Читать книгу Processing of Ceramics - Группа авторов - Страница 12

Human beings have used ceramics symbolized by tableware from ancient times, but the modernization of ceramics and the “ceramic science” based on sintering began since the middle of the twentieth century. In recent years, engineering ceramics used for bearing parts, milling media, surface plate for semiconductor steppers, minor parts of automobile engines, pyroelectric materials for infrared detection, PTC (positive temperature coefficient), NTC (negative temperature coefficient) thermistors as temperature sensors, inkjet printer, touch panel, moreover, sonar for fish finder in fishery and military application, piezoelectric material as ultrasonic diagnosis in medical field, ionic conductors for air‐fuel ratio control in automobile and gas sensor for oxygen detection in molten steel in steel production, magnetic materials used in general motor and servomotor, translucent ceramics as functional materials used in high‐pressure sodium discharge lamps and optical shutters, and so on. Without ceramics, the economic activity of modern society is impossible now.

The development of the translucent ceramics mentioned above was initiated by the development of translucent alumina ceramics by Dr. Coble in the 1950s and its application to high‐pressure sodium lamps [1]. Principally, ceramics has been considered to be opaque, but he controlled the microstructure of ceramics (that is, by controlling the migration rate of pores and grain boundaries in the process of sintering alumina), and was able to reduce the volume of residual pores, characteristic light scattering sources in ceramics, and finally, he succeeded in developing alumina ceramics which can transmit visible light for the first time in the world. By applying this idea, PLZT (lead lanthanum zirconate titanate) ceramics for flash protection, MgF₂ ceramics for infrared ray transmission, scintillation optical ceramics such as Pr,Tb:GOS (Gd₂O₂S) ceramics and Eu:(GdY)₂O₃ ceramics for X‐ray CT (computed tomography) etc. were successively developed, and some of them are already applied in industrial field. About 50 years have passed since the invention of translucent alumina ceramics by Dr. Coble, and it has been applied to high‐pressure sodium discharge lamp until today (2018).

Figure 1.1 Schematic design and transmission mechanism of sodium lamp using translucent Al₂O₃ ceramic tube.

As shown in Figure 1.1, the high‐pressure sodium lamp is illuminated by discharging the metal sodium (Na) inside the alumina tube by applying a high voltage and radiating the luminescent line to the outside of the discharge tube. The radiation efficiency of high‐pressure sodium lamp is about 14–20% which is very high compared to 1–3% of the normal light bulb, has high power, and has a long life (8–9000 hours) so it is used for lighting in tunnels and express highway. The wall thickness of this alumina tube is at most about 1 mm, and the in‐line transmittance is only about 20–30%, and the other discharge lights are radiated from the inside to the outside by diffuse transmission.

Radiated light certainly generates Fresnel loss of about 7% inside the discharge tube, and it is also radiated while repeating transmission and reflection inside the tube. The total transmittance (i.e., amount of light radiated to the outside of the discharge tube/total radiation amount) of the alumina discharge tube is about 97–95%, and the light energy of 3–5% is lost. That is, if calculated simply, optical loss of 3–5%/mm (discharge tube thickness) is generated. The optical quality of these alumina tubes is not a major problem for discharge lamp application even if the in‐line transmission and scattering characteristics are not sufficient. Because it is sufficient as long as the total transmission (total of in‐line & diffuse transmission) is high. Even in the translucent alumina ceramics that has been continuously improved, there are still many scattering sources inside; for example, residual pores, pinning agents such as MgO, Y₂O₃ situated at the grain boundary portions, and also grain boundary phases (generally spinel and YAG [Y₃Al₅O₁₂]) due to the reaction between the pinning agents and the host material. The optical loss of a laser gain medium which requires extremely high optical properties such as high optical homogeneity and extremely low optical scattering should be preferably less than 0.1%/cm (basically scattering with nearly zero). Even for crystal materials, it is very difficult to meet this strict requirement. Therefore, it should be considered impossible to apply ceramic laser with extension of Dr. Coble's technology developed in the 1950s.

In 1974, Dr. Greskovich developed Nd:Y₂O₃‐ThO₂ ceramics and demonstrated laser oscillation, but the concentration of scatterers (especially residual pores and segregated phases) inside the material was too high and lasing efficiency was only less than 0.1% (pulse oscillation only) because of lamp excitation system at that time. In the 1980s, Dr. With of Philips developed translucent YAG (Y₃Al₅O₁₂), and in 1990, Dr. Sekita of NIRIM demonstrated Nd‐doped YAG ceramics, but laser oscillation was not achieved. Therefore, it was considered that significant laser oscillation by polycrystalline ceramics is impossible in principle.

In 1991, the main author was not an expert in laser or ceramics, but just a refractory engineer. I asked Japanese lasers and material scientists, “Can laser oscillation with polycrystalline materials be theoretically possible?” However, the laser scientist says, “Even with glass or single crystals, homogeneity and scattering are being a problem, so ceramic materials are out of question.” Material scientists answered, “ceramics with many scattering sources in the material are impossible to generate laser.” Judging from the level of ceramic production technology at the time in 1991, their answers were correct. However, my thinking is that “exploration on the truth of natural science and prediction of the future is the mission of a scientist” (at that time I was a refractory engineer and not holding a Ph.D.), and I could not simply accept their opinion. So, I decided to work on “potential of ceramics for future exploration in the optical field.” The idea “laser oscillation by ceramics” started in the summer of 1991 and confirmed the success of production in December of that year, but since the author belonged to the private company, it could be published in 1995.

In 1995, the author demonstrated highly efficient laser oscillation by using polycrystalline ceramic materials with performance that could match or surpass high‐quality single crystal [2], but materials and laser experts at the time were highly skeptical about our report. One of the reasons is that I was not an expert in ceramics and lasers, and the invention was by a person from a different field (i.e., a refractory‐related engineer working for steel smelting). Generally, a large number of scattering sources (such as residual pores and heterogeneous phases) are present in the common ceramics, causing significant Mie scattering. Especially from the technical point of view, no one proposed to remove those residual pores completely in ceramics. The density of the transparent ceramics is much higher than the opaque ceramics used for other applications, and it shows good transparency, but even in this case, numbers of pores of more than 1000 ppm are remained inside the ceramic material. Another hurdle to overcome is even if we were able to remove those residual pores completely, nobody had answers to “the problem of Rayleigh scattering from grain boundary phase and grain boundary,” which is the biggest technical problem in terms of technology. There were many reports from ceramists at that time describing that a lot of residual pores (scattering sources) are present in the ceramics material, and also, structure defects certainly generated when the granulated raw materials are pressed in molding process, and these defects are definitely remained even in the final sintered body. They have reported those descriptions with photographs of microstructure of ceramics as evidence. However, the reports only described the observation method for defects in ceramics and observed results, and there were no discussion on the cause of defect formation and how to solve this problem. The authors thought that structure defects in materials were generated mainly by artificial factors, and we should clarify the problem sources and modify the microstructures; finally, we will be able to eliminate those defects including residual pores. Once Mie scattering can be removed from the inside of the material, the remaining problem will be only Rayleigh scattering. When I attended a conference in ceramic society, I saw some researchers reported that “This ceramic has a clean grain boundary,” but the transparency of their ceramics was not as good as single crystals although they showed a clean grain boundary. That was a basic contradiction point noticed to me, and I simply interpreted their “clean grain boundary” that during observation with an electron microscope they could observe only the clean area where secondary phase does not exist. Actually, we should pursue what is the fundamental nature of Rayleigh scattering. Also, we should further pursue that even if we can form a really clean grain boundary, whether or not it will be possible to obtain ceramic material with ultra‐low scattering or without scattering. To acquire such evidences are the essence of good research work, and hence, there can be progress in science and technology and there are roots that can create the next innovation. At that time, I was an amateur (refractory engineer) who knew nothing about lasers and ceramics, but when I reconsidered now probably, I had a challenging spirit pursuing the essence of material science because of lack of knowledge about laser and ceramics. In this chapter, we will describe how the ceramic material, which is a guideline for the development of various optical ceramics, changed from the translucency level [3] to the optical grade material, based on the development of ceramic laser materials.