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.
A universal technique for analysis of acoustic waves in periodic grating sandwiched between multi-layered structures and its application to different types of waves
A numerical technique, which combines FEM analysis of electrode area with matrix formalism applied to spectral domain analysis (SDA) of eigen modes in multi-layered substrate and multi-layered dielectric upper half-space, is described as an efficient tool of investigating the wave characteristics in periodic metal grating sandwiched between two multi-layered half-spaces, with arbitrary thickness of each layer. The universal character of the developed technique is illustrated by few examples of its application to different types of structure providing propagation of common SAW, boundary waves at interface, plate modes, normal modes in a thin film deposited over the grating etc. The transformation between the waves with continuously increasing film thicknesses can be observed.
A concept of wireless and passive very-high temperature sensor
There is a need for sensors capable operating at temperatures above 1000 °C. We describe an innovative sensor that might achieve this goal. The sensor comprises two main elements: a thermocouple and a surface acoustic wave (SAW) strain sensor. The cold junction of the thermocouple is electrically connected to a highly piezoelectric thin layer, deposited on top of a SAW substrate. In operation, the voltage generated by the temperature gradient between the hot (>1000 °C) and cold junction (<350 °C) generates a strain field in the layer, which is mechanically transmitted to the substrate. This modifies the SAW propagation conditions and therefore the sensors' radiofrequency response. The change depends on the applied voltage and thus on the hot junction temperature. As SAW devices are passive elements that can be remotely interrogated, it becomes possible to infer the hot junction temperature from the radiofrequency response, i.e., to remotely read temperatures above 1000 °C, without embedded electronics. In...