Polyimide Aerogel‐Enabled Total Release of Elastic Constraints in 1‐3 Piezoelectric Composites for Ultrahigh Electromechanical Performance

ABSTRACT Thickness electromechanical coupling factor k t and piezoelectric coefficient d 33 determine the energy conversion efficiency as well as the bandwidth and sensitivity of ultrasonic transducers. Due to lateral constrains, k t is only about 65% of k 33 . 1‐3 piezocomposite was invented to resolve this issue, which can increase k t to 90% or more of k 33 if the piezoelectric content is very low. But low piezoelectric content will reduce the piezoelectric coefficient d 33 so that the transducer sensitivity will be compromised. By using 7 vol% polyimide aerogel as the filler, the 1‐3 Pb(Mg 1/3 Nb 2/3 )O 3 ‐PbTiO 3 single crystal aerogel composite exhibits a k t of 93%, the highest ever reported, matching the k 33 of the single crystal while maintaining an uncompromised d 33 of 2145 pC/N. Finite element simulations and experimental results confirmed that the polyimide aerogel strategy is universally applicable to other 1‐3 piezoelectric material. The electromechanical coupling energy density of the composite plate got more than an order of magnitude improvement compared to PMN‐PT single crystal plate resonator, and the theoretical bandwidth limit has been increased to 248%, raising high expectations of transducer engineers to make next‐generation high‐sensitivity and ultrabroadband ultrasonic transducers.