Ferroelectric and Nonferroelectric (Polar) Piezoelectric Glass–Ceramics

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.