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Vacuum Sintering of Alumina Ceramics

In a modern society where science, technology and material civilization are highly developed, the materials that human beings rely on to make various industrial products vary widely, but in general, there are no three categories of non-metals, organic substances and ceramics. Alumina ceramics is one of the ceramic materials with the largest production volume and the widest application in the world. It has high mechanical strength, high resistivity, good electrical insulation, high hardness and melting point, good corrosion resistance, and excellent chemical stability. performance, and under certain conditions, it has good optical properties and ionic conductivity. Based on a series of excellent properties of Al2O3 ceramics, it is widely used in machinery, electronic power, chemical industry, medicine, construction and other high-tech fields. In the production process of alumina ceramics, whether it is raw material preparation, molding, vacuum sintering or cold working, every link cannot be ignored. At present, the preparation of alumina ceramics mainly adopts the vacuum sintering process. After the vacuum sintering of the green body, the microstructure and intrinsic properties of the product are fundamentally changed, and it is difficult to remedy it by other methods. Therefore, in-depth research on the vacuum sintering technology and influencing factors of alumina ceramics, reasonable selection of the ideal vacuum sintering system to ensure the performance of the product, analysis of the vacuum sintering mechanism, research on the working mechanism of additives, etc. It provides a theoretical basis for the wider application of it, which is very important for service production and social needs.

1 Introduction to alumina ceramics

Al2O3 is one of the most widely used raw materials in new ceramic products and has a series of excellent properties. Al2O3 ceramics are usually classified by the content of Al2O3 in the ingredients or in the porcelain body, and are currently divided into two types: high-purity type and ordinary type. High-purity alumina ceramics are ceramic materials with Al2O3 content above 99.9%. Because its vacuum sintering temperature is as high as 1650 ℃ ~ 1990 ℃, and the transmission wavelength is 1 μm ~ 6 μm, it is generally made into molten glass to replace platinum crucible, and it is used as a sodium lamp tube by its light transmittance and alkali metal corrosion resistance; in the electronics industry Can be used as integrated circuit substrate and high frequency insulating material. Al2O3 ceramics have extremely high mechanical strength, good thermal conductivity, high dielectric strength, high resistivity, and low dielectric loss.

There are many isomorphous crystals of Al2O3, and according to research reports, there are at least 10 kinds, and the statement is not consistent. The most common of these variants are α-Al2O3, β-Al2O3 and γ-Al2O3, the rest are mainly transition phases during the thermal decomposition of bauxite. They are almost irreversibly transformed into α-Al2O3 above 1200℃. Its crystal structure is shown in the figure below. It is a trigonal columnar crystal. It is a high-temperature structural ceramic with the most extensive uses, the most abundant raw materials and the cheapest price. Because α-Al2O3 has high melting point, high hardness, chemical corrosion resistance, and excellent dielectric properties, it is the most stable crystal form among various forms of alumina, and it is also the only crystal form of alumina that exists in nature, such as natural corundum. , ruby, etc. The alumina ceramic material prepared with α-Al2O3 as raw material has excellent mechanical properties, high temperature properties, dielectric properties and chemical corrosion resistance.

2 Alumina ceramic vacuum sintering process

Alumina ceramics have strong ionic bonds, resulting in low particle diffusion coefficient (Al3+ diffusion coefficient is only 10-11cm2·S- at 1700℃) and high vacuum sintering temperature (the vacuum sintering temperature of 99 alumina is as high as 1800℃). Such a high vacuum sintering temperature causes the grains to grow rapidly and the residual pores to aggregate and grow, resulting in a decrease in the mechanical properties of the material. At the same time, it also deteriorates the air tightness of the material and increases the damage to the kiln refractory bricks. Therefore, reducing the vacuum sintering temperature of alumina ceramics is a problem that the alumina ceramics industry is concerned about and must solve. For ceramic materials, two ways are generally used to reduce the vacuum sintering temperature. One way is to reduce the vacuum sintering temperature of ceramics by obtaining ultra-fine powders with uniform dispersion, no agglomeration and good vacuum sintering activity; the other way is to reduce the vacuum sintering temperature of ceramic materials. The method of vacuum sintering temperature of ceramic materials is to add an appropriate amount of vacuum sintering aids.

2.1 Refinement of raw material particles

Monodisperse ultra-fine Al2O3 powder with small grain size, large specific surface area and high surface activity is used. Due to the short diffusion distance between particles, only lower vacuum sintering temperature and vacuum sintering activation energy are required. The finer the particles, the easier it is to vacuum Sintering, the lower the vacuum sintering temperature. The relationship between powder particle size and vacuum sintering temperature is shown in Table 1.

Table 1 Relationship between powder particle size and vacuum sintering temperature (vacuum sintering diffusion activation energy Q = 418KJ/mol)

The finer the particles, the shorter the vacuum sintering time. The finer the powder particles, the more defects and the greater the activity, which can promote vacuum sintering, and the strength of the resulting ceramic is also higher. Small particles can also disperse the stress concentration at the grain boundary caused by the different phase line expansion coefficients of corundum and glass, reducing the risk of cracking; fine grains can also hinder the development of micro-cracks and are not easy to cause transgranular fractures, which is conducive to improving 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 for the preparation of high-performance Al2O3 products.

2.2 Adding vacuum sintering aids

Additives can be divided into two categories in terms of their functions: one is to form a solid solution with Al2O3, and the other is to form a liquid phase.

The first type of additives are variable valence oxides, such as TiO2, Cr2O3, Fe2O3 and MnO2. Because its crystal structure and lattice constant are close to Al2O3, it can usually form a solid solution Al2O3 lattice with Al2O3 to generate defects, activate the lattice, and promote vacuum sintering. Studies have shown that such additives promote vacuum sintering and have the following regularities: First, any additives that can form a limited solid solution with Al2O3 have a greater effect than forming a continuous solid solution, which may be the ionic radius and Al3+ ionic radius for forming a limited solid solution. The difference is large, which makes the lattice more easily deformed, thereby promoting vacuum sintering; second, the additive with variable electricity price has a greater effect than the additive that cannot be changed; third, the electronic layer structure of all cations is non-inert gas type , that is, the additive with high cationic price has a greater effect.

The role of the second type of additives is to promote the vacuum sintering of Al2O3 due to the formation of a liquid phase, reducing the sintering temperature. Such additives include kaolin, SiO2, CaO, MgO, etc. The alumina raw material brings impurities such as sodium oxide and silicon oxide into it more or less. In order to lower the vacuum sintering temperature of alumina porcelain, some oxide or silicate liquid phase should be introduced. Oxide additives are easy to form flux during vacuum sintering and promote vacuum sintering. Due to the appearance of the liquid phase, that is, the surface wetting force and surface tension of the liquid to the solid phase, the solid phase particles are brought close together and fill the pores. The added fine-grained admixture can be uniformly adsorbed by Al2O3, reducing the surface energy, thus delaying the grain growth of Al2O3. The more traditional Al2O3 vacuum sintering additive is MgO. Al2O3 and MgO generate binary, ternary or more complex low-melting compounds. The addition of a small amount of MgO (0.05-0.25wt%) during the vacuum sintering of high-purity Al2O3 can effectively inhibit excessive grain growth. In 1961, Coble, a ceramic scientist from GE Company in the United States, first discovered that adding 0.25wt% MgO can reduce the pores generated during the vacuum sintering process of Al2O3, inhibit the growth of grains, and make the vacuum sintering tend to be completely densified.

TiO2 can form a limited substitution type solid solution with Al2O3. Due to the difference in coordination number, electricity price and ionic radius, when Ti4+ replaces Al3+, lattice distortion and cation vacancy occur. TiO2 activates the Al2O3 lattice and promotes vacuum sintering. Obviously; Cr2O3 and Al2O3 have the same lattice type, and the ionic radius of Cr3+ is slightly larger than that of Al3+. The two can form a continuous solid solution, and the lattice is distorted to a certain extent, which promotes vacuum sintering.

For example, when α-Al2O3 is prepared from bismuthite, 0.05%, 0.1%, 0.2% and 5wt% of MgO are added in the ball milling process, and the alumina powder is obtained by roasting after ball milling for 4 hours, and then dry-pressed and roasted at atmospheric pressure. Alumina ceramics were obtained, and their porosity, relative density and water absorption were measured by Archimedes’ method, as shown in Table 2:

Table 2 The effect of different vacuum sintering additives on the density of vacuum sintering

Effect of vacuum sintering aid dosage on green body porosity and relative density

The porosity of the green body is 0.453 when no additives are added, and when it is added to 0.2wt%, the porosity drops to 0.397, the relative density increases from 0.568 to 0.621, and the shrinkage rate increases from 12.59% to 15.90%, but the amount of additives continues to increase. When the porosity, relative density and water absorption rate are not greatly affected, the addition of vacuum sintering aids increases the driving force of vacuum sintering, and at the same time inhibits the growth of grains, so that the rate of sintering of the green body is accelerated. , so the porosity decreases, the relative density increases, and the water absorption increases. However, when the additive amount is 0.5wt%, the second phase MgAl2O4 is formed due to the reaction of excess MgO and Al2O3. Its main function is to inhibit the growth of grains, but it hinders the densification process of vacuum sintering. , and the mechanical properties of the green body are reduced.

It is worth noting that excessive sintering aids will generate a second phase and affect the light transmittance of the ceramic. On the other hand, the sintering aid should be evenly distributed in the material to inhibit the abnormal growth of grains.

Selection of vacuum sintering furnace equipment: RVS series vacuum sintering furnace produced by SIMUWU is a high-quality product for the vacuum sintering process of tooling and molds. Good temperature control accuracy and temperature control uniformity ensure the effective progress of the vacuum sintering process. SIMUWU specializes in the manufacture of vacuum furnaces, has more than ten years of relevant experience, and has a good reputation in the field of vacuum furnace manufacturing. The product line includes vacuum air quenching furnace, vacuum oil quenching furnace, vacuum brazing furnace, etc., which are widely sold in developed and developing countries.