Effects of CaO addition on PTCR properties and room temperature resistance of Ho-doped (Ba,Sr)TiO3

Effects of Ca and Sr addition on the microstructure and positive temperature coefficient of resistance (PTCR) of Ho-doped BaTiO3 have been studied. The specimen was sintered in a reducing atmosphere (Po2≪10⁻⁹ atm) at 1350°C, followed by annealing processes at 1000°C 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 3°C/ mol% of SrO at x≤ 0.15. The substitution of 10 mol% CaO for BaO in (Ba0.992−xSrx Ho0.008)0.997TiO3 caused a negligibly small shift in Tc and enhanced the dielectric constants at Tc. Higher room-temperature resistivities (б25) and larger temperature coefficients (α) were observed in (Ba0.992−xSrxHo0.008)0.997 TiO3. However, the addition of CaO effectively improved the microstructure of (Ba0.992−xSrxHo0.008)0.997TiO3 and lowered the room-temperature resistivity (б25) in the (Ba0.892−x Ca0.1SrxHo0.008)0.997TiO3 systems.