Resumo em Inglês:

The optimization of the energy generated by a MagnetoHydroDynamic (MHD) channel using a superconducting magnet demands the optimization of the magnetic field of the system and of the critical points on the magnet winding. This work must include the development of a high performance superconductor wire suitable for this system. Aiming to the construction of improved performance MHD channel, it was developed a low cost superconductor wire, with the required characteristics. The wire was made using a technology compatible with the assembling steps and heat treatment conditions of the MHD superconducting magnets fabrication. It was used the internal Sn method in Nb-7.5wt%Ta tube to fabricate a 271-filament wire with a diameter of 0.81 mm and a Cu/nonCu ratio of 2.3. The wire was heat treated at 200 °C to diffuse the Sn into the Cu shell, producing bronze, followed by the final reaction at temperatures ranging from 670 °C to 730 °C during 25 to 150 h, to produce (Nb,Ta)3Sn. The superconducting wire characterization was made measuring the critical current Ic versus the applied magnetic field in the range of 5 to 20 T, the critical temperature Tc and the residual resistivity ratio (RRR). The wire transported critical currents above those available in commercial superconducting wires. These values of Ic are higher than the expected values for the optimization of the MHD channel.

Resumo em Inglês:

The aim of this paper is to present the results of characterization of the particle shapes of six standard transition aluminas samples using transmission and scanning electron microscopies; selected area electron diffraction, in parallel with X-ray powder diffraction were used for confirmation of the different transition aluminas types. The transition aluminas - chi; kappa; gamma; theta; delta; and eta were supplied by ALCOA Central Laboratory. The chi-; kappa-;gamma- and delta-Al(2)0(3) microcrystals are pseudomorphs from their respective precursors gibbsite and boehmite. However, theta-Al(2)0(3) microcrystals are not pseudomorphs after the standard delta-Al(2)0(3) sample. Also, eta-Al(2)0(3) are not pseudomorphs after bayerite somatoids.

Resumo em Inglês:

Organo-clays were prepared by exchange cations of the montmorillonite with tetramethylammonium (TMA)- and hexadecylpyridinium (HDP)-chloride. Two different montmorillonite types were studied. Two amounts of each organic substances (1 and 4 cation exchange capacity times of montmorillonite) were contacted with each montmorillonite clay for 1 and 24 h. The solids were characterized by X-ray diffraction and swelling index. The swelling was measured in water and toluene media. The structural characteristics of montmorillonites changed after organo treatment and showed different behaviour in water and toluene media. The type of montmorillonite, the organic cation intercalates (TMA, HDP) and condition of treatment influenced in the different swelling in water and toluene media.

Resumo em Inglês:

The microstructural evolution of the CuZnAl shape memory alloys was studied by indirect techniques relating to the atomic migration rate of grain boundaries. Addition elements were used in a Cu-15,5Zn-8,0Al alloy to provide a comparison with the same alloy without microelement additions. The alloys were melted in an induction furnace of 24 kVA. After casting, the bulk samples of the alloys were homogenized. Then they were solution treated and hot-rolled followed by water-quenching to initiate the recrystallization. Finally, annealing produced at different temperature ranges was made in different samples in order to establish a law for the grain growth. Following the heat treatments, all annealed samples were examined by statistical metallography and the grain sizes were measured. After measurements, the same empirical law of grain growth was found for the different alloys and the ln [D-Do] x 1/T diagrams were plotted in order to establish the kinetic behavior. Based on the estimated values of the activation energy, important conclusions were obtained concerning the addition elements.

Resumo em Inglês:

Commercially available cordierite and mullite powders were used to obtain cordierite and cordierite-30wt% mullite materials by attrition milling, uniaxial pressing and sintering. Cordierite powders were the coarse (D50 = 1.82 mum), medium (D50 = 0.9 mum) and fine (D50 = 0.45 mum) single granulometric fractions and binary mixtures of them. Mullite powder employed in composites was the 7 h-attrition milled one (D50 = 1.3 mum). Hardness (H V) and fracture toughness (K IC) were measured by Vickers indentation techniques. Composites showed higher H V and K IC than cordierite matrices. In both materials, H V and K IC diminish with the increasing porosity. Dielectric constant (epsilon) and losses (tan delta) were determined at 1 MHz. An increase in epsilon and tan delta values was registered when mullite was present in composites with respect to cordierite material. Higher the porosity, epsilon was lower and tan delta increased. The thermal expansion coefficients (alpha) were determined up to 1000 °C resulting the alpha of the composite close to that of the silicon.

Resumo em Inglês:

Microstructure development and phase formation processes during sintering of silicon carbide based materials with AlN-Y2O3, AlN-Yb2O3, and AlN-La2O3 sintering additives were investigated. Densification of the materials occurred by liquid-phase sintering mechanism. Proportion of alpha- and beta-SiC powders in the initial mixtures was a variable parameter, while the molar ratio of AlN/RE2O3, and the total amount of additives (10 vol. %) were kept constant. Shrinkage behavior during sintering in interrelation with the starting composition of the material and the sintering atmosphere was investigated by high temperature dilatometry. Kinetics of b-SiC to a-SiC phase transformation during post-sintering heat treatment at temperatures 1900-1950 °C was studied, the degree of phase transformation being determined by quantitative x-ray analysis using internal standard technique. Evolution of microstructure resulting from beta-SiC to alpha-SiC transformation was followed up by scanning electron microscopy on polished and chemically etched samples. Transformation-controlled grain growth mechanism similar to the one observed for silicon nitride based ceramics was established. Possibility of in-situ platelet reinforced dense SiC-based ceramics fabrication with improved mechanical properties by means of sintering was shown.

Resumo em Inglês:

The pop-in phenomenon, quite common in fracture mechanics tests of welded joints, corresponds to a brittle crack initiation grown from a local brittle zone (LBZ) that is arrested in reaching the higher toughness material that surrounds this LBZ. A methodology to obtain a high percentage of pop-in occurrence in laboratory testing is necessary to study the pop-in significance. Such a method is introduced in this work and includes the consumable combination and welding procedures for the SMAW welding process to generate artificial LBZ. In order to find out the influence of the loading state upon the pop-in phenomenon, laboratory CTOD tests were performed using two specimen configurations: some single edge-notched specimens were loaded on a three-point bending (SE(B)) fixture while others were tested in tensile load (SE(T)). A higher frequency of pop-in occurrence was observed in the SE(B) geometry.

Resumo em Inglês:

CBN grinding wheels are superabrasive tools, with cutting ability higher than the conventional ones, specially used for grinding DTG (Difficult to Grind) materials. Due to the higher hardness and thermal conductivity of the CBN superabrasive grains, when compared with conventional abrasive grains, the use of CBN tools reduce the grinding temperatures decreasing the probability of workipece’s thermal damage. These proprieties of CBN grains permit lowering the energy partition because less heat is generated during grinding and much heat is dissipated through the grinding wheel instead of through the workpiece. This paper shows a research about the microstructural changes in the ground subsurface and the differences in the superficial integrity when using two types of superabrasive grinding wheels, manufactured with vitrified and resin bonds, analyzed in different cutting conditions. The ground material was the VC 131 steel, hardened and tempered. Were not observed microstructural changes in the subsurface ground with the two types of tools. As a result, the use of different types of the bonding do not affect the surface integrity neither resulted in microstructural changes, because the maximum temperatures measured in the grinding zone were lower than 125 °C, much less than the required level to cause microstructural changes.