Resumo em Inglês:

Rapidly solidified Mg based alloys are of interest for industrial applications as a structural material and for hydrogen storage. Mg-Cu-Rare Earth alloys have shown high glass forming ability; full amorphous structure with thickness of mm size can be obtained within these systems. However, their brittle behavior limits their industrial applications. In the present work, the Al effect in substitution of Cu in the Mg65Cu25MM10 (at%, MM: mischmetal) was studied. Samples up to 15at% Al were prepared by splat cooling and their microstructure, stability and mechanical properties were characterised. The crystallization temperature increases with the Al addition; the amorphous phase with different Al content has a Young's modulus of ~55GPa; the microhardness increases with the Al content in the amorphous and crystallized samples and the fracture of the alloy containing 10at% Al showed ductile vein patterns characteristics of ductile metallic glasses. The partial Cu substitution by Al can improve the stability and mechanical properties of the amorphous Mg65Cu25MM10 alloy.

Resumo em Inglês:

Dehydrogenation of a BCC alloy of composition 52Ti-12V-36Cr doped with 4 wt.% Zr7Ni10 was studied by diffraction using synchrotron and neutron radiations. In the case of synchrotron radiation dehydrogenation was imposed by raising the temperature while keeping the sample under vacuum. We found that prior to final dehydrogenation the FCC dihydride phase is becoming metastable. Neutron in-situ experiment was done by keeping the deuterided sample at 200°C and slowly reducing the pressure. This lead to a reduction of the occupation factor of deuterium in the FCC phase and an increase of the isotropic atomic temperature factor of deuterium.

Resumo em Inglês:

The present work deals with the study of some aluminum (Al) composites reinforced with metallized-graphite (MG) particles prepared by mechanical milling and powder metallurgy routes. Density, morphology evolution and mechanical performance of composites were investigated as a function of MG concentration and milling time. The as-milled powders were characterized by X-ray diffraction and optical/electron microscopy; meanwhile, the mechanical testing was carried out on cylindrical specimens prepared from powders by powder metallurgy. Evidence reveals that high-energy ball milling induce a homogeneous dispersion of graphite nanoparticles in the Al matrix; this is related to an enhancement of hardness and strength response of studied composites. The composite sample with 0.5% MG addition (in weight) reached an increase of 40% on hardness and 50% on strength (compared with pure Al sample); nevertheless an adverse effect was observed with longer milling and/or higher MG concentration.

Resumo em Inglês:

Abstract In this paper we report the effect of alloying elements on hydrogen storage properties of melt-spun Mg-based alloys. The base alloys Mg90Si10, Mg90Cu10, Mg65Cu35 (at%) were studied. We also investigated the effect of rare earths (using MM: mischmetal) and Al in Mg65Cu25Al10, Mg65Cu25MM10 and Mg65Cu10Al15MM10 alloys. All the melt-spun alloys without MM show a crystalline structure, and the Mg65Cu25MM10 and Mg65Cu10Al15MM10 alloys showed an amorphous and partially amorphous structure respectively. At 350˚C all the alloys had a crystalline structure during the hydrogen absorption-desorption tests. It was observed that Si and Cu in the binaries alloys hindered completely the activation of the hydrogen absorption. The partial substitution of Cu by MM or Al allowed activation. The combined substitution of Cu by MM and Al showed the best results with the fastest absorption and desorption kinetics, which suggests that this combination can be used for new Mg-alloys to improve hydrogen storage properties.

Resumo em Inglês:

Abstract Nanoparticles nickel-doped cobalt ferrites [NixCo1-xFe2O4 (x = 0.0, 0.25, 0.50, 0.75 and 1.0 of Ni2+)] were prepared by the microwave combustion synthesis, using a stoichiometric mixture of metal nitrates and urea as the oxidizer and fuel to drive the reaction. The effect of microwave irradiation on the phase composition was favorable to promote the formation of fluffy foams and nanoparticles sizes. The fast internal heating with microwaves leads to a reduction in synthesis time, to only 2 min. The structural, chemical and magnetic properties of the nickel-doped cobalt ferrites were analyzed by XRD, TEM, SEM and BET. The XRD results confirmed the formation of pure and single-phase spinel structure. The crystallite size of the nanoparticles was in the range of 38 - 50 nm. SEM images show nanoparticles with spherical shape and homogenous morphology. The TEM analysis shows necked near-spherical particles with an average size of ~30 - 50 nm, reflecting the highly crystalline nature of these nanoparticles. The magnetic measurements of all the samples were recorded using vibrating sample magnetometer (VSM) at room temperature in 10 kOe. Increasing the nickel content directly affects the structural characteristics of the particles, causing a reduction in the coercive field.

Resumo em Inglês:

Abstract The optical band gap energy of ZnO nanorods was estimated by different methods found in literature. These nanorods were deposited onto TiO2 covered borosilicate glass substrates by aerosol assisted chemical vapor deposition. Characterization techniques were employed to reveal the crystalline structure, surface morphology and optical properties of the sample. The analysis proved that hexagonal cross-section ZnO nanorods in the wurtzite phase were grown onto a TiO2 buffer thin film of anatase. Variations in the components of transmittance and reflectance were associated with the development of a rough surface. Such fluctuations were correlated with the equations employed to estimate the optical band gap. The different methods indicated band gap values of 3.17 to 3.24 eV. The energy band gap of 3.24 eV agrees to that reported in the literature for ZnO nanorods. Fluorescence spectroscopy confirmed the accuracy of the mathematical predictions since an emission at 380 nm (3.26 eV) was seen.

Resumo em Inglês:

Abstract The mechanical alloying technique was employed to produce series of high entropy alloys, combining in equiatomic percentage Al, Co, Cr, Cu, Fe, Mo, Ni and Ti. Milling times were 0, 10, 20 and 30 h, and experiments were performed in a high energy ball mill (SPEX-8000M) under argon atmosphere. The structural and microstructural changes due to mechanical alloying process were studied by X-ray diffraction and electron microscopy. Although there is the presence of pure elements with HCP crystalline structure, the XRD patterns of as-milled powders revealed the presence of a mixture of nanocrystalline solid solution with FCC and BCC structure phases. The hardness of the powder samples was evaluated by Vickers microhardness testing. The average microhardness values indicate that the alloy with the greatest hardness is the NiCoAlFeMoCr.

Resumo em Inglês:

ZrC-SiC nanopowders were prepared by carbothermal reduction of ZrO2 and SiO2 with carbon black in argon gas atmosphere, using microwave energy as an alternative heating source. The samples were characterized by XRD, SEM and BET surface area analysis. The results indicate that highly crystalline ZrC-SiC composites with a mean particle size of about 80 nm can be synthesized at a relatively low temperature (~1420 °C). Microwave-assisted heating to obtain the ZrC-SiC nanopowders significantly reduces the synthesis time to only 40 min when compared with the long reaction times involved in conventional carbothermal reduction, which requires about 8 hours to obtain ZrC and 2 hours to obtain SiC.

Resumo em Inglês:

Abstract The modification of the third order nonlinear optical response exhibited by vertically aligned carbon nanotubes in two-wave mixing interactions was analyzed. All-optical switching effects were explored by using a vectorial optical Kerr gate with nanosecond pulses at 532 nm wavelength of excitation. The samples were prepared by a spray pyrolysis method. Morphological and structural characterization of the samples was conducted by Scanning Electron Microscopy, Transmission Electron Microscopy and Raman Spectroscopy studies. Important structural changes dependent on the temperature employed for the preparation of the carbon nanotubes were observed. It was highlighted a remarkable influence of nonlinear energy transference in spatially-resolved optical processes for designing ultrafast low-dimensional devices.

Resumo em Inglês:

The effect of Ni 1-2 wt.% addition on the microstructure and hardness of the aged A319 alloy were studied. Characterization analyses by x-ray diffraction, optical microscopy and scanning electron microscopy suggest clearly that Ni addition forms Al-Ni-Cu-Fe, Al-Cu-Ni and Al-Ni intermetallic compounds that correlates well with equilibria conditions. Analyses by transmission electron microscopy show that aging heat treatment promotes microstructural changes in morphology, size, and spatial distribution of precipitates. Vickers micro-hardness test of Ni 1 and 2 wt.% specimens have a hardness increase from that of A319 alloy of ~6-8% with mean values of 140.98 and 142.93 HV, respectively.

Resumo em Inglês:

The combined processes; rapid solidification, addition of Cu3P compound and heat treatments to improve the mechanical properties of the hypereutectic Al-13Si, Al-20Si and Al-20Si-1.5Fe-0.7Mn alloys (in wt. %) was studied. Optical microscopy and scanning electron microscopy were used to characterize the microstructures. The mechanical properties were evaluated by tensile tests. It was found that the cooling rate (20-50°C/s) used to solidify the alloys plus the addition of Cu3P compound favored the formation of fine primary Si and the transformation of the Al/Si eutectic from acicular to semi-transformed morphology. The spheroidization of the Al/Si eutectic after heat treatment caused an increase in the elongation (%) compared with the as-cast alloys. Al-13Si alloy showed the highest UTS and elongation (%) values, reaching values up to 215 MPa and 9.6%, respectively. On the other hand, the Al-20Si-1.5Fe-0.7Mn alloy showed the lowest UTS (180MPa) and elongation (3.4%) values. The formation of the Fe-intermetallic compounds caused a negative effect on the mechanical properties of the Al-20Si-1.5Fe-0.7Mn alloy.

Resumo em Inglês:

A structural and mechanical characterization of pure aluminum and 2124 T6 aluminum alloy reinforced with quasicrystalline phases of composition Al65Cu20Fe15 and Al70.5Pd21Mn8.5 (%at.) were performed. The quasicrystalline phases were synthesized by arc melting and then milled to produce powder of the alloys, which were then mechanical mixed with the starting powders of aluminum and 2124 aluminum alloy. The composites were produced by hot extrusion of a mechanical mixture containing 20% (%wt.) of the reinforcing phases on the metallic matrix. The structural characterization of the composites was carried out by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. Mechanical characterization was carried out by Vickers hardness measurements and torsion tests at room temperature. The pure aluminum/quasicrystal composite showed the presence of the same phases from the starting powder mixture while for the 2124 aluminum alloy/Al65Cu20Fe15 the quasicrystalline phase transformed to the tetragonal ω-Al7Cu2Fe during the solution heat treatment. Mechanical strength of the composites presented a substantial increase in comparison to the original matrix metal. While the equivalent ultimate tensile strength of the Al/quasicrystal composites reached values up to 215MPa and Vickers hardness up to 60HV, the 2124/quasicrystal composites reached values up to 670MPa and Vickers hardness up to 190HV.

Resumo em Inglês:

In this study, we have investigated the synthesis, microstructure and hydrogen storage properties of Mg-based nanocomposites containing different concentrations of TiCrV and TiCr1.2V0.8 alloys. The Mg-based nanocomposites of Mg containing Ti-Cr-V additives were prepared by reactive milling (RM) under hydrogen atmosphere. The structural characterization revealed the presence of the β-MgH2, γ-MgH2 and BCC phases in the powders samples after RM. In addition, a very refined and homogenous microstructure with average MgH2 crystallite size of around 10-12 nm was observed, including a nanometric dispersion of the additives in the magnesium hydride matrix. The doping with TiCrV and TiCr1,2V0,8 greatly improves the hydrogen desorption behavior of Mg in comparison with the sample without additive, resulting in the lowest onset temperature (240 ºC) for the sample containing 5%mol. of TiCrV. Very fast absorption and desorption kinetics at 275 ºC and 300 ºC (7 minutes and 5 minutes for full desorption and absorption, respectively) were observed in the samples containing TiCrV and TiCr1,2V0,8 without any notable difference between the type of additive used in comparison with the pure sample. However, a slight reduction in hydrogen capacity is observed in the mixtures than for the pure sample (6.7 wt.% against 7.3 wt.%).

Resumo em Inglês:

The effect of the partial substitution of Al for Ag in the glass forming ability (GFA) of the Zr57.52Co21.24Al21.24-xAgx (x = 8, 10, 12, and 14 at. %) family alloy are reported and discussed. Cylindrical and conical ingots were obtained using the suction casting technique. It was found that the Zr57.52Co21.24Al9.24Ag12 alloy showed a glassy structure, with a critical glassy diameter, Dc, of 2 mm and ∆Tx = 41 K. The bulk metallic glass Zr57.52Co21.24Al9.24Ag12 alloy was examined using scanning electron microscopy, differential thermal analysis and mechanical compression test. This alloy showed a Young´s modulus (E) of 76.4 GPa and yield strength (σy) of 1.58 GPa. The Glass Forming Ability of the Zr57.52Co21.24Al9.24Ag12 was explained in terms of the topological model of dense packed clusters and kinetic fragility index of the alloy.

Resumo em Inglês:

Based on the Fe60Cr8Nb8B24 alloy, reported in the literature as good Glass Forming Ability (GFA), in this study the GFA of two new compositions was proposed, Fe68-xCrxNb8B24 (x=10,12). They were evaluated as good candidates to be new compositions of Bulk Metallic Glasses (BMG) with better corrosion resistance due to the high Cr content. Rapidly solidified glassy ribbons were processed, and based on their thermal characteristics, the critical thickness of the glassy structure formation (Zc) was estimated. The critical thickness (Zc) obtained experimentally using the wedge-shaped casting method was evaluated and it presented a much lower value than that estimated theoretically. However, the GFA of the compositions analyzed was ranked and this ranking (i.e. whichever has the most or least GFA) is in agreement with the result predicted theoretically. The GFA of Fe58Cr10Nb8B24, which presented a maximum thickness of the amorphous region of wedge-shaped samples of about 0.6 mm (but estimated it would be 3.56 mm), offered good prospects to be a new Fe-based glass former alloy which has better resistance to corrosion than the Fe60Cr8Nb8B24 alloy reported.

Resumo em Inglês:

In this work, we report the synthesis and microstructural characterization of Tin oxide thin films doped with fluorine for applications such as transparent conductive oxides. Tin oxide doped with fluorine thin films were deposited by aerosol assisted chemical vapor deposition technique onto a borosilicate glass substrate, using a precursor solution of stannic chloride in ethanol and ammonium fluoride as the dopant. Deposition temperature was varied between 623-773 K. Also, other deposition parameters such as concentration of the precursor solution and gas carrier flux were fixed at 0.1 mol∙dm-3 and 5 L min-1 respectively. Results indicate the presence of only the cassiterite phase of Tin oxide in all samples. Thin films obtained were characterized by X-Ray Diffraction. Surface morphology and microstructure were studied by field emission scanning electron microscopy, optical properties of samples were analyzed by total transmittance and reflectance spectra. The resistivity value of the films was measured by the sheet resistance.

Resumo em Inglês:

Globally, water pollution is mainly caused by the presence of heavy metals and metalloids such as arsenic. The majority of the techniques employed in the removal are of low efficiency and high cost. Therefore, in this work it is presented the adsorption processes of arsenic (As III and V) ions employing magnetite nanoparticles (MNPs) synthesized by the aerosol assisted chemical vapor deposition (AACVD) process. The adsorption efficiency was determined at different times and concentrations. The remaining As concentration in the solutions was analyzed by atomic absorption spectroscopy. The adsorbed As ions on the surface of the NMPs was analyzed by high resolution transmission electron microscopy. The results showed an overall removal efficiency of 87% for As+3 and 98% for As+5, in a contact time of 15 minutes. Results suggested the use of NMPs as a promising alternative in the removal of As ions in water.

Resumo em Inglês:

Zinc oxide (ZnO) films were prepared on amorphous quartz substrates by sol-gel spin-coating using Zn(CH3COO)2•2H2O precursor. Isopropanol, ethanol and 2-methoxyethanol were used as solvents. The film drying temperature was kept constant at 200 ºC and the annealing temperature was varied from 400 to 800 ºC. Highly transparent and (002)-oriented hexagonal wurtzite-type ZnO films were obtained using isopropanol solvent. The ZnO films prepared at Tanneal = 600 ºC showed the highest (002) preferential orientation, however it was not the significant feature to achieve low resistivity. The uniform and (002)-oriented ZnO films prepared at Tanneal = 400 ºC display the highest transmittance (>93% in the visible region) and the lowest resistivity (7.1 Ω•cm).

Resumo em Inglês:

Equiatomic AlCoNi alloy matrix were reinforced by the dispersion of SiC nanoparticles (SiCnp) using the mechanical alloying process. The metallic powders were milled for 10, 20 and 30 h and sintered at 1200 0C under vacuum. Through particle size distribution, X-ray diffraction and scanning electron microscopy investigations, the mechanically alloyed powders for 10 h were selected to be reinforced with SiCnp (2.5, 5 and 10 wt.%). The microstructural features, and improvement in microhardness and porosity of sintered AlCoNi-SiC composites were investigated as a function of SiCnp content. Microhardness Vickers test showed that the enhanced microhardness of the AlCoNi-SiCnp composite could be attributed to the decrease of porosity in comparison to the AlCoNi alloy.

Resumo em Inglês:

The sulphoaluminate of strontium, Sr4Al6O12SO4, was synthesized by solid state reaction from mixture of SrCO3, Al2O3 and SrSO4 (3:3:1 molar ratio) as pellets. The kinetics of formation has been studied in the range of temperature between 850ºC and 1100ºC using quantitative X-ray powder diffraction data (XRD) analysis. Likewise, at room temperature, the crystal structure was determined from conventional X-ray powder diffraction data using direct methods and it was refined by the Rietveld method. The kinetics mechanisms that showed the best fit, were identified as geometrical contraction at grain boundary (R1.1) and nucleation and growing by energy law (P1.1). The activation energy values obtained were 145.47KJmol-1 (R1.1) and 151.35KJmol-1 (P1.1), respectively. The resulting crystal structure was orthorhombic type (a: 13.32802Å, b: 13.34430Å and c: 9.38704Å). The observed adjustment parameters were RB: 0.0985, Rwp: 0.137, Rp: 0.0925 and Chi2: 1.94.

Resumo em Inglês:

Composites from polystyrene with hydroxyl end groups and multiwall carbon nanotubes were fabricated to evaluate their properties. The Carbon Nanotubes (CNTs) were synthetized by chemical vapor deposition technique using ferrocene and benzene as precursors. Polystyrene with hydroxyl end groups was prepared by solution polymerization employing styrene as monomer, 2-mercaptoethanol as chain transfer agent and AIBN as initiator. Thin films were obtained by two methods: pouring into petri dishes and dip-coating slides. CNTs were characterized to identify morphology and characteristic spectroscopic signals. The polystyrene with hydroxyl end groups and composites were analyzed by SEM, FTIR, Raman and UV-vis spectroscopies, Vickers microhardness and electrical resistivity. Raman analysis demonstrated chemical interactions between CNTs and polystyrene. Results showed that resistivity and transparency decreased by increasing CNTs concentrations in composites; transmittance was about 80% with 0.8 wt% content of nanotubes. The highest Vickers hardness registered value was at 1.6 wt% CNTs concentration.