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Processing of Ceramics


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(small crystal grains) with random orientations, and there are also many scattering sources. Dr. Coble succeeded in getting translucent Al2O3 ceramics for the first time by aiming at reduction of pores by microstructure control (especially by inhibition of grain growth during sintering process). Even after that, development was carried out by the same method, but many scattering sources remain in the material that cannot be removed by Coble's method, and the application was still limited. Although the idea on the development of laser ceramics is not largely different from the past, firstly “complete removal of macroscopic structure defects causing Mie scattering” is essential. Here, removal of residual pores will be described as a typical example.

Photos depict (a) First demonstration of translucent alumina ceramics by Dr. Coble and (b) topical application for translucent alumina ceramics. Photos depict (a) the appearance of granulated Al2O3-Y2O3 powders by spray drier and (b) internal structure of Al2O3-Y2O3 powder compact after uniaxial press under 20 MPa. Schematic illustration of pore distribution of Al2O3-Y2O3 green body after cold isostatic press (140 MPa) by mercury penetration method.

      The residual pore volume inside the Nd:YAG ceramics at 1995 (by the time the ceramic laser was firstly developed) was at the level of several ppm, but recently it has become possible to control to the residual pore volume below the ppt level, which is essential for larger size with higher quality. It is a remarkable numerical value that the residual pore volume is as low as 10−8 as compared with the translucent alumina. HIP (hot isostatic press) has been used since the 1980s, and it has been confirmed that it is effective for densification of materials. When the sintered material is heat‐treated at high temperature and high pressure, the material is densified, and the density is increased. It seems that HIP is an effective densification (pore removal) process, but in most cases, it compresses residual pores to reduce the pore size inside the materials, and apparently, its density was increased. In the case of only apparently densified samples, rebound of shrunken pores occurs when heated in a pressureless condition, and in the worst case, the transparency returns to the opaque body again. In some cases, the high‐pressure Ar gas which introduced from the grain boundaries or trapped in the pores expands rapidly during the heat treatment process, and finally, the HIP‐treated sintered body may explode. For example, even if the residual pores shrink due to HIP pressure, Mie scattering and Rayleigh scattering certainly occur; therefore, “pore removal process” is essential.