鉑及銀鈀內電極對基層變阻器之微結構與電性的影響

Abstract

晶粒尺寸以及晶粒大小分佈在陶瓷的性質上扮演著一個重要的角色。近來氧化鋅(ZnO)基層變阻器日漸普及。在變阻器的應用上，其單位厚度之崩潰電壓與晶粒尺寸以及晶粒大小分佈有直接之關係。因此，在二電極間之晶粒大小分佈是決定基層變阻器之崩潰電壓的主要因素。本研究主要採用鉑(Pt)與銀鈀(AgPd)內電極於氧化鋅添加氧化鉍(Bi2O3)之基層變阻器中，並探討在內電極間晶粒成長之行為。此外，ZnO-Bi2O3/Pt 或ZnO-Bi2O3/AgPd基層變組器之微結構對其電性之影響也將一併探討。 實驗結果顯示，在ZnO-Bi2O3/Pt 或ZnO-Bi2O3/AgPd系統中，不論氧化鋅晶粒是在二內電極間或是在內電極外側，其晶粒大小均會隨著燒結溫度及持溫時間之增加而增加。然而，在內電極間的晶粒成長速度卻比電極外側的慢。此意謂著，內電極對於晶粒成長之行為扮演著物理束縛的角色。另一方面，隨著持溫時間之增加，當氧化鋅晶粒大小與二內電極間之層厚度相同時，其會形成柱狀結構。而且，視活化能(apparent activation energy)會隨層厚度之減少而增加。 另一方面，ZnO-Bi2O3/AgPd試樣於燒結過程中，氧化鉍和鈀會發生化學反應，生成新的相，PdBi2O4。此外，ZnO-Bi2O3/AgPd試樣於1100℃下持溫1000分鐘或是在1200℃持溫60分鐘後，試樣之銀鈀(AgPd)內電極將失去其連續性。這是因為含有液相氧化鉍之晶界具有高的移動速率，它們輕易地跨過氧化鋅晶粒並留下銀鈀。所以，許多銀鈀被留在氧化鋅晶粒內，使得銀鈀內電極失去其完整性。 至於電性方面，由於氧化鋅基層變阻器的電性是取決於晶界數目的多寡。實驗結果顯示，隨著燒結溫度與持溫時間之增加，氧化鋅晶粒成長，其晶界數目也因此而減少。此現象將降低氧化鋅基層變阻器之崩潰電壓(breakdown voltage, VB)與非線性係數(nonlinear coefficient,α)；然而，其漏電流(leakage current, IL)卻會因為氧化鋅晶粒之成長以及晶界數目之減少而增加。另一方面，在ZnO-Bi2O3/AgPd基層變阻器中，由於在高溫或長持溫時間下進行燒結，其銀鈀內電極會失去其完整性。此現象將導致ZnO-Bi2O3/AgPd基層變阻器之崩潰電壓與非線性係數上升，而漏電流下降的趨勢。

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.