New Insight to the Effects of Heat Treatment in Air on the Permeation Properties of Thin Pd77%Ag23% Membranes.

Nicla Vicinanza,Thijs Peters,Rune Bredesen,Ingeborg-Helene Svenum,Hilde Venvik

doi:10.3390/membranes8040092

Nicla Vicinanza, Thijs Peters + Show 3 more

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https://doi.org/10.3390/membranes8040092

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Journal: Membranes	Publication Date: Oct 10, 2018
Citations: 16	License type: CC BY 4.0

Affiliation: Norwegian University of Science and Technology, SINTEF

Abstract

Sputtered Pd77%Ag23% membranes of thickness 2.2–8.5 µm were subjected to a three-step heat treatment in air (HTA) to investigate the relation between thickness and the reported beneficial effects of HTA on hydrogen transport. The permeability experiments were complimented by volumetric hydrogen sorption measurements and atomic force microscopy (AFM) imaging in order to relate the observed effects to changes in hydrogen solubility and/or structure. The results show that the HTA—essentially an oxidation-reduction cycle—mainly affects the thinner membranes, with the hydrogen flux increasing stepwise upon HTA of each membrane side. The hydrogen solubility is found to remain constant upon HTA, and the change must therefore be attributed to improved transport kinetics. The HTA procedure appears to shift the transition from the surface to bulk-limited transport to lower thickness, roughly from ~5 to ≤2.2 µm under the conditions applied here. Although the surface topography results indicate that HTA influences the surface roughness and increases the effective membrane surface area, this cannot be the sole explanation for the observed hydrogen flux increase. This is because considerable surface roughening occurs during hydrogen permeation (no HTA) as well, but not accompanied by the same hydrogen flux enhancement. The latter effect is particularly pronounced for thinner membranes, implying that the structural changes may be dependent on the magnitude of the hydrogen flux.

Highlights

Palladium-based membranes have been the focus of many studies due to their high hydrogen permeability and selectivity, which may find application in efficient separation technologies [1,2].At temperature below ~300 ◦ C and pressure below ~2 MPa, pure palladium undergoes the α-to-β phase transition that results in irreversible lattice strain
A three-step heat treatment in air has been performed on sputtered Pd77%Ag23% membranes with thickness ranging from 2.2 μm to 8.5 μm
Air oxidation has no apparent effect on the hydrogen solubility, implying that enhancement in permeability may be related to an increase in diffusivity for thinner membranes (≤5 μm)

Summary

Introduction

Palladium-based membranes have been the focus of many studies due to their high hydrogen permeability and selectivity, which may find application in efficient separation technologies [1,2]. At temperature below ~300 ◦ C and pressure below ~2 MPa, pure palladium undergoes the α-to-β phase transition that results in irreversible lattice strain. Over time, cycling of the temperature causes the Pd to become brittle; leading to fractures. In order to prevent hydrogen embrittlement, Pd is conveniently alloyed with other metals [3,4]. Silver is a widely used alloying element, reducing the α-to-β phase transition to below room temperature. Pd-Ag alloys exhibit higher hydrogen permeability than pure palladium [4,5,6], with a maximum at ~23 wt.% of Ag [6]

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