This study describes the nonlinear characteristics of SrCoO 3 -doped ZnO varistors and multilayer ceramic varistors (MLCVs) with copper electrodes, both of which are sintered in a reducing atmosphere. Due to postannealing effects in air or N 2 with low-oxygen concentration (0.02%), bulk disks can be sintered in a reducing atmosphere, with a usable V 1 mA /mm (e.g., 1600 V for bulk bodies or 1200 V for Cu cofiring) and highly nonlinear indices (a 10 lA 5 V 1 mA /V 10 lA , 1.3), regardless of whether cofiring with Cu electrodes on disk surfaces was conducted or not. On the basis of this procedure, Cu-MLCVs were successfully produced, without oxidation of Cu-internal electrodes or structural defects. They exhibited high stability as well as a useful nonlinearity of V 1 mA 5 10.4 V and a 10 lA 5 1.93. The resultant stability against electrostatic discharge (ESD) satisfies the highest standard of level 4 in IEC61000-4-2 (ESD stability test). This is the first report to show that MLCVs with base metals have practical properties, including stability.

Air-sintered SrCoO 3 is strongly reduced at 1020°C in P O2 = 1.6 © 10 ¹9 MPa and then post-annealed at various temperatures in air. The conductive and structural characteristics are examined, comparing them with those of air-sintered. Although annealing in a reducing atmosphere breaks down SrCoO 3 to several compounds, SrCoO 3 is re-synthesized by post-annealing above 800°C. When post-annealed at 1000°C, the oxidation state of SrCoO 3 is believed to be the same higher level as that of sintered in air. The result shows that, enhancement of p-type carriers in SrCoO 3 at grain boundaries leads to nonlinearity improvement of SrCoO 3 -doped ZnO varistors sintered in reducing atmospheres.

The grain-boundary and its electrical characteristics in SrCoO3doped ZnO varistors were studied. The grain-boundary around ZnO-grain is probably composed of SrCoO3, and its electrical behavior is clearly different from two conventional types of Bi-and Pr-based ZnO varitors. The non-linearity and characteristic behavior could be explained by considering the n-p-n hetero-structure at the grain boundary. SrCoO3 in the grain-boundary region should play crucial roles of not only the appearance of non-liner property but also the formation of different hetero-structure from double Schottky-barrier model on conventional varistors.

Nonlinear electrical properties of individual grain boundaries in SrCoO 3 -doped ZnO varistor ceramics are investigated using a nano-prober system. In particular, the effect of post-annealing is clarified for them after sintering in a reducing atmosphere. The nonlinear characteristics are enhanced by annealing in air at above 600°C. The resulting characteristics are attributed to an increase of p-type carrier concentration in SrCoO 3 phase at grain boundaries. By annealing at a higher temperature (e.g., 800°C), the grain boundaries possess nonlinear characteristics similar to conventional Bi-doped ZnO varistors, as well as bulk-bodies.

This would be the first report that transmission line pulse (TLP) analysis for stability against electrostatic discharge (ESD) of CuMLCVs reveals the effect of ZnO grain resistivity under ESD zapping. The influence has been evident from results over a wide range of VI characteristics provided by TLP measurement. ESD stability is found to be closely related to variations of ZnO grain resistivity, greatly depending on oxygen partial pressure (P O2 ) during sintering. The lowering of ZnO resistance leads to high stability against ESD. Thus, the stability is demonstrated to be drastically improved with decreasing ohmic resistance down to the lowest levels by sintering below P O2 of 1.6 © 10 ¹9 MPa. Then, influence of Cu diffusion on their properties of MLCVs was investigated in bulk-disks of SrCoO 3 doped-ZnO with various CuO additions. The enhancement of ZnO grain resistivity is caused by the slight diffusion of Cu + during the sintering in a relatively weak reducing P O2 above 2.2 © 10 ¹9 MPa, resulting in low ESD stability. However, ZnO grain resistivity is not influenced by Cu diffusion below 0.005 mol % as CuO, thus Cu-MLCVs containing below the affected level have been considered bringing high ESD performance. It was confirmed experimentally that sintering of Cu-MLCVs in the optimum reducing atmosphere provides high ESD stability and significant suppression (about 1100 to 150 V) for ESD zapping at 8 kV under IEC61000-4-2. Cu-MLCVs obtained are able to have high protection performance as well as ESD stability at the same level as a device in practical use. Lowering of ZnO resistivity by a decrease of Cu diffusion is believed to give benefits to overall performance of Cu-MLCVs.