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Tungsten metal is used as an electrical conductor in many modern microelectronic devices. One of the primary motivations for its use is that it can be deposited in thin films by chemical vapor deposition (CVD). CVD is a process whereby a thin film is deposited on a solid substrate by the reaction of a gas-phase molecular precursor. In the case of tungsten chemical vapor deposition (W-CVD) this precursor is commonly tungsten hexafluoride (WF6) which reacts with an appropriate reductant to yield metallic tungsten. A useful characteristic of the W-CVD chemical reactions is that while they proceed rapidly on silicon or metal substrates, they are inhibited on insulating substrates, such as silicon dioxide (Si02). This selectivity may be exploited in the manufacture of microelectronic devices, resulting in the formation of horizontal contacts and vertical vias by a self-aligning process. However, reaction parameters must be rigorously controlled, and even then tungsten nuclei may form on neighboring oxide surfaces after a short incubation time. Such nuclei can easily cause a short circuit or other defect and thereby render the device inoperable. If this loss of selectivity could be controlled in the practical applications of W-CVD, thereby allowing the incorporation of this technique into production, the cost of manufacturing microchips could be lowered. This research was designed to investigate the loss of selectivity for W-CVD in an attempt to understand the processes which lead to its occurrence. The effects of passivating the oxide surface with methanol against the formation of tungsten nuclei were studied. It was found that the methanol dissociates at oxide surface defect sites and blocks such sites from becoming tungsten nucleation sites. The effect of reactant partial pressure ratio on selectivity was also studied. It was found that as the reactant partial pressures are varied there are significant changes in the product partial pressure ratios, which are associated with gas phase reactions which contribute to the loss of selectivity.

A dissertation submitted to the Faculty of Engineering and the Built Environment, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Masters of Science in Engineering. Johannesburg, 2017 / This dissertation presents the zinc recycling of tool grade cemented tungsten scrap material generated during commercial production at Pilot Tools Pty Ltd (South Africa), production of powders and alloys from the zinc recycled materials, and evaluation of the properties of the recycled and un-recycled powders and alloys. Tool grade cemented tungsten carbide inserts were subjected to the zinc recycled process under controlled conditions. Tungsten carbide, cubic (TiC, TaC, NbC, TiCN) carbides and Co were recovered from the recycled scrap material. Two recycled alloys, R and RA, and two un-recycled alloys NS and N were produced following the conventional powder metallurgy route. Alloy R was made from 100 % zinc recycled powder with stoichiometric adjustment of C only, and alloy RA was made from 100% zinc recycled powder with stoichiometric adjustment of C and Co. Alloy N was produced under the same conditions as the recycled alloys R and RA, while alloy NS was produced at commercial level at Pilot Tools Pty Ltd (South Africa) using un-recycled powders. The alloy properties were evaluated following standard procedures for hardmetals. When the zinc recycled material was mechanically disintegrated, about 70 % of the recycled material was recovered as fine powder, while 30% was recovered as coarse oversize particles. The oversize particles were quite tough due to a high Co content, and it was difficult to disintegrate them through milling or repeated zinc recycling. The recycled powders took twice as much time to mill to the desired size as the new un-recycled powders, and had predominantly angular particles, while the new powders had smaller more rounded particles. A cubic free layer (CFL) was formed in all the alloys during sintering, although the recycled alloys R and RA had a narrower CFL compared to the new alloys NS and N. The recycled alloys R and RA had carbide grain size, carbide contiguity, binder mean free path, hardness, fracture toughness and wear rate which were generally within the same ranges as the new un-recycled alloys. / MT2018

O xerogel de pentóxido de vanádio possui estrutura bi-dimensional que viabiliza a intercalação de diferentes espécies como íons, compostos orgânicos e espécies poliméricas. E, quando associeado a outros óxidos de metais transição, tais como, WO3 e MoO3 apresenta uma importância em reações de catálise, pois estes óxidos atuam como promotores em reações como, por exemplo, a redução catalítica seletiva do NOx a NH3 e a oxidação de hidrocarbonetos. Além disso, estes óxidos melhoram as propriedades eletroquímicas e eletrocrômicas, favorecendo assim sua utilização em células fotoquímicas, células para conversão de energia solar e cátodos de baterias e lítio. Dentre estes óxidos, o óxido de tungstênio hidratado foi escolhido não somente por favorecer a dissociação ácida das moléculas, mas também por apresentar uma estrutura lamelar que favorece reações de intercalação sem afetar significativamente sua estrutura original. Neste contexto, o xerogel de óxido misto V2O5/WO3 foi sintetizado e suas propriedades eletroquímicas bem como sua capacidade de intercalação foi investigada. em relação ao comportamento eletroquímico, a melhor resposta foi obtida para o óxido misto com menor concentração de WO3, ao passo que para os óxidos com alta quantidade de óxido de tungstênio, a intercalação dos íons Li+ não foi eficiente. Estudos de voltametria cíclica revelaram que a resposta eletroquímica do óxido misto é similar à encontrada para o xerogel de pentóxido de vanádio. Em adição, outros resultados revelaram a dependência da resposta eletroquímica com a natureza do eletrólito suporte. Também foi verificado que é possível intercalar anilina e agentes tensoativos (Brometo de Cetiltrimetilamônio - CTAB), mantendo o caráter bi-dimensional. Neste trabalho, o compósito V2O5/WO3/SiO2 foi preparado pelo método sol-gel. A metodologia usada possibilitou a preparação de microesferas de sílica cobertas por xerogel de V2O5/WO3, conservando não somente a estrutura, mas também as propriedades eletroquímicas. A atividade catalítica avaliou a oxidação do estireno em fase líquida. / Vanadium pentoxide xerogel has a bi-dimensional structure which is suitable for intercalation of a variety of ions, organic compounds, and even polymeric species. In addition, when associated with other metal oxides such as WO3 and MoO3, it presents an important role in catalysis, since the resulting oxides act as promoters in catalytic reduction of the NOx to NH3 and hydrocarbons oxidation. Besides, these oxides improve electrochemical and electrochromic properties, leading to their use in photochemical cells, cathodes and in solar energy cells. Among several metal oxides, hydrated tungsten oxide was chosen not only because of favor the acid dissociation of molecles but also because of its lamellar structure that allows intercalation reactions without affect significantly the original structure. In this context, V2O5/WO3 xerogel mixed oxides were synthesized and electrochemical properties as well as intercalation capacity were investigated. Concerning the electrochemical behavior, the best response was obtained for the mixed oxides with low concentration of WO3, whereas for the oxides with high amount of tungsten oxide, the lithium ions intercalation was not efficient. Cyclic voltammetric studies revealed that the electrochemical response of mixed oxide is similar to that found for the vanadium pentoxide xerogel. In addition, our results reveal the dependence of the electrochemical response on the nature of the supporting electrolyte. It was also verified that it is possible to intercalate aniline and surfactant molecules (N-cetyl-N,N,N,-trimethyl-ammonium bromide - CTAB), maintaining the bi-dimensional character. In this work, V2O5/WO3/SiO2 composite was prepared by sol-gel method. The methodology used enables the preparation of silica microspheres covered by V2O5/WO3 xerogel mixed oxides, retaining not only its structure but also the electrochemical properties. the catalytic activity evaluated in the oxidation of the styrene in liquidphase.

Robert Dean Larrabee. / "May 21, 1957." "This report is based on a thesis submitted to the Department of Physics, M.I.T., May 13, 1957, in partial fulfillment of the requirements for the degree of Doctor of Science." / Bibliography: p. 80-81. / U.S. Army Signal Corps Contract No. DA36-039-sc-64637 Dept. of the Army Task 3-99-06-108 Project 3-99-00-100

Thesis (Ph.D.)--University of Delaware, 2006. / Principal faculty advisor: Jingguang G. Chen, Dept. of Chemical Engineering. Includes bibliographical references.

Adherent Cu films were electrodeposited onto polycrystalline W foils from purged solutions of 0.05 M CuSO4 in H2SO4 supporting electrolyte and 0.025 M CuCO3∙Cu(OH)2 in 0.32 M H3BO3 and corresponding HBF4 supporting electrolyte, both at pH = 1. Films were deposited under constant potential conditions at voltages between -0.6 V and -0.2 V vs Ag/AgCl. All films produced by pulses of 10 s duration were visible to the eye, copper colored, and survived a crude test called "the Scotch tape test", which stick the scotch tape on the sample, then peel off the tape and see if the copper film peels off or not. Characterization by scanning electron microscopy (SEM), energy dispersive X-ray (EDX) and X-ray photon spectroscopy (XPS) confirmed the presence of metallic Cu, with apparent dendritic growth. No sulfur impurity was observable by XPS or EDX. Kinetics measurements indicate that the Cu nucleation process in the sulfuric bath is slower than in the borate bath. In both baths, nucleation kinetics do not correspond to either instantaneous or progressive nucleation. Films deposited from 0.05 M CuSO4/H2SO4 solution at pH > 1 at -0.2 V exhibited poor adhesion and decreased Cu reduction current. In both borate and sulfate baths, small Cu nuclei are observable by SEM upon deposition at higher negative overpotentials, while only large nuclei (~ 1 micron or larger) are observed upon deposition at less negative potentials.

Elemental W and Ni powders were mechanically alloyed in a SPEX mill with WC grinding media for durations ranging from 5 to 50 hours, then compacted samples were sintered in hydrogen to generate bulk tungsten heavy alloys with 2, 4 and 6 wt.% Ni. Evidence of a bimodal grain size distribution was seen in X-ray diffractograms of sintered samples and confirmed by scanning electron microscopy. Grain sizes in the small-grained regions ranged from 200–600 nm; those in the large-grained regions ranged from 1–2 µm. Furthermore, the volume fraction of the small-grained region increased linearly as milling time increased. A slice from a sintered sample was prepared for examination by TEM, in which particles 30–100 nm in diameter were regularly observed on the boundaries of the 200–600 nm grains. EDS point analysis showed that the particles are WC. Therefore it is concluded that heterogeneously distributed contamination from the grinding media is continually incorporated during mechanical alloying and, during sintering, inhibits grain growth through Zener pinning. Densities of sintered samples increased as milling time increased to a maximum of almost 96% of the theoretical value. Density increases with respect to milling time were initially great but diminished upon further milling. While the samples with 4 and 6 wt.% Ni both approached 96% of the theoretical density value by 50 hours of milling, densities in the samples with 2 wt.% Ni were considerably lower. Thus it appears that the Ni that becomes incorporated into the bcc W structure during mechanical alloying activates W diffusion during sintering, though there is a limit to the amount of Ni that the W structure can accommodate. This is evinced in W lattice parameter values from the as-milled powders; while the lattice parameter drops considerable from 2 to 4 wt.% Ni, the difference between 4 and 6 wt.% Ni is much smaller and the Ni content limit surely falls between the two values. Otherwise-equivalent samples with added WC powder were also produced, but did not increase the volume fraction of the small-grained region – probably because the particles remained large and were homogeneously distributed. / Ph. D.

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