Yin Huang

Dielectric and Carrier Transport Properties of Silicone Rubber Degraded by Gamma Irradiation

Silicone rubber (SiR) is used as an insulating material for cables installed in a nuclear power plant. Gamma rays irradiated SiR sheets for various periods at temperatures of 145 and 185 °C, and the resultant changes were analyzed by examining complex permittivity spectra and surface potential decay characteristics. Three different processes, namely, instantaneous polarization, electrode polarization due to the accumulation of ions to form double charge layers at dielectric/electrode interfaces, and DC conduction caused by directional hopping of ions, contribute to the complex permittivity. By fitting the spectra to theoretical equations, we can obtain the dielectric constant at high frequencies, concentration and diffusion coefficient of ions and DC conductivity for the pristine and degraded samples. The instantaneous polarization becomes active with an increase of dose and ageing temperature. The thermal expansion coefficient estimated from the temperature dependence of dielectric constant at high frequencies becomes smaller with an increase in dose, which is in good agreement with the experimental results of the swelling ratio. Additionally, trap distributions are calculated from surface potential decay measurements and analyzed to explain the variation in conductivity. Trap energy increases firstly, and then decreases with an increase in dose, leading to a similar change in DC conductivity. It is concluded that generations of both oxidative products and mobile ions, as well as the occurrence of chain scission and crosslinking are simultaneously induced by gamma rays.

Surface flashover performance of epoxy resin microcomposites infulenced by ozone treatment

The influence of ozone surface treatment on surface flashover performance of epoxy resin/AhO3 microcomposites was investigated. Epoxy resin/AhO3 microcomposite samples with a diameter of 50 mm and a thickness of 1 mm were prepared and treated by ozone for various periods. Surface potential decay, surface conductivity, secondary electron emission coefficient, and surface flashover voltage in SF6 of untreated and ozone treated samples were measured. Both the decay rate of surface potential and surface conductivity increase with an increase in the energy of electron beam. Meanwhile, surface flashover voltage increase. There was no obvious change in secondary electron emission coefficient by the ozone surface treatment. It was found that the decrease in trap energy and increase in trap density can enhance the charge transport process on the surface of samples. The changes can weaken the distortion of electric field at the triple junction of electrode-dielectric-gas and reduce the electron emission, which will improve the surface flashover performance.

Trap and carrier transport of pristine and aged silicone rubber by surface potential measurements

Silicone rubber is widely used in power equipment such as cable joint. In order to get better performance of insulator, it is significant to evaluate the insulation condition of aged silicone rubber insulators. Thus in this paper, surface potential decay measurement has been applied to investigate the performance of pristine and aged silicone rubber. Pristine and aged silicone rubber samples are charged by positive and negative corona discharging. Then the surface potentials are measured by a non-contact probe. It is found that surface potential in aged rubber decays faster, which is resulted from more shallow traps generated during ageing process, for electrons are easier to migrate or even trap out of the shallow traps. Additionally, carrier mobility shows an increase in aged rubber for both positive and negative charges, which may be resulted from the increase in shallow traps in the material. As regards to conductivity, it turns out that conductivity in aged rubber is higher than that in pristine one, which indicates charges in aged rubber migrate faster and further proves the conclusion mentioned that shallow traps increase in internal material.

Study on short-term DC breakdown and coronaresistance mechanism of polyimide

Nano-doping has a significant effect on dielectric and electrical insulation for polymer dielectrics. Pure polyimide 100HN and nano-doping polyimide 100CR from DuPont Company are adopted to conduct short-term breakdown and long-term corona-resistant experiments. The results show that, the short-term breakdown strength of 100CR is 6.4% lower than 100HN. For long-term corona resistance, the corona-resistant time of 100CR is 400% longer than 100HN. In order to study the influencing mechanism of nanoparticles on short-time breakdown and long-term corona resistance, isothermal surface potential decay experiments were conducted to investigate traps characteristics of two samples. It is found that the traps energy and density of 100CR is less than that of 100HN. The volume conductivity of 100CR is much higher than 100HN. The eroded surface morphologies of 100HN and 100CR are observed by Scanning Electron Microscope, which are groove channels and different eroded layered rings respectively. The results show that the effect of nanoparticles on short-term breakdown and long-term corona resistance are different. For short-term breakdown, the main consideration is the change of traps and conductance changed by nano-doping. For long-term coronaresistance, the collision scattering effect of nanoparticles against the charged particles plays the dominate role.

Trap and polarization of polyethylene terephthalate influenced by electron beam irradiation

Trap and polarization greatly affect dielectric breakdown and surface flashover behavior of insulating materials. Trap and polarization may be modified by electron beam irradiation via breaking polymeric molecular chains or triggering cross-linking. Polyethylene terephthalate (PET) with a thickness of 75 μm from DuPont was used to investigate the modifications of trap and polarization by electron beam irradiation. PET samples were irradiated by an electron beam with an energy of 30 keV and a beam current of 5 μA for 5 minutes in a vacuum chamber. Experiments of thermally stimulated depolarization current (TSDC) and broadband dielectric relaxation spectrometer (DRS) were conducted on samples before and after the electron beam irradiation. Three TSDC peaks were observed above glass transition temperature on both samples before and after irradiation. A TSDC current fitting method was used to extract trap parameters of as-received and irradiated samples. Trap energy and trap density are clearly increased after irradiation compared to the as-received sample. In addition, a significant crossover can be seen in the spectra of relative dielectric constant before and after irradiation. Dielectric loss factor is clearly increased after irradiation. Comparative experiments proved that trap parameters and polarization are changed by electron beam irradiation. Analyzing the energy deposition properties in PET, we found that the penetration range of electrons with the energy of 30 keV is about 12 μm. Electron beam irradiation transfers energy to molecular chains, causing ionization and producing free radicals, which can change the trap and polarization of PET.

Dielectric relaxation and carrier transport in epoxy resin

Epoxy resin (EP) is widely used as an insulating material in power equipment. Its dielectric relaxation and carrier transport properties are important factors affecting breakdown and surface flashover performance. The dielectric relaxation and carrier transport properties of EP based alumina (Al2O3) microcomposite were investigated through broadband dielectric spectroscopy. The glass transition temperature was measured by differential scanning calorimetry (DSC), which is about 120 °C. Gold electrodes with a diameter of 30 mm were sputtering on two sides of the samples. A broadband dielectric spectrometer (Concept 80 Novocontrol) was used to measure the dielectric relaxation properties at an ac voltage of 1 Vrms in a frequency range from 10-1 to 107 Hz at various temperatures. Above the glass transition temperature, a relaxation peak occurs at high frequencies due to the motion of molecular chains or segmental chains, and a dc conductivity resulted by the migration of charge carriers appears at low frequencies. In addition, molecular chains with different scales have different relaxation times. It was found that EP microcomposite has a very broad distribution of relaxation time. We calculated the distribution of relaxation time at various temperatures. Furthermore, the temperature dependences of molecular relaxation and dc conductivity satisfy the Vogel-Tammann-Fulcher equation. Fitting the experimental results, we obtained the Vogel temperatures and strength parameters of molecular relaxation and dc conductivity. From the Vogel temperatures, we estimated the glass transition temperature to be 117 °C, which is consistent with the DSC result. It means that free volume increases with increasing temperature, facilitating the motion of molecular chains and the migration of charge carriers.