Transparent ceramics for LASER applications

Abstract

The aim of this thesis is to obtain transparent ceramics based on YAG of high optical quality. Ceramics of high optical quality can replace the single crystal in LASER applications, potentially out-performing the latter while reducing the production costs. The aim is clear and simple, but the path to follow is not. A transparent ceramic comparable to a single crystal must have no defects: no pores nor secondary phases. Only if the material is just YAG phase the light can pass through without scattering. To obtain a fully dense flawless transparent YAG ceramic is not easy. Before sintering the pressed sample has density that is around 50 % of the density of YAG, in other words is half powder and half pores. During sintering all the pores have to fully close, to help the process the sintering is in high-vacuum and sintering aid are used. Sintering aids moderate the grain growth, limiting the formation of intragranular pores. The other process that happens during sintering are the solid-state reactions between alumina and yttria. Precise weigh of the starting powders is required to avoid residual secondary phases. Two selected dopants are: ytterbium and chromium. To reduce the presence of defects different approaches were tested. The first approach, was to select powders of alumina and yttria that differ by morphology and manufacturer. The idea was to combine powder with different morphology and make correlation with the obtained microstructure. It was observed that powder morphology also influences the packing efficiency. To better understand the evolution of the microstructure samples were sintered in high-vacuum and in air, for shorter duration. The second approach focused on the ceramic process. Residual pores can result from local inhomogeneities in the pressed sample, that in turn can come from residual aggregates in the starting powders. The ceramic process was modified to increase the disaggregation of the starting powder. In this section is introduced a new way to compare the optical quality. With increasing optical quality, the use of SEM is not enough to characterize the material. A systematic characterization using optical microscope was developed to determine the residual porosity in the samples. The third approach played on the type of sintering aid. TEOS has been the only sintering aid used in the former approaches. Magnesium oxide was used as sintering aid, alone or in combination with TEOS. The ability of MgO to limit the grain size is interesting to increase the thermo-mechanical properties. The fourth approach make use of pressure-assisted densification techniques to force the closure of residual pores. This approach generally yields higher optical quality compared to vacuum sintering alone. The techniques used were hot isostatic pressing, the golden standard for the purpose, and field assisted sintering technique, with a set-up that has never been used before. The main focus in Cr:YAG was the optimization of the concentration of sintering aids. The necessity to oxidize chromium to Cr4+ does not allow to use silica as sintering aid. Divalent metals were used instead, namely calcium and magnesium oxides. Hot isostatic press was used to obtain perfect samples. Annealing cycles were performed to oxidize chromium ions.