Effect of re-oxidation firing on PTCR properties of Sm-doped barium titanate ceramics

The effect of reduction and re-oxidation firing on the PTCR properties of Sm-doped barium titanate ceramics was investigated for the application in multilayered PTC thermistors. The lattice parameters a, and c decreases monotonically with increasing oxygen concentration in the re-oxidation atmosphere, which seems to be related with an electrostatic Coulomb interaction between an oxygen vacancy and the nearest other atoms. With an increase in the oxygen concentration, the resistivity at room temperature and the magnitude of the resistivity jump as a function of temperature increased in the region of oxygen concentrations of 0-10%. However, the resistivity at room temperature and the magnitude of the resistivity jump is nearly constant and saturated in the region of oxygen concentrations of 10-20%. This phenomenon is considered to be related with the variation of oxygen and barium-vacancy concentrations near grain boundaries.