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 BaTiO
₃
have been studied. The specimen was sintered in a reducing atmosphere (Po
₂
<10
^-9
atm) at 1350°C, followed by annealing processes at 1000°C and Po
₂
=1 atm. The lattice constants decreased with the addition of Sr (≤15 mol%) or/and Ca (10 mol%). The substitution of Sr
²⁺
for Ba sites in (Ba
_0.992-x
Sr
_x
Ho
_0.008
)
_0.997
TiO
₃
shifted Curie point (T
_c
) to lower temperatures by 3°C/ mol% of SrO at x≤ 0.15. The substitution of 10 mol% CaO for BaO in (Ba
_0.992-x
Sr
_x
Ho
_0.008)0.997
TiO
₃
caused a negligibly small shift in Tc and enhanced the dielectric constants at Tc. Higher room-temperature resistivities (ϱ25) and larger temperature coefficients (α) were observed in (Ba
_0.992-x
Sr
_x
Ho
_0.008)0.997
TiO
₃
. However, the addition of CaO effectively improved the microstructure of (Ba
_0.992-x
Sr
_x
Ho
_0.008)0.997
TiO
₃
and lowered the room-temperature resistivity (ϱ25) in the (Ba
_0.892-x
Ca
_0.1
Sr
_x
Ho
_0.008)0.997
TiO
₃
systems.