UBC Theses and Dissertations

UBC Theses Logo

UBC Theses and Dissertations

Reactive hot pressing of boehmite Bradbeer, Ross Stanley 1972

Your browser doesn't seem to have a PDF viewer, please download the PDF to view this item.

Item Metadata

Download

Media
831-UBC_1972_A6_7 B73.pdf [ 4.31MB ]
Metadata
JSON: 831-1.0078723.json
JSON-LD: 831-1.0078723-ld.json
RDF/XML (Pretty): 831-1.0078723-rdf.xml
RDF/JSON: 831-1.0078723-rdf.json
Turtle: 831-1.0078723-turtle.txt
N-Triples: 831-1.0078723-rdf-ntriples.txt
Original Record: 831-1.0078723-source.json
Full Text
831-1.0078723-fulltext.txt
Citation
831-1.0078723.ris

Full Text

REACTIVE HOT PRESSING OF BOEHMITE ( a A l ^ - H ^ O ) BY ROSS STANLEY BRADBEER B . S c , U n i v e r s i t y o f B r i t i s h C o l u m b i a , 1965 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n the Department of METALLURGY We a c c e p t t h i s t h e s i s as c o n f o r m i n g t o t h e r e q u i r e d s t a n d a r d THE UNIVERSITY OF BRITISH COLUMBIA January, 1972 In present ing t h i s thes is in p a r t i a l f u l f i l m e n t of the requirements for an advanced degree at the Un ive rs i t y of B r i t i s h Columbia, I agree that the L ib ra ry s h a l l make i t f r e e l y a v a i l a b l e for reference and study. I fu r ther agree that permission for extensive copying o f t h i s thes is for s c h o l a r l y purposes may be granted by the Head of my Department or by h is representa t ives . It is understood that copying or p u b l i c a t i o n o f th is thes is f o r f i n a n c i a l gain sha l l not be allowed without my wr i t ten permiss ion . Department of Metallurgy The Un ivers i ty of B r i t i s h Columbia Vancouver 8, Canada Date February 9, 1972 ABSTRACT The compaction behaviour of boehmite has been studied under isothermal conditions,with s p e c i a l emphasis devoted to hot pressing i n the temperature range 300 to 600PC. The present work indicates that i t i s possible to produce a hard dense compact under ce r t a i n conditions. However, the behaviour of the material during reactive hot pressing appears to be more complicated than can be explained by simple s i n t e r i n g or k i n e t i c theories. To aid i n understanding the mechanisms of compaction during a phase transformation, the behaviour of the system during reactive hot pressing was studied from a purely phenomenological point of view, a v i s c o e l a s t i c model. By using v i s c o e l a s t i c theory i t i s possible to r e l a t e i d e a l e l a s t i c and energy-absorbing or damping viscous parameters to the behaviour of boehmite during R.H.P. While the apparent compact density v a r i e d as a complex function of temperature, i t was found that the o v e r a l l compaction behaviour of boehmite could be adequately described by a second order l i n e a r d i f f e r e n t i a l equation, which i n turn could be re l a t e d to a combination of e l a s t i c (displacement sensitive) and viscous ( s t r a i n rate sensitive) components. The viscous nature of the powder during R.H.P. reached a maximum value j u s t before the boehmite to gamma t r a n s i t i o n (380 to 443°C), suggesting that strong p a r t i c l e i n t e r a c t i o n was occurring. It i s anticipated that R.H.P. from 380 to 443°C w i l l lead to the most favourable p a r t i c l e rearrangement for producing a hard gamma phase at 500°C. - i i i -In producing the "hard phase" i t was important to maintain a c r i t i c a l water concentration. Approximately 4% retained water was necessary for forming a hard dense compact. On the other hand in the presence of an excessive water vapor pressure, the unreacted boehmite powder appeared to transform directly to alpha alumina, resulting in a friable compact. Thus the need for maintaining the correct vapor pressure during R.H.P. i s essential. Production of the "hard phase" material at 500°C appears promising as an intermediate step in producing strong translucent bodies upon subsequent sintering at 1000°C. - i v -ACKNOWLEDGEMENTS The author wishes to express his gratitude f o r the advice and encouragement of his research supervisor, Dr. A.CD. Chaklader. Thanks are also extended to f a c u l t y members, fellow graduate students, and t e c h n i c a l s t a f f , e s p e c i a l l y Professor R.G. Butters f o r h e l p f u l discussions and assistance. F i n a n c i a l assistance from the Defence Research Board, grant #7565-07 i s g r a t e f u l l y acknowledged. - v -TABLE OF CONTENTS Page 1. INTRODUCTION 1 1.1 Alumina ^ 3 1.2 Previous Work 1.3 Objectives of the Present Study 4 2. EXPERIMENTAL 8 2.1 M a t e r i a l 8 2.2 Reactive Hot P r e s s i n g of Boehmite (R.H.P.) 9 2.3 Water Loss Versus Hardness 13 2.4 E l e c t r o n Microscope Studies 13 2.5 X-Ray Studies 15 3. RESULTS 16 3.1 Reactive Hot Pressing-Graphite Sleeve Versus "Canning" 1 6 3.2 Water Loss Versus Hardness 17 3.3 X-Ray A n a l y s i s 2 0 3.4 Thermogravimetric A n a l y s i s (T.G.A.) 32 3.5 D.T.A 3 2 3.5.1 D i s c u s s i o n of Figure 12 37 3.6 Isothermal Compaction Curves f o r Boehmite 37 3.7 The Experimental Determination of the Constants K, a, A, B, and B 39 4. DISCUSSION 4 2 4.1 Formation of the Hard Phase M a t e r i a l 52 4.2 The Dynamic System 55 - v i -Page 4.3 The M e c h a n i c a l A n a l o g 60 4.4 The V i s c o e l a s t i c Model f o r R.H.P. o f Boehmite 62 4.5 C a l c u l a t i o n o f Compaction Curves f o r D i f f e r e n t P r e s s u r e s a t C o n s t a n t Temperature 69 4.6 C a l c u l a t i o n o f n-^j M ^> 2^ a n c * M 2 ^ 4.6.1 Stage (a) - R.T. t o 275°C 75 4.6.2 Stage (b) - 275°C t o 443°C 75 ( i ) The v i s c o u s r e g i o n 380°C t o 443°C 75 ( i i ) The v i s c o u s r e g i o n and " s t e a d y s t a t e " . . 80 4.6.3 Stage (c) - T r a n s f o r m a t i o n R e g i o n 443°C t o 520°C 81 4.7 The E f f e c t o f P r e s s u r e on R.H.P. o f Boehmite 81 4.8 The V i s c o e l a s t i c M odel and A p p a r e n t E n d - P o i n t D e n s i t y 83 CONCLUSION 87 APPENDICES A The Compaction E q u a t i o n when a„ ,- ^  a. , . , 89 r ^ Ref A p p l i e d B Comparison of the Two Methods 90 BIBLIOGRAPHY 9 5 - v i i -- v i i i -Figure Page 15 (a) Run 33 - Compaction curve f o r boehmite at 270°C and 5860 p s i 44 (b) Run 30 - Compaction curve f o r boehmite at 290°C and 5860 p s i 45 (c) Run 22 - Compaction curve f o r boehmite at 400°C and 5860 p s i 46 (d) Run 26 - Compaction curve for boehmite at 420°C and 5860 p s i 47 (e) Run 29 - Compaction curve f o r boehmite at 529°C and 5860 p s i 48 (f) Run 15 - Compaction curve f o r boehmite at 540°C and 5860 p s i 49 (g) Run 28 - Compaction curve f o r boehmite at 628°C and 5860 p s i 50 (h) Run 23 - Compaction curve f o r boehmite at 622°C and 5860 p s i 51 16 A schematic representation of the behaviour of boehmite during R.H.P 54 17 The "system dynamics" r e l a t i n g output (strain) to input (stress) 57 18 Defining the spring (a s t r a i n s e n s i t i v e device) and dashpot (a s t r a i n - r a t e s e n s i t i v e device) 61 19 E l e c t r i c a l analog 64 20 E l e c t r i c a l to mechanical analog 66 21 The mechanical analog f o r R.H.P. of boehmite 68 22 Mechanical parameters M^, K^, r\^, r^* a n < ^ K versus temperature 74 23 Voigt or Ke l v i n v i s c o e l a s t i c element 72 24 Comparison of the D.T.A. curve and the viscous component ^6 25 D.T.A. for boehmite 77 26 T.G.A. for boehmite 78 27 Compaction curve f o r boehmite at 513°C and 9170 p s i . 84 28 A comparison of apparent "end-point" density to t o t a l e l a s t i c constant M 85 - ix -LIST OF TABLES Table Page I Hardness versus retained water for the indicated locations on specimen I 18 II Correction for CuK^ doublet using Figure 9 25 III Correction for instrumental broadening using Figure 8 30 IV Analysis of T.G.A. data 35 V Conversion from electrical to mechanical analog .... 61 VI Summarized values of coefficients a. and K. for each powder 71 VII Summary of viscoelastic components 73 VIII Average values for mechanical parameters at 513°C... 82 - 1 -1. INTRODUCTION 1.1 Alumina Alumina is the most commonly used oxide ceramic. This is not surprising in view of the fact compounds of aluminum are abundant on the Earth's crust. Because of i t s availability, high melting temperature (2035°C) and chemical inertness, alumina has been used in an ever increasing number of products. In addition to i t s conventional usage such as in refractories, crucibles, furnace tubes etc., i t has been extensively used in a great number of electronic applications. The ab i l i t y to retain high 14 electrical resistance (10 ohm-cm at room temperature) at increasingly high temperatures and voltages, i t s excellent dielectric strength, and low dielectric loss, have made alumina a very good insulator. Among the new ceramic insulators made of alumina are spacers used in vacuum tubes, substrate bases for thin film integrated and hybrid circuits. Because of i t s wide and diversified applications a large number of fabrication techniques are employed to shape and process alumina products. Although cold pressing and subsequent sintering at tempera-tures above 1600°C is the most conventional; s l i p casting, extrusion, and rolling using an organic binder, are also extensively used for specialized products. - 2 -These t r a d i t i o n a l methods o f f o r m i n g s u f f e r from t h e l i m i t a t i o n t h a t t h e p a r t s must be d r i e d f o r l o n g p e r i o d s b e f o r e f i n a l s i n t e r i n g . F u r t h e r m o r e , d u r i n g d r y i n g and f i r i n g , up t o 25-30% volume s h r i n k a g e w i l l o c c u r w h i c h u n l e s s c a r e f u l l y c o n t r o l l e d may p r o d u c e warpage and c r a c k i n g i n t h e p r o d u c t s . O t h e r t e c h n i q u e s w h i c h have been u s e d f o r f a b r i c a t i n g a l u m i n a p r o d u c t s d u r i n g f i n a l f i r i n g a r e i s o s t a t i c p r e s s i n g , h o t p r e s s i n g , h o t e x t r u s i o n and h o t - f o r g i n g ( 1 ) . However, none of t h e s e h i g h t e m p e r a t u r e p r o c e s s e s have been e x t e n s i v e l y used c o m m e r c i a l l y because o f the l i m i t a t i o n s o f d i e m a t e r i a l s , and h i g h t e m p e r a t u r e s i n v o l v e d i n t h e s e p r o c e s s e s . On t h e o t h e r hand, t h e s e h i g h t e m p e r a t u r e p r o c e s s e s have th e c a p a b i l i t y o f f a b r i c a t i n g v e r y h i g h d e n s i t y p r o d u c t s w i t h c l o s e d i m e n s i o n a l t o l e r a n c e . G e n e r a l l y , t h e s e a r e r e l a t i v e l y e x p e n s i v e i t e m s . R e a c t i v e h o t p r e s s i n g i s a low t e m p e r a t u r e h o t p r e s s i n g p r o c e s s , w h i c h has r e c e n t l y been i n v e s t i g a t e d by s e v e r a l w o r k e r s (2) and w h i c h has t h e p o t e n t i a l o f becoming a c o m m e r c i a l p r o c e s s . R e a c t i v e h o t p r e s s i n g i s e s s e n t i a l l y a h o t p r e s s i n g t e c h n i q u e w h i c h i s c a r r i e d o u t i n c o n j u n c t i o n w i t h e i t h e r a d e c o m p o s i t i o n o r a p o l y m o r p h i c r e a c t i o n . Hot p r e s s i n g o f a number o f o x i d e s c o m p o s i t i o n s has been g i v e n p a r t i c u l a r a t t e n t i o n i n r e c e n t y e a r s w i t h r e s p e c t t o a c h i e v i n g dense f i n e g r a i n e d m i c r o s t r u c t u r e . W i t h t h e t r e n d towards f i n e r - g r a i n c e r a m i c s , n o t o n l y have new u l t r a f i n e m a t e r i a l s been d e v e l o p e d b u t e f f o r t s have been d i r e c t e d towards s t a r t i n g w i t h u l t r a f i n e m a t e r i a l s w h i c h have a d i f f e r e n t c r y s t a l s t r u c t u r e from t h a t o f t h e f i n a l h o t - p r e s s e d p r o d u c t . T h i s i s one o f t h e main r e a s o n s f o r i n t e r e s t i n r e a c t i v e h o t p r e s s i n g f o r f a b r i c a t i n g c e r a m i c p r o d u c t s . I t has been s u g g e s t e d t h a t the - 3 -ap p l i c a t i o n of pressure during a phase transformation or a decomposition reaction can produce considerable i n t e r p a r t i c l e bonding, r e s u l t i n g i n the formation of a strong and dense body. In producing translucent dense alumina products f o r use i n high pressure sodium vapor lamps, Bates (3) used boehmite as the precursor material and was able to produce f u l l y dense translucent alumina by f i r s t p r e f i r i n g the material at 400°C then s i n t e r i n g at 1725°C f o r f i v e hours. Bates used three d i f f e r e n t types of boehmite, boehmite "A", which has small c r y s t a l l i t e o s i z e of 100 A; boehmite "B", f i b r i l l a r m aterial of p a r t i c l e dimension o o 50 A by 1000 A; and boehmite "C", which had large grain s i z e (1-5 y ). In contrast to boehmite "A" and "B", boehmite "C" decomposed to alpha alumina d i r e c t l y at 500°C. On the other hand, dense translucent gamma alumina can be produced by reactive hot pressing f i b r i l l a r boehmite at 500°C under 8000 p s i . 1.2 Previous Work 22 MacKenzie was the f i r s t worker to hot press boehmite i n the temperature range 300 to 600°C. He observed enhanced compaction when boehmite was reactive hot pressed during the decomposition reaction (boehmite to gamma alumina) at 500°C. Cook and Chaklader (4) suggested that the degree of compaction should be proportional to the weight loss due to dehydroxylation. However, t h e i r r e s u l t s were inconclusive, i n d i c a t i n g that the true mechanism or mechanisms of compaction could not be determined s o l e l y by weight loss studies. To explain d e n s i f i c a t i o n i n the presence of an applied s t r e s s , they sug-gested that large scale p a r t i c l e rearrangement must e x i s t . They concluded - 4 -that fragmentation during the dehydroxylation process may significantly affect the particle flow. Likewise, equally important mechanisms would be (i) particle sliding and ( i i ) elastic and plastic deformation. St-Jacques (5) investigated under vacuum, the flow behaviour of cold compacted cylindrical specimens of f i b r i l l a r colloidal boehmite by compression creep tests. He suggested that the creep rate was proportional to the applied stress between 350 and 550°C and 31 to 265 psi. By assuming that the same packing geometry was retained during the reaction and that shrinkage was due only to neck growth, St-Jacques derived a simple relationship to express the change of length of the compact with time. The equation had the form j^- = K t n ^ where i s L r i L O 0 the strain, "K" and "n" are constants and"t"is the time. A log-log plot of his data produced a straight line only over a limited interval of time. From the slope, "n" was found to be approximately equal to 5, which according to sintering theory indicated that neck growth was due to the volume diffusion mechanism. One disadvantage of St-Jacques simple approach was that "K" was found to be a function of temperature and pressure. This alone would suggest the problem was far more complex than indicated by his analysis. Moreover, the conclusion that "sintering is due to a volume diffusion mechanism" is not jus t i f i e d and requires further study for confirmation. 1.3 Objectives of the Present Study The objective of this work is to extend the study of reactive hot pressing of boehmite, with special emphasis on hot pressing in the temperature range 300°to 600°C. It has been indicated that the - 5 -behaviour of boehmite during reactive hot pressing is much more complicated than that suggested by previous workers. For this reason, no effort has been made to use sintering or kinetic theories to study the compaction behaviour of boehmite. Instead a more empirical approach, a viscoelastic model" was used to interpret the data. This approach allows one to express the behaviour of boehmite during R.H.P. in terms of ideal elastic and viscous components. By plotting these components as a function of temperature i t should be possible to correlate the behaviour of boehmite during R.H.P. to data obtained from X-ray, T.G.A., D.T.A. and electron microscope analyses. Since a phase transformation is involved during reactive hot pressing operations, i t w i l l be informative to include the transformation sequences for hydrates of alumina, which on heating in air convert to alpha alumina directly or indirectly. This i s shown in Figure 1. i r i { j 1 1 Gibbsite Boehmite g Bayerite Diaspore Chi Eta T T Gamma Kappa Delta Theta Alpha Alumina Alpha Theta Alpha Alpha 400 600 800 1000 Figure 1. Dehydration sequence of alumina hydrates in air. Note: Path"b"is favored by moisture, alkalinity, and coarse particle size (100 micron); path "a" by fine crystal size (below 10 microns) - 6 -The above sequences of t r a n s i t i o n s (Figure 1), i s mainly due to Strumph et a l . and Terian and Papee (6), and i s generally accepted, although there i s controversy about the X-ray i d e n t i f i c a t i o n of some phases and the existence of others. The sequences are affected not only by the s t a r t i n g materials but also by t h e i r degree of c r y s t a l l i n i t y , heating rates, and impurities. Two important t r a n s i t i o n s , relevant to t h i s study are the boehmite t r a n s i t i o n and the diaspore t r a n s i t i o n . Boehmite t r a n s i t i o n (aA^O^.ELjO) 2 Coarse hydrothermal boehmite (surface area less than 15 m /g) prepared from gelatinous aluminum hydroxide, g i b b s i t e , bayerite or higher t r a n s i t i o n phases,by digestion i n H^O at temperatures above 150°C,transforms to gamma (360 to 860°C), to d e l t a , to theta, and f i n a l l y to alpha alumina, on heating progressively at higher and higher temperatures. F i b r i l l a r c o l l o i d a l boehmite on the other hand,transforms to gamma at 500°C,to theta at 1000°C, and to alpha alumina at 1200°C. Diaspore t r a n s i t i o n (BA^O^.H^O) Deflandre (7) i n 1932 showed that diaspore transforms d i r e c t l y to corundum (alpha alumina) by an ordering process and without intermediate products, at about 450 to 600°C. For t h i s reason, when reactive hot pressing boehmite the following factors must be considered. - 7 -(i) Does the precursor material undergo one or more phase transformations in the temperature range used for R.H.P.? ( i i ) What i s the nature of the transformation involved? Does i t depend upon the presence of water etc? If so, what i s the effect of vapor pressure? The vapor pressure could increase or decrease the phase transformation temperature (Clapeyron equation). Secondly, a precursor material that undergoes a transformation sequence, for example, a -> b + c as the temperature is increased, could in the presence of a high pressure environment, transform directly from phase "a" to phase "c" • Under these conditions phase "b" would not appear. - 8 -2. EXPERIMENTAL 2.1 Ma t e r i a l The material used i n t h i s study was c o l l o i d a l boehmite supplied by E.I. du Pont De Nemours and company under i t s trademark name of "Baymal". The c h a r a c t e r i s t i c s of the powder are described by I l e r (8) as having A100H, 83.1%; p h y s i c a l l y adsorbed water, 1.8%; chemically bound water, 3.3%; sulphate as SO^ , 1.75%. The true density of the material i s 2.28 grams per cc. The p a r t i c l e s are f i b r i l l a r , being O 0 about 50 A i n diameter and 1000 to 2000 A long. Boehmite on heating from room temperature to 1200°C undergoes two d i s t i n c t and w e l l known phase transformations, boehmite to gamma alumina at 480°C and gamma to alpha alumina at 1200°C. I t i s possible that the l a t t e r t r a n s i t i o n temperature may be affected by the presence of water vapor pressure i n the system. The dehydroxylation of boehmite to gammaalumina involves only a minor change i n the o v e r a l l structure. Boehmite i s believed to be of orthorhombic symmetry (9). The structure consists of oxygen ion layers within which the oxygen ions are cubic packed, while hydroxy1 ions are situated between these layers to form zig-zag chains. Molecular water i s not present (10) 2 i n boehmite, however, because of i t s large surface area (250 m /gm) i t tends to pick up a measurable amount of water over the formula A1„0„.H 90 as p h y s i c a l l y adsorbed water. - 9 -Gamma alumina has a cubic structure. I t has been concluded (11) that the dehydroxylation scheme of boehmite i s based on the cubic oxygen layer sequence ABC,ABC..., which i s found i n gamma alumina. Although the formula for gamma alumina i s usually written as •y-Al^O^, Glemser and Rieck observed that a l l gamma alumina has water strongly bound as hydroxyl ions, i . e . , y-Al^O^.-xR^^t w n e r e x << 1. 2.2 Reactive Hot Pressing of Boehmite (R.H.P.) For a de t a i l e d study of the hot pressing c h a r a c t e r i s t i c s of boehmite three d i f f e r e n t techniques were used: (a) Reactive hot pressing of boehmite was car r i e d out isothermally at d i f f e r e n t temperatures, i n each case the pressure was maintained constant at 5860 p s i . A s t e e l die was used i n which only one ram was moveable. The clearance between the rams and die w a l l was s u f f i c i e n t to allow the gas phase produced during decomposition to escape. For heating,a P h i l i p s 12 KW induction unit generator was used. The die i t s e l f acted as the susceptor. I d e a l l y one would l i k e to s t a r t with a loose powder. However, t h i s was i m p r a c t i c a l f o r the following reasons. Loosely packed boehmite powder compacts reduce i n the r a t i o 8:1 when cold pressed to a green density of approximately 1.5 gm/cc (0.60 r e l a t i v e density). Thus to produce a 0.5 inch compact approximately 4.0 inches ram displacement would be required. Although i t i s possible to measure accurately such a large displacement, due to the f i n e p a r t i c l e s i z e of boehmite> the loose powder tends to jam between the ram and die w a l l , r e s u l t i n g i n exceedingly high ram-die f r i c t i o n . To avoid t h i s problem weighed amounts of boehmite powder were precompacted at 2000 p s i . The ram was - 10 -was then removed and any excess powder trapped between the die w a l l and rams.was scrapped o f f . The die assembly containing the powder was then placed i n the induction furnace, and further pressed at 5860 p s i . Aft e r mechanical equilibrium had been attained the pressure was lowered to 600 p s i . The powder was then heated to the desired temperature. Aft e r waiting ten to f i f t e e n minutes, to ensure temperature equilibrium was established, the d i a l guage reading was noted. This value being recorded as the i n i t i a l compact length "L ". The pressure was then increased to 5860 p s i and the change i n compaction was recorded as a function of tinie. The pressure was.then released, the specimen a f t e r being cooled to room temperature, was removed from the die. The f i n a l dimensions were then recorded from which the "end point density" could be calculated. Figure 2 shows a diagram of the apparatus and a schematic representation of a t y p i c a l R.H.P. cycle. This configuration was used only to study the compaction behaviour of boehmite. (b) In a second set of experiments a s t e e l die with f l o a t i n g rams was used (Figure 3-A). To f a c i l i t a t e easy removal of the compact a f t e r R.H.P. a graphite sleeve was inserted between the rams and die. Graphite discs were placed between the compact and the rams. The presence of the graphite sleeve, besides f a c i l i t a t i n g easy removal of the compact a f t e r R.H.P. i n h i b i t e d or r e s t r i c t e d the escape of water vapor. More important, i t appeared that a hard c r y s t a l l i n e compact could be produced at temperatures less than 700°C only when a graphite sleeve was used. Figure 4 shows a compact which was R.H.P. at 700°C and 8000 p s i . However, the use of a graphite sleeve had two disadvantages; (i) I t produced non-uniform specimens. Usually only part of the compact - 11 -(thermocouple) (fixed ram) (strain guage) (steel die) (powder compact) (moveable piston) green compact 4 6 0 0 psi 0 psi Figure 2, (expansion during heating . 0 0 5 " ) N elastic spring back .010 compaction profile 600 psi measured compaction (cooling fo) "L — (heating to temperature) applied pressure Diagram of compaction apparatus and a schematic r e p r e s e n t a t i o n of a t y p i c a l R.H.P. c y c l e . - 12 -A . B Figure 3 . A - R.H.P. using a graphite sleeve B - C o n f i g u r a t i o n used f o r "canning". The d i e assembly was heated by i n d u c t i o n heating. - 13 -was converted to a dense "hard phase",and(iQ R e p r o d u c i b i l i t y of the r e s u l t s was generally poor. In many cases the "hard phase" material could not be reproduced, even under the same experimental conditions. (c) In an attempt to obtain a more uniform product, precompacted boehmite p e l l e t s (50% green density) were completely encapsulated or canned. The configuration for canning i s shown i n Figure (3-B). R.H.P. experiments for both the canning operation and hot pressing using a graphite sleeve were repeated for temperatures ranging between room temperature and 700°C and pressures ranging from 4fJ00 to 20,000 p s i . 2.3 Water Loss Versus Hardness To determine i f there may be a c o r r e l a t i o n between the hardness of the material obtained during R.H.P. and retained water, water loss versus hardness experiments were performed. F i r s t , the hardness at indicated locations on specimen 1.(see Figure 4), was measured using a Tukon Tester (136 DPH) microhardness machine. Then samples ( 5 mg) were scraped from the corresponding locations and placed i n a Dupont 950 Thermogravimetric Analyzer. The change i n weight due to water loss was then measured upon heating each sample to 600°C. 2.4 Electron Microscope Studies To observe how boehmite behaves during heat treatment, Dupont Baymal C o l l o i d a l alumina (boehmite) was f i r s t heated to 600°C for 24 hours,then placed i n a Hi t a c h i HU-11A electron microscope. Both electron transmission and d i f f r a c t i o n photographs were taken. The very f i n e p a r t i c l e s were mounted on a carbon support f i l m by suspending the powder i n alcohol, then the alcohol was allowed to evaporate, leaving "A". Numerals i n d i c a t e l o c a t i o n s from which w a t e r l o s s and hardness measurements were t a k e n . Figure 4. Boehmite compact R.H.P. at 700°C and 8000 psi (using graphite sleeve). Figures A and B show the top and bottom of specimen 1. - 15 -the p a r t i c l e s deposited on the f i l m . The d i f f r a c t i o n patterns were obtained. The camera constant was determined using a gold f i l m . The "d spacing" can be calculated from the powder patterns using , camera constant d _ where "R" i s the distance from the center of the d i f f r a c t i o n pattern to a d i f f r a c t i o n spot or a r i n g . 2.5 X-Ray Studies X-ray studies on powder specimens were c a r r i e d out using a Norelco X-ray diffractometer. When only small quantities of material were a v a i l a b l e , Debye-Scherrer cameras of the Buerger design were used. A l l d i f f r a c t i o n patterns were taken using the copper r a d i a t i o n . In addition to i d e n t i f i c a t i o n of the powder pattern using the A.S.T.M. d i f f r a c t i o n data f i l e , l i n e broadening techniques were used to determine c r y s t a l l i t e s i z e . For t h i s purpose, the method d e t a i l e d i n Klug and Alexander (12) was used. In each case, the d i f f r a c t i o n peak to be studied was scanned at the rate of 1/4° per minute using the maximum time constant. Both the source and receiver s l i t s were 1°. "B" the t o t a l l i n e broadening due to both instrument broadening "b" and c r y s t a l l i t e s i z e were obtained by measuring the angular width at h a l f maximum i n t e n s i t y . Instrument l i n e broadening was determined by using o a s i l i c o n standard f o r which the grain si z e was greater than 1000 A. - 16 -3. RESULTS 3.1 Reactive Hot Pressing Graphite Sleeve Versus "Canning" As previously discussed, i f a graphite sleeve was used during R.H.P., i t was possible to produce a very hard, apparently non-porous phase (apparent density 2.2 gm/cc). The bulk of this phase was generally limited to the outer edges of the specimen (Figure 4). The "hard phase" exhibited the following properties; (i) thin sections of the material were translucent while thicker sections appeared black, ( i i ) the material was b r i t t l e and fractured easily, and ( i i i ) the material showed a pronounced preferential cracking, in that i t contained a large number of vertical cracks whose fracture planes ran parallel to the stress axis. After a number of unsuccessful attempts to produce a uniform product using a graphite sleeve, pellets of boehmite powder were pre-compacted to 50% green density, "canned" and then hot pressed at various temperatures and pressures. The results can be summarized as follows: (i) Due to "canning" too much water was retained, and as a result, the f i n a l compact was a wet friable powder. To eliminate this problem the powder was f i r s t calcined at various temperatures to remove part of the s t r u c t u r a l water. The powder was then "canned" and R.H.P. as before. ( i i ) Generally, because of the c a l c i n i n g operation too much water was l o s t . This resulted i n a very poor f r i a b l e white compact. I t was possible only once to r e t a i n the correct amount of water, the product i n this case was black i n appearance and very b r i t t l e . Unfortunately, i t was also highly fractured due to incomplete compaction which was a res u l t of the "canning" prodedure used during hot pressing. While "canning" looked o p t i m i s t i c , the operation was given up i n favor of other studies. 3.2 Water Loss Versus Hardness It i s apparent from the preceding section that there may be a d e f i n i t e c o r r e l a t i o n between hardness and retained water. Therefore, hardness and water loss studies were made on specimen 1 (Figure 4) at the indicated l o c a t i o n s . The r e s u l t s are summarized i n Table I and Figure 5,which i s a plo t of hardness versus percent retained water. (i) The s o f t white material contained less than 1% retained water (point 18, Figure 4-B). ( i i ) The "hard phase" material contained from 4% to 5% retained water (point 3, Figure 4-A). Although no exact c o r r e l a t i o n between retained water and hardness could be determined, the microhardness of the material increased as one approached a fracture surface. For example, at point 16, Figure 4-B, a fra c t u r e surface i s running from the edge to the center of the specimen. The material at the edges of the fracture surface exhibited the maximum hardness observed (400 DPH). - 18 -Poi n t Average hardness DPH 136° Retained water % X-ray s t r u c t u r e C r y s t a l -l i t e s i z e 1 340 7.2 2 390 3 390 3.6 4 390 5.5 5 360 -6 260 -7 120 8 200 1.4 9 210 _ 10 290 -11 340 -12 420 3.5 14 420 - gamma alumina 0 53 A d i a 15 270 10.7 16 400 17 290 — 18 0 0.7 alpha alumina 1.0 u Edge - - gamma alumina Soft white m a t e r i a l (hardness could not be measured, assumed equal to zer o ) . Table I . Hardness versus r e t a i n e d water f o r the i n d i c a t e d l o c a t i o n s on specimen 1. - 19 -% Retained Water Figure 5 . Hardness versus retained water. - 20 -3.3 X-Ray Analysis From X-ray analyses of specimen 1 (Figure 4) the following r e s u l t s were obtained: Point 18 - the so f t white material was alpha alumina. Point 14 - material taken from point 14 was gamma alumina. S i m i l a r l y samples taken from the edge of specimen 1 were gamma alumina. Because of the method of heating the specimen (induction heating, the die acted as the susceptor and the rams as the heat sink) the outer edges would be hotter than the center region. Thus alpha alumina should be formed near the edges and gamma alumina i n the central region, j u s t opposite to what was observed. In order to explain the anomalous behaviour, further studies were made by heat t r e a t i n g boehmite at d i f f e r e n t temperatures. Boehmite powder was heat treated (one atmosphere) at the following temperatures for 24 hours and the phases were i d e n t i f i e d by X-ray d i f f r a c t i o n techniq The r e s u l t s are as follows: Boehmite heated to 400°C f o r 24 hours boehmite " 500°C " gamma alumina " 600°C " gamma alumina " 750"C " gamma alumina I l e r (8) obtained exactly the same re s u l t s i n a s i m i l a r study with f i b r i l l a r boehmite. The X-ray diffractometer curves of heat treated boehmite as obtained by I l e r are shown i n Figure 6. I t i s noted that at atmospheric pressure boehmite i s not converted to alpha alumina u n t i l 1200°C. A small sample of boehmite which had been heat treated at 600°C (gamma alumina) was examined under the electron microscope. In the - 21 -70 50 30 ( 2 9 ) Figure 6. X-Ray d i f f r a c t i o n curves of aluminas from f i b r i l l a r c o l l o i d a l boehmite. (A) c o l l o i d a l boehmite, (B) gamma (700°C) , (C) gamma (900°C), (D) theta (1000°C), (E) alpha alumina (1200°C). presence of a high vacuum (assuming the heating e f f e c t of the electron beam was n e g l i g i b l e ) the p a r t i c l e s were seen to undergo a morphological change. Electron transmission photographs showed that p a r t i c l e o growth took place (up to 13,000 A diameter) (Figure 7), while d i f f r a c t i o n patterns taken of the larger p a r t i c l e s showed that they were alpha alumina. This suggests that the gamma phase i s less stable at low pressures. The observation of p a r t i c l e growth during the gamma to alpha transformation has been supported by I l e r who found that, the alpha c r y s t a l s once nucleated, grew ra p i d l y to 0.5 to 1.0 u i n s i z e . To determine i f p a r t i c l e growth took place during the formation of the "hard phase" material produced during R.H.P., X-ray l i n e broadening studies were made for - 22 -Photo A Photo B F i g u r e 7. Photo A i s an e l e c t r o n m i c r o s c o p e photo o f h e a t t r e a t e d boehmite (600°C f o r 2 h o u r s ) . X-ray a n a l y s i s of the m a t e r i a l i n d i c a t e d gamma a l u m i n a , however, when o b s e r v e d under the e l e c t r o n m i c r o s c o p e p a r t i c l e growth had a p p a r e n t l y taken place. E l e c t r o n d i f f r a c t i o n a n a l y s i s i n d i c a t e d t h e s t r u c t u r e o f the l a r g e r p a r t i c l e s t o be a l p h a a l u m i n a w h i l e the - m a i l e r n a r t i c l e s gave r i n g p a t t e r n s i n d i c a t i n g one of the t r a n s i t i o n a l u m i n a s . Photo B shows a r e p l i c a o f the f r a c t u r e s u r f a c e o f the h a r d phase m a t e r i a l ( x l O K ) . - 23 -(i ) Bohemlte powder which had been heat treated for two hours at 600°C (gamma alumina), ( i i ) and compared with the "hard phase" material (gamma alumina, point 14, Figure 4), which was formed during R.H.P. For the X-ray diffTactometer (13) used i n these experiments, the f o c a l spot was viewed l a t e r a l l y and the v e r t i c a l divergence was l i m i t e d by using S o l l e r s l i t s . Therefore, Figure 8 can be used to correct X-ray diffractometer l i n e breadth for instrumental broadening (12). Figure 8 i s v a l i d to the extent that the d i f f r a c t i o n l i n e p r o f i l e s 2 2 are of the form 1/(1 + K e ). 1.0 1.0 Figure 8. Curve f o r correcting X-ray spectrometer l i n e breadths f o r instrumental broadening. K doublet broadening can be corrected for by using Jones' method a (Figure 9). In Figure 9 curve "B" was used which can generally be applied to both Debye-Scherrer and X-ray d i f f r a c t i o n patterns when "b" and "B" are defined as h a l f maximum breadth. The following nomencla-ture has been used. - 24 -Figure 9. Curve for correcting line breadths for K doublet broadening. "B" = total line breadth due to instrument and crystallite size "b" = instrumental broadening "3" = breadth of purer diffraction profile "B 11 = total line breadth due to instrumental, crystallite size o and "K" doublet "b " = total instrumental broadening due to "K" doublet. To determine the instrument broadening the si l i c o n line at 20 = 47.34' was used where b Q = 0.28° (see Figures 10a to d). First this line was corrected for "K" doublet broadening. From table (II) A26 = 0.126° = Ad. From Figure 9 using curve B - 25 -^-=0.77 giving b = [0.76][0.28] --0.21 o Comparing Figure 10a, t h i s i s approximately the same value one would obtain by extrapolating along the dashed curve (b = 0.18°). Therefore, b = 0.20° has been used as an average. To study the p a r t i c l e s i z e of o the heat treated boehmite and hard phase material, the d = 1.997 A and o d = 1.395 A l i n e s were used. The correction f or K doublet broadening a i s shown i n Table I I . heat treated boehmite gamma alumina 600°C for 2 hours "hard phase" material formed during R . H . P . 700°C "d" "29" Ad= A29 B o Ad/B 0 B B 0 Ad/B B 1.98 45.86 0.12 6 2.62 0.725 2.60 1.04° 0.115 1.02° 1.395 67.0 0.19° - - - 1 . 8 3 ° 0.104 1.8° Table I I . Correction for doublet using Figure 9. The l i n e can now be corrected for instrument broadening using Figure 8. The r e s u l t s are shown i n Table I I I . - 26 -silicon standard / S i l i c o n Standard 1° source s l i t 1° counter s l i t Time constant = maximum Scanning speed = 1/4° per minute 49 Figure 10a. h.mi.= 0.28° I I 47 46 2 0 The s i l i c o n l i n e at 26 = 47.34°, used to o b t a i n instrument l i n e broadening. 1° source s l i t 1° counter s l i t 1/4° scan per minute Figure 10b. The diffraction peak at 20 = 67° for boehmite powder which has b heat treated at 650°C for 2 hours [gamma alumina]. R e c e i v i n g s l i t 1° Source s l i t 1° Scan r a t e 1/4° per minute Time constant = maximum 4 8 47 4 6 45 4 4 43 2 0 Figure 10c. The d i f f r a c t i o n peak at 29 = 45.3° f o r the hard c r y s t a l l i n e m a t e r i a l from specimen 1. S l i t width 1° Sburce width 1° Scan rate 1/4° per minute hmi = 1.83° 69 68 67 66 65 20 gure lOd. The diffraction peak at 29 = 67° for the hard crystalline material from specimen 1. - 30 -heat treated boehmite gamma alumina 600°C for 2 hours "hard phase" material formed during R.H.P. 700°C "d" B b/B b=0.20 B/B B B b/B B/B B 1.98 2.60 0.77 0.97 2.52 1.02° 0.196 0.90 0.92° 1.395 - - - - 1.8° 0.111 0.98 1.77° Table I I I . Correction for instrumental broadening using Figure 8. The c r y s t a l l i t e s i z e was determined using the Scherrer formula B-cosCe) The value of "K" ranges from 0.70 to 1.70 depending upon a. the c r y s t a l l i t e shape b. p a r t i c u l a r d e f i n i t i o n of (h.m.i. or i n t e g r a l breadth) and c. indices of (h, k, 1) r e f l e c t i o n planes. For t h i s work and the nature of the p a r t i c l e s being studied, K = 0.9, was used. The r e s u l t s of the l i n e broadening c a l c u l a t i o n s are - 31 -For gamma powder [heat treated boehmite - 600°C for 2 hours] n [0.9][1.542][57.3] , Q - * D440 = [2.52]cos(33.7°) = 3 8 (^ameter of fi b e r ) "hard phase" material n = (0.9)(1.542)(57.3) _ ° 400 . (0.92)cos(22.9°) ^ A n (0.9)(1.542)(57.3) ° ... , ... . D./r, = —yi—zr—r , p. = 53 A (diameter of f i b e r ) 440 (1.77)cos(33.7 ) In summary, boehmite powder when heat treated at atmospheric pressure was converted to gamma alumina at approximately 500°C. Once i t was converted to the gamma form, i t did not transform to alpha alumina u n t i l 1200°C. During the boehmite to gamma alumina transforma-t i o n very l i t t l e p a r t i c l e growth, i f any, took place. Excessive p a r t i c l e growth did not occur u n t i l the material was converted to the alpha form. Examination of the small gamma alumina p a r t i c l e s under the electron microscope appeared to induce a morphological change (high vacuum environment), transforming the gamma to much larger alpha p a r t i c l e s . On the other hand, X-ray l i n e broadening studies of the "hard phase" material produced at the outer edges of specimen 1, showed that no p a r t i c l e growth took place. X-ray studies of both the heat treated boehmite powder and the "hard gamma phase" could not explain why the center of the hot pressed compact had converted to alpha alumina at 700°C. This w i l l be discussed i n Section 4.1. - 32 -3.4 Thermogravimetfic Analysis (T.G.A.) For t h i s a n a l y s i s , a Dupont 950 T.G.A. unit was used. The instrument consists of a d e l i c a t e balance positioned i n s i d e a furnace which can be heated at a co n t r o l l e d rate. The device measures the weight loss of an unknown sample as a function of temperature, or under isothermal conditions. The chemical analysis for Baymal boehmite i s A100H 83.1 weight % Acetate 9.8 " Chemical bond water 3.3 " Phys i c a l adsorbed water 1.8 " SO. 3 1.7 " 4 The f r a c t i o n a l weight loss (due to dehydroxylation) f or the conversion of pure boehmite (AlOOH) to gamma alumina i s 0.15 (A^O^'^O -»-^]"2^3 + R^O). Therefore the t o t a l weight loss f o r boehmite and impurities to gamma alumina should be 29% [9.8 + 3.3 + 1.8 4- 1.7 + 12.4 (15% of 83.1)]. The t o t a l measured weight loss can be determined from the T.G.A. (Figure 11) and i s F r a c t i o n a l wt. loss [upon heating to 800°C] = 0.304 Assuming that above 200°C a l l impurities (acetate, b.p. = 118.5°C and S0^ ) have been driven o f f , one can calculate the f r a c t i o n a l weight loss due to water only. Wt. of sample which i s A^O^'XH^O = [ t o t a l wt.]-[wt. of impurities] = 5.40 - [0.529 + 0.0917J = 4.78 mg - 33 -Where Acetate = 9.8% x 5.4 = 0.529 mg SO 4 = 1.7% x 5.4 = 0.0918 mg therefore Wt. loss (due to water only) = 4.78-3.76 4.78 = 0.2113 for the reaction A1„0„.XH 0 — y Al 0 + XH 0 gives X = [102][0.2113] [18][1 - 0.2113] = 1.52 suggesting the formula for Baymal boehmite is A^O^" 1. 5^0. The fractional weight loss for each region a, b, c, and d can be determined from the T.G.A. curve and the corresponding x's can be calculated using the formula The results are summarized in Table IV. Table IV shows that the rate of water loss is greatest during the boehmite to gamma alumina transition at 480°C. It should also be pointed out that during stage "d" 4.0% water is lost. This i s after boehmite has been transformed to gamma alumina. Houber and de Boer (14) have suggested that the crystalline structure of gamma alumina probably contains about 3.4% water. Additional water may react with the surface, thus the surface composition Fractional wt. loss = Al 20 3.yH 20 Figure 11. 1.G.A. curve for boehmite. Region F r a c t i o n a l wt. l o s s ( f ) y v f[102+18y] X 18 Chemical formula d(wt) dT° mg/°C A Ur! i" - 0.0167 1.52 0.120 A l 0 *1.5H 0 -»• A l 0 -1.4H 0 + 3 1 0.1H20 -B ^W1 - ••»» 1.40 0.676 A l 0 -1.4H 0^-ALO • 0.7 H 20 1 J + 0.7 H 20 3.08 x i o - 3 C * ^ - - o » » 0.724 0.4659 A l 0 • 0.7 H 0 + A1.0 • 0.8 H 20 + 0.4 H 20 6.84 x IO" 3 D ^ 1 . .043! 0.258 0.2554 A 1 2 0 3 . 0.3 H 20->A1 20 3 + 0.3H2O 0.625 x 1 0 - 3 Table IV. A n a l y s i s of T.G.A. data. A l 2 0 3 . y H 2 0 -»• A 1 2 0 3 > (y-x)H 20 + xH 20 - 36 -is given by A100H while the internal structure is HAl^Og. While others have suggested that any water associated with gamma alumina must exist on the surface only. 3.5 D.T.A. D.T.A. stands for differential thermal analysis. Differential thermal analysis was used for studying the physical and chemical changes of boehmite during heating. The principle of the device i s simple. Thermocouple "A" i s placed in a sample of material to be analyzed. sample + reference Thermocouple "B" is placed in an inert reference material, which has been selected so that i t w i l l not undergo any thermal transformation over the temperature range being studied. When the temperature of the sample equals the temperature of the reference material, the two thermal couples produce identical voltages (emf), the net voltage output i s zero. However, when the sample goes through a transformation where heat i s evolved or absorbed, the thermocouple "A" either reads higher or lower than thermocouple "B", this results in a net voltage - 37 -differential which i s recorded as a temperature differential. 3.5.1 Discussion of Figure 12 Figure 12 i s a D.T.A. curve for Baymal boehmite. The f i r s t endothermic peak at approximately 108°C i s due to decomposition of impurities such as acetate (acetic acid, b.p. = 118°C). The peak at 275°C i s a baseline peak corresponding to a change in the rate of dehydroxylation of boehmite and not an exothermic peak as one might conclude at a f i r s t glance. Therefore the region from 275°C to 381°C must be endothermic. This is more r e a l i s t i c since in this region water i s being lost, which is an endothermic reaction (see Figure 12). Regions I and II are characterized by the same rate of weight loss, however, from 381°C to 443°C there i s an exothermic baseline shift. A shift of baseline in the exothermic direction may be due to an increase in thermal conductivity, a decrease in heat capacity of the sample, or i t may result from the sintering of particles (15). From 443°C to 481°C, the transformation region, there exists a large endothermic peak. This is due to the phase transformation of boehmite to gamma alumina with the attendant loss of water. 3.6 Isothermal Compaction Curves for Boehmite Figure 13 shows a series of isothermal compaction curves (~ versus o time), which were obtained by applying a constant pressure (o r ef) at a given temperature. Each curve can be expressed as a SUB of exponentials of the form ( 4 4 3 ° C exothermic base l ine shift) (AT- 0) (base l ine peak) ( 4 8 1 ° C endothermic peak) endothermic dehydroxy la t ion reg ion i a n d l ( I 0 7 ° C endothermic peak) a c e t a t e being r e l e a s e d D.T.A, " T " scale = 0.8 m v / i n . " A T " s c a l e = 0 . 0 2 m v / i n . hea t ing ra te = ! 5 ° / m i n . 121 216 301 381 Temperature °C 459 532 Figure 12. D.T.A. f o r boehmite (pl a t , versus p l a t . +13% rhodium thermocouple). - 39 -^ = K(l - A e - a t - Be _ B t) (1) o where K, A, a, B and 6 are constants which can be uniquely determined (see Table VII). " A L " -— is the strain (compaction) = z o 't the time . AL K = {~} or the f i n a l compaction ° t -A and B are constants for which the following condition must be satisfied at time = 0, (7^)«._ft= 0 i.e., A + B = 1 O 3.7 The Experimental Determination of the Constants K, a, A, B, and 8 The constant "a" can be uniquely determined by assuming that 6 >> a. Thus for time much greater than zero, the contribution from one of the exponential terms w i l l be approximately equal to zero. Taking the derivative of equation (1) one obtains '-# - K{Aae"at + BBe' B t} (2) i f 8 >> a then for t >> 0 - 40 -- j — - KAae •at or l n ( f e = -at + ln{KAa} (3) Figure 14a shows "-—" versus " t " for run 27, an isothermal compaction i-i o curve for boehmite at 498°C. From t h i s graph the natural logarithm of the slope versus time was p l o t t e d (see Figure 14b). Data was only us for time > 3 minutes. From the slope of Figure 14b the value of "a" can be determined, and from the intercept "A" can be calculated since, intercept = ln{KaA}. The constant "B" can then be determined using A + B = 1. To determine "3", the constants K, a, A, B and the corres-ponding experimental are substituted i n t o equation (1) for time equal to one minute. Hence, the compaction of boehmite at 498°C can be expressed mathematically as o j^- = (0.112)(l-0.472e -0.19t - 0.528e -2.87t ) (4) o The same procedure was repeated for Figures 15a to 15h. The assumption 3 >> a i s j u s t i f i e d , since i n Figure 14a the natural log of the slope f i t s a s t r a i g h t l i n e for t > 3 minutes. Time (min) Figure 13. Isothermal compaction curves for boehmite (R.H.P. at 5860 p s i ) . 100 (experimental curve) € ' (O.II2)(l-0.472e'0•'S!o.528e"2'87 ,) CO I o >J o < 1-3 II C o • r l 4J O cd & 6 o o 80 60 40 / / mk=o.M2 m = i.o 20 JL 10 12 14 Figure 14a. Time (min) Run 27 (R.H.P. compaction curve f o r boehmite at 498°C and 5860 p s i ) 16 - J 1 1 1 1 1_ 4 6 8 10 12 14 Time (min) Figure 14b. ln(slope) versus time for run 27 at 498°C. 120 100 ro I O r J l o ll C O 80 € M O . I 2 6 ) ( l - 0 . 3 l 9 e ° 2 5 t - 0 . 6 8 l e 4 8 l t ) (experimental curve) m.k = 0.126 m= l.O c o •H •u u CO s O o 60 40 L_ 10 L_ 12 Time (min) i_ 14 16 Figure 15a. Run 33 (compaction curve for boehmite at 270°C and 5860 psi). 140 120 € = ( 0 . l 4 4 ) ( l - 0 . 2 l 8 e 0 2 6 2 t - 0 . 7 8 2 e 2 , 6 6 t ) (experimental curve) 100 80 mk ° 0.144 m = i.O 60 40 V 10 12 14 Time (min) 16 Figure 15b. Run 30 (compaction curve for boehmite at 290°C and 5860 psi). 100 I o o <3 H J II U) C o •H 4-1 CJ ct) o. e o u 90 80 € = ( 0 . 9 9 ) ( l - 0 2 4 9 e ° 3 0 2 t - 0.75ie 2 9 8 t , 70 + (experimental curve) m-k = 0.99 m=i.o 60 0^  ~"S W Time (min) 12 14 Figure 15c. Run 22 (compaction curve for boehmite at 400°C and 5860 psi) 16 Figure 15d. Run 26 (compaction curve for boehmite at 420°C and 5860 psi). 120 € = (O . I20 ) ( l -0 .5l le°- 3 L 4 T -0 .489e 2 - 9 l t ) 100 7 • (experimental curve) 80 / 60 L m.k = 0.120 m» l.O 4>-00 40 20 h 10 12 Time (min) Figure 15e. Run 29 (compaction-curve for boehmite at 529°C and 5860 psi) 14 16 € = ( O . I 2 l ) ( . - 0 . 5 2 5 i a 2 3 6 t - 0 . 4 7 5 e , 9 9 t ) (experimental curve) mk = 0.121 m = i.o 4 6 8 10 12 14 Time (min) Run 15 (compaction curve for boehmite at 540°C and 5860 psi). 110 € = (0.103) (1-0.188 e 0 - 4 0 5 - * 0.812 e 4 - 3 4 ^ 100 ro I o >J o II C o •H 4J CJ ca o 80 60 40 (experimental- curve) m-k = 0.103 m« l.O 20 h i 16 10 12 Time (min) 14 Figure 15g. Run 28 (compaction curve for boehmite at 628°C and 5860 psi). Time (min) gure 15h. Run 23 (compaction curve for boehmite at 622°C and 5860 psi). - 52 -4. DISCUSSION 4.1 Formation of the Hard Phase Material When boehmite was R.H.P. at 700°C and under 8000 psi, i t was found that a "hard phase" material could be produced. X-ray analyses indicated that i t s structure was gamma alumina and that no apparent grain growth had taken place during i t s formation. For the same compact, (Figure 4) the central region had been converted to alpha alumina and apparently there was excessive particle growth. Contrary to what was expected, alpha alumina- the high temperature phase,was formed in the cooler or central regions of the compact. The phenomenon can be explained i f the effect of water vapor pressure i s taken into account. Since the vapor pressure would be greatest at the center of the compact, pressure could either reduce the gamma to alpha transition temperature according to Clapeyron's equation. Or secondly, the vapor pressure causes the boehmite to transform directly to alpha alumina without the formation of the intermediate gamma phase. The latter explanation is preferred on the basis of a calculation which showed that to lower the gamma to alpha transition to 500°C, more than a million psi pressure i s needed in the system. Wheat and Carruthers (16) observed the same phenomenon when pressing B-A1 20 3 at 20,000 p s i and 750°C. They found that the edge of t h e i r compacts were composed e n t i r e l y of gamma alumina, whereas the center consisted of the alpha form only. Also, i f one re f e r s to Figure 4, point 16. i s a d i r e c t evidence of the e f f e c t of pressure on the formation of the alpha phase. The fracture crack running from the edge to the center of the compact would act as a pressure sink, allowing the formation of the hard phase closer to the center of the specimen. Also, i t becomes obvious that the hard gamma phase was f i r s t nucleated at the outer edges where the temperature was highest. As the "hard phase" material started to grow towards the center of the compact, i t formed an impermeable b a r r i e r preventing the escape of water vapor from the i n t e r i o r of the compact. Due to the increase i n water vapor pressure, boehmite i n the ce n t r a l region might have converted d i r e c t l y to alpha alumina. To v e r i f y that gamma alumina formed from boehmite at a lower pressure w i l l not convert to alpha alumina below 700°C during R.H.P., boehmite powder was heat treated to 600°C &t atmospheric pressure)to convert i t to the gamma form. Then water was added and the material was l e f t to age f o r 2 hours. A f t e r which the material was vacuum dried then R.H.P. at 700°C under 8000 p s i . The f i n a l compact consisted e n t i r e l y of gamma alumina, as was expected. The behaviour of boehmite during R.H.P. i s schematically represented i n Figure 16. One of the e s s e n t i a l findings of th i s i n v e s t i g a t i o n i s that, i t i s the boehmite to gamma alumina transformation that i s important i n producing "hard phase" material. However, i t should be mentioned that when boehmite powder i s heat treated (at one atmosphere) to produce gamma alumina, - 54 -outer edge of compact [Low Vapor Pressure] 500° 1000°C 1200°C Boehmite Gamma Theta Alpha [Hard Phase Material] No Particle growth [High Vapor] Pressure Alpha (500 to 700°C) [Large Particle] Growth center of compact Figure 16. Behaviour of boehmite during R.H.P. i t s powder characteristics are exactly the same as the i n i t i a l starting material. That i s , the gamma material is s t i l l a finely divided white powder. But when the "hard gamma phase" was formed during R.H.P. i t always had a blackish appearance, suggesting that carbon might somehow be involved in the reaction. Carbon was always present during R.H.P., especially when a graphite sleeve was used. However, for the "canning" operation carbon was only present as the impurity acetate. Alternatively, i t i s also possible that the black appearance - 55 -of the "hard phase" i s due to an o p t i c a l phenomenon and not carbon. Other experimenters have observed the same phenomenon i n d i f f e r e n t systems. Their explanation i s that the "hard phase" material i s translucent, but due to the presence of porosity nests, l i g h t i s scattered i n such a way that the material appears black. Although, the crystallography of boehmite to gamma alumina change has been worked out i n d e t a i l , very l i t t l e i s known about the p h y s i c a l and morphological changes associated with t h i s transformation, except that p a r t i c l e growth does not occur. To obtain a better understanding of the behaviour of boehmite during R.H.P.,isothermal compaction curves were obtained by R.H.P. boehmite i n a s t e e l die with s u f f i c i e n t clearance to allow a l l water vapor to escape. While under these c o n d i t i o n s , i t was hot possible to produce the hard phase mat e r i a l , i t did y i e l d more information relevant to the behaviour of boehmite j u s t before and during the gamma t r a n s i t i o n . These r e s u l t s are discussed i n part II i n terms of a v i s c o e l a s t i c model. 4.2 The Dynamic System - Part II As stated before i n Section 4.1, data obtained from isothermal compaction curves would be used as an aid i n understanding the behaviour of boehmite before and during the gamma transformation. While the compaction data as shown i n Figure 13, appears to be somewhat complex i n nature, an attempt has been made to r e l a t e the data to a simple v i s c o e l a s t i c system as follows. If a mathematical model i s to su i t a b l y describe reactive hot pressing, i t must be dynamic, that i s time varying to any given input (stress, s t r a i n , voltage e t c . ) . The simplest dynamic system which - 56 -could possibly be related to R.H.P. of boehmite i s a second order system (refer to Section 3.6). The output (strain, stress, voltage etc.) can be effectively described by a second order linear differential equation. The response of a second order system to a unit step input can be described by an equation of the form (19) e = K{l-Ae" a t - Be B t} (5) It has been shown i n Section 3.6 that the R.H.P. of boehmite can be expressed by a second order system which has the exact form of equation (5) where "e" would be the strain AL/L o " t " the time K = (7 -^) or (j^-) at time equal to i n f i n i t y o o a and 3 are constants A and B are constants such that the following conditions must be satisfied. At time = o, ^\=Q = 0 o or 0 = K[l - A - B] giving A + B = 1 As the data was obtained as a function of time by heating the powder to a given temperature and applying a constant stress (or pressure), i t can be said that the system is responding as described by equation (5) to an input which can be considered a unit step input. This is explained schematically in Figure 17, INPUT (stress) OUTPUT (stroin) SYSTEM DYNAMICS J cr=o-e u_,(t) € = K( l - A e ' ^ - B c ^ ) Figure 17. Schematic representation of "system dynamics" relating output (strain) to input (stress). and mathematically by e = xo (6) where e = strain (output) a = stress (input) x = transfer function The applied step input was normalized to a given reference stress or Note that..when a £ = cr •, . , the input is -just a unit step function ref applxed • r J r U ^ ( t ) . The reference stress chosen for this work was a . --1 ref 5860 psi. - 58 -U^Ct ) where u^Ct) = 0 f o r t < 0 i 1 f o r t > 0 + i Since the response of the system i s known f o r a given i n p u t , the t r a n s f e r f u n c t i o n f o r the system can now be determined simply by using Laplace transforms. The procedure i s as f o l l o w s : (a) From Feedback theory (17), i t i s known that i f the system i s l i n e a r , then the response of a system to a u n i t pulse input ^ ( t ) i s j u s t the d e r i v a t i v e of the system response f o r a u n i t step input U . ( t y . For our system s i n c e e = K{l-Ae -at - Be } i s the response f o r a u n i t step i n p u t , then the d e r i v a t i v e i s the response f o r a u n i t impulse, or -T— = AKae -at + KgBe -f3t (8) (b) Using equation (6) and t a k i n g the Laplace transform E = XO t (9) t Note c a p i t a l l e t t e r s w i l l be used f o r the Laplace transform. The Laplace transform of any time f u n c t i o n i s w r i t t e n as £[f(t)] and -59 -where rde, = a b (a+b)S+ag+ba " £ l d t J = S+a S+B = s 2 + ( a + 3 ) s + a g and a = KAa b = KB 3 a3+ba = KaB . (boundary condition A + B = 1) but CT- £'[U0(t>] = 1 Therefore on substitution one obtains the transfer function v K[Aa+Bg]S+Ka3 RS+D A - ~~2—: ~ : — = ~j v -LU; S +(a+8)S+a3 S +HS+T In other words,the transfer function which describes R.H.P. of boehmite must have the form of equation (10) where R = K[Aa+B3] (11a) D = Ka3 (lib) H = a+B (11c) T = 'aB (lid) The second order linear differential equation which has a transfer function of the form (10) can easily be determined by rewriting equation (10).as [S2+HS+T]E = [RS+D]CT (12) - 60 -and then taking the inverse Laplace transform, one obtains 2 d e H d e R do ,.. ~2 It + T e " dF + D a ( 1 3 ) dt Equation (13) i s the sacond order d i f f e r e n t i a l equation which describes R.H.P. of boehmite. Since boehmite goes through a phase transformation between room temperature and 600°C i t i s l i k e l y that the constants H, T, R, and D w i l l be functions of temperature. Moreover,these constants may be also functions of pressure. I t would be simpler i f the constants were found to be pressure independent. Then,for a given temperature i f H, T, R, and D are known, the compaction curve for boehmite could be calculated f o r any pressure. The next step i s to develop a model from which H, T, R, and D can be re l a t e d to mechanical parameters. 4.3 The Mechanical Analog A mechanical model based on l i n e a r devices l i k e springs and dashpots can be developed, i n which the constants H, T, R, and D can be expressed i n terms of these devices. A spring being an i d e a l device for which the stress i s d i r e c t l y proportional to the s t r a i n , while a dashpot i s a hypothetical device which i s s t r a i n rate s e n s i t i v e only (Figure 18). At a l a t e r stage i n the development of a v i s c o e l a s t i c model f o r R.H.P. of boehmite an e l e c t r i c a l analog rather than a mechanical analog w i l l be used, since i t i s less cumbersome to work i n terms of e l e c t r i c a l components rather than mechanical components. I t i s r e l a t i v e l y easy to convert from one system to the other using the following r u l e s : - 61 -SPRING (STRAIN SENSITIVE) DASHPOT (STRAIN RATE) SENSITIVE cr = M€ cr=7] d€ dt M (ELASTIC SHEAR MODULUS) 7) (NEWTONIAN VISCOSITY) Figure 18. Defining the spring (a s t r a i n s e n s i t i v e device) and dashpot (a s t r a i n rate s e n s i t i v e device). (a) Mechanical models represented by a v i s c o e l a s t i c theory may be converted to the e l e c t r i c a l analog by replacing springs by capacitors, dashpots by r e s i s t o r s , and by changing p a r a l l e l coupling to serie s and ser i e s to p a r a l l e l (18) . (b) Using the following table (19). MECHANICAL ELECTRICAL spring component (i/M) capacitor (C) dashpot ( 77 ) resistor (R) stress (a ) voltage (v) strain (e ) • charge (q) Table V. Conversion from e l e c t r i c a l to mechanical analog. - 62 -To demonstrate that the e l e c t r i c a l and mechanical analogs are e q u i v a l e n t , both g i v i n g the same answer, the equation of motion f o r one of the simplest v i s c o e l a s t i c systems, the Voig t or K e l v i n element has been c a l c u l a t e d using both methods. These c a l c u l a t i o n s are shown i n Appendix B. 4.4 The V i s c o e l a s t i c Model f o r R.H.P. of Boehmite I t has been shown i n Secti o n 4.2 that the dynamics f o r R.H.P. of boehmite can be expressed by a second order l i n e a r d i f f e r e n t i a l equation of the form dt Where e = s t r a i n . a = s t r e s s H, T, R, and D are constants w i t h the corresponding t r a n s f e r f u n c t i o n X=""*f-+J> (15) S + HS + T The next step i s to de r i v e an e l e c t r i c a l analog from which the mechanical equivalent can be determined. The constants H, T, R, and D can then For constant temperature and pressure. be expressed i n terms of spring (M) and dashpot (n) parameters. Replacing e ->• q and a -*• v (Section 4.3) and by d e f i n i t i o n i = 4^" dt Equation 14 can be rewritten as £ + Hi + T /Id. - + »v taking the d e r i v a t i v e d 2 i , H d i , _ R d 2v , D dv d7 ^ " d7 " for which the Laplace transform is S 2 I + HSI + TI = RS 2V + DSV + I . M R S + D L ( 1 6 ) S +HS+T Therefore,the e l e c t r i c a l analog which i s required must have a transfer function of the form (16). At t h i s juncture one considers various combinations of r e s i s t o r s and condensors and calculates the trans f e r function for each combination. C a p i t a l l e t t e r s are being used for the Laplace transformation. - 64 -The combination which has the same t r a n s f e r f u n c t i o n as equation ( 1 6 ) i s the combination r e q u i r e d . The simple s t arrangement of r e s i s t o r s and condensors found to have the c o r r e c t t r a n s f e r f u n c t i o n i s shown i n Figure 1 9 . R 2 o—vAAA V i Figure 1 9 . E l e c t r i c a l analog. Since R 2 +" ( 1 7 ) where — = S C 2 + R T T - T c^s c 1 s S C 2 + R C S + 1 R 1 C 1 C 2 S + ( C 1 + C 2 ) S R l C l S + l s u b s t i t u t i n g f o r JJ. Z = ( R 1 R 2 C 1 C 2 ) S .+ . [ R 2 ( C 1 . + .C 2 ). . H - . R ^ J S .+ 1 ( R 1 C 1 C 2 ) S 2 + ( C 1 + C 2 ) S - 65 -or 1 = 1 V Z (R^cps* + (C 1 + C 2)S (R 1R 2C 1C 2)S + [ R 2 ( C ^ + C 2) + R 1C 1]S + 1 di v i d i n g the denominator and numerator by R 1 R 2 C ^ C 2 I V BU s 2 + C l + C2 R 1 R 2 C 1 C 2 s 2 + R 2 ( c 1 + C2) + R I C I ] R 1 R 2 C 1 C 2 S + R 1 R 2 C 1 C 2 (18) which i s the form required. Now i = o r I = £[i] = SQ therefore — = 2^-Substituting Q V s + C l + C2  R 1 R 2 C 1 C 2 S 2 + R 2 ( C 1 •+ V + R 1 C 1 R 1 R 2 C 1 C 2 (19) S + R 1 R 2 C 1 C 2 The e l e c t r i c a l analog can now be converted to the mechanical equivalent by applying the rules given i n Section 4.3 or - 66 -R2 °—vAAA V I o R. c=0 M, £ 77, ///////////////// Figure 20. Electrical to mechanical analog. and replacing R -> n C 1/M q -> e v -> a equation (19) becomes E CT S + l / l ^ . + . l / j ^ n l n 2 MXM2 ' s 2 + n2 (M^ + M2") + ^ n l n 2  M1 M2 S + M1M2  n l n 2 (20) - 67 -or expressing as a differential equation and rearranging the coefficients dt 2 n2 ( M l + V + M2 n l 1 n 1 n 2 dt + L n l n 2 . e = '2 J da dt + •M1 + M2 n . n l n 2 (21) Equation (21) can now be compared with the original reactive hot pressing equation 2 de H de „ R do —TT- + — + Te = — + Do (13) dt 2 d t d t therefore one obtains M., + M M H = — + — (22a) -1.M T = -±-=- (22b) n 1 n 2 R = l / n 2 (22c) D = (M1 + M 2 ) / n ; L T i 2 (22d) but from Section 4.2 R = K[Aa + BB] D = KaB H = a+B T = aB - 68 -i t f o l l o w s '2 K[Aa + Bp] (23a) M„ n 2 [ H - Dn 2] (23b) n 2[DM 2 - T] (23c) M, (23d) As a r e s u l t of the analyses c a r r i e d out i n the preceding s e c t i o n , t h e mechanical model which s a t i s f i e d the R.H.P. c o n d i t i o n s i s shown i n Figure 21. M. Mi ir/n///i//i///)ii//i Figure 21. The mechanical analog f o r R.H.P. of boehmite. The parameters n-, , r u , M, and M„ can now be c a l c u l a t e d from K, a, A, 3, 1 2 1 2 and B. - 69 -4.5 Ca l c u l a t i o n of Compaction Curves f o r D i f f e r e n t Pressures at  Constant Temperature If the c o e f f i c i e n t s i n the d i f f e r e n t i a l equation are independent of pressure, then knowing R, D, T, and H f o r a given temperature, i t should be possible to calculate the compaction curve at d i f f e r e n t pressures, assuming the same temperature i s maintained. For example, boehmite was R.H.P. at 498°C and 5860 p s i and the experimental data gives an excellent f i t to the equation [see Figure 14a]. e = = (0.112)(1 - 0.472e"°* 1 9 t - 0.528e" 2' 8 7 t) (24) o The compaction curve for 498°C and 9170 p s i can be calculated by-su b s t i t u t i n g "a" i n t o equation (13) where ''applied , , 9170 . . ' , , " a f U - l ( t ) = 5860 U - l ( t ) " 1 ' 5 6 u - l ( t ) ref °a l i e d In general l e t t i n g — = M, i t can be shown (Appendix A) that the ref compaction curve at any pressure can be expressed by e = MK[1 - A e " a t - Be~ B t] (25) E f f e c t i v e l y , t h i s means that the t o t a l compaction at time equal to i n f i n i t y i s d i r e c t l y proportional to the applied pressure (stress) or - 70 -AL L t=» p-applied "AL' L t=°° CT=ref MK K = M = applied a c ref (26) It i s u n l i k e l y that such a simple r e l a t i o n s h i p i s going to hold, e s p e c i a l l y at higher pressures. Previous i n v e s t i g a t i o n on the cold compaction behaviour of oxides by Cooper and Eaton (20) showed that * the f r a c t i o n a l volume compaction V can be expressed as . V . - V o V - V O °o _ K i / a ~V C T a^e + L - L o L - L o 0 0 (27) where a^, a^, K-^  and are constants V q = i n i t i a l volume ( " L " i n i t i a l length) V = f i n a l volume at pressure a V = volume at t h e o r e t i c a l density. Therefore [AL-] L o App rALi L o Ref a^e + a^e -K 1/a R -K 2/a R a^e + a 2e (28) By using Table V I i n which Cooper and Eaton have summarized values of a_. and K. for four d i f f e r e n t materials, one can obtain some idea of how much error would be introduced i f equation (26) i s used. - 71 -Mat e r i a l K^(psi) K^Cpsi) a^ a l + a 2 Alumina 3100 50,000 0.50 0.85 S i l i c a 2400 54,000 0.60 0.85 Magnesia 2400 49,000 0.65 0.90 C a l c i t e 1450 42,000 0.68 1.00 Table VI. Summarized values of c o e f f i c i e n t s a. and K. for each powder. For example, f or alpha alumina, using an applied pressure of 6000 p s i and comparing to a reference pressure of 3000 p s i , equation (26) gives M = 2, while equation (28) gives M = 1.68. Therefore, by assuming that the f i n a l compaction was proportional to the applied stress for alpha alumina would have l e d to 16% error. Equation (25) can therefore be written i n a more exact form by s u b s t i t u t i n g equation (28) for "M" or = K a^e + a^e -K 2/a R - K 2 / a R a 1e + a 2e (1-Ae a t - Be~ 3 t) (29) This i s a very complicated expression, where the constants a^, a 2 , and K 2 would have to be evaluated experimentally,using the same procedures as used by Cooper and Eaton. Fortunately for the R.H.P. of boehmite and f o r the range of applied pressures used, the assumption that the t o t a l compaction i s proportional to the applied stress gave reasonable agreement with experimental observations,as shown i n Figure 27. Using the expression M = — " L thus proved to be a good approximation. CTRef - 72 -4.6 C a l c u l a t i o n of n^, M^, ru and In Table VII the values of K, A, a, B and g are summarized, f o r each compaction curve shown i n Figure 13. From t h i s information and using equations (23a-d) the value of n^, M^, and M 2 c a n ^ e calculated for each curve. The f i n a l r e s u l t s are gr a p h i c a l l y shown i n Figure 22, where these parameters are plo t t e d as a function of temperature. Figure 22 allows one to v i s u a l i z e the e n t i r e R.H.P. behaviour of boehmite as a function of temperature. More important the viscous and e l a s t i c nature of boehmite during R.H.P. can be e a s i l y distinguished. The e l a s t i c component and the viscous component n^ are small compared to M^ and n^ > an& are nearly temperature independent. For th i s reason both M 2 and n 2 can be assumed to be n e g l i g i b l e . The R.H.P. model for boehmite thus can be approximated by a simple Voigt or K e l v i n v i s c o -e l a s t i c element (Figure 23) which assumes that any changes i n the s t r a i n of the e l a s t i c component are vis c o u s l y damped. The e f f e c t of temperature on the viscous component ru and the e l a s t i c component M.. M Figure 23. Voigt or Kelvin v i s c o e l a s t i c element. Run No. Temp. °C K & o A a B 3 Dens-i t y P E l a s -t i c comp. M l V i s c -ous comp. '• n l E l a s -t i c comp. " M 2 V i s c -ous comp. n 2 V M 1M 2 M1+M2 33 270 0.126 0.319 250 .681 4.81 1.42 28 108 11 2 7.9 .127 30 290 0.144 .218 .262 .782 2.66 1.47 39 146 8 3 6.7 .149 22 400 0.099 .249 .302 .751 2.98 1.41 50 162 13 4 10.3 .097 26 420 0.080 .368 .093 .632 2.1 1.37 37 391 19 9 12.5 .080 27 498 0.112 .422 .190 .528 2.87 1.45 22 108 15 6 8.9 .112 29 529 3.120 .511 .314 .489 2.91 1.49 21 60 14 5 8.4 .119 15 540 3.121 .525 .236 .475 1.99 _ 20.6 78 14 8 8.3 .120 23 622 3.135 .259 .260 .741 2.81 35 130 9 3 7.2 .139 28 628 3.103 .188 .405 .812 4.34 1.42 63 152 11 3 9.4 .106 Table VII. Summary of v i s c o e l a s t i c components. - 74 -400 300 200 100 I p r e s s u r e = 5 8 6 0 p s I (77 viscous component) (M e l a s t i c component) (M^ e l a s t i c component) (Tylg v i s c o u s c o m p j — 1 — 100 "200 300 Temperature "C Figure 22. Mechanical parameters versus temperature. - 75 -i s very d r a s t i c . In f a c t , there i s excellent c o r r e l a t i o n between the behaviour of and the D.T.A. for boehmite (see Figure 24). The R.H.P. behaviour of boehmite as a function of temperature w i l l be discussed i n four stages. 4.6.1 Stage (a) - R.T. to 275°C Referring to both the D.T.A. (Figure 25) and the T.G.A. (Figure 26) one can see a large endothermic peak at 107°C corresponding to the impurities (acetate and SO^ ) being driven o f f . In stage (a) approximately 0.10 H^ O molecules per Al^O^ molecule i s l o s t due to dehydroxylation. At 275°C, the D.T.A. shows a baseline peak. This corresponds to a change i n the rate of dehydroxylation and the s t a r t of stage (b). 4.6.2 Stage (b) - 275°C to 443°C Stage (b) has been divided into two sub-regions, region I and the viscous region I I . At 275°C both the e l a s t i c component and the viscous component are comparatively small (Figure 22). However, as the temperature i s increased both and s t a r t to increase. The e l a s t i c component reached i t s peak value at approximately 380°C. This i s the beginning of the viscous region I I . (i ) The viscous region 380°C to 443°C The viscous region i s introduced with no apparent change i n the rate of dehydration (Figure 26), however, at t h i s temperature approximately 0.5 ^ 0 molecules of water have been removed due to dehydration. Noting that the o r i g i n a l Dupont Baymal c o l l o i d a l allumina ( 4 4 3 ° C exothermic base l ine shift ) (AT= 0) (base l ine peak) (481 ° C endothermic peak) ( 3 8 1 ° 0 endothermic dehydroxy la t ion I (viscous region 1 ) t - v t v i • ( I 0 7 ° C endothermic peak) ace ta te be ing r e l e a s e d ( t ransformat ion region) D.T.A, " T " scale = 0 . 8 m v / i n , " A T " s c a l e = 0 . 0 2 m v / i n . hea t ing ra te = ! 5 ° / m i n . 121 216 301 381 459 Temperature °C 532 F i g u r e 25. D.T.A. f o r boehmite ( p l a t , v e r s u s p l a t . +13% rhodium t h e r m o c o u p l e ) . Temperature °C Figure 26. T.G.A. for boehmite. - 79 -(boehmite) consisted of A^O^. 1.5^0 or 0.5 H^O molecules/Al^O^ must have existed as p h y s i c a l l y absorbed water. At 380°C a l l the p h y s i c a l l y absorbed water has been driven o f f . What i s l e f t i s A^O^-H^O, the empirical formula usually given for boehmite. Therefore,it i s expected that any further removal of water w i l l r e s u l t i n free surface bond formation or active s i t e s . This i s v e r i f i e d by the increase i n v i s c o s i t y , shown i n Figure 22. For temperatures above 380°C the viscous component increases d r a s t i c a l l y , peaking at 443°C, while there i s a corresponding exothermic base s h i f t on the D.T.A. curve. The viscous nature of the boehmite supports the idea of a very active surface (large surface free energy) above 380°C. The small f i b r i l l a r boehmite p a r t i c l e s w i l l coalesce together, the shear force f o r i n t e r p a r t i c l e s l i d i n g w i l l increase. This i s equivalent to s i n t e r i n g . However, s i n t e r i n g suggests p a r t i c l e volume changes to compensate f o r neck growth. I t i s possible that i n t h i s case, however, the p a r t i c l e s s t i c k together not unlike two oppositely charged sheets of paper rather than s i n t e r i n g with associated volume change. I t has been suggested, that surface melting may be taking place, but surface melting would be an endothermic reaction. Also, i f the surface consists of only a few monolayers then i t i s meaningless to t a l k about a l i q u i d zone. On the other hand,it should be possible to detect large amounts of melting i n t h i s region by X-ray methods ( p a r t i c l e s i z e determination). * According to T e r t i a n and Papee, excess water represented by 2 A^O^.1.6^0 i f retained as a monolayer would cover about 265m /g, e s s e n t i a l l y the t o t a l a ctive surface of pseudoboehmite. The surface 2 area for Baymal c o l l o i d a l alumina i s 250 m /g. - 80 -From the D.T.A. Curve i t can be seen that the onset of the viscous region i s associated with a large exothermic base sh i f t . It is unlikely that recrystallization between particles could account for such a large AT change. More likely,the change is due to a change in the effective thermal conductivity of the material (7)}which would result from sintering. It i s interesting to note, that the D.T.A. was repeated at different rates of heating 15°C/min and 3°C/min. The shape of the D.T.A. curve did not change throughout the viscous region. This suggests that coalescence of the particles must take place very quickly, the degree of coalescence being proportional to the temperature; as the temperature is increased more water i s driven off resulting in a greater number of active sites. ( i i ) The viscous region and "steady state" Referring to Figure 13, i t i s noticed that for the experiment carried out at 400°C (run 27), the compaction behaviour approaches a steady state during the latter stages. The corresponding compaction _3 rate is 1.1 x 10 /min. This i s indicative of the predominant viscous nature of the powder at this temperature. A simple attempt can be made as follows to predict the slope of the "steady state" curve for any temperature (380 to 443°C) within the viscous region using the results from the D.T.A. Comparing both the D.T.A. and the viscous component n^, one w i l l immediately see that they behave in a similar manner. Generalizing one can write - 81 -But a = n (by d e f i n i t i o n ) de K" Substituting = ------However, i n the viscous region from the D.T.A. dT = £T + y d K" Therefore - r — = —: where K", £ and v a r e constants. q. T That i s , the s t r a i n rate i s temperature s e n s i t i v e only. 4.6.3 Stage (c) - Transformation Region 443°C to 520°C From 443°C to 520°C both the viscous component n-^  a n ^ the e l a s t i c component decrease to a minimum. This i s the temperature at which boehmite transforms to gamma alumina. The transformation i s accompanied by the largest rate of dehydroxylation. At 520°C, the s t a r t of stage "d", i f i t i s assumed that boehmite has been completely converted to gamma alumina, then gamma alumina s t i l l contains approximately 4% water. 4.7 The E f f e c t of Pressure on R.H.P. of Boehmite To test the theory at a d i f f e r e n t pressure,boehmite was hot pressed at 9170 p s i and 513°C. In theory one could obtain M^, M^, and r\2 at 513°C and 5860 p s i from Figure 22, then c a l c u l a t e a, $, A and B. From t h i s data the compaction behaviour at 9170 p s i could be calculated using the theory i n Section 4.5. Since i t was d i f f i c u l t experimentally to achieve the same temperature between d i f f e r e n t runs, the average values from run 29 at 529°C and run 27 at 498°C were used (see Table V I I I ) . - 82 -Run 29 at 529°C (5860 psi) . Run 27 at 498°C (5860 psi) Average K 0.120 0.112 0.116 A 0.511 0.472 0.491 a 0.314 0.190 0.252 B 0.489 0.528 0.508 6 2.91 2.87 2.89 20.7 21.8 21.7 M 2 13.8 15.1 14.4 r)x 60.1 108.6 84.3 n 2 5.26 5.56 5.41 Table VIII. Average values for mechanical parameters at 513°C. From the theory, the compaction curve for 9170 ps i is given by e = MK {1-Ae" a t - Be 3 t} (25) where M = J^EP- = 9170 a R e f 5860 (30) therefore at 9170 psi and 513°C e = (0.182)(1 - 0 . 4 9 1 e ~ ° , 2 5 2 t - 0.508e 2 ' 8 9 t ) (31) e = — is plotted in Figure 27 along with the experimental R.H.P. curve o at 9170 ps i . The agreement between the two is reasonable. Also, i t jus t i f ies using the simple assumption that the f ina l compaction is - 83 -proportional to the applied s t r e s s . I t should be r e a l i z e d that under the experimental conditions used,the f i n a l compact den s i t i e s were always less than 75% t h e o r e t i c a l density (based on the t h e o r e t i c a l density of gamma alumina). 4.8 The V i s c o e l a s t i c Model and Apparent End-Point Density One of the most d i f f i c u l t parameters to measure accurately i s the t o t a l compaction "K". During R.H.P. one must i n i t i a l l y s t a r t with a green compact, therefore, i t i s imperative that the density of the green compact be c a r e f u l l y c o n t r o l l e d . Any v a r i a t i o n s i n green density between specimens would r e s u l t i n a measureable err o r i n "K". St-Jacques solved t h i s problem by prepacking at a constant rate using an Instron machine. However, f o r the present work i t was t e c h n i c a l l y impossible to achieve such c o n t r o l . For t h i s reason,the r e s u l t s as shown i n Figure 22 are only q u a l i t a t i v e i n nature, but accurate enough to show the general trend i n the behaviour of boehmite during reactive hot pressing. The degree of accuracy can be checked by measuring the f i n a l compact density. The end-point density (under stress) f o r a compact should be inv e r s e l y proportional to the t o t a l e l a s t i c component. From Figure 22 the t o t a l e l a s t i c component i s j u s t two springs, of e l a s t i c constant M^ and i n s e r i e s . This i s equivalent to a s i n g l e spring with e f f e c t i v e e l a s t i c constant M = M ^ — . 1/Mj, has been p l o t t e d as a function of temperature, along with the apparent end'-point density i n Figure 28. These two curves * Note, the end-point density (under stress) i s approximately equal to the f i n a l compact density (see Figure 2,. a schematic diagram of a t y p i c a l R.H.P. c y c l e ) . The spring back on cooling i s less than 0.005 inches. 180 160 ro I O H i-4 O CJ O •r-T •U U CTJ cu s o u 140L 120 100L (experimental curve) (calculated curve from dafa obtained at) 5860 psi m k= 0.182 m • 1.56 8 10 Time (min) Figure 27. Compaction curve for boehmite at 513°"C and 9170 psi, 12 I 14 CO _1 16 - 85 -'—•• 1 i 1 1_ 200 300 400 500 600 Temperature °C Figure 28. A comparison of apparent "end-point" density to total elastic constant M„, - 86 -are similar, demonstrating that apparent density i s approximately inversely proportional to M^ . Also, this calculation checks the self-consistency of the viscoelastic model used in the interpretation of the hot-pressing data. - 87 -CONCLUSION 1. It has been possible to produce a hard dense phase (2.2 gm/cc) by reactive hot pressing f i b r i l l a r boehmite during i t s transformation to gamma alumina at 500°C. Formation of the "hard phase" material however, depended on achieving and maintaining the correct vapor pressure during the R.H.P. operation. At least 4% retained water must be maintained i n order for the "hard phase" to form. On the other hand, i f the vapor pressure was allowed to become too high(because of gas trapped within the die assembly during R.H.P.), the boehmite powder appears to transform directly to alpha alumina. Under these conditions, i t i s impossible to produce a dense strong compact at 500°C. 2. The isothermal compaction data was analysed using a viscoelastic model. It was possible to correlate adequately experimental data obtained from the D.T.A. and T.G.A. to the behaviour of boehmite during hot pressing by this model. Although the isothermal behaviour of boehmite was a complex function of temperature i t was possible to arrive at the following conclusions: (i) The fi n a l compaction was not proportional to the total weight loss, contrary to the findings of Cook. ( i i ) The total compaction at a given temperature was proportional to the applied stress within the range of applied pressures used in this study (9200 psi max.). ( i i i ) Within the temperature range 380 to 443°C the powder was found to behave in a viscous fashion, suggesting particle interaction - 88 -was taking place. It i s expected that reactive hot pressing at any temperature within this range should produce the most favourable particle alignment for converting a powder compact into the hard dense material at 500°C. (iv) During the viscous deformation the strain rate (to a f i r s t approximation) was found to be temperature sensitive only, indicating pseudo-Newtonian behaviour. 3. Production of the hard phase material at 500°C seems promising as an intermediate step in producing strong sintered alumina products at low temperatures. Although previously not mentioned,it was found that i f the "hard phase" material i s sintered at 1000°C (< T ,„) m/2 for 6 hours i t i s possible to produce a hard, strong and translucent body. - 89 -APPENDIX A The compaction curve for aA , . , 7* a„ can be calculated by r Applied Ref 3 expanding equation (6) by partials, or E . XLT=> MRS + MP m MRS + MP S(S +HS+T) S(S+a)(S+B) since rj = £{Mu_1(t)} = M / S equating equal powers of S gives 0 P q + f + w MR = qH + gf + wa MD = Tq or q = MK w = -MKB f = -MKA Therefore MK MKA MKB E = S S+a S+B or e = £_1(E) = MK{1-Ae~at - Be & t} (25) - 90 -APPENDIX B Comparison of the Two Methods To demonstrate how a viscoelastic system can be calculated using either a mechanical or an elec t r i c a l approach, the following system w i l l be considered. One of the simplest viscoelastic systems i s the Voigt or Kelvin element. It assumes that any changes in the strain of the elastic component are viscously damped and can be modelled as a parallel combination of springs and dashpots (18) (Figure 23). where a T = a E + (stress) for the spring a- Me„ M (elastic shear modulus) de for the dashpot = TTJ^" n (Newtonian viscosity) Now a T = a E + a v Substituting de V °T = M e E + n e V Note eTT = dt But for a parallel coupling e = e = e, therefore a T = Me + r\e, or integrating - 91 -where e = strain at time = 0 o T = n/M The response of the system to a constant stress i s therefore :T ~ M + [ eo M ] O . - t / T (a) and for the following boundary conditions, at time = 0 e T = 0, then E Q = 0 equation (a) becomes ao r, - t / T , eT = M~ [ 1 _ e ] Now the same system w i l l be developed using the electrical analog method, from section 4.3 rule (a) M — A / V V 777777777777 - 92 -The next step is to work out the complex impedance of the elec t r i c a l circuit (the complex impedance i s determined by calculating the Laplaciah of the ratio v to i . ) • • V _ . 1 RCS •+ 1 or Z = — = R + I SC SC Therefore I(RCS + 1) = VSC The differential equation i s therefore RC di = C dv dt 1 dt Substituting ,Jl RC dt R dt dt and integrating dq , 1 1 dt + RC q = R V from Table V , Section 4.3 v -> a R + n C •*• 1/M - 93 -The mechanical equivalent i s therefore de . M 1 , . , t . __ 4. _ £ = _ a a n c j f o r 0 = CT (constant) dt n n o The solution for this equation i s e " ehomogeneous + esteady state Homogeneous de H -1- M - n d t ~ + n £H " ° let P = Ae a t e H e substituting a = -M/n therefore t " M / n -t/T e = Ae = Ae where T = n/M rl Steady State Solution M l 1 — e 0 0 = —r a or c„a = — a n SS n o SS M o therefore e = Ae + - o M o - 94 -from the boundary condition e = 0 at t = 0 A = ~ h a M o . the solution i s therefore 1 r i - t / T , e = M CTo " 6 ] which is the same as obtained previously using a mechanical analog. - 95 -BIBLIOGRAPHY 1. J. Burke, N. Reed, V. Weiss, U l t r a f i n e Grain Ceramics, Syracuse University Press (Lib UF 5263). 2. A.CD. Chaklader, L.G. McKenzie, "Reactive Hot Pressing of Clays", American Ceramic B u l l e t i n , 4_3, No. 12 (1964). 3. C.H. Bates, C.J. Drew, R.C. K e l l , " S i m p l i f i e d Process f o r Preparing Translucent Alumina Tubes from Boehmite Powder", Transactions and Journal of the B r i t i s h Ceramic Society, 70, 4 (1971) page 128. 4. A.CD. Chaklader and R.C. Cook, "Kinetics of Reactive Hot Pressing of Clays and Hydroxides", American Ceramic B u l l e t i n , 47, No. 8 (1968) page 715. 5. R.G.St-Jacques, "Creep of Compacts of C o l l o i d a l Boehmite (A100H) during Dehydroxylation, Master Thesis, Department of Metallurgy, U.B.C, November (1968). 6. (a) J.W. Newsome, H.W. Heisser, A.S. Russell and H.C Stumpf, "Alumina Properties" Tech, paper No. 10, page 46. Alcoa Research Laboratories, Pittsburgh, 1960. (b) R. Ter t i a n and D. Papee, J. Chem. Phys., 55_, 541-53 (1958). 7. M. Deflandee, "The C r y s t a l Structure of Diaspore", B u l l . Soc. Franc. Min 55, 140^65 (1932). 8. R.K. I l e r , " F i b r i l l a r C o l l o i d a l Boehmite - Progressive Conversion to Gamma, Theta and Alpha Aluminas", J . Am. Ceramincs Soc. 44, (12), 618-24 (1961). 9. H. Achenbach, "Thermal Decomposition of Synthetic H y d r a r g i l l i t e " , Chem. Erde 6, 307-56 (1931). 10. W. Gitzen, "Alumina as a Ceramic M a t e r i a l " , The Am. Ceramic Soc., 1970 (page 32). 11. H. S a a l f e l d , "The Structures of Gibbsite and of the Intermediate Products of i t s Dehydration", N. 5b. Miner Abh. 95, 1-87 (1960). 12. H.P. Klug and L.E. Alexander, X-ray D i f f r a c t i o n Procedures, John Wiley and Sons, Inc., (1967) page 507. 13. N o r e l c o i i n s t r u c t i o n and operating manual for X-ray d i f f r a c t i o n unit type No. 8 (2045-2046), (page 2A). 14. J.H. de Boer, CM. Houbee, Proc. Intern. Sympos. Re a c t i v i t y of Solids (Gothewberg I, 237 (1952)). - 96 -15. Dupont differential thermal analyzer (900) operators manual. 16. T.G. Carruthers, T.A. Wheat, Proc, Bri t . Ceram. Soc, Transaction and Journal pf the British Ceramic Society, A5253/C525, (1969). 17. Distefano, Stubberud, Williams, Feed back and Control Systems, Schaums outline series (page 101-problem 6.7). 18. A.J. Kennedy, Processes of Creep and Fatique in Metals, Oliver and Boyd page 9 [Lib. TA450]. 19. J.E. Goldberg, Automatic Controls: Principles of System Dynamics, Allyn and Bacon Inc., (1964). 20. A.R. Cooper, L.E. Eaton, "Compaction Behaviour of Several Ceramic Powders", Amer. Cer. Moc. 45, No. 3, March 1962. 21. J.H. Fancy, F. Donald Bloss, X-Ray Diffraction, Southern I l l i n o i s University Press (Lib Q094). 22. L.G. MacKenzie, A.CD. Chaklader, "Reactive Hot Pressing of Clays and Alumina", Journal of American Ceramics Spciety, 49j, 490 (1966). 

Cite

Citation Scheme:

        

Citations by CSL (citeproc-js)

Usage Statistics

Share

Embed

Customize your widget with the following options, then copy and paste the code below into the HTML of your page to embed this item in your website.
                        
                            <div id="ubcOpenCollectionsWidgetDisplay">
                            <script id="ubcOpenCollectionsWidget"
                            src="{[{embed.src}]}"
                            data-item="{[{embed.item}]}"
                            data-collection="{[{embed.collection}]}"
                            data-metadata="{[{embed.showMetadata}]}"
                            data-width="{[{embed.width}]}"
                            async >
                            </script>
                            </div>
                        
                    
IIIF logo Our image viewer uses the IIIF 2.0 standard. To load this item in other compatible viewers, use this url:
http://iiif.library.ubc.ca/presentation/dsp.831.1-0078723/manifest

Comment

Related Items