Preparation of SiC Particle-dispersed Alumina Powders by Spray Pyrolysis Method Using an Ultrasonic Atomizer

Abstract

It is well known that using the powder preparation technique to obtain homogeneously mixed powders is one of the key manufacturing processes to fabricate composites with superior properties. With the various synthesis processes of powders, the spray pyrolysis technique has such advantages as a single continuous process of quick precipitation and pyrolysis from independent droplets of raw solutions to ®nal form of oxides or compounds, high dispersibility of particles, small primary particle size due to quick precipitation and homogeneous distribution of elements, at least in a droplet, due to direct solidi®cation from a solution. An extensive overview of ceramic powder preparation by spray pyrolysis was given by Messing et al. [1]. While there are many reports on the fabrication of oxides or compounds of oxides by spray pyrolysis method, as cited in [1], and there are some reports on oxide composites [2±4], there are a few reports on the fabrication of non-oxide ceramic composite powders. Mizutani et al. [5] reported that Si3N4 powder can be obtained from a precursor powder made by pyrolysis of polysilazane in N2 atmosphere and subsequent heat treatment at 1400 8C in N2. Lindquist et al. [6] obtained BN powder from a poly(borazinylamine) dissolved solution containing NH3 further calcined at 1600 8C. Amorphous gel precursor particles, which transformed into SiC by heating at 1500 8C, were synthesized by means of spray pyrolysis from a dispersion of colloidal silica, saccharose and boric acid [7]. Recently, formation of ZnS and CdS particles from metal nitrate and thiourea, SC(NH2)2, solution [8]. In these studies, except for the last one, non oxide ceramic powders were obtained after calcination of as-sprayed powders. In this letter, therefore, the fabrication of SiC particle-dispersed alumina as a raw powder for nanocomposite fabrication by the spray pyrolysis method is presented. To avoid further calcination after pyrolysis, we used a suspension of ®ne SiC particles into aluminum nitrate solution as a raw material. Aluminum nitrate enneahydrate (Al(NO3)3 . 9H2O, Nacalai Tesque, Japan) was dissolved in distilled water at room temperature. The concentration of Al(NO3)3 was ®xed as 0:3 mol y1. Two kinds of SiC powders were dispersed into the aluminum nitrate solution. One powder was Ultra®ne Grade (Ibiden Co., Japan) with an average particle size of 0:28 im and a speci®c surface area of 24:1 m gy1 (SiC-A), and the other was made by Sumitomo-Osaka Cement Co. (Japan) with an average particle size of 0.03 im and a speci®c surface area of 50:2 m gy1 (SiC-B). The amount of SiC added was 5, 10 and 20 vol % for the case of SiC-A and 5 and 10 vol % for SiC-B, respectively. Both SiC powders were the â-form of SiC. Two kinds of dispersing agents, sodium hexametaphosphate, (NaPO3)6 (Kanto Chemical, Japan), and SN-Dispersant 5468 (Sun Nopco, Japan) were added to an amount of 0.6 wt % of SiC-A and 3 wt % of SiC-B powders, respectively. Fig. 1 shows the schematic drawing of the spraypyrolysis apparatus used in this study. The atomizer consisted of a pair of small pots with an ultrasonic vibrator (1.6 MHz) at the bottom, suspension circu-