William George Carlson

Improved varistor nonlinearity via donor impurity doping

Zinc oxide (ZnO) varistors exhibit a voltage upturn at higher current densities which reduces their effectiveness as over‐voltage protection devices. This effect has been attributed to a limiting series impedance offered by the ZnO grains. Based on literature data, the varistors were made for this investigation wherein the grain impedance has been reduced by doping with Al2O3 or Ga2O3. The high current surge and high frequency impedance data combined with microstructural investigation indicated an improved varistor nonlinearity via donor doping. A simple defect model developed for a pure ZnO crystal has been used to explain the doping behavior of the varistor grain.

Current instability phenomena in ZnO varistors under a continuous ac stress

A zinc oxide (ZnO) varistor subjected to a 60‐Hz voltage below the characteristic turn‐on voltage exhibits an increase in the resistive component of current with time. The rate of current increase with time can be increased with an increase in applied voltage and/or temperature. An analysis of current rise with an applied 60‐Hz voltage stress and current decay with no voltage stress data obtained for ZnO varistors indicates that the current instability phenomena is associated with the diffusion of interstitial zinc in the depletion layer regions adjacent to the ZnO‐ZnO grain boundary.

Barrier voltage and its effect on stability of ZnO varistor

The major voltage drop in ZnO varistors occurs across the grain boundaries, which behave generally as Schottky barriers supporting a barrier voltage Vg. This voltage is proportional to the device voltage. Experimental evidence shows that there is a time‐dependent reduction in the barrier voltage combined with an increase in resistive current iR when the varistor is subjected to a continuous ac voltage stress. The phenomena is reversed when the applied voltage is removed, showing nearly complete recovery. Instabilities of the resistive current and of the barrier voltage are shown to be manifestations of the same phenomenon and are attributed to a metastable component in the Schottky barriers. It is proposed that this metastable component is due to interstitial zinc ions that are capable of migration under thermal and electrical driving forces. When these ions are removed or stabilized by a suitable heat treatment, the instability of the device is reduced. This paper presents experimental data and analysis ...

Low voltage ZnO varistor: Device process and defect model

A new process is described for achieving the current‐voltage characteristics of a ZnO varistor with a breakdown voltage less than 20 V. The structure has been made by using Ag contacts containing oxides of Bi and Pb which serve both as a diffusion source for grain and/or grain boundary doping, and as metallic contacts on polycrystalline ZnO substrate. The structure was also made by sputtering a layer of various metal oxides such as Bi2O3, Sb2O3, Co3O4, etc. and then evaporating contact metals. The device shows a nonlinear current‐voltage characteristic when annealed at 800–900 °C in air. Annealing in N2 ambient however, results in linear I‐V characteristics. The presence of certain critical additives such as Bi2O3 was found essential for the development of nonlinearity. Based on these observations and the known defect structure of pure ZnO, a defect model which takes into account the role of oxygen in developing the nonlinear behavior of the proposed device is presented.

A Procedure for Estimating the Lifetime of Gapless Metal Oxide Surge Arresters for AC Application

The development of the gapless metal oxide (ZnO) surge arrester has presented the arrester engineer with new materials and an opportunity for new designs. This situation arises because the gapless surge arrester is electrically active throughout its lifetime whereas its predecessor, the silicon carbide arrester, was electrically passive since it was electrically isolated with gap structures. The prime consideration is one of reliably estimating the lifetime of a gapless ZnO surge arrester under continuous ac stress while maintaining the capability not only to limit surge voltages but also to absorb energy inputs resulting from lightning or switching surges and temporary overvoltages. In this paper we establish a procedure for estimating the lifetime of gapless ZnO surge arresters for ac application by incorporating the device characteristics into design requirements. This method is illustrated for ZnO-surge-arrester elements that exhibit a predictable linear resistive current versus time?? behavior as a function of applied voltage and temperature.