Jean-Pierre Petit

Characterization of Chromia Scales Grown on Pure Chromium in Different Oxidizing Atmospheres

Abstract Scanning electron microscopy, X-ray diffraction, photoelectrochemistry and microphotoelectrochemistry have been used to characterize thin chromia scales, in the micrometer range, grown on pure chromium at 900°C in oxygen and in water vapour. The duplex structure formed, more easily observable in water vapour grows by the opposite transport of chromium and of oxide/hydroxide ions. The external chromia subscale exhibits the usually reported 3.5 eV bandgap whereas the internal subscale presents a reduced gap possibly due to impurity contribution. Imaging the photocurrent generated in this subscale allows the observation of good metal-oxide interface properties of samples grown in H2O, whereas samples grown in O2, (liable to cracking during cooling), exhibit partially disrupted zones.

Biotin grafting on boron-doped diamond.

Grafting of biotin on top of a polycrystalline boron-doped diamond layer was achieved by surface oxidation followed by an esterification reaction and revealed by fluorescently labelled streptavidin.

Twin universes cosmology

Starting from the field equationS =χ(T -A(T)), presented in a former paper, we present a test result, based on numerical simulations, giving a new model applied to the very large structure of the Universe. A theory of inverse gravitational lensing is developed, in which the observed effects could be due mainly to the action of surrounding ‘antipodal matter’. This is an alternative to the explanation based on dark matter existence. We then develop a cosmological model. Because of the hypothesis of homogeneity, the metric must be a solution of the equationS = 0, although the total mass of the Universe is non-zero. In order to avoid the trivial solutionR = constant ×t, we consider a model with ‘variable constants’. Then we derive the laws linking the different constants of physics:G, c, h, m; in order to keep the basic equations of physics invariant, so that the variation of these constants is not measurable in the laboratory, the only effect of this process being the red shift, due to the secular variation of these constants. All the energies are conserved, but not the masses. We find that all of the characteristic lengths (Schwarzschild, Jeans, Compton, Planck) vary like the characteristic lengthR, from where all the characteristic times vary like the cosmic timet. As the energy of the photonhν is conserved over its flight, the decrease of its frequencyν is due to the growth of the Planck constanth ≈ t. In such conditions the field equations have a single solution, corresponding to a negative curvature and to an evolution law:R ≈ t2/3.The model is no longer isentropic ands ≈ logt. The cosmologic horizon varies likeR, so that the homogeneity of the Universe is ensured at any time which constitues an alternative to the theory of inflation. We re-find, for moderate distances, Hubbles law. A new law: distance =f(z) is derived, very close to the classical one for moderate red shifts.

Interfacial Structure and Photocatalytic Activity of Magnetron Sputtered TiO2 on Conducting Metal Substrates

The photocatalytic behavior of magnetron sputtered anatase TiO2 coatings on copper, nickel, and gold was investigated with the aim of understanding the effect of the metallic substrate and coating-substrate interface structure. Stoichiometry and nanoscale structure of the coating were investigated using X-ray diffraction, Raman spectroscopy, atomic force microscope, and scanning and transmission electron microscopy. Photocatalytic behavior of the coating was explored by using optical spectrophotometry and electrochemical methods via photovoltage, photocurrent, and scanning kelvin probe microscopy measurements. The nature of the metal substrate and coating-substrate interface had profound influence on the photocatalytic behavior. Less photon energy was required for TiO2 excitation on a nickel substrate, whereas TiO2 coating on copper showed a higher band gap attributed to quantum confinement. However, the TiO2 coating on gold exhibited behavior typical of facile transfer of electrons to and from the CB, therefore requiring only a small amount of photon energy to make the TiO2 coating conductive.

Ionic conduction in (PEO)nKCuxI1 + x complexes

We introduce a new family of solid polymer electrolytes where the Cu+ cation is coordinated with an anionic ligand I−, resulting in a complex carrying at least one negative charge. In the PEOnKCuxI1 + x electrolyte, potassium iodide acts as a supporting electrolyte where copper cations Cu+ are carried as CuI−2 complex anions. The transport of Cu+ thus occurs via a vehicular mechanism. The conductivity of these electrolytes is around 5 × 10−6 Ω−1 cm− at 40°C and the transference number of copper component has been found to be 0.4.

Optimization Of The Conductivity And Passivity Properties Of Passive Films On Stainless Steels Used As Bipolar Plates In PEFC

The properties responsible for electrical contact resistance between a stainless steel bipolar plate and a carbon support in a PEFC has been investigated. Roughness of the bipolar plate is useful but its topography should be adapted to the type of used GDL. Bulk composition of the alloy also plays a role, because it defines the quantity of alloying elements susceptible to participate to the passive film growth, but this role appears secondary. Indeed, passivation pre-treatment can deeply modify both structure and composition of the passive layer as investigated on AISI 316L and 904L steels, until reducing the initial electrical contact resistance at a level comparable to graphite. A low-cost surface treatment could thurs represent a good compromise between using as-received stainless steel plates and deposition of precious metals. Compared to industrial bright annealed state, initial ECR is reduced by a factor 20 but raised slightly after 500 hours of operation.

Photosensitization of Boron-Doped Diamond by Surface Grafting of Pyrene Groups

Boron-doped diamond samples were exposed successively to singlet oxygen and pyrene-alkylcarboxylic acid molecules. The resulting samples were studied by photoelectrochemistry. It is shown that the expected esterification reaction indeed allowed the grafting of pyrene groups onto the diamond surface. Cathodic photocurrents have been obtained with quantum yields as high as η=0.29% (λ=351 nm, U=-0.38 V vs. a saturated calomel electrode) in oxygen saturated electrolyte. The pyrene coverage is estimated from both photocurrent and photopotential measurements. The role of dissolved oxygen in the photocurrent generation is discussed.

Development of a Reference Electrode for a PEMFC Single Cell Allowing an Evaluation of Plate Potentials

Increasing lifetime and performance is critical for proton exchange membrane fuel cell (PEMFC) using stainless steel plates. A good compromise between passivity and electrical contact resistance of the plate material is required. Measuring the potential of each plate during fuel cell operation is of paramount importance to lead to relevant ex situ tests in order to investigate new materials. From a review on methods used for potential measurements, the present work focused on the realization and use of a dynamic hydrogen electrode (DHE) device as a reference electrode in a PEMFC single cell and its evaluation in terms of accuracy and drift. With classic reference electrodes introduced into the flow field, measurements were shown to be irrelevant because of the impossibility to ensure good and stable ionic conductivity between the reference electrode and the plate when operating the cell. Several examples of DHE found in the literature were reviewed and used to realize a DHE, which showed correct accuracy and stability of its potential under fully humidified conditions. The experimental device was shown to be reliable and easily adaptable for different single cells. It was used to investigate transient phenomena while cycling a cell, but needs some improvement when the cell is operated with unsaturated gases.

Optical modulation of ionic conductivity of PEO and azobenzene-based electrolytes

By mixing short polyethyleneoxide (PEO) chains end-grafted with azobenzene groups with long PEO chains together with a lithium salt, we obtained ionically conducting films whose conductivity can be modulated by UV light up to 90% of its dark value on a wide temperature range. From electrical (impedance spectroscopy) and structure measurements (SAXS), we can conclude that this behavior is essentially resulting from a localized process of freeing of charge carriers via the conformational change of the azobenzene molecules. This article mainly compares the results obtained with two different photosensitive derivatives.

A PEO salt complex exhibiting photoresponsive ionicconductivity

A polymer electrolyte film made of lithium perchlorate dissolved in a mix of high molecular mass poly(ethylene oxide) (PEO) and shorter PEO–azobenzene diblocks, shows a reversible 70% conductivity increase under UV light irradiation at room temperature.