Bi2O3與Sb2O3對ZnO變阻器的微結構及電性之影響

Abstract

ZnO變阻器(varistor)之電性與微結構息息相關。本研究主要在探討，當添加Bi2O3 或Bi2O3+Sb2O3時，對其微結構及電性之影響，尤其是對ZnO晶粒大小分佈之影響因素。 實驗結果顯示，Bi2O3從800℃開始劇烈揮發，使用與試樣相同組成之粉體床(powder bed)時，則可以非常有效地減少揮發，當沒有使用粉體床時，暫時的液相出現在Bi2O3-doped ZnO的系統中，當添加量到達某ㄧ臨界值時，此暫時之液相，有助於ZnO之緻密化及晶粒成長。燒結後，沒有發現殘留的液相，從dihedral angle的改變及電性的量測結果，可以證明Bi3+ 離子在晶界的偏析行為。但是，偏析對晶粒大小分佈之影響不大。 Zn7Sb2O12 spinel phase及大量inversion boundaries (IBs)，在添加Bi2O3+Sb2O3之系統中生成，不使用粉體床時，可以減少晶粒大小及其分佈。 當使用粉體床(powder bed)時，Bi-rich phase存在Bi2O3-doped ZnO之系統中，燒結過程中，則Bi-rich liquid phase更容易促進晶粒的成長。但是，溫度與添加量的增加，卻使得晶粒較均勻。同時添加Bi2O3與Sb2O3時，在降溫過程中，Zn2Bi3Sb3O14 pyrochlore phase會大量析出，對電性是不利的。 不管是Bi-segregation或是Bi-rich liquid phase，都增加了晶界的能障，因而造成非線性I-V特性。本研究以微結構分析為基礎，提出ㄧ個對單一晶界的崩潰電壓(Vgb)之計算方法。

The electrical properties of ZnO varistors depend strongly on their microstructure. In the present study, microstructural evolution of ZnO, Bi-doped ZnO and Bi,Sb-codoped ZnO during sintering is investigated. Emphasis is made on the factors to control the grain size distribution of ZnO grains. Experimental results indicated that the vaporization of Bi2O3 is dramatical above 800℃. A powder bed with the same composition as that of the specimens can significantly reduce the vaporization. A transient liquid phase is present in the Bi2O3-doped ZnO systems as no powder bed is used. Such transient liquid phase enhances the densification and grain growth as its amount is higher than a critical value. After sintering, no residual liquid phase is found. The segregation of Bi3+ ion is evidenced by the change of dihedral anagle and electrical properties. The segregation has little effect on the grain size distribution of ZnO grains. Zn7Sb2O12 spinel and inversion boundaries (IBs) are found in the Bi,Sb-codoped ZnO specimens when they are sintered without a powder bed. The presence of the spinel and IBs reduces the grain size and its distribution. By using a powder bed, a Bi-rich phase is existed in the Bi-doped ZnO specimens. The presence of the Bi-rich liquid phase enhances significantly the size of ZnO grains. In the Bi,Sb-codoped ZnO specimens, apart from the Bi-rich liquid phase, both pyrochlore and spinel are formed after sintering. The presence of the pyrochlore is detrimental to the electrical properties. Both the Bi-segregation and Bi-rich liquid phase could provide an electrostatic barrier at grain goundaries and result in a nonlinear I-V characteristic. In the present study, based on the microstructure analysis, a methodology to calculate the breakdown voltage per grain boundary (Vgb) is proposed.