Composite ceramics with a positive temperature coefficient of electrical resistivity effect
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Composite ceramics with a positive temperature coefficient of electrical resistivity effect Darja Lisjak, Miha Drofenik, and Drago Kolar Institute “Jozˇef Stefan,” Jamova 39, 1000 Ljubljana, Slovenia (Received 3 August 1999; accepted 1 December 1999)
Composite ceramics with compositions within the ZnO–NiO–TiO2, ZnO–MgO, and ZnO–Ln2O3 (Ln ⳱ Nd, Sm) systems were found to exhibit an anomalous positive temperature coefficient of electrical resistivity (PTCR) effect. The investigations revealed, that in all cases when the PTCR effect was identified, the composite ceramics were found to be composed of phases with different electrical resistivities and linear thermal expansion coefficients. Thermal mismatch between the phases in the composites leads to a disconnection of the grains of the low resistivity constituent phase on account of the high thermal expansion of the other high resistivity constituent phase, resulting in a PTCR effect.
I. INTRODUCTION
Positive temperature coefficient resistivity (PTCR) effect is a term used for an anomalous positive temperature coefficient of electrical resistivity in nonmetalic materials. It was first observed in donor-doped BaTiO3.1 Other materials known to exhibit a PTCR effect are polymer composites,2–7 inorganic composites,8–11 and compounds based on V2O3.12,13 Some of these materials are employed as PTCR thermistors for temperature sensors and regulators, overcurrent protection, fluid-flow sensors, and TV degaussers. Polymer composites exhibiting the PTCR effect are composed of a conductive phase, being carbon black,3 a metal,4,5 or other highly conductive material,6,7 and an insulating polymer phase. The polymer usually expands abruptly at a temperature near its melting point and can disconnect the contacts between the conductive phase particles, which leads to an increase in the overall composite resistivity; i.e., the PTCR effect. Similar behavior was also observed, for example, with SiC/SiO28 and graphite/SiO29,10 composites, where the volume expansion of the insulating phase can be assigned to the SiO2phase transition. In contrast, the PTCR effect observed in C/NaCl11 composites is attributed to a thermal mismatch between the C and NaCl phases. The linear thermalexpansion coefficient of the insulating NaCl is much larger than that of the conductive C constituent phase. A PTCR effect in ceramics based on the ZnO–NiO– TiO214,15 and ZnO–NiO16 systems has been reported. Recently we have proposed a model that attempts to explain the origin of the PTCR effect in the ZnO–NiO two-phase ceramics.16,17 The ZnO–NiO two-phase ceramics are ceramic composites of a low-resistivity constituent phase ZnOss and a high-resistivity constituent J. Mater. Res., Vol. 15, No. 2, Feb 2000
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phase NiOss, with a significant thermal mismatch (difference in linear thermal-expansion coefficients, ␣). The thermal mismatch and the resistivity difference between the constituent phases, ZnOss and NiOss, indu
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