@article{WegenerBauerGerhard1999,
  author    = {Wegener, Michael and Bauer, Siegfried and Gerhard, Reimund},
  title     = {Apparent dielectric hysteresis without significant dipolar movement?},
  year      = {1999},
  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{BergweilerBergnerGoerneetal.2003,
  author    = {Bergweiler, Steffen and Bergner, Andr{\´e} and G{\"o}rne, Thomas and Wegener, Michael and Gerhard, Reimund},
  title     = {Breathing modes and sound radiation of metallic organ pipes},
  year      = {2003},
  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{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{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{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{PaajanenWegenerGerhard2001,
  author    = {Paajanen, Mika and Wegener, Michael and Gerhard, Reimund},
  title     = {Charging of cellular space-charge electret films in various gas atmospheres},
  isbn      = {0-7803-7053-8},
  year      = {2001},
  language  = {en}
}
@article{MontanariFabianiCianietal.2007,
  author    = {Montanari, Gian Carlo and Fabiani, Davide and Ciani, Franco and Motori, Antonio and Paajanen, Mika and Gerhard, Reimund and Wegener, Michael},
  title     = {Charging properties and time-temperature stability of innovative polymeric cellular ferroelectrets},
  issn      = {1070-9878},
  doi       = {10.1109/TDEI.2007.302892},
  year      = {2007},
  abstract  = {After appropriate mechanical and electrical treatments, some cellular polymers become able to retain space charge for a long time, i.e. they acquire electret behavior. The electrical treatment consists of charging under high levels of DC electric field. The mechanical treatment, based on the application of stretching forces to cellular polymer slabs that were before expanded under pressurized gas, affects the cavity size and shape, and therefore also the effectiveness of the charging process itself. An investigation of charging mechanisms, as well as of mechanical treatment, is therefore fundamental for optimizing the ferro- and piezo-electret properties. The aim of this paper is to discuss the effect of the physical dimension of the cavities on the charging behavior of cellular ferroelectrets and to focus on the time-temperature stability for two families of polymeric cellular ferroelectrets based on polypropylene (PP) and on a cyclo-olefin copolymer (COC). Emphasis will be given to the stretching process and in particular to the expansion rate applied during the manufacturing process (which affects the radial dimension and the height of the cavities, respectively). Space-charge and partial-discharge measurements as a function of time and temperature are the main tools to infer the influence of the cavity size on charging and stability characteristics.},
  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{WegenerWirgesGerhardetal.2004,
  author    = {Wegener, Michael and Wirges, Werner and Gerhard, Reimund and Dansachm{\"u}ller, M. and Schw{\"o}diauer, R. and Bauer-Gogonea, Simona and Bauer, Siegfried and Paajanen, Mika and Minkkinen, Hannu and Raukola, J.},
  title     = {Controlled inflation of voids in cellular polymer ferroelectrets : optimizing electromechanical transducer properties},
  year      = {2004},
  abstract  = {When exposed to sufficiently high electric fields, polymer-foam electret materials with closed cells exhibit ferroelectric-like behavior and may therefore be called ferroelectrets. In cellular ferroelectrets, the influence of the cell size and shape distributions on the application-relevant properties is not yet understood. Therefore, controlled inflation experiments were carried out on cellular polypropylene films, and the resulting elastical and electromechanical parameters were determined. The elastic modulus in the thickness direction shows a minimum with a corresponding maximum in the electromechanical transducer coefficient. The resonance frequency shifts as a function of the elastic modulus and the relative density of the inflated cellular films. Therefore, the transducer properties of cellular ferroelectrets can be optimized by means of controlled inflation. (C) 2004 American Institute of Physics},
  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{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{FruebingWegenerGerhardetal.1999,
  author    = {Fr{\"u}bing, Peter and Wegener, Michael and Gerhard, Reimund and Buchsteiner, Alexandra and Neumann, Werner and Brehmer, Ludwig},
  title     = {Different molecular mechanisms for the dielectric hysteresis and pyroelectricity in a poly(vinyl alcohol) with azobenzene-alkoxy side chains},
  issn      = {0277-786X},
  year      = {1999},
  abstract  = {By means of pyroelectrical measurements and dielectric spectroscopy as well as structural information from differential scanning calorimetry, it is shown that, in a poly(vinyl alcohol) with azobenzene-alkoxy side chains, pyroelctricity and dielectric hysteresis which are usually related to each other have different origins. The pyroelectric effect is explained with reversible dipole-density changes upon thermal expansion, whereas the dielectric hysteresis is proposed to result from a charge-carrier polarisation.},
  language  = {en}
}
@article{TuncerWegenerGerhard2005,
  author    = {Tuncer, Enis and Wegener, Michael and Gerhard, Reimund},
  title     = {Distribution of relaxation times in alpha-phase polyvinylidene fluoride},
  issn      = {0022-3093},
  year      = {2005},
  abstract  = {In this paper, a recently developed numerical method to analyze dielectric-spectroscopy data is applied to alpha-phase polyvinylidene fluoride (PVDF). The numerical procedure is non-parametric and does not contain any of the extensively used empirical formulas mentioned in the literature. The method basically recovers the unknown distribution of relaxation times of the generalized dielectric function representation by simultaneous application of the Monte Carlo integration method and of the constrained least-squares optimization. The relaxation map constructed after the numerical analysis is compared to a-phase PVDF data presented in the literature and results of the parametric analysis with a well- known empirical formula. (c) 2005 Elsevier B.V. All rights reserved},
  language  = {en}
}
@article{WegenerTuncerGerhardetal.2006,
  author    = {Wegener, Michael and Tuncer, Enis and Gerhard, Reimund and Bauer, Siegfried},
  title     = {Elastic properties and electromechanical coupling factor of inflated polypropylene ferroelectrets},
  isbn      = {1-4244-0547-5},
  year      = {2006},
  language  = {en}
}
@article{TuncerWegener2004,
  author    = {Tuncer, Enis and Wegener, Michael},
  title     = {Elastic properties of highly anisotropic thin poly(propylene) foams},
  issn      = {0167-577X},
  year      = {2004},
  abstract  = {In this letter, elastic properties of highly anisotropic cellular poly(propylene) films are reported. The material shows peculiar elastic properties compared to other foams in the literature. The data is displayed as the relative Young's modulus E*/E-s versus relative density rho*/rho(s). Almost all the data from the literature are located on the region E*/E-s = (rho*/rho(s))(n) with 1 less than or equal to n less than or equal to 6. The introduced material on the other hand has lower relative Young's modulus at high relative densities, n greater than or equal to 6. (C) 2004 Elsevier B.V. All rights reserved},
  language  = {en}
}
@article{WegenerGerhard2003,
  author    = {Wegener, Michael and Gerhard, Reimund},
  title     = {Electric poling and electromechanical characterization of 0.1-mm-thick sensor films and 0.2-mm-thick cable layers from piezoelectric poly(vinylidene fluoride-trifluoroethylene)},
  issn      = {0885-3010},
  year      = {2003},
  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{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{KussmaulRisseKofodetal.2011,
  author    = {Kussmaul, Bjoern and Risse, Sebastian and Kofod, Guggi and Wache, Remi and Wegener, Michael and McCarthy, Denis N. and Kr{\"u}ger, Hartmut and Gerhard, Reimund},
  title     = {Enhancement of dielectric permittivity and electromechanical response in silicone elastomers molecular grafting of organic dipoles to the macromolecular Network},
  series = {Advanced functional materials},
  volume    = {21},
  journal   = {Advanced functional materials},
  number    = {23},
  publisher = {Wiley-VCH},
  address   = {Weinheim},
  issn      = {1616-301X},
  doi       = {10.1002/adfm.201100884},
  pages     = {4589 -- 4594},
  year      = {2011},
  abstract  = {A novel method is established for permittivity enhancement of a silicone matrix for dielectric elastomer actuators (DEAs) by molecular level modifications of the elastomer matrix. A push-pull dipole is synthesized to be compatible with the silicone crosslinking chemistry, allowing for direct grafting to the crosslinker molecules in a one-step film formation process. This method prevents agglomeration and yields elastomer films that are homogeneous down to the molecular level. The dipole-to-silicone network grafting reaction is studied by FTIR. The chemical, thermal, mechanical and electrical properties of films with dipole contents ranging from 0 wt\% to 13.4 wt\% were thoroughly characterized. The grafting of dipoles modifies the relative permittivity and the stiffness, resulting in the actuation strain at a given electrical field being improved by a factor of six.},
  language  = {en}
}
@article{WegenerKuenstlerRichteretal.2002,
  author    = {Wegener, Michael and K{\"u}nstler, Wolfgang and Richter, Kristin and Gerhard, Reimund},
  title     = {Ferroelectric polarization in stretched piezo- and pyroelectric poly(vinylidene fluoride-hexafluoropropylene) copolymer films},
  year      = {2002},
  language  = {en}
}
@phdthesis{Wegener2007,
  author    = {Wegener, Michael},
  title     = {Ferroelektrete und Polymersch{\"a}ume: Pr{\"a}paration, Eigenschaften und Anwendung als piezoelektrische Wandler},
  address   = {Potsdam},
  pages     = {129 S. : Ill., graph. Darst.},
  year      = {2007},
  language  = {de}
}
@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{RyabchunKolloscheWegeneretal.2016,
  author    = {Ryabchun, Alexander and Kollosche, Matthias and Wegener, Michael and Sakhno, Oksana},
  title     = {Holographic Structuring of Elastomer Actuator: First True Monolithic Tunable Elastomer Optics},
  series = {Advanced materials},
  volume    = {28},
  journal   = {Advanced materials},
  publisher = {Wiley-VCH},
  address   = {Weinheim},
  issn      = {0935-9648},
  doi       = {10.1002/adma.201602881},
  pages     = {10217 -- 10223},
  year      = {2016},
  abstract  = {Volume diffraction gratings (VDGs) are inscribed selectively by diffusive introduction of benzophenone and subsequent UV-holographic structuring into an electroactive dielectric elastomer actuator (DEA), to afford a continuous voltage-controlled grating shift of 17\%. The internal stress coupling of DEA and optical domain allows for a new generation of true monolithic tunable elastomer optics with voltage controlled properties.},
  language  = {en}
}
@article{GerhardWegenerKuenstleretal.2000,
  author    = {Gerhard, Reimund and Wegener, Michael and K{\"u}nstler, Wolfgang and Wirges, Werner and G{\"o}rne, Thomas and Urayama, D. and Neher, Dieter},
  title     = {Inverse piezoelectricity of porous PTFE films with bipolar space charge},
  year      = {2000},
  language  = {en}
}
@article{KussmaulRisseWegeneretal.2012,
  author    = {Kussmaul, Bj{\"o}rn and Risse, Sebastian and Wegener, Michael and Kofod, Guggi and Kr{\"u}ger, Hartmut},
  title     = {Matrix stiffness dependent electro-mechanical response of dipole grafted silicones},
  series = {Smart materials and structures},
  volume    = {21},
  journal   = {Smart materials and structures},
  number    = {6},
  publisher = {IOP Publ. Ltd.},
  address   = {Bristol},
  issn      = {0964-1726},
  doi       = {10.1088/0964-1726/21/6/064005},
  pages     = {6},
  year      = {2012},
  abstract  = {The properties of dielectric elastomer actuators can be optimized by modifying the dielectric or mechanical properties of the dielectric elastomer. This paper presents the simultaneous control of both dielectric and mechanical properties, in a silicone elastomer network comprising cross-linker, chains and grafted molecular dipoles. Chains with two different molecular weights were each combined with varying amounts of grafted dipole. Chemical and physical characterization showed that networks with stoichiometric control of cross-linking density and permittivity were obtained, and that longer chain lengths resulted in higher electrical field response due to the reduction in cross-linking density and correspondingly in mechanical stiffness. Both actuation sensitivities were enhanced by 6.3 and 4.6 times for the short and long chain matrix material, respectively.},
  language  = {en}
}
@article{TuncerWegenerGerhard2005,
  author    = {Tuncer, Enis and Wegener, Michael and Gerhard, Reimund},
  title     = {Modeling electro-mechanical properties of layered electrets : application of the finite-element method},
  issn      = {0304-3886},
  year      = {2005},
  abstract  = {We present calculations on the deformation of two- and three-layer electret systems. The electrical field is coupled with the stress-strain equations by means of the Maxwell stress tensor. In the simulations, two-phase systems are considered, and intrinsic relative dielectric permittivity and Young's modulus of the phases are altered. The numerically calculated electro-mechanical activity is compared to an analytical expression. Simulations are performed on two- and three-layer systems. Various parameters in the model are systematically varied and their influence on the resulting piezoelectricity is estimated. In three-layer systems with bipolar charge, the piezoelectric coefficients exhibit a strong dependence on the elastic moduli of the phases. However, with mono-polar charge, there is no significant piezoelectric effect. A two-dimensional simulation illustrated that higher piezoelectric coefficients can be obtained for non-uniform surface charges and low Poisson's ratio of phases. Irregular structures considered exhibit low piezoelectric activity compared to two-layer structures. (C) 2004 Elsevier B.V. All rights reserved},
  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{SaarimaekiPaajanenSavijaervietal.2006,
  author    = {Saarim{\"a}ki, Eetta and Paajanen, Mika and Savij{\"a}rvi, Ann-Mari and Minkkinen, Hannu and Wegener, Michael and Voronina, Olena and Schulze, Robert and Wirges, Werner and Gerhard, Reimund},
  title     = {Novel heat durable electromechanical film : processing for electromechanical and electret applications},
  year      = {2006},
  abstract  = {New ferroelectrets were developed on the basis of foams from cyclo-olefin polymers and copolymers. The results obtained on the cyclo-olefin polymer foam demonstrate a significant improvement of the service temperature for ferroelectret transducer materials. Suitable compounding and preparation led to cyclo-olefin ferroelectrets with an electromechanical activity of around 15 pC/N, which is thermally stable at least up to 110 degrees C. The properties in sensor and actuator applications are strongly dependent on the processing parameters related to film-making, sensor and actuator preparation, gas content and electric charging. The processing window for the film stretching was very narrow compared to the earlier developed polypropylene ferroelectrets. The film porosity, softness and thus the electromechanical activity are adjusted by gas-diffusion expansion. The activity of the electromechanically operating sensors and actuators was increased by stacking several layers of cellular cyclo-olefin film. For applications such as flat loudspeakers, the foamed films are tuned by tensioning them on a support frame. Correct tensioning was essential also for reducing the distortion levels.},
  language  = {en}
}
@article{WirgesWegenerVoroninaetal.2007,
  author    = {Wirges, Werner and Wegener, Michael and Voronina, Olena and Zirkel, Larissa and Gerhard, Reimund},
  title     = {Optimized preparation of elastically soft, highly piezoelectric, cellular ferroelectrets from nonvoided poly(ethylene terephthalate) films},
  doi       = {10.1002/adfm.200600162},
  year      = {2007},
  abstract  = {Electrically charged cellular polymer films can exhibit very high piezoelectric activity and are therefore more and more often employed in advanced electromechanical and electro-acoustical transducers. In this paper, we report an optimized sequence of steps for preparing such ferroelectrets from commercial nonvoided ploy(ethylene terephthalate) (PETP) films by means of foaming with CO2 biaxial mechanical stretching, controlled void inflation, and bipolar electric charging. The nonvoid PETP films foamed with supercritical CO2 at a suitably high pressure and subsequently annealed for stabilization. The cellular foam structure was further optimized by means of well controlled biaxial stretching in a commercial stretcher and sometimes subsequent inflamation in a pressure chamber. Bipolar electric charging of the internal voids was achieved through the application of high electric fields in an SF0 atmosphere. The new optimized PETP ferroelectric exhibit quite large piezoelectric coefficients up to almost 500 pCN(-1), for which unusually low elastic stiffness of only around 0.3 MPa are essential. The PETP foam ferroelectrics posses unclamped thickenss-extension resonance frequences between approximately 120 and 250 kHz, and are thus highly suitable for several established as well as novel ultrasonic-transductant applications.},
  language  = {en}
}
@article{TuncerWegenerFrubingetal.2005,
  author    = {Tuncer, Enis and Wegener, Michael and Frubing, Peter and Gerhard, Reimund},
  title     = {Origin of temperature dependent conductivity of alpha-polyvinylidene fluoride},
  issn      = {0021-9606},
  year      = {2005},
  abstract  = {The conductivity of alpha-polyvinylidene fluoride is obtained from dielectric measurements performed in the frequency domain at several temperatures. At temperatures above the glass-transition, the conductivity can be interpreted as an ionic conductivity, which confirms earlier results reported in the literature. Our investigation shows that the observed ionic conductivity is closely related to the amorphous phase of the polymer. (C) 2005 American Institute of Physics},
  language  = {en}
}
@article{WegenerHesseWegeneretal.2002,
  author    = {Wegener, Michael and Hesse, J. and Wegener, T. and Gerhard, Reimund},
  title     = {Patterned ferro-, pyro- and piezoelectricity in poly(vinylindene fluoride) by means of a laser-induced irreversible beta to alpha phase transformation},
  year      = {2002},
  language  = {en}
}
@article{WegenerHesseWegeneretal.2002,
  author    = {Wegener, Michael and Hesse, J. and Wegener, T. and Gerhard, Reimund},
  title     = {Patterned ferro-, pyro- and piezoelectricity in poly(vinylindene fluoride) by means of a laser-induced irreversible beta to alpha phase transformation [Erratum]},
  year      = {2002},
  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{WegenerKuenstlerGerhard2004,
  author    = {Wegener, Michael and K{\"u}nstler, Wolfgang and Gerhard, Reimund},
  title     = {Piezo-, pyro- and ferroelectricity in poly (vinylidene fluoride-hexafluoropropylene) copolymer films},
  year      = {2004},
  abstract  = {Thin films of poly(vinylidene fluoride-hexafluoropropylene) P(VDF-HFP) show significant electroactive properties, such as piezoelectricity, pyroelectricity and electrostriction. Suitable polar P(VDF-HFP) copolymer films can be prepared by melt-pressing or solution-casting. Dipolar orientation causes the macroscopic polarization and thus also the symmetry breaking necessary for electroactive properties. We discuss the polarization build-up in thin, stretched and non-stretched, films of P(VDF-HFP) copolymer with a HFP content of 15\%. Poling currents measured in-situ during electric poling are analyzed and the polarization is calculated. Suitable electric poling leads to hysteresis phenomena of the polarization as a function of the electric field as well as to significant polarization during switching experiments. Our results indicate dipolar orientation also in non-stretched P(VDF-HFP) films},
  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{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{WegenerKuenstlerGerhard2006,
  author    = {Wegener, Michael and K{\"u}nstler, Wolfgang and Gerhard, Reimund},
  title     = {Poling behavior and optical absorption of partially dehydrofluorinated and uniaxially stretched polyvinylidene fluoride},
  doi       = {10.1080/00150190600694761},
  year      = {2006},
  abstract  = {Polyvinylidene fluoride was dissolved together with solid sodium hydroxide as catalyst in a dimethylsulfoxide/ acetone mixture and moderately dehydrofluorinated. The dehydrofluorination leads to a partial degradation of the fluorohydrocarbons, and in particular to main-chain scission and to formation of carbon double or triple bonds. This enhances the absorption at UV-vis frequencies. The degradation process also generates a large amount of excess charges in the polymer, which influence the electrical polarization behavior of the dehydrofluorinated polymer. Uniaxial stretching of moderately dehydrofluorinated polyvinylidene fluoride leads to films in a polar phase. Dipole polarization in the degraded and stretched films is demonstrated by means of switching experiments},
  language  = {en}
}
@article{Wegener2002,
  author    = {Wegener, Michael},
  title     = {Poling of piezoelectric polymer cables and assessment of their sensor properties},
  isbn      = {0-7803-7560-2},
  year      = {2002},
  language  = {en}
}
@phdthesis{Wegener1999,
  author    = {Wegener, Michael},
  title     = {Polungsverhalten und Depolarisierung ferroelektrischer und nicht ferroelektrischer Polymere},
  address   = {Potsdam},
  pages     = {120 Bl. : graph. Darst.},
  year      = {1999},
  language  = {de}
}
@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{WeinholdWegenerKuenstleretal.2000,
  author    = {Weinhold, Till and Wegener, Michael and K{\"u}nstler, Wolfgang and Seiß, Martin and G{\"o}rne, Thomas and Gerhard, Reimund},
  title     = {Porous polytetrafluoroethylene (PTFE) single-film space-charge electrets with high piezoelectric coefficients},
  issn      = {0537-9989},
  year      = {2000},
  language  = {en}
}
@misc{NeumannHorstkemperKruegeretal.2002,
  author    = {Neumann, Bernhard and Horstkemper, Marianne and Kr{\"u}ger, Wolfgang and Wilkens, Martin and Bohlen, Andreas and Fr{\"u}bing, Peter and Wegener, Michael and Scheff, Ullrich and Neher, Dieter and Brehmer, Ludwig and Kleinpeter, Erich and Wolf, Gunter and Koetz, Joachim and Peter, Martin G. and Senkbeil, Sigrid and Meyer, Till},
  title     = {Portal = Im Trend: Physiker und Chemiker erforschen "Soft Matter"},
  volume    = {2002},
  issn      = {1618-6893},
  doi       = {10.25932/publishup-50144},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-501441},
  pages     = {40},
  year      = {2002},
  abstract  = {Aus dem Inhalt: Im Trend: Physiker und Chemiker erforschen „Soft Matter" -Brandenburger Netzwerk f{\"u}r Existenzgr{\"u}nder erh{\"a}lt F{\"o}rderung -Universit{\"a}t leistet Beitrag zum Romantik-Jahr -Musiksender MTV und Bryan Adams auf dem Campus},
  language  = {de}
}
@article{WegenerSeissGerhard2001,
  author    = {Wegener, Michael and Seiß, Martin and Gerhard, Reimund},
  title     = {Prepariation and assessment of piezo- and pyroelectric poly- (vinylidene fluoride-hexafluoropropylene) copolymer films},
  year      = {2001},
  language  = {en}
}
@article{FruebingWegenerGerhardetal.1999,
  author    = {Fr{\"u}bing, Peter and Wegener, Michael and Gerhard, Reimund and Buchsteiner, Alexandra and Neumann, Werner and Brehmer, Ludwig and Dietel, Reinhard},
  title     = {Pyroelectric properties and dielectric hysteresis of a poly(vinylalcohol) with azobenzene alkoxy side chains},
  year      = {1999},
  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{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{FloresSuarezMellingerWegeneretal.2006,
  author    = {Flores Su{\´a}rez, Rosaura and Mellinger, Axel and Wegener, Michael and Wirges, Werner and Gerhard, Reimund and Singh, Rajeev},
  title     = {Thermal-pulse tomography of polarization distributions in a cylindrical geometry},
  series = {IEEE transactions on dielectrics and electrical insulation},
  volume    = {13},
  journal   = {IEEE transactions on dielectrics and electrical insulation},
  number    = {5},
  publisher = {IEEE},
  address   = {Piscataway},
  issn      = {1070-9878},
  doi       = {10.1109/TDEI.2006.258210},
  pages     = {1030 -- 1035},
  year      = {2006},
  abstract  = {Fast, three-dimensional polarization mapping in piezoelectric sensor cables was performed by means of the novel thermal-pulse tomography (TPT) technique with a lateral resolution of 200 mum. The active piezoelectric cable material (a copolymer of polyvinylidene fluoride with trifluoroethylene) was electrically poled with a point-to-cable corona discharge. A focused laser was employed to heat the opaque outer electrode, and the short-circuit current generated by the thermal pulse was used to obtain 3D polarization maps via the scale transformation method. The article describes the TPT technique as a fast non-destructive option for studying cylindrical geometries.},
  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{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{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{PaajanenWegenerGerhard2001,
  author    = {Paajanen, Mika and Wegener, Michael and Gerhard, Reimund},
  title     = {Understanding the role of the gas in the voids during corona charging of cellular electret films : a way to enhance their piezoelectricity},
  year      = {2001},
  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{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}
}