Study Of PVFD-based Dielectric Membranes

With the development of electronic industry，dielectric materials withhigh permittivity （k） and low dielectric loss are attracted much attentions.Traditional ceramic materials have high permittivity, but they are brittle, hardto fabricate, and have high dielectric loss. Besides, they will generatemechanical resonance during charging and discharging and thus reduced theirlife-span. Polymers are light-weight, flexible, and easily integrated. However,they show a relatively low permittivity. Much work has been focused onraising the k of polymer-based composites. One effective method is to addhigh-k ceramics into the polymers to form organic-ceramic composites. Highloadings of ceramic fillers can increase the k of the composites, butdramatically decrease the flexibility of the composites. Another method is tointroduce conductive materials into the polymer matrix to form percolativecomposites. The k of such composites is at least two orders of magnitudehigher than that of the polymer matrix. However, the dielectric loss increasesrapidly with increasing amount of conductive materials. Therefore, it is ofgreat importance to prepare polymer-based composites with high dielectricperformance.In this paper, Poly（vinylidene fluoride）（PVDF） and poly（vinylidene -trifluoroethylene）（PVDF-TrFE） were chosen as the polymer matrix anddifferent membranes were prepared through solution cast method. Themicrostructure， crystallization behavior， and dielectric properties werecharacterized and the corresponding reasons were studied. The detailedinformation is listed as follows:（1） Titanium dioxide nanoparticles with two different phases wereprepared by the sol-gel method. Poly（vinylidene-trifluoroethylene）/titaniumdioxide （TiO₂/PVDF-TrFE） membranes were prepared by the solution castmethod. Scanning electron microscope （SEM） results showed that the TiO₂nanoparticles were well dispersed in the polymer matrix and did not affect thestructure of the PVDF-TrFE matrix. Differential scanning calorimeter （DSC）results showed that crystallinity of TiO₂/PVDF-TrFE membranes increased asthe addition of TiO₂with two different phases. Besides, the crystallinity ofTiO₂/PVDF-TrFE membrane with the rutile TiO₂is higher than that of themembrane with the anatase TiO₂. Dielectric property testing showed that thepermittivity of TiO₂/PVDF-TrFE membrane increased rapidly with theincrease of TiO₂content, but the dielectric loss remains at a low level. Theincrease in permittivity is attributed to the interfacial polarization. At the sameTiO₂content, the permittivity of TiO₂/PVDF-TrFE membrane with the rutileTiO₂is higher than that of the membrane with the anatase TiO₂. Hightemperature is beneficial to the molecular polarization of PVDF-TrFE and alsoleads to the increase in conductivity of the semiconducting TiO₂nanoparticles, and thus increasing the permittivity and dielectric loss of the membranes.（2） Nickel-doped titanium dioxide （Ni-TiO₂） nanoparticles were preparedby the sol-gel method, the particle size is about30nm. TheNi-TiO₂/PVDF-TrFE membranes waere prepared and then characterized bySEM, X-ray diffraction （XRD）, and dielectric property testing. XRD resultsshowed that the phase of PVDF-TrFE was changed from the polar β crystalphase to non-polar phase with the addition of Ni-TiO₂nanoparticles. Thedielectric property testing showed that the permittivity of the membranes wereimproved gradually with the increase of Ni-TiO₂content, and the dielectricloss keeps at a low level （0.046⁰.064）. Besides, the permittivity of themembranes increased with the increase of nickel content in Nix-TiO₂nanoparticles, while the dielectric loss has not been changed。（3） Na₂Ti₂O₄（OH）₂nanotube was prepared through a hydrothermalmethod and the structure of Na₂Ti₂O₄（OH）₂nanotube were characterized.Then the Na₂Ti₂O₄（OH）₂/PVDF membranes were prepared and thecorresponding properties were studied. The cross-sectional SEM images of themembranes showed that the Na₂Ti₂O₄（OH）₂nanotube were well dispersed inthe PVDF matrix and did not affect the structure of PVDF. The gooddispersion is attribute to the hydrogen bond through the oxhydryl group fromthe Na₂Ti₂O₄（OH）₂nanotube and the fluorine atom from PVDF. With theincrease of Na₂Ti₂O₄（OH）₂, Na₂Ti₂O₄（OH）₂connect with each other andformed network in PVDF matrix. DSC results showed that the crystallinity of the membranes were affect by the addition of Na₂Ti₂O₄（OH）₂nanotube. Smallamount of nanotube were embedded by the PVDF matrix and beneficial to thecrystallization of PVDF on the surface because of its large surface area, thusincreased the crystallinity of membranes. High content of nanotube willdestroy the conformation of PVDF molecular chain and thus reduced thecrystallinity of the membranes. The permittivity of Na₂Ti₂O₄（OH）₂/PVDF andNa₂Ti₂O₄（OH）₂/PVDF-TrFE membranes were improved quickly with theincrease of Na₂Ti₂O₄（OH）₂. When the content of Na₂Ti₂O₄（OH）₂nanotube isabout18vol%, the permittivity of the Na₂Ti₂O₄（OH）₂/PVDF membrane is upto178.7at100Hz, which is about20times of the PVDF. At the sameNa₂Ti₂O₄（OH）₂nanotube content, the permittivity of thePVDF-TrFE/Na₂Ti₂O₄（OH）₂membrane is up to330, about25times ofPVDF-TrFE. By calculation, the results are well fit the percolative model.Therefore, the dielectric properties of the membranes were improveddramatically with the addtion of Na₂Ti₂O₄（OH）₂nanotube, and thecorresponding literature has not been reported since now.（4） The Ag@C core-shell nanoparticles were prepared by a hydrothermalmethod. A typical core-shell structure can be observed, the silver cores withdiameters in the range of100-120nm are each covered with a carbon shellabout60-80nm thick. Ag@C/PVDF and Ag@C/PVDF-TrFE membranes wereprepared through the solution cast method. The cross-sectional SEM imagesshowed that the Ag@C particles are well dispersed in both membranes. The good dispersion is attributed to the carbon shell which not only act asinter-particle barriers to prevent direct connection of Ag nanoparticles, butalso produce excellent compatibility between the fillers and the polymermatrix. DSC results showed that the crystallinity of the two membranesdecreased with the increase of Ag@C content. The dielectric propertiesshowed that the permittivity of the Ag@C/PVDF and Ag@C/PVDF-TrFEmembranes improved obviously over the pure polymer with increasingcontent of Ag@C particles due to the enhanced interfacial polarization, butThe dielectric loss keeps low lever. The permiitivity and the dielectric loss ofthe two membranes are affected by the temperature. High temperature isbenefit to the molecular polarization of the polymer and also accelerates themigration of free electron from the Ag core, and thus increasing thepermittivity and dielectric loss of the composites.