Development of a Coal-like Material for Microwave Fracturing: Dielectric Characteristics, Thermal Response, and Mechanical Properties

Microwave-induced borehole fracturing of coal seams for pressure relief and permeability enhancement provides a novel approach for preventing coal and gas outburst disasters. However, large-sized natural coal samples suffer from sampling difficulties, complex processing, and poor repeatability, which restrict further related research. In this study, a 2.45 GHz microwave irradiation system with an adjustable power range of 0–2 kW was used, and the specimens were irradiated at a power of 1 kW. Taking bituminous coal as the target, this study used coal powder, river sand, cement, and iron powder to determine and verify the optimal mixing ratio of a coal-like material through orthogonal experiments and multiple linear regression analysis. Taking the imaginary part of the complex permittivity, the temperature non-uniformity coefficient, the uniaxial compressive strength, and the elastic modulus as core indicators, and controlling the error of each indicator relative to the target coal sample within 10%, the optimal ratio was determined as an aggregate-binder ratio of 1.45:1, a coal-to-sand mass ratio of 4:1, and an iron powder mass fraction of 0.75%. The coal-like specimens prepared accordingly showed good consistency with the target coal sample in complex dielectric properties, microwave non-uniform temperature rise characteristics, and mechanical properties, providing a suitable coal-like material for large-scale open-ended microwave borehole fracturing experiments.