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Low temperature vacuum hot pressing sintering of aluminum nitride ceramics

Aluminum nitride (AIN) ceramics have excellent comprehensive properties and are ideal candidates for high-density electronic substrates and packaging, as well as high-temperature structural materials. They also have broad application prospects in the fields of acoustics and optoelectronics. However, AIN is a covalent compound with a small self-diffusion coefficient, which usually requires high sintering temperature or addition of sintering aids to obtain AIN dense bodies. It has been reported that hot-pressing sintering, reaction sintering and other atmospheric-pressure sintering methods that require sintering aids are difficult to densify. In order to obtain the highest possible density, it is usually necessary to keep it at a high temperature above 1800 °C for 1 to 6 hours. The production efficiency is low and the cost is high. Moreover, the high temperature and long-term heat preservation lead to excessive growth of the grains, resulting in an unsatisfactory microstructure and greatly reduced mechanical properties. The vacuum hot pressing sintering with the addition of sintering aids can obtain highly dense AlN ceramics at low temperature, and reduce the influence of oxygen impurities on the properties of ceramics. In this study, AIN powder produced by high temperature self-propagating method was used as raw materials, Y2O3, Dy2O3, and La2O3. As an additive, the vacuum hot pressing sintering technology was used to realize the low temperature sintering of AIN ceramic materials, and the vacuum hot pressing sintering properties of AIN powder were studied.

According to the analysis results of the phase and microstructure of the sintered body, the process of vacuum hot-pressing sintering AIN can be divided into three stages:

1) Before 1 500 ℃, under the action of external stress, the relative density increases rapidly, and the local fragmentation or plastic flow mass transfer caused by particle rearrangement and grain boundary slip fills the large accumulation voids.

2) In the stage of 1 500 ℃～1 550 ℃, the additive and Al2O3 on the surface of AIN powder. The iridium aluminate grain boundary phase is formed by solid-phase reaction, which reduces the activation energy of sintering, accelerates the mass transfer process, and promotes sintering, and the microstructure changes from loose to dense.

3) When the temperature is higher than 1550 °C, the grain boundary phase formed moves and gradually aggregates at the boundary of AIN particles, and its morphology changes, indicating that the liquid grain boundary phase is formed at higher temperature. The existence of the liquid phase promotes the mass transfer and growth between the grains, makes the grains develop and fill up rapidly, increases the surface contact between the AlN grains, and causes the grain boundary phase to be distributed at the junction of the AlN grains, forming the structure and composition. It is developed in the direction of improving the thermal conductivity of AlN.

AlO on the surface. The solid-phase reaction occurs to form the aluminate grain boundary phase, which reduces the activation energy of sintering, accelerates the mass transfer process, and promotes sintering, and the microstructure changes from loose to dense.

In conclusion

1) The size of the powder particle size and the sintering process have a great influence on the hot pressing sintering properties of AlN. The AIN powder synthesized by self-propagating combustion was used to obtain high-density AIN ceramics at a sintering temperature of 1550℃ under the action of additives Y2O3, Dy2O3 and La2O3. The AIN ceramics sintered by vacuum hot pressing of nano-powders have fine grains and uniform structure, and the second phase mostly exists at the grain boundaries and is continuously distributed. After sintering at 1 550 ℃ for 90 min, the transgranular fracture mode appeared in the Y2O3 sample.

2) The sintering aid has a great promotion effect on the low temperature sintering of AIN ceramics. The sintering aids La2O3, Y2O3, Dy2O3 react with Al2O3, and the second phase materials are LaAlO3, YAlO3, Dy4Al2O9, etc. The self-diffusion coefficient is greater than that of AlN, which is beneficial to the densification of the powder and the improvement of the performance of AIN ceramics.

3) In the initial stage of hot pressing sintering of AIN ceramics, the material migration mechanism is mainly particle rearrangement. This grain rearrangement mechanism includes grain fragmentation, grain slip on the interface, grain fracture rearrangement, and grain plastic deformation and flow. In the late stage of AlN hot pressing sintering, the further densification of AlN mainly depends on The pressure-enhanced diffusion creep process (Nabarro-Herring vacancy migration mechanism) can promote the growth of grains and reduce the voids between grains.

Selection of vacuum sintering equipment: The RVP series vacuum hot pressing furnace produced by SIMUWU is a high-quality product for the vacuum heat treatment process of tooling and molds. A good vacuum system and temperature control system ensure the effective progress of the vacuum heat treatment process. It is used for hot pressing and sintering of metal composite materials or ceramic powder composite materials. The equipment adopts resistance heating, and the cylinder head is driven with pressure, which can realize automatic compression and pressure, with slow displacement loading and fast rising speed. Realize automatic control, unique head structure, long service life.