Hybrid Microstrip Device for Hydrogen Detection at Microwave Frequencies

Abstract

This article describes the analysis, design, and construction of a microstrip device capable of detecting hydrogen at microwave frequencies. The proposed structure is a hybrid microstrip line, 10-cm-long, having part of the copper (Cu) line replaced by a strip of palladium (Pd) film 10-nm-thick. A simple formulation has been developed to estimate the device’s insertion loss, as well as the spectral dependence of the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${S}_{{21}}$ </tex-math></inline-formula> parameter. For a device having a 2-cm-long Pd section, exposed to 1.6% hydrogen at a 0.4-bar pressure in nitrogen gas, detection was accomplished by measuring the changes produced on one of the resonances of the scattering parameter <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${S}_{{21}}$ </tex-math></inline-formula> in the frequency region around 3.2 GHz. The experimental results, corroborated by the theoretical modeling of the device’s response, indicated that, if on one hand, a Pd thickness much smaller than the skin depth, yields a negligible change in attenuation due to hydrogen absorption, on the other, it favors the phase of the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${S}_{{21}}$ </tex-math></inline-formula> parameter to become a highly sensitive function of Pd conductivity, in turn facilitating hydrogen detection. This finding opens the possibility of constructing simple hydrogen sensors by incorporating ultrathin Pd films into planar microwave circuits.