Production and performance of thin and thick film NTCR thermistors based on NiMn(_2)O(_4)+δ

Abstract

In this study reliable film type NTCR thermistors based on NiMn(_2)O(_4)+δ were produced and their electrical properties were studied in detail. Electron-beam evaporation procedures have been applied to produce thin film NTCR thermistors. Phase pure NiMn(_2)O(_4)+δ target material was produced via a traditional ceramic precursor oxide route and thin films were deposited in an optimised procedure. The thickness distribution of evaporated films showed good agreement with a theoretical model, derived from evaporation theory and the sticking coefficient of the vapour on the substrates was approximately 80% ± 1.5%. The composition of electron-beam evaporated films was found to be not controllable in terms of the phase purity and the Ni : Mn ratio. In order to avoid these problems thick film NiMn(_2)O(_4)+δ NTCR thermistors were developed using direct screen-printing techniques. Detailed Rietveld refinement analysis was carried out for the source powder used for screen-printing. The main focus of the work was the measurement of resistance-temperature (R-T) characteristics of thin and thick films and pellets. In the temperature range of concern (77 K -550 K) conduction was found to be by variable-range hopping (VRH) and nearest-neighbour hopping (NNH); R ~ exp (TʆT)(^p), where the index p depends on the mode of hopping. Detailed analysis of R-T data showed that screen-printed films and pellets exhibited a p-value of 0.5, which was identified with VRH with a parabolic density of states (DOS) with an exponential dependence of resistance: R ~ exp (TʆT)(^0.5). For electron-beam evaporated films the mechanisms detected were NNH: R ~ exp (TʆT); and VRH with a constant DOS {p = 0.25) following: R ~ exp (TʆT)(_0.25). For screen-printed films with incorporated glass phase the electrical conduction mechanism was analysed using a.c. impedance spectroscopy and at low frequencies the hopping conduction was in agreement with the d.c. behaviour. The time constant of this mechanism could be described by an equivalent circuit containing a RC element. For higher frequencies a second mechanism was found, best described by a CRL element. [brace not closed]