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According to solid-state physics, refractory products can be regarded as solid materials that do not react with flux (or essentially do not react with flux). Therefore, high-temperature calcination is necessary in the production of refractory products. Even for the increasingly developed amorphous refractory materials and unfired products, although there is no sintering process during the production process, its use process can be regarded as a calcination process, that is to say, the sintering process is realized during use.
Firing is the last process in the production of refractory products. A series of physical and chemical changes will occur during the sintering process. With these changes, the porosity decreases, the bulk density increases, and the undesirable body becomes a product with a certain size, shape, and structural strength. At the same time, through a series of physical and chemical changes during the sintering process, stable microstructure and mineral phases are formed to provide the required performance for products under different usage conditions.
Xinhongji Refractory tells everyone that many factors affect sintering, mainly including the following aspects:
1. Liquid phase recrystallization: Under the action of high temperature, an appropriate amount of liquid phase is generated inside the product to promote the combustion of the product. The liquid phase can close and tighten the solid particles, fill the pores, and increase the density of the defective body; the liquid phase helps to buffer the stress caused by uneven sintering; the liquid phase is relatively strong against some small crystals with lattice defects. The solubility is large, and the large crystals in the solution are saturated while the small crystals are unsaturated, resulting in the appearance of small crystals. This process is called recrystallization. Generally speaking, the more liquid phase components there are and the more complex the liquid phase is, the less conducive it is to crystal recrystallization; the lower the liquid phase viscosity, the faster the recrystallization rate. However, the liquid phase produced by co-dissolution with the main crystalline phase in refractory products has poor high-temperature performance and is often damaged by erosion first. In actual production, the ingredients in this part are usually adjusted to improve product performance.
2. Solid-state reaction: Refractory raw materials and products have a crystal structure, and these crystals have structural defects. When a crystal is in a solid state at low temperatures, particles can only vibrate near the lattice nodes but cannot move. If the temperature gradually increases, these particles will gain energy and increase their amplitude. When the temperature rises to a certain level and the fluidity of the particles can overcome the force of the surrounding particles, displacement (migration) occurs. This migration of particles is also called diffusion. Due to the diffusion of particles, contacting materials interact to form new reaction products. This is often called a solid-state reaction. The key to solid-phase reactions is the migration and diffusion of material particles. If diffusion is accelerated, the reaction is accelerated and sintering is promoted. The crystal structure, grain size, and defects of the reactants, and the interfaces between the reactants, all affect diffusion capabilities. In production, fine grinding or even mixing methods are often used to destroy the complete structure of the crystal, promote lattice activity, increase the contact surface of reactants, promote diffusion, and accelerate solid phase reactions. Sometimes small amounts of mineralizing substances are added to the reactants to promote solid-state reactions.
3. Recrystallization and aggregation recrystallization: In natural or artificially calcined raw materials, there are defects both inside and on the surface of the crystals. In particular, the surface defects of finely crushed and highly dispersed grains are more prominent and have greater surface activity. This defect in the crystal makes the crystal very unstable and highly reactive. It attempts to reduce surface activity and strive for stability. Therefore, under the action of high temperature, if it is carried out in the same crystal, the vacancies of the lattice particles can be eliminated and the defects of the crystal can be corrected; if it is carried out on the indirect contact surface of the grains, the contact interface gradually expands, achieving aggregation and aggregation between grains. Adhesion. This is the result of recrystallization and agglomeration recrystallization. The result is to promote crystal growth and increase the density of defective bodies, which leads to the sintering of bad bodies.
The main measures to promote recrystallization and aggregation recrystallization are fine grinding to increase grain structure defects and expand the contact surface; adding a small amount of mineralized substances to react with the main crystal phase to form a solid solution to accelerate recrystallization; increasing the sintering temperature to promote the reaction strengthen.
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