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Process optimization of alumina ceramic sintering technology

Vacuum sintering technology of alumina ceramics. Pure alumina ceramics have a very high melting point, and the sintering temperature can be as high as 1800°C. High sintering temperature not only consumes a lot of energy, but also has many defects in the structure of the sintered ceramic parts, which greatly reduces the performance of the material.

In addition to being used as a grinding medium, alumina can be used to make vacuum devices, circuit boards, etc. by taking advantage of its high mechanical strength and high insulation resistance; it can be used to make crucibles, sodium lamps, etc. by taking advantage of its high temperature resistance. ; Utilizing its stable chemical properties, it can be used as bioceramic, catalytic carrier, etc.

At present, in order to realize the vacuum sintering of alumina ceramics, we mainly start from these three aspects: ① use raw material powder with small particle size and high surface activity; ② adopt a suitable sintering process; ③ reduce the sintering temperature of ceramic materials through additives; Explain separately.

1.What kind of raw material is conducive to lowering the sintering temperature?

The finer the powder particles, the more defects and the greater the activity, which can promote sintering and the higher the strength of the ceramic made. Small particles can also disperse the stress concentration at the grain boundary caused by the different linear expansion coefficients of corundum and glass phases, reducing the risk of cracking; fine grains can also hinder the development of micro-cracks, and are not easy to cause transgranular fracture, which is beneficial to improve Fracture toughness; in addition, it can improve the wear resistance of the material. Therefore, reducing the particle size of Al₂O₃ powder is of great significance to the preparation of high-performance Al₂O₃ products. At present, the methods for preparing ultrafine activated and easily sintered Al₂O₃ powders are divided into two categories, one is mechanical method and the other is chemical method.

The mechanical method is to use mechanical external force to refine the Al2O3 powder particles. Commonly used crushing processes include ball milling, vibrating milling, sand milling, and gas milling.

Wet chemical method is an important technical method for preparing ultrafine high-purity powder technology, and the more mature method is the sol-gel method. The particle size of the powder prepared by the wet chemical method can reach nanometer level, the particle size distribution range is narrow, the chemical purity is high, and there are many crystal defects. Therefore, the surface energy and activity of the chemical method powder are much higher than that of the mechanical method powder. Using this kind of ultrafine powder as raw material can not only significantly reduce the sintering temperature of alumina porcelain (can be lowered), but also obtain microcrystalline high-strength high-alumina materials.

2.Reasonable selection of sintering method

Reasonable selection of sintering method is the key to make alumina ceramics have ideal structure and predetermined properties. At present, the more mature sintering methods include: atmospheric pressure sintering, hot isostatic pressing, vacuum sintering; in addition, there are some special sintering methods (electric field sintering, ultra-high pressure sintering, activation sintering and activation hot pressing sintering, etc.).

The normal pressure sintering method is often used in traditional ceramic production. This sintering method has lower production costs than the special sintering method and is the most common sintering method. However, its sintering temperature is high, and the requirements for the kiln are also high, resulting in a large waste of energy. The hot pressing sintering process is used to pressurize the green body while heating it. The sintering is not only completed by diffusion and mass transfer. At this time, plastic flow plays an important role. The sintering temperature of the green body will be much lower than that of normal pressure sintering, so Hot pressing sintering is one of the important technologies to reduce the sintering temperature of ceramics. Hot pressing sintering technology not only significantly reduces the sintering temperature of alumina ceramics, but also can better inhibit the growth of grains, and can obtain dense microcrystalline high-strength alumina ceramics, especially suitable for sintering of transparent alumina ceramics and microcrystalline corundum ceramics .

In addition, since the sintering process of alumina is related to the diffusion rate of anions, the reducing atmosphere is conducive to the increase of anion vacancies, which can promote the sintering. Therefore, vacuum sintering and hydrogen atmosphere sintering are effective auxiliary means to realize vacuum sintering of alumina porcelain.

3.Reduce the sintering temperature of ceramic materials through additives

The cost of promoting material densification and vacuum sintering by using ultra-fine particles, no agglomeration, and uniformly dispersed powder with good sintering activity is very high. By introducing an appropriate amount of sintering aids, that is, adding a suitable admixture to the material, and forming a liquid phase or solid solution with the matrix, the diffusion is enhanced to achieve the purpose of promoting the densification of the material and reducing the sintering temperature. The requirements for raw material powder can be reduced. , the cost is relatively low, and this method is widely adopted in the industrial production of ceramics in many application fields.

Mechanism: The sintering aid can form an intergranular liquid phase or form a solid solution with the matrix. A consensus has been reached on the mechanism of action of sintering aids, that is, the intergranular liquid phase formed by sintering aids is conducive to the migration of grain boundaries and can accelerate the transport rate of substances during sintering; while the solid solution formed by sintering aids and the matrix can be Promote the increase of the number of defects to achieve the purpose of activating the lattice to promote densification.

In fact, according to the different performance requirements of the product, different types and different amounts of additives are often added to reduce the firing temperature and promote sintering. As far as additives are concerned, they can be summarized into the following three categories:

The first type of additives are variable price oxides, including TiO₂, Cr₂O₃, and Fe₂O₃. And MnO₂ etc. Such additives can usually form solid solutions with Al2O3 because their lattice constants are close to those of Al₂O₃. At the same time, they are variable-valence oxides. Due to the variable-valence effect, they can cause defects in alumina ceramics, activate the lattice, and promote sintering.

The second type of additives are rare earth oxides, such as La2O3, Y2O3, Sm2O3 and so on. The study found that rare earth oxides La₂O₃, Y₂O₃, Sm₂O₃, etc. are good surfactants, which can improve the wetting properties of alumina composite materials and reduce the melting point of ceramic materials.

The third category is composite additives. The commonly used composite additives include CaO-MgO-SiO₂ system, MnO₂-TiO₂-MgO system and CuO-TiO₂ system.