We review theoretical and experimental studies of the AC dielectric response of inhomogeneous materials, modelled as bond percolation networks, with a binary (conductor-dielectric) distribution of bond conductances. We first summarize the key results of percolation theory, concerning mostly geometrical and static (DC) transport properties, with emphasis on the scaling properties of the critical region around the percolation threshold. The frequency-dependent (AC) response of a general binary model is then studied by means of various approaches, including the effective-medium approximation, a scaling theory of the critical region, numerical computations using the transfer-matrix algorithm, and several exactly solvable deterministic fractal models. Transient regimes, related to singularities in the complex-frequency plane, are also investigated. Theoretical predictions are made more explicit in two specific cases, namely R-C and RL-C networks, and compared with a broad variety of experimental resul...

The electrical conductivity of binary disordered systems, percolation clusters, fractals and related models

Surface forces, deformation and adherence at metal microcontacts

The paper reviews the dependence of electrical constriction 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 contact 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.

Electrical contact resistance: properties of stationary interfaces

Contact of characterised metal surfaces at very low loads: Deformation and adhesion

Nanoindentation of single crystals has been a topic of several recent investigations. This is a result of their ability to withstand near theoretical stresses without showing signs of plastic deformation. When plasticity occurs, it produces a yield point, a sudden discontinuous increase in indenter displacement and decrease in contact pressure. This study combines atomic force microscopy (AFM) with nanoindentation to focus on the roles that oxide and asperities play in the yield point process. Time dependent and instantaneous yield point properties were investigated for single crystals of tungsten and Fe–3 wt% Si in variable temperature and variable humidity environments. AFM observations indicate that the presence of asperities has a dramatic effect on the time dependent yield point properties. AFM measurements also provide evidence for plasticity in the absence of a yield point, suggesting that dislocation nucleation can occur well before a yield point is observed. Measurements on the dependenc...

Surface constrained plasticity: Oxide rupture and the yield point process

We describe equipment and report results of a study of the mechanics of closure for gold electric contacts cleaned by sputtering and closed with precise control of mechanical and environmental variables. The ultrahigh‐vacuum system, in which measurements are performed, is isolated from building vibrations. Our measurements include contact resistance, load, cold welding, creep, anelastic strain recovery, stress strain, yield stress, and strain hardening. We calculated an activation energy of 1200 ± 100 cal/mole for low‐temperature creep in gold.

R. E. Cuthrell

Papers

Electric contacts. II. Mechanics of closure for gold contacts