@article{KofodWirgesPaajanenetal.2007, author = {Kofod, Guggi and Wirges, Werner and Paajanen, Mika and Bauer, Siegfried}, title = {Energy minimization for self-organized structure formation and actuation}, issn = {0003-6951}, doi = {10.1063/1.2695785}, year = {2007}, abstract = {An approach for creating complex structures with embedded actuation in planar manufacturing steps is presented. Self-organization and energy minimization are central to this approach, illustrated with a model based on minimization of the hyperelastic free energy strain function of a stretched elastomer and the bending elastic energy of a plastic frame. A tulip-shaped gripper structure illustrates the technological potential of the approach. Advantages are simplicity of manufacture, complexity of final structures, and the ease with which any electroactive material can be exploited as means of actuation. (c) 2007 American Institute of Physics.}, 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} } @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{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{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{LancaWirgesNeaguetal.2007, author = {Lanca, M. C. and Wirges, Werner and Neagu, Eugen R. and Gerhard, Reimund and Marat-Mendes, Jos{\´e} Narciso}, title = {Influence of humidity on the electrical charging properties of cork agglomerates}, issn = {0022-3093}, doi = {10.1016/j.jnoncrysol.2007.03.037}, year = {2007}, abstract = {Cork is a natural cellular and electrically insulating material which may have the capacity to store electric charges on or in its cell walls. Since natural cork has many voids, it is difficult to obtain uniform samples with the required dimensions. Therefore, a more uniform material, namely commercial cork agglomerate, usually used for floor and wall coverings, is employed in the present study. Since we know from our previous work that the electrical properties of cork are drastically affected by absorbed and adsorbed water, samples were protected by means of different polymer coatings (applied by spin-coating or soaking). Corona charging and isothermal charging and discharging currents were used to study the electrical trapping and detrapping capabilities of the samples. A comparison of the results leads to the conclusion that the most promising method for storing electric charges in this cellular material consists of drying and coating or soaking with a hydrophobic, electrically insulating polymer such as polytetraflouroethylene (Teflon (R)). (c) 2007 Elsevier B.V. All rights reserved.}, language = {en} } @article{FangWegenerWirgesetal.2007, author = {Fang, Peng and Wegener, Michael and Wirges, Werner and Gerhard, Reimund and Zirkel, Larissa}, title = {Cellular polyethylene-naphthalate ferroelectrets : foaming in supercritical carbon dioxide, structural and electrical preparation, and resulting piezoelectricity}, issn = {0003-6951}, doi = {10.1063/1.2738365}, year = {2007}, abstract = {Polymer foams with electrically charged cellular voids, the so-called ferroelectrets, are soft piezoelectric transducer materials. Several polymers such as polyethylene terephthalate or cyclo-olefin copolymers are under investigation with respect to their suitability as ferroelectrets. Here, the authors report an additional ferroelectret polymer, cellular polyethylene-naphthalate (PEN), which was prepared from commercial uniform polymer films by means of foaming in supercritical carbon dioxide, inflation, biaxial stretching, electrical charging, and metallization. Piezoelectric d(33) coefficients of up to 140 pC/N demonstrate the suitability of such cellular PEN films for transducer applications. Their piezoelectricity is partially stable at elevated temperatures as high as 100 degrees C.}, language = {en} } @article{WirgesWegenerVoroninaetal.2007, author = {Wirges, Werner and Wegener, Michael and Voronina, Olena and Zirkel, Larissa and Gerhard, Reimund}, title = {Optimized preparation of elastically soft, highly piezoelectric, cellular ferroelectrets from nonvoided poly(ethylene terephthalate) films}, doi = {10.1002/adfm.200600162}, year = {2007}, abstract = {Electrically charged cellular polymer films can exhibit very high piezoelectric activity and are therefore more and more often employed in advanced electromechanical and electro-acoustical transducers. In this paper, we report an optimized sequence of steps for preparing such ferroelectrets from commercial nonvoided ploy(ethylene terephthalate) (PETP) films by means of foaming with CO2 biaxial mechanical stretching, controlled void inflation, and bipolar electric charging. The nonvoid PETP films foamed with supercritical CO2 at a suitably high pressure and subsequently annealed for stabilization. The cellular foam structure was further optimized by means of well controlled biaxial stretching in a commercial stretcher and sometimes subsequent inflamation in a pressure chamber. Bipolar electric charging of the internal voids was achieved through the application of high electric fields in an SF0 atmosphere. The new optimized PETP ferroelectric exhibit quite large piezoelectric coefficients up to almost 500 pCN(-1), for which unusually low elastic stiffness of only around 0.3 MPa are essential. The PETP foam ferroelectrics posses unclamped thickenss-extension resonance frequences between approximately 120 and 250 kHz, and are thus highly suitable for several established as well as novel ultrasonic-transductant applications.}, language = {en} }