Phase Structures And Electrocaloric Effect Of PVDF-Based Composites

Refrigeration is an essential part of daily life.Refrigeration is needed from the cooling of fruits and vegetables to the use of air conditioners in everyday life.At present,when chlorofluorocarbons released by traditional refrigerating modes rise to the ozone layer,they react with ozone to destroy the ozone layer.The ozone layer plays a role in protecting the living organisms and plants on the earth.Therefore,to look for new refrigeration ways has become an urgent task.The electrocaloric effect is a change of the adiabatic temperature or the isothermal entropy caused by the change of the external electric field in the polar material resulting in the change of the polarization state.The technology using in electrocaloric effect is believed possessing the advantages of low cost,high energy conversion efficiency and environmental protection.Among the series of electrocaloric effect materials,perovskite ceramic material has high dielectric constant and large modulus,but it is not easy to process molding and low breakdown field strength;the dielectric constant of fluoropolymer is low and its modulus is low,but its plasticity is good,and the breakdown field is high.In this work BST ceramic powders are prepared by solid-state reaction method and sintered.And P(VDF-HFP)/BST composites are prepared by solution casting method.The composite material is found to have a high dielectric constant and a low brittleness density.The main works of the dissertation is as follows:1)P(VDF-HFP)thin films were prepared by solution casting mothed which is proved possessing high dielectric properties.2)In order to obtain BST ceramic powders with a fine granularity and the improving electric properties,it was prepared by solid-state sintering method.After pressing and sintering,the BST samples show a high the dielectric constant and loss of 5100 and 0.0014,Curie temperature is 28 degrees centigrade,a dielectric breakdown strength of 35kV/cm,a polarization value of 16uc/cm2,and the favorable electrocaloric effect(AT = 0.93?).3)A series of P(VDF-HFP)/BST composite films with different mass fraction were prepared by solution casting.The effects of the introduction of different mass fraction of BST ceramics on the electrical properties and the electrocaloric effect of the composites were investigated.The results showed that the dielectric constant and dielectric loss were increased first and then decreased with the increase of the ceramic mass fraction.When the mass fraction of ceramic BST is 20%wt,the maximum value is reached.The dielectric constant is 26 and the dielectric loss is 0.07.Under the condition of variable temperature,the measured mass ratio of 20%P(VDF-HFP)/BST composite material was reduced from 4.34?C/cm2 to 3.85 ?C/cm2,and the electrocaloric effect was calculated to be 3.9?.The results show that the introduction of perovskite ceramics can greatly improve the dielectric properties and electrocaloric effect of PVDF-based fluoropolymer materials,providing a reference for the application of composite materials in the field of improving the electrocaloric effect.