Lionel Boyer

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.

Low wear metallic fibre brushes

Metal fibre brushes were tested against stationary surfaces over a wide range of relative speed (up to 100 m s−1) for various mean (spatial) current densities J in dry argon or nitrogen. The experimental results presented are valid for several thousands of hours of test duration under “natural cooling” conditions. The various test facilities are described and results on voltage drop, friction coefficient, electrical and mechanical power losses and wear rate are given. For a pair of anodic and cathodic brushes under the working conditions described above, a total loss of 74 W cm−2 was obtained for J = 50 Acm−2 at a relative speed of 100 m s−1 and for a packing density of 11.8%. The wear rate was 10−11. Some experimental results obtained with a recently built apparatus which are described here indicate that the brushes are superior to the best commercially available brushes for a wide range of applications. Ongoing studies at the Laboratoire de Genie Electrique de Paris are briefly reported.

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. >

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.

Electrochemically activated wear of metal fibre brushes

Abstract The electrical and tribological properties of metal fibre sliding contacts are studied. The Cu-Cd fibres have a diameter of 50 μm. They slide on Cu-Cr tracks with optional surface treatments (gold electroplated on a nickel underplate). The working atmosphere is generally wet pure nitrogen. Several types of experimental results are reported in the first section. A microcomputer-controlled test bench is described first. It allows long duration tests on devices fitted with a large number of brushes. Results obtained with this set-up show that the temperature and the polarity of the electric field at the brush-track interface have an influence on the mechanisms and speed of wear. The electric field part is confirmed by complementary experiments where the current polarity is reversed. The electrochemical nature of the phenomena occurring at the sliding interface is then shown. In the second section the authors present a model adapted from Cabrera and Motts copper oxidation theory. The part played by temperature and by the electrochemical electric field are described. After defining values for these two parameters, results obtained with the model are presented and are in very good agreement with the experimental results.

Electrical and physical modeling of contact defects due to fretting

When the two members of an electrical contact are driven in low-amplitude alternating motion relative to each other, the resulting tribological phenomena may cause an interface to become poorly conductive. If the contact carries low-level signals such as a pulse train, and reaches this stage of degradation, it may deform the signals enough to make them unusable. The present article first describes an experiment in which a signal transmitted through a vibrating contact is compared with an undistorted reference. It is then shown, using theory supported by experiment, that a faulty contact can be modeled by an equivalent resistor and capacitor in parallel. We then attempt to develop a physical model of a contact interface as a third body of variable thickness and find analytical expressions for the contact resistance and capacitance, providing a basis for analyzing the experimental results. This analysis shows that, for the faults observed, the contact interface probably consists of an insulating layer with or without a metallic bridge of very small lateral dimensions passing through it. >

Copper sample analyzed with an n-doped silicon tip using conducting probe atomic force microscopy

A study of the electrical properties of the nanocontact between a conducting tip made of n-doped silicon, and the surface of a cleaned copper sample has been carried out. Current–voltage (I–V) curves have been performed in order to clarify the electrical nature of the nanocontact. Apart from the breakdown phenomenon and the noticeable laser diode effect on the I–V curves, the nanocontact can be modeled by a Schottky diode, in series with a resistance of a few kΩ, which probably reflects the resistance of the semiconducting probe. This fact is partially confirmed by a simple calculation of the resistance of the tip/cantilever system using a rough model. Topographical and electrical images have also been recorded for a given tip/sample bias. For a positive sample–tip voltage, the copper surface appears uniformly conducting over 20×20 μm2, and remains stable during the whole period of study (three weeks). This study thus allows one to have a better understanding of the effects which must be kept in mind when...

Mechanical and electromechanical behaviour of fibre brushes

Abstract Investigations of the mechanics of metal fibres capable of carrying very high currents are presented. The role played by the Laplace force is emphasized. This force appears whenever a fibre carrying a current is subjected to a magnetic field whose direction is different from that of the fibre. The role played by the coefficient of friction is also taken into account. An exact non-linear differential equation is used to describe the problem, i.e. no assumptions are made about the magnitudes of the deflections. When the normal force exerted on the track by the fibre is an increasing function of the Laplace force, it is found that the maximum stress within the fibre has a non-linear dependence on the Laplace force. When the same normal force is a decreasing function of the Laplace force, it is shown that the normal force vanishes for a critical value of the Laplace force. A perturbation method is used to explain the behaviour of the fibre.

Behaviour of fibre brushes under transient atmospheric conditions

Abstract Voltage drop and friction measurements were made on fibre brushes working under transient atmospheric conditions. These transient conditions correspond to the introduction of ball-bearing greases or of water into the chamber containing the experimental set-up under a dry nitrogen atmosphere. It is shown that vapours from a particular grease have a contaminating effect on the sliding contacts. When the same grease is heated up to 100 °C and when the disc on which the brushes are rubbing is gold plated, very low frequency oscillations in the cathodic voltage drop are observed. This phenomenon is interpreted by assuming that the surface film resistance and the surface temperature are dependent on each other. In addition, a short and simplified linear stability study of sliding contacts is given. It is shown that the behaviour of brushes in an atmosphere having a variable relative humidity strongly depends on the surface material ( i.e. gold or copper) of the disc. The major conclusion emerging from the results presented is that, under certain circumstances, water vapour may play the role of an adsorption inhibitor for organic molecules whereas, in other cases, it may act as a lubricant.

Apparent tunnel barrier heights of PtIr–Au interfaces in relation to the Au surface composition

In the literature, the phenomenon of barrier height lowering in tunneling experiments is mainly assigned to the presence of contaminants at the interface. In this article we present the results of barrier height measurements made between a PtIr tip and various types of gold surfaces. Vacuum deposited and galvanic gold samples are investigated, as received, after ultraviolet (UV)/ozone treatment and covered with a fluid used to lubricate electrical contacts. It is found that the value of the barrier height depends both on the gold deposition process and on the treatment. Results of surface analyses performed on the nonlubricated samples are also reported. They show that the galvanic gold is more contaminated by carbon than the vapor deposited gold. The effects of the ultraviolet (UV)/ozone treatment are then described. It is observed that for nonlubricated surfaces, the higher the oxygen content, the higher the mean barrier height, and that, for each kind of gold, the lubricated surface exhibits the larges...