B. Castaing

Nonlinear electrical conductivity in a 1D granular medium

Abstract.We report on observations of the electrical transport within a chain of metallic beads (slightly oxidized) under an applied stress. A transition from an insulating to a conductive state is observed as the applied current is increased. The voltage-current (U-I) characteristics are nonlinear and hysteretic, and saturate to a low voltage per contact (0.4 V). Our 1D experiment allows us to understand phenomena (such as the “Branly effect”) related to this conduction transition by focusing on the nature of the contacts instead of the structure of the granular network. We show that this transition comes from an electro-thermal coupling in the vicinity of the microcontacts between each bead - the current flowing through these contact points generates their local heating which leads to an increase of their contact areas, and thus enhances their conduction. This current-induced temperature rise (up to 1050

Electrical conductivity in granular media and Branly's coherer: A simple experiment

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“Turbulent” electrical transport in copper powders

C) results in the microsoldering of the contact points (even for voltages as low as 0.4 V). Based on this self-regulated temperature mechanism, an analytical expression for the nonlinear U-I back trajectory is derived, and is found to be in very good agreement with the experiments. In addition, we can determine the microcontact temperature with no adjustable parameters. Finally, the stress dependence of the resistance is found to be strongly non-hertzian due to the presence of the surface films. This dependence cannot be usually distinguished from the one due to the disorder of the granular contact network in 2D or 3D experiments.

Ultimate regime in Rayleigh–Bénard convection: The role of plates

We show how a simple laboratory experiment can illustrate certain electrical transport properties of metallic granular media. At a low critical external voltage, a transition from an insulating to a conductive state is observed. This transition comes from an electro-thermal coupling in the vicinity of the microcontacts between grains where microwelding occurs. Our apparatus allows us to obtain an implicit determination of the microcontact temperature, which is analogous to the use of a resistive thermometer. The experiment also helps us explain an old problem, Branly’s coherer effect, which was used as a radio wave detector for the first wireless radio transmission, and is based on the sensitivity of the conductivity of metal filings to an electromagnetic wave.

Some aspects of electrical conduction in granular systems of various dimensions

Compressed copper powder has a very large electrical resistance (1 MΩ), due to the oxide layer on grains (100 μm). We observe that its voltage-current U-I characteristics are nonlinear, and undergo an instability, from an insulating to a conductive state at relatively small applied voltages. Current through the powder is then noisy, and the noise has interesting self-similar properties, including intermittency and scale invariance. We show that heat dissipation plays an essential role in the physics of the system. One piece of evidence is that the instability threshold always corresponds to the same Joule dissipated power whatever the applied stress. In addition, we observe long-time correlations which suggest that thermal expansion locally creates or destroys contacts, and is the driving mechanism behind the instability and noise observed in this granular system.

Effects of electromagnetic waves on the electrical properties of contacts between grains

The ultimate regime of convection, long ago predicted by Kraichnan [Phys. Fluids 5, 1374 (1962)], could be called elusive because some apparently equivalent experiments showed it while others did not, with no apparent reasons for this discrepancy. In this paper, we propose a model which accounts for the finite heat conductivity and heat capacity of real active boundaries. Bad thermal characteristics of the plates can explain differences between various experiments, in agreement with recent numerical simulations.

Pressure dependence of the electrical transport in granular materials

Abstract.We report on measurements of the electrical conductivity in both a 2D triangular lattice of metallic beads and in a chain of beads. The voltage/current characteristics are qualitatively similar in both experiments. At low applied current, the voltage is found to increase logarithmically in good agreement with a model of widely distributed resistances in series. At high enough current, the voltage saturates due to the local welding of microcontacts between beads. The frequency dependence of the saturation voltage gives an estimate of the size of these welded microcontacts. The DC value of the saturation voltage ( ≃ 0.4 V per contact) gives an indirect measure of the number of welded contact carrying the current within the 2D lattice. Also, a new measurement technique provides a map of the current paths within the 2D lattice of beads. For an isotropic compression of the 2D granular medium, the current paths are localized in few discrete linear paths. This quasi-onedimensional nature of the electrical conductivity thus explains the similarity between the characteristics in the 1D and 2D systems.

Experiment and Theory of the Electrical Conductivity of a Compressed Granular Metal

A DC electrical current is injected through a chain of metallic beads. The electrical resistance of each bead-bead contacts is measured. At low current, the distribution of these resistances is large and log-normal. At high enough current, the resistance distribution becomes sharp and Gaussian due to the creation of microweldings between some beads. The action of nearby electromagnetic waves (sparks) on the electrical conductivity of the chain is also studied. The spark effect is to lower the resistance values of the more resistive contacts, the best conductive ones remaining unaffected by the spark production. The spark is able to induce through the chain a current enough to create microweldings between some beads. This explains why the electrical resistance of a granular medium is so sensitive to the electromagnetic waves produced in its vicinity.

Flow reversals in a vertical channel

Abstract.We report on systematic measurements of the electrical resistance of one- and three-dimensional (1D and 3D) metallic and oxidized granular materials under uni-axial compression. Whatever the dimension of the packing, the resistance follows a power law versus the pressure (

Transmission of sound at the liquid-solid interface of helium : a new probe of melting kinetics

R\propto P^{-\alpha}