Abstract
The positive temperature coefficient (PTC) of resistivity characteristics of donor-doped BaTiO3 fired at 1350 °C in a reducing atmosphere of 1×10-7 MPa at the partial pressure of oxygen is investigated. The result reveals that BaTiO3 ceramics fired in such a reducing atmosphere usually exhibit low PTC characteristics, whereas La-doped BaTiO3 with compositions in the range of (Ba+La)/Ti≧1.02 exhibit pronounced PTC characteristics. A La content of less than 0.4 mol % is suitable for the PTC characteristics, while samples doped with more than 0.8 mol % La do not show the PTC characteristics. The PTC characteristics of semiconducting BaTiO3 fired in a reducing atmosphere are enhanced with a decrease in the ion radius of a donor ion in the order La<Nd<Sm, therefore, semiconducting BaTiO3 doped with Sm shows pronounced PTC characteristics with a resistivity increase of more than 6 orders of magnitude. However, the resistivity of BaTiO3 ceramics doped with Y, whose ion radius is less than that of Sm, is extremely high resistivity that decreases with increasing temperature, thereby indicating only a negative temperature coefficient (NTC) characteristics.
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