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Platinum Metals Rev., 2001, 45, (4), 174

Hydrogen Detection with a Palladium-Nickel HSGFET

There are now vehicles on our roads run by hydrogen (H)-powered fuel cells, and H2 is predicted to have a far more important role as an energy source in the future. Hydrogen is a highly flammable gas, and being odourless and colourless is not detectable by human olfactory and other senses. Therefore to have sensors able to detect it rapidly and accurately in the atmosphere is very important. This is particularly critical at high H2 concentrations before ignition concentrations are reached.

One method of sensing could be to measure the work function change due to H physisorption or chemisorption on a gas sensitive layer. As the response to bulk effects is almost negligible, the measurements could be carried out at low temperatures, in contrast to the usual conductance sensors which work at very high temperatures. A new type of gas sensor may then be constructed, utilising a hybrid suspended gate field effect transistor (HSGFET), of low power demand (> 10 mW). Pd-MOS based H sensors are already known, but when operated at high H2 concentrations the Pd film blisters. If Pd alloys are used instead of Pd there is a possibility that blistering could be eliminated.

A team of researchers from Germany, Sweden and India have now attempted to improve the stability of HSGFETs up to high H2 concentrations, by replacing Pd by a thin layer of Pd-Ni or Pd-Ag alloy (K. Scharnagl, M. Eriksson, A. Karthigeyan, M. Burgmair, M. Zimmer and I. Eisele, Sens. Actuators B, Chem ., 2001, 78, (1-3), 138-143).

Pd-Ni and Pd-Ag films were grown onto titaniumcoated silicon wafers in UHV chambers at a base pressure of 1 x 10-10 Torr by co-evaporation techniques. Work function measurements and time responses on exposure to H2 and other commonly found environmental gases were then carried out.

It was found possible to detect H2 concentrations of up to 2% at room temperature without blister formation, when Pd-Ni alloy was used as the gas sensitive material. The response to 2% H2 was ∼ 500 mV in dry conditions, but less than half this value with moistened carrier gas, however then the desorption time was lowered. The Pd-Ag alloy was not stable and is not suitable as a sensor material. The Pd-Ni alloy, in addition to having a low cross-sensitivity to other gases, seems to be a promising material for room temperature H monitoring.

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