Principle and application of ball surface acoustic wave (SAW) sensor(<Special issue>Commemoration of the Japan-China Joint Conference on Acoustics 2007 (JCA2007))

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Detection of hydrogen gas is a crucial task for establishing safety and reliability of fuel cells, a key technology for the environment and our society. However, hydrogen is difficult to detect and various hydrogen sensors have many drawbacks. Here we report a novel hydrogen gas sensor, the ball surface acoustic wave (SAW) sensor, using Pd or PdNi sensitive film. The ball SAW sensor is based on a novel phenomenon, diffraction-free propagation of collimated beam along an equator of sphere. The resultant ultra-multiple roundtrips of SAW makes it possible to achieve highest sensitivity among SAW sensors. Moreover, it enables to use a very thin sensitive film, and consequently the shortest response time (2s) was realized. In terms of the sensing range, it has the widest range of 10ppm to 100% among any hydrogen sensors including FET or resistivity sensors. The response time was less than 1s for 3.0% hydrogen concentration in nitrogen gas, evaluated by using a newly developed digital quadrature detector.