Overview:
According to the definition of Japanese scholars, certain shapes of blanks, produced by using inorganic powders with a certain molding method, under a certain temperature, atmosphere, and pressure conditions, ceramics with a certain degree of transparency are sintered to be transparent ceramics. It is generally stipulated that in the case where the in-line transmittance is greater than 40%, it becomes a transparent ceramic, and some scholars also call it a translucent ceramic. Compared to the single-crystal transparent ceramics of the melting method, the sinterable transparent ceramics, due to its multi-element composition and influencing factors, have crystal structures that are composed of more than two kinds of crystals and polycrystals with irregular geometries, and are therefore called “polycrystals. Transparent ceramic.” Transparent ceramics made of high-purity alumina powder are generally polycrystalline and are called “polycrystalline transparent alumina ceramics”.
In 1957, some ceramics scientists from the United States, according to the principle of crystal transparency, using ceramic production methods, successfully prepared the first transparent alumina ceramic – “Lucalox”. Open up new application areas of ceramic materials, since then, the research and development of ceramic materials has entered a new stage, triggered the upsurge of research and application of transparent ceramics. With the deepening of the research and exploration of the ceramic material’s sintering aids, sintering process, crystal structure, light transmission mechanism, thermodynamic properties, and specific applications, the performance of transparent ceramics has come a long way. Nowadays, ceramic gold halides have been prepared. Transparent aluminum oxide ceramic discharge tube in the lamp, transparent zirconia ceramic lens, yttrium aluminum garnet laser transparent ceramic, magnesium aluminum spinel fairing, transparent ceramic armor, and so on.
First, the preparation method of polycrystalline transparent alumina ceramics
The preparation process of transparent ceramics is not much different from the preparation process of ordinary ceramics, but from the perspective of preparation technology, the preparation of transparent ceramics requires more rigorous technical means.
1 original powder
In the preparation of raw materials for transparent alumina ceramics, the original powder must meet the following requirements: (1) The powder has high purity and dispersibility, the purity must be higher than 99.9%; (2) the powder has good sintering Activity; (3) Powder particles have good dispersibility, can’t appear serious agglomeration phenomenon, uniform in size and can exhibit better spherical shape; powder particle size generally requires submicron or even nanometer grade, and should be α phase powder. When the original powder is self-made, α-Al 2 O 3 powder is usually prepared by pyrolysis using ammonium aluminum sulfate or ammonium aluminum carbonate.
2 Sintering additives
The role of the sintering aid is mainly to promote the liquid phase in the sintering process of the powder, thereby reducing the sintering temperature, inhibiting the abnormal growth of crystal particles, and shortening the diffusion path of the blowhole. When high-purity alumina powders are used to sinter transparent ceramics, a small amount of MgO (0.05-0.25wt%) is generally added as a sintering aid, which can effectively suppress abnormal grain growth.
3 transparent alumina ceramic sintering process
Similar to ordinary ceramics, the preparation process of the transparent ceramic material includes the synthesis of the precursor powder. In addition to the compaction molding, the heat treatment and post-processing (annealing, mechanical processing, and polishing) are included. However, the preparation process of transparent ceramics has its more stringent requirements, especially for powder synthesis and post-molding sintering processes. The sintering process of transparent alumina ceramics is generally the same as that of other ceramics, and mainly includes atmosphere and vacuum sintering, atmospheric pressure sintering, hot isostatic pressure sintering, spark plasma sintering and microwave rapid sintering, etc. Sintering in combination with hot isostatic pressing.
Second, the application of transparent alumina ceramics
Since the first research and preparation of transparent alumina ceramics by US doctor in the late 1950s, the research and application of transparent alumina ceramics have received extensive attention. Compared with glass, transparent alumina ceramics have higher strength, hardness and toughness, and its excellent surface abrasion resistance is also not comparable to glass; compared with single crystal materials, the preparation temperature of transparent alumina ceramics is more Low, shorter production cycle. It is precisely because of the properties of transparent alumina ceramics that it has become a research hotspot. It has been widely used in the fields of optics, special instrumentation, lighting, electronic technology, high-temperature technology, defense and military, and aerospace. application. For example, transparent aluminum oxide ceramics can be made into a light-emitting arc tube for use in high-pressure sodium lamps by utilizing light transmission, corrosion resistance, and high-temperature stability. With statistics, there are more than 70 million aluminum oxide arc tubes produced each year worldwide. In the 1990s, a Dutch company further developed the use of transparent alumina, which was used as an arc tube for metal halide lamps. The ceramic metal halide lamp exhibited good color rendering properties, high light efficiency, and long life. For more than half a century, the United States, Japan, Russia, France and other countries have made great progress in the study of transparent ceramics. In addition to preparing alumina ceramics with high light transmittance, many other transparent ceramics have been developed. The system includes oxide transparent ceramics and non-oxide transparent ceramics. As workers further explored and studied the raw material synthesis, sintering process, crystal structure, performance, light transmission mechanism, and application of transparent ceramic materials, as well as the development of science and technology, the practical application of the performance of transparent ceramics was proposed. More demanding requirements, a large number of more high-performance transparent ceramic materials came into being.