What are the characteristics of refractory properties?
Characteristics of refractory materials: 1. Mechanical properties
Special refractories have large elastic modulus. Most have high mechanical strength, but compared to metallic materials, the impact strength is very low due to brittleness. Most of the special refractory materials have high hardness, so wear resistance, airflow resistance or dust scouring is better. Most special refractory materials have relatively low temperature creep, the largest being molybdenum disilicide. The magnitude of the creep value is related to the crystal size, grain boundary material, porosity, and the like.
2. Thermal properties (1) Thermal expansion: Thermal expansion refers to the reversible increase and decrease of the linearity and volume temperature of the material. Often expressed as the number of linear expansion or volume expansion coefficient. Most of the special refractory materials have relatively large linear expansion coefficients, and only the fused silica, boron oxide, and silicon oxide have relatively small linear expansion coefficients.
3. Nature of use
(1) Fire resistance: The melting point of special refractories is almost above 2000 °C, and the highest cesium carbide (HfC) and tantalum carbide (TaC) are 3887 ° C and 3877 ° C. The degree of refractoriness is also high, and in an oxidizing atmosphere, the use temperature of the oxide is even close to the melting point. Nitrides, carbides have a higher use temperature than oxides in a neutral or reducing atmosphere, for example, TaC can be used up to 3000 ° C in an N 2 atmosphere, and BN can be used up to 2800 ° C in an Ar atmosphere. The high temperature resistance performance is: carbide > boride > nitride > oxide. Their high temperature oxidation resistance is: oxide > boride > nitride > carbide.
(2) Thermal shock resistance: In special refractory materials, due to the low thermal conductivity of cerium oxide, the thermal conductivity of most borides is not high, and the linear expansion coefficient of fused silica is particularly small, so the thermal shock resistance is very high. it is good. Some fiber products and fiber reinforced composite products have high porosity and tensile strength, and these materials have better thermal shock resistance. Silicon carbide, silicon nitride, boron nitride, molybdenum disilicide, etc. also have good thermal shock resistance.
4. Electrical properties
Most of the high melting point oxides are insulators, in which yttrium oxide (ThO2) and stabilized zirconia (ZrO2) have electrical conductivity at high temperatures, as shown in Table 3; carbides and borides have small electrical resistance; some nitrides are electrically Good conductors, while others are typical insulators. For example, TiN has a conductivity of metal (ρ is 30 × 10 -6 Ω·cm), and BN is an insulator (ρ is 1018 Ω·cm). All silicides are good conductors of electricity.