Ceramic sintering is a general term for densification process and phenomenon of green body at high temperature. With the increase of temperature, there are powder particles with larger specific surface area and higher surface energy in the ceramic body, which try to change in the direction of reducing the surface energy, continuously migrate the material, move the grain boundary, gradually eliminate the pores, and produce shrinkage, so that the body becomes a dense ceramic body with a certain strength. The driving force of sintering is surface energy. Sintering can be divided into two types: liquid phase sintering and solid phase sintering. The sintering process is of great significance to ceramic production. In order to reduce sintering temperature and expand sintering range, some additives are usually added as flux to form a small amount of liquid phase to promote sintering. For example, adding a small amount of silicon dioxide to promote the sintering of barium titanate ceramics; Another example is to add a small amount of magnesium oxide, calcium oxide and silicon dioxide to promote the sintering of alumina ceramics. (Baidu Encyclopedia)
Overview of sintering
Sintering is a technology that uses heat energy to densify powder green bodies. Its specific definition is the densification process in which porous green bodies reduce their surface area, porosity and mechanical properties (mechanical strength, etc.) at high temperatures. During the sintering process, the green body will undergo a series of physical and chemical changes, such as expansion, contraction, gas generation, liquid phase appearance, old crystal phase disappearance, new crystal phase formation, etc. Under different temperature and atmosphere conditions, the content and extent of changes are different, thus forming different crystal phase composition and microstructure, which determines the different quality and performance of ceramic products. The glaze layer on the surface of the body will also undergo various physical and chemical changes during the sintering process, and finally form a glassy substance, thus having various physical and chemical properties and decorative effects.
Driving force of sintering
There is only point contact between green bodies and particles, and the strength is very low. Through sintering, although there is no external force or chemical reaction during sintering, the particles in point contact can be tightly bonded into a hard and high strength porcelain body.
What is the power of sintering? It is the surface energy of powder particles. In the process of powder preparation, mechanical properties such as crushing, ball milling or other energy are stored in the powder in the form of surface energy, resulting in many lattice defects on the powder surface, which makes the powder have high activity. Excess surface energy of powder: it is the driving force for sintering (the total surface area will be reduced by more than 3 orders of magnitude after sintering). The sintering cannot be carried out automatically. The powder must be heated and energy must be added to make it change into the non sintered body. In addition to the driving force, there must also be a material transfer process to gradually fill the pores and make the green body compact from the transport.
1. Evaporation and condensation
2. Diffusion
3. Viscous flow and plastic flow
4. Dissolution and precipitation may have several mass transfer mechanisms at work when sintering is too heavy. Under certain conditions, some mechanism is at work. If conditions change, the mechanism of conduction may change accordingly.