Thermodielectric Properties of Polyurethane Composites with Aluminium Nitride and Wurtzite Boron Nitride Microfillers: Analysis Below and near Percolation Threshold

This study explores microcomposites' thermodielectric properties-thermal conductivity (keff) and dielectric permittivity (εr)-across filler concentrations from 1 wt% (φ≈0.0035) to 60 wt% (φ≈0.45) spanning the pre- (φ<0.16) and within-percolation threshold (0.16≤φ≤0.29). Thermal measurements were conducted using a newly designed, cost-effective thermal measurement setup. The setup utilised a transient heat pulse methodology with a heater and NTC thermistors, with a precision better than ±0.01 W·m-1·K-1. Dielectric properties were measured using a three-electrode system over a broad frequency and temperature range. The measurements demonstrate an effective thermal conductivity keff of 0.72 W·m-1·K-1 for AlN at φ=0.36 and 0.65 W·m-1·K-1 for wBN already at φ=0.12. Although theoretical models suggest that, considering interfacial Kapitza resistance, it can yield a keff corresponding to approximately 1-3% of the conductivity of pure material filler, the experimental measurements indicate a maximum of around 0.5%. Dielectric measurements show that in comparison to pure polyurethane, the presence of 60% AlN or 40% wBN at 60 °C decreased the loss tangent by 20 times in the condition of a quasistatic electric field.