Joule Heating a Palladium Nanowire Sensor for Accelerated Response and Recovery to Hydrogen Gas

The properties of a single heated palladium (Pd) nanowire for the detection of hydrogen gas (H-2) are explored. In these experiments, a Pd nano wire, 48-98 mu m length, performs three functions in parallel: 1) Joule self-heating is used to elevate the nano wire temperature by up to 128 K, 2) the 4-contact wire resistance in the absence of H-2 is used to measure its temperature, and 3) the nano wire resistance in the presence of H-2 is correlated with its concentration, allowing it to function as a H-2 sensor. Compared with the room-temperature response of a Pd nano wire, the response of the heated nano wire to hydrogen is altered in two ways: First, the resistance change (Delta R/R-0) induced by H-2 exposure at any concentration is reduced by a factor of up to 30 and second, the rate of the resistance change observed at the beginning (response) and at the end (recovery) of a pulse of H-2 - is increased by more than a factor of 50 at some H-2 concentrations. Heating nearly eliminates the retardation of response and recovery seen from 1-2% H-2, caused by the alpha -> beta phase transition of PdHx, a pronounced effect for nano wires at room temperature. The activation energies associated with sensor response and recovery are measured and interpreted.