Knowledge Of Thermal Expansion Of Refractory Materials

Knowledge of thermal expansion of refractory materials
Thermal expansion refers to the property of refractory materials where their volume or length increases with rising temperature. It is commonly expressed in terms of linear expansion rate and average linear expansion coefficient, or alternatively, in terms of volumetric expansion rate and volumetric expansion coefficient.
The coefficient of thermal expansion is actually not a constant value; it varies with temperature. The commonly mentioned coefficient of thermal expansion refers to an average value within a specified temperature range. When applying this concept, it is important to pay attention to the applicable temperature range.
The thermal expansion properties of materials are closely related to their structure and bond strength. Materials with high bond strength exhibit low thermal expansion coefficients (such as SiC materials). For materials with the same composition, their thermal expansion coefficients vary due to different structures. Generally, crystals with compact structures have relatively large thermal expansion coefficients, while amorphous glasses typically have smaller thermal expansion coefficients. For oxides with a dense packing structure of oxygen ions, the linear expansion coefficient is generally large. In non-isotropic crystals (non-equiaxed crystal systems), the anisotropy of their thermal expansion is particularly evident, with different thermal expansion coefficients in different crystal axis directions. Materials with highly anisotropic structures exhibit very small volumetric expansion coefficients.
The thermal expansion properties of refractory materials depend on their chemical composition, mineral composition, and microstructure, and also vary with changes in temperature range.
The thermal expansion of refractory materials has a direct impact on their thermal shock resistance and volumetric stability, making it one of the important properties to consider during the production (establishment of firing system) and use of refractory materials. For refractory materials with high thermal expansion and polycrystalline transformation, expansion joints should be reserved to offset the stress caused by thermal expansion due to their large expansion at high temperatures. Linear expansion rate and linear expansion coefficient are key parameters for the design and calculation of the total size structure of reserved expansion joints and masonry.
There are two test methods for the thermal expansion of refractory materials, namely the national standard GB/T 7320.1-2000 (plunger method) and the national standard GB/T 7320.2-2000 (telescope method). The test principles of both methods are as follows: heat the sample to a specified test temperature at a predetermined rate of temperature rise, measure the change in sample length as the temperature increases, and calculate the linear expansion rate of the sample as the temperature rises and the average linear expansion coefficient within a specified temperature range.