After the powder is formed, a well-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 main driving force for the sintering of alumina ceramic bodies is the change in the surface energy of the powder, that is, the surface energy of the powder decreases, the surface area decreases, and the ceramic achieves densification. In the process of ceramic sintering and densification, material transfer can be carried out by solid-phase diffusion, including surface diffusion, grain boundary diffusion, and lattice diffusion. In industry, atmospheric pressure sintering is generally used.
1. Atmospheric Pressure Sintering
Atmospheric pressure sintering, that is, sintering the material under atmospheric pressure without pressurizing it, is the most commonly used sintering method at present. It includes atmospheric pressure sintering under air conditions and atmospheric pressure sintering under certain special gas atmosphere conditions. This method has higher sintering temperature, higher requirements for the furnace, and a relatively large waste of energy.
AI2O3 has a high melting point, so the preparation of AI2O3 ceramics often requires the addition of sintering aids and densification through liquid phase sintering. This method usually promotes the sintering of AI2O3 ceramics. The liquid phase sintering of AI2O3 ceramics generates a liquid phase through chemical reactions, which promotes diffusion and viscous flow to achieve particle rearrangement and mass transfer processes, reduce the sintering temperature of AI2O3 ceramics, and accelerate effective sintering.
Since there is no external driving force for normal pressure sintering, it is very difficult to eliminate all the pores in the ceramic to reach the theoretical density. The special sintering process refers to the addition of a sintering driving force during the sintering process of alumina ceramics to promote the densification of the ceramics. At present, the common special sintering processes mainly include hot pressing sintering, hot isostatic pressing sintering, microwave heating sintering, microwave plasma sintering, discharge plasma sintering, etc.
2. Hot Pressing Sintering
Hot-pressing sintering is the application of unidirectional pressure to the sample at high temperature to promote full densification of the ceramic. Compared with conventional sintering, sintering at a pressure of 15 MPa reduces the sintering temperature of the ceramic by 200°C and increases the density by 2%, and this trend increases with increasing pressure. For pure alumina ceramics, conventional sintering requires temperatures above 1800°C; while 20MPa hot-pressing sintering requires only 1500°C.
The pressure provided by hot-pressing sintering promotes the flow of atoms within the particles, while the pressure and surface energy act together as a driving force to enhance diffusion.
Because hot pressing sintering can be sintered at a lower temperature, the growth of grains is suppressed, and the obtained samples are dense and uniform, with small grains and high strength. However, it is not suitable for the production of over-high, over-thick and complex-shaped products, with small production scale and high cost.
3. Hot Isostatic Pressing Sintering
Hot isostatic pressing sintering is a sintering process in which pressure is applied to the ceramic body in all directions at the same time to reduce the sintering temperature of the ceramic, and at the same time, the sintered ceramic has a uniform structure and good performance. Although HIP sintering can successfully reduce the sintering temperature of ceramics and obtain objects with complex shapes, HIP sintering needs to encapsulate or pre-sinter the green body in advance, and the pressure conditions are also harsh.
4. Ultra-High Pressure Sintering
Ultra-high pressure sintering is sintering under high pressure conditions. Due to the high pressure, atomic diffusion is inhibited and the nucleation barrier is relatively small. Therefore, high density (>98%) high density can be obtained at low temperature. Pure alumina ceramic. In the process of ultra-high pressure sintering, the existence 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, which enhances the diffusion effect. Ultra-high pressure sintering usually only needs to be carried out at a relatively low temperature, which inhibits the abnormal growth of grains, thereby obtaining high-purity alumina ceramics with high densification, fine grain size and uniform distribution.
5. Microwave Heating Sintering
Microwave sintering uses the interaction between microwaves and ceramics, because the dielectric effect makes the interior and surface of the ceramic sinter 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 body; at the same time, the microwave improves the activity of the grains and makes it easier to migrate to promote densification. Compared with other sintering methods, microwave sintering can heat up and sinter rapidly, with uniform temperature field, small thermal stress and no pollution. The sintering temperature of microwave sintering is 100°C to 150°C lower than that of conventional sintering, and the sintering time is nearly an order of magnitude shorter than 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 ceramics have small internal grains, good uniformity and good fracture toughness.
6. Microwave Plasma Sintering
Compared with conventional sintering, microwave plasma sintering can reduce the sintering temperature by 200°C under the same conditions, and has 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 grain growth caused by surface diffusion and provides a stronger driving force and shorter path for bulk diffusion and grain boundary diffusion. , thereby reducing the sintering temperature of alumina ceramics and refining the 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 inside the ceramic to activate the crystal grains. Because of this sintering The method has the advantages of rapid heating, cooling and short holding time, which inhibits the growth of crystal grains, shortens the preparation period of ceramics, and saves energy.
Spark plasma sintering is actually a new hot pressing sintering method. The obtained ceramic samples have uniform grains, high density and good mechanical properties. It is a very valuable and promising sintering method. In the process of preparing high-purity alumina ceramics by spark plasma sintering, the heating rate has a great influence on the sintering and densification of samples at different stages. In the initial stage of sintering, a faster heating rate can increase the density of the sintered body, while in the later stage of sintering, a faster heating rate can lead to a decrease in the density of the sintered body.
8. Two-Step Sintering Method
The two-step sintering method heats the sample to a specific temperature (T1) to eliminate subcritical pores in the green body, and then drops to a lower temperature (T2) to densify the green body. In the low-temperature sintering stage of the two-step sintering method, the grain boundary diffusion is the main factor in this stage because the activation energy required for grain boundary migration is higher than that for grain boundary diffusion.
Therefore, in the second stage of the two-step sintering process, the green body is continuously densified, but the grains do not grow too fast. In the low-temperature sintering stage of the two-step sintering method, the prerequisite for the complete densification 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
The two-step sintering can be carried out in a traditional sintering furnace, the equipment cost is low, and it has strong application value. However, the two-step sintering process is a relatively slow sintering process due to the need for a long time holding at the second temperature point. Microwave heating usually has the advantages of integral heating and rapid heating, and few studies have combined microwave heating with two-step heating. However, microwave heating can reduce the sintering temperature and shorten the sintering time, which will be beneficial to the further refinement of the grains and effectively shorten the production cycle of the two-step method.
10. High Vacuum Sintering
High vacuum sintering is a sintering technology that sinters ceramic bodies in a high vacuum state. Vacuum sintering has attracted the attention of many scholars in the preparation of low-porosity and small-sized grain ceramics due to its advantages of reducing the heating rate, inhibiting abnormal grain growth, and reducing irregular porosity. High vacuum sintering can not only enhance some properties of high-purity alumina ceramics, but also reduce impurities at grain boundaries and pores in the sintered body. In the process of preparing high-purity alumina ceramics by vacuum sintering, the oxygen ions in the alumina lattice are easily lost, forming a large number of oxygen ion vacancies, and the aluminum ion concentration increases relatively, which leads to the accelerated aluminum ion diffusion process, which is conducive to the sintering.