Ball SAW device for hydrogen gas sensor

We propose a novel gas sensor using surface acoustic waves (SAW) on a sphere, with a typical velocity change of 10 ppm, and sensitivity of 10 ppb. In the first implementation for hydrogen sensor, a long propagation path of 1.3 m at 41th turns of 45 MHz SAW around a 10-mm-φ quartz ball realized the propagation time as long as 403 μs. Consequently, a sampling period of 25 ps realized relative temporal resolution of 0.06 ppm. By virtue of this high resolution, when sensing film was as thin as 20 nm, 7 ppm velocity change when exposed to 3% hydrogen in Ar gas was precisely measured. The response time was 60s, shortest among reported SAW hydrogen sensors, due to the small Pd film thickness. We also found a SAW attenuation increase which may be used to enhance the gas selectivity.