Huankiat Seh

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.

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 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.

Thin Film Stoichiometry Determination by High Temperature Microbalance Technique

The in-situ determination of small mass changes of thin films became feasible with the availability of high temperature stable microbalances. With this technique, changes of the mechanical properties of thin films deposited on piezoelectric resonators are investigated at temperatures above 500 °C by monitoring the resonance behavior of the resonators. The results are valuable for fundamental understanding of the ionic and electronic transport processes in ceramic materials and for applications such as high temperature gas sensors. This work correlates the electrical and the mechanical properties of TiO 2-x at different oxygen partial pressures. TiO 2-x films are deposited onto high temperature resonators by laser ablation and characterized by the high temperature microbalance technique as well as electrical impedance spectroscopy at 600 °C. The oxygen partial pressure dependent resonance behavior cannot be attributed solely to mass changes of the TiO 2-x film. Changes of the films mechanical stiffness have to be taken into consideration to explain the resonance behavior. The simultaneous electrical impedance measurements indicate a n-type conduction behavior of the TiO 2-x films.