High-temperature piezoelectric crystals and devices

Conventional piezoelectric materials such as quartz are widely used as high precision transducers and sensors based on bulk acoustic waves. However, their operation temperature is limited by the intrinsic materials properties to about 500°C. High-temperature applications are feasible by applying materials that retain their piezoelectric properties up to higher temperatures. Here, langasite (La3Ga5SiO14) and compounds of the langasite family are the most promising candidates, since they are shown to exhibit bulk acoustic waves up to at least 1400°C. The mass sensitivity of langasite resonators at elevated temperatures is about as high as that of quartz at room temperature. Factors limiting potential use of those crystals include excessive conductive and viscous losses, deviations from stoichiometry and chemical instability. Therefore, the objective of this work is to identify the related microscopic mechanisms, to correlate electromechanical properties and defect chemistry and to improve the stability of the materials by e.g. appropriate dopants. Further application examples such as resonant gas sensors are given to demonstrate the capabilities of high-temperature stable piezoelectric materials. The electromechanical properties of langasite are determined and described by a one-dimensional physical model. Key properties relevant for stable operation of resonators are found to be shear modulus, density, electrical conductivity and effective viscosity. In order to quantify their impact on frequency and damping, a generalized Sauerbrey equation is given. Mass and charge transport in single crystalline langasite are correlated with langasite’s defect chemistry and electromechanical properties. First of all, the dominant charge carriers are identified. Undoped langasite shows predominant ionic conduction at elevated temperatures. As long as the atmosphere is nearly hydrogen-free, the transport is governed by oxygen movement. A dominant role of hydrogen is observed in hydrogenous atmospheres since the diffusion coefficient of hydrogen is orders of magnitude higher than that of oxygen. The loss in langasite is found to be governed up to about 650°C by viscoelastic damping related to the above mentioned movement of oxygen ions. Donor doping is shown to lower the loss contribution. Above 650°C the impact of the conductivity related loss becomes pronounced. Here, lowering the conductivity results generally in decreased losses. The evaluation of langasite’s applicability is focused on mapping the regimes of gas insensitive operation. The most relevant feature with respect to frequency fluctuations of resonator devices is the formation of oxygen vacancies. In nominally hydrogen free atmospheres the calculated frequency shift becomes pronounced below oxygen partial pressures of 10 − 17, 10 − 24 and 10 − 36 bar at 1000, 800 and 600°C, respectively. Water vapor is found to shift the resonance frequency at higher oxygen partial pressures. In the hydrogen containing atmospheres applied here, langasite can be regarded as a stable resonator material above 10 − 13 bar and 10 − 20 bar at 800 and 600°C, respectively. The incorporation of OH-groups determines the frequency shift.