Holger Fritze

Langasite for high-temperature bulk acoustic wave applications

Langasite (La3Ga5SiO14) and related compounds are promising candidates for high-temperature piezoelectric applications. To determine the stable operation of langasite as a high-temperature bulk acoustic wave (BAW) resonator, we characterized electrical conductivity [σel=1.56×exp(−1.07 eV/kT)S/cm] and oxygen diffusivity [DoxT=5×10−5 exp(−1.45 eV/kT)cm2/s] up to elevated temperatures. Langasite was successfully operated up to 900 °C as a BAW resonator with an average mass sensitivity of about 0.17 μg Hz−1 cm−2. This enables, in principle, high-temperature monitoring of deposition rates and thermogravimetry with high sensitivity.

Mullite based oxidation protection for SiC-C/C composites in air at temperatures up to 1900 K

For an industrial Si-SiC coated C/C material (reference material) the temperature dependence of the linear rate of mass loss is interpreted in the temperature range 773 < T < 1973 K. The Arrhenius plot of the thermogravimetrically determined oxidation rate shows four typical regimes. Only in the temperature range 1323 < T < 1823 K is the oxidation rate close to or lower than the limit for long-term application. Pulsed Laser Deposition (PLD) allows the ablation of nonconductive and high melting targets and the preparation of films with complex composition. High energy impact CO 2 laser pulses (j= 3.10 7 W cm -2 ) lead to melting and evaporation of the target material in a single step. Therefore the flux of the metal components is stoichiometric. Deposited green layers did not show IR peaks typical for mullite. After a short oxidation treatment (15 min at 1673K) the formation of mullite in the coating was completed as was confirmed by IR spectroscopy and XRD investigations. Thin PLD-mullite layers (900 nm) did not markedly improve the oxidation resistance of the reference material in the high temperature range 1473 < T < 1973 K. However, a preoxidation treatment of the substrate material and mullite coatings with a thickness of 2.5 μm improved the oxidation behaviour significantly. Because of SiO 2 formation at the mullite-SiC interface all samples exhibited a mass increase on oxidation. The inward diffusion of oxygen across the outer mullite-containing layer controlled the kinetics of the reaction as was deduced from 18 O diffusivity measurements in PLD mullite layers. The calculated oxidation rates resulting from the diffusion parameters in SiO 2 and mullite are close to the thermogravimetric data. For oxidation durations of three days only amorphous SiO 2 is formed at the mullite-SiC interface.

High-temperature bulk acoustic wave sensors

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.

Operation limits of langasite high temperature nanobalances

Abstract The high temperature properties of langasite (La 3 Ga 5 SiO 14 ) are presented in order to evaluate its ability to serve as a high temperature nanobalance. Langasite resonators exhibit bulk oscillations at temperatures of up to 900°C. At 800°C, the mass load response for 780 μm thick resonators is approximately −6.5 cm 2 Hz μg −1 . The temperature dependent frequency shift, about −100 Hz K −1 at 600°C, may be effectively compensated by monitoring the difference frequency of closely mounted resonators. As an example, the response of a TiO 2− x coated langasite nanobalance to different oxygen partial pressures at elevated temperatures is presented. The strong frequency shift due to switching from oxidizing to reducing conditions cannot be attributed to mass changes of the sensor film. Mechanical stress caused by changes in the oxygen stoichiometry is the most likely explanation for the frequency changes.

High temperature bulk acoustic wave properties of langasite

High temperature stable piezoelectric materials including langasite (La3Ga5SiO14, LGS) are, in principle, suitable for high temperature bulk acoustic wave applications such as resonant microbalances and gas sensors using specific surface affinity layers. The resonator material LGS has been shown to exhibit bulk oscillations at temperatures of up to at least 935 °C. However, the knowledge of the high temperature bulk acoustic wave properties is required for the evaluation of mass load dependent changes of the resonator characteristics, and, consequently, for the application of these materials as sensors. This paper presents and discusses the bulk acoustic wave resonator behaviour of LGS in the temperature range of up to 935 °C. Test devices are Pt contacted LGS resonators. Based on a one-dimensional description of the resonance behaviour, the material constants are determined as a function of temperature and oxygen partial pressure. The effective viscosity, η, is found to control the resonator quality. At 600 °C and oxygen partial pressures down to ca. 10−20 bar, an environment independent operation of LGS resonators is demonstrated. The mass sensitivities at 800 °C are of the same order of magnitude as for quartz at room temperature.

Precise measurements of BAW and SAW properties of Langasite in the temperature range from 25°C to 1000°C

There is a high demand for wireless sensing devices in harsh environments for industrial applications. For temperatures above 250degC, silicon-based sensors cannot be used. In contrast, bulk acoustic wave (BAW) and surface acoustic wave (SAW) devices are still suitable for this purpose. Further high-temperature applications include thermogravimetry on small volumes and gas sensing based on stoichiometry change of thin sensor films. Langasite (La3Ga5SiO14) is a piezoelectric single crystal that preserves its piezoelectric properties and is chemically stable up to its melting point at 1470degC without any phase transition and, therefore, is a promising material for high-temperature devices. Using a resonance-antiresonance method based on bulk oscillations, all components of elastic and piezoelectric tensors of langasite have been determined at temperatures up to 900deg C. Resonance spectra of several langasite samples have been measured and fitted with the impedance calculated from a one-dimensional physical model of piezoelectric bodies vibrating in several modes. In order to extract the electromechanical parameters, different resonator geometries and orientations are used. Also, the results of measurements are presented for SAW devices on langasite at temperatures from 25 to 750deg C. Two cuts with Euler angles (0deg, 138.5deg,26.6deg) and (0deg,30.1deg,26.6deg) have been studied. The devices were fabricated with a platinum (Pt) layer with different heights on a zirconium (Zr) adhesion layer. The main material parameters relevant for SAW devices such as phase velocity vp, propagation loss alpha and coupling coefficient k2 have been obtained. The measured SAW phase velocities compare well with those calculated with the elastic and piezoelectric tensors obtained by bulk-oscillation measurements.

High-Temperature Properties of Langasite

Materials such as langasite (La 3 Ga 5 SiO 14 ) and related compounds are promising candidates for piezoelectric applications at high temperatures. In particular, langasite does not exhibit phase transformations up to the melting point of 1470 °C. Langasite was investigated with respect to potential applications in high temperature resonator devices. In contrast to current resonator materials, we have observed bulk oscillations at temperatures of up to 750 °C in langasite devices. At 700 °C the mass load response for 0.78 mm thick resonators is approximately 0.10 µg/Hz. At elevated temperatures, the bulk resistivity of the resonator devices cannot be neglected due to attenuation of the resonance signal. Therefore, the temperature dependence of the electrical properties of langasite resonator devices, including bulk resistivity, capacity and resonance frequency were measured and are presented. The electrical conductivity is characterized by an activation energy of 105 kJ/mol. In order to confirm langasites stability with respect to oxidation-reduction reactions, we examined the oxygen diffusivity by measuring 18 O tracer profiles by SIMS. The diffusivity along the Y-axis is given by D = 5-10 −5 exp(-140 kJ/mol / RT) cm 2 /s in the temperature range from 500 to 800 °C. Langasite shows low oxygen diffusion coefficients with respect to other materials which might be investigated using a langasite microbalance. This would, for example, enable oxygen diffusion kinetics to be examined in YBa 2 Cu 3 O 6 at 600 °C by means of 18 O/ 16 O exchange.

Diffusion-related implications for langasite resonator operation

Oxygen and gallium diffusiveness in langasite were experimentally determined by analysis of diffusion profiles of /sup 18/O and /sup 71/Ga tracers by SIMS analysis as functions of temperature and doping. Strontium-enhanced diffusiveness and activation energies of /spl sim/1.2/spl plusmn/ 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.

Defect chemistry, redox kinetics, and chemical diffusion of lithium deficient lithium niobate

High-temperature optical in situ spectroscopy was used to investigate the defect absorption, redox kinetics, and chemical diffusion of a lithium deficient (48.4 mol% Li(2)O) congruent melting lithium niobate single crystal (c-LN). Under reducing atmospheres of various oxygen activities, a(O(2)), UV-Vis-NIR spectra measured at 1000 °C are dominated by an absorption band due to free small polarons centered at about 0.93 eV. The polaron band intensity was found to follow a power law of the form a(O(2))(m) with m = -1/4. A chemical reduction model involving electrons localized on niobium ions on regular lattice sites can explain the observed defect absorption and its dependence on oxygen activity. The kinetics of reduction and oxidation processes upon oxygen activity jumps and the associated chemical diffusion coefficients are found in close agreement over a range from -0.70 to -14.70 in log a(O(2)), indicating a reversible redox process. Assuming coupled fluxes of lithium vacancies and free small polarons for the attainment of stoichiometry, the diffusion coefficients of lithium vacancies as well as of lithium ions in the lithium deficient c-LN have been determined at 1000 °C.

Defect properties of langasite and effects on BAW gas sensor performance at high temperatures

Abstract The electrical and defect properties of langasite (La 3 Ga 5 SiO 14 ) were studied as a function of temperature, oxygen partial pressure and dopants in order to characterize its electrical behavior in relation to its performance as a bulk acoustic wave (BAW) gas sensor operating at elevated temperatures. Undoped, 5%–Nb donor doped, and 1%–Sr acceptor doped langasite specimens were studied. A defect model, shown to be consistent with experimental observations, was used to extract key defect parameters for the system. Implications for device operation are discussed.