Processing of Ceramics. Группа авторов

Figure 1.21 Optical inspection of polycrystalline Spinel Ceramics by sintering method and Spinel crystal by Verneuil and Czochralski (Cz) methods.

      Figure 1.21a‐1–4 show the appearance of polycrystalline spinel ceramics of ϕ20 × t10 mm produced by the sintering method. It is very transparent and has a lower optical loss than the Czochralski single crystal even in the visible region. The transmitted wavefront image by the interferometer showed a straight fringe (<0.1λ/cm [λ = 633 nm]). In the measurement using the polarizer, the optical stress was below the detection limit. Furthermore, there is no inhomogeneous part in the Schlieren observation. Finally, the reason why the absorption edge of polycrystalline ceramics becomes shorter and the band gap shows a larger value than that of a single crystal will be described in the following. As shown in the Figure 1.21b,c, the spinel single crystals have a domain structure with nonuniform refractive index. The chemical formula of the spinel is MgAl₂O₄, and this material can also be a solid solution with “a large composition tolerance.” When the chemical composition of the spinel single crystal is analyzed, MgO/Al₂O₃ (M/A Ratio) was 1.00, and the average composition of the material is almost stoichiometric.

It is considered that the cause of the nonuniform refractive index domain structure observed by Schlieren observation is that the M/A ratio in each domain is different. When materials having different spinel M/A ratios are formed, cation or anion defect structures are induced to form inside the material, and these defect structures certainly affect the band gap. Therefore, it is considered that the absorption edge of the single crystal shifted to the longer wavelength side. Since the refractive index domain structure could not be detected in the spinel ceramics, the shift of the absorption edge due to defects inside the material was small, and this result was similar to the transmission spectrum behavior by computational science. This result breaks the conventional outline that the optical uniformity of single crystals is absolutely superior to ceramics.

      The optical uniformity of ceramics reported to date (2020) is far superior to single crystals regarding different types of materials, and the superiority of optical quality of single crystal material which has been regarded as common sense began to collapse. It is extremely difficult to explain by classical theory about the optical properties of ceramics produced by modern science. Rayleigh's scattering theory was built in the nineteenth century, but it is a mathematical expression of the atmospheric scattering phenomena by calculation. The author thinks that the Rayleigh's scattering theory is still correct in the sense of its theory, but when applying the theory by computational science to a practical material, we should consider again the contradictions existing in theory and in reality, etc., and therefore, it is essential to predict the future of material science in this way.

      Some technological examples which changed the material science will be described in Chapter 4. It is difficult to produce TAG (Tb₃Al₅O₁₂) and YIG (Y₃Fe₅O₁₂) single crystals by FZ method, and it is not easy to synthesize high‐quality crystals because these single crystals are incongruent materials. The optical quality of ceramics prepared by sintering method for these materials is equal to or superior to those of single crystals, demonstrating the scale‐up and high productivity of optical performance and size that cannot be realized with single crystals. For optical isolators used in laser processing in the 1 μm band (especially for fiber lasers), TO (Tb₂O₃) and TYO (Tb₂O₃‐Y₂O₃) ceramics could be expected to become Faraday rotation elements having the largest Verdet constant, but their melting point is as high as around 2400 °C and also have phase transition point just below the melting point; therefore, the synthesis of single crystals is virtually impossible. For this reason, the representative of the Faraday rotation element is TGG (Tb₃Ga₅O₁₂) crystal. But the TO and TYO ceramics which the author first demonstrated in the world show Verdet constants which are nearly four times that of the TGG single crystal, and this fundamental performance makes the isolator device much smaller. An isolator device using this material has already been put to practical use and contributes to the high reliability of a fiber laser for processing.

1.5 Conclusions

      As mentioned above, it is a translucent ceramic pioneered by Coble in the 1950s, but the technical potential since 1995 has significantly improved since the development of ceramic laser materials. Indeed, ceramics became a paradigm of optical materials, and in the future industrial application, it is beginning to change the era from “optical single crystal to optical ceramics.” There are some technical issues, but one of them is that the crystal structure of current ceramic (polycrystalline) optics is limited to cubic system. Since the anisotropic material has birefringence, there is no possibility as an optical material from the viewpoint of the current science and technology. However, if it is possible to create true nano‐sized materials without birefringence which has never been experienced, there is the possibility of initiating the same technological innovation as cubic system ceramics. On the contrary, if it is possible to create a single crystal from compositionally homogeneous fine grains, there is also the possibility of an ideal crystal that can breakthrough an anisotropic single crystal synthesized from the conventional melt solidification method. The synthesis of laser grade ceramics by sintering method suggests the development of infinite material science in the future.

      Finally, what the scientist should not forget is “What is the truth and what should we do for the future?” the authors themselves will continue to try new challenges based on this philosophy.

References