Uncoated SAW delay lines as thermal gas detector

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 Δλ, flow rate Δu, dynamic viscosity Δμ, and the density Δρ 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 SiO₂, LiNbO₃, Bi₁₂ GeO₂₀, and Bi₁₂SiO₂₀ substrates. The gases under test (H₂, He, Ar, CH₄, NH₃ , N₂, O₂, dry air) are used within concentrations of 0.1-100% and flow rates of 50-2000 mL/min at 20-150°C and atmospheric pressure. The SAW response Δφ/φ, is measured as function of Δλ, Δu, operating temperature, and gas concentration. The SAW prototype has a good sensitivity: Δφ/φ≡15 ppm to 0.7% CH₄ in N₂ for a Bi₁₂SiO₂₀ delay line heated at 120°C; Δφ/φ≡15 ppm to 0.4% NH₃ in N₂ for a YZ-LiNbO₃ delay line heated at 120°C. Some selective SAW gas responses are discussed