Analysis of Thermodielectric Properties of Polyurethane Composites Containing a Hybrid Microfiller

This study investigates the thermodielectric properties of polyurethane-based microcomposites filled with hybrid microfiller systems based on combinations of wurtzite boron nitride (wBN), aluminium nitride (AlN), and aluminium hydroxide (Al(OH)3). The dielectric properties (εr, tanδ) in the mHz–MHz frequency range and the effective thermal conductivity (keff) were experimentally characterised for filler loadings up to 40 wt.%. The hybrid systems (wBN + AlN, wBN + Al(OH)3, and AlN + Al(OH)3) yielded thermal conductivities in the range of 0.40–0.50 W·m−1·K−1 at 40 wt.% total loading (0.19≤ϕ≤0.23), showing modest synergistic enhancement and remaining within the quasi-linear regime of the Nan model. These results demonstrate that the overall thermal transport in the composites depends far more on the formation of particle percolation networks than on the intrinsic thermal conductivity of the individual fillers, even when accounting for Kapitza interfacial resistance, as confirmed by simulations. Importantly, even at high filler loadings, the electrical insulation properties remain suitable for highly energy-dense applications in electric aircraft. In particular, tanδ values are comparable to or better than those of unfilled polyurethane, while dielectric strength results lie within the industrially relevant range.