Effects of CaO and SrO addition on PTCR properties and room temperature resistances of Ho-doped BaTiO3

Effects of Ca and Sr substitution on the microstructure, room temperature resistances and positive temperature coefficient of resistances (PTCR) of Ho-doped BaTiO3 have been studied. The specimen was sintered in a reducing atmosphere (Po2<10-9 atm) at 1350oC, followed by annealing processes at 1000oC and Po2=1 atm. The lattice constants decreased with the addition of Sr (_15 mol%) or/and Ca (10 mol%). The substitution of Sr2+ for Ba sites in (Ba0.992-xSrxHo0.008)0.997TiO3 shifted Curie point (Tc) to lower temperatures by 3oC/ mol% of SrO at x_ 0.15. The substitution of 10 mol% CaO for BaO in (Ba0.992-xSrxHo0.008)0.997TiO3 caused a negligibly small shift in Tc and enhanced the dielectric constants at Tc. Higher room-temperature resistivities (_25) and larger temperature coefficients (_) were observed in (Ba0.992-xSrxHo0.008)0.997TiO3. However, the addition of CaO effectively improved the microstructure of (Ba0.992-xSrxHo0.008)0.997TiO3 and lowered the room-temperature resistivity (_25) in the (Ba0.892-x Ca0.1SrxHo0.008)0.997TiO3 systems.