Processing of Ceramics. Группа авторов
Figure 1.11 (a) Image of removing pores from polycrystalline ceramics by hot isostatic press. (b) Typical transmittance spectrum by “Mie” and “Rayleigh” scattering.
In synthesizing Nd:YAG ceramics with high transparency, a trace amount of SiO2 is added as a sintering aid, but even if only a slight excess of sintering aid (SiO2) is introduced, the grain boundary phase tends to be precipitated. Figure 1.12a is a TEM image of the grain boundary part. Although the grain boundary phase is only 70 nm, Rayleigh scattering certainly occurs, and the transmission spectrum behavior will be like Type B. Detection of the grain boundary phase of nm size is difficult with SEM observation, but even by the transmission polarizing microscope, it is possible to detect nano‐sized heterogeneous phase (grain boundary phase) observed by TEM. Figure 1.12b is a transmission microscope (open nicol) photograph. Only transmitted light through the inside of the material can be observed. Since the crystal structure of Nd:YAG is cubic, there is no birefringence unless there is a heterogeneous phase. Therefore, when it is observed under polarized light (cross nicol condition), the whole image gets black. However, as shown in Figure 1.12c, needle shape birefringence was detected in this material; its position corresponds to the grain boundary area, so it can be judged as a grain boundary phase. Also, when the specimen stage of the optical microscope is rotated, light and dark (angle dependence of birefringence) are repeated alternately, so that the crystal structure of the grain boundary phase can be easily found out. In addition, since the distribution of grain boundary phases can also be understood, it is a promising method for inspecting Rayleigh scattering sources. In general, the grain boundary phase is determined by many factors such as impurities incorporated in raw material, kind, and amount of sintering aids, and sintering process (especially cooling process condition), so it is necessary to set up procedures to control these parameters so that grain boundary phases do not generate inside the materials.
Figure 1.12 (a) TEM image of Y3Al5O12(YAG) ceramics including excess SiO2 near grain boundary, (b) transmission microscopy image of YAG ceramics under open nicol, and (c) transmission polarizing microscope image of YAG ceramics. We can clearly see secondary phase on grain boundaries.
Rayleigh scattering arises from nano‐sized scatterers near grain boundaries. Figure 1.13 shows transmission spectra of single crystal and polycrystalline ceramics of the same composition (0.6% Nd:YAG). Laser generation is a process of extracting laser light (coherent light) with a wavelength of 1064 nm by exciting a gain medium with an LD (laser diode) with a wavelength of 808 or 885 nm; hence, scattering characteristics around 1 μm is particularly important. If there is grain boundary scattering in the transmission spectrum, the wavelength dependence of the transmittance occurs (that is, the transmittance of the ceramic decreases as the wavelength becomes shorter). But in this case, the transmission spectrum of both shows exactly the same behavior and does not obey the Rayleigh scattering law. Even when compared with a material with a thickness of 100 mm, the transmittance of both of them in the 1 μm region is nearly the theoretical limit (about 83%), and it can be understood that there exist “material sciences” having different concepts from the conventional ceramics.
Figure 1.14a shows a SEM photograph of the fracture surface of the fabricated Nd:YAG ceramics, and Figure 1.14b shows the HR‐TEM image near the grain boundary of this material. Although the material undergoes intergranular fracture, the presence of fine pores and grain boundary phases cannot be recognized, and even in HR‐TEM, the grain boundary has a clean structure with no grain boundary phase (it is not just a clean grain boundary that has been told so far, and in fact it has been proven in terms of its properties). Since Nd:YAG grains have different crystal orientations, dislocations are present near grain boundaries, so it is important to verify whether or not that dislocation parts cause optical scattering in the laser oscillation wavelength region.
Figure 1.13 Transmission spectra of Nd:YAG single crystal and polycrystalline ceramics with same Nd content and thickness between 400 and 1200 nm wavelength.
Figure 1.14 (a) Fracture surface of Nd:YAG ceramics by SEM and (b) lattice structure of Nd:YAG ceramics around grain boundary by high‐resolution transmission electron microscopy.
Figure 1.15 (a) Schematic setup of laser tomography with 1 μm light source, (b) sample with grain boundary phase, and (c) sample with no including anisotropic phase measured by tomography.
Figure 1.15a shows the setup of the laser tomography used for detecting the light scattering inside the material (the light source has a wavelength of 1 μm). Here, the material is irradiated with a laser beam, and scattered light inside the material is captured with a CCD camera installed in a perpendicular direction. Figure 1.15b shows a material with a grain boundary phase, Figure 1.15c shows a sample with no grain boundary phase and a scattering coefficient equivalent to that of a single crystal. In the sample shown in Figure 1.15b, scattering from the grain boundary area can be easily detected, but in the sample of Figure 1.15c it is difficult to detect scattered light. Recently, we fabricated Yb:YAG ceramics (size 80 × 190 × t6 mm) for high power laser oscillation. After optical polishing and AR coating, then its optical loss at 1 μm was measured. Its scattering loss was as low as 0.02%/cm. We confirmed extremely low scattering. Generally, the optical loss of a single crystal is 0.1%/cm, but nowadays, the optical loss of polycrystalline ceramics is significantly lower than that of the single crystal. Although the obtained results cannot be understood from the conventional material science, actually how to consider these real facts is to open the “further science doors.” As