GRAIN-BOUNDARY MODIFICATIONS OF MANGANESE FERRITES

The effect of minor additions (<1 wt%) of SiO2 and CaCO3 to manganese ferrites was investigated. Complex impedance analysis showed that the resistivity was modified by altering the electrical response of the gr ain boundaries. The measured resistivity of the grain boundaries went through a maximum at between 0.2 and 0.4 wt% additions. DC conductivit y measurements showed a shift from grain boundary to bulk control of t he conductivity at fields > 10(3) V/m. An equivalent circuit model was used to calculate the coefficient of non-ohmic response for Mn ferrit es which were less than 3.0 for all compositions investigated. Differe ntial thermal, analysis in a magnetic field showed that Mn ferrite dec omposed when subjected to post sintering thermal anneals at temperatur es >600 degrees C in air. The addition of less than 1 wt% SiO2 and CaC O3 inhibited this phase decomposition, by preventing the rapid diffusi on of oxygen at the grain boundaries. The control of oxidation in ferr ites by dopants is in addition to the previously recognized role in th e control of the electrical and magnetic properties.