Defect and Diffusion Forum Vols. 297-301

Abstract: MEMS technology requires low cost techniques to permit large scale fabrication for production. Porous silicon (PS) can be used in different manner to replace standard expensive etching techniques like DRIE (Deep Reactive Ion Etching). To perform same process quality as the latter, one need to understand how different parameters can influence porous silicon properties. We investigate here local formation of macroporous silicon on 2D and 3D silicon substrates. The blank substrate is a low doped (26–33 Ω cm) n type 6 inches silicon wafer. Then, an in situ phosphorus-doped polycrystalline silicon (N+ Poly-Si) is deposited on a thermal oxide layer to delimit the regions to be etched. Porous silicon is obtained afterwards using electrochemical anodization in a hydrofluoric acid (HF) solution. The effect of the temperature process on Si-HF electrochemical system voltamperometric curves, macropores morphology and electrochemical etch rates is more specifically studied. Moreover, permeation of porous substrates to hydrogen is studied after various anodization post-treatments such as KOH and HF wet etching or after a thin gold layer deposition used as current collector in micro fuel cells.

Abstract: The low carbon steels, used for the production of car bodies by deep drawing, are gradually substituted by high strength steels for vehicle weight reduction. The drawn car body components are joined by welding and the welded points undergo a reduction of the local tensile strength. In developing an accurate welding process model, able to optimized process parameters and to predict the final local microstructure, a significant improvement can be given by the knowledge of the welded steels thermal diffusivity at different temperatures. The laser-flash method has been used to compare the thermal diffusivity of two traditional deep drawing steels, two high strength steels already in common usage, i.e. a Dual Phase (DP) steel and a TRansformation Induced Plasticity (TRIP) steel, and one experimental high-Mn austenitic TWIP (Twinning Induced Plasticity) steel. The low carbon steels, at low temperatures, have a thermal diffusivity that is 4-5 times larger than the TWIP steel. Their thermal diffusivity decreases by increasing temperature while the TWIP steel shows an opposite behaviour, albeit with a lesser slope, so that above 700°C the TWIP thermal diffusivity is larger. The different behaviour of the TWIP steel in respect to the ferritic deep drawing steels arises from its non ferro-magnetic austenitic structure. The DP and TRIP steels show intermediate values, their diffusivity being lower than that of the traditional deep drawing steels; this latter fact probably arises from their higher alloy content and more complex microstructure.

Abstract: The study of composite electroless Ni-P coatings is pursued in order to obtain coatings on a metal substrate with advanced properties i.e. with high wear and corrosion resistance for particular engineering uses. Composite NiP-ZrO2 layers were prepared by simultaneous electroless co-deposition of Ni-P and ZrO2 on steel, from a reducing solution in which ZrO2 particles were kept in suspension by stirring. The particles load in the bath was 0, 0.5, 1.0, 2.0 and 5.0 g/l. The deposits of 30 m in thickness, were characterized for structure, morphology and hardness by scanning electron microscopy and microanalysis as well as X-ray diffraction. It was found that the maximum ZrO2 particle incorporation attained was 7-10% of Zr. Vickers microhardness was measured for the plain deposit and found to be 720 HV and for the maximum zirconia content of 10%, 820 HV. After a vacuum heat treatment for 10 min, the microhardness of the composite is found to be 1500 HV. The electrochemical linear polarization measurements in a corrosive NaCl 3.5% solution shows at 2.0 g/l bath load favourable conditions for the formation of a defects free composite deposit could be created.

Abstract: The surface conditions of LiFePO4 powder were modified by adding AlF3 and Al2O3 by using the sol-gel process to improve its electrochemical properties. The surface of LiFePO4 powders was partially covered with nano-sized AlF3 and Al2O3, which is confirmed by using a transmission electron microscope image. The states of coated Al materials were examined by using X-ray photoelectron spectrometer results. The nano-sized AlF3- and Al2O3-coated LiFePO4 powders showed no difference in the bulk structure compared with the pristine one. However, the AlF3- and Al2O3-coating on LiFePO4 powders improved the overall electrochemical properties such as the discharge capacity, the cyclability, and the rate capability compared with those of a pure LiFePO4. Such enhancements were attributed to the presence of a stable AlF3 and Al2O3 layer which acts as an interfacial stabilizer on the surface of LiFePO4 powders.

Abstract: Electrolytical deposition of CdSe on Ti substrates from CdSO4 - SeO2 solutions is investigated. The effect of the bath temperature was investigated and how it affects the CdSe deposits. According to the XRD spectra, the obtained CdSe films exhibit the cubic zinc-blende structure that remains unaffected by vacuum heat treatment (650°C). The surface morphology of the films differs significantly with bath solution temperatures. When Au contacts are used, the Ti/CdSe/Au structure may exhibit rectifying properties depending on the temperature during the electrodeposition. High temperature baths make the deposits to obtain ohmic properties.

Abstract: The utilization of Al2O3 and Li2O as dopants that promote the anatase-to-rutile (A-R) phase transition in TiO2 nanoparticles during calcinations is studied. X-Ray Diffraction and SEM techniques were employed for the evaluation of phase transformation and particle size coarsening in pure TiO2, TiO2-Al2O3 and TiO2-Li2O mixtures. For the Li-Ti-O pseudobinary systems some complex oxides may be formed during phase transformation that occurs at significantly lower temperatures compared to pure TiO2 or TiO2-Al2O3 mixtures. Al2O3 doping in TiO2 only increases the anatase-to-rutile transition rate once the phase transformation has been initiated.

Abstract: The present work consists in a numerical examination of the dispersion of pollutants discharged from a bent chimney and crossing twin similar cubic obstacles placed in the lee side of the source. The resulting flow is assumed to be steady, three-dimensional and turbulent. Its modelling is based upon the resolution of the Navier Stokes equations by means of the finite volume method together with the RSM (Reynolds Stress Model) turbulent model. This examination aims essentially at detailing the wind flow perturbations, the recirculation and turbulence generated by the presence of the twin cubic obstacles placed tandem at different spacing distances (gaps): W = 4 h, W = 2 h and W = 1 h where W is the distance separating both buildings.

Abstract: The electrolytic codeposition has drawn interest as an alternative method for the preparation of metal matrix composite electrocoatings with improved properties. Moreover, the application of a current periodically varied by time during electrodeposition, i.e. pulse current (p.c.), instead of the commonly used direct current (d.c.), has been proved to lead to the preparation of surfaces with well-defined properties. In this work, Ni/TiO2-ZrO2 composite electrocoatings were produced under direct and pulse current conditions. The synergetic action of both the pulse application and the simultaneous codeposition of the two different types of particles on the structure, the morphological characteristics and the microhardness of the composite electrocoatings, were studied. Concerning their mechanical behaviour, it was found that the Ni/TiO2-ZrO2 deposits prepared by pulse electrolysis are characterized by Vickers microhardness threefold harder than the d.c. ones.

Abstract: This paper deals with the interaction of twin elliptic jets with a cooler oncoming crossflow. The jet nozzles are placed tandem with the main flows, three diameters one from another and initially inclined with a 60° angle. The exploration of the resulting flowfield was carried out numerically by means of the finite volume method together with the RSM (Reynolds Stress Model) second order turbulent closure model and non uniform grid system that was particularly refined near the nozzles. After validation with reference to PIV (particle image velocimetry) experimental data, the model was upgraded by discharging a non reactive fume from the jet nozzles and introducing a variable temperature gradient between the interacting flows. We focused mainly in the present work on the determinant role of the temperature gradient on the dispersion of the discharged pollutants within the domain. Such a study is likely to optimize (control, reduce, eliminate, etc.) one of the most alarming nowadays’ environmental concern: the atmospheric pollution.

Abstract: The micro arc oxidation process (MAO) was applied to a 2024 ingot aluminium alloy by an AC MAO equipment using an alkali based electrolyte. The processing parameters of the process were positive and negative voltage pulse durations. Structural and morphological characterization of the coating were made by a scanning electron microscope (SEM), an X-ray diffractometer (XRD), a surface profilometer and a thickness gage operating according to the Eddy current principle. Cross sectional hardness of the coatings was measured, and reciprocating wear and immersion corrosion tests were performed. XRD analysis showed that an oxide layer comprising - and -Al2O3 phases was produced on the surface, whose thickness and surface roughness varied by the processing parameters applied. Wear and corrosion resistance of the original alloy significantly improved upon the MAO process. Variation of hardness, wear and corrosion resistance with respect to the processing parameters was discussed based on the experimental data obtained.