A Theoretical Study of Love Wave Sensors Based on ZnO–Glass Layered Structures for Application to Liquid Environments

Abstract

The propagation of surface acoustic Love modes along ZnO/glass-based structures was modeled and analysed with the goal of designing a sensor able to detect changes in the environmental parameters, such as liquid viscosity changes and minute amounts of mass supported in the viscous liquid medium. Love mode propagation was modeled by numerically solving the system of coupled electro-mechanical field equations and Navier–Stokes equations. The phase and group velocities and the attenuation of the acoustic wave propagating along the 30° tilted c-axis ZnO/glass structure contacting a viscous non-conductive liquid were calculated for different ZnO guiding layer thicknesses, added mass thicknesses, and liquid viscosity and density. The three sensor responses, i.e., the wave phase and group velocity, and attenuation changes are calculated for different environmental parameters and related to the sensor velocity and attenuation sensitivities. The resulted sensitivities to liquid viscosity and added mass were optimized by adjusting the ZnO guiding layer thickness corresponding to a sensitivity peak. The present analysis is valuable for the manufacture and application of the ZnO-glass structure Love wave sensors for the detection of liquid properties, such as viscosity, density and mass anchored to the sensor surface.

Guided acoustic wave sensors for liquid environments

“…The peak of the ℎ sensitivity decreases with increasing the mode order and the highest value corresponds to the LM1 mode; the mass sensitivity increases rapidly with the layer thickness, and it can be larger than the former as its peak can be sharper. Both the LMs group and phase velocity can represent a sensor response [58]: the phase velocity can be experimentally estimated by measuring the operating frequency f = ℎ /λ of the sensing device at the minimum insertion loss of the scattering parameter S 12 . The group velocity can be estimated by measuring the group time delay τ = L/v gr of the sensing device at the minimum insertion loss of the scattering parameter S 12 in the time domain, being L the acoustic wave delay path (the IDTs centreto-centre distance)…”

“…The IDTs can be located only onto the piezoelectric substrate surface, under the overlayer, and thus they are isolate from the liquid environment. ZnO antibody-antigen immunoreactions in aqueous solutions [66] The LMs also propagate along a non-piezoelectric halfspace (such as Si, glass, BN, a-SiC,...) covered by a piezoelectric layer (such as c-axis tilted ZnO or AlN) [58,[67][68][69].…”

“…The LM sensor based on a piezoelectric layer/non piezoelectric substrate has a remarkable advantage over their counterpart based on piezoelectric half-space/non-piezoelectric layer, as well as over PSAW and HVPSAW-based sensors: four coupling configurations can be investigated to enhance the K 2 The K 2 of the LM device is affected by the mode order, the crystallographic orientation of both the halfspace and layer, the layer thickness, and also the coupling configuration (the electrical boundary conditions). As an example, figure 3.9 shows the K 2 dispersion curves for the first Love mode (LM1) in ZnO/glass for the four coupling configurations and different ZnO c-axis tilt angle (from 10° to 90°) [58]. With increasing the Love mode order, ever decreasing K 2 values can be reached by the 4 coupling configurations.…”

“…The four coupling configurations for the non-piezoelectric half-space/piezoelectric layer structure[58].…”

Love Wave Sensor Based on PMMA/ZnO/Glass Structure for Liquids Sensing

“…The propagation of Love modes along 30° tilted c-axis ZnO/glass-based structures has been modeled and analysed aimed to the design of a sensor able to operate in liquid environment [1]. The sensor velocity and attenuation sensitivities to the changes of liquid viscosity and mass loading were calculated for different ZnO layer thicknesses and the peak sensitivity was achieved at the ZnO thickness to/wavelength ratio h/λ = 0.05, with electromechanical coupling coefficient is K 2 around 0.2% for substrate-transducer-piezolectric film (STF) configuration.…”