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Factors Affecting Vacuum Sintering of Alumina Ceramics

Among industrial special ceramics, Al2O3 products are widely used in chemical phosphate compound fertilizer, non-ferrous metal industry and other industries because of their excellent acid and alkali resistance, wear resistance, electrical resistance and high mechanical strength. In the production process of alumina ceramics, the microstructure and intrinsic properties of the green body after vacuum sintering will undergo fundamental changes, and it is difficult to remedy it by other means. Therefore, it is very necessary to study the vacuum sintering process of alumina ceramics, choose a reasonable firing system, and ensure the performance and product quality of alumina ceramics. This paper discusses the mechanism of vacuum sintering and the factors affecting the performance of vacuum sintering.

1 Vacuum sintering mechanism and factors affecting vacuum sintering performance

1.1 Mechanism of vacuum sintering

Vacuum sintering is the process of obtaining dense and hard ceramic products due to the physical and chemical reactions of the green body due to temperature changes. Its physical and chemical changes include the elimination of residual mixing water in the green body, the elimination of compound bound water in the material and the decomposition of organic matter, the crystal transformation of alumina isomorphism, and the solid-state reaction between solid particles. The solid-state reaction occupies an important position in the vacuum sintering technology of alumina ceramics. It is carried out through the migration and diffusion of material particles. The form diffuses outwards and reacts to produce new substances.

1.2 Effect of firing atmosphere on vacuum sintering

Vacuum sintering is a process of reducing the total energy of the system. The blank powder has a large specific surface area, and the surface atoms have much higher energy than the internal atoms. At the same time, during the molding process, there are various lattice defects inside the blank powder. Any system has a tendency to transform to the lowest energy state, which is the driving force of the vacuum sintering process. That is to say, the transition from compact to vacuum sintered compact is a process in which the system transitions from a metastable state to a stable state. However, vacuum sintering generally cannot be carried out automatically, because its own energy is difficult to overcome the energy barrier, and it must be raised to a certain temperature to proceed. Different vacuum sintering atmospheres will have different effects on the vacuum sintering process of alumina ceramics, which may promote vacuum sintering or hinder vacuum sintering.

Choose the mixed atmosphere of carbon monoxide and hydrogen, the easiest vacuum sintering and the most dense vacuum sintered body. The second is the pure hydrogen atmosphere, because the CO+H2 atmosphere is used, during the vacuum sintering process, the oxygen ions in the alumina lattice will be easily lost, forming vacancies, accelerating the diffusion of cations, and effectively promoting vacuum sintering. We have seen from experiments that the lower the partial pressure of oxygen ions in the atmosphere, the more favorable it is for vacuum sintering of alumina. In a vacuum sintering atmosphere, such as an atmosphere containing a large amount of CO2 and water vapor (H2O) for vacuum sintering, vacuum sintering can be completed at a relatively low temperature and a good density can be obtained.

1.3 Vacuum sintering pressure

In the process of vacuum sintering, if the migration direction of particles or vacancy defects is appropriate, the greater the speed and efficiency, the more conducive to vacuum sintering. The effect of promoting the migration and diffusion efficiency of particles or vacancies is called migration driving force, which includes surface energy, environmental pressure and grain boundary free energy. For example, the isostatic vacuum sintering method effectively promotes the vacuum sintering process by increasing the environmental pressure of the vacuum sintering process, reduces the energy consumption of the vacuum sintering process, improves the density of the vacuum sintered body, and is capable of performing tasks that cannot be completed under normal temperature conditions. The green body is sintered in vacuum and provides equipment conditions for the low-temperature sintering process of alumina ceramics.

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