Chiang Y, Wang H, Lee J
Department of Materials Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, U.S.A.
J Microsc. 1998 Sep;191(3):275-285. doi: 10.1046/j.1365-2818.1998.00377.x.
Grain boundaries in model ZnO-Bi2O3 and ZnO-Bi2O3-CoO varistors and a commercial multicomponent varistor have been characterized by high-resolution electron microscopy (HREM) and scanning transmission electron microscopy (STEM), in order to determine the relationship between Bi grain boundary segregation and formation of thin intergranular films. By controlling Bi2O3 content, applied pressure and temperature, the grain boundary Bi excess has been systematically varied from nearly zero to GammaBi = 1 x 1015 cm-2 ( approximately 1 monolayer), as measured by HB 603 STEM using an area-scan method. HREM shows that intergranular amorphous films are clearly distinguishable in samples with GammaBi > 8 x 1014 cm-2. These films range in thickness, depending on the Bi excess, from 0.6 to 1.5 nm. Similar films of approximately 1 nm thickness are widely observed in the commercial varistor. The composition of the films is a ZnO-Bi2O3 solid solution, which is in all cases more enriched in ZnO than the bulk eutectic liquid. The Bi-doped grain boundaries in ZnO varistors therefore contain an intergranular amorphous film which has not only an equilibrium thickness, but also a distinct equilibrium composition.
通过高分辨率电子显微镜（HREM）和扫描透射电子显微镜（STEM）对模型ZnO-Bi2O3和ZnO-Bi2O3-CoO压敏电阻以及一种商用多组分压敏电阻中的晶界进行了表征，以确定Bi在晶界的偏析与薄晶间膜形成之间的关系。通过控制Bi2O3含量、施加压力和温度，利用HB 603 STEM采用面积扫描法测量，晶界处的Bi过量已系统地从近零变化到ΓBi = 1×1015 cm-2（约1个单分子层）。HREM表明，在ΓBi > 8×1014 cm-2的样品中，晶间非晶膜清晰可辨。这些膜的厚度取决于Bi的过量情况，范围为0.6至1.5 nm。在商用压敏电阻中广泛观察到厚度约为1 nm的类似薄膜。这些薄膜的成分是ZnO-Bi2O3固溶体，在所有情况下，其ZnO含量都比共晶熔体中的含量更高。因此，ZnO压敏电阻中Bi掺杂的晶界包含一个晶间非晶膜，该膜不仅具有平衡厚度，而且具有独特的平衡成分。
