Hydrogen in tungsten: Absorption, diffusion, vacancy trapping, and decohesion
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Emily A. Cartera) Department of Mechanical and Aerospace Engineering, and Program in Applied and Computational Mathematics, Princeton University, Princeton, New Jersey 08544 (Received 14 August 2009; accepted 3 November 2009)
Understanding the interaction between atomic hydrogen and solid tungsten is important for the development of fusion reactors in which proposed tungsten walls would be bombarded with high energy particles including hydrogen isotopes. Here, we report results from periodic density-functional theory calculations for three crucial aspects of this interaction: surface-to-subsurface diffusion of H into W, trapping of H at vacancies, and H-enhanced decohesion, with a view to assess the likely extent of hydrogen isotope incorporation into tungsten reactor walls. We find energy barriers of (at least) 2.08 eV and 1.77 eV for H uptake (inward diffusion) into W(001) and W(110) surfaces, respectively, along with very small barriers for the reverse process (outward diffusion). Although H dissolution in defect-free bulk W is predicted to be endothermic, vacancies in bulk W are predicted to exothermically trap multiple H atoms. Furthermore, adsorbed hydrogen is predicted to greatly stabilize W surfaces such that decohesion (fracture) may result from high local H concentrations.
I. INTRODUCTION
One of the most crucial aspects to the development of a viable fusion power reactor is the choice of materials to act as plasma-facing components (PFCs). A large amount of current research is aimed at characterizing how various materials can withstand the high-energy particle flux imposed on the first walls. For example, the divertor plate (in divertor-type reactors) may be subjected to particle fluxes up to 1024 m2s1 and heat fluxes up to 10 MWm2,1–3 while fluxes at other plasma-facing walls can be orders of magnitude smaller. High energy particles can sputter surface atoms or penetrate micrometers deep, leading to defect formation (e.g., vacancies or interstitials) and transmutations (due to neutron bombardment). These processes will slowly weaken and corrode the plasma-facing material (PFM). In addition, PFCs must retain structural integrity at high temperatures (up to 900 C). While both low-Z (e.g., C, Be) and high-Z (e.g., W) materials have been considered for use in PFCs, tungsten has emerged as one of the most promising materials for use in PFCs.2,4,5 Tungsten has several properties that make it well suited for use as a PFM. Current technology employs W as a a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2010.0036 J. Mater. Res., Vol. 25, No. 2, Feb 2010
thin (1–3 mm) coating on graphite or carbon fiber composite tiles, but future PFCs might be composed simply of bulk W.2,5 Tungsten retains strength at high temperatures and can sufficiently conduct heat away from the surface, although underlying Cu-based components will likely act as future heat sinks.2 Ingress, transport, and retention of radioactive tritium in PFMs is of great concern and the amount of hydroge
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