Positive temperature coefficient of resistivity effect in Pb-doped KnbO 3
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Positive temperature coefficient of resistivity effect in Pb-doped KNbO3 Irena Pribosˇicˇ and Darko Makovec Jozˇef Stefan Institute, Jamova 39, Ljubljana, Slovenia
Miha Drofenik Jozˇef Stefan Institut, Jamova 39, Ljubljana, Slovenia, and Faculty for Chemistry and Chemical Engineering, University of Maribor, Slovenia (Received 23 May 2002; accepted 9 September 2002)
KNbO3 is a ferroelectric material with a Curie temperature (TC) at 415 °C, thus giving it the potential to be a material for high-temperature positive temperature coefficient of resistivity (PTCR) applications. In this study, we investigated the PTCR effect in donor-doped KNbO3 ceramics containing 0, 0.1, 0.2, and 0.3 mol% PbO. The donor-doped KNbO3 ceramics exhibited a PTCR anomaly with a relatively low room-temperature resistivity. The temperature of the tetragonal-to-cubic phase transition (TC) of the KNbO3 decreased with the amount of added PbO, while the orthorhombic-to-tetragonal phase transition (TOT) remained unchanged.
Donor-doped BaTiO3 exhibits an anomalous positive temperature coefficient of resistivity (PTCR), also called the PTCR effect. Adjusting the Curie temperature (TC) of BaTiO3, which can be easily done with isovalent substitutions, can shift the onset temperature of this PTCR effect. At present, positive temperature coefficient thermistors with a TC up to approximately 350 °C are commercially available;1 however, there is a great demand from industry for PTCR materials with higher TCs. Several articles reporting the high-temperature PTCR effect have been published.2–10 The majority of them are based on Pb-containing ceramics such as (Ba,Sr)PbO3,2 (Ba,Pb)TiO3,3 and PbTiO3.4,5 However, the PTCR effect was also noted in Pb-free ceramics such as K0.5Bi0.5TiO36 and in KNbO3.7–10 KNbO3 is a ferroelectric material with perovskite structure and the same phase symmetries and same sequence of structural phase transitions as BaTiO3. With increasing temperature, the sequence of the ferroelectric modifications—rhombohedral, orthorhombic, and tetragonal—is continued by the paraelectric cubic modification. The differences between the two perovskites are the structural-phase-transition temperatures: the TC of BaTiO3 is 128 °C, whereas the TC of KNbO3 is 415 °C. Because of its high TC, KNbO3 is a potential candidate for the preparation of a high-temperature PTCR material. In spite of the fact that the PTCR effect in KNbO3-based ceramics has been known for 20 years,7–10 J. Mater. Res., Vol. 17, No. 12, Dec 2002
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the applications of this material are limited because it is difficult to prepare KNbO3 ceramics with a low roomtemperature resistivity. Er and Ishida used the doping of KNbO3 ceramics with 0.2 mol% W-donors to prepare PTCR ceramics with a low room-temperature resistivity.10 Here we used the same strategy of donor doping to design a semiconducting KNbO3 PTCR ceramic with a low room-temperature resistance. Pb2+ in the form of PbO was chosen as the donor dopant because it could be easily
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