Light-to-thermal Properties And Shape Memory Characteristics Of Paraffin-based Nanometallic Composite Phase Change Materials

Paraffin wax（PW）-based phase change materials（PCMs）with moderate thermal storage density,good chemical stability,a wide melting temperature range and no subcooling have great potential for use in solar energy storage.However,paraffin waxes have two outstanding problems in use:low thermal conductivity and leakage in the molten state,which very much limit their further development.If paraffin-based phase change materials can be endowed with optical and thermal driving properties,this can help to expand their application scope.In this paper,the thermal conductivity and light absorption properties of composite phase change energy storage materials are improved by adding nanoparticles（NPs）to paraffin wax to optimise their thermal storage properties.The vacuum impregnation method is used to adsorb and encapsulate liquid paraffin wax with NPs in melamine foam（MF）,which solves the problem of paraffin phase change leakage and obtains a new flexible paraffin-based composite phase change energy storage material with light-to-thermal conversion performance and shape memory properties.Among them,the addition of NPs includes:direct addition of copper nanoparticles（Cu NPs）;composite materials of silver nanoparticles（Ag NPs）and paraffin wax obtained by thermal decomposition of silver acetate-oleamine complexes in molten paraffin wax system through organic precursors;complexes formed by thermal decomposition of silver acetate and carboxylated multi-walled carbon nanotubes（MWCNTs）in molten paraffin wax system to obtain The composites of multi-walled carbon nanotubes（Ag-MWCNTs）with silver nanoparticles modified with paraffin wax were obtained.In this paper,oleylamine was used as a dispersing stabiliser and molten liquid paraffin as a protective agent and heating solvent to form an air-insulated protective layer on the surface of the NPs to prevent oxidation of the metallic NPs.The effects of different NPs additions and the amount of additions on the morphology,structure,thermal properties,light-to-thermal conversion properties,shape memory properties,memory reliability and cycling stability of PCMs were investigated using transmission electron microscopy,X-ray diffraction,infrared spectroscopy,differential scanning calorimetry and other analytical methods as well as a home-made light-to-thermal conversion performance test system.The main studies in this paper include.（1）Investigating the effect of Cu NPs content on the properties of composites,and extending the discussion on the dispersion state of general metal NPs in molten liquid paraffin,the optimum content of Cu NPs added to composite phase change materials was screened out to 0.6 wt%.A home-made test system for light-to-thermal conversion performance was developed,and the test results combined with X-ray diffraction,differential scanning calorimetry and thermal infrared imager concluded that the MF/Cu/PW PCMs obtained have large latent heat and good light-to-thermal conversion capacity when the content of Cu NPs is at 0.6wt%.（2）Oleylamine modified Ag NPs/PW composites were prepared in a single step by thermal decomposition of silver acetate-oleylamine complexes in a molten paraffin system.It was obtained that the oxidation of Ag NPs was prevented by paraffin and oleylamine during the thermal decomposition reaction,and Ag NPs were stably dispersed in the liquid paraffin.The effect of Ag NPs content on the performance of the composites was investigated.The results showed that when the Ag NPs content was at 0.6 wt%,the prepared MF/Ag-0.6/PW PCMs had a large latent heat(about 150.4 J·g^-1)and good thermal and shape stability below 200°C,exhibiting a very good light-to-thermal conversion（conversion efficiency of 87.8%）and light/thermal driven shape memory properties.（3）Ag-MWCNTs/PW composites were prepared in a single step using a complex of silver acetate and MWCNTs as precursors.The test results showed that Ag atoms coordinated with the carboxyl groups on the surface of MWCNTs,and the Ag NPs-modified MWCNTs were obtained after thermal decomposition.The effects of adding equal amounts of Ag NPs,MWCNTs and Ag-MWCNTs on the properties of PCMs were investigated comparatively.The results showed that the latent heat of PCMs with the addition of Ag-MWCNTs increased to 201.7 J·g^-1,with good light-to-thermal conversion performance（94.61%conversion efficiency）,photo/thermally driven shape memory properties and stability.