Surface modification of Si3N4 with alumina was tried. It was achieved by simply mixing Si3N4 powder with an alumina sol up to ∼2 wt% as alumina in an aqueous medium, dried, and followed by calcination at 400°C for 1 h. A TEM micrograph showed a coating layer of ∼15 nm thickness. The isoelectric point of the modified Si3N4 powder with porous alumina was at pH 7.8, which is different from 5.8 and 8.6 for Si3N4 and amorphous alumina, respectively.

A method is introduced to prepare almost-spherical submicrometer-sized ␣-alumina via surface modification of ␥-alumina with an alumina sol. Milled ␥-alumina, in the presence of 3 wt% of ␣-alumina with a median particle size (d 50 ) of 0.32 m (AKP-30), produced irregularly shaped ␣-alumina with d 50 Ϸ 0.3 m after heat treatment at 1100°C for 1 h. ␥-alumina that had been surface-modified by milling in the presence of 3 wt% of the alumina sol resulted in almostmonosized, spherical ␣-alumina ϳ0.3 m in size after heat treatment at 1100°C for 1 h. Furthermore, almost-spherical ␣-alumina 0.1-0.2 m in size was obtained by milling ␥-alumina with 3 wt% of AKP-30 alumina in the presence of 3 wt% of the alumina sol, followed by heat treatment at 1100°C for 1 h. The alumina sol that has been introduced in this work seems to act as a dispersant, in addition to helping to form a spherical shape.