Aging of iron manganite negative temperature coefficient thermistors
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Aging of iron manganite negative temperature coefficient thermistors T. Battaulta) , R. Legros, and A. Roussetb) Laboratoire de Chimie des Mat´eriaux Inorganiques, U.R.A. C.N.R.S. 1311, Universit´e Paul Sabatier – 118, route de Narbonne, 31062 Toulouse Cedex, France (Received 6 April 1996; accepted 26 June 1997)
“Aging,” defined as the drift of resistance with temperature after 1000 h, was investigated for iron manganite temperature coefficient thermistors. For these devices, aging is relatively large, about 10%. The cationic distributions before and after aging were determined by M¨ossbauer spectroscopy. These distributions explain all the x-ray diffraction and correlated electrical data. The origin of the aging observed on iron manganites thermistors has been identified. It is due to the migration of Fe31 ions from tetrahedral to octahedral sites of the spinel structure in order to reach a structural equilibrium.
I. INTRODUCTION
Transition-metal manganites with spinel structure were first mentioned for use as thermally sensitive resistors as early as 1979 by Macklen.1 Because electrical resistance decreases exponentially with increasing temperature, it is referred to as a negative temperature coefficient (NTC). This is due to thermally activated electron transfer between Mn31 and Mn41 ions on the octrahedral sublattice1 (hopping mechanism). However, under thermal stress the thermistor’s resistivity increases with time and this is harmful to their use. Standard stability is measured as the relative variation (drift) DRyR of the resistance observed after aging of the ceramic at 398 K for 1000 h in air. Extensive studies have been carried out to minimize this drift of resistivity and thereby stabilize the thermistors. In a previous paper it was shown that polyphasic ceramic thermistors have better stability than monophasic ones. Thus, stability can be greatly improved by quenching or doping the ceramic with Ba21 ions because precipitated phases then appear in the vicinity of the spinel phase.2–4 However, the so-called “varistor effect” could not be detected.3 Thus, the phases precipitated at the grain boundaries don’t seem to play any role in the conduction process.3,6 What causes this aging? Several possible explanations have been put forth: ion oxidation, ionic and/or electronic migration, and influence of the metal-ceramic interface among others. Metz,6 studying nickel-cobalt manganite, discards any participation of the electrodes and moreover shows that thermal treatment at 1123 K a)
Present address: Centre National d’Etudes Spatiales (French Space Agency), 18, Av. Edouard Belin-31401, Toulouse Cedex 4, France. b) Author to whom correspondence should be addressed. e-mail: [email protected] 1238
http://journals.cambridge.org
J. Mater. Res., Vol. 13, No. 5, May 1998
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used to bond electrodes triggers the aging phenomenon. Because of their high mobility, Mn31 ions migrate at low temperature to octahedral sites, creating Mn31 clusters
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