After the powder is formed, a good-shaped green body is obtained, and then the green body is heated at a certain temperature, and the green body shrinks in volume, and finally becomes a dense sintered body. This process is called sintering. The driving force for the sintering of the alumina ceramic body is mainly the change of the surface energy of the powder, that is, the surface energy of the powder is reduced, the surface area is reduced, and the ceramic is densified. In the process of ceramic sintering and densification, the transfer of substances can be carried out through solid phase diffusion, including surface diffusion, grain boundary diffusion, lattice diffusion and so on. Normal pressure sintering is generally used in industry.
1. Atmospheric sintering
Atmospheric sintering is to sinter the material under atmospheric pressure without applying pressure. It is currently the most commonly used sintering method. It includes atmospheric sintering under air conditions and atmospheric sintering under certain special gas atmosphere conditions. This method has a higher sintering temperature, higher requirements on the furnace, and relatively large waste of energy.
Al2O3 has a high melting point, so the preparation of Al2O3 ceramics often requires the addition of sintering aids to achieve compactness through liquid phase sintering. This method usually promotes the sintering of Al2O3 ceramics. The liquid phase sintering of Al2O3 ceramics generates a liquid phase through chemical reactions, promotes diffusion and viscous flow, so as to achieve particle rearrangement and mass transfer processes, lowers the sintering temperature of Al2O3 ceramics, and accelerates effective sintering.
Since atmospheric sintering has no external driving force, it is very difficult to eliminate all the pores in the ceramic to reach the theoretical density. The special sintering process refers to adding sintering driving force during the sintering process of alumina ceramics to promote the densification of ceramics. At present, the common special sintering processes mainly include hot pressing sintering, hot isostatic pressing sintering, microwave heating sintering, microwave plasma sintering, spark plasma sintering and so on.
2. Hot pressing and sintering
Hot-pressing sintering is the application of unidirectional pressure to the sample at high temperature to promote the ceramics to achieve full densification. Compared with conventional sintering, sintering under a pressure of 15 MPa reduces the sintering temperature of the ceramic by 200 ℃ and increases the density by 2%, and this trend increases with the increase of pressure. For pure alumina ceramics, conventional sintering requires a temperature above 1800°C; while hot-press sintering of 20MPa only requires 1500°C.
The pressure provided by hot press sintering promotes the flow of atoms in the particles, and at the same time, the pressure and the surface energy together act as a driving force to strengthen the diffusion effect. Since hot pressing sintering can be sintered at a lower temperature, the growth of crystal grains is inhibited, and the obtained samples are dense and uniform, with small crystal grains and high strength. However, it is not suitable to produce products that are too high, too thick, and complex in shape, and the production scale is small and the cost is high.
3. Hot isostatic pressing sintering
Hot isostatic pressing sintering is a sintering in which pressure is applied to all directions of the ceramic body at the same time to reduce the sintering temperature of the ceramic, and at the same time, the ceramic obtained by sintering has a uniform structure and good performance. Although hot isostatic pressing sintering can successfully reduce the sintering temperature of ceramics and obtain objects with complex shapes, hot isostatic pressing sintering requires encapsulation or pre-sintering of the green body in advance, and the pressure conditions will be relatively harsh.
4. Ultra-high pressure sintering
Ultra-high pressure sintering is sintering under higher pressure conditions. Due to the higher pressure, the diffusion of atoms is inhibited, and the nucleation barrier is relatively small. Therefore, high density (>98%) can be obtained at a lower temperature. Purity alumina ceramics. During the ultra-high pressure sintering process, the presence of pressure increases the diffusion rate of vacancies and atoms in the particles, and the pressure and surface energy together act as the driving force for sintering to increase the diffusion effect. Ultra-high pressure sintering usually only needs to be carried out at a relatively low temperature, which suppresses the abnormal growth of crystal grains, so as to obtain high-purity alumina ceramics with high densification, fine grain size and uniform distribution.
5. Microwave heating method sintering
Microwave heating method sintering uses the interaction between microwaves and ceramics, because the dielectric effect makes the ceramic interior and surface sintered at the same time. Microwave sintering is different from other sintering methods. Its hot gas flow is from the inside to the outside, which is conducive to the outward diffusion of the gas inside the green body; at the same time, the microwave increases the activity of the crystal grains, and is easier to migrate to promote densification.
Compared with other sintering methods, microwave sintering can quickly heat up and sinter, with uniform temperature field, low thermal stress and no pollution. The sintering temperature of microwave sintering is lower than that of conventional sintering by 100°C to 150°C, and the sintering time is nearly an order of magnitude shorter than that of conventional sintering. Sintered under the same conditions, the density of microwave sintering is obviously higher than that of conventional sintering. Microwave sintering can sinter objects with complex shapes, and the sintered ceramic has small internal grains, good uniformity, and good fracture toughness.
6. Microwave plasma sintering
Compared with conventional sintering, microwave plasma sintering can lower the sintering temperature by 200°C under the same conditions, and has a fast sintering speed, small grain size, and high mechanical strength. One of the reasons that microwave plasma sintering promotes densification is the rapid heating, which reduces the growth of grains caused by surface diffusion, and provides a strong driving force and a shorter distance for volume diffusion and grain boundary diffusion, thus Lower the sintering temperature of alumina ceramics and refine the crystal grains.
7. Spark plasma sintering
Spark plasma sintering is a relatively new sintering method developed in recent years. It uses the instantaneous high temperature field generated by pulse energy and pulse pressure to realize the spontaneous heating of the crystal grains in the ceramic to activate the crystal grains. Due to this sintering The method heats up, cools quickly, and has a short heat preservation time, which inhibits the growth of crystal grains, shortens the ceramic preparation cycle, and saves energy. Spark plasma sintering is actually a new hot pressing sintering method. The obtained ceramic samples have uniform crystal grains, high density and good mechanical properties. It is a very valuable and promising sintering method.
In the process of spark plasma sintering to prepare high-purity alumina ceramics, the heating rate has a great influence on the densification of samples at different stages. In the initial stage of sintering, a faster heating rate can increase the density of the sintered body, and in the later stage of sintering, a faster heating rate can cause the density of the sintered body to decrease.
8. Two-step sintering method
The two-step sintering method is to heat the sample to a specific temperature (T1) to eliminate the subcritical pores in the green body, and then reduce it to a lower temperature (T2) to make the green body compact. In the low-temperature sintering stage of the two-step sintering method, since the activation energy required for grain boundary migration is higher than that required for grain boundary diffusion, this stage is mainly based on grain boundary diffusion. Therefore, in the second stage of the two-step sintering method, the green body is continuously densified, but the crystal grains will not grow too fast. In the low-temperature sintering stage of the two-step sintering method, the prerequisite for complete compaction of the green body is that the pores in the green body gradually become closed pores during the shrinkage of the green body.
9. Microwave two-step sintering
Two-step sintering can be carried out in a traditional sintering furnace, with low equipment cost and strong application value. However, two-step sintering is a relatively slow sintering process because it needs to be kept at the second temperature for a long time. Microwave heating usually has the advantages of overall heating and rapid heating. Few studies have combined microwave heating with two-step heating. However, the characteristics of microwave heating that can reduce the sintering temperature and shorten the sintering time will help to further refine the crystal grains and effectively shorten the production cycle of the two-step method.
10. High vacuum sintering
High-vacuum sintering is a sintering technology for sintering ceramic bodies in a high-vacuum state. Vacuum sintering has received much attention from many scholars in preparing ceramics with low porosity and small size due to its advantages such as reducing the heating rate, suppressing abnormal growth of crystal grains, and reducing irregular porosity.
High vacuum sintering can not only enhance some of the properties of high-purity alumina ceramics, but also reduce impurities at the grain boundaries and pores in the sintered body. In the process of preparing high-purity alumina ceramics by vacuum sintering, oxygen ions in the alumina crystal lattice are easily lost, forming a large number of oxygen ion vacancies, and the aluminum ion concentration is relatively increased, which leads to the acceleration of the diffusion process of aluminum ions, which is beneficial to the progress of sintering.
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