Thermal Property Of Composite Form-stable Phase Change Materials Enhanced By Reduced Graphene Oxide/carbon Nano-felts

Our country and even the globe has landed itself into energy crisis at present,therefore,raising the efficiency of energy utilization and conserving energy are matters of extreme urgency.As an environment-friendly renewable-energy utilization technique,phase change materials?PCMs?are able to absorb and release enormous amount of thermal energy at a constant temperature in the form of latent heat during physical phase transition and have become one of the most important materials to improve the efficiency of thermal energy utilization.However,the lower thermal conductivity and leakage of the fatty acids in the melting state influence their further develop and applications seriously.In this study,we through combining the technique and principle of electrospinning,formation of hydrogen bonds,pre-oxidation/carbonization and amidoximation/carbonization prepared reduced graphene oxide/carbon nano-felts?RGO/CNFs?successfully,which possessed high thermal conductivity and good support structure.Then,we further use the RGO/CNFs to absorb lauric-myristic-stearic acid?LA-MA-SA?ternary eutectic mixture phase change materials to obtain form-stable phase change materials?FSPCMs?including excellent thermal storage density,high thermal conductivity and superior shape-stabilization.Firstly,the PAN nano-felts obtained by electrospinning were pre-oxidized to prepare OPAN nano-felts,then by virtue of the hydrogen bonding interactions,graphene oxide?GO?prepared through the modified Hummers'method was combined with OPAN nano-felts to acquire stable GO/OPAN nano-felts,GO/OPAN nano-felts were further converted into preoxidized carbonization RGO/CNFs by the high temperature calcination and subsequently were immersed into heat storage unit LA-MA-SA for preparation of preoxidized carbonization FSPCMs?PC-FSPCMs?.The characterization on the structure and morphology as well as properties of the samples suggested that because of the existence of hydrogen bonding,preoxidized carbonization RGO/CNFs possessed excellent structural morphology.Simultaneously,the intimate connection of RGO and CNFs can decrease the thermal contact resistance and facilitate the rapid transportation of heat,so that the thermal conductivity of PC-FSPCMs increased to 1.13 W/mK,which was 304%and 169%higher than that of the LA-MA-SA and control sample,respectively;the thermal energy storage rates were increased by 37.1%and 32.5%in comparison to that of LA-MA-SA and control sample,respectively;the thermal energy retrieval rates were increased by 57.5%and 37.3%in comparison to that of LA-MA-SA and control sample,respectively.Then,the amidoxime surface-functionalized polyacrylonitrile?ASFPAN?nano-felts that was derived from electrospinning of PAN nano-felts and thus amidoxime groups?H₂N-C=N-OH?were introduced onto the surface of PAN nano-felts.By immersion ASFPAN nano-felts into the GO aqueous dispersion,GO/ASFPAN obtained successfully by the effect of hydrogen bonds,formed between the–COOH/-OH on the edge of GO sheets and H₂N-C=N-OH in ASFPAN.GO/ASFPAN nano-felts were further converted into RGO/CNFs with superior structural morphology properties by the high temperature calcination and subsequently physical adsorption LA-MA-SA ternary fatty acid eutectic mixture for preparation of aminoximation carbonization FSPCMs?AC-FSPCMs?.The comparison and characterization on the structure and morphology as well as properties with samples prepared by preoxidized carbonization indicated that as a result of the larger BET specific surface area and more stable degree of graphitization of RGO/CNFs,the melting/crystallization enthalpies of AC-FSPCMs up to 126.4/121.1 kJ/kg,the thermal conductivity arrived at 1.73 W/mK,the thermal energy storage/retrieval rates were increased by 40.4/62.1%,36.0/44.1%and 5.2/10.9%in comparison to that of LA-MA-SA,control sample and PC-FSPCMs,respectively.Finally,we based on the technology of amidoximation/carbonization and physical adsorption,the influences of GO concentration and calcination temperature on the structural properties of RGO/CNFs as well as AC-FSPCMs were further discussed.Analyses about their morphological structure,thermo gravimetric and thermal properties showed that with the increment of the concentration of GO,the thermal stabilities and thermal energy storage density of AC-FSPCMs displayed the tendency declining at the begining and rising up in late.The superior concentration of GO that made AC-FSPCMs with better structure and performance was 2 mg/ml.The morphological structure of RGO/CNFs improved gradually with the increased calcination temperature.When the calcination temperature reached 950 ^oC,the BET specific surface area of RGO/CNFs arrived 277.03 m²/g,which showed excellent three-dimensional networks,thus the adsorption rate reached to 72.9%,corresponding melting/crystallization enthalpies of AC-FSPCMs up to 128.1/127.9 kJ/kg.Moreover,synergistic enhancement of thermal conductivity was further strengthened;the thermal conductivities also reached up to 1.88 W/mK,which was 583%higher than that of pure LA-MA-SA,the thermal energy storage/retrieval rates were increased by 55.1/62.7%in comparison to that of pure LA-MA-SA.The fabricated AC-FSPCMs also possessed excellent thermal cycle stability and shape-stabilization,while no leakage of LA-MA-SA on the surfaces of AC-FSPCMs was found during the heating process.The FSPCMs can effectively improve the utilization rate of heat energy,which guaranteed the promising practical application.