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Thickness Shear Mode Resonators

Fact Sheet

[Cross-sectional view] Figure 1. Cross-sectional view of a thickness shear mode resonator with the upper surface contacted by a liquid and a mass layer.
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
[Fluid Monitoring Testcell] Figure 2. A miniature fluid monitoring testcell used for measuring deposition of jet fuel degradation products at elevated temperatures.

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.

[Density/Viscosity Sensor] Figure 3. Schematic diagram of Sandia's density/viscosity sensor using dual TSM resonator sensors.

Acoustic TSM resonator microsensors can be used for the following applications:

[Frequency Response] Figure 4. The frequency response (solid black line) for a poly (vinyl- acetate)-coated quartz resonator sensor exposed to increasing concentrations of chloroform (solid blue line). The response of an uncoated resonator (solid red line) is used as a reference.

Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy.
Kent Pfeifer

(505) 844-8105

Last modified: August 23, 1999

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