Alumina ceramics are one of the most widely used ceramics in modern engineering technology. From daily kitchenware close to people to high-end aerospace technology, their low cost and excellent performance can be seen. However, although they are all alumina ceramics, different applications still have different requirements. For example, in electronic ceramics, 99 ceramics and 97 ceramics are mainly used as integrated circuit substrates.
Because integrated circuits must have a highly flat and smooth plane, to ensure that the ceramic substrate has a very high surface finish after being carefully polished, the substrate itself must be dense enough, and the grain size should be fine so that the grain bonding performance can be very good.
High density brings many benefits. For example, if the grains are closely arranged, they can better withstand external loads or corrosion from corrosive substances, and it is not easy to form destructive breakthrough points. However, it is not so simple to obtain dense alumina ceramics. Since the performance of ceramic materials is closely related to their manufacturing technology to a large extent, it is necessary to control the production conditions to obtain satisfactory results. What specific attention should be paid to?
Selection of raw materials
When selecting ceramic powders, special attention should be paid to purity. Although organic impurities will burn during sintering, irregular pores will be formed during densification. However, inorganic impurities may react with ceramic powder at high temperature or remain in the matrix to form microcracks. These potential defects in the microstructure will have a significant impact on the densification of alumina ceramics. Therefore, the use of high-purity Al2O3 powder is an important prerequisite for the preparation of alumina ceramics with excellent performance.
Consistency of raw materials composition
To reduce the sintering temperature of the ceramic body, appropriate additives should be added to the powder before sintering, so the quality of the mixture is also an important factor affecting the ceramic sintered body. If the mixture is not uniform, some parts of the composition will deviate from the total proportion, resulting in some parts with less additives, and alumina is difficult to sinter at low temperatures, while places with lower melting points have more additives, which are prone to liquid phase and rapid grain growth, ultimately resulting in uneven microstructure and low-density products.
Particle size of raw materials
Generally, the finer the raw material particles, the shorter the sintering time. This is because the finer the particles, the closer the contact between them, the shorter the diffusion path during sintering, and the greater the sintering driving force-surface energy. However, too fine is unacceptable, because too high particle surface activity may adsorb impurities, resulting in impure powder, and at the same time, this will lead to difficulties in molding. Therefore, the powder selected for the production of high-density alumina ceramics is usually in the range of 0.1um ~ 1um.
Formation of alumina green body
The acquisition of high density is largely affected by the molding pressure. In order to ensure high density, the molding pressure is usually high. At present, the molding methods of high-performance alumina ceramics mainly include isostatic pressing, slip casting, hot pressing, extrusion, and calendering.
Sintering of alumina ceramics
Sintering is a key stage in the preparation of ceramics. Due to the strong chemical bond of Al2O3, sintering can only be achieved at very high temperatures (generally 1800). However, even with ordinary high-temperature sintering, the sintering properties of the sintered body cannot meet people’s requirements. To reduce production costs, achieve dense sintering of alumina ceramics, and improve the performance of the sintered body, special sintering methods are used in production. Currently, commonly used sintering methods include hot pressing sintering, atmosphere sintering, vacuum hot pressing sintering, etc.
Effect of sintering aids on alumina sintering
Since the chemical bonds between Al2O3 atoms are covalent bonds and ionic bonds with large binding energy, it is difficult to transfer mass during sintering. If sintering aids are not added, the sintering temperature of alumina will be as high as 1800. At such a high sintering temperature, grain growth will be promoted, pores will be difficult to eliminate, the mechanical properties of the material will be reduced, and the air tightness will become worse. This not only increases the preparation cost of ceramics, but also does not meet technical requirements.
Therefore, appropriate sintering additives are usually added to Al2O3 to reduce the sintering temperature, improve the microstructure of alumina ceramics, achieve high density and low porosity, thereby improving its strength and toughness, and then achieve the purpose of obtaining ceramics with excellent performance at a lower temperature. Additives can generally be divided into two categories: one is to form a solid solution with Al2O3, Al2O3 is generally a variable valence oxide, mainly including titanium dioxide, chromium trioxide, iron trioxide and manganese dioxide; the other is to produce a liquid phase, reduce the sintering temperature and promote the sintering of Al2O3, mainly including kaolin, silicon dioxide, calcium oxide, magnesium oxide, etc.