Membrane patterned by pulsed laser micromachining for proton exchange membrane fuel cell with sputtered ultra-low catalyst loadings

International audienceProton exchange membranes were nano-and micro-patterned on their cathode side by pressing them against stainless steel molds previously irradiated by a Ti:Sapphire femtosecond laser. The membranes were associated to ultra-low loaded thin catalytic layers (25 µgPt cm-2) prepared by plasma magnetron sputtering. The Pt catalyst was sputtered either on the membrane or on the porous electrode. The fuel cell performance in dry conditions were found to be highly dependent on the morphology of the membrane surface. When nanometric ripples covered by a Pt catalyst were introduced on the surface of the membrane, the fuel cell outperformed the conventional one with a flat membrane. By combining nano-and micro-patterns (nanometric ripples and 11-24 µm deep craters), the performance of the cells was clearly enhanced. The maximum power density achieved by the fuel cell was multiplied by a factor of 3.6 (at 50 °C and 3 bars): 438 mW cm-2 vs 122 mW cm-2. This improvement is due to high catalyst utilization with a high membrane conductivity. When Pt is sputtered on the porous electrode (and not on the membrane), the contribution of the patterned membrane to the fuel cell efficiency was less significant, except in the presence of nanometric ripples. This result suggests that the patterning of the membrane must be consistent with the way the catalyst is synthesized, on the membrane or on the porous electrode

Investigation of pulsed laser induced dewetting in nanoscopic metal films

Hydrodynamic pattern formation (PF) and dewetting resulting from pulsed laser induced melting of nanoscopic metal films have been used to create spatially ordered metal nanoparticle arrays with monomodal size distribution on SiO_{\text{2}}/Si substrates. PF was investigated for film thickness h\leq7 nm < laser absorption depth \sim11 nm and different sets of laser parameters, including energy density E and the irradiation time, as measured by the number of pulses n. PF was only observed to occur for E\geq E_{m}, where E_{m} denotes the h-dependent threshold energy required to melt the film. Even at such small length scales, theoretical predictions for E_{m} obtained from a continuum-level lumped parameter heat transfer model for the film temperature, coupled with the 1-D transient heat equation for the substrate phase, were consistent with experimental observations provided that the thickness dependence of the reflectivity of the metal-substrate bilayer was incorporated into the analysis. The spacing between the nanoparticles and the particle diameter were found to increase as h^{2} and h^{5/3} respectively, which is consistent with the predictions of the thin film hydrodynamic (TFH) dewetting theory. These results suggest that fast thermal processing can lead to novel pattern formation, including quenching of a wide range of length scales and morphologies.Comment: 36 pages, 11 figures, 1 tabl

Direct measurements of the energy flux due to chemical reactions at the surface of a silicon sample interacting with a SF6 plasma

Energy exchanges due to chemical reactions between a silicon surface and a SF6 plasma were directly measured using a heat flux microsensor (HFM). The energy flux evolution was compared with those obtained when only few reactions occur at the surface to show the part of chemical reactions. At 800 W, the measured energy flux due to chemical reactions is estimated at about 7 W.cm\^{-2} against 0.4 W.cm\^{-2} for ion bombardment and other contributions. Time evolution of the HFM signal is also studied. The molar enthalpy of the reaction giving SiF4 molecules was evaluated and is consistent with values given in literature.Comment: 3 page

Real-Time Laser Based Diagnostics for Complex Thin Films Thermo-physical and Thermo-electrical Properties Characterization

International audienceFast optical methods were successfully used for the thermo-physical characterization of various samples, from metallic films to complex films such as vertically aligned carbon nanotube (CNT) carpets and mesoporous silicon (MeSi) substrates. Contribution of real-time reflectometry (RTR) to laser induced processes is highlighted through several applications with Excimer and Nd:YAG laser beams processing a high number of materials (sc-Si, sc-ZnO, etc.) related to sensors fabrication [1-2]. The pulsed photo-thermal technique (PPT, with less than 2ns response time, see Fig.1) allows the evaluation of thermal properties of a wide range of thin film materials. Due to their promising microelectronic applications in design and packaging, thermo-physical properties of AlN, CNT and MeSi films will be discussed leading to predict the best materials fabrication process for a targeted application. As reducing the size of active surfaces, laser-based nanostructuration of copper thin films was also successfully monitored using RTR at picosecond time scale [3]. This last topic is also illustrated in this paper and connected to the enhancement of the thermo-electric properties of thin films, particularly in the case of oxides and polymers. The detailed description of the measurement setup (Fig.2) and the instrumental concept behind will also be illustrated. The complete characterization of selected materials (polymer based, MeSi, etc.) will be discussed [4] namely on their figure of merit and efficiency

Metallic thin films heated by pulsed lasers. Numerical simulation of the thermal field and the melting kinetics

This modeling is especially applied to the pulsed laser induced heating and melting of a metallic film deposited on a substrate. Study of the thermal field over a surface is usually performed by considering the assumption of ‘semi-infinite medium’. However, a thin film deposited on a rough substrate surface induces bad thermal contacts commonly known as ‘thermal contact resistance’. This interfacial thermal resistance affects the melting kinetics mainly when the film thickness (Z) is small comparatively to the heat diffusion length (ZT). In this work the heat conduction equation and related boundary conditions are resolved by using the implicit finite differences method. The heat source (i.e. the laser intensity) is treated as a surface boundary layer. The thermal contact resistance is introduced in the computation procedure when the heat wave propagation reaches the thin film/substrate interface. It is then possible to calculate the critical temperatures and the melting threshold fluence for high and low contact resistance values. Under these conditions, the temperature profile and melting depth are plotted considering different thickness.. Finally, for 750 mJ/cm² excimer laser fluence and 0.1 cm²/s thin film apparent diffusivity results show that for Z/ZT higher than 0.5, there is no sensitive effect of the thermal contact resistance on the melting kinetics. </jats:p

Performances d'une centrale à concentrateur de Fresnel de moyenne puissance sur sites Algériens

International audienceL'objectif du présent travail est de caractériser une centrale à concentrateur de Fresnel de moyenne puissance (5MWe) sur différents sites Algériens (Sahara), ces sites ont été choisis pour comparaison en déplaçant la centrale dans ces derniers, à savoir Hassi-R'mel, Tamanrasset, Béni-Abbes, et El Oued. Une modélisation théorique a permis la détermination du nombre optimum des miroirs à ne pas dépasser pour éviter les effets d'ombre, blocage et l'effet cosinus. Le nombre optimum de miroirs a été estimé à 40 miroirs. Par ailleurs, une simulation a été réalisée à l'échelle horaire : elle concerne l'ensoleillement, le système 'champ', et le système 'récepteur'. Les résultats de la simulation sont représentés en moyenne annuels. Les rendements, la production énergétique annuelle, et le coût du kWh sont évalués. En effet, le calcul du rendement diffère d'un site à un autre avec des valeurs de 15.7%, 11.8%,11.6%, et 10.4% respectivement pour les sites Tamanrasset, El Oued, Béni-Abbes, Hassi R'mel. Les performances thermiques et les résultats technico-économiques sont évalués aussi pour ces mêmes sites. Mots clés : Énergie solaire, concentrateur solaire à miroirs de Fresnel, les centrales solaires thermiques, performance, les centrales thermoélectriques