Thermal conductivity gas sensor based on LiNbO3 SAW resonators

A new thermal conductivity detector based on a lithium niobate surface acoustic wave (SAW) resonator has been fabricated for use in a gas chromatograph. The device exploits the large temperature coefficient of LiNbO₃ to detect low concentrations of CO₂ in air by sensing the change in the thermal conductivity of a helium carrier gas and the CO₂ exiting the capillary gas chromatograph column. The LiNbO₃ resonator is maintained at 150°C and is held in close proximity (0.2-0.3 mm) to the exit of the capillary column which is maintained at 50°C. Heat loss from the LiNbO₃ is primarily due to the thermal conductivity of the gas exiting the capillary column such that changes in the constituency of the gas result in temperature changes on the surface of the resonator. The surface temperature change is monitored by recording the resulting frequency change in a 342-MHz SAW resonator-controlled oscillator circuit. The entire system is small, battery powered, and can monitor greenhouse gases in remote field sites throughout the world