Processing of Ceramics. Группа авторов
microscope, and when it is irradiated with a He–Ne laser (from the left side of the image), the light is scattered from the vicinity of the interface between the base material and the residual pores (the portion where the refractive index fluctuation exists). In this state, when the light of the transmission microscope is turned off and the observation is continued while irradiating the He–Ne laser, circular Mie scattering can be clearly confirmed scattered from the interface of residual pores and the base material. This scattering conditions are shown in Figure 1.19e. Ceramic materials have numerous grain boundaries, and even though the He–Ne laser passes through the grain boundaries, the scattered light cannot be detected by a normal tomography or a tomography using an optical microscope. This means that scattering from the grain boundaries is very insignificant or almost nonexistent (not limited to the spinel ceramics). Therefore, it is still necessary to seriously discuss grain boundary scattering phenomenon in ceramics, which has been doubted to be used as an optical material.
Figure 1.19 (a) Optical loss at 1064 nm and laser tomography at 633 nm of polycrystalline and single crystal, (b) residual pore and Mie scattering from pore by He–Ne laser.
By the way, Rayleigh scattering is expressed as follows.
where θ is the scattering angle, I0 is the light intensity before transmission, n is the refractive index; R is the distance between the measurement point and the scattering source, d is the scatterer size, and λ is the measuring wavelength. Basically, light scattering increases in proportion to the power of the scatterer size to the sixth power, the reciprocal of the measurement wavelength to the fourth power. In common sense which is well recognized by almost all material scientists until now, “There are many dislocations at the grain boundaries in the ceramics and their dislocations become scattering sources causing grain boundary scattering, so the transmittance of the ceramics increases as the wavelength becomes shorter.” However, the obtained result is opposite to the conventional common sense that “polycrystalline ceramics having grain boundaries are superior in optical properties to single crystals, and in particular, in the short wavelength region, they show a significant difference in optical properties.”
It is important that the optical material must be “extremely low scattering,” but optical uniformity is also a very important parameter. Laser beam patterns after passing through the spinel ceramic (25 mm‐thick), Czochralski and Verneuil single crystals irradiated with a He–Ne laser having a Gaussian mode are observed with a beam profiler, and these results are summarized in Figure 1.20. When laser irradiation is performed, no scattering line is detected inside the spinel ceramic material (see Figure 1.20a), and only Fresnel scattering (surface scattering due to the difference in refractive index between air and the base material) is observed at the input and output surface of laser irradiation. As a reference, the original beam pattern is shown in Figure 1.20b‐1. The beam that has passed through the Verneuil spinel single crystal with significant optical inhomogeneity showed the greatest distortion (see Figure 1.20b‐2). The beam that has passed through the spinel single crystal by the Czochralski method is also deformed into an elliptical (vertical) shape (see Figure 1.20b‐3). Only the beam that has passed through the ceramic maintains a concentric shape similar to the original beam (see Figure 1.20b‐4) (because the material surface is not AR‐coated, the laser beam that has passed through any material is attenuated by about 24% compared to the original beam due to surface reflection). Beam quality is a critical parameter for optical materials, and the superiority of ceramics, which is different from the conventional understanding, has been proved. The fact that spinel ceramics exhibit extremely low scattering and high beam quality is closely related to the microstructure of the material. As can be seen from the laser tomography shown in above Figure 1.19, a clean grain boundary in which no grain boundary phase exists reduces Rayleigh scattering. In addition, since there is almost no residual pore as a main scattering source, Mie scattering can be almost completely eliminated.
Figure 1.20 (a) He–Ne laser irradiation test and (b‐1) original and (b‐2‐4) changing of beam pattern via various specimens.
The following results clearly indicate that high beam quality, one of the lifelines of optical materials, can be guaranteed. The internal optical stress of the spinel single crystals prepared by the Verneuil method and the Czochralski method was observed using a polarizer. In addition, the uniformity of the refractive index inside these crystalline materials was observed with a Schlieren imaging system. These observation results are summarized in Figure 1.21b‐1, b‐2, c‐1 and c‐2. The Verneuil single crystal shows significant optical inhomogeneity as well as significant optical stress. Since the single crystal of the Czochralski method has a core at the center of the grown crystal ingot, a high‐quality peripheral portion is used as a sample. However, optical distortion is also observed in this part. Although the optical uniformity is better than the crystal grown by the Verneuil method, “a domain structure of around Φ1 mm having a nonuniform refractive index” is still observed inside the material.
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