Study On Modification Of Poly(vinylidene Fluoride)-based Polymer Used In Electrocaloric Refrigeration

With the aggravation of global warming and environmental pollution,improving energy efficiency and reducing environmental pollution have become the consensus of all countries.Electrocaloric refrigeration technology is a new solid-state refrigeration technology which has great potential to replace the traditional vapor compression refrigeration technology.In this paper,the doping modification of polyvinylidene fluoride(PVDF)materials in electrocaloric cooling was studied.P(VDF-Tr FE)/P(VDF-Tr FE-CFE)mixed polymers were prepared by solution evaporation method.The introduction of copolymer can effectively reduce the Curie temperature of the mixed polymers.On this basis,P(VDF-Tr FE)/P(VDF-Tr FE-CFE)/graphene composites were prepared by introducing two-dimensional graphene.The polarization strength of the composites was greatly improved under the condition of low electric field strength when the dielectric loss is small.The main research contents and conclusions are as follows:(1)The comparison of XRD and SEM surface morphology shows that the doping of the copolymer can reduce the domain size of the mixed polymer and improve the crystallinity of the material.When the copolymer mass fraction is 10wt.%,the crystallinity is the highest,and the introduction of graphene can further increase the crystallinity.The mechanical properties of the material were studied,At the tensile rate of 0.08 N?s^-1,the elastic modulus of the mixed polymer with mass fraction of 10wt.%can reach 844.5 MPa.Under the same experimental conditions,when the mass fraction of graphene is 0.5wt.%,the maximum elastic modulus can reach 1431.84 MPa.(2)The electrical properties of the materials were studied.The dielectric constant of the mixed polymer reaches a maximum of 12.21 when the mass fraction is10wt.%,and the dielectric loss is 0.45.The dielectric constant and dielectric loss of graphene composites are positively correlated with the mass fraction.When the mass fraction is 0.5wt.%,the dielectric constant and dielectric loss are 50.25 and 0.71, respectively.The two systems of materials show good dielectric properties in the low frequency region(<10³Hz).The hysteresis loops of mixed polymers at three electric field intensities of 60 MV?m^-1,120 MV?m^-1and 200 MV?m^-1were tested.The maximum polarization intensity was obtained when the mass fraction was 10wt.%,which were 1.213?C?cm^-2,6.2117?C?cm^-2and 9.1327?C?cm^-2,respectively.Graphene composites leak current under high electric field strength due to the addition of graphene,the maximum polarization strength at 60 MV?m^-1is 4.2304?C?cm^-2,which is nearly four times higher than basis material under the same conditions.(3)The thermogravimetric curves and DSC curves of the two systems of materials were tested.DSC curves show that both systems have good characteristics of relaxed ferroelectric materials.The melting temperature of the mixed polymer is about167?,and the melting temperature of the graphene composites fluctuates around170?.Each component material begins to lose weight at 400?,which is much higher than its melting temperature.The results can ensure the stable work of electrocaloric materials in the working temperature range.(4)The temperature change?T,entropy change?S and the electrocaloric coefficient?T/?E,?S/?E of the materials were calculated by phenomenological theory.The maximum?T and?S of the mixed copolymers can reach 1.5 K and 7.1 J?kg^-1?K^-1respectively.When the mass fraction is 0.5wt.%,the?T and?S of the graphene composite are 4.8 K and 22.6 J?kg^-1?K^-1.Under the same condition,the?T and?S of the graphene composite is three times than the basis materials.The electrocaloric effect coefficient also showed the same trend.