## Materials and Methods
### Materials and Preparation
#### 2.1.2 Preparation of the Novel Self-Healing Polyethylene Insulating Composite  
At present, there is no research on self-healing microcapsule/polyethylene insulating composites. Thus, it is novel research to use a microcapsule system to automatically repair the structural damage of polyethylene insulating material. Low-density polyethylene (LDPE), as the main raw material of XLPE, has few impurities. Moreover, LDPE is also the main raw material of various functional polyethylenes. Therefore, the microcapsule/LDPE insulating composite was taken as the research object. The raw material of polyethylene is 2426H LDPE. The LDPE particles were ground into LDPE powder (15 meshes), to ensure the uniform mixing of the polyethylene and microcapsule system. According to relevant literature [17] and my previous work [35] (as shown in Table 1), the basic properties (such as thermal stability and tensile strength) of a composite doped with about 1 wt.% microcapsule are more in line with the application requirements of the cable. It has the best thermal stability and better tensile strength, on the premise of a higher repair efficiency. Preliminary analysis showed that the effect of microcapsules on the basic properties of polyethylene is mainly related to the interface and impurities introduced by the microcapsule. Moreover, an appropriate number of interfaces can improve the properties of the material (such as thermal stability), but excessive impurities and interface defects can reduce its properties (such as tensile strength and thermal stability) [35]. Therefore, the performance of the composite is affected by the concentration of microcapsule. Overall, when the concentration of the microcapsule is about 1 wt.%, the comprehensive properties of the composite are better. Thus, the more appropriate concentration (1 wt.%) was selected to study the effect mechanisms of microcapsules on the electrical performance of polyethylene and exclude the influence of other secondary factors. In addition, Grubbs' second-generation catalyst, with better stability and catalytic efficiency, was selected to improve the effect of self-healing, and its concentration accounted for 10 wt.% of the microcapsule. The microcapsule/LDPE composite was prepared by the melt blending method [36]. Due to the demand of self-healing behavior, the microcapsule needed to have the characteristic of rupturing under the stress condition. In the original preparation condition, the microcapsule could be ruptured (as shown in Figure 1a, thus, the self-healing ability could not be realized. However, if the thickness and strength of the microcapsule wall structure are excessively increased, the microcapsule may not respond to the damage of matrix in time. Therefore, the thermal stress and mechanical stress in the original preparation process have been adjusted in this paper. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Basic Performances of Composite**} & \multicolumn{5}{c}{**Concentrations of Microcapsule in Composite**} \\  & **0 wt.\%** & **0.5 wt.\%** & **1 wt.\%** & **5 wt.\%** & **10 wt.\%** \\ \hline Repair efficiency for scratches about 100 & 0\% & \(\geq\)56.3\% & \(\geq\)82.4\% & \(\geq\)85.1\% & \(\geq\)90.0\% \\ Thermal decomposition temperature & 442.9 \({}^{\circ}\)C & 445.1 \({}^{\circ}\)C & 456.7 \({}^{\circ}\)C & 438.0 \({}^{\circ}\)C & 432.0 \({}^{\circ}\)C \\ Tensile strength & 11.62 MPa & 11.58 MPa & 11.14 MPa & 9.11 MPa & 3.61 MPa \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Basic performance of the composite with different concentrations of microcapsule. The composite was obtained by the Hangfa YJ8F flat vulcanizing machine. The working pressure was 3 MPa, which was designed to avoid damage to the microcapsule, while ensuring sample formation. The working temperature was 160 \({}^{\circ}\)C, and the pressing time was 30 minutes, to further enhance the morphology of sample.