Dielectric relaxation and carrier transport in epoxy resin and its microcomposite

Epoxy resin and its microcomposite are widely used as electrical insulation in electrical equipment. Surface flashover and breakdown properties of these materials can be influenced by dielectric relaxation process and charge carrier behavior. The charge transport characteristics and dielectric relaxation process for neat EP and its microcomposites were studied by means of dielectric relaxation spectroscopy. The glass transition temperatures Tg of the two kinds of samples were investigated with differential scanning calorimetry measurement, which were approximately 105 °C and 120 °C, respectively. Both the two kinds of samples were sputtered with gold electrodes on two sides, and electrodes' diameter was 30 mm. The properties of dielectric relaxation was measured by Concept 80 Novocontrol broadband dielectric spectrometer at various temperatures from 100 °C to 180 °C. There is a relaxation peak which labeled α in the range of high frequencies on account of the molecular chain movement or motion of segmental chains when the temperature is above Tg. At the same time, the dc conductivity attributed by the charge carrier transport occurs in the range of low frequencies. Besides, there are different relaxation times for molecular chains with different length scales. Moreover, there is a broad distribution of relaxation time for both neat EP and its microcomposites and the relaxation time distribution at different temperatures was calculated. In addition, it was found that the relation between peak frequency of relaxation process and temperature follow the Vogel-Fulcher-Tammann (VFT) law, and temperature dependences of dc conductivity also obey the VFT equation. The Vogel temperatures of dielectric relaxation process and charge carrier transport were calculated by fitting the results of permittivity and conductivity. The glass transition temperatures for neat EP and its microcomposite were estimated to be 102 °C and 117 °C through Vogel temperatures, which are in consistence with DSC measurement results. It represents that free volume in the two kinds of samples increases with the increase of temperature, thus enhancing molecular chain movement and charge carrier transport process.

1 Introduction

The materials of epoxy resin (EP) and its microcomposites are widely used as insulating materials and mechanical support in electrical equipment especially in gas insulated switchgear (GIS) due to their excellent mechanical, electrical and thermal performance [1]–[6]. Applications of EP indicates that the dielectric relaxation and charge carrier properties of neat EP is significant for the stable and safe operation of electrical equipment. Dielectric relaxation process is usually regarded as one of the main properties for polymers, and its dielectric response in different ranges of frequencies and temperatures can be used to characterize the relaxation of dipoles, motion of molecular chains, thermal expansion coefficient, carrier mobility and some other parameters [7], [8]. These characteristic parameters can help to establish the relationship between microstructure and macroscopic electrical properties.