Michal Schulz

Sr diffusion in undoped and La-doped SrTiO3 single crystals under oxidizing conditions

Strontium titanate SrTiO3(100), (110), and (111) single crystals, undoped or donor doped with up to 1 at% La, were isothermally equilibrated at temperatures between 1523 and 1773 K in synthetic air followed by two different methods of Sr tracer deposition: ion implantation of 87Sr and chemical solution deposition of a thin 86SrTiO3 layer. Subsequently, the samples were diffusion annealed under the same conditions as before. The initial and final depth profiles were measured by SIMS. For strong La-doping both tracer deposition methods yield similar Sr diffusion coefficients, whereas for weak doping the tracer seems to be immobile in the case of ion implantation. The Sr diffusivity does not depend on the crystal orientation, but shows strong dependency on the dopant concentration supporting the defect chemical model that under oxidizing conditions the donor is compensated by Sr vacancies. A comparison with literature data on Sr vacancy, Ti, and La diffusion in this system confirms the concept that all cations move via Sr vacancies. Cation diffusion is several orders of magnitude slower than oxygen diffusion.

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.

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.

Material and resonator design dependant loss in langasite bulk acoustic wave resonators at high temperatures

Single crystalline langasite (La3Ga5SiO14) resonators exhibit piezoelectrically exited bulk acoustic waves up to temperatures close to its melting point at 1470 °C. The loss observed in bulk acoustic wave devices depends on the materials properties and the resonance frequency. Anticipated operation at extremely high temperatures requires the understanding of both influences and enables tailoring of both properties to reduce the loss. Electrical impedance spectroscopy and diffusion runs using stable isotopes are the key methods used to study the atomistic transport processes and the electromechanical properties of langasite. At elevated temperatures, electrical as well as mechanical loss contributions are found. In particular, oxygen vacancies are responsible for strong losses which can be, however, suppressed by light donor doping. Above 650 °C, the impact of the conductivity related loss becomes pronounced. Further, the coupling of mechanical and electrical properties due to the piezoelectric effect causes a loss maximum at the dielectric relaxation frequency. Doping of langasite modifies the electrical conductivity and shifts, thereby, the dielectric relaxation frequency. Consequently, the choice of appropriate dopants and/or of the resonance frequency far off the latter frequency minimizes the loss. The concept is demonstrated and leads to an improved performance of resonant sensors at high temperatures.

High-temperature electroacoustic characterization of Y-cut and singly-rotated Ca 3 TaGa 3 Si 2 O 14 resonators

Synthetic piezoelectric crystals in the P321 crystal class have been a focus of substantial research that is largely driven by applications in high-temperature resonant BAW and SAW sensing. Fully ordered crystals in this class, such as Ca3TaGa3Si2O14 (CTGS), have been suggested as offering the potential of electroacoustic performance that is superior to more extensively studied langasite (LGS) and langatate (LGT), which are partially disordered. In this study, the resonant frequencies, acoustic damping, and electrical conductivity of CTGS bulk acoustic resonators with V-cut and (VXl)-30° crystal orientations and fundamental frequencies near 5 MHz are investigated at temperatures between ambient and 1100°C. (VXl)-30° resonators are found to have turnover temperatures near 200°C for the third and fifth overtones, in contrast to a monotonic decrease in resonant frequencies of V-cut crystals with increasing temperature. The maximum temperature derivative of fractional changes in fifth-overtone frequency of (VXl)-30° CTGS is 40 × 10-6K-1 (near 1100°C), and this value is not greatly different from the temperature derivative of V-cut CTGS frequencies over a broader range of temperatures. At ambient temperatures, the acoustic loss Q-1 of CTGS with both crystal orientations is found to be greater than the lowest values previously reported for LGS and LGT. The electrical conductivity of the CTGS specimens between 500°C and 1100°C is substantially lower than that previously reported for LGS. Corresponding to this lower conductivity, the piezoelectric/conductive contribution to Q-1 at elevated temperatures is reduced. Additional anelastic relaxation peaks observed between 100°C and 700°C are similar to those previously reported for LGS and LGT.

Solid State Sensors for Selective Gas Detection at High Temperatures—Principles and Challenges

To satisfy demands for in-situ monitoring of high-temperature processes, ever more sophisticated sensors are required. Advances in achieving improved sensitivity and selectivity are expected from miniaturization and integration of active electronic components thereby enabling new sensor principles or simultaneous application of different sensor principles. First, conventional resistive and potentiometric sensors are reviewed with respect to their stability and gas selectivity at temperatures up to 1000°C. Limitations caused by e.g. atomistic transport processes such as mixed ionic conduction are summarized. Subsequently, the improvement of resonant gas sensors by miniaturization and related material issues are discussed. The technology used here to prepare miniaturized structures enables to accommodate arrays of resonators in a single device and to reduce cost. For exemplification, miniaturized bulk acoustic wave resonators based on single crystalline piezoelectric langasite (La3Ga5SiO14) are machined by wet chemical etching and coated with gas sensitive films. Those devices could be operated up to at least 1000°C and are demonstrated to be selective in-situ gas sensors for carbon monoxide and hydrogen. The underlying concept includes monolithic structures in order to minimize thermal stress. For example, locally doped areas showing high electrical conductivity are used as electrodes at high temperatures. Further, field emission diodes are prepared and demonstrated to be operational. The radius of the langasite tips is estimated to be as low as 30 nm. Those diodes and other active electronic elements are intended to process e.g. sensor signals already close to the sensor element.

High-Temperature Acoustical and Electrical Properties of LGS, LGT and CTGS Resonators

Acoustic characteristics and electrical conductivity of CTGS, LGT and LGS bulk acoustic wave resonators operated at the fundamental mode in the temperature range of 20-1470°C are studied. It is shown that LGS and CTGS resonators can be excited piezoelectrically up to 1470 and 1270°C, respectively, which is close to their melting temperatures. The electrical conductivity of CTGS is found to be by at least two and three orders of magnitude lower than that of LGS and LGT, respectively, over the temperature range 400-1000°C. Measurements of temperature dependent electromechanical losses show, that they are at least by two orders of magnitude lower in CTGS comparing to that in LGS within the measured temperature range.

Stability and applications of high-temperature piezoelectric crystals

High-temperature piezoelectric crystals such as gallium phosphate (GaPO4) and langasite (La3Ga5SiO14) have been demonstrated to show piezoelectric properties at fairly high temperatures. For example, langasite can be exited piezoelectrically up to temperatures close to its melting point at about 1470 °C. The resonance behavior including damping and temperature dependent frequency of resonators made of these crystals are investigated and reviewed to evaluate the applicability of such devices as high-temperature gas sensors or film deposition monitors.

Acoustic and electrical properties of Ca 3 Ta G a 3 Si 2 O 14 piezoelectric resonators at elevated temperatures

Synthetic piezoelectric crystals in the P321 crystal class have been a focus of substantial research in relation to their application in high-temperature resonant bulk-acoustic-wave (BAW) and surface-acoustic-wave (SAW) sensors. Most of this research has been on partially disordered langasite (LGS) and langatate (LGT), but fully ordered crystals in this class, such as Ca₃TaGa₃Si₂O₁₄ (CTGS), have been suggested as offering potentially superior performance. In this study, acoustic characteristics and electrical conductivity of CTGS bulk acoustic resonators with a crystal orientation of (YXl) -30° are investigated at the fundamental mode of 5 MHz and overtones of 15 MHz and 25 MHz in the temperature range from room temperature to 1100 °C. Magnitudes of the fractional changes in frequency with temperature are found to be less than 41 × 10^-6 K^-1 over this range, with turnover temperatures near 200 °C for the third and fifth overtones. The acoustic loss μ^-1 at ambient temperatures is greater than the lowest values previously reported for LGS and LGT. Between 100 °C and 700 °C, μ^-1 has two anelastic relaxation peaks that are similar to those previously reported for LGS and LGT. The electrical conductivity over the range from 500 °C to 1100 °C is found to be approximately an order of magnitude lower than that previously reported for LGS, and this leads to a reduction in μ^-1 at elevated temperatures.

Langasite based high-temperature bulk acoustic wave sensors

The objective of the work is to review the high-temperature properties, the operation limits and the sensor principles related to high-temperature stable langasite (La3Ga5SiO14, LGS) resonators. The electromechanical properties of such resonators including mechanical stiffness, piezoelectric and dielectric constant, effective viscosity and electrical conductivity are determined up to 900 °C and described using a one-dimensional physical model. Insights from defect chemical models are correlated with the electromechanical properties of the resonators. Further, application examples are given which demonstrate the capabilities of high-temperature stable piezoelectric resonators. For example, the simultaneous determination of mechanical and electrical properties of thin sensor films by resonant sensors enables the detection of CO in hydrogen-containing atmospheres. Further, the ability of langasite resonators to detect soot at elevated temperatures is demonstrated.