Conductivity data and preparation routes for NiMn2O4 thermistor ceramics

Requirements for the formation of NiMn2O4 thermistor ceramics with highly reproducible electrical parameters are reported. The application of the defect spinel powder NiMn2□3°/4O4 + δ prepared by low temperature decomposition of (Ni, Mn)-oxalate mixed crystals together with a small addition of Pb5Ge3O11 as sintering aid is shown to be most advantageous. Thus single-phase NiMn2O4 ceramics of sufficient density are obtained on sintering at 1000°C (40h, O2 atmosphere) and annealing at 800°C (40h) for reoxidation. The electrical conductivity of NiMn2O4 is found to be σ20 C = 5·0 ± 0·2 × 10−4Ω−1cm−1. The activation term, B = 3775 ± 7 K is deduced from simple Arrhenius analysis in the range 273 < T < 343 K allowing possible exchange of thermistors with an accuracy ΔT ≤ ± 0·1 K. A more accurate linearization of the experimental data succeeds from the equation σ(T)=σ(T1)(T1/T)k exp[−Bk(1/T−1/T1)] with k = 2·91 ± 0·05, yielding Bk = 2853 ± 15 K = 0·246 ± 0·001 eV as the activation energy representative for NiMn2O4. Ageing behaviour and the influence of decomposition in air with rising temperature on the electrical properties are studied.