Intrinsic Defects and the Inducing Conduction Mechanism of Langasite-Type High-Temperature Piezoelectric Crystals

Increasing the crystal resistivity is critically important for enhancing the signal-to-noise ratio and improving the sensing capability of high-temperature piezoelectric sensors based on langasite-type crystals. The resistivity of structural ordered langasite-type crystals is much higher compared to that of the disordered crystals. Here, we selected structural ordered Ca₃TaGa₃Si₂O₁₄ (CTGS) and disordered La₃Ga₅SiO₁₄ (LGS) as representatives to investigate the microscopic conduction mechanism and further reveal the origin of the different resistivities of the ordered and disordered langasite-type crystals at elevated temperatures. By combining first-principles calculations and experimental investigations, we found that the different conductivity behaviors of the ordered and disordered crystals originate from different types of point defects formed in the crystal and their different contributions to the conductivity. For the disordered LGS crystal, the oxygen vacancies are apt to be formed at high temperatures, promoting the transition of valence electrons and yielding high conductivity. For the ordered CTGS crystal, the dominant Ta_Ga antisite defects can introduce an electron-hole recombination center in the electronic band gap, significantly shortening the carrier lifetime and thus reducing the conductivity. This provides effective guidance to improve the resistivity performance of langasite-type crystals at high temperatures by optimizing the experimental conditions, such as oxygen atmosphere treatment, antisite defect modification, etc.

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.

High performance piezoelectric crystal cut designed using LiNbO3 for high temperature acoustic emission sensing application

An optimal piezoelectric crystal cut designed using LiNbO3 possessed stable electro-elastic performance over a wide temperature range of 20–500 °C.

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.

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.

High-temperature piezoelectric crystals ReCa4O(BO3)3: a review

High-temperature sensors are desirable for structural health monitoring and/or nondestructive evaluation of next-generation turbines, more efficient jet engines, and the furnace components of electrical power plants. Of all the investigated high-temperature piezoelectric materials, rare-earth calcium oxyborate crystals ReCa4O(BO3)3 (ReCOB, Re: rare-earth) exhibit attractive advantages for high-temperature piezoelectric sensing. In this paper, the electroelastic properties of different ReCOB piezoelectric crystals are investigated. The crosstalk between various vibration modes are discussed, from which the optimized crystal cuts are achieved. Furthermore, temperature dependences of the electrical resistivity, dielectric, elastic, piezoelectric, and electromechanical properties of ReCOB crystals are studied. Finally, the thermal properties, including thermal expansion, specific heat, and thermal conductivity at elevated temperatures are studied and compared with commercially available high-temperature piezoelectric crystals.

Wideband impedance spectrum analyzer for process automation applications

For decades impedance spectroscopy is used in technical laboratories and research departments to investigate effects or material characteristics that affect the impedance spectrum of the sensor. Establishing this analytical approach for process automation and stand-alone applications will deliver additional and valuable information beside traditional measurement techniques such as the measurement of temperature, flow rate, and conductivity, among others. As yet, most of the current impedance analysis methods are suited for laboratory applications only since they involve stand-alone network analyzers that are slow, expensive, large, or immobile. Furthermore, those systems offer a large range of functionality that is not being used in process control and other fields of application. We developed a sensor interface based on high speed direct digital signal processing offering wideband impedance spectrum analysis with high resolution for frequency adjustment, excellent noise rejection, very high measurement rate, and convenient data exchange to common interfaces. The electronics has been implemented on two small circuit boards and it is well suited for process control applications such as monitoring phase transitions, characterization of fluidal systems, and control of biological processes. The impedance spectrum analyzer can be customized easily for different measurement applications by adapting the appropriate sensor module. It has been tested for industrial applications, e.g., dielectric spectroscopy and high temperature gas analysis.

Diffusion-related implications for langasite resonator operation

Oxygen and gallium diffusivities in langasite were experimentally determined by analysis of diffusion profiles of 18O and 71Ga tracers by SIMS analysis as functions of temperature and doping. Strontium-enhanced diffusivities and activation energies of approximately 1.2+/-0.2 eV confirm the predominant role of oxygen vacancies in controlling the electrical conductivity of langasite at elevated temperature and oxygen partial pressure. The potential impact of high levels of porosity and the use of an oxygen primary ion beam on the accuracy of some of the data is discussed. The gallium diffusivity, with activation energy of 3.13 eV, was found to be more than two orders of magnitude lower than that of oxygen. Surface exchange measurements enabled estimation of gallium loss at elevated temperatures and oxygen partial pressure; the level is not believed to be of major concern for resonator performance.