As an important part of engineering materials and functional materials, advanced ceramic materials have broad application prospects in new energy, communication electronics, semiconductor, aerospace and other industrial fields. However, because ceramic powder is mostly ionic bond or covalent bond compound, the traditional sintering process to prepare dense ceramic materials requires higher sintering temperature and longer holding time, which inevitably leads to grain coarsening and porosity residue, and then affects the properties of ceramic materials injection molding. In order to reduce sintering temperature, shorten sintering time, improve sintering density and material properties, researchers have developed a variety of new sintering technologies.
Spark plasma sintering(SPS)
SPS technology is a new type of rapid sintering technology that has been widely concerned and studied in the academic world. FIG. 1 shows its working principle.
SPS technology pioneered the introduction of a dc pulse current into the sintering process. The indenter acts as a carrier for the current to pass through while applying pressure to the material. Unlike traditional sintering techniques, which typically use radiant heating of a heating body, SPS uses the thermal effect of a large electric current passing through a mold or conducting sample to heat the material. For insulating samples, graphite with good electrical conductivity is usually used as the mold material, and the resistance heat of the mold is used to make the sample rise rapidly. For conductive samples, an insulating mold can be used to heat a current directly through the sample. The heating rate can reach 1000 ℃ /min. When the sample temperature reaches the set value, sintering can be completed after a short time of heat preservation.
SPS technology has the advantages of low sintering temperature, short holding time, fast heating rate, adjustable sintering pressure, and multi-field coupling (electric-mechanical-thermal).
Flash sintering (FS)
FS technology was first reported in 2010 by Cologna et al from the University of Colorado, based on the study of field-assisted sintering technology (FAST)
FS technology mainly involves three process parameters, namely furnace temperature (Tf), field intensity (E) and current (J). Figure 3C shows the variation trend of each parameter in the traditional FS process. In this mode, a steady 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. With the increase of furnace temperature, the sample resistivity decreases and the current increases gradually. 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 limit of the power supply, and the system will change from voltage control to current control, which is called FS stage. When the resistivity of the material does not rise, the field strength stabilizes again, and sintering enters the stable stage, that is, the insulation stage of FS. After the insulation stage, a complete FS process ends.
Compared with traditional sintering, FS has the following advantages: shorten sintering time and reduce the furnace temperature required for sintering, inhibit grain growth, can achieve non-equilibrium sintering, simple equipment, low cost.
Post time: Jun-07-2022