@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{LangMellinger2005, author = {Lang, Sidney B. and Mellinger, Axel}, title = {A package of Mathematica programs for training students in the use of LIMM and its analysis}, year = {2005}, abstract = {A package of programs written using the symbolic mathematics program, Mathematicatrademark, has been developed. Its principal usage is in teaching the LIMM method to students and demonstrating data analysis by means of the Polynomial Regularization Method (PRM). A large number of variables can be changed in the program and their effects can be shown graphically. Students at the University of Potsdam and Monash University have used the program successfully}, 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{MellingerMellinger2011, author = {Mellinger, Axel and Mellinger, Olena}, title = {Breakdown threshold of dielectric barrier discharges in ferroelectrets where Paschen's law fails}, 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.2011.5704491}, pages = {43 -- 48}, year = {2011}, abstract = {The piezoelectric activity of charged cellular foams (so-called ferroelectrets) is compared against simulations based on a multi-layer electromechanical model and Townsend's model of Paschen breakdown, with the distribution of void heights determined from scanning electron micrographs. While the calculated space charge hysteresis curves are in good agreement with experimental data, the onset of piezoelectric activity is observed at significantly higher electric fields than predicted by Paschen's law. One likely explanation is that the commonly accepted Paschen curve for electric breakdown in air poorly describes the critical electric field for dielectric barrier discharges in micrometer-size cavities.}, 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{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{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} } @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{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{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} }