基于陷阱密度的BOPP薄膜耐γ辐照积累剂量阈值评估

doi:10.19595/j.cnki.1000-6753.tces.221456

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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 as the characterization of radiation degradation of polymer films, including conductivity, dielectric loss, breakdown strength, etc. 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 characteristics and space charge characteristics, can reflect the deterioration more accurately.
In this paper, biaxially oriented polypropylene film (BOPP) for capacitors was taken as the object and was irradiated in air. The γ irradiation dose is 0.2kGy, 0.5kGy, 1kGy, 5kGy, 10kGy, 50kGy, 100kGy, 500kGy and 1000kGy. 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 differential scanning calorimetry (DSC) method, and the trap parameters of the films were measured by 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 10kGy. When D exceeds 10kGy, 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=100kGy, the crystallinity and melting point decreased by 7.3% and 5.7% respectively compared with those without irradiation. When D=1000kGy, 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>10kGy, 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 N_t 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 respectively used for fitting. The fitting results show that the goodness of fit of the total trap density is higher.
The following conclusions can be drawn: 1) The crystallinity and melting point of BOPP film decreases with the increase of the accumulated γ irradiation dose. 2) Based on the irradiation-induced trap generation model, the coefficient k_trap of the increase of trap density 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≤500kGy, the trap density with accumulated irradiation dose satisfies the piecewise function. When k_trap≤0.25 (D≤10kGy), the film degradation is not significant, and the film can be used normally. When 0.25<k_trap≤ 2.0 (10kGy<D≤ 80kGy), the degradation degree of the film is relatively low, and 10kGy is taken as the initial threshold of accumulated irradiation dose for BOPP film degradation. When k_trap>2.0 (D>80kGy), the degradation of BOPP film is severe, and 80kGy is taken as the threshold of significant degradation.

Key words： BOPP film γ irradiation crystallinity trap parameters threshold evaluation

Received: 27 July 2022

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TM852

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Wang Yucheng,Li Hua,Wang Zhehao等. Threshold Evaluation of γ Irradiation Accumulated Dose of BOPP Film Based on Trap Density[J]. Transactions of China Electrotechnical Society, 0, (): 230619-230619.

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https://dgjsxb.ces-transaction.com/EN/10.19595/j.cnki.1000-6753.tces.221456 OR https://dgjsxb.ces-transaction.com/EN/Y0/V/I/230619

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