Preparation and thermal energy storage properties of shaped composite phase change materials with highly aligned honeycomb BN aerogel by freeze-vacuum drying under the control of a temperature gradient

Both the low thermal conductivity and liquid leakage of phase change materials (PCMs) during its phase change limit their applications in thermal energy storage. In this paper, a three-dimensional boron nitride aerogel (3D-BN) with highly aligned honeycomb structure was synthesized by a newly proposed method utilizing in-situ freeze-vacuum drying under the control of a temperature gradient. 3D-BN/paraffin shaped composite phase change materials (CPCMs) were prepared and their thermal energy storage properties were analyzed. Its thermal conductivity was significantly enhanced to be 0.854 W/(m·K) when the mass fraction of 3D-BN was 18 wt%. The possessed thermal conductivity was 3.69 times higher than that of pure paraffin and was comparable with that of known vertical aligned 3D-BN/paraffin CPCMs prepared by the liquid nitrogen freezing. The heating-cooling cycle tests also showed that the prepared CPCMs at the present work possessed both excellent thermal stability and antileakage performance. After 120 cooling and heating cycles, the CPCMs still had a high latent heat of phase transition of 213.08 J/g. With the predefined thermal performance evaluation experiments, the thermal conductivity of CPCMs and its heat storage properties were verified. Based on our proposed method, the production of 3D-BN could be simplified and improved; moreover, the proposed method could serve as a new design concept for many 3D aerogels and their based CPCMs for heat storage and thermal management.