Polycrystalline barium titanate that has been doped to give a positive temperature coefficient of resistance (PTCR) effect is an inhomogeneous material electrically. Analysis of ac impedance data using the complex impedance plane representation gives the dc resistance of PTCR ceramics. By additional use of the complex electric modulus formalism to analyze the same data, the inhomogeneous nature of the ceramics may be probed. This reveals the presence of two, sometimes three elements in the equivalent circuit. Grain‐boundary and bulk effects may be distinguished from capacitance data: grain‐boundary effects have temperature‐independent capacitances, whereas bulk effects show a capacitance maximum at the Curie point and Curie–Weiss behavior above the Curie point. Both grain‐boundary and bulk effects appear to contribute to the PTCR effect. These results reveal limitations in current theories of the PTCR effect.

Impedance and modulus spectroscopy of semiconducting BaTiO3 showing positive temperature coefficient of resistance

Se
discute y revisa la teoria basica de la espectroscopia de impedancia y se exponen varios ejemplos de electromateriales/dispositivos, donde las medidas de impedancia compleja dan una excelente informacion, que no puede ser obtenida por tecnicas ac y de convencionales. Se hace enfasis en las tecnicas avanzadas de adquisicion y elaboracion de datos; por ejemplo la combinacion de los diagramas de espectroscopia de impedancia con la espectroscopia del modulo electrico, para caracterizar electricamente materiales ceramicos inhomogeneos como los materiales basados en BaTi03 que son resistencias con coeficiente de temperatura positivo. Se presenta un analisis electrico detallado de monocristales ferroelectricos de LiTa03 en funcion de la orientacion cristalografica. Los resultados demuestran la utilidad de la tecnica de impedancia para visualizar procesos tan complejos como la inversion de dominios en materiales ferroelectricos. Se discute tambien el uso de estas tecnicas in-situ para monotorizar variaciones en las capas intercaras en las intercaras electrodo-electrolito en las baterias de litio de estado solido.

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Characterization of electro-materials using ac impedance spectroscopy

BaTiO3 positive temperature coefficient of resistance (PTCR) ceramics were prepared using commercially available BaTiO3 which was doped with 0.4 mol.% Ho to make it semiconducting. The samples were sintered in air at 1320 degrees C and while cooling to room temperature, different samples were annealed at 1220 degrees C for various periods ranging from 0 to 27 h. The annealing process was found to result in the formation of a resistive grain boundary region in series with the space charge layer. Dielectric measurements in the audio- and radio-frequency regions revealed that the resistive layer did not diffuse throughout the entire grain bulk, even during the longest annealing period. It was also found that annealing at 1220 degrees C results in (i) an increase in the value of rho max, (ii) a reduction in the temperature corresponding to rho max and (iii) an increase in the steepness of the resistivity-temperature plots in the region of the transition from low to high resistance. These changes were explained in terms of an increase in the acceptor state density.

https://iopscience.iop.org/article/10.1088/0022-3727/20/12/016/pdf

The effect of annealing on the characteristics of semiconducting BaTiO3 positive temperature coefficient of resistance devices

Electrical characterization of strontium titanate borosilicate glass ceramics system with bismuth oxide addition using impedance spectroscopy

Several positive temperature coefficient (PTC) thermistors made of barium titanate with an excess of titania and containing additives such as yttria, and eventually silica, have been prepared following two different routes. The electrical properties of the ceramic samples have been studied at room temperature, i.e., below the transition temperature, using complex impedance spectroscopy. The latter proved to be very useful to measure separately the grain and grain-boundary resistivities which have been followed as a function of the yttrium concentration. They behave very similarly and go through a minimum for the same composition. From both electrical resistivity measurements and local chemical analysis, it is inferred that the average dopant concentration in the grains is lower than the nominal content in the starting powders. An overall interpretation is given, emphasizing the importance of liquid-phase sintering.

Effects of yttrium doping on the grain and grain-boundary resistivities of BaTiO3 for positive temperature coefficient thermistors

The effects of dopants, e.g. oxides of La, Sb, Nd, Gd, Y, Nb etc have been studied on the PTC behaviour of polycrystalline BaTiO3. Sb-doped material showed the best characteristics under our experimental conditions. A systematic study on the effects of different electrode materials like Ag, In, In-Ga and Ni has also been carried out. The contact resistance was exceptionally high for Ag and In, but negligible in case of either In-Ga or Ni. Impedance measurement at different frequencies has been used to distinguish between the intra-grain, inter-grain and electrode-contact resistances. Power-limiting capability of the prepared specimens has been tested by measuring I-V characteristics.

H. S. Maiti

Papers

PTC Behaviour of Semiconducting BaTiO3 Ceramics