A wide range of hydrogen-air compositions explode on ignition, and may cause serious accidents. To avoid such situations arising a variety of hydrogen sensors has been developed. At present, the hydrogen concentration in an atmosphere is usually measured by monitoring changes in an electrical property of the sensor as reaction with the hydrogen gas takes place. To enhance the sensitivity of such sensors it is necessary to maintain clean surfaces, and therefore they are generally heated to temperatures above 150°C. Thus there has been a need to develop an accurate hydrogen sensor which could operate at room temperature.

Researchers at Nagaoka University of Technology and at the University of Tokyo, Japan, had observed that the optical transmittance of palladium thin films depends strongly on the presence of hydrogen in the ambient atmosphere; now they have developed a hydrogen sensor which utilises this phenomenon and operates at room temperature. (Y.-S. Oh, J.-I. Hamagami, Y. Watanabe, M. Takata and H. Yanagida, J. Ceram. Soc Jpn., 1993, 101, (6), 618–620).

The sensor consists of a palladium thin film deposited on a glass substrate by radio-frequency magnetron sputtering. When a sample is exposed to hydrogen the optical transmittance to light of wavelength 780 nm increases and becomes saturated at a constant value, but on exposure to dry air the transmittance returns to its original value. It is reported that these reactions are perfectly reversible even after multiple repeats. For a thin film of specified thickness, the optical transmittance of the sensor was found to depend on the hydrogen concentration; with an increase in hydrogen concentration the relative change in transmittance increased, the response time decreased, and the recovery time increased.

This new optically readable palladium thin-film hydrogen sensor has excellent sensitivity and is compatible with fibre optics.

It is suggested that this novel palladium sensor may be selective only to hydrogen. Furthermore, the dependence of optical transmittance on the hydrogen concentration in the ambient atmosphere has also been observed for radio-frequency sputtered platinum films.