Study on the Effects of (Bi1/2Na1/2)TiO3 Content on the Curie Temperature of (Bi1/2Na1/2)TiO3-BaTiO3 PTCR Ceramics

Perovskite-structured (Bi1/2Na1/2)TiO3 (BNT) ferroelectric with Curie temperature about 320 degrees C is considered to be a good candidate of high temperature lead-free materials with a positive temperature coefficient of resistance (PTCR). In this study, lead-free PTCR ceramics with compositions of (1-x)BaTiO3-x(Bi1/2Na1/2)TiO3 (BT-BNT x=0.5-55mol%) were successfully prepared without any additional donor or acceptor dopants. The effects of BNT content on the Curie temperature T-c and the PTC effect of the ceramic materials were investigated. The X-ray diffraction data indicated that BT phase and BNT phase formed a solid solution during sintering even though the x value was up to 55mol%. The Curie temperature T-c of the samples increased from similar to 130 degrees C to similar to 216 degrees C with the increase of the BNT amount (x value) from 0.5mol% to 40mol%. However, it decreased when excess BNT was added (x value increased from 40mol% up to 55mol%), which was resulted from the volatile effects of Bi3+ and Na+ ions during sintering. The sample of 0.6BaTiO(3)- 0.4(Bi1/2Na1/2)TiO3 with T-c =216 degrees C, room-temperature resistivity 104 Omega.cm and R-max/R-min= 10(2.7) was obtained. There is a relationship between the Curie temperature and the cell volume of the perovskite structured lattice.