Mullite (Aluminum silicate) is a general term for a series of minerals composed of aluminosilicates, named after the earliest discovery of the Isle of Mull in Scotland in 1924. Although the earth's crust is rich in aluminum, silicon and oxygen, there are few naturally occurring mullite. Except for Mull Island, no deposits of industrial value have been seen so far. Mullite belongs to the SiO2-Al2O3 binary system, and the only crystalline compound stably exists under normal pressure. The mullite used in the industry is all artificially synthesized, and the conventional synthesis temperature is above 1650 ℃. Mullite also belongs to a high-quality refractory material, mainly including high-purity fused mullite, ordinary fused mullite, all natural bauxite concentrate sintered mullite and lightly burned mullite.
Mullite ceramics and their properties
Mullite Ceramic (mullite Ceramic) is a general term for a class of ceramics whose main crystal phase is mullite (3Al2O3 · 2SiO2). If it is made of synthetic ultra-fine high-purity mullite powder and does not contain glass phase mullite porcelain, it is also called neo-mullite ceramic / high-purity mullite ceramic. Mullite ceramics have high melting point, good chemical stability at high temperature, corrosion resistance, low expansion coefficient, low thermal conductivity and good high temperature creep characteristics. What is more remarkable is that its flexural strength and fracture toughness increase with temperature. The height does not drop but rises, and the intensity at 1300 ° C is 1.7 times that at normal temperature. The disadvantage of this material is that the strength at normal temperature is too low and the toughness is poor.
Application of high-purity mullite ceramics
① Mullite refractories:
Mullite has high refractoriness and excellent corrosion resistance at high temperature, as well as high temperature load resistance and spalling resistance. It can be used as a lining for various high temperature kilns, including furnace kilns, blast furnaces, hot iron pouring troughs and continuous Lining materials such as casting furnaces. Japan has used mullite refractory materials to line the upper and lower parts of the rotary valve outlet that controls the flow of molten steel. And it is also widely used in the lining of cement high temperature calcination area.
②High temperature engineering materials:
Mullite ceramics have good high-temperature stability and thermal shock resistance, so they are also widely used in heat-resistant materials such as high-temperature protection tubes, thermocouple protection tubes and metal melting crucibles, which proves that their corrosion resistance to high-temperature gases is better than alumina ceramics.
In the early 1990s, when the United States developed advanced ceramic engines with low thermal expansion and low heat loss, mullite was listed as the main candidate material; the mullite ceramic cylinder head bottom plate successfully developed in China was successfully tested in desert vehicles; Germany adopted aluminum The silicate long fiber toughened mullite composite material layer was successfully tested and tested on the exhaust device of the propeller aircraft; the Japanese Chi chbu company has developed a high-purity mullite ceramic conveyor belt used in the tunnel kiln, which replaced The original stainless steel conveyor belt is used to prepare electronic ceramic components by sintering above 1000 ℃.
③Optical materials:
Mullite belongs to the orthorhombic crystal system and has low birefringence. Under certain conditions, transparent ceramics can be produced. Its light absorption performance is higher than spinel, sapphire and other materials. Among the window materials in the application environment, mullite is also an excellent material that transmits infrared rays. Su Chunhui and others used the sol-gel process to synthesize the mullite ceramic film, expanding the optical application range of mullite ceramics.
④Microelectronic materials:
Mullite's low expansion coefficient and dielectric constant are precisely what the IC technology and SMT technology in the microelectronics industry require. A low dielectric constant can cause rapid signal transmission. In the signal transmission of the IC substrate, the signal transmission time is proportional to the relative dielectric coefficient. Due to the low dielectric coefficient of mullite, the signal transmission speed of the pure mullite substrate is faster than that of the commonly used alumina substrate. The addition of mullite ceramics to the absorbing material not only expands the absorption band of the absorbing material, but also significantly improves the absorbing efficiency of the absorbing material.