Caracterização de compósitos Nb-20%cu obtidos por moagem de alta energia e sinterizados por fase líquida

In this work, was studied the formation of a composite of the refractory metal niobium with copper, through the process of high-energy milling and liquid phase sintering. The HEM can be used to synthesize composite powders with high homogeneity and fine size particle distribution. It may also produce the solid solubility in immiscible systems such as Nb-Cu, or extend the solubility of systems with limited solubility. Therefore, in the immiscible system Cu-Nb, the high-energy milling was successfully used to obtain the composite powder particles. Initially, the formation of composite particles during the HEM and the effect of preparation technique on the microstructure of the material was evaluated. Four loads of Nb and Cu powders containing 20%wt Cu were synthesized by MAE in a planetary type ball mill under different periods of grinding. The influence of grinding time on the metal particles is evaluated during the process by the withdrawal of samples at intermediate times of milling. After compaction under different forces, the samples were sintered in a vacuum furnace. The liquid phase sintering of these samples prepared by HEM produced a homogeneous and fine grained. The composite particles forming the sintered samples are the addition of a hard phase (Nb) with a high melting point, and a ductile phase (Cu) with low melting point and high thermal and electrical conductivities. Based on these properties, the Nb-Cu system is a potential material for many applications, such as electrical contacts, welding electrodes, coils for generating high magnetic fields, heat sinks and microwave absorbers, which are coupled to electronic devices. The characterization techniques used in this study, were laser granulometry, used to evaluate the homogeneity and particle size, and the X-ray diffraction, in the phase identification and to analyze the crystalline structure of the powders during milling. The morphology and dispersion of the phases in the composite powder particles, as well the microstructures of the sintered samples, were observed by scanning electron microscopy (SEM). Subsequently, the sintered samples are evaluated for density and densification. And finally, they were characterized by techniques of measuring the electrical conductivity and microhardness, whose properties are analyzed as a function of the parameters for obtaining the composite

Caracterização de compósitos Nb-20%cu obtidos por moagem de alta energia e sinterizados por fase líquida

In this work, was studied the formation of a composite of the refractory metal niobium with copper, through the process of high-energy milling and liquid phase sintering. The HEM can be used to synthesize composite powders with high homogeneity and fine size particle distribution. It may also produce the solid solubility in immiscible systems such as Nb-Cu, or extend the solubility of systems with limited solubility. Therefore, in the immiscible system Cu-Nb, the high-energy milling was successfully used to obtain the composite powder particles. Initially, the formation of composite particles during the HEM and the effect of preparation technique on the microstructure of the material was evaluated. Four loads of Nb and Cu powders containing 20%wt Cu were synthesized by MAE in a planetary type ball mill under different periods of grinding. The influence of grinding time on the metal particles is evaluated during the process by the withdrawal of samples at intermediate times of milling. After compaction under different forces, the samples were sintered in a vacuum furnace. The liquid phase sintering of these samples prepared by HEM produced a homogeneous and fine grained. The composite particles forming the sintered samples are the addition of a hard phase (Nb) with a high melting point, and a ductile phase (Cu) with low melting point and high thermal and electrical conductivities. Based on these properties, the Nb-Cu system is a potential material for many applications, such as electrical contacts, welding electrodes, coils for generating high magnetic fields, heat sinks and microwave absorbers, which are coupled to electronic devices. The characterization techniques used in this study, were laser granulometry, used to evaluate the homogeneity and particle size, and the X-ray diffraction, in the phase identification and to analyze the crystalline structure of the powders during milling. The morphology and dispersion of the phases in the composite powder particles, as well the microstructures of the sintered samples, were observed by scanning electron microscopy (SEM). Subsequently, the sintered samples are evaluated for density and densification. And finally, they were characterized by techniques of measuring the electrical conductivity and microhardness, whose properties are analyzed as a function of the parameters for obtaining the composite

Estudo da moagem de alta energia e sinterização de pós compósitos W-Cu

The Tungsten/copper composites are commonly used for electrical and thermal objectives like heat sinks and lectrical conductors, propitiating an excellent thermal and electrical conductivity. These properties are dependents of the composition, crystallite size and production process. The high energy milling of the powder of W-Cu produces an dispersion high and homogenization levels with crystallite size of W very small in the ductile Cu phase. This work discusses the effect of the HEM in preparation of the W-25Cu composite powders. Three techniques of powder preparation were utilized: milling the dry with powder of thick Cu, milling the dry with powder of fine Cu and milling the wet with powder of thick Cu. The form, size and composition of the particles of the powders milled were observed by scanning electron microscopy (SEM). The X-ray diffraction (XRD) was used to analyse the phases, lattice parameters, size and microstrain of the crystallite. The analyse of the crystalline structure of the W-25Cu powders milled made by Rietveld Method suggests the partial solid solubility of the constituent elements of the Cu in lattice of the W. This analyse shows too that the HEM produces the reduction high on the crystallite size and the increase in the lattice strain of both phases, this is more intense in the phase W

Estudo de duas pseudoligas de Nb-Cu obtidas pela rota da metalurgia do pó

The Nb-Cu pseudoalloys present themselves as potential substitutes for the alloys from a well known system and already commercially applied, as the W-Cu alloys, used in applications such as heat sinks, electrical contacts and coils for the generation of high magnetic fields. Because it is an immiscible system, where there is mutual insolubility and low wettability of the liquid Cu on the Nb surface, the processing route used in this work was the Powder Metallurgy. Two Nb alloys were used, with additions of 10% and 20% in weight of Cu, and times of 20, 30 and 40 hours for the high energy milling of the starting powders. The milling evolution of the powders is presented through the characterization techniques, such as the LASER diffraction for particle size, XRD, SEM, EDS, DSC, dilatometry, TEM and chemical analysis. After the milling, portions of the loads were submitted to the annealing heat treatment. The process used for the samples consolidation was the hot pressing, which has been applied both on some milled powders samples, as on the annealed powders. Subsequent heat treatments were performed in the samples at temperatures of 1000ºC (solid phase) and 1100ºC (in the Cu liquid phase). All sets of consolidated samples, and also the two sets of the heat treated, were analyzed by XRD, SEM, EDS, density and Vickers microhardness. Moreover, other Nb powder samples with 10% and 20% in weight of Cu obtained by simple mechanical mixing, were consolidated, thermally treated and characterized with the same techniques applied to the others, and the results were compared among themselves. Despite the difficulty of consolidation and densification of the two pseudoalloys of the Nb-Cu system of this study, on the route that passes through the HEM, samples were obtained with densities around 90% of the theoretical density. And, on the processing route of which were only mixed, the values reached up to 97%. Therefore, in this work are also emphasized the processes that made possible these results.

Estudo da moagem de alta energia e sinterização de pós compósitos W-Cu

The Tungsten/copper composites are commonly used for electrical and thermal objectives like heat sinks and lectrical conductors, propitiating an excellent thermal and electrical conductivity. These properties are dependents of the composition, crystallite size and production process. The high energy milling of the powder of W-Cu produces an dispersion high and homogenization levels with crystallite size of W very small in the ductile Cu phase. This work discusses the effect of the HEM in preparation of the W-25Cu composite powders. Three techniques of powder preparation were utilized: milling the dry with powder of thick Cu, milling the dry with powder of fine Cu and milling the wet with powder of thick Cu. The form, size and composition of the particles of the powders milled were observed by scanning electron microscopy (SEM). The X-ray diffraction (XRD) was used to analyse the phases, lattice parameters, size and microstrain of the crystallite. The analyse of the crystalline structure of the W-25Cu powders milled made by Rietveld Method suggests the partial solid solubility of the constituent elements of the Cu in lattice of the W. This analyse shows too that the HEM produces the reduction high on the crystallite size and the increase in the lattice strain of both phases, this is more intense in the phase W

Estudo da síntese e sinterização de pós compósitos do sistema Ta-Cu

Ta-Cu bulk composites combine high mechanical resistance of the Ta with high electrical and thermal conductivity of the Cu. These are important characteristics to electrical contacts, microwave absorber and heat skinks. However, the low wettability of Ta under Cu liquid and insolubility mutual these elements come hard sintering this composite. High-energy milling (HEM) produces composite powders with high homogeneity and refines the grain size. This work focus to study Ta-20wt%Cu composite powders prepared by mechanical mixture and HEM with two different conditions of milling in a planetary ball mill and then their sintering using hydrogen plasma furnace and a resistive vacuum furnace. After milling, the powders were pressed in a steel dye at a pressure of 200 MPa. The cylindrical samples pressed were sintered by resistive vacuum furnace at 10-4torr with a sintering temperature at 1100ºC / 60 minutes and with heat rate at 10ºC/min and were sintered by plasma furnace with sintering temperatures at 550, 660 and 800ºC without isotherm under hydrogen atmosphere with heat rate at 80ºC/min. The characterizations of the powders produced were analyzed by scanning electron microscopy (SEM), x-ray diffraction (XRD) and laser granulometry. After the sintering the samples were analyzed by SEM, XRD and density and mass loss tests. The results had shown that to high intense milling condition produced composite particles with shorter milling time and amorphization of both phases after 50 hours of milling. The composite particles can produce denser structure than mixed powders, if heated above the Cu melting point. After the Cu to arrive in the melting point, liquid copper leaves the composite particles and fills the pores

Estudo da síntese e sinterização de pós compósitos do sistema Ta-Cu

Ta-Cu bulk composites combine high mechanical resistance of the Ta with high electrical and thermal conductivity of the Cu. These are important characteristics to electrical contacts, microwave absorber and heat skinks. However, the low wettability of Ta under Cu liquid and insolubility mutual these elements come hard sintering this composite. High-energy milling (HEM) produces composite powders with high homogeneity and refines the grain size. This work focus to study Ta-20wt%Cu composite powders prepared by mechanical mixture and HEM with two different conditions of milling in a planetary ball mill and then their sintering using hydrogen plasma furnace and a resistive vacuum furnace. After milling, the powders were pressed in a steel dye at a pressure of 200 MPa. The cylindrical samples pressed were sintered by resistive vacuum furnace at 10-4torr with a sintering temperature at 1100ºC / 60 minutes and with heat rate at 10ºC/min and were sintered by plasma furnace with sintering temperatures at 550, 660 and 800ºC without isotherm under hydrogen atmosphere with heat rate at 80ºC/min. The characterizations of the powders produced were analyzed by scanning electron microscopy (SEM), x-ray diffraction (XRD) and laser granulometry. After the sintering the samples were analyzed by SEM, XRD and density and mass loss tests. The results had shown that to high intense milling condition produced composite particles with shorter milling time and amorphization of both phases after 50 hours of milling. The composite particles can produce denser structure than mixed powders, if heated above the Cu melting point. After the Cu to arrive in the melting point, liquid copper leaves the composite particles and fills the pores

Estudo de um compósito Nb-15%pCu obtidos por moagem de alta energia e sinterizados por fase líquida

There is great difficulty in forming a composite refractory metal niobium with copper. This is due to the fact that Nb-Cu system is almost mutually immiscible and may be neglected solubility between them. These properties hinder or prevent obtaining homogeneous and high-density structures, conventionally prepared. This study aims to analyze the use of high-energy milling process (MAE) to implement these natural difficulties, with regard to the densification of the sintered bodies. The MAE and the press were used in the preparation of powders, to obtain a fine and homogeneous distribution of the grain size. Four loads Nb and Cu powders containing 15% by weight of Cu were then milled for MAE in a planetary type ball mill under various milling times and speeds. The results obtained by MAE were analyzed by scanning electron microscopy (SEM), according to the parameters of time and grinding speed. The samples were compacted under pressure of 200 MPa, were then sintered in liquid phase in a vacuum furnace at temperatures of 1100 ° C / 60 min and 1200 ° C / 60 min. Then it was used to characterize diffraction of X-rays to identify the phases. The microstructures of the sintered samples were observed and evaluated using scanning electron microscopy (SEM). Vickers Microhardness tests were performed, obtaining higher values for the sintered bodies in the largest of the post milling times and the larger grinding speeds. It was found that the liquid phase sintering of the samples that were processed by MAE produced at the end of a homogeneous and densified structure in 77,4% relative to the value of the theoretical density of the composite

Efeito da moagem de alta energia na densificação e microestrutura do compósito AI2O3-Cu

The Cu-Al2O3 composite ceramic combines the phase of alumina, which is extremely hard and durable, yet very brittle, to metallic copper phase high ductility and high fracture toughness. These characteristics make this material a strong candidate for use as a cutting tool. Al2O3-Cu composite powders nanocrystalline and high homogeneity can be produced by high energy milling, as well as dense and better mechanical structures can be obtained by liquid phase sintering. This work investigates the effect of high-energy milling the dispersion phase Al2O3, Cu, and the influence of the content of Cu in the formation of Cu-Al2O3 composite particles. A planetary mill Pulverisatte 7 high energy was used to perform the experiments grinding. Al2O3 powder and Cu in the proportion of 5, 10 and 15% by weight of Cu were placed in a container for grinding with balls of hard metal and ethyl alcohol. A mass ratio of balls to powder of 1:5 was used. All powders were milled to 100 hours, and powder samples were collected after 2, 10, 20, 50 and 70 hours of grinding. Composite powders with compact cylindrical shape of 8 mm diameter were pressed and sintered in uniaxial matrix resistive furnace to 1200, 1300 to 1350 °C for 60 minutes under an atmosphere of argon and hydrogen. The heating rate used was 10°C/min. The powders and structures of the sintered bodies were characterized by XRD, SEM and EDS. Analysis TG, DSC and particle size were also used to characterize the milled powders, as well as dilatometry was used to observe the contraction of the sintered bodies. The density of the green and sintered bodies was measured using the geometric method (mass / volume). Vickers microhardness with a load of 500 g for 10 s were performed on sintered structures. The Cu-Al2O3 composite with 5% copper density reached 61% of theoretical density and a hardness of 129 HV when sintered at 1300 ° C for 1h. In contrast, lower densities (59 and 51% of the theoretical density) and hardness (110 HV and 105) were achieved when the copper content increases to 10 and 15%.

Geoquímica de cenizas volcánicas a lo largo de dos transectas en Sudamérica: implicaciones ambientales

VIII Congreso geológico de España, Oviedo, 17-19 julio 2012

Optimization of Heat Treatment Profiles Applied to Nanometric-Scale Nb(3)Sn Wires With Cu-Sn Artificial Pinning Centers

Nb(3)Sn is one of the most used superconducting materials for applications in high magnetic fields. The improvement of the critical current densities (J(c)) is important, and must be analyzed together with the optimization of the flux pinning acting in the material. For Nb(3)Sn, it is known that the grain boundaries are the most effective pinning centers. However, the introduction of artificial pinning centers (APCs) with different superconducting properties has been proved to be beneficial for J(c). As these APCs are normally in the nanometric-scale, the conventional heat treatment profiles used for Nb(3)Sn wires cannot be directly applied, leading to excessive grain growth and/or increase of the APCs cross sections. In this work, the heat treatment profiles for Nb(3)Sn superconductor wires with Cu(Sn) artificial pinning centers in nanometric-scale were analyzed in an attempt to improve J(c) . It is described a methodology to optimize the heat treatment profiles in respect to diffusion, reaction and formation of the superconducting phases. Microstructural, transport and magnetic characterization were performed in an attempt to find the pinning mechanisms acting in the samples. It was concluded that the maximum current densities were found when normal phases (due to the introduction of the APCs) are acting as main pinning centers in the global behavior of the Nb(3)Sn superconducting wire.

Electrodeposition and characterization of Cu-Nb composite coatings

Copper coatings containing well-distributed Nb particles were obtained by co-electrodeposition in an acidic sulfate bath. Nb particle concentration in the bath was the most significant factor for the incorporation of Nb particles in copper, followed by stirring rate, whereas current density presented low significance. High Nb particle concentration and low stirring rate led to a higher incorporated Nb particle content. The microhardness of the composite layers was higher than that of pure copper deposits obtained under the same conditions due to copper matrix grain refinement and increased with the increase of both current density and incorporated Nb particle volume fraction. The corrosion resistance of Cu-Nb composites in 0.5 wt.% H(2)SO(4) solution at room temperature was higher than that of pure copper and increased with the increase of the Nb content. (C) 2010 Elsevier B.V. All rights reserved.

Heat treatment influence on the superconducting properties of nanometric-scale Nb(3)Sn wires with Cu-Sn artificial pinning centers

Since the discovery of Nb(3)Sn superconductors many efforts have been expended to improve the transport properties in these materials. In this work, the heat treatment profiles for Nb(3)Sn superconductor wires with Cu(Sn) artificial pinning centers (APCs) with nanometric-scale sizes were analyzed in an attempt to improve the critical current densities and upper critical magnetic field. The methodology to optimize the heat treatment profiles in respect to the diffusion, reaction and formation of the superconducting phases is described. Microstructural characterization, transport and magnetic measurements were performed in an attempt to relate the microstructure to the pinning mechanisms acting in the samples. It was concluded that the maximum current densities occur due to normal phases (APCs) that act as the main pinning centers in the global behavior of the Nb(3)Sn superconducting wire. The APC technique was shown to be very powerful because it permitted mixing of the pinning mechanism. This achievement was not possible in other studies in Nb(3)Sn wires reported up to now.

Magnetic properties of Fe-Cu-Nb-Si-B nanocrystaliine magnetic alloys

Several ribbons of composition Fe73.5Cu1Nb 3Si16.5B6 and Fe73.5Cu1 Nb3Si13.5B9 were prepared by annealing the as-quenched samples between 525°C and 700°C; which induced nucleation of nanocrystallites of Fe bcc-type composition. Mean grain sizes were obtained from X-ray diffraction. Static magnetic properties were measured with both a Magnet Physik Hysteresis-Graph (up to 200 Oe) and a SHE SQUID magnetometer (up to 50 kOe). Soft magnetic parameters (coercive field and initial permeability) were very sensitive to grain size. The ZFC magnetization at low field showed a broad peak at a temperature TM, thus signalling a certain distribution of nanocrystalline sizes, and TM strongly decreased when the mean grain size decreased. Isothermal magnetization curves at low temperature showed the expected asymptotic behavior of a random magnet material at low and high fields.

Depinning do fluxo de Josephson em compósitos YBa 2 Cu 3 O 7-delta/Ag

Apresenta-se neste trabalho um estudo experimental da dinâmica do fluxo magnético intergranular no cuprato supercondutor Y Ba2Cu3O7 e nos compósitos Y Ba2CU3O7 fi/Ag para várias concentrações de prata. Verificou-se que sob certas condições é possível separar os efeitos da dinâmica do fluxo de Josephson daqueles resultantes da dinâmica de Abrikosov. Através da técnica de medidas de magnetização DC foram obtidas as linhas de depinning inicial e de irreversibilidade magnética em campo magnético aplicado até 120 Oe. O revestimento dos grãos supercondutores aparentemente não afeta a energia de acoplamento dos elos fracos entre os grãos e o potencial de pinning até 16.1 % em peso de prata. A vantagem da mistura da prata é que esta melhora as propriedades mecânicas dos óxidos supercondutores além de melhorar as correntes críticas do sistema. Observamos que o depinning do fluxo de Josephson começa numa temperatura bem definida, dependendo do campo magnético aplicado e gradualmente aumenta com a temperatura até ou muito próximo do limite de irreversibilidade, acima do qual todo o fluxo intergranular (e intragranular) está livre. As linhas de irreversibilidade das nossas amostras podem ser entendidas dentro do modelo do Vidro supercondutor.