TY - JOUR A1 - Wegener, Michael A1 - Wirges, Werner A1 - Paajanen, Mika A1 - Gerhard, Reimund T1 - Charging behaviour and thermal stability of porous and non-porous polytetrafluoroethylene (PTFE) electrets Y1 - 2007 SN - 978-1-4244-1482-6 ER - TY - JOUR A1 - Wegener, Michael A1 - Wirges, Werner A1 - Gerhard, Reimund A1 - Dansachmüller, M. A1 - Schwödiauer, R. A1 - Bauer-Gogonea, Simona A1 - Bauer, Siegfried A1 - Paajanen, Mika A1 - Minkkinen, Hannu A1 - Raukola, J. T1 - Controlled inflation of voids in cellular polymer ferroelectrets : optimizing electromechanical transducer properties N2 - When exposed to sufficiently high electric fields, polymer-foam electret materials with closed cells exhibit ferroelectric-like behavior and may therefore be called ferroelectrets. In cellular ferroelectrets, the influence of the cell size and shape distributions on the application-relevant properties is not yet understood. Therefore, controlled inflation experiments were carried out on cellular polypropylene films, and the resulting elastical and electromechanical parameters were determined. The elastic modulus in the thickness direction shows a minimum with a corresponding maximum in the electromechanical transducer coefficient. The resonance frequency shifts as a function of the elastic modulus and the relative density of the inflated cellular films. Therefore, the transducer properties of cellular ferroelectrets can be optimized by means of controlled inflation. (C) 2004 American Institute of Physics Y1 - 2004 ER - TY - JOUR A1 - Wegener, Michael A1 - Paajanen, Mika A1 - Wirges, Werner A1 - Gerhard, Reimund T1 - Corona-induced partial discharges, internal charge separation and electromechanical transducer properties in cellular polymer films Y1 - 2002 SN - 0-7803-7560-2 ER - TY - JOUR A1 - Saarimäki, Eetta A1 - Paajanen, Mika A1 - Savijärvi, Ann-Mari A1 - Minkkinen, Hannu A1 - Wegener, Michael A1 - Voronina, Olena A1 - Schulze, Robert A1 - Wirges, Werner A1 - Gerhard, Reimund T1 - Novel heat durable electromechanical film : processing for electromechanical and electret applications N2 - New ferroelectrets were developed on the basis of foams from cyclo-olefin polymers and copolymers. The results obtained on the cyclo-olefin polymer foam demonstrate a significant improvement of the service temperature for ferroelectret transducer materials. Suitable compounding and preparation led to cyclo-olefin ferroelectrets with an electromechanical activity of around 15 pC/N, which is thermally stable at least up to 110 degrees C. The properties in sensor and actuator applications are strongly dependent on the processing parameters related to film-making, sensor and actuator preparation, gas content and electric charging. The processing window for the film stretching was very narrow compared to the earlier developed polypropylene ferroelectrets. The film porosity, softness and thus the electromechanical activity are adjusted by gas-diffusion expansion. The activity of the electromechanically operating sensors and actuators was increased by stacking several layers of cellular cyclo-olefin film. For applications such as flat loudspeakers, the foamed films are tuned by tensioning them on a support frame. Correct tensioning was essential also for reducing the distortion levels. Y1 - 2006 ER - TY - JOUR A1 - Paajanen, Mika A1 - Wegener, Michael A1 - Gerhard, Reimund T1 - Charging of cellular space-charge electret films in various gas atmospheres Y1 - 2001 SN - 0-7803-7053-8 ER - TY - JOUR A1 - Paajanen, Mika A1 - Wegener, Michael A1 - Gerhard, Reimund T1 - Understanding the role of the gas in the voids during corona charging of cellular electret films : a way to enhance their piezoelectricity Y1 - 2001 ER - TY - JOUR A1 - Montanari, Gian Carlo A1 - Fabiani, Davide A1 - Ciani, Franco A1 - Motori, Antonio A1 - Paajanen, Mika A1 - Gerhard, Reimund A1 - Wegener, Michael T1 - Charging properties and time-temperature stability of innovative polymeric cellular ferroelectrets N2 - After appropriate mechanical and electrical treatments, some cellular polymers become able to retain space charge for a long time, i.e. they acquire electret behavior. The electrical treatment consists of charging under high levels of DC electric field. The mechanical treatment, based on the application of stretching forces to cellular polymer slabs that were before expanded under pressurized gas, affects the cavity size and shape, and therefore also the effectiveness of the charging process itself. An investigation of charging mechanisms, as well as of mechanical treatment, is therefore fundamental for optimizing the ferro- and piezo-electret properties. The aim of this paper is to discuss the effect of the physical dimension of the cavities on the charging behavior of cellular ferroelectrets and to focus on the time-temperature stability for two families of polymeric cellular ferroelectrets based on polypropylene (PP) and on a cyclo-olefin copolymer (COC). Emphasis will be given to the stretching process and in particular to the expansion rate applied during the manufacturing process (which affects the radial dimension and the height of the cavities, respectively). Space-charge and partial-discharge measurements as a function of time and temperature are the main tools to infer the influence of the cavity size on charging and stability characteristics. Y1 - 2007 UR - http://ieeexplore.ieee.org/servlet/opac?punumber=94 U6 - https://doi.org/10.1109/TDEI.2007.302892 SN - 1070-9878 ER - TY - JOUR A1 - Kofod, Guggi A1 - Wirges, Werner A1 - Paajanen, Mika A1 - Bauer, Siegfried T1 - Energy minimization for self-organized structure formation and actuation N2 - 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. Y1 - 2007 UR - http://apl.aip.org/ U6 - https://doi.org/10.1063/1.2695785 SN - 0003-6951 ER - TY - JOUR A1 - Kofod, Guggi A1 - Paajanen, Mika A1 - Bauer, Siegfried T1 - Self-organized minimum-energy structures for dielectric elastomer actuators Y1 - 2006 UR - http://www.springerlink.com/content/100501 U6 - https://doi.org/10.1007/s00339-006-3680-3 SN - 0947-8396 ER - TY - JOUR A1 - Gerhard, Reimund A1 - Wegener, Michael A1 - Wirges, Werner A1 - Giacometti, J. A. A1 - Altafim, Ruy Alberto Pisani A1 - Santos, Lucas F. A1 - Faria, Roberto M. A1 - Paajanen, Mika T1 - Electrode polling of cellular polypropylene films with short high-voltage pulses Y1 - 2002 SN - 0-7803-7502-5 ER -