Introduction to Common Sintering Methods of Aluminum Nitride Industrial Ceramics

Kezhong Ceramic Factory has introduced several preparation processes of aluminum nitride industrial ceramics and aluminum nitride. In fact, the sintering of aluminum nitride ceramics is also a crucial step in the entire ceramic processing and forming process. Let's take a look together.

Common AlN sintering methods:

Sintering refers to the densification process of ceramic powder formed by pressure pressing at high temperatures. At the sintering temperature, ceramic powder particles bond with each other, grain size grows, grain boundaries and voids in the body gradually decrease, and the volume of the body shrinks, resulting in an increase in density, until a polycrystalline sintered body with a certain strength is formed. As a covalent bond compound, aluminum nitride industrial ceramics are difficult to be solid-phase sintered.

Usually, a liquid phase sintering mechanism is used, which involves adding sintering aids that can generate a liquid phase to the aluminum nitride raw material powder and promoting sintering by dissolving to produce a liquid phase. AlN sintering power: the specific surface energy of the powder, lattice defects, capillary force between solid and liquid phases, etc.

To prepare AlN ceramics with high thermal conductivity, two problems must be solved in the sintering process: firstly, to improve the density of the material, and secondly, to avoid oxygen atoms from dissolving into the lattice during high-temperature sintering. The common sintering methods are as follows:

1. Atmospheric sintering

Normal pressure sintering is a traditional preparation process for AlN ceramics. In the process of atmospheric pressure sintering, the green body is not subjected to external pressure and is only transformed from an aggregate of powder particles into a grain combination by heating under general air pressure. Atmospheric pressure sintering is the simplest and most widely used sintering method. The general temperature range for atmospheric sintering of aluminum nitride ceramics is 1600-2000 ℃. Increasing the sintering temperature and extending the insulation time appropriately can improve the density of aluminum nitride ceramics. Due to the covalent bond structure of AlN, it is difficult for pure aluminum nitride powder to carry out solid phase sintering, so sintering additives are often added to raw materials to promote ceramic sintering densification. Common sintering aids include alkaline earth metal compound aids, rare earth compound aids, etc. In general, the preparation of AlN ceramics by atmospheric sintering requires high sintering temperature and long insulation time, but its equipment and process flow are simple and easy to operate.

2. Hot pressing sintering

In order to reduce the sintering temperature of aluminum nitride ceramics and promote ceramic densification, hot pressing sintering can be used to prepare aluminum nitride ceramics, which is currently one of the main process methods for preparing high thermal conductivity densified AlN ceramics. The so-called hot pressing sintering refers to the sintering of ceramics under a certain pressure, which can enable both heating sintering and pressure forming to occur simultaneously. An AlN ceramic sintered body with a density of 3.26g/cm3 and a thermal conductivity of 200W/(m.K) was prepared by sintering at 1700 ℃ for 4 hours under a high pressure of 25MPa. The AlN lattice oxygen content was 0.49wt%, which was more than 60% lower than the lattice oxygen content (1.25wt%) of the AlN sintered body sintered at 1800 ℃ for 8 hours, and the thermal conductivity was improved.

3. High-pressure sintering

High pressure sintering of AlN ceramics is similar to hot pressing sintering, except that the applied external pressure is higher. Generally, sintering at a high pressure greater than 1 GPa is called high pressure sintering. It can not only rapidly achieve high density of materials, with fine grains, but also cause the crystal structure and even atomic and electronic states to change

4. Atmosphere sintering

Atmosphere sintering generally involves a chemical reaction between the AlN body and the gas phase at sintering temperature, resulting in an increase in body mass and a decrease in porosity. Atmosphere sintering of aluminum nitride ceramics utilizes the nitriding reaction of aluminum powder in nitrogen to form aluminum nitride powder and sinter it together at high temperature. The reaction process of atmosphere sintering AlN ceramics is essentially the preparation of aluminum nitride powder by direct nitridation of aluminum powder. This reaction is exothermic reaction and very intense. The atmosphere sintering method is often used to manufacture corrosion-resistant and high-strength products such as crucibles due to its difficulty in obtaining dense sintered bodies, but it is not suitable for manufacturing high thermal conductivity substrates.

5. Discharge ion sintering

Spark Plasma Sintering (SPS) is a new type of rapid sintering technology that developed in the 1990s and has gradually matured. It integrates plasma activation, hot pressing, resistance heating and other technologies, and has the characteristics of fast sintering speed and uniform grain size. The equipment diagram is shown in Figure 4. In addition to the Joule heat generated by the pulse current passing through the graphite mold and the plastic deformation caused by the pressure during the hot pressing sintering process, according to the traditional sintering theory, the pulse current can also generate voltage at the tip between AlN green particles, and generate partial discharge phenomenon. The generated plasma hits the particle surface, causing evaporation of the material, which can achieve the role of purifying the particle surface and activating the particles. Using discharge ion sintering technology at 1730 ℃ and 50MPa, AlN industrial ceramic materials with a relative density of 99.3% can be sintered in just 5 minutes.

6. Microwave sintering

Microwave sintering has received widespread attention from researchers since its introduction into the field of ceramics in the 1970s. A sintering method that utilizes the interaction between microwaves and the medium to generate dielectric loss and heat the entire body; Microwave simultaneously increases the activity of powder particles, which is beneficial for material transfer. Figure 5 shows the actual image of the microwave sintering furnace. Microwave sintering is also a rapid sintering method. Although the mechanisms are different, both microwave sintering and discharge ion sintering can achieve overall heating, greatly reducing sintering time and inhibiting grain growth, resulting in fine and uniform ceramic crystals. AlN ceramics with a thermal conductivity of 66.4W/(m • K) can be obtained by using Nd2O3-CaF2-B2O3 as a sintering agent and sintering at low temperature of 1250 ℃ under microwave irradiation.

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