# Effect of Environmental Temperature on the Insulating Performance of Epoxy/MgO Nanocomposites
## 4 Conclusions  
Nano-MgO/epoxy composites with different doping concentrations were prepared. The influence factors of thermal conductivity, volume resistivity, dielectric property and breakdown property of composite materials were analyzed. The conclusions are as follows:  
(1) The thermal conductivity of the composites increased slowly in the early stage and more rapidly in the later stage with an increasing MgO nanoparticle content. When the content was 7%, the thermal conductivity of the composites increased by 75% compared with the pure epoxy resin. It is proposed that the increase in thermal conductivity is due to the contact between the particles to form a local thermal conductive chain and network.  
Figure 6: Weibull distribution breakdown strength of h-BN/epoxy composites at (a) 20 \({}^{\circ}\)C; (b) 150 \({}^{\circ}\)C.  
(2) As the content of MgO nanoparticles increased, the volume resistivity of the composite dielectric first increased and then decreased. When the content of MgO nanoparticles was 1%, the volume resistivity of the composite dielectric reached the maximum, which was higher than that of the pure epoxy resin. After reaching 26.8%, the volume resistivity began to decrease. When the content of MgO nanoparticles was 7%, the volume resistivity reached the minimum, which was 6% lower than that of the pure epoxy resin. It was hypothesized that the concentration and mobility of carriers affected the dielectric resistivity.  
(3) The dielectric constant under different temperatures increased with the increase in temperature. This is because as the temperature increased which led to an increase in the degree of polarization, thereby increasing the dielectric constant. Moreover, the dielectric constant of the epoxy composites decreased with frequency. The reason may be that the dipoles are turned by the electrostatic force. The dielectric loss first decreased and then increased with the content of MgO nanoparticles. When the content was 1%, the dielectric loss reached the minimum. The reason may be that nano-MgO and the epoxy resin were tightly bound in the interaction zone, which inhibited the steering polarization process, thereby reducing steering polarization loss. The dielectric loss of the epoxy composites at low temperatures increased with frequency. This may be because the dielectric loss was mainly attributed to relaxation polarization loss at low temperatures and conductive loss at high temperatures.  
(4) With an increasing MgO nanoparticle content, the electrical strength first increased and then decreased. When the content of MgO nanoparticles was 1%, the electrical strength of the nano-MgO/EP composite dielectric was higher than that of the pure epoxy resin. The reason may be that the added MgO nanoparticles introduced a large number of heterogeneous crystal nuclei. At low temperatures, the composites were in a glassy state, with a stable internal molecular structure. When the temperature was higher than the glass transition temperature of the composites, the maximum breakdown strength was reduced by 51.3% compared with the maximum breakdown strength at 20 \({}^{\circ}\)C. It is proposed that as the temperature increased, the binding of the molecular structure inside the material weakened and the material exhibited viscous flow characteristics.  
[PERSON], [PERSON] conceived the presented idea; [PERSON] and [PERSON] prepared the material samples; [PERSON], [PERSON] measured the experimental data; [PERSON] and [PERSON] processed and analyzed the data; [PERSON] and [PERSON] wrote the paper. All authors have read and agreed to the published version of the manuscript.  
The research was funded by the \"Major research program of national natural science foundation of China (Grant No. 91848206)\".  
The authors declare no conflict of interest.