SiC is widely used in key parts of the steel and nonferrous smelting industry due to its stable high-temperature chemical properties, excellent high-temperature strength, high wear resistance and good thermal shock resistance, such as blast furnace tuyere, inner wall and ceramic cup, various The lining material of the furnace wall and the material of kiln furniture, etc. Compared with metals and intermetallic compounds, it has higher high-temperature strength and creep resistance. Compared with oxide ceramics, it has higher thermal conductivity and thermal shock resistance. The high temperature performance of silicon carbide refractories is as follows:
1. High temperature oxidation resistance of SiC refractories
Although SiC has excellent properties, it is easily oxidized during long-term use. Scholars at home and abroad have done a lot of research on the oxidation process of SiC. The results show that high-temperature oxidation of SiC can be divided into inert oxidation and active oxidation. When the partial pressure of O2 is lower than 10-4Pa, the SiC product is the active oxidation of SiO gas phase and the net weight is reduced; when the partial pressure of O2 is higher than 10-4Pa, the SiC product is the inert oxidation of SiO2 and the net weight is increased. The formation can prevent the further occurrence of oxidation; but when the oxidation temperature is above 1473K, SiO2 is converted into cristobalite at high temperature, and the volume expansion occurs, which destroys the structure of the oxide film and generates cracks, which in turn leads to internal oxidation of the material, which seriously affects The service life of silicon carbide materials. Therefore, improving the oxidation resistance of SiC materials is a factor that must be considered in the design and preparation of silicon carbide refractories.
Huang Qingwei et al. Studied the high-temperature oxidation behavior of self-bonded SiC materials with a porosity of 11.5% in 1573K air atmosphere. The results show that the amorphous SiO2 formed in the early stage of oxidation can passivate the pores and crack tips in the material, so that the room temperature strength of the material increases with the oxidation time. When the oxidation time is 22.5h, the strength of the refractory material is the highest, up to 293MPa; as the oxidation time continues to increase, the amorphous SiO2 crystallizes to form cristobalite, which destroys the structure of the oxide film and produces a new surface during the cooling process Cracks, causing a reduction in the strength of the material at room temperature.
Cong Lina et al. Studied the effect of adding different amounts of calcium oxide, aluminum oxide and zirconium oxide on the anti-oxidation performance of SiC materials at different temperatures. The experimental results show that when 2wt% alumina is added, the SiC material has the best anti-oxidation effect. Lu Zhenlin et al. Used a sol-gel method to generate a mullite coating on the surface of recrystallized SiC materials with different particle sizes, and studied the effect of coating thickness and particle size on the high-temperature oxidation behavior of recrystallized SiC materials at 1773K. The results show that the formation of mullite coating can significantly improve the high-temperature oxidation resistance of the recrystallized SiC material, and as the thickness of the coating increases, the oxidation resistance of the recrystallized SiC material also increases. Zhang Zongtao and others applied Al2O3, SiO2 and Mullite (mullite) coating on the surface of SiC whiskers by sol-gel method. The results of anti-oxidation experiments show that the presence of the three coatings is beneficial to the improvement of the anti-oxidation performance of SiC.
In short, although silicon carbide refractories have relatively good oxidation resistance, when they are oxidized to a certain extent, the damage to the material structure is fatal. Therefore, the mechanism and control of the oxidation process of silicon carbide refractories and their effects on the material structure and The impact of performance is of great significance.
2. Thermal shock resistance of SiC refractories
As an important high-temperature industrial structural material, silicon carbide refractories also have high requirements for thermal shock resistance. The thermal shock resistance of SiC material is not only related to its microstructure, grain size, shape and distribution of internal defects, but also to the strength, elastic modulus, thermal conductivity, thermal expansion coefficient, Poisson's ratio and porosity of the material And other physical properties. Improving and improving the thermal shock resistance of SiC materials is of great significance for its safe and stable use.
Liu Chunxia and others outlined the effects of different bonding methods on the thermal shock resistance of SiC kiln furniture materials. The results show that the thermal shock resistance of Si2N2O combined SiC kiln furniture materials is better than that of mullite and Si3N4 combined SiC kiln furniture materials. When the Si2N2O content ≤20%, the thermal shock resistance of the Si2N2O combined SiC sample increases with the increase of the Si2N2O content. When the Si2N2O content exceeds 20%, the thermal shock resistance of the sample decreases.
Ma Bin et al. Prepared Si3N4-SiC and Sialon-SiC materials by reactive sintering. The results show that the in-situ Si3N4 or Sialon combination can increase the toughness of the SiC material, affect the crack propagation, adjust the stress distribution at high temperature, improve the plastic deformation ability of the material at high temperature, and thus enhance the thermal shock resistance of the SiC material. performance.
Zhu Lihui et al. Studied the thermal shock resistance of reaction sintered SiC materials. The results show that the thermal shock resistance of the material with low residual Si content and small SiC particles is superior to the material with high residual Si content and large SiC particles.
3. Resistance to cryolite erosion of SiC refractories
Because aluminum liquid does not wet SiC, and has high thermal conductivity, high chemical stability and excellent oxidation resistance, it is used as the bottom material of aluminum electrolytic cells. Aluminum smelting is generally carried out by using cryolite (Na3AlF6) as a flux, through electrolytic reduction of alumina in an electrolytic cell, and the electrolysis temperature is usually from 1173 to 1273K. Therefore, it is of great practical significance to study the anti-erosion performance of cryolite by SiC materials.
Sun Ju and others studied the erosion of cryolite on different non-oxides, such as BN, SiC, Si3N4, A1N and TiN. The results show that these non-oxides have good resistance to cryolite erosion, and cryolite melt will enter Si3N4 binds the pores of SiC refractory material and reacts with the bonding phase. The erosion of the bonding phase will cause the SiC particles to fall off and fall into the cryolite melt, causing the material to be eroded.
Miao Lifeng et al. Used the crucible method to study the resistance to cryolite melt erosion of Si3N4 combined with SiC refractory materials. The results show that after the erosion of Si3N4 combined with SiC refractory materials prepared under air atmosphere at 1273K for 20h, only a small amount of inner wall Corrosion, indicating that the material has good resistance to cryolite attack.
Han Bo et al. Used the static crucible method to study the cryolite resistance of alumina-based Sialon combined with corundum-SiC composite refractories at 1273K. The results show that under this condition, the cryolite has less erosion to the composite material, the thickness of the erosion layer is about 1mm, the erosion product is NaAlSiO4, the penetration layer depth is about 6mm, and the penetration rate decreases with the increase of Sialon content.
In short, silicon carbide refractory has good resistance to cryolite erosion. From the perspective of its erosion mechanism, erosion mainly occurs between cryolite and the oxide in the bonding phase. If the oxide in the bonding phase can be reduced or avoided The content can further improve the resistance to cryolite attack of silicon carbide refractories.