THE INFLUENCE OF GRAIN BOUNDARIES IN METAL OXIDE VARISTORS ON THEIR AC ELECTRICAL BEHAVIOR (ZINC OXIDE, MOVS)
The zinc oxide varistor is a two terminal polycrystalline device with an excellent symmetric nonlinear current-voltage characteristic with respect to the polarity of the applied voltage. This device has high breakdown field (>1000 V/cm) and power handling capability, thus, it is now widely used for the protection of electronic circuit elements and electric power systems (in the form of arresters) against transient overvoltage due to switching surges, lightning, and circuit disturbances in AC applications. The presence of a reverse-biased Schottky barrier within the device is documented via linear dependence of 1/C ('2) upon DC voltage (capacitance-voltage characteristic); i.e., Mott-Schottky behavior. The varistors electrical response is characteristic of a back-to-back depletion layer configuration. Single frequency C-V studies have clearly established that depletion layer capacitance plays a prominent role in MOV AC response. The observed dispersion in the AC small-signal Admittance data leads to a complicated Mott-Schottky response wherein both its magnitude and linear slope vary with measurement frequency. Analysis of this frequency-dependent Mott-Schottky behavior in ZnO-based varistor system using lumped parameter/complex plane analysis leads to an equivalent circuit model whose components are representative of phenomena operative in the device. The transformation of this equivalent circuit under high values of DC stress and at elevated temperatures were observed, and are interpreted in terms of a zero bias-room temperature model. This study leads to a comprehensive characterization of MOV grain boundaries in terms of traps in the depletion layers of the device. This information should prove to be quite valuable in the development of this device. Two-probe small-signal AC immittance data for the ZnO-based varistor system have been analyzed via lumped parameter/complex plane analysis in the frequency range 10('-3) Hz < f < 10('9) Hz. This technique revealed several contributions to the measured polarization comprising MOV grain boundary response. The resolved polarization processes yield a better understanding of the frequency-dependent Mott-Schottky behavior observed in this MOV system. The present investigation has concentrated in the frequency range 5 Hz < f < (TURN)10('7) Hz. (Abstract shortened with permission of author.)
Mohammad Anwarul Alim,
"THE INFLUENCE OF GRAIN BOUNDARIES IN METAL OXIDE VARISTORS ON THEIR AC ELECTRICAL BEHAVIOR (ZINC OXIDE, MOVS)"
(January 1, 1986).
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