Processing of Ceramics. Группа авторов
(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.
Dr. Coble outlined the removal of residual pores when he developed translucent alumina by pressureless sintering. In the case of translucent alumina, when the relationship between (1) grain growth rate and (2) moving speed of pores is (1) > (2), pores are trapped inside the grains as explained in Figure 1.8a, and these residual pores become main scattering sources. As he introduced a trace amount of MgO as a grain growth inhibitor (pinning center), the above relationship is changed to (1) ≤ (2) and he succeeded to reduce the number of residual pores in the ceramics. However, in the case of laser materials even if there is “a small number of pores” remained inside, it severely affects the laser function and therefore it is still necessary to develop the materials with “zero pores.” Also, it is necessary to control the grain boundary phase in nano‐size and dislocations at grain boundaries. Figure 1.8b shows the product of the research applied to the discharge tube of a high‐pressure sodium lamp. From here, it will be a different part from Coble.
Figure 1.9a shows granules obtained by spray drying an Al2O3‐Y2O3 system (composition is YAG) slurry with a spray dryer. Figure 1.9b‐1 and b‐2 show the fracture surface after the granules were molded in a metal mold observed by SEM. Granules of about 50 μm diameter exhibit a real spherical structure in which Al2O3 and Y2O3 are homogeneously mixed. When these granules are press‐molded inside the metal mold, the granules collapse, and finally, the spherical granule shape disappears, and a green powder compact with a uniform structure is obtained. Figure 1.9b‐1 shows a granule using a binder with good crushing property (a binder having excellent plastic deformability is used in this case), and it has a homogeneous structure with a molding pressure of 20 MPa. Figure 1.9b‐2 shows the fracture surface structure of the powder compacted body when granules with poor crushing characteristics (i.e., binders having high elastic deformability are used in this case) are similarly molded at 20 MPa. Since it is a hard granule, the strength of powder compact after molding is strong, but the form of granules (the part where the granules are not collapsed) can be confirmed inside the green powder compact.
Figure 1.8 (a) First demonstration of translucent alumina ceramics by Dr. Coble (he explained how to remove pores from inside of polycrystalline ceramics) and (b) topical application for translucent alumina ceramics.
Figure 1.9 (a) Appearance of granulated Al2O3‐Y2O3 powders by spray drier and (b) internal structure of Al2O3‐Y2O3 powder compact after uniaxial press under 20 MPa.
The powder compact showed in the above figure is further pressed in CIP (cold isostatic press) machine at 140 MPa. Then, the pore size distribution of this sample after CIP was measured by using the mercury penetration method. The result is shown in Figure 1.10. The pore diameter of the sample using a binder with good crushing characteristics is concentrated at 30–40 nm, but large voids of μm size are detected in the other samples without a binder, resulting in structure defects (which induce light scattering sources at the end product). Once large structural defects are formed, it is difficult to completely remove them even by high‐pressure sintering techniques such as HP (hot press) and HIP (hot isostatic press) treatments.
In order to effectively eliminate residual pores on the fracture surface of the green powder compacts, it is necessary to prepare before sintering the green compact with pore size smaller than the particle size of the raw powder materials to be used and to have a narrow and sharp distribution in the pore diameter. By subsequent preliminary sintering, it is necessary to prepare a sintered body having a relative density 98–99%, and further, it must be subjected to HIP treatment to prepare a sintered body having almost no residual pores. It depends on the sintering properties of the raw materials.
Figure 1.10 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.
Figure 1.11 shows the image of pore removal by HIP (hot isostatic pressing) treatment. The relationship between the pore size and the grain size is important, and the key point is timing of HIP treatment. This point is determined by the sintering properties of the raw materials, type of sintering additives, and its adding amount. In addition, the most important point is to clarify that the residual pores are totally removed from the materials after the HIP treatment or whether the pores were merely shrunk and still remained inside the materials. If the pores can be completely removed, the transmittance of the material will be improved from Type A (Mie scattering) to Type B (Rayleigh scattering), and hence, the remaining technical issue will be only how to control Rayleigh scattering inside the transparent