Metal-loaded polymer films for chemical sensing of ES&H-related pollutants Page: 4 of 74

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INTRODUCTION
In this project, we proposed tle use of metal-loaded polymer films as chemically-sensitive
layers on surface acoustic wave (SAW) devices. Small metal particles can be dispersed in polymer
films. When the concentration of these particles is such that adjacent particles touch, the electrical
conductivity of the films is high. When the film is exposed to a vapor that is absorbed the polymer,
swelling of the polymer occurs, resulting in a greater separation between particles and a significant
reduction in film conductivity. This can be used as a basis for constructing chemical sensors.
In this project, we proposed using the SAW device as a "platform" for detecting changes in
metal-loaded polymer films due to vapor absorption. These devices are extremely sensitive to
changes in film conductivity. As the film passes through a critical conductivity, attenuation goes
through a maximum, while wave velocity changes drastically. By incorporating these devices in an
oscillator circuit, very minute changes in film conductivity can be measured.
In coating SAW devices with polymer films, we found that there were other effects, aside
from film conductivity changes, that lead to a sensor response. As a vapor is absorbed into a
polymer, the mass density of the polymer changes. In addition, very large changes in the film's
viscoelastic properties arises due to film "plasticization" by the absorbed vapor. The majority of the
activity in this project was spent in characterizing the response of the SAW device to changes in film
viscoelastic properties. A model was derived for the response of the SAW device with .a viscoelastic
film overlay. The response of polymer-coated SAW devices was also measured as viscoelastic
properties were changed due to (1) temperature changes, and (2) vapor absorption. This report
describes our efforts to characterize the response of the polymer-coated SAW device.
BACKGROUND
Surface acoustic wave (SAW) devices use interdigital transducers photolithographically
patterned on the surface of a piezoelectric crystal (Fig. 1) to excite and detect surface waves'. These
waves are excited in a frequency band centered at f0 = v/d, where v is the SAW propagation velocity
and d is the transducer period. The SAW wavelength A generated at fa matches the transducer period
d. SAW devices typically operate in the 30 - 300 MHz range and use ST-cut2 quartz substrates for
high temperature stability near room temperature. In the delay-line device configuration, input and

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Martin, S.J. & Frye, G.C. Metal-loaded polymer films for chemical sensing of ES&H-related pollutants, report, March 1, 1997; Albuquerque, New Mexico. (https://digital.library.unt.edu/ark:/67531/metadc675600/m1/4/ocr/: accessed May 19, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.

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