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Overview
In situ monitoring of physical and chemical properties of fluids --
such as density, viscosity, precipitates, trace contaminants, phase changes,
non-Newtonian behavior — can be performed using thickness shear mode (TSM)
resonator microsensors developed at Sandia National Laboratories. Many
laboratory and industrial applications of the sensor have been investigated and
tested, leading to several technology demonstration prototypes. The
sensor’s small size, low cost components, and absence of macroscopic
moving parts enable cost effective, in situ measurements in confined
spaces and adverse environments.
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Figure 1. Cross-sectional view of a thickness shear mode resonator
with the upper surface contacted by a liquid. |
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Figure 2. A miniature fluid monitoring test cell used for measuring
deposition of jet fuel degradation products at elevated temperatures. |
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Figure 3. Schematic diagram of Sandia’s density/viscosity
sensor using dual TSM resonator sensors. |
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Figure 4. The frequency response (solid line) for a poly(vinyl
acetate)-coated quartz resonator sensor exposed to increasing concentrations of
chloroform (dashed line). The response of an uncoated resonator (dotted line)
is used as a reference. |
Technical Approach
Thickness shear mode (TSM), or bulk acoustic wave, resonators consist of
piezoelectric quartz wafers with metal electrodes on opposite faces.
Application of a radio frequency signal to the electrodes causes excitation of
a shear mechanical resonance (see Figure 1). Materials in contact with the
quartz surface interact mechanically and perturb the resonant frequency and
crystal damping. Changes in these parameters can be measured using a network
analyzer or a Sandia-developed oscillator circuit. Mass accumulation on the
crystal surface produces a shift in resonant frequency, while contacting
liquids shift both the resonant frequency and the oscillation magnitude
proportional to the density and viscosity. Some materials are viscoelastic,
producing changes in the frequency and magnitude as their properties vary.
Chemical sensors consist of TSM resonators with thin sensitive and selective
surface layers. Analytes in the liquid or vapor phase are sorbed onto or into
the coating, changing the film mass or perturbing the film viscoelastic
properties. Device electrical response is related to the chemical interaction
through several sophisticated mechanical and equivalent electric circuit
models.
Applications
Acoustic TSM resonator microsensors can be used for the following
applications:
- In situ measurement of surface mass accumulation to ascertain
degradation in (1) hydrocarbon fluids such as jet fuel (see Figure 2) or (2)
process fluids such as plating or photographic baths.
- On-line monitoring of density-viscosity of industrial fluids to improve
process control and increase throughput rates.
- In situ measurements of liquid density and viscosity for laboratory
and industrial purposes; this technique uses dual resonators, one with a smooth
surface and the other with a corrugated surface (see Figure 3).
- In situ monitoring of lubricant or fluid viscosity to determine
optimum maintenance intervals.
- Monitoring of fluid density and viscosity to indicate state-of-charge of
lead-acid batteries or the freezing/boiling point of coolant mixtures.
- Determination of non-Newtonian fluid properties (Maxwellian
characterization) in lubricants and polymer melts.
- Sensing material phase changes in processes such as epoxy curing, onset of
icing on aircraft wings and road surfaces, precipitation or gelation in
slurries and solutions, and cloud point in crude oils and hydrocarbon
products.
- Characterization of film viscoelastic properties such as polymer elastic
moduli, glass transition points, frequency dependencies, and temperature
profiles.
- In situ detection of trace chemical contaminants in air or water
(e.g., volatile organic compounds using polymer films, see Figure 4) or
detection of trace amounts of water in fuel or oil.
- In situ monitoring of electrochemical processes such as deposition
of electroactive polymers from solution.
- In situ monitoring of pH changes in aqueous solutions and acidity in
non-aqueous environments.
Resources
- Facilities for fabricating bulk acoustic wave resonators, including quartz
polishing, metal electrode deposition, photolithographic electrode patterning,
and electroforming of surface corrugations.
- Facilities for deposition and characterization of chemical sensing surface
layers, including spin coaters, evaporation chambers, curing ovens, atomic
force microscope (AFM), FTIR spectrometer, and a profilometer.
- Network and impedance analyzers for electrical characterization of devices;
computer models for extracting equivalent circuit parameters from device
response; and models for extracting mass, fluid, and viscoelastic properties
from device response.
- Oscillator electronic circuits to operate the sensor devices at their
fundamental resonance and to measure changes in the resonant frequency and
damping magnitude.
- Liquid test facilities for determining device response: uncoated devices
used to measure surface mass accumulation and contacting liquid properties; and
coated devices used for measuring fluid chemistry.
- Gas test facilities for evaluating coated device response to gases bearing
trace chemical contaminants.
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