Z.A. Shana

Analysis of piezoelectric bulk-acoustic-wave resonators as detectors in viscous conductive liquids

An analytical solution for the resonance condition of a piezoelectric quartz resonator with one surface in contact with a viscous conductive liquid is presented. The characteristic equation that describes the resonance condition and accounts for all interactions including acoustoelectric interactions with ions and dipoles in the solution is obtained in terms of the crystal and liquid parameters. A simple expression for the change in the resonance frequency is obtained. For viscous nonconductive solutions, the frequency change is reduced to a relationship in terms of the liquid density and viscosity. For dilute conductive liquid, the change in frequency is derived in terms of the solution conductivity and dielectric constant. The boundary conditions for the problem are defined with and without the electrical effects of electrodes. Experiments were conducted with various viscous and conductive chemical liquids using a fabricated miniature liquid flow cell containing an AT-cut quartz crystal resonator. The results, which show good agreement with the theory, on the use of quartz crystal resonators as conductivity and/or viscosity sensors are reported.<<ETX>>

Theory and application of a quartz resonator as a sensor for viscous liquids

Abstract A comprehensive analysis of the interaction between an AT-cut quartz crystal resonator and a viscous fluid is presented. The analysis, which includes peizoelectric effects, assumes a liquid of finite extent and therefore could also be used to study thin film of viscous liquids. A novel continuous flow cell system was designed and fabricated to monitor viscosity using an 11-MHz quartz crystal resonator. Measured data for frequency shifts of aqueous solutions of alcohols and sugars are in excellent agreement with theory.

On the use of ZX-LiNbO3 acoustic plate mode devices as detectors for dilute electrolytes

Abstract Acoustic plate mode (APM) devices on Z-cut, X-propagating LiNbO3, a high piezoelectric coupling material, are investigated as detectors in dilute electrolyte or metal-ion solutions. The sensing wave is an APM, a slow shear wave coupled to the interdigital transducer, which results in a relatively strong acoustic-ionic interaction. The resulting electrical loading leads to a measurable perturbation in the wave-propagation characteristics, which can then be related to the liquid electrical properties. Experiments conducted at different frequencies with various solutions that compare well with theoretical results show a detector that is at least two orders of magnitude more sensitive than similar APM sensors on quartz. These promising results have also led to the investigation of the detection of aqueous transition-metal ions (Fe3+, Cu2+, etc.) using solution conductivity changes rather than the commonly used mass sensitivity. The method involves the bonding of metal ions in soluiton to a ligand-coated silica or LiNbO3 support, placed adjacent to the APM device and in the path of the liquid flow. The resulting frequency increase is then interpreted in terms of the concentration of the metal ions. X-ray photoelectron spectroscopy analysis of the coated surface is performed to confirm the binding.

Electrical surface perturbation of a piezoelectric acoustic plate mode by a conductive liquid loading

Surface loading of a piezoelectric crystal supporting acoustic plate modes (APMs) by a dilute conductive liquid is analyzed using a perturbation theory. The formulation of the problem is such that only the electrical loading is relevant, and the mass loading and viscous entrainment caused by the solute are ignored. The perturbation in the propagation characteristics is then obtained relative to the solvent and is described in terms of the coupling coefficient, the capacitive loading, and the conductivity of the liquid. The results are compared to measurements made on Z-cut X-propagating LiNbO/sub 3/ APM device loaded with various conductive liquids of different concentrations. While an interpretation of the results can be given on the use of the APM device as a detector of the liquid properties, it is shown that a conductive liquid loading of the piezoelectric surface can be used as a means of assessing the electromechanical coupling coefficient of APMs.<<ETX>>

Analysis of shear horizontal surface waves at the boundary between a piezoelectric crystal and a viscous fluid medium

Surface shear horizontal wave propagation at the boundary of a piezoelectric substrate with a viscous fluid is theoretically analyzed. The analytical solution clearly shows the dependence of the velocity change, the wave propagation loss, and the wave amplitude profile (thus the energy confinement near the surface), in terms of the liquid viscosity and density, the layer thickness, and the wave frequency. It is shown that the viscous fluid loading produces some guidance near the surface but also some damping of the wave. The propagation loss is due only to viscous coupling and not to a mode conversion and viscous coupling as is the case with Rayleigh surface acoustic waves (SAWs). Closed‐form expressions are derived for the attenuation coefficient and the fractional velocity change (thus the frequency change) in terms of the piezoelectric crystal and viscous liquid parameters. The theory, applicable to both a Bleustein–Gulyaev (BG) wave and a surface skimming bulk wave (SSBW), indicates that surface shear...

Acoustoionic interaction of SH surface waves with dilute ionic solutions

A theory describing the acoustoionic interaction of shear horizontal (SH) surface waves with viscous conductive ionic liquid is presented. A Greens function formulation that accounts for the acoustoelectric interaction with ions and dipoles in the solution is obtained for the surface potential in terms of the liquid and piezoelectric crystal parameters. For dilute ionic solutions, simple closed-form expressions for the velocity change and attenuation are obtained in terms of liquid conductivity and dielectric constant and the piezoelectric coupling coefficient. It is shown that SH surface waves in particular and acoustic waves in general can be used to perform microanalysis of dilute ionic solutions, detecting conductivity, dielectric constant, and relaxation frequency. The analysis, which was done for a simple crystal class, the hexagonal (6 mm), shows results which compare very well with exact numerical computations.<<ETX>>

Identification of metal ion solutions using acoustic plate mode devices and pattern recognition

The temperature dependence of acoustic plate mode (APM) devices used as probes for dilute electrolytes is described. Specifically, the probe responses that consist of the frequency change and device loss were studied for dilute aqueous solutions of alkali metal ions. It is shown that by integrating the temperature dependence of the APM probe responses with pattern recognition techniques, valuable information about the solutions can be obtained that include identification and quantification. A preliminary investigation of the feasibility of identifying binary mixtures of the alkali metal ion solutions using only the temperature responses showed good results.<<ETX>>

Reflection of bulk waves at a piezoelectric crystal–viscous conductive liquid interface

The reflection of obliquely incident shear horizontal (SH) bulk acoustic waves at the interface between a piezoelectric crystal and a viscous conductive liquid is theoretically investigated. In addition to viscous entrainment and mass loading effects, it is shown that the presence of a reflection‐generated external electric field offers the possibility of acoustoelectric interaction with charged particles and dipoles in the solution. These interaction mechanisms could be exploited for liquid chemical analysis and biosensor applications. Expressions are obtained for the reflection coefficients (amplitude and energy) and phase shift upon reflection in terms of the angle of incidence, wave frequency, crystal parameters, and liquid properties (density, viscosity, dielectric constant, and conductivity) by using the normal wave impedance and the input impedance of the interface. It is shown that the sensitivity of such acoustic wave detectors could be tailored by properly choosing the interdigital transducer (I...

Propagation of acoustic waves at the interface between a piezoelectric crystal and an isotropic viscoelastic conductive medium (SAW sensors)

Liquid-phase acoustic wave biosensors consist of a piezoelectric substrate, an isotropic film used for selectivity or as a molecular recognition element, and a liquid medium providing the environment for the species to be detected. At any point in time (during the chemistry being conducted at the surface) both film and liquid may be modeled as isotropic viscoelastic conductive media. A general theory describing the effects of isotropic viscoelastic conductive media on the propagation characteristics of acoustic waves excited by interdigital transducers (IDTs) is presented for arbitrary piezoelectric crystal orientations. Results and discussion are given relative to chemical sensors and biosensor applications.<<ETX>>

Sensitivity of various critical frequencies of a QCR sensor in dilute electrolytes

For a quartz crystal resonator (QCR) loaded with a conductive solution, two groups of closely spaced resonance frequencies (fm , fs, fr) and antiresonance frequencies (f a, fp, fn) can be used to characterize the QCR sensor. The variation of all critical frequencies of the QCR sensor in terms of the electrical properties of dilute electrolytic solution is studied. Theoretical expressions, which include the QCR electrode geometrical factor, are obtained for the various frequencies. Experimentally, it is shown that by using a network analysis method, the impedance/admittance can be monitored to accurately track frequency variations in terms of liquid properties. The appropriate frequency of operation, hence the rate of change can then be selected to ensure highest sensitivity and reproducibility