@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{WegenerWirgesGerhardetal.2004,
  author    = {Wegener, Michael and Wirges, Werner and Gerhard, Reimund and Dansachm{\"u}ller, M. and Schw{\"o}diauer, R. and Bauer-Gogonea, Simona and Bauer, Siegfried and Paajanen, Mika and Minkkinen, Hannu and Raukola, J.},
  title     = {Controlled inflation of voids in cellular polymer ferroelectrets : optimizing electromechanical transducer properties},
  year      = {2004},
  abstract  = {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},
  language  = {en}
}
@article{WegenerPaajanenWirgesetal.2002,
  author    = {Wegener, Michael and Paajanen, Mika and Wirges, Werner and Gerhard, Reimund},
  title     = {Corona-induced partial discharges, internal charge separation and electromechanical transducer properties in cellular polymer films},
  isbn      = {0-7803-7560-2},
  year      = {2002},
  language  = {en}
}
@article{SaarimaekiPaajanenSavijaervietal.2006,
  author    = {Saarim{\"a}ki, Eetta and Paajanen, Mika and Savij{\"a}rvi, Ann-Mari and Minkkinen, Hannu and Wegener, Michael and Voronina, Olena and Schulze, Robert and Wirges, Werner and Gerhard, Reimund},
  title     = {Novel heat durable electromechanical film : processing for electromechanical and electret applications},
  year      = {2006},
  abstract  = {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.},
  language  = {en}
}
@article{PaajanenWegenerGerhard2001,
  author    = {Paajanen, Mika and Wegener, Michael and Gerhard, Reimund},
  title     = {Charging of cellular space-charge electret films in various gas atmospheres},
  isbn      = {0-7803-7053-8},
  year      = {2001},
  language  = {en}
}
@article{PaajanenWegenerGerhard2001,
  author    = {Paajanen, Mika and Wegener, Michael and Gerhard, Reimund},
  title     = {Understanding the role of the gas in the voids during corona charging of cellular electret films : a way to enhance their piezoelectricity},
  year      = {2001},
  language  = {en}
}
@article{MontanariFabianiCianietal.2007,
  author    = {Montanari, Gian Carlo and Fabiani, Davide and Ciani, Franco and Motori, Antonio and Paajanen, Mika and Gerhard, Reimund and Wegener, Michael},
  title     = {Charging properties and time-temperature stability of innovative polymeric cellular ferroelectrets},
  issn      = {1070-9878},
  doi       = {10.1109/TDEI.2007.302892},
  year      = {2007},
  abstract  = {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.},
  language  = {en}
}
@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{KofodPaajanenBauer2006,
  author    = {Kofod, Guggi and Paajanen, Mika and Bauer, Siegfried},
  title     = {Self-organized minimum-energy structures for dielectric elastomer actuators},
  issn      = {0947-8396},
  doi       = {10.1007/s00339-006-3680-3},
  year      = {2006},
  language  = {en}
}
@article{GerhardWegenerWirgesetal.2002,
  author    = {Gerhard, Reimund and Wegener, Michael and Wirges, Werner and Giacometti, J. A. and Altafim, Ruy Alberto Pisani and Santos, Lucas F. and Faria, Roberto M. and Paajanen, Mika},
  title     = {Electrode polling of cellular polypropylene films with short high-voltage pulses},
  isbn      = {0-7803-7502-5},
  year      = {2002},
  language  = {en}
}