Abstract:
We report on observations of the
electrical transport within a chain of metallic beads (slightly
oxidised) 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°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.
PDF file : here (560 KB)
(Version Preprint on cond-mat/0311453
or ccsd-00000853
)