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Processing of Ceramics


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defects in a common ceramic is illustrated in Figure 1.5. As shown in the figure, ceramics is composed of fine grains (single crystals) with random crystal orientation. Light scattering factors in ceramics are (i) existence of grain boundaries, (ii) residual pores localized at grain boundaries or inside grains, (iii) inclusions, (iv) reflection or double refraction due to grain boundaries, and (v) surface roughness. However, the fundamental differences on the microstructure affecting the light scattering between the single crystal material and the ceramic material are as follows: (i) the presence of grain boundaries and (ii) volume of residual pores. It is believed that these are the main sources of scattering and cause a large difference in optical performance.

      On the other hand, Dr. Greskovich of GE developed a transparent Nd‐doped 10%ThO2‐Y2O3 ceramics, and in 1974, he succeeded in laser oscillation at room temperature with this polycrystalline ceramic for the first time in the world. 10%ThO2 was added as sintering aid to Y2O3, and after sintering it at high temperature of 2200 °C for about 100 hours, he succeeded to obtain transparent Nd:ThO2‐Y2O3 ceramics (about 10 years before his success in room temperature laser oscillation, there was a report on laser oscillation in cryostat system using Dy:CaF2 ceramics, but details of the physical properties of these ceramics are unknown).

Schematic illustration of optical scattering caused by various microstructure defects in common polycrystalline ceramics. Photos depict (a) Reflection and (b) transmission microscopic photograph of 1%Nd:ThO2-Y2O3 ceramics by Greskovich.

      Source: Akio Ikesue, Yan Lin Aung, Voicu Lupei (2013), Ceramic Lasers, Cambridge University Press. https://doi.org/10.1017/CBO9780511978043.

Schematic illustration of the relationship between optical scattering loss and amplifying number of optical resonators depend on laser power.

      The translucent ceramics that began to be developed in the late 1950s are used in high‐pressure sodium vapor lamp, and PLZT ((PbLa)ZrO3) ceramics are applied to flash‐protecting shutters utilizing the electro‐optic effect, after that development on various types of materials and application research were conducted, but research and development stagnated because fundamental problems related to optical technology cannot be solved. The purpose of writing this book is to describe; (i) how the author challenged with what kind of idea to develop the “optical grade polycrystalline ceramics” which is far superior to the conventional translucent ceramics and comparable to the optical grade single crystalline materials, (ii) how to evaluate the optical quality of fabricated optical ceramic materials, and (iii) how to feedback the obtained information from the evaluation into the fabrication process and so on.

      First, I would like to mention how I was able to challenge the development of laser ceramics which was considered “thoughtless.” Please refer Figure 1.6 again. This figure shows a microstructure image