High-temperature piezoelectric actuators are demanded in automotive, aerospace, and related industrial applications. However, the application of common piezoelectric materials is limited at elevated temperatures. On the one hand, polycrystalline ceramic actuators exhibit thermal instability above about 200°C. On the other hand, the piezoelectric coefficients of high-temperature stable piezoelectric crystals such as langasite are too small for actuating applications.
Lithium niobate (LiNbO₃) and lithium tantalate (LiTaO₃) possess high piezoelectric coefficients, however their usage is limited by thermal instability (LiNbO₃) and low Curie temperature (LiTaO₃) above about 300°C. In this respect Li(Nb,Ta)O₃ solid solutions are expected to overcome the above-mentioned restrictions of the individual compounds. Further, it is known that the intrinsic properties of LiNbO₃, LiTaO₃ and, consequently, of Li(Nb,Ta)O₃ solid solutions are strongly dependent on lithium stoichiometry. In this work the electrical and acoustic properties of Li(Nb,Ta)O₃ solid solutions with different Nb/Ta ratios and variable Li-stoichiometry are studied at high temperatures and in a wide oxygen partial pressure (p_O2) range by means of impedance spectroscopy and resonant piezoelectric spectroscopy.
The bulk single crystals are grown by two different methods namely micro-pulling-down and Czochralski techniques. The off-congruent samples were prepared by the vapor transport equilibration technique (VTE). The high-temperature experiments are performed on platinum-electroded samples in a gas-tight tube furnace, which allows working temperatures up to 1000 °C. The adjustment of p_O2 is realized using an oxygen ion pump in the range from 10^-15 to 10^-3 bar by well-defined Ar/H₂/O₂ gas mixtures.
The investigations revealed that at 930°C the decrease of p_O2 leads to the conductivity increase in all measured samples, since the conductivity at low p_O2 is governed by the transport of electrons. However Ta-rich samples show much smaller increase, comparing to LiNbO₃ indicating that increased Ta content improves the stability of Li(Nb,Ta)O₃ (Fig.1).
Moreover, the conductivity of near stoichiometric Li(Nb,Ta)O₃ samples was found substantially lower in the measured range, compared to the Li-deficient samples with the same Nb/Ta ratio.