Love Wave In Ferroelectric Single Crystal Layered Structure And Application In Gravimetric Sensor

Relaxor-based ferroelectric single crystals lead magnesium niobate-lead titanate(PMN-PT) and lead zinc niobate-lead titanate(PZN-PT) exhibit extremely high piezoelectric properties and ultrahigh electromechanical properties. These ferroelectric single crystals could be used in next generation high sensitivity ultrasonic wave sensor as the core material. In all kinds of ultrasonic wave devices, Love wave gravimetric sensor has the advantages of simple manufacture technology, high sensitivity and low noise, so it becomes a current hot topic of research in biosensor in liquid phase and gas sensor. Love wave is a result of the interferation of Shear-Horizontal waves(SH waves) guided by an elastic layer, which is closely attached on a semi-infinite medium. At present, almost all Love wave devices use quartz crystal as the substrate material. In general, the waveguide of Love wave devices made of fused quartz, polymethylmethacrylate(PMMA), Zn O, etc. In order to design higher sensitivity gravimetric sensors, PMN-PT single crystals and PZN-PT single crystals be used in piezoelectric substrate of Love wave devices, and unidirectional carbon fiber epoxy composite(CFEC) was introduced in the waveguide layer. A detail analysis has been performed on the Love wave propagating in ferroelectric single crystal layered structure by employing partial wave theory. In addition, based on perturbation theory, the sensitivity and optimal design parameters have been derived for Love wave devices with different configuration.Partial wave method was performed to investigate the dispersion properties and the evolution of displacement distribution of Love wave propagating in ferroelectric single crystal layered structure. The results indicate that, there is no cross coupling among different modes, which is conducive to eliminate cross interference between different modes. More of the acoustic energy would be concentrated inside waveguide with the increase of layer thickness for a given frequency. Compared with traditional piezoelectric ceramics, ferroelectric single crystal has a slower attenuation of amplitude of Love wave. This means that the single crystal has a lower acoustic absorption. Therefore, the single crystal is much suitable for making low loss acoustic wave devices with a high operating frequency. The effect of the compositions and polarization direction of single crystals on propagation characteristic of Love wave was analyzed. The results indicate that, the optimal compositions near MPB regions, and the [011]c poled single crystal near the MPB is much suitable for making transverse wave mode ultrasonic sensors.Perturbation theory was performed to investigate Love wave gravimetric sensor with the PMMA/PMN-PT and Epoxy/PMN-0.29 PT configurations. The PMMA and epoxy layers are very lossy and as their thickness increases so does the ultrasonic absorption. As a result the optimal thickness cannot be reached. Therefore, unidirectional CFEC waveguide layer was presented. By analyzing the dispersion properties and sensitivity curves, it was found that the maximum frequency shift occur in the most rapidly changing area of Love wave phase velocity. For the CFEC waveguide/[011]c poled PMN-0.29 PT single crystal sensor configuration with l=24 mm, the maximum sensitivity of 2max|| 1230(cm /g)fmS=, which is the highest sensitivity achieved up to date. By analyzing the devices with different PMN-PT ferroelectric single crystal substrate, it was found that a substrate material having larger elastic constant 66Ec and lower density can improve the sensitivity of the gravimetric sensor. The maximum relative scaled frequency sensitivity max(| |)fmSl exhibits an increasing tendency with the decrease of the optimal normalized layer thickness(h/l)opt. This phenomenon is due to the increasing operation frequency.The relationship between acoustic impedance and acoustic attenuation for unidirectional CFEC waveguide was investigated. The results indicate that, the acoustic attenuation coefficient exhibits a decreasing tendency with the increase of acoustic impedance. Therefore, the waveguide material should have high acoustic impedance and low porosity. The effect of carbon fibers orientation on the propagation properties of Love wave was analyzed. The results indicate that, the distribution of dispersion curves was widened due to the carbon fibers perpendicular to the propagation direction compared with the fibers parallel to the propagation direction of acoustic guided wave. This phenomenon is due to the elastic constant c44 was increased, while c66 remains unchanged.In order to optimize the design of Love wave gravimetric sensor, two general guideline for the improvement of the device sensitivity has been summarized:(1) the waveguide composite should have low shear velocity, low density, low acoustic absorption, and the carbon fibers parallel to the propagation direction of acoustic guided wave;(2) the substrate material should have larger elastic constant(good elastic properties, high hardness), lower density, higher shear horizontal(SH) wave velocity, and excellent piezoelectric properties.