The Voltage and Composition Dependence of Switching in a Polymer Current Limiter Device
- PDF / 403,317 Bytes
- 6 Pages / 420.48 x 639 pts Page_size
- 65 Downloads / 199 Views
INTRODUCTION Recently it was demonstrated that conductor-filled polymer composites can be fashioned into "polymer current limiter" devices that rapidly and reversibly switch from a low to high resistance state when high current densities flow through them.[1-3] A typical device consists simply of two metal electrodes pressure contacted to the polymer composite material. Such a device holds great promise for the power distribution industry because it is fast-acting, reusable, and relatively inexpensive. Figure IA shows typical current and voltage waveforms when a polymer current limiter device is placed in a high power circuit. The device utilizes a nickel-filled epoxy composite material and the circuit consists of a high power amplifier system attempting to apply a 300V voltage pulse. With the application of the pulse, the current initially increases, but then, within 200 microseconds, decreases to a low level as the device switches to a high resistance state. The switching event is accompanied by a loud noise, visible light generation, and ablation of material at the material/electrode interfaces. In spite of this, when the voltage pulse is terminated, the device regains its low initial resistance and, under low power conditions, exhibits ohmic behavior. Previous studies have brought understanding to some aspects of the high power switching phenomenon. It has been determined that the phenomenon is not a bulk effect and is not based on the positive temperature coefficient of resistance (PTCR) effect which is exhibited by many conductor-filled polymer composite materials.[3] With pressure-contacted electrodes, the switching is localized at the electrode/material interfaces and it has been shown that the time required to initiate switching can be explained quantitatively with an adiabatic contact heating model.[4] The current qualitative model for device operation is based on polymer ablation and dielectric breakdown or arcing. Specifically, it is postulated that with a high power input, polymer ablation occurs at the material/electrode interfaces due to the excess constriction resistance heating that occurs in these regions. The ablation causes separation of filler-electrode and filler-filler contacts which tends to increase the device resistance. At the same time, transient arcs form between separated contacts and this tends to mitigate the increase in resistance caused by the contact separations. The overall high device resistance state is then postulated as a dynamic equilibrium that includes contacts continually separating and arcs continually forming 287 Mat. Res. Soc. Symp. Proc. Vol. 600 0 2000 Materials Research Society
and extinguishing between these separating contacts. With the cessation of the high power input, the ablation-induced separation of contacts ceases and, since the electrodes are under pressure, the contacts reform and the device regains its low resistance. In this work, switching experiments as a function of voltage and material parameters aimed at better characterizing the switched state o
Data Loading...
