IMPROVED CURRENT LIMITERS BASED ON MIXED-PHASE BATIO3 CERAMIC SEMICONDUCTORS

Very stable and highly reproducible current-limiting characteristics have been observed for polycrystalline ceramics prepared from sintering mixtures of coarse-grained, donor-doped BaTiO3 (tetragonal) as the major phase and ultrafine, undoped cubic perovskite such as BaSnO3, BaZrO3, SrTiO3 or BaTiO3 (cubic) as the minor phase. The initial linear current-voltage (I-V) relationship becomes current-limiting with increase in applied potential and the consequent onset of thermal equilibrium. The strong current maximum of the I-V curve of donor-doped BaTiO3 can be eliminated when the ceramics are constituted of mixed phases. The voltage drop at the insulating grain boundaries minimizes the temperature gradient between the interior and the surface, and subdues the thermal runaway. The magnitude of the limiting current, and hence the power-handling capacity, can be varied with the controlled addition of grain boundary layer modifiers and by optimizing the processing parameters. The dielectric constant versus temperature or voltage variation in power dissipation with ambient temperature and resistivity-temperature relations point to the necessity of the mixed phase character for the current-limiting property.