High-temperature piezoelectric crystals and devices

Abstract

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 (La 3 Ga 5 SiO 14 ) 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 hightemperature 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 general-H. Fritze (B)

High temperature measurement and characterisation of piezoelectric properties

“…Resonance measurements have been reported at temperatures up to 1400 °C for a variety of materials including lead titanate based relaxors up to 150 °C [35], commercial lead-and bismuth-based ceramics up to 500 °C [36], lithium-sodium niobate ceramics up to 800 °C [37], ordered langasite crystals up to 900 °C [38], rare earth calcium oxyborate single crystal (YCOB) up to 950 °C [39] and 1000 °C [40], langasite and -quartz crystals up to 1400 °C [41] [42]. For high quality crystals at moderate temperatures, the standard (low loss) method can be applied directly with no account taken of the effects of the losses, but this is unlikely to be the case particularly for many perovskite lead and bismuth based piezoelectrics even at moderately elevated temperatures.…”

Correlation of BAW and SAW properties of langasite at elevated temperatures

“…The crystals exhibit mixed electronic and ionic conductivity, which contributes to the loss at high temperatures. The oxygen partial pressure-dependent conductivity impacts the performance of the resonators in a minor way as long as the oxygen partial pressure is kept, e.g., above 10 −20 bar at 600 • C Fritze, 2006Fritze, , 2011a.…”

Development of 2" AlN Substrates Using SiC Seeds

“…Long AlN crystals grow for several dozens of hours and often in several runs. The evolution of the crystal quality via the improvement of the growth regime from the 2-inch diameter 10 mm long bulk AlN crystal having a single-crystal core of about 40 mm diameter and a polycrystalline rim (Avdeev et al, 2010;2011) to the "good enough" single crystal (and, hence, the substrate) is illustrated by Fig. 12.…”