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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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