Thermal expansion of refractory raw materials refers to its volume or length increases with the temperature of the component, there are volume expansion coefficient and linear expansion points, in the performance of refractory raw materials, usually use linear expansion rate and linear expansion coefficient. Linear expansion rate refers to the relative rate of change of sample length from room temperature to set temperature: linear expansion coefficient refers to the relative rate of change of sample length for every 1℃ increase from room temperature to set temperature.
The measurement methods of linear expansion include the indirect method of top bar and the direct reading method of telescope. It should be noted that the coefficient of thermal expansion is not a constant value, but changes with the test temperature, so it is the average value over the specified temperature range. Therefore, when using this data, it is necessary to indicate its temperature range.
The thermal expansion of refractory raw materials is closely related to the crystal structure and chemical bond strength of the minerals contained in it, and the thermal expansion of the minerals formed by ionic or covalent bonds is larger than that of the minerals bound by molecular bonds.
The chemical composition of the same material, due to the difference in structure, thermal expansion is different, usually the closer the structure of the mineral crystal, the greater the thermal expansion; And similar to amorphous glass, the thermal expansion is often small, like SiO2, polycrystalline quartz thermal expansion coefficient is 12×10-6℃-1; Quartz glass is only 0.5×10-6℃-1. For oxides with close packing structure, the thermal expansion increases due to the close contact of oxygen ions. In the non-equiaxed crystal system, the thermal expansion anisotropy is particularly obvious, such as the layered structure of graphite, parallel to the C axis of the vertical inter-layer expansion coefficient is 27×10-6℃-1, and perpendicular to the C axis of the expansion coefficient is only 1×10-6℃-1, this is because the layer is a strong connection, and the molecular bond between the layers is much weaker. In the structure of the temperature anisotropy of the material, its comprehensive performance of the volume expansion coefficient is very small, such as cordierite as a material with excellent thermal shock stability and widely used in the ceramic kiln industry.
The thermal expansion of refractory raw materials depends on its chemical mineral composition, the thermal expansion coefficient of basic refractory raw materials is larger than that of acidic raw materials, and the high aluminum raw materials are between the two. When the mineral crystal transformation of raw materials occurs, the thermal expansion coefficient will cause uneven changes, and the phase transition point will occur.
Thermal expansion is an important performance of refractory raw materials, which has an obvious impact on the strength and thermal shock stability of the refractory products. The coefficient of thermal expansion of common refractory raw materials is listed in Table 1. The thermal expansion coefficient of raw materials is very important for the study of thermal stress size and distribution, crystal transformation, micro-crack generation and healing.
In the research and production of refractories, it is very important to adjust the performance of refractories by using the difference of thermal expansion coefficient of raw materials. For example, add kyanite and other original materials into the amorphous refractory, and use its significant expansion at high temperature to offset the shrinkage of the amorphous refractory. There are four cases of the combination of aggregate and base thermal expansion coefficient. The difference between aggregate and matrix thermal expansion coefficient can be used to balance the strength and thermal shock stability of the material.
