Tungsten

Summary

Tungsten (also called wolfram) is a chemical element with the symbol W and atomic number 74. Tungsten is a rare metal found naturally on Earth almost exclusively as compounds with other elements. It was identified as a new element in 1781 and first isolated as a metal in 1783. Its important ores include scheelite and wolframite, the latter lending the element its alternate name. The free element is remarkable for its robustness, especially the fact that it has the highest melting point of all known elements, melting at . It also has the highest boiling point, at . Its density is , comparable with that of uranium and gold, and much higher (about 1.7 times) than that of lead. Polycrystalline tungsten is an intrinsically brittle and hard material (under standard conditions, when uncombined), making it difficult to work into metal. However, pure single-crystalline tungsten is more ductile and can be cut with a hard-steel hacksaw. Tungsten occurs in many alloys, which have numerous applic

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Heavy impurity transport in the presence of 3D MHD ideal perturbations

Eduardo José Lascas Neto

Heavy impurity accumulation poses a problem for the operation of tokamaks featuring tungsten plasma facing components. Early termination of the plasma due to tungsten accumulation is often observed following long living 3D MHD perturbations. Such scenarios areoften observed in present tokamaks like JET and ASDEX-U, and may be of concern for futuremachines like ITER and the European DEMO. Finding a way of designing high performancescenarios while preventing tungsten accumulation is therefore crucial. This thesis aims atunderstanding and modelling heavy impurity transport in tokamak plasmas in the presenceof long living 3D MHD ideal perturbations. In the first part of the thesis, we develop thetheoretical framework to treat the problem, building on stellarator theory for the main ions,while including the effect of strong toroidal rotation that is only present in tokamaks. Theorderings of the background ion species and the heavy impurity species are developed indetail. The background ions are subsonic which allows for the calculation of their flows usingthe stellarator 1/Îœ collisional regime, while the heavy impurities flow supersonically whichrequires the inclusion of centrifugal effects for the impurity description. An expression forthe neoclassical heavy impurity flux is obtained which helps identify the contrasting physicsinvolved. In the second part of the thesis, we present the development of the numerical toolsused to model the problem according to the theoretical framework presented. The usage ofthe VMEC code to obtain suitable 3D MHD equilibria is explained. Also, the development ofnew codes for calculating the background ion and heat flux flow of the ions, and the impurityflow from such magnetic equilbria is described. The heavy impurities are followed in thisbackground plasma using the VENUS-LEVIS code. This code describes the guiding-centermovement of the heavy impurities, accounting for the centrifugal and Coriolis drifts, as wellas for the correct effect of friction and thermal forces exerted by the background ions onthe impurities through a newly implemented collision operator. Finally, the numerical toolsdeveloped in this thesis are used to model the impact of long living 1/1 internal kink modeson heavy impurity transport. Heavy impurity accumulation is observed to occur rapidly inthe presence of a 1/1 internal kink mode, contrary to what is observed in axisymmetry, inwhich off-axis accumulation occurs due to the strong rotation. These cases agree well witha JET pulse where tungsten accumulates following rapid growth of a continuous 1/1 mode.In the weakly 3D phase of the pulse, off-axis accumulation of tungsten is observed, whilstin the strong 3D phase of the pulse, strong tungsten on-axis accumulation is observed. Thetheoretical developments allow us to break down all the relevant physics effects. It is seen thatsuch on-axis accumulation is due to the synergetic effect of the 1/1 mode, the strong toroidalrotation and the NTV ambipolar electric field.

EPFL2022

Pulse plating of nickel-tungsten alloys has been studied from two citrate ammonia based model electrolytes containing either an excess of nickel or an excess of tungsten. The alloy composition and current efficiency were determined for pulse periods ranging from 1 ms to 50 s at a constant duty cycle of 0.4. Deposits were analysed by X-ray fluorescence and by gravimetry. In the nickel rich electrolyte the rate of co-deposition of tungsten was found to be controlled by mass transport as previously observed with Ni-Mo alloys. In the tungsten rich electrolyte the tungsten concentration in the deposited alloys was the highest at short pulse periods. This suggests that the deposition rate of tungsten was affected by the rate of transport of nickel which is the highest at short times. As the pulse on time exceeded the transition time for nickel the current efficiency for alloy deposition dropped owing to hydrogen formation.

2008

The chemical-mechanical polishing process (CMP) is an essential part of the production of integrated circuits. Metal to be polished in the CMP reacts with the oxidants from the aqueous suspension (slurry) and the passive film is formed on the metal surface. Suspended abrasive nano particles from the slurry cause detachment of the formed friable passive films. The CMP represents the tribocorrosion process where material deterioration results from the interaction of wear and corrosion which takes place in tribological contact exposed to an aggressive environment. Due to increasing number of the materials to be polished in the modern integrated circuits it is necessary to gather fundamental understanding of the removal mechanism of each material. The aim of this work is to elucidate the removal mechanisms of tungsten (W), as a typical microelectronic metal (chosen as a model system), under different oxidising conditions (represented by electrochemically imposed potential). Also, we want to evaluate the effect of the slurry composition (chelating agents lactic and phosphoric acid) on material removal. To achieve this electrochemical techniques (chrono amperometry, cyclic voltametry, passivation kinetics measurements under mass transport control), and tribocorrosion techniques (tests on tribometer with electrochemical cell with reciprocating sliding motion of alumina ball) were used. Surface analysis techniques (SEM, XPS, AES) were used to assess worn and unworn surfaces' morphology and chemistry. In addition, electrochemical tests and polishing tests on the CMP machine were performed with technical CMP slurries. The sliding action of the alumina ball on tungsten in the 0.01 M H2SO4 solution was found to cause three distinct wear responses depending on imposed electrode potential. Below the first threshold potential the wear is low due to negligible oxide film formation. In the following wear region, up to the second threshold potential, materials deterioration in the rubbed area proceeds by cyclic mechanical removal of the WO3 passive film followed by repassivation of tungsten. The amount of anodic oxidation corresponds to the overall removed metal volume. Beyond the second threshold potential a compact tribolayer of WO3 (more than 400 nm thick), probably formed by agglomeration of oxide particles detached from the metal surface, forms and covers the wear track. Similar tribocorrosion behavior of tungsten was also observed in the presence of 0.1 M H3PO4. The presence of chelating agents, lactic acid and phosphoric acid, in the 0.01 M H2SO4 solution does not change the volume of material wastage at potentials below the second threshold potential. Beyond this electrode potential, lactic acid promotes WO3 dissolution and thus impedes the third body formation and enhances wear. Electrochemical factors have a strong influence on the tungsten CMP removal rate. It was found that CMP removal rate increases with the open circuit potential (electrode potential) and with passivation charge density. A good correlation of tribocorrosion results and CMP practice was observed. Tribocorrosion models can be successfully applied to the tribocorrosion of tungsten and as well can be fairly used for modelling the material removal in CMP.

EPFL2009

Heavy impurity transport in the presence of 3D MHD ideal perturbations

Eduardo José Lascas Neto

EPFL2022

EPFL2009