CORRELATION BETWEEN MICROSTRUCTURE AND AGING OF IRON MANGANITE THERMISTORS

Negative Temperature Coefficient (NTC) thermistors made of spinel stru cture transition metal manganites usually display ageing phenomena und er thermal stress. Their resistance drift depends on their composition , crystal structure (cubic or tetragonal) and heat treatments. We have previously shown in iron manganite thermistors, Mn3-xFexO4 (with 0 le ss than or equal to l x less than or equal to 1.51), that the ageing i s due to the migration of Fe3+ and Mn2+ ions between tetrahedral and o ctahedral sites of the spinel structure. Iron manganites were investig ated by Transmission Electron Microscopy (TEM) in order to relate micr ostructure to electrical stability. For iron manganites with iron cont ent x less than or equal to 0.78, tao dimensional defects result in a domain microstructure (microtwins). As x increases and exceeds 0.78, t he domain structure gradually vanishes and transforms into a tweed mic rostructure (x = 1.05) and, for x > 1.30, no bidimensional defects are observed. Thus it is suggested that the microstructural disturbance p lays an important role in the kinetics of the ion migration during the ageing of the studied ceramics.