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The age of ore deposits constitutes a decisive element in understanding their formation. Deciphering their precise chronology may be a challenge in the absence of mineral phases that can be dated by conventional geochronometers. Fluorite is very common either as the major or accessory mineral in a wide variety of ores and may provide information regarding the origin and timing of mineralizing fluid flows. In this contribution, we explore U–Pb dating on fluorite crystals from the world-class carbonate strata-bound fluorite ore of Pierre-Perthuis in Burgundy (Morvan massif, France). The uranium distribution within fluorite is mapped using induced fission-track and synchrotron radiation X-ray fluorescence nano-imaging, showing that higher U content is measured in an overgrowth of fluorite (Flog) as a discrete band. Preservation of a micrometer-thick zonation in U, associated with other substituted elements such as Sr, Y, Fe and Zr, implies that neither solid-state diffusion nor dissolution–recrystallization occurred. These U-bearing external fluorite overgrowths contain solid inclusions of about 30 µm globular pyrite crystals with a mean δ34S of −23.6 ± 0.4 ‰V-CDT. We propose that the U incorporation in the fluorite lattice results from the development of a redox front during bacterial sulfate reduction. Flog generation sampled and analyzed by laser ablation–inductively coupled plasma mass spectrometry (LA-ICP-MS) on four different crystals provides identical U–Pb ages within the limits of analytical uncertainty. Considered altogether, these four crystals yield an age estimate of 40.0 ± 1.7 Ma, not corrected for matrix-related elemental fractionation. Our results show that fluorite LA-ICP-MS U–Pb geochronology has potential for dating distinct crystal growth stages, although further research should be conducted to evaluate its accuracy.

International audience; It is well known that good wear performance can be obtained with Au-flashed palladium-nickel sphere on flat contacts simulating various connector applications. It has also been shown that these good wear properties could be recorded for asymmetrical coatings such as a noble metal sphere on Au-flashed palladium-nickel. In this paper we investigate the fretting properties of such asymmetrical coatings particularly of Ag versus Au flashed PdNi. The contacts are submitted to severe fretting wear by applying imposed sine displacements of amplitude similar to the Hertz contact radius. The contact resistance, the friction force and wear evolutions were analyzed and correlated to 3D profilometry measurements allowing wear volumes quantification and EDS elemental identification of the wear tracks.Very different behaviors are observed whether the silver coating is on the sphere or on the plane though in both case the embedding of the oxidized wear debris by silver improves the fretting behavior. The transfer of silver from the sphere contact onto the palladium nickel flat is seen to be beneficial to the electrical stability of the interface for a long time, leading to more stable results than even silver/silver contacts. Nevertheless an unusual phenomenon of the galling type is observed for which more investigations are needed.

International audience; The properties of n‐GaP/p‐Si interface as well as their influence on solar cell performance are studied for GaP layers grown by low‐temperature (380 °C) plasma‐enhanced atomic layer deposition (PE‐ALD). The influence of different plasma treatments and RF power values are explored. The increase of RF power leads to a growth transition from amorphous (a‐GaP) to microcrystalline GaP (μc‐GaP) with either amorphous‐GaP/Si or epitaxial‐GaP/Si interface, respectively. However, when continuous hydrogen plasma is used the amorphous‐GaP/Si interface exhibits better photovoltaic performance compared to the epitaxial one. Values of open circuit voltage, Voc = 0.45–0.55 V and internal quantum efficiencies, IQE > 0.9 are obtained for amorphous‐GaP/Si interfaces compared to Voc = 0.25–0.35 V and IQE

International audience; This paper presents a thin nanocomposite film, composed of graphene and of a polymer acting as a binder. The challenge was to realize a conducting, homogeneous and adherent film on a copper based material substrates (CuSn8) used for electrical contacts applications with the aim of reducing corrosion. The first step was to find the right formulation, the deposition process and the post-treatment for the composite. Flakes of graphene were mixed to two different types of high molecular weight polymers in ethanol. The films were deposited by dip-coating of the substrates in the solution; they were then grafted to the substrate by UV curing. Films between 100 nm to 300 nm thick could be deposited. Good adhesion of the film to the substrate was obtained with poly-4-vinylpyridine (P4VP) acting as a binder. The weight ratio of graphene/polymer was important for the homogeneity of the deposited films. The next step was to plate the samples with a nickel layer. Very thin nanometric nickel films were deposited on the graphene/polymer composite by an electroless nickel-boron process (NiB). The nickel nanofilm, a few tens of nanometres thick, was observed to be covering the graphene flakes embedded in the composite. Various techniques were used to characterize the samples: SEM, AFM, conducting AFM, Raman and 4 point sheet resistance measurements. Finally the samples were coated with 1.2 μm of electrolytic nickel and 0.4 μm of electrolytic gold. They were then exposed to nitric acid vapor (NAV) and hypochlorite bleach corrosion tests for several hours. The graphene composite layer was observed to hinder the pore and pitting corrosion on the samples. This process is very versatile and can find many applications in the field of corrosion protection.

International audience; Fretting is a well known phenomena due to unwanted micro displacements leading to drastic electrical contact resistance increase. Much work has been done to understand and evaluate the critical parameters in view of finding solutions to alleviate the problem. Several paths have been proposed involving for example connector design and surface modification. The simplest surface modification process is depositing an organic thin film with specific properties. The effect of liquid lubricants on fretting of electrical contacts has been well studied and the lubrication efficiency has been shown to depend on the degradation modes of the contacts which involve the nature and type of metallic surface (Au, Ni, Sn, Ag, plating thickness). Another type of lubrication involves semi liquid films such as greases. While much work has been done on the effect of greases on the fretting-wear of mechanical devices, less has been devoted to electrical contacts. In such field the grease films must also maintain a low electrical resistance for durations corresponding to field service. The work presented here was performed with a standard fretting device imposing controlled constant amplitude sine vibrations under constant normal load to pin on flat tinned contacts. This laboratory test allows comparing the behaviour of different types of greases for a given set of experimental conditions. Different commercial greases are compared involving different thickeners and different base oils. Tests at room temperature show little variations in the electrical behaviour of the grease coated contacts. Nevertheless wear particles dispersion seems to vary. A very high increase of contact resistance is observed in some cases after heat treatment 10 days at 150°C. Polymerization was observed to occur with one of the formulations and it is suggested that the base oil/thickener choice might be unfavourable to contact application.

We develop a fully analytical model in order to describe the temperature dependence of the low frequency capacitance of heterojunctions between hydrogenated amorphous silicon (a-Si:H) and crystalline silicon (c-Si). We demonstrate that the slope of the capacitance-temperature (C-T) curve is strongly enhanced if the c-Si surface is under strong inversion conditions compared to the usually assumed depletion layer capacitance. We have extended our analytical model to integrate a very thin undoped (i) a-Si:H layer at the interface and the finite thickness of the doped a-Si:H layer that are used in high efficiency solar cells for the passivation of interface defects and to limit short circuit current losses. Finally, using our calculations, we analyze experimental data on high efficiency silicon heterojunction solar cells. The transition from the strong inversion limited behavior to the depletion layer behavior is discussed in terms of band offsets, density of states in a-Si:H, and work function of the indium tin oxide (ITO) front electrode. In particular, it is evidenced that strong inversion conditions prevail at the c-Si surface at high temperatures down to 250 K, which can only be reproduced if the ITO work function is larger than 4.7 eV.

International audience; We briefly review the basic concepts of junction capacitance and the peculiarities related to amorphous semiconductors, paying tribute to Cohen and to his pioneering work. We extend the discussion to very high efficiency silicon heterojunction (SiHET) solar cells where both an amorphous semiconductor, namely hydrogenated amorphous silicon, and heterojunctions are present. By presenting both modeling and experimental results, we demonstrate that the conventional theory of junction capacitance based on the depletion approximation in the space charge region, cannot reproduce the capacitance data obtained on SiHET cells. The experimental temperature dependence is significantly stronger than that of the depletion-layer capacitance, while the bias dependence yields underestimated values of the diffusion potential, leading to strong errors if applied to the determination of band offsets using the procedure proposed precedingly in the literature. We demonstrate that this is not related to the amorphous nature of a-Si:H, but to the existence of a strongly inverted c-Si surface layer that requires minority carriers to be taken into account in the analysis of the junction capacitance.

International audience; The temperature dependence of the capacitance of very high efficiency silicon heterojunction solar cells exhibits an anomalously large increase with temperature that cannot be explained under the usual depletion approximation. Based on a full calculation of the capacitance, we show that this large increase of capacitance with temperature of p-type hydrogenated amorphous silicon (a-Si:H)/n-type crystalline silicon (c-Si) heterojunctions occurs when a strong inversion layer at the c-Si surface appears. It is further shown that due to the promotion of inversion as the temperature increases, the temperature at which strong inversion appears depends on the valence band offset and position of the Fermi level in a-Si:H. Therefore, a simple analysis of the temperature dependence of silicon heterojunction solar cells' capacitance can be used to reveal the presence of a strong inversion, to study details of the band diagram and to get insight into the heterointerface.

International audience

International audience

International audience; Fretting remains a major cause of connector failure and can impair reliability of complex system. Much work is devoted to minimise the effects of cyclic displacements in the contact interface. One of the options is to choose proper materials for the final layer and eventually select an underlayer. Another option is to apply a lubricant compatible with the electrical function of the connecting device. Its effect must be the reduction of friction with the preservation of conduction. In this work we have investigated the properties of nickel electrodeposited on cuprous coupons submitted to an imposed sine displacement fretting test (1 Hz, 25 μm, 2.5 N). The contacts are of the sphere on plane type. The plating conditions are shown to have a large effect on the microstructure, roughness and composition of the nickel coatings. Consequently the fretting behaviours of the compared coating are different. Fretting degradation is evaluated for our application as the number of cycles for which the build-up of oxidised debris gives a value of electrical resistance superior to a threshold. During the fretting test the rough nickel with large grains is observed to be prone to very high friction and partial slip. Fretting of the smoother coating with small grains is characterised by gross slip. Contacts were lubricated with solutions of perfluorinated polyether (PFPE). It is shown that a strong friction reduction takes place and that the fretting degradation is postponed. Surface analysis shows that fluorine is grafted in the interface and protects the interface. The electrodeposition process and the properties of the nickel coating are shown to have an effect on the lubricant behaviour in the contact interface. These results show the strong potential of lubricated nickel for applications in the field of electrical devices.

National audience

Hydrogenated amorphous silicon/crystalline silicon (a-Si:H/c-Si) heterojunctions are promising candidates for high efficiency solar cells. The performance of such cells is strongly dependent on the interfacial properties namely defect states and band discontinuities. Capacitance spectroscopy of (p) a-Si:H/ (n) c-Si heterojunctions has been used to study fundamental properties of such structures. Numerical calculations of a-Si:H/c-Si heterojunction capacitance (C) vs temperature (T) and frequency (f) show a capacitance step. We have studied the dependence of the step activation energy (Ea) on various heterojunction parameters such as the position of the Fermi level in a-Si:H, capture cross sections of gap states in a-Si:H, values of band offsets, carrier mobility. It is shown that Ea generally corresponds to the energy difference between the Fermi level and the bottom of the valence band in the bulk a-Si:H and is related to the transport in a-Si:H and to the response of gap states at the Fermi level through capture and emission of holes. However, if very large values of hole mobility in a-Si:H are introduced in the modeling, larger values of Ea, close to that of the valence band offset, are obtained, revealing a contribution from the modulation of free holes in the strong inversion layer at the c-Si surface. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Solar cells made of heterojunctions between hydrogenated amorphous silicon (a-Si:H) thin films and crystalline silicon (c-Si) require good passivation of both front and back surface defects of the crystalline silicon wafers and low recombination at the interfaces. A good indicator of the interface quality is given by the effective lifetime that can be deduced from a modulated photoluminescence (MPL) technique by recording the frequency dependence of the photoluminescence phase shift with respect to the time-modulated light excitation. We apply the MPL technique to assess the passivation quality of different kinds of amorphous layers, for both p- and n-type silicon wafers as well as their evolution upon annealing. The MPL effective lifetimes are also compared to those deduced from the photoconductance technique.