Dielectric capacitors are essential energy storage devices with high power density. The dielectric films of capacitors will age at working temperatures, a leading their performance to degrade. 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 have not been discussed. Considering the operating temperature of commercial dielectric capacitors, the aging characteristics of PU are investigate by being exposed to 80 °C for different durations. 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. Combining experimental method and simulation method, the correlation between molecular structure, trap properties and breakdown strength during thermo-oxidative aging is studied and established. The results show that the thermal-oxidative aging of PU can be divided into three stages. In the early stage of aging, the bridging effect of oxygen promotes the order arrangement of molecular chains. In this stage, the molecular chain spacing decreases, but the H-bonding interaction between adjacent urea groups is enhanced slightly as shown in Fig. (a). As a result, the dielectric constant decreases, while the breakdown strength is almost unchanged. In the middle stage of aging, ether bonds break and inducing the formation of biphenyl structures and resulting in a disordered structure as illustrated in Fig. (b). The enhanced mobility effect increases the dielectric constant. Meanwhile, the biphenyl structures deepen the trap depth, resulting in the decrease of carrier mobility and the increase of the breakdown strength. In the late stage of aging, oxygen promotes the decomposition of urea groups, thereby reducing the number of urea groups that form deep traps. At the same time, the main chain undergoes breakage, releasing small molecules such as CO2 and H2O, which is shown in Fig. (c). These factors collectively lead to a significant reduction in the breakdown strength of PU. In addition, the variations of dielectric constant, breakdown strength and energy density in the three stages are summarized in Fig. (d).
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