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Vacuum sintering of transparent ceramics

In the preparation process of transparent ceramics, in addition to the key link of preparing nano-powders with high purity, uniform particle size and good activity, the molding and vacuum sintering processes are also crucial. The most commonly used molding methods for transparent ceramics are dry pressing and isostatic pressing.

Dry pressing molding is a simple and easy molding method, and it is also a commonly used molding method at present. The main process of dry pressing is to put the powder into the mold after ball milling and granulation (you can add a certain solvent, usually water or organic solvent), and apply a certain pressure to make the powder form a certain shape and strength. Ceramic green body. Dry pressing has the advantages of simple operation, low cost, and high production efficiency, and is suitable for producing ceramic samples with small size and simple shape.

1.Transparent ceramic vacuum sintering method

At present, the main vacuum sintering methods for preparing transparent ceramics include vacuum sintering, vacuum hot-pressing sintering and other methods.

(1) Vacuum sintering

Vacuum sintering refers to the sintering process of ceramic blanks under vacuum conditions, and vacuum sintering belongs to the method of pressureless sintering. In a vacuum environment, a pressure difference is formed between the interior of the ceramic and the outside world, which is helpful for the discharge of pores, reduces the porosity of the ceramic, makes the grain grow, and forms a ceramic material with higher density. Under the decomposition or volatilization of elements. Vacuum sintering has the advantages of simple operation, low cost, and high production efficiency. It is currently the most widely used transparent ceramic sintering technology. Some oxide-based ceramic materials, such as rare earth activated YAG, Y₂O₃ and Al₂O₃, can be vacuum sintered. Method preparation.

(2) Vacuum hot pressing sintering

Vacuum hot pressing sintering is to put the powder or ceramic body in a mold, usually a high-melting point metal or graphite mold, and apply a certain pressure to the ceramic body while sintering at high temperature, and complete the vacuum sintering process in a short time. Ceramics have high density and fine grains. This method can directly sinter the ceramic green body, and can also combine the forming process and the vacuum sintering process into one and carry out simultaneously.

2.The main factors affecting the vacuum sintering of transparent ceramics

(1) Powder particle size

The excellent degree of powder particle size is the prerequisite for ceramic vacuum sintering. Reducing the particle size of powder is beneficial to promote vacuum sintering. Nano powder is more beneficial to vacuum sintering. This is because nano powder has small particle size, high surface energy, and powder The activity of the body is good. When the nano-powder is used for the ceramic vacuum sintering process, a lower sintering temperature can prepare a ceramic material with a low porosity and a high degree of densification.

(2) Vacuum sintering temperature and sintering time

Sintering temperature and sintering time are the most important factors affecting ceramic sintering. They largely determine the size and distribution of grains in ceramics. By controlling the size and distribution of grains, the porosity and density inside ceramics can be controlled.

(3) Vacuum sintering atmosphere

The atmosphere in the sintering process can affect the number of pores inside the ceramic, which in turn affects the degree of densification of the ceramic. For some materials, when the sintering process is carried out in air, the presence of oxygen will cause defects inside the crystal and increase the porosity, resulting in a decrease in the densification of the ceramic material. Sintering in a vacuum atmosphere is more conducive to the discharge of pores inside the ceramic and the progress of the diffusion and sintering process.

(4) Vacuum sintering pressure

The pressure has a great influence on the discharge of pores during the sintering process. Usually, the ceramic body contains air, which is left in the molding process. The environment of vacuum sintering or decompression sintering can cause a pressure difference between the inside and outside of the ceramic and promote the internal and external pressure of the ceramic. The discharge of air reduces the porosity of ceramics and increases the densification of ceramics.

(5) Additives

The sintering aid acts as a flux in the vacuum sintering process of transparent ceramics. The melting point of the sintering aid is low, which will distort the crystal lattice, increase the activation energy of the ceramic, facilitate the diffusion and sintering, and increase the densification of the ceramic.