Selection of materials for multilayered structures to be used in packageless sensors
Recently a multilayered structure with a low-velocity layer sandwiched between two high-velocity materials, one of which also works as a supporting substrate and another one as a coat isolating the wave from the influence of undesired external factors, was suggested for application in packageless sensors [1,2]. The thickness of a middle piezoelectric layer must be sufficient to confine the wave within this layer. The suggested combinations of materials included ZnO or GaN as a waveguiding layer, sapphire or diamond as a supporting substrate and Al2O3 (alumina) or AlN as a coating. In the present work, two previously reported structures were analyzed and compared to find the selection criterion for optimal combination of materials enabling high coupling of the useful mode and suppression of parasitic modes.
Transformation of acoustic waves in periodic metal grating sandwiched between piezoelectric and dielectric
The mechanism of SAW transformation with variation of film thickness is investigated in a piezoelectric substrate with a metal grating overlaid by a dielectric film, via simulation and visualization of the acoustic fields. By way of example, two orientations of lithium niobate substrates are analyzed, YX-LN and 128°YX-LN, with a Cu grating and an isotropic silica glass overlay. The motions, which follow the wave propagation in the sagittal plane, are visualized within two periods of the grating, with added contour plots showing the shear horizontal displacements. The continuous transformation of the wave's nature is investigated for each wave propagating in the analyzed material structures when the film thickness is increased from zero to a few wavelengths. The examples of the SAW transformation into boundary waves and into plate modes of different polarization have been found and investigated. The behavior of the SAW characteristics in the grating is correlated with transformation of the wave structure with increasing overlay thickness.
High temperature ultrasonic transducer up to 1000 °C using lithium niobate single crystal
Structural health monitoring (SHM) techniques are needed to maintain the reliability of aging power plants. The high temperature transducers are necessary to realize SHM under the working condition of power plants. In this paper, a high temperature transducer was developed using lithium niobate (LiNbO3) single crystal, which is well known as a high Curie temperature piezoelectric material. The LiNbO3 was bonded onto a stainless steel substrate. The transducer was heated in an electric furnace while measuring the bottom echoes from the substrate. We confirmed that the high temperature transducer could work up to 1000 °C.