Phosphate-passivated AlN/NFC-reinforced cellulose composite paper with coupled thermal, dielectric, and aging stability for oil-immersed insulation

Cellulose paper is a renewable fibrous composite material, but its low thermal conductivity and aging-induced embrittlement limit its use in oil-immersed electrical insulation. Here, a wet-formed cellulose composite paper was reinforced with phosphate-passivated aluminum nitride (AlN) and nanofibrillated cellulose (NFC) to couple heat dissipation, dielectric stability, and mechanical reliability. Molecular dynamics (MD) simulations and experiments were combined to screen AlN loading and clarify structure–property relationships. With NFC fixed at 15 wt%, the 9 wt% AlN composite paper (P9) showed the best property balance. Relative to pristine cellulose paper, P9 increased tensile strength, thermal conductivity, volume resistivity, and breakdown strength by 40.2%, 97.5%, 59.6%, and 32.7%, respectively, while reducing the relative permittivity and dielectric loss to 2.73 and 0.28% at 50 Hz. After 90 days of oil-immersed aging at 130 °C, P9 showed retention ratios of 67.0%, 38.2%, and 85.5% for tensile strength, degree of polymerization, and breakdown strength, respectively, all higher than those of commercial T4 paper. The balanced performance is attributed to hydrolysis-resistant AlN, NFC-assisted filler dispersion, and a dense multiscale fiber network. This work demonstrates a multiscale co-reinforcement strategy for designing multifunctional cellulose composite papers for thermally and electrically demanding service environments.