O presente trabalho tem por objetivo principal explorar a produção, a dispersão e a consolidação de zircônia (óxido de zircônio – ZrO2), em sua forma hidratada, obtida a partir do tungstato de zircônio (ZrW2O8), em meio alcalino e em condições de temperatura de até 100°C. A produção dos materiais particulados foi realizada sob diferentes condições experimentais (concentração de NaOH, tempo e temperatura) de modo a investigar como as características cristalográficas, morfológicas, químicas e térmicas dos pós produzidos são afetadas e, com base nesses resultados, buscou-se inferir o mecanismo pelo qual o ZrW2O8, pó micrométrico e insolúvel em água, originou partículas nanométricas de zircônia em condições brandas de síntese. Neste trabalho também se explorou a dispersão das partículas de zircônia, no momento de sua síntese, com o uso de trietanolamina (TEOA) como surfactante, de modo a obter soluções coloidais estáveis que pudessem ser ultracentrifugadas visando à obtenção de corpos de zircônia transparentes de forma controlada. Verificou-se, de um modo geral, que os materiais particulados produzidos sem a adição de TEOA são compostos por aglomerados de nanopartículas constituídas majoritariamente por Zr e O, com partículas primárias de tamanho próximo a 5 nm e cristalitos inferiores a 3 nm. Os pós apresentaram uma estrutura cristalina semelhante à da ZrO2 cúbica (ou de uma mistura de fases tetragonal e cúbica dependendo da condição de síntese), que formam aglomerados de elevada área superficial, mesoporosidade e capacidade para adsorção de água e dióxido de carbono em diferentes sítios superficiais. O mecanismo da síntese parece ser constituído, primeiramente, por uma reação química de substituição entre os tetraedros de WO4 e íons hidroxila, com posterior solubilização de parte da estrutura, pelo excesso de hidroxilas no meio, formando íons zircônio coloidais que polimerizam/condensam para a formação de núcleos cristalinos para posterior crescimento, em um processo facilitado por nucleação heterogênea e supersaturação. Além disso, a presença de tungstênio remanescente em todas as amostras parece ser um fator importante para a estabilização do tamanho e da estrutura cristalina dos materiais produzidos. A utilização de TEOA no processo de síntese permitiu a obtenção de corpos amarelados e transparentes, por ultracentrifugação, semelhantes a géis, que podem ser entendidos como um híbrido contendo material orgânico e WOx/ZrO2. / Fundação de Amparo à Pesquisa do Estado do Rio Grande do Sul, FAPERGS / The aim of the present work is to explore the production, dispersion and consolidation of zirconia (zirconium oxide – ZrO2), in its hydrous form, obtained from zirconium tungstate (ZrW2O8) in alkaline medium using temperatures up to 100°C. The production of particulate materials was carried out under different experimental conditions (NaOH concentration, time and temperature) in order to investigate how the crystallographic, morphological, chemical and thermal characteristics of the powders are affected and, based on these results, it was possible to infer the mechanism by which micrometric ZrW2O8 powder, water insoluble, yielded nanosized zirconia particles under mild synthesis conditions. This work also explored the dispersion of zirconia particles using triethanolamine (TEOA) as a surfactant, to obtain stable colloidal solutions which could be ultracentrifuged in order to obtain transparent zirconia bodies. It was found, generally, that the particulate materials produced without the addition of TEOA are composed of agglomerated nanoparticles composed mainly of Zr and O, with primary particle sizes near 5 nm having crystallites of less than 3 nm. The powders had a similar crystalline structure to cubic ZrO2 (or a mixture of tetragonal and cubic phases depending on the synthesis conditions), which create high surface area agglomerates with mesoporosity and capacity for water and carbon dioxide adsorption onto different surface sites. The mechanism of synthesis seems to be related to a chemical reaction of substitution between WO4 tetrahedra and hydroxyl ions, with subsequent solubilization of some portions of the structure due to the hydroxyl groups excess in the medium, originating colloidal zirconium ions which polymerize/condense to form crystal nuclei for further growth, in a process facilitated by heterogeneous nucleation and supersaturation. Furthermore, the presence of the remaining tungsten in all samples seems to be an important factor for stabilizing the size and crystalline structure of the materials produced. The use of TEOA in the synthesis process was responsible for the production of yellowish and transparent bodies by ultracentrifugation, similar to gels, which can be understood as a hybrid containing organic material and WOx/ZrO2.

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Une installation de mélange rapide entièrement automatisée permet d'étudier l'influence des conditions de mélange sur la co-précipitation d'oxydes mixtes de cérium et de zirconium. L'intensité du mélange est contrôlée par le débit d'entrée des solutions réactives. Un modèle d'engouffrement à iso-volume a permis d'estimer le temps de mélange à partir de la mesure d'un indice de ségrégation par le système de Villermaux Dushman pour trois mélangeurs Hartridge Roughton de géométries différentes. Pour une même puissance spécifique dissipée, le mélange est plus intense lorsqu'un rétrécissement est présent. L'intensification du mélange diminue la température maximale de réductibilité et augmente les contraintes du réseau cristallin des oxydes mixtes synthétisés et calcinés à 1100 °C. Cela est interprété par une meilleure homogénéité des particules. L'étude des particules directement en sortie du mélangeur rapide montre que pour les débits étudiés le mélange parfait avant précipitation n'est pas atteint. Une partie des particules se forme en milieu acide, incorporant des nitrates dans leur structure. Une modélisation simple des phénomènes de mélange couplée à une prise en compte des équilibres chimiques confirme ce résultat expérimental. / An automated experimental set-up with rapid mixers is used to study the influence of mixing conditions on the co-precipitation of cerium-zirconium mixed oxides. The intensity of mixing is controlled by the inlet flowrates of the reacting solutions. An engulfment model is used to estimate a mixing time from the measurement of a segregation index by the Villermaux-Dushman reaction system. Three geometries of Hartridge Roughton mixers are compared. Mixing performance is better when a separate mixing chamber upstream of a narrower outlet pipe is present. A better mixing decreases the maximal reducibility temperature of the material and increases the crystal strains of the particles calcined at 1100 °C. This is probably due to a better homogenization of the particles content. The important incorporation of nitrates in the particle at the outlet of the mixers shows precipitation occurs while the mixing process is not finished. This experimental result was confirmed by numerical simulation and an estimation of sursaturations during the mixing process.

Cette thèse de doctorat traite du comportement géochimique de Zr, Hf et Terres Rares dans des environnements aqueux extrêmes. Les études ont été effectuées dans les eaux hyper-salines long de la faille de la Mer Morte (Israël), les eaux hyper-acides dans le système volcanique hydrothermal du Nevado del Ruiz (Colombie) et les eaux riches en CO2 du système volcanique hydrothermal de l'île de Pantelleria, en comprenant le lac alcalin “Specchio di Venere”. Haute appauvrissement en Terres Rares légères a été trouvé dans les eaux acides dominées par le sulfate où on a reconnu la formation d’alunite et jarosite. Certaines eaux long de la faille de la Mer Morte montrent des enrichissements en Terres Rares intermédiaires, causés par la dissolution des minéraux évaporitiques. Les grandes variations redox et de pH observées dans ces systèmes hydrothermaux sont la cause des anomalies de Eu et Ce. Les eaux sulfates acides (1 < pH < 3.6) se caractérisent par des relations de Zr/Hf sous-condritiques et des relations condritiques de Y/Ho. Les rapports de Zr/Hf augment à l'augmentation du rapport Cl/SO4 en suggérant un comportement différent de Zr et Hf. Contrairement aux eaux acides, les relations de Y/Ho et Zr/Hf dans les eaux proches de la neutralité avec valeurs de Eh positives augmentent vers des valeurs super-condritiques, en raison de l'élimination préférentielle par les oxyhydroxydes de fer, de l'Hf et Ho que de Zr et Y. Le distribution des Terres Rares, avec les rapports de Y/Ho et Zr/Hf du “Specchio di Venere” montrent l’interaction entre les particules atmosphériques qui viennent du désert du Sahara et le lac “Specchio di Venere”, démontrent qu’ils sont de traceurs géochimiques. / This thesis concerns the geochemistry of Zr, Hf and REE (Rare Earth Elements) in extreme water environments. The investigations were carried out in hypersaline waters covering a wide range of Eh values along Dead Sea Fault (Israel), in hyperacid waters circulating in Nevado del Ruiz volcano-hydrothermal system (Colombia) and in CO2-rich waters in Pantelleria volcano-hydrothermal system (Italy), including the alkaline lake “Specchio di Venere” within a calderic depression. The acidic sulphates waters characterized by the precipitation of alunite and jarosite show a strong LREE depletion. The REE in waters along Dead Sea Fault show MREE enrichments in waters with relative high Ca and SO4 concentrations due to the water interaction with MREE-enriched salt minerals. In the natural waters, changing of pH and Eh induce variations of Ce and Eu anomalies, due to the different behaviour of these elements with respect to the neighbours REE. In sulphate acidic waters, Zr/Hf ratios are very low down to 4.7, while quite constant Y/Ho ratio (close to the local rock value) indicates the lack of decoupling. Zr/Hf ratio increases as Cl/SO4 ratio increases. On the contrary, Zr/Hf and Y/Ho ratios in near-neutral pH waters with positive Eh values change from near-chondritic to super-chondritic. The precipitation of Fe-oxyhydroxides removes preferentially Hf and Ho with respect to Zr and Y. The interaction of atmospheric fallout from the nearby Sahara Desert with the water of the lake “Specchio di Venere” was recognized by the Zr, Hf and REE distribution. Zr, Hf and REE show the capability to trace the interaction process between open water bodies and atmospheric fallout.

Les travaux présentés dans ce manuscrit ont trait à la catalyse de polymérisation des α-oléfines sont présentés en 4 parties distinctes. La première est consacrée à l'étude d'un système catalytique pour la production de polypropylène élastomère. L'analyse poussée des polymères produits et la caractérisation complète des catalyseurs utilisés a permis de montrer la présence de deux homopolymères sous forme de blende. La seconde partie porte sur la copolymérisation de monomères bifonctionnels vinyl-vinylidène avec le propylène. La caractérisation des polymères a permis de révéler la réactivité particulière des liaisons vinylidène et d'étudier l'influence du catalyseur utilisé sur le mécanisme de la polymérisation. La troisième partie s'intéresse à la caractérisation des espèces active en polymérisation et à l'étude des mécanismes d'activation et de désactivation des catalyseurs métallocènes. La synthèse et la caractérisation d'espèces cationiques, l'étude de leur comportement dynamique en solution, ainsi que l'évaluation de leur productivité en polymérisation ont permis d'établir un lien entre les propriétés électrophiles de ces espèces et de leur activité en polymérisation. La dernière partie porte sur l'homopolymérisation d'α-oléfines encombrées. La recherche d'un catalyseur suffisamment productif nous a amené à tester plusieurs catalyseurs présentant des structures différentes. L'absence de catalyseur productif soulève l'hypothèse d'interactions désactivantes entre le catalyseur et le monomère. / The work presented in the manuscript focus on α-olefin polymerization catalysis, and is divided into four distinct parts. The first part is dedicated to the study of catalytic systems for the production of elastomeric polypropylene. The analysis of the produced polymers and the characterization of the catalysts showed the presence of two homopolymers as a blend in the elastomeric polypropylene. The second part focuses on the copolymerization of bifunctionnal vinyl-vinylidene monomers with propylene. The characterization of the polymers revealed the reactivity of the vinylidène bonds and showed different polymerization mechanisms for the different catalysts. The third part reports a study on the activation and deactivation pathways of the active species in polymerization. The characterization of model cationic species and the study of their behavior in solution and in polymerization showed the relationship between the electrophilicity of the species and its productivity in propylene polymerization. The last part is dedicated to the polymerization of hindered α-olefins. The quest for a productive catalyst led to test various single site catalysts with different structures. Deactivating interactions between the monomers and the catalyst are supposed to explain the low productivity of the tested catalysts.

Zirconium tetrachloride (ZrF4) is extensively used in the manufacturing of zirconium metal. The concept of producing zirconium tetrafluoride from dissociated zircon and ammonium bifluoride is well established at the South African Nuclear Energy Corporation (Necsa) State Owned Company (SOC) Limited. Zirconium and hafnium are always found in the same minerals. In nuclear application zirconium is used for structural construction and as a cladding material for fuel, because of the low thermal neutron absorption, while hafnium is used as control rod in nuclear reactor, because of the high thermal neutron absorption. The methods of separating hafnium from zirconium prefer the use of ZrCl4 than ZrF4. This is because of the high solubility in both aqueous solutions and organic solvents and low sublimation temperature of ZrCl4, while ZrF4 is almost insoluble in organic solvent and has a high sublimation temperature. Thermodynamic evaluations showed that chlorinating ZrF4 with either CaCl2, KCl, LiCl or NaCl respectively was not favourable, while chlorinating ZrF4 with either BeCl2 or MgCl2 was favourable. But due to cost consideration chlorinating ZrF4 with BeCl2 was not investigated. A thermogravimetric apparatus was used to investigate the isothermal and the non-isothermal kinetics of chlorinating analytical grade ZrF4 with MgCl2. The thermogravimetric apparatus revealed that chlorination of ZrF4 commence at temperature above 350°C. Isothermal kinetics of chlorinating analytical grade ZrF4 with MgCl2 was investigated at temperatures of 400, 450, 480, 500°C. The reaction progressed towards completion prematurely before the isothermal temperatures were reached, due to a low heating rate of 20 °C/minutes was used to heat up the reaction mixture to the desired isothermal temperatures. As a result, the isothermal kinetics could not be determined. Heating rates of 5, 10, 15 and 20 °C/minutes were used to investigate the non-isothermal kinetics. The apparent activation energy of chlorinating ZrF4 with MgCl2 varied significantly when the non-isothermal kinetics was investigated. The variation was due to changes in the reaction mechanism. As a result, rate law of chlorinating ZrF4 with MgCl2 could not be determined due to variation of the apparent activation energy. Crude ZrF4 prepared at Necsa SOC ltd. was chlorinated with MgCl2, a mixture of MgCl2 and KCl, a mixture of MgCl2 and LiCl, and a mixture of MgCl2 and NaCl respectively. Chlorination of the crude ZrF4 was conducted at temperatures of 400, 450 and 500°C respectively. The aim of chlorinating the crude ZrF4 was to investigating the effect of the chlorinating on the purity of the produced ZrCl4. A batch reactor was used in this study. The reactor was divided into two sections, namely the reaction zone and the condensation zone. The diameter of the condensation zone was larger than that of the reaction zone. Reactants were placed into the reaction zone and the products were collected at the reaction zone and the condensation zone. Samples were collected from these products and analysed using for X-Ray Diffraction analysis (XRD) and Inductive Coupled Plasma Optical Emissions Spectroscopy (ICP-OES). XRD was used to identify the compounds that were present in the products and ICP-OES was used to determine the concentration of the elements that were present in the products. The analysis of the results obtained showed that the highest recovery of zirconium in the products collected from the condensation zone, the sublimed products, was achieved by chlorinating ZrF4 with MgCl2 at 500°C. About 80% was recovered. About 96% of the concentration of the impurities in the sublimed products was reduced when ZrF4 was chlorinated with a mixture of MgCl2 and LiCl at 450°C. About 36% of hafnium in the sublimed products was reduced when ZrF4 was chlorinated with a mixture of MgCl2 and NaCl at 400°C. / Chemical Engineering / M.Tech. (Chemical Engineering)

Sharp leading edges and nose caps on hypersonic vehicles, re-entry vehicles and reusable launch vehicles are items of current research interest for enhanced aerodynamic performance and maneuverability. The unique combination of mechanical properties, physical properties, thermal / electrical conductivities and thermal shock resistance of ZrB2 make it a promising candidate material for such applications. In the recent past, a lot of work has been carried out on ZrB2-based materials towards processing as well as characterization of their mechanical, oxidation and thermal behaviour. ZrB2 based materials have been successfully processed by conventional hot pressing, pressureless sintering, reactive hot pressing and spark plasma sintering. Densification of ZrB2 gets activated when the oxide impurities (B2O3 and ZrO2) were removed from particle surfaces, which minimized coarsening. B4C is widely used as a sintering additive for ZrB2 because it reduces ZrO2 at low temperature. It is found that full densification in ZrB2 based materials by hot pressing is achieved either at 2000 C and higher temperatures with moderate pressure of 20-30 MPa or at reduced temperature (1790-1840 C) with much higher pressure (800-1500 MPa). But no study is available that identifies the dominant hot pressing mechanism at different temperatures and pressures. On the other hand, reinforcement of SiC in ZrB2 is known to increase flexural strength, fracture toughness and oxidation resistance. It has been shown that oxidation resistance of ZrB2-SiC composites is superior to that of monolithic ZrB2 and SiC. For high temperature applications in air, the residual strength (room temperature strength after exposure in air at high temperatures) of non oxide ceramics after oxidation is important. A few reports are available on residual strength of ZrB2 –SiC composite after thermal exposure at high temperatures. In contrast to the literature on composites, there are no reports available on the residual strength of monolithic ZrB2 after exposure to high temperatures. Also, previous studies on residual strength of ZrB2-SiC composites have been limited to a single temperature of exposure. But there is a need to measure the residual strength after exposure to a range of temperatures since the oxide layer structure changes with temperature. The room temperature thermal conductivity data for ZrB2 and ZrB2-SiC composite shows a wide scatter in value as well as a dependence on microstructural parameters, especially porosity and grain size. Also, there is insufficient data available for the high temperature thermal conductivity of ZrB2-SiC. Therefore, it is difficult to evaluate the effect of SiC content on thermal conductivity of ZrB2-SiC composites at high temperatures. The present thesis seeks to address some of these gaps to better understand the suitability of ZrB2 and ZrB2-SiC composites for ultra-high temperature applications. In the present work, hot pressing is used for densification of ZrB2 and ZrB2-SiC composites. Different amounts of B4C (0, 0.5, 1, 3 & 5 wt %) were used as sintering additives in ZrB2 and hot pressed at 2000 C with 25 MPa applied pressure. The hot pressed samples are characterized for their microstructural, mechanical properties and oxidation behaviour. By addition of B4C, density as well as micro-hardness increased. For lower B4C content (0.5 & 1 wt %), hot pressed ZrB2 has shown considerable improvement in flexural strength after exposure in air at 1000 C for 5 hours, while higher B4C content (3 & 5 wt %) leads to marginal or no improvement. Due to the better mechanical and oxidation behavior of composites containing SiC, the densification behavior during hot pressing was studied. The densification behaviors as well as the microstructures for hot pressing of ZrB2-20 % SiC composite were found to change in a very 0 narrow temperature range. During hot pressing at 1700 C, the densification was found to be mechanically driven particle fragmentation and rearrangement. On the other hand, thermally activated mass transport mechanisms started dominating after initial particle fragmentation and rearrangement after hot pressing at 1850 C and 2000 C. At 2000 C, the rate of grain boundary diffusion was enhanced which resulted into annihilation of dislocation. The effect of SiC contents (10, 20 & 30 vol %) on mechanical and oxidation behavior of ZrB2-SiC composite were also studied. The average micro-hardness and fracture toughness of ZrB2-SiC composites increased with SiC content. But the flexural strength of ZrB2-20 vol % SiC composites was found to be the highest. Oxidation and residual strength of hot pressed ZrB2 -SiC composites were evaluated as a function of SiC contents after exposure over a wide temperature range (1000-1700 C). Multilayer oxide scale structures were found after oxidation. The composition and thickness of these multilayered oxide scale structures were found to depend on exposure temperature and SiC content. After exposure to 1000 C for 5 hours, the residual strength of ZrB2 -SiC composites improved by nearly 60 % compared to the as-hot pressed composites with 20 & 30 vol % SiC. On the other hand, the residual strength of these composites remained unchanged after 1500 C for 5 hours. A drastic degradation in residual strength was observed in composites with 20 & 30 vol % SiC whereas strength was retained for ZrB2-10 % SiC composite after exposure to 1700 C for 5 hours in ZrB2 –SiC. Therefore, residual strength of ZrB2-10 % SiC composite was measured at different exposure times (up to 10 hours) at 1500 0C. An attempt was made to correlate the microstructural changes and oxide scales with residual strength with respect to variation in SiC content and temperature of exposure. Since the ZrB2-20 vol % SiC composite showed the maximum strength, the dependence of strength on various microstructural as well processing parameters was also studied. It was found that porosity, grain size as well as surface residual stress due to grinding influenced the strength of ZrB2-20 vol % SiC composites. Finally, thermal diffusivity and conductivity of hot pressed ZrB2 with different amounts of B4C and ZrB2-SiC composites were investigated experimentally over a wide temperature range (25 – 1500 C). Both thermal diffusivity as well as thermal conductivity was found to decrease with increase in temperature for all hot pressed ZrB2 and ZrB2-SiC composites. At around 200 C, thermal conductivity of ZrB2-SiC composites was found to be composition independent. Thermal conductivity of ZrB2-SiC composites was also correlated with theoretical predictions of the Maxwell-Eucken relation. The dominated mechanisms of heat transport for all hot pressed ZrB2 and ZrB2-SiC composites at room temperature were determined by Wiedemann-Franz analysis using measured room temperature electrical conductivity of these materials. It was found that the electronic thermal conductivity dominated for all monolithic ZrB2 whereas the phonon contribution to thermal conductivity increased with SiC contents for ZrB2-SiC composites. The heat conduction mechanism at high temperature was also studied by measuring the high temperature electrical conductivity of ZrB2 and ZrB2-SiC composites. The effect of porosity on thermal diffusivity and conductivity was also studied for ZrB2-20 vol % SiC composites.

Zirconium tetrachloride (ZrF4) is extensively used in the manufacturing of zirconium metal. The concept of producing zirconium tetrafluoride from dissociated zircon and ammonium bifluoride is well established at the South African Nuclear Energy Corporation (Necsa) State Owned Company (SOC) Limited. Zirconium and hafnium are always found in the same minerals. In nuclear application zirconium is used for structural construction and as a cladding material for fuel, because of the low thermal neutron absorption, while hafnium is used as control rod in nuclear reactor, because of the high thermal neutron absorption. The methods of separating hafnium from zirconium prefer the use of ZrCl4 than ZrF4. This is because of the high solubility in both aqueous solutions and organic solvents and low sublimation temperature of ZrCl4, while ZrF4 is almost insoluble in organic solvent and has a high sublimation temperature. Thermodynamic evaluations showed that chlorinating ZrF4 with either CaCl2, KCl, LiCl or NaCl respectively was not favourable, while chlorinating ZrF4 with either BeCl2 or MgCl2 was favourable. But due to cost consideration chlorinating ZrF4 with BeCl2 was not investigated. A thermogravimetric apparatus was used to investigate the isothermal and the non-isothermal kinetics of chlorinating analytical grade ZrF4 with MgCl2. The thermogravimetric apparatus revealed that chlorination of ZrF4 commence at temperature above 350°C. Isothermal kinetics of chlorinating analytical grade ZrF4 with MgCl2 was investigated at temperatures of 400, 450, 480, 500°C. The reaction progressed towards completion prematurely before the isothermal temperatures were reached, due to a low heating rate of 20 °C/minutes was used to heat up the reaction mixture to the desired isothermal temperatures. As a result, the isothermal kinetics could not be determined. Heating rates of 5, 10, 15 and 20 °C/minutes were used to investigate the non-isothermal kinetics. The apparent activation energy of chlorinating ZrF4 with MgCl2 varied significantly when the non-isothermal kinetics was investigated. The variation was due to changes in the reaction mechanism. As a result, rate law of chlorinating ZrF4 with MgCl2 could not be determined due to variation of the apparent activation energy. Crude ZrF4 prepared at Necsa SOC ltd. was chlorinated with MgCl2, a mixture of MgCl2 and KCl, a mixture of MgCl2 and LiCl, and a mixture of MgCl2 and NaCl respectively. Chlorination of the crude ZrF4 was conducted at temperatures of 400, 450 and 500°C respectively. The aim of chlorinating the crude ZrF4 was to investigating the effect of the chlorinating on the purity of the produced ZrCl4. A batch reactor was used in this study. The reactor was divided into two sections, namely the reaction zone and the condensation zone. The diameter of the condensation zone was larger than that of the reaction zone. Reactants were placed into the reaction zone and the products were collected at the reaction zone and the condensation zone. Samples were collected from these products and analysed using for X-Ray Diffraction analysis (XRD) and Inductive Coupled Plasma Optical Emissions Spectroscopy (ICP-OES). XRD was used to identify the compounds that were present in the products and ICP-OES was used to determine the concentration of the elements that were present in the products. The analysis of the results obtained showed that the highest recovery of zirconium in the products collected from the condensation zone, the sublimed products, was achieved by chlorinating ZrF4 with MgCl2 at 500°C. About 80% was recovered. About 96% of the concentration of the impurities in the sublimed products was reduced when ZrF4 was chlorinated with a mixture of MgCl2 and LiCl at 450°C. About 36% of hafnium in the sublimed products was reduced when ZrF4 was chlorinated with a mixture of MgCl2 and NaCl at 400°C. / Chemical Engineering / M.Tech. (Chemical Engineering)

Group IV transition metal carbides are promising materials for high temperature structural application, due to their unique sets of properties such as high melting temperature, high temperature strength, hardness, elastic modulus, wear and corrosion resistance, metal-like thermal and electrical conductivity and thermal shock resistance. This group includes zir-conium carbide, which, along with its composites, are potential candidates for applications such as nose cones for re-entry vehicle, engines, wear resistant parts and in nuclear fuel cladding. Such structural applications demand high strength material with minimal flaws, in order to achieve the required reliability. Attainment of high strength calls for fully dense material with as small a grain size as possible. Producing fully dense zirconium carbide requires very high temperature, which is a direct consequence of its high melting point. Higher processing temperatures increase grain size, thereby also causing a loss in strength, along with the increased cost. Therefore, there is always a driving force to produce such a material in fully densified form at as low a temperature as possible. There have been a number of studies on processing and densification of zirconium carbide. Pressureless sintering of zirconium carbide requires temperature of 2400oC-3000oC to reach reasonably high density. At such high temperatures, abnormal grain growth limits the final density, as pores get entrapped inside the grains. Hot pressing of zirconium carbide also requires upwards of 2000oC to reach high density and is the primary route to produce densified zirconium carbide product. Reactive hot pressing (RHP), is a relatively new processing approach. Here, the reaction between zirconium and carbon to produce zirconium carbide and the densification of the porous mass, occurs simultaneously. Study on reactive hot pressing of zirconium carbide have shown that, it is possible to achieve very high density at much lower temperatures 1600oC. Clearly, reactive processing is an exciting new technique to process zirconium carbide. However, there has been a lack of studies to understand why it provides better densification than conventional hot pressing. Such understanding is of paramount importance, as it can lead to better optimization of RHP and perhaps even lower the process temperature further. The objective of the present study is to understand the densification process in RHP of zirconium carbide through systematic and carefully designed experiments. A model of reactive hot pressing is also constructed to get more insight into the phenomenon. 0.1 Pressureless Reaction Sintering of Zirconium Car-bide Pressureless reaction sintering (RS) of zirconium carbide is studied to understand the role of stoichiometry and zirconium metal in densification. ZrC of four different stoichiometries are chosen for these sets of experiments which are conducted in vacuum at 1200oC and 1600oC for 1 hour to understand the role of stoichiometry. One sample of pure Zr is also sintered to elucidate the role of zirconium in densification. After reaction sintering, all the samples are characterized by density measurement, x-ray diffraction and microstructure, using scanning electron microscopy. After pressureless sintering at 1600oC, zirconium metal reaches the highest relative density of ~ 95%. Densification decreases monotonically with increasing stoichiometry. Zr+0.5C composition reaches the next best relative density (of 90%), while Zr+0.67C composition shows much lower densification. The other two compositions, Zr+0.8C and Zr+C, in contrast, display de-densification rather than densification. Since the pure zirconium sample reaches high density, it can, in principle, help in densification of the mixed powders before getting fully reacted. Non-stoichiometric carbides also exhibit higher diffusivity of carbon, which aids the densification and the greater the deviation from stoichiometry, the smaller the deleterious effects of de-densification from reaction. This troika of factors is responsible for the substantially better densification in non-stoichiometric carbide, compared to stoichiometric carbide. 0.2 Reactive Hot Pressing of Zirconium and Carbon Reactive hot pressing of zirconium carbide is explored with the emphasis on finding the underlying densification mechanism. The earlier proposed densification mechanism for RHP is the plastic flow of transient non-stoichiometric carbide. To differentiate the effect of transient phases from that of zirconium, RHP is carried out at 800oC. At this low temperature, transient phases cannot take part in plastic flow and subsequent densification. Thus, any densi cation at this temperature can be totally attributed to zirconium and the role of zirconium thus can be separated from that of transient phases. A combination of RHP and RS experiments are carried out at 1200oC to better understand the phenomenon. Again, ZrC carbide of four different stoichiometries are investigated in this RHP study. After RHP at 800oC, all the four different ZrC compositions reached more than 90% RD through plastic flow of the Zr leading to a continuous matrix with embedded graphite particles. Since the reaction remains incomplete at this temperature, it is clear that Zirconium alone is responsible for enabling densification at such a low temperature. It is therefore argued that any unreacted Zr would, at higher temperature, be able to drive densification even more. Thus, zirconium does not only participate in densification; it is a dominant factor enabling low temperature densification. Pressureless reaction sintering at 1200oC following the RHP at 800oC, results in de-densi fication, as the reaction between zirconium and carbon occurs with significant volume shrinkage. Since such shrinkage increases with stoichiometry of the carbide, the higher stoichiometry carbides are more susceptible to de-densification. RHP at 1200oC, mostly completes the reaction, but only ZrC0:5 reaches near theoretical density. Thus, the final density of the fully reacted mixture is arrived at through a combination of processes in which the more stoichiometric carbides suffer from not only the smaller metal content but also a greater volume shrinkage during reaction. Thus, ZrC0:5 reaches 99% RD whereas ZrC reaches only 85% RD. The interplay between these two processes may be controlled by a two step RHP begin-ning at 800oC followed by a ramp up to 1200oC. The higher RD achieved at 800 C results in a higher final density for all the four compositions. Thus, two step RHP is a novel way to get better densification in RHP of zirconium carbide. 0.3 Hot Pressing of Zirconium Carbide Powders of Different Stoichiometry In the literature, densification in RHP is mostly attributed to the presence of transient non-stoichiometric carbides. To examine this hypothesis, ZrC of three different stoichiometries are prepared and then subjected to hot pressing at the same temperature and pressure as the previous RHP experiments (i.e. 1200oC and 40MPa for 30 min). After the hot pressing experiments, ZrC0:5 composition shows significant densification (95% RD), whereas ZrC0:67 composition shows very limited densification (70% RD) and ZrC composition shows little or no densification (50% RD). Evidently, the transient phase formed with stoichiometry close to ZrC0:5 can certainly contribute substantially to densification. But for the more carbon-rich compositions, the transient phases do not appear to play a significant role and the benefit of RHP, wherein ZrC can reach 90% RD, must come from the contribution of metal plasticity. 0.4 Reactive Hot Pressing of Zirconium and Zirconium Carbide Two limiting factors for densification during RHP are, de-densification (courtesy of the reaction) and the gradual increase in volume fraction of a rigid, non-sintering phase. To investigate the role of these factors further, two compositions of mixed metal and carbide powders, namely Zr+ZrC and 0.5Zr+ZrC, are subjected to RHP. When reaction is complete, the compositions after RHP will correspond to ZrC0:5 and ZrC0:67, respectively, but with the following difference with respect to the metal-carbon mixtures investigated earlier: these new compositions do not experience de-densification due to reaction and they contain significantly more amount of hard phase (53 and 69%) in the starting composition than their zirconium and carbon mixture counterparts i.e. Zr+0.5C and Zr+0.67C (16 and 20%). These two compositions are subjected to the same process schedules, i.e., RHP at 800oC, pressureless reaction sintering at 1200oC following RHP at 800oC and two step 800oC and 1200oC RHP. After 800oC RHP, Zr+ZrC and 0.5Zr+ZrC compositions reach much lower density than Zr+0.5C and Zr+0.67C compositions as a direct consequence of the larger amount of hard phase hindering densification at the lower temperature. After the 1200oC pressureless sintering following the RHP at 1200oC, the RD of Zr+ZrC and 0.5Zr+ZrC compositions increase (which is opposite to the behaviour of Zr+0.5C and Zr+0.67C com-positions) as they do not su er from reaction derived de-densification. After two step RHP, Zr+ZrC and 0.5Zr+ZrC compositions reach a final RD that is higher than the Zr+0.5C and Zr+0.67C compositions, even though after the first RHP at 800oC, they were much less densified. Thus, the absence of de-densification during reaction is able to more than compensate for the increase in hard phase content. 0.5 Reactive Hot Pressing: Low temperature process-ing route Based on the major factors of densification identified earlier, it was investigated whether RHP temperatures could be brought down further while being supplemented by a free sintering step to complete the reaction without de-densification. From a practical standpoint, such a process would allow dense products to be made by hot pressing with low temperature dies and fixtures while carrying out a more economical pressureless sintering at higher temperatures Therefore, Metal-carbide mixtures, Zr+ZrC and Ti+ZrC, are chosen, along with a temperature of 900 C which is above the allotropic phase transformation temperature for Zr around 880oC, thereby utilizing a zirconium phase that is softer than the hexagonal Zr. For completion of reaction, pressureless reaction sintering is done at 1300oC and 1400oC. It is found that after 1400oC reaction sintering, both the compositions reach almost full density and the Ti+ZrC composition also shows a higher hardness (13 vs 10 GPa) than the Zr+ZrC composition, due to the formation of a binary carbide with consequent solid solution hardening. 0.6 Effect of Particle Size on Reactive Hot Pressing During RHP, premature exhaustion of zirconium by reaction can limit densification. One way to have better densification is to slow down the reaction, so that significant amount of densification takes place before the metal zirconium is exhausted. One way to reduce reaction rates is to increase particle size. Larger particles are expected to slow down the reaction without affecting sintering, as densification is controlled by power law creep of Zr which is grain size independent. Because of lack of availability of Zr with different particle sizes, two different graphite particle sizes, i.e. 7-10 m and 50-60 m, were studied and it was shown that after 1200oC RHP, indeed the larger particle size improves densification. 0.7 Modelling of Reactive Hot Pressing Reactive hot pressing is a complicated phenomenon, and to get an insight and also to optimize the parameters, the availability of a computational model is of paramount importance. Keeping that in mind, a model of RHP has been constructed based on four different parts, namely: 1. Densification of zirconium under pressure 2. Reaction of zirconium and carbon 3. The constraint on sintering from a rigid phase and, finally, 4. The volume contraction during reaction. The model uses published data for the 4 steps and shows reasonable qualitative and quantitative agreement with the experimental results. Further experiments are done with the model to optimize the processing parameters. Results from the virtual experiments consolidates our earlier conviction gained from experimental results, by showing zirconium is the principal factor in densification and exhaustion of zirconium coupled with reaction derived de-densification prevent the higher stoichiometric carbide from achieving full densification. It also shows, RHP gives best densification when reaction is 70-80% complete. So two step RHP where the first RHP will only complete the reaction 70-80%, and a final RHP at temperature which will complete the reaction, will possibly be the way to achieve best densification. 0.8 Conclusions The study on RHP of zirconium carbide led to the following conclusions: • Zirconium plays the most crucial role in densification. • Transient phases only play a role when the final stoichiometry of RHPed carbide is close to that of ZrC0:5. • De-densification from reaction prevents higher stoichiometric carbide from reaching full densification. • Two step RHP, with one RHP at lower temperature at which reaction will remain incomplete, and the other at higher temperature to complete the reaction, yields best densification. • For lower stoichiometric carbide (ZrC0:5,ZrC0:67), full densification can be achieved at 1200oC. For higher stoichiometric carbide, even though large amount of densification upward of 90% RD is achieved at 1200oC, full densification will be out of reach. • RHP shows better densification than conventional hot pressing for all stoichiometries.