Effect Of Sintering Aids On The Positive Temperature Coefficient Performance Of BaTiO₃/K_0.5Bi_0.5TiO₃ Ceramics

BaTiO3-based positive temperature coefficient resistivity (PTCR) ceramics are mainly applied to the aspects of electronic communication and household electric appliance. With the development of electronic devices tending to be lead-free and miniaturization, the performance of the PTC ceramics is required to have more excellent behaviors with low room temperature resisitivity, obvious resistance jump and high Curie temperature (TC). In this paper, the sintering aids (CuO, Na2Ti6O13 and SiO2) were added to the matrix of BaTiO3/K0.5Bi0.5TiO3 ceramics respectively. The ceramics were sintered in reducing atmosphere (N2, Ar) and then reoxidized in air. The effect of sintering aids on the phase composition, microstructure and PTC performances were investigated. In order to further improve the TC, the amount of Ko.sBio.sTiO3 was increased. The ceramics were firstly sintered in Hydrogen atmosphere, and then reoxidized in the atmosphere with different oxygen partial pressure. The effects of H2 atmosphere on the semiconducting properties and different oxygen partial pressure on the PTC properties were investigated. This paper is aimed at finding a new formulation and preparation process of lead-free BaTiO3-based PTCR ceramics.Firstly, the CuO added Bao.96(Bi0.5K0.5)0.04Ti03 ceramics were fabricated by the solid-state reaction technique, and the ceramics were sintered in nitrogen atmosphere. The results showed that when the content of CuO was in the range of 0.05%?x?0.20%, the grains were very uniform, well formed and the grain boundaries were clear cut. Meanwhile, the ceramics displayed a marked PTC effect. When x=0.10%, the ceramic possessed a relative low room-temperature resistivity about 3.1 ×103 ?·cm,TC=137.3 ? and lg(pmax/?min)=2.65. The reason was that the Cu2+ was partly reduced to Cu+ when sintered in nitrogen atmosphere, and they were all incorporated into the Bao.96(Bi0.5K0.5)0.04TiO3 lattice on the Ti-site as acceptors. When x?0.4%, the grain growth was promoted, the room temperature resistivity increased and all the ceramics showed a NTC effect.Secondly, the effect of Na2Ti6O13 addition on the PTC performance of Bao.94(Bi0.5K0.5)o.o6Ti03 ceramics was investigated. All the ceramics were sintered in argon atmosphere and then reoxidized in air. The results showed that the addition of Na2Ti6O13 could increase the Tc from 132.9 ? to 145.6 ?. With the increasing Na2Ti6O13 content, the ?R-T increased. The reason was that the Na-ions at the grain boundaries could form suface acceptor states. Through controlling the reoxidation time and reoxidation temperature, the PTC performance could be optimized. When x=0.15%, the obtained ceramic reoxidized at 1000 ? for 3 h possessed a relative low room temperature resistivity about 2.27×103 ?·cm, the TC=145.4 ?, and a obvious PTC effect of ?max/pmin=2.39.Thirdly, the effect of SiCh addition on the PTCR characteristics of Bao.92(Bi0.5K0.5)0.08TiO3ceramics was investigated. The results showed that when x?0.01, the secondary phases of Ba2TiSi2Os and BaSiO3 were found. EDS spectra revealed that they existed at intergranular regions. The influence of SiO2 on the Curie temperature was slight. The grain size did not change greatly for all samples, but the porosity deseased and the densification was enhanced. With the increasing SiO2 content, the room temperature resistivity increased. When x=0.01, the obtained ceramic possessed a room temperature resistivity about 1.23×104 ?·cm, the TC about 147.4 ? and a good PTC effect of pmax/pmin=3.40.In order to obtain higher 7c, the amount of K0.5Bi0.5TiO3 was increased. The ceramics were sintered in hydrogen atmosphere and then reoxidized in atmosphere with different oxygen partial pressure. The results showed that when x?40%, the mobile efficiency of the Curie temperature was 2.5 ?/mol%; When 40%? x? 60%, the Curie temperature had changed little; When x=S= 80%, the mobile efficiency was 8.25 ?/mol%. For the ceramics sintered in hydrogen atmosphere, when x? 60%, the room temperature resistivity ?RT? 2000 ?·cm; when x=70?80 mol%, the resistivity were 0.8?5×105 ?·cm. The ceramics with 60% K0.5Bi0.5TiO3 were reoxidized in atmosphere with different oxygen partial pressure. It was found that the ceramics had resistance jumps at Curie temperature, but the resistivity increased abruptly at about 290 ?, and the ceramics all changed from semiconducting to insulation.