Pascal Brault

Plasma sputtering deposition of platinum into porous fuel cell electrodes

Platinum is deposited into porous carbon materials relevant for fuel cell electrodes using plasma sputtering techniques. The resulting platinum concentration profile extends up to 2 µm into the porous carbon and is well fitted by a generalized stretched Gaussian function, which displays the non-thermal nature of the penetration process. Platinum deposits are observed to grow as clusters. On the outermost carbon particles, platinum nano-cluster sizes of 3.5 nm have been measured. In tests using actual PEM fuel cells, current densities as high as 1000 mA cm−2 have been obtained at 400 mV with 25 cm2 plasma electrodes. This compares favourably with commercially available electrodes but the present electrodes have a platinum density 4.5 times lower and hence can be considered to be significantly more efficient.

Molecular dynamics for low temperature plasma?surface interaction studies

The mechanisms of physical and chemical interactions of low temperature plasmas with surfaces can be fruitfully explored using molecular dynamics (MD) simulations. MD simulations follow the detailed motion of sets of interacting atoms through integration of atomic equations of motion, using inter-atomic potentials that can account for bond breaking and formation that result when energetic species from the plasma impact surfaces. This paper summarizes the current status of the technique for various applications of low temperature plasmas to material processing technologies. The method is reviewed, and commonly used inter-atomic potentials are described. Special attention is paid to the use of MD in understanding various representative applications, including tetrahedral amorphous carbon film deposition from energetic carbon ions, the interactions of radical species with amorphous hydrogenated silicon films, silicon nanoparticles in plasmas, and plasma etching.

Silicon columnar microstructures induced by an SF6/O2 plasma

An inductively coupled SF6/O2 plasma is used to form a columnar microstructure (CMS) on silicon samples cooled at very low temperature (~ −100 °C). The formation of this CMS is studied as a function of bias voltage, temperature, RF power and gas pressure. The characteristic mean diameter and mean height of the microstructure are evaluated by image processing tools from SEM micrographs. A crystallographic effect is also observed at very low temperature, which induces a needle-shaped structure. A physical mechanism is proposed to explain the formation of this CMS.

Nucleation and initial growth of platinum islands by plasma sputter deposition

The nucleation and islands growth of platinum have been followed from an early stage through to complete substrate coverage. Thus, this study of initial stages of thin film growth has allowed to improve the understanding of growth mechanisms and to control the final nanomaterials structure. In the presented deposition method, the metal atom source is a negatively biased metal wire submitted to the bombardment of ions created in HF plasma. The originality of this plasma sputtering technique is that in addition to the metal atom flux, the substrate surface is submitted to a high flux of low energy ions, inducing a high atomic surface mobility and modifying the growth mode with respect to conventional deposition techniques (magnetron sputtering or evaporation). Platinum deposits have been investigated by ex situ characterisation techniques: grazing incidence-small angle X-ray scattering (GISAXS) and X-ray diffraction (GIXD) in conjunction with transmission electron microscopy (TEM). Using morphological parameters as coverage rate, islands number density and islands size, different mechanisms (scale laws, islands mobility, coalescence, etc.) governing the growth are described vs. Ar ion and Pt atom energies and fluxes.

Anomalous diffusion mediated by atom deposition into a porous substrate.

Constant flux atom deposition into a porous medium is shown to generate a dense overlayer and a diffusion profile. Scaling analysis shows that the overlayer acts as a dynamic control for atomic diffusion in the porous substrate. This is modeled by generalizing the porous diffusion equation with a time-dependent diffusion coefficient equivalent to a nonlinear rescaling of time.

Deposition and structure of W?Cu multilayer coatings by magnetron sputtering

W–Cu–W multilayer metallic coatings are designed and deposited by dc magnetron sputtering on an Fe substrate. Correlations between the deposition parameters, such as target power and Ar gas pressure, and the film characteristics are investigated. Especially, deposition parameters for a dense W–Cu multilayer coating are discussed. The coatings exhibit small grain sizes and a dense surface structure for high target power and low argon pressure, leading to dense and well adhesive films.

Effect of Nafion and platinum content in a catalyst layer processed in a radio frequency helicon plasma system

A helicon plasma sputtering system is used to deposit small amounts of platinum on microporous carbon support composed of Vulcan XC 72 carbon particles (known as gas diffusion layer) to form Pt catalyzed electrodes for proton exchange membrane fuel cells. Electrodes with low Pt loading are prepared, assembled in custom-made membrane electrode assemblies (MEAs) and tested for the hydrogen oxydation and the oxygen reduction. Initially, the Nafion ® loading spread on these plasma prepared electrodes is optimized by measuring the MEA performance. It is found that the optimum Nafion ® loading is 1 mg cm −2 for an electrode previously covered with 0.1 mgPt cm −2 using the helicon plasma system. For a commercial electrode prepared by ink processes with 0.5 mg Pt cm −2 , the optimized Nafion ® loading is 2m g cm −2 . Using the respective optimized Nafion ® loading, the electrical performance of the custom-made MEA with one plasma prepared electrode (either anode or cathode) is compared with that of a reference MEA from Electrochem Inc. (Pt loading per electrode of 0.5 mg cm −2 and maximum power density of 425 mW cm −2 ) without gas humidification. The custom-made MEA fitted with an anode covered with 0.005 mg Pt cm −2 leads to the same performance as that of the reference MEA at low current density (<500 mA cm −2 ) and high gas backpressure (3 bar). This result indicates that the catalyst utilization efficiency in the plasma prepared anode is 100 times higher than that in the commercial anode (85 kW g −1 Pt versus 0.85 kW g −1 Pt ). For plasma prepared cathodes with 0.1 mgPt cm −2 , the cathodic Pt utilization efficiency is 2.7 kW g −1 Pt , which is 3 times higher than that obtained in the commercial cathode. (Some figures in this article are in colour only in the electronic version)

Pd nanoclusters grown by plasma sputtering deposition on amorphous substrates

Abstract In this work, we study the deposition of palladium by a non-conventional plasma sputtering technique. The metal atom source is a biased wire which is sputtered by argon ions present in an HF-excited plasma. Two energetic plasma species (ions and metastable atoms) impinge onto the substrate surface during deposition and thus, may influence the growth. The flux of metastable atoms can be evaluated to 1014 atoms/cm2 with a potential energy of 12 eV. Electrical measurements of the plasma performed by the Langmuir probe allow the evaluation of the ion flux. It is of the order of 1014 ions/cm2 s for a kinetic energy of 50 eV. The respective effect of both species cannot be separated but, since ions carry higher energy, they are expected to play the major role. Thus, comparison is made with conventional ion beam-assisted deposition techniques, for which the ion flux vs. metal atom flux ratio is low (values ≤0.1 against 10 in plasma sputtering technique) and the incoming ion energy is high: of the order of hundreds or thousands of electron volts. Information on the film growth in these particular conditions are obtained by transmission electron microscopy (TEM) analysis of deposits performed on carbon membrane (coating copper grids) and by grazing incidence small angle X-ray scattering (GISAXS) characterization of Pd/amorphous SiO2 deposits. In the present plasma conditions (100 mTorr argon pressure and −100 V wire bias), 3D clusters are found to be formed which grow in size and coalesce to form meandering islands. After the coalescence step, when the fractional covered area is sufficiently high, it is evidenced that the simultaneous energetic species flux causes a reorganization of the meandering aggregates, and further, the formation of more compact islands composed of bigger elementary clusters. This is attributed to a rise of the mobility of metal atoms and small clusters under bombardment.

Percolative growth of palladium ultrathin films deposited by plasma sputtering

Palladium ultrathin films have been deposited by a plasma sputtering technique. In a first step, small particles (mean diameter d<3nm). homogeneously distributed on amorphous carbon membrane surfaces. are grown by surface diffusion of Pd atoms. Coalescence starts to form meandering compound clusters. Such Pd clusters. composing the islands. are still enlarging during the deposition. During the coalescence. a percolating network builds up with a measured percolation threshold around p c =0.7. The time evolutions of the mean size of the elementary and compound clusters are slightly modified when compared to vacuum deposition and theoretical predictions.

Roughness scaling of plasma-etched silicon surfaces

Atomic force microscopy reveals scaling behaviour of silicon surfaces etched by plasma. The experimental results are compared with some theoretical models. It is shown that plasma-induced roughness is driven by a phenomenon that can be described by shadowing instabilities resulting in columnar microstructure growth. The same scaling properties as are predicted by a growth model are obtained.