Improving the Sensitivity of Palladium-Based Fiber Optic Hydrogen Sensors

This paper reports on the sensitivity improvement of palladium (Pd)-based fiber optic hydrogen sensors. The expansion of Pd undergoing hydrogen absorption transduces strain in a suitable sensing element. The sensors reported here are based on fiber Bragg grating (FBG) strain sensors with a Pd foil hydrogen sensing element. The hydrogen sensitivity is dependent upon; strain sensor sensitivity, the Pd body geometry, the volumetric expansion of Pd, and the strain transfer between Pd and the sensor element. We have investigated three options to improve the sensitivity: First, improve the strain transfer by decreasing the bond line thickness with new manufacturing methods. Second, increasing the volumetric expansion by using a Pd silver (Ag) alloy (Pd75/Ag25) foil with increased hydrogen solubility. Third, concentrating the strain of the expanding Pd on an FBG by using a new sensor concept. All sensors were tested in hydrogen concentrations of 2,500, 10,000, and 50,000 ppm at 90 °C and 1060 mbar. Using new manufacturing methods, the strain transfer is improved by up to 72% compared to previously reported sensors and the sensitivity is 0.062 pm/ppm. With the PdAg alloy foil, the sensitivity is amplified by a factor of 17 compared to pure palladium foil sensor and the sensitivity is 0.77 pm/ppm. With the new sensor concept with strain concentration, the signal is amplified by a factor 2.5 compared to sensors without strain concentration and the sensitivity is 0.18 pm/ppm.