383 / 2018-08-06 16:54:06

Electrical characteristics of micro-surface of coal with different degrees of metamorphism and their influencing factors

micro-surface of coal,electrical characteristics,metamorphism degree,functional group,electromagnetic radiation

矿井瓦斯与煤与瓦斯突出 > 煤与瓦斯突出

It has been generally believed that coal and rock material can generate electromagnetic radiation (EMR) due to their rupture, which can be used to early warning dynamic hazards of underground spaces. Researchers propose some mechanisms of this EMR, but most of them are based on empirical hypotheses, lacking direct experimental verification methods. Experimental study on the micro-scale coal composition and their electrical characteristics is the premise of revealing the EMR mechanism. This study examined the electrical characteristics of the micro-surface of coals with different metamorphism using atomic force microscopy, analyzed their changing regularities from the aspects of element composition, surface functional groups and Raman structural parameters, and elucidated the mechanisms underlying theses changes. The results showed that the surface potential and surface electrical charging density (SECD) of coal vary in the range of -28.5–-87.4 μV and 58.98–312.97 μC/m2, respectively, and demonstrate V-shaped trends with the coal rank increasing. At the nanoscale, both positive and negative electrical potential zones were observed on the coal surface, indicating the surface has a weak electrical property, while at the micrometer scale (5 μm × 5 μm), only very weak negative or neutral electrical potential zone was observed at the coal surface. The results also showed that SECD 1) non-linearly changes with the carbon and oxygen contents increasing, showing a trend of slow decline before dramatic increase, 2) is positively correlated with the contents of C=C and C=O groups ad negatively correlated with the contents of C-O and OH groups, but barely correlated with the contents of CH3 and CH2 groups. Moreover, the negative electrical property of coal surface is closely related to the electronegativity of functional groups. The OH group has higher cumulative electronegativity than the other three groups in lower ranking coals and is the dominant group affecting the electronegativity of low-ranking coals. By contrast, the cumulative electronegativity of C=C group on benzene rings sharply increases with coal ranking increasing. As a consequence, the electronegativity of high-ranking coals is determined by the combination of C=C, OH, and C=O groups. Overall, our results may provide a new way to reveal the mechanisms of EMR generated by the deformation and fracture of coal from the microscopic scale and have important theoretical and practical significance for ensuring the underground mining safety.