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Sapphire whiskers from boehmite gel seeded with α-alumina
Journal of Crystal Growth 85 (1987) 527-534 North-Holland, Amsterdam
527
SAPPHIRE WHISKERS FROM BOEHMITE GEL S E E D E D WITH a-ALUMINA
S. J A G O T A and R. RAJ Department of Materials Science and Engineering, Bard Hall, Cornell University, Ithaca, NY14853-1501 New York 14853-1501, USA Received 1 May 1987; manuscript received in final form 9 July 1987
Sapphire whiskers were grown in thin films of boehmite (A1OOH) gel seeded with c~-alumina seeds at temperatures from 900 to 1200°C. The whiskers were 20-60 nm in diameter and had lengths up to 10 /~m. The diameter was found to increase with temperature. The crystallographic growth direction of the whiskers was at 46 o to the c-axis of a-alumina. The sides of the whiskers were faceted, exposing the pyramidal planes. A special technique used to prepare the films allowed us to characterize them directly by transmission electron microscopy. Details of the technique are described.
1. Introduction
1.1. Whiskers Whiskers are filamentary single crystals with aspect ratios greater than 10 : 1. They are remarkable for their high strength which can approach the theoretical tensile strength of the crystal. Ceramic whiskers are, therefore, attractive materials for fiber reinforcement of ceramic composites for applications at high temperature and stresses. Aluminum oxide whiskers are usually grown from the vapor phase either by the oxidation of aluminum [1], reacting hydrogen with aluminum oxide [2,3], or by the hydrolysis of halides [4,5]. The reaction temperatures lie in the range 1 2 0 0 - 1 7 0 0 ° C and A1 or A1203 can be employed to nucleate the crystal growth process [5]. Morphology is controlled through the supersaturation of the reactants. Nearly always, the fiber growth direction is the c-axis of the hexagonal a-alumina crystal. In the present work we show that sapphire whiskers can also be grown from boehmite (A1OOH) gels. This technique is simpler than the vapor phase technique of growing fibers. It consists of seeding the gels with crystals of a-alumina and heat treating these gels at temperatures between 900 and 1200 ° C. The fibrous morphology of crystal growth results from the redistribution of
the porosity in the alumina gel. The width of the whiskers can be controlled by means of the crystallization temperature. We believe that ours is the first example where boehmite gels have been transformed into sapphire whiskers by crystallization. Gels have been used in earlier studies to grow whiskers, but, in those cases, the porous structure of the gel served as a framework for constraining the morphology of the crystallization process [6]. In one case, however, silica gels have been converted into silicon-carbide whiskers by a controlled reaction between the gel and carbon black [71.
1.2. C~stallization of boehmite gels A gels is a fibrous polymerized network of the hydroxide and m a n y hydroxides form gels under suitable conditions of temperature and p H [8,9]. Boehmite, A1OOH, forms a gel which, on that treatment loses water and transforms into several phases of alumina and finally crystallizes to aalumina. The dehydration and transformation of boehmite to a-alumina through metastable phases has been studied extensively in the literature from various points of view [10-25] as discussed below. The structure of the gel is characterized by high porosity. The porosity arises from physically and chemically adsorbed water. Pierre and Uhlmann [10] have studied differences in properties of
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S. Jagota, R. Raj / Sapphire whiskers frorn boehmite gel seeded with ~-alumina
boehmite gels under different conditions of preparation. Wilson and coworkers [11-14] used porosimetry and T E M to study (a) changes in pore morphology during dehydration and (b) subsequent phase transformations in boehmite. They [11-14] and Lippens and De Boer [15] have studied the crystal structure of boehmite and transition aluminas by electron diffraction and found that the structure of these aluminas consists of close packed oxygen planes held together by zig-zag O - H bonds. The arrangement of the bonds is such as to accommodate the porosity of the structure. The transformations to the successive phases of alumina may be considered to be topotactic since all have a spinel lattice. The unit cell dimensions for different phases can be derived from each other by a rearrangement of the bonds between the close packed oxygen planes. The formation of a-alumina, however, requires a change in the crystal structure from a spinel lattice to a hexagonal lattice. This formation to a-alumina is believed to take place by nucleation and growth with an incubation time for nucleation [13,16,19[. The transformation of boehmite to a-alumina can be accelerated by seeding the gel with small crystallites of a-alumina. Seeding reduces the temperature for transformation of the gel to a-alumina since self nucleation of a-crystals is no longer necessary. For example, Kumagai and Messing [25] found the transformation temperature to decrease by 170 ° C. Various morphologies of a-crystals grown from gels have been reported and discussed in the literature. At 1200°C, colonies of large (10/~m), porous, vermicular, a-grains are seen [16 23]. Dynys, Ljungberg and Halloran [16,17] also report formation of discrete a-grains 250-500 m m in diameter, when the gel is held at 1000°C. Lamber [22] report equiaxed and columnar grains of a-alumina in thin boehmite films at 1200-1300 ° C. A mechanism for the formation of the vermicular grain has been discussed by Badkar and Bailey [19]. They explain the structure in terms of pinning of the transition alumina matrix by the porosity. Curiously, Kumagai and Messing [25] reported obtaining an equiaxied a-alumina grain structure when seeding was used to promote the phase transformation of the gel.
In the present paper we report results of a study where thin films of boehmite were seeded with a-alumina and then crystallized at temperatures ranging from 900 1200°C. Interestingly, the crystals grew in the form of whiskers. The whiskers were 1 - 1 0 / L m long and 20-60 nm wide. The conditions under which the whiskers grew are reported and possible mechanisms of growth are discussed. The advantage of the thin film technique is that samples can be directly observed in the transmission electron microscope. These observations are reported.
2. Experimental The boehmite gel was prepared by controlled hydrolysis of Al-tri-sec butoxide and repeptization by adjusting the p H to 3.00(_+0.2). The method followed is due to Yoldas [8] and Klein [9]. The initial sol had 40 g/liter of boehmite and formed a gel on drying at 60 o C for 2 - 3 days. The a-alumina seeds had an average diameter of 0.2 /~m. The seeds were dispersed in water at a p H of 2.5 with nitric acid and ultrasonicated to break up agglomerates. The films were made by two methods: (i) a drop of the sol (10/11) and a drop (10/~1) of 0.1 wt% a-alumina seed suspension was placed in the cell of an ultracentrifuge and spun onto T E M grids placed in the cell (W or Ni TEM grids were used). The films were spun at 90,000 rpm for 5 min.
(ii) The T E M grids were dipped in the gel and then the seeds spun onto the film in the centrifuge. Unseeded films were made by the same technique except that no seeds were used. The films were then freeze dried under vacuum overnight. Both techniques resulted in boehmite films thin enough to be viewed directly in the transmission electron microscope. These films were heat treated in an argon atmosphere at temperatures of 700, 900, 1000, 1120 or 1200°C for 20 min after reaching the temperature. The argon atmosphere was used to prevent oxidation of the metal T E M grids. The specimens for T E M were coated with a 10 nm carbon layer, by evaporation, to make them conducting to the electron beam. SEM specimens were sputtered with a 12 nm layer of gold-pal-
S. Jagota, R. R a j / Sapphire whiskers from boehmite gel seeded with c~-alumina
ladium. The whisker dimensions were determined from the T E M micrographs. About 50-100 measurements of the width were made for each temperature. The crystallized phase was identified by X-ray powder diffraction. Fiber orientation was identified using electron diffraction. 3. Results
3.1. Crystallization of unseeded films: Debye-Scherrer X-ray powder diffraction patterns of unseeded films showed that the gel transformed from poorly crystalline boehmite to poorly crystalline g a m m a alumina at 700 °C, to crystalline &alumina at 8 0 0 - 1 0 0 0 ° C and finally to aalumina above 1200 o C. This result is in general agreement with earlier studies of this type [24]. The microstructure of an unseeded film which had been heat treated at 1200°C for 20 min is shown in fig. 1. The fine grained structure in the upper half of the picture consists of &alumina which is a transition alumina. The fine grain size is a result of the pre-existing nuclei in the boehmite gel. Since the starting boehmite is partially crystalline to begin with, the growth of topotactic phases of transition aluminas do not require any nucleation.
529
The lower half of the picture in fig. 1 shows a large single crystal of a-alumina. The crystal contains porosity of a vermicular morphology. This morphology probably results from the redistribution of the porosity in the fine grained transition aluminas as the a-alumina grain grows and consumes the smaller grains of the transition aluminas [19]. The large difference in the grain size between the transition and a-alumina is evidence of a nucleation barrier for the formation of a-alumina. Electron diffraction from the a-alumina crystal shown in fig. 1 revealed that it was oriented with its c-axis normal to the plane of the film.
3.2. Seeded films The seeded films were heat treated at temperatures from 900 to 1200 o C, for a period of 20 min. When the temperature was much less than 900 o C, the film did not transform into a-alumina. A T E M micrograph of the a-alumina seeds taken from a film which was heat treated at 7 0 0 ° C for
ALPHA e~LUMINA
Fig. 1. TEM micrograph of an unseeded gel film calcined at 1200°C showing a vermicular o~-alumina grain growing in a fine grained polycrystalline transition alumina matrix. Marker represents 100 nm.
Fig. 2. TEM mlcrograph of a seeded gel film calcined at 7 0 0 ° C showing seed agglomerates in the gel. Marker represents 200 nm.
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S. Jagota, R. Raj / Sapphire whiskers from boehmite gel seeded with a-alumina
Fig. 3. T E M m i c r o g r a p h of a seeded gel film calcined at 900 ° C showing a - a l u m i n a whiskers w i t h gel r e m a n e n t s a l o n g the sides of the whiskers and c a p p i n g the tips. M a r k e r represents 50 nm.
Fig. 4. T E M m i c r o g r a p h of a seeded gel film calcined to 1200 o C showing a - a l u m i n a whiskers. The whiskers are b r o a d e r t h a n in fig. 3 and gel r e m a n e n t s are not seen. M a r k e r represents 100 nm.
20 min is shown in fig. 2. The seeds are seen to be agglomerated in clusters which are about ten times the particle size. The spacing between these agglomerates varied between 2 to 10 #m. Transmission electron micrographs of films which were heat treated at 9 0 0 ° C and at 1200°C are shown in figs. 3 and 4 respectively. The transformation of the gel into a-alumina is incomplete at the lower temperature since some residual gel material can be seen on the stem of the crystal fibers (see fig. 3). The layer of the gel remaining on the side of the whiskers suggests that the crystals grow much faster along the length direction than in the lateral direction. The sharp edges of the fully crystallized whiskers in fig. 4 shows that the side planes of the whiskers were faceted. Electron diffraction of the whiskers revealed that the growth direction is at 46 o to the c-axis of a-alumina. Fig. 5 shows the diffraction pattern. The (0111) plane axis is along the electron beam direction. Thus, the pyramidal planes [{0111} type planes] are exposed along the lengths of the whiskers. In contrast, the vermicular c~-grain of the unseeded film had the close packed (0001)
S. Jagota, R. Raj / Sapphire whiskers from boehmite gel seeded with a-alumina
531
Fig. 5. D i f f r a c t i o n p a t t e r n f r o m a w h i s k e r f o r m e d at 1120 ° C. B = (0111).
basal plane along the plane of the foil. A SEM micrograph of a film heat treated at 1000 ° C for 20 rain is shown in fig. 6. The regions where the fibers are short and dense are probably where the seeds are located. The fibers which are oriented along the plane of the film are seen to grow to long lengths. Those normal to the plane of the film are short and more closely spaced. Thus, it appears that many whiskers are nucleated from the seed c]hster and some of them, especially those lying close to the plane of the film grow to a length which is approximately equal to the spacing between the seed clusters.
4. Discussion The whisker length can be estimated from the average distance between the seeds in the film. The distance between the seeds can be calculated from the volume of seeds added to the film and by using an average size of the seed clusters, obtained from the TEM micrographs such as the one shown in fig. 2. We assume that the agglomerates are arranged in a square array with distance l between centers of two agglomerates. Let the agglomerates have an average radius r and thickness t equal to the film
S. Jagota, R. Raj / Sapphire whiskers from boehmite gel seeded with a-alumina
532
Fig. 6. SEM micrograph of a seeded gel film calcined at 1 0 0 0 ° C showing whiskers growing on and around seed agglomerates. Marker represents 1.0 #m.
thickness. Then, if the total volume of seeds is Vs, the total volume of the film Vr, volume of each agglomerate ILa, and V~ the volume of a cell of the square array, we must have,
7°f
K/v-. Since I,~ = ~rr2t
6O
vj r , =
v 50 :]2 a
eq. (1)
--
40
,"IN
30
l= r( ~rVf/Vs) 1/2.
tu
20
10
1073
I
I
I
L
II 7 3
1275
1373
[473
TEMP.(K)
Fig. 7. Variation of fiber width with crystallization temperature (20 min).
(1) and V~=
IZt,
we obtain from (2)
F r o m the a m o u n t of seed suspension used per film i.e. 10 ffl of a 0.1 wt% suspension, we get the total volume of seeds V ~ = 2 . 5 X 1 0 6 cm 3. The total volume of the film Vf can be obtained from the dimensions of the cell of the centrifuge and the a m o u n t of gel used. Taking the film thickness as 0.2 # m we get Vf to be 5.16 x 1 0 6 c m 3. A s s u m ing the average agglomerate radius to be 3 /~m, and substituting these values in eq. (2), we esti-
S. Jagota, R. Raj / Sapphire whiskers from boehrnite gel seeded with a-alumina
mate the average length of the whisker to be 7.6 /am. The experimental value lies in the range 2 to 10/am. Thus, a reasonable agreement is obtained. The analysis given here implies that the length of the whiskers can be increased by decreasing the amount of a-alumina seed in the gel. The width of the whiskers was measured from transmission electron micrographs. The change in width with the crystallization temperature is shown in fig. 7. Each point represents 50-100 measurements of the width. The error bars represent the standard deviation of the width distribution. The width at the lower temperatures, where the crystallization was not quite complete, does not include the gel layer remaining on the sides of the whiskers. However, the increase in the width with temperature reported in fig. 7 cannot be accounted for simply in terms of residual gel at the lower temperatures. It is not clear to us why the fiber width depends on the crystallization temperature. Our view is that the fiber growth morphology is determined by the relative kinetics of crystal growth and the relaxation of the viscous gel ahead of the crystallizing interface. The rate at which the gel flows and redistributes the porosity may be the key factor in determining the width of the whiskers. This mechanism would suggest that the rate of viscous relaxation increases more quickly with temperature than does the rate of crystal growth. Crystallization of a-alumina from a seeded gel brings together a number of factors which result in a filamentary growth mechanism. The gel contains physically and chemically adsorbed water about 40% by weight. This water is released, on heat treatment, up to 1000°C. The water loss causes shrinkage in the gel as well as the creation of porosity and high surface area. The transformation of A1OOH to A1203 involves the loss of water and the mechanism of water removal must be coupled to the rate of crystal growth of A1203. A decrease in volume is also caused by the change in crystal structure on going from transition aluminas to a-alumina. Therefore, the porosity, water content and interfacial energy are believed to be the primary variables which control the fiber morphology. The following findings are of particular interest: (i) Seeding reduces the transformation tempera-
533
ture for a-alumina formation from 1200 to approximately 900 ° C. (ii) We think that the crystal morphology is influenced by water loss in the gel and the stresses at the gel-crystal interface caused by the decrease in volume on crystallization. Since the transformation temperature, in the presence of seeds, is reduced to 900 ° C, the temperature range in which transformation a-alumina takes place and that where water loss from the gel occurs overlap. Therefore, the growth of a-alumina must be coupled with the mechanism of water loss from the gel and the subsequent rearrangement of bonds. Also, shrinkage caused by the decrease in volume on crystallization of the gel would lead to stress at the gel-crystal interface and the response of the gel to these volume changes would depend on its viscous flow properties and porosity. Thus, the growth of a whisker may be regarded as a perturbation which grows in response to all the above mentioned conditions. The width of the whisker may be regarded as a characteristic wavelength of the perturbation at that temperature. The increase of this wavelength with temperature would be reflected in the increase of whisker width with temperature which is observed. The temperature dependence of the crystallographic growth velocity in the lateral direction would also be determining factor for the equilibrium whisker width. (iii) The whiskers grow exposing pyramidal {0111 } planes. This direction of whisker growth seems to be one of the preferred growth directions for a-alumina under hydrothermal conditions. Alumina whiskers grown from vapor phase reactions grow to expose either the pyramidal {0111 } or the prismatic (1010} planes [1-5]. Kuznetsov, in experiments on hydrothermal growth of sapphire [26,27], at 500-600 °C, finds the growth velocity of the basal plane to be about 100 times smaller than that of the pyramidal planes and about 20 times smaller than that of the prismatic planes. This could explain why the pyramidal and prismatic planes are exposed preferentially over the basal plane. Kuznetsov found that the growth rates of the pyramidal and prismatic planes approached each other as the crystallization temperature was increased.
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S. Jagota, R. Raj / Sapphire whiskers from boehmite gel seeded with a-alumina
Acknowledgements This research was supported by the Semiconductor Research Corporation under Contract No: 83-01-039 and by the National Science Center at Cornell University.
References [1] W.W. Webb and W.D. Forgeng, J. Appl. Phys. 28 (1957) 68. [2] W.B. Campbell, Chem. Eng. Progr. 62 (1966) 1449. [3] R.C. DeVries and G.W, Sears, J. Chem. Phys. 31 (1959) 1256. [4] G.W. Sears and R.C. DeVries, J. Chem. Phys. 39 (1963) 2837. [5] I. Yamai and H. Scuto, J. Crystal Growth 45 (1978) 511. [6] R.H. Doremus, B.W. Roberts and D. Turnbull, Eds., Growth and Perfection of Crystals (Wiley, New York, 1958). [7] A. Yamamoto, Process for preparing silicon carbide whiskers, US Patent No. 4,500,504, Feb. 1985. [8] B.E Yoldas, J. Mater. Sci. 10 (1975) 1856. [9] L.C. Klein, Ceram. Eng. Sci. Proc. 5 (1984) 379. [10] A.C. Pierre and D.R. Uhlmann, in: Better Ceramics through Chemistry, Mater. Res. Soc. Proc., Vol. 32, Eds. C.J. Brinker, D.E. Clark and D.R. Ulrich (North-Holland, New York, 1984) pp. 119-124.
[11] S.J. Wilson, Proc. Brit. Ceram. Soc. 28 (1979) 281, [12] S.J. Wilson, J. Solid State Chem. 30 (1979) 247. [13] S.J. Wilson and J.D.C. McConnell, J. Solid State Chem. 34 (1980) 315. [14] S.J. Wilson and M.H. Stacey, J. Colloid Interface Sci. 82 (1981) 507. [15] B.C. Lippens and J.H. de Boer, Acta Cryst. 17 (1964) 1312, [16] F.W. Dynys and J.W. Halloran, Alpha alumina formation in AlzO 3 gels, in: Ultrastructure Processing of Ceramics, Glasses and Composites, Eds. L.L.. Hench and D.R. Ulrich (Wiley, New York, 1984) p. 142. [17] F.W. Dynys, M. Ljungberg and J.W. Halloran, in: Better Ceramics through Chemistry, Mater. Res. Soc. Symp. Proc., Vol. 32, Eds. C.J. Brinker, D.E. Clark and D.R. Ulrich (North-Holland, New York, 1984) p. 32. [18] F.W. Dynys and J.W. Halloran, J. Am. Ceram. Soc. 65 (1982) 442. [19] P.A. Badkar and J.E. Bailey, J. Mater. Sci. 11 (1976) 1794. [20] B.E. Yoldas, J. Am. Ceram. Soc. 65 (1982) 387. [21] R.K. Iler, J. Am. Ceram. Soc. 44 (1961) 618. [22] R. Lamber, J. Mater. Sci. Letters 5 (1986) 177. [23] R. Lamber, L. Kepinski and G. Jablonska, J. Mater. Sci. 21 (1986) 2775. [24] R.K. Diwedi and G. Gowda, J. Mater. Sci. Letters 4 (1985) 331. [25] M. Kumagai and G,L. Messing, J. Am. Ceram. Soc. 68 (1985) 500, [26] V.A. Kuznetsov, Soviet Phys.-Cryst. 10 (1966) 561. [27] V.A. Kuznetsov, Kristallografiya 9 (1964) 123.
527
SAPPHIRE WHISKERS FROM BOEHMITE GEL S E E D E D WITH a-ALUMINA
S. J A G O T A and R. RAJ Department of Materials Science and Engineering, Bard Hall, Cornell University, Ithaca, NY14853-1501 New York 14853-1501, USA Received 1 May 1987; manuscript received in final form 9 July 1987
Sapphire whiskers were grown in thin films of boehmite (A1OOH) gel seeded with c~-alumina seeds at temperatures from 900 to 1200°C. The whiskers were 20-60 nm in diameter and had lengths up to 10 /~m. The diameter was found to increase with temperature. The crystallographic growth direction of the whiskers was at 46 o to the c-axis of a-alumina. The sides of the whiskers were faceted, exposing the pyramidal planes. A special technique used to prepare the films allowed us to characterize them directly by transmission electron microscopy. Details of the technique are described.
1. Introduction
1.1. Whiskers Whiskers are filamentary single crystals with aspect ratios greater than 10 : 1. They are remarkable for their high strength which can approach the theoretical tensile strength of the crystal. Ceramic whiskers are, therefore, attractive materials for fiber reinforcement of ceramic composites for applications at high temperature and stresses. Aluminum oxide whiskers are usually grown from the vapor phase either by the oxidation of aluminum [1], reacting hydrogen with aluminum oxide [2,3], or by the hydrolysis of halides [4,5]. The reaction temperatures lie in the range 1 2 0 0 - 1 7 0 0 ° C and A1 or A1203 can be employed to nucleate the crystal growth process [5]. Morphology is controlled through the supersaturation of the reactants. Nearly always, the fiber growth direction is the c-axis of the hexagonal a-alumina crystal. In the present work we show that sapphire whiskers can also be grown from boehmite (A1OOH) gels. This technique is simpler than the vapor phase technique of growing fibers. It consists of seeding the gels with crystals of a-alumina and heat treating these gels at temperatures between 900 and 1200 ° C. The fibrous morphology of crystal growth results from the redistribution of
the porosity in the alumina gel. The width of the whiskers can be controlled by means of the crystallization temperature. We believe that ours is the first example where boehmite gels have been transformed into sapphire whiskers by crystallization. Gels have been used in earlier studies to grow whiskers, but, in those cases, the porous structure of the gel served as a framework for constraining the morphology of the crystallization process [6]. In one case, however, silica gels have been converted into silicon-carbide whiskers by a controlled reaction between the gel and carbon black [71.
1.2. C~stallization of boehmite gels A gels is a fibrous polymerized network of the hydroxide and m a n y hydroxides form gels under suitable conditions of temperature and p H [8,9]. Boehmite, A1OOH, forms a gel which, on that treatment loses water and transforms into several phases of alumina and finally crystallizes to aalumina. The dehydration and transformation of boehmite to a-alumina through metastable phases has been studied extensively in the literature from various points of view [10-25] as discussed below. The structure of the gel is characterized by high porosity. The porosity arises from physically and chemically adsorbed water. Pierre and Uhlmann [10] have studied differences in properties of
0022-0248/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
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S. Jagota, R. Raj / Sapphire whiskers frorn boehmite gel seeded with ~-alumina
boehmite gels under different conditions of preparation. Wilson and coworkers [11-14] used porosimetry and T E M to study (a) changes in pore morphology during dehydration and (b) subsequent phase transformations in boehmite. They [11-14] and Lippens and De Boer [15] have studied the crystal structure of boehmite and transition aluminas by electron diffraction and found that the structure of these aluminas consists of close packed oxygen planes held together by zig-zag O - H bonds. The arrangement of the bonds is such as to accommodate the porosity of the structure. The transformations to the successive phases of alumina may be considered to be topotactic since all have a spinel lattice. The unit cell dimensions for different phases can be derived from each other by a rearrangement of the bonds between the close packed oxygen planes. The formation of a-alumina, however, requires a change in the crystal structure from a spinel lattice to a hexagonal lattice. This formation to a-alumina is believed to take place by nucleation and growth with an incubation time for nucleation [13,16,19[. The transformation of boehmite to a-alumina can be accelerated by seeding the gel with small crystallites of a-alumina. Seeding reduces the temperature for transformation of the gel to a-alumina since self nucleation of a-crystals is no longer necessary. For example, Kumagai and Messing [25] found the transformation temperature to decrease by 170 ° C. Various morphologies of a-crystals grown from gels have been reported and discussed in the literature. At 1200°C, colonies of large (10/~m), porous, vermicular, a-grains are seen [16 23]. Dynys, Ljungberg and Halloran [16,17] also report formation of discrete a-grains 250-500 m m in diameter, when the gel is held at 1000°C. Lamber [22] report equiaxed and columnar grains of a-alumina in thin boehmite films at 1200-1300 ° C. A mechanism for the formation of the vermicular grain has been discussed by Badkar and Bailey [19]. They explain the structure in terms of pinning of the transition alumina matrix by the porosity. Curiously, Kumagai and Messing [25] reported obtaining an equiaxied a-alumina grain structure when seeding was used to promote the phase transformation of the gel.
In the present paper we report results of a study where thin films of boehmite were seeded with a-alumina and then crystallized at temperatures ranging from 900 1200°C. Interestingly, the crystals grew in the form of whiskers. The whiskers were 1 - 1 0 / L m long and 20-60 nm wide. The conditions under which the whiskers grew are reported and possible mechanisms of growth are discussed. The advantage of the thin film technique is that samples can be directly observed in the transmission electron microscope. These observations are reported.
2. Experimental The boehmite gel was prepared by controlled hydrolysis of Al-tri-sec butoxide and repeptization by adjusting the p H to 3.00(_+0.2). The method followed is due to Yoldas [8] and Klein [9]. The initial sol had 40 g/liter of boehmite and formed a gel on drying at 60 o C for 2 - 3 days. The a-alumina seeds had an average diameter of 0.2 /~m. The seeds were dispersed in water at a p H of 2.5 with nitric acid and ultrasonicated to break up agglomerates. The films were made by two methods: (i) a drop of the sol (10/11) and a drop (10/~1) of 0.1 wt% a-alumina seed suspension was placed in the cell of an ultracentrifuge and spun onto T E M grids placed in the cell (W or Ni TEM grids were used). The films were spun at 90,000 rpm for 5 min.
(ii) The T E M grids were dipped in the gel and then the seeds spun onto the film in the centrifuge. Unseeded films were made by the same technique except that no seeds were used. The films were then freeze dried under vacuum overnight. Both techniques resulted in boehmite films thin enough to be viewed directly in the transmission electron microscope. These films were heat treated in an argon atmosphere at temperatures of 700, 900, 1000, 1120 or 1200°C for 20 min after reaching the temperature. The argon atmosphere was used to prevent oxidation of the metal T E M grids. The specimens for T E M were coated with a 10 nm carbon layer, by evaporation, to make them conducting to the electron beam. SEM specimens were sputtered with a 12 nm layer of gold-pal-
S. Jagota, R. R a j / Sapphire whiskers from boehmite gel seeded with c~-alumina
ladium. The whisker dimensions were determined from the T E M micrographs. About 50-100 measurements of the width were made for each temperature. The crystallized phase was identified by X-ray powder diffraction. Fiber orientation was identified using electron diffraction. 3. Results
3.1. Crystallization of unseeded films: Debye-Scherrer X-ray powder diffraction patterns of unseeded films showed that the gel transformed from poorly crystalline boehmite to poorly crystalline g a m m a alumina at 700 °C, to crystalline &alumina at 8 0 0 - 1 0 0 0 ° C and finally to aalumina above 1200 o C. This result is in general agreement with earlier studies of this type [24]. The microstructure of an unseeded film which had been heat treated at 1200°C for 20 min is shown in fig. 1. The fine grained structure in the upper half of the picture consists of &alumina which is a transition alumina. The fine grain size is a result of the pre-existing nuclei in the boehmite gel. Since the starting boehmite is partially crystalline to begin with, the growth of topotactic phases of transition aluminas do not require any nucleation.
529
The lower half of the picture in fig. 1 shows a large single crystal of a-alumina. The crystal contains porosity of a vermicular morphology. This morphology probably results from the redistribution of the porosity in the fine grained transition aluminas as the a-alumina grain grows and consumes the smaller grains of the transition aluminas [19]. The large difference in the grain size between the transition and a-alumina is evidence of a nucleation barrier for the formation of a-alumina. Electron diffraction from the a-alumina crystal shown in fig. 1 revealed that it was oriented with its c-axis normal to the plane of the film.
3.2. Seeded films The seeded films were heat treated at temperatures from 900 to 1200 o C, for a period of 20 min. When the temperature was much less than 900 o C, the film did not transform into a-alumina. A T E M micrograph of the a-alumina seeds taken from a film which was heat treated at 7 0 0 ° C for
ALPHA e~LUMINA
Fig. 1. TEM micrograph of an unseeded gel film calcined at 1200°C showing a vermicular o~-alumina grain growing in a fine grained polycrystalline transition alumina matrix. Marker represents 100 nm.
Fig. 2. TEM mlcrograph of a seeded gel film calcined at 7 0 0 ° C showing seed agglomerates in the gel. Marker represents 200 nm.
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S. Jagota, R. Raj / Sapphire whiskers from boehmite gel seeded with a-alumina
Fig. 3. T E M m i c r o g r a p h of a seeded gel film calcined at 900 ° C showing a - a l u m i n a whiskers w i t h gel r e m a n e n t s a l o n g the sides of the whiskers and c a p p i n g the tips. M a r k e r represents 50 nm.
Fig. 4. T E M m i c r o g r a p h of a seeded gel film calcined to 1200 o C showing a - a l u m i n a whiskers. The whiskers are b r o a d e r t h a n in fig. 3 and gel r e m a n e n t s are not seen. M a r k e r represents 100 nm.
20 min is shown in fig. 2. The seeds are seen to be agglomerated in clusters which are about ten times the particle size. The spacing between these agglomerates varied between 2 to 10 #m. Transmission electron micrographs of films which were heat treated at 9 0 0 ° C and at 1200°C are shown in figs. 3 and 4 respectively. The transformation of the gel into a-alumina is incomplete at the lower temperature since some residual gel material can be seen on the stem of the crystal fibers (see fig. 3). The layer of the gel remaining on the side of the whiskers suggests that the crystals grow much faster along the length direction than in the lateral direction. The sharp edges of the fully crystallized whiskers in fig. 4 shows that the side planes of the whiskers were faceted. Electron diffraction of the whiskers revealed that the growth direction is at 46 o to the c-axis of a-alumina. Fig. 5 shows the diffraction pattern. The (0111) plane axis is along the electron beam direction. Thus, the pyramidal planes [{0111} type planes] are exposed along the lengths of the whiskers. In contrast, the vermicular c~-grain of the unseeded film had the close packed (0001)
S. Jagota, R. Raj / Sapphire whiskers from boehmite gel seeded with a-alumina
531
Fig. 5. D i f f r a c t i o n p a t t e r n f r o m a w h i s k e r f o r m e d at 1120 ° C. B = (0111).
basal plane along the plane of the foil. A SEM micrograph of a film heat treated at 1000 ° C for 20 rain is shown in fig. 6. The regions where the fibers are short and dense are probably where the seeds are located. The fibers which are oriented along the plane of the film are seen to grow to long lengths. Those normal to the plane of the film are short and more closely spaced. Thus, it appears that many whiskers are nucleated from the seed c]hster and some of them, especially those lying close to the plane of the film grow to a length which is approximately equal to the spacing between the seed clusters.
4. Discussion The whisker length can be estimated from the average distance between the seeds in the film. The distance between the seeds can be calculated from the volume of seeds added to the film and by using an average size of the seed clusters, obtained from the TEM micrographs such as the one shown in fig. 2. We assume that the agglomerates are arranged in a square array with distance l between centers of two agglomerates. Let the agglomerates have an average radius r and thickness t equal to the film
S. Jagota, R. Raj / Sapphire whiskers from boehmite gel seeded with a-alumina
532
Fig. 6. SEM micrograph of a seeded gel film calcined at 1 0 0 0 ° C showing whiskers growing on and around seed agglomerates. Marker represents 1.0 #m.
thickness. Then, if the total volume of seeds is Vs, the total volume of the film Vr, volume of each agglomerate ILa, and V~ the volume of a cell of the square array, we must have,
7°f
K/v-. Since I,~ = ~rr2t
6O
vj r , =
v 50 :]2 a
eq. (1)
--
40
,"IN
30
l= r( ~rVf/Vs) 1/2.
tu
20
10
1073
I
I
I
L
II 7 3
1275
1373
[473
TEMP.(K)
Fig. 7. Variation of fiber width with crystallization temperature (20 min).
(1) and V~=
IZt,
we obtain from (2)
F r o m the a m o u n t of seed suspension used per film i.e. 10 ffl of a 0.1 wt% suspension, we get the total volume of seeds V ~ = 2 . 5 X 1 0 6 cm 3. The total volume of the film Vf can be obtained from the dimensions of the cell of the centrifuge and the a m o u n t of gel used. Taking the film thickness as 0.2 # m we get Vf to be 5.16 x 1 0 6 c m 3. A s s u m ing the average agglomerate radius to be 3 /~m, and substituting these values in eq. (2), we esti-
S. Jagota, R. Raj / Sapphire whiskers from boehrnite gel seeded with a-alumina
mate the average length of the whisker to be 7.6 /am. The experimental value lies in the range 2 to 10/am. Thus, a reasonable agreement is obtained. The analysis given here implies that the length of the whiskers can be increased by decreasing the amount of a-alumina seed in the gel. The width of the whiskers was measured from transmission electron micrographs. The change in width with the crystallization temperature is shown in fig. 7. Each point represents 50-100 measurements of the width. The error bars represent the standard deviation of the width distribution. The width at the lower temperatures, where the crystallization was not quite complete, does not include the gel layer remaining on the sides of the whiskers. However, the increase in the width with temperature reported in fig. 7 cannot be accounted for simply in terms of residual gel at the lower temperatures. It is not clear to us why the fiber width depends on the crystallization temperature. Our view is that the fiber growth morphology is determined by the relative kinetics of crystal growth and the relaxation of the viscous gel ahead of the crystallizing interface. The rate at which the gel flows and redistributes the porosity may be the key factor in determining the width of the whiskers. This mechanism would suggest that the rate of viscous relaxation increases more quickly with temperature than does the rate of crystal growth. Crystallization of a-alumina from a seeded gel brings together a number of factors which result in a filamentary growth mechanism. The gel contains physically and chemically adsorbed water about 40% by weight. This water is released, on heat treatment, up to 1000°C. The water loss causes shrinkage in the gel as well as the creation of porosity and high surface area. The transformation of A1OOH to A1203 involves the loss of water and the mechanism of water removal must be coupled to the rate of crystal growth of A1203. A decrease in volume is also caused by the change in crystal structure on going from transition aluminas to a-alumina. Therefore, the porosity, water content and interfacial energy are believed to be the primary variables which control the fiber morphology. The following findings are of particular interest: (i) Seeding reduces the transformation tempera-
533
ture for a-alumina formation from 1200 to approximately 900 ° C. (ii) We think that the crystal morphology is influenced by water loss in the gel and the stresses at the gel-crystal interface caused by the decrease in volume on crystallization. Since the transformation temperature, in the presence of seeds, is reduced to 900 ° C, the temperature range in which transformation a-alumina takes place and that where water loss from the gel occurs overlap. Therefore, the growth of a-alumina must be coupled with the mechanism of water loss from the gel and the subsequent rearrangement of bonds. Also, shrinkage caused by the decrease in volume on crystallization of the gel would lead to stress at the gel-crystal interface and the response of the gel to these volume changes would depend on its viscous flow properties and porosity. Thus, the growth of a whisker may be regarded as a perturbation which grows in response to all the above mentioned conditions. The width of the whisker may be regarded as a characteristic wavelength of the perturbation at that temperature. The increase of this wavelength with temperature would be reflected in the increase of whisker width with temperature which is observed. The temperature dependence of the crystallographic growth velocity in the lateral direction would also be determining factor for the equilibrium whisker width. (iii) The whiskers grow exposing pyramidal {0111 } planes. This direction of whisker growth seems to be one of the preferred growth directions for a-alumina under hydrothermal conditions. Alumina whiskers grown from vapor phase reactions grow to expose either the pyramidal {0111 } or the prismatic (1010} planes [1-5]. Kuznetsov, in experiments on hydrothermal growth of sapphire [26,27], at 500-600 °C, finds the growth velocity of the basal plane to be about 100 times smaller than that of the pyramidal planes and about 20 times smaller than that of the prismatic planes. This could explain why the pyramidal and prismatic planes are exposed preferentially over the basal plane. Kuznetsov found that the growth rates of the pyramidal and prismatic planes approached each other as the crystallization temperature was increased.
534
S. Jagota, R. Raj / Sapphire whiskers from boehmite gel seeded with a-alumina
Acknowledgements This research was supported by the Semiconductor Research Corporation under Contract No: 83-01-039 and by the National Science Center at Cornell University.
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[11] S.J. Wilson, Proc. Brit. Ceram. Soc. 28 (1979) 281, [12] S.J. Wilson, J. Solid State Chem. 30 (1979) 247. [13] S.J. Wilson and J.D.C. McConnell, J. Solid State Chem. 34 (1980) 315. [14] S.J. Wilson and M.H. Stacey, J. Colloid Interface Sci. 82 (1981) 507. [15] B.C. Lippens and J.H. de Boer, Acta Cryst. 17 (1964) 1312, [16] F.W. Dynys and J.W. Halloran, Alpha alumina formation in AlzO 3 gels, in: Ultrastructure Processing of Ceramics, Glasses and Composites, Eds. L.L.. Hench and D.R. Ulrich (Wiley, New York, 1984) p. 142. [17] F.W. Dynys, M. Ljungberg and J.W. Halloran, in: Better Ceramics through Chemistry, Mater. Res. Soc. Symp. Proc., Vol. 32, Eds. C.J. Brinker, D.E. Clark and D.R. Ulrich (North-Holland, New York, 1984) p. 32. [18] F.W. Dynys and J.W. Halloran, J. Am. Ceram. Soc. 65 (1982) 442. [19] P.A. Badkar and J.E. Bailey, J. Mater. Sci. 11 (1976) 1794. [20] B.E. Yoldas, J. Am. Ceram. Soc. 65 (1982) 387. [21] R.K. Iler, J. Am. Ceram. Soc. 44 (1961) 618. [22] R. Lamber, J. Mater. Sci. Letters 5 (1986) 177. [23] R. Lamber, L. Kepinski and G. Jablonska, J. Mater. Sci. 21 (1986) 2775. [24] R.K. Diwedi and G. Gowda, J. Mater. Sci. Letters 4 (1985) 331. [25] M. Kumagai and G,L. Messing, J. Am. Ceram. Soc. 68 (1985) 500, [26] V.A. Kuznetsov, Soviet Phys.-Cryst. 10 (1966) 561. [27] V.A. Kuznetsov, Kristallografiya 9 (1964) 123.