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Charakterisation and modelling of the visco-elastic, elektro-mechanical, electro-acoustical and electret properties of cellular polymer ferroelectrets

Dipl.-Ing. Dr. Mario Hubert Dansachmüller

 Dipl.-Ing. Dr. Mario Hubert Dansachmüller

Supervisory committee:

Univ.-Prof. Dr. Siegfried Bauer
Univ.-Prof. Dipl.-Ing. Dr. Bernhard Zagar

Final exam:

November 17, 2008

Charged, cellular polymer ferroelectret films have emerged recently as an interesting class of materials for electromechanical applications. Characteristic properties, typical for ferroelectrics, such as a swichable permanent polarization have motivated the introduction of the concept of a 'ferroelectret'.

This work concentrates on the investigation of strongly anisotropic polypropylene (PP) ferroelectrets (FEs), since they are available on the market in a high quality. The aim is to draw conclusions from the investigations that allow to optimize the structure of FEs and their performance in applications. Among other things, the mechanical properties and the stability of FE-PP films against ionizing alpha radiation was investigated.

PP-FEs, due to the inherent viscoelastic properties of PP, exhibit longterm deformation and creep upon mechanical loading. Thereby, the deformation of the sample is deduced from its capacitance (see Figure 1). In the course of deformation the air within the cells diffuses slowly out of the material. Therefore a diffusion barrier can be used to control longterm deformation.

The shape of the stress-strain curve of a PP-FE film depends strongly on the coupling of the force to the sample (see Figure 2). Whereas coupling with a uniform pressure distribution over the surface of the sample yields a nearly constant stiffness in the small strain regime, a strongly nonlinear stress-strain curve results when the force is coupled to the sample by rigid, smooth contact faces.
The same applies for the piezoelectric response. For applications it is therefore favourable to supply the PP-FE films with a coupling layer.

The suitability of PP-FE films for ionizing alpha radiation dosimetry was tested. The decay of the piezoelectric coefficient and therefore the internal charge was measured in-situ during irradiation with alpha particles using nonlinear capacitive dilatometry. FE-PP films of a density of about 350 kg/m³ are about 5 orders of magnitude less sensitive than commercial electret ionization chambers.
Information about the distribution of surface charge density as a function of cell height can be extracted from the characteristics of the charge decay.

Figure 1: Setup for the quasistatic measurement of the strain of, and of the electrode charge on soft, cellular PP-FE samples as a function of applied pressure.

Figure 2: Ratio of compressive strain over pressure and charge versus pressure of a PP-FE sample for the coupling with to rigid pressing faces (black symbols), and for a uniform pressure distribution over the area of the sample.