High-Temperature Electromechanical Properties of CTGS
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High-Temperature Electromechanical Properties of CTGS Michal Schulz1, Holger Fritze1, Ward L. Johnson2 1
Institute of Energy Research and Physical Technologies, Clausthal University of Technology, Am Stollen 19 B, 38640 Goslar, Germany. 2
National Institute of Standards and Technology, 325 Broadway St., MS 647, Boulder, CO 80305, USA*
ABSTRACT CTGS (Ca3TaGa3Si2O14) is a commercially available, Czochralski-grown piezoelectric material from the langasite family that has an ordered crystal structure. It can be excited piezoelectrically up to at least 1285 °C, which is very close to the melting temperature of 1350 °C. In order to determine the loss at elevated temperatures, two different resonance techniques are used. A contactless transduction method is employed up to about 600 °C, whereas transduction involving standard keyhole-shaped film electrodes is employed up to 1285 °C. Comparison of the temperature-dependent inverse Q factor shows that contactless measurements are best suited for the lower temperature range, where sample clamping and losses caused by the electrodes contribute significantly to the total loss. However, at higher temperatures, measurement of the electrical impedance of samples with film electrodes in the vicinity of the resonance frequency proves to be suitable. Even at 1100 °C, 5 MHz CTGS resonators are found to have a Q factor of about 1200, which is great enough to enable numerous bulk-acoustic-wave applications. Further, a nearly linear temperature dependence of the resonance frequency with a temperature coefficient of 210 Hz/K makes Y-cut CTGS well suited for temperature-sensing applications. INTRODUCTION High-temperature piezoelectric materials with crystal structures similar to langasite (LGS, La3Ga5SiO14) have been demonstrated to show piezoelectric properties even above 1400 °C [1]. Thus, these crystals are, in principle, applicable in high-temperature bulk-acousticwave (BAW) and surface-acoustic-wave (SAW) devices [2]. Related applications include temperature, pressure and gravimetric mass sensing. However, a potential drawback of many members of the langasite family is a partially disordered crystal structure, which involves some types of lattice sites being occupied randomly by one of two atomic species. Such statistical occupancy distributions may lead to increased loss and decreased frequency stability at elevated temperatures, which, in turn, will introduce uncertainties in frequency determination of resonant devices. In contrast, a number of less-studied crystals in the langasite family have fully ordered crystal structures and, therefore, potentially superior electromechanical properties at elevated temperatures. One such crystal is CTGS (Ca3TaGa3Si2O14), with a melting point of 1350 °C [3]. Czochralski-grown CTGS is commercially available. The focus of this paper is on acoustic loss, electrical conductivity, and the dielectric constant of CTGS, as determined through the use of two different measurement techniques at temperatures ranging from room temperature to 1285 °C. Acous
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