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

S S Das,A Gerber,G Kopnov

doi:10.1063/5.0009194

Abstract

Resistive solid state sensors are widely used in multiple applications, including molecular and gas detection. The absorption or intercalation of the target species varies the lattice parameters and an effective thickness of thin films, which is usually neglected in the analyses of 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 the 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 the 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 contents of silica, and Pd-rich CoPd alloys where resistivity depends on the Co concentration. The superposition of the bulk resistivity increase due to hydride formation and the decrease in the film resistance due to the thickness expansion provides a consistent explanation of the hydrogenation response in both continuous and discontinuous films with different structures and compositions.

Highlights

Volume of the unit cell changes in multiple physical phenomena: due to thermal expansion, under external stress, at phase transitions, due to intercalation and hydrogenation, due to magnetostriction, and many others
We fabricated three different Pd-based systems with variable resistivity: thin Pd films, thick Pd–SiO2 granular films with different contents of SiO2, and Pd-rich CoPd alloys where the resistivity depends on the Co concentration
We developed a simple model of the hydrogenation induced changes of resistance in Pd based materials, which takes into account an expansion of the effective film thickness under the lateral in-plane stress

Summary

INTRODUCTION

Volume of the unit cell changes in multiple physical phenomena: due to thermal expansion, under external stress, at phase transitions, due to intercalation and hydrogenation, due to magnetostriction, and many others. When the material is in the form of a thin film grown on a rigid substrate, the lateral in-plane changes are restricted by adhesion to the surface, and the lattice modifications are oriented normal to the plane. This leads to the variation of the effective film thickness. Scitation.org/journal/adv of nanometer void spacers that allow expansion and the very ability of the material to expand laterally within the film plane This condition is not met in at least a number of cases when a negative resistivity response to hydrogen was observed in solid continuous materials, e.g., in thick Pd films at elevated temperatures[18] or in thick PdCuSi metallic alloys.[16]. We shall demonstrate in the following that the reduction of the resistance in both continuous and discontinuous hydrogenated Pd-based films is the result of the expanding film thickness

THE MODEL

EXPERIMENTAL

Thin Pd films

Pd–SiO2 granular mixtures

CoPd alloys

SUMMARY