(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 materials.
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.
