1. Overview of microcrystalline ceramics
After the heat treatment at a certain temperature, the basic glass composed of a specific nucleating agent becomes a composite material with uniform distribution of microcrystalline and glass, which is called microcrystalline ceramics or microcrystalline glass.
Low-expansion microcrystalline ceramics mainly include Li2O-Al2O3-SiO2 system and MgO-Al2O3-SiO2 system. They can separate β-spodumene or eucryptite and cordierite crystals in the base glass, respectively, and have low expansion characteristics.
2. Manufacturing technology of microcrystalline ceramics
(1) Preparation of microcrystalline ceramics
Since microcrystalline ceramics are made of glass after certain heat treatment, the properties of microcrystalline glass are closely related to the composition of glass. In order to ensure its crystal precipitation, a certain nucleating agent is often introduced.
(2) Glass melting and forming
The melting point of low-expansion microcrystalline ceramics are generally higher, so the melting temperature is also higher, reaching above 1500~1600℃. In general, Li2O-Al2O3-SiO2 and MgO-Al2O3-SiO2 can use neutral or oxidizing atmosphere, while Cu2O-Al2O3-SiO2 must use reducing atmosphere, otherwise Cu2O will be oxidized to CuO and lose low expansion.
The forming method of microcrystalline ceramics can adopt the forming method of ordinary glass, such as blowing, pressing, drawing, calendering, centrifugal pouring, etc.
(3) Processing before crystallization
The hot working and cold working of microcrystalline ceramics should be completed before crystallization as much as possible, because the glass at this time is less hard, the softening temperature is low, and processing is easy.
The hot and cold machining of microcrystalline ceramics should be completed before crystallization. Because at this time the glass hardness is small, softening temperature is low and processing is easy.
(4) Crystallization heat treatment
Heat treatment is a key process for microcrystalline ceramics to produce predetermined crystal phases and glass phases. The structure of microcrystalline ceramics depends on the heat treatment temperature system. During heat treatment, the process of phase separation, crystal nucleation, crystal growth, and secondary crystal growth occurs successively in the glass. The heat treatment system of low-expansion microcrystalline ceramics is generally carried out in two stages. That is to keep the temperature at a certain temperature to produce as many crystal nuclei as possible in the glass, and then grow the crystal at a higher temperature, and finally convert the basic glass into microcrystalline ceramic.
3. Performance and application of microcrystalline ceramics
The best properties of low-expansion microcrystalline ceramic is its low thermal expansion coefficient and high bending strength. The general Li2O-Al2O3-SiO2 system microcrystalline ceramics can be produced with various expansion coefficients by using different compositions and heat treatment systems.
MgO - Al2O3 - SiO2 system are non-alkali microcrystalline ceramic system , its crystal phase is cordierite, good electrical insulating properties, resistivity is 108.65 Ω.cm, can be used for radome, integrated circuit base board and screen printing the dielectric particle.
Low-expansion microcrystalline ceramics are mainly used in astronomical mirrors, supporting rods for gas laser resonators, cooking utensils, glass for high-temperature light sources, heating appliances for laboratories, high-temperature heat exchangers instead of quartz glass, etc.
Microcrystalline ceramic products