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 NiMn 2 O 4+δ thermistor material have been analysed. Electron transport in spinel NTCR thermistors is commonly described by an Arrhenius hopping model (R ∼ Texp(T 0 /T) for nearest neighbour hopping (NNH) or by different variable range hopping (VRH) models (R ∼ T 2p exp(T 0 /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 To ranged from 1.90 x 10 5 K to 1.97 x 10 5 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 x 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.