Transparent Ceramics: From Protective Windows to Laser Gain Media, Relations Between the Use Dopants and Sintering Aids, The Microstructure and the Properties

Transparent ceramic materials are polycrystalline materials with perfect or close-to-perfect microstructure that are used in cutting edge technologies and products. They find use both as structural and functional materials. Structural ceramics represented mostly by MgAl2O4 spinel, AlON or Al2O3, exploit their superior thermal, mechanical and tribological properties in comparison to transparent glasses, and find applications as transparent windows, protective domes and others,. Functional transparent ceramics are mostly based on garnets (e.g. YAG, LuAG), sesquioxides (Y2O3, Sc2O3, Lu2O3) or fluorides (CaF2), with the addition of active dopants (lanthanides or transition metals), and find applications as laser gain media, scintillators or phosphors. In order to achieve a full transparency, the material has to be free of pores and secondary phases, which would act as scattering centers. Such stringent requirements imply that these materials must attain density close to the theoretical value, along with the material properties. In comparison to single crystals, transparent ceramics offer a major advantage in the shaping process, because they can be produced by a variety of ceramic shaping techniques. This allows the production of near-net-shape components that do not require further machining, or even in-situ production of components with controlled variations in composition. The latter aspect is particularly interesting in laser gain media. The production process of transparent ceramics requires the use of high-purity raw materials and a careful control of stoichiometry to prevent the formation of secondary phases, and approaches leading to a complete elimination of porosity. To eliminate pores, advanced sintering processes are usually used, e.g. vacuum sintering or pressure-assisted sintering. Moreover, sintering aids are often used to control the microstructure. This presentation will introduce both the production and application possibilities of transparent ceramics. The relation between the microstructure and properties will be illustrated, along with the use of sintering aids and the effect of dopants on material properties.