Electromechanical properties and defect chemistry of high-temperature piezoelectric materials

Abstract

Langasite and gallium phosphate are shown to exhibit piezoelectrically stimulated bulk acoustic waves up to at least 1,400 and 900°C, respectively. Most critical issues are stoichiometry changes due to, e.g. low oxygen partial pressures, and high losses. Therefore, the paper discusses the atomistic transport and defect chemistry of those crystals and correlates them with the electromechanical properties. First, the defect chemistry of langasite is investigated. As long as the atmosphere is nearly hydrogen-free, the transport of charge carriers is governed by oxygen movement. A dominant role of hydrogen is observed in hydrogenous atmospheres. Based on the developed defect model, donors are expected to suppress the oxygen vacancy concentration and, thereby, the loss in langasite. The prediction is proven by niobium doping and found to be valid. A one-dimensional physical model of thickness shear mode resonators is summarized. The analysis of the resonance spectra showed that the loss of the resonators can be described satisfactorily by mechanical and electrical contributions expressed as effective viscosity and bulk conductivity, respectively. The mechanical loss in langasite is significantly impacted by the electrical conductivity due to the piezoelectric coupling. The effect of the piezoelectric coupling on the loss is negligible for gallium phosphate since it shows an extremely low electrical conductivity.

Langasite based miniaturized functional structures: Preparation, high‐temperature properties and applications

“…An increase in conductivity by some orders of magnitude for heavily acceptor doped langasite corresponds to predictions of the defect model 19, 20. Since langasite is a mixed ionic conductor 5, 14, the increase in conductivity is correlated to the increased mobility of oxygen ions by the formation of oxygen vacancies. In other words, the ionic contribution to the conductivity is increased by the strontium doping.…”

“…Previous investigations of the transport phenomena in langasite confirmed the high‐temperature stability and showed the operation limits of langasite‐based devices such as microbalances 12–14. The investigations focused, e.g.…”

“…Hence, acceptor and donor levels should be created, which might result in the formation of mobile charge carriers such as electrons, holes or oxygen vacancies and, thereby, in an increase of the electrical conductivity. Finally it turned out, that high electrical conductivity as required to form monolithic electrodes can be achieved by acceptor doping 14. Additionally, a change in the chemical properties of doped areas leading to modified chemical etch behaviour is expected.…”

High-temperature electromechanical characterization of AlN single crystals

“…GaPO 4 shows features similar to quartz, such as high electrical resistivity and mechanical quality factor, but it also exhibits high electromechanical coupling and greater piezoelectric sensitivity until the α to β phase transition occurs (<970°C). However, decreased mechanical quality factor due to increased structural disorder at temperatures above 700°C limits its usage [8]- [10]. Langasite crystal, which does not undergo a phase transition before its melting point (1470°C), also has been extensively studied for high-temperature applications.…”

The solid-solution region for the langasite-type Ca3TaGa3Si2O14 crystal as determined by a lever rule

“…Crystalline langasite (La 3 Ga 5 SiO 14 ) is a well-known piezoelectric material and will be potentially used in pressure sensors [5,6] and surface acoustic wave (SAW) devices [7]. The electro-mechanical coupling factor of langasite is approximately three times that of quartz and this material also has a wider working temperature range [8].…”