Transparent, Flexible and Thermal Stable Al ₂ O ₃ /Aramid Insulating Nanopaper for Extra-High Voltage Transmission

The rapid advancement of ultrahigh voltage (UHV) transmission technology, commonly referred to as the “power highway,” has significantly increased the demand for dry-type transformers. This trend imposes more stringent requirements on the internal insulating materials used in dry-type transformers, particularly on insulating paper. Herein, a series of transparent, flexible and thermal stable Al2O3/aramid insulating nanopaper were successfully fabricated through highly dispersed spherical nano- Al2O3 within a three-dimensional network structure formed by aramid nanofibers. Among all composite samples, ANF/Al2O3-3 exhibited the maximum breakdown strength of 216.7 kV/mm, which is approximately 8.6 times that of commercial aramid paper (25.3 kV/mm). In contrast, ANF/Al2O3-5 showed outstanding comprehensive performance, with a tensile strength of 149.86 MPa, an elongation at break of 3.62%, and a volume resistivity of 3.53 × 1013 Ω·cm. Moreover, slight drop of breakdown strength for ANF/Al2O3-5 was found when subjected to thermal aging at 300 °C for 24 h, only 1.0% from 199.11 kV/mm to 197.03 kV/mm. Three-dimensional finite element simulation results showed that the enhanced dielectric breakdown strength of ANF/Al2O3 composite paper is attributed to the introduction of energy traps at the ANF/Al2O3 interface, which impede electron transport through repeated charge trapping-detrapping processes. This study establishes a morphology–property relationship paradigm, demonstrating that the synergistic integration of spherical filler geometry with quantitative interfacial analysis provides a approach for developing high-performance composite insulating materials.