Effects of Pt and AgPd Inner Electrodes on the Microstructure and Electrical Properties of Multilayer Varistors
Date Issued
2007
Date
2007
Author(s)
Kuo, Shu-Ting
DOI
en-US
Abstract
The grain size and its distribution play a key role on the performance of ceramics. Recently, ZnO-based multilayer varistors (MLVs) have become available. For the applications of varistors, the breakdown voltage per unit thickness is related to grain size and its distribution. The grain size distribution between two inner electrodes is thus the most important factor in determining the breakdown voltage in the multilayer varistors. In the present study, the Pt or AgPd inner electrodes are used in the multilayered ZnO-Bi2O3 varistors. The grain growth behavior within inner electrodes is investigated. In addition, the effect of microstructure on the electrical properties in ZnO-Bi2O3/Pt or ZnO-Bi2O3/AgPd MLVs is also discussed.
In the ZnO-Bi2O3/Pt or ZnO-Bi2O3/AgPd systems, the size of ZnO grains within or outside the electrodes increases by increasing the sintering temperature and dwell time. However, the grain growth rate within the inner electrodes is slower than that outside the electrodes. The inner electrodes thus act as physical constraints on the grain growth behavior. By prolonging the dwell time, as the size of ZnO grains approaches the thickness between inner electrodes, the column structure is then developed. In addition, the apparent activation energy increases with the decrease of layer thickness.
On the other side, the chemical interaction between Bi2O3 and Pd occurs during co-firing of ZnO-Bi2O3 and AgPd system. The reacted phase, PdBi2O4, forms during sintering. Furthermore, after sintering at 1100℃ for 1000 min or at 1200℃ for 60 min, the AgPd inner electrodes are no longer continuous. It is speculated that the grain boundaries with Bi2O3-rich liquid phase have higher mobility during sintering. They easily sweep through the ZnO grains and leave behind many AgPd. Lots of AgPd may be trapped within the ZnO grains; the AgPd inner electrodes thereby lose their integrities.
With respect to the electrical properties, since the number of grain boundaries is an important role in determining the electrical properties of ZnO-based MLVs, the decrease of breakdown voltage and nonlinear coefficient and the increase of leakage current are resulted from an increase the sintering temperature or dwell time. These results are resulted from the increase of grain size and the decrease of number of grain boundaries. However, in the ZnO-Bi2O3/ AgPd MLVs, due to the lost of integrity of AgPd electrodes after sintering at higher temperature or sintering for a longer time, the multilayer structure is destroyed to produce a single layer structure. The breakdown voltage and nonlinear coefficient are thus increased.
Subjects
氧化鋅
氧化鉍
銀鈀
鉑
基層變阻器
電極
晶粒成長
微結構
電性
ZnO
Bi2O3
AgPd
Pt
Multilayer varistor
Electrode
Grain growth
Microstructure
Electrical properties
Type
thesis
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