Fumed silica (FS)-filled polydimethylsiloxane (PDMS) composite is widely used as the external insulating material in power grid. The function of PDMS composite is to keep the core of an insulation device being dry by preventing the permeation of outside water. To study water diffusion in PDMS composite helps to shed light upon the failure mechanism of the insulation devices. In this paper, positron annihilation lifetime spectroscopy (PALS) and electrochemical impedance spectroscopy (EIS) were applied to study the FS/PDMS interface structure and water diffusion in PDMS composite. It is found that the FS has two basic existing states in PDMS matrix: dispersed state and percolated state, and the percolation threshold of FS is fitted to be 6.0 wt.%. In the dispersed state, the FS particles are randomly dispersed in PDMS matrix. The values of τ3\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\tau _3$$\end{document} remain stable, being approximately 1.0 ns, indicating no overlaps of the FS/PDMS interfaces. Moreover, water cannot diffuse through the sample after corona aging. In the percolated state, the FS particles are tightly packed in the PDMS matrix. The values of τ3\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\tau _3$$\end{document} decrease with the weight fraction of FS, suggesting the overlapping of the FS/PDMS interfaces. As the FS percolates in sample bulk, these overlapped FS/PDMS interfaces provide continuous diffusion tunnels for water, thus resulting in failure of the composite after corona aging. It is found that polarization and discharge of the electrons at FS/PDMS interface under corona discharge is the main mechanism that leads to the failure of the composite bulk.

中文翻译：

---

气相二氧化硅填充的聚二甲基硅氧烷复合材料中的水扩散率转变：与正电子湮没寿命光谱表征的界面自由体积的相关性

气相二氧化硅（FS）填充的聚二甲基硅氧烷（PDMS）复合材料被广泛用作电网的外绝缘材料。PDMS 复合材料的功能是通过防止外部水分渗透来保持绝缘装置的核心干燥。研究 PDMS 复合材料中的水扩散有助于阐明绝缘装置的失效机制。在本文中，正电子湮没寿命谱（PALS）和电化学阻抗谱（EIS）被应用于研究FS/PDMS界面结构和PDMS复合材料中的水扩散。发现FS在PDMS矩阵中有两种基本存在状态：分散态和渗流态，FS的渗流阈值拟合为6.0 wt.%。在分散状态下，FS 颗粒随机分散在 PDMS 基质中。τ3\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{ 的值\oddsidemargin}{-69pt} \begin{document}$$\tau _3$$\end{document} 保持稳定，大约为 1.0 ns，表明 FS/PDMS 界面没有重叠。此外，水在电晕老化后不能扩散通过样品。在渗透状态下，FS 颗粒紧密堆积在 PDMS 基质中。τ3\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{ 的值\oddsidemargin}{-69pt} \begin{document}$$\tau _3$$\end{document} 随着 FS 的重量分数减少，建议 FS/PDMS 界面重叠。由于 FS 在样品中渗透，这些重叠的 FS/PDMS 界面为水提供了连续的扩散通道，从而导致复合材料在电晕老化后失效。发现电晕放电下FS/PDMS界面电子的极化和放电是导致复合体失效的主要机制。