The positive temperature coefficient (PTC) of resistivity characteristics of donor-doped BaTiO 3 fired at 1350 C in a reducing atmosphere of 1 Â 10 À7 MPa at the partial pressure of oxygen is investigated. The result reveals that BaTiO 3 ceramics fired in such a reducing atmosphere usually exhibit low PTC characteristics, whereas La-doped BaTiO 3 with compositions in the range of ðBa þ LaÞ=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 BaTiO 3 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 BaTiO 3 doped with Sm shows pronounced PTC characteristics with a resistivity increase of more than 6 orders of magnitude. However, the resistivity of BaTiO 3 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.
The positive temperature coefficient of resistivity (PTCR) characteristics of donor-doped BaTiO 3 fired in a reducing atmosphere and reoxidized in air are investigated. The result reveals that conventional semiconducting BaTiO 3 ceramics fired in a reducing atmosphere and reoxidized at a low temperature of 8001C in air show minimal PTCR characteristics, as reported earlier; however, Ca-doped BaTiO 3 with compositions in the range of 1.005r(Ba1Ca1La)/Tir1.010 exhibit pronounced PTCR characteristics, even when reoxidized at such a low temperature. The semiconducting BaTiO 3 ceramics with {(Ba1 Ca1La)/Ti} 5 1.005 and Ca-doped to 20 mol% exhibit remarkable PTCR characteristics with a resistivity jump of two orders of magnitude when they have been reoxidized at 8001C after firing in a reducing atmosphere.C. Randall-contributing editor
A multilayer semiconducting BaTiO 3 ceramics having positive temperature coefficient of resistivity (PTCR) characteristics is investigated with the aim of lowering its resistance. The multilayer BaTiO 3 ceramics was fired in reducing atmosphere and reoxidized at 700 C in air. The result reveals that the multilayer semiconducting BaTiO 3 ceramics with Ni inner electrodes shows high resistance even when the thickness is lowered; however, excessive donor doping markedly decreases the resistance of BaTiO 3 ceramics to a very low value. The donor in excessive amounts is considered to cancel the Ni acceptors occupying the Ti sites; these acceptors diffuse from the Ni inner electrode and increase resistivity. As a result, a multilayer BaTiO 3 ceramics element (3:2 Â 2:5 mm 2 ) with a low resistance of 0.05 is realized by using excess donor doping (Sm ¼ 0:35 mol %).
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