Research On Pyroelectric Energy Harvesting And Electrocaloric Properties Of PVDF-based Composites

PVDF(Polyvinylidene Fluoride)-based ferroelectric polymers have limitless applications in wearable electronic products,medical sensors,portable electronic devices in terms of their typical advantages,e.g.,lightweight,mechanical flexibility,environment friendly and biocompatibility.PVDF-based polymers have attracted interests in room-temperature pyroelectric energy harvesting and electrocaloric cooling due to their particular chain conformation which can produce pyroelectric effect.But the limited pyroelectric energy harvesting density(ND)and relatively low cooling power density of ferroelectric materials pose challenging obstacles for the practical application of the low grade heat harvesting and electrocaloric cooling.Compared to other ferroelectric materials,the polymer nanocomposites are the base and precursor of the further development of pyroelectric energy harvesting and electrocaloric cooling.To improve the properties of pyroelectric and electrocaloric materials,we firstly studied the pyroelectric energy harvesting property of P(VDF-TrFE-CFE)(Poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene)matrix and(Ba1-xSrx)TiO3(BST)fillers,respectively,and then focused on the electrocaloric performance of the nanocomposites with P(VDF-TrFE-CFE)as matrix and BST NWs(BST nanowires)as fillers.Our main works are listed as follows:(1)Ter-xCo(Terpolymer-Copolymer)and Co-xTer(Copolymer-Terpolymer)polymer blends have been prepared to improve the ND according to the Maxwell-Wagner-Sillars(MWS)interfacial polarization theory.Ter-xCo and Co-xTer polymer blends mean the P(VDF-TrFE-CFE)/P(VDF-TrFE)(Poly(vinylidene fluoride-trifluoroethylene)blends with P(VDF-TrFE-CFE)as the matrix or additives.As a result,two kinds of interfacial effect were existed in the polymer blends at the low content(15 wt.%)of additives,e.g.,intermolecular interactions among polymer chains and MWS interfacial polarizations.The synergistic integration of the interfacial effect are benefit for improving ND.The Tc of ferroelectric materials have a great effect on the operation temperature windows,e.g.,as the temperature changes from 25℃to 50℃,Ter-10Co blends exhibits the highest N_D of 0.86J/cm~3 at 150 MV/m,three times higher than the pure P(VDF-TrFE-CFE),while Co-10Ter blends possess optimum N_D of 1.2 J/cm~3 during temperature range from 25℃to 65℃,more than 20%of pure P(VDF-TrFE).(2)BST ceramics with various Tc have prepared by the traditional solid state sintering methods.The property of dielectric,ferroelectric and pyroelectric energy harvesting of BST ceramics have been studied systematically.The results suggest that the value of Tc determined the working temperature range of the harvesters.As the electric field changes from 0 MV/m to 7 MV/m,the maximum ND of 0.48 J/cm~3 is obtained in BST72/28(Tc=43℃)during higher temperature range from 25℃to 85℃.But the optimized N_D of 0.36J/cm~3 is obtained in BST66/34(T_c=23℃)during lower temperature range from 25℃to65℃.Based on the previous research,BST NWs with designed T_c used for nanocomposites preparation were synthesized successfully using the two-step hydrothermal reaction by tuning the addition amount of Ba(OH)2?8H2O and Sr(OH)2?8H2O.(3)The nanocomposites of BST NWs with various Tc and P(VDF-TrFE-CFE)have been produced by solution casting method.The influence of the inclusions with single or multiple BST NWs on electrocaloric strength and the temperature stability have been studied.As a result,substantial improvements in electrocaloric strength have been achieved in the nanocomposites with single BST NWs in comparison to that of the neat P(VDF-TrFE-CFE).Optimized electrocaloric properties are always obtained during the narrow temperature window near Tc.Thus the temperature stability of electrocaloric properties were not better than the nanocomposites with multiple BST NWs which exhibited the same giant electrocaloric strength.For instance,the multiple nanocomposite exhibits a remarkable temperature stability of>90%in the temperature range from 0°C to 60°C,which is significantly superior to the single-composition BST NWs filled nanocomposites with a temperature stability of～50%.Thus,the nanocomposites used for room temperature cooling possess multifunctional properties of large electrocaloric strength and remarkable temperature stability.(4)BNNSs(Boron Nitride Nanosheets)were firstly prepared by liquid exfoliation,and then added to the P(VDF-TrFE-CFE)-BST NWs matrix to form multiple P(VDF-TrFE-CFE)-BNNSs-BST NWs nanocomposites.It is found that the introduction of BNNSs in the binary nanocomposites resulted in remarkable enhancements in dielectric breakdown strength.As a result,the safety of multiple nanocomposites for low-field application and the feasibility for high-field application have been greatly improved.Under a relatively low electric field of 75 MV/m,a sizable electrocaloric strength,that is,Q of 25 MJ/m~3,ΔS of 70kJ/(m~3K),andΔT of 8°C,is obtained in the nanocomposites,which is comparable to that of the P(VDF-TrFE-CFE)-BST NWs binary nanocomposites.In addition,high temperature stability of～90%was also obtained in temperature range from 0°C to 60°C.As analyzed by the finite element method,the influence of the BST NWs and BNNSs on thermal conductivity property,cooling power densities and efficiencies were investigated.As a result,the multiple nanocomposites with high thermal conductivity are benefit for generating high cooling power densities and efficiencies due to the high heat transfer rate.Electrocaloric heating and cooling energy can be absorbed unimpededly by the regenerator within low time interval of 0.4 s.With an operating frequency of 16 Hz and electric field of 75 MV/m,the cooling power density and efficiency of the multiply-nanocomposites were 150 W/cm~3 and7,respectively,which is more than 3 times higher than that of pure P(VDF-TrFE-CFE).