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本文采用热氧老化方式模拟聚脲(polyurea,PU)薄膜在高温工况下的老化行为，根据PU薄膜介电储能特性的变化规律将其老化过程划分为3个阶段.结果表明：老化初期，氧气的桥接作用促使分子链规整排列，相邻脲基团之间的氢键作用稍有增强，分子链间距减小，介电常数减小，而击穿场强变化较小；老化中期，醚键断裂并诱导形成联苯结构，体系无定形程度加剧，介电常数增大，但联苯结构加深了陷阱深度，导致载流子迁移率降低，这提高了薄膜的击穿场强；老化后期，氧气促使脲基分解，造成贡献深陷阱能级的基团数量减少，同时主链发生裂解，并释放CO2与H2O等小分子物质，这些因素共同导致了PU的击穿场强显著降低.热氧老化过程中PU的储能密度表现出与击穿场强相同的时间依赖性，证明了计及陷阱演变的PU储能性能退化机理：氧气对醚键和脲基的分解作用分别诱发了联苯结构的形成和主链的裂解，这改变了陷阱深度，其中陷阱深度越浅，PU性能退化越显著.Dielectric capacitors are essential energy storage devices with high power density. The dielectric films of capacitors will undergo aging at working temperatures and cause performance degradation. Polyurea (PU) is a potential working dielectric for capacitors with high energy density and low dielectric loss. However, the aging characteristics and underlying mechanism of PU has never been discussed. Given the operating temperature for commercially available dielectric capacitors, PU is exposed to 80℃ for different durations to investigate its aging characteristics. Compared with dielectric constant, breakdown strength changes significantly with aging time, which can be used as a characteristic parameter to evaluate the aging degree of PU. Combing the experimental and simulation methods, the correlation between molecular structure, trap properties and breakdown strength during thermo-oxidative aging has been studied and established. The results show that: the thermal-oxidative aging of PU can be divided into three stages. In the early stage, the bridging effect of oxygen promotes the order arrangement of molecular chains, which is shown in Fig. (a). It not only reduces the molecular chain spacing, but also slightly enhances the H-bonding interaction between adjacent urea groups. As a result, the dielectric constant decreases, while the breakdown strength are almost unchanged. In the middle stage, ether bond cleavage induces the formation of biphenyl structures, leading to a disordered structure, which is illustrated in Fig. (b). The enhanced mobility effect increases the dielectric constant. Meanwhile, the biphenyl structures deepen the trap depth, reduce carrier mobility and increase the breakdown strength. In the late stage, oxygen promotes the decomposition of urea groups, which reduces the number of urea groups that contributes to deep traps. At the same time, the main chain undergoes cleavage, releasing small molecules such as CO2 and H2O, which is revealed in Fig. (c). These factors collectively lead to a significant reduction in the breakdown strength of PU. In addition, the variation of dielectric constant, breakdown strength and energy density in the three stages is summarized, which is shown in Fig. (d).
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Keywords:
- polyurea /
- energy storage characteristics /
- thermal-oxidative aging /
- degradation mechanism
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