@article{MellingerSinghGerhard2005,
  author    = {Mellinger, Axel and Singh, Rajeev and Gerhard, Reimund},
  title     = {Fast thermal-pulse measurements of space-charge distributions in electret polymers},
  issn      = {0034-6748},
  year      = {2005},
  abstract  = {Space-charge depth profiles in various electret polymers have been measured in both the time and the frequency domain using thermal pulses and waves, respectively. A comparison of the two techniques on corona-charged polytetrafluoroethylene showed that the thermal-pulse method yielded similar results as the thermal-wave technique, but approximately 20-50 times faster. The article discusses sensitivity limitations as well as possible applications, including the real-time monitoring of space-charge decay under UV irradiation. (C) 2005 American Institute of Physics},
  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{Gerhard2006,
  author    = {Gerhard, Reimund},
  title     = {Electret science has a long history and a bright interdisciplinary future},
  year      = {2006},
  language  = {en}
}
@article{GerhardFariaOliveiraJretal.2006,
  author    = {Gerhard, Reimund and Faria, Roberto M. and Oliveira Jr., O. N. and Giacometti, Jose A.},
  title     = {Useful effects of space charge and dipole polarization: Recent developments in polymer electrets and organic semiconductors (Editorial)},
  issn      = {1070-9878},
  doi       = {10.1109/TDEI.2006.247817},
  year      = {2006},
  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{BassoAquinoAltafimetal.2006,
  author    = {Basso, Heitor Cury and Aquino, C. V. and Altafim, Ruy Alberto Pisani and Altafim, Ruy Alberto Pisani and Gerhard, Reimund},
  title     = {Piezoelectricity of a single bubble formed by two oppositely charged Teflon-FEP films},
  isbn      = {1-4244-0547-5},
  year      = {2006},
  language  = {en}
}
@article{FruebingKremmerGerhardetal.2006,
  author    = {Fr{\"u}bing, Peter and Kremmer, Alexander and Gerhard, Reimund and Spannoudaki, Anna and Pissis, Polycarpos},
  title     = {Relaxation processes at the glass transition in polyamide 11 : From rigidity to viscoelasticity},
  issn      = {0021-9606},
  doi       = {10.1063/1.2360266},
  year      = {2006},
  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{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{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{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{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{AbelBergweilerGerhard2006,
  author    = {Abel, Markus and Bergweiler, Steffen and Gerhard, Reimund},
  title     = {Synchronization of organ pipes : experimental observations and modeling},
  issn      = {0001-4966},
  doi       = {10.1121/1.217044},
  year      = {2006},
  abstract  = {We report measurements on the synchronization properties of organ pipes. First, we investigate influence of an external acoustical signal from a loudspeaker on the sound of an organ pipe. Second, the mutual influence of two pipes with different pitch is analyzed. In analogy to the externally driven, or mutually coupled self-sustained oscillators, one observes a frequency locking, which can be explained by synchronization theory. Further, we measure the dependence of the frequency of the signals emitted by two mutually detuned pipes with varying distance between the pipes. The spectrum shows a broad '' hump '' structure, not found for coupled oscillators. This indicates a complex coupling of the two organ pipes leading to nonlinear beat phenomena.},
  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{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{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{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{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{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}
}