Effect of Ag on the micro structure and electrical properties of ZnO

Various amounts of silver particles, 0.08-7.7 mol%, are mixed with zinc oxide powder and subsequently co-fired at 800-1200 degrees C. The effects of Ag addition on the microstructural evolution and electrical properties of ZnO are investigated. A small Ag doping amount (<0.76 mol%) promotes the grain growth of ZnO; however, a reversed trend in grain growth is observed for a relatively larger Ag addition (>3.8 mol%). It is evident that a tiny amount of Ag (similar to 0.08 mol%) may dissolve into the ZnO lattice. High-resolution TEM observations give direct evidences on the segregation of Ag solutes at the ZnO grain boundaries. The grain boundary resistance of ZnO increases 35-fold with the presence of Ag solute segregates. The Ag-doped ZnO system exhibits a nonlinear electric current-voltage characteristic, confirming the presence of an electrostatic barrier at the grain boundaries. The barrier is approximately 2 V for a single grain boundary. (C) 2007 Elsevier Ltd. All rights reserved.