@article{AltafimQiuWirgesetal.2009,
  author    = {Altafim, Ruy Alberto Pisani and Qiu, Xunlin and Wirges, Werner and Gerhard, Reimund and Altafim, Ruy Alberto Pisani and Basso, Heitor Cury and Jenninger, Werner and Wagner, Joachim},
  title     = {Template-based fluoroethylenepropylene piezoelectrets with tubular channels for transducer applications},
  issn      = {0021-8979},
  doi       = {10.1063/1.3159039},
  year      = {2009},
  abstract  = {We describe the concept, the fabrication, and the most relevant properties of a piezoelectric-polymer system: Two fluoroethylenepropylene (FEP) films with good electret properties are laminated around a specifically designed and prepared polytetrafluoroethylene (PTFE) template at 300 degrees C. After removing the PTFE template, a two-layer FEP film with open tubular channels is obtained. For electric charging, the two-layer FEP system is subjected to a high electric field. The resulting dielectric barrier discharges inside the tubular channels yield a ferroelectret with high piezoelectricity. d(33) coefficients of up to 160 pC/N have already been achieved on the ferroelectret films. After charging at suitable elevated temperatures, the piezoelectricity is stable at temperatures of at least 130 degrees C. Advantages of the transducer films include ease of fabrication at laboratory or industrial scales, a wide range of possible geometrical and processing parameters, straightforward control of the uniformity of the polymer system, flexibility, and versatility of the soft ferroelectrets, and a large potential for device applications e.g., in the areas of biomedicine, communications, production engineering, sensor systems, environmental monitoring, etc.},
  language  = {en}
}
@article{SborikasQiuWirgesetal.2014,
  author    = {Sborikas, Martynas and Qiu, Xunlin and Wirges, Werner and Gerhard, Reimund and Jenninger, Werner and Lovera, Deliani},
  title     = {Screen printing for producing ferroelectret systems with polymer-electret films and well-defined cavities},
  series = {Applied physics : A, Materials science \& processing},
  volume    = {114},
  journal   = {Applied physics : A, Materials science \& processing},
  number    = {2},
  publisher = {Springer},
  address   = {New York},
  issn      = {0947-8396},
  doi       = {10.1007/s00339-013-7998-3},
  pages     = {515 -- 520},
  year      = {2014},
  abstract  = {We report a process for preparing polymer ferroelectrets by means of screen printing-a technology that is widely used for the two-dimensional patterning of printed layers. In order to produce polymer-film systems with cavities that are suitable for bipolar electric charging, a screen-printing paste is deposited through a screen with a pre-designed pattern onto the surface of a polymer electret film. Another such polymer film is placed on top of the printed pattern, and well-defined cavities are formed in-between. During heating and curing, the polymer films are tightly bonded to the patterned paste layer so that a stable three-layer system is obtained. In the present work, polycarbonate (PC) films have been employed as electret layers. Screen printing, curing and charging led to PC ferroelectret systems with a piezoelectric d (33) coefficient of about 28 pC/N that is stable up to 100 C-a similar to. Due to the rather soft patterned layer, d (33) strongly decreases already for static pressures of tens of kPa. The results demonstrate the suitability of screen printing for the preparation of ferroelectret systems.},
  language  = {en}
}
@article{KofodWirgesPaajanenetal.2007,
  author    = {Kofod, Guggi and Wirges, Werner and Paajanen, Mika and Bauer, Siegfried},
  title     = {Energy minimization for self-organized structure formation and actuation},
  issn      = {0003-6951},
  doi       = {10.1063/1.2695785},
  year      = {2007},
  abstract  = {An approach for creating complex structures with embedded actuation in planar manufacturing steps is presented. Self-organization and energy minimization are central to this approach, illustrated with a model based on minimization of the hyperelastic free energy strain function of a stretched elastomer and the bending elastic energy of a plastic frame. A tulip-shaped gripper structure illustrates the technological potential of the approach. Advantages are simplicity of manufacture, complexity of final structures, and the ease with which any electroactive material can be exploited as means of actuation. (c) 2007 American Institute of Physics.},
  language  = {en}
}
@article{WegenerWirgesTiersch2006,
  author    = {Wegener, Michael and Wirges, Werner and Tiersch, Brigitte},
  title     = {Porous polytetrafluoroethylene (PTFE) electret films : porosity and time dependent charging behaviour of the free surface},
  doi       = {10.1007/s10934-006-9015-0},
  year      = {2006},
  abstract  = {Electrically charged porous polytetrafluoroethylene (PTFE) films are often discussed as active layers for electromechanical transducers. Here, the electric charging behavior of open-porous PTFE films with different porosities is investigated. Optimized electric charging of porous PTFE films is determined by variation of charging parameters such as electric fields and charging times. Maximum surface potentials are depending on the porosity of the PTFE films. Suitable charging leads to high surface potentials observed on non-stretched or slightly stretched porous PTFE films. Further increase of charging fields yields decreasing values of the surface potential accompanied with an increase of conductivity.},
  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{WirgesRaabeQiu2012,
  author    = {Wirges, Werner and Raabe, Sebastian and Qiu, Xunlin},
  title     = {Dielectric elastomer and ferroelectret films combined in a single device how do they reinforce each other?},
  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/s00339-012-6833-6},
  pages     = {583 -- 588},
  year      = {2012},
  abstract  = {Dielectric elastomers (DE) are soft polymer materials exhibiting large deformations under electrostatic stress. When a prestretched elastomer is stuck to a flat plastic frame, a complex structure that can be used as an actuator (DEA) is formed due to self-organization and energy minimization. Here, such a DEA was equipped with a ferroelectret film. Ferroelectrets are internally charged polymer foams or void-containing polymer-film systems combining large piezoelectricity with mechanical flexibility and elastic compliance. In their dielectric spectra, ferroelectrets show piezoelectric resonances that can be used to analyze their electromechanical properties. The antiresonance frequencies ( ) of ferroelectret films not only are directly related to their geometric parameters, but also are sensitive to the boundary conditions during measurement. In this paper, a fluoroethylenepropylene (FEP) ferroelectret film with tubular void channels was glued to a plastic frame prior to the formation of self-organized minimum-energy DEA structure. The dielectric resonance spectrum (DRS) of the ferroelectret film was measured in-situ during the actuation of the DEA under applied voltage. It is found that the antiresonance frequency is a monotropic function of the bending angle of the actuator. Therefore, the actuation of DEAs can be used to modulate the of ferroelectrets, while the can also be taken for in-situ diagnosis and for precise control of the actuation of the DEA. Combination of DEAs and ferroelectrets brings a number of possibilities for application.},
  language  = {en}
}
@article{MellingerWegenerWirgesetal.2006,
  author    = {Mellinger, Axel and Wegener, Michael and Wirges, Werner and Mallepally, Rajendar Reddy and Gerhard, Reimund},
  title     = {Thermal and temporal stability of ferroelectret films made from cellular polypropylene/air composites},
  year      = {2006},
  abstract  = {Ferroelectrets are thin films of polymer foams, exhibiting piezoelectric properties after electrical charging. Ferroelectret foams usually consist of a cellular polymer structure filled with air. Polymer-air composites are elastically soft due to their high air content as well as due to the size and shape of the polymer walls. Their elastically soft composite structure is one essential key for the working principle of ferroelectrets, besides the permanent trapping of electric charges inside the polymer voids. The elastic properties allow large deformations of the electrically charged voids. However, the composite structure can also possibly limit the stability and consequently the range of applications because of, e. g., penetration of gas and liquids accompanied by discharge phenomena or because of a mechanical pre-load which may be required during the application. Here, we discuss various stability aspects related to the piezoelectric properties of polypropylene ferroelectrets. Near and below room temperature, the piezoelectric effect and the stability of the trapped charges are practically independent from humidity during long-time storage in a humid atmosphere or water, or from operating conditions, such as continuous mechanical excitation. Thermal treatment of cellular polypropylene above -10 degrees C leads to a softening of the voided structure which is apparent from the decreasing values of the elastic modulus. This decrease results in an increase of the piezoelectric activity. Heating above 60 degrees C, however, leads to a decrease in piezoelectricity},
  language  = {en}
}
@article{QiuMellingerWirgesetal.2007,
  author    = {Qiu, Xunlin and Mellinger, Axel and Wirges, Werner and Gerhard, Reimund},
  title     = {Spectroscopic study of dielectric barrier discharges in cellular polypropylene ferroelectrets},
  doi       = {10.1063/1.2786597},
  year      = {2007},
  abstract  = {The transient light emission from the dielectric barrier discharges (DBDs) in cellular polypropylene ferroelectrets subjected to high electric poling fields was spectroscopically measured. The spectrum shows strong emission from the second positive system of molecular nitrogen, N-2(C (3)Pi(u))-> N-2(B (3)Pi(g)), and the first negative system of N-2(+), N-2(+)(B (2)Sigma(+)(u))-> N-2(+)(X (2)Sigma(+)(g)), consistent with a DBD in air. When a dc voltage is applied stepwise to the ferroelectret film, light emission starts above a threshold, coinciding with the threshold voltage in obtaining piezoelectricity. From selected vibronic band strength ratios, the electric field in the discharge was determined and found to agree with Townsend breakdown.},
  language  = {en}
}
@article{MellingerFloresSuarezSinghetal.2006,
  author    = {Mellinger, Axel and Flores Su{\´a}rez, Rosaura and Singh, Rajeev and Wegener, Michael and Wirges, Werner and Lang, Sidney B. and Gerhard, Reimund},
  title     = {High-resolution space-charge and polarization tomography with thermal pulses},
  isbn      = {3-8007-2939-3},
  year      = {2006},
  abstract  = {Die Arbeit wurde am 13.03.2006 mit dem "BEST PAPER AWARD" des deutschen IEEE Instrumentation and Measurement (I\&M) Chapter ausgezeichnet.},
  language  = {en}
}
@article{BassoAltafimAltafimetal.2007,
  author    = {Basso, Heitor Cury and Altafim, Ruy Alberto Pisani and Altafim, Ruy Alberto Pisani and Mellinger, Axel and Fang, Peng and Wirges, Werner and Gerhard, Reimund},
  title     = {Three-layer ferroelectrets from perforated Teflon-PTFE films fused between two homogeneous Teflon-FEP films},
  isbn      = {978-1-4244-1482-6},
  year      = {2007},
  language  = {en}
}
@article{WegenerWirgesPaajanenetal.2007,
  author    = {Wegener, Michael and Wirges, Werner and Paajanen, Mika and Gerhard, Reimund},
  title     = {Charging behaviour and thermal stability of porous and non-porous polytetrafluoroethylene (PTFE) electrets},
  isbn      = {978-1-4244-1482-6},
  year      = {2007},
  language  = {en}
}
@article{QiuMellingerWirgesetal.2007,
  author    = {Qiu, Xunlin and Mellinger, Axel and Wirges, Werner and Gerhard, Reimund},
  title     = {Dielectric barrier discharges during the generation of ferroelectrets : optical spectroscopy for process monitoring},
  isbn      = {978-1-4244-1482-6},
  year      = {2007},
  language  = {en}
}
@article{QiuMellingerWegeneretal.2007,
  author    = {Qiu, Xunlin and Mellinger, Axel and Wegener, Michael and Wirges, Werner and Gerhard, Reimund},
  title     = {Barrier discharges in cellular polypropylene ferroelectrets : how do they influence the electromechanical properties?},
  year      = {2007},
  language  = {en}
}
@article{MellingerFloresSuarezSinghetal.2007,
  author    = {Mellinger, Axel and Flores Su{\´a}rez, Rosaura and Singh, Rajeev and Wegener, Michael and Wirges, Werner and Gerhard, Reimund},
  title     = {Zerst{\"o}rungsfreie Tomographie von Raumladungs- und Polarisationsverteilungen mittles W{\"a}rmepulsen},
  issn      = {0171-8096},
  doi       = {10.1524/teme.2007.74.9.437},
  year      = {2007},
  abstract  = {Non-destructive, three-dimensional imaging of space-charge and polarization distributions in electret materials has been implemented by means of laser-induced thermal pulses. In pyroelectric films of poled poly(vinylidene fluoride), images of up to 45 x 45 pixels with a depth resolution of less than 0.5 mu m and a lateral resolution of 40 mu m were recorded, the latter being limited by fast thermal diffusion in the absorbing metallic front electrode. Initial applications include the analysis of polarization distributions in corona-poled piezoelectric sensor cables and the detection of patterned space-charge distributions in polytetrafluoroethylene films.},
  language  = {de}
}
@article{WegenerBergweilerZscherpeletal.2006,
  author    = {Wegener, Michael and Bergweiler, Steffen and Zscherpel, Detlef and Wirges, Werner and Gerhard, Reimund},
  title     = {Detection of elliptical oscillations and monopole breathing of organ-pipe bodies with piezoelectric polymer sensors},
  year      = {2006},
  abstract  = {In this paper, a measuring technique is presented for the detection of radial oscillations of tube walls excited by changes in internal air pressure. On organ pipes, the oscillations were investigated by means of piezoelectric polymer films slightly tensioned around the pipe bodies. Employing sensors with patterned electrodes, the well-known elliptical oscillation of the cross section as well as an additional monopole breathing of the organ-pipe body were detected. For the monopole breathing, a close relationship between the pressure distribution of the air-column resonances inside the pipe and the circumference variations along the pipe was observed},
  language  = {en}
}
@article{MellingerWegenerWirgesetal.2006,
  author    = {Mellinger, Axel and Wegener, Michael and Wirges, Werner and Mallepally, Rajendar Reddy and Gerhard, Reimund},
  title     = {Thermal and temporal stability of ferroelectret films made from cellular polypropylene/air composites},
  year      = {2006},
  language  = {en}
}
@article{QiuWegenerWirgesetal.2005,
  author    = {Qiu, X. L. and Wegener, Michael and Wirges, Werner and Zhang, X. Q. and Hillenbrand, J. and Xia, Zhongfu and Gerhard, Reimund and Sessler, G. M.},
  title     = {Penetration of sulfur hexafluoride into cellular polypropylene films and its effect on the electric charging and electromechanical response of ferroelectrets},
  issn      = {0022-3727},
  year      = {2005},
  abstract  = {Cellular polypropylene (PP) films were treated with sulfur hexafluoride (SF6) gas in order to study the SF6 penetration behaviour and optimize the electric charging conditions. There were differences in the penetration of SF6 for different cellular PP materials, depending on the microscopic properties, which manifest themselves in the voided structure as well as in the mechanical stiffnesses of the cellular films. The penetration of SF6 after long-term pressure treatment is confirmed in strongly inflated cellular PP films with a low mechanical stiffness of about 1 MPa. No SF6 penetration occurs for slightly inflated cellular PP films with smaller void sizes and higher mechanical stiffnesses of around 5.8 MPa. The observed thickness variations, the higher charging fields during corona charging because of SF6 penetration and the SF6 environment, as well as the resulting electromechanical properties are discussed},
  language  = {en}
}
@article{MellingerSinghWegeneretal.2005,
  author    = {Mellinger, Axel and Singh, Rajeev and Wegener, Michael and Wirges, Werner and Gerhard, Reimund and Lang, Sidney B.},
  title     = {Three-dimensional mapping of polarization profiles with thermal pulses},
  issn      = {0003-6951},
  year      = {2005},
  abstract  = {High-resolution, large-area three-dimensional mapping of polarization profiles in electret polymers was carried out by means of a fast thermal pulse technique with a focused laser beam. A lateral resolution of 38 mu m and a near- surface depth resolution of less than 0.5 mu m was achieved. At larger depths, fast thermal diffusion in the metal electrode rather than the laser spot size becomes the limiting factor for the lateral resolution. (c) 2005 American Institute of Physics},
  language  = {en}
}
@article{WegenerWirgesGerhard2005,
  author    = {Wegener, Michael and Wirges, Werner and Gerhard, Reimund},
  title     = {Piezoelectric polyethylene terephthalate (PETP) foams : specifically designed and prepared ferroelectret films},
  year      = {2005},
  language  = {en}
}
@article{BauerGogoneaBauerWirgesetal.2004,
  author    = {Bauer-Gogonea, Simona and Bauer, Siegfried and Wirges, Werner and Gerhard, Reimund},
  title     = {Dielectric investigation of photo-induced chromophore degradation in nonlinear optical side-chain polymer electrets},
  year      = {2004},
  abstract  = {Organic materials with non-centrosymmetric chromophores are known to be susceptible to a number of photochemical processes, including reversible isomerization reactions as well as irreversible photo-oxidation or photo- reduction reactions. Reversible isomerization is the basis for a variety of applications, such as photo-induced poling, optical data storage and optical grating formation. The irreversible processes that involve the destruction of the chromophores have been found useful for the fabrication of optical waveguides, but they also limit the life times of polymeric photonic devices. In this paper, it is demonstrated that dielectric measurements allow for an in-depth investigation of non-reversible chromophore degradation processes in a typical side-chain polymer. The time- and temperature-dependent dielectric function of the polymer at 1 kHz enables us to follow the chromophore-degradation kinetics and to monitor the bleaching depth as a function of time at room and elevated temperatures},
  language  = {en}
}
@article{WegenerBergweilerWirgesetal.2005,
  author    = {Wegener, Michael and Bergweiler, Steffen and Wirges, Werner and Pucher, Andreas and Tuncer, Enis and Gerhard, Reimund},
  title     = {Piezoelectric two-layer stacks of cellular polypropylene ferroelectrets : transducer response at audio and ultrasound frequencies},
  issn      = {0885-3010},
  year      = {2005},
  abstract  = {Piezoelectric cellular polypropylene films, so-called ferroelectrets, are assembled in a stack with two active transducer layers. The stack is characterized with respect to its linear and quadratic response in a frequency range from 1 kHz to 80 kHz. A relatively smooth frequency response in the sound-pressure level is found for the individual layers as well as for both layers driven in phase. The piezoelectric response of the two-layer stack is twice the response of an individual layer over a rather broad frequency range. Furthermore, the influence of the preparation conditions on the resonance frequency and the effect of the quadratic distortion on the radiated sound are investigated both for the individual transducer films in the stack and for the stack system as a whole},
  language  = {en}
}
@article{WegenerBergweilerWirgesetal.2004,
  author    = {Wegener, Michael and Bergweiler, Steffen and Wirges, Werner and Pucher, Andreas and Gerhard, Reimund},
  title     = {Voided space-charge electrets : piezoelectric transducer materials for electro-acoustic applications},
  year      = {2004},
  language  = {en}
}
@article{WegenerWirgesFohlmeisteretal.2004,
  author    = {Wegener, Michael and Wirges, Werner and Fohlmeister, Jens Bernd and Tiersch, Brigitte and Gerhard, Reimund},
  title     = {Two-step inflation of cellular polypropylene films: Void-thickness increase and enhanced electromechanical properties},
  year      = {2004},
  abstract  = {In cellular, electromechanically active polymer films, the so-called ferroelectrets, the cell size and shape distributions can be varied through a controlled inflation process. Up to now, high-pressure treatments were usually performed at elevated temperatures. There are, however, significant experimental limitations and complications if the pressure and temperature treatments are performed at the same time. Here, we demonstrate the controlled inflation of cellular polypropylene films by means of sepal-ate pressure and temperature treatments. Separate procedures are Much easier to implement. Excellent electromechanical properties were achieved with Such a two-step inflation process. The technique has significant potential for inflating large-area transducer films for electromechanical and electroacoustical applications},
  language  = {en}
}
@article{WegenerGerhardWirgesetal.2003,
  author    = {Wegener, Michael and Gerhard, Reimund and Wirges, Werner and Bergner, Andr{\´e} and Bergweiler, Steffen},
  title     = {Breathing modes of organ-pipe bodies : experimental detection with ring-shaped piezoelectric-polymer sensors},
  year      = {2003},
  language  = {en}
}
@article{WegenerPaajanenWirgesetal.2002,
  author    = {Wegener, Michael and Paajanen, Mika and Wirges, Werner and Gerhard, Reimund},
  title     = {Corona-induced partial discharges, internal charge separation and electromechanical transducer properties in cellular polymer films},
  isbn      = {0-7803-7560-2},
  year      = {2002},
  language  = {en}
}
@article{GerhardWegenerWirgesetal.2002,
  author    = {Gerhard, Reimund and Wegener, Michael and Wirges, Werner and Giacometti, J. A. and Altafim, Ruy Alberto Pisani and Santos, Lucas F. and Faria, Roberto M. and Paajanen, Mika},
  title     = {Electrode polling of cellular polypropylene films with short high-voltage pulses},
  isbn      = {0-7803-7502-5},
  year      = {2002},
  language  = {en}
}
@article{WegenerWirgesRichteretal.2001,
  author    = {Wegener, Michael and Wirges, Werner and Richter, Kristin and K{\"u}nstler, Wolfgang and Gerhard, Reimund},
  title     = {Charge stability and piezoelectric properties of porous fluoro-polymer space-charge electrets in layer systems},
  year      = {2001},
  language  = {en}
}
@article{MellingerWegenerWirgesetal.2001,
  author    = {Mellinger, Axel and Wegener, Michael and Wirges, Werner and Gerhard, Reimund},
  title     = {Thermally stable dynamic piezoelectricity in sandwich films of porous and non-porous amorphous fluoropolymer},
  year      = {2001},
  language  = {en}
}
@article{EllingPinnowDanzetal.2001,
  author    = {Elling, B. and Pinnow, M. and Danz, Rudi and Wegener, Michael and Wirges, werner and Gerhard, Reimund},
  title     = {Coating of porous polytetrafluoroethylene films with other polymers for electret applications},
  isbn      = {0-7803-7053-8},
  year      = {2001},
  language  = {en}
}
@article{StrackeBayerZimmermannetal.1999,
  author    = {Stracke, A. and Bayer, A. and Zimmermann, S. and Wendorff, Joachim Heinz and Wirges, Werner and Bauer-Gogonea, Simona and Bauer, Siegfried and Gerhard, Reimund},
  title     = {Relaxation behaviour of electrically induced polar orientation and of optically induced non-polar orientation in an azo-chromophore side group polymer},
  issn      = {0022-3727},
  year      = {1999},
  language  = {en}
}
@article{BauerGogoneaChengWirgesetal.1998,
  author    = {Bauer-Gogonea, Simona and Cheng, Z. Y. and Wirges, Werner and Bauer, Siegfried and Gerhard, Reimund and Das-Gupta, D. K.},
  title     = {Dielectric investigation of thermally-induced chromophore degradation in nonlinear optical polymer electrets},
  year      = {1998},
  language  = {en}
}
@article{DonvalBerkovicYilmazetal.1996,
  author    = {Donval, Ariela and Berkovic, Garry and Yilmaz, S{\"u}kr{\"u} and Bauer-Gogonea, Simona and Brinker, Walter and Wirges, Werner and Bauer, Siegfried and Gerhard, Reimund},
  title     = {Spatial and thermal analysis of nonlinearity created by asymmetric charge injection},
  year      = {1996},
  language  = {en}
}
@article{DingerYilmazBrinkeretal.1996,
  author    = {Dinger, Claudia and Yilmaz, S{\"u}kr{\"u} and Brinker, Walter and Wirges, Werner and Bauer, Siegfried and Gerhard, Reimund},
  title     = {Ellipsometry and Michelson interferometry for fixed- and variable- frequency electro-optical measurements on poled polymers},
  year      = {1996},
  language  = {en}
}
@article{BauerGogoneaWirgesBaueretal.1996,
  author    = {Bauer-Gogonea, Simona and Wirges, Werner and Bauer, Siegfried and Gerhard, Reimund and Liang, J. and Zyss, Joseph},
  title     = {Electrical determination of the degree of cross-linking in a poled nonlinear optical polymer},
  year      = {1996},
  language  = {en}
}
@article{BrinkerYilmazWirgesetal.1995,
  author    = {Brinker, Walter and Yilmaz, S{\"u}kr{\"u} and Wirges, Werner and Bauer, Siegfried and Gerhard, Reimund},
  title     = {Phase-shift interference microscope for the investigation of dipoleorientation distributions},
  year      = {1995},
  language  = {en}
}
@article{BauerGogoneaBauerWirgesetal.1995,
  author    = {Bauer-Gogonea, Simona and Bauer, Siegfried and Wirges, Werner and Gerhard, Reimund},
  title     = {Preparation and pyroelectrical investigation of bimorph polymer layers},
  year      = {1995},
  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{FloresSuarezGanesanWirgesetal.2010,
  author    = {Flores Su{\´a}rez, Rosaura and Ganesan, Lakshmi Meena and Wirges, Werner and Gerhard, Reimund and Mellinger, Axel},
  title     = {Imaging liquid crystals dispersed in a ferroelectric polymer matrix by means of thermal-pulse tomography},
  issn      = {1070-9878},
  doi       = {10.1109/TDEI.2010.5539683},
  year      = {2010},
  abstract  = {A new arrangement of the optical elements in a Thermal-Pulse-Tomography (TPT) setup allows to scan micrometer structures in composite and heterogeneous samples such as polymer-dispersed liquid crystals (PDLCs). The non-destructive TPT technique allows the determination of three-dimensional profiles of polarization and space charge in dielectrics. The samples under study were 12 mu m thick films of a copolymer of vinylidene fluoride with trifluoroethylene P(VDF- TrFE) (65/35) with embedded liquid-crystal droplets. The poling process was performed in direct contact well above the coercive field of the copolymer. The 3D map obtained from scanning with a 10 mu m wide spot shows elliptically shaped areas with liquid-crystal droplets. Considering the droplets as oblate spheroids, their major axis lies in the x-y plane, while their minor axis in the z direction measures 0.5 mu m or more. This result is in good agreement with scanning electron micrographs. It is believed that the major axis is overestimated due to imaging of liquid-crystal clusters.},
  language  = {en}
}
@article{PhamPetreBerquezetal.2009,
  author    = {Pham, Cong Duc and Petre, Anca and Berquez, Laurent and Flores Su{\´a}rez, Rosaura and Mellinger, Axel and Wirges, Werner and Gerhard, Reimund},
  title     = {3D high-resolution mapping of polarization profiles in thin poly(vinylidenefluoride-trifluoroethylene) (PVDF- TrFE) films using two thermal techniques},
  issn      = {1070-9878},
  doi       = {10.1109/TDEI.2009.5128505},
  year      = {2009},
  abstract  = {In this paper, two non-destructive thermal methods are used in order to determine, with a high degree of accuracy, three-dimensional polarization distributions in thin films (12 mu m) of poly(vinylidenefluoride- trifluoroethylene) (PVDF-TrFE). The techniques are the frequency-domain Focused Laser Intensity Modulation Method (FLIMM) and time-domain Thermal-Pulse Tomography (TPT). Samples were first metalized with grid-shaped electrode and poled. 3D polarization mapping yielded profiles which reproduce the electrode-grid shape. The polarization is not uniform across the sample thickness. Significant polarization values are found only at depths beyond 0.5 mu m from the sample surface. Both methods provide similar results, TPT method being faster, whereas the FLIMM technique has a better lateral resolution.},
  language  = {en}
}
@article{LancaWirgesNeaguetal.2007,
  author    = {Lanca, M. C. and Wirges, Werner and Neagu, Eugen R. and Gerhard, Reimund and Marat-Mendes, Jos{\´e} Narciso},
  title     = {Influence of humidity on the electrical charging properties of cork agglomerates},
  issn      = {0022-3093},
  doi       = {10.1016/j.jnoncrysol.2007.03.037},
  year      = {2007},
  abstract  = {Cork is a natural cellular and electrically insulating material which may have the capacity to store electric charges on or in its cell walls. Since natural cork has many voids, it is difficult to obtain uniform samples with the required dimensions. Therefore, a more uniform material, namely commercial cork agglomerate, usually used for floor and wall coverings, is employed in the present study. Since we know from our previous work that the electrical properties of cork are drastically affected by absorbed and adsorbed water, samples were protected by means of different polymer coatings (applied by spin-coating or soaking). Corona charging and isothermal charging and discharging currents were used to study the electrical trapping and detrapping capabilities of the samples. A comparison of the results leads to the conclusion that the most promising method for storing electric charges in this cellular material consists of drying and coating or soaking with a hydrophobic, electrically insulating polymer such as polytetraflouroethylene (Teflon (R)). (c) 2007 Elsevier B.V. All rights reserved.},
  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{FangWegenerWirgesetal.2007,
  author    = {Fang, Peng and Wegener, Michael and Wirges, Werner and Gerhard, Reimund and Zirkel, Larissa},
  title     = {Cellular polyethylene-naphthalate ferroelectrets : foaming in supercritical carbon dioxide, structural and electrical preparation, and resulting piezoelectricity},
  issn      = {0003-6951},
  doi       = {10.1063/1.2738365},
  year      = {2007},
  abstract  = {Polymer foams with electrically charged cellular voids, the so-called ferroelectrets, are soft piezoelectric transducer materials. Several polymers such as polyethylene terephthalate or cyclo-olefin copolymers are under investigation with respect to their suitability as ferroelectrets. Here, the authors report an additional ferroelectret polymer, cellular polyethylene-naphthalate (PEN), which was prepared from commercial uniform polymer films by means of foaming in supercritical carbon dioxide, inflation, biaxial stretching, electrical charging, and metallization. Piezoelectric d(33) coefficients of up to 140 pC/N demonstrate the suitability of such cellular PEN films for transducer applications. Their piezoelectricity is partially stable at elevated temperatures as high as 100 degrees C.},
  language  = {en}
}
@article{RosenhahnFinlayPettitetal.2009,
  author    = {Rosenhahn, Axel and Finlay, John A. and Pettit, Michala E. and Ward, Andy and Wirges, Werner and Gerhard, Reimund and Callow, Maureen E. and Grunze, Michael and Callow, James A.},
  title     = {Zeta potential of motile spores of the green alga Ulva linza and the influence of electrostatic interactions on spore settlement and adhesion strength},
  issn      = {1559-4106},
  doi       = {10.1116/1.3110182},
  year      = {2009},
  abstract  = {The zeta potential of the motile spores of the green alga (seaweed) Ulva linza was quantified by video microscopy in combination with optical tweezers and determined to be -19.3{\~n}1.1 mV. The electrostatic component involved in the settlement and adhesion of spores was studied using electret surfaces consisting of PTFE and bearing different net charges. As the surface chemistry remains the same for differently charged surfaces, the experimental results isolate the influence of surface charge and thus electrostatic interactions. Ulva spores were demonstrated to have a reduced tendency to settle on negatively charged surfaces and when they did settle the adhesion strength of settled spores was lower than with neutral or positively charged surfaces. These observations can be ascribed to electrostatic interactions.},
  language  = {en}
}
@article{GerhardBauerBauerGogoneaetal.1995,
  author    = {Gerhard, Reimund and Bauer, Siegfried and Bauer-Gogonea, Simona and Brinker, Walter and Dinger, Claudia and Molzow, Wolf-Dietrich and Wirges, Werner and Yilmaz, S{\"u}kr{\"u}},
  title     = {Electro-optical investigation of the dipole orientation in poled polymers},
  year      = {1995},
  language  = {en}
}
@article{BauerBauerGogoneaYilmazetal.1995,
  author    = {Bauer, Siegfried and Bauer-Gogonea, Simona and Yilmaz, S{\"u}kr{\"u} and Dinger, Claudia and Wirges, Werner and Gerhard, Reimund and Michelotti, F. and Toussaere, E. and Levenson, R. and Liang, J. and Zyss, Joseph},
  title     = {Pyroelectric, dielectric and electro-optical investigation of crosslinking in Red Acid Magly},
  year      = {1995},
  language  = {en}
}
@article{WirgesPrzyrembelBrinkeretal.1995,
  author    = {Wirges, Werner and Przyrembel, G. and Brinker, Walter and Gerhard, Reimund and Klemberg-Sapieha, J. and Martinu, L. and Poitras, D. and Wertheimer, M. R.},
  title     = {Metallised viscoelastic control layers for light-valve projection displays},
  year      = {1995},
  language  = {en}
}
@article{BauerGogoneaBauerWirgesetal.1995,
  author    = {Bauer-Gogonea, Simona and Bauer, Siegfried and Wirges, Werner and Gerhard, Reimund and Wintler, H. J.},
  title     = {Physical aging after photo-induced or thermally assisted poling for enhancing the stability of polymeric dipole glasses},
  year      = {1995},
  language  = {en}
}
@article{BrinkerWirgesMolzowetal.1995,
  author    = {Brinker, Walter and Wirges, Werner and Molzow, Wolf-Dietrich and Gerhard, Reimund and Melcher, R. and Budde, W. and Fiedler, H.},
  title     = {Active silicon CMOS addressing matrices for light-valve projection displays},
  year      = {1995},
  language  = {en}
}
@article{DonvalBerkovicBauerGogoneaetal.1995,
  author    = {Donval, Ariela and Berkovic, Garry and Bauer-Gogonea, Simona and Wirges, Werner and Bauer, Siegfried and Gerhard, Reimund},
  title     = {Pyroelectric depth profiling of the optical nonlinearity caused by charge injection},
  year      = {1995},
  language  = {en}
}
@article{BauerBauerGogoneaYilmazetal.1995,
  author    = {Bauer, Siegfried and Bauer-Gogonea, Simona and Yilmaz, S{\"u}kr{\"u} and Wirges, Werner and Gerhard, Reimund},
  title     = {Pyroelectrical investigation of nonlinear optical polymers with uniform or patterned dipole orientation},
  year      = {1995},
  language  = {en}
}