FIELD

The present disclosure relates to metal oxide varistors formulations, which may be produced at reduced cost and provide improved thermal stability compared to conventional formulations.

BACKGROUND

Surge protection in both high and low-voltage circuits is typically afforded by surge arresters that utilize one or more metal oxide varistors (MOVs). A MOV acts as a voltage-dependent resistor that reversibly switches between low and high resistivity states over a narrow voltage range. Near the switching voltage, the voltage-current relationship is often characterized using a power law relationship I=CVαwhere α is a nonlinearity coefficient and C is constant. The peak value of α may exceed 40-50 in some materials. MOVs are typically rated by the manufacturer for various properties that may include the maximum continuous operating voltage (MCOV), temporary overvoltage (TOV) capability and residual voltage during a specified surge event (e.g., 8×20 μs surge at 10,000 A). Commercially available MOVs for high voltage application are rated for particular levels of duty per guidelines given in the US IEEE C62.11 standard or the international IEC 60099-4 standard.

The nonlinear V-I behavior of the MOV originates from the additive elements. The MOV microstructure consists of n-type semiconductive ZnO grains and resistive grain boundaries. The grain boundaries are compositionally distinct, containing Bi2O3as well as other modifying elements such as Mn, Co, and Sb. Interface states located at the grain boundary behave as electron traps, and capture conduction band electrons within the adjacent n-type ZnO. This process results in a double Schottky barrier at the grain boundary that impedes the flow of electrons. Current leakage through the Schottky barrier is thermally activated, and the power dissipation of a MOV in the range of 1.0-1.25*MCOV typically has a strong temperature dependence. If the MOV is overheated by repeated exposure to surge duty, the power dissipation at MCOV may exceed a level that can be dissipated by the arrester housing, causing the arrester to fail by thermal runaway.

Thus, there remain a need for alternative MOV formulations improve thermal stability and nonlinear behavior.

SUMMARY

In one aspect, the present disclosure provides a metal oxide varistor comprising:a sintered ceramic comprising, by weight:about 91.0% to about 97.0% ZnO;at least 0.3% Mn;at least 0.4% Bi;at least 1.0% Sb; and0.50% or less Co.

In another aspect, the present disclosure provides a surge arrester for protecting one or more conductors against a power surge, the surge arrestor comprising a metal oxide varistor as described herein, wherein surge current is diverted from the conductor(s) to ground through the metal oxide varistor.

The metal oxide varistors as described herein exhibit reduced power dissipation, improved thermal stability, and may be produced at a lower cost relative to conventional MOVs.

Other aspects of the application will become apparent by consideration of the detailed description and accompanying drawings.

