CT - DEMAT - Artigos publicados em periódicos
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Artigo Neutron diffraction and finite-element analysis of thermal residual stresses on diffusion-bonded silicon carbide–molybdenum joints(Wiley, 1999) Martinelli, Antonio Eduardo; Drew, Robin A.L.; Fancello, Eduardo A; Rogue, Ronald; Root, John H.Various approaches can be used to minimize residual stresses in ceramic–metal joining, such as a refractorymetal interlayer in a hot-pressed joint. Nonetheless, it is still necessary to characterize the stresses at and near the interface between the interlayer and the ceramic, as a function of the hot-pressing parameters. This study combines two techniques to assess the stress distribution of hotpressed silicon carbide–olybdenum joints: neutron diffraction and finite-element (FEM) analysis. The results demonstrate that the joining temperature greatly influences the final stress distribution, and that significant stress accommodation is achieved by controlling the cooling rate of the diffusion couples. FEM analysis provides a broad view of stress distribution profiles, whereas experimental stress values that are obtained via neutron diffraction allow a better assessment of the effects of parameters that are not easily reproduced using a mathematical modelArtigo Microstructure and mechanical strength of diffusion-bonded silicon nitride–molybdenum joints(Elsevier, 1999-10) Martinelli, Antonio Eduardo; Drew, Robin A.L.Solid-state bonding of reactive systems, such as Si3N4–Mo often results in the formation of excessively thick intermetallic layers that can be detrimental to the final strength of the joint. The objective of this work was to study the microstructural evolution of Si3N4–Mo interfaces, aiming at maximum joint strength via a balance between the fraction of bonded material and the amount of interfacial reaction. Joining was carried out under vacuum or nitrogen atmosphere for temperatures between 1100 and 1800°C. Microstructural analyses of the interfaces revealed the presence of Mo3Si and Mo5Si3 along with residual pores. The results from shear strength tests revealed a strong relationship between the microstructure of the interface and the mechanical strength of the jointArtigo Brazing Al2O3 to sintered Fe-Ni-Co alloys(Springer, 1999-12) Nascimento, Rubens Maribondo do; Martinelli, Antonio Eduardo; Bushinelli, Augusto José de Almeida; Klein, A. N.Active metal brazing has been widely used to join ceramics to metals, as sound joints are usually achieved in a single step process without special equipment. However, residual stresses may be a potential problem especially upon joining ceramics to alloys with relatively high thermal expansion coefficients. This work investigates the brazing behavior of Alumina (Al2O3) to a sintered Fe-Ni-Co alloy, specially designed to match its coefficient of thermal expansion to that of the ceramic counterpart. The results indicate the presence of an interfacial zone whose microstructure depends on the filler alloy employed. A relationship was established between the microstructure of the interface and the flexural strength of the jointsArtigo The effect of pulse width on the microstructure of d.c.-plasma-nitrided layers(Elsevier, 1999-12-15) Alves Júnior, Clodomiro; Rodrigues, José de Anchieta; Martinelli, Antonio EduardoModern ion nitriding equipment are powered by pulsed sources in order to prevent arcing and overheating. In industrial applications, the pulse frequency is usually adjusted to set the working temperature. However, frequency variations may drastically affect the plasma species population, and consequently, the microstructure of the final nitrided layer. In this study, AISI-409 steel was nitrided in a mixture of 20% N2–80% H2 using a d.c power source. The depth and microstructure of the nitrided layer were assessed as a function of the time that the voltage was off, toff, in the range of 1–4 ms during a 10 ms pulse cycle. The results suggested that both the microstructure and mechanical behavior of the nitrided layer were affected by toff. Long toff values imply a deleterious effect on the mechanical properties of the nitrided layersArtigo Growth of nitrided layers on Fe–Cr alloys(Elsevier, 2000-02-29) Alves Júnior, Clodomiro; Rodrigues, José de Anchieta; Martinelli, Antonio EduardoChromium is an important alloying element present in numerous commercial steels. A systematic study on the nitriding behavior of Fe–Cr alloys is helpful in predicting the properties of nitrided Cr-alloyed steels. Aspects such as microstructural evolution, growth kinetics, and mechanical properties should be particularly emphasized. Fe–Cr alloys containing 5, 10, and 20 wt.% Cr have been arc melted and subsequently plasma nitrided under a N2–80% H2 atmosphere. The microstructure of the resulting nitrided layers was characterized and the microhardness profiles evaluated. Thicker layers developed on low chromium alloys. Differences in hardness profiles were also observed as a function of chromium contents. Nitriding Fe–5% Cr alloys resulted in two discrete fronts, refereed to as the diffusion front and the transformation front. Transformed regions sustained a decrease in hardness from 1000 down to 600 HV, associated with the conversion of homogeneously dispersed fine precipitates into coarser needle like particles immersed in the ferritic matrix. Similar behavior was not observed for the other alloys, where both fronts developed simultaneouslyArtigo Effect of Y2O3 addition on the densification and mechanical properties of alumina±niobium carbide composites(Elsevier, 2000-04-21) Acchar, Wilson; Greil, P.; Martinelli, Antonio Eduardo; Vieira, F. A.; Bressiani, A. H. A.; Bressiani, José CarlosAlumina-based composites reinforced with refractory carbides are potential cutting tool materials. They exceed the capabilities of cemented carbides with respect to hot hardness and thermal stability, resulting in faster cutting speeds. Liquid-phase sintering of Al2O3±NbC composites was investigated as an alternative to pressure-assisted processes. Al2O3 reinforced by NbC (5±40 wt.%) was sintered with 3 wt.% Y2O3. In order to assess the e ect of the formation of a liquid phase on the properties of the composites, sintering was carried out either below or above the Al2O3±Y2O3 eutectic temperature, at 1650 and 1800 C, respectively. Density, hardness, fracture toughness and wear resistance of the composite materials were evaluated. Liquid phase sintering did not affect the fracture toughness, but improved both the density and the hardness of the material, regardless of its NbC contents. Higher concentrations of NbC increased the wear resistance of the compositeArtigo Sintering behaviour of alumina–tungsten carbide composites(Elsevier, 2000-05-31) Acchar, Wilson; Martinelli, Antonio Eduardo; Vieira, F. A.; Cairo, Carlos Alberto AlvesAlumina reinforced with tungsten carbide has been investigated as an alternative material for metalworking, combining resistance to high service temperatures and improved toughness. Pressureless sintered and hot-pressed Al2O3–WC composites were manufactured and characterised. The use of Y2O3 as a sintering additive has also been evaluated. Additions of up to 30 wt.% WC resulted in limited grain boundary pinning and corresponding high densification. Although the addition of Y2O3 improved sintering, the presence of a residual grain boundary phase (YAG) was harmful to the fracture toughness of the composites, as it affected the effectiveness of the crack deflection mechanism that takes place at the interfaces between Al2O3 and WC grains. Hot-pressing resulted in hardness ∼17.5 GPa and fracture toughness ∼7 MPa m−1/2, which is an improvement compared to alumina reinforced by other refractory carbidesArtigo Reinforcing Al2O3 with W–Ti mixed carbide(Elsevier, 2000-11) Acchar, Wilson; Martinelli, Antonio Eduardo; Cairo, Carlos Alberto AlvesRecent advances in high-speed cutting materials have focused on reinforcing alumina with different carbides and nitrides in order to improve hardness and fracture toughness. However, data on mixed carbides is still scarce. The potential use of this type of material for cutting tool applications has yet to be determined. The present study reports some preliminary results obtained reinforcing Al2O3 with WTiC in the range of 5–30 wt.%. The material was hot-pressed at 1650°C for 30 min and then characterized. Vicker's microhardness (HV) and fracture toughness (KIC) were evaluated by the indentation method. The addition of WTiC did not result in any significant change in the fracture toughness of alumina, however hardness values in excess of 22 GPa were obtainedArtigo Effect of workpiece geometry on the uniformity of nitrided layers(Elsevier, 2001-05-01) Alves Júnior, Clodomiro; Silva, Eronildes Felisberto da; Martinelli, Antonio EduardoThe growth behavior of plasma-nitrided layers on workpieces with complex geometry was systematically investigated. AISI 316 stainless steel pellets with different heights were nitrided under a mixture of N2–80% H2 at different temperatures (673, 773 and 843 K) and pressures (100 and 500 Pa). Significant differences in thickness and hardness of the resulting nitrided layers were observed as a function of nitriding parameters. The thickness of nitrided layers increased with sample height, excepted those nitrided at 843 K. The diameter of eroded rings, commonly observed on nitrided samples, varied with coupon height. Changes in both layer thickness and eroded ring diameter are presently addressed based on the thermal balance and charge density that take place near the edges of the samplesArtigo Low-temperature synthesis of single-phase crystalline LaNiO3 perovskite via Pechini method(Elsevier, 2002-03) Fernandes, Joana D. G.; Melo, Dulce Maria de Araújo; Zinner, Léa Barbieri; Salustiano, Clayton de Matos; Silva, Z. R.; Martinelli, Antonio Eduardo; Nasar, Marinalva Cerqueira; Alves Júnior, Clodomiro; Silva, Elson Longo da; Bernardi, Maria Inês BassoLaNiO3 powders were prepared from polymeric precursors. The material was calcinated either under flowing oxygen or in air at temperatures ranging from 300 to 800 °C. The formation of new phases upon calcination was investigated using X-ray diffraction, thermal gravimetric analysis, Fourier-transform infrared spectroscopy and CHN analysis. The results revealed the formation of a single-phase powder having the perovskite structure at temperatures as low as 600 °C. The atmosphere did not significantly affect the final productArtigo Ceramic crucibles: a new alternative for melting of PbO–BiO1.5–GaO1.5 glasses(Elsevier, 2002-07-10) Santos, Ieda Maria Garcia dos; Moreira, Rafael Carlos Martins; Souza, Antônio Gouveia de; Lebullenger, Ronan; Hernandes, Antônio Carlos; Leite, Edson Roberto; Paskocimas, Carlos Alberto; Silva, Elson Longo daPbO–Bi2O3–Ga2O3 glasses present interesting properties such as good transmission in the mid-infrared region, high magnetic Verdet constant and non-linear properties. The processing of these heavy-metal-oxide (HMO) glasses is limited by the high corrosive nature of the melt, even in relation to noble metal crucibles. In this work, three kinds of ceramic crucibles (alumina, tin oxide and zirconia) were tested for melting HMO glasses. The main physical properties of the prepared glasses, such as the characteristic temperatures, optical transmission were studied in function of the crucible nature, time/temperature melting parameters. The incorporation of crucible material in the glasses was determined by ICP and atomic absorption. Themaximumglass contamination fromthe crucible was 2.9, 1.6 and 3.6mol% forAl2O3, SnO2 and ZrO2 crucibles, respectively, whenmelting was done at 900 C/240min, for zirconia crucibles and at 1000 C/60 min, for the other two crucibles. The evolution of the physical properties was discussed as a function of contamination degreeArtigo Preparation and characterization of lanthanum palygorskite clays as acid catalysts(Elsevier, 2002-10-07) Melo, Dulce Maria de Araújo; Ruíz, Juan Alberto Chavez; Melo, Marcus Antônio de Freitas; Vitor Sobrinho, Eledir; Martinelli, Antonio EduardoPalygorskite clays containing 2, 4, 6 or 10% lanthanum (w/w) were prepared by ion-exchange from acid-leached natural clays. Samples were characterized by chemical and thermogravimetric analysis, X-ray diffraction, infrared spectroscopy, and N2 adsorption. Isopropyl alcohol was transformed in order to improve the acid behavior of the material. The introduction of La3+ did not change the structure of the palygorskite. Surface areas as high as 177.6 m2 g−1 were measured for catalysts containing 2% lanthanum. This material also showed a very good activity for alcohol dehydrationArtigo Artigo revisão: avanços recentes em brasagem metal-cerâmica(ABCERAM, 2003) Nascimento, Rubens Maribondo do; Martinelli, Antonio Eduardo; Bushinelli, Augusto José de AlmeidaO interesse no estudo de métodos de junção-cerâmica para aplicações industriais tem crescido gradativamente ao longo dos anos. A evolução dos processos de união tem permitido a utilização de cerâmicas em conjunto com metais na fabricação de diversos componentes híbridos incluindo lâmpadas tradicionais, juntas para vácuo, ferramentas de corte de alto desempenho e modernos dispositivos eletrônicos de medição e monitoramento. Novos métodos de união e aprimoramentos de métodos convencionais têm sido estudados com o intuito de produzir-se juntas com alta confiabilidade e interfaces capazes de suportar altas temperaturas de trabalho com o mínimo de tensões residuais. O presente trabalho apresenta um panorama dos recentes avanços em técnicas alternativas de união, incluindo união por laser e junção direta com fases líquidas transientes. Além disso, apresenta-se uma discussão sobre novas tendências em brasagem de cerâmicas metalizadas e seleção de ligas de adição, já que este processo continua sendo amplamente utilizado por seu baixo custo de fabricação de grandes lotes de peças. Por fim, métodos de análise de resistência mecânica e tensões residuais são apresentados juntamente com alternativas para melhoria de confiabilidade de juntas por meio da redução de tensõesArtigo The origin of photoluminescence in amorphous lead titanate(Springer, 2003-03) Leite, Edson Roberto; Paris, Elaine Cristina; Pontes, F. M.; Paskocimas, Carlos Alberto; Silva, Elson Longo da; Sensato, F.; Pinheiro, Carlos Davidson; Varela, José A.; Pizani, Paulo Sérgio; Campos, C. E. M.; Lanciotti Júnior, FranchescoWe discuss the nature of visible photoluminescence at room temperature in amorphous lead titanate in the light of the results of recent experimental and theoretical calculations. Experimental results obtained by XANES and EXAFS revealed that amorphous lead titanate is composed of a Ti-O network having fivefold Ti coordination and NBO-type (non-bridging oxygen) defects. These defects can modify the electronic structure of amorphous compounds. Our calculation of the electronic structure involved the use of first-principle molecular calculations to simulate the variation of the electronic structure in the lead titanate crystalline phase, which is known to have a direct band gap, and we also made an in-depth examination of amorphous lead titanate. The results of our theoretical calculations of amorphous lead titanate indicate that the formation of fivefold coordination in the amorphous system may introduce delocalized electronic levels in the HOMO (highest occupied molecular orbital) and the LUMO (lowest unoccupied molecular orbital). A comparison of the experimental and theoretical results of amorphous compounds suggests the possibility of a radiative recombination (electron-hole pairs), which may be responsible for the emission of photoluminescenceArtigo Determination of relative acid strength of La/paligorskyte by n-butylamine(Elsevier, 2003-03-01) Melo, Dulce Maria de Araújo; Ruiz, Juan Alberto Chavez; Vitor Sobrinho, Eledir; Melo, Marcus Antônio de Freitas; Martinelli, Antonio Eduardo; Zinner, Léa BarbieriThe relative acid strength of lanthanum catalysts supported on paligorskyte was determined by thermodesorption of n-butylamine and infrared absorption spectroscopy. The results indicated that the desorption reaction took place in four temperature ranges. The catalyst containing 2% lanthanum particularly showed a considerable number of acid sites probably as a result of the relatively high specific area of the materialArtigo Systems involving cobalt and cerium oxides: characterization and catalytic behavior in the C6–C7 n-alkanes combustion(Elsevier, 2003-05) Pedrosa, Anne Michelle Garrido; Sousa, Marcelo José Barros; Melo, Dulce Maria de Araújo; Araújo, Antônio S.; Zinner, Léa Barbieri; Fernandes, Joana D. G.; Martinelli, Antonio EduardoThe objective of this work is to report the characterization and catalytic behavior of Co3O4, CeO2, and Co3O4–CeO2 catalysts in the C6–C7 n-alkanes combustion. X-ray diffraction indicated that crystalline oxides of Co3O4 and CeO2 are obtained by heat treatment at temperatures higher than 600 °C. The FTIR spectra are characteristic of cobalt and/or cerium oxides. The reactions were processed in a fixed bed continuous flow reactor, at 450 °C varying F/W ratio and n-alkanes. The results obtained showed that the catalysts give good activity in the C6–C7 n-alkanes combustion with high selectivity to carbon dioxideArtigo Evaluation of CoAl2O4 as ceramic pigments(Elsevier, 2003-09-02) Melo, Dulce Maria de Araújo; Cunha, Jardel Dantas da; Fernandes, Joana D. G.; Bernardi, Maria Inês Basso; Melo, Marcus Antônio de Freitas; Martinelli, Antonio EduardoCoAl2O4 powder was obtained from a mixture of Co and Al oxalates at a ratio of 1:8 (Co:Al). The material was calcinated at different temperatures, established from TG data, and characterized using FTIR spectroscopy, X-ray diffraction, BET surface area, and thermal analysis. The dyeing characteristics of CoAl2O4 were established by coating ceramic substrates with different concentrations of the powderArtigo O papel dos modificadores de rede na produção da fotoluminescência no CaWO4(ABCERAM, 2004-01) Ciaco, F. R. C.; Pontes, F. M.; Pinheiro, C. D.; Leite, Edson Roberto; Lázaro, S. R. de; Varela, José A.; Pizani, Paulo Sérgio; Paskocimas, Carlos Alberto; Souza, Antônio Gouveia de; Silva, Elson Longo daDiscutiu-se a fotoluminescência para o tungstato de cálcio amorfo, observada à temperatura ambiente. Verificou-se que há concordância entre os resultados experimentais e teóricos. Neste trabalho foram simuladas as estruturas cristalinas e amorfas do tungstato de cálcio (CaWO4), comparando-se as respectivas estruturas eletrônicas. Os resultados dos cálculos teóricos indicam a formação de novos níveis de energia na banda de valência e de condução do amorfo. Estes níveis eletrônicos extras são os responsáveis pela formação da cauda na curva do espectro de absorção. Correspondentemente, medidas experimentais de absorção óptica mostraram a presença da cauda, com relação ao espectro do cristalino. Desta forma, pode-se interpretar a formação da cauda, como sendo associada aos defeitos promovidos na estrutura desordenada do material amorfoArtigo An investigation of metal oxides which are photoluminiscent at room temperature(Elsevier, 2004-01) Leite, Edson Roberto; Pontes, Fenelon M.; Lee, Eduardo J. H.; Aguiar, Rosiana; Silva, Elson Longo da; Pontes, D. S. L.; Nunes, M. S. J.; Pizani, Paulo Sérgio; Lanciotti Júnior, Franchesco; Boschi, Tania M.; Varela, José A.; Paskocimas, Carlos Alberto; Pinheiro, Carlos Davidson; Taft, Carlton AnthonyAmorphous thin films, based on different network formers, were processed by a soft chemical process called the polymeric precursor method. The resultant amorphous metal oxides, displayed intense photoluminescence (PL) at room temperature. Heat treatment increases the PL intensity of these materials. Theoretical ab initio calculations are correlated with the observed experimental trendsArtigo Thermal transformations of tile clay before and after kaolin addition(Springer, 2004-02) Albuquerque, F. R.; Parente, B.; Lima, Severino Jackson Guedes; Paskocimas, Carlos Alberto; Silva, Elson Longo da; Souza, Antônio Gouveia de; Santos, Ieda Maria Garcia dos; Fernandes Júnior, Valter JoséThe use of clays for ceramic filter processing may reduce its cost, leading to different applications, as water treatment. In this work, a low cost tile clay mixed with kaolin, for use in ceramic filters, were evaluated. Mineralogical and thermal changes occurring during sintering were characterized by differential thermal analysis, thermogravimetry, thermomechanical analysis and X-ray diffraction. An increase in the initial melting temperature of samples due to kaolin addition was observed. Mullite formation in kaolin was observed by DTA and in other samples by XRD. TMA analysis permitted the observation of pre-sintering step, around 850°C. This step is difficult to observe in other types of analysis