## 3 Experimental Results  
Figure 2 shows the FTIR spectra of the silicone rubbers on the cable terminals operated for different times. At the same time, the absorption peaks at two wave numbers of \(785\) and \(1005\) cm\({}^{-1}\) were selected for comparison. Figure 2b,c shows the relationship between the absorbance of the sample and the aging time of materials. First, we found that the strong absorption peak near the wave number of \(750\)-\(850\) cm\({}^{-1}\) is the Si-(CH\({}_{3}\))\({}_{3}\) group, and the peak around the wave number of \(1000\) to \(1100\) cm\({}^{-1}\) is Si-O-Si on the main chains; the peak at a wave number of \(1260\) cm\({}^{-1}\) is the Si-CH\({}_{3}\); and the absorption peak of the carbon-hydrogen bond is near \(2955\) cm\({}^{-1}\)[6]. According to the FTIR spectra of the samples, it can be obtained that the silicone rubber material used for the test is methyl vinyl silicone rubber [11]. Comparing the difference in the FTIR spectra between different silicone rubber samples, it can be found that the intensity of each absorption peak on the surface of the aged silicone rubbers is changed compared to the pristine silicone rubber. The possible reason is that the oxidation causes the methyl  
Figure 1: Schematic diagrams of surface potential decay measurement system (**a**) and volume conductivity measurement system (**b**) for dielectric materials. group to break and form new Si-O-Si crosslinks [6]. When the oxygen content is low, Si-CH\({}_{2}\)-Si crosslinks may also be formed [12]. In addition, when the silicone rubber insulation is aged for a long time, chemical bond rupture will occur. These reactions will affect the dielectric, mechanical, and thermal characteristics of silicone rubber insulation [13]. Figure 3 shows the results of broadband dielectric spectroscopy of pristine and aged silicone rubber samples. The dielectric constant of silicone rubber varies little with frequency, which is consistent with its symmetrical molecular chain structure. With the increase of the aging time, the dielectric constant of silicone rubber shows a rising trend, which may be related to the oxidation reaction of silicone rubber to generate polar groups under an electric field and heating [8]. The dielectric loss of silicone rubber at high frequencies is very small. Due to the limitation of the test accuracy of the instrument, accurate dielectric loss cannot be measured at some frequency points. As the voltage frequency decreases, the dielectric loss of silicone rubber gradually increases, and the dielectric loss at low frequencies has a great relationship with the conductivity. The dielectric loss of the pristine silicone rubber insulation does not increase significantly with the decrease in frequency, which indicates that the pristine silicone rubber insulation has a low conductivity and does not have a large impact on the dielectric loss, which is consistent with the conductivity experimental results below. After aging for a period of time, the dielectric loss of the silicone rubber increases with the decrease of the frequency, indicating that the electrical conductivity of the aged silicone rubber is much higher than that of the pristine sample. This is also consistent with the experimental results of conductivity. Figure 4 shows the surface potential curves of pristine and aged silicone rubber samples. Aged samples show a faster decaying rate than that of the pristine one under both positive and negative corona charging, which suggests that silicone rubber samples may generate more shallow traps during the ageing process where charge carriers can easily detrap and migrate along the electric  
Figure 3: Frequency-dependent dielectric constants (**a**), dielectric losses (**b**), and real part of conductivity (**c**) of a pristine silicone rubber sample and samples aged by ac voltage with a harmonic voltage component for different times. Figure 2: (**a**) FTIR spectra of the pristine silicone rubber sample and samples aged by ac voltage with a harmonic voltage component for different times.