PRODUCT CATEGORIES

Vacuum Heat Treatment Furnace

Vacuum Sintering Furnace

Vacuum Brazing Furnace

Alumina ceramic densely sintered

With the improvement of the production level, alumina ceramics have also been favored in the optical field in recent years. When the alumina ceramics are fully densified, the light transmittance is greatly improved and it is translucent. It can be used to replace single crystal sapphire and make arc tubes for high-pressure sodium lamps. Infrared optical components, microwave integrated circuit substrates and other devices. Not only that, the increase in density can also improve the mechanical properties of alumina ceramics.

Densification is a degassing process

Pores are common in ceramic materials, and the densification process of ceramics is actually a process of continuous reduction of pores. According to different sintering processes, pores often remain in the interior of ceramic materials in two ways, one is in the interior of ceramic grains, and the other is in the grain boundaries.

Therefore, it is generally believed that there are two main ways to improve the density of alumina ceramics. First of all, by increasing the sintering temperature or providing a reducing atmosphere, the ceramics are easily sintered by atom diffusion under high temperature conditions, and at the same time, the gas phase in the ceramics is easy to diffuse out of the grains and sintered into dense ceramics; secondly, the ceramics are improved by additives. sex.

In addition, in actual production, the selection of raw materials and production process are also key points that affect the densification of alumina ceramics.

Raw materials: select high-purity, ultra-fine alumina

1.Select high-purity alumina powder

It is inevitable that impurities will be introduced during the preparation of ceramic powder. The organic impurities will be burned during the sintering process, but irregular pores will be formed during the densification process; while the inorganic impurities may react with the ceramic powder at high temperature or remain in the matrix to form microcracks . These microstructural defects caused by impurities are bound to have a significant impact on the densification of alumina ceramics. Therefore, the use of high-purity Al₂O₃ powder is an important prerequisite for the preparation of alumina ceramics with excellent properties.

2.Reduce the particle size of alumina powder

The finer the particles, the shorter the sintering time. This is because the finer the particles, the closer the contact between them, the shorter the diffusion path during sintering, and the greater the sintering driving force – the surface energy. The emergence of ultrafine powder preparation technology has opened up a new way to reduce the sintering temperature of ceramic materials, improve the microstructure of products, and improve the mechanical properties of materials (such as hardness, strength, toughness and wear resistance, etc.).

However, due to the large surface energy of superfine powder particles, the grains grow rapidly or grow abnormally during the high temperature sintering process. will be more difficult. Therefore, the powder selected for the production of high-density ceramics is generally in the range of 0.1 μm to 1 μm in particle size.

3.The uniformity of the mixture

In order to reduce the sintering temperature of alumina ceramics, appropriate additives should be added to the powder before sintering. Therefore, the level of mixing is also an important factor affecting the ceramic sintered body. The purpose of mixing is to make the powder composition uniform. If the composition distribution is uneven, the local composition will deviate from the overall ratio, and there will be less local additives, and it is difficult to sinter alumina at low temperature, while the melting point of the place with more additives is lower, and the liquid phase is easy to appear, and the crystal grains grow rapidly, eventually resulting in the product The microstructure is uneven and the density is not high.

Reasonable molding method

Forming is one of the important processes that directly affects the sintering process and the performance of the sintered body. Relevant experiments have verified that at the same sintering temperature, the higher the relative density of the green body, the higher the relative density of the corresponding sintered body. Therefore, in order to ensure high density, the general molding pressure is relatively high. At present, the forming methods of high-performance alumina ceramics are divided into two types: dry method and wet method.

1.Dry forming

In dry molding, we will first think of cold isostatic pressing. The isostatic pressing method is to put alumina powder into an elastic mold and seal it, and then put the mold into a container filled with high-pressure gas or liquid. After sealing, the powder is pressed into a green body with higher density by using the characteristics of equal three-dimensional force. The main molding method of domestic high-pressure sodium lamp tubes is isostatic pressing.

In order to further improve the density of the molded body, people have developed high-pressure molding and ultra-high pressure molding on the basis of ordinary isostatic pressing. Using these two molding methods, ceramic green bodies with relatively high density have been prepared.

2.Wet forming

For wet forming, in recent years, it has developed rapidly because of its ability to control the agglomeration and impurity content of particles in the green body, reduce the defects of the green body and form ceramic parts with complex shapes. Such as centrifugal grouting, the powder is evenly dispersed in the liquid by adjusting the pH value and other means, and then the particles are settled by high-speed centrifugation to obtain the green body. There are also gel direct molding that makes the powder preparation and molding process complete in one go, as well as the gel casting molding that completes the solidification of the green body by the polymerization of organic monomers, and the infiltration curing method that relies on chemical potential molding. These methods have been used in some nanometers. It has been well applied in the synthesis of materials.

The importance of sintering method

The sintering quality directly affects the microstructure and a series of mechanical properties of ceramics. With traditional sintering methods, it is difficult to suppress the growth of grains, and the excessive growth of grain size may have a negative impact on the performance of ceramics. Therefore, some special sintering methods must be adopted, such as pressureless sintering and pressure sintering. Pressureless sintering is divided into reaction sintering and atmosphere sintering, pressure sintering is divided into hot pressing, hot isostatic pressing sintering, ultra-high pressure sintering and so on.

1.Hot pressing sintering

Hot pressing sintering starts from the driving force of sintering. The combination of external pressure (10~40MPa) and the surface energy of the powder promotes the plastic flow and rearrangement of the green body particles, which reduces the pores in the green body, shrinks the green body, and improves the density. The process of increasing the bonding strength between particles and improving the mechanical strength. Compared with the ordinary sintering method (pressureless sintering), the alumina ceramics prepared by this method have higher density and less grain growth, and fine-grained alumina ceramic materials can be obtained. Now it has become a high-performance ceramic material. common sintering method.

2.Vacuum sintering

Vacuum sintering sinters the sample under low pressure by vacuuming the vacuum furnace. Compared with ordinary solid-phase sintering, vacuum pressureless sintering can not only inhibit the growth of grain size but also effectively discharge the gas in the green body under a longer holding time, and the obtained grain size is more uniform.

3.Hot isostatic pressing (HIP)

Hot isostatic pressing sintering is essentially a special hot pressing sintering method. It refers to placing the sintered body in a gas medium under high temperature conditions, so that it is subjected to equal pressure in all directions, thereby promoting the densification process of ceramic materials. Hot isostatic pressing is an advanced material densification process, which has the characteristics of low sintering temperature, short firing time and uniform green body shrinkage. High-performance and complex-shaped ceramic parts with uniform microstructure and almost no pores can be prepared.

Adding sintering aids – simple and effective

Intentionally introducing an appropriate amount of additives into raw materials is an important way to effectively reduce the sintering temperature of ceramics, improve the microstructure of samples, and thus achieve densification of ceramic materials. So far, the additive method is still the most active research in the field of ceramics, and it is also one of the most concise and feasible methods to reduce the sintering temperature and improve the densification.

1.Types of additives

A class of additives can introduce lattice vacancies, facilitate diffusion, reduce sintering activation energy, and form solid solutions. Commonly used additives are mainly variable-valence metal oxides such as CuO, TiO₂, and MnO₂;

The second type of additive is to generate a liquid phase, so that the mass transfer mechanism changes from solid phase diffusion to liquid phase diffusion. Usually, SiO₂, CaO, MgO, etc. are used as additives to form a glass phase to reduce the sintering temperature.

Generally speaking, a small amount of additives can have a certain impact on the densification and microstructure of Al₂O₃ ceramics, especially when introduced in a certain proportion of composite form.

For example: binary composite additive CuO-TiO₂, when the mass ratio of CuO and TiO₂ is 1:2, the relative density of Al₂O₃ can reach 99%.

2.Pre-sintering of additives

CaO, MgO, SiO₂ simple oxides and their pre-sintered bodies were added as additives to alumina with a mass fraction of 95%, and sintered at 1600°C. The results show that compared with the alumina ceramics with simple oxide additives, the alumina ceramics with pre-sintering additives can achieve more sufficient liquid phase sintering, the sintering time is significantly shortened, and the relative density of the ceramics can reach more than 98%. The grain size is about 5µm, and the bending strength is greater than 300MPa.