Piezoelectric materials that can function at high temperatures without failure are desired for structural health monitoring and/or nondestructive evaluation of the next generation turbines, more efficient jet engines, steam, and nuclear/electrical power plants. The operational temperature range of smart transducers is limited by the sensing capability of the piezoelectric material at elevated temperatures, increased conductivity and mechanical attenuation, variation of the piezoelectric properties with temperature. This article discusses properties relevant to sensor applications, including piezoelectric materials that are commercially available and those that are under development. Compared to ferroelectric polycrystalline materials, piezoelectric single crystals avoid domain-related aging behavior, while possessing high electrical resistivities and low losses, with excellent thermal property stability. Of particular interest is oxyborate [ReCa4O (BO3)3] single crystals for ultrahigh temperature applications (>1000°C). These crystals offer piezoelectric coefficients deff, and electromechanical coupling factors keff, on the order of 3–16 pC/N and 6%–31%, respectively, significantly higher than those values of α-quartz piezocrystals (~2 pC/N and 8%). Furthermore, the absence of phase transitions prior to their melting points ~1500°C, together with ultrahigh electrical resistivities (>106 Ω·cm at 1000°C) and thermal stability of piezoelectric properties (< 20% variations in the range of room temperature ~1000°C), allow potential operation at extreme temperature and harsh environments.

Piezoelectric Materials for High Temperature Sensors

Inorganic lanthanide compounds with complex anions.

This paper reviews the interaction between coherently stimulated acoustic phonons in the form of surface acoustic waves with light beams in semiconductor based photonic structures We address the generation of surface acoustic wave modes in these structures as well as the technological aspects related to control of the propagation and spatial distribution of the acoustic fields The microscopic mechanisms responsible for the interaction between light and surface acoustic modes in different structures are then reviewed Particular emphasis is given to the acousto-optical interaction in semiconductor microcavities and its application in photon control These structures exhibit high optical modulation levels under acoustic excitation and are compatible with integrated light sources and detectors

https://iopscience.iop.org/article/10.1088/0034-4885/68/7/R02/pdf

Modulation of photonic structures by surface acoustic waves

Operation at temperatures well above ambient is desired for applications such as smart structures integrated within aircraft and space vehicles. Piezoelectric yttrium calcium oxyborate single crystal YCa4O(BO3)3 (YCOB) was found to exhibit no phase transition until its melting temperature around ∼1500°C. The temperature characteristics of the resonance frequency, electromechanical coupling, and dielectric permittivity were studied in the temperature range of 30–950°C for different orientations. The electrical resistivity at 800°C was found to be greater than 2×108Ωcm. Together with its temperature independent electromechanical coupling factor (∼12%) and engineered resonance frequency behavior, these make YCOB crystals excellent candidates for sensing applications at ultra high temperatures.

Characterization of piezoelectric single crystal YCa4O(BO3)3 for high temperature applications

Piezoelectric crystals like langasite (La3Ga5SiO14, LGS) and gallium orthophosphate (GaPO4) exhibit piezoelectrically excited bulk acoustic waves at temperatures of up to at least 1450 °C and 900 °C, respectively. Consequently, resonant sensors based on those materials enable new sensing approaches. Thereby, resonant high-temperature microbalances are of particular interest. They correlate very small mass changes during film deposition onto resonators or gas composition-dependent stoichiometry changes of thin films already deposited onto the resonators with the resonance frequency shift of such devices. Consequently, the objective of the work is to review the high-temperature properties, the operation limits and the measurement principles of such resonators. The electromechanical properties of high-temperature bulk acoustic wave resonators such as mechanical stiffness, piezoelectric and dielectric constant, effective viscosity and electrical conductivity are described using a one-dimensional physical model and determined accurately up to temperatures as close as possible to their ultimate limit. Insights from defect chemical models are correlated with the electromechanical properties of the resonators. Thereby, crucial properties for stable operation as a sensor under harsh conditions are identified to be the formation of oxygen vacancies and the bulk conductivity. Operation limits concerning temperature, oxygen partial pressure and water vapor pressure are given. Further, application-relevant aspects such as temperature coefficients, temperature compensation and mass sensitivity are evaluated. In addition, approximations are introduced which make the exact model handy for routine data evaluation. An equivalent electrical circuit for high-temperature resonator devices is derived based on the one-dimensional physical model. Low- and high-temperature approximations are introduced. Thereby, the structure of the equivalent circuit corresponds to the Butterworth–van Dyke equivalent circuit extended by a finite bulk resistance. Assignments of the lumped elements to the physical properties are given. Finally, an application example demonstrates the capabilities of high-temperature stable piezoelectric resonators. The simultaneous determination of mechanical and electrical properties of thin sensor films by resonant sensors enables the detection of CO in hydrogen-containing atmospheres.

https://iopscience.iop.org/article/10.1088/0957-0233/22/1/012002/pdf

High-temperature bulk acoustic wave sensors

Czochralski growth and characterization of piezoelectric single crystals with langasite structure: La3Ga5SiO14 (LGS), La3Ga5.5Nb0.5O14 (LGN) and La3Ga5.5Ta0.5O14 (LGT) II. Piezoelectric and elastic properties

We have determined the elastic constants of langasite-type crystals (La3Ga5SiO14, La3Ga5.5Nb0.5O14, and La3Ga5.5Ta0.5O14) from measurements of the sound velocity of acoustic waves. Starting with the elastic tensor determined from bulk acoustic waves, we optimized the data set by investigating the influence of the elastic tensor components on the angular dispersion of surface guided waves. This procedure is particularly useful for accurate determination of the nondiagonal elements of the elastic tensor. Phase velocity calculations based on the new set of constants show an increased agreement with experimental data compared to the data set derived from bulk waves and previously published material data.

Elastic properties of langasite-type crystals determined by bulk and surface acoustic waves

The surface acoustic wave (SAW) velocity with respect to propagation angle on a crystal substrate is dependent on the elastic constants of the crystal. Measurement of the SAW velocity dispersion with angle gives us information about these constants but one needs to solve the inverse problem of SAW propagation in order to obtain the elastic constants (C11, C12, C44). In this work we investigate, both experimentally and theoretically, SAW velocity behaviour on (001), (111) and (311) GaAs. Several different acoustic modes can be measured on each surface. Using the measured velocity dispersion curves on each surface we derive elastic constants by inversion, and examine the potential of the inversion method to accurately derive elastic constant values for measurements on each cut. We find that, by combining multiple mode curves, this method allows measurement of elastic constants with high accuracy, particularly for the off-axis constant C12.

http://ieeexplore.ieee.org/iel5/6852/18412/00849449.pdf

Elastic properties of GaAs obtained by inversion of laser-generated surface acoustic wave measurements

We have determined the elastic constants of langasite-type crystals LGS (langasite), LGN (langanite), and LGT (langataite) from measurements of the sound velocity of bulk and surface guided acoustic modes. Surface acoustic waves (SAW) and leaky waves were measured by thermoelastic laser excitation. The average measurement error was about 1 m/s. The angular dispersion of the propagation velocity was obtained by mounting the sample on a computer controlled rotation stage. Starting with the elastic tensor determined from bulk acoustic waves, we optimized the data set by investigating the influence of the elastic tensor components on the angular dispersion of surface guided waves. This procedure is particularly useful for accurate determination of the non-diagonal elements of the elastic tensor. Acoustic velocity calculations based on the new set of elastic constants show an increased agreement with experimental data compared to the data set derived from bulk waves and previously published material data.

Elastic constants of langasite-type crystals determined by bulk and surface guided acoustic modes

Surface acoustic wave (SAW) velocities were measured as a function of angle on both GaAs (001) and GaAs (111) crystal cuts. Surface acoustic wave packets were generated by a pulsed nitrogen laser beam focused into a line shape on the sample surface. The SAWs were detected by a PVDF foil with steel wedge transducer at different propagating distances. The system was automated to measure SAW velocity at a range of angles, and thus produces a velocity‐angle (or slowness) curve. From the slowness curve the three independent elastic constants may be derived. The SAW velocity measurement setup is presented and aspects of the propagation of SAW modes for different cuts and angles are discussed. The accuracy of the elastic constants derived from SAW measurements on both cuts is examined, and other detected acoustic wave modes are discussed with regard to their origin and suitability for elastic constant measurement.

C. M. Flannery

Papers

Czochralski growth and characterization of piezoelectric single crystals with langasite structure: La3Ga5SiO14 (LGS), La3Ga5.5Nb0.5O14 (LGN) and La3Ga5.5Ta0.5O14 (LGT) II. Piezoelectric and elastic properties

Elastic properties of langasite-type crystals determined by bulk and surface acoustic waves

Elastic properties of GaAs obtained by inversion of laser-generated surface acoustic wave measurements

Elastic constants of langasite-type crystals determined by bulk and surface guided acoustic modes

Measurement of elastic properties of GaAs with laser‐generated surface acoustic waves