Thin alloy films of Pd and Ni (8%<Ni<20% in at. %) formed by dual-electron-beam evaporation techniques have been found to give durable and quickly reversible detectors of high H2 concentrations (pH2 0.1%–100%, 0.7–700 Torr) near 1 atm and 300 K, including accurate determinations of pH2 around the lower explosive limit of 4% in air. The addition of Ni suppresses the α to β phase transition found in pure Pd under these conditions. The measurement of resistivity changes in the thin films along with flatband shifts of metal-oxide-semiconductor capacitors on the same Si wafer gives accurate values of pH2 over more than six decades.

The nonlinear partial differential equations which describe transient photoconductivity in insulators are solved numerically. Trapping and recombination are included which allows photoconduction to be studied in the presence of large space charge. Use of methods for ‘‘stiff’’ differential equations insures stable time dependent solutions even for problems which have widely different time constants. The program is applied to a thin film (1000 Å) of silicon dioxide to illustrate the salient features of the numerical solutions and the capabilities of the program.

This report presents a review of sensors and technologies that are capable of detecting and monitoring volatile organic compounds. The scope of this review was limited to those sensors that have the potential to be used in geologic environments for long-term monitoring applications. Four general categories of sensor technologies were reviewed: (1) chromatography and spectrometry; (2) electrochemical sensors; (3) mass sensors; and (4) optical sensors. Based on the review criteria set forth in this report, the most viable sensors for in-situ chemical sensing appear to be electrochemical sensors (specifically conductometric sensors), fiber-optic sensors, and surface-acoustic-wave sensors. However, very few chemical sensors have been successfully demonstrated in realtime, continuous, in-situ applications.iv Acknowledgments

The addition of Ag to Pd in the gate metal of a metal-insulator-semiconductor gas sensing diode can improve the performance and change the selectivity of the sensors for a variety of reactions. Data on the response of diodes with 12 different ratios of Ag to Pd in alloys and layers of Pd and Ag to hydrogen and other gases are reported. Diodes with as much as 32% Ag respond very well to H2 gas and the films are much more durable to high hydrogen exposure than pure Pd films. Improvements in the rate of response and aging behavior are found for certain Ag combinations; others give poorer performance. The presence of Ag on the surface changes the catalytic activity in some cases and examples of H2 mixed with O2 and/or NO2, propylene oxide, ethylene, and formic acid are given. Such selectivity forms the basis for miniature chemical sensor arrays which could analyze complex gas mixtures.

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