Investigating Helium Bubble Nucleation and Growth through Simultaneous In-Situ Cryogenic, Ion Implantation, and Environmental Transmission Electron Microscopy.

Caitlin A Taylor,Joshua D Sugar,Emily Aradi,Jonathan A Hinks,David B Robinson,Samuel Briggs,Anthony Monterrosa,Khalid Hattar,Graeme Greaves

doi:10.3390/ma12162618

Abstract

Palladium can readily dissociate molecular hydrogen at its surface, and rapidly accept it onto the octahedral sites of its face-centered cubic crystal structure. This can include radioactive tritium. As tritium β-decays with a half-life of 12.3 years, He-3 is generated in the metal lattice, causing significant degradation of the material. Helium bubble evolution at high concentrations can result in blister formation or exfoliation and must therefore be well understood to predict the longevity of materials that absorb tritium. A hydrogen over-pressure must be applied to palladium hydride to prevent hydrogen from desorbing from the metal, making it difficult to study tritium in palladium by methods that involve vacuum, such as electron microscopy. Recent improvements in in-situ ion implantation Transmission Electron Microscopy (TEM) allow for the direct observation of He bubble nucleation and growth in materials. In this work, we present results from preliminary experiments using the new ion implantation Environmental TEM (ETEM) at the University of Huddersfield to observe He bubble nucleation and growth, in-situ, in palladium at cryogenic temperatures in a hydrogen environment. After the initial nucleation phase, bubble diameter remained constant throughout the implantation, but bubble density increased with implantation time. β-phase palladium hydride was not observed to form during the experiments, likely indicating that the cryogenic implantation temperature played a dominating role in the bubble nucleation and growth behavior.

Highlights

Helium is insoluble in almost all solids and precipitates into nanometer-sized bubbles that can result in mechanical property degradation and eventually fracture
Palladium wire was purchased from Alfa Aesar (Alfa Aesar, Haverhill, MA, USA) and was annealed prior to Transmission Electron Microscopy (TEM) sample preparation to cause pre-existing voids identified near the surface to coalesce into larger voids that could not be confused with He bubbles
To determine the effects of an H2 atmosphere on the Pd sample at cryogenic temperatures, a was subjected to an H2 environment at temperatures between −100 ◦ C and −20 ◦ C

Summary

Introduction

Helium is insoluble in almost all solids and precipitates into nanometer-sized bubbles that can result in mechanical property degradation and eventually fracture. Materials 2019, 12, 2618; doi:10.3390/ma12162618 www.mdpi.com/journal/materials (fcc) crystal structure as a metal hydride [4,5,6] One of these applications is solid-state tritium storage, where 3 H will decay to 3 He with a half-life of 12.3 years, causing rapid accumulation of 3 He in the. Studying 3 He evolution in PdTx is difficult due to the safety constraints of radiological work, though some microscopy has been performed investigating early stage 3 He bubble formation (less than one year of aging) [7,8,9] Over these short aging times, 3 He bubbles reached 1–2 nm in diameter. If the over-pressure is removed, most H will diffuse out of the material [4,5,6]

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