Quartz Resonator Fluid Monitor
Thickness Shear Mode Resonators
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.
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.
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| Figure 1. Cross-sectional view of a thickness shear mode resonator with the upper surface contacted by a liquid. |
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.
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| Figure 2. A miniature fluid monitoring test cell used for measuring deposition of jet fuel degradation products at elevated temperatures. |
- 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.
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| Figure 3. Schematic diagram of Sandia’s density/viscosity sensor using dual TSM resonator sensors. |
- 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.
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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. |
- 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.
For additional information or questions, please email us at Microsystems Gas Analysis
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