PRODUCT CATEGORIES

Vacuum Heat Treatment Furnace

Vacuum Sintering Furnace

Vacuum Brazing Furnace

Vacuum sintering of Al2O3 ceramic products

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 product after sintering the green body will change fundamentally, and it is difficult to remedy it by other means. Therefore, it is very necessary to study the sintering process of alumina ceramics, choose a reasonable firing system, and ensure the performance and product quality of alumina ceramics. In this paper, the mechanism of vacuum sintering, the factors affecting the performance of vacuum sintering and the addition of sintering aids are discussed.

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 in the vacuum environment of the vacuum furnace due to temperature changes. Its physical and chemical changes include the elimination of residual mixture moisture 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 sintering technology of alumina ceramics. It is carried out through the migration and diffusion of material particles. As the temperature increases, the thermal defects of the crystal continue to increase. Diffusion to the outside, resulting in a reaction to produce new substances.

1.2 There are many factors affecting Al2O3 and ceramic vacuum sintering, the main performances are as follows:

1.2.1 Crystal structure

Compounds (crystals) with strong chemical bonds have higher lattice energy, and the lattice structure is firm. Even at higher temperatures, the vibration migration of particles is weak, and only when the temperature is close to the melting point, significant physical and chemical reactions will occur. . Therefore, the green body composed of such compounds is not easy to sinter. Compared with the single crystal, the polycrystal composed of fine crystals has many internal grain interfaces, and the grain boundary is the place where vacancy defects are relatively concentrated and easy to eliminate, and it is also a fast channel for the diffusion and migration of atoms and ions. The latter are easy to sinter.

1.2.2 Dispersion of materials

The higher the dispersion of the material, the higher the surface energy, so it has a strong effect of promoting migration and diffusion, which is beneficial to sintering. When the material is fully pulverized, the defects inside and on the surface of the particles increase, which can effectively improve the mobility of the particles.

1.2.3 Bulk density of materials

The close contact of the particles can greatly shorten the migration distance of the particles at high temperature, accelerate the diffusion of the particles, increase the ability of the particles to move relative to each other and combine with each other, which is beneficial to shorten the firing time and ensure that there are no defects such as large pores in the sintered body.

1.2.4 Add firing aids

In the Al2O3 ceramic batching system, the introduction of appropriate sintering aids can promote the generation of liquid phase at a lower temperature, which is beneficial to sintering. For example, the function of V2O5 can reduce the sintering temperature of the product, and at the same time it is an excellent ceramic mineralizer, which has a positive effect on promoting the formation of mullite crystal phase in ceramics. Considering the cost, it can also be added in small amounts. After adding the sintering aid, the vacancies and defects inside the alumina lattice increase, which is conducive to the migration and diffusion of particles and accelerates the sintering process. For example, the introduction of Ti4+ can increase the cation vacancies in the alumina lattice, activate the particles near the vacancy, and thus facilitate the vibrational migration of the particles. At the same time, Ti4+ will change in price at high temperature, and change into low-priced Ti3+ to further increase the anion defects in the crystal lattice, which greatly promotes sintering.

The test is as follows: 1% TiO2 is added to 95% Al2O3 powder, and the sintering is completed at 1600±10°C. The performance parameters are shown in Table 1.

However, the additives introduced are different, and their effects are also different. For example, introducing a small amount of MgO into sintered talc can limit the recrystallization phenomenon in the sintering process, so that the sintered body has a crystalline microstructure and high mechanical strength. If the amount of MgO added is too large, it will also hinder sintering, and the ideal amount is about 0.04 wt%~0.07 wt%.

1.2.5 Firing temperature and holding time, over-firing and firing temperature range

Usually, the temperature at which performance parameters such as porosity, density, and mechanical strength of the green body tend to be stable is called the firing temperature, and the holding time at the highest temperature point is called the holding time.

When the ambient temperature exceeds a certain limit, the bulk density and mechanical strength of the green body will not increase but decrease, which is called overburning. The above-mentioned temperature range is often referred to as the firing temperature range.

The ambient temperature and the holding time at high temperature are important external conditions for whether the green body can be completely sintered. With the increase of the ambient temperature and the extension of the holding time, the migration and diffusion of the material particles are sufficient, the green body will continue to shrink, and the bulk density will continue to increase.

Generally speaking, when the firing temperature is lower than the firing temperature, no matter how long the holding time is, it will still burn. If the firing temperature is too high or the holding time is extended too much at the firing temperature point, it will cause over-burning phenomena such as coarse grains and a decrease in bulk density. The mechanical strength of the sintered body is reduced, so in the case of stable ingredients, it is very important to determine the firing curve of the firing temperature and holding time of the alumina product. Figure 1 is the firing curve of 95% alumina ceramics.

2 Conclusion

The factors affecting the performance of vacuum sintering described above depend on different conditions in the sintering process, and the leading role is also different. It is necessary to conduct specific analysis, make directional choices, design the best process route and equipment conditions, and formulate a scientific firing system.

(1) The particle size of the formed raw materials should be similar or relatively concentrated, and the grain boundaries should preferably be planar. This requires that the pulverized powder has a good particle distribution curve, and it is not allowed to mix large particles or aggregated particles from the raw material.

(2) In the process of raw material processing and molding, it is not possible to mix large particle additives or impurities, even combustible organic substances, so as to avoid low-temperature melting holes, pinholes and large pores.

(3) Selectively increase or decrease the firing elements according to the shape and specification of the green body, strictly record the firing, pay attention to analysis, and constantly improve.

(4) The sintering process is irreversible, and the process control must be done carefully, and unqualified products cannot enter the kiln. Layered and cracked green bodies are not allowed to enter the kiln, otherwise it will cause bursting, waste and defective products, and kiln inverted accidents. It is necessary to strengthen product inspection before entering the kiln.

Vacuum sintering furnace selection: RVS series vacuum sintering furnace produced by SIMUWU company is a high-quality product for vacuum sintering process of ceramic products. Good temperature control accuracy and temperature control uniformity ensure the effective vacuum heat treatment process. SIMUWU company focuses on the manufacture of vacuum furnaces, has more than ten years of relevant experience, and has a good reputation in the field of vacuum furnace manufacturing. The product line includes vacuum air quenching furnace, vacuum oil quenching furnace, vacuum brazing furnace, etc., which are widely sold in developed and developing countries.