New piezoelectric materials (electrets)

Electrets are materials which maintain electric charge for long time after polarisation process. They are used to realize piezoelectric transducer (i.e. they give an electrical signal related to the force applied to them) and capacitors of elevated specific capacitance. As an example, they are optimal for large-area sensors for monitoring e.g. human functions when implemented on a floor, chair or bed. They can be also used e.g. in security systems, for flat loudspeakers and active panels to reduce sound levels in noisy environments like vehicles. Since few years ago, materials suitable for these purposes were constituted by rare-earth electrets, but these materials are expensive and hard to work. In the last years, the possibility to realize electrets by means of cellular polymeric materials was discovered; these materials are easy workable. Cellular polymeric electrets are obtained introducing filler particles (as talc, mica, calcium carbonate) in a polymer during polymerisation. Subsequently, during the phase of extrusion of the material, these particles evolve gas, which expands the material producing many cavities. At last, cavities are lengthened and deformed by means of a bi-axial stretching. Figure 1 reports a photography of a cellular polymeric electret, realized by means of a Scanning Electron Microscope; both filler particles and cavities are easily recognizable in this picture.

The aim of the tests carried out at LIMAT is to characterize a cellular polymer electret films with giant electro-mechanical transducer coefficients, and with features stable with time and temperature (up to 110C). In order to reach this objective, the following activities have been developed: 

  • investigate the presence of space charge and polarisation, by means of the PEA (Pulse Electro-Acoustic) system;

  • investigate the variation of space charge by application of electric field;

  • investigate the possibility of injecting charge by application of a suitable DC field;

  • infer the poling field value at which material starts to store charge (Eta threshold for charge accumulation, see Fig. 2), and the poling field value at which material becomes electret (Ete threshold for electret behaviour, see Fig. 2);

  • investigate the behavior of material as a function of time and temperature;

  • investigate charge features by partial discharge measurements;

  • model charging features by analytical models.

References

[1] G.C. Montanari, "Electrical measurements for the characterisation of electret properties of cellular polymers", IEEE Trans. on Dielectrics and Electrical Insulation, Vol. 10, n. 4, pp., agosto 2003.