Microstructural and Nonohmic Properties of ZnO center dot Pr6O11 CoO Polycrystalline System

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

The microstructure and electrical properties of varistors composed of (95-x) ZnO + x Pr6O11+ 5 CoO (ZPC), (x = 0.1, 0.5 and 1.0) and sintered at 1300 and 1350 degrees C, were investigated. According to X-ray diffraction, several phases (ZnO, Pr2O3 and Pr2CoO4) are present when x = 1.0. Using Scanning Electron Microscopy, all of these compositions contain precipitates. These phases are important regarding the development of the microstructure and the electrical properties. The samples with x = 0.1 introduce the best nonohmic behavior (alpha = 9.0), however when x = 0.5 the electrical properties are highly degraded due to the small quantity of effective barriers. The density of superficial states N-IS and donor concentration Nd decreases with Pr6O11 addition. The decrease in the donor concentration is attributed to the annihilation of the donor defects according to the transformation of praseodymium oxides from Pr6O11 to Pr2O3.