Large and broadband piezoelectricity in smart polymer-foam space-charge electrets

Charged closed-cell microporous polypropylene foams are shown to exhibit piezoelectric resonance modes in the dielectric function, coupled with a large anisotropy in the electromechanical and elastic material properties. Strong direct and converse dynamic piezoelectricity with a piezoelectric $d_{33}$ coefficient of 140 pC/N at 600 kHz is identified. The piezoelectric $d_{33}$ coefficient exceeds that of the ferroelectric polymer polyvinylidene fluoride by a factor of 5 and compares favorably with ferroelectric ceramics. Applications of similar concepts should provide a broad class of easily fabricated “soft” piezoelectric materials.