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Publications (2)3.79 Total impact

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    ABSTRACT: Nonlinearities in ferroelectret polymer foam capacitors arise from voltage-dependent thickness changes. Such thickness changes are caused by the converse piezoelectric and electrostrictive effects in these soft materials. The authors show that the higher harmonics of the current response during application of a sinusoidal voltage to ferroelectret capacitors provide information on the elastic and electromechanical properties of the foam. The authors demonstrate the potential of this versatile measurement technique by investigating the temperature dependence of the piezoelectric response and by monitoring the changes in the elastic and electromechanical properties during inflation of cellular polypropylene.
    Applied Physics Letters 10/2007; · 3.79 Impact Factor
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    ABSTRACT: Ferroelectrets are closed-cell polymer foams that behave like ferroelectric materials during and after internally charging the voids of the cellular structure. In order to optimize the electromechanical properties of ferroelectrets, the voids within the cellular polymer are expanded in a controlled manner in a high pressure gas chamber. Thereby, the gas pressure is first increased, subsequently released and finally the expanded structure is stabilized by a heat setting treatment at elevated temperatures. Dielectric dilatometry provides an easy means for the in-situ monitoring of the multi-step foaming process, and therefore allows for the control of the degree of foaming. The mechanical stabilization of the expanded cellular polymer structure is explained in terms of the enhanced crystallinity as a result of the heat setting process. A simple model for the gas exchange process between the gas in the voids and the gas in the high pressure gas chamber is developed. For the model a simplification of the cellular structure is used, where an air-gap is separated from the electrodes by two polymer layers. The gas transport through the polymer is modelled by Fick's law with permeabilities taken from the solution diffusion gas transport model. Good agreement between model calculations and experiment is obtained for the diffusion expansion of cellular polypropylene with CO<sub>2</sub> and He as blowing agents.
    Electrets, 2005. ISE-12. 2005 12th International Symposium on; 10/2005