On the crucial influence of some supporting electrolytes during electrocoagulation in the presence of aluminum electrodes

The influence of some supporting electrolytes on aluminum electrode oxidation and pH variation during electrocoagulation of an unskimmed milk sample and a cutting oil emulsion has been investigated. Among the electrolytes studied, sulfate anions were found to be quite harmful both for electrical consumption and electrocoagulation efficiency. At the opposite, chloride and ammonium ionswere particularly benefic respectively for aluminum corrosion and pH regulation, whereas sodium cations were observed to have a neutral role. The results indicate that electrocoagulation can be realized at lowanodic potential even in the presence of sulfate ions when the [Cl−]/[SO4 2−] ratio is around or greater than 1/10. The detrimental effect of sulfates on electrocoagulation efficiency can be thwarted by the use of the ammonium salt thanks to its related buffer effect

Influence of the oxyanion nature of the electrolyte on the corrosion/passivation behaviour of nickel

The electrochemical behaviour of nickel in the presence of various electrolyte solutions at 0.1 mol/L concentration exhibits a distinction according to the oxyanion nature of the investigated anions. Passivity is achieved with oxyanions whereas it fails with anions not containing oxygen. SIMS and XPS measurements performed from isotopic and non isotopic KNO₃ electrolytes indicate that the oxygen and nitrogen atoms from nitrate oxyanions are incorporated into the passive film during anodic polarization and with evidence of a direct bonding between nitrogen and nickel surface

Multi-scale modelling of silicon nanocrystal synthesis by Low Pressure Chemical Vapor Deposition.

A multi-scale model has been developed in order to represent the nucleation and growth phenomena taking place during silicon nanocrystal (NC) synthesis on SiO2 substrates by Low Pressure Chemical Vapor Deposition from pure silane SiH4. Intrinsic sticking coefficients and H2 desorption kinetic parameters were established by ab initio modelling for the first three stages of silicon chemisorption on SiO2 sites, i.e. silanol Si―OH bonds and siloxane Si―O―Si bridges. This ab initio study has revealed that silane cannot directly chemisorb on SiO2 sites, the first silicon chemisorption proceeds from homogeneously born unsaturated species like silylene SiH2. These kinetic data were implemented into the Computational Fluid Dynamics Fluent code at the industrial reactor scale, by activating its system of surface site control in transient conditions. NC area densities and radii deduced from Fluent calculations were validated by comparison with experimental data. Information about the deposition mechanisms was then obtained. In particular, hydrogen desorption has been identified as the main limiting step of NC nucleation and growth, and the NC growth rate highly increases with run duration due to the autocatalytic nature of deposition

Towards multiscale modeling of Si nanocrystals LPCVD deposition on SiO2: From ab initio calculations to reactor scale simulations

A modeling study is presented involving calculations at continuum and atomistic (DFT, Density Functional Theory) levels so as to better understand mechanisms leading to silicon nanocrystals (NC) nucleation and growth on SiO2 silicon dioxide surface, by Low Pressure Chemical Vapor Deposition (LPCVD) from silane SiH4. Calculations at the industrial reactor scale show that a promising way to improve reproducibility and uniformity of NC deposition at short term could be to increase deposition time by highly diluting silane in a carrier gas. This dilution leads to a decrease of silane deposition rate and to a marked increase of the contribution to deposition of unsaturated species such as silylene SiH2. This result gives importance to our DFT calculations since they reveal that only silylene (and probably other unsaturated species) are involved in the very first steps of nucleation i.e. silicon chemisorption on silanol Si–OH or siloxane Si–O–Si bonds present on SiO2 substrates. Saturated molecules such as silane could only contribute to NC growth, i.e. chemisorption on already deposited silicon bonds, since their decomposition activation barriers on SiO2 surface are as high as 3 eV

Stability of TiO2 Promoted PtCo/C Catalyst for Oxygen Reduction Reaction

International audienceThis work was carried out to explore the effect of TiO2 on activity and stability of a PtCo/C catalyst for an oxygen reduction reaction (ORR). Two types of TiO2, including commercial TiO2 (TCOM) and a home-prepared TiO2 by chemical vapor deposition (TCVD), were incorporated on the PtCo/C catalyst layer. The activity of all prepared-catalysts was tested in a single proton exchange membrane (PEM) fuel cell under an H2/O2 environment at ambient pressure, while their stability was tested by the linear sweep voltammetry (LSV) in 0.5 M H2SO4. The preliminary results demonstrated that the TCVD promoted PtCo/C catalyst (TCVD-PtCo/C) exhibited the highest activity in a PEM fuel cell for both activation polarization and ohmic polarization regions, which can produce the current density of 434 mA/cm2 (277 mW/cm2 or 1,847 W/gPtcm2) at 0.6 V. It also exhibited the highest stability in 0.5 M H2SO4 with performance loss of around 40% after 6,000 LSV-cycles

Multiscale modelling of low-pressure CVD of Silicon based materials in deep submicronic trenches: a continuum feature scale model

The ability to predict feature profile evolution across wafers during filling from equipment scale operating conditions is one important goal of process engineers for power component fabrication. We develop an integrated approach for simulating the multiple length scales needed to address this problem for Low Pressure CVD processes of silicon based materials in deep submicronic trenches (aspect ratio can exceed 50). In this approach, continuum models at the reactor (100m) and feature (10-7m) scales are tightly coupled in order to predict micro- and macro- loading effects in a transient environment. First, the main principles and assumptions of the reactor and trench scale models are presented. Then, some characteristic examples of numerical results at the trench scale are analysed and compared with the predictions of the deterministic Ballistic Transport-Reaction Model (BTRM) EVOLVE. This comparison shows that our continuum approach gives results as accurate as those of the BTRM one even for highly non conformable layers, for computations times up to 3 times lower

Graphene in silicon photovoltaic cells

Graphene is an allotrope of carbon. Its structure is one-atom-thick planar sheets of carbon atoms that are densely packed in a honeycomb crystal lattice [1]. The richness of optical and electronic properties of graphene attracts enormous interest. Its true potential seems to be in photonics and optoelectronics, where the combination of its unique optical and electronic properties can be fully exploited. The optical absorption of graphene layers is proportional to the number of layers, each absorbing A=1-T=πα=2.3% over the visible spectrum [2].The rise of graphene in photonics and optoelectronics is shown by several recent results, ranging from solar cells and light emitting devices, to touch screens, photodetectors and ultrafast lasers. Current photovoltaic (PV) technology is dominated by Si cells, with an energy conversion coefficient up to 25% [3]. Such an inorganic PV consists in a current transparent conductor (TC) replacing one of the electrodes of a PIN photodiode. The standard material used so far for these electrodes is indium-tinoxide, or ITO. But indium is expensive and relatively rare, so the search has been on for a suitable replacement. A possible substitute made from inexpensive and ubiquitous carbon is graphene. Being only constituted of carbon, it will become cheap and easily recyclable. But at the moment, the major difficulty consists in its fabrication and/or transfer. Our project consists in synthetizing graphene by CVD (Chemical Vapor Deposition) on Cu and in transferring the obtained layer on silicon PV cells, and then in testing their energy conversion efficiency

Modeling a MOCVD process to apply alumina films on the inner surface of bottles

Deposition inside a cavity with a narrow neck, i.e. a hollow body, is a challenge, in particular for the supply and the evacuation of the reactive phase and of the effluents through the same entrance. High gradients (pressure, velocity, species concentrations) and important convective mechanisms related to the impacting jet of the gaseous phase on the bottom inner wall of the cavity are created in that region. Given the increasing number of experimental parameters,modeling helps efficient process optimization with better understanding of physical and chemical phenomena occurring inside the hollow body. This study presents a numerical modeling using the CFD code Fluent to simulate a direct liquid injection metal organic chemical vapor deposition of amorphous alumina coatings from aluminum tri-isopropoxide (ATI) on the inner walls of a glass bottle. The model is used to predict local profiles of gas velocity, temperature and species concentrations into the reactor as well as local growth rate based on an apparent heterogeneous kinetic law of ATI decomposition. Comparison with experimental thickness profiles allows validating the model and leads to a better insight in the phenomena which impact film thickness inhomogeneity along the inner surface. The model contributes to the improvement of the reactor configuration by increasing the distance between nozzle and bottle and the outlet section of the nozzle

Effect of MO2 (M = Ce, Mo, Ti) layer on activity and stability of PtCo/C catalysts during an oxygen reduction reaction

The performance of PtCo/C catalysts in the presence of a metal oxides layer for an oxygen reduction reaction (ORR) was investigated. Different types of metal oxides (CeO2, MoO2 and TiO2) and metal loadings (0.03–0.45 mg/cm2) were incorporated on the PtCo/C catalyst layer. Their activity was analyzed in acid solution and proton exchange membrane (PEM) fuel cell under a H2/O2 environment at 60 °C and ambient pressure, while the stability was tested in an N2-saturated H2SO4 solution using repetitive potential cycling. It was found that the addition of metal oxides on a catalyst layer had no influence for PtCo/C morphology. However, they significantly affected the electrochemical surface area (ESA), internal contact resistance (ICR) and hydrophilic/hydrophobic properties of the catalysts layer. Furthermore, they significantly affected the ORR activity and stability in acid solution and PEM fuel cell operation. Among all studied metal oxides, the TiO2 exhibited the best property for use as the catalyst interlayer in PEM fuel cell for both activity and stability enhancement