Sandwich-Structured Aramid Composite Papers with Heterogeneous Dielectric Layers for Integrated Corona Resistance and Thermal Conductivity.

Aramid dielectric papers are indispensable for insulation in high-performance motors for electric vehicles, aircraft, and high-speed rail. However, the ongoing electrification trend toward higher voltage, frequency, and power density imposes severe electrothermal stresses that conventional aramid papers cannot withstand. Their inherently low thermal conductivity leads to localized heat accumulation while poor corona resistance accelerates electrothermal aging, both compromising long-term motor reliability. Here, we report a sandwich-structured poly(m-phenylene isophthalamide) (PMIA) composite dielectric paper that decouples thermal conductivity from corona resistance through a functional multilayer architecture. Fluorinated graphene (FG)/PMIA surface layers provide interface passivation and charge dissipation, while boron nitride nanotube (BNNT)/PMIA core layers enable thermal conduction and dielectric reinforcement. Electrospinning followed by hot-pressing achieves spatially oriented functional filler arrangements. FG constructs continuous charge-transfer barrier to suppress surface discharge and BNNT constructs continuous thermal pathways in the bulk without compromising insulation integrity. The optimized dielectric paper achieves a thermal conductivity of 10.22 W·m- 1·K- 1 and a corona lifetime of 147.42 min, representing 1.23-fold and 14.7-fold improvements over neat PMIA paper, while maintaining a high breakdown strength of 199.52 kV/mm. This specific synergy originates from hierarchically oriented architecture rather than simple property addition, offering a universal strategy to design dielectric materials with decoupled multifunctionality.