Hank Wohltjen

Mechanism of operation and design considerations for surface acoustic wave device vapour sensors

Abstract Surface acoustic wave (SAW) devices offer many attractive features for application as vapour phase chemical microsensors. This paper describes the characteristics of SAW devices and techniques by which they can be employed as vapour sensors. The perturbation of SAW velocity by polymeric coating films is investigated both theoretically and experimentally. Highest sensitivity can be achieved when the device is used as the resonating element in a delay line oscillator circuit. A simple equation has been developed from theoretical considerations which offers reasonably accurate quantitative predictions of SAW device frequency shifts when subjected to a given mass loading. In this mode the SAW device behaves very like conventional bulk-wave quartz crystal microbalances except that the sensitivity can be several orders of magnitude higher and the device size can be several orders of magnitude smaller. Detection of mass changes of a few femtograms by a SAW device having a surface area of 10−4 cm2 is theoretically possible.

A vapor-sensitive chemiresistor fabricated with planar microelectrodes and a Langmuir-Blodgett organic semiconductor film

A vapor-sensing method has been developed which is compatible with monolithic silicon microelectronics technology. Specifically, electronic conductance changes caused by vapor interactions with very thin films of organic semiconductors are shown to be sensitive, reproducible, rapid, and stable chemical detectors. Functionalized copper phthalocyanine multilayer films deposited by the Langmuir-Blodgett technique onto planar microelectrode arrays can easily detect ammonia at sub-ppm concentration levels.

Derivatives of phthalocyanine prepared for deposition as thin films by the Langmuir-Blodgett technique☆

Abstract Film pressure versus area data for a series of organic derivatives of phthalocyanine are presented in detail. Classical monomolecular films were not observed. Compounds studied include tetraphenoxy, dicumylphenoxy and tetracumylphenoxy, tetraoctadecoxy and tetraneopentoxy phthalocyanine. Spread films of metal-substituted tetracumylphenoxy phthalocyanines that contained iron, cobalt, nickel, copper, zinc, palladium, platinum and lead were also examined along with 1:1 mole ratio mixed films of these compounds with octadecanol. Mixed films with varying mole ratios of nickel tetracumylphenoxy phthalocyanine and octadecanol are also described. On the basis of these data and considerations of recent studies of tetra- tert -butyl phthalocyanine by other investigators, a stacked phthalocyanine structure is indicated. The stack axis is not parallel to the plane of the film.

Trace Chemical Vapor Detection Using SAW Delay Line Oscillators

The resonant frequencies of SAW delay line oscillators are shown to be very sensitive to the presence of thin organic films that are deposited onto the delay line surface. Theory suggests that the sensi- tivity of the SAW device to mass loading depends on the square of the resonant frequency. This dependence has been studied experimentally using devices resonating at 31, 52, and 112 MHz that were coated with organic films of precisely controlled composition and thickness. The mass sensitivity of SAW oscillators can be exploited to make very sen- sitive chemical sensors if an appropriate sorptive coating is applied to the device. Results obtained from a delay line oscillator operating at 290 MHz (the highest frequency SAW chemical sensor reported to date) suggest that the rapid detection of organic vapors at concentra- tions substantially below 100 parts per billion (by volume) is readily achievable.

An investigation of conductivity in metal-substituted phthalocyanine Langmuir-Blodgett films

Abstract Conductivity data for Langmuir-Blodgett films of cobalt, nickel, copper and zinc tetracumylphenoxy phthalocyanine complexes are presented as well as the changes in conductivity of the films induced by exposure to ammonia. An explanation is given for these results, supported by discrete variational X-α calculations of simple cobalt, nickel, copper and zinc phthalocyanine complexes and on calculations of these metals complexed with tetraazaporphyrin and ammonia. It was found that to some extent the conductivity can be correlated with the energy gap between the highest occupied and lowest unoccupied orbitals.

An automated vapor-generation and data collection instrument for the evaluation of chemical microsensors

Abstract An instrument has been built which can generate up to twelve different vapors from bubbler or permeation tube sources. These vapors can be individually selected, diluted to concentrations typically in the range of 1 to 10 000 mg/m3 volume and delivered at a programmable flow rate to the system output. Mixtures of two vapors can also be easily generated. The system is fully automated using a small personal computer and permits completely unattended operation during elaborate vapor-exposure sequences. Parameters describing the vapor test conditions are automatically stored in the computer disk memory, along with the sensor responses to the test. This instrument has proved to be extremely valuable for rapidly testing prototype chemical sensors.

Use of SAW devices to monitor viscoelastic properties of materials

Preliminary data are presented which demonstrate the potential sensitivity of SAW (surface acoustic wave) to the elastic properties of polymer coatings. 158-MHz dual SAW devices were used to demonstrate the sensitivity of the SAW to changes in coating elastic properties, and to identify the T/sub g/ and/or T/sub m/ of several polymer coatings. Data for PCAP (poly(caprolactone)) and FPOL (fluoropolyol) indicate that the SAW frequency is sensitive to changes in the shear modulus of the coatings. The steady decrease in frequency with increased temperature observed for FPOL-coated devices may be the result of increased surface coverage due to expansion of the film. Increased surface coverage would result in an increase in surface wave interaction with the film and a corresponding steady decrease in frequency. This indicates that these polymer films may not be true lossless films. It is suggested that in light of this, previous assumptions made in the development of predictive equations using SAW data must be reevaluated. It is concluded that the sensitivity of SAWs to elastic properties is indicative of their potential as monitors for cure processes, or other applications where changes in the elasticity of materials are of interest.<<ETX>>

Chemical microsensors ‐ a new approach for the detection of hazardous agrochemicals

Abstract A number of novel analytical devices are being pursued at the Naval Research Laboratory for potential application as very small, inexpensive and sensitive detectors of hazardous agricultural chemicals. Techniques are being investigated for both solution and gas phase analysis. The new detectors include surface acoustic wave devices, optical waveguides and interdigital electrode arrays coated with chemically sensitive organic semiconducting thin films. The organic semiconducting films can also be used in other devices such as chemically sensitive field‐effect transistors. Low cost and small size are achieved through the use of electronic planar microfabrication technology to construct the various sensing elements. Chemical sensitivity and specificity are achieved by developing chemical coatings for the different devices that have specific reactivity to selected chemicals. By combining the various sensors and coatings with integrated electronics and microprocessors small chemical detectors can be d...

A Study of SAW Delay Line Behavior in Liquids

SAW delay line oscillators designed for operation at 10, 30, 31, and 50 MHz have been used to study the role of liquid loading on Rayleigh surface wave propagation. Experimental data clearly show that the dominant mode of energy transport in thin, liquid loaded SAW devices is not due to Rayleigh waves. These results have important implications for the design of SAW chemical sensors intended for operation in solutions.

Electronic Detection of Synthetic Lubricant Oxidative Breakdown

The oxidative breakdown of lubricating oils is accompanied by the liberation of vapor-phase oxidation products. A simple apparatus which uses a semiconducting tin oxide gas sensor has been employed to monitor the evolution of these products and determine the length of the induction period. The method provided reliable end-point detection with ester-base lubricants of moderate stability, simplifying the oxidation testing procedure and providing useful data for studies of lubricant stability. A sensor calibration technique was developed, and the sensors were found to have adequate sensitivity, response speed, and reliability. Presented at the 40th Annual Meeting in Las Vegas, Nevada May 6–9, 1985