Study of the structural changes in oxide ceramics using polarization microscopy

The mechanical and thermal properties of heat-resistant ceramic materials based on refractory oxides largely depend on the defect-free nature of their crystal structure, the presence of extraneous phases, the size and orientation of the grains. We present the results of studying the structure of ceramic fibers of mullite-corundum composition during high-temperature heating using optical polarization microscopy. The structural features of a continuous fiber of Al₂O₃ – SiO₂ system heat-treated in a free state and under tension, as well as the structure of short fibers of a similar composition obtained by injecting a solution with subsequent heat treatment of the fiber mass at different temperatures were studied. It is shown that the structure of a continuous fiber after heat treatment in the free state has a multidirectional orientation of the grains, whereas heat treatment at the same temperatures under tension leads to the formation of an oriented uniaxial structure. Thus, the fibers obtained from the same solution and heat-treated at the same temperatures are characterized by different strength values. Moreover, the strength of a fiber with an oriented uniaxial structure is one and a half times higher than that of a fiber with a multidirectional grain orientation, which greatly facilitates its further processing. It is also revealed that structural defects like incomplete crystallization, collective recrystallization, and inhomogeneous inclusions of the second phase, characteristic of short fibers obtained by nozzle spraying, negatively affect the thermal properties of thermal insulation made of such fibers. The results obtained can be used to improve the methods of analyzing the microstructure of aluminum oxide fibers using the method of optical polarization microscopy.

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