CORRELATION BETWEEN THE STRUCTURE, THE MICROSTRUCTURE AND THE ELECTRICAL-PROPERTIES OF NICKEL MANGANITE NEGATIVE TEMPERATURE-COEFFICIENT (NTC) THERMISTORS

The structure and the microstructure of nickel manganite NTC thermisto rs have been determined and correlated with their electrical propertie s in order to understand the 'ageing' phenomenon observed in these sem iconductor devices. It has been shown that the heat treatment of metal lization by serigraphy is responsible for structural and microstructur al modifications. During this annealing some of the Ni2+ cations migra te from the octahedral sites to the tetrahedral sites. At the same tim e, the ceramic is oxidized and precipitates of a cationic defect spine l phase appear. These modifications lead to a decrease in resistivity. During the ageing test at 125 degrees C the cationic distribution doe s not change but the precipitates disappear. It induces an increase in resistivity. An improved serigraphy process is proposed to achieve st able NTC thermistors. (C) 1998 Elsevier Science B.V. Ail rights reserv ed.