Multiphysics coupled sensing mechanism of Pd/Ni alloy thin-film coated SAW hydrogen sensor

Multiphysics coupled sensing mechanism of palladium/nickel (Pd/Ni) alloy thin-film coated surface acoustic wave (SAW) hydrogen (H₂) sensor is demonstrated theoretically and experimentally to allow the optimization of the sensing device in this work. The resistor-capacitance circuit model is used to describe the interaction between Pd/Ni film and H₂. Referring to the perturbation theory, the relationship between the changes in SAW velocity/phase and the multi-physical field quantities of the Pd/Ni film are analyzed. To verify the theoretical model, the Pd/Ni film is sputtered on the Y35°X quartz substrate to build the delay-line patterned SAW H₂ sensor. Experimental results have well verified the theoretical predictions. That is, the main response mechanism is the mass loading effect, and the contribution of the acoustoelectric effect can be neglected. The expansion effect induced by hydrogen adsorption is completely different from the mass loading effect, which causes the sensing response failure, but it can be effectively improved by increasing the working temperature or decreasing the thickness of the Pd/Ni thin-film. Wide detection range (100 ppm ∼ 38 v/v %), rapid response (t ₉₀ ∼ 7 s), and good humidity stability are achieved from the optimized SAW H₂ sensor.