Low Temperature Sintering Technology Of Alumina Ceramics

At present, the low temperature sintering technology of various alumina ceramics can be summarized in three aspects: raw material processing, formula design and sintering process. The following are respectively summarized.

1. By reducing the particle size of alumina powder and increasing the activity of powder, the sintering temperature of porcelain body can be reduced.

Powder has higher surface free energy.The surface energy of the powder is the internal driving force of sintering.Therefore, the finer the particles of Al2O3 powder are, the higher the activation degree is, the easier the powder is sintered, and the lower the sintering temperature is.In the low temperature sintering technology of alumina ceramics, it is one of the important means to use high activity alumina powder as raw material, so the powder preparation technology becomes a basic link in the low temperature sintering technology of ceramics.

At present, the methods of preparing ultrafine activated easily sintered alumina powder are divided into two categories, one is mechanical method, the other is chemical method.

Mechanical method

Al2O3 particle is refined by mechanical external force. The commonly used crushing technology includes ball grinding, vibration grinding, sand grinding, air flow grinding and so on.Although it is effective to increase the specific surface area of powder by means of mechanical crushing, it is usually only able to reduce the average particle size of powder to about 1 m or a little bit finer. Moreover, it has a wide particle size distribution range and is easy to bring in impurities.

Chemical method

 In recent years, the production of ultrafine and high purity Al2O3 powder by wet chemical method has developed rapidly, among which sol-gel method is more mature.Due to the high stability of the sol, a variety of metal ions can be evenly and stably distributed in the colloid, through further dehydration to form a uniform gel (amorphous body), and then after appropriate treatment can be obtained extremely high activity of ultrafine powder mixed oxide or uniform solid solution.

2. Reduce the sintering temperature of porcelain body through the design and doping of porcelain material formula

The sintering temperature of alumina ceramics is mainly determined by the content of alumina in its chemical composition. The higher the alumina content is, the higher the sintering temperature of the porcelain materials will be. In addition, it is also related to the composition system of porcelain materials, the composition ratio and the species added.Therefore, on the premise of ensuring that the ceramic body can meet the purpose and technical requirements of the product, we can choose a reasonable ceramic material system through formula design, and add appropriate additives to help firing, so as to reduce the sintering temperature of alumina ceramics as far as possible.

At present, the additives added in the formulation design can be divided into two categories according to their different mechanism of promoting alumina ceramic sintering, namely, the additives forming new phase or solid solution and the additives forming liquid phase.

First category: additives forming new phase or solid solution with alumina

Such additives are oxides close to the lattice constants of alumina, such as TiO2, Cr2O3, fe2O3, MnO2, etc.This kind of additive promotes the sintering of alumina porcelain with certain regularity:

A. The additives that can form finite solid solution with alumina have A greater cooling effect than those that form continuous solid solution;

B. Additives of variable valence ion have a greater effect than additives of constant valence;

C. Cationic additives with high electricity price have a greater cooling effect.

It should be noted that because this kind of additive is sintered in the absence of liquid phase (recrystallization sintering), the pores in the crystal are difficult to fill, the air tightness is poor, and the electrical properties decline more, which should be taken into account in the formulation design. 

The second kind: the additive that forms liquid phase in firing

The chemical components of this kind of additive are mainly SiO2, CaO, MgO, SrO, BaO, etc. They can form binary, ternary or multiple eutectic substances with other components in the firing process.The sintering temperature of alumina ceramics is greatly reduced due to the low formation temperature of liquid phase.When a considerable amount (about 12%) of the liquid phase appears, the solid particles have a certain solubility in the liquid phase and the solid particles can be wetted by the liquid phase, the promotion of sintering is also more significant.The mechanism of action lies in the wetting force and surface tension of the liquid phase on the surface of the solid phase, which make the solid particles close to and fill the pores.

In addition, because of the high surface activity of the fine and defective crystals in the sintering process, the solubility in the liquid phase is much greater than that of the large crystals.In this way, in the sintering process, the small and medium crystals grow continuously, the pores decrease and recrystallization occurs.In order to prevent excessive grain growth due to recrystallization and affect the mechanical properties of ceramics, it is necessary to select some additives that have no effect on or even inhibit grain growth in formula design, such as MgO, CuO and NiO. 

3. Adopt special sintering process to reduce sintering temperature of porcelain body

In the process of hot pressing sintering, when the green body is heated and pressurized at the same time, sintering is not only completed by diffusion and mass transfer. At this time, plastic flow plays an important role. The sintering temperature of the green body will be much lower than atmospheric sintering, so hot pressing sintering is one of the important technologies to reduce the sintering temperature of Al2O3 ceramics.

At present, there are two kinds of hot pressing sintering methods: pressure sintering and high temperature isostatic pressing sintering (HIP).HIP method can make the body subjected to isotropic pressure, and the microstructure of ceramics is more uniform than pressure sintering method.

In the case of alumina porcelain, if ordinary sintering under normal pressure must be burned to a high temperature above 1800℃, hot pressure sintering at 20MPa, at a low temperature around 1000℃ has been densified.Hot pressing sintering technology can not only significantly reduce the sintering temperature of alumina ceramics, but also inhibit the grain growth and obtain dense alumina ceramics with high strength microcrystalline, especially suitable for the sintering of transparent alumina ceramics and corundum ceramics.In addition, the sintering process of alumina is related to the diffusion rate of anions, and the reduction atmosphere is conducive to the increase of anion vacancy, which can promote the sintering process.Therefore, vacuum sintering and hydrogen atmosphere sintering are effective auxiliary means to realize low temperature sintering of alumina ceramics.

In the actual production process, in order to obtain the best comprehensive economic benefits, the low-firing techniques mentioned above are often used in cooperation with each other, among which the method of adding additives for firing has the characteristics of low cost, good effect and simple and practical process compared with other methods.It is widely used in the production of aluminium porcelain, alumina porcelain and corundum porcelain.

In addition, from the perspective of materials, through doping modification technology, the mechanical and electrical properties of alumina ceramics can be greatly improved, and the ceramic body with low alumina content can replace the ceramic body with high alumina content, which is also an effective technical means commonly used by enterprises to reduce the sintering temperature of alumina ceramic products.