A pinboard by
Xinyue Chen

I am a Ph.D. student in polymer engineering at Case Western Reserve University.


Multilayer films serve as dielectrics for high electric constant and low loss film capacitors

Polymer materials are now a everwhere in our lifes! With recent developments, polymer film capacitors become increasingly important for advanced power applications such as DC-link capacitors in electric/hybrid vechicles, defribrillators and pulsed power. "State-of-the-art" biaxially oriented polypropylene (BOPP) film is the most commonly used commercial capacitor film. BOPP film has the advantage of high breakdown strength, and extremely low loss. However, its low operating temperature (up to 85°C), low dielectric constant (2.25) limit its use in applications requiring high temperature operation and compact design. Therefore researchers are working hard to develop polymer films with high dielectric constant, low loss and high operation temperature. However, it's a delimma that polymers with high dielectric constant (e.g. Polyvinylidene fluoride, nylon) usually have high loss, whereas linear dielectrics (e.g. Polycabobate, polysulfone) with low loss ofen have low dielectric constant. At Case Western Reserve University , we multilayer coextrude a high breakdown strength linear dielectric material and high dielectric constant material, which combines materials' good properties.The obtained nanolayered polymer films also have interfacial polarization that could enhance the insulation property of polymer films.


Semicrystalline Structure-Dielectric Property Relationship and Electrical Conduction in a Biaxially Oriented Poly(vinylidene fluoride) Film under High Electric Fields and High Temperatures.

Abstract: Poly(vinylidene fluoride) (PVDF)-based homopolymers and copolymers are attractive for a broad range of electroactive applications because of their high dielectric constants. Especially, biaxially oriented PVDF (BOPVDF) films exhibit a DC breakdown strength as high as that for biaxially oriented polypropylene films. In this work, we revealed the molecular origin of the high dielectric constant via study of a commercial BOPVDF film. By determination of the dielectric constant for the amorphous phase in BOPVDF, a high value of ca. 21-22 at 25 °C was obtained, and a three-phase (i.e., lamellar crystal/oriented interphase/amorphous region) semicrystalline model was proposed to explain this result. Meanwhile, electronic conduction mechanisms in BOPVDF under high electric fields and elevated temperatures were investigated by thermally stimulated depolarization current (TSDC) spectroscopy and leakage current studies. Space charge injection from metal electrodes was identified as a major factor for electronic conduction when BOPVDF was poled above 75 °C and 20 MV/m. In addition, when silver or aluminum were used as electrodes, new ions were generated from electrochemical reactions under high fields. Due to the electrochemical reactions between PVDF and the metal electrode, a question is raised for practical electrical applications using PVDF and its copolymers under high-field and high-temperature conditions. A potential method to prevent electrochemical degradation of PVDF is proposed in this study.

Pub.: 30 Jun '15, Pinned: 28 Jun '17

Effects of Interphase Modification and Biaxial Orientation on Dielectric Properties of Poly(ethylene terephthalate)/Poly(vinylidene fluoride-co-hexafluoropropylene) Multilayer Films.

Abstract: Recently, poly(vinylidene fluoride) (PVDF)-based multilayer films have demonstrated enhanced dielectric properties, combining high energy density and high dielectric breakdown strength from the component polymers. In this work, further enhanced dielectric properties were achieved through interface/interphase modulation and biaxial orientation for the poly(ethylene terephthalate)/poly(methyl methacrylate)/poly(vinylidene fluoride-co-hexafluoropropylene) [PET/PMMA/P(VDF-HFP)] three-component multilayer films. Because PMMA is miscible with P(VDF-HFP) and compatible with PET, the interfacial adhesion between PET and P(VDF-HFP) layers should be improved. Biaxial stretching of the as-extruded multilayer films induced formation of highly oriented fibrillar crystals in both P(VDF-HFP) and PET, resulting in improved dielectric properties with respect to the unstretched films. First, the parallel orientation of PVDF crystals reduced the dielectric loss from the αc relaxation in α crystals. Second, biaxial stretching constrained the amorphous phase in P(VDF-HFP) and thus the migrational loss from impurity ions was reduced. Third, biaxial stretching induced a significant amount of rigid amorphous phase in PET, further enhancing the breakdown strength of multilayer films. Due to the synergistic effects of improved interfacial adhesion and biaxial orientation, the PET/PMMA/P(VDF-HFP) 65-layer films with 8 vol.% PMMA exhibited optimal dielectric properties with an energy density of 17.4 J/cm3 at breakdown and the lowest dielectric loss. These three-component multilayer films are promising for future high energy density film capacitor applications.

Pub.: 11 May '16, Pinned: 28 Jun '17