivcness and hinders More extensive sinlcring. up to 90% of the iheorelical sintering of the material. density, was achieved by hot pressing. The powder was com• pacted into a graphite mould fitted with graphite pistons and Sintering of materials heated inductively by a 32 kW heater at 450 kHz. while Sintering of solids is enhanced by increases in temperature continuous pressure up to a maximum of 3000 kg cm"- was and lime of calcination, which promote surface and crystal applied during the heating cycle of 1 h. The mould was in• lattice diffusion.'' These processes are assisted further by the sulated by a packing of burnt lime. Temperatures up to

J. appl. Chem., 1969, Vol. 19, May Glasson <& Jones: Fonuaiion ami Reactivity of Borises, Carbides and Silicides. II 139

1900" were recorded by a iherniocouplc. The cooled compact was removed easily from the mould and sectioned with a diamond-impregnated saw, before determination of its phase composition and density. l~or more detailed studies, hot presses designed by Scholt/ ei al. are referenced and dis• cussed in Part 1.^

Phase composition identification Samples were examined for phase composition and crysial- linity using an emission spectrograph, an A'-ray powder camera and a Solus-Schall ^'-ray diffractometer (Cu Kot- radiation) with Geiger counter and Panax ratemeter. Certain samples were examined further by optical and elcciron- microscopes ( Philips KM-100).

Results Emission spcctrographic and A'-ray analysis indicated that the boron carbide was a high-purity stoicheionieiric B^C; batches contained less than I % free carbon. Trial hot 7 i. B 10O0 UOO 1800 pressings showed a material capable of being sintered to CAUaNAllON IIME.h CALCIMAtlON lEMPERilURE. "c better than 90% theoretical density without any additives. Particle size and shape analysis by A'-ray line-broadening" hig. 1. Variaiion of average crysfalliie size and specific surface of and electron-micrography indicated sub-micron sizes ranging t)ofon carhiile with calcination lime anit leniperasure from O Ol-l ^im and regular shape. This was in accord (a) and (c): A 1000°. A 1^00^ B 1400^ • I600^ O ISOO^ with gas sorption and surface area measurement. Addition of 10% Cr shown by broken lines for 1600= and 1800" In Fig. I, variations in specific surface, 5, and average (b) and (d): O 2 h. • 5 h calcinations crystallite size (equivalent spherical diameter) are shown for boron carbide .samples calcined /'// vacuo for difTerent lengths of lime at each of a number of fixed temperatures, (a) and as calcium oxide, it sintered much less extensively at 1000- (c). and for fixed times of 2 h and 5 h at difTerent tempera• 1200°, which is near the Tammann temperature (half m.p. in tures, (b) and (d). °K) for lime.'^ Other related ionic compounds also show appreciable crystal lattice diffusion at temperatures of about Discussion half m.p. (in "K).' ' The covaleni-bond character and crystal structure of boron carbide (discussed in Part 1^) confer low rormation of boron carbide plasticity and great resistance to sliding on the grain bounda• Boron carbide from the magnesium reduction method ries up to temperatures near to the m.p., combined with a gave an extremely fine powder of submicron size. Neverthe• low surface tension in the solid state. At about 1800". less, the material could be filtered readily after acid leaching chromium metal completely wets the boron carbide surface."^ and washing. It tended to aggregate in acidic or neutral The adhesion between the metal and the carbide is weak, for media, probably because of some acidic oxide coating of the the chromium can be removed readily from the surface of the particles. Accordingly, the material could be dispersed carbide cakes. However, it is sufficient to accelerate carbide readily in alkaline media and then became impossible to sintering. No zone of interaction of boron carbide with filter. The extremely fine granularity is ascribed to the chromium was found previously.and no crystalline chro• simultaneous and rapid formation of two refractory pro• mium boride or carbide has been detected by A'-ray examina• ducts, boron carbide and magnesium oxide, at temperatures tion in the present work. Some graphitisation of carbon has well below their m.p. (2350° and 2800° respectively). The been noted at 1800^, which could have left some boron in magnesium oxide inhibits any subsequent sintering of the solid solution with the remaining boron carbide or chro• boron carbide. On the other hand, boron carbide produced by mium. This behaviour of chromium contrasts with that of the electro-thermal carbon reduction at very high tempxTra- iron which forms zones of interaction when it accelerates tures is coarse crystalline material, which requires ball- sintering of boron carbide. Other transition metals, e.g. Co milling to give suitable grain-size compositions for hot and Ni. give similar zones, apparently consisting of carbide pressing cfifectively. This material has irregular shape and is and boride alloys of the corresponding metals."* compacted only with difficulty, requiring much higher tem• More extensive sintering of boron carbide requires hot peratures for sintering without additives. pressing, i.e. heating under a pressure exceeding the critical stresses at temperatures relatively close to the m.p. Sintering of boron carbide Fine powders of less than 0-5-1 pm grain size, as in Fig. I. The changes in specific surface, 5. and average crystallite hoi pressed to almost zero porosity even at 1650—1700° and size in Fig. 1 show that sintering of boron carbide was en• pressures of about 200 kg cm.The boron carbide grains hanced by increased temperature and time of calcination. increased to 3-15 um. The extra cost of the magnesium Addition of 10% chromium (broken-lined cur\'es) accelera• required to replace part of the carbon for reducing B2O., ted sintering at temperatures above 1600° and espcciallv at can be counterbalanced mainly by the simplification of the 1800^ plant equipment and the elimination of ball milling, besides Although boron carbide has approximately the same m.p. producing a superior material for sintering.

J. appi, Chem., 1969, Vol. 19, May 140 Cila.s.son & .lone.s: Fornialion and Reacfivily of BorUlcs, Carbides and Silicides. //

Ackiiowledt^nients Priiig. J. N., & Fielding, W., J. client. .Soc. 1909. 95, 1497 The authors thank Dr. S. J. Gregg and Mr. L. Bullock for Powell. C. F., O.vlcy. J. 11.. .*c Blocher. J. M., 'Vapour de• position", 1966, p. 359 (London: Wiley) their interest and encouragement in this work, and for Lowell. C. F.. J. Am. Ceram. Soc, 1967, 50 (3), 142 facilities al the Bullock Research Laboratories; Mrs. M. A. Samsonov. G. V., Markovskiy, L. Va., Zhigash, A. F.. & Shcppard for her assistance in the A'-ray analytical work; Valyashko. M. G.. "Bor ego soedineniva i splavy". 1960 the University of London. Imperial Chemical Industries Ltd.. (Kiev: Akad. Nauk Ukr. SSR). USAFC translation, 1962, and the Science Research Council for grants for apparatus No. 5032 (1). p. 179 and a S.R.C. Research Technicianship (for M.A.S.). Samsonov, G. V.. Zaiivanskiy. I. L., & Popova, N. V.. Dokl. Akad. Naak SSSR, 1950, 74, 723 John Graymorc Chemistry Laboratories, Fajana, C. & Barber, S.. J. Am. client. Sac, 1952. 74. 2761 Miitlig, G. F., KoUoidzeii.'ichrift, 1941. 97, 281 College of Technology, Glasson, D. R., J. appl. Chem., Land., 1967. 17, 91 Plymouth Wheiidon, W. M.. & King, A. G., Ceramic Ind., 1967. 88 (6), 56 Brunaucr. S.. Emmctt, P. II.. & Teller, F., J. Am. chcm. Soc, Received 7 November, 196S 1938, 60, 309 Gregg, S. J., J. chem. Soc, 1946, p. 561 References Sanorius-Wcrke, 'Flectrono-vacuum balances*. 1963 (Gottingcn: ' Houlton. J. F., & Eardley. R. P.. Analv.si, Land. 1967. 92. 271 ^ Glasson, D. R.. & Jones, J. A.. J. appl. Chem.. Land.. 1969, 19. Sartorius-Werke, A.-G.) 125 Glasson, D. R., J. appl. Chem.. Land., 1964, 14, 121 Storms. E. K.. The refractory carbides*. Refractory materials llamijan, I L, & Lidman, W., J. Am. Ceram. Soc. 1952. 35. 44 monogrs (Ed. J. L. Margrave). 1967, p. 225 (London: Aca• DawihI. W., Z. Metallk.. 1952. 43, 138 demic Press): U.S.P. 3,284.178 Brvjan, E.. Missol, W.. & Bojarski, Z.. Hutnik. Katowice, 1956 •^JANAF thcrmochemical tables. 1960-65, P.H. 168,370 (New 23. 11 7 York: Dow Chemical Co.)

.1. appl. Chcm., 1969, Vol. 19, May