Effect of crystallization of alumina hydrate in preparation of alumina-coated composite particles

Ceramics International 18 (1992) 201-206

Effect of Crystallization of Alumina Hydrate in Preparation of AluminaCoated Composite Particles H i r o y u k i N a k a m u r a & A k i o Kato Department of Chemical Science and Technology, Faculty of Engineering, Kyushu University, 6-10-1, Hakozaki, Higashi-ku, Fukuoka, 812, Japan (Received 1 July 1991; accepted 1 August 1991)

Abstract: Fine alumina hydrate-coated silica particles, as a precursor of mullite

(3A12Oa" 2SIO2), could be made from the system of colloidal silica suspension in aluminium sulphate aqueous solution with a homogeneous precipitation technique using urea. In this method, it was important to keep the alumina hydrate in a noncrystalline state. When the reaction was continued until high pH to allow the crystallization of alumina hydrate, the hydrate separated from SiO2 particles and the intimate composite structure of both phases was broken. Alumina hydratecoated SiC whiskers could also be prepared from the system of SiC whiskers suspended in aluminium sulphate aqueous solution by using the same method. In this case, it was also important to keep alumina hydrate in a non-crystalline state to keep the intimate composite structure of SiC whiskers and alumina hydrate.

1 INTRODUCTION

ever, in the preparation of alumina hydrate-coated particles, crystallization of alumina hydrate is found to have a remarkable effect on the morphology of composite particles. In the present paper, we studied the influence of experimental conditions, especially the crystallization of alumina hydrate, on the morphology of composite particles.

Recently, composite particles having a structure where a core particle is coated with a second phase have attracted increasing attention from various sources. 1'2 The main objectives of the coating are improvement of chemical stability and dispersion nature of particles, and homogeneous addition or mixing of second phase. When the coating materials are oxides, the solution method is useful. Using the solution method, composite particles of mullite4 Y ' Z r O 2 , 3 ~ - A 1 2 O a - T i O 2 , 4 and Cr2Oa-alumina hydrate 5 have been prepared. We have prepared composite particles in which SiO 2 particles are coated with alumina hydrate from silica particles and aluminium sulphate by a homogeneous precipitation method. This composite powder gave highly sinterable mullite powder by calcination. 6"v The present authors have also found that SiC whiskers can be coated with alumina hydrate by using homogeneous precipitation. How-

2 EXPERIMENTAL PROCEDURE 2. 1 Preparation of mullite

Experimental procedures are shown in Fig. 1. Colloidal silica (Snowtex-OS, 8nm in diameter, Nissan Kagaku Industries, Ltd (Fig. 2)) was dispersed in distilled water and aluminium sulphate (reagent grade, Wako Pure Chemical Industries, Ltd) corresponding to mullite composition was dissolved in the suspension. This mixture was heated to 90°C and then urea (reagent grade, Wako Pure Chemical) was added to start the reaction. During 201

Ceramics International 0272-8842/92/$05.00 © 1992 Elsevier Science Publishers Ltd, England. Printed in Great Britain

Hiroyuki Nakamura, Akio Kato

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the reaction, the solution was stirred by magnetic stirrer, and p H and a m o u n t of CO2 evolved were measured. The precipitate was washed, with distilled water initially and finally with ethanol using a centrifugal separator, and subsequently dried in vacuum at 70°C. The dried powder was heat treated at various temperatures for 1 h to investigate the mullitization.

2.2 Preparation of alumina hydrate-coated SiC whiskers Experimental procedures are shown in Fig. 3. SiC whisker (Tokai Carbon, TWS-400 (1.2/~m in diameter and 40 Fm in length)) were treated with 55% H F solution to remove surface SiO2, and passed through a sieve (65 mesh). After SiC whiskers were dispersed in distilled water, AI2(SO4) 3 was dissolved in this suspension, and agitated with supersonic waves for 15 min and then stirred with a wing-type stirrer for 30rain to reduce e n t a n g l e m e n t o f whiskers. During the stirring, the suspension was heated to 90°C. SiC whiskers picked up from the solution at this point are shown in Fig. 4. SiC

l Reaction J 90"C I l(~ent ri f ugation Washing I i IVacuumdrying J 70"C Fig. 3. Preparation procedures for alumina hydrate-coated

SiC whiskers. whiskers seem to be dispersed well. After this treatment, urea was added to start the reaction. During the reaction, the suspension was stirred with a wing-type stirrer. The precipitate was washed similarly to mullite and dried in vacuum at 70°C. 3 RESULTS A N D D I S C U S S I O N

3. 1 Preparation of alumina hydrate-coated silica particles U n d e r the applied conditions [colloidal S i O 2 ] = 0"05 M, [Al 3 + ] = 0" 15 M and reaction temperature = 90°C, aluminium sulphate was the most suitable aluminium salt for preparing alumina hydratecoated silica particles. W h e n aluminium nitrate and aluminium chloride were used as an aluminium source, alumina hydrate deposited separately from the silica particles. 6'7

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O.Ipm Fig. 2. Colloidal silica.

20,um Fig. 4. Dispersion state of SiC whiskers in reacting solution.

Crystallization of alumina-coated composite particles

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3.1.1 Calcination of precipitate

The change of pH, amount of C O 2 evolved, yield of aluminium hydrate, and morphology of precipitate are shown in Fig. 5. Numbers on photographs correspond to numbers on the pH curve. The yield of aluminium hydrate was measured by chelate titration of the remaining aluminium ion in the supernatant. In the initial precipitate, 0 , particles of 10-12 nm in diameter could be seen. The particle size of the precipitate increased gradually as the pH became higher, showing alumina hydrate deposited on silica particles. These particles coagulated heavily around pH 4 or at the point of 80% yield and the particle diameter was about 0-1 ~m.

The powder from the aluminium sulphate had a structure such that aluminium hydrate coated silica particles could be converted to single phase mullite with calcination at 1250°C for I h. In contrast, the powder from aluminium nitrate had a morphology such that aluminium hydrate deposited separately from the silica particles could not be converted to single phase mullite by calcination at 1300°C for 3 h. X-ray diffraction patterns of the powder calcined at 900-1250°C for 1 h are shown in Fig. 6. At 900°C, no crystalline phase could be seen except a halo of amorphous silica. At 1000°C, an AI-Si spinel phase

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Hiroyuki Nakamura, Akio Kato

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[AI2(SO,)a] = 0-075M, [SiO2] = 0"05M, [Urea] = l'8M, reaction temperature = 90°C.

appeared. At 1200°C, the mullite phase appeared and finally the A1-Si spinel phase disappeared to give single phase mullite at 1250°C. For the formation of mullite from silica and alumina, two routes have been proposed; s one is direct formation and another is the formation by way of the A1-Si spinel phase. The present result supports the latter process.

3.1.2 Effect of co,stallization of alumina hydrate on mullitization When aluminium sulphate was used as the aluminium salt, except for the slight difference in the degree of aggregation, there were no remarkable changes in morphology under the reaction conditions examined, i.e. urea concentration (0"9-3"6M), reaction temperature (80-95°C) and aluminium sulphate concentration (0.0375-0.15M). All precipitates prepared under these conditions were amorphous and converted to single phase mullite by calcination at 1250°C for 1 h. Although the yield of alumina hydrate was 100% at p H = 5 . 5 , the morphology of the precipitates changed with final pH, as shown in Fig. 7, When the reaction was stopped at pH=7.2, the alumina hydrate deposited was XRD amorphous. On the other hand, when the reaction was continued up to pH=8-6, the alumina hydrate crystallized into pseudo-boehmite. The amorphous alumina hydrate which coated silica particles at pH = 7-2 separated from the silica particles after crystallization into pseudo-boehmite at pH = 8.6. When such segregation of silica particles and alumina hydrate occurs,

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0.2jurn Fig. 7. Effect of final pH on morphology of precipitates. (a) Reaction was stopped at pH = 7.2; (b) reaction was stopped at pH = 8.6. [AI2(SO4)a] = 0"075M, [SiO2] = 0"05M, [Urea] = l'8M, reaction temperature = 90°C.

the powders can not be converted to single phase mullite by calcination at 1300°C for 3 h. Therefore, one may conclude that one must stop the reaction before crystallization of alumina hydrate in the preparation of alumina hydrate-coated silica particles as the precursor of mullite powders. It should be noted that when amorphous precipitates prepared from SiO2 and Al2(SO,) 3 at 90°C were washed with distilled water and suspended in aqueous ammonia (pH 8"8) at room temperature for 18 h, alumina hydrate crystallized into bayerite, but the crystallization did not occur from suspension in distilled water at room temperature or 90°C for 18 h. It was also observed that when the precipitation was carried out at low concentrations of the reactants s u c h a s 0 . 0 l 5 i aluminium sulphate, 0-01 i silica and 0"038M urea at 90°C, the alumina hydrate crystallized into pseudo-boehmite even when reaction

Crystallization of alumina-coated composite particles

205

4 CONCLUSIONS

was stopped at pH 7.2. All powders in which alumina hydrate crystallized into pseudo-boehmite or bayerite could not be converted into single phase mullite by calcination at 1250°C for 1 h.

As a precursor of mullite (3A120 a • 2SIO2), alumina hydrate-coated SiO2 particles can be prepared from the system of colloidal SiO2 suspension in aluminium sulphate solution with a homogeneous precipitation technique using urea. In this method, it is important to avoid the crystallization of initially deposited amorphous alumina hydrate because the crystallization breaks down the composite structure of the hydrate-coated SiO 2 particles. Similarly, the composite structure of alumina hydrate-coated SiC whiskers can be prepared with a homogeneous precipitation method using urea. Also, in this case, suppression of the crystallization of amorphous alumina hydrate deposited on SiC whiskers is important to hold the composite structure.

3.2 Preparation of alumina hydrate-coated SiC whiskers Alumina hydrate-coated SiC whiskers were prepared under the conditions of [A12(SO4)3] = 0"075M [SiC whisker] =0"21M and [Urea] = l'8M at 90°C. SEM photographs of the products obtained at a final pH of 7.0 and of 8-7 are shown in Fig. 8. Alumina hydrate obtained at pH 7.0 was X R D amorphous and had a structure such that alumina hydrate coated SiC whiskers. In contrast, alumina hydrate crystallized into pseudo-boehmite and separated from SiC whiskers at pH 8-7. From these results, we know that alumina hydrate-coated SiC whiskers can be prepared by a homogeneous precipitation method. In order to prevent the destruction of the composite structure, however, it is also important to stop the reaction before crystallization of alumina hydrate occurs.

A C K N O W L E D G EM ENTS The present authors thank Nissan Chemical Industries Ltd for supplying colloidal silica. T E M photographs were taken in Research Laboratory of High Voltage Electron Microscope, Kyushu University.

(a)

(b) Fig. 8. Alumina hydrate-coated SiC whiskers. (a) Reaction was stopped at pH=7.0; (b) reaction was stopped at pH = 8.7. [Al2(SO,t)3 ] ---0"075M, [SIC-] = 0"21M,[Urea] = l'8M, reaction temperature= 90°C.

206

REFERENCES 1. KATO, A., Seramikkusu, 26 (1991) 187-90. 2. HIRANO, S. Seramikkusu, 25 (1990) 8-10. 3. HIRANO, S., HAYASHI, S. & KATO, C., Funtai-OyobiFunmatsuyakin, 37 (1990) 371-5. 4. OKUMURA, H., BARRINGER, E. A. & BOWEN, H. K., J. Mater. Sci., 24 (1989) 1867-80.

Hiroyuki Nakamura, Akio Kato 5. KIM, B. & YASUI, I., Yogyo-Kyokai-Shi, 95 (1987) 442-9. 6. KATO, A., KATATAE, Y. & N A K A M U R A , H., Research paper of Asahi Grass Foundation, 56 (_1990) 39-46. 7. NAKAMURA, H. & KATO, A., Ceramic Transactions, Vol. 22, In Ceramic PowderScience IV, Am. Ceram. Soc., 1991, pp. 463-8. 8. KANZAKI, S., Seramikkusu, 23 (1988) 1060-4.