Low-Temperature Fabrication of Transparent Yttrium Aluminum Garnet (YAG) Ceramics without Additives

A carbonate precursor of yttrium aluminum garnet (YAG) with an approximate composition of NH₄AlY_0.6(CO₃)_1.9(OH)₂·0.9H₂O was synthesized via a coprecipitation method from a mixed solution of ammonium aluminum sulfate and yttrium nitrate, using ammonium hydrogen carbonate as the precipitant. The precursor precipitate was characterized using chemical analysis, differential thermal analysis/thermogravimetry, X-ray diffractometry, and scanning electron microscopy. The sinterability of the YAG powders was evaluated by sintering at a constant rate of heating in air and vacuum sintering. The results showed that the precursor completely transforms to YAG at ∼1000°C via the formation of a yttrium aluminate perovskite (YAP) phase. YAG powders obtained by calcining the precursor at temperatures of ≤1200°C were highly sinterable and could be densified to transparency under vacuum at 1700°C in 1 h without additives.