No abstract

Hydrogen leak detection sensors must have high sensitivity and a short response time of 1 s or less. A ball surface-acousticwave (SAW) hydrogen sensor has a high sensitivity and can detect hydrogen in a very wide concentration range of 10 ppm to 100%. Moreover, a fast response can be expected because of the very thin sensitive film used. In this study, we developed a digital quadrature detector (DQD) to measure responses of less than 1 s, and measure phases in 1 ms intervals with excellent sensitivity. We evaluated the response time of the ball SAW hydrogen sensor where the signal was averaged 256 times in 0.256 s using the DQD. As a result, the response time was found to be 1 s or less for 3.0 vol % hydrogen gas in nitrogen.

In order to improve the performance of ball surface acoustic wave (SAW) devices, we have developed an orientation control apparatus to find the Z axis of a quartz crystal ball by an optical birefringence measurement, and to find the X and Y axes by measuring the propagation characteristics of a SAW using an interdigital transducer (IDT), which is moved close to the ball using a translation stage. The soft spring of the vacuum chuck of the ball manipulator prevented damage to the ball or IDT during the contact. The concave shape of the ball holder of the chuck enabled repeatable alignment of the ball center and the manipulator axis. As a preliminary result, we confirmed that the SAW velocity variation had 60° periodicity and succeeded in identifying the -Y axis-equivalent direction on the quartz ball.

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 (2 s) was realized. In terms of the sensing range, it has the widest range of 10 ppm to 100% among any hydrogen sensors including FET or resistivity sensors. The response time was less than 1 s for 3.0% hydrogen concentration in nitrogen gas, evaluated by using a newly developed digital quadrature detector.

A ball surface acoustic wave (SAW) hydrogen gas sensor employing a sensitive nanoscale PdNi thin film has been developed. It has been observed that the response time of the sensor depends upon the thickness of the nano film. The authors have performed computer simulations employing fast Fourier transforms (FFTs) and Hilbert transforms for the SAW acoustic waveforms for the sensor. It has been observed that it is possible to forecast the frequency-dependent SAW behavior with the deposition of nanoscale PdNi thin film and also under the influence of hydrogen.