methods are also established, and relatively high‐quality crystal materials can be easily obtained but there are also various disadvantages. The optical quality of conventional optical ceramics represented by translucent alumina ceramics is remarkably poorer than that of the above mentioned single crystals, but if there is a technology that can produce a ceramic material that can compete with single crystal, then the fabrication of new materials that was impossible by the conventional single crystal growth technology and also other advantages such as fabrication of large size and mass productivity will become possible. The question is what kind of idea is to carry out with material development. It is said that the single crystal fabrication method has limitations on the homogeneity of the grown single crystalline material because segregation tends to occur at the solid–liquid interface during crystal growth under gravitational field, and it is difficult to synthesize more homogeneous materials unless it is synthesized under zero gravity (for example, outer space). In addition, since the conventional method basically is a process of solidifying from a temperature higher than the melting point, there is also a problem that lattice defects inside the material are likely to be involved. However, it is also a fact that the optical performance of a single crystal is much higher than the conventional transparent ceramics. Despite the fact that ceramic specialists have carried out various researches so far, they were in the dark and had no direction where they should open the key of technology.
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 of common ceramics, but the material is composed of microcrystals (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
