Elsevier

Solid State Ionics

Volume 177, Issues 19–25, 15 October 2006, Pages 2191-2194
Solid State Ionics

Effect of annealing in O2 or N2 on the aging of Fe0.5Mn1.84Ni0.66O4 NTC-ceramics

https://doi.org/10.1016/j.ssi.2006.01.037Get rights and content

Abstract

Fe0.5Mn1.84Ni0.66O4 NTC ceramic thermistors were annealed in nitrogen or oxygen atmosphere at 600 °C. The change of the electrical properties of the thermistors with time was reduced sharply by annealing in N2, whereas it was enhanced upon annealing in O2. N2-annealed samples exhibited a less degree of redistribution of cations in the lattice as compared with O2-annealed samples. The aging of the electrical properties is believed to result from the cation redistribution which in turn is favoured by the presence of cation vacancies. And the improved aging behaviour of the N2-annealed thermistor is explained by the reduction of the concentration of the cation vacancy upon annealing under lower oxygen partial pressure and the suppression of cation redistribution.

Introduction

Negative temperature coefficient (NTC) ceramic thermistors are increasingly used in various industrial and domestic applications, such as elements for suppression of in-rush currents, for temperature measurement and control, and for the compensation of other circuit elements [1]. The specific resistivity of these ceramics follows the well known Arrhenius relation: ρ = ρ0 exp(Ea / kT), in which ρ is the specific resistivity, Ea the activation energy for electrical conduction, k Boltzmann's constant and T the absolute temperature. In practice, the NTC thermistors are routinely characterized by two parameters, B, the thermal constant (unit in Kelvin) which is related to B = Ea / k, and ρ 25 °C , the specific resistivity at 25 °C. Aging, i.e., the change of the electrical properties with time is the main problem limiting the application of the spinel-structured ceramic NTC thermistors. It is generally accepted that exchange of cations between the sublattice tetrahedral (A-sites) to the octahedral (B-sites) gives rise to the aging of the thermistors [2]. It is also suggested that the aging involves migration of cation vacancies from the grain boundaries to the grain interior [3]. In our previous work [4], it was found that Fe0.5Mn1.84Ni0.66O4 exhibits the severest aging in the FexMn2.34−xNi0.66O4 (0 < x < 1) series. And this paper is to investigate the effect of annealing atmosphere on the aging behaviour of that composition.

Section snippets

Experimental

Fe0.5Mn1.84Ni0.66O4 powders were prepared by the Pechini method [5]. Appropriate amounts of Fe(NO3)3 · 9H2O, Ni(NO3)2 · 6H2O and Mn(NO3)2 were dissolved in deionized water, and citric acid (CA) and ethylene glycol (EG) were added to the solution with a molar ratio [Fe + Mn + Ni] : CA : EG = 1 : 2 : 2. The pH of the solution was adjusted to about 3 with ammonia. The solution was first heated at 90 °C under stirring to yield a viscous solution, and then heated to 140 °C to transform to a resin. Upon further heating

Results and discussion

The relative density of the sintered ceramic was measured to be about 95%. SEM examination shows that the sintered ceramic is well densified, and consists of grains of 5∼8 μm (Fig. 1). XRD analysis reveals that the ceramic retains the single-phase spinel structure after annealing in O2 or N2 (Fig. 2). Table 1 summarizes the electrical properties of the as-sintered and annealed ceramic thermistors. It can be seen that in comparison with the as-sintered ceramic, the drift in electrical resistivity

Conclusions

Fe0.5Mn1.84Ni0.66O4 NTC ceramic thermistors annealed in nitrogen exhibit much improved aging behaviour, and undergo a less degree of cation redistribution among the A- and B-sites in the spinel, as compared with thermistors annealed in O2. Annealing of the spinel-structured ceramic at a proper oxygen partial pressure leads to the reduction of cation vacancies, inhibiting the cation redistribution and thereby suppressing the aging of the electrical properties.

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