Electrical-Thermal Switching in Carbon-Black–Polymer Composites as a Local Effect

Following the lack of microscopic information about the intriguing well-known electrical-thermal switching mechanism in carbon-black–polymer composites, we applied atomic force microscopy in order to reveal the local nature of the process and correlated it with the characteristics of the widely used commercial switches. We conclude that the switching events take place in critical interparticle tunneling junctions that carry most of the current. The macroscopic switched state is then a result of a dynamic-stationary state of fast switching and slow reconnection of the corresponding junctions.

Figure 1

Typical current-voltage characteristics of an ET switching device made of a composite of 50 vol % low structure CB and a polymer. The current is read 1 (open circles), 30, or 300 sec (full circles) after bias onset.

Figure 2

Two $5\times 5{\mathrm{m}\mathrm{m}}^2$ current images taken on the same area of a 46 vol % low structure CBP sample, for two different voltages, as indicated. Note the disappearance of some of the conducting islands (encircled for 5 V), and the general current reduction, upon bias increase (the current scale is shown to the right).

Figure 3

$1\times 1{\mathrm{m}\mathrm{m}}^2$ current image (a) and friction image (b) taken simultaneously on a sample of (high structure) 23 vol % CBP composite.

Figure 4

Two typical kinds of $I\mathrm{\text{−}}V$ characteristics that exhibit a local ET switching. In the first, (a), there is a single switching event and in the other, (b), the first switching is followed by another one. These characteristics were obtained on an 18 vol % high structure CBP composite.

Figure 5

The dependence of the first threshold voltage on the CB content (for high structure CB-polymer composites). The inset shows the dependence of the total area of conducting islands on bias voltage, for a 15 vol % CB sample, and the arrow marks the first threshold voltage.