Advanced Materials Research Vol. 976

Abstract: This work presents a study of the corrosion resistance as a function of the holding time on the interface generated during the process of brazing an AISI 304 to AISI 316L stainless steels by using a non-commercial Fe₆₀Ni₁₂Cr₈P₁₃B₇ metallic glass alloy ribbon by induction heating at 1000 °C into a chamber with an Ar controlled atmosphere. Samples of the austenitic stainless steels were joined in a sandwich-like arrangement using the Fe-based metallic glass ribbon. Corrosion experiments carried out in distilled water and 3.5 wt. % sodium chloride solution revealed that the corrosion resistance was higher for samples tested in the distilled water than the latest medium for all dwell joining times, since in the former medium the samples passivated. It was also found that the highest corrosion resistance was achieved for samples joined for a dwelling time of 4 minutes.

Abstract: In this work, the dissolution of iron from a kaolinitic mineral (Veracruz, México) is compared with that from an iron ore (Michoacán, México), a specular hematite (México), a magnetite (southeastern Utah, USA) and a hematite (from Arizona, USA), using sodium thiosulfate and citric acid, as the reducing and complexing agents, respectively. The minerals were dry sieved to obtain particle sizes between-100 +250 mesh. The minerals were then characterized by X-ray diffraction (XRD), energy dispersive spectroscopy EDS and atomic absorption (AA). Leaching experiments were using 0.5 M of both reactants at temperature of 363.15 °K and pH = 3; this one last parameter was controlled with ammonium hydroxide. The experimental results showed removal of dissolved iron above 99% in the kaolinitic clay and the magnetite, but only 79% for the iron ore, 46% for the hematite and 30% for the hematite, in 180 minutes.

Abstract: The effect of the powder quantity on the effectiveness of the mechanical alloying process using different ductile powder systems was studied. X-ray diffraction, inductively coupled plasma-optical emission spectroscopy and energy dispersive x-ray spectrometry-scanning electron microscopy were the techniques employed to characterize the mechanically alloyed powders. Results showed that a same volume of powders, which represented different powder mass quantities for each system and composition, was used to mechanically alloy in an effective way in horizontal ball mills.

Abstract: Hafnium Nitride (HfN) thin films were fabricated by sputtering technique reaching a thickness of 250 nm. The substrate temperature was varied in 25, 100, 200, 300, 400 and 500 °C. Hardness, coefficient of friction, electrical resistivity and corrosion behavior of HfN thin films were studied. The structural changes were analyzed by X ray diffraction (XRD). The film growth at room temperature showed the highest hardness value (15.8 GPa), higher electrical resistivity (1.23´10¹³ mW-cm) and the lowest coefficient of friction (0.37), as well as the best polarization resistance. Increased substrate temperature tends to decrease the corrosion resistance (i_corr: 17.50 nA/cm² to 254.9 nA/cm²) and friction coefficient (0.37 to 0.58), however, all values are above of corrosion resistance and friction coefficient reported by TiSiNO and TiSiN films. Images of scanning electron microscopy (SEM) show that films presented low surface damage with no evidence of corrosion products.

Abstract: The effect of annealing temperature on the properties of c-Si wafer/SiO_x interface (x = 1.15 and 1.3) is studied by Transmission Electron Microscopy and Capacitance/Conductance-Voltage measurements. Furnace annealing for 60 min at 700 and 1000 °C is used to grow amorphous or crystalline Si nanoparticles. The high temperature process leads to an epitaxial overgrowth of the Si wafer and an increase of the interface roughness, 3-4 monolayers at 700 °C and 4-5 monolayers at 1000 °C. The increased surface roughness is in correlation with the higher density of electrically active interface states.

Abstract: The use of aniline as carbon source has two advantages for the synthesis of carbon materials exhibiting conducting properties: the aromatic ring contributes to the graphitic character and the presence of nitrogen could work as n-dopant, decreasing the band gap of the carbon materials. These conditions contribute to improve their efficiency as electrocatalysts, for example in fuel cells. The objective of this work is to correlate physicochemical characteristics and electrical behavior of carbon samples prepared by nanocasting of aniline in mesoporous silica and its subsequent carbonization under controlled conditions.

Abstract: In the present work, it was analyzed the palladium electrodeposition onto High Oriented Pyrolitic Graphite (HOPG) electrode from an aqueous solution (0.001 M PdCl₂ + 1M NH₄Cl (pH 5)) through cyclic voltammetry and chronoamperometry. The analysis of voltammetric data showed that palladium electrodeposition is controlled by mass transfer. From the potentiostatic study it was calculated the diffusion coefficient, the number of active nucleation sites (N₀) and the rate constant of the proton reduction process (k_PR). It was seen that an increment of N₀ and k_PR values is obtained when the overpotential applied is increased.

Abstract: Cobalt electrodeposition on palladium and glassy carbon was studied at different temperatures by using voltammetric techniques. Temperature effect on the diffusion coefficient value was analyzed. The results clearly showed that cobalt electrodeposition is a diffusioncontrolled process. The temperature effect on the values of the diffusion coefficient was analyzed through the Arrhenius equation. The value of the activation energy was calculated as 21.56 kJ mol^-1 and 25.73 kJ mol¹ for palladium and glassy carbon respectively.

Abstract: Copper based composites with 30, 40, 50 and 60 vol.% Al₂O₃ were fabricated by powder metallurgy and consolidated by pulsed electric current sintering (PECS). For the purpose of determining the advantage of using coated fillers, composite alumina particles with 18 vol.% copper were prepared by electroless copper plating. Coatings were continuous and homogeneous through alumina surface. Thus, composites consolidated by the modified process increased contact between the matrix and filler, which resulted in superior thermo-physical properties. Thermal conductivities of 210-99 and 227-114 W/mK were obtained for Cu/Al₂O₃ made by the admixture and the coated filler method, respectively. Such superiority is mainly attributed to the continuity in the matrix phase; the thermal conductivity values observed are similar to those shown by the traditional materials used in electronic packaging. The coefficient of thermal expansion was slight lower in composites fabricated by the coated filler method; values in the ranges of 14-11 and 13-10.5 μm/m°C were obtained for the admixture and the coated filler method, respectively.

Abstract: Aluminum alloys are important in aerospace industry, due to their mechanical properties, low specific weight and good corrosion resistance. Such properties are achieved due to a heat treatment of solubilization, quenching and aging, in order to precipitate metastables phases, which act as dislocation obstacles, increasing the strength of the alloy. In the present study, the precipitation sequence of Al-8%Ag alloy was analyzed via Vickers hardness and Transmission Electron Microscopy. The size and morphology of the precipitated particles, involved in the stages of precipitation process was characterized. It was determined the microstructure at the peak hardness, which is mainly composed of spherical GP zones with about 6 nm average diameter, which are responsible for the alloy achieve a value of 72 HVN. It was observed that this hardness value does not compete with others well known alloys, like AA 6061 and AA 2024, which can be precipitation hardened. The main reason for the low values of HVN, is because of there is no enough difference between the matrix and the precipitated particles lattice parameters, and dont cause a significant elastic strain by coherence in the matrix lattice, that could produce a substantial hardening. To ascertain this assumption, the aged material was severely plastic deformed, achieving 94 HVN, and the grain refinement and high dislocations density were the major hardening mechanisms, since the precipitates behavior was similar as the matrix, because particles were distorted instead of acting as impediment to material flow.