Study On The Construction Of Directional Thermally Conductive Composite Phase Change Materials By Biomass Template Method And Their Thermal Properties

Organic phase change materials have high thermal storage density and chemical stability,which can be applied to low temperature thermal storage and thermal management of electronic devices.However,the poor thermal conductivity and shapelessness of organic phase change materials after melting severely limit their wide application.In order to solve these problems,the present paper constructs directional thermally conductive phase change composites based on the biomass template method.The effects of the three-dimensional porous skeleton constructed based on biomass templates on the leakage resistance and thermal conductivity of the phase change composites were investigated.In addition,its effects on the thermal storage performance and photothermal conversion performance of the phase change composites were investigated.The main findings are as follows:1.To address the problems of poor thermal conductivity of paraffin and shapelessness after melting,a three-dimensional porous carbon skeleton composed of Si C-coated biomass carbon fibers was firstly prepared by using cotton scrolls constructed from cotton blankets as templates,and then the phase change composites were prepared by vacuum impregnation of paraffins.The results show that the phase change composite has good thermal conductivity,thermal storage,shape stability and chemical stability.In particular,the porous carbon skeleton consists of laminarly distributed one-dimensional oriented carbon fibers,which makes the phase change composites show anisotropic thermal conductivity.When the carbon skeleton filling rate is 10.4 wt%,the thermal conductivity of the phase change composites in the lateral and axial directions are 0.48 and 0.61 W m^-1 K^-1,respectively,which are 2.4 and 3.05times that of paraffin wax,and the melting enthalpy is 186.5 J g^-1 at this time.2.To further improve the thermal conductivity,insulating properties and strength of the skeleton of the paraffin complex,firstly,a biomass template of cotton-based cotton scroll with a woven structure was used,and a Si C skeleton with woven mesh structure and bionic three-dimensional porosity was prepared by high-temperature carbothermal reduction reaction using Si and Si O₂ powder as the silicon source,and then the phase change composite was prepared by vacuum impregnation of paraffin.The results show that the phase change composites have good thermal conductivity,thermal storage,shape stability and electrical insulation properties.In particular,the phase change composite with a bionic porous Si C ceramic skeleton has good electrical insulation properties,which greatly improves its scope for thermal management applications in electronic devices.Furthermore,In addition,the bionic Si C cotton scrolls exhibit a laminar spiral structure,and by laminating them with paraffin wax,it not only enhances the thermal transfer properties of the complexes,but also makes the composites have certain anisotropic thermal conductivity.When the Si C filling ratio is32.6 wt%,the lateral and axial thermal conductivities of the phase change composites are 0.90 and 1.05 W m^-1 K^-1,respectively,which are 4.5 and 5.25 times higher than those of paraffin wax,and the melting enthalpy is 148.8 J g^-1 at this time.3.To further improve the thermal conductivity of paraffin complexes,firstly,a novel two-dimensional directional porous carbon coil was successfully designed by carbonizing cotton scrolls with a sandwich graphite layer structure.A two-dimensional graphite layer is attached to the cloth-based carbon sheet of the carbon scroll,and a fast thermal conductivity channel can be formed along the cloth-based carbon sheet and the graphite layer.The phase change composites were obtained by vacuum impregnation of paraffin wax in the porous carbon scrolls.The results show that the phase change composites have excellent thermal conductivity,thermal storage,shape stability and photothermal conversion.In particular,the structural characteristics of the vertically aligned two-dimensional thermally conductive carbon layers ensure the unique anisotropic thermal conductivity characteristics of the phase change composites,and the thermal conductivity properties along the carbon scroll axis are greatly improved.When the carbon filling rate is 28.5 wt%,the lateral and axial thermal conductivity of the phase change composites are 4.04 and 7.38 W m^-1K^-1,respectively,which are 20.2and 36.9 times that of paraffin wax,and the enthalpy is 157.2 J g^-1 at this time.In conclusion,the directional thermally conductive composite phase change material constructed based on biomass template method not only solves the problem of low thermal conductivity of organic phase change material represented by paraffin wax,but also effectively solves the problem of non-formation of paraffin wax in the process of heat by shaping porous skeleton.In addition,the phase change composites developed in this thesis have the advantages of high thermal conductivity and high thermal storage capacity,which have great potential for applications in low-temperature thermal storage and thermal management of electronic devices.