Made available in DSpace on 2014-10-09T12:43:36Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T13:58:20Z (GMT). No. of bitstreams: 1 06531.pdf: 4466032 bytes, checksum: a6e28ab0187aa788035c1d0e3ed8764f (MD5) / Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) / Dissertacao (Mestrado) / IPEN/D / Instituto de Pesquisas Energeticas e Nucleares - IPEN/CNEN-SP / FAPESP:96/08958-1

Made available in DSpace on 2014-10-09T12:33:49Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:03:57Z (GMT). No. of bitstreams: 0 / Neste trabalho, investigou-se a elaboração mecânica do composto intermetálico TiFe por moagem de bolas com alta energia. Uma forte aderência do material moído, particularmente nas paredes do recipiente de moagem, foi o principal problema verificado com tempos de moagem superiores a 1 hora (moinho agitador). Tentativas para resolver este problema foram realizadas inicialmente com o emprego de agentes controladores de processo (ACPs), como etanol, ácido esteárico, polietileno de baixa densidade, benzeno e ciclohexano, em diferentes quantidades (1 a 20% em massa) e tempos (1 a 40 h), mantendo-se constantes outros parâmetros de moagem como a razão bola:pó em massa (10:1) e o tamanho das bolas (=7mm). Os rendimentos mais elevados (em termos da massa de pó não aderido) foram obtidos quando se utilizaram grandes quantidades de benzeno e ciclohexano (101 e 103% em massa, respectivamente), porém com a formação de TiC ao invés de TiFe em razão da decomposição do ACP e reação do carbono com as partículas de titânio. As moagens foram realizadas posteriormente sem o emprego de qualquer ACP e também utilizando um moinho planetário. Várias estratégias foram investigadas para se tentar mitigar a aderência incluindo-se: (a) moagem de uma pequena quantidade da mistura de pós de Ti e de Fe, revestindo as paredes do recipiente e as bolas de moagem, antes da moagem da carga principal, (b) moagem pausada com aberturas intermediarias do recipiente em atmosfera ambiente, (c) moagem pausada para rotação e inversão da posição do recipiente de moagem (apenas no moinho agitador), (d) moagem isolada dos pós de Ti e de Fe, antes da moagem da mistura, e (e) moagem do pó de Fe com o Ti hidretado. Os melhores resultados, em termos de diminuição da aderência combinada com a formação majoritária do composto TiFe durante a moagem, foram obtidos quando se adotou o procedimento de inversão/rotação, juntamente com o processo de revestimento preliminar do recipiente e das bolas de moagem (26% em massa). Rendimentos maiores foram obtidos com a utilização do TiH2 no moinho planetário, porém sem a formação majoritária do TiFe durante a moagem. / Dissertação (Mestrado) / IPEN/D / Instituto de Pesquisas Energéticas e Nucleares - IPEN-CNEN/SP

Made available in DSpace on 2014-10-09T12:54:37Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:07:21Z (GMT). No. of bitstreams: 0 / Neste trabalho, a moagem com alta energia foi utilizada para a ativação mecânica de misturas de pós de alumínio e nióbio, na proporção de 75% atômico de alumínio, para a síntese por reação de combustão do NbAl3. O objetivo foi investigar os mecanismos de ativação atuantes e a eventual preponderância de um deles. A moagem foi realizada tanto nos pós de alumínio e de nióbio separadamente (pré-ativação), como nas misturas. O processo de síntese por reação foi realizado no modo combustão simultânea, em pastilhas compactadas a partir de misturas com e sem ativação mecânica. O comportamento térmico das pastilhas foi registrado durante todo o ciclo térmico de aquecimento e, as principais características térmicas da reação de combustão, foram determinadas. O parâmetro de rede, o tamanho de cristalito e a microdeformação elástica do alumínio e do nióbio foram determinados por difratometria de raios X, mediante análise pelo método de Rietveld. A microscopia eletrônica de varredura foi utilizada para caracterização microestrutural dos pós moídos e da pastilha reagida. Constatou-se que o mecanismo preponderante de ativação mecânica é o aumento da área de interface, que ocorre durante a formação de agregados de partículas de alumínio e nióbio. A eficiência na formação de interfaces diminuiu com a utilização de nióbio pré-ativado (encruado) e com o aumento da quantidade de ácido esteárico (utilizado como agente controlador de processo durante a moagem). O efeito principal da ativação mecânica na síntese por reação de combustão foi a redução da temperatura de ignição com o aumento do tempo de moagem. A alta densidade de defeitos cristalinos, gerada durante a pré-ativação dos pós de alumínio e nióbio e na ativação mecânica das misturas, não produziu efeitos mensuráveis sobre o comportamento térmico das pastilhas. / Tese (Doutoramento) / IPEN/T / Instituto de Pesquisas Energéticas e Nucleares - IPEN/CNEN-SP

Mechanical Alloying is a manufacturing process that produces alloys by cold welding of powders. Usually, a vial containing both the powder and steel balls is agitated. Due to impact between the balls and balls and the vial, the powder is mechanically deformed, crushed, and mixed at nano-scales. In this thesis, a numerical model is developed to simulate the dynamics of the vial and the grinding balls of the SPEX 8000 ball milling device, a standardized equipment in both industrial and academic investigations of ball milling. The numerical model is based on the Event Driven Method, typically used to model granular flows. The method implemented is more efficient than the discrete element method used previously to study ball milling dynamics. The numerical tool obtained is useful for scale-up and optimization of mechanical alloying of various materials. An optimization study is presented for the SPEX 8000.

Les travaux présentés dans cette thèse portent sur la synthèse et la caractérisation structurale et physico-chimique de composés sulfures à propriétés thermoélectriques. Un intérêt a été porté sur plusieurs familles de composés sulfures avec pour objectif le développement et/ou l’optimisation des performances thermoélectriques de ces composés.Un premier composé binaire, TiS2, a été élaboré par mécanosynthèse suivi d’une étape de densification par Spark Plasma Sintering (SPS). Les caractérisations structurales ont démontré un effet du processus d’élaboration sur la microstructure ainsi que sur la stœchiométrie du composé. Ce procédé induit une réduction considérable de la conductivité thermique mais aussi électrique du matériau ne permettant pas d’optimiser la figure de mérite du composé. Un second composé a ensuite été développé selon deux voies de synthèses (conventionnelle et mécanosynthèse), le composé ternaire Cu4Sn7S16. Il a été mis en évidence que ce composé semi-conducteur possède une structure complexe qui favorise une conductivité thermique intrinsèquement faible. Les propriétés thermoélectriques ainsi que l’influence de la non-stœchiométrie sur ce composé ont été rapportées. Enfin les composés CuCoxTi2-xS4 et Cu26V2Sn6S32 ont été au cœur des derniers résultats présentés. Ces composés présentent des propriétés de transport plus métalliques propices à l’obtention de facteurs de puissance plus élevés que dans composé Cu4Sn7S16. D’une part, l’élaboration du matériau et l’influence du taux de Co sur le transport électronique ont été discutées sur le composé CuCoxTi2-xS4. D’autre part, l’élaboration par la mécanosynthèse ainsi que les conditions de densification ont été reliés aux propriétés de transport du composé Cu26V2Sn6S32. Une amélioration significative des performances thermoélectriques de ce dernier a été rapportée.Ces différentes études ouvrent des perspectives intéressantes dans l’élaboration et l’optimisation des composés sulfures en vue d’applications industrielles. / The work presented in this thesis focuses on the synthesis and the structural/physicochemical characterizations of sulfide compounds with thermoelectric properties. Several families of sulphide compounds have been studied with the aim of developing and/or optimizing their thermoelectric performances.A binary compound, TiS2, was synthesized by mechanical alloying followed by a densification using Spark Plasma Sintering (SPS). The structural characterizations have revealed the effect of the elaboration on the microstructure and stoichiometry of the compound. This process induces a considerable reduction in the thermal and electrical conductivity of the material which hindered the optimization of the figure of merit. The ternary compound Cu4Sn7S16 was then developed according to two synthetic routes (conventional and mechanical alloying). It has been demonstrated that this semiconductor compound has a complex structure which promotes an intrinsic low thermal conductivity. The influence of the non-stoichiometry on the thermoelectric properties has been reported. Finally, the CuCoxTi2-xS4 and Cu26V2Sn6S32 compounds were the last interesting results presented. These compounds show metallic transport properties with high power factors. The synthesis and the influence of the Co content on the electronic transport properties have been discussed on the CuCoxTi2-xS4 compound. The effect of mechanical alloying and densification conditions were related to the transport properties of the Cu26V2Sn6S32 compound. Substantial improvement of the thermoelectric performances as reported.These various studies open interesting perspectives for the development and optimization of sulfide compounds for industrial application.

Aluminum-alumina nanocomposites were synthesized using mechanical alloying of blended component powders of pure constituents. This study was performed on various powder mixtures with aluminum as the matrix and alumina as the reinforcement with volume fractions of 20, 30, and 50 % and Al2O3 particle sizes of 50 nm, 150 nm, and 5 µm. X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques were used for the crystal structure and microstructural characterization of the powders at different stages of milling. Al2O3 powders with 50 nm and 150 nm particle size were predominantly of γ-type, while Al2O3 of 5 µm size was of α-type. The main goal was to achieve uniform distribution of the Al2O3 ceramic particles in the Al matrix, which was achieved on milling for 24 h in a SPEX mill or 100 h in a Fritsch Pulverisette planetary ball mill. The powders were consolidated in two stages: pre-compaction at room temperature followed by vacuum hot pressing (VHP) or hot isostatic pressing (HIP) techniques to a fully dense condition. The effect of reinforcement particle size and volume fraction on the stress-strain response, elastic modulus and yield strength of the composites was investigated. Nanoindentation and compression tests were performed to characterize the composite material. Yield strength of 515 MPa, compressive strength of 685 MPa and elastic modulus of 36 GPa were obtained from compression tests. Nanoindentation results gave the yield strength of 336 MPa, maximum shear stress of 194 MPa and an elastic modulus of 42 GPa. The low elastic modulus values obtained from the above tests might be because of localized yielding possibly due to residual stresses.

Aluminum and metal-oxide powders were mixed using mechanical alloying. Exothermic displacement reactions could be initiated in the powders either by mechanical alloying alone or by heat treating the mechanically alloyed powders. Exponential relationships developed between the initiation time of the reaction and the mechanical alloying charge ratio. The exponential relationships were the result of changes in the intensity and quantity of collisions occurring during mechanical alloying. Differential thermal analysis of the mechanically alloyed powders indicated that increased milling time inhibited the initiation of the displacement reactions. It is believed that the reactions were inhibited because of heat dissipation from reacting oxide particles in the surrounding metal. Determining the effects of mechanical alloying on displacement reactions will lead to a more thorough understanding of the kinetics of mechanical alloying. Reacted powders were densified by uniaxial compaction and extrusion. Metallographic analysis of the reacted specimens confirmed the findings of the thermal analysis. Increased mechanical alloying inhibited the chemical reactions. Densified specimens from longer-milled mechanically alloyed specimens showed finer, more uniformly dispersed reaction products. These samples also showed increased mechanical properties as a result of their finer microstructure. Current particle strengthening models were used to accurately predict room temperature properties. Because of the fine microstructures produced, it may be possible to use similar techniques to yield new high-temperature aluminum alloys. / Ph. D.

This work investigates processing-microstructure-property relationships of a model cryogenically mechanically alloyed polymer-polymer system: polycarbonate (PC) and poly (ether ether ketone) (PEEK). Mechanically milled and alloyed powders were characterized using a variety of techniques including microscopy and thermal analysis. Cryogenically mechanically alloyed powders processed for 10 hours were shown to have a sub-micron level two-phase microstructure. These powders were processed into testable coupons using a mini ram-injection molder; microstructure and bulk mechanical properties of the coupons were investigated as a function of mechanical alloying and injection molding parameters. Atomic force microscopy, transmission electron microscopy, and scanning transmission X-ray microscopy revealed that the intimate blending achieved during the mechanical alloying process is not retained upon post-processing using a conventional polymer processing technique. Injection molded coupons were tested in 3-point bend mode via dynamic mechanical and quasi-static mechanical testing. Results demonstrated that no improvement in energy to break, strain at failure, or failure strength was achieved in coupons made from cryogenically mechanically alloyed powders compared to those of coupons made from non-mechanically alloyed samples. / Ph. D.

The effects of mechanical alloying milling time and carbon concentration on microstructural evolution and hardness of high-carbon Fe-C alloys were investigated. Mechanical alloying and powder metallurgy methods were used to prepare the samples. Mixtures of elemental powders of iron and 1.4, 3, and 6.67 wt.% pre-milled graphite were milled in a SPEX mill with tungsten milling media for up to 100h. The milled powders were then cold-compacted and pressure-less sintered between 900°C and 1200°C for 1h and 5h followed by furnace cooling. Milled powders and sintered samples were characterized using X-ray diffraction, differential scanning calorimetry, Mossbauer spectroscopy, scanning and transmission electron microscopes. Density and micro-hardness were measured. The milled powders and sintered samples were studied as follows: In the milled powders, the formation of Fe_3 C was observed through Mossbauer spectroscopy after 5h of milling and its presence increased with milling time and carbon concentration. The particle size of the milled powders decreased and tended to become more equi-axed after 100h of milling. Micro-hardness of the milled powders drastically increased with milling time as well as carbon concentration. A DSC endothermic peak around 600°C was detected in all milled powders, and its transformation temperature decreased with milling time. In the literature, no explanation was found. In this work, this peak was found to be due to the formation of Fe_3 C phase. A DSC exothermic peak around 300°C was observed in powders milled for 5h and longer; its transformation temperature decreased with milling time. This peak was due to the recrystallization and/or recovery α-Fe and growth of Fe_3 C . In the sintered samples, almost 100% of pearlitic structure was observed in sintered samples prepared from powders milled for 0.5h. The amount of the pearlite decreased with milling time, contrary to what was found in the literature. The decrease in pearlite occurred at the same time as an increase in graphite-rich areas. With milling, carbon tended to form graphite instead of Fe_3 C. Longer milling time facilitated the nucleation of graphite during sintering. High mount of graphite-rich areas were observed in sintered samples prepared from powders milled for 40h and 100h. Nanoparticles of Fe_3 C were observed in a ferrite matrix and the graphite-rich areas in samples prepared from powders milled for 40h and 100h. Micro-hardness of the sintered samples decreased with milling time as Fe_3 C decreased. The green density of compacted milled powders decreased with milling time and the carbon concentration that affected the density of sintered samples. / Ph. D.

Elemental W and Ni powders were mechanically alloyed in a SPEX mill with WC grinding media for durations ranging from 5 to 50 hours, then compacted samples were sintered in hydrogen to generate bulk tungsten heavy alloys with 2, 4 and 6 wt.% Ni. Evidence of a bimodal grain size distribution was seen in X-ray diffractograms of sintered samples and confirmed by scanning electron microscopy. Grain sizes in the small-grained regions ranged from 200–600 nm; those in the large-grained regions ranged from 1–2 µm. Furthermore, the volume fraction of the small-grained region increased linearly as milling time increased. A slice from a sintered sample was prepared for examination by TEM, in which particles 30–100 nm in diameter were regularly observed on the boundaries of the 200–600 nm grains. EDS point analysis showed that the particles are WC. Therefore it is concluded that heterogeneously distributed contamination from the grinding media is continually incorporated during mechanical alloying and, during sintering, inhibits grain growth through Zener pinning. Densities of sintered samples increased as milling time increased to a maximum of almost 96% of the theoretical value. Density increases with respect to milling time were initially great but diminished upon further milling. While the samples with 4 and 6 wt.% Ni both approached 96% of the theoretical density value by 50 hours of milling, densities in the samples with 2 wt.% Ni were considerably lower. Thus it appears that the Ni that becomes incorporated into the bcc W structure during mechanical alloying activates W diffusion during sintering, though there is a limit to the amount of Ni that the W structure can accommodate. This is evinced in W lattice parameter values from the as-milled powders; while the lattice parameter drops considerable from 2 to 4 wt.% Ni, the difference between 4 and 6 wt.% Ni is much smaller and the Ni content limit surely falls between the two values. Otherwise-equivalent samples with added WC powder were also produced, but did not increase the volume fraction of the small-grained region – probably because the particles remained large and were homogeneously distributed. / Ph. D.