Knowledge of thermal expansion of refractory materials

Thermal expansion refers to the performance of refractory materials that increases in volume or length with increasing temperature. It is commonly expressed by linear expansion rate and average linear expansion coefficient, but can also be expressed by volume expansion rate and volume expansion coefficient.

The coefficient of thermal expansion is not actually a constant value, it varies with temperature. The commonly referred coefficient of thermal expansion has the concept of an average value within a specified temperature range, and attention should be paid to its applicable temperature range when applied.

The thermal expansion performance of materials is closely related to their structure and bond strength. Materials with high bond strength have a low coefficient of thermal expansion (such as SiC materials). For materials with the same composition, their thermal expansion coefficients vary due to different structures. Typically, crystals with tight structures have a relatively high coefficient of thermal expansion, while amorphous glasses generally have a lower coefficient of thermal expansion. For oxides with tightly packed oxygen ions, the coefficient of linear expansion is generally large. In non isotropic crystals (non equiaxed crystal systems), the anisotropy of thermal expansion is particularly evident, with varying coefficients of thermal expansion in different crystal axis directions. Materials with highly anisotropic structures have small coefficients of volume expansion.

The thermal expansion of refractory materials depends on their chemical composition, mineral composition, and microstructure, and also varies with temperature ranges.

The thermal expansion of refractory materials has a direct impact on their thermal shock resistance and volume stability, and is one of the important properties that should be considered in the production (formulation of firing system) and use of refractory materials. For refractory materials with large thermal expansion and polycrystalline transformation, expansion joints should be reserved to counteract the stress caused by thermal expansion when used at high temperatures due to the large expansion. Linear expansion rate and coefficient of linear expansion are key parameters for the design and calculation of reserved expansion joints and overall dimensions of masonry structures.

There are two testing methods for the thermal expansion of refractory materials, namely the national standard GB/T 7320.1-2000 (top rod method) and the national standard GB/T 7320.2-2000 (telescope method). The experimental principle of the two is to heat the sample to the specified test temperature at a specified heating rate, measure the change in sample length with increasing temperature, calculate the linear expansion rate of the sample with increasing temperature and the average linear expansion coefficient within the specified temperature range.