There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

Electrochromic tungsten oxide films: Review of progress 1993–1998

Mesoporous semiconducting films consisting of preferentially orientated monoclinic-phase nanocrystals of tungsten trioxide have been prepared using a novel version of the sol−gel method. Transformations undergone by a colloidal solution of tungstic acid, stabilized by an organic additive such as poly(ethylene glycol) (PEG) 300, as a function of the annealing temperature have been followed by means of a confocal Raman microscope. The shape and size of WO3 nanoparticles, the porosity, and the properties of the films depend critically on preparation parameters, such as the tungstic acid/PEG ratio, the PEG chain length, and the annealing conditions. Well-crystallized WO3 films combine excellent photoresponse to the blue region of the solar spectrum, up to 500 nm, with good transparency at wavelengths larger than 550 nm. Particular applications of these nanocrystalline WO3 films include photoelectrochemical and electrochromic devices.

Crystallographically oriented mesoporous WO3 films: synthesis, characterization, and applications.

In the recent past a great deal of research efforts were directed toward the development of miniaturized gas-sensing devices, particularly for toxic gas detection and for pollution monitoring. Though various techniques are available for gas detection, solid state metal oxides offer a wide spectrum of materials and their sensitivities for different gaseous species, making it a better choice over other options. In this article a critical parameter analysis of different metal oxides that are known to be sensitive to various gaseous species are thoroughly examined. This includes phase of the oxide, sensing gaseous species, operating temperature range, and physical form of the material for the development of integrated gas sensors. The oxides that are covered in this study include oxides of aluminum, bismuth, cadmium, cerium, chromium, cobalt, copper, gallium, indium, iron, manganese, molybdenum, nickel, niobium, ruthenium, tantalum, tin, titanium, tungsten, vanadium, zinc, zirconium, and the mixed or...

Oxide Materials for Development of Integrated Gas Sensors—A Comprehensive Review

http://cau.ac.kr/~jjang14/BioMEMS/Ivnitski_BSBE_Bacteria_Detection_Bio_Sensors_Review_1999.pdf

Biosensors for detection of pathogenic bacteria

The theory describing the operation of a surface acoustic wave (SAW) hydrogen sulfide (H 2 S) sensor was examined. This sensor employs a tungsten trioxide (WO 3 ) chemiresistive overlay deposited as the sensing element on a SAW delay line. Previous work has shown that a change in the film's DC conductivity upon exposure to a target gas, such as H 2 S, has far more influence on the SAW than the film's mechanical properties. However, early work also pointed out that higher sensitivities had been experienced experimentally than the current theory suggests. The objective of the current work was to reexamine the theory describing the effect of a film's electrical properties on SAW velocity. Perturbation theory has been employed to characterize the effects of the film's DC conductivity, dielectric constant and device operating frequency on the SAW gas sensor. Experimental work was conducted on a SAW delay line residing on a cut of quartz optimized for stability at high temperature.

The integration of a chemiresistive film overlay with a surface acoustic wave microsensor

The quartz crystal microbalance (QCM) has been used extensively as a bulk acoustic wave (BAW) platform for applications such as chemical and biological sensors and rate monitors in thin film deposition systems. Although the QCM is capable of measuring mechanical property changes critical in many thin film deposition systems, it cannot measure electrical property changes that can occur in many sensor applications. In this paper we review the recent developments of two novel transducer configurations for BAW sensors. In the first sensor, called the lateral field excited (LFE) sensor, the transverse shear mode (TSM) in AT-cut quartz is excited by two electrodes on the reference surface, resulting in a bare sensing surface which allows both electrical and mechanical properties of target analytes to be measured. In the second sensor, called the monolithic spiral coil acoustic transduction (MSCAT) sensor, the TSM is excited by a photolithographically deposited spiral antenna on the reference surface which can excite high-order harmonics in the substrate, and potentially lead to increased sensitivity. The responses of both the LFE and MSCAT sensors to electrical and mechanical property changes of liquids have been examined and compared to the response of the standard QCM. In addition, results relating to the detection of chemical and biological target analytes using the LFE and MSCAT sensor platforms are presented.

Recent advances in lateral field excited and monolithic spiral coil acoustic transduction bulk acoustic wave sensor platforms

Tungsten trioxide (WO3) thin films were r.f. magnetron sputtered onto quartz substrates in various argon/oxygen (Ar/O2) atmospheres at various deposition rates and temperatures. The electrical conductivity of the WO3 films was monitored as they were subjected to annealing cycles of successively higher temperatures with gas testing performed after each cycle. This work demonstrates that the electrical conductivity of the WO3 films is a function of deposition parameters such as argon/oxygen ration, deposition rate and substrate temperature and annealing conditions. Furthermore, it is shown that by a judicious choice of deposition parameters and annealing conditions, WO3 films whose electrical conductivity falls within the optimum range of operation for a surface acoustic wave (SAW) gas sensor may be produced.

The optimization of a tungsten trioxide film for application in a surface acoustic wave gas sensor

The elastic, piezoelectric, and dielectric constants of single‐crystal BaLaGa3O7, which belongs to the 42m class of the tetragonal system, were measured. The six independent elastic constants, two piezoelectric constants, and coupling coefficients were determined using standard resonance‐antiresonance measurements and the following results were received (cij in 1011 N/m2): c11=1.78, c12=1.03, c33=1.17, c13=0.88, c44=0.39, c66=0.54, e14=0.29 C/m2, e36=0.10 C/m2, kl14=0.141, and kl36=0.055. The two dielectric constants were measured at low frequency using a three electrode configuration. The relative values of these constants were determined to be eT11/e0=12.7, eS11/e0=12.4, eT33/e0=9.5, eS33/e0=9.5.

Elastic, piezoelectric, and dielectric properties of the BaLaGa3O7 crystal

An acoustic wave sensor having a piezoelectric substrate shaped to generate lateral electric fields. The sensor has a pair of electrodes deposited upon a reference surface of the substrate. A sensing surface that is opposite from the reference surface is adapted to be immersed in an environment, such as a gas or a liquid.

John F. Vetelino

Papers

The integration of a chemiresistive film overlay with a surface acoustic wave microsensor

Recent advances in lateral field excited and monolithic spiral coil acoustic transduction bulk acoustic wave sensor platforms

The optimization of a tungsten trioxide film for application in a surface acoustic wave gas sensor

Elastic, piezoelectric, and dielectric properties of the BaLaGa3O7 crystal

Lateral field excited acoustic wave sensor