| Electromagnetic radiation(EMR)technology has been well applied in many engineering practices such as coal and rock dynamic disaster monitoring and early warning,and has provided a certain safety technical guarantee for the engineering practice.At present,EMR mechanism of coal/rock deformation and fracture is still unclear,which makes the application of this technology lack rigorous theoretical support.Experimental researches on EMR mechanism of coal/rock deformation and fracture mostly stay on the macro-micro scale,and has not really advanced to the micro-nano scale.Only through precise microscopic experiments to truly understand the electromechanical characteristics of coal/rock at the micro-nano scale can we essentially grasp the EMR phenomenon generated from coal/rock deformation and fracture.Therefore,for the purpose of revealing the micro-nano scale electromechanical characteristics of coal,an experimental system for in-situ testing under dynamic loading conditions of coal micro-surface mechanical and electrical characteristics was developed.Micro-surface morphology,mechanical and electrical characteristics and their changing rules of coals with different metamorphisms under static and loading conditions were analyzed,and the mechanism of the influence of load on the micro-surface mechanical and electrical characteristics was studied.The linear relationship between load and micro surface potential was revealed,and the micro surface charge generation mechanism under load was explained.On this basis,combined with fracture mechanics and electromagnetism,the EMR model of charge oscillation was established,and macroscopic experiments were conducted to further reveal the mechanism of EMR induced by coal fracture.This thesis mainly draws the following conclusions:(1)The surface morphology,mechanical and electrical characteristics of coal at the nano-scale under static conditions and their changing rules were tested and studied,and the formation mechanism of the micro surface potential was studied.Results show that there are many round and elliptical protrusions and irregular depressions on the micro surface of the coal,which increases the micro surface roughness and surface area;mechanical and electrical parameters of the micro surface all show obvious non-uniformity,such as elastic modulus,viscous force,deformation variable and electric potential,and their statistical distribution conforms to the characteristics of normal distribution.With the increase of metamorphic degree,the elastic modulus,viscous force and surface potential of coal micro-surface decrease first and then increase,and the micro-surface deformation increases and then decreases.As the degree of metamorphism of coal increases,the charged polar functional groups in the macromolecules on the surface gradually decrease,while the aromatic structure gradually increases,the non-chemical bonds decrease first and then increase,and the chemical dangling bonds decrease gradually,resulting in the gradual enhancement of electrical properties,which in turn makes the surface potential changes regularly;polarization also plays a role in the formation of micro-surface potentials.(2)Micro-surface electromechanical characteristics of coal under dynamic loading conditions and their changing rules were tested in situ,and the effect and mechanism of the load on the micro-surface electromechanical characteristics of coal were studied.Linear relationship between the load and the micro surface potential was revealed,and the mechanism of micro surface charge generation under load was explained.Results show that with increasing load,the micro-surface elastic modulus of coals with medium and low metamorphic degree(R0≤1.06)shows an increasing trend and then decreases,while that of coal with high metamorphic degree(R0≥1.76)shows a gradually decreasing trend.Deformation variables show opposite characteristics corresponding to the elastic modulus.Effect of loading changes the gaps,molecular bond lengths,bond angles and bond energies of coal macromolecules,which in turn changes the micro-surface mechanical properties of coal.Micro-surface potential of coal exhibits a linear relationship that decreases with increasing load.Due to the work done by the load on the coal body,the stress field and temperature field of the coal body are changed,which promotes friction,electricity,thermal electron emission,and field-induced electron emission,etc.Electrons are thus excited,and some of the electrons then interact with positive charges on the coal surface,which reduces the potential of the coal surface obtained by the test.(3)Micro-crack formation mechanism of coal was analyzed,and the charge movement on micro-crack wall with crack propagation was studied,which is equivalent to the electric dipole model of charge movement.Results show that the formation of microcracks in coal is a structural instability that occurs when the local stress concentration exceeds the load limit of micro structures.Formation and propagation of microcracks is accompanied by the separation and movement of positive and negative charges.Mechanism of free charge generation in the process of micro-crack propagation can be expressed as follows:When the micro-cracks in coal are propagated by tensile stress,chemical bonds between molecules and molecules at the crack tip are destroyed,which separates the positive and negative charges to form free charges.During the separation of micro-crack wall,force between the molecules causes the molecular structure to shift and do work,which in turn increases the energy of molecules,and then excites electrons.Effect of shear stress on the micro-crack wall causes friction between molecules on the surface.At the same time,the energy of molecules increases and the temperature increases,resulting in thermionic emission electrons.Microcracks continue to oscillate during the propagation process so that charge on the surface is forced to oscillate.The oscillating motion of electric dipole groups formed by a large number of charges excites electromagnetic waves and radiates outward.Based on this,the EMR model of charge oscillation on the crack surface is established.(4)A macro coal Brazilian test is carried out,the response mechanism between the load,EMR and AE(acoustic emission)signal was studied,and the characteristics of EMR waveform and AE waveform were analyzed.Correlation between theoretical EMR value and experimental results was studied to verify the micro EMR model established.Results show that EMR signals and AE signals are generated and propagated at the same time,and the waveform of the two are similar.The theoretically calculated EMR values are well correlated with the experimental measurement results,with a significant or highly correlated ratio of 75%.Thus,the rationality of the microscopic EMR model is verified,that is,the EMR model of charge oscillation with the crack reveals the mechanism of EMR generated from coal fractures.Research results have important theoretical and practical significance for deeply revealing the EMR mechanism of coal deformation and fractures,and can provide theoretical support and scientific basis for the application of EMR monitoring and early warning technology.The thesis has 90 pictures,10 tables,and 256 references. |
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