Electrical properties of screen-printed NiMn2O4+δ

The direct current (dc) resistance versus temperature (R–T) characteristics and the alternating current (ac) impedance of thick screen-printed films made of ceramic NiMn2O4+δ thermistor material have been analysed. Electron transport in spinel NTCR thermistors is commonly described by an Arrhenius hopping model (R ∼ T exp(T0/T)) for nearest neighbour hopping (NNH) or by different variable range hopping (VRH) models (R ∼ T2p exp(T0/T)p). In screen-printed films dc conduction was well described by VRH with p ∼ 0.5, indicating that the shape of the density of states (DOS) was parabolic. Values of T0 ranged from 1.90 × 105 K to 1.97 × 105 K. Impedance spectroscopy was carried out at a frequency range of 5 Hz–6 MHz between 60 °C and 220 °C. For frequencies below ∼2.7 MHz the complex impedance was well described by a standard equivalent circuit, based on one parallel resistance–capacitance (RC) element. Plots on the complex plane of the imaginary versus real parts of the impedance (Z″–Z′ loci) showed one regular semicircle at each temperature, from which the capacitance and resistance of the RC element were determined. The capacitance was found to be in the order of 1 × 10−12 F at all temperatures, indicating a grain effect. The ac resistance versus temperature characteristics were in very good agreement with the dc results and followed the same VRH model with p ∼ 0.5. At frequencies above 2.7 MHz the impedance could not be described by a standard RC element.