Changes of PTCR properties during annealing in reducing gases in Nb(W)-Mn-codoped BaTiO3 ceramics

The degradation behaviors of Nb(W)-doped BaTiO3 and Nb(W)-Mn-codoped positive temperature coefficient of resistivity (PTCR) BaTiO3 during annealing in reducing gases were studied by measuring their electrical properties, diffuse reflectance and ESR spectroscopies. The decrease of PTCR effects during the annealing is consistent with the decrease of the grain-boundary resistivity and density of surface acceptor states, and the increase of conduction electrons. The results also indicated that durability to reducing-atmosphere of Nb (W)-Mn-codoped PTCR BaTiO3 was enhanced by adding Mn after calcination of the Nb (W)-doped BaTiO3 powders. It is assumed that Mn ions concentrated in the grain boundaries contribute to the enhancement of the durability to reducing-atmosphere by stabilizing the chemisorbed oxygens through Nb5+(W6+)-Mn pairs or forming high valence Mn(Mn3+, Mn4+) which trap electrons released from the chemisorbed oxygens. Thus Mn ions in grain boundaries suppress the lowering of the potential barrier during annealing in reducing atmosphere.