High-Temperature Ultrasound Attenuation in Langasite and Langatate

Evaluation of Ru-Ti Electrode-Based TSM Langasite Resonators for High-Temperature Applications

High-temperature (HT) properties of a thickness-shear mode (TSM) langasite resonator with Ru-Ti electrodes are reported for the first time. Resonators with 300 nm Ru and 15 nm Ti films as the primary and adhesive electrode layers, respectively, were investigated and compared against those with Au-Cr and Au-Ti electrodes. HT stability of the fabricated samples under continuous excitation were examined up to 750 °C by monitoring their morphological changes, sheet resistance, resonance parameters, and their equivalent circuit elements. Results indicate that for Ru-Ti electrodes, a polycrystalline RuO₂ cover layer was formed on the surface of Ru, which protected the underlying layer from further oxidation. Consequently, the electrical and motional resistances of the Ru-Ti sample experienced the least change post-annealing, which was also reflected in its ability to retain the highest Q -factor after heat treatment. Ru-Ti-based resonator also exhibited comparable performance to other samples in terms of resonant frequency shifts and second-order temperature coefficients, further strengthening the position of Ru as a suitable alternative to other electrode materials.

Microwave Acoustic Attenuation in CTGS Single Crystals

This article presents a study of special material properties of the single crystalline material Ca₃TaGa₃Si₂O₁₄ (CTGS = Catangasite). The comparatively highly ordered crystal structure and acceptable piezoelectric strength make it a candidate for microacoustic applications under extreme conditions. Obviously, low-loss dynamic behavior is typical for this crystal which consequently enables high-temperature use. As a particular challenge, the behavior at gigahertz frequencies is investigated here. For that, High overtone Bulk Acoustic wave Resonator (HBAR) type measurements in the range of 1-6 GHz are performed. The selection of five distinctive propagation directions for exclusively pure or quasi-longitudinal modes enables to derive the dynamic viscosities from the quality factors of HBAR results. The observed frequency dependences exhibit Akhiezer behavior as the predominant loss mechanism in the cases examined.

Optimal Design of TSM Langasite Resonator for High-Temperature Applications: A Review

In this review article, we address two vital design considerations that govern the high-temperature operation of a thickness-shear mode langasite resonator: 1) electrode design and 2) electrode material. Optimal electrode designs to mitigate unwanted spurious modes and achieve a high Q-factor for fundamental and higher overtone modes have been discussed in great detail. Governing equations that determine the size, shape, and orientation of these electrodes have also been presented. In addition, the suitability of six platinum-group metals as electrode materials for high-temperature resonators have been assessed and summarized. Furthermore, the adhesion to the substrate, electrical conductivity, thermal stability, and various other temperature-dependent properties of these metals have been discussed. Finally, several combinations and operating ranges of these electrode materials have been thoroughly evaluated.

Correlation of Electrical Properties and Acoustic Loss in Single Crystalline Lithium Niobate-Tantalate Solid Solutions at Elevated Temperatures

Electrical conductivity and acoustic loss Q−1 of single crystalline Li(Nb,Ta)O3 solid solutions (LNT) are studied as a function of temperature by means of impedance spectroscopy and resonant piezoelectric spectroscopy, respectively. For this purpose, bulk acoustic wave resonators with two different Nb/Ta ratios are investigated. The obtained results are compared to those previously reported for congruent LiNbO3. The temperature dependent electrical conductivity of LNT and LiNbO3 show similar behavior in air at high temperatures from 400 to 700 °C. Therefore, it is concluded that the dominant transport mechanism in LNT is the same as in LN, which is the Li transport via Li vacancies. Further, it is shown that losses in LNT strongly increase above about 500 °C, which is interpreted to originate from conductivity-related relaxation mechanism. Finally, it is shown that LNT bulk acoustic resonators exhibit significantly lower loss, comparing to that of LiNbO3.

Modeling and Electrode Design Optimizations of Plano-Plano Langasite Crystal Resonator

The 3-D finite-element model (FEM) of a Y-cut plano-plano langasite crystal thickness shear mode (TSM) resonator is presented, and Mindlin's theory is used to investigate the optimal electrode shapes and sizes for langasite crystal resonator. Circular and elliptical electrodes of various arc lengths are studied to identify the most optimal electrode design configuration in order to achieve TSM vibration free from any anharmonic modes. Simulation results show that resonators with elliptical electrodes have noticeably better suppression of spurious modes compared to that of circular electrodes. Moreover, spurious mode suppression is accomplished for multiple electrode sizes for the same shape, which greatly differs from Mindlin's theory. Hence, three optimized designs are shortlisted and their mass loading sensitivities are investigated. Circular and elliptical electrodes of the same area show similar responses to added mass, indicating that elliptical electrodes have no apparent advantage over circular electrode in mass sensing applications.

Order/disorder processes and electromechanical properties of monoclinic GdCa4O(BO3)3

Large single crystals of GdCa4O(BO3)3 (space group Cm) were grown by the Czochralski method. Dielectric, piezoelectric and elastic coefficients at room temperature as well as specific heat capacity, thermal expansion and cation disorder were studied employing a variety of methods including resonant ultrasound spectroscopy, differential scanning calorimetry, dilatometry and X-ray diffraction techniques. The electromechanical parameters (4 dielectric, 10 piezoelectric and 13 elastic stiffness coefficients) obtained on different samples are in excellent agreement indicating high internal consistency of our approach, whereas the values reported in literature differ significantly. The elastic behaviour of GdCa4O(BO3)3 resembles the one of structurally related fluorapatite, i.e. the elastic anisotropy is relatively small and the longitudinal effect of the deviations from Cauchy-relations exhibit a pronounced minimum along the direction of the dominating chains of cation polyhedra. GdCa4O(BO3)3 exhibits a maximum longitudinal piezoelectric effect of 7.67 × 10−12 CN−10, a value in the order of that of langasite-type materials. Significant changes of the calcium/gadolinium distribution on the 3 independent cation sites accompanied by characteristic anomalies of heat capacity and thermal expansion suggest processes of nonconvergent cation ordering above about 900 K in GdCa4O(BO3)3.