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This study focused on the kinetic data measurements, such as diffusion coefficient D and exchange current density i_0 of the electrochemical reactions of corrosion products (Fe, Cr and Ni ions) and corrosive species (OH-), and corrosion studies of structural materials (SS 316H and Alloy 617), including static corrosion and galvanic corrosion, in molten MgCl2-NaCl-KCl and/or NaF-KF-UF4-UF3 salts in a temperature range of 600 to 800C. The study applied the semi-differential (SD) analysis method and innovative fitting method for the kinetic property data measurements in the multicomponent system of NaF-KF-UF4-UF3 salts. In molten MgCl2-NaCl-KCl salts, the measured D_(OH^- ) has the largest value followed by D_(〖Cr〗^(2+) ), D_(〖Fe〗^(2+) ), D_(〖Cr〗^(3+) ) and D_(〖Ni〗^(2+) ) at the studied temperatures, and none of the diffusion coefficients depends on the ion concentration in the studied concentration range and all of them followed the Arrhenius law. At the same temperature, the measured D_(Fe^(2+) ) and D_(〖Cr〗^(2+) ) values in molten NaF-KF-UF4-UF3 salts were slightly smaller than those obtained in molten MgCl2-NaCl-KCl salts. The non-linear curve fitting technique was applied to determine the exchange current density i_0, charge transfer coefficient α, limiting current density i_L and standard rate constant k^0 values. i_0 and k^0 followed the Arrhenius law. The obtained fundamental data can be applied to corrosion models which make the corrosion rate prediction possible in a static system from the experimental kinetic data. Corrosion studies of SS 316H and Alloy 617 in thermal purified molten NaF-KF-UF4-UF3 salts were performed for 120 hours. Based on the post-test analysis, the major metal species corrosion products were Cr, Fe and Mn in SS 316H tests, and Cr, Co, Ni in Alloy 617 tests. The measured UF4/UF3 ratio increased after corrosion tests because some of the U3+ was oxidized to U4+ by corrosive impurities and corrosion products during tests. Cr depletion and salt penetration were observed at grain boundaries (GBs) for both SS 316H and Alloy 617. For Alloy 617 specimens, the corroded area could be divided into two parts: the first part (near the surface) where Cr was completely depleted, and the second part (underneath the first part) where Cr was partially depleted. For SS 316H specimens, the average attack depth was larger than that of Alloy 617. Mo segregation was observed in the matrix of SS 316H specimens but was found to be enriched at GBs in the second part of Alloy 617 specimens. The corrosion study of Alloy 617 with time was also conducted for 72 hours and 32 hours, respectively. A thin layer composed of Fe, Co, Ni and Mo was found on the surface of the specimen, which was different from the previous 120-hour tests. In the salt, the concentration of Cr kept increasing with time, while for the other identified corroded elements, i.e., Fe, Co, Ni and Mo, their concentrations increased first, then decreased until becoming zero or stable. In the galvanic corrosion study of Alloy 617/graphite in molten NaF-KF-UF4-UF3 salts, the galvanic corrosion rate of Alloy 617 at 750C was about four times of that at 650C in the 2-hour tests, which indicated that temperature has a significant effect on the galvanic effect. In the 120-hour galvanic corrosion test, the galvanic corrosion rate became slightly larger with time in the studied system. Similar to the previous 120-hour Alloy 617 corrosion test, the corroded area of the post-test specimen was divided into two parts. The measured attack depth in both parts were much smaller compared with that in the 120-hour Alloy 617 test. This was because of the lower corrosive impurity concentrations in the salt used in the test. The salt in the galvanic corrosion test has been used in the previous corrosion test, during which the corrosive impurities were consumed, which made the salt less corrosive. Finally, it is necessary to point out that all the salts used in the present work were only thermally purified, which is effective in the removal of moisture but not in the removal of oxide impurities. Therefore, further studies are needed to understand the oxides' impacts on the corrosion behavior, especially on the salt penetration. / Doctor of Philosophy / Molten salt is a promising candidate that can be used as fuel and coolant in the molten salt reactors (MSRs). Besides, it can also be used as thermal energy storage, heat transfer fluid in the concentrated solar power plants, because it has high heat capacity, low vapor pressure, and high thermal conductivity. However, materials corrosion is a key concern of molten salt applications, and it is known that the corrosion by molten salts is mainly impurity driven. The impurities, such as moisture in the salts, can make the salt more oxidized, thus becoming more corrosive to corrode the structural materials. The present work focus on the kinetic property of metal specie corrosion products and non-metal impurity in the molten fluoride and chloride salts, which were directly related to the mass transfer and charge transfer process during the corrosion. Especially in the measurements of fluoride salts, innovative methods were applied which were confirmed to perform well in the multicomponent system (Fe and Cr ions coexisted). The static corrosion tests of SS 316H and Alloy 617 were conducted in molten fluoride salt at high temperatures. The main purpose was to study their corrosion behavior and understand the corrosion mechanisms. The corrosion rate of SS 316H was also estimated, which could be a crucial criterion in the material selection. In addition, the corrosion of Alloy 617 with time was also investigated. The metal specie corrosion product concentration change trends were obtained, and the corrosion behavior over the different corrosion stages was analyzed. Different corrosion phenomenon was observed in different corrosion test. Thus, they shed lights on the study of how the corrosion was developed during the corrosion process. Moreover, galvanic corrosion was another major corrosion type when two or more dissimilar materials were electrically contacted. The galvanic corrosion of Alloy 617/graphite was studied in the molten fluoride salts. The galvanic corrosion rate increased with the rise of temperature, which verified that temperature was a key factor that affected the galvanic corrosion. And the galvanic effect was also turned out to increase with time in the present study.

No description available.

[pt] Neste trabalho é realizada a determinação dos parâmetros cinéticos críticos da redução eletroquímica do dissulfeto de ferro numa mistura de haletos clorados fundidos. Este catodo é empregado como material alternativo em sistemas de elevado grau tecnológico, por exemplo, componente em coletores de energia solar, anodo despolarizador para a produção de hidrogênio e material catódico em baterias e pilhas de alta densidade de energia. Cabe ressaltar que o par eletroquímico Li / FeS2 vem sendo testado em novas configurações com diversos eletrólitos, especialmente com sais fundidos em pilhas térmicas e polímeros orgânicos em veículos elétricos / híbridos. Os ensaios desta pesquisa foram realizados em uma célula de teste num forno vertical com leitura digital em tempo real da temperatura e dos dados eletroquímicos. A estabilidade de diversos eletrodos de referência de primeira espécie foi avaliada em testes em branco de longa duração, sendo analisados os seguintes materiais: prata, platina, níquel, molibdênio. A célula eletroquímica teve a configuração de três eletrodos: prata como referência; dissulfeto de ferro, na forma de pó compactado, de trabalho e grafite sendo o contra-eletrodo. A metodologia empregada foi a voltametria linear cíclica com taxa de varredura lenta (0,002 Vs-1), garantindo quasi equilíbrio. O cálculo dos potenciais padrão em circuito aberto, de equilíbrio termodinâmico, indicou 0,3306 ± 0,014 V (773 K) em relação ao eletrodo de referência de Ag / AgCl. O coeficiente de transferência catódico ficou determinado como valendo 0,48, comprovando a reversibilidade do processo e apontando para a possibilidade de utilização deste sistema eletroquímico em baterias. Foi estudado o comportamento eletrocatalítico do eletrodo de FeS2 pelo levantamento das curvas de Tafel a partir dos voltamogramas. O parâmetro indicador desta espontaneidade reacional foi as correntes de transferência, que para o sistema foram determinadas como 14,75 ± 2,73 kA m-2. A avaliação dos produtos reacionais e intermediários foi realizada aliando dados eletroquímicos e técnicas de caracterização. O mecanismo de reação proposto é iniciado pela redução do FeS2 a Fe metálico, como etapa controladora da reação, envolvendo a troca de um elétron, seguida de duas reações envolvendo íons enxofre e uma etapa final puramente química com a formação de Li2S. Uma série de reações químicas e eletroquímicas são propostas para explicar a formação de polissulfetos intermediários, sendo o mais importante o Li2FeS2 ( fase X ), caracterizado neste estudo através de micrografias com a formação de cristais de hábito acicular. / [en] In this work the measurement of the critical kinetics parameters of iron disulphide electrochemical reduction in molten chloride halides mixture was made. This cathode is applied as alternative material in high technology systems, such as, solar energy collector`s components, anode depolariser for hydrogen production and cathodic materials for high energy density primary and secondary batteries. It should be notice that the Li / FeS2 electrochemical pair is being tested in new configurations together with several electrolytes, specially molten salts in thermal batteries and organic polymers in hybrid / electrical vehicles. The experiments in this research were carried in a test cell placed inside a vertical furnace having a real time data acquisition system for temperature and electrochemical data. The stability of many first kind reference electrodes was evaluated in long duration blank tests, being selected the following materials: silver, platinum, nickel and molybdenum. The chosen three- electrode cell configuration was: silver as reference, iron disulphide compacted powder as working electrode and graphite as counter-electrode. The applied methodology was the cyclic linear voltammetry at slow sweep rate (0,002 Vs-1), ensuring quasi equilibrium conditions. For the thermodynamical equilibrium the standard potential determinations for open circuit resulted 0,3306 +- 0,014 V (773 K) with respect to the Ag / AgCl reference. The cathodic transfer coefficient measured to be 0,48 indicates the reversibility of the electrode process and points at its possible application as secondary battery. The FeS2 electrocatalytical behaviour was evaluated though the Tafel curves extracted from the voltammograms. The indicating parameter for this reaction spontaneity, the transfer currents, for this systems were measured to be 14,75 +- 2,73 kA m-2. The evaluation of the reaction intermediaries and products were made allying electrochemical data and characterization techniques. The proposed reaction mechanism is initiated by the reduction of FeS2 to metallic iron as the controlling step, followed by two reactions involving sulphur ions and terminated by the chemical formation of Li2S. A series of chemical and electrochemical processes are proposed to explain formation of intermediary polisulphides, being the most important Li2FeS2 (phase X) spotted here though micrographies displaying it`s characteristic crystals of needle-like morphology.

O espinélio MgAl2O4 é utilizado prioritariamente na indústria de refratários. Atualmente há várias técnicas de síntese, visto que não é encontrado na natureza. A técnica de sais fundidos apresenta grande potencial para otimização e redução do custo energético na produção deste óxido. Este trabalho teve como objetivo o desenvolvimento da técnica de síntese via sais fundidos para obtenção do espinélio MgAl2O4, e a caracterização do pó produzido via diferentes técnicas. A dissertação explora as variáveis da produção do espinélio a partir de Al2O3 e MgO em KCl, e também alternando o sal para MgCl2 e o eutético KCl-MgCl2. As seguintes alterações foram testadas para verificar o resultado no pó produzido: redução da temperatura para 1150°C, aumento do MgO nos precursores em 10% e 30%, redução da granulometria da Al2O3 precursores para 0,60 μm, variação da razão sal/precursor (S/P) para 1:1 e 9:1. Foi testado o sal MgCl2 nas proporções de precursores estequiométrica, -50% de MgO e -100% de MgO. Por fim, o sal KCl-MgCl2 na razão eutética foi testado. Para caracterização do pó foram utilizadas as técnica de análise cristalográfica por difração de raios X (DRX) e pela técnica de RAMAN, granulometria por dispersão a laser (GDL), microestrutura por microscopia eletrônica por varredura (MEV) e análise de cristalitos por mircroscopia eletrônica de transmissão (MET). Os pós produzidos apresentaram as fases MgAl2O4, Al2O3 e MgO. A redução de temperatura reduziu a quantidade de espinélio em pequena proporção, a alteração de estequiometria dos precursores no sal KCl reduziu a quantidade de Al2O3 em um primeiro momento e também sobrou MgO na reação para um aumento maior de MgO. A redução de granulometria da Al2O3 propiciou aumento da proporção de espinélio na reação. A proporção S/P 1:1 não mostrou alteração, contudo para 9:1 houve sedimentação e um efeito de diluição/precipitação. Para o sal MgCl2, houve precipitação de MgO a partir do sal, alterando o perfil granulométrico dos pós e as fases obtidas: MgO e MgAl2O4. A redução de MgO aumentou a conversão para MgAl2O4, reduzindo também a granulometria. O sal na proporção eutética apresentou boa conversão de espinélio e pouco aumento de granulometria. / The MgAl2O4 spinel is mainly applied at the refractory industry. There are many techniques to produce that oxide, since it is not found in the nature. The molten salts technique shows potencial energy savings and cost reduction to produce spinel. This work aimed at the development of the molten salts synthesis technique to obtain the MgAl2O4 spinel, and characterization of the powder produced via different techniques. The dissertation explores the variables of production of spinel from Al2O3 and MgO in KCl, and also alternating salt to MgCl2 and KCl-MgCl2 eutectic. The following changes were tested to verify the result of the powder produced: reducing the temperature to 1150 ° C, an increase in the MgO precursor at 10% and 30% reduction in particle size of the Al2O3 precursor to 0.60 micrometres variation ratio of salt / precursor (S / P) to 1: 1 and 9: 1. MgCl2 salt was tested in the stoichiometric proportion of precursors, -50% of MgO and -100% MgO. Finally, the KCl-MgCl2 salt eutectic ratio was tested. For characterization of the powder was used the crystallographic analysis technique of X-ray diffraction (XRD) and RAMAN technique, laser scattering for particle size analisys, scanning electron microscopy (SEM) and crystallite analysis by transmission electron mircroscopy (TEM). The powders produced presented the MgAl2O4 phase, Al2O3 and MgO. The reduction in temperature reduced the amount of spinel in small proportion, the stoichiometry change of the precursors in the KCl salt reduced the amount of Al2O3 at first and also left MgO in reaction to a greater increase of MgO. The reduction of the Al2O3 particle size resulted in an increase in the proportion of the spinel reaction. The ratio S/P 1: 1 showed no change, however to 9: 1 was sedimentation and dilution / precipitation effect. To MgCl2 salt, there was precipitation of MgO from the salt, changing the physical profile of the powder and the obtained phases: MgO and MgAl2O4. The reduction of MgO increased the conversion MgAl2O4, by reducing the grain size. The salt in the proportion eutectic showed good conversion spinel and little increase in particle size.

Ce travail de thèse a pour objet le développement d'une nouvelle voie de synthèse de nanomatériaux métalliques à base d'éléments légers : bore et phosphore. L'intérêt porté à ces composés s'explique par les propriétés variées qu'ils présentent, tels que la supraconductivité, la thermoélectricité ou le stockage d'énergie. Dans le cadre de ce travail, les domaines de la catalyse et de l'électrocatalyse sont explorés. Les borures de différents métaux de transition, en particulier le nickel, le palladium et un composite nickel-cobalt, ont tout d'abord été étudiés. Pour cela une synthèse a été mise au point, reposant sur la réactivité de nanoparticules métalliques avec un précurseur de bore en milieu sels fondus inorganiques. Elle a notamment permis d'obtenir des nanoparticules de borures de nickel avec un bon contrôle de composition, structure, morphologie et taille. Les propriétés de ces nanomatériaux ont par la suite été étudiées en catalyse dans la réaction d'hydrodésoxygénation, et en électrocatalyse dans les réactions de génération d'hydrogène ou d'oxygène à partir de l'eau. Enfin la réactivité du phosphore rouge en milieu sels fondus a été abordée, ouvrant ainsi une nouvelle voie vers l'élaboration de phosphures de métaux de transition. / This PhD work deals with a novel synthesis of metal boride and metal phosphide nanomaterials. Nanostructures of these solids are subject to an increasing interest due to their exciting properties for various applications fields such as superconductivity, high temperature thermoelectricity, energy conversion and storage. In this work, the catalytic and electrocatalytic properties of these nanomaterials are explored. First of all, borides of various transition metals, such as nickel, palladium or a nickel-cobalt composite are studied. To do so, a new liquid-phase synthesis was developed, based on the reactivity of already formed metal nanoparticles with a boron precursor in inorganic molten salts. This new synthesis allowed a precise control over the nanoparticle morphology, size, composition and crystalline structure. By accessing such nanoscale objects, we were able to investigate their properties and performances, especially in the fields of catalysis with the hydrodeoxygenation reaction and electrocatalysis for the hydrogen evolution reaction and oxygen evolution reaction. Finally, the reactivity of red phosphorus in molten salts was addressed, thus paving the way to the extension of this synthetic pathway to metal phosphides.

La thèse s’inscrit dans le cadre d’un projet de recherche visant à élaborer une anode permettant le dégagement d’oxygène en milieu de cryolithe fondue à 960°C et dont l’utilisation supprimerait les émissions de dioxyde de carbone du procédé de production d’aluminium Hall-Héroult. Le type d’anode envisagée est un composite Cermet (CERamique – METal), généralement constitué d’une phase métallique à base de nickel, cuivre et fer et d’une ou plusieurs phase(s) oxyde(s) conductrice(s) du type NixFe3-xO4 et Ni1-xFexO. Deux types de cermet, biphasé et triphasé, ont été étudiés dans le cadre de ce projet. L’objectif principal a été de comprendre les mécanismes de dégradation de telles anodes durant les électrolyses. Les cermets ont été testés sous une densité de courant de 0,80 A/cm² pour différentes durées, puis analysés par microscopie optique et microscopie électronique à balayage couplée à une analyse dispersive en énergie. Les premières électrolyses, effectuées dans un électrolyte synthétique, sans aluminium métallique et sous atmosphère argon ont révélé à la fois une dégradation chimique et électrochimique. L’étude de la dégradation chimique a été réalisée grâce à des immersions dans l’électrolyte sans polarisation. Les résultats ont mis en évidence une dissolution de la phase spinelle, liée à un phénomène de substitution entre les ions Al3+ de l’électrolyte et Fe3+ du spinelle, entrainant la formation d’un aluminate du type NixFe3-x-yAlyO4. Lorsque x = 0 et y proche de 2, la phase spinelle est dissoute alors que lorsque x est proche de 1, la phase spinelle est stabilisée et sa dissolution ralentie. Les électrolyses menées jusqu’à la fin de vie des matériaux ont révélé l'attaque préférentielle de la phase métallique pour former des composés fluorés et oxydes. Grâce aux caractérisations micrographiques et aux calculs thermodynamiques (logiciel FactSage 7.1), un mécanisme de dégradation global des anodes a été proposé. La compréhension des mécanismes de dégradation chimique et électrochimique des matériaux a permis de proposer des voies d'amélioration concernant à la fois le matériau d'anode et la chimie du bain cryolithique. / The thesis is a part of a research project which develop an anode permitting the release of oxygen in molten cryolite medium at 960°C, in order to eliminate carbon dioxide emissions from the aluminium production process called Hall-Héroult process. The type of anode envisaged is a Cermet composite (CERamic - METal), generally consisting of a metallic phase based on nickel, copper and iron and one or more conductor oxide phase(s) such as NixFe3-xO4 and Ni1-xFexO type. Two types of cermet, two-phase and three-phase, have been studied in this project. The main objective was to understand the degradation mechanisms of such anodes during electrolysis. The cermets were tested at a current density of 0.80 A/cm² for different durations and analyzed by optical microscopy and scanning electron microscopy coupled with dispersive energy analysis. The first electrolyses, carried out in a synthetic electrolyte, without metallic aluminum and under argon atmosphere revealed both chemical and electrochemical degradations. Study of chemical degradation was carried out thanks to immersions in the electrolyte without polarization. The results showed a dissolution of the spinel phase, linked to a substitution phenomenon between the Al3+ ions from the electrolyte and Fe3+ from spinel, resulting in the formation of a NixFe3-x-yAlyO4 aluminate type. When x = 0 and y close to 2, the spinel phase is dissolved whereas when x is close to 1, the spinel phase is stabilized and its dissolution slowed down. Electrolyses conducted to the end of life of the materials revealed the preferential attack of the metal phase to form fluorinated compounds and oxides. Thanks to micrographic characterizations and thermodynamic calculations (FactSage 7.1 software), a global degradation mechanism of the anodes has been proposed. The understanding of the chemical and electrochemical degradation mechanisms of the materials made it possible to propose ways of improvement concerning both the anode material and the cryolithic bath chemistry.

In order to make thermal solar power compete with the traditional sources of energy, the efficiency must increase and one way of doing it is by changing the operating fluid. Among the alternate fluids is the use of molten salts as a part of the process; either for thermal storage and later utilization for electrical production during the hours without sun or as a substitute of the operating fluid to provide higher temperatures resulting in better efficiency. The difficulty of using molten salts is the lack of physical properties in literature; such as viscosity, boiling point, vapor pressure and volumetric absorption of solar radiation, thus making the selection of a suitable salt a very difficult endeavor. As a part of the Multidisciplinary Research Initiative (MURI) of the Department of Energy in the project of High Operating Fluids, this work will focus on the optical properties of the molten salts (volumetric absorption). The objective of this Thesis is to design, build and test a device capable of measuring the light attenuation coefficient; which is directly related to volumetric absorption of solar radiation, as well as determine the attenuation coefficient of various eutectic systems for the ternary salt mixture of ZnCl2, NaCl and KCl. Based on the little existent literature, a device capable of measuring the attenuation coefficient was designed, built, validated and tested. This was done by projecting a stable beam of light simulating sun radiation through the molten salt sample and to a spectrometer with a wavelength range going from 400 nm to 1000 nm with operating temperatures going from 350oC to 600oC. This device is capable of controlling the thickness, from 1 to 60 mm, of the molten salt sample by a computer controlled linear stage with an accuracy of 0.1mm. Quartz was used as a container for the molten salts because of its high melting point and transparency. A ceramic heater was used as a heat source, which can heat up the sample to temperatures up to 1200 oC if necessary. Two validation tests for the device were done by measuring the light attenuation coefficient of clear water and extra virgin olive oil and then they were compared to the ones in literature. The eutectic systems were tested next; the results characterized the attenuation coefficient as a function of wavelength and temperature, something that no other experimental work has done before for this specific fluid. These values will help to determine an optimal operating fluid for high temperature thermal applications.