Nonlinear property of SrCoO 3 -doped ZnO ceramics sintered in a reducing atmosphere and multilayer ceramic varistors wit
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Eiichi Koga Circuit Components Business Unit, Automotive & Industrial Systems Company, Panasonic Co., Ltd., 1037-2 Kamiosatsu, Chitose City, Hokkaido 066-8502, Japan (Received 6 February 2015; accepted 16 June 2015)
This study describes the nonlinear characteristics of SrCoO3-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 N2 with low-oxygen concentration (0.02%), bulk disks can be sintered in a reducing atmosphere, with a usable V1 mA/mm (e.g., 1600 V for bulk bodies or 1200 V for Cu cofiring) and highly nonlinear indices (a10 lA 5 V1 mA/V10 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 V1 mA 5 10.4 V and a10 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.
I. INTRODUCTION
Contributing Editor: Gary L. Messing a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2015.199
conventional lower limit for MLCVs, have excellent nonlinearity and stability against ESD.3 Conventional MLCVs are produced using precious metals, such as Ag, Pd, Au, and Pt, as internal electrodes, which can be sintered with ZnO varistors in air. As numbers of stacked layers increase, to assist miniaturization and higher performance, the proportional cost of the internal electrodes to the overall cost sharply increases. This makes reducing their cost in MLCVs a major challenge. To reduce the cost of internal electrodes, base metals, such as Cu or Ni, should be used for MLCVs instead of precious metals. However, to use base metals as internal electrodes, since Ni and Cu are easily oxidized in air at high temperatures, MLCVs need to be sintered in a reducing atmosphere so as not to cause oxidation of the electrodes. Usually, the nonlinearity of ZnO varistors is very sensitive to any variation in oxygen or composition at grain boundaries.5 After sintering in a reducing atmosphere, conventional Pr- and Bi-doped ZnO varistors become semiconductive and show linearity, owing to the reduction of grain boundaries and evaporation of additives, such as Bi2O3. To resolve the issue for cofiring with base metal electrodes, ZnO-based varistors will need to show high nonlinearity after sintering in a reducing atmosphere. However, to the best of the authors’ knowledge, no research on MLCVs cofired with base metal electrodes has yet been carried out. This prompted us to study MLCVs, which can be cofired with base metal
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Ó Materials Research Society 2015
Because of their excellent nonlinearity, ZnO-based ceramic varistors are
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