@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{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.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{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{FruebingKremmerGerhardetal.2006, author = {Fr{\"u}bing, Peter and Kremmer, Alexander and Gerhard, Reimund and Spanoudaki, Anna and Pissis, Polycarpos}, title = {Relaxation processes at the glass transition in polyamide 11: From rigidity to viscoelasticity}, series = {The journal of chemical physics : bridges a gap between journals of physics and journals of chemistr}, volume = {125}, journal = {The journal of chemical physics : bridges a gap between journals of physics and journals of chemistr}, number = {12}, publisher = {American Institute of Physics}, address = {Melville}, issn = {0021-9606}, doi = {10.1063/1.2360266}, pages = {8}, year = {2006}, abstract = {Relaxation processes associated with the glass transition in nonferroelectric and ferroelectric polyamide (PA) 11 are investigated by means of differential scanning calorimetry, dynamic mechanical analysis, and dielectric relaxation spectroscopy (DRS) in order to obtain information about the molecular mobility within the amorphous phase. In particular, the effects of melt quenching, cold drawing, and annealing just below the melting region are studied with respect to potential possibilities and limitations for improving the piezoelectric and pyroelectric properties of PA 11. A relaxation map is obtained from DRS that shows especially the crossover region where the cooperative alpha relaxation and the local beta relaxation merge into a single high-temperature process. No fundamental difference between quenched, cold-drawn, and annealed films is found, though in the cold-drawn (ferroelectric) film the alpha relaxation is suppressed and slowed down, but it is at least partly recovered by subsequent annealing. It is concluded that there exists an amorphous phase in all structures, even in the cold-drawn film. The amorphous phase can be more rigid or more viscoelastic depending on preparation. Cold drawing not only leads to crystallization in a ferroelectric form but also to higher rigidity of the remaining amorphous phase. Annealing just below the melting region after cold drawing causes a stronger phase separation between the crystalline phase and a more viscoelastic amorphous phase.}, 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{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{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{AltafimBassoAltafimetal.2006, author = {Altafim, Ruy Alberto Pisani and Basso, Heitor Cury and Altafim, Ruy Alberto Pisani and Lima, L. and de Aquino, C. V. and Goncalves Neto, Luiz and Gerhard, Reimund}, title = {Piezoelectrets from thermo-formed bubble structures of fluoropolymer-electret films}, issn = {1070-9878}, doi = {10.1109/TDEI.2006.247822}, year = {2006}, abstract = {The acetone extracts of the root bark and stem bark of Erythrina sacleuxii showed antiplasmodial activities against the chloroquine-sensitive (D6) and chloroquine-resistant (W2) strains of Plasmodium falciparum. Chromatographic separation of the acetone extract of the root bark afforded a new isoflavone, 7-hydroxy-4 -methoxy-3'- prenylisoflavone (trivial name 5-deoxy-3' - prenylbiochanin A) along with known isoflavonoids as the antiplasmodial principles. Flavonoids and isoflavonoids isolated from the stem bark of E. sucleuxii were also tested and showed antiplasmodial activities. The structures were determined on the basis of spectroscopic evidence.}, language = {en} }