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What is vacuum sintering

Vacuum sintering is a method of sintering ceramic blanks under vacuum conditions. The gases such as water vapor, hydrogen and oxygen in the pore of oxide ceramic blanks can be dissolved and diffused along the grain boundary of the blanks or escaped from the pore through grains during sintering. However, carbon monoxide and carbon dioxide, especially nitrogen, are not easy to escape from the stomata due to their low solubility, resulting in the products containing stomata, which reduces the density. If the green body is sintered in vacuum, all gases will escape from the pore before the green body is fully sintered, so that the product does not contain the pore, thus increasing the density of the product.

Under high temperature and certain vacuum, the ceramic body with certain shape is transformed into a compact, hard, volume-stable and sintered body with certain properties by physicochemical process. Physicochemical processes include viscous flow, diffusion, evaporation, condensation, formation of new phases, dissolution and precipitation, and formation of solid solutions. During the process, the total surface energy of the powder decreases, the bulk shrinks and the strength increases on the macro level, while the number of pores decreases, the shape and size of the pores change, the grain size and morphology change, the grain grows, the grain boundary decreases and the structure becomes dense on the micro level. Porcelain billet contains a certain amount of pore, in which water vapor, hydrogen and oxygen can escape from the closed pore through the process of dissolution and diffusion. Carbon monoxide, carbon dioxide and nitrogen are not easy to escape from the closed pore due to their low solubility. When the sample is placed in vacuum and a certain driving force is applied to make the gas escape from the closed pore, the density of the ceramic can be improved.

The sintering under negative pressure in furnace is called vacuum sintering. There are batch vacuum sintering and continuous vacuum sintering. Continuous vacuum sintering is very complicated, the vacuum degree is not guaranteed, there is no way to pass through gas, the sintering process curve is not well adjusted, and so on. It is seldom used. There are special vacuum sintering furnaces for batch vacuum sintering, equipped with dewaxing furnaces and dewaxing-sintering integrated furnaces. At present, the mainstream vacuum sintering is an integral furnace in which dewaxing (forming agent) and vacuum sintering are completed at one time in the same furnace.

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