Tantalum carbide (TaC) and hafnium carbide (HfC) possess extremely high melting points, around 3900 oC, which are the highest among the known materials. TaC and HfC exhibit superior oxidation resistance under oxygen deficient and rich environments, respectively. A versatile material can be expected by forming solid solutions of TaC and HfC. However, the synthesis of fully dense solid solution carbide is a challenge due to their intrinsic covalent bonding which makes sintering challenging. The aim of the present work is to synthesize full dense TaC-HfC solid solutions by spark plasma sintering with five compositions: pure HfC, HfC-20 vol.% TaC (T20H80), HfC- 50 vol.% TaC (T50H50), HfC- 80 vol.% TaC (T80H20), and pure TaC. To evaluate the oxidation behavior of the solid solutions carbides in an environment that simulates the various applications, an oxygen rich, plasma assisted flow experiment was developed. While exposed to the plasma flow, samples were exposed to a temperature of approximately 2800 oC with a gas flow speed greater than 300 m/s. Density measurements confirm near full density was achieved for all compositions, with the highest density measured in the HfC-contained samples, all consolidated without sintering aids. Confirmation of solid solution was completed using x-ray diffraction, which had an excellent match with the theoretical values computed using Vegard’s Law, which confirmed the formation of the solid solutions. The solid solution samples showed much improved oxidation resistance compared to the pure carbide samples, and the T50H50 samples exhibited the best oxidation resistance of all samples. The thickness of the oxide scales in T50H50 was reduced more than 90% compared to the pure TaC samples, and more than 85% compared to the pure HfC samples after 5 min oxidation tests. A new Ta2Hf6O17 phase was found to be responsible for the improved oxidation performance. Additionally, the structure of HfO2 scaffold filled with molten Ta2O5 was also beneficial to the oxidation resistance by limiting the availability of oxygen.

Made available in DSpace on 2014-12-17T14:07:16Z (GMT). No. of bitstreams: 1 LeilianeAO_TESE.pdf: 4802035 bytes, checksum: bde6bc8d17e2550b03283ed5a4ab3acc (MD5) Previous issue date: 2013-05-17 / Conselho Nacional de Desenvolvimento Cient?fico e Tecnol?gico / Steel is an alloy EUROFER promising for use in nuclear reactors, or in applications where the material is subjected to temperatures up to 550 ? C due to their lower creep resistance under. One way to increase this property, so that the steel work at higher temperatures it is necessary to prevent sliding of its grain boundaries. Factors that influence this slip contours are the morphology of the grains, the angle and speed of the grain boundaries. This speed can be decreased in the presence of a dispersed phase in the material, provided it is fine and homogeneously distributed. In this context, this paper presents the development of a new material metal matrix composite (MMC) which has as starting materials as stainless steel EUROFER 97, and two different kinds of tantalum carbide - TaC, one with average crystallite sizes 13.78 nm synthesized in UFRN and another with 40.66 nm supplied by Aldrich. In order to improve the mechanical properties of metal matrix was added by powder metallurgy, nano-sized particles of the two types of TaC. This paper discusses the effect of dispersion of carbides in the microstructure of sintered parts. Pure steel powders with the addition of 3% TaC UFRN and 3% TaC commercial respectively, were ground in grinding times following: a) 5 hours in the planetary mill for all post b) 8 hours of grinding in the mill Planetary only for steel TaC powders of commercial and c) 24 hours in the conventional ball mill mixing the pure steel milled for 5 hours in the planetary mill with 3% TaC commercial. Each of the resulting particulate samples were cold compacted under a uniaxial pressure of 600MPa, on a cylindrical matrix of 5 mm diameter. Subsequently, the compressed were sintered in a vacuum furnace at temperatures of 1150 to 1250 ? C with an increment of 20 ? C and 10 ? C per minute and maintained at these isotherms for 30, 60 and 120 minutes and cooled to room temperature. The distribution, size and dispersion of steel and composite particles were determined by x-ray diffraction, scanning electron microscopy followed by chemical analysis (EDS). The structures of the sintered bodies were observed by optical microscopy and scanning electron accompanied by EDS beyond the x-ray diffraction. Initial studies sintering the obtained steel EUROFER 97 a positive reply in relation to improvement of the mechanical properties independent of the processing, because it is obtained with sintered microhardness values close to and even greater than 100% of the value obtained for the HV 333.2 pure steel as received in the form of a bar / O a?o EUROFER ? uma liga promissora para utiliza??o em reatores nucleares, ou em aplica??es onde o material ? submetido a temperaturas de servi?o at? 550?C devido sua menor resist?ncia sob flu?ncia. Uma forma de aumentar essa propriedade, para que o a?o trabalhe a temperaturas mais altas ? necess?rio impedir o deslizamento de seus contornos de gr?o. Fatores que influenciam nesse deslizamento dos contornos s?o a morfologia dos gr?os, o ?ngulo e a velocidade dos contornos de gr?o. Esta velocidade pode ser diminu?da com a presen?a de uma fase dispersa no material, desde que seja fina e distribu?da de forma homog?nea. Neste contexto, este trabalho apresenta o desenvolvimento de um novo material Comp?sito de Matriz Met?lica (CMM), que tem como materiais de partida o a?o inoxid?vel EUROFER 97; e dois tipos diferentes de Carbeto de T?ntalo TaC, um com tamanhos m?dios de cristalitos de 13,78 nm sintetizado na UFRN e outro com 40,66 nm fornecido pela Aldrich. Objetivando melhorar as propriedades mec?nicas da matriz met?lica foi adicionado, atrav?s da metalurgia do p?, part?culas nanom?tricas desses dois tipos de TaC. Este trabalho discute o efeito da dispers?o desses carbetos na microestrutura das pe?as sinterizadas. P?s de a?o puro, com adi??o de 3% de TaC UFRN e 3% de TaC comercial respectivamente, foram mo?dos nos seguintes tempos de moagem: a) 5 horas, no moinho planet?rio para todos os p?s; b) 8 horas de moagem no moinho planet?rio somente para os p?s de a?o com TaC comercial; e c) 24 horas no moinho convencional de bolas da mistura do a?o puro mo?do durante 5 horas no moinho planet?rio com 3% de TaC comercial. Cada uma das amostras particuladas resultantes foram compactadas a frio sob uma press?o uniaxial de 600MPa, em uma matriz cil?ndrica de 5 mm de di?metro. Posteriormente, os compactados foram sinterizadas em forno a v?cuo, em temperaturas de 1150 e 1250? C com incremento de 20 ?C e 10?C por minuto, sendo mantidas nestas isotermas por 30, 60 e 120 minutos e resfriadas ? temperatura ambiente. A distribui??o, tamanho e dispers?o dos a?os e comp?sitos particulados foram determinadas por difra??o de raios x, microscopia eletr?nica de varredura seguida de uma an?lise qu?mica (EDS). As estruturas dos corpos sinterizados foram observadas por microscopia ?tica e eletr?nica de varredura acompanhada de EDS al?m da difra??o de raios x. Os estudos iniciais de sinteriza??o com o a?o EUROFER 97 obteve uma resposta positiva em rela??o a melhoria das propriedades mec?nicas independente do processamento, pois se obteve sinterizados com valores de microdureza pr?ximo e at? maior que 100% do valor de 333,2 HV obtidos para o a?o puro como recebido, em forma de barra

Transition metal carbides (TMCs) are of increasing interest for catalytic processes. Their performance and stability under common oxidative conditions in catalytic reactions are crucial for several applications, including catalysis and electrochemical reactions. In this work, we report a detailed XPS study of the interactions of stoichiometric NbC and TaC surfaces with common oxidizing agents like O2 and H2O, which are important media in many chemical processes. Experimental results showed that NbC reacts with O2 to produce Nb sub-oxrides, while TaC is inert to O2 exposure. TaC surfaces are more sensitive to H2O vapor, with a greater surface oxidation and hydroxylation. Atmospheric oxidation of NbC and TaC was also studied, and results showed that both films oxidized yielding to the formation of Nb2O5 and Ta2O5, hydroxylated/ oxide carbon species, and some adventurous carbon build-up. TMCs are catalytically active in many reactions, especially those involving electrochemical nitrogen reduction reactions (NRR) to ammonia. Experimental and DFT calculations were used to provide insight on how carbide surface structures change electrochemically and how that evolution relates to NRR activity. Results showed that NbC has NRR activity at pH 3.2 after immersion in 0.3 M NaOH, leaving niobium suboxides. However, photoemission data showed that the Nb2O5 overlayer is restored after polarization to -1.3 V vs. Ag/AgCl, inhibiting NRR activity. TaC, on the other hand, is inactive for NRR at potentials more positive than -1.0 V, as NaOH treatment fails to remove the Ta2O5 surface layer induced by ambient exposure. The study also found that the formation and stabilization of intermediate oxidation states on the surface of transition metal ions are crucial for N≡N bond activation and NRR activity.

Made available in DSpace on 2014-12-17T14:06:49Z (GMT). No. of bitstreams: 1 LeilianeAO.pdf: 1040153 bytes, checksum: 7b51c6820dc4a457d8c742b3fc25c179 (MD5) Previous issue date: 2008-08-22 / Conselho Nacional de Desenvolvimento Cient?fico e Tecnol?gico / The present work shows a contribution to the studies of development and solid sinterization of a metallic matrix composite MMC that has as starter materials 316L stainless steel atomized with water, and two different Tantalum Carbide TaC powders, with averages crystallite sizes of 13.78 nm and 40.66 nm. Aiming the metallic matrix s density and hardness increase was added different nanometric sizes of TaC by dispersion. The 316L stainless steel is an alloy largely used because it s high resistance to corrosion property. Although, its application is limited by the low wear resistance, consequence of its low hardness. Besides this, it shows low sinterability and it cannot be hardened by thermal treatments traditional methods because of the austenitic structure, face centered cubic, stabilized mainly in nickel presence. Steel samples added with TaC 3% wt (each sample with different type of carbide), following a mechanical milling route using conventional mill for 24 hours. Each one of the resulted samples, as well as the pure steel sample, were compacted at 700 MPa, room temperature, without any addictive, uniaxial tension, using a 5 mm diameter cylindrical mold, and quantity calculated to obtain compacted final average height of 5 mm. Subsequently, were sintered in vacuum atmosphere, temperature of 1290?C, heating rate of 20?C/min, using different soaking times of 30 and 60 min and cooled at room temperature. The sintered samples were submitted to density and micro-hardness analysis. The TaC reforced samples showed higher density values and an expressive hardness increase. The complementary analysis in optical microscope, scanning electronic microscope and X ray diffractometer, showed that the TaC, processed form, contributed with the hardness increase, by densification, itself hardness and grains growth control at the metallic matrix, segregating itself to the grain boarders / O presente trabalho apresenta uma contribui??o ao estudo do desenvolvimento e sinteriza??o s?lida de um Comp?sito de Matriz Met?lica CMM que tem como materiais de partida um a?o inoxid?vel 316L atomizado a ?gua, e duas partidas diferentes de Carbeto de T?ntalo TaC, com tamanhos m?dios de cristalitos de 13,78 nm e 40,66 nm. Objetivando aumentar a densidade e dureza da matriz met?lica foi adicionado, por dispers?o diferentes part?culas nanom?tricas de TaC. O a?o inoxid?vel 316L ? uma liga largamente utilizada pela sua propriedade de alta resist?ncia ? corros?o. Contudo, sua aplica??o ? limitada pela baixa resist?ncia ao desgaste, conseq??ncia da sua baixa dureza. Al?m disso, apresenta baixa sinterabilidade e n?o pode ser endurecido pelos m?todos tradicionais de tratamentos t?rmicos, devido a sua estrutura austen?tica, c?bica de face centrada, estabilizada principalmente pela presen?a do N?quel. Amostras de a?os adicionadas com 3% em peso de TaC (cada amostra com carbetos de partidas diferentes), seguiram uma rota de moagem mec?nica em moinho convencional por 24 horas. Cada uma das amostras resultantes, assim como amostras do a?o puro foram compactados a 700 MPa, a frio, sem nenhum aditivo, uniaxialmente, em uma matriz cil?ndrica de 5 mm de di?metro, em quantidade calculada para ter uma altura m?dia final do compactado de 5 mm. Posteriormente, foram sinterizadas em forno a v?cuo, em temperatura de at? 1290? C com incremento de 20 ?C por minuto, sendo mantidas neste patamar por 30 ou 60 minutos e resfriadas ? temperatura ambiente. As amostras sinterizadas foram submetidas aos ensaios para a medi??o da densidade e da micro-dureza. As amostras contendo o refor?o de TaC apresentaram maiores valores de densidade e um aumento significativo na sua dureza. As an?lises complementares no microsc?pio ?tico, no microsc?pio eletr?nico de varredura e no difrat?metro de raios-X, mostram que o TaC, na forma processada, contribuiu com o aumento da dureza, pela densifica??o, pela sua pr?pria dureza e pelo controle do crescimento dos gr?os da matriz met?lica, segregando-se nos seus contornos

Made available in DSpace on 2014-12-17T14:07:09Z (GMT). No. of bitstreams: 1 Maria JSL_DISSERT.pdf: 2126864 bytes, checksum: 5f05e8062e1e0a0ae6a41666703f3c86 (MD5) Previous issue date: 2013-06-20 / Coordena??o de Aperfei?oamento de Pessoal de N?vel Superior / The research and development of nanostructured materials have been growing significantly in the last years. These materials have properties that were significantly modified as compared to conventional materials due to the extremely small dimensions of the crystallites. The tantalum carbide (TaC) is an extremely hard material that has high hardness, high melting point, high chemical stability, good resistance to chemical attack and thermal shock and excellent resistance to oxidation and corrosion. The Compounds of Tantalum impregnated with copper also have excellent dielectric and magnetic properties. Therefore, this study aimed to obtain TaC and mixed tantalum oxide and nanostructured copper from the precursor of tris (oxalate) hydrate ammonium oxitantalato, through gas-solid reaction and solid-solid respectively at low temperature (1000 ? C) and short reaction time. The materials obtained were characterized by X-ray diffraction (XRD), Rietveld refinement, Scanning Electron Microscopy (SEM), Spectroscopy X-Ray Fluorescence (XRF), infrared spectroscopy (IR), thermogravimetric (TG), thermal analysis (DTA) and BET. Through the XRD analyses and the Reitiveld refinement of the TaC with S = 1.1584, we observed the formation of pure tantalum carbide and cubic structure with average crystallite size on the order of 12.5 nanometers. From the synthesis made of mixed oxide of tantalum and copper were formed two distinct phases: CuTa10O26 and Ta2O5, although the latter has been formed in lesser amounts / A pesquisa e o desenvolvimento de materiais nanoestruturados v?m crescendo significativamente nos ?ltimos anos. Estes materiais apresentam propriedades significativamente modificadas em compara??o ?s dos materiais convencionais, devido ?s dimens?es extremamente reduzidas dos cristalitos. O carbeto de t?ntalo (TaC) ? um material extremamente duro, apresentando elevada dureza, elevado ponto de fus?o, elevada estabilidade qu?mica, boa resist?ncia ao ataque qu?mico e choque t?rmico e excelente resist?ncia ? oxida??o e corros?o. Os compostos de T?ntalo impregnados com Cobre tamb?m possuem excelentes propriedades diel?tricas e magn?ticas. Desta forma este trabalho teve como objetivo a obten??o de TaC e do ?xido misto de t?ntalo e cobre nanoestruturado a partir do precursor tris(oxalato)oxitantalato de am?nio hidratado, atrav?s de rea??o g?s-s?lido e s?lido-s?lido, respectivamente,a baixa temperatura (1000?C) e curto tempo de rea??o. Os materiais obtidos foram caracterizados atrav?s de Difra??o de Raios-X (DRX), Refinamento Rietveld, Microscopia Eletr?nica de Varredura (MEV), Espectroscopia por Fluoresc?ncia de Raios-X (FRX), Espectroscopia de Infravermelho (IV), Termogravim?trica (TG), Analise Termodiferencial (DTA) e BET. Atrav?s das analises de DRX e do refinamento Reitiveld para o TaC com S= 1,1584 observou-se a forma??o do carbeto de t?ntalo puro com estrutura c?bica e tamanho m?dio de cristalitos na ordem de 12,5 nan?metros. Para a s?ntese realizada do ?xido misto de t?ntalo e cobre houve a forma??o de duas fases distintas: CuTa10O26 e Ta2O5, embora esta ?ltima tenha sido formada em menor quantidade

Hypersonic aerospace vehicles are severely limited by the lack of adequate high temperature materials that can withstand the harsh hypersonic environment. Tantalum carbide (TaC), with a melting point of 3880°C, is an ultrahigh temperature ceramic (UHTC) with potential applications such as scramjet engines, leading edges, and zero erosion nozzles. However, consolidation of TaC to a dense structure and its low fracture toughness are major challenges that make it currently unviable for hypersonic applications. In this study, Graphene NanoPlatelets (GNP) reinforced TaC composites are synthesized by spark plasma sintering (SPS) at extreme conditions of 1850˚C and 80-100 MPa. The addition of GNP improves densification and enhances fracture toughness of TaC by up to ~100% through mechanisms such as GNP bending, sliding, pull-out, grain wrapping, crack bridging, and crack deflection. Also, TaC-GNP composites display improved oxidation behavior over TaC when exposed to a high temperature plasma flow exceeding 2500 ˚C.