Investigation of micro-surface potential of coals with different metamorphism by AFM

Abstract

Micro-surface potential of different coals (Reflectance range; Ro = 0.90% ~ 2.47%) was measured by atomic force microscopy (AFM) at five scanning scales, attempting to gain clearer insights into the electrical characteristics of coal micro-surface. Results show that coal micro-surface potential is affected by metamorphism and has significant scale effect. Generally, the distribution of coal micro-surface potential is heterogeneous (showing Gaussian distribution) and the potential is mostly positive, which can reach more than 800 mV. The potential difference of various points measured in certain scanning area can exceed 100 mV. The mean potential is more likely to decrease with increasing scanning scale, while the variance increases obviously. Analysis indicates that the scale effect is mainly due to the heterogeneity of coal and the metamorphism can affect surface potential by changing functional groups, distribution of surface microstructures, micro-morphologies and internal connectivity. Moreover, the contribution of coal micro-surface potential to electromagnetic radiation (EMR) produced by coal fracture has been discussed, revealing that the existence of coal micro-surface potential especially its scale effect provides a physical basis for the generation of EMR.