Thin flakes of yttrium hydroxide precipitates coagulated in a manner similar to a house of cards at temperatures below room temperature. Increasing the aging temperature not only resulted in an increase the flake thickness but also breaking of the coagulation. Fine disks of yttrium hydroxide appeared above 50 deg C. Sulfate ions enhanced the sinterability of yttria. The fabricated yttria powder was mixed with a commercially available alumina powder in the ratio of yttrium aluminum garnet, YAG. Many inclusions and pores were noticed in the powder compact sintered at 1600 deg C. Yttrium ions were rich in some inclusions and poor in others. The inclusions and pores disappeared on firing at 1700 deg C for 1 h. Transparency was achieved in the case where highly sinterable yttria powder was mixed with alumina powder with a particle size of 0.3 mu m.
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