Threshold Evaluation of γ Irradiation Accumulated Dose of BOPP Film Based on Trap Density
Wang Yucheng1, Li Hua1,2, Wang Zhehao1, Lin Fuchang1,2
1. State Key Laboratory of Advanced Electromagnetic Engineering and Technology Huazhong University of Science and Technology Wuhan 430074 China; 2. Key Laboratory of Pulsed Power Technology Huazhong University of Science and Technology Ministry of Education Wuhan 430074 China
Abstract：The service life of electrical systems usually depends on the endurance limit of the polymer dielectric. After irradiation, the macromolecular structure changes, and the electrical performance deteriorates, which may cause the electrical system to terminate its service life prematurely due to dielectric degradation before electrical failure. The previous studies mostly used macroscopic electrical parameters to characterize the radiation degradation of polymer films, including conductivity, dielectric loss, and breakdown strength. These parameters are easily affected by working conditions such as applied voltage and temperature. Compared with the macroscopic properties, the intrinsic microscopic properties, such as crystallization and space charge characteristics, can reflect the deterioration more accurately. In this paper, biaxially oriented polypropylene film (BOPP) as the object for capacitors was irradiated in the air. The γ irradiation dose is 0.2, 0.5, 1, 5, 10, 50, 100, 500 and 1 000 kGy. First, the surface morphology of the films before and after γ irradiation was observed. Then the heat flow curves of the films were measured by the differential scanning calorimetry (DSC) method, and the trap parameters of the films were measured by the thermally stimulated depolarization current (TSDC) method. The effects of accumulated γ irradiation dose on crystallinity, melting point, and trap parameters were studied. Finally, the threshold evaluation method of the accumulated γ irradiation dose was proposed based on the irradiation-induced trap generation model. The surface morphology observation shows that γ irradiation does not affect the shape and size of the grain on the surface of the film. The DSC test results show that the melting point and crystallinity of the films after irradiation have no significant changes when the accumulated γ irradiation dose D does not exceed 10 kGy. When D exceeds 10 kGy, the crystallinity and melting point decrease gradually with the increase of the accumulated irradiation dose. The variation of crystallinity with the accumulated irradiation dose satisfies the exponential law. When D=100 kGy, the crystallinity and melting point decreased by 7.3% and 5.7%, respectively, compared with those without irradiation. When D=1 000 kGy, the crystal region is seriously damaged, and the crystallinity and melting point are reduced by 30.7% and 15.1%, respectively, compared with those without irradiation. The TSDC test results show that the peaks of thermally stimulated depolarization current of BOPP film can be divided into three categories: the low-temperature peak whose peak temperature is near the glass transition temperature, medium-temperature peak with peak temperature lower than 90℃, high-temperature peak with peak temperature higher than 90℃. When D>10 kGy, the overall trap depth increases with the increase of D. In this paper, the trap density was taken as the characteristic quantity of film degradation, and the mathematical model of trap density Nt and accumulated γ irradiation dose D, namely, the irradiation-induced trap generation model, was established. The discrete trap density and the total trap density were used for fitting. The fitting results show that the goodness of fit of the total trap density is higher than the discrete trap density. The following conclusions can be drawn: (1) The crystallinity and melting point of BOPP film decrease with the increase of the accumulated γ irradiation dose. (2) Based on the irradiation-induced trap generation model, the coefficient ktrap of the trap density increases after irradiation was defined to characterize the degradation degree of the electrical properties of BOPP film. The total trap density was used to evaluate the threshold of accumulated γ irradiation dose to reflect the overall deterioration of the film. When D≤500 kGy, the trap density with accumulated irradiation dose satisfies the piecewise function. When ktrap≤0.25 (D≤10 kGy), the film degradation is insignificant and can be used normally. When 0.25<ktrap≤2.0 (10 kGy<D≤80 kGy), the degradation degree of the film is relatively low, and 10 kGy is taken as the initial threshold of accumulated irradiation dose for BOPP film degradation. When ktrap>2.0 (D>80 kGy), the degradation of BOPP film is severe, and 80 kGy is taken as the threshold of significant degradation.
王雨橙, 李化, 王哲豪, 林福昌. 基于陷阱密度的双向拉伸聚丙烯薄膜耐γ 辐照积累剂量阈值评估[J]. 电工技术学报, 2023, 38(18): 5039-5048.
Wang Yucheng, Li Hua, Wang Zhehao, Lin Fuchang. Threshold Evaluation of γ Irradiation Accumulated Dose of BOPP Film Based on Trap Density. Transactions of China Electrotechnical Society, 2023, 38(18): 5039-5048.
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