Nucleation of Helium clusters in metals studied by positron annihilation combined with ion beam analysis and temperature programmed desorption
Synthesis and Properties of Functional Materials
Final Report Abstract
This final report presents the findings of a comprehensive study that aimed to (i) establish a procedure to prepare defect-free tungsten single crystals and tungsten with well-defined vacancy concentration, (ii) determine the concentration of vacancies in single crystalline tungsten, (iii) explore the impact of helium loading on defects in tungsten, and (iv) characterize the micro-pores in thin surface films of tungsten-oxide formed on tungsten. Within the project we hence focused on the targeted preparation of single crystalline tungsten samples and on helium loading of tungsten. Electron beam irradiation was used to create vacancies as dominant defect type with a defined concentration, and helium has been implanted ex situ at energies below the displacement damage threshold. In addition to the main focus on helium-induced processes, we extended our investigation to tungsten-oxide thin films and so-called self-damaged tungsten. The characterization techniques employed include depth resolved Positron Annihilation Lifetime Spectroscopy (PALS) and Doppler Broadening Spectroscopy (DBS) for defect characterization as well as Elastic Recoil Detection Analysis (ERDA) and Nuclear Reaction Analysis (NRA) to study the distribution of helium in the nearsurface region. The project sheds light on the role of lattice defects and their interaction with helium in tungsten. This knowledge is crucial for understanding the atomic-scale processes governing helium-induced phenomena. We found that helium loading did not induce changes in defect types, but it led to a significant increase in helium retention by defects. The vacancies introduced acted as effective barriers, limiting helium penetration into deeper layers beyond 34 nm. The IBA techniques (NRA and ERDA) provided detailed information about the distribution of helium in the near-surface region. It could be shown for the first time experimentally, that for relevant helium fluxes expected in a future fusion device so-called self-trapping and trap mutation (as predicted by density functional theory calculations) does not dominate helium retention but pre-existing defects play the major role. This insight is pivotal for comprehending the interaction between helium and defects in tungsten. Our study and characterization of tungsten-oxide thin films contribute to a more comprehensive understanding of tungsten-based materials, addressing a gap in existing research. Through systematic experimentation and characterization, our project has made significant strides in unravelling the mysteries of the interaction between vacancies and helium in tungsten. Benefitting from the expertise of both research groups at TUM and IPP our findings not only contribute to fundamental knowledge in materials science but also hold implications for the development of robust materials in the context of nuclear fusion reactors.
Publications
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The influence of displacement damage on helium uptake and retention in tungsten. Nuclear Materials and Energy, 34, 101370.
Kärcher, A.; Schwarz-Selinger, T.; Burwitz, V.V.; Mathes, L.; Hugenschmidt, C. & Jacob, W.
