Iowa State University Capstones, Theses and Retrospective Theses and Dissertations Dissertations

1958 Preliminary sintering phenomena in aluminum oxide David Randolph Wilder Iowa State College

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This Dissertation is brought to you for free and open access by the Iowa State University Capstones, Theses and Dissertations at Iowa State University Digital Repository. It has been accepted for inclusion in Retrospective Theses and Dissertations by an authorized administrator of Iowa State University Digital Repository. For more information, please contact [email protected]. PRELIMINARY SINTERING PHENOMENA IN ALUMINUM OXIDE

by

David Randolph Wilder

A Dissertation Submitted to the Graduate Faculty in Partial Fulfillment of The Requirements for the Degree of DOCTOR OF PHILOSOPHY

Major Subject: Ceramic Engineering

Approved*

Signature was redacted for privacy.

Signature was redacted for privacy.

Signature was redacted for privacy.

Signature was redacted for privacy.

Iowa State College 1958 il

TABLE OF CONTENTS

\ Page

I. INTRODUCTION 1 II. STATEMENT OF PROBLEM lj_ III. LITERATURE REVIEW 5 A. General Mechanism of Sintering in Ceramic Refractories 7 B. Initiation of Sintering in Aluminum Oxide 12 IV. MATERIALS CONSIDERED 16 V. EXPERIMENTAL METHODS 35 A. Fractionation of Fused Aluminum Oxide Powders with the Water Elutriator 35 B» Determination of Particle Size 37 1. Microscopic measurements 37 2. Nitrogen adsorption 39 C. Forming of Specimens 44 D. Heating Methods 44 E. Measurement of Physical Dimensions 4? F. Slaking Technique 4t Gr. Infrared Spectroscopy 48 H. Thermogravimetric Analysis 49 VI. RESULTS 52 A. Slaking Characteristics 52 B# Thermogravimetric Analyses 52 C» Infrared Spectroscopic Examination of Surface Anions 59 D. Sintering as indicated by Shrinkage 82 1» Variation with particle size 82 2» Variation with firing temperature 82 VII. DISCUSSION OF RESULTS 87 A. Slaking Characteristics 87 I

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TABLE OF COUTENT S (Continued) Page B. Thermogravimetric Analyses 91 C. Infrared Spectroscopic Analyses 91 Do Sintering as indicated by Shrinkage 93 VIII. C01ÎCLUSI0HS 10k IX. LITERATURE CITED 105 X. ACKNOWLEDGMENTS 111 iv

LIST OP TABLES

Table Page 1 Correlation of material transport mechanism with n 11 2 Chemical analysis of fused aluminum oxide 18 3 Chemical analysis of tabular alumina T-61 19 4 Chemical analysis of calcined alumina A-14 20 5 Chemical analysis of calcined alumina KS 21 6 Materials considered 22 7 Slaking characteristics of fractionated fused aluminum oxide 53 8 Slaking characteristics of calcined aluminum oxide and ball milled fused aluminum oxide 54 9 Results of infrared spectroscopic analyses 59 V

LIST OF FIGURES

Figure Page 1 Particle size distribution of sample N-M 23 2 Photomicrograph of Sample T-61, lj.60X 26 3 Photomicrograph of Sample A-14, 4&0X 27 4 Photomicrograph of Sample ES, 4&0X 28 5 Photomicrograph of Sample N-M, 4^0X 29 6 Photomicrograph of Sample IT-AX, 4&0X 30 7 Photomicrograph of Sample Ii-A, 460X 31 8 Photomicrograph of Sample U-B, 4^0X 32 9 Photomicrograph of Sample N-C, I4.6OX 33 10 Photomicrograph of Sample Ïï-D, 4&0X 34 11 Water elutriator 38 12 Nitrogen adsorption apparatus with oxygen thermometer 42 13 Furnace used for heating specimens IpS IÎ4. Thermogravimetric analysis device 50 15 Change in weight with increasing temperature for sample KS 55 16 Change in weight with increasing temperature for sample N-M 57 17 Infrared transmlttance curve for sample H-AX; no heat treatment 60 18 Infrared transmittance curve for sample H-AX; heated at 800° C. 62 vi

LIST OF FIGURES (Continued)

Figure Page 19 Infrared transmlttance curve for sample N-AX; heated at 1000 C, 64 20 Infrared transmlttance curve for sample N-AX; heated at 1200 C. 66 21 Infrared transmlttance curve for sample N-B; no heat treatment 68 22 Infrared transmlttance curve for sample N-B; heated at 800° G. 70 23 Infrared transmlttance curve for sample A-14; no heat treatment 72 24 Infrared transmlttance curve for sample A-llj.; heated at J4.OO0 C» 74 25 Infrared transmit tance curve for sample A-14; heated at 800 C* 76 26 Infrared transmlttance curve for sample KS; no heat treatment 78 27 Infrared transmlttance curve for sample KS; heated at 800° G. 80 28 Linear shrinkage vs. time at temperature for samples H-AX, N-A, N-B; 1200° C. 83 29 Linear shrinkage vs. time at various tempera­ tures for sample N-AX 85 30 Correlation of particle size and maximum slaking temperature 89 31 Logarithm of shrinkage vs. time at temperature for samples N-AX, N-A; 1200° C. 94 32 Logarithm of shrinkage vs. time at temperature for sample N-B; 1200® C. 97 33 Logarithm of shrinkage vs. time at temperature for sample N-AX; 1100° G. 99 vil

LIST OP FETEES (Continued)

Figure Page 3I1 Herring analogy for samples N-A and N-AX; 1200° C, 101 1

I. INTRODUCTION

Sintering phenomena, if the broadest interpretation may be considered, include all of the processes and techniques employed to produce bonding, adhesion, or consolidation be­ tween particles. In the narrow, technical sense, sintering can be defined as the mechanism whereby particles of a single, pure material consolidate when subjected to the ex­ ternal influences of pressure, temperature, and time less than sufficient to cause complete disruption of the initial molecular structure, i.e., melting. Sintering of fine powders was perhaps first utilized by Wollaston (1) in preparation of tungsten compacts from metal sponge. In this application, the temperature required to work the metal by conventional metallurgical techniques was excessive, hence sintering was the only successful method by which the sponge could be consolidated into a dense, useful, compacted object. A parallel situation exists in all highly refractory materials, whether metallic or non-metallic. In the area of the non-metallic ceramic refractories, sintering is the accepted technique for enduration. It has found uni­ versal utility in the fabrication and stabilization of the ceramic materials considered super refractories, those materials which are stable at temperatures In excess of 2

approximately lj?00° C. A definite need for such, super refractories has arisen in several areas, such as aerodynamic structures, nuclear component and structural materials, and metallurgical shapes. Due to the hardness "which, generally accompanies the refrac­ tory characteristics, such materials are not readily cut in massive forms, nor do they lend themselves to conventional molten casting, or assemblage of components. It is to fill such a need in fabrication that the ceramic forming processes of slip casting, dry-pressing, plastic extrusion, and hot pressing have been employed followed by sintering to a density approaching that of a theoretical single crystal. The forming processes all depend upon the compaction of finely ground powders into the shape desired, with proper allowances for shrinkage during later heat treatment, and then sintering or heating to a temperature sufficiently high to develop a dense, hard, durable shape. This heating causes the occurrence of rapid sintering phenomena, I.e.* » densifi- cation, shrinkage, grain growth, and general enduration of the compact. In all cases the temperatures employed are be­ low the melting temperature of the material being sintered, and the sintering frequently takes place in the total absence of impurities. While most; of the ceramic refractories are ionic in nature, similar phenomena te those found in ceramics have long been recognized in metals. This latter recognition 3 has formed the basis of the powder metallurgy industry. There are still many questions to be answered in the area of powder sintering; most of these questions are specifically about sintering mechanisms of refractory ceramic materials. While qualitative descriptions and explanations have been given for the influence of sintering time, tempera­ ture, particle size, impurities present, forming pressure, sintering pressure, and sintering atmosphere, efforts to fully define the mechanisms involved have been only partially successful. Such discussion and explanations will be fully considered in the literature review. One of the unanswered questions which has received only cursory attention has been the question of what conditions prevail at the initiation of sintering. To paraphrase, when does sintering start, or when do the ionic bonds actually start to break and reform to develop a consolidated, semi- continuous structure, bonding one grain to the next? If the answer to this question may be obtained, it may be possible to initiate sintering at lower temperatures or under variable conditions. Mille sintering is employed in a wide group of the super refractories, aluminum oxide was chosen as the material for this study. The choice was based on stability, availability, classic ionic crystal structure, and its very prevalent application as a super refractory. II. STATEMENT OF PROBLEM

The problem concerns the initiation of sintering on an ionic scale in aluminum oxide* The conditions under which sintering commences should be defined and some information determined concerning why sintering does not commence under other conditions considered. Limitations of the investigation will be isobaric conditions (one atmosphere), sintering atmospheres will be limited to air, and only representative samples of high purity refractory aluminum oxide will be considered. 5

III. LITERATURE REVIEW

Aluminum oxide, or alumina as it is frequently termed, has been very widely studied as a refractory (2), abrasive (3, if.), catalyst support (5), surface active material for gas chromatography (6), and for other applications (2). It has come to be of major importance in many industries ranging from heavy refractories to artificial gem stones for orna­ mentation or bearing materials* It has been shown to exist in a number of crystalline modifications; this is the reason for the great variation in properties and likewise the variety of uses. An excellent review of the nomenclature of the alumina-water system and alumina in general has been prepared by Russell (?)• While there are many hydrated and semi- hydrated forms of alumina, all of these forms transform to the alpha structure when heated for a minimum of one hour at a maximum temperature of II4.OO0 C» Since this transformation has been found to be irreversible (8), and since the alpha form is. quite stable with respect to air, steam, and most re­ act ants, this form has been most widely employed as a refrac­ tory. This is likewise the crystal form, which has been studied in this investigation and is the only form of com­ mercial significance as a refractory material• The crystal system of alpha alumina is hexagonal (7). 6

It has a measured and theoretical density of 3.97 gra/cc (9). The crystal structure has been studied extensively by Pauling and Hendricks (10)• The alumina lattice is made up of oxygen ions in a hexagonal close packed structure, with two out of three of the available octahedral holes in the structure filled by aluminum ions. Each aluminum ion is then co-ordinated to, or surrounded by, six oxygens. Each oxygen is co-ordinated to four aluminum ions. Alpha alumina has a melting point of 2015° 0. (12) and a boiling point of 3f>00° Co (13* lip » The mechanical proper­ ties have been summarized by Russell (7)• The thermodynamic properties have been summarized by Runck (2) » Most of the work reported in the literature concerned with sintering of aluminum oxide pertains to the macro phenomena which occur at 0.5 to 0.8 Ta or higher, where T& is the absolute melting point (11). In this temperature region, shrinkage is readily measured and consolidation has increased to a point sufficient to give some utility to the fabricated shapes. This area of sintering is roughly 1000° C. to 1800° G m for aluminum oxide. The literature concerned with sintering in general will be considered first as background material followed by the literature review of sintering initiation in aluminum oxide» 7

A. General Mechanism of Sintering in Ceramic Refractories

While the early work on sintering of ionic materials dates to Tammann (11) and to Wollaston (1) for metals and perhaps before, the recent interest is reported between the years 1930 and the present. Taylor (15) noted that many materials could be fabricated by reaction of solid powders, in the absence of a liquid phase, and that the reactivity of an ionic type of solid is enhanced by factors which weaken the interionlc forces. Examples of such factors are larger cation imparities, surface reactants, and introduction of strain. Dawihl (16) at about the same time reported the sintering characteristics of aluminum oxide alone and with additions of MgO and Fe^O^. Rapid sintering was found to occur with pure alumina at temperatures above about 1850° C. and at slightly lower temperatures when the additives were present. Ernest and von Warteriberg (17) reported a phenom­ enon In slip cast and sintered alumina which concerned orientation of the sintered crystals with respect to the sur­ face of the casting. They stated the c-axis occurred per­ pendicular to the surface due to the greater adsorptive capacity for water of the basal face of alumina crystals and their consequent orientation on a wet surface. At about this same date Byscfckewitsch (18} began a long series of 8 investigations on sintering which has been reported in both the German and United States technical literature* This early work by Ryschkewitsch concerned the growth of grains of alumina at elevated temperatures, often mistakenly termed re crystallization by ceramists* The above articles are all primarily concerned with observation of the phenomena of sintering and are representative of the work of that time» Following "World War II, considerable work was published and is still being presented concerning the exact mechanism of material transport involved in sintering* Bangham (19) offered a fundamental explanation that shrinkage, i*e*, sintering, is a manifestation of. the tendency of the par­ ticles to reduce their surface energy to a minimum* This explanation has found wide support and agreement* Bangham further states the surface forces act locally at particle contact points and tend to broaden contact areas* The elastic forces resident in the particles tend to resist such broadening, but become less powerful as the temperature is increased* As particle size is decreased, the surface forces become relatively greater and sintering may be accomplished with greater ease or at a lower temperature* At about the same time Baumann and Benner (20) patented the use of several oxides as additions to aluminum oxide in the manu­ facture of sintered abrasives* Oxides included were TiOg, "^®2®3* M11O2, Cr^O^, ZrOg, and VgO^* Frerikel noted the applicability of the diffusion theory to ionic crystals (21) and observed that diffusion flow, or movement of lattice vacancies, was entirely separate and distinct from plastic or viscous deformation» Much of the more recent work reported since 1950 has also pertained to the mechanism of material transport during sintering» Perhaps most notable are the studies by Clark and White (22) and Clark, Cannon, and White (23)• This work con­ sidered the mechanism to be one of the four possible; viscous flow, plastic flow involving diffusion of dislocations, and surface and/or volume diffusion of lattice vacancies. Using simplified models, these investigators obtained equations which fit the experimental data rather well within certain regions » Similar work was performed by Mackenzie and Shut tie worth. (2ij.) assuming a plastic flow to be the mechanism of material transport in ionic crystals# The latest work by Clark, Cannon, and White (23) showed fair agreement of the derived equations if a plastic deformation with a temperature dependent yield point was assumed» They concluded that sintering is not a simple continuous phenomenon but is made up of several mechanisms, the occurrence and dominance of each depending upon the conditions which prevail. A great quantity of closely related literature has been published recently, some of the more notable work is by Murray et al. (25), and the investigations by Marshall et al. 10

(26), and Hedvall (27) on material transport mechanisms occurring during macro sintering. Smothers and Reynolds (28) and Gahoon. and Christensen (29) have reported on some of the effects of additives to aluminum oxide. Walker (30), Weyl (31), and Schwarzkopf (32) have all contributed to the general knowledge of the sintering phenomena. An excellent review by Allison and Murray (33) analyzes sintering from data obtained from fluoride model systems. They conclude plastic flow is occurring, at least in the early stages of sintering. Burke (34) has authored a summary concerning the sintering phenomena, combining much of the concept currently applied to metals and the later ideas that have been applied to ceramic and ionic materials. His comments on the mechanism of material transport are of particular interest since, on the basis of micro structures and observations during sintering of metal powders, Burke supports a diffu­ sion mechanism rather than a plastic flow model. Of the two currently popular schools of thought with respect to the mechanism of material transport, plastic flow appears to be as possible as the diffusion mechanism. It may well be that each type of transport occurs, depending upon the circumstances. Evidence of such multiple occurrence has been given by Wilder and Fitzsimmons (35>) In work with, carefully sized aluminum oxide powders. Following the 11

analysis by Herring (36) the influence of particle size on sintering can be ezpressed as follows:

n At2 = X Atx

where At^ and Atg are the intervals of time at tempera­ ture required to accomplish a given fractional shrinkage in two powder compacts where the ratio, X , of the particle radii is given by

x = # where rg and r^ are the respective radii of the particles. She constant n is determined by measurement of actual shrinkage curves. Herring concludes that the values of n as given below in Table 1 are Indicative of the respective modes of material transport. The work by Wilder and Fitzsimmons

Table 1. Correlation of material transport mechanism with n

n = 1 Plastic flow n = 2 Evaporation and condensation n = 3 Volume diffusion n = k Surface diffusion

(35) with aluminum oxide shows the mechanism to be plastic flow at early stages of sintering and some other mechanism, with values of n > 5, occurring during the later stages of sintering. 12

A general bibliography of sintering phenomena has been prepared by Wilder (37» 38) and further discussion of the literature of this field may be obtained from these sources, therefore it need not be pursued further here. The conclusions drawn from the literature at this time indicate a complex mechanism, changing in nature as tempera­ ture, time, and particle size vary. A combination of several phenomena apparently make up the sintering mechanism. Several of these have been isolated and studied with a fair degree of success.

B. Initiation of Wintering in Aluminum Oxide

If a finely ground aggregate of aluminum oxide crystals is compacted at room temperature, a noticeable amount of ad­ hesion between the particles will be developed. This adhe­ sion may well be termed sintering, with the driving forces reduction of surface energy and the forming pressure. However, the nature of this adhesion has been observed to be non-continuous at relatively low temperatures, indicating the room temperature adhesion might be due to an adsorbed surface layer which acts as a binder. It has also been noted that binders, i.e., waxes, oils, water, and other ad- hesives, increase the degree of adhesion between the particles to a much greater degree than increases in forming pressure or low temperature heat treatment. For these reasons, it 13

appears likely that the room temperature strength is more of a cementing action than an actual ionic sintering. Ionic sintering infers that ionic or perhaps Vander Waal bonds are broken between the cations and the anions involved. This breaking may occur within the crystal or on the surface. To produce the sintering effects these bonds are reformed be­ tween different cations and anions. Some of the earliest investigators that reported an abrupt change from the cemented bond obtained at room temperature to some sort of stronger bond at elevated temperatures were Clark, Cannon, and White (23). Parallel work at an earlier date by Huttig (39) and Kingston and Huttig (40) described the effects of adsorbed surface layers on metallic aggregates. Clark et al. (23) noted that an appreciable increase in mechanical strength occurred when dry-pressed bars were heated in air to between 500° C. and 600° C. To further describe the change, the broken specimens from strength determinations were submerged in water. It was observed that the specimens fired to £00° c<> slaked readily in water, but the specimens heated to 600° C. remained in­ tact. Similar results were obtained with MgO, but here there is always a strong possibility of simple macro hydration taking place. It was suggested by these investigators that hydrogen bonds between layers of hydrate produced a cementing action accounting for the room temperature strength. I

Hi-

Studies of surface charge also yield some Information concerning this question; the work by O'Connor, Street, and Buchanan (IjJL) and Buchanan and Heymann (1*2) and 0*Connor, Johansen, and Buchanan (43) being representative* O'Connor, Johansen, and Buchanan measured zeta potentials by the streaming cell technique and found some very interesting changes in the sign of the potential. The initial charge was found to be negative, .i.e., indicative of an anion (~QE) bond at the surface. If the naturally occurring corundum had been heated above 1150° C., they found the charge would change from negative to positive, indicating a broken cation bond at the surface. Two hours contact with water at 80° C. would reverse the surface charge, and it would again be negative. Similar reversal was produced by light grinding with a mortar and pestle in air, or by treating with HC1. Parallel results were obtained with silica-free sapphire. The above data, based on naturally occurring corundum, is in agreement with data reported by Kipling and Peak all (44, 45) tAio worked with activated alumina. Their work indi­ cates, according to the sign of the zeta potential, a possible hydroxide layer on the surface of the alumina when in tiie mixed corundum-aluminum hydroxide, i.e.* activated, form. There has been no similar work reported on calcined or fused alumina, alpha form, at this time. Summarizing the literature search with respect to 15 initiation of sintering, one may conclude the literature is scant, there is a distinct probability of surface adsorbed ions, and changes are known to occur in the slaking characteristics of the aluminum oxide powders at fairly low temperatures. 16

IV, MATERIALS CONSIDERED

The investigation was limited to refractory grades of aluminum oxide by definition, so only the alpha forms of alumina were considered in the choice of raw materials• Alpha alumina, or corundum, is offered commercially in two types, one the product of arc fusion, the other the product of calcination at temperatures below the melting point. The arc fusion process involves loading the basic raw material, bauxite, along with coke, and either metallic iron or iron pyrite into a vertical furnace with top and bottom electrodes, A large electrical potential is imposed across the top and bottom graphite electrodes, this causes an arc to form, and the mixture is fused. Any silica present in the bauxite forms an alloy with the iron and slags out of the mixture. The aluminum metal can form aluminum sulfide which lowers the melting point of the aluminum oxide. For this reason, if the aluminum oxide product is to be used as a refractory, metallic iron is used in the mixture. After fusion is completed, and solidification has occurred, the entire furnace shell is removed and the fused charge is broken away from the slag (46, 47, 48, 49) * The above mentioned calcination process, the Bayer process, starts with the same naturally occurring bauxite, 17

but depends upon chemical extraction for obtaining the oxide desired (50). The bauxite is crushed and ground to less than 100 mesh, dissolved by caustic soda solution in the presence of soda ash and lime, and the silica thus removed as a precipitate of sodium aluminum silicate. The sodium aluminate product is filtered from the "red mud" residue, and is then hydrolyzed to aluminum hydroxide by cooling and washing with water » Seeding of the solution aids the crystallization process. After filtration and further washing, the aluminum hydroxide is calcined in a rotary kiln to 1150° c. or higher (lj.9, 51 52). The product of calcination is compared to the product of arc fusion by Partridge and Lait (53) and Kirby

(5W* A representative sample of the arc fusion type of aluminum oxide was supplied by the Morton Company. This material was ball-milled in a one gallon steel mill with steel balls for 16 hours, thoroughly washed with hydrochloric acid and water to remove the iron from the milling operation, and then fractionated into various particle sizes using a short column water elutriator. The procedure of milling and cleaning employed has been fully described by Hauth (55)• The elutriator and the techniques involved are described in the section on experimental methods.

"Norton Company, Worcester, Massachusetts. Refractory Grain Alumina. Grade 38%, Specification 220F. 18

Chemical analysis of the fused aluminum oxide after grinding and fractionation by elutriation is tabulated in Table 2. This material was studied in fractionated form as well as in a ball-milled mixed distribution. The particle

Table 2* Chemical analysis of fused aluminum oxide

SiO, 0.01$

Ti02 0.02 MgO 0.07 C&0 0.05

F@20^ 0.02

Na20 Nil

k2o Nil

A12°3 99.8 (by difference)

sizes, as determined by microscopic measurements and nitrogen adsorption measurements are given in Table 6, Materials considered. Comparison of these and other various diameters, and the operating characteristics of the elutriator are given in an evaluation by Wilder and Fitzsimmons (56). The other materials chosen for study were all calcined, and had not therefore passed through the molten state during their manufacture. In every case, while the particles were agglomerated to a free-flowing powder, the actual particle size was quite small, definitely no greater than the average 19

size of the finest of the fused particle fractions * The materials chosen from the available calcined or tabular aluminas consisted of two grades supplied by the Aluminum Company of America,"®" and one by the Kaiser Aluminum Company^.

Table 3» Chemical analysis of tabular alumina T-6l

T-61 AI2O3 99.5 % plus

ffa20 0.02

Fe2°3 0.03

Si02 0.02

Ti°2 0.002 LOI 0.000

The chemical analyses were given by the suppliers as noted in Tables 3, l|. and 5. This material is -widely employed as a low soda fliTmvtnp for electrical insulators, and as a high tempera­ ture catalyst support*^ The calcined alumina is widely used in abrasive manufacture and in small quantities as an addi­ tive to enamels, glazes, and porcelain bodies. This material is not calcined at as high, a temperature as the tabular

Aluminum Company of America, Pittsburgh, Pennsylvania. Alcoa Tabular Alumina. Grade T-61. Alcoa Calcined Alumina. Grade A-1%. p Kaiser Aluminum and Chemical Sales, Inc., Chicago, Illinois. Kaiser Calcined Alumina. Grade I. Standard. 20

Table !}.« Chemical analysis of calcined alumina A-14

A-11}. AV, 99.6 %

Na20 O.Olj.

Si02 0.10

Fe2°3 0.0%.

Ti02 0.002 LOI 0.15

alumina, as evidenced by the high loss on ignition. It is calcined at a temperature sufficiently elevated to produce a material containing all the alumina in the alpha form, i»e>, above Uj.00° C. (?)• The actual temperature at which the transformation takes place depends on the material being cal­ cined, varying from lj.5>0° 0. to lij.000 C. for original materials of alpha trihydrate and alpha monohydrate, respectively» The alumina grains discussed above are pro­ duced by treating aluminum hydroxide which has been obtained from the Bayer process. A mixture of the various hydrates would be expected upon decomposition of the hydroxide. The third calcined sample had the following chemical analysis, as supplied by the manufacturer• This material is quite similar to the two listed previously, being calcined to the alpha form. I

21

Table 5* Chemical analysis of calcined alumina KS

Si02 0.02 % 0.02

TiG2 0.005

Ua20 0.72 LOI 0.69 98.55 (by difference)

Each of the four materials chosen for study was sub­ jected to spectrographs analysis which substantiated the chemical analyses. Each sample was then examined by X-ray diffraction, using the powder technique, and a Debye-Scherrer powder camera. Only alpha alumina was found in every case, indicating only a slight probability that any other form of alumina was present. A tabulation of the manufacturer, manufacturer * s desig­ nation, the designation used in this thesis, particle size, method of manufacture, and treatment after delivery to this investigator is given in Table 6, Materials considered. Figure 1, Particle size distribution of Sample N-M shows the particle distribution of the fused alumina after ball milling, as determined by microscopic measurement. The methods employed are discussed in the section on experi­ mental methods. Table 6. Materials considered Manufac­ Manufac­ Manufac­ Thesis Treatment BET Microsoople turer turing turer1 s desig­ on diameter diameter method designa­ nation delivery (microns) (microns) tion

Alcoa Calcined T-61 T-61 None 0.787 m Alcoa Calcined A-lij. A-ll*. None 0.51+3 m Kaiser Calcined Std KS None 0.030 Norton Fused 38x N-M Ball milled aold leeohed 0.301*. 1.10

Norton Fused 38X N-AX Ball milled, aold leeohed, fractionated 0.262 Norton Fused 38x N-A n t« 0.550 1.20 Norton Fused 38x N-B il it 0.708 2.98 Norton Fused 38% N-C H ft 1.633 3.79 Norton Fused 38X N-D H ii 2.930 5.21 % Figure 1* Particle size distribution of sample N-M PERCENT FINER

"C

o 25

Photomicrographs of each of the grains studied are given on the following pages in Figures 2 through 10. These were all taken on Kodak Contrast Process Ortho film, using a Leitz CMU Research Polarizing microscope with transmitted light. Individual counts using the microscope were not made for the calcined grains due to the agglomeration which can be readily observed in the photomicrographs. The bulk of this investigation was devoted to the first mentioned raw material, fused aluminum oxide. The purity is higher than that of the calcined grades and the particles are more discreet and homogeneous. Some of the variations observed between the different materials during the investi­ gation are related to these differences. A"hTnrtTmTw oxide which has been calcined from the hydroxide has become agglomerated to such an extent it is impossible to measure an exact particle size distribution, hence it is impossible to fully define the nature of the surface. The latter three materials considered are therefore included to compliment the investigation of the sintering characteristics of the fused material. 26

Figure 2. Photomicrograph, of Sample T-6l* 1+60X Pigore 3. Photomicrograph of Sample A-llj., 1+60X 28

Figure !+• Photomicrograph of Sample KS, â$.60X 29

Figure 5» Photomicrograph, of Sample Ef-M, 1+60X 30

Pigore 6. Photomicrograph of Sample N-AX, 1+60X

32

Figure 8 Photomicrograph of Sample N-B, 33

Figure 9* Photomicrograph of Sample H-C, lj.60Z 3k

Figure 10. Photomicrograph of Sample H-D, fy60X 35

V, EXPERIMENTAL METHODS

A. Fractionation of Fused Aluminum Oxide Powders with the Water Elutriator

The water elutriator employed is a device which has been employed by many investigators to fractionate fine powders into different particle size groups, and also to measure particle size distribution* An evaluation and general dis­ cussion of its performance has been published elsewhere (56) so it will suffice to comment briefly on its construction. The water elutriator depends on the variation of settling velocity through a liquid with variation in particle size of the material falling. Stoke*s Law is applicable for a first approximation of the size which will be produced by a given upward flow of fluid. The elutriator itself consists of a glass cylinder approximately three inches in diameter, topped by a metal cylinder upon which is mounted a spiral discharge trough. All three of these rest on, and are bolted to, a metal cone. Water flows downward through the central hollow shaft, out into the cone where the material to be fractionated is held, and up through the glass cylinder carrying upward and over the lip and into the discharge trough the various sized particles. The size of the particles 36 removed is thus dependent upon the velocity of flow, i.e»» all particles having a settling velocity greater than the flow rate upward will be retained in the elutriator# The flow of water originates in a storage system, of three 12- gallon glass bottles. The water flows by gravity from these to a two liter flask so arranged that the level in the flask is maintained by a constant overflow* The flask discharges at the base to a tube which leads to a calibrated spigot» This spigot is drawn from tubing and then ground on a stone to the proper discharge opening size. To eliminate the difficulty of maintaining and drawing a spigot for each flow rate desired, a system of cutters is used to yield a frac­ tion of the spigot flow. These cutters consist of portions of cones, fitted into a funnel in the top of the hollow shaft. They divert one-half, one-fourth, one-eighth, or other predetermined fraction of the flow into the elutriator as the shaft revolves. The remainder of the flow may be either recycled or discarded to a drain. These cutters have been described by Clemmer and Coghill (57)* An inrpellor made of perforated one-eighth inch thick sheet metal is fastened to the central hollow shaft and agitates the material to be fractionated continuously. A grid above the impeller reduces to streamline flow any turbulence caused by the stirring action. The central hollow shaft is rotated continuously by a belt drive from a l/20 h.p. motor and gear reducer. The 37

shaft rotates at approximately 20 r.p.m. Since the aluminum oxide was fractionated in acidic media, all parts of the elutriator that come in contact with the liquid are of stainless steel, glass, or neoprene. A photograph of the elutriator is shown in Figure 11, Water elutriator• The funnel, cutters, motor drive, and the water supply and spigot system are not shown. While this device is slow, it has been quite reliable and relatively trouble -free. Approximately 10 days are re­ quired to fractionate 100 gnu of aluminum oxide into the fractions noted previously. However, only a few minutes attention each day are required for continuous operation.

B, De termination of Particle Size

1. Microscopic measurements

To determine the average particle size of the grain size fractions by microscopic measurements, a smear of the powder to be analyzed was first made on a microscope slide with water and a glass rod. If care was taken in application, a uniform smear of well separated particles resulted when the evaporation of the water was complete. This technique has been described by Green (f>8). After the smear was dry, a cover glass was sealed over the powder with Permount, a com­ mercial mount ant. To eliminate any possibility of unequal 38

'M/.'

Figure 11. Water elutriator 39

distributions of particles near the edge of the slide, only the center sections were evaluated. Arbitrary areas were chosen before the field was observed, and all particles with­ in the predetermined field were measured with a filar micro­ meter* Martin*s diameter (59) was chosen to evaluate each particle. This diameter is the length of a line which forms a horizontal bisector of the projected area of the particle in question. White incandescent light was employed with a blue filter to reduce eyestrain. Due to the tedious nature of this work, and the eyestrain involved, it was found that a maximum of two hours daily could be spent at the microscope bench with no resultant ill effects. Approximately 1$0 particles can be evaluated each hour. The results of microscopic measurement are invariably of greater diameter than the measured diameter obtained on the same material by nitrogen adsorption. This is due to the in­ ability to see particles below approximately 0.5 microns using an ordinary bench microscope. In each detexamination reported, at least lj.00 particles were measured. This number gives a highly reproducible average value.

2. Nitrogen adsorption

Nitrogen adsorption has been used extensively to study the surface of fine particles (60). The principles involved are fairly simple and readily applied. If a powder is ko

completely covered by a monomolecular layer of a gas, and the number of molecules involved in forming such a layer is known, and the area covered by each molecule is known, it is possible to multiply and determine the surface area of the powder• In actual practice, the volume of a gas adsorbed on an evacuated surface is measured at low temperatures and various pressures* If the volumes adsorbed are then plotted against the respective pressures, one can deduce the mono- molecular volume from the resultant S-shaped isothem* Such deduction is only possible if the curve has the characteristic S-shape • Such a shape was found to occur in all of the materials examined for this study. Brunauer et al. (6l) have shown that the isotherm could be plotted in such a manner that a straight line results, the slope of which gives directly the volume of gas required to form a monomole cular layer on the particles. The classic BET equation, named after Brunauer, Bramett, and Teller, is as given below.

p _ 1 + (C-l)p

v(po-p) vàc vmc p0 where V = volume adsorbed at pressure p at a temperature at

which the vapor pressure of the liquefied gas is

Po-

Vm = volume of gas to form a monolayer. C = a constant. Thus if , P . is plotted versus P a straight line V(PO-P) Po 41 c-i results, the slope of -which, is tT"c and the intercept will 1 m be —— • The constant C can be evaluated and also the value *mc for the volume Vm determined. Prom this volume, knowing the weight of the sample, the specific surface or surface per unit of weight tested, can be calculated. Then assuming a spherical particle, the average diameter may be calculated. This diameter has been widely employed in sintering studies and is probably the best of the various measured diameters to use, because it considers the roughness of the surface and any fissures or breaks in the surface which undoubtedly con­ tribute to slit ering effects. In the measurements of specific surfaces and average diameters reported in this thesis, liquid nitrogen was em­ ployed with aluminum oxide sample weights of approximately $0 ga. After out-gassing at a pressure of 10""-* mm. Eg, the sample In the sample bulb was immersed in the liquid nitrogen bath, and gaseous nitrogen was allowed to adsorb onto the surface of the particles within the sealed system. The pressure and temperature of the nitrogen bath were observed, and more nitrogen gas was allowed to enter the system. The pressure and temperature were again observed, and so on. The results were subjected to the above equations and the data plotted. The pressures were measured with a mercury manometer and vernier scale. The apparatus is shown in Figure 12, Hitrogen adsorption apparatus with oxygen thermometer# The 42

Figure 12. nitrogen adsorption apparatus with oxygen thermometer 43

oxygen thermometer is a unique device employed to determine the temperature of the liquid nitrogen bath at the time of adsorption (62). This part of the apparatus consists of a sealed tube, one portion of which contains oxygen, the other portion being a vacuum, and the two portions being separated by a mercury manometer so situated it will measure the pressure difference between the two portions. Mien the oxygen-filled portion is immersed in the liquid nitrogen bath, since the temperature is below the boiling point of oxygen, condensation takes place and a vapor pressure exists above the liquid oxygen. This vapor pressure may be measured with the manometer, and knowing the relation between vapor pressure and temperature from independent measurements, one can calculate the temperature of the bath. From a similar relation for nitrogen, it is then possible to calculate the saturation pressure of the nitrogen adsorbate at the tempera­ ture previously determined. This thermometer has been re­ moved from the vacuum flask nitrogen bath in Figure 12. This scheme of temperature measurement has been very satisfactory and is less cumbersome and expensive than a resistance thermometer. It will be noted in Table 6 that the results of nitrogen adsorption yield diameters for the samples which are somewhat less than the diameters from microscopic measurements. This is due to the inability to see the smallest particles with an kk

optical microscope, as mentioned above, and due to the ability of the adsorbed monolayer to enter crevices and other irregularities on the particle surfaces» Such a tendency has been found to exist in other materials tested with this equipment, and is also noted by Dalla Valle (63)•

C. Forming of Specimens

All specimens were prepared by dry pressing in the absence of water or any other binder. The sample powders were dried for at least lp8 hours at 105° C. prior to pressing. Hardened steel simple two plunger dies were used to produce two sizes of pellets. The smaller were 0.25 inches in diameter and 0.25 inches in height. The larger were 0.752 inches in diameter and 0.25 inches in height. All specimens were formed on a hydraulic press at a calculated load of 20,000 psi. Variation in forming pressure in this range has been shown to have little influence on the sintering mechanism occurring in aluminum oxide (35)•

D. Heating Methods

In all instances where heating is reported, an electric furnace with silicon carbide heating elements was employed* This furnace was activated by a saturable reactor, which, was controlled by a Celectray indicating potentiometer temperature k$ controller^" and suitable amplifier* She control was always within C. as determined by measurements with, an inde­ pendent thermocouple and potentiometer. Temperature dis­ tribution within the furnace was quite uniform, largely due to the one-fourth inch thick aluminum oxide tube which acted as a muffle» Figure 13, Furnace used for heating specimens, shows the arrangement of the four silicon carbide heating elements, the aluminum oxide muffle tube, and the thermo­ couple lead. The furnace was operated with the lid down and loading was through the end of the muffle tube. Platinum, Platinum-ID^ Bhodium thermocouples were used in all cases to measure the temperatures, with an ice bath, cold junction and compensating lead wires. Since sintering is a rate phenomenon, the specimens were placed in th.e furnace while it was at the temperature of interest. This minimized the effect of the heating rate by decreasing the heat-up time to a few minutes. Since in most cases the interval involved at temperature is one hour or longer, this heat-up time will be ignored and it will be assumed the specimens reached temperature instantaneously. Following the same reasoning, the specimens were allowed to cool immediately upon removal from the furnace • Uhile this does not duplicate industrial practice, it certainly minimizes

^Model Ho. lj.8302. C. J. Tagllabue Corporation, Newark, Few Jersey. V>

Figare 13» Furnace used for heating specimens kl

the influence of the heating and cooling cycles, which are very difficult to reproduce accurately with a small specimen. Thermal failure, due to the rapid heating and cooling, did not appear in any of the specimens.

E. Measurement of Physical Dimensions

Many of the dry pressed specimens described above were measured for linear dimensions. The specimens were allowed to cool to room temperature throughout, and were then measured with an ordinary machinists micrometer. Three or more diameters were determined on each specimen and any with, rough or broken edges were discarded. Points as plotted on the curves are the results from measurements of at least three specimens. The specimens having the larger diameters were used wherever measurements of this type were to be made.

F. Slaking Technique

Specimens which were to be subjected to slaking tests were chosen from the smaller diameter compacts. After heating, they were allowed to cool to room temperature, and were then completely immersed in distilled water* If slaking occurred, it was found it would be apparent within 30 seconds, therefore this was taken as the time interval of immersion. After this interval, the specimen was declared slaked or not k& slaked. Prolonged immersion, up to seven days, would not produce slaking in a specimen that had remained intact for the first 30 seconds.

G. Infrared Spectroscopy

A study of the adsorbed ions on the surfaces of the particles was undertaken using the technique of infrared spectroscopy. Samples of most of the materials considered were examined with transmitted radiation. To determine what effect heating might have upon the surface ions, loose powders were heated to various temperatures in dense aluminum oxide boats. The boats were placed in and removed from a hot ftirnace. After this heat treatment, the boats containing the powders were exposed to the air for 48 hours, at approximately 78° F. and 55 per cent relative humidity. Following this interval, the powders were placed in a drier and held for 72 hours at 105° C. The pellet technique intro­ duced by Stimson and 01 Donne 11 (6%) was employed for the actual infrared exposure. Samples of aluminum oxide weighing 3 mg. were dispersed in 600 mg. of potassium bromide which had been thoroughly dried at lij.00 C. The mixture was placed in a steel die, the die was then evacuated, and pressure was applied. The resultant pellet was placed in a Baird infrared recording spectrophotometer, and the spectrum analyzed# k9

There are two absorption bands of interest, one occurring at a wave length of about 6.3 microns, the other at a wave length of 7*1 microns* The former absorption is usually attributed to ~0H groups, the latter to HOH groups (65)» The extent of the absorption at these points was measured, cor­ rected to 100 per cent background level and is given with the other results.

H. Thermogravimetric Analysis

Several measurements were made to determine the nature of the losses in weight which, occur when most fine powders are heated* For such measurements a thermogravimetric analysis device was employed which permits continuous control of heating rate and continuous recording of sample weight. This device is shown in Figure Uj., Thermogravimetric analysis device* It consists of a balance with a conventional chain- weight loading mechanism, a photocell and ligit source which detects any variation from balance by means of an optical lever activated by the balance arm, and an amplifier to in­ crease the photocell signal. This increased signal drives a reversible motor which operates a gear drive correcting the chainweight load to bring the balance arm back into equilibrium. Geared directly to the balance chainweight drive is a potentiometer, the position of which activates a recording potentiometer» 50

Figure Thermogravimetric analysis device 51 % A Brown cam-type automatic temperature controller and recorder measures the furnace temperature and with the aid of an amplifier controls the voltage output of a saturable re­ actor. The output voltage is applied to a vertical furnace with silicon carbide heating elements, similar to the one used to beat the pellet specimens, shown in Figure 13. This furnace contains four heating elements, as does the one shown in Figure 13» and the heating elements are equally spaced around an aluminum oxide tube which acts as a muffle. Con­ vection currents are sometimes a problem in a furnace of this type, but proper placing of shields and rock wool insulation eliminates the bulk of any chimney effects. The 10 gat. sample is suspended on a platinum wire from the arm of the balance and is held in an aluminum oxide crucible that has been fired to a much higher temperature than any temperature employed in the thermogravimetric analyses. From the temperature and weight records, appropriate plots of the two variables may be obtained. Similar equipment has been described by Hyatt, et al. (66), and by Duval (67).

^Brown KLectronik Temperature Controller, Model Ho. 152G15PS-239-101. Minneapolis-Honeywell Regulator Company, Minneapolis, Minnesota. 52

VI. RESULTS

A. Slaking Characteristics

In order to folly establish the nature of the slaking phenomena, a number of specimens were prepared# heated# and subjected to the slaking action of water. All of the materials considered in the study were evaluated by this technique. First# results were obtained on fused alumina which had been fractionated into different particle sizes. The results of this are given in Table 7» Slaking character­ istics of fractionated fused aluminum oxide. The results of the investigation of the other materials are reported in Table 8, Slaking characteristics of calcined and ball milled aluminum oxide.

B. Thermogravimetric Analyses

The two extreme conditions encountered in these analyses are given in Figure 15» Change in weight with increasing temperature for sample ES, and in Figure 16, Change in weight with increasing temperature for sample H-M. Fote the con­ trast between the two, which were plotted purposely on the same scale. The heating rate in both instances was 100° C./hour. Table 7» Slaking characteristics of fractionated fused aluminum oxide

a Material Temperature (° 0e) 25 150 200 300 350 400 5oo 6Ô0 700 800 900 1000

N-AX Sb s s s s N N N N N-A S s s s s s s N N N N N N-B S s s s s s s s S N N N N-0 s s s s S S S N N-D s s s s S S S S

*Time at temperature is one hour in all oases,

indicates free slaking pellet, N indicates non-Rlaking pellet. Table 8* Slalclng characteristic a of calcined aluminum oxide and ball milled fused aluminum oxide

Material Temperature (° C») 25 150 200 300 350 400 500 700 850 1000 1200

T-61 Sb S s S a s 3 N À—14 S s S s N N N N H N-M S s s s a S S a N N N KS s s s s N N N N N

&Tlme at temperature is one hour in all cases.

bS indicates a free slaking pellet, N indicates a non-slaking pellet* Figure 15» Change in weight with increasing temperature for sample ES 56

-0.4

•Q8 w

UJ

UJ

UJ

-2.0

•2.4

2.8 200 400 600 800 1000 1200 1400 TEMPERATURE °C Tq

Figure 16» Change in weight with increasing temperature for aanple ÏT-M 58

-0.4

-0.8 H Z UJ ccu LU a oUJ z < X ° -1.6 I- oX- w

•2.0

•2.4

2.8 0 200 400 600 800 1000 1200 1400 TEMPERATURE °C 59

C. Infrared Spectroscopic Examination of Surface Anions

The results of this examination are given in Figure 17 through Figure 27» infrared transmittance curves for the various materials and various heat treatments. The numerical results which were calculated from these curves are given in Table 9» Results of infrared spectroscopic analyses»

Table 9. Results of infrared spectroscopic analyses

Material Heat H0H "OH Total of H0H treatment absorption absorption and "OH absorption ° G. % % %

H-âX Hone 7.3 12.0 19.3 800 11.0 26.4 37.4 1000 12.0 26.5 38.5 1200 12.0 26.0 38.0 M Hone 6.0 13.0 19.0 800 12.2 22.1 34.3 A-14 Hone 13.4 18.5 31.9 400 6.4 23.5 29.9 800 14.4 15.5 29.9 KS Hone 10.8 24.0 34.5 800 10.6 22.1 32.7 Figure 17» Infrared transmit tance curve for sample N-AX; no heat treatment 61

100

<

< 50

5 6 7 8 9 WAVE LENGTH IN MICRONS Figure 18. Infrared transmittance curve for sample H-AX; heated at 800° C. 63

100

< 50

LU

25 —

5 6 7 8 9 WAVE LENGTH IN MICRONS Figure 19» Infrared transmittance curve for sample N-AX; heated at 1000° c. 65

100

75

< 50

UJ

UJ s- 25

5 6 7 8 9 WAVE LENGTH IN MICRONS Figure 20. Infrared transmittance curve for sample ÏÏ-AX; heated at 1200° C. 67

6 7 8 WAVE LENGTH IN MICRONS Figure 21. Infrared transmit tance curve for sample IT-B no heat treatment 69

100

75

< 50

û- 25

5 6 7 8 9 WAVE LENGTH IN MICRONS OT

Figure 22. Infrared transmittance curve for sample H-B: heated at 800 C. 71

100

75

< 50

Q- 25

0 5 WAVE LENGTH IN MICRONS Figure 23* Infrared transmit tance curve for sample A-lit; no heat treatment 100

CO Z 50

25

5 6 7- 8 9 WAVE LENGTH IN MICRONS 4ÂV

Figure 21}.. Infrared transmittance curve for sample A-lit» heated at ij.00 C. 75

100

ui 75

< 50

lli Û- 25

5 6 7 8 9 WAVE LENGTH IN MICRONS Figure 25» Infrared transmittance curve for sample A-lit; heated at 800° C. PERCENT TRANSMITTANCE en

z oo Figure 26» Infrared transmittance curve for sample KS no heat treatment 79

100

75

CO < 50

UJ LU 25

5 6 7 8 9 WAVE LENGTH IN MICRONS 05

Figure 27. Infrared trangmittance curve for sample KS; heated at 800 C. 81

100

o 75

I— CO

25

5 6 7 8 9 WAVE LENGTH IN MICRONS 82

D. Sintering as indicated by Shrinkage

1. Variation with particle size

Linear shrinkage of dry pressed compacts is shorn in Figure 28, Linear shrinkage vs. time at temperature. Shrinkage of compacts prepared from three particle sizes of fused aluminum oxide are shown. Hote especially the influence of decreasing particle size on the extent of the shrinkage.

2. Variation with firing temperature

In Figure 29» linear shrinkage vs. time at various temperatures, sample ÏÏ-AX, the influence of firing tempera­ ture on the finest fused aluminum oxide fraction is shown= Hote especially that no shrinkage occurs at 1000° C., and that shrinkage becomes much more evident as the firing temperature is increased. ro Figure 28. Linear shrinkage vs. time at tenmerature for samples N-AX, c? N-A, N-B; 1200° C« LINEAR SHRINKAGE (PERCENT) m

ro

m > o

m

m

œ Figure 29» Linear shrlnlcage vs. time at various temperatures for sample N-AX LINEAR SHRINKAGE (PERCENT) en ro

ro

m

m m

en

|o

98 87

VU. DISCUSSION OF RESULTS

A. Slaking Characteristics

Sintering would not logically be expected to occur be­ tween particles unless broken bonds are present, either from loss of surface anions, or breaking of internal bonds, .i.e., by thermal effects. At a freshly broken surface, the normal co-ordination of six oxygen ions around each aluminum ion and four aluminum ions around each oxygen ion must be incom­ plete. In the presence of water or water vapor the aluminum ions may take up hydroxyl ions and the oxygen ions may take up a hydrogen ion. In either instance, surface hydroxyl groups will be produced which will be loosely bonded to the surface. They may in torn add another hydrogen ion and form water molecules, more or less completely satisfying the broken bonds on the surface of the particle. Therefore, if these surface broken bonds are somewhat satisfied, it would not be expected that sintering would commence until the sur­ face adsorbed hydroxyl and water groups had been lost or driven off. After these are lost, but not necessarily immediately following the loss, the sintering will commence. Once these bonds are tied to other aluminum oxide particles, slaking is not likely. The variation in slaking temperature 88

limits may be due more to particle size and the subsequent intimacy of contact or state of activity on the surface, than the loss of hydroxy 1 and water groups. It would be expected these groups would all leave at about the same temperature for all of the materials considered, and with respect to the infrared spectroscopic data to be discussed later, such appears to be the occurrence* That the hydroxyl and water groups are actually lost during the heating process is fairly apparent, because if they were retained, it is difficult to conceive that slaking would halt abruptly* Also the work cited previously by O'Connor et al» (28) indicates positively the loss and rapid return of surface hydroxyl and water groups in natural corundum. A similar loss would be expected in calcined and fused aluminum oxide of the types being herein considered. It also appears quite significant that slaking tempera­ tures are related to particle size as noted in Tables 7 and 8. This would be indicative of a sintering phenomenon, rather than a simple change in the character of the surfaces of the particles. The correlation between particle size and limit of slaking temperature is shown also in Figure 30. The curve shorn is exponential with respect to particle size, indicating a relation parallel to sintering characteristics. The variation of slaking temperature limits with the various raw materials can also be explained on the basis of •CO o

Figure 30e Correlation of particle size and maximum slaking temperature O) z o ££ O S LU M V) UJ_J g h— or S 0.5

100 200 300 400 500 600 700 800 900 iôoo

MAXIMUM SINTERING TEMPERATURE AFTER WHICH SLAKING OCCURS 91 sintering, rather than loss or presence of surface anions. The failure of specimens to slake even after long periods under water is also a strong argument for the above.

B. Thermogravime trie Analys e s

The thermogravime trie analyses need little explanation and discussion. The losses concerned are much greater in the more active, smaller partie led, KS sample than in the sample H-M. These losses are most likely hydroxyl and water losses, and some adsorbed gases from the atmosphere. Unfortunately a partial layer of hydroxyl ions on the surface of such particles is below the sensitivity of the instrument, so it is impossible to study anything but the most general macro . losses. Such losses, however, do follow a general pattern, differing only in amount between the different materials.

C. Infrared Spectroscopic Analyses

The results of these analyses are most surprising. One would expect the surface adsorbed hydroxyl and water groups to decrease with increase in temperature of heat treatment. The contrary seems to be the occurrence. However, if the nature of a freshly heated and cooled surface is considered, it is highly possible this surface would tend to pick up more hydroxyl groups on cooling than it had originally retained. 92

It is quite probable the aluminum-hydroxyl bonds are broken at the elevated temperatures, thus placing the aluminum oxide particles in a highly active state for sintering with one another, or, when cooling occurs, to adsorb hydroxyl or other groups which might be present in the atmosphere* It is not logical that the heat treatment should add hydroxyl or water groups to those already adsorbed, for if this were so, tilere would not be a large loss upon heating after drying, nor would there be an abrupt change in the slaking characteristics, as shown in Table 7 and Table 8, The question may well be asked, that if the above re ad­ sorption occurs within two days in loose powders that have been heated to higher temperatures than the limiting slaking temperature, why shouldn't the compacts slake if left in immediate contact with water for several days? The greater intimacy of contact between the particles produced by dry pressing is also a promoter of sintering* Loose powders will sinter and shrink (56), but not nearly as readily as compacted powders• Hence the variation in surface adsorption of hydroxyl and water groups between the loose powders being considered here and the compacts considered under the section on slaking. 93

D. Sintering as indicated by Shrinkage

To further explain and clarify the nature of the sintering curves, the data from two of the fused aluminum oxide fractions were plotted in Figure 31, Logarithmic plot of shrinkage vs. time at temperature, sample N-AX, and sample N-A. The temperature considered was 1200° C. Ex­ cellent straight line plots were obtained for these data, indicating the phenomenon follows a simple exponential plot, e.£.,

—— = Et» Lo where L = the linear change in dimensions of a compact. = the original linear dimension. t = the time interval at sintering temperature, m = a constant having an experimentally determined value of 0.1}. to 0.5» K = a temperature dependent proportionality constant. If log K is plotted against l/T, where T is the absolute temperature at which sintering is taking place, a straight line should be obtained if an Arrhenius type of equation is obeyed. The Arrhenius equation would be of the following form: K = A e'^RI where A = an experimentally determined constant. Figure 31* Logarithm of shrinkage vs. time at temperature for samples N-AX, N-A; 1200° 0, LINEAR SHRINKAGE (PERCENT)

• O

> O

CO

56 96

Q = the activation energy. R = the gas constant. T = the absolute sintering temperature. K = a temperature dependent proportionality constant. This type of equation will be fairly descriptive, or at least fit the experimental data, of the curves given in Figure 31. However, with a snail variation in particle size, the equation will no longer fit the data, e.g.* Figure 32. This is most certainly indicative of a complex phenomenon, or phenomena. Similar lack of obedience of an Arrhenius equation is obtained if data are taken at a lower temperature, for example, the data plotted in Figure 33* Therefore, while some correlation by such equations has been obtained by many of the investigators cited in the literature review, it does not give an overall description of the data presented here, and seems to describe only a limited portion of the sintering mechanism or mechanisms without a definition of the limits. To further elucidate the mechanism involved, Figure 34» Herring analogy, samples ST-A, IT-AX, was plotted for the 1200° C. data. This shows the relation between the time intervals required to produce identical amounts of shrinkage in the two samples. According to the Herring analysis of sintering phenomena, the other straight lines would be expected for plastic flow, evaporation and condensation, surface diffusion —1

Figure 32* Logarithm of shrinkage vse time at temperature for sample N-Bj 1200° 0. LINEAR SHRINKAGE (PERCENT)

1 1—T 1 I 1 T \ X \ o

ol J 1 _!_Q 1 _l 1 1—I—L o

86 'O'Xi

Figure 33* Logarithm of shrinkage vs. time at temperature for sample N-AX: 1100° 0. LINEAR SHRINKAGE (PERCENT)

m

o

OOT Figure 34e Herring analogy for samples N-A and N-AX; 1200° 3s O X

X

20 30 40 50 60 70 80 90 100 N-A At (HOURS) 103 and volume diffusion, respectively, as noted in Table 1. There are portions of the experimental curve which approxi­ mate the predicted slopes, bat the correlation is not sharp and a combination of continuously overlapping phenomena is indicated. Curiously enough, data has been reported at llj.00o G. for the same two particle sizes which supports a plastic flow for the first several hours using the same method of analysis (56). 10k

VIII. CONCLUSIONS

1. Aluminum oxide becomes more active when heated due to loss of surface adsorbed material# thought to be hydroxy! and water groups. Such losses occur in all types of refrac­ tory grade aluminum oxide, but are more pronounced in cal­ cined and tabular grades. If they are heated at higher temperatures, e>£., 800° C», all refractory grades of aluminum oxide tend to approach a maximum adsorption capacity or activity. This capacity is exhibited within a few hours after cooling in air and is more pronounced in the absence of sintered bonds* 2* Changes in slaking characteristics which are ob­ served when aluminum oxide is heated are due to the initia­ tion of sintering, rather than the loss of the surface adsorbed ions. 3. Sintering commences with a complex mechanism which cannot be described by an Arrhenius equation. Analysis of the sintering changes by Herring.* s analogy gives a reasonable explanation oaly if a complex mechanism is assumed. This mechanism may include all or some of the modes of material transport thought to occur during sintering. This is contrary to evidence noted in similar work at higher temperatures. 105

IX. LITERATURE CITED

Smith., C. S. History of powder metallurgy. In Waif, J., ed. Powder metallurgy, pp. I-I4.O. Cleveland, Ohio, American Society for Metals. 191*2. Runck, R. J. Oxides. In Campbell, I. E., ed. High temperature technology, pp. 29-91. New York, John Wiley and Sons, Inc. 1956. Schr ewelius, N. G. Constitution and microhardœ ss of fused corundum abrasives. J. Am. Ceram. Soc. 31: 170-175. 191*8. Ridgeway, R. R. Manufactured abrasives, old and new. Chem. Engr. News. 21: 858-862» 1943* Weyl, W. A. A new approach to surface chemistry and to heterogeneous catalysis* State College, Pennsyl­ vania, The Pennsylvania State College. 1951* Harwood, J. and Davies, W. C. Activated alumina. Chem Age (London) 50: 223-228. 19#.. Russell, A. S. Alumina properties. Technical Paper No. 10. Pittsburgh, Pennsylvania, The Aluminum Company of America. 1953» Ervin, G. and Osborn, E. P. The system aluminum-water. J. Geol. 59: 381-394. 1951. Silverman, A. Data on chemicals for ceramic use. Washington, D. C., Natl. Research Council, Natl. Academy of Science. 1949. Pauling, L. and Hendricks, S. 5. Crystal structures of hematite and corundum. J. Am. Chem. Soc# 4?5 781-790. 1925. Tamnann, G> and Mansuri, Q. A. Zur rekristallisation von Hetallen and Salzen. Z. anorg. u. allgem. Chem. 126: 119-123. 1923. Geller, R. P. and Yavorsky, P. J. Melting point of alpha-alumina. J. Res. Natl. Bur. Stds. 34! 395. 1945. 106

13• Brewer, L. and Searcy, A. W. The gaseous species of the aluminum-alumina system. J*. Am. Chem. Soc. 73: 5308-5314. 1951. 11*.. Wartenberg, H. von. Zur kenntnis der Toner de. Z. anorg. u. allgem. Chem. 269: 76-85. 1952. 15. Taylor, #. W. Reactions between solids in the absence of a liquid phase. J. Am. Ceram. Soc. 17: 155- 163. 1934. 16. Dawihl, W. Researches in high temperatures. Tonind. Z. 58: (37) 449-451; (39) 463-465; (39) 477-478; (40) 485-487. 1934* (Original not available for examination, abstracted in Chem. Abs. 28: 7454* 1934») 17. Ernst, Th. and Wartenberg, H. von. Grain arrangement in sintered alumina. Z. angew. Mineral. 1: 115- 124. 1938. (Original not available for examina­ tion, abstracted in Chem. Abs. 33: 3226. 1939• ) 18. Ryschkewitsch, E. Ceramic materials of pure oxides. Chem. and Ind. 57: 965. 1938. 19. Bangham, D. H. Physical processes involved in sintering. J. Soc. Glass Tech. 31: 264-266. 1947. 20. Banmann, H. H., Jr. and Benne r, R. C. Aluminous material. United States Patent 2,360,841. October 24» 1944* 21» Frenkel, Ta. Surface motion of particles in crystals and natural roughness of crystalline faces. (In Russian, English summary.) J. Phys. (U.S.S.R.) 9: 385-391. 1945. (Abstracted in Chem. Abs. 4O: 3321. 1946.) 22. Clark, P. ¥. and White, J. Some aspects of sintering. Trans. Brit. Ceram. Soc. 49: 305-333» 1950.

23. Clark, P. ¥., Cannon, J. H. and White, J. Further in­ vestigations on the sintering of oxides. Trans. Brit. Ceram. Soc. 52: 1-49» 1953. 24. Mackenzie, J. K. and Shnttleworth, R. A phenomenologi- cal theory of sintering. Proc. Phys. Soc. 62: 833-852. 1949. 107

25. Murray, P., Rodgers, E. P. and Williams, A* E. Practical and theoretical aspects of the hot pressing of refractory oxides. Atomic Energy Research Establishment TS/B. 893. Harwell, Berk­ shire, England* 1952. 26. Marshall, D* À., Enright, D. P. and Weyl, W. A. On the mechanism of sintering and the reczystallization of oxides. Int. Symposium Reactivity of Solids Proceedings. 1: 273. 1954. 27. Hedvall, J. A. Methods of regulating the surface activity and their principal importance for powder metallurgy and powder ceramics. Arkiv. Kemi 6: 241-250. 1953. 28. Smothers, W« J. and Reynolds, H. J. Sintering and grain growth in alumina. J. Am. Ceram. Soc. 37: 588-595. 1954. 29« Cahoon, H. P. and Christensen, C. J. Effect of tempera­ ture and additives on the creep properties and re- crystallization of aluminum oxide. Tech. Report No. XLII. Salt Lake City, Utah. Institute for the Study of Rate Processes, University of Utah. 1955. 30. Walker, R. P. Mechanism of material transport ch.ring sintering. J. Am. Ceram. Soc* 38: 187-197. 1955. 31* Weyl, ¥. A. Atomic interpretation of the mechanism of solid-state reactions and of sintering* Ceram* Age 60; (5) 28-38. 1952. 32. Schwarzkopf, P. Sintering of refractory powders. Am. Ceram. Soc. Bui. 34: 45-47. 1955. 33» Allison, E. B. and Murray, P. Fundamental investigation of sintering phenomena. Atomic Energy Research Establishment Vi/B. 1099. Harwell, Berkshire, England. 1953. 34» Burke, J. E. Recryst alliz at ion and sintering in ceramics. In Kingery, W. D., ed. Ceramic fabrica­ tion processes, pp. 120-131. Cambridge, Massa­ chusetts, Technology Press of Massachusetts Institute of Technology. 1958. 35. Wilder, D. R. and Fitzsimmons, E. S. Further study of sintering phenomena. J. Am. Ceram. Soc. 38: 66- 71. 1955. 108 36. Herring, G* Effect of changes of scale on sintering phenomena. J. Applied Phys. 21: 301-303. 1950. 37» Wilder, D» R. Sintering mechanism as applied to refrac­ tory oxides, a bibliography, 1937-1952. Supplement I. 1952-1955. TJ. S. Atomic Energy Commission Report, JSC-288. Iowa State College. 1956. Office of Technical Services, Washington 25, D. C.

38. Wilder, D. R. Sintering of metallic and nonmetallic refractory materials. In Campbell, I. E., ed. High temperature technology, pp. 235-250. New York, John Wiley and Sons, Inc. 1956. 39. Huttig, G-. F. Die Frittungsvorgange innerhalb von Pulvern, welche aus einer einzigen Komponente bestehen—Bin Beitrag zur Aufklarung der Prozesse der Met all-Keramik und Oxyd-Keramik. Kolloid-Z. 98: 263-286. 1942. U-0. Kingston, W. E. and Huttig, G. F. The physics of powder metallurgy. New York, McGraw-Hill Book Co. 1951. 41. 0*Connor, D. J., Street, N. and Buchanan, A. S. Charge densities and surface conductances at solid- solution interfaces. Austral. J. Chem. 7: 245- 255. 1954. 42. Buchanan, A. S. and Hermann, E. The electrokinetic potential of barium sulfate. Proc. Royal Soc. (London) A195: 150-162. 1948. 43. 0! Connor, D. J., Johansen, P. G. and Buchanar», A. S. Electrokinetic properties of surface reaction of corundum. Trans. Faraday Soc. 52: 231-236. 1956. 44. Kipling, J. J. and Peakall, D. B. Adsorption from bi­ nary liquid mixtures on activated alumina. J. Chem. Soc. (London) 944: 4828-4840. 1956. 45« Kipling, J. J. and Peakall, D. B. Reversible and irre­ versible adsorption of vapours by solid oxides and hydrated oxides. J. Chem. Soc. (London) 157: 834- 842. 1957. 46. Cooper, H. C. Modern abrasives. J. Chem. Ed. 19: 122-124. 1942. 47. Upper, A. L. The manufacture of abrasives. J. Chem. Ed. 26: 676. 1949. 109

Perry, J. E., ed. Chemical engineer*s handbook. 3rd ed. Hew York, McGraw-Hill Book Company, Inc. 1950. Shreve, R. N. The chemical process industries. 2nd ed. New York, McGraw-Hill Book Company, Inc. 1956. Frary, F. C. Alumina problem in peace and war. Chem. Bngr. News 23: (15) 1324-1327. 1945. Gould, B. F* Alumina from low grade bauxite. Ind. Engr. Chem. 37: 796-802. 1945. Frary, F. C. Adventures with alumina. Tad. Engr. Chem. 38: 129-31. 1946. Partridge, J. H. and Lait, J. R. Manufacture of refrac­ tory articles from pure oxides of high melting point. J. Soc. Glass Tech. 20: 200-217. 1936. Kirby, D. Pure oxide refractories. Metallurgia. 30: 65-69. 1944. Hauth, W. E., Jr. Slip casting of aluminum oxide. J. Am. Ceram. Soc. 32: 394-399» 1949» "Kllder, D. R. and Fitzsimmons, E. S. Evaluation of a short column water elutriator. Am. Ceram. Soc. Bui. 34: 109-114. 1955» Clemmer, J. B. and Cogbi.ll, W. E. Improved laboratory elutriator and its application to ores. Engr. Mining J. 129: 551-554» 1930. Green, H. Microchemistry of white pigments and inerts as they occur mixed in paints. Ind. Engr. Chem. 16: 677-680. 1924. Her dan, G> and Snith, M. C. Snail particle statistics- Statistical methods for the investigation of finely divided materials. Houston, Texas, The Elsevier Press, Inc. 1953» Bugge, P. E. and Kerlogue, R. E. The determination of the surface area of powders by means of low temperature adsorption isotherms. J. Soc. Chem. Ind. (London) 66: 377-381. 1947. 110

61. Brunauer, S», Emmett, P. H. and Teller, E. Adsorption of gases in multimolecular layers• J. Am. Chem. Soc. 60: 309-319. 1938. 62. Parkas, A* and Melville, H, W. Experimental methods in gas reactions. Few York, Macmillan Company. 1939.

63. Dalla Valle, J. M. Micromeritics-technology of fine particles* 2nd ed. New York, Pitman Publishing Corporation. 1948. 64. Stimson, M. M. and 0'Donne 11, M. J. Infrared and ultraviolet absorption spectra of eytosine and iso- cy to sine in the solid state. J. Am. Chem. Soc. 74: 1805-1808. 1952. 65* Wilson, E. B. Jr., Decius, J. C. and Cross, P. C* Molecular vibrations ; the theory of infrared and Raman vibrational spectra. New York, McGraw-Hill Book Company* 1955* 66. Hyatt, E. P*, Cutler, I. B. and Wadsworth, M. E* An apparatus for thermogravimetrie analyses. Am. Ceram. Soc. Bui. 35: l80-l8l* 1956. 67. Duval, C. Inorganic thermogravimetric analysis. Houston, Texas, Elsevier Press, Inc. 1953. X. ACOO|££DŒ1ENTS

The author wishes to express appreciation to Professor

G* M* Dodd for his cooperation and helpful criticisms and to Dr* M* Siautz for his encouragement during the preparation of this thesis.