Processing of Ceramics. Группа авторов
defects in a common ceramic is illustrated in Figure 1.5. As shown in the figure, ceramics is composed of fine grains (single crystals) with random crystal orientation. Light scattering factors in ceramics are (i) existence of grain boundaries, (ii) residual pores localized at grain boundaries or inside grains, (iii) inclusions, (iv) reflection or double refraction due to grain boundaries, and (v) surface roughness. However, the fundamental differences on the microstructure affecting the light scattering between the single crystal material and the ceramic material are as follows: (i) the presence of grain boundaries and (ii) volume of residual pores. It is believed that these are the main sources of scattering and cause a large difference in optical performance.
On the other hand, Dr. Greskovich of GE developed a transparent Nd‐doped 10%ThO2‐Y2O3 ceramics, and in 1974, he succeeded in laser oscillation at room temperature with this polycrystalline ceramic for the first time in the world. 10%ThO2 was added as sintering aid to Y2O3, and after sintering it at high temperature of 2200 °C for about 100 hours, he succeeded to obtain transparent Nd:ThO2‐Y2O3 ceramics (about 10 years before his success in room temperature laser oscillation, there was a report on laser oscillation in cryostat system using Dy:CaF2 ceramics, but details of the physical properties of these ceramics are unknown).
Reflection micrograph and transmission micrograph of 1%Nd:ThO2‐Y2O3 ceramics are shown in Figure 1.6 [6, 7]. In the reflection micrograph, residual pores are observed on the surface, and microstructure defects (voids and grain boundary phases) are observed in the transmission micrograph. Also, in visual observation, it is a heterogeneous structure naming “orange peel.” Optical quality of this transparent ceramic material is remarkably higher than the conventional translucent ceramics. Although the level of optical loss of this ceramic reached about several %/cm, it is poor optical characteristics as laser material. It was succeeded at pulsed laser oscillation at room temperature by lamp excitation, but the slope efficiency was 0.1%, which is a very low result. His technology of successful laser oscillation at room temperature was the world's highest level in the 1970s, and laser oscillation by ceramics is a great dream for material scientists, but the next challenger did not appear for nearly 20 years afterward. This fact is also evidence that no one could exceed his technology and ideas in 20 years.
Figure 1.5 Illustration of optical scattering caused by various microstructure defects in common polycrystalline ceramics.
Let us discuss about the very strict requirement of optical properties for laser material in the following. Figure 1.7 shows the optical loss of material and the relative value of laser amplification caused by the optical loss. By continuously adding external energy (incoherent light) into the laser material, it is converted into “coherent light” in the laser gain medium by light amplification.
For this reason, the generated laser light repeats light amplification while reciprocating in the medium many times. At this time, if the optical loss in the medium is zero, it is 1n (n is the number of reciprocations in the medium). If the optical loss of the medium is 0.1 and 1%/pass and light amplification is carried out 10 times, then the output is computationally (0.999)10 = 0.99 and (0.990)10 = 0.90 (i.e., reduction of 1 and 10% output power, respectively). In the case that the light amplification is carried out 100 times, then the output is (0.999)100 = 0.90 and (0.990)100 = 0.36 (i.e., reduction of 10 and 64% output power, respectively), which leads to a significant reduction in lasing efficiency; therefore, laser amplification using a laser material with an extremely low optical loss is essential (this calculation simplifies the laser power subtracted by optical loss inside the gain medium).
Figure 1.6 (a) Reflection and (b) transmission microscopic photograph of 1%Nd:ThO2‐Y2O3 ceramics by Greskovich.
Source: Akio Ikesue, Yan Lin Aung, Voicu Lupei (2013), Ceramic Lasers, Cambridge University Press. https://doi.org/10.1017/CBO9780511978043.
Figure 1.7 Relationship between optical scattering loss and amplifying number of optical resonators depend on laser power.
1.4 Objective of Optical Grade Ceramics
The translucent ceramics that began to be developed in the late 1950s are used in high‐pressure sodium vapor lamp, and PLZT ((PbLa)ZrO3) ceramics are applied to flash‐protecting shutters utilizing the electro‐optic effect, after that development on various types of materials and application research were conducted, but research and development stagnated because fundamental problems related to optical technology cannot be solved. The purpose of writing this book is to describe; (i) how the author challenged with what kind of idea to develop the “optical grade polycrystalline ceramics” which is far superior to the conventional translucent ceramics and comparable to the optical grade single crystalline materials, (ii) how to evaluate the optical quality of fabricated optical ceramic materials, and (iii) how to feedback the obtained information from the evaluation into the fabrication process and so on.
Conventionally, optical grade single crystalline materials are produced by various methods such as Verneuil (VN), floating zone (FZ), Bridgman (BM), and Czochralski (Cz), and basically, the crystals are grown after melting the raw materials. Industrial manufacturing 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