In-situ AFM measurement on the micro-surface potential of coal under variable loads

Abstract

The electromagnetic radiation (EMR) method has long been utilized in monitoring and early warning of coal mine dynamic disasters globally. However, the governing mechanism of coal fracture-induced EMR remains unclear. Clarifying the generation of coal fracture-induced charge is essential as it underpins EMR's physical basis. This study employs a novel micro-loading machine coupled with atomic force microscope (AFM) to in-situ characterize the continuous evolution of coal micro-surface potential during loading. Results show that coal micro-surface topography exhibits load-dependent fluctuations and localized deformations, with shift along the loading direction reaching the micron scale. Local micro-surface potentials undergo transfer and redistribution, with tens of millivolt-scale staggered variations at various scanning points. The mean surface potential (average electricity) of each coal in the same region exhibits distinct changing trends, showing no clear linear relationship with the load. The analysis reveals that the micro-surface pores of coal facilitate the storage of charges, and the charged functional groups and carriers on the micro-surface also serve as important sources of charges. Non-uniform deformation of the micro-surface during loading induces the non-uniform movement of charge. Additionally, the activation of dormant carriers and the piezoelectric effect also contribute to complex changes in the micro-surface potential throughout the loading process. Due to the heterogeneity of coal and the property difference in various coals, micro-surface potential responses to load are non-linear and vary significantly across coal types. The experimental results demonstrate the persistence of charge on the coal surface during loading and its accumulation during fracturing, forming the physical basis for EMR's generation. These findings further clarify the electrical source of EMR and provide new equipment for revealing the mechanism of EMR.