Elsevier

Advanced Powder Technology

Volume 26, Issue 5, September 2015, Pages 1300-1305
Advanced Powder Technology

Original Research Paper
Synthesis of tungsten nanopowders: Comparison of milling, SHS, MASHS and milling-induced chemical processes

https://doi.org/10.1016/j.apt.2015.07.004Get rights and content

Highlights

  • Tungsten nanopowders were synthesized by various methods.

  • An oxygen content lower than 10 at% has been obtained.

  • Maximal purity was obtained with a 10’ milling before SHS.

  • Introducing mechanical activation before SHS improves the yield.

Abstract

Synthesis of tungsten nanopowders was studied using milling of micrometric tungsten, then using the WO3–Mg thermitic reaction, using SHS (Self-propagating High-temperature Synthesis), milling induced chemical reaction (MICR), and MASHS (Mechanically Activated SHS). Reactions are studied by measuring temperature and pressure inside the milling jar (during MICR), or by analyzing the temperature profile along the sample during the reaction propagation by infrared thermography (SHS, MASHS). After reaction, samples were analyzed by AFM or SEM, by XRD, and BET. MASHS seems to possess optimum conditions with a pre-milling before SHS of 10 min, which yielded the highest tungsten purity, together with a grain size corresponding to our aim.

Introduction

The ITER project, being the most important international project on energy, aims at satisfying the ever increasing demand on energy, through the use of thermonuclear fusion of hydrogen isotopes to helium. In the presently considered configuration of the reactor, the lower part, called the divertor, will be made of tungsten and is aimed at receiving the high energy particles resulting from the reaction, and from this interaction will result sputtering which will yield the creation of tungsten nanoparticles.

Due to a potential risk due to the presence of these nanoparticles, e.g. if a Loss Of Vacuum Accident (LOVA) occurs, whether it is due by a water leak from the cooling system or an air leak from the surroundings of the reactor chamber, periodical cleanings are scheduled, during which the tungsten nanoparticles will be removed. To prevent contamination to the environment and/or to people supervising the cleaning operations, High Efficiency Particulate Air (HEPA) filters will be used, but these filters always have a smaller retention capability for particles in the 100–200 nm range. Because these particles are the ones to which operational staff will be exposed, they therefore are the most potentially dangerous ones. Unfortunately, the availability of tokamak-retrieved powders is quite low. Therefore, in order to conduct the different studies to assess the potential dangerousness of tungsten nanopowders, including their behavior when in presence of tritium, or when facing a hydrogen plasma [1], [2], [3], or their cytotoxicity or their genotoxicity (study currently under progress), other means of production must be sought for. Moreover, as ITER will be a significantly larger tokamak than the presently existing ones, different morphologies might occur during its operation, and thus it would be preferable to obtain a set of different morphologies to make sure the worst case scenario can be handled. Indeed, our aim here will be to synthesize such ITER-relevant particles through other means, to study their behavior in order to know how to deal with them, either in case of an accident or during maintenance operations.

In previous papers we started to study the behavior of different kinds of tungsten particles towards 3H [1], [2], and in presence of hydrogen plasma [3]. The present paper aims at providing more insights on their preparation.

Beside SHS-based methods, classical chemistry [4], solvothermal decomposition of tungsten hexacarbonyl [5], electric wire explosion [6] or even sonoelectrochemistry [7] have been used successfully to synthesize tungsten nanopowders. As for SHS-based methods, one can make the distinction between pure SHS e.g. using WO3 and Mg, NaN3, NaBH4 [8], [9] or Zn, together with a preheating of the reacting mixture [10], [9], and mechanically induced thermitic reaction of WO3 with Mg [11] or Li3N [12]. All the above mentioned papers have been a great help for the determination of our experimental protocol, and we are here aimed at studying the influence of parameters which have not been studied previously, especially the use of Mechanically Activated Self-propagating High-temperature Synthesis (MASHS) [13], [14], i.e. to use as starting reactants powders which have been subjected to a short-time preliminary co-milling.

Finally, it must be underlined that the synthesis of tungsten nanoparticles have some use in other fields, mainly in metallurgy, in machinery and for catalytic reactions. Indeed, it has been proved that addition of small quantities of tungsten nanoparticles greatly enhance the sinterability of otherwise micrometric tungsten, thus reducing the temperature required for sintering, and the final cost [15]. On the other hand, the use of tungsten nanoparticles has been used successfully to reinforce various materials, including metallic and intermetallic systems [16], [17], [18] or alumina [19]. Finally, tungsten nanoparticles have attractive reactivity and appropriate dimension for the synthesis of tungsten compounds nanoparticles, such as tungsten disulfide [20] used for its catalytic properties [21]. Such compounds, due to their graphitic crystal structure are also of prime interest for lubrication applications [22].

Section snippets

Milling and milling-induced chemical reaction

Our first tries to produce tungsten nanopowders have been made by milling, using a Fristsch Pulverisette 7 premium line planetary ball mill, with tungsten carbide balls and jar, a ball-to-powder ratio (BPR) of 40:1, and a velocity of 350 RPM for times ranging from 2 to 64 h, from tungsten powders with an initial diameter range of 3–13 μm [3]. Results presented here only concern a milling time of 24 h. Millings were performed under ethanol, in order to reduce firstly the sticking/welding of W

Milling

From X-ray diffraction results, Fig. 1, neither crystallized oxide phase nor tungsten carbide impurities from the milling media could be detected, although X-ray photoelectron spectroscopy (XPS) revealed the presence of oxide films on the surface of W particles [1]. Using Rietveld refinement, the obtained average crystallites size is 4 nm with a micro-strains level around 0.3%.

The surface area has then be measured using the Brunauer–Emmett–Teller analysis (BET) [24] by measuring the physical

Conclusion

As milling produced a wide size and shape range within the particles produced, nanometric tungsten particles were prepared using the thermitic reduction of tungsten trioxide by magnesium using SHS, MASHS and MICR processes. Considering the particles sizes, the larger particles were obtained for the pure SHS process, but best conditions are obtained for MASHS conditions where a 10 min milling time was used: indeed, with these conditions we have the lowest residual oxides amount, and a particle

Acknowledgments

The authors wish to thank the transversal toxicology program from the CEA DSV (France) – TWEET project and the A∗MIDEX project (n° ANR-11-IDEX-0001-02) funded by the “Investissements d’Avenir” French Government program, managed by the French research Agency (ANR) for financial support.

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