Influence of CaO and SiO2 additives on the sintering behavior of Cr,Ca:YAG ceramics prepared by solid-state reaction sintering

Optical ceramics of yttrium aluminum garnet (Y3-xCaxAl4.95Cr0.05O12) doped with 0.1 at.% of chromium ions and 0.5, 0.8, and 1.2 at.% of calcium ions were prepared by reactive sintering in vacuum at 1750 degrees C. The influence of SiO2 and CaO sintering aids on the optical properties and microstructure of Cr,Ca:YAG ceramics was investigated. A decrease in CaO concentration led to a reduction of Cr,Ca:YAG transparency from 81 to 0 percent at 1064 nm; moreover, ceramics with lower CaO content exhibited abnormal grains and high porosity. These features were attributed to interactions between silica and calcium oxide during vacuum sintering. Two mechanisms are proposed to explain the effect of CaO and SiO2 on the formation of Ca,Cr:YAG ceramics. The first involves the formation of a liquid phase due to the CaO-SiO2 interaction during the heating stage, resulting in abnormal grain growth. The second mechanism is the mutual consumption of Si4+ and Ca2+ ions during the isothermal stage of sintering, which leads to a decrease in Cr4+ concentration. Both mechanisms negatively affect the optical and laser properties of the ceramics.