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https://hdl.handle.net/2142/22304
Description
Title
Grain boundary modifications of manganese ferrite
Author(s)
Boy, Jeffrey H.
Issue Date
1991
Doctoral Committee Chair(s)
Wirtz, Gerald P.
Department of Study
Engineering, Materials Science
Discipline
Engineering, Materials Science
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Engineering, Materials Science
Language
eng
Abstract
Complex impedance spectroscopy of Mn ferrite with $\le$1.0 wt% SiO$\sb2$ and CaCO$\sb3$ additions, cooled in air, showed two distinct arcs which were attributed to bulk and grain boundary phenomena. The bulk component was invariant while the grain boundary component varied by less than two orders of magnitude and went through a maximum between 0.2 wt% and 0.4 wt% additions. Only the spinal phase was detected by X-ray diffraction for Mn ferrit with both SiO$\sb2$ and CaCO$\sb3$.
Mn ferrite with only CaCO$\sb3$ additions, sintered at 1250$\sp\circ$C for 4 h and cooled at $-$10$\sp\circ$C/min in air, showed a maximum in the grain boundary component of impedance at 0.4 wt%. Microstructural and X-ray diffraction analyses for compositions with $\ge$0.2 wt% CaCO$\sb3$ additions cooled at $-$1$\sp\circ$C/min showed formation of bixbyite as a second bulk phase.
At fields $>$10$\sp3$V/m, D.C. conductivity measurements showed a shift from grain boundary to bulk control of the conductivity for Mn ferrite with SiO$\sb2$ and CaCO$\sb3$ additions cooled in air. The difference in the conductivity between the bulk and grain boundaries was less than two orders of magnitude. The coefficients of non-ohmic response were less than 3.0 for all compositions investigated, insufficient for most varistor applications. The magnitude of the non-ohmic transition in Mn ferrite was found to be limited to 10$\sp4$ ohms.
Simple Arrhenius response with single activation energies all greater than 35 kJ/mole was obtained from D.C. conductivity measurements of Mn ferrite between 30$\sp\circ$ and 300$\sp\circ$C. Despite the high temperature coefficient of resistance, the domination of the resistance by the highly resistive grain boundaries limits the applicability as a NTCR device.
Differential thermal analysis in a magnetic field showed that Mn ferrite decomposed when subjected to post sintering thermal anneals at temperatures below 600$\sp\circ$C in air. The addition of less than 1.0 wt% SiO$\sb2$ and CaCO$\sb3$ inhibited this phase decomposition. The rapid oxidation of Mn ferrite without additions was attributed to rapid diffusion of oxygen along the grain boundaries. The presence of dopants at the grain boundaries inhibited this grain boundary diffusion.
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