Dissertations / Theses on the topic 'Chromium'
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1
Yalcin, Sezgin. "Modeling Chromium Leaching From Chromite Ore Processing Waste." Master's thesis, METU, 2003. http://etd.lib.metu.edu.tr/upload/4/1053412/index.pdf.
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Chromium has been widely used in many industrial applications. As a result of chromite ore processing, large amounts of chromite ore processing waste (COPW) material that can be classified as hazardous have been produced and released into the environment.
Therefore, knowledge of migration behavior and leaching rates of chromium through waste materials and soils are of primary concern for environmentally sound management
of land-disposal hazardous wastes. Haskök (1998) experimentally studied leaching rates of total Cr and Cr(VI) using laboratory columns packed with chromium COPW material produced by a sodium chromite plant. Based on the experimental results of Haskö
k (1998), present study aim, through mathematical modeling, to understand the dissolution kinetics of chromium during leaching of COPW material and to investigate the effectiveness of intermittent leaching involving a sequence of batch (dissolution) and leaching (mass flushing) operational modes. Obtained results show that a coupled system of two first order differential equations was able to capture the essential characteristics of leaching behavior of COPW material. In addition, the kinetics of chromium dissolution from COPW appeared to be controlled by the difference between aqueous phase concentration and a saturation concentration, by the mass fraction of dissolvable chromium remaining in the solid phase, and finally by the contribution of a constant dissolution rate manifested as a steady-state tailing behavior. As a result of performed simulations it was seen that intermittent leaching could be 65%and 35% more effective than continuous leaching for total Cr and Cr(VI), respectively.
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Eadie, Edward Norman. "Pattern of international trade in chromite and ferrochromium : an historical perspective /." Title page, table of contents and abstract only, 1997. http://web4.library.adelaide.edu.au/theses/09PH/09phe11.pdf.
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Hall, Mitchell Scott. "Emissions of hexavalent chromium from hard chromium plating operations." Master's thesis, University of Central Florida, 1992. http://digital.library.ucf.edu/cdm/ref/collection/RTD/id/20511.
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University of Central Florida College of Engineering ThesisThis research addressess emissions of hexavalent chromium mist from hard chromium electroplating operations. Most of these emissions are typically captured by a ventilation stack and directed to a pollution control device; those which escape capture are called fugitive emissions. Releases of toxic materials such as hexavalent chromium must be reported annually to the Environmental Protection Agency (EPA) under provisions of the Superfund Amendments and Reauthorization Act (SARA) Title III of 1986 via the Toxic Release Inventory (TRI), Form R. The objectives were: (1) to provide estimates of fugitive hexavalent chromium emissions for the completion of Form R; and (2) to develop a predictive model for stack and fugitive emissions versus process and ventilation parameters. The database for stack emissions included published results from EPA studies. Fugitive release data were generated by field characterization at two operating facilities. Supplemental data for stack releases were also obtained during this field activity. The fugitive releases were documented to represent a small portion of the total atmospheric discharge; in most instances, the fugitive releases were less than the detection capability of the smapling/analytical protocols. Stack releases were successfully correlated with a measure of production activity (ampere-hours), production capacity (mass of chromium in the process bath), tank dimensions (plating bath surface area), and ventilation efficiency (ventilation slot area). This effort was supported by the EPA in the form of a cooperative agreement with the American Electropaters and Surface Finishers Society (AESF).
M.S.;
Civil and Environmental Engineering
Engineering;
Environmental Engineering
112 p.
viii, 112 leaves, bound : ill. ; 28 cm.
4
Villalobos, Aragon Alejandro. "Using chromium stable isotopes to monitor chromium reactive transport oxidation experiments and field studies /." To access this resource online via ProQuest Dissertations and Theses @ UTEP, 2009. http://0-proquest.umi.com.lib.utep.edu/login?COPT=REJTPTU0YmImSU5UPTAmVkVSPTI=&clientId=2515.
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Capozzi, Alexander. "Combustion synthesis of chromium-chromium sulfide CERMETS for ballistic protection." Thesis, McGill University, 2013. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=119535.
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A method was developed to synthesize chromium-chromium sulfide (Cr-CrS) cermets (ceramic-metal composites) without the use of a hot isostatic press. This synthesis method allowed for the production of materials that range from almost pure ceramics to highly metalized cermets. These materials were then impacted under ballistic loading to determine if a change in properties occurred as the cermet became more metalized. Six techniques were developed to increase the quality of cermet samples: controlling the rheology of the molten precursor mixture, vacuum degassing the molten precursor mixture, preheating the mould before pouring in the molten green mixture, using the vortex mixer to process mixtures of higher viscosity, reacting the precursor mixture under high pressure of an inert gas, and directional solidification of the cermet after reaction. Ballistic trials revealed that under certain conditions ductile-like behaviour was observed in Cr-CrS cermets. Perhaps even more interestingly, these tests revealed that even the ceramic phase seemed to support this ductile-like behaviour.Une technique de synthèse a été développée pour fabriquer des cermets (composites en céramique et métal) de chrome et sulfide de chrome (Cr-CrS) sans avoir besoin d'une presse isostatique. Cette technique peut être employé pour fabriquer des matériaux qui sont presque des céramiques purs jusqu'aux cermets avec une concentration de métal élever. Des échantillons ont été soumis à des tests balistiques pour déterminer si les propriétés du cermet changent avec la concentration de métal. Six techniques ont été développées pour améliorer la qualité des échantillons : contrôle de la rhéologie du mélange précurseur fondu, dégazage sous vide du mélange précurseur fondu, préchauffage de la moule, utilisant un agitateur vortex pour les mélanges de viscosité plus élevée, synthèse du cermet sous haute pression de gaz inerte, et la solidification dirigée du cermet après la réaction. Des essais balistiques ont révélé que, sous certaines conditions, les cermets Cr-CrS—y compris la phase céramique—ont démontré une certaine ductilité.
6
Xie, Hong. "Misrepair of Particulate Chromium(VI)-Induced DNA Double Strand Breaks Leads to Neoplastic Transformation." Fogler Library, University of Maine, 2007. http://www.library.umaine.edu/theses/pdf/XieH2007.pdf.
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Hume, W. Ewan. "Chiral chromium complexes." Thesis, University of Oxford, 1994. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.240480.
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Michalik, Marek Hänsel Michael Quadakkers Willem Joseph. "Effect of water vapour on growth and adherence of chromia scales on pure chromium /." Jülich : Forschungszentrum, Zentralbibliothek, 2007. http://www.loc.gov/catdir/toc/fy0803/2007464877.html.
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HajiAli, Afzali Saba. "Reactivity of Chromium with Aluminates: The Quest for Monovalent Chromium Complexes." Thèse, Université d'Ottawa / University of Ottawa, 2013. http://hdl.handle.net/10393/26078.
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Progress in catalytic ethylene oligmerization and polymerization processes creates an enormously valuable and active field in both industry and academic perspective.
From the naive point of view, gathering ethylene molecules to generate carbon carbon bonds can be assumed as the no electron, no energy and facile chemical process which results from interaction of several π-electrons but this process never can afford macro- and micromolecule without applying catalysts. It is no wonder that major efforts have been committed to designing novel catalysts to perform poly- and oligomerization processes, both recently and in the past which provide the extensive library of catalysts today. Among all the metals applied in preparation of homogeneous catalysts for ethylene oligo- and polymerization, chromium has been ideal in providing versatile systems in terms of selectivity and activity for oligomer and polymer productions.
In the first part of the current work we chose two different ligand systems (iminopyridine and bisiminophosphine) to investigate the activity of their chromium complexes for ethylene oligmerization and polymerization at different conditions. In addition we took on the substantial challenge of explaining the activities through isolation of intermediates during the catalytic runs. The major focus in this part was rationalizing catalytic behaviour of the catalysts through elucidation of metal oxidation state. To fulfill this goal, syntheses of chromium-aluminate complexes in different valence states as well as description ligand-metal interactions were presented.
In the second part of this thesis, our mainly focus of study was on the development in the well- established field of radical chemistry of alkylaluminum. Preferential cation radical and diradical formation and exploring their stabilities through both experimental and computational methods will be discussed. Also continuation of this work led us to report unprecedented double alkylation on the ligand system.
10
Gould, Peter John. "The high temperature oxidation of chromium and chromium implanted with cerium." Thesis, Imperial College London, 1991. http://hdl.handle.net/10044/1/46790.
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Puzon, Geoffrey J. "Biochemistry of chromium(VI) reduction formation, fate, and implications of soluble organo-chromium(III) complexes on the biogeocycle of chromium /." Online access for everyone, 2004. http://www.dissertations.wsu.edu/Dissertations/Spring2004/g%5Fpuzon%5F042204.pdf.
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McIntosh, Monique Sandra. "Mechanisms and Factors Affecting Chromium Oxide Particle reduction in Iron-Chromium Honeycombs." Diss., Georgia Institute of Technology, 2005. http://hdl.handle.net/1853/6934.
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In the production of iron chromium honeycombs, iron oxide and chromium oxide mixtures are reduced by hydrogen at elevated temperatures to produce a metallic alloy. The complete reduction of the iron oxide occurs prior to the reduction of the chromium oxide. The reduction of the chromium oxide particles within the iron matrix is affected by factors that include the diffusion of the reduced chromium away from the chromium oxide particle into the iron matrix, the diffusion of the gaseous reactants and products to and from the chromium oxide particles, and the porosity of the iron matrix, which changes as a result of sintering. The type of heat-treatment used, (isothermal or non-isothermal, i.e., holding at a specific temperature versus using a steadily increasing temperature) plays a vital role in how these factors will affect chromium oxide reduction.
Experimental data were used in conjunction with sintering and dissolution models to obtain an understanding of the environment in which the chromium oxide particles reduce as a function of heat-treatment. This understanding will assist in the development of more effective processing steps for the reduction of metallic honeycombs from oxide mixtures.
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Brooke, Ian William. "The electrodeposition of Tin/Zinc, Tin/Chromium and Tin/Zinc/Chromium alloys." Thesis, Glasgow Caledonian University, 2005. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.426444.
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The electrodeposition of tin/chromium and tin/zinc/chromium is a new field of research. This field was investigated because it may be of commercial, as well as scientific, interest if a range of alloy compositions could be sustainably deposited from environmentally acceptable electrolytes. With no published work to guide this research difficulties were encountered in producing sustainable deposition procedures. A novel electrolyte that deposited tin/chromium alloys in a range of compositions was produced. This electrolyte was adjusted to produce novel tin/zinc/chromium deposits in a range of compositions. However the thickness of deposited tin/zinc/chromium was limited to 300nm. It was hypothesised that polymerised chromium glycine could be used as a weak brightener to increase deposit thickness. This produced another novel electrolyte and it was confirmed that chromium could be codeposited from polymerised chromium glycine with tin and zinc. This electrolyte produced deposits in a range of compositions and produced tin/zinc/chromium deposits up to 3Jlm thick. The ability to deposit chromium from the polymerised electrolyte was enhanced by the codeposition oftin. This induced codeposition of chromium with tin had not previously been reported. It was observed that tin metal was dissolved by the chromium glycine electrolyte and that this dissolved tin was not precipitated out. It was suggested that the dissolved tin could be electrodeposited with chromium. Experimentation confirmed this hypothesis. The alloy produced was rich in chromium, 6Jlm thick and of a bright appearance. Tin/chromium and tin/zinc/chromium deposits were analysed by Mossbauer and X-ray Diffractometry and p-tin identified in all samples examined. Significantly tin only phases could not explain the Mossbauer and X-ray diffraction results, the presence of other tin microenvironments are required. However the lack of standards for comparison made identification of such microenvironments difficult.
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Shipton, Mark Ralph. "Pyridine chromium tricarbonyl complexes." Thesis, University of Oxford, 1990. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.276844.
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Abdol, Rahim Kartini. "Chromium dynamics in soil." Thesis, University of Nottingham, 2016. http://eprints.nottingham.ac.uk/37605/.
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Due to increasing awareness of potential Cr toxicity, there is a pressing need to establish sensitive and robust Cr fractionation and speciation methodologies that will be enable separation of the two redox Cr species (CrIII and CrVI) from different environmental phases and their quantification. The intention of this work was to assess the behaviour of Cr species, especially CrVI, in soils and the factors controlling Cr solubility, fractionation, redox transformation rates and uptake by plants. The analysis methods relied on alkaline extraction in TMAH, liquid chromatography (LC) to separate the chromium species and inductively couple plasma mass spectrometry (ICP-MS) for quantification of chromium. The interference of 40Ar12C+ background peak at mass 52 was reduced by using the CCT-KED facility of the ICP-MS. A solution of 50 mM TRIS buffer, 40 mM NH4NO3, 10-5 M ammonium-EDTA at pH 7.0 was used as the chromatographic eluent. The method developed is suitable for determining CrVI in soil, following alkaline extraction in TMAH, but not for CrIII due to poor recovery, redox transformation and strong binding of CrIII with humic acid despite attempts to preserve the trivalent species using EDTA and heating. The extraction method was applied to assessing Cr speciation and fractionation in a wide range of soil ecosystems collected from urban sites in Wolverhampton, Nottingham, London and a historical sewage sludge disposal farm in Nottinghamshire. To predict soil CrVI content the use of TMAH-extractable Cr (CrTMAH) was better (R=0.911) compared to total soil Cr content (Crtotal; R=0.554). The same analytical approaches were also applied to the development of a method to determine isotopically exchangeable CrVI in soils. This employed isotopically enriched 50CrVI as a ‘spike’ isotope added to soils suspended in varying concentrations of TMAH in an attempt to resolve a consistent fraction of isotopically exchangeable, or ‘labile’, CrVIO42- in soil. It was apparent that, because of the slow exchange kinetics of CrVI in soils, it was difficult to determine a consistent isotopically exchangeable fraction. Nevertheless, the investigation did suggest a refinement of the simple TMAH extraction protocol could enable direct determination of labile soil CrVI. The kinetics of CrVI interaction with a geocolloid (humic acid) was assessed and humic acid was found capable of both reducing CrVI and binding with the resulting CrIII species. Finally, Cr uptake by maize grown on a historical sewage sludge disposal farm was assessed with several approaches to finding a correlation between Cr in soil and Cr uptake by plants. The concentration of CrVI in soil, and its solubility, could be reasonably well predicted from Crtotal or CrTMAH and soil properties. However, restricted uptake of CrVI by the maize plants, and probably reduction of CrVI to CrIII in the root system, made it impossible to predict Cr transfer to shoots or the speciation of the Cr in maize shoots. Overall, due mainly to the apparent ability of the maize plants to control uptake and speciation of CrVI, the produce was considered safe to be consumed by ruminants as regards CrVI content.
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Miles, Jessica. "Atomic Nanofabrication with Chromium." Thesis, University of Manchester, 2010. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.516404.
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Кваша, О. П. "Chromium-plating of components." Thesis, Київський національний університет технологій та дизайну, 2018. https://er.knutd.edu.ua/handle/123456789/10694.
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Hu, Xianfeng. "Fundamental Studies on Direct Chromium Alloying by Chromite Ore with Designed Alloying Precursor." Licentiate thesis, Luleå tekniska universitet, Mineralteknik och metallurgi, 2014. http://urn.kb.se/resolve?urn=urn:nbn:se:ltu:diva-18117.
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Chromium is an important alloying element for stainless steels and other Cr-bearing steels. During the steelmaking process chromium is added to the steels mainly in the form of ferrochrome, which is largely produced by the energy-intensive smelting reduction process of chromite ore in the submerged arc furnace. To reduce the overall energy consumption during the ferrochrome production process and the chromium alloying process, direct chromiumalloying by chromite ore has been proposed. The application of this process will integrate the processes for ferrochrome production and chromium alloying, and thus has the potential to cut the production costs of the Cr-bearing steels by avoiding, or at least partially avoiding, the usage of ferrochrome. Further, this new alloying process has the capacity to improve therecovery of chromium from chromite ore. This thesis presents fundamental studies on the carbothermic reduction of synthetic iron chromite (FeCr2O4) and chromite ore, which aim at designing a direct alloying precursor to be applied in the industrial process. Thermogravimetric Analysis (TGA) experiments have been carried out to investigate the carbothermic reduction processes of FeCr2O4 in the absence/presence of metallic iron, and of chromite ore in the absence/presence of mill scale. In the case of using the mixture ‘FeCr2O4 +iron powder + graphite’, it is found that the presence of metallic iron enhances the reduction of FeCr2O4, and this enhancing effect increases with increasing iron addition. The enhancing effect of iron addition on the reduction of FeCr2O4 is due to the fact that the reduction of component Cr2O3 in FeCr2O4 is enhanced, and this effect is attributed to the presence of solidiron which can decrease the activity of chromium by having chromium in situ dissolved in the iron. In the case of using the mixture ‘chromite ore + petcoke’, it is found that the reduction of iron ions in the chromite ore starts before that of chromium ions in the ore and the reduction of iron ions and chromium ions in the ore overlaps to some degree. (Cr,Fe)7C3 is found to bethe intermediate phase during the reduction and a chromium gradient is found in the spinel phase of the fractional reduced sample at 1673 K. A four-stage reduction process is proposed: one stage involving the reduction of iron ions in the chromite ore and three stages involving the reduction of chromium ions in the ore. The activity aspects of component FeCr2O4 and component MgCr2O4 in the chromite ore have been considered. The difficulty in the reductionof the chromite ore is attributed to the fact that, as the reduction proceeds, the activity of component MgCr2O4 in the fractional reduced ore will decrease to a very low level, which makes the further reduction very difficult. In the case of using the mixture ‘chromite ore + mill scale + petcoke’, it is found that mill scale is reduced to iron before 1573 K. The asreduced iron is disseminated around chromite ore particles and, at the same time, some carbonis dissolved in the iron via diffusion. Reduction of chromite ore is enhanced with the addition of mill scale at temperatures higher than 1623 K, and the enhancing effect increases with increasing mill scale addition. The enhancing effect, in this case, is attributed to the presence of molten Fe-Cr-C phase in the vicinity of chromite ore, which can decrease the activity of chromium by having chromium in situ dissolved into the melt. Induction furnace experiments have been carried out to investigate the effectiveness of some different alloying mixtures. The experimental results have confirmed the necessity ofadjusting the composition of the slag to ensure high chromium yield in the final product and the experimental results show that, by using iron scrap, chromium yield can reach 90%. The present findings have led to the proposal of using ‘chromite ore + mill scale + petcoke’ as alloying precursor for direct chromium alloying. The effectiveness of this alloying precursor needs to be further explored by induction furnace experiments, followed by full scale Electric Arc Furnace experiments.Godkänd; 2014; 20141027 (andbra); Nedanstående person kommer att hålla licentiatseminarium för avläggande av teknologie licentiatexamen. Namn: Xianfeng Hu Ämne: Processmetallurgi /Process Metallurgy Uppsats: Fundamental Studies on Direct Chromium Alloying by Chromite Ore with Designed Alloying Precursor Examinator: Professor Bo Björkman Institutionen för samhällsbyggnad och naturresurser Avdelning Mineralteknik och metallurgi Luleå tekniska universitet Diskutant: Professor Timo Fabritsius University of Oulu Finland Tid: Fredag den 15 december 2014 kl 10.00 Plats: E246, Luleå tekniska universitet
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Shaddick, Lindsay Raymond, University of Western Sydney, of Science Technology and Environment College, and of Science Food and Horticulture School. "The geochemistry of chromium in the supergene environment : chromium (VI) and related species." THESIS_CSTE_SFH_Shaddick_ L .xml, 2003. http://handle.uws.edu.au:8081/1959.7/474.
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A description of the role of chromium in aqueous solution with respect to its geochemistry and the formation of secondary chromium minerals in the supergene environment is developed. Secondary chromium minerals are relatively rare in Nature and apart from the lead chromate crocoite which is by far the most common and also the most keenly sought after by collectors due its great beauty, little is known of related species. In attempt to redress this situation the initial aim of this study is to present an up-to-date list and description of secondary chromium minerals. It has long been recognised that secondary chromium mineralisation, in the form of chromates and compound chromates, occurs as a minor component of total secondary mineralisation in oxidised zones of some sulphide ore bodies. A thermochemical approach is adopted and a model to describe the geochemistry of chromium in aqueous solution as applied to mineral formation in the supergene environment and its transport in near surface waters is established. The model confirms that species distribution of chromium in aqueous solution is quite pH dependent and that at concentrations appropriate to those experienced in Nature, chromate is the only important species in basic solutions under highly oxidising conditionsMaster of Science (Hons.)
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Ma, Shufang. "Sustained deposition of thick coatings of chromium from environmentally acceptable chromium (III) electrolytes." Thesis, Glasgow Caledonian University, 1996. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.308560.
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Bauman, Stephanie. "ACUTE CHROMIUM (CHROMIUM(+VI)) TOXICITY IN THE ESTUARINE SHRIMP, MYSIDOPSIS BAHIA (CRUSTACEA: MYSIDACEA)." Thesis, The University of Arizona, 1985. http://hdl.handle.net/10150/291245.
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Shaddick, Lindsay Raymond. "The geochemistry of chromium in the supergene environment : chromium (VI) and related species." Thesis, View thesis, 2003. http://handle.uws.edu.au:8081/1959.7/474.
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A description of the role of chromium in aqueous solution with respect to its geochemistry and the formation of secondary chromium minerals in the supergene environment is developed. Secondary chromium minerals are relatively rare in Nature and apart from the lead chromate crocoite which is by far the most common and also the most keenly sought after by collectors due its great beauty, little is known of related species. In attempt to redress this situation the initial aim of this study is to present an up-to-date list and description of secondary chromium minerals. It has long been recognised that secondary chromium mineralisation, in the form of chromates and compound chromates, occurs as a minor component of total secondary mineralisation in oxidised zones of some sulphide ore bodies. A thermochemical approach is adopted and a model to describe the geochemistry of chromium in aqueous solution as applied to mineral formation in the supergene environment and its transport in near surface waters is established. The model confirms that species distribution of chromium in aqueous solution is quite pH dependent and that at concentrations appropriate to those experienced in Nature, chromate is the only important species in basic solutions under highly oxidising conditions
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Shaddick, Lindsay Raymond. "The geochemistry of chromium in the supergene environment : chromium (VI) and related species /." View thesis, 2003. http://library.uws.edu.au/adt-NUWS/public/adt-NUWS20050921.140829/index.html.
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Ustaoglu, Emre. "Optimization Of Conditions In Sulfuric Acid Leaching Of Turkish Chromite Concentrates." Master's thesis, METU, 2006. http://etd.lib.metu.edu.tr/upload/2/12607344/index.pdf.
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In this thesis work, a high grade chromite concentrate obtained from PinarbaSi, Kayseri region of Turkey was reacted with sulfuric acid solution to determine the optimum conditions of leaching of chromite ores.
Conventional methods for producing chromium are still valid in industry. The main process in production includes soda melting and taking it into solution in the form of chemical compounds such as sodium dichromate. Three and six valence
states of chromium have importance in production. Trivalent chromium, chrome ore or chromite have no harmful effects. However, compounds of hexavalent chromium
show toxic, irritating and corrosive action to people and environment. In the mentioned conventional method, main products consist of hexavalent chromium compounds. In this study, only trivalent chromium remained in the leach solution
and did not change to hexavalent state. Obtained product after leaching was chromium(III) sulfate.
The maximum extraction of chromium in the absence of perchloric acid was 94.1 % under the conditions of 175 °C, 6 hours and 84.6 wt % sulfuric acid. The maximum extraction of chromium in the presence of perchloric acid was 98.7 % under the conditions of 175 °
C, 2 hours, 84.6 wt % sulfuric acid and ½
perchloric acid / chromite ratio. The latter one was also the highest recovery value obtained during the experiments. Moreover, in none of the analyzed samples, appreciable amounts of hexavalent chromium was found during analyses.
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El-Sharif, M. R. "Continuous codeposition of chromium-nickel and chromium-nickel-iron alloys from aqueous amide electrolytes." Thesis, University of the West of Scotland, 1987. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.376533.
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There are numerous reports of attempts to deposit thick coatings of chromium-nickel and chromium-nickel-iron and although high quality deposits have been obtained these have been limited to very thin coatings only. The purpose of the present work was to identify and understand the controlling factors preventing continuous deposition and the build-up of thick coatings, and to develop a suitable process for such thick coatings. An amide electrolyte system was chosen asworking model for its relative simplicity and the high quality chromium-nickel deposits obtained from this system. This bath was investigated initially using conventional operating techniques (Constant Current) to establish its general characteristics. High quality deposits were obtained for thin coatings only. The need for a major change of operating philosophy was identified as essential. A number of published hypotheses explaining the failure of continuous deposition were experimentally tested and shown not to apply. Electrolytically generated chromium (II) has been identified to be an important controlling factor on the quality and composition of chromium-nickel alloys codeposited from amide electrolytes. The control of the low transient chromium (II) level, necessary for the maintenance of high quality codeposition with constant composition, can be achieved as follows. A controlled potential is required, in combination with circulation and agitation of the electrolyte and the generation of the chlorine gas as the preferred anodic product. The combination of the three techniques is necessary for the control of chromium (II) and if any of the three techniques are not employed sustained deposition fails. Chromium (II) has further been identified to catalyse an electrochemical ageing of the bath. Reproducible high quality true metallic deposits of constant composition of chromium-nickel and chromium-nickel-iron can be obtained to a thickness of several hundred microns. These have shown to display a laminar structure on etching and possible causes have been considered. Mossbauer spectroscopy and x-ray diffraction have shown that chromium-nickel-iron deposits contain a significant quantity of micro-crystalline austenite phase ~, fcc) with a dominant ferrite (e<, bcc) phase.
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Loveridge, Tracey. "Chiral chromium complexes in synthesis." Thesis, University of Oxford, 1995. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.296868.
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Christofi, Anna Maria. "Polycationic arene chromium tricarbonyl complexes." Thesis, University College London (University of London), 1999. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.327058.
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Kloppers, Marius Jacques 1962. "Electrochemistry of iron-chromium alloys." Thesis, Massachusetts Institute of Technology, 1991. http://hdl.handle.net/1721.1/106706.
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Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1991.Vita.
Includes bibliographical references (leaves 307-314).
by Marius Jacques Kloppers.
Ph.D.
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Бойчук, Д. М. "Chromium VI in leather updated." Thesis, Київський національний університет технологій та дизайну, 2018. https://er.knutd.edu.ua/handle/123456789/10629.
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Liu, Yuhang. "Chromium Cluster on Defected Graphene." Thesis, Uppsala universitet, Materialteori, 2017. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-326390.
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Michalik, Marek. "Effect of water vapour on growth and adherence of chromia scales on pure chromium." [S.l.] : [s.n.], 2007. http://deposit.ddb.de/cgi-bin/dokserv?idn=985669020.
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Nasser, Fatima. "Conversion of toxic hexavalent chromium to trivalent chromium by rhamnolipid stabilized zero valent iron nanoparticles." Thesis, University of Birmingham, 2013. http://etheses.bham.ac.uk/4347/.
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Zero valent iron nanoparticles (nZVI) are being used in environmental remediation of contaminants due to their reducing properties. Toxic chemicals such as hexavalent chromium (Cr(VI)) are released into environmental waters from industrial effluent and are capable of triggering life threatening diseases in humans. Solution phase Cr(VI) has can be converted to less harmful trivalent chromium (Cr(III)) using nZVI, which decreases the oxidation state. Cr(III) binds to iron and can be removed, thereby decreasing the concentration of solution phase Cr(VI). The nZVI have a high specific surface area though aggregate in solution which decreases specific surface area. In this study rhamnolipid surfactant capped nZVI of 50 nm were synthesized and were compared to nZVI without any incorporated surfactant using various characterization methods. At 10mg/L rhamnolipid incorporated nZVI and maintained a core size of 50 nm whilst raw nZVI z-average diameter increases from 341 nm to 1993 nm forming micron sized nZVI aggregates. It was determined that 1 gram of micro sized nZVI was able to convert 3 mg of Cr(VI) whilst 1 gram of nZVI was able to convert 36 mg in the same time period indicating that rhamnolipid was able to disperse particles and increase efficiency of the reaction.
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Dubarle, Offner Julien. "Novel heterobimetallic manganese-chromium and rhenium-chromium complexes : coordination, cyclometallation, benzannulation and haptotropic metal migration." Paris 6, 2009. http://www.theses.fr/2009PA066040.
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Une propriété caractéristique des complexes formés par des métaux de transition et des ligands polycycliques aromatiques est leur capacité à se soumettre à des réarrangements haptotropiques ; le fragment métallique est considéré comme le groupe fonctionnel déplaçable qui migre entre deux cycles non-équivalents du ligand hydrocarboné. Nous avons décidé de coordiner d’autres métaux de transition aux complexes (polyarène)Cr(CO)3 afin d’étudier leurs différentes influences sur la migration du tricarbonylchrome. Dans ce contexte, les premiers complexes hétérobimétalliques syn et anti Cr(CO)3-Mn(CO)3 et Cr(CO)3-Re(CO)3 ont été synthétisés. De plus, la synthèse de nouveaux complexes du tricarbonylchrome avec un polyarène N-hétérocyclique a été étudiée, ainsi que leur équivalents cyclomanganatés. Dans toutes ces synthèses, la benzannulation [3+2+1] de carbène de Fischer portant une unité métallique additionnelle a été employée afin d’introduire l’atome de chrome sur le ligand. Les migrations haptotropiques thermiquement induites du chrome le long des plateformes aromatiques ont été exécutées sur tous les complexes et ont conduit dans la plupart des cas aux haptotropomères thermodynamiques souhaités. Les déplacements intramoléculaires du fragment chromique ont été confirmés avec des études cinétiques par RMN qui ont montré une cinétique de premier ordre. De plus, toutes les structures hétérobimétalliques ont été déterminées par des analyses cristallographiques.
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Lindahl, Bonnie. "Equilibrium Study of Chromium Containing Cemented Carbides : Solubility of chromium in tungsten carbide and η-phase." Thesis, KTH, Materialvetenskap, 2010. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-49974.
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This project treats the W-C-Co-Cr quaternary system at low carbon contents. The main goals with this project were to find a four phase equilibrium between WC, Co-binder phase, η-phase (M6C) and a fourth unknown phase, to discover which the fourth phase is and to establish whether or not WC dissolves any chromium. In order to answer these questions a number of alloys were prepared, the compositions were chosen using thermodynamic calculations and observations from previous alloys. The samples were prepared using a powder metallurgical route and they were sintered at 1410oC. The samples were prepared for analysis via grinding and polishing and then analyzed using LOM, SEM, EDS, WDS and XRD. The results showed that no four phase equilibrium could be found in the analyzed spectra and therefore neither could the fourth phase. However other interesting phenomena were found. The η-phase showed a much higher solubility of chromium than previously thought. According to the current Sandvik-Seco database the η-phase should dissolve a maximum of 0.21 wt% and the maximum value detected using WDS was 8.42 wt%. The solubility of chromium in WC was found to be approximately 5 wt%. The most important conclusion that could be drawn from this project was that the Sandvik-Seco database for the W-C-Co-Cr system still needs to be improved, especially concerning the solubility of chromium in the η-phase.
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Lazauskas, Algirdas. "Surface morphology, cohesive and adhesive properties of physical vapor deposited chromium and chromium composite thin films." Doctoral thesis, Lithuanian Academic Libraries Network (LABT), 2014. http://vddb.library.lt/obj/LT-eLABa-0001:E.02~2014~D_20141105_162006-35788.
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Chromium thin film on glass substrate is the backbone of lithographic mask production as well as in the manufacturing of precision scale gratings and reticles. The lithographic masks, precision scale gratings and reticles, which are widely used in metrology instruments, motion systems and high precision machining tools ranging from digital calipers to coordinate measuring machines, typically consist of the patterned chromium film on the top of the glass substrate. In proportion to the development of such systems and devices, as a rule, high quality is requested altering the requirements for chromium thin films.
This work aims to develop thin films with low surface roughness and improved scratch/wear resistance as compared to conventional chromium thin films. It covers – the systematic study of substrate surface preparation methods for improved chromium thin film adhesion to substrate; properties of chromium thin films deposited on glass using different physical vapor deposition techniques; investigation of diamond-like carbon (DLC) films and DLC-polymer composites on chromium thin films; and development of novel chromium composite films.
The systematic study showed that soda–lime–silica float glass substrate surface activation and contamination removal by O2 plasma treatment process and wet chemical processing with NH4OH and H2O2 in H2O solution can effectively improve chromium thin film adhesive bonding to the substrate. It was determined that two-step thermal deposition... [to full text]Plona chromo plėvelė ant stiklo pagrindo yra pagrindinis litografinių kaukių, precizinių optinių liniuočių ir limbų gamybos komponentas, nes plėvelėje formuojami didelės skiriamosios gebos topografiniai piešiniai. Šie piešiniai yra plačiai naudojami moderniausiuose mikro- ir nanotechnologijų procesuose, įvairiuose metrologiniuose instrumentuose, judesio sistemose, precizinio tikslumo medžiagų apdirbimo staklėse, fotoelektriniuose linijinių ir kampinių poslinkių matavimo keitikliuose. Tokių sistemų ir įrenginių technologinis progresas kelia naujus aukštos kokybės reikalavimus plonoms chromo plėvelėms. Šiame darbe buvo siekiama sumažinti chromo plėvelių paviršiaus šiurkštumą bei pagerinti jų mechanines savybes (atsparumą įbrėžimams/dilimui). Disertacija apima stiklo (naudojamo litografinių kaukių, precizinių optinių liniuočių ir limbų gamyboje) paviršiaus paruošimo metodų efektyvumo sisteminę studiją, skirtingais fizikiniais metodais nusodintų plonų chromo plėvelių bei apsauginių deimanto tipo anglies (DLC) ir DLC-polimero kompozicinių plėvelių savybių tyrimą, o taip pat naujų chromo kompozicinių plėvelių sukūrimą. Sisteminiai tyrimai parodė, kad plonos chromo plėvelės adheziją su silikatiniu plukdytuoju stiklu labiausiai pagerina apdorojimas deguonies jonų plazmoje ir cheminis paviršiaus paruošimas amonio hidroksido ir vandenilio peroksido vandeniniame tirpale. Nustatyta, kad dviem žingsniais termiškai garintos plonos chromo plėvelės, po pirmo garinimo jas apdorojant... [toliau žr. visą tekstą]
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CORREIA, THIAGO OLIVEIRA FERREIRA. "SYNTHESIS OF NANOSTRUCTURED FE0-NI0/SIO2 PARTICLES FOR THE REDUCTION OF HEXAVALENT CHROMIUM TO TRIVALENT CHROMIUM." PONTIFÍCIA UNIVERSIDADE CATÓLICA DO RIO DE JANEIRO, 2018. http://www.maxwell.vrac.puc-rio.br/Busca_etds.php?strSecao=resultado&nrSeq=36037@1.
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PONTIFÍCIA UNIVERSIDADE CATÓLICA DO RIO DE JANEIROCOORDENAÇÃO DE APERFEIÇOAMENTO DO PESSOAL DE ENSINO SUPERIOR
CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICO
PROGRAMA DE SUPORTE À PÓS-GRADUAÇÃO DE INSTS. DE ENSINO
No presente trabalho, foram abordadas duas formas alternativas de materiais para a redução do cromo hexavalente. Para um dos materiais foram sintetizadas nanopartículas de ferro metálico, níquel metálico e sílica como suporte, e o outro material visando menor custo e maior acessibilidade foi uma lã de aço comercial. Ambos têm como objetivo a diminuição ou remediação de cromo hexavalente através de soluções de dicromato de potássio em concentrações que visam simular contaminações geradas por diversas operações industriais. A função destes materiais é de atuar como um forte agente redutor, doando elétrons para reduzir o cromo hexavalente em cromo trivalente. As nanopartículas de ferro zero valente, níquel zero valente suportadas em sílica foram sintetizadas empregando sulfatos em uma técnica de redução com borohidreto de potássio em solução aquosa. As nanopartículas foram caracterizadas com o auxílio da técnica de difração de raio-X para determinar as fases presentes, e de microscopia eletrônica de varredura com espectroscopia por dispersão de energia, definindo a composição química elementar e a morfologia das superfícies, e ICP para quantificar a composição dos elementos ferro e níquel. Finalmente, foram realizados os testes de redução do cromo hexavalente em cromo trivalente através do Espectrofotômetro UV-VIS, sendo quantificada a concentração de cromo hexavalente presente na solução através da técnica colorimétrica com o reagente altamente seletivo, o 1,5-difenilcarbazida (DFC). As nanopartículas e a lã de aço demonstraram eficiência na redução do cromo hexavalente em concentrações de 8,32 ppm (K2Cr2O7 80 Mmol/L), 6,241 (K2Cr2O7 60 Mmol/L) e 4,161 ppm (K2Cr2O7 40 Mmol/L) com tempos inferiores a 5 minutos. Contudo, as nanopartículas de Fe0-Ni0 suportadas em SiO2 reduziram o cromo hexavalente com uma quantidade em massa de Fe0 aproximadamente 110 vezes menor em relação a lã de aço, considerando que este é constituído unicamente por ferro metálico. Credita-se esta efetividade à maior área superficial por unidade de massa das partículas nanoestruturadas.
In the present work, two alternative forms were approached using different materials. For one of the materials, nanoparticles of metallic iron, metallic nickel and silica as support were synthesized, and the other material aiming at lower cost and greater accessibility will be steel wool. Both have the objective of reducing or remediation of hexavalent chromium through solutions of potassium dichromate in concentrations that aim to simulate contaminations generated by various industrial operations. The function of these materials is to act as a strong reducing agent by donating electrons to reduce the hexavalent chromium to trivalent chromium. The nanoparticles of zero-valent Iron, zero-valent nickel supported on silica was synthesized using sulfates in a reduction technique with potassium borohydride in aqueous solution. The nanoparticles were characterized with the help of the XRD technique, from the diffraction pattern generated and quantifying the present phases, and SEM / EDS, defining elemental chemical composition and surface morphology and ICP to quantify the elemental composition. Finally, the tests of reduction of the hexavalent chromium in trivalent chromium through the UV-VIS spectrophotometer were carried out, and the concentration of hexavalent Chromium present in the solution was quantified through the Colorimetric technique with the highly selective reagent, 1,5-diphenylcarbazide (DFC). Nanoparticles and steel wool showed efficiency in the reduction of hexavalent chromium at concentrations of 8,32 ppm (K2Cr2O7 80 Mmol/L), 6,241 (K2Cr2O7 60 Mmol/L) e 4,161 ppm (K2Cr2O7 40 Mmol/L) with times less than 5 minutes. However, as Fe0-Ni0 nanoparticles supported in SiO2, they reduced the hexavalent content of a quantity of steel mass in relation to steel wool, considering that it consists only of metallic iron. The effectiveness in the most superficial part by the mass unit of the nanostructured particles is credited.
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Du, Preez Rudolph C. "Effect of lime additions and bulk chromium content on chromium deportment in smelter matte-slag systems." Thesis, Stellenbosch : Stellenbosch University, 2010. http://hdl.handle.net/10019.1/17459.
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Thesis (MScEng) –Stellenbosch University, 2010ENGLISH ABSTRACT: More mining houses are turning their attention to the processing of ore mined from the UG2 reef. This is mainly due to the depletion of the Merensky reef or the low availability of ore from the Platreef. With the higher UG2 ore concentration in the process feed, companies experience more problems with the processing of the ore due to its high chrome content. Although there are various possible solutions to the problems experienced in the processing of UG2 ore, very little of thise solutions are actually implemented in the industry. When smelting chrome-bearing ore, the chrome can go to any of three phases: matte phase, spinel phase or glass phase. If it reports to the furnace matte phase it can lead to problems in the down stream processing. When chrome forms part of the spinel phase it forms a solid, refractory-like material which, to an extent, is an unwanted material inside the furnace. Lastly the chrome can report to the glass phase (liquid phase) which is the more desireable phase to which chrome should report to since it will have little or no impact on downstream processing. There were four main objectives for this research project namely to conduct a literature review to understand the problem of chrome in the smelting process, to do an experimental simulation of a matte and slag phase in one crucible, to interpret the experimental results and compare the experimental results to thermodynamic predictions obtained using FactSageTM. From literature the following aspects were found to be important: Silica structures Research has shown that silica incorporates different metal ions into its structures. When the metal ions are incorporated into the silica structures they are seen as part of the glass phase. By incorporating cations such as chromium into the silica structure the structure is able to maintain its neutral electrical charge. Basicity of slag Oxides once melted will either donate or accept oxide ions. The group of oxides that are in excess will determine whether that specific slag is acidic or non acidic. This is important to our study since the basicity can determine the stability of the spinel phase. Partial pressures The partial pressure of the system is important since it forms part of the equilibrium constant calculations - meaning that partial pressures determine the stability of certain species. Currently different arguments exist as to how the sulphur pressure inside a molten bath is maintained. What is important, however, is that for the system in this research project a log oxygen partial pressure of -8 and a log sulphur partial pressure of -3.5 was chosen. These values were based on previous research done on PGM and copper smelting processes. Chrome deportment Previous research on chrome deportment shows a relationship between temperature and the amount of chromium dissolving into the glass phase as well as an increased chromium(II)oxide solubility (when compared to chromium(III)oxide) in a silicate melt. Regarding slag chemistry and chrome deportment very little work has been published for the system found in the PGM industry but it is mentioned that by adjusting certain slag additives (alumina, lime and silica) the chrome deportment can be manipulated. The scope of this project was to investigate the effect of chromium, lime and silica on chrome deportment. More specifically, the effect of lime. The reason being that lime was originally added as 10%wt of the feed stream in the processing of Merensky ore to act as a fluxing agent. With the new furnace design and higher power densities the slag are maintained at 200oC to 350oC higher than when Merensky ore was smelted. With these higher slag temperatures fluxing agents will play a smaller role meaning that lime additions become less important. Literature studies also showed that additional lime in a slag system can stabilize the spinel structure (which is an unwanted phase). Removing lime would be advantageous out of a chrome presepective as well as an economic point of view. The research was conducted in three sections namely the determination of the time required for this particular system to reach equilibrium, the investigation of different additives on chrome deportment using a controlled atmosphere and synthetic slags and, lastly, a comparison of the experimental results obtained to thermodynamic predictions. For the equilibration studies reaction time periods of 4,7,11 and 16 hours were used. From the results it was found that the alumina crucible dissolved into the glass phase continually. This indicated that equilibrium was not reached. However, a reaction time, rather than an equilibrium time, was chosen where the species and phases had enough time to react. This was based on literature, on observations of species diffusing between the matte and slag phase as well as on two-point analysis (diffusion gradients). A reaction time of nine hours was chosen With the reaction time fixed, the effect of different slag additives on chrome deportment was investigated. For the addition of chrome it was seen that an 1.5%wt increase in the starting material increased the chrome content of the slag phase by 0.025%wt. For the same increase in chromium in the starting materials the chrome content of the spinel phase increased by 2.1% indicating that chromium has a tendency to report to the spinel phase. The increase in chromium had a minor effect on the chrome content of the matte phase, however, since the chrome content only increased from 0.025%wt to 0.028%wt. Plotting the results showed that increasing the lime in the starting material decreases the chromium content in the glass phase. For the 39%wt silica system the chromium content in the glass phase decreased from 0.75%wt to 0.46%wt for an increase in the lime content from 1.7%wt to 7.3%wt. The same trend was seen for the 33%wt silica system. For the spinel phase an increase of 1.27%wt was seen when the lime content of the starting materials was increased from 1.7%wt to 10.1%wt for a 33%wt silica system. The same increase in lime increased the chrome content of the matte phase from 0.03%wt to 0.06%wt for a 33%wt silica system. Silica also proved to affect chrome deportment. Increasing the silica content of the starting materials from 25%wt to 39%wt increased the amount of chromium in the glass phase from 0%wt to 0.46%wt for a 10%wt lime system. The same effect is seen for a lower lime content except that more chromium were incorporated into the silica structure. A silica increase from 32.4%wt to 39%wt resulted in a chromium decrease from 5.2%wt to 0% in the spinel phase for a 10%wt lime system. The same trend was seen for the 1.5%wt lime system. An increase in the silica levels lead to an increase in the chrome level of the matte phase. When silica is increased from 32%wt to 39%wt the chrome content of the matte phase increased from 0.06%wt to 0.07%wt. The last part of the research project entailed the comparison of the trends observed with the experimental results to trends obtained from thermodynamic predictions. FactSageTM is a program that uses model equations to predict the Gibbs free energies for different phases. The program is therefore also able to predict the amount of different phases present at equilibrium. This is called thermodynamic ”optimization”. In section 6 trends that were observed from FactSageTM results are compared to the trends found in the experimental results. It is important to note that it is only trends that are evaluated and not actual values since FactSageTM calculations are for a system that is at equilibrium and (as explained above) this system is not at complete equilibrium. The comparison however was good. Several trends found in the experimental results were confirmed by the results from FactSageTM. These included the relationships of chromium fed versus chromium spinel, lime fed versus chromium spinel, lime fed versus chromium in glass, silica fed versus chromium in glass, silica fed versus chromium in spinel and silica fed versus chromium in matte. To conclude, slag additions can be used to manipulate chrome deportment to an extent. Secondly, FactSageTM can be used for thermodynamic predictions but a proper understanding as well as some form of validation of the specific system investigated is still needed. Due to time constraints and the difficulty of experimentally simulating this multi-phase system in the lab, only a few parameters were investigated. In order to obtain a more complete understanding of the system the effect of partial pressures and temperature should also be investigated.
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Lockyer, D. M. "Adsorption and reaction of cyanogen with copper oxide, chromium oxide and copper oxide-chromium oxide surfaces." Thesis, University of Bath, 1988. https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.383244.
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Wall, Martin. "Temper embrittlement in 9% chromium steels." Thesis, Imperial College London, 1986. http://hdl.handle.net/10044/1/38179.
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Ganguli, Satyajit Nimu. "A kinetic study of chromium etching /." Thesis, McGill University, 1988. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=63943.
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Prasad, Rajesh. "Metastable phases in chromium-titanium alloys." Thesis, University of Cambridge, 1991. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.386323.
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Sebhat, Iyassu Kenneth. "Enantioselective deprotonation of arenetricarbonyl chromium complexes." Thesis, Imperial College London, 1996. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.243310.
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Blagg, J. "Arene chromium tricarbonyl complexes in synthesis." Thesis, University of Oxford, 1986. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.376895.
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Coote, S. J. "Arene chromium tricarbonyl complexes in synthesis." Thesis, University of Oxford, 1989. http://ora.ox.ac.uk/objects/uuid:318503a4-e288-4eea-9e11-9f82d26f1b8c.
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The last chapter describes an extension of this cyclisation reaction enabling the synthesis of homochiral l-aryl-l,2,4,5-tetrahydrobenzazepines. The stereoselectivity observed in the acid-mediated cyclisation of homochiral N-(3,4-dimethoxyphenethyl)halostachine to Nmethyl- l-phenyl-l,2,4,5-tetrahydrobenzazepine is assessed. The chromium tricarbonyl complex of this cyclisation precursor undergoes a stereoselective cyclisation to furnish the homochiral 1-aryl benzazepine after decomplexation. The diastereoselectivity observed in the acid-mediated cyclisation of homochiral N-3,4-dimethoxyphenethyl phenylpropanolamines is also investigated and the mechanism by which they occur is discussed. Coordination of one of these precursors to the chromium tricarbonyl unit renders the cyclisation stereoselective to give homochiral N-methyl-1 -phenyl-2-methyl-7,8-dimethoxy-1,2,4,5-tetrahydrobenzazepine after decomplexation.
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Kaluwin, C. "Analytical exploration of chromium waste deposits." Thesis, University of Salford, 1987. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.376866.
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Wararatananurak, Puchong. "Fractionation of chromium toxicity in water." Thesis, University of Aberdeen, 2000. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.327394.
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Yan, Shi. "Strengthening Aluminum By Zirconium and Chromium." Digital WPI, 2013. https://digitalcommons.wpi.edu/etd-theses/3.
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The Al-Zr system is used to form a thermally stable strengthening phase in high temperature aluminum-base casting alloys. These alloys have good strength at elevated temperature due to the precipitation of coherent metastable Al3Zr particles upon decomposition of the supersaturated Al-Zr solid solution by a carefully designed heat treatment. Formation of the Al3Zr particles occurs by a peritectic reaction, which decrees that once formed, the particles cannot be dissolved by a solid-state homogenization process. Accordingly, melting the alloy must serve as the homogenization step of the precipitation hardening process; and solidification during casting must serve as the quenching step. Unfortunately, a prohibitively fast solidification rate is necessary to obtain a solid solution with as little as 0.4% Zr in Al. It is found that adding Cr to Al-0.4wt%Zr binary alloy makes it easier to form the supersaturated solid solution, and the ternary Al-0.4wt%Zr- 0.8wt%Cr alloy has better room and elevated temperature tensile properties than the binary Al- 0.4wt%Zr alloy. Various one-step and two-step isothermal aging cycles were investigated in order to arrive at the optimum aging schedule for the Al-0.4wt%Zr-0.8wt%Cr. It is found that soaking the alloy at 400C for 24 hours is optimum; and employing a two-step aging schedule reduces the aging time without sacrificing strength. The two- step aging schedule includes soaking the alloy at 375C for 3 hours and then at 425C for an additional 12 hours. Examination of the precipitates that form in the Al-0.4wt%Zr-0.8wt%Cr with High Resolution Transmission Electron Microscopy (HRTEM) shows that they have the L12 crystal structure. Energy Dispersive Spectrometry (EDS) shows that the particles contain only aluminum and zirconium whereas the matrix is a solid solution of chromium in aluminum. Hence, it is suggested that zirconium strengthens the Al- 0.4wt%Zr-0.8wt%Cr alloy by a precipitation hardening mechanism and chromium further enhances the strength by solid solution strengthening.
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Sreekanthan, Pradeep. "Study of chromium in welding fume." Thesis, Massachusetts Institute of Technology, 1997. http://hdl.handle.net/1721.1/40245.
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Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1997.Vita.
Includes bibliographical references (leaves 52-54).
by Pradeep Sreekanthan.
M.S.
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Moos, Simone Beatrice. "The marine biogeochemistry of chromium isotopes." Thesis, Massachusetts Institute of Technology, 2018. http://hdl.handle.net/1721.1/115788.
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Thesis: Ph. D., Joint Program in Oceanography/Applied Ocean Science and Engineering (Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 2018.Cataloged from PDF version of thesis.
Includes bibliographical references.
In the ocean, chromium (Cr) is a redox-sensitive trace metal. The reduction of Cr(VI) to Cr(III) occurs in oxygen deficient zones (ODZs), and Cr reduction in general has been identified as a significant Cr isotope fractionation mechanism. This thesis presents the first Cr isotope variations (653 Cr) in ODZs of the ocean and adds to the sparse Cr isotope data published for modern seawater. I developed a precise and accurate Cr isotope method for seawater samples. Seawater acidification converts total Cr to Cr(III) which is preconcentrated by Mg(OH) 2 coprecipitation. A three-column anion exchange chromatography scheme separates Cr from isobaric and polyatomic interferences present in the seawater and reagent matrixes. Isotope analysis is performed on a MC-ICP-MS IsoProbe. The addition of a 50Cr-54Cr double spike allows for accurate correction of procedural and instrumental Cr mass fractionations. The first Cr isotope ratio data for a full water column profile in the Pacific Ocean is presented. This station serves as a fully oxic counterpart to stations located within the ODZ of the Eastern Tropical North Pacific. At one station, Cr concentrations are lower and [delta]53Cr values are heavier within the ODZ. This is consistent with Cr reduction resulting in isotopically lighter, particle-reactive Cr(III), which is scavenged and exported from the water column. A strong correlation of [delta]53Cr and [delta]15 NNo3- at this station suggests that Cr reduction may be microbially mediated instead of simply being a product of thermodynamic equilibrium. Alternatively, Cr may be reduced by Fe(II). In the anoxic bottom waters of the Santa Barbara Basin a strong Cr reduction signal (lower [Cr], heavier [delta]53Cr) is observed, which may result from the same aforementioned Cr reduction mechanisms. A shift to the heaviest seawater Cr isotope signatures yet observed was detected in the oxic bottom waters of the shallow Arctic Chukchi shelf, while Cr concentrations decreased. This extreme isotope signal may result from Cr reduction by a reduced species (e.g. Fe(II)), which was released from the underlying anoxic shelf sediments. Cr in the Atlantic layer and in the bottom water of a central Arctic station appears to be shaped by a novel, unidentified process.
by Simone Beatrice Moos.
Ph. D.
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Tilley, Richard David. "Sulphidisation of nickel and chromium nanoparticles." Thesis, University of Cambridge, 2001. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.621422.
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