Abstract:
Compressed copper powder has a very
large electrical resistance (1MOhm), 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.
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