Enhanced Sensitive Surface Acoustic Wave Device Designed for Nitric Oxide Gas Detection

Surface acoustic wave (SAW) devices equipped with an electrode-width-controlled (EWC)/single-phase unidirectional transducer (SPUDT) and fabricated on ST-X quartz were proposed and found to have low insertion loss and single-mode capability. The devices were then used to determine parts-per-billion levels of nitric oxide (NO). A new modified equivalent circuit (EC) model was used to model and analyze the characteristics of SAW devices prior to their fabrication. The small-signal frequency responses (S-21) obtained by simulation and measurement were consistent. A feedback-loop oscillator was used to design SAW-resonator-stabilized oscillators that provided stable oscillation. A SAW oscillator with an EWC/SPUDT structure was used to provide fundamental signals with high output power, induce high harmonic suppression, and realize long-term frequency stability. SAW devices with an EWC/SPUDT structure were coated with a Cu2+/PANI/WO3-sensitive layer to successfully determine parts-per-billion levels of NO. For a NO concentration of 50 ppb in dry air, the frequency shift in the SAW sensor with the EWC/SPUDT structure is 502 Hz. Moreover, the response and recovery time are 40 and 112 s, respectively. The sensitivity of NO detection for a concentration range of 20-80 ppb is approximately 4.2 Hz/ppb (similar to 3.43 kHz/mg/m(3)).