Wall Forum 2017

Recent experiments and modeling on the influence of helium on deuterium transport and retention in tungsten

Wall Forum
Recent laboratory experiments revealed that He admixture to deuterium (D) plasmas reduces D retention at elevated temperature for tungsten [1]. This change in retention is accompanied by reduced blistering and the growth of He nano bubbles below the surface. While there are several speculative attempts to explain these observations, the actual cause for the reduced retention remains unclear. One possibility is that He might act as diffusion barrier for D. Likewise nano-sized bubbles might open up additional pathways for D to reach the surface thereby decreasing its transport into the bulk. Contrary to these experimental findings density functional theory (DFT) calculations show strong attraction between He and hydrogen [2], indicating increased trapping and hence retention of D around He clusters should be expected. In order to unravel this mystery we took a novel experimental approach. We separated the surface effect from the He effect by moving the He interaction zone into depth. Moreover, He implantation and D transport was decoupled. With this we show for the first time unambiguously that the presence of He does locally increase D trapping. He does not act as diffusion barrier. Rate equation modelling is presented which can explain the observed effects without the need for free fitting parameters. ([1] M. J. Baldwin et al. Nucl. Fusion 51 (2011) 103021 [2] H-B. Zhou, Nucl. Fusion 50 (2010) 115010) [mehr]

Recent Progress in the Understanding of H Transport and Trapping in W

Wall Forum
The retention of hydrogen isotopes (H, D and T) in the first, plasma exposed wall isone of the key concerns for the operation of future long pulse fusion devices. It affects theparticle-, momentum and energy balance in the scrape-off layer as well as the retention ofhydrogen isotopes (HIs) and their permeation into the coolant. The currently acceptedpicture that is used for interpreting current laboratory and tokamak experiments is thatof diffusion hindered by trapping at lattice sites. This paper summarises recent resultsthat show that this current picture of how HIs are transported and retained in W needsto be extended: The modification of the surface (e.g. blistering) can lead to the formationof fast loss channels for near surface H. Trapping at single occupancy traps with fixed de-trapping energy fails to explain isotope exchange experiments, instead a trapping modelwith multi occupancy traps and fill level dependent de-trapping energies is required. Thepresence of interstitial impurities like N or C affects the solute HI diffusion coefficient. Thepresence of HIs during damage creation by e.g. neutrons stabilises defects and reducesdefect annealing at elevated temperatures. [mehr]

Medium energy ion scattering in Uppsala: Highlights and application to preferetial sputtering studies in EUROFER

Wall Forum
  • !!! Changed location: L1 Seminar Room (Tandem Accelerator) !!!
  • Datum: 10.05.2017
  • Uhrzeit: 15:30 - 16:30
  • Vortragende(r): Petter Ström
  • Uppsala
  • Ort: Garching
  • Raum: Seminarraum L1
  • Gastgeber: IPP
[mehr]

Research on Tungsten for Fusion Reactors at Oak Ridge National Laboratory

Wall Forum
[mehr]

Hydrogen in the Near-Surface Region of Materials Studied by Nuclear Reaction Analysis and Thermal Desorption Spectroscopy

Wall Forum
  • !!! Unusual date: Tuesday afternoon !!!
  • Datum: 04.07.2017
  • Uhrzeit: 15:30 - 16:30
  • Vortragende(r): Prof. Markus Wilde
  • Institute of Industrial Science, The University of Tokyo, Japan
  • Ort: Garching
  • Raum: Seminarraum D3
  • Gastgeber: IPP
Hydrogen (H) in the near-surface region of solids is of great relevance in science and technology. Hydrogen diffusion across the gas/solid interface of H-absorbing metals is exploited routinely in metal hydride H storage and H2 permeation membranes to produce purified H2 for fuel cell utilization. While palladium (Pd) absorbed hydrogen enables olefin hydrogenation catalysis [1], H in the interior of metals may also cause embrittlement and pose other problems, such as the retention of hydrogen isotopes in plasma-facing walls of fusion reactor devices. This talk introduces our unique combination of 1H-specific depth profiling via 1H(15N,aγ)12C nuclear reaction analysis (NRA) [2] with thermal desorption spectroscopy (TDS) as a powerful technique to identify and evaluate the thermal stability of H states in the near-surface region of solids in a highly depth-resolved fashion. I describe our recent research applying this method and isotope-labeling (H, D) of surface hydrogen to structurally well-defined Pd single crystal surfaces, which clarifies the atomic-level H transportation mechanisms at H2-exposed Pd surfaces during H2 absorption and thermal desorption [3]. I will further demonstrate that the insight thus obtained into the surface structure-sensitive H ingress can be used to achieve control over the thermal desorption of Pd-dissolved H in a wide range of temperatures [4] and discuss implications for industrial olefin hydrogenation over Pd catalysts [1, 5]. The NRA(/TDS) technique is equally applicable to nanostructured metals [6], ultrathin oxide films, and semiconductor/dielectric material stacks relevant to the microelectronics industry [7]. (References: [1] M. Wilde, K. Fukutani, W. Ludwig, B. Brandt, J.H. Fischer, S. Schauermann, H.J. Freund, Angew. Chem. Int. Ed., 47 (2008) 9289-9293. [2] M. Wilde, K. Fukutani, Surf. Sci. Rep., 69 (2014) 196-295. [3] S. Ohno, M. Wilde, K. Fukutani, J. Chem. Physics, 140 (2014) 134705. [4] S. Ohno, M. Wilde, K. Fukutani, J. Phys. Chem. C, 119 (2015) 11732-11738. [5] S. Ohno, M. Wilde, K. Mukai, J. Yoshinobu, K. Fukutani, J. Phys. Chem. C, 120 (2016) 11481-11489. [6] M. Wilde, K. Fukutani, M. Naschitzki, H.J. Freund, Phys. Rev. B, 77 (2008) 113412. [7] Z. Liu, S. Fujieda, H. Ishigaki, M. Wilde, K. Fukutani, ECS Transactions, 35 (2011) 55-72.) [mehr]

Elemental maps in large areas on non-flat surfaces by ion beam analysis

Wall Forum
The elemental mapping of large areas using ion beam techniques is a desirable capability of several scientific communities, involved on topics ranging from geoscience to cultural heritage. Usually, the constraints for large-area mapping are not met in setups employing micro- and nano-probes implemented all over the world. In this sense, a novel setup for mapping large sized samples in an external beam was recently built at the University of São Paulo employing a broad MeV-proton probe with sub-millimeter dimension, coupled to a high-precision large range XYZ robotic stage (60 cm range in all axis and precision of 5 um ensured by optical sensors). An important issue on large area mapping is how to deal with the irregularities of the sample's surface, that may introduce artifacts in the images due to the variation of the measuring conditions. In our setup, we implemented an automatic system based on machine vision to correct the position of the sample to compensate for its surface irregularities. As an additional benefit, a 3D digital reconstruction of the scanned surface is also obtained. [mehr]

Deformation and damage mechanisms of oxide dispersion strengthened steel under high temperature cyclic loading

Wall Forum
  • !!! CANCELLED - new date will be announced !!!
  • Datum: 13.09.2017
  • Uhrzeit: 15:00 - 16:00
  • Vortragende(r): Ankur Chauhan
  • Institute for Applied Materials, Karlsruhe Institute of Technology (KIT)
  • Ort: IPP Garching
  • Raum: Seminarraum D3
  • Gastgeber: IPP
Oxide dispersion strengthened (ODS) steels are promising structural material candidates for both generation four (GEN-IV) fission reactors and breeding blanket material in future fusion power plants. Within the framework of FP7 project MatISSE (Material's innovations for safe and sustainable nuclear energy in Europe), characterization and mechanical testing of several Fe-Cr alloys was undertaken. One of the alloys under investigation is a tempered martensitic 9Cr-ODS steel. The microstructure typically consists of a high density of hierarchically organized internal interfaces such as prior austenitic grain, packet or block boundaries, lath boundaries and sub-grains with high dislocation density. Coarse non-regular M23C6 carbides decorate different boundaries and are rich in Fe, Cr, and W. The complex nano-sized Y-Ti-O particles were observed embedded in the matrix. Tensile tests at various temperatures showed a good compromise between strength and ductility. However, for application, the behavior of ODS steels under cyclic loading is of decisive importance.The fully reversed strain-controlled low-cycle fatigue tests at elevated temperatures revealed a distinctive cyclic response. Apart from the higher cyclic stress levels, the steel manifests complex cyclic softening which is significantly lower in comparison to that observed for similar non-ODS steels. The presence of nano-sized particles was found beneficial in terms of mitigating the well-known cyclic softening in non-ODS steels. Testing includes creep-fatigue investigations by introducing a hold-time at the peak tensile strain. Such a hold period simulates the loading of a component under stationary operation. Introducing creep into the fatigue loading results in lower fatigue life. The tests are accompanied by microstructural investigations for the identification of main deformation and damage mechanisms. [mehr]

Hydrogen and H diffusion barriers in Metal-Oxide-Semiconductor (MOS) devices

Wall Forum
The H diffusion in intact model MOS structures is probed by H depth profiling via resonant 15N-H nuclear reaction analysis (NRA) combined with a variety of surface-sensitive spectroscopies. It is discovered that a specific oxynitride layer observed on the near-surface region of nitride films is able to function as a potential H diffusion barrier. This ultra-thin layers are desirable in the integrated circuit processing of silicon transistors. [mehr]

What we have learned about preferential sputtering with the ToF-RBS

Wall Forum
With the well-succeeded commissioning of the ToF-RBS, we performed a series of experiments aiming new insights on the variation of the surface composition of Tungsten-containing steel alloys due to the preferential sputtering induced by low energy Deuterium irradiation. We exposed W-Fe models systems with three different Tungsten concentrations and EUROFER to a Deuterium beam with 200 eV energy at fluences of 10^23 D/m^2. The ToF-RBS with a 2.0 MeV Silicon ion beam achieved enough depth-resolution to reveal the resulting profiles after irradiation. The combination of the ToF-RBS data with ToF-ERD data, together with surface morphology information obtained by AFM, provided an improved computational modeling of the samples using the MultiSIMNRA software. The profiles revealed that the light elements present in the near-surface regions, i.e. Oxygen and Carbon, may play some hole of importance either in the dynamics of the process (considering the presence of these elements prior the irradiation) or in the interpretation of the results (considering the incorporation of these elements after the exposure). The obtained profiles are in good agreement with the ones obtained by XPS, with the advantage of a direct assessment of the W-profiles with no need for corrections. The analysis of samples irradiated at a high temperature indicates a different behavior for the EUROFER when compared to the model systems, possibly due to Tungsten segregation that occurs in the latter. SEM-EDX measurements confirm segregation on model systems and not on EUROFER. [mehr]

Temperature Programmed Desorption of Helium in Tungsten

Wall Forum
  • !!! Unusual date: Friday afternoon !!!
  • Datum: 03.11.2017
  • Uhrzeit: 12:30 - 13:30
  • Ort: Garching
  • Raum: Seminarraum D3
  • Gastgeber: IPP
During the course of this masters thesis the existing temperature programmed desorption (TPD) setup TESS was upgraded with an electron beam heater in order to achieve high temperatures (> 2000 K) needed for He TPD from W. First results were obtained and cross-checked by elastic recoil detection analysis with O ions. [mehr]

First results on the use of tungsten heavy alloys in the divertor of ASDEX Upgrade

Rehearsal for an invited talk at the ICFRM conference (Aomori, Japan) 2017. [mehr]

Interdiffusion and Phase Formation at Iron-Tungsten Interfaces

Wall Forum
During the course of this master's thesis, the interdiffusion coefficient between tungsten and iron was determined. From data obtained by means of Rutherford backscattering spectrometry, an effective diffusion coefficient was determined via finite element simulations and the Boltzman-Matano method. SEM images on FIB-cuts into a layered tungsten-iron systems reveal the formation of an intermediate phase. From its growth with respect to time, a diffusion coefficient could be determined. A simulation code, describing the formation and growth of such intermediate phases, was developed. [mehr]
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