Improvement of piezoelectric activity of cellular polymers using a double-expansion process

Abstract

Expansion of cellular polypropylene films through an increase in gas pressure and subsequent pressure release at elevated temperatures prior to charging is known to enhance the piezoelectric d33-coefficient of the material. By means of a second pressure expansion the piezoelectric activity can be further increased by more than 40% in comparison with samples subjected to only a single expansion. The effectiveness of the double-expansion process must be attributed to the gain in thickness through the second expansion, following the charging and metallization processes. This thickness change causes a decrease in Young's modulus and thus an increase in d33. Typical d33-coefficients of 1400 pC N−1 at 0.01 Hz and about 500 pC N−1 at 25 kHz have been achieved.