Rozprawy doktorskie na temat „Synthesis in molten salts”

Compared with conventional synthesis techniques, the so-called molten salt synthesis (MSS) technique has attracted substantial interest and has been used extensively to synthesise a range of advanced materials because it offers several advantages: (1) the synthesis reaction can be completed at a relatively low temperature and within a short time; (2) the resultant product powders are generally well dispersed and have high surface reactivity; (3) the grain shapes (spheroidal, platelet-shaped or rod-like) and sizes (nanoscale to microscale) can be controlled; and (4) the process is easy to perform, scalable and cost effective. In this thesis, a molten-salt-mediated magnesiothermic reduction technique was used to synthesise high-quality boron-based fine powders, including titanium diboride (TiB2), hafnium diboride (HfB2), lanthanum hexaboride (LaB6), calcium hexaboride (CaB6), amorphous boron and alumina-titanium diboride (Al2O3-TiB2) nanocomposites, at relatively low temperatures, from relatively cheap oxide-based raw materials. The effects of the processing parameters, such as salt type, starting batch composition, and firing temperature and time, on the reaction extents were investigated, based on which, the synthesis conditions optimised and the responsible underlying mechanisms proposed. Among the three chloride salts (NaCl, KCl and MgCl2), MgCl2 showed the best accelerating effect on the MSS of amorphous boron, TiB2, HfB2 and LaB6.This finding could be explained by the higher solubility levels of Mg and MgO in molten MgCl2 than in the other two salts. When using appropriately excessive amounts of Mg and/or B2O3 to compensate for their evaporation losses at reaction temperatures, phase-pure TiB2, HfB2 and LaB6 fine powders of 100-200 nm were synthesised after 4-6 h firing in MgCl2 at 900-1000°C. In the MSS of amorphous boron, 15 mol% excessive Mg was used. High-purity amorphous boron fine powders with sizes of 100-200 nm were achieved after 6 h firing in MgCl2 at 900°C and further leaching with hot H2SO4 solution. In the MSS of CaB6, CaCl2 facilitated the overall synthesis more effectively than NaCl, KCl or MgCl2. Upon using 20 mol% excessive Mg, phase-pure CaB6 nanoparticles of ~50 nm were formed in CaCl2 after 6 h at 800°C. Unlike in the molten-salt-mediated magnesiothermic reduction technique, Al2O3-TiB2 nanocomposite powders were synthesised via the aluminothermic reduction in molten salt. In this case, NaCl was regarded as the most appropriate reaction medium. When using appropriately excessive amounts of Al and B2O3, phase-pure Al2O3-TiB2 nanocomposite powders were synthesised after 4 h at 1150°C or 5 h at 1050°C. These synthesis conditions were much milder than those required by many other techniques reported previously. The “dissolution-precipitation” mechanism was found to be more dominant in the overall MSS processes than the “template-growth” mechanism. The success of this work indicates that the MSS technique could be a promising alternative approach to low-temperature synthesis of a range of nanomaterials. In addition, the MSS route opens new possibilities for the synthesis of known materials as well as new materials with complex structures, such as new zeolitic materials, metal organic frameworks and polymer chemistry by utilising tailored salt systems.

Pure submicron-sized zirconium diboride (ZrB2) powder was synthesised via a molten salt mediated reduction route using ZrO2, Na2B4O7, and Mg powders as the starting raw materials and MgCl2 as the reaction medium. By using appropriately excessive amounts of Mg and Na2B4O7 to compensate for their evaporation losses, ZrO2 can be completely converted into ZrB2 after 3 h of firing at 1200 °C. This synthesis temperature is 100-500 °C lower than that of other conventional synthesis techniques. In addition, the formation of undesirable Mg3B2O6 can be effectively suppressed. To a large extent, the prepared ZrB2 particles preserved the shapes and sizes of the original ZrO2 particles, indicating a template growth mechanism for their formation in which ZrO2 functions as the reaction template. Using this developed synthesis method, submicron-sized and nanosized zirconium carbide (ZrC) powders were synthesised in the reaction system of ZrO2-Mg-carbon black using NaCl-KCl as the reaction medium. The synthesis temperatures were 950 and 850 °C for the former and the latter powder size, respectively, which are much lower temperatures than those used in most of the reported methods. Compared with the submicron-sized ZrO2 powders, the finer ZrO2 particles considerably enhanced the reaction rate and thus the completion of the reaction at a lower temperature. The resulting ZrC particles exhibited two different morphologies: one retained the shapes and sizes of the original ZrO2, and the other retained those of the starting carbon black, suggesting that both ZrO2 and carbon black had acted as reaction templates. In addition, the 2D-nanostructure of a non-layered structure material, titanium carbide (TiC), was fabricated. The novel TiC nanosheets (TNS) and TiC-coated graphite nanosheets (TCNS) were produced at 950 °C for 8 h and 900 °C for 5 h, respectively, in KCl molten salt using graphite nanosheets (GNS) as both a carbon source and reaction templates. The produced TNS and TCNS retained the shapes, sizes and thickness of the original GNS to a high degree, indicating that the GNS had acted as the reaction template. For TCNS, a lower molar ratio of Ti/C required a lower synthesis temperature and/or a shorter holding time. This effective processing technique was also employed to produce TiC foams at 1050 °C for 4 h using carbon foam as the reaction template. This synthesis temperature is significantly lower than that (>1450 °C) used in most of the other techniques. The resultant pores were clear of any undesired phases such as impurities and/or membranes, and the cell-networks were free of surface cracks and holes. The cell-networks, pore sizes, and cell sizes of the synthesised foams were well defined by those of the original carbon foam, suggesting a template growth mechanism of the formation of the TiC foam.

Les siliciures de métaux de transition sont une famille de composés intermétalliques, qui ont été largement étudiés en tant que matériaux fonctionnels dans les circuits intégrés, la thermoélectricité, la supraconductivité, le magnétisme et la catalyse hétérogène. La nanostructuration offre la possibilité repousser les frontières de la science de ces matériaux avec de nouvelles phases et des propriétés diverses. Cependant, l'énergie de liaison relativement élevée des siliciures de métaux de transition nécessite généralement une température élevée pour leur formation, ce qui n'est pas propice à la conception de nanomatériaux et n'est pas compatible avec les méthodes de synthèse colloïdales traditionnelles. Dans cette thèse, des synthèses en sels fondus basées sur l'insertion d'éléments dans des nanoparticules sont développées. Des nanoparticules de siliciure de métal de transition (M-Si, M=Ni, Fe, NiFe, Co) et un silicophosphure de nickel ternaire sont cristallisés dans des solvants inorganiques à haute température, où un environnement dilué et sans carbone est fourni. Les nanoparticules de siliciures obtenues sont étudiées en électrocatalyse de l'oxydation de l'eau alcaline et du magnétisme. Les siliciures de NiFe démontrent une activité et une stabilité exceptionnelles résultant d'une structure originale cœur-coquille-coquille générée in situ, tandis que les nanoparticules de CoSi riches en défauts présentent un ferromagnétisme inhabituel. De plus, l'étude des nanoparticules de silicophosphure donne un aperçu de la conception de matériaux multinaires dans les sels fondus et du rôle des éléments non métalliques dans l’électrocatalyse de l’électrolyse de l’eau
Transition metal silicides are a family of intermetallic compounds, which have been widely studied as functional materials in integrated circuits, thermoelectricity, superconductivity, magnetism and heterogeneous catalysis. Nanostructuration offers the opportunity to extend the frontier of silicon-based materials science with novel phases and diverse properties. However, building transition metal silicides encompassing relatively high energy bonds usually requires high temperatures, which are not conducive for nanomaterial design and not compatible with the traditional colloidal synthesis methods. In this thesis, molten salts syntheses based on element insertion into nanoparticles are developed. Transition metal silicide nanoparticles (M-Si, M=Ni, Fe, NiFe, Co) and a ternary nickel silicophosphide are crystallized in high temperature inorganic solvents, where a diluted and carbon-free environment is provided. The obtained silicide nanoparticles are investigated in electrocatalysis of alkaline water oxidation and magnetism. NiFe silicides demonstrate outstanding activity and stability arising from an original in situ generated core-shell-shell structure, while defect-rich CoSi nanoparticles show an unusual surface related ferromagnetism. Moreover, the study of silicophosphide nanoparticles provides an insight in multinary material design in molten salts and the role of nonmetal elements in overall alkaline water splitting electrocatalysis

Kyoto University (京都大学)
0048
新制・課程博士
博士(エネルギー科学)
甲第11699号
エネ博第115号
新制||エネ||29(附属図書館)
23342
UT51-2005-D448
京都大学大学院ｴﾈﾙｷﾞｰ科学研究科ｴﾈﾙｷﾞｰ基礎科学専攻
(主査)教授 尾形 幸生, 教授 片桐 晃, 助教授 萩原 理加
学位規則第4条第1項該当

The increasing miniaturization of electric and mechanical components makes the synthesis and assembly of nanoscale structures an important step in modern technology. Functional materials, such as the ferroelectric perovskites, are vital to the integration and utility value of nanotechnology in the future. In the present work, chemical methods to synthesize one-dimensional (1D) nanostructures of ferroelectric perovskites have been studied. To successfully and controllably make 1D nanostructures by chemical methods it is very important to understand the growth mechanism of these nanostructures, in order to design the structures for use in various applications. For the integration of 1D nanostructures into devices it is also very important to be able to make arrays and large-area designed structures from the building blocks that single nanostructures constitute. As functional materials, it is of course also vital to study the properties of the nanostructures. The characterization of properties of single nanostructures is challenging, but essential to the use of such structures. The aim of this work has been to synthesize high quality single-crystalline 1D nanostructures of ferroelectric perovskites with emphasis on PbTiO3 , to make arrays or hierarchical nanostructures of 1D nanostructures on substrates, to understand the growth mechanisms of the 1D nanostructures, and to investigate the ferroelectric and piezoelectric properties of the 1D nanostructures. In Paper I, a molten salt synthesis route, previously reported to yield BaTiO3 , PbTiO3 and Na2Ti6O13 nanorods, was re-examined in order to elucidate the role of volatile chlorides. A precursor mixture containing barium (or lead) and titaniumwas annealed in the presence of NaCl at 760 °C or 820 °C. The main products were respectively isometric nanocrystalline BaTiO3 and PbTiO3. Nanorods were also detected, but electron diffraction revealed that the composition of the nanorods was respectively BaTi2O5/BaTi5O11 and Na2Ti6O13 for the two different systems, in contradiction to the previous studies. It was shown that NaCl reacted with BaO(PbO) resulting in loss of volatile BaCl2 (PbCl2 ) and formation and preferential growth of titanium oxide-rich nanorods instead of the target phase BaTiO3 (or PbTiO3 ). The molten salt synthesis route may therefore not necessarily yield nanorods of the target ternary oxide as reported previously. In addition, the importance of NaCl(g) for the growth of nanorods below the melting point of NaCl was demonstrated in a special experimental setup, where NaCl and the precursors were physically separated. In Paper II and III, a hydrothermal synthesis method to grow arrays and hierarchical nanostructures of PbTiO3 nanorods and platelets on substrates is presented. Hydrothermal treatment of an amorphous PbTiO3 precursor in the presence of a surfactant and PbTiO3 or SrTiO3 substrates resulted in the growth of PbTiO3 nanorods and platelets aligned in the crystallographic <100> orientations of the SrTiO3 substrates. PbTiO3 nanorods oriented perpendicular to the substrate surface could also be grown directly on the substrate by a modified synthesis method. The hydrothermal method described in Paper II and III was developed on the basis of the method described in Appendices I and II. In Paper IV, a template-assisted method to make PbTiO3 nanotubes is presented. An equimolar Pb-Ti sol was dropped onto porous alumina membranes and penetrated into the channels of the template. Single-phase PbTiO3 perovskite nanotubes were obtained by annealing at 700 °C for 6 h. The nanotubes haddiameters of 200 - 400 nm with a wall thickness of approximately 20 nm. Excess PbO or annealing in a Pb-containing atmosphere was not necessary in order to achieve single phase PbTiO3 nanotubes. The influence of the heating procedure and the sol concentration is discussed. In Paper V, a piezoresponse force microscopy study of single PbTiO3 nanorods is presented. The piezoelectric properties were studied in both vertical and lateral mode. Piezoelectric activity and polarization switching was observed in the vertical mode, demonstrating the ferroelectric nature of the nanorods. The nanorods decomposed after repeated cycling of the dc bias at one spot on the nanorod, which resulted in parts of the nanorod disappearing and/or accumulation of particles on the surface of the nanorod. In Paper VI, a method to contact single nanorods by electron beam induced deposition of platinum is presented. An organometallic compound, (trimethyl)-methylcyclopentadienylplatinum(IV), was used as precursor. A home-made apparatus was constructed for the purpose and was mounted onto a scanning electron microscope. Calculations based on apparatus geometry and molecular flow were used to estimate the deposition time and the height of the deposits. The location and height of the deposits were controlled so that single nanorods could be successfully contacted at the ends of the nanorods. Fabrication of a sample device for piezoresponse force microscopy studies of single nanorods using an axial dc bias setup is described in Appendix IV. A proposed experimental setup for such studies is also presented.

Les composés contenant du bore présentent plusieurs propriétés physiques exploitables pour les besoins industriels actuels, à savoir l'activité catalytique, le magnétisme, la supercapacité, la capacité de stockage élevée du Li+ et d'excellentes propriétés mécaniques. La plupart de ces propriétés peuvent être adaptées et idéalement optimisées en façonnant le matériau en morphologies nanostructurées. Cependant, la nature fortement covalente des liaisons chimique impliquant le bore entrave la synthèse des nanostructures, car un apport d'énergie élevé est nécessaire pour former ces liaisons. Cela se traduit par des températures de synthèse élevées qui, en fin de compte, déclenchent également la croissance des grains. La synthèse de sels fondus a suscité un intérêt considérable en tant qu'outil de synthèse pour produire des nanostructures. Les sels fondus permettent d'effectuer des réactions chimiques dans un milieu liquide à des températures suffisamment élevées pour déclencher la cristallisation des borures, mais suffisamment douces pour limiter leur croissance. Malgré son succès, le contrôle de la morphologie du produit reste un défi important. Dans certains cas, ce problème peut être surmonté par des méthodes isomorphes, c'est-à-dire en utilisant des nanoparticules comme précurseurs, qui subissent une restructuration interne, de sorte qu'elles puissent également servir de nano-gabarits. Dans ce travail de thèse, l'utilisation de nano-gabarits couplée à la synthèse de nanomatériaux par sels fondus a été explorée pour deux systèmes difficiles à base de bore. Tout d'abord, des nanoparticles de carbure de bore ont été synthétisées à partir de gabarits de carbaborure de sodium, eux-mêmes synthétisés dans des sels fondus. L'intérêt de produire des nanostructures de carbure de bore a été largement reconnu dans la littérature comme un moyen d'améliorer sa dureté et son utilisation durable en tant que matériau de structure. La synthèse du gabarit a été réalisée grâce à la réaction entre une source de carbone polymère (polyéthylène) et NaBH4 dans du NaI fondu, ce qui a fourni des nanoparticules de ~ 5 nm. Ces nanoparticules ont alors pu être transformées avec succès en carbure de bore par décomposition thermique tout en conservant la morphologie nanométrique . En outre, la transformation du carbure de bore en monolithes denses a également été étudiée au moyen du frittage par plasma en courant pulsé. Une fois la densification et la consolidation réalisées, les propriétés mécaniques des nanostructures de carbure de bore ont été étudiées. Nous avons ainsi mis en évidence une dureté et une résistance à l'amorphisation significativement supérieures à celles de leur homologue massif. En parallèle, un système de borure métallique lamellaire a également été étudié avec des procédures analogues. Le système en question est Fe2AlB2, constitué de couches de Fe2B2 intercalées par des couches d'Al. Cette phase suscite un énorme intérêt en tant que précurseur possible vers le Fe2B2 bidimensionnel. La synthèse du Fe2AlB2 présente cependant plusieurs difficultés. Nous avons ici exploité l'approche du gabarit dans les sels fondus à partir d'un précurseur de FeB. Nous avons démontré que l'insertion d'Al en milieu LiCl/KCl fondu fourni bien Fe2AlB2. La délamination de la phase Fe2AlB2 vers 2D-Fe2B2 a été étudiée par oxydation sélective des atomes d'Al. Bien que la délamination n'ait pas été atteinte, nous avons mis en évidence un comportement thermique anormal dans Fe2AlB2. Lorsqu'il est traité thermiquement, Fe2AlB2 expulse des atomes de Fe et de B de la structure, générant ainsi des lacunes. Ce mécanisme a été démontré par diffraction des rayons X in situ et par des analyses post mortem
Boron-containing compounds exhibit several physical properties exploitable for current industrial needs, i.e. catalytic activity, magnetism, supercapacitance, high Li+ storage capacity and excellent mechanical properties. Most of these properties can be tailored and ideally optimized by shaping the material into nanostructured morphologies. However, the strong covalent nature of boron bonding hurdles the synthesis of nanostructures, as high input energy is needed to form such bonds. This translates in elevated synthesis temperatures, which ultimately also trigger grain growth. Molten-salts synthesis has gained considerable attention as a synthetic tool to yield nanostructures. Molten-salts permit to perform chemical reactions under a liquid media in a range of temperatures sufficiently large to trigger borides crystallization, but soft enough to limit their growth. Despite its success, the control over the product’s morphology remains a significant challenge. In some cases, this can be overcome by isomorphic methods, i.e., using nanoparticles as precursors, which undergo internal restructuration, so that they could also act as nanotemplates. In this thesis work, the use of nanotemplates coupled to molten salts synthesis of nanomaterials has been explored for two challenging boron-based systems. Firstly, boron carbide nanostructures were synthesized from sodium carbaboride templates, themselves synthesized in molten salts. The interest behind producing boron carbide nanostructures has been largely recognized in the literature, as a way to ameliorate its hardness and durable use as a structural material. The template synthesis is achieved thanks to the reaction between a polymeric carbon source (polyethylene) and NaBH4 in molten NaI, which yield ~ 5 nm nanoparticles. These nanoparticles can be successfully transformed to boron carbide while maintaining the nanoscale morphology by thermal decomposition. Furthermore, the processing of boron carbide into dense monoliths was also studied by means of spark-plasma sintering. Once proper densification and consolidation were achieved, the mechanical properties of the boron carbide nanostructures were investigated. We then highlighted a significantly higher hardness and amorphization resistance than the bulk counterpart. In parallel, a layered metal boride system has also been investigated with analogous procedures. The system in question is Fe2AlB2, consisting of Fe2B2 layers intercalated by Al layers. This phase has raised enormous interest as a possible parent phase towards bidimensional Fe2B2. The synthesis of Fe2AlB2 presents several difficulties though. We have herein exploited the templating approach in molten salts from a bulk FeB template, which we demonstrate that upon Al insertion in molten LiCl/KCl yields Fe2AlB2. The Fe2AlB2 phase delamination towards 2D-Fe2B2 was investigated by selective oxidation of the Al atoms. Although delamination did not occur, we evidenced an abnormal thermal behaviour in Fe2AlB2. When thermally treated, Fe2AlB2 expulses Fe and B atoms out of the structure, generating vacancies. This mechanism was demonstrated by in situ X-ray diffraction and post mortem analyses

Les borures et phosphures de métaux de transition présentent des propriétés intéressantes en électrocatalyse pour la production de dihydrogène. Dans ces matériaux, l’élément du bloc p modifie la densité électronique des atomes métalliques, ce qui rend non seulement ces matériaux résistants à l’oxydation et à la corrosion, mais modifie aussi les propriétés catalytiques. Le développement de catalyseurs requiert la conception d’objets de rapport surface/volume élevé, donc à l’échelle nanométrique. Il est alors essentiel de développer des voies de synthèse adaptées à l’obtention de ces matériaux et à la recherche de nouveaux composés. Notamment la combinaison du bore avec du phosphore et/ou un ou plusieurs métaux de transition au sein d'un même composé pourrait engendrer des modifications profondes, notamment de la densité électronique des sites métalliques et de la géométrie des sites de surface en raison de la formation de structures cristallines spécifiques, contraintes par la présence de liaisons covalentes. Au vu des propriétés des phases binaires, l’ensemble des modifications en termes de morphologie, géométrie et composition permet d’envisager un contrôle de la réactivité et des propriétés (électro)catalytiques. Ce travail de thèse a ainsi porté sur le développement de borures et borophosphures de métaux de transition avec pour objectifs un contrôle morphologique à l'échelle nanométrique, le développement de nouveaux composés, y compris métastables, et la caractérisation de leurs propriétés électrocatalytiques. Les approches de synthèse choisies reposent sur deux piliers. D’abord les sels fondus, qui permettent de réaliser des synthèses à l'état liquide et à des températures intermédiaires entre celles de la chimie du solide et celles des voies colloïdales. Ainsi, les milieux réactionnels sont homogènes, permettent une réactivité accrue entre précurseurs solubles ou dispersés, et limitent la coalescence des particules. L’autre pilier est l'utilisation de nanoparticules métalliques comme nanoréacteurs dans lesquels le bore et/ou le phosphore sont incorporés, de façon à maintenir la morphologie des particules. Nous obtenons ainsi des nanomatériaux de phases pures dans le cas des borures de nickel et de fer, avec un contrôle de la composition et de la morphologie jamais obtenu auparavant. Ce contrôle n’a pu être atteint qu’en comprenant les mécanismes de formation des composés et nano-objets correspondant. Cette compréhension a pu être obtenue en développant le suivi in situ des synthèses grâce à la diffraction des rayons X sous rayonnement synchrotron. Nous avons ainsi pu mettre en évidence des intermédiaires réactionnels sous forme de nanoparticules amorphes, dont la nature a été élucidée grâce à la diffusion totale des rayons X couplée à la fonction de distribution de paires. Par extension, cette voie particulière de synthèse permet aussi la recherche de phases rares ou jamais reportées et pouvant présenter une importante activité électrocatalytique pour la dissociation de l'eau. Le choix des domaines de composition d'intérêt, guidé par l’apprentissage automatique pour la minimisation des énergies de formation, a ainsi permis de réaliser la synthèse de nanoparticules d’un borure bimétallique de nickel et cobalt bore. Nous avons aussi progressé dans l'étude de nouvelles voies de synthèse de phases ternaires borophosphures de nickel, rares et difficilement atteignables. Enfin, nous avons abordé l’exploration d'une voie de synthèse de nanoparticules métalliques exemptes de ligands organiques de surface, afin d'éviter des réactions secondaires ou des mécanismes compétitifs de diffusion au sein des métaux à partir des ligands, lors des synthèses en sels fondus. En nous basant sur des travaux démontrant la stabilité colloïdale de nanomatériaux dans des sels fondus, nous décrivons une méthode rapide et simple de synthèse de nanoparticules métalliques basés sur les sels fondus avec le nickel comme cas d'étude
Transition metal borides and phosphides present interesting properties in electrocatalysis for the production of dihydrogen. In these materials, the p-block element modifies the electron density of the metal atoms, which not only makes these materials resistant to oxidation and corrosion, but also modifies the catalytic properties. The development of catalysts requires the design of objects with a high surface/volume ratio, i.e. at the nanoscale. It is therefore essential to develop synthesis routes adapted to obtaining these materials and to the search for new compounds. In particular, combining boron with phosphorus and/or one or more transition metals in the same compound could lead to profound changes, notably in the electron density of the metal sites and the geometry of surface sites, due to the formation of specific crystalline structures constrained by the presence of covalent bonds. In view of the properties of the binary phases, all the modifications in terms of morphology, geometry and composition make it possible to control the reactivity and (electro)catalytic properties. This thesis work focused on the development of transition metal borides and borophosphides, with the objectives of morphological control on the nanometric scale, the development of new compounds, including metastable ones, and the characterisation of their electrocatalytic properties. The synthesis approaches chosen are based on two pillars. Firstly, molten salts, which enable synthesis to be carried out in the liquid state and at temperatures intermediate between those of solid-state chemistry and those of colloidal routes. Thus, the reaction media are homogeneous, enabling greater reactivity between soluble or dispersed precursors, and limiting the coalescence of particles. The other pillar is the use of metallic nanoparticles as nanoreactors in which boron and/or phosphorus are incorporated, so as to maintain the morphology of the particles. In this way, we obtain pure-phase nanomaterials in the case of nickel and iron borides, with a degree of control over composition and morphology that has never been achieved before. This control could only be achieved by understanding the formation mechanisms of the corresponding compounds and nano-objects. This understanding was achieved by developing in situ monitoring of the syntheses using X-ray diffraction under synchrotron radiation. We were thus able to identify reaction intermediates in the form of amorphous nanoparticles, the nature of which was elucidated using total X-ray scattering coupled with the pair distribution function. By extension, this particular synthesis route also enables us to search for rare or never reported phases that may exhibit significant electrocatalytic activity for the dissociation of water. The choice of compositional domains of interest, guided by machine learning to minimise formation energies, led to the synthesis of nanoparticles of a bimetallic boride of nickel and cobalt boron. We have also made progress in studying new synthesis routes for nickel borophosphide ternary phases, which are rare and difficult to obtain. Finally, we explored a route for synthesising metal nanoparticles without organic surface ligands, in order to avoid secondary reactions or competitive diffusion mechanisms within the metals from the ligands during molten salt synthesis. Based on work demonstrating the colloidal stability of nanomaterials in molten salts, we describe a rapid and simple method for synthesising metal nanoparticles based on molten salts using nickel as a case study

Les oxydes pérovskites sont étudiés depuis des décennies pour leurs propriétés magnétiques et plus récemment leurs propriétés électrocatalytiques dans le contexte de la conversion d’énergie, dans les piles à combustibles et les électrolyseurs, notamment. Dans ce travail, nous avons cherché à développer de nouveaux électrocatalyseurs à base de pérovskites opérant en milieu aqueux, en visant des objets nanométriques capables d’interagir avec leur environnement et donc de présenter des propriétés électrocatalytiques dépendant de stimuli extérieurs. Ce travail se situe donc au croisement entre le développement de méthodes de synthèse, la conception de nanomatériaux, l’étude de leurs propriétés électrochimiques et magnétiques et du lien entre ces propriétés. En premier lieu, nous décrivons une stratégie de synthèse adaptée à l’obtention de pérovskites quaternaires en utilisant des milieux sels fondus, un chauffage micro-ondes innovant, et l’ajustement des conditions d’oxo-basicité du milieu fondu. Nous étudions ensuite la relation entre magnétisme et électrocatalyse dans le cas d’école de nanocristaux de La0.7Sr0.3MnO3 obtenus en sels fondus. Nous développons ainsi un diapositif permettant de déclencher des changements de propriétés électrocatalytiques pour la réduction du dioxygène en appliquant un champ magnétique. Nous mettons en avant l’importance de la composition de l’électrolyte pour l’amélioration des propriétés électrocatalytiques sous champ magnétique. Enfin, nous abordons un autre moyen de modifier par un stimulus les propriétés de pérovskites nanométriques, en développant la synthèse de nanocristaux de pérovskite dopées avec divers métaux, puis en déclenchant pour la première fois l’exsolution de ces cations métalliques à la surface des matrices pérovskites nanométriques
Perovskite oxides are studied since decades for their magnetic properties and more recently for their electrocatalytic properties in the context of energy conversion, within fuel cells and water electrolyzers, for instance. In this work, we have aimed at developing new perovskite-based electrocatalysts operating in aqueous media, by targeting nanoscaled perovskites able to interact with their environment and then to exhibit stimuli-dependent electrocatalytic properties. This work was then at crossroad between the development of synthesis strategies, the design of nanomaterials, the study of their electrochemical and magnetic properties and of their interplay. We first describe a synthesis strategy suitable to reach quaternary perovskites by using molten salt media, innovative microwave heating and precise tuning of the oxo-basicity of the melt. We then address the relationship between magnetism and electrocatalysis on the case study of La0.7Sr0.3MnO3 nanocrystals derived from molten salts, through the development of an original set-up enabling triggering changes in the oxygen reduction reaction electrocatalysis by application of a magnetic field. We especially stress out the important of the composition of the electrolyte for magnetic enhancement of electrocatalytic activity. Finally, we address another way to trigger “externally” the properties of nanoscaled perovskite oxides, by developing a synthesis of doped perovskite nanocrystals and then by triggering under reducing conditions the exsolution of the metal cation dopants as metal nanoparticles at the surface of perovskite nanocrystal hosts, for the first time

Kyoto University (京都大学)
0048
新制・課程博士
博士(エネルギー科学)
甲第14961号
エネ博第204号
新制||エネ||46(附属図書館)
27399
UT51-2009-M875
京都大学大学院エネルギー科学研究科エネルギー基礎科学専攻
(主査)教授 萩原 理加, 教授 尾形 幸生, 准教授 野平 俊之
学位規則第4条第1項該当

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Fósforos a base de silicatos de Ca e Mg foram preparados pelo método sol-gel combinado com o processo de sais fundidos. O gel de sílica foi obtido a partir da solução de Na2SiO3 usando soluções de cloretos de európio, disprósio, cálcio e magnésio. Assim, estes cloretos foram homogeneamente distribuídos no gel. O gel obtido foi seco e tratado termicamente a 900°C por 1h para permitir a fusão dos sais presentes. Em seguida o material foi lavado com água até teste negativo para íons Cl- e seco em estufa a 80°C. A redução do európio para Eu2+ foi realizada em um forno sob atmosfera de 5% de H2 e 95% de Ar a 900°C por 3h para obter os fósforos de CaMgSi2O6:Eu2+ e CaMgSi2O6:Eu2+:Dy3+. Nos difratogramas de DRX das amostras, a diopsita foi identificada como fase cristalina principal e quartzo, como a secundária. Micrografias obtidas por MEV (microscopia eletrônica de varredura), das amostras, mostraram morfologia acicular, esférica, folhas e bastonetes das partículas e dos aglomerados . Curva de análise térmica (TGA-DTGA) revelou que a temperatura de cristalização do CaMgSi2O6:Eu2+ é próxima de 765°C. Estudos de espectroscopia de fotoluminescência foram baseados nas transições interconfiguracionais 4fN → 4fN-1 5d do íon Eu2+. O espectro de excitação apresentou banda larga relativa à transição de transferência de carga ligante metal (LMCT) O2- (2p) → Eu3+ na região de 250 nm e bandas finas oriundas das transições 4f → 4f do íon Eu3+ , mostrando a transição 7F0 → 5L6 em 393 nm quando a emissão é monitorada em 583,5 nm. E o espectro de emissão com excitação monitorada em 393 nm apresentou picos finos entre 570 e 750 nm característicos das transições 5D0 → 7 FJ (J = 0 - 5) do íon Eu3+ , indicando que o íon Eu3+ se encontra em um sítio com centro de inversão. Os resultados obtidos indicam que o método desenvolvido é viável na síntese de fóforos, CaMgSi2O6:Eu2+ e CaMgSi2O6:Eu2+:Dy3+ como foi proposto.
Dissertação (Mestrado em Tecnologia Nuclear)
IPEN/D
Instituto de Pesquisas Energéticas e Nucleares - IPEN-CNEN/SP

PONTIFÍCIA UNIVERSIDADE CATÓLICA DO RIO DE JANEIRO
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.
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.

As the currently available methods for recycling of valuable metals from batteries and old electronics (commonly called eWaste) are in need of improvement, this project focuses on the development of a novel valuable metals recovery method by electrolysis in molten salts. The process proposed consists of three steps: metal oxides dissolution in borate salts, liquid-liquid interface ion transfer between the borate and chloride layer, and electrodeposition from the chloride phase. Inherent borate salts stability and its affinity to metals, coupled with the chloride salts large electrochemical window enables a stable and efficient (semi)-continuous process concept to be explored. Two electrolytic cell concepts akin to an industrial set-up were designed. The first composed of three interconnected chambers each for one of the three steps of the process, or a simpler, single-vessel solution relying on the immiscibility of the molten phases. For the needs of a laboratory scale testing the smaller, one vessel solution has been assembled. The proposed recycling method is a novel solution for the recovery of valuable metals considered and evaluated in this work; Co, Cu, Ni, and Mn, present in most Li-ion and Ni-MH batteries, but also other metals suitable for electrodeposition present in the eWaste or other metal-rich waste streams. The process proposed was designed, evaluated and resulted in a successful recovery of all of the metals considered. Novel and promising experimental data on the metal oxides dissolution in molten borate salts is reported. Boron oxide salts were assessed, with the sodium borate achieving significant metals concentrations ranging from 4-20 wt%. Metals distribution between the oxide and halide layers was evaluated, and was found to be biased towards the borate layer due to its structure resulting in high metal affinity, with the metal ions concentration in the chloride layer around 1 wt% for the evaluated salts combination. This enables the sodium borate phase to work as a buffer, feeding the dissolved metal required for the electrodeposition into the chloride layer sustaining the process. Liquid-liquid interface transfer and diffusion phenomena in the melt as well as the metal electrodeposition parameters were studied using a range of (electro)-analytical methods, validating the main steps of the proposed metal recovery process. The system was evaluated in a three-electrode set-up (WE: tungsten, CE; graphite, QRE: tungsten) and the formal redox reaction potentials were reported for the following feedstock: Co2O3 [-0.733/-1.848 V], CuO [-1.297/-2.375 V], Mn2O3 [-1.552 V] and NiO [-1.734 V] versus chlorine evolution. The recovered metals were analysed and found to form high purity (~99 %) dendritic deposits (SA/V of 950 cm-1), which also supports the assumption of a diffusion controlled process. This marks the successful outcome of this proof-of-concept process, providing a feasible, alternative valuable metals recovery method design.

This study focused on molten salt purification processes to effectively reduce or eliminate the corrosive contaminants without altering the salt's chemistry and properties. The impurity-driven corrosion behavior of HAYNES® 230® alloy in the molten KCl-MgCl2-NaCl salt was studied at 800 ºC for 100 hours with different salt purity conditions. The H230 alloy exhibited better corrosion resistance in the salt with lower concentration of impurities. Furthermore, it was also found that the contaminants along with salt's own vaporization at high temperatures severely corroded even the non-wetted surface of the alloy. The presence of Mg in its metal form in the salt resulted in an even higher mass-loss possibly due to Mg-Ni interaction. The study also investigated the corrosion characteristics of several nickel and ferrous-based alloys in the molten KCl-MgCl2-NaCl salt. The average mass-loss was in the increasing order of C276 < SS316L < 709-RBB* < IN718 < H230 < 709-RBB < 709-4B2. The corrosion process was driven by the outward diffusion of chromium. However, other factors such as the microstructure of the alloy i.e. its manufacturing, refining, and heat-treatment processes have also shown to influence the corrosion process. Lowering the Cr content and introducing W and Mo in the alloy increased its resistance to corrosion but their non-uniform distribution in the alloy restricted its usefulness. To slow-down the corrosion process, and enhance the material properties, selected alloys were boronized and tested for their compatibility in the molten KCl-MgCl2-NaCl salt. The borided alloys exhibited better resistance to molten salt attack, where the boride layer in the exposed alloy was still intact, non-porous, and strongly adhered to the substrate. The alloys also did not show any compensation in their properties (hardness). It was also found that the boride layer always composed of an outermost silicide composite layer, which is also the weakest and undesired layer as it easily cracks, breaks, or depletes under mechanical and thermal stresses. Various different grades of "virgin" nuclear graphites were also tested in the molten KF-UF4-NaF salt to assist in the selection of tolerable or impermeable graphites for the MSR operational purposes. It was found that molten salt wettability with graphite was poor but it still infiltrated at higher pressure. Additionally, the infiltration also depended on the pore-size and porosity of the graphite. The graphite also showed severe degradation or disintegration of its structure because of induced stresses.
Doctor of Philosophy
Molten salts are considered as potential fuel and coolant candidates in MSRs because of their desirable thermophysical properties and heat-transfer capabilities. However, they pose grave challenges in material selection due to their corrosive nature, which is attributed to the impurities and their concentration (mostly moisture and oxygen-based) in the salt. This study focused on purifying the salt to reduce these contaminants without compromising its composition and properties. The influence of purification processes on the corrosion behavior of HAYNES® 230® alloy was studied in the molten chloride salt with different purity conditions. Various nickel and ferrous-based alloys were also studied for their compatibility in the molten chloride salt. This will assist in expediting the material selection process for various molten salt applications. It was observed that several factors such as alloy composition, its microstructure, impurities in the salt attribute to molten salt corrosion. It was also quite evident that corrosion in molten salts is inevitable and hence, the focus was shifted on slowing down this process by providing protective barriers in the form of coatings (i.e. boronization). The borided (coated) alloys not only improved the corrosion resistance but also enhanced and retained their properties like hardness after exposure to molten salts. Since these studies were conducted under static conditions, a more detailed investigation is needed for the selected alloys by subjecting them to extreme flow-conditions and for longer a duration of time. To achieve this objective, a forced circulation molten salt loop was designed and fabricated to conduct flow corrosion studies for alloys in molten chloride salt. Graphite is another critical component of the MSR where it is used as a moderator or reflector. Generally, molten salts exhibit poor wettability with graphite, but they can still infiltrate (graphites) at higher applied pressures, and result in the degradation or disintegration of graphite's structure, and eventually its failure in the reactor. This study provides infiltration data, and understanding of the degradation of various 'virgin' nuclear graphite grades by the molten fluoride salt. This should assist in the selection of tolerable or impermeable graphite grades for MSR operational purposes.

Les préoccupations actuelles concernant l'exploitation sécuritaire des centrales nucléaires existantes et la conception d’architectures spéciales envisagées pour la quatrième génération de réacteurs nucléaires se combinent avec l’intérêt plus prononcé pour l'efficacité et la fiabilité de l'équipement d'un système énergétique. Cela fait que dans un souci de meilleure compréhension et optimisation basée sur des moyens modernes de conception assistée par l’ordinateur, l'étude de différentes parties de ces systèmes fait de plus en plus l’objet de recherches approfondies. . Parmi les types proposés pour la quatrième génération de réacteurs nucléaires font partie ceux ayant comme agent de refroidissement les sels fondus, respectivement le sodium fondu. En raison de leurs propriétés physiques, les sels fondus et le sodium liquide ont le potentiel d'être déplacés par les pompes électromagnétiques. Cet ouvrage est à la fois une étude approfondie des phénomènes qui se produisent en raison de l'écoulement du fluide dans le champ électromagnétique d’une pompe électromagnétique - interaction magnétohydrodynamique - et un rapport sur les capacités et les avantages des outils informatiques modernes pour faciliter la conception et l'optimisation des pompes électromagnétiques. Afin d’atteindre l'objectif principal de la thèse, notamment une meilleure compréhension des phénomènes interdépendants spécifique à l’opération des pompes électromagnétiques, deux objectifs secondaires ont été considérés. Le premier objectif concerne la pleine exploitation des modèles électromagnétiques numériques en éléments finis afin d'obtenir autant d'informations que possible sur le comportement des pompes électromagnétiques, dans l’hypothèse où on ne tient pas compte de l'écoulement des fluides. Le deuxième objectif était la construction des modèles numériques qui réunissent l’électromagnétisme et la dynamique des fluides, respectivement des modèles numériques qui regroupent les deux phénomènes caractérisant l’écoulement magnétohydrodynamique dans les canaux des pompes électromagnétiques. Dans l'étude dédiée au pompage électromagnétique de sels fondus, la thèse met en évidence des problèmes spécifiques liés à la génération de forces électromagnétiques dans les fluides faibles conducteurs d'électricité et fournit des résultats sur les applications où le pompage électromagnétique de sels fondus peut être efficace. À l’aide des modèles électromagnétiques nous avons obtenu des informations importantes sur l'influence du nombre de pôles électromagnétiques et de la fréquence d’alimentation sur la caractéristique Pression - Vitesse des pompes annulaires à induction. Ils ont été analysés le phénomène de blindage créé par les parois métalliques - avec des répercussions négatives sur les performances de la pompe, les effets de freinage exercés à l'entrée et à la sortie du canal de la pompe et la relation entre la capacité de surcharge hydrodynamique et la caractéristique Pression - Vitesse des pompes à induction. Une section spéciale a été consacrée à l'analyse de la dépendance de la force électromagnétique du temps et de l’espace et à l’étude de non-uniformitées des quantités de nature électromagnétique en direction azimutale dans les pompes annulaires à induction.Dans le chapitre qui traite de l'interaction magnétohydrodynamique à base de modèles couplés, ils sont proposés deux modèles qui couplent l'électromagnétisme et l’écoulement. Ils sont présentés les avantages des modèles couplés, en matière de la précision des résultats, par rapport aux modèles électromagnétiques. Il est également présenté l'évolution des profils de vitesse, de densité de force et de courant sous l'influence du champ électromagnétique et de l’écoulement de sodium à vitesses différentes. Les contributions de la thèse sont complétées par des observations importantes sur les méthodes de travail et les logiciels utilisés tout au long de l'étude
The actual concerns with respect to safe operation of existing nuclear plants and to designing special architectures envisaged for the fourth generation of nuclear reactors, corroborated with the increasing interest for efficiency and reliability of any equipment belonging to an energetic system, make that more and more research endeavors to be devoted to the study of various parts of these systems for a better understanding and optimization based on modern techniques of computer aided design. Among the types proposed for the fourth generation of nuclear reactors belong those that have as cooling agent molten salts, respectively liquid sodium. Many reactors of previous generations use mechanical pumps of special design for driving the coolants. Molten salts and liquid sodium, thanks to their physical properties, have the potential to be driven using electromagnetic pumps. Although the technology of electromagnetic pumping of electroconductive fluids was developed since the first half of the last century, currently it undergoes a revival due to the reconsideration of its multiple technological and security advantages. This work is both an intimate study of the phenomena that occur as a result of the electroconductive fluids flow in the electromagnetic field of an electromagnetic pump – magnetohydrodynamic interaction - and a report on the capabilities and advantages of modern computational tools to facilitate design and optimization of electromagnetic pumps.To achieve the principal goal of deeper understanding of the interdependent phenomena specific to electromagnetic pumps operation, two auxiliary objectives were considered. The first is related to the full exploitation of electromagnetic finite element models in order to retrieve as much information as possible about electromagnetic pumps behavior in a simplifying hypothesis that does not take into account the fluid dynamics. The second auxiliary objective is to build numerical models that couple the electromagnetism and the fluid dynamics, namely the two interdependent physics that govern the magnetohydrodynamic flow through channels of electromagnetic pumps.In the section dealing with the study of electromagnetic pumping of molten salts, the thesis highlights specific problems related the generation of electromagnetic forces in fluids with low electrical conductivity and provides results with respect to applications where electromagnetic pumping of molten salts can be effective. With the electromagnetic numerical models were obtained important data about the influence of the number of electromagnetic poles and supply frequency on the Pressure – Velocity characteristic of annular linear induction pumps. Were analyzed the shielding effect generated by the metallic walls - with negative repercussions on pumps performances, braking effects exerted at pump inlet and pump outlet and the connection between the overload capacity and Pressure – Velocity characteristic of induction pumps. A special portion was devoted to the analysis of the time and space dependence of the electromagnetic force and to the study of the non-uniformities of electromagnetic quantities in azimuth direction of annular linear induction pumps.In the chapter devoted to the magnetohydrodynamic interaction through coupled models, the thesis proposes two models that couple the electromagnetism and the fluid flow, one realized using multiphysic software and the second by coupling two different softwares. There are presented the advantages of the coupled models with respect to the results accuracy in comparison with electromagnetic models. It is presented the evolution of velocity, force and current densities profiles under the influence of the electromagnetic field and of different sodium mean velocities.The contributions of the thesis are completed with significant observations related to the study methods and software tools used along the study process
Preocupările actuale în legătură cu exploatarea în siguranță a centralelor nucleare existente și în legătură cu proiectarea arhitecturilor speciale de reactoare nucleare pentru generația a patra,coroborate cu interesul tot mai pronunțat pentru eficiența și fiabilitatea oricărui echipament dintr-un sistem energetic, fac ca tot mai multe demersuri de cercetare să se îndrepte spre studiul diverselor subansamble ale acestor sisteme pentru o mai buna înțelegere și optimizare pe baza mijloacelor moderne de proiectare asistată de calculator. Din rândul tipurilor propuse pentru generația a patra de reactoare nucleare fac parte și cele care au ca agent de răcire săruri topite, respectiv sodiu topit. Multe reactoare nucleare de generație anterioară folosesc ca mijloc de antrenare a fluidelor de răcire pompe mecanice de construcție specială. Sărurile topite ăi sodiul lichid, datorită proprietăților fizice,au potențialul de a fi recirculate cu ajutorul pompelor electromagnetice. Deși tehnologia pompării electromagnetice a fluidelor electroconductoare a fost dezvoltată încă din prima parte a secolului trecut, în prezent cunoaște o renaștere datorită reconsiderării multiplelor avantaje tehnologice și de securitate în exploatare. Lucrarea de față este atât un studiu al fenomenelor intime ce au loc ca urmare a curgerii fluidelor electroconductoare în câmpul electromagnetic al pompelor electromagnetice – interacțiune magnetohidrodinamică - cât și un raport despre capabilitățile și avantajele uneltelor moderne de calcul de a înlesni proiectarea și optimizarea pompelor electromagnetice. Pentru a atinge scopul principal al tezei, și anume o înțelegere mai adâncă a fenomenelor interdependente specifice funcționării pompelor electromagnetice, au fost considerate două obiective secundare. Primul obiectiv se referă la exploatarea la maximum a modelelor electromagnetice numerice în element finit cu scopul de a obține cât mai multe informații cu putință despre comportamentul pompelor electromagnetice în ipoteza care nu ia în considerare curgerea fluidelor. Al doilea obiectiv a fost construirea unor modele numerice care cuplează electromagnetismul și curgerea, respectiv, cuplează modelele numerice ale celor două fenomene ce caracterizează curgerea magnetohidrodinamică din canalele pompelor electromagnetice. În partea dedicată studiului pompării electromagnetice a sărurilor topite, teza evidențiază problemele specifice legate de generarea forțelor electromagnetice în fluide slab conductoare electric și oferă rezultate cu privire la aplicațiile unde pomparea electromagnetică a sărurilor topite poate fi eficientă. Cu ajutorul modelelor electromagnetice s-au obținut date importante despre influența numărului de poli electromagnetici și frecvența de alimentare asupra caracteristicii Presiune – Viteză a pompelor electromagnetice inelare de inducție. Au fost analizate fenomenul de ecranare creat de peretii metalici – cu repercursiuni negative asupra performanțelor pompelor, efectele de frânare exercitate la intrarea și la ieșirea din canalul de pompare și legătura dintre capacitatea de suprasarcină hidrodinamică și caracteristica Presiune – Viteză a pompelor de inducție. O secțiune specială a fost consacrată analizei dependenței de timp și a variației de la punct la punct a forței electromagnetice, precum și studiului neuniformităților mărimilor de natură electromagnetic în direcție azimutală în pompele inelare de inducție. În capitolul despre interacțiunea magnetohidrodinamică pe baza modelelor cuplate, se propun două modele ce cuplează electromagnetismul și curgerea fluidelor, unul realizat cu ajutorul unui singur software și al doilea realizat prin cuplarea a două software-uri diferite. Sunt prezentate avantajele modelelor cuplate din punctul de vedere al acurateței rezultatelor în comparație cu modelele electromagnetice …

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2010.
Includes bibliographical references.
Molten salts are important heat transfer fluids used in nuclear, solar and other high temperature engineering systems. Dispersing nanoparticles in molten salts can enhance the heat transfer capabilities of the fluid. High temperature and high ionicity of the medium make it difficult to make a colloidally stable dispersion of nanoparticles in molten salts. The aggregation and sedimentation kinetics of different nanoparticles dispersed in molten salts is studied, and trends of settling rates with system parameters like particle size, temperature and concentration are observed. Finally, a hypothesis based on ultra low values of Hamaker coefficient is suggested in order to achieve long term colloidal stability in molten salts medium.
by Vaibhav Somani.
S.M.

Technological advances results in the fact that quite a large number of electronic equipment interacts with its environment leading to the malfunction of the devices. This brings about the necessity of using proper electromagnetic (EM) wave absorbers/shields to avoid such interactions. In order to absorb EM waves in a large frequency band from several MHz to GHz, barium hexaferrite (BaHF) ceramics which are produced as textured ceramics as well as in multilayered form can be used. Textured ceramics are processed by tape casting using templated grain growth (TGG) phenomenon. In order to obtain textured ceramics, BaHF powders and platelets are required as raw materials in such a way that during sintering small size powders are directioned by large platelet surfaces. In this study, ferrite powders were synthesized by mixed oxide technique while the platelets were produced by both molten salt synthesis (MSS) and reactive templated grain growth (RTGG) methods. In the case of platelet synthesis by MSS, effects of calcination temperature and time as well as type and composition of the flux on the formation and morphology of platelets were investigated based on the XRD and SEM results. Studies have shown that KCl flux led to the formation of sharper platelet morphology, while NaCl resulted in more round shapes. However, extent of BaHF formation in the case of NaCl was higher when compared to KCl flux due to its higher wettability characteristic, and hence faster interaction with the raw materials. Since the aspect ratio of the synthesized platelets was only ca. 2-4, these platelets were not efficient for further TGG studies. Alternatively, BiFeO3 (BiF) particles having ~30-40 &mu
m average size were synthesized as seed crystals for the synthesis of BaHF platelets by RTGG method. After the washing of these platelets with dilute HNO3, pure BaHF powders and platelets were directed by tape casting which was followed by sintering using TGG phenomenon. Degree of achieved texturing in the processed ceramics was studied using Rietveld analysis, pole figure measurement and electron backscattered diffraction (EBSD).

The behaviour of sulphur containing species has been investigated during the last few decades in various molten chloride and fluoride electrolytes but their effect on the performance of the cells producing aluminium still remains a subject of controversy. In the present work the electrochemical behaviour of sulphur containing species was studied in various molten chlorides and fluorides with the aim to contribute to a better understanding of the behaviour of sulphur impurities in the Hall-Héroult process.

The sulphur electrochemistry and chemistry in the systems containing molten electrolytes are of interest for aluminium electrowinning because of the serious impact of the sulphur containing gases on the environment and the effect of sulphur compounds on the efficiency of the electrolysis. The complexity of the sulphur chemistry is briefly summarised in chapter 2.1.

The literature review focused on the electrochemistry of sulphur species in molten salts is discussed in chapter 2 while chapter 3 is dealing with the description of the electrochemical and analytical methods used in this study.

Preliminary experiments in electrolytes consisting of molten NaCl and mixtures of NaF-AlF₃ revealed the necessity to find an inert electrode material for electrochemical studies. Platinum in the chloride melts and gold in the fluoride systems were selected as the most suitable materials for the working electrodes.

The behaviour of the anions containing sulphur in various oxidation states was investigated by means of cyclic voltammetry and chronoamperometry and is described in chapter 4.3. The behaviour of sulphate anions was studied in a single NaCl melt and compared with the molten CaCl₂-NaCl (10-90 mol%) mixture at 840 ºC. It was found that the sulphate reduction proceeds differently in these two electrolytes which was ascribed to the difference in the oxoacidity of the melts. The number of electrons transferred during sulphate (S^VI) reduction in pure NaCl melt was found to be two with probable sulphite formation which possibly decomposed to sulphur oxide. In the molten CaCl₂-NaCl mixture sulphate seemed to be transformed to SO₃ which is further reduced in a one-electron exchange. The electrochemical signals recorded in the voltammograms also suggested that the character of the reduction products involved in those two melts is different. However, the “ec” mechanism, where a charge transfer is followed by a chemical reaction seemed to be the common feature in both electrolytes. The sulphate reduction was also studied in the eutectic LiCl-KCl mixture in the range of temperatures from 450 ºC to 840 ºC. In spite of the fact that the sulphate solubility seemed to rapidly decrease at temperatures below 700 ºC, the possible presence of a chemical reaction taking place after the reduction process of sulphate anions was suggested to be very similar to that found in molten NaCl.From the analysis of the data obtained by cyclic voltammetry followed that the reduction of sulphate anions to a lower oxidation state is probably diffusion controlled in all the three above mentioned systems at the sweep rates where the effect of the coupled chemical reaction can be neglected. Chapter 4 also includes an additional investigation of the behaviour of the sulphite and sulphide anions in the eutectic LiCl-KCl mixture.

Chapter 5 is devoted to the investigation of the sulphate species in the NaF-AlF₃ mixtures saturated by Al₂O₃ with varying cryolite ratio, i.e. CR = 3, 2, 1.5 and 1.2. It contains a description of sulphate modification at various experimental conditions and revealed its complicated and complex electrochemical behaviour which was monitored by cyclic voltammetry, chronoamperometry and square wave voltammetry. For the electroactive species in the electrolyte with CR equal to 3 two reduction steps were suggested involving three and two electrons respectively. The “ce” reaction mechanism was assumed where a chemical reaction seems to precede a charge transfer. The chemical step might be the partial decomposition of sulphate to SO₂. The sulphate reduction in the electrolyte with CR = 2 appeared to be considerably different. Since the temperature difference in these two systems is only 40 ºC the reason was likely to be due to the higher concentration of AlF₃ in the electrolyte. In this electrolyte only one cathodic process was observed and the reaction mechanism involving three electrons with a preceeding chemical reaction was suggested. Because the production of aluminium metal in the electrolysis cells operating with inert anodes is an attractive idea and have opened a question about the behaviour of sulphur species in very acidic electrolytes (high AlF₃ content) a part of the present studies was devoted to the study of the electrochemical behaviour of the sulphate anions in the electrolytes with CR = 1.5 and 1.2. Cyclic voltammetry and square wave voltammetry revealed that the sulphate is then reduced in two cathodic steps. Simulations and the obtained square wave voltammograms indicated that the cathodic process probably involves more electrons in the very acidic NaF-AlF₃ mixtures than in electrolytes with higher CR equal to 3 or 2. Observation of volatile species formed in the NaF-AlF₃ mixtures with CR = 1.5 and 1.2 was revealed as another important difference. This could be due to the formation of sulphur which at high temperature is present in the gaseous form and thus likely to escape from the laboratory furnace. The difference in the electrochemical behaviour of the sulphate anions in the cryolitic based melts was related to the difference in the bath composition and experimental temperature.

The present study shows the complexity and variability in the electrochemical behaviour of sulphur containing species as well as the difficulties and limitations resulting from the sulphur chemistry in the molten salts at high temperatures.

Aluminium can be electrodeposited on tungsten, aluminium, platinum, nickel and glassy carbon from 2: 1 A1C13-TMPAC (trimethylphenylammonium chloride) molten salts at room temperature. The A1C13-TMPAC melts diluted with 1,2-dichlorobenzene of had a beneficial effect on increasing aluminium deposition and stripping currents by decreasing viscosity and increasing the conductivity of the melt. The reductions of A12C17 ions were quasi-reversible and diffusion controlled, The irreversibility of the reductions was due to the relatively low conductivity and high viscosity of the melts. The aluminium deposition was associated with the reduction of the resulting product A1C14 ions following the locally changing acidity of the melts, and a corrosion reaction between fresh aluminium deposits and the melt. The mechanism for aluminium electrodeposition from 2: 1 A1C13-TMPAC was realised likely similar to that for the acidic A1C13- BPC melts. The bulk depositions of aluminium on all electrode except platinum involved three-dimensional instantaneous nucleation process followed by hemispherical diffusion-controlled growth of the developing nuclei. On platinum the nucleation process was characteristic of instantaneous at short time and then tended to progressive. The phenomena of aluminium UPD were found on all electrodes but not on aluminium itself, highly dependent upon substrates, and involved alloying effects on metal substrates. The UPD layers on the surface of the electrodes corresponded to about 2 -- 10 aluminium monolayer equivalents. The UPD was surface constrained on tungsten, whereas were diffusion controlled on nickel and platinum. The processes on tungsten and platinum were under the limitation of kinetics. On nickel and platinum, the electrodes and the nickel and platinum components in fresh deposits were likely to react with A12C17 and Cl' ions in the melt to form corresponding metal complexes. 11 The passivation phenomena were observed to occur on all electrodes. These could resulted from a black layer formed by AIC13 precipitates. Particularly, on aluminium the stripping of aluminium was involved in an active dissolution-passivation process.

Molten salts have been widely used as thermal energy storage (TES) materials as they offer favourable specifications which enable them to be employed in TES applications. Finding a cost-effective method to enhance the energy storage capability of molten salts has caught the attention of many researchers. It was reported that by adding a small amount of nanoparticles, a major enhancement of the specific heat capacity was observed in molten salts. Though different studies argued that the enhancement was not found in other thermal storage materials, the observation of the enhancement was continuously reported. This work studied the thermal properties of molten salt based nanosuspensions synthesized with a novel method modified based on other studies. Molecular dynamics (MD) simulations were employed to study the thermodynamic properties of the nanosuspension systems. By the analysis of the effect on the internal energy of the nanosuspensions I draft general conclusions and explain why molten salt have this specific heat enhancement while other materials (e.g. water) does not. I use MD simulation to support, for the first time, a theory that can explain the apparently contradictory behaviour of the experimental data. Moreover, the main impact factor affecting the enhancement was investigated and discussed.

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.

This thesis examines the electrochemical reduction of metal oxides in molten salts for nuclear reprocessing applications. The objective of this research is to characterise and understand the direct electrochemical reduction of UO2 to U metal in a LiCl-KCl molten salt eutectic, as part of the nuclear pyroprocessing scheme, following a similar approach to the FFC Cambridge for the reduction of TiO2 to Ti metal. The voltammetric behaviour of reduction processes of metal oxides were evaluated using electroanalytical techniques such as cyclic voltammetry and chronoamperometry on different precursor types, such as thermally grown thin oxide films, metallic cavity electrodes, and ‘a fluidised cathode’, a novel process that was developed within this work. Material was characterised before and after the electroanalytical experiments using scanning electron microscopy, X-ray energy dispersive spectroscopy and X-ray diffraction. Predominance phase diagrams, using recent thermodynamic data, for metal-molten salt systems, relating the potential to the negative logarithm of the activity of O2− ions (E-pO2−), were produced for the range of spent nuclear materials in both LiCl-KCl and NaCl-KCl. The bulk of this research was on investigating the electrochemical reduction of WO3 to W metal. Tungsten was selected as a chemical surrogate for uranium to investigate specific process parameters. Nonetheless, tungsten is an important and desirable refractory metal, and the electrochemical route for producing it of high purity might prove viable. The electrochemical reduction of UO2 to U metal using the fluidised cathode process was investigated. Voltammetry studies were conducted, and alongside a predominance diagram that was constructed, the reaction path-way was studied, and a Faradaic efficiency was established. The fluidised cathode is a robust, three-phase, high efficiency process. It was studied here for the electrochemical reduction of WO3 and UO2, however, it is likely applicable for other spent fuel oxides, and in the production of refractory metals.

Pour pallier la demande croissante de batteries Li-ion, il existe aujourd’hui le besoin urgent de recycler les composants de ces dispositifs. Recycler les matériaux cathodes qui contiennent des oxydes des métaux de transition est stratégique. Ces trois dernières années les recherches dans ce domaine ont augmenté de manière très significative. Dans le contexte du recyclage des batteries, il existe actuellement deux méthodes utilisées dans l’industrie : l’hydrométallurgie et la pyrométallurgie. L’objectif de ce projet a été de proposer une méthode alternative permettant de recycler les composants organiques et inorganique, en utilisant des mélanges de sels fondus comme milieu réactionnel. Les carbonates et chlorures fondus ont été choisis comme solvants pour leur efficacité dans le traitement des déchets. Le cobalt est un des matériaux critiques dans les batteries, rare sur la planète et toxique. Dans ce projet, nous avons étudié la dissolution et récupération du cobalt dans les carbonates et chlorures fondus. Des techniques électrochimiques (voltammetrie cyclique, chronoampérométrie) et la spectroscopie des rayons X ont été utilisés pour mener les investigations. Les résultats montrent une lente et faible dissolution du cobalt dans les carbonates fondus et il est présent sous la forme de Co (II). Les chlorures fondus ont été choisi comme deuxième alternative de solvant. La dissolution du cobalt et sa récupération ont été réussis dans ce milieu en ajoutant des additifs
To meet the increasing demands of lithium-ion batteries, there is an urgent need to recycle the batteries components. In particular, electrode materials containing transition metal oxides such as LiCoO2, LiNi1/3Mn1/3Co1/3O2, and LiNi0.8Co0.15Al0.05O2 are of strategic importance. Over the last three years, this field research has rocketed. In the frame of batteries recycling, there are two main methods that are used in industry nowadays: hydrometallurgy and pyrometallurgy. The aim of our research project is to propose an alternative method to recycle organic compounds and metals of electrode materials based on molten salts as reactive medium. Molten carbonates and molten chlorides have been chosen for their great efficiency on wastes treatment. Cobalt is one of the critical raw materials in batteries and it is rare on the Earth-crust and toxic for environment. In this work, we study cobalt dissolution and recovery in molten carbonates and chlorides. Electrochemical techniques (cyclic voltammetry, chronoamperometry) and X-Ray spectroscopy have been used for the investigations. Results show that a low and slow dissolution of cobalt is obtained in molten carbonates and in the form of Co (II). Molten chlorides have been used as a second alternative of solvent. Cobalt dissolution increase and its recovery have been achieved in this solvent when using additives

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Dissertacao (Mestrado)
IPEN/D
Instituto de Pesquisas Energeticas e Nucleares - IPEN/CNEN-SP

Thesis: S.B., Massachusetts Institute of Technology, Department of Nuclear Science and Engineering, 2017.
DISCLAIMER NOTICE: The pagination in this thesis reflects how it was delivered to the Institute Archives and Special Collections. Cataloged from student-submitted PDF version of thesis.
Includes bibliographical references (pages 46-48).
The mechanism by which tellurium causes intergranular corrosion (IGC) of structural alloys in molten salt reactors is currently poorly understood. Limited corrosion testing has been performed on a few select alloys in simulated reactor conditions. In this thesis, the results of performing 50 h, 100 h, and 150 h corrosion tests on alloys Hastelloy N, Nickel-201, Incoloy 8ooH, and 316L Stainless Steel are presented. Upon inspection of the corroded surfaces of each alloy after its immersion in molten LiF-NaF-KF (FLiNaK) salt at 700 °C using scanning electron microscopy (SEM) and energy-dispersive x-ray spectroscopy (EDS), a consistent corrosion rate could not be determined for any of the alloys, nor could confident identification of telluride compounds within the corrosion layer or grain boundaries of any alloy be made. However, the results did appear to confirm the importance of using a low oxygen environment and avoidance of galvanic corrosion during testing. Furthermore, preliminary results from EDS analysis of one alloy sample implied that, with improved count rates taken during the elemental identification process, tellurium may be more clearly revealed in the corrosion layers and grain boundaries of the alloys tested.
by Natasha Skowronski.
S.B.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Nuclear Science and Engineering, September, 2020
Cataloged from student-submitted PDF version of thesis.
Includes bibliographical references (pages 122-142).
Various advanced nuclear reactors including fluoride high-temperature salt-cooled reactors (FHRs), molten salt reactors (MSRs) and fusion devices have proposed to use molten salt coolants. However, there remain many uncertainties in the chemistry, dynamics and physicochemical properties of many salts, especially over the course of reactor operation, where impurities are introduced, and compositional and thermodynamic changes occur. Density functional theory (DFT) and ab initio molecular dynamics (AIMD) were used for property, structure and chemistry predictions for a variety of salts including LiF, KF, NaF, BeF2, LiCl, KCl, NaCl, prototypical Flibe (66.6%LiF-33.3%BeF2), and Flinak (46.5%LiF-11.5NaF-42%KF). Predictions include thermophysical and transport properties such as bulk density, thermal expansion coefficient, bulk modulus, and diffusivity, which were compared to available experimental data.
DFT consistently overpredicted bulk density by about 7%, while all other properties generally agreed with experiments within experimental and numerical uncertainties. Local structure was found to be well predicted where pair distribution functions showed similar first peak distances (+ 0.1 A) and first shell coordination numbers (+ 0.4 on average), indicating accurate simulation of chemical structures and atomic distances. Diffusivity was also generally well predicted within experimental uncertainty (+20%). Validated DFT and AIMD methods were applied to study tritium in prototypical salts since it is an important corrosive and diffusive impurity found in salt reactors. It was found that tritium species diffusivity depended on its speciation (TF vs. T2), which was related to chemical structures formed in Flibe and Flinak salts. Further, predictions allowed comparison with and interpretation of past contradictory experimental results found in the literature.
Lastly, robust neural network interatomic potentials (NNIPs) were developed for LiF and Flibe. The LiF NNIP accurately reproduced DFT calculations for pair interactions, solid LiF and liquid molten salt. The Flibe NNIP was developed for molten salt at the reactor operating temperature of 973K and was found to reproduce local structures calculated from DFT and showed good stability and accuracy during extended MD simulation. Ab initio methods and NNIPs can play a major role in advanced reactor development. Combined with experiments, these methods can greatly improve fundamental understanding and accelerate materials discovery, design and selection.
by Stephen Tsz Tang Lam.
Ph. D.
Ph.D. Massachusetts Institute of Technology, Department of Nuclear Science and Engineering

In this thesis, the compatibility of some structural alloys with the most recent high-temperature candidates as storage media (molten salts) to be used in thermal energy storage (TES) for the next generation of concentrating solar power (CSP) plants is investigated. This study endeavours to investigate the thermal and corrosive impacts of molten salts on commercial alloys by developing methods to measure and track the material degradation in contact with the corrosive medium. The role of material microstructure on corrosion and/or oxidation behaviour is studied and discussed in detail.

In recognition of the shortcomings inherent to the operating temperature ranges of current fuel cell systems, namely the“temperature gap" between 200C and 600C, an effort to develop an intermediate-temperature molten-salt electrolyte fuel cell (IT-MSFC) was undertaken. In this type of fuel cell, the molten salt electrolyte is supported on a porous support, in a planar or other geometry similar to that used in existing fuel cell technologies, such as phosphoric acid fuel cell (PAFC) and molten carbonate fuel cells (MCFC). Such a fuel cell using a molten hydroxide electrolyte and Pt/C catalyst was constructed and tested using hydrogen and oxygen as fuel. The performance was comparable to that which has been obtained from PEM fuel cells at the low end of the voltage range, reaching 950ma/cm2 at 0.4 V in the highest performing test. Performance was superior to PEM fuel cells at the high end of the voltage range, due to the more favorable kinetics at the higher temperatures, with an open circuit voltage (OCV) of 1.0 V with a linear performance curve between 1.0 V and 0.6 V, which is characteristic of fuel cells with low kinetic overpotentials. Longevity of the fuel cell was very poor, however a number of experiments were undertaken to improve it, enabling extension of operating life from 5 minutes to 30 minutes, which is still far too low for practical use. The key problem was identified as electrolyte retention by the support matrix and possible degradation of the gas diffusion layer and catalyst. Experiments were also conducted using methanol vapor as fuel, and it was found to provide performance close to that recorded with pure hydrogen. Experiments were also conducted using several alternative molten salts, including nitrate and chloride eutectics. Combinations of nitrates with hydroxides added to act as a charge carrier produced a working fuel cell, however performance was greatly reduced. Though preliminary, the work described herein demonstrates the great potential of IT-MSFC, and outlines the work needed to make this type of fuel cell practical.

There are many kinds of elements, especially heavy metallic elements, presentin the industrial slags. These elements bring big environmental problems ifthey are directly used in land filling. And the recovery of these elements canalso have benefits for the resource conservation. This paper reports the use ofelectrochemical method to extract the metal elements from both industrial slagand pure oxide. The mixture of NaCl-KCl was used as the electrolyte for thisprocess. Some proposals are alsomentioned for the further work.

KIVCET smelter is a modern direct smelting process which has been used to replace conventional sinter/blast furnace technology in pyrometallurgical process of lead and zinc. Although heavy metals including Cd, Pb, Zn, Fe as well as S, O and Cl, are the main elements playing a leading role in the formation of molten phase on the waterwall tubes of the KIVCET waste heat boiler, hot corrosion behavior and electrochemical properties of weld overlay and wrought alloy 625 have not been studied yet in the molten salt environments containing the above-mentioned elements. The present study was carried out to study hot corrosion behavior of the weld overlay and wrought alloy 625 as well as failure mechanism of the weld overlay alloy 625 under the corrosive conditions of the radiant boiler in the KIVCET smelter. It was found that the deposited salt mixtures on the waterwall tubes of the radiant boiler had a strong tendency to form a molten phase at the operating temperature range of the radiant boiler. Presence of the deposited salt mixtures and the formation of the molten phase led to the occurrence of the hot corrosion attack in the waterwall and the ultimate failure of the weld overlay. The dilution and, consequently, the presence of a significant amount of Fe in the weldment composition were the key issues in the alloy 625 weld overlay. High concentrations of sulfur and oxygen in the grain boundaries of the wrought alloy 625 are to blame for the occurrence of the intergranular corrosion together with the internal attack. An electrochemical model (a porous and non-protective barrier layer model) was used to explain the corrosion/electrochemical behavior of the wrought alloy which fit into the obtained EIS data well.