Positive versus negative resistance response to hydrogenation in palladium and its alloys

TL;DR

This paper investigates how hydrogen absorption causes thickness expansion in palladium-based thin films, significantly affecting their resistance response, and develops a model that explains both positive and negative resistance changes across various Pd systems.

Contribution

It introduces a thickness-expansion model to explain hydrogenation resistance responses in palladium films, validated across multiple Pd-based materials with different structures and resistivities.

Findings

Thickness expansion influences resistance response polarity.

The model explains both positive and negative resistance changes.

Validation across different Pd systems confirms model robustness.

Abstract

Resistive solid state sensors are widely used in multiple applications, including molecular and gas detection. Absorption or intercalation of the target species varies the lattice parameters and an effective thickness of thin films, which is usually neglected in analyzing their transport properties in general and the sensor response in particular. Here, we explore the case of palladium-based thin films absorbing hydrogen and demonstrate that expansion of thickness is an important mechanism determining the magnitude and the very polarity of the resistance response to hydrogenation in high resistivity films. The model of the resistance response that takes into account modifications of thickness was tested and confirmed in three Pd-based systems with variable resistivity: thin Pd films above and below the percolation threshold, thick Pd-SiO2 granular composite films with different content of silica, and Pd-rich CoPd alloys where resistivity depends on Co concentration. Superposition of the bulk resistivity increase due to hydride formation and decrease of film resistance due to thickness expansion provides a consistent explanation of the hydrogenation response in both continuous and discontinuous films with different structures and compositions.