@article{HeWangHeetal.2022, author = {He, Yushuang and Wang, Feipeng and He, Li and Wang, Qiang and Li, Jian and Qian, Yihua and Gerhard, Reimund and Plath, Ronald}, title = {An insight Into the role of Nano-Alumina on DC Flashover-Resistance and surface charge variation of Epoxy Nanocomposites}, series = {IEEE transactions on dielectrics and electrical insulation}, volume = {29}, journal = {IEEE transactions on dielectrics and electrical insulation}, number = {3}, publisher = {Inst. of Electr. and Electronics Engineers}, address = {Piscataway}, issn = {1070-9878}, doi = {10.1109/TDEI.2022.3173510}, pages = {1022 -- 1029}, year = {2022}, abstract = {The addition of nano-Al2O3 has been shown to enhance the breakdown voltage of epoxy resin, but its flashover results appeared with disputation. This work concentrates on the surface charge variation and dc flashover performance of epoxy resin with nano-Al2O3 doping. The dispersion of nano-Al2O3 in epoxy is characterized by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The dc flashover voltages of samples under either positive or negative polarity are measured with a finger-electrode system, and the surface charge variations before and after flashovers were identified from the surface potential mapping. The results evidence that nano-Al2O3 would lead to a 16.9\% voltage drop for the negative flashovers and a 6.8\% drop for positive cases. It is found that one-time flashover clears most of the accumulated surface charges, regardless of positive or negative. As a result, the ground electrode is neighbored by an equipotential zone enclosed with low-density heterocharges. The equipotential zone tends to be broadened after 20 flashovers. The nano-Al2O3 is noticed as beneficial to downsize the equipotential zone due to its capability on charge migration, which is reasonable to maintain flashover voltage at a high level after multiple flashovers. Hence, nano-Al2O3 plays a significant role in improving epoxy with high resistance to multiple flashovers.}, language = {en} } @article{WangZhangYanetal.2022, author = {Wang, Feipeng and Zhang, Zheng and Yan, Yuyang and Shen, Zijia and Wang, Qiang and Gerhard, Reimund}, title = {Surface reconstruction on electro-spun PVA/PVP nanofibers by water evaporation}, series = {Nanomaterials}, volume = {12}, journal = {Nanomaterials}, number = {5}, publisher = {MDPI}, address = {Basel}, issn = {2079-4991}, doi = {10.3390/nano12050797}, pages = {7}, year = {2022}, abstract = {Tailoring the secondary surface morphology of electro-spun nanofibers has been highly desired, as such delicate structures equip nanofibers with distinct functions. Here, we report a simple strategy to directly reconstruct the surface of polyvinyl alcohol/polyvinylpyrrolidone (PVA/PVP) nanofibers by water evaporation. The roughness and diameter of the nanofibers depend on the temperature during vacuum drying. Surface changes of the nanofibers from smooth to rough were observed at 55 degrees C, with a significant drop in nanofiber diameter. We attribute the formation of the secondary surface morphology to the intermolecular forces in the water vapor, including capillary and the compression forces, on the basis of the results from the Fourier-transform infrared (FTIR) and X-ray photoelectron (XPS) spectroscopy. The strategy is universally effective for various electro-spun polymer nanofibers, thus opening up avenues toward more detailed and sophisticated structure design and implementation for nanofibers.}, language = {en} } @article{WangFruebingWirgesetal.2010, author = {Wang, Feipeng and Fr{\"u}bing, Peter and Wirges, Werner and Gerhard, Reimund and Wegener, Michael}, title = {Enhanced Polarization in Melt-quenched and Stretched Poly(vinylidene Fluoride-Hexafluoropropylene) Films}, issn = {1070-9878}, doi = {10.1109/TDEI.2010.5539679}, year = {2010}, abstract = {beta-phase poly(vinylidene fluoride-hexafluoropropylene) (P(VDF-HFP)) copolymer films were prepared by uniaxially stretching solution-cast or melt-quenched samples. Different preparation routes lead to different amounts of the crystalline alpha and beta phases in the films, as detected by means of Fourier-transform infrared spectroscopy and X-ray diffractometry. The beta phase is significantly enhanced in melt-quenched and stretched films in comparison to solution-cast and stretched films. This is particularly true for copolymer samples with higher HFP content. The beta- phase enhancement is also observed in ferroelectric-hysteresis experiments where a rather high polarization of 58 mC/ m(2) was found on melt-quenched and stretched samples after poling at electric fields of 140 MV/m. After poling at 160 MV/m, one of these samples exhibited a piezoelectric d(33) coefficient as high as 21 pC/N. An electric-field-induced partial transition from the alpha to the beta phase was also observed on the melt-quenched and stretched samples. This effect leads to a further increase in the applications-relevant dipole polarization. Uniaxially stretched ferroelectric- polymer films are highly anisotropic. Dielectric resonance spectroscopy reveals a strong increase of the transverse piezoelectric d(32) coefficient and a strong decrease of the transverse elastic modulus c(32) upon heating from 20 to 50 degrees C.}, language = {en} } @article{ChinagliaGregorioStefanelloetal.2010, author = {Chinaglia, Dante Luis and Gregorio, Rinaldo and Stefanello, Josiani Cristina and Altafim, Ruy Alberto Pisani and Wirges, Werner and Wang, Feipeng and Gerhard, Reimund}, title = {Influence of the solvent evaporation rate on the crystalline phases of solution-cast poly(vinylidene fluoride) films}, issn = {0021-8995}, doi = {10.1002/App.31488}, year = {2010}, abstract = {The influence of the solvent-evaporation rate on the formation of of. and P crystalline phases in solution-cast poly(vinylidene fluoride) (PVDF) films was systematically investigated. Films were crystallized from PVDF/N,N- dimethylformamide solutions with concentrations of 2.5, 5.0, 10, and 20 wt \% at different temperatures. During crystallization, the solvent evaporation rate was monitored in situ by means of a semianalytic balance. With this system, it was possible to determine the evaporation rate for different concentrations and temperatures of the solution under specific ambient conditions (pressure, temperature, and humidity). Fourier-Transform InfraRed spectroscopy with Attenuated Total Reflectance revealed the P-phase content in the PVDF films and its dependence on previous evaporation rates. Based on the relation between the evaporation rate and the PVDF phase composition, a consistent explanation for the different amounts of P phase observed at the upper and lower sample surfaces is achieved. Furthermore, the role of the sample thickness has also been studied. The experimental results show that not only the temperature but also the evaporation rate have to be controlled to obtain the desired crystalline phases in solution-cast PVDF films.}, language = {en} } @article{FangWangWirgesetal.2011, author = {Fang, Peng and Wang, Feipeng and Wirges, Werner and Gerhard, Reimund and Basso, Heitor Cury}, title = {Three-layer piezoelectrets from fluorinated ethylene-propylene (FEP) copolymer films}, series = {Applied physics : A, Materials science \& processing}, volume = {103}, journal = {Applied physics : A, Materials science \& processing}, number = {2}, publisher = {Springer}, address = {New York}, issn = {0947-8396}, doi = {10.1007/s00339-010-6008-2}, pages = {455 -- 461}, year = {2011}, abstract = {A process for preparing three-layer piezoelectrets from fluorinated ethylene-propylene (FEP) copolymer films is introduced. Samples are made from commercial FEP films by means of laser cutting, laser bonding, electrode evaporation, and high-field poling. The observed dielectric-resonance spectra demonstrate the piezoelectricity of the FEP sandwiches. Piezoelectric d (33) coefficients up to a few hundred pC/N are achieved. Charging at elevated temperatures can increase the thermal stability of the piezoelectrets. Isothermal experiments for approximately 15 min demonstrate that samples charged at 140A degrees C keep their piezoelectric activity up to at least 120A degrees C and retain 70\% of their initial d (33) even at 130A degrees C. Acoustical measurements show a relatively flat frequency response in the range between 300 Hz and 20 kHz.}, language = {en} } @article{WangLackXieetal.2012, author = {Wang, Feipeng and Lack, Alexander and Xie, Zailai and Fr{\"u}bing, Peter and Taubert, Andreas and Gerhard, Reimund}, title = {Ionic-liquid-induced ferroelectric polarization in poly(vinylidene fluoride) thin films}, series = {Applied physics letters}, volume = {100}, journal = {Applied physics letters}, number = {6}, publisher = {American Institute of Physics}, address = {Melville}, issn = {0003-6951}, doi = {10.1063/1.3683526}, pages = {4}, year = {2012}, abstract = {Thin films of ferroelectric beta-phase poly(vinylidene fluoride) (PVDF) were spin-coated from a solution that contained small amounts of the ionic liquid (IL) 1-ethyl-3-methylimidazolium nitrate. A remanent polarization of 60 mC/m(2) and a quasi-static pyroelectric coefficient of 19 mu C/m(2)K at 30 degrees C were observed in the films. It is suggested that the IL promotes the formation of the beta phase through dipolar interactions between PVDF chain-molecules and the IL. The dipolar interactions are identified as Coulomb attraction between hydrogen atoms in PVDF chains and anions in IL. The strong crystallinity increase is probably caused by the same dipolar interaction as well.}, language = {en} } @article{GrecoWangWegener2010, author = {Greco, Tonino and Wang, Feipeng and Wegener, Michael}, title = {Multifunctional silver poly(vinylidene fluoride) nanocomposites : nanoparticle synthesis, film processing, and structural characterization}, issn = {0015-0193}, doi = {10.1080/00150193.2010.482896}, year = {2010}, abstract = {Scope of this work was the synthesis of homogeneously dispersed silver nanoparticles in the ferroelectric polymer poly(vinylidene fluoride) (PVDF) and the study of the resulting properties affecting both the electro-active matrix and the optically-active nanofiller. In the nanocomposites surface plasmon resonances can be tuned across the UV- vis to the NIR spectral range. From IR spectra and DSC measurements it is concluded that the - to -phase transformation is observed and no degradation of the polymer matrix occurs. Finally, electrical poling was performed in order to investigate the influence of the embedded silver particles on the polarization behavior of the ferroelectric polymer.}, language = {en} } @article{FruebingWangWagener2012, author = {Fr{\"u}bing, Peter and Wang, Feipeng and Wagener, Michael}, title = {Relaxation processes and structural transitions in stretched films of polyvinylidene fluoride and its copolymer with hexafluoropropylene}, series = {Applied physics : A, Materials science \& processing}, volume = {107}, journal = {Applied physics : A, Materials science \& processing}, number = {3}, publisher = {Springer}, address = {New York}, issn = {0947-8396}, doi = {10.1007/s00339-012-6838-1}, pages = {603 -- 611}, year = {2012}, abstract = {Relaxation processes and structural transitions in nonstretched and uniaxially stretched films of poly(vinylidene fluoride-hexafluoropropylene) (P(VDF-HFP)) and its homopolymer polyvinylidene fluoride (PVDF) for comparison were investigated with the aim of understanding the electromechanical properties of this lower-modulus ferroelectric copolymer. The mechanical and the dielectric response at the glass transition ( relaxation) exhibit similar temperature dependence of the relaxation time, whereas mechanical and dielectric processes above the glass transition are not related. They represent a continuous softening process within the amorphous phase and the dielectric relaxation, respectively. The latter is attributed to conformational changes of VDF segments in lamellae of spherulites constituting the nonpolar crystalline phase. Furthermore, there is a contribution from melting of secondary crystallites formed in the amorphous phase during annealing or storage. Mechanically, this transition appears in nonstretched and stretched films as an accelerated decrease of the elastic modulus that terminates the rubber plateau. Dielectrically, this transition becomes visible as a frequency-independent loss peak only in stretched films, because stretching removes the relaxation, which superimposes the transition in nonstretched films. Melting of secondary crystallites is shown to appear in the homopolymer, too, though less pronounced because of more complete primary crystallisation. Stretching increases the modulus above the glass transition only slightly, and it does not significantly influence the softening process. On the other hand, stretching causes a spontaneous polarisation and introduces order within the amorphous phase, rendering it more polar. Melting of secondary crystallites provides an additional contribution to the polarisation. These findings may explain the relatively high electromechanical activity of P(VDF-HFP) but also its relatively low thermal stability. Moreover, they may be important for correct procedure and analysis of temperature-dependent dielectric measurements on partially crystalline polymers, in particular on those with less favourable sterical conditions for primary crystallisation.}, language = {en} } @article{XieJelicicWangetal.2010, author = {Xie, Zai-Lai and Jelicic, Aleksandra and Wang, Feipeng and Rabu, Pierre and Friedrich, Alwin and Beuermann, Sabine and Taubert, Andreas}, title = {Transparent, flexible, and paramagnetic ionogels based on PMMA and the iron-based ionic liquid 1-butyl-3- methylimidazolium tetrachloroferrate(III) [Bmim][FeCl4]}, issn = {0959-9428}, doi = {10.1039/C0jm01733g}, year = {2010}, abstract = {The iron-containing ionic liquid (IL) 1-butyl-3-methylimidazolium tetrachloroferrate(III) [Bmim][FeCl4] has been used as a building block in the synthesis of transparent, ion-conducting, and paramagnetic ionogels. UV/Vis spectroscopy shows that the coordination around the Fe(III) ion does slightly change upon incorporation of the IL into PMMA. The thermal stability of the PMMA increases significantly with IL incorporation. In particular, the onset weight loss observed at ca. 265 degrees C for pure PMMA is completely suppressed. The ionic conductivity shows a strong temperature dependence and increases with increasing IL weight fractions. The magnetic properties are similar to those reported for the pure IL and are not affected by the incorporation into the PMMA matrix. The resulting ionogel is thus an interesting prototype for soft, flexible, and transparent materials combining the mechanical properties of the matrix with the functionality of the metal-containing IL, such as magnetism.}, language = {en} }