| Epoxy resin is extensively used as insulation material for power system equipment,but its poor thermal conductivity makes the heat dissipation problem of equipment increasingly serious,so it is of great importance to study the insulation material with high thermal conductivity.In order to prepare composites with high thermal conductivity under low filling amount,the way of applying external electric field inducing the orientation arrangement of filler in the matrix can effectively improve the thermal conductivity of composites.Nanosecond pulsed electric fields(field strength greater than 200 k V/mm)have been used to induce the orientation of fillers to prepare polymer-based film composites of 120-250 ?m with high thermal conductivity.If we want to prepare bulk composites suitable for power system insulation materials,extremely high nanosecond pulse voltages are required to cause the orientation of the boron nitride nanofiller.However,it is difficult to manufacture a nanosecond pulse generator with extremely high output voltage,which also brings limitations and difficulties to engineering applications.Therefore,in this paper,boron nitride nanosheets(BNNS)was used as the heat conducting filler,and the microsecond pulsed electric field with pulse width of 1 ?s was used for the first time to prepare the bulk epoxy nanocomposites with the thickness of1.7 mm.The orientation degree of boron nitride nanosheets in the epoxy matrix was characterized by scanning electron microscopy(SEM)and X-ray diffraction(XRD).The effects of pulse repetition frequency and field strength on the orientation of BNNS and the thermal conductivity of the epoxy composites were also studied.The results showed that the thermal conductivity of the composites and the orientation degree of the BNNS increased with the increase of the pulse repetition frequency and the electric field strength.At higher repetition frequency and higher electric field strength,the degree of orientation of the filler tends to be saturated,but the increase in the thermal conductivity of the composite material does not appear to be saturated.When the filling amount of BNNS was 10wt%,the pulsed electric field strength was about 12 k V/mm,and the repetition frequency was 100 Hz,the thermal conductivity of the oriented composite material was 0.588 W/(m·K),which was more than twice as much as the composites prepared without applying pulsed electric field(0.286 W/(m·K)),and more than three times as much as pure epoxy(0.19 W/(m·K)).At the same time,based on the stress of the BNNS during the curing,the corresponding mechanisms of the pulsed electric field repetition frequency and electric field strength on the orientation degree of fillers and the thermal conductivity of the composite were analyzed.Besides,based on the effective medium theory(EMT)model,the thermal conductivity of composites under different loadings was fitted,and through fitting,it is found that the thermal resistance value of the interface between the epoxy compound and boron nitride in the oriented composite material is only about 1/4 of the random composite material.This shows that the fundamental reason for the improvement of thermal conductivity of oriented BNNS/EP composites prepared under the induction of pulsed electric field is the reduction of interfacial thermal resistance.Finally,the dielectric properties,AC breakdown field strength,thermal weight loss performance,glass transition temperature,and dynamic thermomechanical properties of the pure epocy resin and its composittes were measured to fully reflect the insulation performance and thermal stability of the oriented BNNS/EP composites.The experimental results showed that the AC breakdown field strength of oriented composites was 42.16 k V/mm,which is lower than random BNNS/EP composites prepared without the application of an electric field,but still higher than the breakdown filed strength of pure epoxy resin(39.42 k V/mm).In addition,the glass transition temperature and thermal decomposition temperature of the oriented composites were not only higher than those of pure epoxy resins,but also higher than that of random composite materials.This showed that the thermal stability of the oriented composites prepared under the action of a pulsed electric field was excellent.In general,this research is a preliminary exploration of microsecond pulse-induced filler orientation to improve the thermal conductivity of composites,and can provide some guidance for the preparation of insulating materials with high thermal conductivity and good insulation properties at low loadings. |
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