Heat transfer, crystallization and properties of Bayan Obo tailings-based nanocrystalline glass-ceramics processed with microwave radiation

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Using Bayan Obo tailings containing rare earth element as raw material to prepare nanocrystalline glass-ceramics by microwave heating is a feasible scheme for the sustainable development of resources and low-carbon production. In this paper, the effects of conduction, infrared, microwave and plasma heating on nucleation were studied in order to obtain evidence for a thermal and non-thermal microwave effect. The differences in the mechanical properties, high-temperature softening properties and microstructures of the glass-ceramics prepared by conventional and microwave heating were analyzed. The results showed that microwave heating had a higher heating rate than conventional heating. The volumetric heating reduced the dependence of heat conduction within the sample, forming a uniform thermodynamic system for nucleation. The polymeric character of the silicon-oxygen network could be reduced by microwave radiation to promote nucleation, and fine nanocrystals with globular crystals and a uniform microstructure were produced after continuous nucleation-crystallization. In contrast to conventional heat treatment, the flexural strength of the samples increased from 169 MPa to 219.9 MPa under microwave radiation, and the thermal stability improved. The excellent mechanical properties were attributed to the increased Si–O stretching vibration frequency and bond force. In addition, the structure of the diopside cell tended to be dumpy with a decreased β angle, and the crystal orientation was affected. Tailings-based nanocrystalline glass-ceramics with excellent properties can be prepared in a short processing time by microwave heating.