Negative Temperature Coefficient Of Resistance Behavior Research Articles

Temperature dependent resistivity study on zinc oxide and the role of defects

The temperature dependent (30–550°C) resistivity of zinc oxide (ZnO) has been studied by the standard four probe resistivity method. The room-temperature resistivity of the sample is measured as 0.75MΩm. Resistivity versus temperature plot of the sample shows normal NTCR (negative temperature coefficient of resistance) behavior up to 300°C. However, a crossover from NTCR to a PTCR (positive temperature coefficient of resistance) behavior is observed at ∼300°C. The origin of the PTCR behavior is explained with the defects present in the ZnO annealed up to 550°C. Temperature dependent S-parameter (positron annihilation line-shape parameter) indicates the formation of oxygen vacancy like defects in this temperature region. At the PTCR region, the activation energy for the electron conduction is calculated ∼2.6eV. This value is very close to the theoretically predicted defect level energy of 2.0eV for oxygen vacancies present in ZnO.

A phenomenological model of conduction in Ho-doped barium titanate sensor

PTCR effect in doped barium titanate (BaTiO3) sensors originates from the grain boundary resistance and is attributed to the existence of potential barriers at these boundaries. Height of the potential barrier at the grain boundary is a function of the dielectric constant which is strongly temperature dependent. The height of the barriers between the grains has been investigated in semiconducting BaTiO3 samples prepared by doping with 0.3mol% Ho2O3 using the conventional ceramic methods. The results show a Positive Temperature Coefficient of Resistance (PTCR) for temperatures above the Curie temperature (TC) followed by a region of Negative Temperature Coefficient of Resistance (NTCR). The dielectric properties of the samples were studied in the temperature interval of 300–570K using capacitance measurements at 1.2kHz from which the permittivity and the barrier height were determined. It was found that the barrier height increases linearly in a narrow range of temperature where the PTCR effect is observed beyond which it continues to increase slowly. The barrier height does not become constant but varies from 0.06eV to 0.18eV over a temperature interval of 410–570K. A phenomenological model based on Thermionic Emission (TE) and Thermally Assisted Field Emission (TFE) mechanism has been proposed to account for the NTCR behavior.

NTCR behavior of Sm-substituted barium zirconium titanate nanocrystalline solid solution

Ceramic solid solution of nanocrystalline barium zirconium titanate in the form of Ba(Zr 0.52Ti 0.48)O 3 substituted by samarium (Sm 3+) was prepared using the conventional solid state reaction method. The phase assemblage analyzed by the X-ray diffraction technique was fitted for cubic-crystal-symmetry. The change in the grain size depicted the influence of Sm 3+ ions on the microstructure. The electrical behavior was studied in the temperature range from 323 to 773 K. The sintered samples exhibited a negative temperature coefficient of resistance (NTCR) and superior semiconducting behavior above 513 K. Addition of Sm 3+ increased the room temperature resistivity of Ba(Zr 0.52Ti 0.48)O 3 solid solution. The results obtained from the thermoelectric power measurement confirm electrons as the majority charge carriers.

Dielectric relaxation and ac conductivity study of Ba(Sr 1/3Nb 2/3)O 3

Single phase perovskite Ba(Sr 1/3Nb 2/3)O 3 ceramics was prepared using the columbite precursor method. X-ray diffraction (XRD) technique is used to verify the single phase formation. It stabilizes in hexagonal phase with lattice constants a=12.1243 Å and c=15.3747 Å. Impedance analysis shows distributed relaxation time. Single semicircular arc observed in complex impedance plot confirms that only semiconducting grains are contributing to the polarization. The scaling behavior of both Z″ and M″ infers that the dynamical processes are temperature independent. Comparison of the impedance and electrical modulus data shows that the bulk response has contributions from both localized, i.e. defect relaxation, and non-localized conduction, i.e. ionic or electronic conductivity. The ac conductivity of the ceramics at higher temperatures indicates NTCR (negative temperature coefficient of resistance) behavior like semiconductors. The ac conduction activation energies are estimated from Arrhenius plots and conduction is largely due to hopping process.