Gonzalo Gonzalez

Production and characterisation of carbon nitride thin films produced by a graphite hollow cathode system

Abstract Carbon nitride thin films have been prepared by plasma enhanced chemical vapour deposition of CH 4 and N 2 gas mixtures, by chemical transport from a hollow graphite cathode. The deposits prepared on pieces of single crystal silicon substrates were characterised using FTIR, Raman, SEM, EDX, SIMS, X-ray and electron diffraction. Single, double and triple carbon nitrogen bonds were detected in the FTIR spectra; the relative intensities of the associated bands being a function of the plasma power and substrate bias. Elemental analysis of the deposit showed that the nitrogen content was ∼57 at%. A polycrystalline deposit identified as hexagonal β -C 3 N 4 was obtained at low substrate temperatures. The deposit was found to grow preferentially on scratch defects on the silicon substrate surface and was stable upon annealing under vacuum up to 700°C.

Hardness-lattice parameter correlation for aged Al-Zn-Mg alloys

Two ternary Al-2.2Zn-0.95Mg and AI-5.5Zn-2.1Mg (in wt pct) alloys, with Zn:Mg ratios close to 2.5 were produced by conventional ingot casting metallurgy. The ingots were solution heat treated at 500°C for 0.5 h and aged at 180°C for times between 0.5 and 80 h. The structural characterization was carried out by X-ray diffraction (XRD), transmission electron microscopy (TEM), selected area electron diffraction (SAED) and Vickers microhardness measurements (HV). The study was focused on the investigation of the precipitates formation and the relationship between hardness and lattice parameter for α-Al. The results showed that there was an inverse correlation for all the experimental conditions, and the aged peaks coincided with lattice parameter minima. Significant precipitates formation only occurred for the alloy containing 5.5 wt pet Zn and 2.1 wt pet Mg, provoking an important strengthening and variations in the lattice parameter, however, this was not observed for the alloy containing 2.2 wt pet Zn and 0.95 wt pet Mg. A plausible explanation of the increment of hardness values could be the presence of a well distributed η phase (MgZn 2 ). At initial stages of the precipitation process, η′ was the most abundant precipitate while the phase τ was observed at overaged conditions. These results showed that the aging response of the conventionally cast Al-Zn-Mg alloys could be obtained using the lattice parameter of the α-AI matrix, even for alloy systems with low precipitates formation.

Fractal analysis of the heat treatment response for multiphase Al alloys

The present work shows the fractal analysis for the microstructure of Al-6Si-3Cu-xMg (x = 0.59, 3.80 and 6.78 wt.%) alloys, solution heat treated at 480 °C for times ranging from 4 to 72 h. Microstructural changes were examined using Optical Microscopy (OM) and Scanning Electron Microscopy (SEM), and quantified by measuring conventional characteristics of second phases. The fractal analysis of the microstructures was also carried out using fractal dimension (Df). Results shown important variations in the microstructure when Mg content is increased, being the eutectic Al-Si and Cu-rich phases the predominant phases for the alloys with low Mg content, while for higher Mg contents predominant phases were Mg2Si and Cu-rich phases. The analysis of Df showed that this parameter could be used in order to compare the microstructural solution response of the Al alloys. Besides, it was possible to correlate the fractal dimension with the shape factor of the second phases, mainly for the alloys with only one second phase remaining after heat treatment.

Deformation heterogeneity study of a 6061-T6 aluminum alloy processed by equal channel angular pressing

Among the severe plastic deformation techniques, the equal channel angular pressing (ECAP) has drastically improved the mechanical properties of the processed alloys. However, information regarding friction phenomenon, which modifies the deformation at the surface and the heterogeneity microstrain state produced by the process itself, is still scarce. In the present work, the deformation heterogeneity and the friction effect, at the surface in the bulk material of the 6061-T6 aluminum alloy processed by ECAP, is presented and discussed. The residual stress (RS) measurements were performed by means of X-Ray diffraction. By means of synchrotron diffraction, volumetric sections of the ECAPed samples were characterized. Finite element analysis showed a good agreement with the experimentally obtained residual stress and microhardness mapping results. The study also showed that the highest deformation zones were located at the outer parts of the deformed samples (top and bottom), while the inner zone showed strain oscillations of up to 49±2 MPa.

Study on the structural and magnetic properties of Ru 1−x Mo x Sr 2 GdCu 2 O 8±z system

The structural and magnetic properties of Ru_1-xMo_xSr₂GdCu₂O₈ with 0.0 ≤ x ≤ 0.50 were investigated through Mo substitution in the Ru sites. Samples were synthesized by solid-state reaction at ambient pressure at temperature between 960 °C and 1060 °C. X-ray diffraction results indicate that the partial substitution of Ru by Mo within the studied level of doping takes place isostructurally in a tetragonal structure (space group P4/mmm) with a solubility up to x = 0.50. Rietlveld- refinement analysis indicates that the Cu-O(2) and Ru-O(3) bond lengths increase with Mo content, while the Cu-O(1) and Ru-O(1) bond lengths, as well as Cu-O(1)-Ru bond angles decrease with increasing x with a minimum at approximately x = 0.30, and then increase for x > 0.30 resulting in inverse bell-type curves. Results of the DC magnetic susceptibility measurements on samples indicate that all samples exhibit ferromagnetic ordering, with T^Curie values depend on Mo content with a dependence that seems to be similar to Cu-O(1)-Ru bond angle. These results suggest a relationship between the structural and magnetic properties in the Ru_1-xMo_xSr₂GdCu₂O₈ system.

Study on the Structural, Electrical and Magnetic Properties of the

Samples of the Ru(Sr2-xBax)GdCu2O8±z system, with x = 0.00, 0.05, 0.10, 0.15, 0.20, 0.25 and 0.50 were prepared through solid-state reaction at ambient pressure at 980-1025°C. From the X-ray diffraction (XRD) patterns, it was observed that the substitution of Sr by Ba takes place iso-structurally into a tetragonal structure (space group P4/mmm). Rietveld refinement analysis indicates that both Cu-O(1) and Ru-O(1) bond lengths strongly depend on Ba content. The temperature dependence of the electrical resistance for the samples with 0.0 ≤ × ≤ 0.25 annealed in flowing oxygen at 960°C for 2 hrs. show a semiconducting behavior, whereas samples annealed during 30 hrs. present a semiconducting-to-metal transition. Results of the DC magnetic susceptibility measurements on samples annealed for 2 hrs. indicate that all samples exhibit ferromagnetic ordering, with magnetic transition temperature TCurrie that depend on Ba content with a dependence that seems to be similar to one found for the bond lengths.