Gregory C. Frye

Characterization of SH acoustic plate mode liquid sensors

Abstract Two-port acoustic wave sensors have been fabricated, which utilize shear horizontal (SH) acoustic plate modes (APMs) to probe a solid/liquid interface. These modes, excited and detected by interdigital transducers on thinned quartz plates, propagate efficiently with liquid contacting the device and allow sensing to be performed on the side of the device opposite the transducers. A number of sensing mechanisms have been discovered, including mass loading, viscous entrainment of the contacting liquid, and acoustoelectric coupling between evanescent plate mode electric fields and ions and dipoles in solution. The mass sensitivity of the APM device enables it to function as a microbalance in a number of sensing applications. A chemical sensor capable of detecting low concentrations of Cu 2+ ions in solution has been constructed by chemically modifying the device surface with ethylenediamine ligands. The treated devices bind Cu 2+ ions in a manner that can be reversed by the addition of acid to the solution.

Sensing liquid properties with thickness-shear mode resonators

Abstract The responses of smooth- and textured-surface thickness-shear mode (TSM) resonators in liquid are examined. Smooth devices, which viscously entrain a layer of contacting liquid, exhibit a response that depends on the product of liquid density and viscosity. Textured-surface devices, which also trap liquid in surface crevices, pores, etc., exhibit an additional response that depends on liquid density alone. Combining smooth- and textured-surface resonators in a monolithic sensor enables the liquid density and viscosity to be extracted simultaneously.

Dual output acoustic wave sensors for molecular identification

It is pointed out that online chemical monitoring systems can help ensure safe, environmentally sound operation of industrial processes using hazardous chemicals. Using polymer-coated surface acoustic wave sensors, the authors have demonstrated monitors that are capable of detecting dilute concentrations of volatile organic species. Using changes in both wave velocity and wave attenuation, the identity and concentration of an isolated chemical species can be determined. A polysiloxane coating has been found to provide unique properties for monitoring chlorinated hydrocarbons (CHCs) such as trichloroethylene: good discrimination of CHCs from most organic species, rapid and reversible sensor response, and low detection limits. Using this technology, a portable acoustic wave sensor (PAWS) system has been constructed.<<ETX>>

Acoustoelectric interaction of plate modes with solutions

Acoustic plate mode (APM) devices operating in liquids have an associated electric field which extends several micrometers into the liquid and is capable of interacting with ions and dipoles in solution. Experiments have been performed using quartz APM devices which demonstrate these acoustoelectric interactions. Changes in the dielectric constant or conductivity of solutions in contact with the device were found to perturb the propagation velocity of the plate mode. An equivalent circuit is proposed to describe the interaction of the electric field with ions and dipoles. From this simple model we obtain the velocity shift and attenuation which should occur as a result of this interaction and compare the model predictions to the experimentally observed trends.

Gas sensing with acoustic devices

A survey is made of acoustic devices that are suitable as gas and vapor sensors. This survey focuses on attributes such as operating frequency, mass sensitivity, quality factor (Q), and their ability to be fabricated on a semiconductor substrate to allow integration with electronic circuitry. The treatment of the device surface with chemically-sensitive films to detect species of interest is discussed. Strategies for improving discrimination are described, including sensor arrays and species concentration and separation schemes. The advantages and disadvantages of integrating sensors with microelectronics are considered, along with the effect on sensitivity of scaling acoustic gas sensors to smaller size.

Polymer film characterization using quartz resonators

The dynamic behavior and the electrical response of a thickness-shear mode resonator with a polymer film of a few microns thickness coating one face are considered. With glassy polymers, this film moves synchronously with the oscillating resonator surface; with rubbery polymers, the upper portions of the film lag behind the driven resonator-film interface, causing shear deformation of the film. Interesting dynamic film behavior results, notably a film resonance when the acoustic phase shift across the film is an odd multiple of pi /2. This dynamic behavior influences the electrical response of the resonator due to the coupling between shear displacement and the electric field in the piezoelectric quartz. An equivalent circuit model is derived that relates the near-resonant electrical characteristics of a polymer-coated resonator to the film properties. Measurements made on a polyisobutylene-coated resonator were interpreted with the model to obtain the films sheer storage and loss moduli, giving good agreement with previously reported values. In addition, changes in resonant frequency and damping observed at film resonance agree with model predictions.<<ETX>>

Measuring liquid properties with smooth- and textured-surface resonators

The response of thickness shear mode (TSM) resonators in liquids is examined. Smooth-surface devices, which viscously entrain a layer of contacting liquid, respond to the product of liquid density and viscosity. Textured-surface devices, which also trap liquid in surface features, exhibit an additional response that depends on liquid density alone. Combining smooth and textured resonators in a monolithic sensor allows simultaneous measurement of liquid density and viscosity. >

DETECTION OF ORGANIC CHEMICALS BY SAW SENSOR ARRAY

Abstract A portable SAW sensor array instrument with integrated chemometric software has been developed to identify a variety of chemical classes like paraffinics, aromatics, chlorinated hydrocarbons, ketones and alcohols. The system was trained with known chemicals and a chemometric model was developed for two applications: bulk VOC identification and environmental monitoring of trace level VOCs. The instrument was then successfully tested in the lab and in the field for these two applications. The ability to identify and quantify these VOCs over a dynamic range from low ppm trace levels to high concentrations (close to the chemicals vapor pressure) was demonstrated.

Materials Characterization Using Surface Acoustic Wave Devices

Abstract Thin film materials are currently used in a wide variety of industrial applications. For example, thin films are used as protective or passivating layers to prevent corrosion and increase lifetime, as conductive and photoactive (e.g., photoresists) layers in the preparation of electronic components and integrated circuits, and as selectively permeable layers for use in gas separations. These thin film materials can be formed by a wide variety of processes, including spraying, spin-coating, dipcoating, chemical vapor deposition (CVD), evaporation, and sputtering. The applications of thin film technology continue to expand due to the ability to decrease cost and increase performance using thin film materials whose properties have been tailored to meet the specifications of the application.

Characterization of the Surface Area and Porosity of Sol-Gel Films Using Saw Devices

A novel technique for accurately obtaining nitrogen adsorption isotherms on thin porous films has been developed. These isotherms are useful for characterizing the surface area and pore size distribution of porous samples. The sensitivity to adsorbed nitrogen is increased by several orders of magnitude over conventional techniques by forming the test film on the substrate of a surface acoustic wave (SAW) device. This device functions as a microbalance able to detect less than 100 pg of adsorbed nitrogen per cm{sup 2} of film. Surface areas and pore size distributions were calculated from adsorption isotherms obtained with this technique on silicate sol-gel films. The results are compared to those for bulk samples prepared from similar sol-gel solutions. 18 refs., 5 figs., 1 tab.)