Processing of Ceramics. Группа авторов
of clearing the technical and economic problems of current single crystal isolators. Development and practical application of blue‐violet LED and LD are advancing. In addition, LED lighting that can convert blue‐violet light into white light was also commercialized by using organic–inorganic composite phosphors in which Ce:YAG powder was dispersed in silicone resin. However, the development of all‐ceramic phosphors (Ce:YAG ceramics as a representative example) has been carried out in response to the demand for long durability and high power application, and some applications have begun to be applied in automobile head lamps and projectors. In addition, for military applications, the development of high strength and highly transparent ceramic dome and ceramic armor to replace sapphire single crystal has also been advanced, and the basic technology of laser ceramics will be applied in various fields in the near future. It has been thought that grain boundary scattering (Rayleigh scattering) cannot be avoided even if scattering sources, excluding grain boundaries, are completely eliminated in ceramics. It should be noted that Rayleigh scattering is the theory of the scattering phenomenon in the atmosphere and is not premised on scattering in real substances (ceramics). Of course the theory is correct, but it seemed that we, the material scientists, wrongly imagined the limits of material development just by assumption, without confirming the truth of natural science. Polycrystalline ceramics with optical properties superior to high‐quality single crystals have been developed in the wavelength range from ultraviolet to visible to infrared recently, and the concept of optical material development based on the conventional theory has been completely overturned. This “technological innovation” has caused the trend in optics to move from single crystal to polycrystalline ceramics.
This book “Breakthroughs in Optical Materials” not only introduces research and development examples starting from the development of ceramic lasers that broke through conventional common sense, but also mentions historical background, theory, manufacturing process, and applications. This book is a compilation of the transparent ceramics revolution that I started working on since 1991.
Akio Ikesue
1 Introduction
Akio Ikesue and Yan Lin Aung
World‐Lab Co., Ltd. Mutsuno, Atsutaku, Nagoya, Japan
1.1 Introduction
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,