@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{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{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{MellingerCamachoGonzalezGerhard2004, author = {Mellinger, Axel and Camacho Gonz{\´a}lez, Francisco and Gerhard, Reimund}, title = {Photostimulated discharge in electret polymers : an alternative approach for investigating deep traps}, year = {2004}, abstract = {The stability of space charge in electrets such as polytetrafluoroethylene (PTFE), polyethylene terephthalate (PETP) and polypropylene (PP) under ultraviolet irradiation has been investigated using photostimulated discharge spectroscopy. While only weak discharge currents were observed in PTFE coated with semitransparent gold electrodes, up to 15 pA/cm(2) were found in PETP around the UV absorption edge near 310 nm. Space charge profiles obtained with the piezoelectrically generated pressure step method indicate that near-surface charges were almost completely removed. In PP foam, recent findings of a UV-reduced d(33) coefficient were confirmed for exposure times of up to 3.5 h, and a discharge peak at 200 urn could be assigned to the charges stored on the surfaces of the voids. The unique morphology and the (quasi-) piezoelectric properties of cellular PP make it a role model for the future investigation of charge storage in electrets}, language = {en} } @article{MellingerGonzalezGerhard2003, author = {Mellinger, Axel and Gonzalez, Francisco Camacho and Gerhard, Reimund}, title = {Ultraviolet-induced discharge currents and reduction of piezoelectric coefficient in cellular polypropylene films}, year = {2003}, language = {en} } @article{GonzalezMellingerGerhardetal.2002, author = {Gonzalez, Francisco Camacho and Mellinger, Axel and Gerhard, Reimund and Santos, Lucas F. and Faria, Roberto M.}, title = {Photo-stimulated discharge of highly insulating polymers (PTFE and PETP)}, isbn = {0-7803-7502-5}, year = {2002}, language = {en} } @article{MellingerGonzalezGerhardetal.2002, author = {Mellinger, Axel and Gonzalez, Francisco Camacho and Gerhard, Reimund and Santos, Lucas F. and Faria, Roberto M.}, title = {Phototstimulated discharge of corona and electron-beam charged electret polymers}, isbn = {0-7803-7560-2}, year = {2002}, 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{MellingerFruebingPrescheretal.1999, author = {Mellinger, Axel and Fr{\"u}bing, Peter and Prescher, Dietrich and Gerhard, Reimund}, title = {Dielectric spectra of a partially fluorinated chromophore / amorphous Teflon AF guest-host system}, year = {1999}, 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{QiuGerhardMellinger2011, author = {Qiu, Xunlin and Gerhard, Reimund and Mellinger, Axel}, title = {Turning polymer foams or polymer-film systems into ferroelectrets dielectric barrier discharges in voids}, series = {IEEE transactions on dielectrics and electrical insulation}, volume = {18}, journal = {IEEE transactions on dielectrics and electrical insulation}, number = {1}, publisher = {Inst. of Electr. and Electronics Engineers}, address = {Piscataway}, issn = {1070-9878}, doi = {10.1109/TDEI.2011.5704490}, pages = {34 -- 42}, year = {2011}, abstract = {Polymer foams and void-containing polymer-film systems with internally charged voids combine large piezoelectricity with mechanical flexibility and elastic compliance. This new class of soft materials (often called ferro-or piezoelectrets) has attracted considerable attention from science and industry. It has been found that the voids can be internally charged by means of dielectric barrier discharges (DBDs) under high electric fields. The charged voids can be considered as man-made macroscopic dipoles. Depending on the ferroelectret structure and the pressure of the internal gas, the voids may be highly compressible. Consequently, very large dipole-moment changes can be induced by mechanical or electrical stresses, leading to large piezoelectricity. DBD charging of the voids is a critical process for rendering polymer foams piezoelectric. Thus a comprehensive exploration of DBD charging is essential for the understanding and the optimization of piezoelectricity in ferroelectrets. Recent studies show that DBDs in the voids are triggered when the internal electric field reaches a threshold value according to Townsend's model of Paschen breakdown. During the DBDs, charges of opposite polarity are generated and trapped at the top and bottom internal surfaces of the gas-filled voids, respectively. The deposited charges induce an electric field opposite to the externally applied one and thus extinguish the DBDs. Back discharges may eventually be triggered when the external voltage is reduced or turned off. In order to optimize the efficiency of DBD charging, the geometry (in particular the height) of the voids, the type of gas and its pressure inside the voids are essential factors to be considered and to be optimized. In addition, the influence of the plasma treatment on the internal void surfaces during the DBDs should be taken into consideration.}, 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} } @phdthesis{Mellinger2004, author = {Mellinger, Axel}, title = {Charge storage in electret polymers: mechanisms, characterization and applications}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus-5689}, school = {Universit{\"a}t Potsdam}, year = {2004}, abstract = {Electrets are materials capable of storing oriented dipoles or an electric surplus charge for long periods of time. The term "electret" was coined by Oliver Heaviside in analogy to the well-known word "magnet". Initially regarded as a mere scientific curiosity, electrets became increasingly imporant for applications during the second half of the 20th century. The most famous example is the electret condenser microphone, developed in 1962 by Sessler and West. Today, these devices are produced in annual quantities of more than 1 billion, and have become indispensable in modern communications technology. Even though space-charge electrets are widely used in transducer applications, relatively little was known about the microscopic mechanisms of charge storage. It was generally accepted that the surplus charges are stored in some form of physical or chemical traps. However, trap depths of less than 2 eV, obtained via thermally stimulated discharge experiments, conflicted with the observed lifetimes (extrapolations of experimental data yielded more than 100000 years). Using a combination of photostimulated discharge spectroscopy and simultaneous depth-profiling of the space-charge density, the present work shows for the first time that at least part of the space charge in, e.g., polytetrafluoroethylene, polypropylene and polyethylene terephthalate is stored in traps with depths of up to 6 eV, indicating major local structural changes. Based on this information, more efficient charge-storing materials could be developed in the future. The new experimental results could only be obtained after several techniques for characterizing the electrical, electromechanical and electrical properties of electrets had been enhanced with in situ capability. For instance, real-time information on space-charge depth-profiles were obtained by subjecting a polymer film to short laser-induced heat pulses. The high data acquisition speed of this technique also allowed the three-dimensional mapping of polarization and space-charge distributions. A highly active field of research is the development of piezoelectric sensor films from electret polymer foams. These materials store charges on the inner surfaces of the voids after having been subjected to a corona discharge, and exhibit piezoelectric properties far superior to those of traditional ferroelectric polymers. By means of dielectric resonance spectroscopy, polypropylene foams (presently the most widely used ferroelectret) were studied with respect to their thermal and UV stability. Their limited thermal stability renders them unsuitable for applications above 50 °C. Using a solvent-based foaming technique, we found an alternative material based on amorphous Teflon® AF, which exhibits a stable piezoelectric coefficient of 600 pC/N at temperatures up to 120 °C.}, subject = {Elektret}, language = {en} } @article{GanesanFruebingMellingeretal.2009, author = {Ganesan, Lakshmi Meena and Fr{\"u}bing, Peter and Mellinger, Axel and Gerhard, Reimund}, title = {Dielectric relaxation behaviour of nematic liquid crystals dispersed in poly(vinylidene fluoride- trifluoroethylene)}, issn = {0022-3727}, doi = {10.1088/0022-3727/42/9/092006}, year = {2009}, abstract = {Polymer-dispersed liquid crystals (PDLCs) are prepared from poly(vinylidene fluoride-trifluoroethylene) (P(VDF- TrFE)) and a nematic liquid-crystal (LC). The anchoring effect was studied using dielectric relaxation spectroscopy. Two dispersion regions are observed in the dielectric spectra of the pure P(VDF-TrFE) film. They are related to the glass transition and to a space-charge relaxation. In PDLC films containing 10 wt\% of LC, an additional, bias field-dependent relaxation peak is found that can be attributed to the motion of LC molecules. Due to the hindered movement of the LC molecules, this relaxation process is considerably slowed down, compared with the related process in the pure LC.}, language = {en} }