A precursor for Y 3 Al 5 O 12 was synthesized as a YAG sol by simply dissolving Y 2 O 3 powder in an alumina sol. Phasepure Y 3 Al 5 O 12 powder was obtained by precipitating the YAG sol with an aqueous dilute ammonia solution followed by calcination at 1100°C. TG/DTA analysis showed an exotherm at 938°C attributed to formation of YAG phase and weight loss of 44% at 1000°C. XRD and FT-IR analysis showed that phase-pure YAG can be formed through noncrystalline and metastable hexagonal YAlO 3 without forming either yttrium or aluminum formate intermediate.

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.