Synthesis of nanocrystalline yttria powder and fabrication of transparent YAG ceramics

Synthesis of nanocrystalline yttria powder from Y(NO3)3 solution and ammonia water was investigated. It was found that the precursor precipitate is Y2(OH)5(NO3).H2O. The addition of small amount of ammonia sulfate in yttrium nitrate solution can reduce the agglomeration and particle size of the produced yttria powders. Nanocrystalline yttria powder (60 nm in average size) was obtained by calcining the precursor at 1100 °C for 4 h. Using this yttria powder and a commercial ultrafine Al2O3 powder, fully transparent YAG ceramics was fabricated by vacuum sintering at 1700 °C for 4 h through a solid-state reaction method. It was found that the addition of 0.5 wt.% tetraethyl orthosilicate (TEOS) is suitable for the fabrication of transparent YAG ceramics. If the amount of TEOS is less than 0.05 wt.%, abnormal grain growth occurs, and pores are entrapped in the grains. If the amount of TEOS is more than 3 wt.%, large amount of liquid phase is yielded, leaving some residual inclusions at grain boundaries. These are detrimental for the transparency of YAG ceramics.