Thin flacks of yttrium hydroxide coagulated in a manner similar to houses of cards at again temperatures lower than room temperature. Increase of againg temperature resulted in not only increasing the thickness of the flacks concerned but also breaking the coagulation. Fine disks of yttrium hydroxide appeared above 50 deg C. Sulfate ions enhanced sinterability of yttria. The yttria powder fabricated 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 the rest. Such inclusions and pores disappeared by firing at 1700 deg C for 1h, and transparency was achieved in the case where the highly sinterable yttria powder was mixed with the alumina powder with a particle size of 0.3 #mu#m.
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