Research on the variation law of coal electrical parameters under different temperatures and detection frequencies

Most of the existing research on the influencing factors of coal electrical parameters are carried out under fixed frequency or low frequency and high temperature conditions. There is a lack of research on the influencing factors of coal electrical parameters under ultra-wideband frequency and normal temperature conditions. Therefore, there are certain limitations in the research on the influencing factors of coal electrical parameters. To address this problem, in order to study the influence of coal electrical parameters on the selection of ultra-wideband radar wave mine detection frequency band under different conditions, the Concept-80 ultra-wideband dielectric spectrum test system is applied. The system is set to detect the relative permittivity, electrical conductivity and loss angle tangent values of coals with different metamorphic degrees (lignite, long-flame coal and lean coal) at 0-80 ℃ and 500-1 000 MHz detection frequency and analyze their variation laws. The analysis results show that the relative permittivity of coal increases with the increase of coal metamorphic degrees, and the electrical conductivity and loss angle tangent values of coals decrease with the increase of coal metamorphic degrees. The relative permittivity of coal samples with different metamorphic degrees decreases, then increases and decreases again with the increase of detection frequency, and the relative permittivity is positively correlated with temperature. The electrical conductivity is positively correlated with both detection frequency and temperature. The loss angle tangent value is positively correlated with temperature. However, the detection frequency does not reflect the variation of loss angle tangent value well. Based on the variation law of three electrical parameters at different temperatures and detection frequencies, and the comprehensive comparison of influence of three electrical parameters on the transmission of ultra-wideband radar waves in coal, it is pointed out that the frequency of 550-650 MHz performs best when using 500-1 000 MHz ultra-wideband radar waves to penetrate the coal body for detection at 0-80 ℃, and the detection frequency should decrease as the temperature increases. The conclusion of this study can provide a reference for the selection of ultra-wideband radar wave mine detection frequency band under different conditions.