Unidirectional thermal conduction in electrically insulating phase change composites for superior power output of thermoelectric generators

Owing to their ability to absorb excess heat within narrow temperature ranges, phase change materials (PCMs) have attracted significant interest in the thermal management of power equipment and energy devices. However, the low thermal conductivity of PCMs severely limit their application in high-power devices. Current methods for increasing the thermal conductivity are generally at the cost of lowering the phase change enthalpy (i.e., latent heat) of PCMs. In this study, we developed phase change composites (PCCs) based on highly oriented aramid nanofiber/boron nitride nanosheets (ANF/BNNS) aerogels impregnated with paraffin; the aerogels were prepared by unidirectional freeze-drying method and were impregnated with the PCM, namely, paraffin, by a vacuum assisted method. The ANF/BNNS PCCs exhibited high thermal conductivity enhancement efficiency (TCEE), ultra-high latent heat retention, and superior electrical insulation. For a loading of 2.21 wt% BNNS, the TCEE and the latent heat retention of the PCCs reached impressive values of 89.2%, and 93.64%, respectively. In addition, the PCCs are highly electrically insulating with an electrical conductivity of <1.17 × 10−14 S cm−1. The PCCs also maintain good shape stability even at temperatures higher than the melting point of the PCM (i.e., paraffin). The ANF/BNNS PCCs were shown to reduce the cold-side temperature of thermoelectric generators by more than 10 °C in comparison to that achieved by those operated by air-cooling, resulting in an approximately 300% increase in power output. These PCCs, with both high latent heat retention and TCEE, offer new opportunities for use as advanced thermal management materials in future high-power devices.