Abstract: Nanocrystalline yttria powders were synthesized from Y(NO₃)₃ solution and ammonia water by a precipitation method. The chemical composition of the hydroxide precursor was Y₂(OH)5NO₃·H₂O. The effects of pH values and striking process on the properties of hydroxide precursor and resultant yttria powders were studied. At low pH conditions (pH=7.9) of the normal striking method, the
precipitation precursor has a layered structure. If the final pH value is comparatively high (pH=10.0), the layered nature of the precipitation precursor decreases, and the crystallites are more fine. By using reverse-strike technique, the layered nature of the flocs disappears, and it is mainly composed of more equiaxed crystallites. More sinterable yttria powders can be synthesized by
calcining the hydroxide precursor with platelet-shaped structure. A small amount of ammonium sulfate doping can reduce the agglomeration of the resultant yttria powders. Resultant Y₂O₃ powders synthesized at a low pH (below 9) show much better sinterability than those from higher pH value derived precursor. By using the nanocrystalline yttria powder, transparent yttria ceramics were produced by vacuum sintering at 1700℃ for 4h without any additives.

Key words: Y₂O₃, transparent ceramics, optical properties, nanopowders