What are the characteristics of refractory materials?
2019-12-27
Characteristics of Refractory Materials:
1. Mechanical Properties
Special refractory materials generally have a high elastic modulus. Most have high mechanical strength, but compared to metal materials, their impact strength is very low due to brittleness. The vast majority of special refractory materials have high hardness, so they are wear-resistant and resistant to airflow or particle erosion. Most special refractory materials have small high-temperature creep, with molybdenum disilicide being an exception. The magnitude of the creep value is related to crystal size, grain boundary material, and porosity.
2. Thermal Properties
(1) Thermal Expansion: Thermal expansion refers to the property of a material's linear and volumetric dimensions increasing or decreasing reversibly with temperature changes. It is often expressed as a linear expansion coefficient or a volumetric expansion coefficient. Most special refractory materials have relatively large linear expansion coefficients, with only fused quartz, boron oxide, and silicon oxide having relatively small linear expansion coefficients.
3. Usage Properties
(1) Refractoriness: The melting points of special refractory materials are almost all above 2000℃, with hafnium carbide (HfC) and tantalum carbide (TaC) being as high as 3887℃ and 3877℃, respectively. The refractoriness is also very high; in an oxidizing atmosphere, the service temperature of oxides is even close to their melting point. In neutral or reducing atmospheres, nitrides and carbides have higher service temperatures than oxides. For example, TaC can be used up to 3000℃ in an N2 atmosphere, and BN can be used up to 2800℃ in an Ar atmosphere. The high-temperature performance is in the order of: carbides > borides > nitrides > oxides. However, their high-temperature oxidation resistance is in the order of: oxides > borides > nitrides > carbides.
(2) Thermal Shock Resistance: Among special refractory materials, beryllium oxide has low thermal conductivity, and most borides also have low thermal conductivity. Fused quartz has a particularly small linear expansion coefficient, so its thermal shock resistance is very good. Certain fiber products and fiber-reinforced composite products have high porosity and tensile strength, so these materials have good thermal shock resistance. Silicon carbide, silicon nitride, boron nitride, molybdenum disilicide, etc., also have good thermal shock resistance.
4. Electrical Properties
Most high-melting-point oxides are insulators, among which thorium oxide (ThO2) and stabilized zirconia (ZrO2) are conductive at high temperatures, as shown in Table 3; carbides and borides have very low resistance; some nitrides are good conductors of electricity, while others are typical insulators. For example, TiN has the electrical conductivity of a metal (ρ is 30×10-6Ω·㎝), while BN is an insulator (ρ is 1018Ω·㎝). All silicides are good conductors of electricity.
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