Novel lead-free composites with two porosity levels: large piezoelectric anisotropy and high sensitivity

Abstract

A novel lead-free 1–3-type composite based on a ferroelectric domain-engineered single crystal is put forward. In the porous polymer matrix of this composite, two different porous structures are observed, and the effect of these structures on the piezoelectric performance, electromechanical coupling and related anisotropy parameters of 1–3-type composites is first studied. New diagrams that link the volume fractions of the single-crystal component and of the porous regions in the polymer medium are built to show validity of conditions for a large anisotropy of piezoelectric coefficients d_3j* and electromechanical coupling factors k_3j*, k_t* and k_p*. In the composites based on the complex alkali niobate – alkali tantalate single crystal with small piezoelectric anisotropy ( d_33 / | d_31 | = 2.1), the three anisotropy factors d_33* / |d_31*| 5, k_33* / |k_31*| 5 and k_t* / |k_p*| 5 hold simultaneously due to the presence of layers with heavily prolate and heavily oblate air pores in the porous polymer matrix. The two porosity levels influence the elastic anisotropy of the porous matrix, and this leads to an increase in the three anisotropy factors across wide volume-fraction ranges. Of independent interest is the high piezoelectric sensitivity of the composites for which the condition g-33* > 1 mV m N^-1 holds at their piezoelectric coefficient d_33 ≈ (200–500) pC N^-1 and electromechanical coupling factors k_33* ≈ k_t* ≈ 0.8–0.9. The studied parameters of the novel piezo-active 1–3-type composites are of value for various applications such as active elements of piezoelectric transducers, energy-harvesting devices and sensors.