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Both complex refractories and rare earth corundum materials can significantly enhance the thermal shock resistance of corundum-based materials. The thermal shock resistance of refractories is typically evaluated through parameters such as hot-end area loss, residual flexural strength, or the strength retention rate after a specific thermal cycling test, such as "room temperature to 950°C with water cooling and forced convection air." These methods are particularly suitable for materials that undergo bending stress during operation.
To improve thermal shock resistance, refractory materials should first aim to increase their workability and reduce the coefficient of thermal expansion (α). Additionally, increasing the E/α ratio while maintaining moderate mechanical strength is crucial. When the microstructure consists of fine grains, fewer grain boundary defects, smaller pores, and uniform dispersion, the material exhibits better thermal shock resistance. For instance, M-Si can help achieve a fine-grained structure in corundum, but it may leave uneliminated pores at the grain boundaries, leading to higher defect density and reduced thermal shock performance. However, introducing elements like Yao Chou or M4 can effectively minimize these grain boundary defects, thereby enhancing the material’s ability to withstand thermal shocks. Products made from A12 field plates also demonstrate excellent thermal shock resistance due to their favorable microstructural characteristics.
Tabular alumina, with its sheet-like structure, acts as a reinforcing skeleton in refractories and ceramics, significantly boosting the product's mechanical strength. Its unique structure helps distribute stress evenly, reducing the damage caused by rapid heating or quenching, which in turn enhances thermal shock resistance. This type of alumina is known for its high thermal conductivity, good thermal stability, high-temperature strength, and strong erosion resistance, making it an ideal refractory material. However, its production process tends to be costly.
From the perspective of thermal expansion coefficients, when two-phase materials have a moderate difference in their thermal expansion behavior, micro-cracks formed near the phase boundaries during firing can actually improve thermal shock resistance without compromising the material’s strength. However, if the thermal expansion differences between the phases are too large, destructive cracks may form, leading to a decline in mechanical properties.
Professor Shen Jiyao proposed that multiphase materials composed of two or more phases with controlled thermal expansion coefficients and proportions often exhibit superior thermal shock resistance compared to single-phase materials. Multiphase modification is thus an effective strategy for improving the thermal shock resistance of refractories. Using plate-like alumina as a matrix in refractory products not only enhances bending strength but also improves thermal shock resistance, contributing to better overall performance.
The addition of rare earth oxides can promote the sintering of corundum, resulting in highly dense materials with an isogranular microstructure and excellent thermal shock resistance. Transparent alumina tubes, thermocouple protective sheaths, and compounds doped with meta-magnesium or ytterbium also show remarkable thermal shock resistance. This indicates that there are multiple approaches to improving the thermal shock resistance of corundum materials. This study considers both the promotion of sintering and the enhancement of thermal shock resistance, using full powder processing.
The samples were press-molded on a Y71-LOA hydraulic press, with flat pieces of 50x10 mm molded under 80 MPa pressure to measure bulk density. Bar-shaped specimens of 2.5x5x25 mm were used for flexural strength and thermal shock tests. After molding, the samples were dried in a 63-D thermostatic drying oven at 90–105°C for about four hours. All fired samples were processed in a silicon-carbon rod high-temperature box furnace. Flexural strength was tested using a three-point bending method, with the help of a SJ-AZ type three-axis shear instrument, YJ-16 static resistance strain meter, and a desktop automatic balance recorder. The test specimen dimensions were 2.5x5x25 mm.