This dissertation has been 65—9337 microfilmed exactly as received

BEARD, William Clarence, 1934- PHASE RELATIONS IN THE SYSTEMS TITANIA AND TITANIA— BORIC OXIDE.

The Ohio State University, Ph.D., 1965 M in eralogy

University Microfilms, Inc., Ann Arbor, Michigan PHASE RELATIONS IN THE SYSTEMS TITANIA

AND TITANIA—BORIC OXIDE

DISSERTATION

Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy in the Graduate School of the Ohio State University

by

William Clarence Beard, B. S.

The Ohio State University 1965

Approved by

n Adviser Department of Mineralogy ACKNOWLEDGMENTS

The author wishes to thank the people who have contributed to the preparation of this dissertation. First, to his adviser,

Dr. Wilfrid Raymond Foster, for advice and suggestion of the problem; and second to other members of the faculty of the

Department of Mineralogy, Drs. Henry Edward Wenden, Ernest

George Ehlers, and Rodney Tampa Tettenhorst, for discussions of the problem and the reading of this dissertation, the author extends his thanks. Thanks also go to his colleague, William

Charles Butterman, for assistance in the construction and main­ tenance of many pieces of experimental apparatus.

Acknowledgment is made for financial support received under contract No. AF 33(616)-6509 (twenty-four months), spon­ sored by Aeronautical Research Division, Wright-Patterson Air

Force Base, Ohio, as well as for a Mershon National Graduate

Fellowship awarded (1962-63) by the Mershon Committee on

Education in National Security; and a William J. McCaughey

Fellowship (1963-64).

The author is indebted to his wife, Ursula, for her constant help and encouragement. ii VITA.

June 9, 1934 Born - G allipolis, Ohio

195 2-1956. . United States Air Force

1960 .... B. S. , The Ohio State University, Columbus, Ohio

1960-1961. . Teaching Assistant, Department of Geology, The Ohio State University, Columbus, Ohio

1961-1964. . Research Assistant, Department of Mineralogy, The Ohio State University, Columbus, Ohio

196 2-1963. . Mershon National Graduate Fellow

FIELDS OF STUDY

Major Fieid: Mineralogy

Thermochemical Mineralogy, Advanced Thermochemical Mineralogy. Professor Wilfrid R. Foster

Advanced Crystallography, X-ray Crystallography, Crystallochemical Mineralogy, Descriptive Mineralogy. Professor Henry E. Wenden

Microscopic Mineralogy, Microscopic Petrography, Advanced Optical Mineralogy, Crystallography. Professor Ernest G. Ehlers

Clay Mineralogy. Professor Rodney T. Tettenhorst CONTENTS

Page

I. INTRODUCTION ...... 1

II. THE SYSTEM TITANIA...... 3 A. Introduction ...... 3 B. Literature Survey ...... 4 1. Natural occurrences ...... 4 2. Rutile s y n th e s e s ...... 7 3. Anatase syntheses ...... 14 4. Brookite syntheses ...... 15 5. Amorphous Ti0 2 s y n th e s e s ...... 18 6. Polymorphic transformations ...... 19 7. Summary ...... 27

C. Experimental ...... 27

1. Grinding experiments ...... 29 2. Heating of Ti02...... 31 3. Heating of titanium sulfate ...... 31 4. Titanium-halide experiments ...... 32 5. Flux runs ...... 33 6 . Hydrothermal bomb ru n s ...... 34 7. Parallel firing runs ...... 35 8 . Strip furnace experiments ...... 37 9. Titania-enamel experiments ...... 41 10. Plasma-arc flame spraying . . . . . 43 experiment

D. Discussion of Literature and Experiments .... 43

1. Possible phase diagrams for Ti02 . . 43 2. Discussion of pertinent data ...... 50 3. Selection of a probable diagram . . . 53

E. Suggested Future W ork...... 58

iv CONTENTS - Continued

Page

III. THE SYSTEM T i0 2-B20 3 ...... 59

A. Introduction ...... 59

B. Literature Survey ...... 60

C. Experimental ...... 62

1. Procedure ...... 62 2. Experimental ru n s ...... 75

D. Discussion of Experimental Results ...... 88

1. Liquid immiscibility ...... 88 2. Phase relations of Ti0 2-B20 3...... 95

IV. SUMMARY . 99

REFERENCES...... 100

v TABLES

Table Page

1. Reported Temperatures for the Anatase-Rutile Transition ...... 23

2. Reported Temperatures for the Brookite-Rutile Transition...... 25

3. Data from Grinding Experiments ...... 30

4. Heating of TiOz (Anatase) ...... 31

5. Heating of Titanium Sulfate ...... 32

6. Flux Runs with Anatase and R utile ...... 34

7. Hydrothermal Bomb Runs ...... 36

8 . Parallel Firings of Anatase and Rutile with B £>3 .... 37

9. Strip Furnace Platinum Tube Heating Experiments . . . 38

10. Strip Furnace Heating Experiments ...... 40

11. Titania-Enamel Firing Experiments ...... 42

12. Bursting Pressure Data for Platinum Tubing ...... 7 0

13. Preliminary Firings: Pressed Disks ...... 76

14. Preliminary Firings: Crucible Runs ...... 78

15. Preliminary Firings: Platinum Envelopes ...... 80

16. Sealed Capsule Firing Runs ...... 84

17. Prediction of Liquid Immiscibility ...... 93

vi ILLUSTRATIONS

Figure Page

1. Graphic Summary of Ti0 2 S y n th ese s ...... 28

2. Possible Phase Diagrams: One Stable, Two Meta stable Phases ...... 45

3. Possible Phase Diagrams: One Stable, Two Metastable Phases ...... 46

4. Possible Phase Diagrams: Two Stable, One Metastable Phase ...... 47

5. Possible Phase Diagrams: Three Stable Phases . . . 48

6. Proposed Phase Diagram for Ti02 ...... 55

7. Metastable Precipitation of High-Temperature Phases from Solution ...... 57

8 . Apparatus for Producing Anhydrous B^D 3...... 64

9. Heat-Sink Blocks for Welding Platinum Capsules . . 66

10. P-T Plots for Various Amounts of W ater ...... 69

11. Electron Micrograph of B20 3 G l a s s ...... 89

12. Electron Micrograph of T i02-B 20 3 G la ss...... 90

13. Electron Micrograph of T i02-B 20 3 G la s s...... 91

14. Prediction of Liquid Immiscibility ...... 94

15. Tentative Phase Diagram Ti02-B20 3 ...... 96

16. Tentative Phase Diagram Ti02-B20 3 ...... 97

vii I. INTRODUCTION

The purpose of this investigation is to determine the high temperature phase relations existing between titanium dioxide and boric oxide at one atmosphere. The polymorphism of titanium dioxide is presented in terms of the pressure-temperature relations which best explain the diverse behavior of titanium dioxide modifications reported in the literature.

Mineralogically, titanium ranks ninth in atomic abundance in the igneous rocks of the upper lithosphere. It is nearly always present in the analyses of hgneous rocks, although it does not occur in any great concentration. Titanium is present in the minerals rutile, ilmenite, perovskite, sphene, arizonite, geikielite and pyrophanite of which the first four are the most important sources. Besides rutile, the oxide Ti0 2 also occurs as the minerals anatase and brookite.

Boron occurs less abundantly than titanium. However, it does appear in many silicates, e. g. , tourmaline, datolite and danburite, as well as the borates themselves: boracite, borax, kernite, ulexite, and colemanite.

1 Technologically, both titanium dioxide and boric oxide are important. Titania is used as a stain for ceramic bodies and glazes; as a constituent in welding rod coatings; as a nucleating agent in glass-ceramics; in glass and porcelain enamels; in self- opacifying enamels; and in glass compositions partly for its high refractive index. Because of its high index, titania is the

strongest white pigment known, and therefore is used as a paint

pigment as well as in enamels.

In the electronics industry, titania is used to make capa­

citors and piezoelectric porcelain bodies. Slightly reduced

single crystals of rutile, produced by the Verneuil process, are

n-type semiconductors, and single crystals doped with transition

metal ions are suitable as maser materials.

Boric oxide, because of the low stability of B0 3 triangles

in three-dimensional arrangement, readily disorders and yields a

glass. Because of this tendency, B/D 3 is regarded as a network

former, and hence its main use as a constituent in glass compo­

sitions. II. THE SYSTEM TITANIA

A. Introduction

Titania exists in three crystalline modifications: anatase, rutile, and brookite. Three separate forms of anatase and of rutile have been reported (Lashenko, 1913) as well as a and p forms of anatase (Schroeder, 1928). The anatase to rutile transition has been located as low as 100°C (Weiser et al. , 1941) and as high as

1200°C (Schossberger, 1942). The synthesis of brookite has been claimed as low as 300°C (Daubree, 1849) and as high as 1300°C

(Junker, 1936), with one author (Hautefeuille, 1864) assigning an

800°-1040°C range. Many investigators, like Osborn (1953) and

De Vries and Roy (1954), believe anatase and brookite to be com­ pletely metastable with respect to rutile at all temperatures.

Others, like Boeke and Eitel (1932), believe anatase the low-tem- perature, brookite the intermediate-temperature, and rutile the high- temperature phases, based on reported laboratory syntheses and natural occurrences. Obviously, additional study is called for in order to obtain clarification of these confused and conflicting relationships.

3 4 B. Literature survey

This survey will mention only a few of the numerous reports on natural occurrences of the various modifications of TiOz. The first reports of the different species and papers which suggest temperatures and conditions of formation are among these. Mainly, this survey is concerned with syntheses and studies on the trans­ formations of the polymorphs of 1 TiOz. For information on early work in mineralogical chemistry, the reader is referred to von Meyer

(1891). In the following list of syntheses it is well to keep in mind that the earlier reports of rutile, anatase, and brookite for­ mation were based on identification by measurement of crystal form or density, and often only on one of these criteria (Gmelin, 1951).

According to Groth (1906) these are not to be regarded as suf­ ficient proof. Barblan (1943) said that x-ray-proved synthesis of brookite had not yet been done at the time of his report.

1. Natural occurrences. The first report of rutile was under the name schorl rouge by Rome de l'Isle (1783). The Comte de Bournon (17 83) described anatase by the name schorl bleu indigo. Brookite was first noted as jurinite by Soret (1822).

Niggli (1926), in discussing the occurrence and chemistry of the titania polymorphs, said that rutile appears pseudomorphically after anatase, brookite, ilmenite, hematite, hornblende, etc. Large, well-formed crystals of rutile appear in hydrothermally formed geodes; rutile is frequently a secondary product of titanium-bearing materials and almost always contains a small amount of Fe 203. With respect to brookite, Niggli says that it

is obvious from the color that it is hardly pure Ti02. Practically

every specimen contained several weight percent of Fe 20 3. He

also says that it is interesting that all three minerals, brookite,

anatase, and rutile, can appear side by side. The variety

arkansite is probably a contact metamorphism mineral and anatase

a hydrothermally deposited mineral, Niggli believes.

Brammalland Harwood (1928), in studying the occurrence of

rutile, brookite, and anatase in the Dartmoor granite, suggested

that rutile has a high temperature of formation because of its oc­

currence as "an inclusion in apparently unaltered biotite, but more

frequently as an obvious by-product of reactions occurring at

magma temperatures. " Brookite and anatase were postulated to

have a temperature of formation with an upper limit of about

575°C, with anatase having the lower formation temperature.

Their temperature determinations were based on the works of

Wright and Larsen (1909) who proposed the use of quartz as a

geologic thermometer.

Dana's System of Mineralogy, I, 7th Ed. (1944) lists rutile

as being typically formed at high temperatures, and anatase and Large, wall-formedcrystals of rutile appear in hydrothermally formed geodes;rutile is frequently a secondary product of. titanium-bearing materials and almost always contains a small amount of FejOs* With respect to brookite, Niggli says that it is obvious fromthe color that it is hardly pure TiOz. Practically every specimen contained several weight percent of Fe 20 3. He also says that itis Interesting that all three minerals, brookite, anatase, and rutile,can appear side by side. The variety arkansite is probablya contact metamorphism mineral and anatase a hydrothermallydeposited mineral, Niggli believes.

BrammallandHarwood (1928), in studying the occurrence of rutile, brookite, and anatase in the Dartmoor granite, suggested

that rutile has ahigh temperature of formation because of its oc­

currence as Man Inclusion in apparently unaltered biotite, but more

frequently as an obviousby-product of reactions occurring at

magma temperatures." Brookite and anatase were postulated to

have a temperatureof formation with an upper limit of about

575°C, with anatasehaving the lower formation temperature.

Iheir temperature determinations were based on the works of

Wright and Larsen(1909) who proposed the use of quartz as a

geologic thermometer.

Dana's System ofMineralogy, I, 7th Ed. (1944) lists rutile

as being typically formedat high temperatures, and anatase and 6 brookite as being hydrothermally formed by the leaching of TiOz from country rock.

In a more recent study, Sigvaldsson (1959) observed rock decomposition due to post-volcanic activity. He noted a high concentration of H ^ 0 4 in the water and,

... close to the fumarole, the original minerals of the basalt were completely dissolved while calclte, montmorillonite, kaolinite, opal and anatase were formed... action of the acid solutions results in the removal of most elements and a consequent enrichment in Ti, Zr and Nb. Added only were S and water.

Lebedev (1961), in studying altered rocks in a hotspring region, said,

... under the action of comtemporary hydro- thermal solutions, these primary ore minerals are undergoing alterations and new deposits of magnetite, pyrite, anatase and brookite are forming.. . the anatase and brookite are forming in an even smaller interval of the gypsum horizon, where the highest temperatures and lowest pH values were registered.

Concerning anatase, Platonov (1962) described three

categories of crystals and nine types of habits. He said that

the color of the anatase crystals seems to be related to their

morphology. 2. Rutile syntheses. Synthesis of rutile by oxidation of

Ti has, in many cases, been done unintentionally. Mixter (1912) formed rutile by the oxidation of Ti in a calorimeter bomb.

Hickman and Bulbransen (1948) studied the formation of oxide films on Ti metal. Their specimens were heated in the range

300°-700°C, and the rutile modification of TiOz was the oxide obtained over the complete time-temperatvre range of their experi­ ments. In explaining their results they said there was disagree- ment with the report (Schroeder, 1928) that anatase should be the oxide obtained below 915°C, and suggested that the regions of stability of the modifications of TiOz are probably influenced by the presence of the metallic substrate. Studies of Morton and

Baldwin (1952) on Ti-oxide films were carried out in the tem­ perature range of 5 00°-1300°C, from six minutes to 600 hours.

Rutile was the Ti0 2 phase. Between 850° and 1000°C they observed an isothermal transition from the low parabolic scaling rate previously noted to a very much higher rate characteristic of a new form of oxide scale. Jenkins (1954), in further studies of

Ti-oxide films says, "Although a large number of titanium

specimens oxidized for various times in the range 600°-950°C were examined, rutile was the only oxide observed. " Conjeaud

(1954) found that thin-films of TiOz produced by evaporating either anatase or rutile, under high vacuum, at white heat, onto cold substrates, were amorphous. When these amorphous films were heated above 800°C, they gave a rutile diffraction pattern.

Mizushlma et al. ( 1962) studied the anodic oxidation of titanium.

Their findings showed, "... electron diffraction measurements of the oxide films when heated above 300° and 800°C showed the patterns of anatase and rutile respectively, but no crystalline features could be observed at room temperature. "

Rutile has been formed by crystallization from a Ti0 2-m elt.

Hautefeuille (1865) obtained rutile crystals from TiOz melted with an oxyhydrogen blowpipe. The crystalline character of the solidified mass was noticeable only as striations on the surface of cavities. Grieve and White (1939) more recently confirmed the formation of rutile from the melt. Using the well-known Verneuil flame-fusion technique, Moore (1949) produced single crystals of rutile several centimeters long. The crystals produced in this way were black, due to reduction of the TiOz, but heating of the boules in an oxygen stream produced colorless rutile.

There are numerous early reports concerned with the formation of rutile through the heating of precipitated titanic acid.

Rose (1823, 1844) produced rutile crystals, too small to be measured on the goniometer, by heating at white heat, thoroughly 9 washed and dried NHS-precipitated titanic acid. Ebelmen (1851 a, b, c,d) obtained rutile needles up to one centimeter long by heating one part titanic acid with four to five parts microcosmic salt. By heating titanic acid at bright red heat in the presence of HC1 as a

mineralizer, Deville (1861) formed small, unmeasurable crystals of rutile. The same sort of crystals were formed by Hautefeuille (1865), using both dry and moist HC1 as a mineralizer. He also used a

stream of HF as a mineralizer in producing rutile crystals.

Hautefeuille and Perrey ( 1890 ) formed rutile crystals by treating titanic acid with HC1 at three atm pressure and bright red heat.

Concerning the use of mineralizers in syntheses, Gmelin (1951) says,

One must assume that the syntheses, of rutile... described by Hautefeuille (1863, 1864, 1865) through the treatm ent of titanic acid melts with KF, CaF2, or KaSiF6, or CaCl2 + S i0 2 as well as with K2TiOj plus KC1, or K2TiF6 and KF, or with CaF 2 CaT103 with HC1 or HF, alone or in the presence of air, at bright-red heat depend on the crystal­ lization of Ti0 2 liberated by means of these mineralizers. Since, however, in the case of these syntheses, both of the hydrogen halide gases are used partially in mixture with air and sometimes in addition to water vapor, it is not sure whether all of these syntheses are to be ascribed exclusively to the mineralizing action of the hydrogen halides.

According to Doss (1894), rutile forms in microcosmic

salt and borax beads at high temperatures. 10

Treatment of TiCl* with oxygen has been used by some

investigators to make rutile. Chudoba and Wisfield (1933) produced rutile prisms by passing a mixture of TiCl* vapor and dry 0 2 in a

1:1 mole ratio through a porcelain tube furnace. Merwin and

Hostetter ( 1919) obtained crystals of rutile in the course of investi­

gations on the volatilization of iron from clay pots used for the

melting of optiGal glass. Chlorine gas was passed into covered pots

at temperatures from 1000° to 1100°C for several hours. On the

inside of one of the pots were found minute crystals of rutile. A

more recent patent (du Pont, 1961) describes the production of

fibrous TiOz by bubbling air through TiCl4 and passing it over a

KC1 melt. In addition to TiCl*, TiBr 4 and TH 4 can be used. It

is not stated whether the TiOz thusly produced is rutile, anatase,

brookite, or amorphous.

Through the hydrolysis of Ti-halide vapors at high temper­

atures, Hautefeuille (1864, 1865) produced well-formed rutile

crystals, measurable on the goniometer. He treated TiCl* vapor

with H P vapor at bright-red heat. Hydrolysis is also involved in

the experiments of Hautefeuille (1863, 1864, 1865) described

previously. According to the reactions,

K2Ti03 + 2HC1 -*> TiOz + 2KC1 + HzO

K2TiF6 + 2HC1 - * TiF 4 + 2KC1 + 2HF 11 one sees that T1F4, KC1, HF, and water are formed along with the production of crystalline TiOz. Holgersson and Herrlin (1931) confirmed the TiCl* vapor and-water vapor experiment at 1000° to

1200°C.

Parravano (1938) states that Ti0 2 modifications can be produced from Ti-salt solutions in many different ways, but that rutile is only formed by transformation from primarily formed anatase.

On the other hand, Weiser et al. (1941) claim to have produced rutile from Ti-chloride or nitrate solutions at 100°C.

Titanium dioxide has been used as an opacifying agent in enamels for some time, and, recently, as a nucleating agent in the production of glass-ceramics. Friedberg et al. (1947) made frits by smelting, at 1150°C, mixtures which contained TiOz. Firings were made at 830° and 843°C for four minutes with the precipitated Ti0 2 phase identified as anatase. Anatase was only present in a small number of the firings and the authors could not say what governed the crystallization of anatase or rutile. Styhr and Beals (1948) made frits (containing TiOz) at temperatures in the range 135 0° to 155 0°C.

These were fired in the range 840° to 1110°C. They stated that with the relatively simple base glasses, anatase appeared to be the predominant crystalline phase which precipitated during firing. As the complexity of the glass increased, there appeared to be a greater tendency for rutile to form. Yee and Andrews (1956) fired enamels on thin platinum sheets to prepare specimens for x-ray analysis. The samples were fired at 741°, 804°, 841°, 904°, 942°,

1000°, 1043°, and 1105°C for four minutes. The presence of anatase alone was noted at 704°C, with rutile first appearing in the 804°C sample. Between 1000° and 1043°C, all traces of anatase had disappeared.

Bunting (1933) used several low-melting (310°-7 00°C) fluxes to produce rutile from anatase and brookite. His anatase was pre­

pared by hydrolysis of TiCl*, and the brookite used was natural

mineral. He ran experiments of rutile seeded with anatase and

brookite; brookite seeded with anatase and rutile; and anatase

seeded with brookite and rutile, in the presence of fluxes in the

temperature range 320°-1400°C. Anatase was changed as low as

400°C and brookite as low as 5 25°C, both in the presence of

K^Cr^07, to rutile. He did not succeed in changing rutile to either

anatase or brookite. Phase identification was by x-ray diffraction.

He concluded, "The data indicate that if there is a temperature

where rutile is in stable equilibrium with anatase or brookite, it

must be below 400°C. " Junker's (1936) experiments in the system

Na20 - T i0 2 led him to believe that brookite is stable up to 1300°C,

where a transition to rutile takes place. His observations were 13 made on melts in a high frequency furnace. Hautefeuille (1864) produced rutile by the prolonged heating of titanic oxide with sodium tungstate or vanadate as a flux.

The formation of rutile from anatase or brookite through solid-state transitions has been reported by many writers. These reports as well as the temperatures of transition will be tabulated below.

Among other methods of producing rutile, Devllle (1961) heated a mixture of titanic acid and SnO in a clay crucible and produced rutile crystals. Hautefeuille (1880) produced alkali silicotitanates, e.g., 2 Nai0:5Ti02* 4Si02, Na 20* 2Ti02* 3SiOz which formed clear, colorless glasses upon melting, and, upon slow cooling, yielded needle-shaped rutile crystals. According to

Michel (189 2), if one heats a mixture of one part iron titanate with two and one-half parts pyrite in a graphite crucible at 1200°C, then one obtains a crystalline mass of pyhrrotite penetrated with blue rutile needles. If heated in oxygen, the rutile needles become colorless.

Kubo et al. (1963) report the formation of rutile from anatase by means of mechanical grinding. They state that grinding

TiO* obtained from TiCl*, for 96 hours results in rutile formation.

Grinding anatase obtained by the H ^04-method produced an almost amorphous TiOz. 3. Anatase syntheses. According to Gmelin (1951), anatase Is formed by the hydrolysis of Ti-halides at high temper­ atures (at dark-red heat). Anatase has also been formed from Ti- salt solutions (Weiser et al. , 1941; Weiser and Milligan, 1934,

1942; Milligan and Weiser, 1936). Jamil et al. (1963) produced anatase by hydrolysis of titanium sulfate. Sullivan and Cole (1959) also obtained anatase from a Ti(S04) 2 solution. Anatase has been precipitated from solution in glass frits, at enameling temperatures

(Friedberg et a l., 1947; Styhr and Beals, 1958; Yee and Andrews, m 1956). Anatase was prepared from butyl titanate through hydrolysis by Jamil et al. (1963). Knoll (1961) also synthesized anatase by the hydrolysis of titanium acid esters. Gmelin (1951) cites syntheses of anatase due to the heating of precipitated titanic acid.

Czanderna et al. (1957) prepared a "highly purified" anatase by dissolving silica-free titanium metal in an ammoniacal solution of

90 per cent H 20 2; filtering- out impurities such as Fe, Mn, Mg,

Sn, Ni, Al, and Ag; and drying off the peroxide to form a titania gel. After heating this gel at 200°C, small anatase crystals,

l l 3 mm on each edge, were formed. Wohler (1849) obtained anatase by heating, in a stream of water gas, cubic crystals of cyanogen- nitride-titanium formed in a blast furnace. Burgers et al. (1932) observed anatase formed in a layer on Ti metal sheet by electrolytic oxidation in H 3P04. Anatase was confirmed by x-rays. By treating TiO with hydrogen chloride gas, Hagenmuller et al. (1959) were able to form both anatase and rutile, and said, "...the two varieties of TiOz are not simultaneously formed in the destruction of the TiO lattice: the rutile form results from transformation of the anatase." According to Parravano and Cagliotti (1934), mineralizers for the transformation of amorphous Ti0 2 to anatase are: KOH, NaF,

Ba-borate, Ba-phosphate, barite, and NH3.

4. Brookite syntheses. By the hydrolysis of TiCl*,

Daubree (1849, 1850) formed brookite. He passed TiCl* and steam through a red hot porcelain tube and obtained "a wart-shaped mass covered with microscopic crystals. " He also produced brookite crystals, besides others, through the action of TiCl* on CaO at red heat (Daubree, 1854). Hautefeuille .(1865) obtained Lavender blue, transparent crystals of brookite be treating TiF 4 with steam at about

1000°C. More recently, Inazuka (1941) reported the formation of brookite when a clean glass plate was dipped into an aqueous solution of TiCl 3 or TiCl* containing HC1, dried at 100°C, and heated at 300° to 400°C for one hour. His identification was by electron diffraction of the thin film. Schossberger (1942) reported,

but with uncertainty, the formation of brookite from sulfuric acid-

ilmenite solutions or from hydrochloric-TiCl* solutions. This same

uncertainty holds for the formation of brookite as an intermediate product in the thermal transformation of other TiO 2-modifications, as reported by Parravano (1938). From 300°-7 00°C, Shchegrov (1963) obtained anatase by the hydrolysis of TiCl*.

Since 1956, there have appeared several reports on the synthesis of brookite from titanium-organic compounds. Glemser and Schwarzmann (1956) first formed amorphous TiOz by hydrolysis of titanic acid-tetraethylester at room temperature, then, as 100°C, the hydrolysis of titanic acid tetraethyl- or tetrabutylester formed brookite along with anatase. Knoll and Kuehnhold (1957) confirmed the brookite synthesis of Glemser and Schwarzmann by performing the synthesis and identifying the phases through x-ray analysis.

Knoll (1961) says that the formation of brookite was dependent on

the water content of the hydrolysis products. Schroeder (1962),

used an ester of titanic acid as a coating for glass. After heating

to 400°C and forming an oxide layer, an electron diffraction

examination showed patterns for anatase, brookite, and a third,

unknown group, thought to be a titanium suboxide. Jamil et al.

(1963) hydrolyzed butyl titanate in a BuO solution with H20 at room

temperature. The precipitate was fired and examined by DTA, In

their report they say that titanyl hydroxide decomposed at 128°C,

and the amorphous Ti0 2 changed to anatase at 320°C. Furthermore,

hydrolysis in BuOH by steam yielded some brookite. Bach (1964) 17 also produced thin films of Ti0 2 In a manner similar to that of

Schroeder (1962).

Yamaguchi (1961) produced an oxide film about 3000 A.

oil pure titanium plate by an anodic process. The electrolyte

used was H^0 4 (50 wt. %). The oxide was Identified by trans­ mission electron diffraction, and the d-spaclngs they report are in

good agreement with those of brookite.

Hautefeuille (1864, 1865) formed the arkansite variety of

brookite in rhombohedral crystals by decomposing a melt of S102,

TiOz and K*3iF 6 In a HC1 gas stream at dark red heat In a graphite crucible. In a slow-moving gas stream of air and HC1, plate­

shaped brookite was formed.

Knoll (1961) reported partial transformation of anatase to

brookite by grinding in a ball mill. Complete transformation of

anatase to brookite finally succeeded with the grinding of titanium

dioxide specimens which already contained brookite and were pre­

viously heated for several hours at 600°C. After thirty hours

grinding, all anatase peaks disappeared from the x-ray pattern.

The synthesis of brookite from an aqueous solution of

titanium potassium oxalate and H^ 0 2 in a hydrothermal bomb was

done by Wenden (1961). 18

5. Amorphous T1Q2 syntheses. Concerning the formation of glasses, Smekal (1951) says,

The efficiency of cooling melts depends on the rate of heat removal, and is very limited even for small quantities. Vitreous products may sometimes be obtained more effectively by starting from gaseous or crystalline states. This has been done recently for SiO, Al 2Os and TiOz, by condensation of the oxidation products of vaporized metals on well-cooled surfaces.

He also predicted that if TiOz forms a glass, the Ti ion would be

four-coordinated rather than six-coordinated as in the crystalline

state. The production of amorphous TiOz referred to by Smekal was

II done by Hiesinger and Konig (1951). They evaporated metallic

titanium, in pure oxygen, onto glass surfaces. When these films

were dissolved off with diluted HF, they gave a rutile diffraction

pattern. If, however, a better heat conductor such as rocksalt is

used as a substrate instead of glass, one obtains a broadly diffuse

electron diffraction pattern, which changes to the previous sharp

pattern after heating to 450°C. In conclusion, they say, "Thus

titanium dioxide in analogy to aluminum oxide, exists in a disordered

state which, by the addition of energy, transforms into crystals. "

Conjeaud (1954) evaporated the oxides, anatase and rutile,

under high vacuum onto cold surfaces and produced amorphous thin

films of TiOz. 19

Glemser and Schwarzmenn (1956), Knoll (1961), Knoll and Kuehnhold (1957), and Bach (1964) reported synthesis of x-ray- amorphous Ti0 2 by the hydrolysis of titanium tetraethylester.

Krylova and Bagdyk (I960) reported that TiOz produced by the hydrolysis of HCI 4 is amorphous.

6. Polymorphic transformations. There has been a great

deal of experimental work concerning the effect of foreign ions on

the transformation rates and temperatures of the three modifications

of TiOz. Parravano and Caglioti (1934) reported a variation of

transition temperature of anatase to rutile depending on the amount

of sulfate ion present. Huttig and Kosterhon (1939) studied the

anatase-rutile transition in the presence of HC1 gas. Weiser et al.

(1941) also suggested that the sulfate ion stabilizes the anatase

phase. The stabilizing effect of sulfate ions was also discussed

by Schossberger (194 2), and the influence of stabilizing foreign

ions was suggested to account for varying transition temperatures.

In the case of Ti02-opacified enamels, Friedberg et al. (1947) found

that CaO promoted the formation of anatase,' i.e. , it retarded the

anatase-rutile transition. Cowan (1956) found that the addition of

MgO acted to suppress the transition of anato rutile in enamels.

Yee and Andrews (1956) reported that addition of P 20 5 stabilizes

the anatase phase in titania-enamels. Knoll and Kuehnhold (1957) %

20 investigated the anatase-rut lie transition by DTA on specimens con­ taining nitrate, chloride, sulfate and fluoride ions. The nucleation of rutile was impeded by the above ions in increasing order, respectively. Rao et al. (1959) reported that the addition of five atomic per cent of Sb+^, Al+3, Zn+2, P0 4 S04 2, and Cl 1 all stabilized the anatase structure. According to Kubo and Shinriki

(1953), the anatase-rutile transition is promoted by the presence of

HC1 or H£04, while H3B03 inhibits the transition. The addition of foreign chemicals lowers the transition temperature. The most favorable substances for lowering the transition temperature are

LiCl, ZnCl2, and ZnO. Iida and Ozaki ( 1961), depending on the method of preparation of anatase, transformed anatase to rutile at temperatures varying from 700° to 900°C. They studied the effects of additives on grain growth and transformation, and found that

CuO promoted the transformation, whereas W0 3 and Na20 retarded it. According to Yoganarasimhan and Rao (1962), the presence of the sulfate ion progressively inhibits the anatase-rutile transformation with increasing concentration, and the activation energy also increases with increasing sulfate ion concentration. Yoganarasimhan

(1963) also studied the effects of the chloride ion on the anatase- rutile transformation. Rao and Lewis (I960) examined the effects of impurities on the anatase-rutile transformation. Sullivan and Coleman (1962), however, had different findings than the numerous accounts up until now. They used radioactive sulphur 35 as a tracer in their experiments and concluded that no evidence was found to support the theory of S0 3 2 stabilization of the anatase structure. Back (1964) found that Ti0 2 films precipitated on glass slides are dependent upon the chemical composition of the glass with respect to the TiOz modification which forms upon heating.

He found that films precipitated onto alkali-free glass substrates produced anatase when heated, and those precipitated onto alkali- containing glasses, e.g. window glass, when, heated, produced brookite. Klbshev et al. (1964) investigated the anatase-rutile transformation with regard to NiO, CuO, WOj, and Fe 20 3 impurities.

They found that NiO and CuO lowered the transformation temperature, w hereas W 0 3 and F e 20 3 raised it. The effect of the former two was ascribed to their crystallochemical similarity to TiOz.

Several workers have investigated the kinetics of the transitions of the Ti0 2 polymorphs. Czanderna et al. (1958) used spectroscopically pure anatase in their study. They found that below 6 1 0 ° the transition of anatase to rutile is immeasurably slow, and above 7 30°C it occurs extremely rapidly. According to their account, the conversion is second order with respect to remaining anatase and is characterized by an activation energy of ca. 100 Kcal 22 per mole. They concluded that the anatase-rutile transition is strongly dependent on time and temperature. Rao's (1961) work showed essentially the same results as that of Czanderna et al.

According to Suzuki and Kotera (1962), the anatase-rutile transition kinetic data fit a first-order equation reasonably well. They say that the transition rate and activation energy are significantly af­ fected by the heating atmosphere and the flux.

Several investigators have studied the anatase-rutile transition by means of DTA. Sullivan and Cole (1959) described a typical DTA curve as follows:

During heating, loss of water occurs at about 150°C, loss of sulphur trioxide at about 650°C, and the transformation of anatase to rutile in the range 700° to 950°C, depending on the method of preparing the samples.

On their thermograms, "...an exothermic pip at 900°C is associated with the transformation of anatase to rutile. " Ricker and Hummel

(1951) also used DTA as a means of investigating the anatase- rutile inversion. They say that the inversion is ". . . sluggish, be­ ginning at 750°C, but requiring as much as 18 hours at 1080°C for completion. " Olympia (1953) examined the crystallization of TiOz from titania-opacified enamels. An endothermic peak at 425°-460°C was attributed to the formation of crystallite nuclei, and an exo­ thermic peak at 620°-720°C was ascribed to crystallization of titania. Various temperatures reported in the literature for the transition of anatase to rutile have been assembled in Table 1.

TABLE 1

REPORTED TEMPERATURES FOR THE ANATASE-RUTILE TRANSITION

Author Temperature (°C)

W eiser et a l (1941) 100 Osborn (195 3) 250 Bunting (1933) 400 Czanderna et al (1958) 610 Yoganarasimhan and Rao (1962) 695 Iida and Ozaki (1961) 7 00 This work 700-800 Barblan et al (1944) 800 Huttig and Kosterhon (1939) 850 Sullivan and Cole (1959) 700-950 Parravano and Caglioti (1934) 830-950 Kleshev et al (1964) 900-1000 Schroeder (1928) 915 Pamfilov and Shikher (1937) 1000 Naylor and Cook (1946) 1050 Brusilovskii et al (19 39) 1100 Schossberger (1942) 600-1200

The brookite-rutile transition has not received as much

attention in the literature as the anatase-rutile transition, per­

haps because of the difficulty of readily synthesizing suitable

brookite. Yoganarisimhan (1962) says that smaller particle size

and greater purity favor the brookite-rutile transformation. He 24 suggested that grinding of the material produces dislocations which act as nucleation sites.

Rao et al. (1961) used natural brookite in their experiments.

They report that below 715°C the transformation is extremely slow, and that the activation energy for the brookite-rutile transition is lower than for the anatase-rutile transition. Schroeder (1928) says that the monotropic transformation of brookite to rutile is very sluggish, and the speed is an exponential function of the temperature which first attains measurable values at about 65 0°C.

Junker (1936) reported the transition of brookite to rutile at 1300°C, in his study of the system NA£>-TiOz.

In Table 2 are some of the temperatures for the brookite- rutile transition which have been reported in the literature.

Knoll (1963) reported the transformation of anatase to brookite after thirty hours grinding in a ball mill. The anatase was heated at 600°C for several hours before grinding. 25

TABLE 2

REPORTED TEMPERATURES FOR THE BROOKITE-RUTILE TRANSITION

Author Temperature (°C)

Schroeder (1928) 650 Barblan et al. (1944) 700 Rao et al. (1961) 715 Brusilovskii (1939) 1100 Junker (1936) 1300

Schroeder ( 1928 ) lists two forms of anatase which invert enantiotropically at 642°C. Thus low (p) anatase changes enantiotropically to high (a ) anatase, and high anatase inverts monotropically to rutile as follows:

642°C „ 9 15±15 °C _ p anatase x s. a anatase ------> rutile.

Structural considerations have played a part in many of the discussions of TiOz polymorphism. Schossberger (1942), in con­ sidering the stabilities of the three modifications of titania, note +4 that the three forms are similar insofar as all have the Ti ion surrounded by six 0 2 ions at the apices of a somewhat deformed octahedron. Invoicing Pauling's rules, Schossberger says that although anatase should be least stable, brookite more stable, and rutile the most stable form, experiments have shown that 26 apparently anatase is more stable than brookite. He attributes this disparity to the presence of foreign ions which tend to stabilize the anatase lattice. It is interesting to note that

Pauling (i960) used the polymorphs of titania in explaining the significance of the polyhedral-element-sharing rule. He says,

In rutile, brookite, and anatase, for example, each atom is common to three titanium octahedra, but the number of edges shared by each octa­ hedron with adjoining octahedra is two in rutile, three in brookite, and four in anatase. The significance of this structure is contained in the following rule: The presence of shared edges and especially shared faces in a coordi­ nated structure decreases its stability; this effect is large for cations with large valency and small ligancy.

Barblan et al. (1944) found that whereas anatase converted to an aggregate of rutile crystals, brookite can be converted in an orderly fashion. They considered the brookite-rutile transformation from a structural point of view and found that the ( 100) planes of brookite and rutile remain parallel. Also, the rutile c-axes were either parallel to the c-axes of brookite, or both b-axes were parallel or in the direction of the [027] and [ 023] zones of brookite. Along contact regions of twinned rutile the octahedral cells are arranged similar to brookite. 27

There has not been a great deal of work done concerning the polymorphism of Ti0 2 based on thermodynamic calculations.

Skinner et al. ( 1954) say that it has not been possible to calculate the transition temperature from the thermal functions of anatase and rutile, because the free energy functions are no nearly the same.

Czanderna et al. ( 1961) studying the brookite-rutile transition, found the activation energy to be about 60 Kcal mole *, the 13 -1 frequency factor to be of the order of 10 hr , and the heat of transformation to be about -100*75 cal mole Arthur (1950),

Kelley and Mak (1959), Lietz (1956), Naylor and Cook (1946),

Nilson and Pettersson (1887), and Wicks and Block (1963) have investigated the thermodynamic properties of TiOz.

7. Summary. In summarizing the data from the literature regarding syntheses of the various modifications of titania, an effort has been made to present the data in a graphic form which, it is hoped, will add clarity to the preceding survey (see Figure 1).

C. Experimental

The experiments described in this section were done to test new hypotheses related to the system titania and to confirm data reported in the literature. Amorphous TiOa

solutions

4,5

4,5 6,10, II ANATASE RUTILE

7,11 BROOKITE

Figure 1. Graphic Summary of Ti02 Syntheses.

1. Precipitation 7 . Heating 2. Hydrolysis (cold) g, ignition 3. Hydrolysis (warm) 9.. Oxidation 4. Hydrolysis (presence SO 4 , P04~ ) 10. Grinding 5. Hydrolysis (presence Cl , N0 3 ) 11. Fluxing 6 . Dehydration 1. Grinding experiments. Failure to convert rutile directly to anatase under any laboratory conditions largely accounts for belief in the monotropic nature of the anatase-to-rutile inversion.

In the first of a series of grinding experiments, rutile, formed by heating anatase, was ground in liquid nitrogen for five minutes in an attempt to convert it to either anatase or brookite. Burns and

Bredig (1956) were able to transform calcite to argonite by grinding at room temperature. Previously, aragonite had widely been regarded as having a monotropic relation towards calcite, similar to the relation generally believed to exist between anatase and rutile.

It was thought that if anatase had a low temperature stability range, grinding might initiate a conversion, even though the density rela­ tions of anatase (3.84) and rutile (4.26) would appear unfavorable.

However, no conversion of rutile to anatase was effected.

In another experiment, anatase was ground for 220 hours in a

mechanical mortar. X-ray diffraction patterns made during and at the

conclusion of the experiment showed a slight decrease in anatase

peak intensities with no indication of transformation.

Rutile was heated to 900°C for several hours and quenched

in water to induce thermal shock. In this experiment, the material was ground for seventy-one hours under water in a mechanical

mortar. No change in the rutile was observed by means of x-ray

diffraction. Anatase was given a heat treatment of 600°C for nine hours,

In accordance with Knoll (1963), previous to ball-milling under water for 150 hours. No transformation was indicated from x-ray evidence.

Finally, anatase, with a small amount of brookite "seeds'* was ball-milled for fifty-six and one-half hours with no noticeable transformation to brookite.

The data concerning grinding experiments are summarized in

Table 3.

TABLE 3

DATA FROM GRINDING EXPERIMENTS

Starting Length of Products m aterial run (x-ray) Remarks

rutile 5 mins. rutile mechanical mortar, under liquid nitrogen

rutile 71 hrs. rutile mechanical mortar, heat treated

anatase 220+ hrs. anatase mechanical mortar

anatase 150 hrs. anatase ball mill, heat treated, under water

anatase, brookite "seeds 56-1/2 hrs. anatase ball mill 31

2. Heating of T102. Experiments on the heating of T10 2

(anatase) were carried out in the initial period of the Investigation to gain some knowledge of the transformation rate and temperature.

It is evident from the data in Table 4 that the anatase-rutile trans­ formation is rather sluggish.

TABLE 4

HEATING OF TiOz (ANATASE)

Starting Temperature Time Products m aterial (°C) (h rs.) (x-ray)

anatase 600 9 anatase (quenched in water)

anatase 640 41 anatase

anatase 700 26-1/2 anatase (from preceding run)

anatase 950 116 rutile (from preceding run)

3. Heating of titanium sulfate. When titanium sulfate is heated and decomposed, TiOz is formed. Several samples of titanium

sulfate, in small platinum envelopes, were heated in the quench furnace at different temperatures to see at what temperature the change from anatase to rutile formation occurs. Table 5 is a list

of data concerning these runs. 32

TABLE 5

HEATING OF TITANIUM SULFATE

Temperature Time Products (°C) (h rs .) (Ti02 phase, x-ray)

851 2- 1/2 anatase 861 7 -1 /2 anatase, rutile (weak) 870 4 anatase, rutile (weak) 898 2 anatase, rutile 951 2 rutile

4. Titanlum-halide experiments. In the first of these experiments, the reaction of TiF 4 with water vapor was an attempt to produce crystals of the various forms of TiOz, namely anatase, brookite and rutile, through the following reaction:

TiF4 + 2HP—► TiOz + 4HF.

The TiF4 was placed in a combustion boat inside a fused silica tube which was heated in a tubular furnace. Water vapor was carried over the boat of TiF 4 by argon gas. After reacting with the TiF4, excess water vapor and reaction products were carried to the cooler end of the tube where they were either deposited on the walls, or went on through the gas train and bubbled through an aqueous solution of CaOH. In the CaOH solution, the HF reacted to form CaF2. 33

The first experiments were conducted with the combustion tube just below the decomposition temperature of TiF 4 (284°C), and later raised to 65 0°C. X-ray diffraction patterns of material scraped from the walls of the tube and from the combusion boat proved to be anatase or TiOF2.

Heating a solution of TiClt and NH4OH to 100°C and drying on a glass slide yielded a rutile diffraction pattern.

A glass slide was dipped in TiClt, dried at 100°C, and heated at 400°C for one hour, as described by Inazuka (1941).

An x-ray diffraction pattern showed only anatase present.

5. Flux runs. Flux runs with the anatase and rutile forms of T i0 2 were made at several temperatures and with five different fluxes, in a series of experiments similar to those of Bunting (1933).

An attempt was made to effect the rutile-to-anatase transition by using these fluxes. On the assumption that anatase rather than rutile is the phase stable at low temperatures, it was hoped that the more soluble (because less stable) rutile would dissolve in the flux and that the less soluble (because more stable) anatase would precipitate.

Rutile was heated in a platinum crucible with a large excess of flux, the flux dissolved to concentrate the TiOz phase, and the

TiOz phase identified by x-ray diffraction. 34

Anatase was heated in flux runs to see the effect of the flux on the anatase-to-rutile transformation temperature. The data on these runs are presented in Table 6 .

TABLE 6

FLUX RUNS WITH ANATASE AND RUTILE

Temperature Products Reactants r c) Time (x-ray) rutile, LiN03 268 + 3 dys. rutile rutile, LiN03 500 2 dys. rutile rutile, Li/KF 504 19 hrs. rutile rutile, V20 5 717 7 hrs. rutile rutile, Na2W04 719 7 hrs. rutile anatase, LiN0 3 650 19 hrs. crucible destroyed anatase, B 20 3 650 19 hrs. anatase, rutile anatase, V^ 0 5 700 19 hrs. could not dissolve flux anatase, Na 2W04 700 19 hrs. do.

6. Hydrothermal bomb runs. Because of Wenden's (1961) successful hydrothermal synthesis of brookite, a series of runs were made in stainless steel, hot-seal, hydrothermal bombs to reproduce his results and establish brookite-anatase relations *in the Ti02-H P system, comparable to those reported by Osborn (1953) .for the anatase-rutile system. The experiments were mainly unsuccessful in the production of brookite, with two exceptions where the x-ray diffraction patterns show peaks which might possibly be attributed 35 i to the presence of brookite. The results of these experiments are presented In Table 7.

7. Parallel firing runs. This series of experiments was directed at finding the transformation temperature of anatase to rutile In the presence of B^D3, since a study of the system TiOz-

B20 3 was being done at the same time. Also, the reversibility of the anatase-rutile transformation had not been determined.. Therefore,

parallel sets of runs, one Involving rutile as the initial form of

T102, the other Involving anatase, mixed with 70 weight per cent

B20 3 were carried out. The pairs of mixtures were contained in

platinum-foil packets and suspended side-by-side in a quench fur­

nace for firing. The data of these runs, as presented in Table 8 ,

indicate that the transformation of anatase to rutile, in the presence

of B20 3 begins between 702° and 7 24°C. TABLE 7

HYDROTHERMAL BOMB RUNS

Temperature Products Reactants (°C) Time (x-ray)

Ti(S04)2, HPz 200 3 d. anatase, rutile K-Ti-oxalate, Fe, H2Q2 209 17 d. anatase, rutile, brookite (?) rutile, H3BO3 247 3 d. rutile anatase, H 3BO3 250 3 d. 6 hrs. anatase Ti(S04) 2, H2P 2 250 10 d. (leaked) K-Ti-oxalate, H20 2 250 10 d. anatase, rutile rutile, H3BO3 254 87 hrs. rutile - K-Ti-oxalate, H^Oz 274 3 d. rutile, K-titanate (?) Ti(S04) 2, H2P 2 284 5 d. anatase, rutile Ti(S04) 2, Na2C 0 3 390 9 d. anatase, brookite ( ? ) , Na 2Ti5On (?) Ti(S04) 2, Na2COj 400 7 d. anatase, Na2Ti5On , Na 2C 0 3 (leaked)

u> O' 37

TABLE 8

PARALLEL FIRINGS OF ANATASE AND RUTILE WITH B20 3

Products (x-ray) Temperature Time Initially Initially (°C) (h rs.) Anata se Rutile

700 2 anatase rutile 702 4 -1 /2 anatase rutile 724 2- 1/2 anatase, rutile rutile 743 3 -1 /2 anatase, rutile rutile 786 2 anatase, rutile rutile 810 4 rutile rutile 836 2 rutile rutile 844 2 rutile rutile

8 . Strip furnace experiments. The strip furnace (Roberts and

Morey, 1930) was used to make exploratory runs early in the investigation of the system Ti0 2-B203. Due to the B^D3 volatilization problem, a few runs were tried using short lengths of platinum tubing to both contain the sample and serve as the resistance heater. After being loaded with the sample, the ends of the tubing were crimped flat and folded over twice. These folded ends were further held shut by the electrode clamps of the strip furnace. Most of the early attempts at heating mixtures in tubes failed because of ruptures in the tubing wall. It is now known that this was due to non- anhydrous Bj£>3. However, the fact that the ruptures occurred midway between the ends of the tubing attests to the sealing qualities of the folded tubing, clamping arrangement. This technique, then, has something to recommend it for systems where volatilization is a problem if the vapor pressures of the materials are not exces­ sively high and if the materials do not readily become hydrated.

The results of these tubing experiments are presented in Table 9.

TABLE 9

STRIP FURNACE PLATINUM TUBE HEATING EXPERIMENTS

Composition (wt. %) Temperature Time Products TiOz B p 3 (°C) (mins.) (x-ray)

15 85 1175 2 tube ruptured 15 85 1130 2 do. 20 80 1245 3 do. 20 80 1320 5 rutile, glass *

Another larger group of runs was made in the conventional manner using either platinum-40 per cent rhodium, or iridium, heating strips, depending on the operating temperature. The first runs were made with mixtures of Ti0 2 + SiOz and TiOz + B p 3 in unknown proportions, because of volatilization losses in both of these mixtures. Later, the starting composition of the mixtures was 39 fixed, so that if the temperature and heating time were the same for a series of runs, the runs could be placed on a temperature horizontal in a way relative to their initial compositions.

The mixtures of TiOz + SiOz were fired to obtain a view of the "spectacular" separation of two liquids as described by De Vries et al. (1954). The features of liquid immiscibility were observed for use in detecting possible liquid separation in the system TiOz-

B20 3. However, when the high temperature runs (17 00°C and higher) were examined by x-ray diffraction, anatase was present, alone or with rutile, in a glass. This was found in mixtures of TiOz + Si0 2 as well as Ti02 + B20 3.

In the original design of the strip furnace (Roberts and Morey,

1930), the U-shaped strip approximated a black body radiator closely enough that an optical pyrometer could be used to measure temper­ atures. The temperatures in these experiments, however, is not known with certainty since the iridium strips could not be convenient­ ly bent to a sharp radius without breaking. The temperatures re­ corded are those read from the pyrometer scale and, according to

Weber (1950), could be low by a maximum value of 240°C. In spite of this uncertainty, these experiments are significant in that they show the formation of anatase at high temperatures, be it stable or metastable, from mixtures of Ti0 2 with S i0 2 or B20 3 where the 40 initial form of TiOz was rutile. The data for this series of runs are presaited in Table 10.

TABLE 10

STRIP FURNACE HEATING EXPERIMENTS

Composition Temperature Time Products (wt. %) (°C) (mins.) (x-ray) " ■ i . . —.i ■ . . . ._ ,| ■ Titanium dioxide-boric oxide compositions T i0 2 B2O3

50 50 1550 2 rutile, glass 40 60 1850 1/2 rutile (B4O3 volatilized) 30 70 17 00 1 a nata se, rutile, gla s s 30 70 1710 1 anata se, rutile, gla s s 20 80 1660 1 anatase, rutile, glass 10 90 1675 1 anatase,rutile, glass 10 90 950 60 rutile glass 8 92 825 30 rutile glass 5 95 1750 1 anatase glass 5 95 1700 5 rutile(B 20 3 volatilized) 3 97 1750 1- 1/2 anatase, glass 3 97 1700 1- 1/2 rutile(B 20 3 volatilized) unknown 1700 3 anatase,rutile, glass Titanium dioxide - silicon dioxide compositions TiOz S i0 2 35 65 17 25 30 anatase, rutile, glass 35 65 1700 120 anatase,rutile, glass 30 70 ------(strip burned out) unknown 1700 3 anata se, rutile, glas s 100 0 1850 1/2 rutile 41 9. Titania-enamel experiments. Titania-enamel compos­ itions were made and fired at various temperatures to determine whether anatase or rutile was the TiOz form precipitated from

solution in the glass frit. Reports in the literature (Friedberg et a l.,

1947; Yee and Andrews, state that anatase is initially precipitated from solution and later transforms to rutile.

The composition used in the first series of runs was the

same as Friedberg et al. 's composition AH6 , with this exception;

one batch was made using anatase as the TiOz ingredient and

another using rutile as the form of TiOz. The frits were prepared as

described by Friedberg et al. and fired on small strips of copper.

A second, smaller group of enameling runs was made by using

the frit composition and firing procedures outlined by Yee and

Andrews. These frits were fired and heat treated on small strips

of platinum. Phase identification of the opacifying crystals in the

enamels of both firing series was done by x-ray diffraction.

The formation of anatase in the first group of firings was not

definitely established. However, the firings after Yee and Andrews

indicated that anatase can be precipitated, with proper heat treat­

ment, from a clear glass solution containing TiOz. Firing data

for these runs are presented in Table 11. 42

TABLE 11

TTTANIA-ENAMEL FIRING EXPERIMENTS

Temperature Time Opacifying crystals (°C) (m in s.) (x-ray) Composition AH 6 , Friedberg et al. (1947) TiOz * rutile 7 07 4 no crystals detected 745 4 rutile 798 4 rutile 800 4 an atase (?), rutile 844 4 rutile 906 4 rutile 948 4 rutile 996 4 no x-ray record

Composition AH 6 , TiOz ■ an atase

707 4 no crystals detedted 745 4 do. 798 4 an atase (?), rutile 800 4 an atase (?), rutile 844 4 rutile 906 4 rutile 948 4 rutile 996 4 no x-ray record Composition, Yee and Andrews (1956) 750 5 no crystals detected 750 4 anatase (?) , rutile 750 2 an a tase 43 10. Plasma-arc flame spraying experiment. In this experiment, finely grouns (325 mesh) anatase was sprayed through the flame (8000°-11000°C) of a Plasmadyne plasma-arc torch into water. By using this technique, it was hoped that a rapid enough quench could be obtained to produce a TiOz-glass or at least retain the high temperature form(s) of Ti02 as a quench product.

Microscopic examination revealed small, blackish spheres.

(. 004-. 04 mm diameter) which were mostly opaque. After grinding the above material, it was somewhat translucent and reddish brown.

X-ray diffraction examination of the material showed anatase and rutile peaks.

The blackish color of the quenched material is probably due to oxygen loss, because heating of material overnight at 600°C changed the color to gray, and further heating at 700°C, overnight, changed the color to a light yellow.

D. Discussion of literature and experiments

1. Possible phase diagrams for TiQ2. In considering the

polymorphic relations of the three forms of TiOz, it was deemed

necessary to first arrive at the combinations of phases, stable and

metastable, which could exist. Next, these possible arrangements were examined in the light of knowledge gained by experiment and

from the literature. Some arrangements were retained as probable and others discarded as unfeasible, on the basis of the above know­ ledge. These possible arrangements are represented, schematically, in the combinations for: one stable and two metastable phases

(Figure 2,3), two stable and one metastable phases (Figure 4), and three stable phases (Figure 5). The alpha and beta forms of anatase are not shown. The boundary curves for stable phases are indicated by solid lines and for metastable phases by broken lines. As men­ tioned previously, these diagrams are schematic, and there are no

special values attached to the pressure and temperature coordinates, except that they increase respectively upward and to the right.

Also, a word is in order concerning the slope of the boundary curves,

especially those separating solid phases.

The relation of solid and vapor phases at equilibrium, with respect to pressure and temperature, is given by the Clausius-

Clapeyron equation

= — k s------, (i) dT T(Vv - Vs )

where P = pressure, T = temperature, Ls = heat of sublimation,

Vv = volume vapor, and Vs = volume solid. By using this form of

the equation, one may obtain the slope of the curve if the heat of

sublimation is known, or vice versa. 45

a. b. c.

L -A * ?

d. T f.

Figure 2. Possible phase diagrams for the system titania: one stable, two metastable phases.

A ■ anatase, B = brookite, R = rutile, L = liquid', V = vapor, P = pressure, and T = temperature.

✓ 46

b.a. c.

-I atm R

d.T f.

Figure 3. Possible phase diagrams for the system titania: one stable, two metastable phases. See Figure 2 for symbols. 47

a. b. c.

I atm

P

T f.

Figure 4. Possible phase diagrams for the system titania: two stable, one metast&le phase. See Figure 2 for symbols. 48

b. e.

I atm

P

T d. f.

Figure 5. Possible phase diagrams for the system titania: three stable phases. See Figure 2 for symbols. 49

When written in the inverted form, equation (1) represents the influence of external pressure on the melting point of a solid, that is, it gives the solid-liquid boundary curve for temperatures and pressures corresponding to equilibria between solid and liquid.

Thus, in inverted form,

= T(VL - Vs) dP Lj [ ' where Lf = heat of fusion. If one is dealing with another form of transition, namely a solid-solid polymorphic change, then the volume change in equation ( 2) is (Vj - V2), where V2 and Vj are the volumes of two polymorphs, and instead of Lf one must measure the heat of transition, Lt, in order to examine the effect of external pressure on Tf, the transition temperature. In other words, if an increase in pressure causes a given phase to transform to a higher temperature phase, the boundary curve, separating the two stability regions will have a negative slope, and vice versa. Thus it would appear that if one had sufficient data on heats of fusion, sublimation, transition, and vaporization for different conditions of T and P, a phase diagram could be derived. And, indeed, this is the case. However, for some materials, especially refractory oxides, little thermodynamic data, or at least usable data, are available. This was mentioned earlier, above, and the possible solution for this lack of data on 50 oxides, etc. will be discussed later. The curves shown in Figure 2,

3, 4, and 5 have been drawn to clearly show the slope and thus the density relations of solid phases, etc. to one another.

Therefore, because of the lack of sufficiently accurate thermodynamic data, this discussion will be of a qualitative nature considering the relative stabilities of the various forms of titania, rather than stipulating values of T and P for boundary conditions.

2. Discussion of pertinent data. For considering the pos­

sible phase relations in the system titania, it is helpful to summarize the pertinent facts concerning TiOz and its transitions.

a) Both anatase and brookite can be converted to rutile by heating.

b) The anatase-to-rutile and brookite-to-rutile tran­

sitions are not sharp, but occur over a wide range of temperatures

(see Tables 1, 2).

c) Rutile, however, has never been reported as being

transformed to either anatase or brookite.

d) According to Sullivan and Cole (1959), the anatase-

to-rutile transition was detected as an exothermic peak. A change

from a stable low-temperature form to a stable high-temperature form

should entail an endothermic reaction, whereas a monotropic change

of a thermodynamically unstable (metastable) form to a lower 51

temperature form would account for liberation of heat in the tran­

sition.

e) By grinding, Knoll ( 1963) has transformed anatase

to brookite, and Kubo et al. (1963)- have transformed anatase to

rutile. As mentioned earlier, these relations seem analagous to

those in the transformation of calcite to aragonite by grinding

(Burns and Bredig, 1956).

. f) Anatase has been precipitated from solution in glass

frits at moderate temperatures (7 00°-800°C) as the initial phase of

TiOz to form. In quench runs from about 1400° to 1600°C, anatase

has been identified by x-ray diffraction. These two observations

just mentioned may be due to a high-temperature, metastable phase

precipitating from solution first in accordance with Ostwald's step­

wise inversion rule. However, formation of anatase in the plasma-

arc flame spraying experiment, due to high temperature and rapid

cooling (quenching), could be explained as the retention of a high-

temperature phase. Also, the presence of anatase in equilibrium

(?) with liquid in the Ti0 2-S i0 2 system, held at about 17 00°C

for 30 and 120 minute periods, does not seem explained in terms

of metastable precipitation alone.

4 52

g) According to Pauling's (1960) rules, the stability of the T i0 2 modifications should decrease in this order: rutile, brookite, anatase, because two, three, and four edges, respectively, are shared by the octahedra which make up the crystal structures of the three forms.

h) Several minerals have the rutile structure; however, none of them, as far as is known, has two other polymorphs with both the anatase and brookite structures. On the other hand, TeO^ which has the rutile structure as a low-temperature form, has the brookite structure at high temperatures. Although not being the strongest of evidence, the analogy of Te0 2 with Ti02 could lead one toward the belief that brookite is a higher temperature form than rutile. Only brookite and Te0 2 have the brookite structure.

i) The densities of the three forms of Ti0 2 are these: rutile, 4.26; brookite, 4.17; and anatase, 3.86. Usually, the higher temperature forms of a substance have a more open structure and hence, a lower density. Considering this, one could conjecture that rutile is the low-, brookite the intermediate-, and anatase the

high-temperature form of titania.

j) Much study has been devoted to the effect of

foreign ions on the transition of anatase and brookite to rutile.

It was argued by Buerger (1935) that impurities enter the interstices 53 of the trldymlte and crlstoballte lattices thus stabilizing the higher temperature forms and preventing their inversion to quartz. The role of impurities in anatase and brookite has not been settled and is discounted by Sullivan and Coleman (1962), at least with respect to sulfur trioxide. If, however, impurities do play a role in the anatase and brookite forms of Ti02, their appearance at low tem­ peratures may be a result of stabilization by impurities rather than a manifestation of the Ostwald rule.

3. Selection of a probable diagram. In selecting a phase diagram for the system titania, the pertinent data summarized in the previous section was used to test the various combinations of phases shown in Figures 2, 3, 4, and 5. Even when some criteria, such as the high temperature brookite-form of Te02 analog, are not used as tests, Figure 5a best explains the observed

phenomena associated with the polymorphs of Ti02. Figure 5a, in the author's opinion, can explain the reported behavior of the

titania polymorphs—heating effects, inversions induced by grinding,

low- and high-temperature appearance of anatase, etc. Figures 2b

and 3b are both about equally good as second choices, as can be

seen in the similarity of the stable-metastable phase relations when

compared with 5a. The three most unlikely arrangements, in order

of decreasing probability, are 4d, 3e, and 2e. 54

Figure 5a has been enlarged in Figure 6 , the proposed phase diagram (P-T) showing schematically the relations of rutile, brookite, and anatase. The enantiotropic p -<* anatase inversion (Schroeder,

1928) has been added in Figure 6. The omission of this inversion from the other series of diagrams (Figures 2, 3, 4, and 5) was for the sake of clarity, and because of its enantiotropic nature does not invalidate the choices made.

Since Figure 6 is a schematic representation, the temperatures and pressures indicated by boundary curves have no values. It is not intended that the stability range of any one form be greater, equal to, or less than that of any other form, although this may appear to be the case, graphically. Indeed, the stable regions of anatase and brookite may exist only over a short temperature span at high temperature—above the melting point of rutile.

From Figure 6 , it can be seen that the densities of the three forms decrease with increasing temperature, and, for a given tem­ perature, the stability decreases: rutile, brookite, anatase in ac­ cordance with Pauling's (I 960) prediction. The inversion of anatase and brookite to rutile, over a wide range of temperatures, can be explained as a monotropic inversion from a metastable state to the more stable rutile. The exothermic reaction of the anatase-rutile

inversion (Sullivan and Cole, 1959) is also explained by the 55

atm

P

T

Figure 6. Proposed phase diagram for TiOz. P and T hypothetical, phase relations schematic. foregoing statement. Likewise, metastability of anatase, at lower temperatures, explains its inversion to the next more stable phase, brookite, and to rutile when triggered by grinding at room temper­ ature (Knoll, 1963; Kubo et al., 1963). Precipitation of anatase from glass frits at relatively low temperatures (Friedberg et al.,

1947; Yee and Andrews, 1956) is explainable by invoking O stw ald's rule. The arrearance of anatase at high temperatures from solution with B^03 and S i0 2 can also be explained on the basis of Ostwald's rule. Figure 7 shows how high-temperature polymorphs might be precipitated from solution metastably, below their stability ranges.

In Figure 7, cooling of composition X could cause the least stable phase, anatase, to be the initial form of TiOz crystallizing from solution. Fusion of rutile without the formation of brookite or anatase can be explained as metastable melting, a phenomenon which occurs with quartz. The formation of rutile from the melt is explained by supercooling. The formation of anatase in the plasma- arc experiment can be explained as retention of the high-temperature phase due to extremely rapid quenching. Finally, brookite occupies a stability region of high temperature with respect to rutile, as one might predict when considering the temperature relation of the rutile and brookite forms of TeOz. 57

T

R+L

B203 w TiO, Si02

Figure 7. Metastable precipitation of high- temperature phases from solution. P and T hypothetical. E. Suggested future work

As mentioned previously, accurate thermodynamic data are not available for many oxides. If the standard free energies of formation for anatase, brookite, and rutile were known with sufficient accuracy, then it would simply be a matter of saying that the stable, less stable, and least stable phases were res­ pectively those with the lowest, intermediate, and highest free energies of formation, Rapp (1963) determined the free-energy of formation of molybdenum dioxide through solid-electrolyte galvanic cell measurements. Rapp (1964) proposed using the above tech­ nique for determining the free energies of formation of Si0 2 poly­ morphs to establish their absolute and relative stabilities. In

Rapp's opinion, a solid-electrolyte galvanic cell could be used to solve the stability relations in the system Ti02, providing a suitable electrolyte and electrode mixture could be found. Per­ haps, then, the definitive test of the Ti0 2 polymorph stabilities awaits the accurate determination of their free energies of formation. III. THE SYSTEM TITANIA—BORIC OXIDE

A. Introduction

As far as is known by the author, no study designed specifically to establish phase relations in the system Ti0 2-B20 3 has yet been made.

Both titania and boric oxide are important constituents in glass making. Titanium dioxide greatly increases the refractive index of glasses and lowers the viscosity and surface tension.

Titania in enamels increases the chemical durability and acts as a flux. In general, clear glasses containing titanium dioxide may be found in all the common glass-forming systems: borates, silicates, and phosphates {Beals, 1963). Boric oxide is one of the major constituents in borosilicate glasses, and is a glassformer in its own right. More recently, B £>3 has been used, experi­ mentally, as a flux for the growth of oxide cyrstals. In this connection, the growth of fibrous TiOz has been tried in several laboratories, using Bp 3 as the flux. With the advent of boron rocket fuels, there has been additional interest in the phase relations between boric oxide and refractory oxides.

59 60

B. Literature survey

Ebelmen (1851) failed to obtain compound formation by pro­ longed heating of titanic oxide with boric acid. Guertler (1904) reported that TiOz is dissolved in molten boric acid anhydride and yields an emulsion on cooling. He said that mixtures from pure

B^Oj to the molar ratio 1:1 form homogeneous melts at high tem­ perature, but on cooling—within a certain composition region—the melts become turbid and segregate into two liquid phases immedi­ ately. Guertler included Ti02, among other oxides, in a group to which the foregoing description applies. Foex (1938, 1939) estab-

, ^ lished the solubility of Ti0 2 in B 20 3 as 4.81 weight per cent at

1200°C. Foex also described separation in B 203-glasses upon cooling. However, he also says,

Frequently, in the course of cooling certain solutions of oxides in boric anhydride, one obtains turbid appearances resembling, at first view those mentioned earlier (emulsions), which are not due to the formation of emulsions, but really to a suspension of crystals of very small dimensions, as microscopic examination reveals. Thus solutions of boric anhydride and oxides of the form XOz (TiOz, ZrOz, Sn02, ThO^ give this sort of result. Dietzel and Tober (1953) were unable to decide whether compound formation is to be expected in the system TiOz-B/D3.

They state that the difference between the field strengths of the cations must be at least 0. 3 for the appearance of compound for­ mation in binary systems. Using the values listed for the cation 2 field strength (Z/a ) by Dietzel and Tober (1953) one obtains a ✓ 2 difference of 0..2 for TiOz and B^03. However, the Z/a values listed in an earlier paper by Dietzel (1942) give a difference of

0. 3, or just at their cutoff point for compound formation. Imaska

(1957) has investigated the range of glass formation of B20 3 with numerous other oxides, including TiOz. Strimple and Giess (1958) C studied glass formation in the system Na 20 -B 20 3-S i0 2-Ti02. In a diagram showing the glass-forming area in the system NazO-B^03-

Ti02, they indicate that about two to three mole per cent TiOz is soluble in B^03. However, in their table of runs for the NBT system, they do not indicate any runs consisting of only TiOz and B 20 3. The smallest amount of NazO, in any run containing all three components, is twenty mole per cent. Nor is the firing temperature stated with any certainty. They state merely, “The batches were melted in a Globar-type furnace that could be heated to 1600°C." Their usual melting times were twenty minutes with an extra thirty minutes allowed for fining. Sholokhovich's (1959) studies on the growth of single crystals of lead metatltanate led him to an examination of the system Pb0-B 20 3-Ti02. He used the visual polythermal fusion method to delineate the surface of crystallization in the aforementioned system. In the diagram for the system, Sholokhovich shows some dashed lines and the desig­ nation "two liquids" towards the B 20 3-T i0 2 side of the ternary diagram. In the paper he says, "The dashed line indicates a region in which there is evidence of immiscibility. Quenching of melts from these regions give milky glasses. " Later, he says with reference to temperature, "In the system B 20 3-Ti02, near

1100°C, there are indications of immiscibility. " The reader is referred to the reports of Guertler (1904) and Foex (1938, 1939) concerning the appearance of "emulsions" and their explanation on the basis of microscopic examination. Rao (1963) came to the conclusion that TiOz was itself a glass former, in alkali binary

systems. He believed that these glasses are three-dimensional networks with an equilibrium between Ti0 4 and Ti0 6 groups.

C. Experimental

1. Procedure. The Ti0 2 used in these experiments was

Baker's C. P. titanium dioxide. X-ray diffraction showed the

material from the jar to be the anatase form of Ti02. When it

was desirable to use rutile as the initial form of Ti0 2 in experiments, heating of anatase at 1000°C, or higher, overnight usually produced the rutile form. The conversion to rutile was checked by x-ray, and if anatase peaks were observed, the material was further heated until anatase peaks were no longer detected.

Fisher's A-76 fused purified boric acid (vitreous B 20 3) was used as the B20 3 component in the earlier, preliminary runs where partial hydration of the B^0 3 was not a great problem. Later, in the sealed platinum capsule runs, especially at high temperatures, it was found necessary to produce anhydrous B;jQ 3 from H3B03. This was done in a drying apparatus constructed by the writer's col­ league, W. C. Butterman. This apparatus, shown schematically in

Figure 8 , was made after the description of Lange (1946) who, it seems, got the idea from a paper published by Tiede and Ragoss

(1923). In the preliminary runs, where hydration was not as great a problem since the mixtures were not sealed in containers, the

T i02 and B 20 3 (vitreous) components were weighed out and mixed by grinding under xylene in a mechanical mortar. The xylene was used to minimize contact between the mixture and the surrounding atmos­

phere. After grinding, the xylene was removed in a drying oven, and the batches were transferred to labelled bottles and stored in a dessicator over P 2Os. Because of the loss of B^03 due to volatilization at high temperatures, sealed platinum capsules had

to be used in the quench runs. These capsules were made by 64

Heating tape

To vacuum pump

Figure 8 . Apparatus for producing anhydrous B 20 3. 65 cutting short lengths (one-half inch to three-fourths inch long) of

tubing, welding one end shut, loading the oxide mixture, and welding the other end shut. At first, an electric arc device was us

used to make these welds, but many failures of the capsules

pointed to defective welds. It appears that the welds were not

continuous across the tubing ends, in some cases, and often, small

specks of carbon were included in the platinum which prevented an

effective weld from being formed. The problem of welding capsules was solved by the use of heat-sink blocks in conjunction with the

oxidizing flame of an oxy-acetylene torch. These blocks are shown

in Figure 9. After being loaded, the tube is crimped flat for about

three-eighths inch on the end, making sure first that the inside of

the tube, near the weld, is cleaned free of the mixture, and placed

in the blocks as shown in the upper right-hand corner of Figure 9.

The blocks are then lightly clamped in a vise which forces them to

hold the crimped capsule shut while also increasing the thermal con­

tact between the blocks and the tubing. About one-fourth inch of

the flattened tubing end should protrude above the blocks. When

the hot, inner cone of the oxidizing oxyacetylene flame is played

against the protruding end of the tubing, it melts back into a

uniform weld all the way across the flattened end. Welds produced

in the above manner have proved very satisfactory. Capsules 66

[CHAMFER PINS 45* ON EACH END

SLIDING FIT DRILL AND REAM FOR PINS HOLES .esoM

FINISH HOLE BOTTOM WITH S /I6 * E N 0 MILL L1 J ‘ SCALE

Figure 9. Heat-sink blocks for welding platinum capsules. 67 which have ruptured have always done so at some place other than the welded ends of the tubes.

Concerning the bursting of the platinum capsules, calculations were made to see what material and how much of it would be neces­

sary to burst the platinum capsules. First, it was necessary to

find the bursting pressure of the capsules being used. The dimen­

sions of the platinum tubing are: 0. 25 mm wall thickness and

2.5 mm outside diameter. For calculating the volume, 20 mm

(about three-fourths inch) was chosen as the size generally used

at high temperatures. These dimensions give a volume of 6 . 28 x

In their work on hydrothermal growth, Laudise and Nielsen

( 1961) give two formulas for calculating the strength of pressure

vessels. For thin-walled cylindrical vessels where k< 1. 1 (k ■

ratio of outer radius to inner radius) they give the following

formula,

P - (k - 1)F (1)

where P = pares sure

k = ratio of outer radius to inner radius

f = yield stress 68 and for thick-walled cylindrical vessels where k > 1. 1,

k 2 - 1 P - V f. (2) 2k

They state that ^ (ultimate tensile stress) may be sub­ stituted for f in ( 1), and a good estimate of the bursting pressure can be obtained for the vessel in question. Using formula (1) for k > 1. 1 will yield a bursting pressure figure which is high, but one which can be used to obtain the magnitude of the amount of water required to produce the pressure for a certain temperature and volume.

Using values for the ultimate tensile strength of platinum

(Metals Handbook 1961; Handbook Chemistry Physics, 1963) at high temperatures and formula ( 1), the bursting pressures of a platinum cylinder with a volume of 6. 28 x 10^1 and k s 1. 25 were calculated for temperatures from 1000° to 1600°C. From the ideal gas law

PV = nRT, (3) the number of moles of water vapor necessary to produce a given bursting pressure can be calculated. These values are presented in

Table 12. Figure 10 shows P-T plots for various amounts of water -4 (g x 10 ) for a constant volume. The bursting pressure of the cap­ sule is also plotted versus temperature. Pressure (atm.) 120 100 0 2 60 40 80 0 00 10* 20 10* 40 10* I600# 1500* 1400* 1300* 1200* 1100* I000# f asl i cntn * constant is capsule of tr Nmee lns ersn g 1-. Volume 10-4. x g represent lines Numbered ater. w Figure 10. P-T plots for various amounts of of amounts various for plots P-T 10. Figure eprtr (*C) Temperature 6 . 28 x x 28 . 10 - - 5 1 . 69 70

TABLE 12

BURSTING PRESSURE DATA FOR PLATINUM TUBING

20 mm long, 0. 25 wall, 2.0 mm ID; volume a 6.28 X 10-5 1 Temperature Ultimate Bursting Pressure Grams Tensile Strength .I!*).. (osi) (psi) (atm) (X io "4i

1273 7400 1850 125.7 12. 6 1373 5600 1400 95. 3 9.6 1473 4000 1000 68 . 0 6.4 1573 3000 750 51. 0 4. 5 1673 2120 530 36. 0 3. 0 1773 1400 350 23. 8 1.9 1873 800 200 13. 6 1. 0

From Table 12 it can be seen that, for the given volume, a -4 platinum capsule cannot tolerate as much as 1 X 10 g water vapor without bursting. The actual amount is probably much less than this figure for, as previously mentioned, the formula used to calculate these bursting pressures would give high figures for k 1:1. Also, the volume occupied by the charge itself, B20 3 and TiOz, has been neglected. This reduces the volume of the capsule, and, for a given amount of water, increases the internal pressure. Adding the vapor -4 7 pressures of Ti02, 10 * atm and 17 20°C (Shaffer, 1964) and B 20 3, _3 2.5 X 10 atm at 1600°C calculated from the equation given by

Speiser et al. (1950) does not significantly change the total pressure 71 within the capsule; thus, these pressures can be neglected.

This discussion of bursting pressures shows that the bursting is due to water vapor, and, moreover, that only very small amounts are necessary to cause rupture of the capsules.

Now one can see why preventing the hydration of B^0 3 is of such importance. Hydrated or H 3B03, gives up its water of hydration when heated. It was with this idea that crystalline B^0 3 was made by other workers (Gielisse, 1961; Rockett, 1963). Accord­ ing to their reports, the crystalline variety of B^ 0 3 hydrates less readily than the vitreous form. However, the crystalline form proved to be not easily synthesized, and when synthesized, extremely dif­ ficult to break and pulverize. On the other hand, using the appa­ ratus shown in Figure 8 , and following the instructions of Lange

(1948), one obtains a very fine-grained anhydrous B 20 3 by dehydrating boric acid. However, upon removal of this fine-grained, anhydrous

B20 3 from the drying apparatus, the process of hydration immediately be­ gins as water from the atmosphere attacks the B20 3. Due to the fine­ grained nature of the B 20 3 powder, hydration is enhanced, because of the high surface area per unit weight ratio. Therefore, it seems that if one can easily dehydrate boric acid with Lange's method, the problem of rapid rehydration seems soluble if one can reduce the surface area

per unit weight before bringing the material into contact with the atmosphere. This was done by increasing the temperature of the drying tube, after the water had been removed, and sintering the fine-grained powder into a somewhat rigid body to reduce the sur­ face area. Next, the sintered B / ) 3 was broken into pieces and fused in a platinum crucible lid to further reduce the surface area.

These thin, fused layers of B £ )3 could be broken into small pieces without much difficulty. In actual practice, for the sealed capsule runs above about 1400°C, the procedure was as follows. First, the platinum capsules were prepared and heated to remove all moisture, and kept hot until loaded. Next, after fusing a piece of anhydrous B^0 3 into a thin layer, the B 20 3 was broken into pieces small enough to drop into the platinum tubing. Four or five small pieces of B20 3 were weighed and then placed in a capsule. From the weight of B20 3 placed in the capsule, the total weight, and hence the weight of TiOz to be added, could be calculated for the desired weight per cent of the mixture. The TiOz used in the higher temperature runs was rutile which was heated before weighing to remove moisture. Before sealing the capsules, they were placed

in the drying tube. The heat of the tube was raised to about 240°C ft while a vacuum was maintained. There was also a boat of P 2Os in

the drying tube. The pumping and heating operation was done for

at least one hour, or longer, depending on the number of capsules

being treated. After this drying operation the capsules were 73 removed, their ends crimped shut, and welded as quickly as possible.

The question of homogeneity of sample mixtures arose, especially in the technique described above. It can be said that when T i0 2 and B 20 3 were heated in the strip furnace, mixing of the two components was readily effected. A piece of B 20 3 was placed on the platinum or iridium strip and heated until it flowed across the strip, then cooled. Finely-ground rutile was placed on top of the B203. When heated to high temperatures, the T10 2 was quickly # dispersed and/or dissolved in the molten B 203. These strip furnace experiments rarely exceeded one and one-half minutes.

The furnaces used to make the firing runs of pressed disk and crucible-contained mixtures were a Blue M Electric Company

Model M10 and a Globar furnace with a Barber-Coleman Wheelco

Model S257 controller. The quench runs in sealed capsules were made in a vertical tube furnace wound with platinum-40% rhodium wire. Temperature control of the quench furnaces was by either a

Barber-Coleman Wheelco Model S257, Celectray, or Leeds-Northrup

Speedomax-H controller. The quench method (Shepherd, Rankin, and

Wright, 1909) was used, but in a modified form. The usual pro­ cedure of fusing a mixture, crushing, refusing, etc., until a homogeneous glass is obtained, was not used. The repeated fusion 74 to obtain a uniform glass would have caused the overall composition to change an unknown amount because of B^D 3 loss through volatili­ zation. Thus the main departure from the quenching method, as originally described, is that of firing a mixture of oxides rather than * a uniform glass made from that oxide mixture. This departure, along with sealing charges in platinum capsules, was necessitated because of the volatilization of B p3.

Identification of starting materials and reaction products was done with the polarizing microscope and x-ray diffraction. Both high- and low-angle Norelco x-ray diffraction units were used.

Because of the small sample size, the x-ray mounts were made by grinding, the fired material under xylene in a mortar and transferring the finely-ground materials to a glass slide with a medicine dropper. The slide was vibrated until a uniform, thin layer was formed, then dried. Microscopic and x-ray examination was done as quickly as possible after opening the capsules, because there was a great tendency for the samples to become hydrated; especially those with mixtures near the B p 3 end-member. 2. Experimental runs. Several preliminary studies were carried out in the system Ti0 2-B£)3. These had a fourfold purpose; to discover whether any intermediate compound is formed, a question left open by Dietzel and.-Tober (1953); to check the possible exis­ tence of a two-liquid area, a feature suggested by the "emulsion" noted by Guertler '(1904); to ascertain whether the TiOz liquidus is close to the B20 3 composition vertical, as suggested by the solubility data of Foex (1939); and to locate approximately the anatase-rutile transition temperature in the presence of B 20 3.

In the first series of experiments seven mixtures were pre­ pared (Table 13) using anatase and fused boric acid. Disks one inch in diameter and one-sixteenth inch thick were pressed with a paraffin- carbon tetrachloride binder. For each mixture two disks, one on top of the other, were placed on coarse zirconia sand on a slab of silica brick. The lower disk was to act as a buffer for the upper test disk.

In the specimens containing over 60 per cent B^03f however, the entire disk-pair flowed into the zirconia placing sand, and rendered the specimens useless. There was also evidence of considerable lo ss of B P 3 through volatilization. The results of these experiments are presented in Table 13. No evidence for intermediate compounds was obtained. The firing temperatures of 800° and 9 0 0°C were too low to reveal any liquid immiscibility. No reliable information on the 76 location of the liquidus curve was obtained, because of the useless­ ness of the B^03-rich specimens for this purpose. Anatase was

Converted to rutile at both 800° and 900°C. The glasses from these runs were clear and colorless.

TABLE 13

PRELIMINARY FIRINGS: PRESSED DISKS

Composition (wt. %) Temperature Time Products TiOz BjP3 (°C) (hrs. ) . (microscope, x-ray)

80 20 900 65-1/2 rutile, glass 80 20 800 24 rutile, glass 60 40 900 6 5 -1 /2 rutile, glass 60 40 800 24 rutile, glass 40 60 900 21- 1 /2 rutile, glass 40 60 800 24 rutile, glass 20 80 900 71 rutile, glass 20 80 800 24 rutile, glass 15 85 900 71 rutile, glass 15 85 800 24 rutile, glass 10 90 900 71 rutile, glass 10 90 800 24 rutile, glass 5 95 900 71 rutile, glass 5 95 800 24 rutile, glass

In the next series of preliminary firings sample mixtures of

TiO2-B^03 were fired in a platinum crucible, a container having been found necessary in the first series of runs. Although volatili­ zation of B^ 0 3 at high temperatures causes the composition to shift to one relatively richer in Ti0 2 than prepared, the data obtained from such runs can still be used in determining the syntectic horizontal (lower limit of immiscibility) , assuming there is liquid immiscibility in the system. The crucibles were covered to mini­ mize volatilization loss of B p3. The runs below 1000°C were fired in a Blue M furnace, and those above 1000°C were fired in the Globar furnace. The firing products were stored in a des- sicator over P^0 5 immediately after removal from the furnaces, until microscopic and x-ray identification of the phases could be made.

These platinum crucible runs are presented in Table 14.

Glasses formed below about 1050°C are generally clear, whereas those above this temperature are brownish. The runs made at

1400°C produced glasses which have a mottled, pebbly appearance suggesting blebs of different refractive index than the matrix. The brownish color of the glasses is probably due to TiOz dissolved in them.

It was mentioned above that some of the glasses had a

*-i pebbly, mottled brownish appearance. In one case the mottled glass was dissolved in a 10 per cent acetic acid solution while being observed under the microscope. An oil immersion objective was used, and, together with the eyepiece, the total magnification was X800. When the fragments of glass were dissolved many

small, blackish specks were released into the solution. 78

TABLE 14

PRELIMINARY FIRINGS: CRUCIBLE RUNS

Composition (wt. %) Temperature Time Products T i02* B2P 3 (°C) (hrs) (microscope, x-ray)

80 20 1070 1 rutile (B^0 3 volatilized) 80 20 1054 2 rutile,do. 75 25 1060 2 rutile, glass 70 30 1065 2-1/2 rutile, glass 60 40 1070 1 rutile, glass 60 40 1063 2 rutile, glass 60 40 1059 2 rutile, glass 55 45 1128 2-1/2 rutile, glass 50 50 1140 14 rutile, glass 50 50 1063 2 rutile, glass 45 55 1220 2-1/2 rutile, glass 45 55 1061 2 rutile, glass 40 60 1059 2 rutile, glass 40 60 1059 1 rutile, glass 40 60 1040 48 rutile, glass 30 70 1140 14 rutile, glass 30 70 1065 2 rutile, glass 20 80 1400 1 anatase, rutile glass 20 80 1088 1 rutile, glass 20 80 996 1 rutile, glass 20 80 902 1 rutile, glass 20 80 782 1 anatase, glass 15 85 1400 1 anatase, rutile glass 15 85 1103 1 ru tile, glass 15 85 1089 rutile, glass 15 85 107 0 rutile, glass 15 85 1060 I • rutile, glass 15 85 1047 rutile, glass 15 85 1012 1 rutile, glass 15 85 907 1 rutile, glass 15 85 785 1 anatase, glass 79

TABLE 14 —CONTINUED

Composition (wt. %) Temperature Time Products T i02* Bjp, (°C) (hrs) (microscope, x-ray)

10 90 1400 1 anatase, rutile (?), g lass 10 90 1140 14 rutile, glass 10 90 1104 1 rutile, glass 10 90 1067 1 rutile, glass 10 90 1066 2 rutile, glass 10 90 1001 1 rutile, glass 10 90 908 1 rutile, glass 10 90 814 1 anatase, glass 5 95 1400 1 anatase, glass 5 95 1246 3 rutile, glass 5 95 1140 14 rutile, glass 5 95 1110 1 rutile, glass 5 95 1000 1 rutile, glass 5 95 912 1 rutile, glass 5 95 790 1 anatase, glass

* Initial form of TiOz was anatase.

A somewhat euhedral shape was barely discernible on some of the particles. The particles were so small that they were tumbled about in the solution due to the Brownian effect. When the polarizers were crossed, these small particles flashed intermit­ tently. Thus, the opacity of these glasses may be due to tiny quench-growth crystals dispersed throughout the glassy matrix in addition to the brownish color of TiOz in solution in the glass.

The problem of quenching Ti02-containing solutions will be dis­ cussed later. 80

To obtain more rapid cooling of the fired mixtures than was possible using a crucible, sample mixtures were fired in small envelopes fashioned from platinum foil. These envelopes measured about one-fourth inch by three-eights inch and were fired in a quench furnace. The data of these runs are presented in Table 15. Mixtures fired in these envelopes, in several cases, bubbled up and ran out of the containers while being heated.

TABLE 15

PRELIMINARY FIRINGS: PLATINUM ENVELOPES

Composition (wt. %) Temperature Time Products Ti02* B2P 3 (°c) (h rs.) (microscope, x-ray)

45 55 908 2 rutile, glass 45 55 898 2 rutile, glass 30 70 908 2 rutile, glass 30 70 898 2 rutile, glass 15 85 908 2 rutile, glass 15 85 898 2 rutile, glass

^Initial form of Ti0 2 was anatase.

The fact that TiOz has never been formed as a glass

from the liquid state attests to its rapid crystallization rate. The

only exception to the formation of a Ti02-glass may be those dis­

cussed in the literature survey under the system titania. These,

however, may be considered as special cases indeed, since they usually involve formation from vapor state, cryogenic temperatures, sputtering of oxides at low pressures, and special substrates. This tendency of TiOz to crystallize seems to carry over to binary solutions involving TiO* That is, it is extremely difficult to form glasses from binary mixtures involving any appreciable amount of TiOz. An exception to the foregoing statement, of course, is the previously mentioned work of Rao (1963), in which he prepared one to five gram quantities of K-, Rb-, and Cs-Ti0 2 g lasses in 1:1 and 1:2 mole ratios. The rapid formation of T i0 2 crystals from solution was first observed by the author in strip furnace studies of Ti0 2-B^03 mixtures.

By slightly decreasing the heating current of the furnace strip, rutile needles could be seen to shoot out from the sides of the melt. The cooler sides of the melt, to be sure, provided seeds for initial growth and eliminated the need for spontaneous nucleation, but the rate of growth was very rapid. In another instance, in an experiment de­ signed to examine crystal growth rate from a melt, a crucible con­ taining a molten T i02- B^0 3 mixture was quickly withdrawn from the

Globar furnace and placed under a stereo-microscope for viewing.

But, before the crucible could be transferred from the furnace to the microscope—perhaps three seconds—the glass became a bluish-white color. X-ray examination showed that the opacification was due to tiny crystals of rutile. This explanation may account for the "emulsions" encountered by Guertler (1904) and Sholokhovich (1958).

It seems to support the findings of Foex (1939). Because of the rapid crystallization of TiOz from solution in B£>3, it was decided to try some experiments to detect the thermal effects of Ti02-crystal- lization from a cooling melt. The freezing out, or crystallization, of a phase from solution should liberate heat; the reverse of fusion where heat is absorbed. Thus, in a plot of temperature versus time, vertical and horizontal coordinates respectively, crystallization from solution of the pure end member, without further reaction, should manifest itself as a change, to a less steep slope, in the curve representing the cooling of the solution. Three experiments were done with Ti02-B203 melts to obtain time-temperature curves. First, a 40% TiO2-60% B^03 (wt.) mixture was heated to 1650°C for ten minutes in a quench furnace, then the heating current was shut off.

The resulting cooling curve showed no significant slope changes.

The foregoing experiment was repeated using an oxyacetylene torch and firebrick furnace for heating a 25% Ti02-75% B203 (wt.) mixture.

Again, no slope changes were observed in cooling from 1150°C to about 600°C. The failure to obtain changes of slope in the heating curves may have been due to the sensitivity of the potentiometer-

recorder. A DTA experiment was made on a 50% TiO2-50% B203 (wt.)

mixture. Since the DTA equipment is much more sensitive than the potentiometer-recorder used previously, it was hoped that any heat effect missed due to sensitivity of the latter could be picked up by the DTA set-up. The DTA strip chart showed three peaks of which, if one assumes the one at 900 °C to be exothermic according to

Sullivan and Cole (1959), two are exotherms at about 900°C and

1130°C and one an endotherm at about 1465°C. The furnace was shut off at 1465 °C. The significance of these data is doubtful, since the alumina sample holder was severely attacked by the

Ti02-B/D3 mixture. Further DTA experiments were not attempted for want of a suitable sample holder for the Ti02-B203 mixture.

The technique of loading and sealing platinum capsules was explained earlier in this section (Procedure). These runs were used to delineate the liquidus curve in the system Ti02-B^03. The runs could be plotted accurately on the temperature and composition coordinates. All runs made in the study of the system have been plotted on diagrams with the value on the composition coordinate being that of the starting composition. However, due to B203 loss through volatilization, only those runs which were sealed in cap­ sules may be used to place boundaries which are dependent upon compositional variation. The temperature coordinates of all runs have been plotted as measured. Because of quench growth of crystalline phases, textural relations of crystalline phases had to 84 be taken into consideration in the placement of the liquidus curve.

This is by way of explanation for the apparent contradiction between the products of some runs and the position of those runs in the phase diagram with relation to the liquidus curve. That is to say, while many runs appeared to be entirely glass by microscopic investigation, x-ray diffraction showed the glass to contain either anatase or rutile or both. The data on these firing runs are pre­ sented in Table 16.

TABLE 16

•SEALED CAPSULE FIRINGS IN THE SYSTEM TiOz- B p 3

Composition (wt. %) Temperature Time Products T i02 B20 3 (°C) (h rs .) (microscope, x-ray)

80 20 1600 3 -1 /2 rutile, glass 80 20 1550 16 anatase, rutile, glass 80 20 1516 3 -1 /2 rutile, glass 80 20 1295 4 rutile, glass 80 20 1250 23 rutile, glass 80 20 1200 4 rutile, glass 80 20 1005 13 rutile, glass 75 25 1030 2 rutile, glass 70 30 1200 4 rutile, glass 60 40 1602 5 -1 /2 anatase, rutile, glass 60 40 1600 3 -1 /2 leaked 60 40 1550 16 anatase, rutile, glass 60 40 1516 3-1/2 anatase, rutile, glass 60 40 1396 2 anatase, rutile, glass 85

TABLE 16 - CONTINUED

Composition (wt. %) Temperature Time Products T i02 BaQj (°C) (h rs .) (microscope, x-ray)

60 40 1295 4 rutile, glass 60 40 1200 ' 4 rutile, glass 60 40 1003 12-1/2 rutile, glass 50 50 1108 2-1/2 rutile, glass 40 60 1604 16-1/2 anatase, rutile glass* 40 60 1602 5 -1 /2 anatase, rutile glass 40 60 1550 16 anatase, rutile g lass 40 60 1501 12 anatase, rutile glass 40 60 1400 23 anatase, rutile glass 40 60 1250 23 rutile, glass 40 60 1006 2 rutile, glass 38.4 63.6 1020 4 rutile, glass 38.4 63. 6 984 2 rutile, glass 35 65 1604 16-1/2 anatase, glass* 35 65 1602 5 -1 /2 anatase, rutile glass 30 70 1604 16-1/2 anatase, glass* 30 70 1551 14 anatase, glass* 30 70 1010 2 rutile, glass 30 70 1000 7 rutile, glass 30 70 700 15 rutile, glass 25 75 1602 5-1/2 anatase, rutile glass* 25 75 1551 14 anatase, rutile (?), glass* 20 80 1603 8 anatase, rutile glass* 20 80 1602 5-1/2 anatase, rutile glass* 20 80 1551 14 an atase, rutile (?), glass* 86

TABLE 16 - CONTINUED

Composition (wt. %) Temperature Time Products TiOz B£>3 (°C) (h rs .) (microscope, x-ray)

20 80 1501 12 anatase (?), ru­ tile (?), glass* 20 80 1010 7 -1 /2 anatase, rutile, g la ss5* 15 85 1501 12 anatase, rutile, g la ss5* 15 85 1400 23 anatase, rutile, g la ss5* 10 90 1404 20 glass 10 90 1303 13 rutile (?), glass 10 90 1295 4 leaked 10 90 1200 4 leaked 8 92 1404 20 glass 7 93 1305 12 glass 7 93 1202 20-1/2 glass 5 95 1600 4 leaked 5 95 1600 3-1/3 leaked 5 95 1495 2 leaked 5 95 1396 2 leaked 5 95 1200 16 rutile (?), g la ss5* 5 95 1101 45 glass 5 95 1002 2 leaked 5 95 997 2 leaked 4 96 1005 39 rutile, glass 3 97 1207 4 rutile, glass 3 97 1105 4 rutile, glass 3 97 1098 4 leaked 3 97 1097 22 leaked 3 97 1000 13 rutile.(?), glass 2 98 1003 24 rutile, glass 1 99 910 25 rutile, glass ^Crystalline phase is submicroscopic; presence of crystals detected by x-ray diffraction. In the preliminary experiments it was noted that TiOz-B p3 mixtures fired above about 1050°C gave brownish glasses, while those fired below this temperature gave essentially clear and color­ less glasses. Also, a pebbly, mottled texture was observed in some glasses, suggesting the possibility of phase separation in the glass. The use of electron microscopy for detecting phase separ­ ation in glasses has been made by many workers recently (Murthy,

1961; Vogel and Gerth, 1962; Ohlberg, Golob, and Strickler, 1962;

Ohlberg, Golob, and Hollabaugh, 1962; Rindone, 1962) , and especially notable is the detection of liquid immiscibility in the system BaO-Si02 by Argyle and Hummel (1963). Thus, a series of electron micrographs were made of glasses in the system Ti02-B£)3.

Mixtures of 50, 7 0, 90, and 95 weight per cent B£)3, rest Ti02, were fired in a platinum crucible at 1140°C for fourteen hours.

After cooling the crucible, the glass was removed in fairly large

chunks and stored over P205 in a dessicator. To minimize hydration

effects, the chunks of glass were broken, and the replicating

solution was quickly applied to the fresh fracture surface. A

solution of two per cent collodion, 98 per cent amyl acetate was

used. After stripping the replicas with cellulose tape, a layer

of carbon was deposited on the collodion, followed by shadow

casting with gadolinium. In all, about fifty micrographs were made. 88

In addition to the glasses mentioned above, a glass of pure B£)3 was made and photographed as a check against attributing exclu­ sively to Ti02-B p3 glasses features which are also characteristic of the B203 glass. Three of these electron micrographs are reproduced in Figures 11, 12, and 13. The features shown in these micrographs are not similar to those associated with liquid immiscibility by the workers mentioned above. The interpretation of electron micrographs is a difficult and somewhat subjective matter. One must keep in mind that these micrographs show the topography of the surface and not the particles themselves which make up the surface. Later experiments made by the author have suggested that electron micro­ graphs of glasses formed at higher temperatures might have yielded more positive results with regard to evidence of liquid immiscibility.

D. Discussion of experimental results

!• Liquid immiscibility. Since the discovery of liquid immiscibility in silicate melts by Grieg (1927), the importance of this phenomenon has been recognized as a factor controlling the melting behavior of oxide mixtures. Because of its importance, many attempts have been made to predict the occurrence and extent of liquid immiscibility in oxide systems as a function of size and charges of the cations. Warren and Pincus (1940) used Z/r, where Z is the valence and r the radius of the cation, as a Figure 11. Electron micrograph of B p3 glass fracture surface, X7 000. . Figure 12. Electron micrograph of T102-B2 glass fracture surface, X7000. 50% (wt.) B£)3. Figure 13. Electron micrograph of Ti02-B203 glass fracture surface, X 22, 000. 95% (wt.) B^03. measure of cation field strength for calculating the extent of liquid miscibility gaps. Dietzel (1942) found Z/a2, where a is the cation field strength. Levin and Block (1957) modified the equations of Warren and Pincus to provide better agreement with experimental determinations. Recently, Glasser et al. (1963) made predictions of the extent of immiscibility in silicate systems by plotting Z/r versus mole percent immiscibility in known systems.

Lines through points of a given valence were roughly parallel to lines through other valence groups. That is, the line through the divalent (Sr, Ca, Zn, Fe, Mn, and Mg) cation points was nearly parallel to that of the trivalent (rare earth, Sc, Fe, and Ga) cation

points. A line parallel to those just mentioned was drawn through the known point of TiOz, and the intersection of the Z/r value of

Zr02 with that line gave the predicted mole per cent miscibility

gap for the system ZrO2-Si02. The choice of the value Z/r by

Glasser et al. for plotting purposes seems to have been on the

basis that it best fits the known data. The writer has tried plot­

ting different values versus liquid immiscibility extent for binary

oxide-borate systems in an attempt to predict the extent of a

possible miscibility gap in the system Ti02-B203. In all, ten 2 2 plots were made. The values l/r, Z/r, Z/r , Z/a and Z/a were

plotted, first using Pauling-, then Goldschmidt-radii, for the cations. The mole per cent of immiscibility for the cations Ba, Pb, Sr, Ca,

Cd, Zn, Co, and Mg, in B ^3 systems, was taken from the work of

Levin and Block (1957). A 70 mole per cent miscibility gap has been recently reported for the system ThOz-B^03 (Rase and Lane,

1964). According to the work of the writer's colleague W. C.

Butterman (1962), the system HfOz-B/D3 has a miscibility gap of approximately 46 mole per cent. A typical plot, in this case in- volving Z/a (Pauling radii), is shown in Figure 14. It can be seen that the Hf and Th points do not lie on a line parallel to that drawn through the divalent cations. Average values of 65 and

77 mole per cent, Pauling and Goldschmidt radii respectively, liquid immiscibility in the system Ti02-B203 were obtained. The dif­ ferent figures obtained for different plots are summarized in Table 17.

TABLE 17

PREDICTION OF LIQUID IMMISCIBILITY

Value Predicted immiscibility (mole %) Dlotted Paulina radii Goldschmidt radii

1/r 57 71 Z /r 68 98 Z /r2 70 —* Z/a 64 78 Z /a 2 65 61 *No value predictable from this plot. 94

.0

0.8

Th

0.6

2

0.4

0.2

0 20 40 60 80 100 Mol«% immiscibility

Figure 14. Prediction of liquid immiscibility in the system Ti02-B p3.

* 2. Phase relations of TiOy-BiQi. No intermediate compounds were found In the system TiO2-B^03, nor was any solid solution of either end member observed. Since below about 900°C almost no

T102 could be dissolved In B^)3, a monotectic has been indicated on the B203 end. The reader Is referred to Figures 15 and 16 for this discussion. The steepness of the llquidus curve shows a great

positive deviation from ideality, and thus, a tendency toward un-

mixlng of the components in the liquid state. Figure 15 was drawn with the liquidus location being based on evidence from microscopic

examination of firing products. The presence of anatase and rutile,

revealed by x-ray diffraction, in the area designated "liquid"

(Figure 15) was attributed to quench growth due to an insufficient

cooling rate. The rapid crystallization of TiOz from solution has

been discussed earlier in this paper. Several authors have sug­

gested that crystallization from a melt may be enhanced when pre­

ceded by liquid-liquid separation. Stookey (1959) proposed that the

precipitation of an immiscible phase proceeds homogeneously because

only small interfacial free energies are involved. The separated

globules are then presumed to crystallize either homogeneously or

heterogeneously. Roy (1960) observed the rather close correlation

between easy nucleation and the proximity of stable or metastable

immiscibility regions. Other evidence besides the rapid 1800* Liquid

1400*

(*C) Rutllt 4* Liquid

1000 *

800*

400H 10 20 30 40 50 80 70 80 80 (wt.%)

Figure 15. Tentative phase diagram for the system Ti02-B p3. Liquid I 1801 ' Liquid Two Liquids

1600*

1400*

1200*

CC) Rutllo 4* Liquid

1000*

600*

R utils + B»0 400* 20 30 60 70 60

Figure 16. Tentative phase diagram for the system Ti02-B203. 98 crystallization of TiOz suggests the possibility of a two-liquid region in the system Ti02-B£)3. The pebbly, mottled texture of the glasses as well as the steepness of the liquidus curve, as determined, also suggest a two-liquid area. Dietzel's (1942) rules also suggest the possibility of a two-liquid area because of the small difference between the cation field strengths of T102 and B^03. Considering the foregoing, an alternative diagram showing a two-liquid field has been drawn (Figure 16). The relationship of the three polymorphs of TiO& as proposed in the first part of this paper, has been in­ corporated in these diagrams as alternate arrangements. The system

TiO2-B^03 has been examined experimentally to 1600°C and the liquidus curve delineated to this temperature. This study has been carried out essentially at one atmosphere. The deviations from this pressure due to B^03 volatilization and the use of sealed capsules are relatively small and can be neglected when considering their effects on solubility curves. The melting points of Ti02 and B203,

1840° and 470°C respectively, were taken from those listed in

Phase Diagrams for Ceramists (Levin et al., 1964). IV. SUMMARY

Two systems have been studied which are of mineralogical and technological significance. The schematic P-T diagram for the system TiOz has been derived from considerations of experimental results and accounts from the literature.

The system TiOz-B^03 has been examined, experimentally, by a modified quench technique, and two alternative versions of the phase relations have been proposed. In this system no inter­ mediate compounds were found, nor was there evidence of solid solution. Anatase was observed to form from solutions at high temperatures—a phenomenon as yet unreported. The question of liquid immiscibility has not been definitely settled.

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