Sophie Noël

Covalent grafting onto self-adhesive surfaces based on aryldiazonium salt seed layers

The chemistry of aryldiazonium salts has been thoroughly used in recent years to graft in a very simple and robust way ultrathin polyphenylene-like films on a broad range of surfaces. We show here that the same chemistry can be used to obtain “self-adhesive surfaces”. This target was reached in a simple way by coating various surfaces with chemisorbed organic films containing active aryldiazonium salts. These “self-adhesive surfaces” are then put into contact with various species (molecules, polymers, nanoparticles, nanotubes, graphene flakes, etc.) that react either spontaneously or under activation with the immobilized aryldiazonium salts. Our self-adhesive surfaces were synthesized following a simple aqueous two-step protocol based on p-phenylenediamine diazotisation. The first diazotisation step results in the robust grafting of thin polyaminophenylene (PAP) layers onto the surface. The second diazotisation step changed the grafted PAP film into a “poly-aryldiazonium polymer” (PDP) film. The covalent grafting between those self-adhesive surfaces and the target species was achieved by direct contact or by immersion of the self-adhesive surfaces in solution. We present in this preliminary work the grafting of multi-wall carbon nanotubes (MWCNTs), flakes of highly oriented pyrolytic graphite (HOPG), various organic compounds and copper nanoparticles. We also tested these immobilized aryldiazonium salts as electropolymerization initiators for the grafting-to process.

Self-assembled monolayers of alcanethiols on nickel surfaces for low level electrical contact applications

Nickel can be used as final coating for separable electrical contacts in various types of applications: batteries, automotive connectors, etc. The possible growth of a poorly conducting layer on the metal due to environmental conditions has limited up to now the use of such coatings to common applications. However durable quality performances can be expected with a proper protective material avoiding corrosion. The new approach we have been working on, is to find compounds that bind to nickel and form well-defined layers. We report here the first results describing the behaviour of self-assembled monolayers of thiol molecules deposited on bare nickel substrates. We briefly describe the deposition method, the type of molecules and of substrates used and summarize the main physicochemical characterisations available. The electrical and tribological properties of the monolayers formed on nickel slabs are investigated in a ball/plane configuration simulating a real contact element. The influence of an electrochemical pretreatment of the nickel surface prior to the layer assembly is shown. Excellent tribological behaviours can already be obtained with corresponding values of the contact resistance varying between 1 /spl Omega/ and 10 m/spl Omega/. The results show that building organised monolayers acting as protective coatings is of high interest for electrical contacts.

Surface modifications of nickel substrates with self-assembled monolayers of alkanethiols for electrical contact applications

Abstract Electrical and tribological properties of alkanethiol monolayers chemisorbed on nickel slabs, bare or electrochemically reduced, are investigated in a ball/plane configuration simulating a real contact. In the case of the electrochemically reduced nickel surfaces, friction coefficients of the order of 0.2 and contact resistances varying between 1 Ω and 10 mΩ are obtained.

Electrical properties of very thin heat-treated polyacrylonitrile layers electropolymerized on nickel for contact application

Abstract Recent investigations concerning weakly pyrolysed thin films of polyacrylonitrile (PAN) grafted and grown by electropolymerization on a nickel underplate are reported, which clearly attest the potential interest of the obtained material for low-level connector coating application. The electrical characterization of these organic layers in a contact situation requires various precautions, principally the use of surfaces as smooth as possible in order to avoid piercing the film and therefore metal/metal junctions. An original device for static contact resistance measurements is presented, simulating the real case of industrial connector parts by a sphere/plane contact with an adjustable normal load. The tested samples consist of Ni-electroplated plane brass strips covered by PAN layers about 550 A thick and heated at different temperatures in the range 350–500 °C for 10 or 30 s; the pressing sphere is a 3.2 mm diameter smooth golden ball. IRRAS and XPS spectra of raw and thermally treated samples reveal a series of drastic changes in the molecular structure of the polymer and the correlative evolution of the electrical properties of the films is unambiguously observed. The most promising results concern the 400 °C/30 s heated coating, for which stable and reproducible contact resistance values lower than 10 mΩ are found under a 50 gf normal load.

Constriction resistance of a multispot contact: an improved analytical expression

When dealing with a multispot metallic contact two different problems are raised: whether the calculation of the constriction resistance R/sub c/ is required from geometrical considerations and whether one calculates the real area of contact from electrical measurements. Attention is focused on the second case, and it is shown that when the total area built up by the elementary spots represents a large fraction of the apparent area of contact, then the usual analytical expressions of the resistance can be imprecise. Considering the case of n elementary spots of radius r regularly spread in a disc of radius R representing the interface of contact of two metals of equal resistivity rho , they give an improved expression of the resistance. When the total area of the spots is equal to the disc area, this expression gives for any value of n the usual R/sub c/= rho /2R. Conversely, when n=1, it gives, for any value of R, the correct expression: R/sub c/= rho /2r. It is shown that this improved expression is quite useful when the number of spots is small and the real area of contact close to the apparent one, i.e. in many practical cases. >

A two-dimensional modeling of the fine-grained polycrystalline silicon thin-film solar cells

A two-dimensional device modeling for polycrystalline silicon thin-film solar cells was performed. A ThRee-dimensional Emitter Based on Locally Enhanced diffusion (TREBLE) concept was applied to evaluate the degree of leveling of the device efficiency. The model assumes a n + -p-p + junction device of a 3-μm grain size and 10-μm-thick polycrystalline Si columnar structure unit, where the n + regions extend along the grain boundaries. The analysis was carried out using ISE-DESSIS, a two-dimensional semiconductor device simulator. It has been found that open circuit voltage could be improved by increasing the base doping level to the optimum value of 10 17 cm 3 . Preferential doping has a beneficial effect on short-circuit current of the cell and a slight influence on open circuit voltage. Conversion efficiency of ∼10% could be expected at the surface, recombination velocity of 10 4 cm/s and dopant diffusion depth along the grain boundary x gb =2.5 μm At base doping ∼ 10 17 cm -3 and well-passivated grain boundaries (S gb =10 3 cm/s), an efficiency of approximately 12% may be obtained when x gb = 3.7 μm.

Determination of the effective contact radius between a conducting sphere and a thin metallic film

A method for studying the contact surface between a sphere and a plane is presented. The contact consists of a conducting sphere pressed against a metallised insulating strip. A model presented in the appendix allows one to calculate either the effective radius of contact or the electric resistance of an eventual interface layer. A set of experiments involving loading-unloading cycles (from 6 gf to 350 gf) of the sphere is described. Experimental values of the electric resistance are in good agreement with those obtained for the ideal case of a Hertz contact. The discrepancy with the Hertz law is found to be due to topographical defects.

Electrical and tribological properties of hot-dipped tin separable contacts with fluorinated lubricant layers

Degradation phenomena occurring during the lifetime of tinned separable electrical contacts remain a factor of loss of reliability to be minimized. In this paper, friction, wear and electrical properties of hot-dipped tin coatings on bronze substrates are analysed with various techniques, in order to show the improvement due to a well-suited fluorinated lubricant layer. Contacts are of the sphere on plane type. Coupons are cut from strips of tinned CuSn/sub 4/. The sphere contacts are obtained by forming the coupons. Macroscopic friction cycles simulating insertion and withdrawal of separable contacts are performed, as well as fretting cycles. Wear is characterized by topographic examination of the tracks and by surface analyses. We first describe macroscopic friction of dry contacts. Then the anti-friction and anti-wear behaviour of two perfluorinated ethers is evaluated for both conditions. Large differences are observed with the two chosen lubricants. Fretting tests are then performed on connector terminals in order to validate the results in closer-to-use conditions. Outstanding friction and wear reduction are recorded which can be linked to the particular behaviour of the fluorinated layers on tin.

Atomic force microscopy study of the topographic evolution of polyacrylonitrile thin films submitted to a rapid thermal treatment

Abstract Atomic force microscopy (AFM) has been used to characterize the modification of the large-scale surface morphology of thin films of electropolymerized polyacrylonitrile (PAN) before and after rapid thermal treatment (RTT) at different temperatures. This study was led together with infrared spectroscopy investigations which provide information about the evolution of the chemical structure of the heated polymer samples. The results bring invaluable clues on how surface changes resulting from RTT occur. For the softest heat treatments, the melting of the polymer clearly leads to a significant leveling of its extreme surface; for the strongest ones, the chemical evolution of the layer is probably accompanied by the release of gaseous products which increases the microroughness of the film.

Fretting Behavior of Nickel Coatings for Electrical Contact Applications

Fretting remains a major cause of connector failure and can impair reliability in complex systems. Oxidizable metals such as tin, copper and nickel are particularly prone to fretting degradation. We report here the first results of an investigation on fretting of nickel contacts with two types of deposits. Sulfate nickel layers are electrodeposited in different conditions and show very different behaviours during fretting tests. The characteristics of the layers are analyzed and show different compositions and microstructures. The compositions are measured by X-Ray Photoelectron Spectroscopy (XPS) which allows determining the chemical nature of the compounds formed during exposure to air. Topography is measured by AFM and the roughness and grain characteristics are assessed. Electrical properties at the micro/nanoscale are measured with the CP-AFM technique. Various loads are applied to the cantilever beam; the electrical characterization is performed versus the load. The results of fretting experiments are analyzed in terms of fretting regimes. The fretting regimes occurring during the test of the matte layers involve partial slip which delays the occurrence of contact resistance (Rc) increase. Gross slip in the interface is shown to create very poorly conducting wear debris leading to drastic increase of Rc. This study is part of a larger one aiming at tailoring coatings allowing the best tribological and electrical behaviors during fretting of nickel contacts.