Laurent Arurault

Controlled growth of CNT in mesoporous AAO through optimized conditions for membrane preparation and CVD operation.

Anodic aluminium oxide (RAAO) membranes with a mesoporous structure were prepared under strictly controlling experimental process conditions, and physically and chemically characterized by a wide range of experimental techniques. Commercial anodic aluminium oxide (CAAO) membranes were also investigated for comparison. We demonstrated that RAAO membranes have lower content of both water and phosphorus and showed better porosity shape than CAAO. The RAAO membranes were used for template growth of carbon nanotubes (CNT) inside its pores by ethylene chemical vapour deposition (CVD) in the absence of a catalyst. A composite material, containing one nanotube for each channel, having the same length as the membrane thickness and an external diameter close to the diameter of the membrane holes, was obtained. Yield, selectivity and quality of CNTs in terms of diameter, length and arrangement (i.e. number of tubes for each channel) were optimized by investigating the effect of changing the experimental conditions for the CVD process. We showed that upon thermal treatment RAAO membranes were made up of crystallized allotropic alumina phases, which govern the subsequent CNT growth, because of their catalytic activity, likely due to their Lewis acidity. The strict control of experimental conditions for membrane preparation and CNT growth allowed us to enhance the carbon structural order, which is a critical requisite for CNT application as a substitute for copper in novel nano-interconnects.

Pilling–Bedworth ratio of thick anodic aluminium porous films prepared at high voltages in H2SO4 based electrolyte

Abstract Thick porous anodic films have been prepared using high voltages in a sulphuric acid based electrolyte. The use of H2SO4 low concentration, low bath temperature and the boric acid as modifier allows the anodic porous film to be significantly thickened preventing its chemical dissolution. A new relation including the Pilling–Bedworth ratio, especially of the thick anodic porous films, is proposed here to take into account the nanoporosity and the anodic current efficiency.

Surface integrity after pickling and anodization of Ti–6Al–4V titanium alloy

The surface integrity of Ti–6Al–4V titanium alloy was studied at different stages of surface treatments, especially pickling and compact anodization, through surface characteristics potentially worsening fatigue resistance. No significant changes of the equiaxe microstructure were detected between sample core and surface, or after the pickling and anodization steps. Surface hydrogen and oxygen superficial contents were found to remain unchanged. Roughness characteristics (i.e. Ra, Rz but also local Kt factor) similarly showed only slight modifications, although SPM and SEM revealed certain random local surface defaults, i.e. pits about 400 nm in depth. Finally internal stresses, evaluated using X-ray diffraction, highlighted a significant decrease of the compressive internal stresses, potentially detrimental for fatigue resistance.

Impact of the type of anodic film formed and deposition time on the characteristics of porous anodic aluminium oxide films containing Ni metal

Porous anodic films containing nickel were prepared by AC electro-deposition. The porosity of the films was controlled by using different working conditions (anodisation electrolyte, voltage, and time). Then nickel was electro-deposited using an alternating voltage. The impact of the anodic film on the current density waveforms and the metal content can largely be explained by the porosity differences, while changing the deposition time caused changes due to over-oxidation of the aluminium substrate, experimentally proved by TEM. Finally, the impact of deposition time on the deposited metal was successfully fitted using an Elovich type law over a large time-span (up to 1800 s), showing the ability to achieve precise control of the metal content.

Oxidation of Stoichiometric Nickel Ferrites Used as Inert Anodes for Aluminium Electrolysis in Molten Cryolite Mixtures

Thanks to their good electronic conductivity and their low solubility in cryolite melts, nickel ferrites are considered to be among the most suitable ceramic materials that could be used as inert anodes in the electrowinning of aluminium. In this work, electrodes composed of single phased and stoichiometric nickel ferrite NixFe3-xO4 (0≤x<1) have been studied by electrochemical techniques (linear voltammetry and potentiostatic electrolysis) in a molten cryolite mixture at 960°C. The aim was to understand the oxidation reactions susceptible to take place inside the material under anodic polarization and oxygen evolution. Ex situ characterization of the electrodes by SEM-EDX and microprobe analysis allows proposing a degradation mechanism of the pure nickel ferrites: the formation of haematite Fe2O3 at the grain boundaries was evidenced and it resulted in a slow degradation. The influence of the Ni content in the ceramic phase was investigated, and it was shown that rich-Ni compositions exhibit a better resistance to corrosion.

Decisive influence of colloidal suspension conductivity during electrophoretic impregnation of porous anodic film supported on 1050 aluminium substrate.

The present paper studies the influence of suspension conductivity on the electrophoretic deposition (EPD) of nanoparticles inside a porous anodic aluminium oxide film. It is shown that an increase in the suspensions conductivity enhances impregnation of the anodic film by the nanoparticles. Two mechanisms are seen to promote the migration of particles into the pores. Indeed an increase in the suspension conductivity leads on the one hand to a strengthening of the electric field in the anodic film and on the other hand to a thinning of the electric double layer on the pore walls. The results of our study confirm that colloidal suspension conductivity is a key parameter governing the electrophoretic impregnation depth.

Simulation Of The Electrical Transient During ThePorous Anodizing Of Pure Aluminium Substrate

Electrical transients were recorded during the anodizing of highly pure aluminium in phosphoric electrolyte, carried out in potentiostatic mode (25-150V) or in galvanostatic conditions (20-1000A/m). The experimental reproductibility is satisfactory, according to the low standard deviations. For the galvanostatic mode, the voltage experimental transients show a “bell shape”, characterized by some significant parameters (S0, tm, Vm, Vss). Two mathematical relations were then proposed to simulate the voltage transient curves considering two parts, i.e. before and after the maximum (Vm, tm). The validity of these computational simulations was next checked by comparison with the corresponding experimental curves. All the corresponding fittings of voltage transients are in good agreement, especially for the first part of the experimental curves, within the current densities range. Then, these computational simulations were correlated with the corresponding experimental FEG-SEM plan-views. By analogy with the nucleation phenomena during the metal electrodeposition, the “bell shaped” curves could be interpreted by the initial formation of a highly resistive oxide layer, followed by the subsequent appearance of the nanopores. The pores formation was in part explained showing that the Vm experimental values obtained with the galvanostatic mode are closed to the previous critical voltage value Uc initializing the anodic dissolution phenomenon.

Alternative black coatings prepared on aluminium alloy 1050

ABSTRACT The present study investigated the preparation and characterisation of alternative black anodic films on aluminium alloy 1050 using three different inorganic colouring processes. The colouring process was performed via the standard chemical reaction and two alternative mechanisms, i.e. electrochemical reaction or a combination of electrochemical and chemical reactions. Black anodic films with high thermo-optical properties (αs ≥ 0.90 and εn ≥ 0.90) were successfully obtained using all three processes. Differences in chemical content and location of the Co-based pigments in the coatings were highlighted and explained according to the colouring reaction mechanisms.

Mechanical behaviour of black anodic films on 7175 aluminium alloy for space applications

Because of their low outgassing and their thermo-optical properties, black anodized aluminium parts are often used near optical instruments to manage thermal control in space applications. However, critical cases of flaking of the film were observed after simulated thermal ageing. To understand the mechanisms leading to flaking, the influence of the initial porosity of the film on its mechanical behaviour during and after the black anodizing process has been investigated. The decrease of limit tensile stress with the porosity, the coloring and the sealing combined to thermal stresses due to the difference of coefficient of thermal extension between film and substrate has been shown to cause crazing in particular conditions. For high initial porosity films, thermal cycling ageing has a detrimental influence on the adhesion measured by scratch testing. Numerical simulation has been used to simulate the combined effects of thermal stresses and film cracking on the stress field at the interface.