Este trabalho teve por objetivo estudar a redução por hidrogênio de sínteres produzidos com coque e carvão vegetal. Para isso, foram realizadas sinterizações utilizando 3 tipos de sinter feeds hematíticos. Os sinter feeds foram caracterizados através de análise granulométrica, análise química, difração de raios-X, microscopia óptica e microscopia eletrônica de varredura. Os primeiros ensaios de sinterização visaram a produção de sínter de retorno para as sinterizações posteriores. Misturas adicionando o sinter feed resíduo do processo de pelotização de minérios (SFRP) foram sinterizadas para avaliar o efeito da adição deste subproduto no processo de sinterização. Um teste foi realizado para verificar o efeito da redução de finos presentes no sínter de retorno no processo de sinterização. Foram realizados 6 ensaios de sinterização substituindo o coque por carvão vegetal como combustível do processo. As substituições foram de 0%, 8%, 16%, 25%, 50% e 100%. As amostras com substituições de 0%, 50% e 100% foram reduzidas em termobalança, durante 6 horas, em temperatura de 1000°C, utilizando uma mistura de gases contendo Argônio 10% Hidrogênio. As amostras que apresentaram a melhor e a pior redutibilidade foram estudadas através de cálculos cinéticos, a fim de determinar os mecanismos de reação envolvidos durante a redução dos óxidos. A etapa de caracterização microscópica dos sínteres e reduzidos foi realizada de modo que tornou possível comparar a amostra de sínter antes e após o processo de redução. Os resultados mostraram que, para as misturas produzidas, o sínter fabricado com carvão vegetal diminuiu os índices de produção da sinterização e de resistência do sínter. Contudo, o sínter produzido utilizando somente carvão vegetal como combustível, apresentou maior redutibilidade. Os cálculos cinéticos indicaram, tanto para o sínter produzido com carvão vegetal quanto para o sínter produzido com coque, que o mecanismo controlador nos primeiros instantes (entre 50 e 55 minutos) é a reação química. No sínter produzido com carvão vegetal, o mecânismo muda para o controle misto. No sínter produzido com coque, o mecânismo muda para a difusão. Ensaios interrompendo o tempo de redução em 40min, 80min e 120min (40min + 40min + 40min), foram realizados com a amostra de sínter produzido com 8% de carvão vegetal. Nesta etapa, foi possível identificar os fenômenos de nucleação e crescimento da fase de ferro metálico sobre a estrutura do sínter. / This work aimed to study the sinter reduction by hydrogen produced with charcoal and coke breeze. Three kinds of hematitic sinter feeds were used. The materials were characterized by particle size analysis, chemical analysis, X-ray diffraction optical and scanning electron microscopy. The inicial sintering tests aimed to produce return fines for subsequent sintering. Mixtures adding the sinter feed residue from the pellets process were sintered to evaluate the addition effect of this product in the process of sintering. A test was conducted to verify the effect of elimination of fines present in the sinter return sintering process. Six sintering tests were conducted replacing coke breeze by charcoal as process fuel. The replacements were 0%, 8%, 16%, 25%, 50% and 100%. The samples with substitutions of 0%, 50% e 100% were reduced in thermobalance for 6 hours at a temperature of 1000°C using a blend of gases containing Argon 10% Hydrogen. The samples which presented the best and the worst reducibility were studied through kinect calculations to determine the reaction mechanisms involved during the reduction of oxide. The step of microscopic characterization of reduced sinters was carried out in a way that made possible to compare the sinter sample before and after the process of reduction. The results showed the sinter made with charcoal decrease the sinter rates of production and resistance. However, the sinter made using only charcoal as fuel presented better reducibility. The kinect calculation denotes both for the sinter made with charcoal and sinter made with coke breeze the controlling mechanism in the initial stages (between 50 and 55 minutes) is the chemical reaction. For the sinter produced with charcoal the controlling mechanism switches for mixed control. The sinter produced with coke breeze, the mechanism switches for diffusion. Tests interrupting the reduction time for 40min, 80min e 120min (40min + 40min + 40min) were produced with sinter sample produced with 8% with charcoal. At this stage it was possible to identify the phenomena of nucleation and growth phase of metallic iron on the sinter structure.

Eletrólitos sólidos cerâmicos de ZrO2:8% mol Y2O3 foram preparados com pós de três origens: pó comercial fabricado pela Nissan (Japão), pó obtido por coprecipitação dos hidróxidos no IPEN e pó obtido por mistura de óxidos (utilizando-se ZrO2 produzido na Usina de Zircônio do IPEN e Y2O3 (USA) de pureza superior a 99,9%). Esses materiais de partida foram analisados pelas seguintes técnicas: fluorescência de raios X para a determinação dos teores de ítrio, difração de raios X para a determinação das fases, sedimentação para a determinação da distribuição do tamanho de partículas, adsorsão gasosa (BET) para a determinação da área de superfície específica e microscopia eletrônica de transmissão para a determinação do tamanho médio de partícula. Amostras compactadas foram analisadas por dilatometria para avaliar os estágios de sinterização. As amostras sinterizadas foram caracterizadas por difração de raios X para análise de fases e microscopia eletrônica de varredura para análise da morfologia dos grãos. Análises por espectroscopia de impedância foram feitas para acompanhar o envelhecimento térmico dos eletrólitos sólidos de zircônia-ítria a 600 ºC, temperatura de trabalho do sensor permanente de oxigênio, e para estudar a sua cinética de sinterização. Os principais resultados mostram que o envelhecimento térmico a 600 ºC diminui a resposta (fem) do sensor nas primeiras 100 h até um valor estável. Além disso, os estudos de sinterização por espectroscopia de impedância permitiram encontrar correlações entre parâmetros elétricos, a cinética de sinterização e os mecanismos de crescimento de grão. / ZrO2:8 mol %Y2O3 solid electrolyte ceramic pellets have been prepared with powders of three different origins: a Nissan (Japan) commercial powder, a powder obtained by the coprecipitation technique at IPEN, and the mixing of powder oxides (ZrO2 produced at a Pilot Plant at IPEN and 99.9% pure Y2O3 of USA origin). These starting powders have been analysed by the following techniques: X-ray fluorescence for yttrium content, X-ray diffraction for structural phase content, sedimentation for particle size distribution, gas adsorption (BET) for surface area determination, and transmission electron microscopy for average particle size determination. Pressed ceramic pellets have been analysed by dilatometry to evaluate the sintering stages. Sintered pellets have been characterized by X-ray diffraction for phase analysis and scanning electron microscopy for grain morphology analysis. Impedance spectroscopy analysis have been carried out to follow thermal ageing of zirconia-yttria solid electrolyte at 600 ºC, the working temperature of permanent oxygen sensor, and to study sintering kinetics. The main results show that ageing at 600 ºC decreases the emf sensor response in the first 100 h to a steady value. Moreover, sintering studies by impedance spectroscopy allowed for finding correlations between electrical parameters, sintering kinetics and grain growth mechanisms.

Le « Frittage Flash » ou « Spark Plasma Sintering (SPS) » est utilisé pour consolider des poudres en des temps relativement courts (quelques minutes). Ce procédé utilise un haut courant continu pulsé (quelques kA), traversant les parties conductrices du système et générant une montée rapide en température induite principalement par effet Joule. L’application d’un chargement mécanique, via des pistons, et d’une rapide montée en température permet d’obtenir une pièce dense sans grossissement excessif des grains. L’objectif de ce travail a été de proposer une simulation numérique thermo-électro-mécanique du procédé « Frittage Flash » sur ABAQUS, afin de suivre in situ les évolutions de température, de porosité et des contraintes difficilement accessibles expérimentalement. Dans ce travail, un modèle de comportement des corps poreux est proposé. Cette approche est basée sur les modèles micromécaniques de la littérature et modifiés de manière heuristique pour reproduire la densification réelle du matériau pour des porosités comprises entre 0 et 50%. Les simulations thermo-électro-mécanique incluant ce modèle, intègrent la dépendance en porosité et température des paramètres matériaux. Quatre cycles d’élaboration de poudre de nickel ont été réalisés avec différentes histoires de température. Les évolutions de la température et de la porosité calculées ont été confrontées avec des résultats expérimentaux. Des analyses post mortem sur des échantillons densifiés confortent la distribution de la température obtenue par le calcul. Ce travail ouvre de nombreuses perspectives, notamment, la possibilité d’optimiser le procédé / Nowadays, Spark Plasma Sintering (SPS) is used to consolidate powders in a relative short time (few minutes). This process uses a pulsed high DC electrical current (few kA) which flows through the conductive part of the device and generates large heating rate mainly due to Joule effect. The application of an uniaxial pressure via punches combined with a rapid heating allow the production of near net shape specimen. The thermal electrical mechanical numerical simulation of SPS process is a powerful tool to capture in situ evolutions of temperature, porosity and stresses which are difficult to obtain in experiments. In this work, a new constitutive model is presented for the description of the behavior of porous medium. This model is based on original viscoplastic micromechanical models of the literature and modified in a heuristic manner to better reproduce the real densification of sintered material for porosity in the range [0;0,5]. The model has been implemented in ABAQUS software. A thermal electrical mechanical simulation of SPS is performed where the dependence of material parameters on temperature and porosity is taken into account. Four processing cycles of nickel have been conducted with different temperature histories. Calculated porosity and temperature evolutions are compared to experimental results. Post-mortem analyses of the material (grain size, yield stress) confirm the temperature distribution obtained by numerical simulations in the sample made of nickel. This simulation is seen to be able to capture experimental trends. The work will permit in a near future the optimization of the sintering conditions to reach prescribed properties

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1982. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND SCIENCE / Vita. / Includes bibliographical references. / by Elaine Paula Rothman. / M.S.

Eletrólitos sólidos cerâmicos de ZrO2:8% mol Y2O3 foram preparados com pós de três origens: pó comercial fabricado pela Nissan (Japão), pó obtido por coprecipitação dos hidróxidos no IPEN e pó obtido por mistura de óxidos (utilizando-se ZrO2 produzido na Usina de Zircônio do IPEN e Y2O3 (USA) de pureza superior a 99,9%). Esses materiais de partida foram analisados pelas seguintes técnicas: fluorescência de raios X para a determinação dos teores de ítrio, difração de raios X para a determinação das fases, sedimentação para a determinação da distribuição do tamanho de partículas, adsorsão gasosa (BET) para a determinação da área de superfície específica e microscopia eletrônica de transmissão para a determinação do tamanho médio de partícula. Amostras compactadas foram analisadas por dilatometria para avaliar os estágios de sinterização. As amostras sinterizadas foram caracterizadas por difração de raios X para análise de fases e microscopia eletrônica de varredura para análise da morfologia dos grãos. Análises por espectroscopia de impedância foram feitas para acompanhar o envelhecimento térmico dos eletrólitos sólidos de zircônia-ítria a 600 ºC, temperatura de trabalho do sensor permanente de oxigênio, e para estudar a sua cinética de sinterização. Os principais resultados mostram que o envelhecimento térmico a 600 ºC diminui a resposta (fem) do sensor nas primeiras 100 h até um valor estável. Além disso, os estudos de sinterização por espectroscopia de impedância permitiram encontrar correlações entre parâmetros elétricos, a cinética de sinterização e os mecanismos de crescimento de grão. / ZrO2:8 mol %Y2O3 solid electrolyte ceramic pellets have been prepared with powders of three different origins: a Nissan (Japan) commercial powder, a powder obtained by the coprecipitation technique at IPEN, and the mixing of powder oxides (ZrO2 produced at a Pilot Plant at IPEN and 99.9% pure Y2O3 of USA origin). These starting powders have been analysed by the following techniques: X-ray fluorescence for yttrium content, X-ray diffraction for structural phase content, sedimentation for particle size distribution, gas adsorption (BET) for surface area determination, and transmission electron microscopy for average particle size determination. Pressed ceramic pellets have been analysed by dilatometry to evaluate the sintering stages. Sintered pellets have been characterized by X-ray diffraction for phase analysis and scanning electron microscopy for grain morphology analysis. Impedance spectroscopy analysis have been carried out to follow thermal ageing of zirconia-yttria solid electrolyte at 600 ºC, the working temperature of permanent oxygen sensor, and to study sintering kinetics. The main results show that ageing at 600 ºC decreases the emf sensor response in the first 100 h to a steady value. Moreover, sintering studies by impedance spectroscopy allowed for finding correlations between electrical parameters, sintering kinetics and grain growth mechanisms.

A research report submitted to the Faculty of Engineering and the Built Environment, University of the Witwatersrand, Johannesburg, in partial fulfilment of the requirements for the degree of Master of Science in Engineering, July 2015 / ABSTRACT Solid titanium (Ti) and Ti-6Al-4V (wt.%) materials were fabricated from powders using spark plasma sintering (SPS), cold isostatic press (CIP) and sinter, layered (rapid) manufacturing, centrifugal and vacuum casing. ASTM Grade 4 Ti, Al and V, 60Al-40V (wt.%) and the pre-alloyed Ti-6Al-4V powders were used as starting materials. The solid Ti and Ti-6Al-4V materials produced by the SPS were compared to the CIP and sinter method on the basis of density, microstructure and chemistry. The materials produced by the CIP and sinter method were also compared to those produced by vacuum casting method on the basis of microstructure, oxygen pick-up, chemistry and room temperature tensile properties. Centrifugal casting was compared to the vacuum casting technique on the basis of microstructural homogeneity. Rapid manufacturing was compared to SPS and CIP and sinter on the basis of microstructural homogeneity, density and tensile properties. The tensile properties of all materials were also compared to their commercial counterparts to investigate the effect of interstitial oxygen. The technology resulting in materials with superior properties was finally identified as most promising for commercial production of Ti-based materials. On the basis of densification, the SPS method appears superior compared to the CIP and sinter and rapid manufacturing method due to the benefit of pressure aided sintering, while the rapid manufacturing method is superior to the CIP and sinter method due to the use of a high power laser resulting in high densification rates. In cases where microstructural homogeneity is the key requirement, the CIP and sinter and rapid manufacturing methods appear superior compared to the SPS method due to longer isothermal holding time and higher sintering temperature and the use of pre-alloyed Ti-6Al-4V powder, respectively. On the basis of oxygen pick-up and additional contamination, the vacuum casting route is inferior due to the tendency of melt-crucible interaction, resulting in the dissociation of ZrO2 and subsequent pick-up of O and Zr. Based on the homogeneity of the microstructure, centrifugal casting is better than vacuum casting. The ductility of vacuum cast Ti was better than that of CIP and sintered Ti, possibly due to limited diffusion of oxygen from the crucible compared to oxygen absorbed from the controlled atmosphere during CIP and sinter. The vacuum casting of the Ti-6Al-4V alloy resulted in dissolution of oxygen and Zr due to melt-crucible interaction. Hence the ductility was worse compared to the alloy produced by CIP and sinter. The rapidly manufactured Ti-6Al-4V specimens exhibited superior ductility and strength compared to all alloys produced by other methods due to the use of high purity starting powder. The tensile properties of these specimens were also comparable to standard requirements. The similarity of the tensile properties of wrought Ti-6Al-4V alloy reported in the literature was an indication of limited oxygen pick-up during rapid manufacturing. Therefore based on low oxygen pick-up, microstructural homogeneity, high density and superior tensile properties, the rapid manufacturing route appears to be the most promising approach for commercial processing of titanium based materials.

Continuous and pulse selective laser sintering and laser powder deposition were used to find a solution to the manufacturing of micro-foil lattice structured components. A full factorial test matrix was used for each process to determine the processes capability to produce continuous tracks for joining the micro-foil materials. The samples were evaluated for dimensional profiles, distortion, and cycle times, to develop selection criteria for implementation of the processes into industry. The selective laser sintering processes were able to join the micro-foil materials into lattice structures with continuous tracks. The laser powder deposition processes were not able to properly join the micro-foil materials into lattice structures. The end results showed that micro-foil lattice structures can be produced using continuous and pulse selective laser sintering.

Increasing concern over the environmental impact of motor vehicles is driving the need for the development of lighter materials to reduce automobile weight and fuel consumption. Sintered aluminium alloys, with their high strength to weight ratios, have potential applications in the automotive industry, but conventional pressed-and-sintered materials have poor mechanical properties due to the presence of residual porosity in the sintered compact. Residual porosity can be eliminated by Hot Isostatic Pressing (HIPing) or combined sinter-HIPing, but these processes are expensive due to the high gas pressures involved, up to several hundred MPa, and also pose a significant safety hazard. There is a limited amount of evidence in the literature suggesting that applied gas pressures as low as a few MPa may be beneficial to the sintering of aluminium alloys, and it is this idea that the present work explores. Compacts of aluminium alloy 2712 (Al-3.8Cu-1Mg-0.7Si-0.1Sn) were prepared from elemental powders and sintered at 590ºC for up to 60 minutes in a horizontal tube furnace under constant flowing nitrogen or argon at pressures up to 600 kPa. Archimedes’ method was used to measure the density of sintered compacts, and the amount of open and closed porosity. Increasing the nitrogen pressure at the start of the isothermal holding stage to 160 kPa increased the sintering rate compared to standard atmospheric pressure sintering. Increasing the nitrogen pressure further, up to 600 kPa, had no additional benefit. The sintering rate was increased further by applying a 600 kPa nitrogen pressure during both heating and isothermal holding. The elevated nitrogen pressure had a negligible effect on the maximum sintered density achieved, and sintering in argon at elevated pressures had no measurable effect on the sintered density or sintering rate. It was shown that the elevated pressure aids in the closure of pores open to the specimen surface, contrary to HIPing and sinter-HIPing, where the pores must be isolated prior to the application of pressure. It was also shown that at 600 kPa nitrogen pressure, the sintered density was independent of the presence of tin in the alloy. The improvements to sintering seem to be related to the formation of aluminium nitride.

Le frittage est un phénomène physico-chimique excessivement complexe et gouverné par de nombreuses variables expérimentales. Nous souhaitions comprendre ce phénomène et identifier les variables expérimentales influençant les propriétés dun matériau ayant subi un cycle de frittage. Les variables expérimentales étant nombreuses et interagissant parfois entre elles, une approche multivariée du problème était indispensable afin de fournir un travail riche dinformations. Nous avons donc utilisé massivement des techniques dites de data-mining (planification expérimentale) afin de mettre en évidence les corrélations existantes entre les paramètres expérimentaux et les propriétés du matériau fritté.

An onset coarsening-coalescence event based on the incubation time of cylindrical mesopore formation and a significant decrease of specific surface area by 50% relative to the dry pressed samples was determined by N2 adsorption-desorption hysteresis isotherm for cassiterite SnO2 nanoparticles (rutile-type structure with bimodal size distribution). In the temperature range of 800-1100oC, the nanoparticles underwent onset sintering coupled with coarsening-coalescence without appreciable polymorphic transformation or decomposition of SnO2. The apparent activation energy of such a rapid process for SnO2 nanoparticles was estimated as 75 ¡Ó 5 kJ/mol, respectively. The minimum temperature for sintering/coarsening/coalescence of the SnO2 nanoparticles is 735oC based on the extrapolation of steady specific surface area reduction rates to null. PLA fragmentation of cassiterite SnO2 powder (rutile type, 20-50 nm in size) in water was conducted under Q-switch mode (532 nm, 400 mJ per pulse) having laser focal point fixed at 5, 10, 15 and 20 mm beneath the water level for an accumulation time of 5, 15, 20 and 30 min at 10 Hz. The 532 nm laser incidence suffered little water absorption and was effective to produce cassiterite nanocondensates as small as 5 nm in diameter and occasional nanocondensates of £\-PbO2-type structure more or less in coalescence. The combined effects of nanosize, internal compressive stress and H+ and Sn2+ co-signature in the lattice may account for a lower minimum band gap.