The sintering union of refractory materials is a complicated and restricted process. The main factors affecting the sintering of ultrafine talc include the nature of talc powder raw materials, additives, sintering temperature and holding time, firing atmosphere, forming method and pressure of the green body, etc.

First, the impact of raw materials

The influence of raw materials on sintering is divided into internal and external factors. The internal cause is the crystalline chemical characteristics of the material, and the external cause is mainly reflected in the particle composition of the raw materials used. The lattice energy of the material crystal is an important parameter that determines the ease of sintering and recrystallization of the material. Crystals with large lattice energy, relatively stable structure, small particle mobility at high temperatures, and sintering are difficult. The type of crystal structure is also an important influencing factor. If the polarity of the material cation is low, the crystal structure of the compound it forms is relatively stable, and it must have significant defects at a temperature close to the melting point. Therefore, the mobility of the particle of this compound is small. Not easy to sinter. The lattice energy of Al2O3 and MgO in refractories is high and the polarity is low, so it is difficult to sinter.

Polycrystals composed of fine grains are easier to sinter than single crystals because polycrystals contain many grain boundaries. This is the main place to eliminate defects, and it may also be a fast channel for atom and ion diffusion. When an ion crystal is sintered, both positive and negative ions must diffuse to cause mass transfer and sintering. The slower diffusion rate controls the sintering rate. It is generally believed that the radius of negative ions is large and the diffusion rate is slow. However, experimental research on Al2O3 and Fe2O3 found that O2- diffuses rapidly through the channels provided by the grain boundary, so that the positive ions Al3 + and Fe3 + diffuse more slowly than oxygen ions and become a sintering process Speed control steps.

From the perspective of preventing secondary recrystallization, if there are a small number of large particles in the fine powder, abnormal growth of crystal grains is likely to occur and sintering is not easy. The suitable powder particle size of general oxide materials is 0.05-0.5 μm.

Crystal growth rate is another crystal chemistry that affects sintering. For example, MgO crystals grow quickly during sintering, and it is easy to grow to 1000 to 1500 times the original grain, but its density can only reach 60% to 80% of the theoretical value. Al2O3 is not the case. Although its grain growth is only 50 to 100 times, it can reach 90% to 95% of the theoretical density, which basically achieves sufficient sintering. In order to increase the density of MgO materials, measures must be taken to suppress grain growth.

Depending on the particle size of the original powder, the sintering mechanism sometimes changes. For example, for sintering of AIN, when the particle size is 0.78 to 4.4 μm, coarse particles are sintered according to the volume diffusion mechanism, while fine particles are sintered according to the grain boundary diffusion or surface diffusion mechanism.

The particle size of the raw materials used is also an important factor affecting the densification of sintering. Whether it is solid phase sintering or liquid phase sintering, fine particles increase the driving force for sintering, shorten the distance of particle diffusion and increase the solubility of particles in the liquid phase. It leads to the acceleration of the sintering process. According to some reports, when the original particle size of MgO is more than 20 μm, even if it is kept at 1400 ° C for a long time, it can only reach 70% of the relative density and cannot be further densified. When the particle size is less than 20 μm, the temperature It is 1400 ° C or the particle size is 1 μm or less. At 1000 ° C, the sintering speed is fast. If the particle size is 0.1 μm or less, the sintering speed is almost the same as that of hot-pressing sintering.

Effects of additives

In solid phase sintering, a small amount of additives (sintering aids) can form a solid solution with the main crystal phase to promote increased defects; in liquid phase sintering, additives can change the properties of the liquid phase (such as clay, composition, etc.), and thus can promote sintering. Role. The role of additives may be in the following aspects

1. Form a solid solution. When the additive forms a solid solution with the sinter, it can increase the wholesale of lattice defect talc powder, activate the crystal lattice, and promote sintering. In general, the formation of a limited displacement solid solution between them is more helpful to promote sintering. The larger the difference between the electricity price and radius of the additive ions and the electricity price of the main lattice ions, the greater the degree of lattice distortion, and the more obvious the effect of promoting sintering. For example, when Al2O3 is sintered, adding 3% Cr2O3 to form a continuous solid solution can be sintered at 1860 ° C, and adding 1% to 2% TiO2 can be densified only at about 1600 ° C.

2. Prevent crystal form transformation. Some oxides undergo crystal form transformation during sintering with large volume effect, which makes it difficult to achieve sintering densification and easily cause cracking of the green body. At this time, if appropriate additives can be selected to suppress, sintering can be promoted. Adding a certain amount of CaO and MgO during sintering of ZrO2 belongs to this mechanism. At about 1200 ° C, m-ZrO2 is transformed into t-ZrO2, accompanied by a volume shrinkage of about 10%, which deteriorates the stability of the product. The introduction of Ca2 + (or Mg2 +) with a lower electricity price than Zr4 + can form a stable cubic fluorite structure solid solution. In this way, not only the cracking of the product is prevented, but also the concentration of defects in the crystal is increased, which accelerates the sintering.

3. Inhibit the grain growth, and the talc powder wholesale in the later stage of sintering plays an important role in promoting sintering densification. However, if secondary recrystallization or intermittent grains grow too fast, the grains will become coarser and the grain boundaries will widen, which will cause the phenomenon of anti-densification and affect the microstructure of the product. At this time, sintering can be promoted by adding additives that can suppress the abnormal growth of the particles. When sintering transparent Al2O3 products, in order to suppress secondary recrystallization and eliminate pores on the grain boundaries, MgO or MgF2 is generally added to form magnesium at high temperatures. Aluminum spinel is wrapped on the surface of Al2O3 grains, which suppresses the speed of grain boundary migration and promotes the discharge of pores, which has a significant effect on promoting the sintering of the green body.

4. Formation of a liquid phase. The additives form a liquid phase during the sintering process. The reasons for the formation of the liquid phase may lie in two aspects: the low melting point of the additive itself and the formation of multiple eutectics between the additive and the sinter; the low mass transfer resistance of the liquid phase and the rapid mass transfer speed reduce the sintering temperature and increase The density of the body. For example, when manufacturing 95% Al2O3 ceramics, CaO and SiO2 are generally added. When CaO: SiO2 = 1, CaO-Al2O3-SiO2 liquid phase is generated, so that the material can be sintered at 1540 ° C. It should be noted that not all additives that can promote the formation of a liquid phase can promote sintering, such as adding some alkali metal oxides to Al2O3 will seriously hinder its sintering. In addition, although the formation liquid is beneficial to sintering, it will seriously affect the high temperature performance of the material, so it must be considered in a unified manner.