Helium nanobubble release from Pd surface: An atomic simulation
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Fei Gaob) and Howard L. Heinisch Pacific Northwest National Laboratory, Richland, Washington 99352
Shilin Hu China Institute of Atomic Energy, Beijing 102413, China (Received 7 June 2010; accepted 22 September 2010)
Molecular dynamic simulations of helium atoms escaping from a helium-filled nanobubble near the surface of crystalline palladium reveal unexpected behavior. Significant deformation and cracking near the helium bubble occur initially, and then a channel forms between the bubble and the surface, providing a pathway for helium atoms to propagate toward the surface. The helium atoms erupt from the bubble in an instantaneous and volcano-like process, which leads to surface deformation consisting of cavity formation on the surface, along with modification and atomic rearrangement at the periphery of the cavity. The present simulation results show that, near the palladium surface, there is a helium-bubble-free zone, or denuded zone, with a typical thickness of about 3.0 nm. Combined with experimental measurements and continuum-scale evolutionary model predictions, the present atomic simulations demonstrate that the thickness of the denuded zone, which contains a low concentration of helium atoms, is somewhat larger than the diameter of the helium bubbles in the metal tritide. Furthermore, a relationship between the tensile strength and thickness of metal film is also determined.
I. INTRODUCTION
Metals used to store tritium are subjected to extensive interactions with helium atoms that are produced by transmutation reactions from energetic tritium fusion neutrons.1 Because of extremely low solubility of helium in materials, helium atoms tend to be trapped at defects, such as vacancies, dislocations, and grain boundaries, and can consequently form helium nanobubbles.2–5 The formation of helium bubbles will lead to high-temperature intergranular embrittlement,6 which can significantly degrade the mechanical properties of materials. At the same time, the release of helium atoms from tritium-storing materials will decrease the purity of tritium and will also influence the plasma performance in a fusion environment. Therefore, the retention and release behavior of helium is an important issue for tritium-storing materials. As such, the helium release from Ti, Pd, and Ni et al.7–13 has received great attention. By analysis similar to that for dislocation loop punching, it was found that further bubble growth ceases by one of two mechanisms, (i) the equilibration of the chemical potentials for gas atoms in the bubble and in interstitial solution or (ii) interbubble fracture when the average
Address all correspondence to these authors. a) e-mail: [email protected] b) e-mail: [email protected] DOI: 10.1557/jmr.2010.49 416
J. Mater. Res., Vol. 26, No. 3, Feb 14, 2011
http://journals.cambridge.org
Downloaded: 07 Sep 2014
ligament stress exceeds the theoretical tensile strength of the material14,15 as the helium content in the host metal reaches critical dimensions.16 Previously, two theoretical models were sugge
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