TY - JOUR A1 - Basso, Heitor Cury A1 - Altafim, Ruy Alberto Pisani A1 - Altafim, Ruy Alberto Pisani A1 - Mellinger, Axel A1 - Fang, Peng A1 - Wirges, Werner A1 - Gerhard, Reimund T1 - Three-layer ferroelectrets from perforated Teflon-PTFE films fused between two homogeneous Teflon-FEP films Y1 - 2007 SN - 978-1-4244-1482-6 ER - TY - JOUR A1 - Flores Suárez, Rosaura A1 - Ganesan, Lakshmi Meena A1 - Wirges, Werner A1 - Gerhard, Reimund A1 - Mellinger, Axel T1 - Imaging liquid crystals dispersed in a ferroelectric polymer matrix by means of thermal-pulse tomography N2 - 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. Y1 - 2010 UR - http://ieeexplore.ieee.org/servlet/opac?punumber=94 U6 - https://doi.org/10.1109/TDEI.2010.5539683 SN - 1070-9878 ER - TY - JOUR A1 - Flores Suárez, Rosaura A1 - Mellinger, Axel A1 - Wegener, Michael A1 - Wirges, Werner A1 - Gerhard, Reimund A1 - Singh, Rajeev T1 - Thermal-pulse tomography of polarization distributions in a cylindrical geometry JF - IEEE transactions on dielectrics and electrical insulation N2 - 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. Y1 - 2006 U6 - https://doi.org/10.1109/TDEI.2006.258210 SN - 1070-9878 VL - 13 IS - 5 SP - 1030 EP - 1035 PB - IEEE CY - Piscataway ER - TY - JOUR A1 - Ganesan, Lakshmi Meena A1 - Frübing, Peter A1 - Mellinger, Axel A1 - Gerhard, Reimund T1 - Dielectric relaxation behaviour of nematic liquid crystals dispersed in poly(vinylidene fluoride- trifluoroethylene) N2 - 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. Y1 - 2009 UR - http://iopscience.iop.org/0022-3727/ U6 - https://doi.org/10.1088/0022-3727/42/9/092006 SN - 0022-3727 ER - TY - JOUR A1 - Gonzalez, Francisco Camacho A1 - Mellinger, Axel A1 - Gerhard, Reimund A1 - Santos, Lucas F. A1 - Faria, Roberto M. T1 - Photo-stimulated discharge of highly insulating polymers (PTFE and PETP) Y1 - 2002 SN - 0-7803-7502-5 ER - TY - JOUR A1 - Harris, Scott A1 - Mellinger, Axel T1 - Towards a better understanding of dielectric barrier discharges in ferroelectrets: Paschen breakdown fields in micrometer sized voids JF - Journal of applied physics N2 - Charged cellular polypropylene foams (i.e., ferro-or piezoelectrets) demonstrate high piezoelectric activity upon being electrically charged. When an external electric field is applied, dielectric barrier discharges (DBDs) occur, resulting in a separation of charges which are subsequently deposited on dielectric surfaces of internal micrometer sized voids. This deposited space charge is responsible for the piezoelectric activity of the material. Previous studies have indicated charging fields larger than predicted by Townsend's model of Paschen breakdown applied to a multilayered electromechanical model; a discrepancy which prompted the present study. The actual breakdown fields for micrometer sized voids were determined by constructing single cell voids using polypropylene spacers with heights ranging from 8 to 75 mu m, "sandwiched" between two polypropylene dielectric barriers and glass slides with semi-transparent electrodes. Subsequently, a bipolar triangular charging waveform with a peak voltage of 6 kV was applied to the samples. The breakdown fields were determined by monitoring the emission of light due to the onset of DBDs using an electron multiplying CCD camera. The breakdown fields at absolute pressures from 101 to 251 kPa were found to be in good agreement with the standard Paschen curves. Additionally, the magnitude of the light emission was found to scale linearly with the amount of gas, i.e., the height of the voids. Emissions were homogeneous over the observed regions of the voids for voids with heights of 25 lm or less and increasingly inhomogeneous for void heights greater than 40 lm at high electric fields. Y1 - 2014 U6 - https://doi.org/10.1063/1.4871678 SN - 0021-8979 SN - 1089-7550 VL - 115 IS - 16 PB - American Institute of Physics CY - Melville ER - TY - JOUR A1 - Lang, Sidney B. A1 - Mellinger, Axel T1 - A package of Mathematica programs for training students in the use of LIMM and its analysis N2 - 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 Y1 - 2005 ER - TY - JOUR A1 - Mellinger, Axel T1 - Unbiased iterative reconstruction of polarization and space-charge profiles from thermal-wave experiments N2 - The thermal-wave technique or laser-intensity modulation method is an important tool for the non-destructive probing of space-charge and polarization profiles in electrets. Analysing the experimental data requires solving a Fredholm integral equation which is known to be an ill-conditioned problem. This paper presents an iterative approach that is capable of reconstructing inherently unsmooth distributions. The deviations from the true profiles are slightly smaller than those obtained with Tikhonov regularization, while the computational burden is not a limiting factor on modem personal computers. The optimum number of iterations is estimated using the randomized generalized cross- validation technique. Results are shown for a number of model distributions, as well as for experimental data from a layered polyvinylidene fluoride film sandwich Y1 - 2004 SN - 0957-0233 ER - TY - THES A1 - Mellinger, Axel T1 - Charge storage in electret polymers: mechanisms, characterization and applications T1 - Ladungsspeicherung in Elektret-Polymeren: Mechanismen, Charakterisierung und Anwendungen N2 - 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. N2 - Elektrete sind Materialien, welche orientierte elektrische Dipole oder eine elektrische Überschussladung über längere Zeit speichern können. Der Begriff wurde 1885 von Oliver Heaviside in Anlehnung an das Wort "Magnet" eingeführt. Zunächst nur als wissenschaftliche Kuriosität betrachtet, wurden sie seit Mitte des 20. Jahrhunderts in zunehmendem Maße für technische Anwendungen interessant. Als bekanntestes Beispiel sei hier das 1962 von Sessler und West entwickelte Elektret-Kondensator-Mikrofon erwähnt, welches in jährlichen Stückzahlen von mehr als 1 Milliarde hergestellt wird und aus der modernen Kommunikationstechnik nicht mehr wegzudenken ist. Trotz der weit verbreiteten Anwendungen in der Sensorik war bisher nur wenig über die mikroskopischen Mechanismen der Ladungsspeicherung bekannt. Allgemein wird davon ausgegangen, dass die Überschussladungen in physikalischen oder chemischen Haftstellen gespeichert sind. Bisherige Experimente zur thermisch stimulierten Entladung ergaben Bindungsenergien unterhalb von 2 eV, was im Widerspruch zu den beobachteten Lebensdauern (extrapoliert wurden Werte von mehr als 100000 Jahren) steht. Mittels photostimulierter Entladung sowie simultaner Messung des Ladungsprofils konnte nun für eine Reihe wichtiger Elektret-Polymere (darunter das unter dem Handelsnamen Teflon® bekannte Polytetrafluorethylen, Polypropylen und Polyethylenterephthalat) erstmals gezeigt werden, dass zumindest ein Teil der Ladungen in tiefen Haftstellen von bis zu 6 eV gespeichert wird, was auf eine tiefgreifende lokale Strukturänderung hinweist. Ausgehend von dieser Information könnten in Zukunft Materialien mit verbesserter Ladungsspeicherung gezielt entwickelt werden. Die neuen Messungen waren erst möglich, nachdem mehrere Verfahren zur Bestimmung elektrischer, elektromechanischer und mechanischer Eigenschaften von Elektreten für einen In Situ-Einsatz weiterentwickelt wurden. So konnten z. B. durch Anregung von kurzen Wärmepulsen in der Polymerfolie Informationen über das Tiefenprofil der Raumladung in Echtzeit gewonnen werden. Die schnelle Abtastung ermöglichte darüber hinaus die dreidimensionale Kartierung von Polarisationsprofilen und Raumladungen. Ein zur Zeit sehr aktives Forschungsgebiet ist die Entwicklung piezoelektrischer Sensorfolien aus geschäumten Elektret-Polymeren. Nach elektrischer Aufladung in einer Korona-Entladung werden Ladungen an der Innenseite der Gasbläschen gespeichert, wodurch das Material piezoelektrische Eigenschaften erhält, welche deutlich besser sind als die der herkömmlichen ferroelektrischen Polymere. Für die bisher gebräuchlichen Polypropylenschäume wurde neben der Temperaturstabilität mittels dielektrischer Resonanzspektroskopie auch das Verhalten unter UV-Bestrahlung untersucht. Aufgrund ihrer beschränkten thermischen Stabilität sind diese Schäume nicht für Anwendungen oberhalb von 50 °C geeignet. Mittels eines Lösungsmittel-basierten Schäumungsverfahrens wurde ein alternativer Werkstoff auf der Basis von amorphem Teflon® AF entwickelt, welcher einen stabilen piezoelektrischen Koeffizienten von 600 pC/N bei Temperaturen von bis zu 120 °C aufweist. KW - Elektret KW - Elektretfolie KW - Polymere / Physik KW - Fluorpolymere KW - Dielektrikum KW - Ladungsspeicherung KW - Ferroelektret KW - electret KW - charge storage KW - dielectrics KW - ferroelectret Y1 - 2004 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus-5689 ER - TY - JOUR A1 - Mellinger, Axel A1 - Camacho González, Francisco A1 - Gerhard, Reimund T1 - Photostimulated discharge in electret polymers : an alternative approach for investigating deep traps N2 - 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 Y1 - 2004 ER -