Showing papers by "John F. Vetelino published in 2003"

A lateral field excited liquid acoustic wave sensor

Abstract: Lateral field excited (LFE) AT-cut quartz acoustic wave sensors in which the electrodes are located on the reference surface have been fabricated and tested in liquid environments. The sensing surface, which is opposite to the reference surface, is free allowing the electric field of the thickness shear mode (TSM) to penetrate into the liquid. This results in increased sensitivity to both mechanical and electrical property changes of the liquid. In the present paper, several 5-MHz LFE sensors with a range of electrode spacings were exposed to liquid environments in which the viscosity, relative permittivity, and conductivity were varied. The LFE sensors demonstrate sensitivity to viscosity that is more than twice that obtained for the standard quartz crystal microbalance (QCM), and sensitivity to relative permittivity and conductivity about 1.5 times that of the QCM sensors with modified electrodes. The present results clearly indicate that the LFE sensors may have a wide range of liquid phase applications in which sensitivity is crucial.

Current/voltage characteristics of a semiconductor metal oxide gas sensor

Abstract: The current/voltage (I/V) characteristics of undoped and gold-doped tungsten trioxide (WO3) thin film sensors were studied experimentally and theoretically. An equivalent circuit, which includes forward and reverse-biased Schottky diodes representing the contact region between the electrodes and film, and an equivalent resistor which represents the resistance of the individual WO3 film crystallites and intercrystallite boundaries was formulated. Depending upon the electron transport mechanisms through the metal–semiconductor (M/S) interfaces and the WO3 film, two different I/V and therefore resistance/voltage (R/V) characteristics are possible, one based on thermionic emission theory and the other on tunneling theory. In order to determine the dominant electron transport mechanism several WO3 films were tested. Amorphous and polycrystalline WO3 and WO3: Au films were RF-sputtered onto a sapphire substrate. A dc voltage ranging from −20 to 20 V was applied to the films in compressed air with and without ethylene gas present. The experimental R/V results are approximately constant except in the low voltage region where they are nonlinear. Comparing the experimental results of the R/V characteristics, with the theoretical curves predicted by thermionic emission and tunneling theories, some interesting conclusions can be made regarding the dominant electron transport mechanism in the WO3 films. The depletion width between the M/S interface and the intercrystallite boundaries is the key parameter, which determines the appropriate electron transport mechanism. Since the depletion width is related to the oxygen vacancies in the film and therefore the argon/oxygen ratio in the sputtering process, one can a priori design a sensor, in which the electron transport will exhibit either thermionic emission or tunneling.

Chemical sensors based on electrically sensitive quartz resonators

Abstract: The sensitivity of a quartz resonant sensor to the change of liquid electrical properties can be significantly enhanced by modifying the electrode geometry of the sensor. Chemical sensors based on electrically sensitive quartz resonators can then be developed for applications where the liquid electrical properties, as well as the mechanical properties change. In the present study, a theoretical model for the electrically sensitive quartz resonant sensor is presented and it is shown that the previous models for liquid phase quartz sensors can be obtained from the present model as special cases. Examples of using the present sensor for monitoring ammonium in aquaria water and detecting liquid permittivity change have been presented.