Refine
Has Fulltext
- no (2)
Document Type
- Article (2)
Language
- English (2)
Is part of the Bibliography
- yes (2)
Institute
Polymer foams with electrically charged cellular voids, the so-called ferroelectrets, are soft piezoelectric transducer materials. Several polymers such as polyethylene terephthalate or cyclo-olefin copolymers are under investigation with respect to their suitability as ferroelectrets. Here, the authors report an additional ferroelectret polymer, cellular polyethylene-naphthalate (PEN), which was prepared from commercial uniform polymer films by means of foaming in supercritical carbon dioxide, inflation, biaxial stretching, electrical charging, and metallization. Piezoelectric d(33) coefficients of up to 140 pC/N demonstrate the suitability of such cellular PEN films for transducer applications. Their piezoelectricity is partially stable at elevated temperatures as high as 100 degrees C.
Cellular polyethylene-naphthalate (PEN) ferroelectrets are useful as soft and flexible electromechanical transducer materials. Improved cellular PEN foams are prepared by means of a "voiding + inflation + stretching" process and investigated with respect to their structure and their applications-relevant properties. It is found that most of the cellular voids have heights below 8 mu m. The polymer walls do not allow sufficient gas exchange between the voids and the ambient atmosphere, when the cellular films are exposed to atmospheric pressures between a millibar and a few bars. As expected for ferroelectrets, a threshold voltage for charging is observed: A reasonable piezoelectric coefficient d(33) is only found when the charging voltage is higher than 4 kV. Furthermore, d(33) increases with charging voltage and reaches saturation at approximately 8 kV. Annealing after charging or charging at elevated temperatures may enhance the thermal stability of the PEN ferroelectrets. The d(33) of properly annealed samples is stable up to the respective annealing temperatures, but the annealing process reduces the piezoelectric activity of charged ferroelectret films to some extent. Samples charged at suitable elevated temperatures show much better thermal stability than those charged at room temperature, but the charging temperature should be limited to values below the material's glass-transition temperature T-g. Furthermore, the relevant elastic modulus c(33) of PEN ferroelectrets may decrease upon thermal treatment.