Roland S. Timsit

Electrical contact resistance: properties of stationary interfaces

The paper reviews the dependence of electrical contact resistance on the shape and dimensions of /spl alpha/-spots and on the magnitude of the mechanical contact load. The range of validity of the classical voltage-temperature relation for electrical contacts is also examined. The paper describes experimental evidence of breakdown of the classical electrical theory when /spl alpha/-spots become too small. One of the interesting and useful properties of relatively small /spl alpha/-spots is that they are subject to large surface stresses. These stresses induce /spl alpha/-spot growth.

Constriction Resistance of Thin-Film Contacts

The electrical constriction resistance of a circular a-spot in a bulk interface stems from convergence of electrical current flow lines from a distance far from the constriction, and the subsequent spreading out of the current from the constriction. The spreading resistance of an a-spot located in a thin film cannot be identical to that generated in a bulk component of the same material since current spreading is constrained by the thin film boundaries. This paper addresses the effect of film thickness on spreading resistance, and hence on constriction resistance, in a thin conducting layer. If a and L are, respectively, the constriction radius and the film thickness, the spreading resistance is found to deviate significantly from that in a bulk solid only for relatively thin film and for values of a/L greater than 0.02. This paper also compares the dc spreading resistance in a thin film to the spreading resistance for high-frequency current in a bulk solid where current flow is limited to the electromagnetic skin depth. It turns out that spreading resistance at high-signal frequencies is comparable with the dc spreading resistance in a thin film if the thickness of the equivalent thin film is identified as the skin depth at the relevant frequency.

On the Spreading Resistance of Thin-Film Contacts

The spreading resistance of a microscopic area of contact (the “-spot”) located in a thin film is studied for both Cartesian and cylindrical geometries. The effect of film thickness on the spreading resistance is evaluated over a large range of aspect ratios. In the limit , the normalized thin-film spreading resistance converges to the finite values, i.e., 2.77 for the Cartesian case and 0.28 for the cylindrical case. An interpretation of these limits is given. Extension to a general α-spot geometry is proposed.

Electrical Conduction Through Small Contact Spots

Earlier literature has reported that the voltage-temperature (V-T) relation for electrical contacts, and classical electrical contact theory in general, are invalid where the a-spots are on the order of nanometers. This paper reviews earlier literature dealing with the breakdown of the V-T relation and presents quantitative arguments to propose that this breakdown stems from the ballistic motion of electrons through nanometer-sized constrictions. Earlier literature also suggested that the breakdown of classical theory stems in part from thermal losses from a-spots to contaminant layers in an electrical interface. Through the use of a simple a-spot model, this paper shows that this cooling mechanism is not sufficiently important to affect the validity of the classical V-T relation and classical electrical contact theory in general

The 'melting' voltage in electrical contacts

An investigation of the melting voltage in Fe/Fe, Ni/Ni, and W/W electrical contacts is presented. The Ni/Ni system is investigated to establish potential operating limits of nickel-plated aluminum conductors. The work on Fe/Fe contacts is undertaken in part to confirm the validity of the experimental approach by confirming observations of contact melting in an earlier investigation. The W/W system is investigated to identify any effect of the growth of electrically conductive oxide around the contact on the melting voltage, since tungsten oxide is known to grow relatively rapidly in oxygen at elevated temperatures. In agreement with earlier work, the melting voltage for Fe/Fe couples is found to be smaller than the value predicted by the conventional voltage-temperature relation. Similarly, Ni/Ni contacts melt at a low potential drop. However, melting is not observed in W/W contacts at contact temperatures as elevated as 890 degrees C. This is contrary to the prediction of the theory that accounts successfully for the melting of Fe and Ni contacts. The reasons for this discrepancy with theory are discussed. >

Interdiffusion at Bimetallic Electrical Interfaces

The compatibility of two dissimilar electric-contact materials in a bimetallic junction is determined in part by the metallurgical stability of the interface generated. Materials which interdiffuse to form significant quantities of (brittle) intermetallics at this interface in the early life of an electrical junction, and hence generate a potentially unreliable electric contact, must obviously be avoided. An investigation is described of the rate of growth, composition, and hardness of interdiffusion layers formed by Zn, In, and Sn platings on brass and phosphor-bronze at temperatures ranging from 80°C to 150°C. Data on iutermctallics formation at Al/brass interfaces at temperatures ranging from 150°C to 450°C are reported. Also discussed is the expected effect of intermetallics growth on the performance of electrical junctions in which these combinations of materials are used.

Electrical instabilities in stationary contacts: Al/Al and Al/brass junctions

Electrical instabilities in Al/Al and Al/brass stationary point junctions has been examined in ultrahigh vacuum and in oxygen gas. It is found that Al/brass junctions are electrically stable in vacuum and in oxygen gas if the surfaces are sufficiently clean. The growth of intermetallic compounds in this interface affects stability only after a sufficiently long exposure of the contact, typically several hours, to a temperature typically higher than 150 degrees C. Surface contaminants, probably in the form of carbonaceous material, are deleterious to the performance of Al/Al and Al/brass junctions. However, the junctions often recover from contaminant-induced degeneration. This recovery is attributed in part to thermal dispersal of the contaminants from the electrical interface during the early stages of degeneration.<<ETX>>

Mechanical and electrical contact properties of wedge-connectors

The paper describes the mechanical and electrical contact properties of a typical fired wedge-connector. The measured forces and displacements in the connector components are compared with the predictions of a Finite Element Analysis (FEA) of a wedge-connector interfaced with solid aluminium conductors. The distribution of mechanical stresses resulting from typical applied mechanical loads are evaluated using a nonlinear FEA code. The nonlinear input data for the FEA calculations were obtained from mechanical testing of metal specimens obtained from connectors used in the investigation. Measurements of contact resistance of the electrical interfaces are also reported. The wipe-off of surface oxide films from the electrical interfaces during connector installation is described.

High-resolution electron microscopy of quasi-amorphous materials

Abstract Conventional methods of interpretation of HREM images rely on structure modeling followed by image simulation. These methods are inapplicable to the interpretation of micrographs of amorphous materials because the atomic structure is unknown a-priori. Previous experimental work by the present authors had indicated that the cross-correlation of intensity fields of different regions of an HREM micrograph can identify regions of structural order in an apparently random matrix. The present paper confirms this initial result by reporting results of cross-correlations on computer-simulated HREM images. The images were generated from a “multislice” calculation of electron-wavefront propagation through a composite material consisting of a Si crystallite affixed to a relatively thick amorphous Si substrate. The cross-correlation technique is shown to be capable of retrieving the c-Si region where the simulated HREM images reveal to evidence of the crystallite. There is an optimum microscope defocus for which retrieval of the ordered structure is most efficacious. The cross-correlation technique is found to be superior to conventional Fourier filtering for identifying regions of atomic order.

Environmental degradation of utility power connectors in a harsh environment

This paper reports on an investigation of the effects of a marine atmospheric environment on the performance of aluminum bolted, compression, and fired wedge type power connectors. These connector types are representative of the three major connector technologies used in electrical distribution systems throughout the world. The atmospheric test site was situated on the seacoast in Daytona Beach, Florida. The test specimens consisted of sections of aluminum and copper conductors joined together by a connector. The paper describes the performance and lifetime of the different connector types exposed to the harsh marine environment. Examinations of failed connectors revealed that failure in all three connector types was ultimately due to galvanic corrosion at the aluminum/copper interfaces within the connectors. The failure rate was largest for compression connections. Bolted connections failed somewhat less rapidly. The failure rate for fired-wedge connections was the lowest.