Bilayer structure for hydrogen detection in a surface acoustic wave sensor system

Presented here are the results concerning a hydrogen sensor based on a bilayer structure in a surface acoustic wave (SAW) dual delay line system. The sensor material consists of two layers performed in two different vapour deposition processes. The first one is a 720 nm copper phthalocyanine (CuPc) layer, the second is a 20 nm thin palladium (Pd) film. This structure was formed in one of the dual delay line systems on a LiNbO3 Y-cut Z-propagation substrate, while the other serves as a reference, permitting easy detection of the arising differential frequency Δf. This frequency, depending on the operating frequency modes, is in the range of 200–400 kHz, whereas the oscillator frequencies are in the range of 43.6 MHz. The wavelength is 80 μm. In such a bilayer structure we can detect hydrogen in a medium concentration range (from 0.5 to 3% in nitrogen), even at room temperature. The sensor is highly sensitive, very stable and is entirely reversible. The response and recovery times defined as 90% of the saturation level are quite good (from 100 s for 0.5% to 1000 s for 1.5%), which is very important from a practical point of view. The sensitivity depends on temperature, and decreases with the increase of the interaction temperature. In addition, the phase transition of the palladium hydride at 30 and 43 °C is distinct and repeatably visible as an “interaction jump”, which is something of a disadvantage in respect of the sensor. This undesirable phase transition shifts in the direction of higher hydrogen concentrations at higher temperatures, and consequently can be avoided; for instance, at 61 °C for the investigated concentration range (0.5–3% of hydrogen in nitrogen).