Advanced piezoelectric materials for sensing, integrated autonomous micro-power sources or frequency-control that could be operated at high temperatures are in particular interest for aerospace, automotive and energy industries. The sensing principles of such piezoelectric resonators are based on resonance frequency shifts that arise from external factors, such as varying temperature or mass load. Practical applications of such materials, especially at elevated temperatures, require minimization of electromechanical losses and, thereby maximization of the application relevant quality factor Q, since low Q results in uncertainty in resonant frequency determination and, therefore, limits the resolution in sensing applications.
Crystals of the langasite (La3Ga5SiO14, LGS) family are promising piezoelectric materials, which possess relatively high electromechanical coupling coefficients and could be operated even at temperatures above 1300 °C. This work focuses on acoustic loss contributions in LGS and catangasite (Ca3TaGa3Si2O14, CTGS) single crystals as a function of temperature up to 900 °C. Obtained results are described in the framework of a model that considers different physical mechanisms including phonon-phonon interactions, anelastic point defect relaxations and conductivity-related relaxations of charge carriers. Finally, steps for loss minimization are discussed.
The crystals for this study were grown by Fomos Materials (Moscow, Russia) and IKZ (Berlin). The high temperature measurements were performed using Y-cut CTGS resonators, operated in the thickness-shear mode (TSM). Two different measurement techniques, namely resonant ultrasound spectroscopy on Pt-electroded samples as well as non-contacting tone-burst excitation technique on samples without electrodes were applied. The study revealed that a superposition of several physical mechanisms, including phonon-phonon interactions (Akhiezer relaxation), point-defect relaxations and conductivity-related relaxation determines the losses in CTGS and in LGS.