Liuqing Yang

Non-destructive diagnosis of degradation of silicone rubber by indenter modulus and scanning probe microscopy

We aged silicone rubber, which is widely used for electrical insulation of cables in nuclear power plants under various conditions. The surface of each sample was measured by an indenter and a scanning probe microscope (SPM) for non-destructive diagnosis. As a result, it has become clear that the surface of silicone rubber becomes hard with the progress of degradation in both cases of thermal aging and simultaneous aging by heat and radiation. The indenter modulus shows a good correlation with the elongation at break (EAB). On the other hand, the delay of phase angle measured by SPM exhibits a rather low correlation with EAB. Through the results of thermal analyses, it seems that the thermal aging without radiation induces cross-linking first, which is followed by bond break. However, bond breakage seems predominant over cross-linking in the simultaneous aging with heat and radiation.

Aging mechanism of silicone rubber by heat and gamma-rays

The aging mechanism of silicone rubber, which is important for electrical insulation of cables, is examined from many aspects such as instrumental, chemical, electrical, and mechanical analyses. As a result, it has become clear that silicone rubber degrades by forming crosslinked structures via the formation of abundant siloxane bonds.

Effect of BN size on AC breakdown performances of PTFE/BN composites

To satisfy the rapid development of energy requirements, power transmission with higher voltage must be adopted. The large improvement in transmission voltage needs more excellent power equipment and better materials to meet its insulation requirement. Polytetrafluoroethylene (PTFE) nozzle used in high voltage circuit breaker faces the issue of breakdown and arc ablation, and thus superior performance nozzles are urgently needed. Previous work has proved that boron nitride (BN) is excellent filler, which can greatly improve the arc ablation performance of nozzles. Moreover, the effect of BN size on arc ablation characteristics has been studied. However, the influence of BN size on breakdown performance is still unknown. In this paper, PTFE filled with BN of different size are prepared to study the effect of BN size on AC breakdown performances of PTFE/BN composites. Scanning electron microscope (SEM) was used to characterize the dispersion condition of BN sheets in PTFE matrix. SEM results show that BN sheets are uniformly dispersed in PTFE matrix. AC breakdown results show that the breakdown strength of PTFE/BN composites increases with the increasing BN size. And the value of breakdown strength of PTFE/BN composites filled with 3 um BN is 63.24 kV/mm, which has been enhanced by 103% compared with that of unfilled PTFE (31.17 kV/mm). Thermally stimulated current (TSC) results show that the trap parameters have been greatly changed after BN is introduced into PTFE matrix. Based on the above experimental results, the relationship between breakdown strength and trap parameters is discussed. The improvement of breakdown performance is considered to be closely related to the change of trap depth and trap density, which can influence the charge transport properties in materials.

Effect of surface state on DC breakdown of LDPE films

Interface between the metal electrode and insulating material affects dielectric performance of the whole insulating system significantly. In this study, low density polyethylene (LDPE) films were treated in ozone atmosphere for different time duration to investigate the effect of interface state on breakdown performance. Fourier transform infrared spectra (FT-IR) results show that carbonyl group (C=O) is introduced into the surface of LDPE film after ozone treatment, and the amount of C=O groups increases when the ozone oxidation time extends. Surface trap distribution characterized by surface potential decay (SPD) reveals that two hole-type traps exist in the surface of LDPE and ozone treated specimens. Both trap levels become shallower and the density of deeper level traps decreases with the increasing ozone oxidation time, while that of shallower level traps increases. Space charge distribution measured by pulsed electro-acoustic (PEA) method suggests that more positive charges are injected into specimens from the anode and accumulated in the vicinity of cathode after ozone treatment. Furthermore, the dc breakdown strength declines monotonously with longer ozone treatment time, and the lowest breakdown strength of specimen occurs after 6 hours, decreasing by 26.9% compared with that of untreated LDPE. Finally, it is concluded that ozone treatment results in changing of surface chemical composition, which determines surface trap distribution (surface state), leading to the variation of charge injection and accumulation properties as well as the subsequent breakdown performance.

Degradation behavior and mechanisms of several polymeric insulating materials exposed to heat and radiation

Sheet-shaped poly(ether-ether-ketone) (PEEK), silicone rubber (SiR), flame-retardant (FR) ethylene-propylene-diene monomer rubber (EPDM), and FR crosslinked polyethylene (FR-XLPE) were aged thermally at various temperatures with or without concurrent irradiation of gamma rays. The degradation mechanism was examined extensively by instrumental analyses, while those in mechanical and dielectric properties were evaluated by measuring indenter modulus, elongation at break, tensile strength, electrical conductivity, and complex permittivity. The behavior and its underlying mechanism of degradation induced by heat and radiation are quite different among these polymeric insulating materials. All the four materials exhibit deterioration in mechanical properties at a much earlier stage of aging than that in dielectric properties. Therefore, mechanical parameters are important indicators that should be considered in the condition monitoring.

Degradation of Silicone Rubber Analyzed by Instrumental Analyses and Dielectric Spectroscopy

Silicone rubber (SiR) was gamma irradiated at 125, 145 and 185 °C or thermally aged at 220, 250 and 280 °C and the resultant changes in performance were evaluated. It has become clear from instrumental analyses that crosslinking via oxidation of silicon atoms and chain scission are induced by gamma rays. Furthermore, from the temperature dependence of real relative permittivity at high frequencies, the thermal expansion coefficient was found to become smaller with the increase in dose. These results can be understood well by the chemical and structural changes in SiR induced by the degradation.

Effect of BN size on arc ablation characteristics of PTFE material

To satisfy the rapid development of energy requirements, power transmission with higher voltage must be adopted. The large improvement in transmission voltage needs more excellent power equipment and better materials to meet its insulation requirement. PTFE nozzle used in high voltage circuit breaker faces the issue of arc ablation, and thus superior performance nozzles are urgently needed. In this paper, polytetrafluoroethylene (PTFE)/boron nitride (BN) micro sheet composites (PTFE/BN) are prepared to study the relationship of BN size and arc ablation characteristics. The thermal transport performance of PTFE/BN composites increases with BN sizes and the highest value has improved by 86.44% compared with that of neat PTFE when PTFE is filled with 3 μm BN. Moreover, PTFE/BN composites indicate higher spectral reflectance properties than that of neat PTFE when the light wavelength is longer than 300 nm. The arc ablation test results show that the PTFE/BN composites present better arc ablation resistance performance, which gets more and more excellent with increase of BN size. It is considered that the increase in thermal transport performance makes the heat form the high temperature arc conduct more quickly and then reduce the heat accumulation on the specimen surface, and thus the arc ablation resistance performance is improved.

Surface flashover performance of phenolphthalein modified LDPE in vacuum

Since surface flashover of insulators in vacuum restricts the operation of the power equipment, improvement of surface flashover performance is of considerable importance in various fields. Previous studies show that surface flashover performance is closely related to the trap parameters of materials. In this paper, the crystallization behavior of low density polyethylene (LDPE) is modified by phenolphthalein. It results in the change of the trap parameters, leading to the enhancements in surface flashover performance. Results of differential scanning calorimeter (DSC) and scanning electron microscope (SEM) show that the crystallization behavior of LDPE is greatly changed by phenolphthalein. The crystallinity increases initially and then decreases with the increase of phenolphthalein concentration, and its maximum value occurs at 0.4 wt%. The spherulite size of LDPE composites decreases obviously with the increasing phenolphthalein concentration. The results of thermally stimulated current (TSC) show that both the shallow trap (γ peak) and the deep trap (α peak) become deeper with the increase of phenolphthalein concentration. Furthermore, the impulse and dc surface flashover voltages present the optimum values at 1 wt% phenolphthalein concentration, which can be enhanced by 30.8% and 48.4%, respectively. It is concluded that LDPE with the smaller spherulite size indicates the deeper trap and the better surface flashover performance. This paper reveals the relationship between surface flashover performance and crystallization behavior of LDPE, and proposes a new method to enhance the surface flashover performance of semi-crystalline polymers in vacuum.