Design and performance of an open-ended converging microwave antenna in fracturing biotite diorite at low microwave power levels

The converging waveguide antennas have great potential in fracturing rocks at relatively low microwave power outputs and short exposure times comparing to the standard waveguide and horn antennas. This article presents the design process of a converging waveguide antenna considering the maximum temperature, the volume characteristics of the high temperature zone and the effective working distance. It is decided that the waveguide with an aperture height of 30 mm is the optimal design. The antenna was then manufactured and used for surface radiation tests on a biotite diorite at low power levels. Non-destructive tests including temperature measurement, crack pattern characterisation, ultrasonic velocity tests, Schmidt hammer rebound tests and rock breakage tests using pneumatic and hydraulic hammers were performed to quantify the thermal damage and the weakening effect. The tests results show the converging waveguide antenna is efficient in fracturing the biotite diorite. The cracks are in the radial pattern. The maximum P-wave velocity and rebound number reductions on the radiation face are 22.7% and 22.6% for the specimens heated at 6 kW for 4 min, and the maximum P-wave velocity reduction is 54% in the lateral direction. Rock breakage tests on microwave specimens using impact hammers demonstrated that microwave treatment substantially improves the mechanical rock breakage efficiency.