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