High Temperature Acoustic Wave Gas Sensors|
For Automotive Exhaust and Industrial Stack Monitoring
A novel area of research and development at Sandia National Laboratories is exploring the utility of acoustic wave-based microsensors for high temperature gas monitoring. Robust coatings on surface acoustic wave (SAW) and bulk wave resonant devices allow sensitive measurement of gas species concentration at temperatures above 250°C. The small size, low cost, and simple implementation of these sensors make them excellent candidates for monitoring vehicle exhaust streams and industrial combustion processes.
Figure 1. Surface acoustic wave (SAW) sensors wire-bonded to a
ceramic substrate in preparation for high temperature testing.
Acoustic wave sensors are constructed on two basic platforms: surface acoustic wave (SAW) devices used as delay lines or resonators and thickness shear mode (TSM) bulk wave resonators (see Figure 1). Other acoustic platforms, such as flexural plate wave (FPW) or beam resonators, can also be utilized. Quartz is commonly used as a piezoelectric substrate for the SAW devices and TSM resonators for operation at temperatures up to approximately 520°C. Above this temperature, higher Curie point piezoelectrics, such as lithium niobate, lithium tantalate, or gallium phosphate, are implemented. Chemical sensing layers consist of pure or mixed noble metal catalytic thin films, binary metal oxide thin films (e.g., zirconia, titania, tin dioxide) with and without metal ion doping, and transition metal ion activated surfactant-templated mesoporous metal oxide films (a Sandia-patented technology).
Figure 2. Measured frequency shift versus temperature for a TSM
resonator coated with a palladium-doped mesoporous silica thin film when exposed to 1% concentrations of propylene (circles) and hydrogen (squares) gases.
The acoustic sensor functions as the control element in an oscillator electronic circuit. Since the sensor chemical films are very rigid, concentration of gas species is directly indicated by changes in the oscillator operating frequency during exposure (see Figure 2). Several interaction mechanisms can create a frequency shift:
- Mass changes produced by sorption of gas molecules,
chemical combination with film ions/atoms, or stripping
of atoms from the film matrix;
- Temperature changes produced by
exothermic/endothermic catalytic reactions;
- Surface stress changes created by atomic or molecular
substitutions at crystal or grain boundary sites; and
- Conductivity changes produced by ion interaction or
Thin film coating materials are tailored to utilize one or more of the interaction mechanisms to sense and discriminate particular gas species. Target gases of interest include hydrogen, carbon monoxide, carbon dioxide, the nitrogen oxides, sulfur dioxide, water vapor and a variety of residual hydrocarbons found in combustion exhaust streams, especially the non-methane organic gases.
Figure 3. An interior view of the high temperature test cell used for
measuring acoustic sensor response to gas species.
This technology is being developed for potential use in the following
- On-vehicle monitoring of exhaust gas species
- to determine efficiency and function of the
- to document compliance with emission regulations, and
- for closed loop engine control to optimize performance.
- Monitoring of gas constituents in industrial exhaust
stacks to determine compliance.
- In situ monitoring of industrial combustion processes for
control and optimization.
- A high temperature gas test system for SAW devices,
TSM resonators, and other sensor components. The
system consists of a multiple gas mixing and flow station,
a temperature-controlled test cell that operates up to
525°C (See Figure 3.), and complete control and data
- Facilities for fabrication of quartz and lithium niobate SAW
devices and bulk wave resonators, including substrate
polishing, photolithographic patterning, metal electrode
deposition, and wire-bonding of devices.
- Facilities for depositing and modifying thin film sensor
coatings: spin-coaters for sol-gel depositions, RF
sputtering chambers, high-temperature curing ovens.
- Techniques for thin film coating inspection and
characterization: X-ray diffraction, ellipsometry, optical
microscopy, and nitrogen sorption using the BET method, AFM, SEM, FTIR spectroscopy, and profilometry.
- Network and impedance analyzers for electrical
characterization of devices and models for extracting
sensor properties from device response.
Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy.
Last modified: August 23, 1999
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