Potential barriers in PTC barium titanate observed by EFM

Abstract

PTC thermistors based on doped barium titanate are ferroelectric materials that show commercial interest due to a large variation in electrical resistance with temperature. The PTC effect in ceramics based on semiconducting barium titanate is due to formation of electric potential barriers in grain boundaries, increasing the electrical resistance [1]. Barium titanate ceramics when doped shows an electric behavior, which consists of an abrupt increase of resistivity with the temperature, close to Curie temperature (Tc). This effect happens in barium titanate during the change of cubic to tetragonal phase when cooled to temperatures below 130 ◦C [2, 3]. The models more accepted attributes the PTC effect to the formation of potential barriers in grain boundaries, increasing the resistivity when temperatures above 130 ◦C are reached [4]. Being basically a grain boundaries effect the PTC behavior depends strongly on microstructure [2–5]. The electric properties of ceramic PTC can be observed through the nature of the grains boundaries, and the levels of energy involved in their process were not well explained [4, 5]. The formation of potential barriers on barium titanate doped with yttrium were investigated using electrostatic force microscopy (EFM) with electric field gradient applications and surface potential images shows the formation of potential along grain boundary at applied voltage of 10 V. Semiconducting barium titanate was obtained by mixing 0.3% of yttrium in the nitrate form with BaTiO3 powder (TAM ceramics). The mixing was made in aqueous solution and after drying, the powder was dispersed and pellets were obtained by compaction with a uniaxial press. Sintering was done at 1350 ◦C by 2 hr and grains were reveled by thermal attack at 1250 ◦C for 1 min. EFM images were obtained using an atomic force microscope (Nanoscope IIIa) operating in electrostatic force mode (EFM) using a silicon probe (NSC15). The images of surface potential were done with applications of 0, 4 and 10 V, in situ. Fig. 1a show the AFM topographic image of 0.3YBaTiO3 reveling microstructure where grains and grain boundaries are clearly defined. Fig. 1b show an EFM