## 3 Experimental Results
Intensities of Si–O–Si (**b**) and Si–(CH\({}_{3}\))\({}_{3}\) (**c**) chemical groups as a function of the aging times. field [14]. Figure 4a shows the time-varying characteristics of the surface potential of pristine and aged silicone rubber samples after positive corona charging. The positive corona is used to deposit positive ions on the surface of the silicone rubber, and the positive ions exchange charges with the traps on the surface of the silicone rubber to transfer the positive charges to the surface hole traps. The positive charges detrap out from the traps gradually by thermal activation, and these charges migrate to the ground electrode under the self-built electric field. The reduction of trapped positive charges will cause the surface potential to decay over time. It can be seen from Figure 4a that as the aging time increases, the decay of the positive surface potential of the silicone rubber gradually accelerates. The surface potential of the pristine silicone rubber decreased from 2961 to 2228 V after decaying for 1000 s, and the surface potentials of the silicone rubber samples after being aged for 1 and 8 months were 1112 and 122 V after decaying for 200 s, respectively. Figure 4b shows the time-varying characteristics of the surface potential of pristine and aged silicone rubber samples under negative corona charging. Negative corona discharge is used to deposit negative ions on the surface of the silicone rubber, and the negative ions transfer electrons to the electron traps on the surface of the silicone rubber. Then, the trapped electrons gradually detrap out under thermal activation and migrate to the ground electrode. The gradual decrease of trapped electrons will cause the surface potential to decay over time. After decaying for 1000 s, the surface potential of the pristine silicone rubber changed from \(-\)3013 to \(-\)1642 V. The surface potentials of the silicone rubber aged for 1 and 8 months were \(-\)1226 and \(-\)185 V after decaying for 200 s, respectively. The decay rate of the negative surface potential gradually accelerated with the aging time of silicone rubber. It shows that the electron and hole migration characteristics of silicone rubber are simultaneously affected by the aging process under ac superimposed harmonic voltage. Figure 5 demonstrates the experimental results of volume conductivity of silicone rubber with different ageing times. It can be found that the volume conductivity of silicone rubber samples varies with the increment of the ageing time. The volume conductivity of all silicone rubbers aged by ac superimposed harmonic voltage is higher than the pristine sample. The electrical conductivity of the pristine silicone rubber is \(7.3\times 10^{-14}\) Sm\({}^{-1}\). The electrical conductivities of the silicone rubber samples aged for 1 and 8 months were \(1.1\times 10^{-13}\) and \(2.4\times 10^{-13}\) Sm\({}^{-1}\), respectively. The conductivity of the silicone rubber aged for 8 months is 3.3 times that of the pristine sample. It is indicated that the silicone rubber insulation of the cable terminal was subjected to the combined effects of the electric field and heating under ac superimposed harmonic voltage. During aging processes, shallow traps are generated inside the material, which promotes the carrier migration and increases the electrical conductivity of the silicon rubber, reducing its insulation performance. Figure 4: Surface penitential decay curves of pristine and aged silicone rubber samples after (**a**) positive corona charging and (**b**) negative corona charging. Generally, a higher volume conductivity indicates that more shallow traps exist in the bulk of the material, which can assist charge transport under the same electric field. Consequently, a larger leakage current can be obtained via the pico-ammeter. According to the experimental results, the variation of volume conductivity shows the same tendency with the surface potential decaying rate of samples with different ageing times.