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

Yttrium aluminum garnet optical ceramics (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 C. The influence of SiO2 and CaO sintering aids on optical properties and microstructure of Cr,Ca:YAG ceramics was investigated. A decrease in CaO concentration led to a reduction of the Cr,Ca:YAG ceramics transparency from 81 to 0% at 1064 nm, moreover, samples with a lower concentration of CaO had abnormal grains and high-level porosity. The obtained features were attributed to the interactions between silica and calcium oxide during vacuum sintering of Cr,Ca:YAG ceramics. We propose and discuss two possible ways in which CaO and SiO2 influence the formation of Ca,Cr:YAG ceramics. The first one involves the appearance of a liquid phase thanks to the CaO-SiO2 interaction during the heating stage of ceramics sintering which results in abnormal growth of ceramics grains. The second mechanism consists on the mutual consumption of Si4+ and Ca2+ ions during the isothermal stage of ceramic sintering, which leads to a decrease in Cr4+ concentration. Both mechanisms have a negative effect on the optical and laser properties of the ceramics.