Surface Acoustic Wave Sensors based on Nanoporous Films for Hydrogen Detection

Surface Acoustic Wave Sensor is one of the most promising detection systems due to smaller size, lower weight, power requirements and great sensitivity. It is known from the literature that Pd and ZnO are the most promising materials for detecting hydrogen. A nanoporous Pd and ZnO based layered SAW sensors have been developed and investigated for hydrogen, at room temperature. The sensors were of a "delay line" type (quartz substrate, 70 MHz central frequency). The nanoporous sensitive layer was directly deposited onto a quartz substrate using a picosecond laser ablation method. Lasers with picosecond pulses or a high repetition rate leads to major changes in the ablation process and implicit in the deposited film structure. Thus, using such a regime, at a certain ambient gas pressure, we have the advantage of directly obtaining a nanoporous film. The sensor performances (sensitivity, limit of detection and response time), for a hydrogen concentration in synthetic air of 0.015-2 % were studied. For a concentration between 0.2 - 0.8 % hydrogen/synthetic airs, the response times was 15 - 44 s and 6 - 27 s in case of Pd films and ZnO respectively. Limit of detection was about 6.3 times better at Pd sensors (50 ppm) than ZnO sensors (315 ppm).