Varistor System Research Articles

Influence of Sintering Temperature on Intrinsic Trapping in Zinc Oxide‐Based Varistors

The ac electrical data of the ZnO‐Bi2O3 varistor system, in the frequency range 5 Hz to 13 MHz, when analyzed in the complex capacitance (C*) plane, yields a distinct depressed semicircular relaxation having an average time constant on the order of 10−6 s. This trapping is attributed to the possible formation of ionized intrinsic/native defects. The decrease in the depression angle and average relaxation time, monitored as a function of the increasing sintering temperature (≥1100°C), indicated an improvement in the degree of uniformity in the loss‐conductance and rapidity of response of the intrinsic trapping. The dependence of the ac parameters related to this trapping response obtained from the C* plane on the sintering temperature has been presented.

Anisotropic Grain Growth of ZnO Grain in the Varistor System ZnO-Bi2O3-MnO-TiO2

From the analysis of selected-area electron diffraction patterns through transmission electron microscopy, the crystallographic orientation of anisotropic ZnO grain growth by the addition of TiO{sub 2} to the varistor system ZnO-Bi{sub 2}O{sub 3}-MnO was determined to be the (10{anti 1}0) and ({anti 2}110) directions. The behavior of anisotropic grain growth was interpreted by the epitaxial growth of prism planes (resulting from the favorable atomic arrangement on these planes compared with other planes) and the increase in reactivity of the Bi-rich liquid phase toward the ZnO grain by the addition of TiO{sub 2}.

Effects of dopants on the deep bulk levels in the ZnO-Bi2O3-MnO2 system

Small amounts of In2O3 and Li2O were added to the ZnO-Bi2O3-MnO2 ternary primitive varistor system. Considering Schottky disorder and Frenkel disorder, oxygen vacancy or zinc interstitial density was expected to decrease with In2O3 doping and to increase with Li2O doping. Using admittance spectroscopy, a 0.32-eV electron trap, which is suggested to be an ionized oxygen vacancy, was observed regardless of the doping conditions. Changes in the peak height, for the 0.32-eV trap, in the admittance spectra are correlated with In2O3 and Li2O doping. Doping with In2O3 reduces the conductance contribution due to the trap, while doping with Li2O enhances it. However, calculations show that small amounts of dopants, less than 100 ppm, do not greatly influence the trap density. The trap density remains within experimental error approximately 1.0×1017 cm−3.

Complex plane analysis of trapping phenomena in zinc oxide based varistor grain boundaries

The origin of the frequency-dependent Mott–Schottky behavior observed in a wide range of ZnO-Bi2O3 varistor systems has been investigated. Lumped parameter/complex plane analysis of two-probe ac electrical data indicates that several trapping relaxations contribute to the measured MOV grain-boundary admittance in the frequency range, 10−2 Hz≤f≤107 Hz. Furthermore, this approach allows the development of an equivalent circuit representation which incorporates these trapping phenomena in a systematic manner.

High‐Temperature/Field Alternating‐Current Behavior of ZnO‐Based Varistors

The ac electrical response of the grain boundary in the ZnO–Bi2O3‐based varistor system (in the frequency range 10−2≤f≤107 Hz) has been analyzed using complex plane analysis techniques as a function of temperature (90°≤T≤150°C) and electric field (E60%lmA/cm2≤E≤ElmA/cm2). The high‐temperature/field data were best represented in the impedance plane. The lumped parameter equivalent circuit representation, containing a voltage variable resistive element associated with the grain boundary, was in agreement with a model previously reported in the literature. The resulting equivalent circuit representation of the grain‐boundary response consisting of a resistance and a capacitance are interpreted in terms of various phenomena occurring at the electrical thickness of the grain boundary.

Nature of an intergranular thin−film phase in a highly non−Ohmic metal oxide varistor

A detailed examination by transmission electron microscopy and selected−area diffraction revealed that the thin−film intergranular phase in a highly non−Ohmic multicomponent metal oxide varistor system based on ZnO with small additions of antimony, bismuth, cobalt, manganese, and tin oxides is amorphous and is chemically related to a Bi2O3−rich pyrochlore coexisting in the ceramic.

Ultra Speed Recording for Acoustical Measurement

The technique and tests herein described were devised to study possible improvement in rating the acoustical excellence of rooms, by using higher pen speeds than had hitherto been available. Recordings of reverberation decay characteristics, made with the new ultra speed technique, are compared with results from older methods of recording reverberation; and it is found that the new method provides a much more adequate diagnosis of recognized acoustical faults. The new technique makes possible ink recording of acoustical levels on a db basis at pen speeds of 5000 to 10,000 db per second, or ten to twenty times that afforded by the high speed level recorder. It uses a new stabilized logarithmic Varistor system for instantaneous conversion to a db basis, followed by a direct writing oscillograph. Hitherto unrecognized requirements for the electronic system are indicated as necessary if adequate stability, linearity, and scale range are to be achieved.