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Forschungsarbeit

Self-passivating W-alloys: armour material for fusion reactors

By Christian Lenser (24.02.2011)

In the search for new energy sources, nuclear fusion is a promising candidate with the potential to replace existing nuclear fission and coal combustion power plants as the suppliers of the base load of energy demand. The absence of carbon dioxide emission and radioactive waste with long half-life makes it an ideal energy source of tomorrow.

A future fusion power plant is based on the fusion of two hydrogen nuclei, deuterium and tritium, into a helium nucleus and a neutron:

D+ + T+ -> He2+ + n + 17.6 MeV

The emitted neutron carries the largest part of the heat of fusion, in terms of an extremely high kinetic energy (14.1 MeV). One prerequisite for this is a temperature of > 108 Kelvin, which can only be achieved in a magnetically confined plasma contained inside a ultra high vacuum (UHV) vessel if a continuous operation is desired. One of the largest remaining hurdles towards a power reactor is the choice of materials [1]. Any material that is exposed to the intense neutron radiation is required to exclusively consist of elements characterized by a short half-life, imposing strong constraints on the composition of the structural components.

One of the most important components of a fusion reactor is the armour, so called first wall, the part of the UHV vessel that is directly exposed to the plasma. This part of the fusion reactor is subject to a bombardment by energetic particles, which creates high thermal and mechanical stresses in the material, along with radiation damage such as nuclear transformation, He and H formation in the wall, induced defects and erosion. Therefore, the first wall needs to have a high thermal conductivity as well as thermal and mechanical stability. In addition, the contamination of the plasma itself by atoms that are sputtered from the wall needs to be below several percent for low-Z materials (C, Be) and in the ppm range for high-Z materials (W).  

Fig. 1: FIB prepared cross-sections of a W-alloys oxidized at 1000°C for 1 hour. Left: HRSEM micrograph of the passivating scale (sample is upside down). Right: HAADF image showing the elemental contrast of the Cr2O3 scale and the Cr-depleted zone. Fig. 1: FIB prepared cross-sections of a W-alloys oxidized at 1000°C for 1 hour. Left: HRSEM micrograph of the passivating scale (sample is upside down). Right: HAADF image showing the elemental contrast of the Cr2O3 scale and the Cr-depleted zone.[Bildunterschrift / Subline]: Fig. 1: FIB prepared cross-sections of a W-alloys oxidized at 1000°C for 1 hour. Left: HRSEM micrograph of the passivating scale (sample is upside down). Right: HAADF image showing the elemental contrast of the Cr2O3 scale and the Cr-depleted zone.

Self-passivating tungsten-based alloys are an attractive material for application in future fusion reactors, because they combine the favorable properties of tungsten with an enhanced passive safety in the case of a loss-of-coolant accident. In this case, the wall temperature would rise to over 1000°C due to the nuclear decay heat of radioactive species, which are created by transmutation reactions under neutron bombardment. Should structural damage cause an oxidizing atmosphere in the reactor chamber, the formation and sublimation of radioactive WO3 is suppressed by the self-passivating alloy.

Ternary and binary alloys have been shown to exhibit superior passivation properties in comparison to pure W [2]. The present work aims to expand the understanding of quaternary tungsten alloys. The oxidation behavior of thin films of self-passivating quaternary tungsten alloys is studied with respect to oxide growth kinetics, oxide phases and microstructure. A compositional screening is done on six alloy compositions prepared by magnetron sputtering containing tungsten, chromium, silicon and either yttrium or zirconium. Three oxidation conditions are chosen: 600°C for 48 hours, 800°C for 8 hours and 1000°C for 1 hour. Activation energies, calculated from parabolic rate constants, indicate that different oxidation processes are at work for different temperatures. The alloy WSi3Cr10Zr5 shows the best passivation behavior and is studied in detail with X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX). The amount of Cr incorporated into the W lattice leads to a distortion of the unit cell that can be quantified by X-ray diffraction.  In addition to a top layer of Cr2O3, various oxide phases such as WCrO4, WO2 and ZrSiO4 can be identified by XRD in the oxidized samples. SEM micrographs of cross-sections prepared by a focused ion beam (FIB) reveal the influence of oxidation on the microstructural evolution at 1000°C and the corresponding demixing of Cr from the W lattice. EDX on FIB-prepared lamellae reveal that Cr is the main diffusing species that depletes in an extended region below the surface.

Fig. 2: SEM cross-section overlaid with EDX linescan-profiles for the constituent elements of the sample.[Bildunterschrift / Subline]: Fig. 2: SEM cross-section overlaid with EDX linescan-profiles for the constituent elements of the sample. The Cr signal emphasizes the role of Cr as the main diffusing species, while the oxygen signal demonstrates the total oxidation depth. The W and Si signal are indistinguishable with the energy resolution of the EDX detector.

Quaternary tungsten alloys exhibit passivation properties superior to ternary alloys, while the W fraction in the quaternary alloys is much higher. This is important for applications in a future fusion reactor. Quaternary alloys show a change of the oxidation mechanism during oxidation, which is connected to the formation of multiple oxide phases.

This research was performed in the “materials synthesis and characterization” group of the materials research department, Max-Planck-Institut für Plasmaphysik, Garching.

Literature:

[1] D. Ward, S. Dudarev, Materials Today, Volume 11, Number 12, 2008.

[2] F. Koch, S. Koeppl, H, Bolt: Journal of Nuclear Materials 386—388 (2009), 572 – 574.

 


Stationen
  • 2009 – present
  • Ph.D. studies at the Institute for Solid State Research (IFF-6), Electronic Materials Resistive switching behavior of Fe-doped SrTiO3
  • 2007 – 2009
  • Technische Universität München, Ludwig-Maximilians Universität München and University of Augsburg: Elitestudiengang “Advanced Materials Science”, M.Sc.
  • Thesis title: “Quaternary self-passivating tungsten alloys”, Max-Planck-Institut für Plasmaphysik, Garching
  • 2004 – 2007
  • RWTH Aachen University: “Materials Science”, B.Sc.