Uncoated SAW delay lines as thermal gas detector

Abstract

A SAW delay line, without any absorbent film between the interdigital transducers, has been developed as a thermal gas detector. The change of the thermal conductivity of the ambient atmosphere produces a change of substrate temperature thereby causing a SAW phase variation (SAW response) at the output of the device. Different effects contribute to the SAW response such as the thermal conductivity /spl Delta//spl lambda/, flow rate /spl Delta/u, dynamic viscosity /spl Delta//spl mu/, and the density /spl Delta//spl rho/ of the test ambient and they are numerically analysed for each test gas. Uncoated SAW delay lines operating at 41-263 MHz are implemented on SiO/sub 2/, LiNbO/sub 3/, Bi/sub 12/GeO/sub 20/, and Bi/sub 12/SiO/sub 20/ substrates. The gases under test (H/sub 2/, He, Ar, CH/sub 4/, NH/sub 3/, N/sub 2/, O/sub 2/, dry air) are used within concentrations of 0.1-100% and flow rates of 50-2000 mL/min at 20-150/spl deg/C and atmospheric pressure. The SAW response /spl Delta//spl phi///spl phi/, is measured as function of /spl Delta//spl lambda/, /spl Delta/u, operating temperature, and gas concentration. The SAW prototype has a good sensitivity: /spl Delta//spl phi///spl phi//spl equiv/15 ppm to 0.7% CH/sub 4/ in N/sub 2/ for a Bi/sub 12/SiO/sub 20/ delay line heated at 120/spl deg/C; /spl Delta//spl phi///spl phi//spl equiv/15 ppm to 0.4% NH/sub 3/ in N/sub 2/ for a YZ-LiNbO/sub 3/ delay line heated at 120/spl deg/C. Some selective SAW gas responses are discussed.