Electrothermal Aging of Polyimide at Kilohertz Frequencies: Multiscale Insights and Physics-Informed Lifetime Model

Polyimide (PI) is widely used as the primary dielectric in solid-state transformers (SSTs), yet rapid degradation under kilohertz, kilovolt pulses at elevated temperature remains insufficiently understood. In practical geometries with gas gaps and exposed edges, partial discharges (PD) localize field at triple points, leading to extrinsic failure. By integrating in situ diagnostics (30 kHz partial discharge monitoring, FTIR, SEM/EDS, and PEA space charge mapping) with a three-tier simulation chain (ReaxFF-MD → TD-DFT → electrothermal phase-field), this work provides an integrated bond-to-breakdown view of PI aging under high-frequency electrical stress. Experiments reveal a nonmonotonic evolution of partial discharge (PD) activity: the PD pulse amplitude and repetition rate first rise and then fall as aging progresses. Reactive molecular dynamics pinpoints C–N/C–C scission in the imide ring, whose CO off-gassing and defect formation halve the PI’s HOMO–LUMO gap (from 5.32 to 2.57 eV). Time-dependent DFT confirms a markedly lower excitation threshold and stronger hole–electron overlap in the defected structure, explaining the trap-assisted charge retention observed by PEA. Using frequency- and temperature-dependent dielectric parameters, a phase-field model incorporates dielectric-loss heating and successfully reproduces the experimentally observed transition from sparse electrical treeing to a continuous breakdown channel with localized thermal runaway. Finally, a physics-informed lifetime model is formulated and fitted to accelerated aging data, quantifying how high-frequency PD damage and dielectric-loss heating together expedite insulation failure. The results establish a defect-mediated, PD-initiated extrinsic electrothermal aging mechanism in PI under kHz excitation and demonstrate a predictive framework for lifetime estimation, thereby guiding the design of polymer dielectrics for high-frequency power equipment.