Effect of Annealing Atmosphere on Electric Conductivity of Mn-Fe-Co-Ni Spinel Type Oxide as a Thermistor Material

Abstract

An electric conduction mechanism and an effect of atmospheres in annealing processes on electric conductivity of Mn-Fe-Co-Ni oxide (Mn: Fe: Co: Ni=5.7: 4.0: 1.5: 1.0 in molar ratio) used for a thermistor material were investigated. The samples were prepared by firing a mixture of oxides, obtained from aqueous solutions of Mn, Fe, Co, and Ni nitrates, at 1400°C for 1 h and cooling in an electric furnace to room temperature after holding at 1000°C for 1 h during cooling.
The samples were annealed at 310°C and 370°C in atmospheres of oxygen, air, and argon in the apparatus as shown in Fig.1. The samples annealed at 370°C under above three atmospheres had the same crystal phases as that of the as-fired sample, that is, the cubic spinel phase. Annealing at 310°C in oxygen and air, on the contrary, yielded an additional tetragonal spinel phase; the phase transition from the cubic to the tetragonal phase occurred in an oxidizing atmosphere. During annealing at 310°C and 370°C in different atmospheres the electric conductivities (σ) and Seebeck potentials (Qs) were measured. While no change in σ a and Qs by annealing in argon was observed at 310°C and 370°C, σ increased due to the increase in the concentration of positive charge carriers and the decrease of σ was followed by annealing in oxygen and air, as shown in Figs.6 and 7. The position of the acceptor level (EA) and the activation energy of the movility (Eμ), as determined from the slopes of the in σT and Qs vs. T^-1 plots in Fig.5, were 27.1 and 28.3 kJ⋅mol^-1, respectively. With the aid of these data the jump frequency ν0 was found to be =5.4×10¹²s^-1, which is in the frequency range expected for optical phonons. These data can be explained satisfactorily in terms of a smallpolaron hopping model.