Preparation and characterization of manganese and cobalt based semiconducting ceramics

Abstract

Abstract The spinel phases Mn3-x Co x O4 are prepared directly and at low temperature (600-700°C) by means of thermal decomposition of mixed oxalate salt precursors in air or controlled atmosphere. The powders so obtained when sintered at 1200°C behave as semiconducting or insulating ceramics, depending on their cobalt content. The electrical properties of these ceramics have been found to be related to the distribution and valencies of the cations in the spinel structure. The study of their crystalline structure and stability, complemented by electrical measurements have permitted us to distinguish two types of compounds. The first type includes those given by Mn3-x Co x O4 with 0 < x < 1. These are stable at high temperature, have a tetragonal structure and are insulators. The cation distributions for them can be deduced from that of hausmannite as given by Mn2+ [Mn3+ 2]O4, by substituting Mn2+ ions with Co2+ ions. The second type of compounds correspond to the cubic phases and occur for 1 < x < 3. These are unstable at high temperature and show a lower resistivity (e.g. 360 ° cm) which is of particular interest for their application in the design of negative temperature coefficient (N.T.C.) thermistor components.