Chaika M. Conditions and formation features of the phase composition and microstructure of YAG:Cr4+ ceramics.

The work is devoted to the study of conditions and regularities of formation of phase composition and microstructure of Cr4+:YAG ceramics. The influence of CaO additive in combination with SiO2 aid on the formation of structural and optical properties of ceramics of (Y,Ca)3 (Al,Cr)5O12, 0.14 wt% SiO2 was studied. The relationship between the SiO2/CaO ratio and the microstructure of the obtained ceramics was determined; the ratio of 37/63 wt.% causes the formation of fusible compounds in the SiO2-CaO system, which leads to abnormal grain growth with the entrapment of pores in the grain volume. An increase in the CaO fraction contributes to both the transparency of ceramics and the increase in the concentration of Cr4+ ions. The interaction of Ca2+ and Si4+ ions results in the formation of a charge-neutral Ca2+_Si4+ complex, which causes the formation of Cr4+ ions at a much lower concentration than in single crystals of YAG:Cr4+. It was concluded that the SiO2 aids is not suitable for the synthesis of YAG:Cr4+ ceramics of high quality. The influence of CaO additive on the formation of microstructure and optical properties of YAG ceramics has been established. Rise concentration of Ca2+ ions from 0.15 at.% to 0.5 at.% lead to increase the average grain size from 1.5 μm to 5 μm without manifestations of abnormal grain growth and the formation of impurity phases. The influence of CaO additive on the formation of structural and optical properties of Cr4+:YAG ceramics was determined. The degree of transparency and microstructure of ceramics depends on the interaction of CaO and Cr2O3. This interaction leads to the modification of the sintering process due to the formation of fusible compounds that cause the formation of porous ceramics with micron pores. The efficiency of Cr4+ ions formation depends on the concentration of Ca2+ ions. Low concentration of charge compensator decreases the probability of formation of Ca2+_Cr4+ complex, excess of Ca2+ ions leads to Ca segregation at the grain boundary. It was established that for the synthesis of high-quality optical ceramics YAG:Cr4+ it is necessary to take well-defined concentrations of CaO additives (C(Ca)=0.5 at.%) and Cr2O3 (C(Cr)=0.1 at.%). Based on the obtained data, the influence of CaO, Cr2O3 and SiO2 additives on the formation of the functional properties of YAG:Cr4+ ceramics was established allowing the synthesis of YAG:Cr4+ ceramics with optical losses at the level of 0.1 cm-1 and an absorption coefficient of 4 cm-1 at λ = 1064 nm (a patent of Ukraine for invention). These characteristics meet the requirements for materials for passive Q-switching solid-state lasers with generation at a wavelength λ = 1064 nm. On bench tests, using a passive Q-switcher made from the developed ceramics of YAG:Cr4+, a generation pulse with a frequency of 8 kHz and a duration of 10 ns was obtained.