Improved Responsivity and Sensitivity of Hydrogen Mapping Technique in Pure Iron Using WO<sub>3</sub> Thin Film by Control of Pd Intermediate Layer

The objective in this study is to improve the responsivity and the sensitivity of the hydrogen mapping technique using the WO3 thin film by the optimization of the Pd intermediate layer. Especially, the effect of the thickness of Pd on the responsivity and the sensitivity of the hydrogen detection during hydrogen charging was investigated. Pd and WO3 thin films were coated on the detection side of the pure iron sheet by a magnetron sputtering system. No color change was observed on the hydrogen detection side of the specimens with the Pd intermediate layer more than 25 nm in thickness during the hydrogen detection test for 7.2 ks. The responsivity for the hydrogen mapping technique using WO3 was essentially improved by decreasing the Pd thickness. In terms of the onset time of the average color change, the earliest response was obtained when the Pd thickness was 4 nm because of the uneven distribution of the color change in the case of the Pd thickness of 2.5 nm. The sensitivity for the hydrogen mapping technique using WO3 was improved by decreasing the Pd thickness. Taking into account the responsivity, the sensitivity, and the spatial resolution comprehensively, the best thickness of the Pd intermediate layer seems to be 4 nm in this study.