Influence of Nitrogen Annealing on Phase Evolution, Microstructure, and Electrical Properties of Mn0.76fe0.87co1.07zn0.3o4 Ntc Ceramic

Abstract

In this work, we have examined effects of nitrogen annealing on phase composition, microstructure, ion valence states and electrical properties of Mn0.76Fe0.87Co1.07Zn0.3O4, a spinel-type NTC thermistor ceramic fabricated via solid-state reaction. Our objective is to explore the underlying mechanism behind the observed shifts in electrical performance of the material. Post-annealing electrical results reveal a time-dependent increase in the resistivity (ρ25) and material constant (B25/50). After a 6-hour annealing period, ρ25 rose from 5430 Ω.cm to 18304 Ω.cm and B25/50 from 4023 K to 4316 K. AC impedance analysis identify changes in grain boundary resistance as the primary driver for the increased ρ25 and B25/50. These electrical modifications can be attributed to several transformations during annealing, including a decline in the lattice oxygen/oxygen vacancy (OL/OV) ratio, a reduction in valence states of transition metals (excluding Zn2+), structural transitions within the spinel. The structural transitions lead to the formation of a low-conductive Co-rich rock salt phase. The emergence of this phase alongside the presence of oxygen vacancies is directly linked to the enhanced ρ25 and B25/50. The overall findings from this study provide fundamental insights into improving the thermal stability of spinel-type NTC thermistors.

Keywords: NTC Thermistor, Nitrogen Annealing, microstructure, Phase Evolution, Electrical Properties, Aging