Radiation damage in nanostructured materials

Plasma-facing materials and components in a fusion reactor are the interface between the 
plasma and the material part. The operational conditions in this environment are probably 
the most challenging parameters for any material: high power loads and large particle and 
neutron fluxes are simultaneously impinging at their surfaces. To realize fusion in a tokamak 
or stellarator reactor, given the proven geometries and technological solutions, requires an 
improvement of the thermo-mechanical capabilities of currently available materials. In its 
first part this article describes the requirements and needs for new, advanced materials for the 
plasma-facing components. Starting points are capabilities and limitations of tungsten-based 
alloys and structurally stabilized materials. Furthermore, material requirements from the 
fusion-specific loading scenarios of a divertor in a water-cooled configuration are described, 
defining directions for the material development. Finally, safety requirements for a fusion 
reactor with its specific accident scenarios and their potential environmental impact lead to 
the definition of inherently passive materials, avoiding release of radioactive material through 
intrinsic material properties. The second part of this article demonstrates current material 
development lines answering the fusion-specific requirements for high heat flux materials. 
New composite materials, in particular fiber-reinforced and laminated structures, as well 
as mechanically alloyed tungsten materials, allow the extension of the thermo-mechanical 
operation space towards regions of extreme steady-state and transient loads. Self-passivating

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Development of advanced high heat flux and plasma-facing materials

A new method for complex metallic architecture fabrication is presented, through synthesis and 3D-printing of a new class of 3D-inks into greenbody structures followed by thermochemical transformation into sintered metallic counterparts. Small and large volumes of metal-oxide, metal, and metal compound 3D-printable inks are synthesized through simple mixing of solvent, powder, and the biomedical elastomer, polylactic-co-glycolic acid (PLGA). These inks can be 3D-printed under ambient conditions via simple extrusion at speeds upwards of 150 mm s –1 into millimeter- and centimeterscale thin, thick, high aspect ratio, hollow and enclosed, and multi-material architectures. The resulting 3D-printed green-bodies can be handled immediately, are remarkably robust, and may be further manipulated prior to metallic transformation. Green-bodies are transformed into metallic counterparts without warping or cracking through reduction and sintering in a H 2 atmosphere at elevated temperatures. It is shown that primary metal and binary alloy structures can be created from inks comprised of single and mixed oxide powders, and the versatility of the process is illustrated through its extension to more than two dozen additional metal-based materials. A potential application of this new system is briefl y demonstrated through cyclic reduction and oxidation of 3D-printed iron oxide constructs, which remain intact through numerous redox cycles.

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Metallic Architectures from 3D-Printed Powder-Based Liquid Inks

Microwave processing of materials has emerged as a new method for processing of a variety of materials in the recent years. Microwaves have been used effectively with significant advantages, particularly in food processing and chemical synthesis. They are also found to be efficient for processing polymers, ceramics, polymeric composites, and ceramic composites. The physics of interaction of microwaves with characteristically different materials is not yet explored well; consequently, there are challenges in microwave processing of metal-based materials. Industrial processing of bulk metal is yet to be popular in spite of the fact that the feasibility of metal powder sintering was demonstrated a few decades ago. This article provides a summary of fundamental aspects of microwave processing of metal-based materials and their interaction with metallic materials. The processing challenges have been surveyed; developments in terms of techniques and tooling have been analyzed. Possible effects of microw...

A Review of Research Trends in Microwave Processing of Metal-Based Materials and Opportunities in Microwave Metal Casting

Transmission Kikuchi diffraction (TKD), also known as transmission electron backscatter diffraction (t-EBSD), has received significant interest for the characterisation of nanocrystalline materials and nanostructures. In this paper, we will review the development of TKD, including forescatter detector imaging and ongoing parameter optimisation, as well as some of the current applications of the technique. A comparison to other microanalysis techniques is also included, highlighting their relative strengths and weaknesses and their complementarity with TKD. Finally, potential applications of the technique and possible future developments are discussed.

Transmission Kikuchi diffraction in a scanning electron microscope: A review

Nanotendril “fuzz” will grow under He bombardment under tokamak-relevant conditions on tungsten plasma-facing materials in a magnetic fusion energy device. We have grown tungsten nanotendrils at low (50 eV) and high (12 keV) He bombardment energy, in the range 900–1000 °C, and characterized them using electron microscopy. Low energy tendrils are finer (~22 nm diameter) than high-energy tendrils (~176 nm diameter), and low-energy tendrils have a smoother surface than high-energy tendrils. Cavities were omnipresent and typically ~5–10 nm in size. Oxygen was present at tendril surfaces, but tendrils were all BCC tungsten metal. Electron diffraction measured tendril growth axes and grain boundary angle/axis pairs; no preferential growth axes or angle/axis pairs were observed, and low-energy fuzz grain boundaries tended to be high angle; high energy tendril grain boundaries were not observed. We speculate that the strong tendency to high-angle grain boundaries in the low-energy tendrils implies that as the tendrils twist or bend, strain must accumulate until nucleation of a grain boundary is favorable compared to further lattice rotation. The high-energy tendrils consisted of very large (>100 nm) grains compared to the tendril size, so the nature of the high energy irradiation must enable faster growth with less lattice rotation.

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Morphologies of tungsten nanotendrils grown under helium exposure.

Transmutation-induced precipitation in tungsten irradiated with a mixed energy neutron spectrum

Effect of starting microstructure on helium plasma-materials interaction in tungsten

Microwave sintering of W/Cu functionally graded materials

Plasma-facing materials in the divertor of a magnetic fusion reactor have to tolerate steady state plasma heat fluxes in the range of 10 MW/m2 for ∼107 s, in addition to fusion neutron fluences, which can damage the plasma-facing materials to high displacements per atom (dpa) of ∼50 dpa. Materials solutions needed for the plasma-facing components are yet to be developed and tested. The material plasma exposure experiment (MPEX) is a newly proposed steady state linear plasma device designed to deliver the necessary plasma heat flux to a target for testing, including the capability to expose a priori neutron-damaged material samples to those plasmas. The requirements of the plasma source needed to deliver the required heat flux are being developed on the Proto-MPEX device which is a linear high-intensity radio-frequency (RF) plasma source that combines a high-density helicon plasma generator with electron- and ion-heating sections. The device is being used to study the physics of heating overdense plasmas i...

Kun Wang

Papers

Morphologies of tungsten nanotendrils grown under helium exposure.

Transmutation-induced precipitation in tungsten irradiated with a mixed energy neutron spectrum

Effect of starting microstructure on helium plasma-materials interaction in tungsten

Microwave sintering of W/Cu functionally graded materials

Plasma source development for fusion-relevant material testing