Langasite-Type Resonant Sensors for Harsh Environments
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Langasite-Type Resonant Sensors for Harsh Environments
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Yuriy Suhak1, Michal Schulz1, Hendrik Wulfmeier1, Ward L. Johnson2, Andrei Sotnikov3, Hagen Schmidt3, Steffen Ganschow4, Detlef Klimm4 and Holger Fritze1 1 Clausthal University of Technology, Am Stollen 19B, Goslar, 38640, Germany. 2 National Institute of Standards and Technology, 325 Broadway St., Boulder, CO 80305, U.S.A. 3 Leibniz Institute for Solid State and Materials Research Dresden, Helmholtzstr. 20, Dresden, 01069, Germany. 4 Leibniz Institute for Crystal Growth, Max-Born-Str. 2, Berlin, 12489, Germany. ABSTRACT Operation of single crystalline Ca3TaGa3Si2O14 (CTGS) and La3Ga5SiO14 (LGS) bulk acoustic wave resonators is demonstrated up to 1270 °C and 1470 °C, respectively. The mass sensitivity of such devices is about 35 cm2 Hz/Pg at 800 °C. Therefore, they are sensitive transducers suited to monitoring, for example, mass deposition processes at high temperatures. The electromechanical loss in CTGS is found to be significantly lower than that in LGS. Platinum coated CTGS samples show a remarkable long-term stability at 1000 °C in air. After an initial period of 300 h, the conductivity is found to remain nearly constant for at least 2400 h. Measurements of resonance frequency of CTGS for 1000 h show a qualitatively similar sequence, with an initial systematic increase followed by nearly constant values. In contrast, measurements on platinum-coated LGS plates show a conductivity decreasing by 15 % over a period of 5000 h. INTRODUCTION Piezoelectric single crystals of the langasite family enable novel sensing mechanisms in harsh environments. In particular, they do not exhibit phase transformations up to temperatures of about 1400 °C [1]. The frequencies of bulk-acoustic-wave (BAW) resonators fabricated from these crystals reflect mechanical and electrical properties even of nanometer-scale films deposited on the transducer. Applications of such resonators include nanogram-resolution mass balances, gas sensors and characterization of phase transformations of thin films. The high quality factors (Q) of BAW resonators enable robust and precise sensor-data acquisition, which is facilitated by data acquisition units of reasonable cost in the low megahertz range [2]. The stability of these crystals is a precondition for their application, and this needs to be determined through high-temperature measurements in relevant gaseous environments. This paper explores fundamental properties such as electrical conductivity, resonance frequency, loss and thermal conductivity as a function of temperature and time. Changes in properties at elevated temperatures over long time periods are a particular focus. EXPERIMENT Circular bulk acoustic wave (BAW) resonators and rectangular plates were prepared from Czochralski-grown Ca3TaGa3Si2O14 (CTGS) and La3Ga5SiO14 (LGS) crystals. The crystal orientations, dimensions, fundamental frequency, and manufacturers of the resonators are listed in Table 1 along with the types of measurement methods that were employed. The high-tempera
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