@article{GulyakovaGorokhovatskyfruebingetal.2017,
  author    = {Gulyakova, Anna A. and Gorokhovatsky, Yuri A. and fr{\"u}bing, Peter and Gerhard, Reimund},
  title     = {Relaxation Processes Determining the Electret Stability of High-Impact Polystyrene/Titanium-Dioxide Composite Films},
  series = {IEEE transactions on dielectrics and electrical insulation},
  volume    = {24},
  journal   = {IEEE transactions on dielectrics and electrical insulation},
  publisher = {Inst. of Electr. and Electronics Engineers},
  address   = {Piscataway},
  issn      = {1070-9878},
  doi       = {10.1109/TDEI.2017.006587},
  pages     = {2541 -- 2548},
  year      = {2017},
  abstract  = {The influence of relaxation processes on the thermal electret stability of high-impact polystyrene (HIPS) free-standing films filled with titanium dioxide (TiO2) of the rutile modification are investigated by means of a combination of dielectric methods (dielectric relaxation spectroscopy (DRS), thermally stimulated depolarization current (TSDC) and thermally stimulated surface-potential decay (TSSPD)), supplemented by differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). Films with 2, 4, 6 and 8 vol.\% TiO2 are compared to each other and to non-filled samples. Filling HIPS with up to 8 vol.\% of TiO2 enhances the elastic modulus below the glass transition and increases the thermal electret stability above the glass transition without significantly increasing the DC conductivity. The improvement of the electret stability is caused by the build-up of an interface polarization which decays only gradually if the glass transition is exceeded. Two kinds of Arrhenius processes are considered in order to explain the decay of the composite-polymer electrets: (1) charge release from chemical traps located at the phenyl rings of the polymer chain with an activation energy of E-a = 1.1 eV after passing the glass transition at about 100 degrees C and (2) charge release from traps formed by the TiO2 particles with E-a = 2.4 eV at temperatures above 130 degrees C. Finally, the activation energies are discussed with respect to their significance.},
  language  = {en}
}
@article{RamanVenkatesanGulyakovaGerhard2020,
  author    = {Raman Venkatesan, Thulasinath and Gulyakova, Anna A. and Gerhard, Reimund},
  title     = {Influence of film stretching on crystalline phases and dielectric properties of a 70/30 mol\% poly(vinylidenefluoride-tetrafluoroethylene) copolymer},
  series = {Journal of advanced dielectrics},
  volume    = {10},
  journal   = {Journal of advanced dielectrics},
  number    = {5},
  publisher = {World Scientific},
  address   = {Singapore},
  issn      = {2010-135X},
  doi       = {10.1142/S2010135X2050023X},
  pages     = {10},
  year      = {2020},
  abstract  = {Polyvinylidene fluoride (PVDF)-based copolymers with tetrafluoroethylene (P(VDF-TFE)), trifluoroethylene (P(VDF-TrFE)) or hexafluoropropylene (P(VDF-HFP)) are of strong interest due to the underlying fundamental mechanisms and the potential ferro-, pyro- and piezo-electrical applications. Their flexibility and their adaptability to various shapes are advantageous in comparison to inorganic ferroelectrics. Here, we study the influence of stretching temperature on the crystalline phases and the dielectric properties in P(VDF-TFE) films by means of Dielectric Relaxation Spectroscopy (DRS), Fourier-Transform InfraRed spectroscopy (FTIR), Wide-Angle X-ray Diffraction (WAXD), Differential Scanning Calorimetry (DSC) and Dynamic Mechanical Analysis (DMA). Especially, the effect of stretching and the influence of the temperature of stretching on the mid-temperature (T-mid) transition are studied in detail. The results show that stretching has a similar effect as that on PVDF, and we observe an increase in the fraction of ferroelectric beta-phase with a simultaneous increment in both melting point (T-m) and crystallinity (chi(c)) of the copolymer. While an increase in the stretching temperature does not have a profound impact on the amount of ferroelectric phase, the stability of the ferroelectric phase seems to improve - as seen in the reduction of the Full Width at Half Maximum (FWHM) of the WAXD peaks in both parallel and perpendicular directions to the molecular chain axis. The observation is also supported by the reduction of dissipation losses with an increase in stretching temperature - as seen in DRS measurements. Finally, both stretching itself and the temperature of stretching affect the various molecular processes taking place in the temperature range of the T-mid transition.},
  language  = {en}
}
@article{RamanVenkatesanGulyakovaFruebingetal.2019,
  author    = {Raman Venkatesan, Thulasinath and Gulyakova, Anna A. and Fr{\"u}bing, Peter and Gerhard, Reimund},
  title     = {Electrical polarization phenomena, dielectric relaxations and structural transitions in a relaxor-ferroelectric terpolymer investigated with electrical probing techniques},
  series = {Materials research express},
  volume    = {6},
  journal   = {Materials research express},
  number    = {12},
  publisher = {IOP Publ. Ltd.},
  address   = {Bristol},
  issn      = {2053-1591},
  doi       = {10.1088/2053-1591/ab5352},
  pages     = {7},
  year      = {2019},
  abstract  = {Dielectric Relaxation Spectroscopy (DRS) and Thermally Stimulated Depolarization Current (TSDC) measurements were employed to study dielectric-relaxation processes, structural transitions and electric-polarization phenomena in poly(vinylidenefluoride-trifluoroethylene-chlorofluoroethylene) (P(VDF-TrFE-CFE)) terpolymer films. Results from DRS confirm the existence of two separate dispersion regions related to a para-to-ferroelectric phase transition and to the glass transition. The dipolar TSDC peak correlates with the loss peak of the ? relaxation that represents the glass transition. The electric polarization calculated from the dipolar TSDC peak (glass transition) shows a non-linear electric-field dependence and saturates at high electric poling fields. As the observed behaviour is essentially the same as that of the electric polarization obtained from direct polarization-versus-electric-field hysteresis measurements, TSDC experiments are also suitable for studying the polarization in relaxor-ferroelectric polymers. A saturation polarization of 44 mC m(?2) was found for an electric field of 190 MV m(?1).},
  language  = {en}
}
@article{RamanVenkatesanGulyakovaFruebingetal.2018,
  author    = {Raman Venkatesan, Thulasinath and Gulyakova, Anna A. and Fr{\"u}bing, Peter and Gerhard, Reimund},
  title     = {Relaxation processes and structural transitions in poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) relaxor-ferroelectric terpolymers as seen in dielectric spectroscopy},
  series = {IEEE transactions on dielectrics and electrical insulation},
  volume    = {25},
  journal   = {IEEE transactions on dielectrics and electrical insulation},
  number    = {6},
  publisher = {Institut of Electrical and Electronics Engineers},
  address   = {Piscataway},
  issn      = {1070-9878},
  doi       = {10.1109/TDEI.2018.007440},
  pages     = {2229 -- 2235},
  year      = {2018},
  abstract  = {Dielectric relaxation processes and structural transitions in Poly(vinylidenefluoride-trifluoroethylene-chlorofluoroethylene) (P(VDF-TrFE-CFE)) terpolymer films with two different monomer compositions were investigated in comparison with Poly(vinylidenefluoride-trifluoroethylene) (P(VDF-TrFE)) copolymer films as reference material. Differential Scanning Calorimetry was employed to assess annealing effects on phase transitions and crystalline structure, while relaxation processes were investigated by means of Dielectric Relaxation Spectroscopy, the results of which indicate the existence of two separate dispersion regions, denoted as processes A and B, respectively. Process A appears at a certain temperature independent of frequency, but is strongly influenced by the crystallisation temperature and the CFE content, while peak B shows typical features of a relaxation process and is less influenced by crystallisation temperature and CFE content. Furthermore, peak B is related to the glass transition which is more pronounced in the terpolymer than in P(VDF-TrFE). A closer analysis indicates that the addition of CFE and thermal annealing gradually shift the ferro-to-paraelectric transition in P(VDF-TrFE) to lower temperatures, while the phase transition is transformed more and more into a relaxation.},
  language  = {en}
}