Yoshio Takada

A mechanism of degradation in leakage currents through ZnO varistors

Thermally stimulated currents through ZnO varistors subjected to the voltage stresses, dc, and square‐wave voltages are analyzed on the base of carrier trappings in the depletion layers of ZnO grains. The analysis indicates that the field ionization of trapped carriers in a reverse‐biased depletion layer plays an important role in the degradation phenomena, thereby suggesting that the current increase with time under the dc and ac stresses is due to the same origin. An Arrhenius relation to predict the life of varistors and a dependence of leakage current Jc on time t, ln Jc ∝t1/2, are derived from a carrier trapping model.

Electrical Conduction of ZnO Varistors under Continuous DC Stress

The degradation mechanism of I-V curves of ZnO varistors is investigated by means of thermally-stimulated currents and photoconductivity measurements. We postulate that the degradation is due to electrons and holes accumulated across the grain boundary face to face with each other such as polarized charges in a condenser. According to the photoconductivity measurements, the degradation of I-V characteristics is found to be due to the increase in carrier density and field-dependent drift mobility. In addition, the varistor after dc stressing shows n-type conduction. The conduction mechanism for the initial I-V characteristics is also discussed. A model similar to the mobility-gap model is proposed, and it is suggested that the intrinsic conduction should be taken into account for the Schottky-type conduction mechanism.

Improvement of ZnO Element Performance by Reducing Spinel Particles

The performance of zinc-oxide (ZnO) elements is strongly dependent on the properties of the boundaries between ZnO grains, which are greatly affected by the formation of spinel particles between ZnO grains during the sintering process. The authors studied the effects of reducing spinel particles on the performance of elements and found that the characteristics of the spinel-reduced elements were improved compared with that of normal elements (e.g., superior voltage-current characteristics, smaller micropores, and larger capacitance). It was also found that the sintering temperature of the spinel-reduced elements could be decreased by more than 100degC than that of normal elements.

Correlation between electrical properties and crystalline phases for ZnO-Bi2O3 based varistor ceramics with rare earth additives

In this study, the correlation between the electrical properties and crystalline phases of ZnO-Bi2O3 based varistor ceramics with rare earth additives was investigated. The additives used were in the form R2O3, where R represents Eu, Ho, Y, Er, Yb, or Lu. It was found that the incorporation of rare earth additives led to an increase in the varistor voltage in the nonlinear region, an increase in the leakage current in the pre-breakdown region, and a reduction in the limiting voltage in the upturn region. In the sample with added Y2O3, a phase referred to as the R-phase, which contained Y, Bi, Sb, Zn, Mn, and O, was observed to be homogeneously distributed at ZnO grain boundaries. X-ray diffraction analysis revealed that the amount of the Zn-Sb-O spinel and Bi-Cr-O phases present was reduced by the rare earth addition, and that rare earth elements were incorporated into a common crystalline phase. It is suggested that the formation of the R-phase at ZnO grain boundaries and the reduction in the Bi2O3 liquid phase during the sintering process might be responsible for suppressing ZnO grain growth, thus leading to an increase in the varistor voltage.

Growth technique and optical characterization of (Cd,Mn)Te for fiber-optic magnetic field sensors

Crystal growth of Cd1−xMnxTe (x > 0.4) with giant Faraday rotation has been studied for applications as fiber-optic magnetic field sensors. These crystals contain two types of inclusions: large-sized (>10 μm) manganese oxide and small-sized (<2 μm) manganese telluride. Both inclusions decreased with purification of the Mn metal surface and optimisation of growth conditions. As a result, single crystals with low extinction ratio ( <10−4) and transmission loss ( < 0.45 dB/mm) at 660 nm were obtained.