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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
41

Tungsten hexacarbonyl mediated C-S bond cleavage reactions.

January 1988 (has links)
Ng Chi Tat. / Thesis (M.Ph.)--Chinese University of Hong Kong, 1988. / Bibliography: leaves 72-76.
42

Desulfur-dimerization of dithioacetals with tungsten hexacarbonyl: synthesis of bifluorenylidenes.

January 1987 (has links)
Yu Chi Yip. / Thesis (M.Ph.)--Chinese University of Hong Kong, 1987. / Bibliography: leaves 74-76.
43

THE SPECIATION, KINETICS, AND ADSORPTION OF TUNGSTEN IN SULFIDIC NATURAL WATERS: FROM PALEO-ENVIRONMENT TO NANOTECHNOLOGY

January 2018 (has links)
acase@tulane.edu / Redox-sensitive trace metals tend to be more soluble under oxidizing conditions and less soluble under reducing conditions resulting in authigenic enrichments in oxygen-depleted sedimentary facies (Algeo and Rowe, 2012; P Ho et al., 2017; Tribovillard et al., 2006). Because of that, redox sensitive trace elements are used as paleoredox proxies to reconstruct redox status of the environment (Tribovillard et al., 2006). Unfortunately, we lack the quantitative understanding of many of these trace elements. One of the most important trace elements is tungsten. To contribute a better understanding of the biogeochemistry of tungsten to our community, I utilize laboratory experiments, geochemical modeling, and statistical methods to investigate the speciation, kinetics, and adsorption of tungsten in the environment. This thesis includes three major chapters. In the first major chapter, I performed a series of chemical experiments to explore the particle reactivity of tungstate and tetrathiotungstate in sulfidic solutions. I found that pyrite is a strong scavenger of W in aquatic environments. Our results indicate that the difference of specific adsorption between WO42– and WS42– may be attributed to their different inner-sphere complexation on the pyrite surface. Our results also show that WS42– is less particle reactive with respect to pyrite than MoS42–. In the second major chapter, I examined effect of acid on tungsten (W) sulfidation process as well as developed the Brønsted acid relationship, which provides a tool to predict the effect of acids on the kinetics of the thiolation reaction of W in natural waters. The results of laboratory experiments show that thiotungstate formation is first order with respect to both H2S and WO42- concentrations, and is catalyzed by acids. Therefore, low pH and high H2S concentrations both favor W thiolation. However, compared to molybdenum (Mo), thiolation of W is kinetically “sluggish”. The modeling results show that full thiolation of Mo requires ca. 110 days, whereas full thiolation of W requires ca. 50 years under a persistent euxinic condition such as the Black Sea. Our results indicate that the longer the period of euxinia, the higher chance of WS42- species in solutions and subsequently be incorporated into euxinic sediments as W-S species. In the last chapter, I successfully tested whether protonated mineral surfaces also catalyze the hydrolysis of tetrathiotungstate anions. Our results show that kaolin (Al2Si2O5[OH]4), aluminum oxide (γ-Al2O3), and titanium dioxide (TiO2) exert an appreciable catalytic effect on tetrathiotungstate hydrolysis. The data suggest that the pH dependent hydrolysis rate of WS42- for kaolin, γ-Al2O3, and TiO2 fall into two distinct groups, which consist two reaction pathways. The pH dependence of the mineral-catalyzed reactions suggest that acid surface sites on the mineral surfaces promote WS42- hydrolysis reactions. In the presence of UV-light, TiO2 substantially enhanced the hydrolysis rate of WS42- compared to identical experiments that were conducted in the absence of UV-light, we suggest the increased hydrolysis rate of WS42- in the presence of UV-light reflects the production of reactive oxygen species by TiO2. Due to the rapid development of nanotechnology, more engineered nanomaterials like TiO2 are introduced into the environment, which can impact the speciation and mobility of trace elements. Combined, the results in this thesis advance our understanding of mechanisms for W biogeochemistry in euxinic systems and will allow to facilitate reconstruction of paleodepositional conditions, paleoproductivity, and paleoredox. / 1 / Minming Cui
44

An experimental study of transition metal halides, directed towards the text of a stereochemical theory

Allison, G. B. (Graham Bruce) January 1968 (has links) (PDF)
Includes bibliography.
45

Transformations of cyclic olefins mediated by tungsten and molybdenum nitrosyl complexes

Buschhaus, Miriam Sarah Anne 11 1900 (has links)
Thermolysis of Cp*W(NO)(CH ₂CMe₃) ₂,CpW(NO)(CH ₂CMe₃)₂, Cp*W(NO)(CH₂SiMe₃)(η²-CPhCH₂), or Cp*W(NO)[CH(Ph)CH₂CH(nPr)CH₂]in cyclic olefins results in the formation of ring-retaining oligomers having lengths up to dodecamers. The main cyclohexene dimer is 3-cyclohexylcyclohexene. A small percentage of oligomers contain neopentyl or CH=CHPh end groups. Turnover frequencies for the Cp*-tungsten precatalysts range from 5.5 to 6.5 mol/h at 100 °C. In room temperature solutions, Cp*Mo(NO)(CH₂CMe₃)₂ generates the alkylidene intermediate [Cp*Mo(NO)(=CHCMe₃)], which couples with cyclic olefins to form cismetallacycles. The isolable cyclopentene-derived cis-metallacycle, Cp*Mo(NO)[cis-η² CH(CH₂)₃CHCHCMe₃], converts in the solid state to the allyl-hydride complex Cp*Mo(NO)(H)(η³-CH(CH₂)3CCHCMe₃). With larger cyclic olefins (cyclohexene through cyclooctene) the initial cis-metallacycles isomerize to trans-metallacycles of the form Cp*Mo(NO)[trans-η²-CH(CH₂)nCHCHCMe₃] (n = 4, 5, 6), and these subsequently convert with loss of dihydrogen to η⁴-diene complexes, Cp*Mo(NO)[η⁴-CHCH(CH₂)n-₁CCHCMe₃]. Thermolysis of the η⁴-diene complexes in cyclohexene results in decomposition of the organometallic complex with small amounts of oligomer formation. Thermolysis of Cp*W(NO)CH₂CMe₃)₂ in cyclic-olefm substrates generates the alkylidene intermediate [Cp*W(N0)(=CHCMe₃)], which couples with cyclic olefins in a manner analogous to the Cp*Mo-system. Tungsten trans-metallacycles are observed by ¹H NMR spectroscopy, but the organometallic subsequently reacts further with loss of the coupled neopentyl-cyclic olefin and coordination of two substrate molecules to form the putative Cp*W(NO)(cyclic olefm)₂ complex. Two additional cyclooctene products are isolated, the 1,4- diene Cp*W(N0)[η⁴-CHCH(CH₂)₅CHCCH(CH₂)₆] and the allyl hydride Cp*W(NO)(H)[η³- CH(CH₂)₆CCCHCH(CH₂)₅], both containing two coupled cyclooctene molecules. A tungsten cis-metallacycle forms with 2,5-dihydrofiiran, but a ring-opened alkoxy-allyl complex forms with 3,4-dihydro-2H-pyran, and 1,2,3,6-tetrahydro-pyridine undergoes N -H bond activation to afford an amido product. CpW(NO)(CH₂CMe₃)₂ produces some oligomers of cyclohexene, but in all other reactions bimetallic decomposition pathways predominate.
46

The research of the high aspect ratio of a micron size round pin

Wang, Yong-siang 09 September 2008 (has links)
In this study, an electrolytic micro-machining tester is employed to investigate the effects of supply voltage and initial machining position on the geometry of the tungsten needle using the single-point and the reciprocating methods. The tungsten needle to be electrolyzed is dipped in an aqueous electrolyte of 2wt % sodium hydroxide as the anode, and the stainless steel needle with a diameter of 100 £gm as the cathode, and the electrode gap is set to be 10 £gm. Experimental results show that it is difficult to control the diameter of the tungsten needle because the reduction rate of its diameter is quite fast and the bubbles are generated violently to cause the breakage of the tungsten needle at the higher supply voltage. At the lower supply voltage, the tungsten needle can be machined to a finer scale, but it takes a long machining time. Under the single-point machining condition, it can be used to manufacture a short, uniform, and smooth tungsten needle with the diameter of 9 £gm at the supply voltage of 12V and the initial machining position of 75 £gm. Under the reciprocating machining condition, a long uniform micro-cylinder tungsten needle can be manufactured, but its surface becomes rough slightly at the supply voltage of 4V and the initial machining position in the range between -50 and 0 £gm. A tungsten needle with the aspect ratio of 30 and the diameter of 9 £gm can be manufactured using the following process: the machining time of 24 min at the supply voltage of 4V, and then the machining time of 28 min at the supply voltage of 2V. Key words¡Gtungsten needle, sodium hydroxide
47

Electrochemistry of multiply metal-metal and metal-ligand bonded building blocks for conjugated materials /

Haines, Daniel E. January 2001 (has links)
Thesis (Ph. D.)--University of Chicago, Dept. of Chemistry, August 2001. / Includes bibliographical references. Also available on the Internet.
48

Synthesis of nano-structured monoclinic WO3 particles /

Lu, Zhixiang, January 2001 (has links)
Thesis (M.S.) in Chemistry--University of Maine, 2001. / Includes vita. Includes bibliographical references (leaves 70-74).
49

Electroanalytical studies of lead and tungsten.

Lai, Ping-chi, Edward, January 1978 (has links)
Thesis (M. Phil.)--University of Hong Kong, 1978.
50

The effect of advanced carburization techniques on the properties of pressed tungsten compacts

Holman, Marshall Graves, 1933- January 1964 (has links)
No description available.

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