Preparation And Thermophysical Properties Of Two-dimensional Nanosheet Based Composite Phase-change Heat Storage Materials

Phase change materials(PCMs)can solve the contradiction between thermal energy supply and demand and alleviate the energy crisis by reversibly storing renewable and sustainable thermal energy.Phase change materials have the advantages of high energy density,strong energy output stability,and wide temperature application range.However,the low thermal conductivity of pure phase change materials and the easy leakage during the phase change process hinder their wide application and development.In order to overcome the shortcomings of phase-change heat storage materials and improve the utilization efficiency of thermal energy,the thermal conductivity of PCMs can be improved by compounding with two-dimensional nanosheets,which can solve the problem of heat transfer rate in the heat storage process.In this paper,two-dimensional nanosheets are added to phase change heat storage materials to prepare composite phase change heat storage materials:(1)Hexagonal boron nitride nanosheets(h-BNNs)with atomic layer thickness were successfully prepared by ultrasonically exfoliating hexagonal boron nitride in methane sulfonic acid using the liquid phase exfoliation method,and the obtained h-BNNs solar salt(Solar Salt 60 wt.%Na NO₃+40 wt.%KNO₃)was compounded.Compared with pure Solar Salt,the thermal conductivity,solid-phase heat capacity,and liquid-phase heat capacity of the prepared nanofluids were increased by 76.79%,29.84%,and 12.82%,respectively.At the same time,the degree of subcooling dropped from 12.2°C to 6.1°C.(2)The surface of atomic layer-thick hexagonal boron nitride nanosheets was functionalized,composited with graphene oxide(GO)as the framework,and divalent calcium ions were added to enhance the cross-linking between the two,and the directional freezing strategy was used.Combining the strategy of reducing the interplanar spacing doubles the mechanical strength.The better adsorption of h-BN on the GO surface reduces the interfacial thermal resistance between nanosheets and improves the heat transfer rate.Meanwhile,the loading of phase-change material(paraffin)can reach an astonishing 95%and remain above90%after 300 cycles.