Thermocycling Ageing of EP/Al2O3 Micron Composites: Phenomena and Insight

Abstract-This paper studied thermocycling ageing phenomenon of $\mathrm{EP} / \mathrm{Al}_{2} \mathrm{O}_{3}$ micron composites that are used as cable terminal insulation. $\mathrm{EP} / \mathrm{Al}_{2} \mathrm{O}_{3}$ micron composites were prepared and thermocycling ageing tests were conducted. Microscopic changes and ageing dielectric characteristics of different ageing periods were tested. The results of scanning electron microscope (SEM) indicated that, during the thermocycling ageing, the matrix-filler interface was in a continuous stretched state under the effect of elevated temperature and alternating temperature stresses, resulting in weakening of the interfacial bond and deterioration of epoxy crosslinking network. Holes and microcracks appeared on the fracture surface of the samples, and the free volume increased. The microstructure of the samples showed obvious deterioration, which can be further supported by the dielectric and traps characteristics and lower AC breakdown strength $\left(\boldsymbol{E}_{\mathrm{b}}\right)$. A 15 % decrease in $E_{\mathrm{b}}$ and a 40 % decrease in deep trap density were found after the ageing of $750 \mathrm{~h}$. The trap distribution results also indicated that the ageing process was delayed by post-curing reaction of epoxy.