Study On Dielectric Storage Energy Properties And Relaxation Behaviors Of Composite Films Based On Poly(Vinylidene Fluoride)

With the electronics and electrical power systems,dielectric materials for energy storage have attracted more and more attention.Desired its properties include high energy storage,high charge-discharge efficiency,high breakdown strength,and high operation temperature.Compared with traditional ceramic materials,polymer dielectrics are low-cost,possess high breakdown strength,low dielectric loss,and fail gracefully.One major challenge in developing dielectric polymers is realizing high energy density while maintaining low dielectric loss,even when high electric fields are applied.The traditional dielectric polymers have a relatively low dielectric constant(about 2 ~ 3)and energy density(below 3 J/cm~3)?Thus,in this paper,P(VDF-HFP)with high dielectric constant is used as matrix,and is blended with different polymer to prepare polymer composite films with high energy density and low ferroelectric loss through different processing technology and controlling microstructure.The detailed information is given as follows:P(VDF-HFP)/PC composite films were fabricated by solution method.The results showed that the spherical phase of PC achieved uniform dispersion in P(VDF-HFP)copolymer.As the content of PC was increased,P(VDF-HFP)was induced phase transition and the formation of less amount of g-2b crystal,the crystallinity and crystalline temperature of P(VDF-HFP)decreased.Moreover,the Curie temperature,dielectric constant and loss of composite films all decreased,and possessed excellent frequency stability and temperature stability.The breakdown strengths of composite films were improved.Furthermore,the electric hysteresis loops of composite films showed that remnant polarization decreased which induced the diminution of hysteresis loops,and tended to linear dielectric.When the content of PC was 7%,the discharge energy density of the composite film was up 5.88 J/cm~3 at 325 MV/m,and discharge efficiency was60.68%.The P(VDF-HFP)/PMMA composite films which had good compatibility were fabricated by solution method.The results showed that the phase transition of P(VDF-HFP)happened,and formed large amount of g-2b crystal.The crystallinity of composite films decreased,but melting temperature increased with the addition of PMMA increasing.Moreover,the dielectric constant and loss of composite films all dropped more or less,and the frequency stability and temperature stability of dielectric properties were enhanced.Meanwhile,the breakdown strengths of composite films were improved significantly,the breakdown strength of 10% PMMA film was up 542.62 MV/m.Furthermore,the storage properties of composite films showed that remnant polarization and coercive field decreased sharply,and the electric hysteresis loop tended to linear dielectric.When the content of PMMA was above 10%,discharge efficiency of composite film was up 80% at300 MV/m,and greater than 45% of P(VDF-HFP)at 267 MV/m.The miscible P(VDF-HFP)/PMMA composite films were prepared by two-way synchronous tensile machine.The results showed that the composite films formed a certain amount of g-2b crystal due to the effect of biaxially oriented and PMMA,and promoted grain refinement and the decrease of crystallinity.Moreover,the dielectric constant and loss of biaxially oriented composite films decreased,and the excellent frequency stability and temperature stability were improved.Meanwhile,the breakdown strengths of composite films increased.The breakdown strength of biaxially oriented 10%PMMA film was up 561.04 MV/m.In addition,the remnant polarization and coercive field of biaxially oriented composite films decreased significantly,and tended to linear dielectric.The discharge energy density of biaxially oriented 10% PMMA film was up17.7 J/cm3 at 575 MV/m,and discharge efficiency maintained above 65%.The results of dielectric relaxation spectroscopy show: For P(VDF-HFP)/PC composite films of solution blending,the imaginary part of complex permittivity linearly decreased with frequency increasing at high temperature,and dc conductivity extended to high frequency with temperature increasing.With the addition of PC,the relaxation time broadened,relaxation strength and activation energy increased,showing that relaxation behaviors of the composite films were multiplicity.For P(VDF-HFP)/PMMA composite films obtained by solution blending,the three relaxation process were found: aa,ac and conductivity relaxation.With the introduction of PMMA,the relaxation time of aa and ac broadened,dc conductivity markedly reduced;the activation energy of aa dropped and ac was enhanced,implying that PMMA had dilution effect at low temperature and restricted segment motion near crystal at high temperature.For biaxially oriented P(VDF-HFP)/PMMA composite films,the aa,ac and conductivity relaxation behaviors also appeared.Under the effects of biaxially oriented and PMMA,the relaxation time of a a and a c broadened,and their relaxation strengths and activation energies significantly decreased,dc conductivity also obviously reduced.These indicated that the biaxially oriented and PMMA had synergistic effect.