When the field strength is 60 V/cm, the sample will be instantly densified when the furnace temperature rises to about 1025°C; when the field strength is increased to 120 V/cm, the sintering furnace temperature can even be reduced to 850°C. This new sintering technology is called "flash sintering", which is a new type of sintering technology that achieves low-temperature and extremely rapid sintering of materials under a certain temperature and electric field. Usually, the following three phenomena will occur with FS: thermal runaway inside the material; sudden drop in the resistivity of the material itself; strong flashing phenomenon.
FS technology mainly involves three process parameters, namely furnace temperature (Tf), field strength (E) and current (J). Figure 4c shows the change trend diagram of various parameters in the traditional FS process. In this mode, a stable electric field is applied to the material, and the furnace temperature rises at a constant rate. When the furnace temperature is low, the material resistivity is high, and the current flowing through the material is small. As the furnace temperature increases, the resistivity of the sample decreases and the current gradually increases. This stage is called the incubation stage, and the system is voltage controlled. When the furnace temperature rises to the critical temperature, the material resistivity drops suddenly, the current rises sharply, and FS occurs. Since the field strength is still stable at this time, the system power (W = EJ) will quickly reach the power upper limit of the power supply, and the system will change from voltage control to current control. This stage is called the FS stage (flash sintering stage). When the resistivity of the material no longer increases, the field strength is stabilized again, and the sintering enters a steady stage (steady stage), that is, the holding stage of FS. After the holding stage, a complete FS process ends.
Compared with traditional sintering, FS has the following advantages: shortens the sintering time and reduces the furnace temperature required for sintering, inhibits grain growth, can achieve non-equilibrium sintering, simple equipment and lower cost.
5 Cold Sintering (CS)
In order to make the density of the ceramic material reach more than 95% of its theoretical density, the sintering temperature of the ceramic material needs to reach 50% to 75% of its melting temperature. Therefore, the sintering temperature of most ceramic materials is greater than 1000 ℃, making the production process of ceramic materials consume more energy, and high-temperature sintering makes ceramic materials limited in terms of material synthesis and phase stability.
In order to reduce the sintering densification temperature of ceramic powder, new sintering technologies such as liquid phase sintering, field-assisted sintering, and FS have been applied. However, due to solid phase diffusion and liquid phase formation, higher temperature heating of ceramic powder is still required. The above technologies do not Reduce the sintering temperature to the "low temperature range". Recently, the Andall research group of Pennsylvania State University in the United States was inspired by the hydrothermal-assisted hot pressing process and proposed a new "ceramic CS process" technology. Different from the traditional high-temperature sintering process, the ceramic CS process adds an instantaneous solvent to the powder and applies a relatively large pressure (350～500 MPa) to enhance the rearrangement and diffusion between particles, so that the ceramic powder is at a lower temperature. Temperature (120 ~ 300 ℃) and a short time to achieve sintering densification, creating the possibility for low-temperature sintering to produce high-performance structural ceramics and functional ceramics.
The basic process of ceramic CS technology is to add a small amount of aqueous solution to the ceramic powder to wet the particles, and the surface material of the powder is decomposed and partially dissolved in the solution, thereby generating a liquid phase at the particle-particle interface. Put the wetted powder into the mold, and heat the mold while applying a relatively large pressure. After the pressure is maintained for a period of time, a dense ceramic material can be prepared. Maria et al. observed and analyzed the preparation process of a variety of ceramic systems, and summarized the internal process of the CS process into two steps:
In the first stage, the mechanical pressure causes the liquid phase between the powder particles to flow, thereby triggering the rearrangement of the powder particles; in the second stage, the pressure and temperature promote the dissolution of the powder surface substances in the liquid phase, and this process The substance is diffused and transported. In the first stage, the driving force of the densification process is mainly provided by mechanical pressure. The role of the liquid phase is to promote the sliding and rearrangement of the particles, and the tip of the particles will dissolve in the liquid phase to make the particles spherical, thereby improving the particles during the compression process. The bulk density. In the second stage, the mechanical pressure and temperature will cause the solution in the system to evaporate instantaneously, and the supersaturation of the solution will increase with the sintering time. The substance diffuses in the liquid phase and precipitates on the surface of the particles away from the pressure zone. Filling in the grain boundaries or pores to densify the ceramics. At this stage, the amorphous precipitates will be pinned to the grain boundaries to inhibit the growth of crystal grains.
6 Oscillating pressure sintering (OPS)
Existing various pressure sintering technologies use static constant pressure. The introduction of static pressure during the sintering process helps to eliminate pores and increase the density of ceramics, but it is difficult to completely bond special ceramics with ionic and covalent bonds. The elimination of pores in the material still has certain limitations for the desired ultra-high strength, high toughness, high hardness and high reliability materials. The main reasons for the limitations of HP static pressure sintering are reflected in the following three aspects: Before the start of sintering and the early stage of sintering, the constant pressure cannot make the powder in the mold fully realize the particle rearrangement to obtain a high bulk density; in the middle and late sintering period, Plastic flow and the elimination of agglomerates are still subject to certain restrictions, and it is difficult to achieve complete uniform densification of the material; in the later stage of sintering, it is difficult to completely eliminate residual pores under constant pressure.
Research on the mechanism of OPS technology to strengthen the densification of ceramics shows that:
First, the continuous oscillating pressure applied during the sintering process shortens the diffusion distance through particle rearrangement and elimination of particle agglomeration; secondly, in the middle and late sintering process, the oscillating pressure provides a greater sintering driving force for powder sintering, which is conducive to accelerating the viscosity. Flow and diffusion creep, stimulate the rotation of grains in the sintered body, grain boundary slip and plastic deformation to accelerate the densification of the green body; in addition, by adjusting the frequency and size of the oscillating pressure to enhance plastic deformation, it can promote the grain boundary at the later stage of sintering The combination and discharge of pores completely eliminates the residual pores inside the material, making the density of the material close to the theoretical density; finally, OPS technology can effectively inhibit the growth of grains and strengthen the grain boundaries. In short, the densification of materials in the OPS process mainly stems from the following two mechanisms:
The first is the traditional mechanisms of grain boundary diffusion, lattice diffusion, and evaporation-condensation under the action of surface energy; the second is the new mechanism conferred by oscillating pressure, including particle rearrangement, grain boundary slip, plastic deformation, and grain movement caused by deformation , Stoma discharge, etc. Therefore, the use of OPS technology can fully accelerate powder densification, reduce sintering temperature, shorten holding time, inhibit grain growth, etc., thereby preparing cemented carbide materials and ceramic materials with ultra-high strength and high reliability to meet extremes The application environment has higher requirements for material performance.
As an important member of engineering materials, advanced ceramic materials are still widely used by traditional sintering technologies, such as high energy consumption, high time consumption, and poor product performance. The new rapid sintering technology has demonstrated its huge advantages in the rapid preparation of special ceramics, material connection, gradient and nano-ceramic preparation.