Kenjiro Mihara

Preparation of Multilayer Semiconducting BaTiO3 Ceramics Co-Fired with Ni Inner Electrodes

A multilayer semiconducting BaTiO3 ceramics having positive temperature coefficient of resistivity (PTCR) characteristics is investigated with the aim of lowering its resistance. The multilayer BaTiO3 ceramics was fired in reducing atmosphere and reoxidized at 700 °C in air. The result reveals that the multilayer semiconducting BaTiO3 ceramics with Ni inner electrodes shows high resistance even when the thickness is lowered; however, excessive donor doping markedly decreases the resistance of BaTiO3 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 BaTiO3 ceramics element (3.2×2.5 mm2) with a low resistance of 0.05 Ω is realized by using excess donor doping (Sm=0.35 mol %).

Effects of Ba/Ti Ratio on Positive Temperature Coefficient of Resistivity Characteristics of Donor-doped BaTiO3 Fired in Reducing Atmosphere

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)/Ti1.02 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.