Ferroelectric and Nonferroelectric (Polar) Piezoelectric Glass–Ceramics

Abstract

An increased need for high‐temperature piezoelectric materials for sensors, some of which must be Pb free due to RoHS regulations, has led to a focused search for suitable materials. Glass–ceramic processing—the controlled crystallization of a precursor glass—offers a unique manner in which to produce partially to wholly crystalline, Pb‐free, and temperature‐stable piezoelectric materials starting with optically homogeneous amorphous materials. Building on previously published work, we have produced NaNbO3‐containing, poled, and pore‐free ferroelectric glass–ceramics that exhibit d33 values of ∼15 pC/N, a dielectric constant of ∼200, an Np frequency constant of ∼3400 Hz·m, and Qm∼60. Nonferroelectric, lithium borosilicate polar glass–ceramics—initially developed by R.E. Newnham and coworkers at Penn State some 20 years ago—have also been produced and yielded d33 values of ∼5 pC/N, although with dielectric constants of <10 they achieved significant g33 values (∼50 × 10−3 V m/N; Np∼4500 Hz·m; Qm∼1500). Room‐temperature planar coupling coefficients of 0.15 and 0.10 were obtained for the polar and ferroelectric varieties, respectively. High‐temperature resonance measurements of both varieties reveal piezoelectricity to at least 600°C for the polar glass–ceramic and up to 300°C for the ferroelectric variety. Excessive conductivity in the polar type, presumably due to high lithium contents, resulted in a strong decrease in resonance amplitude as the temperature was increased. Interestingly, the estimated piezoelectric coefficients for this type showed nearly no temperature dependence and suggest that polar glass–ceramics, lacking a Curie temperature, potentially offer a unique route to high‐temperature piezoelectrics.