Optimizing the sensitivity of palladium based hydrogen sensors
Abstract This paper reports on the sensitivity optimization of palladium based hydrogen sensors. The transduction mechanism is the volumetric expansion of palladium, leading to induced strain in a strain sensing element. Absorption of hydrogen in the palladium lattice causes expansion, which can be measured using strain sensors attached to the palladium. The strain transduced from the expanding palladium is dependent not only on the hydrogen partial pressure and temperature, but also the geometry of the palladium and the mechanical transfer of strain to the sensor. In this paper we demonstrate the design and validation of a sensor for defined hydrogen concentration ranges, selection of a working temperature, and selection of a palladium geometry to achieve a desired sensitivity. We show that operating temperature and concentration range are critical in order to avoid the phase change region, which can cause hysteresis, as well as degradation of the palladium itself and the adhesive interface. In order to quantify the strain transfer between palladium and the sensor, a coefficient of strain transfer k st is introduced. Two empirical methods are demonstrated to calculate k st : (a) via calibration for hydrogen sensitivity, (b) via calibration for temperature sensitivity. Three sensor designs, with fiber Bragg gratings as a strain sensor, are used to correlate the theoretical design assumptions with experimental data. Highlights A method to select the working temperature of a Pd based H 2 sensor to avoid the phase change region is presented. We developed a new diagram which enables the reader to easily pick a working temperature for the sensor. A method to select a palladium geometry to achieve a required sensitivity is presented. We developed new diagrams to help the reader understand the influence of the geometry of the palladium on the sensitivity and select the geometry according to the required sensitivity. Two different methods to calibrate the hydrogen sensitivity of the sensor are presented. Both rely on the strain transfer coefficient quantifying the strain that is transduced from the expanding palladium into the fiber. The strain transfer coefficient is determined via (a) a hydrogen measurement, and (b) a temperature measurement. We found that the strain transfer coefficient is the same for both mehtods. This implies that a temperature calibration can be used to also calibrate for hydrogen sensitivity.
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