Figure 1. Surface acoustic wave (SAW) sensors wire-bonded to a ceramic substrate in preparation for high temperature testing.
Figure 2. Measured frequency shift of a zirconia-coated quartz SAW device when exposed to hydrogen gas at different temperatures.
A new 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.
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 500° C. Above this temperature, higher Curie point piezoelectrics, such as lithium niobate or lithium tantalate, are implemented. Chemical sensing layers consist of pure or mixed noble metal catalytic thin films, binary metal oxide thin films (e.g., TiO2, ZrO2, SnO2) with and without metal ion doping, and metal ion activated surfactant-templated mesoporous metal oxide films (a Sandia-patented technology).
The acoustic sensor functions as the control element in an oscillator electronic loop. 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: (1) mass changes produced by sorption of gas molecules, chemical combination with film ions/atoms, or stripping of atoms from the film matrix; (2) temperature changes produced by exothermic/endothermic chemical reactions; (3) surface stress changes created by atomic or molecular substitutions at crystal or grain boundary sites; and (4) conductivity changes produced by ion interaction or chemical reaction. 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 H2, CO, CO2, NO, NO2, SO2, H2O (water vapor), and HCs (residual hydrocarbons found in combustion exhaust streams, especially the non-methane organic gases).
This technology is being developed for potential use in the following applications:
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