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Spherical ‘chip’ extends H2 sensor range a hundredfold

By Chemical Engineering |

A prototype of what is called the world’s fastest, wide-ranging hydrogen gas sensor has been constructed by a team of researchers, led by Professor Kazushi Yamanaka at the Dept. of Materials Processing, Tohoku University (Sendai, Japan; edlinks.chemengonline.com/5828-531), together with Toppan Printing Co. and Yamatake Corp. (both Tokyo) and support from Ball Semiconductor Inc., (Allen, Tex.). The sensor, based on surface-acoustic wave (SAW) technology, can measure H2 concentrations from 100% down to 0.001 vol. % (100 ppm) — 100 times more sensitive than conventional SAW sensors — in 2–4 s. Other sensitive H2 sensors, such as those based on field-effect transistors, cannot measure high concentrations, due to saturation effects.

The new sensor is based on the same operating principle as conventional SAW-type sensors: Mechanical oscillations are induced on the surface of a piezoelectric quartz substrate by a pulsed radio-frequency generator. As the waves pass through a gas-selective coating, the frequency changes proportionally to the concentration of the absorbed gas. The frequency phase-shift is then measured by an interdigital transducer (IDT). Because conventional SAW sensors are built on planar substrates (diagram, left), the SAWs only pass once through the coating, which limits the sensitivity to 0.1% for H2, says Yamanaka. To get around this limitation, the researchers built the sensor onto a spherical substrate. As a result, the SAWs make multiple passes through the coating, thus multiplying the sensitivity.

The new sensor (diagram, right) is constructed on a 1-mm-dia. piezoelelectric quartz sphere. Part of the sphere is coated with a palladium alloy, which selectively absorbs H2; and the oscillator and IDT are mounted at opposite poles. Under certain conditions, the SAWs can be collimated to circulate, without attenuation, around the circumference of the sphere.

The researchers believe the new sensor will help meet demand for H2 detectors covering a range of concentrations, such as for monitoring fuel-cell batteries and other energy applications. Modules are available for evaluation by firms interested in collaborating to commercialize the technology. (For practical information on handling hydrogen efficiently and safely, see pp. 54–64.)

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