Studies On The BaTiO₃-based PTC Composites

In order to prepare the lead-free PTC ceramics with high Curie Tempreture(TC)and low room temperature resistivity, some BaTiO₃-based PTC composites werestudied in this paper. The properties and influence factor of this composite ceramicswere investigated through many test methods such asρ-t curve measuring,ε-t curvemeasuring, XRD, SEM and so on. The research achievements were as follows:The TC of PTC ceramics could be increased by compounding NBT with high TCinto the BaTiO₃-based PTC composites. When the content of NBT amounted to1.0mol%, the TC of PTC ceramics increased 30°C, and when the content of NBT wasup to 1.5mol%, the TC increased about 50°C, the reason was that introduce of NBTstrengthened Ti-O bond and then enhanced the tetragonality of the ceramic crystallattice. At the same time, the PTC effects were also meliorated which attributed tovast electron capture centers formed by Bi2O₃ acting with MnO and grain growthrestrained by Bi2O₃ volatilized from NBT: The magnitude had been improved greatlyfrom 104to106, and the resistivity temperature factor increased from 8.1%°C-1 to21.9%°C-1. When the content of NBT amounted to 8.0mol%, the Curie point increasedfrom 30°C to 230°C, so it was possible to prepare the lead-free BaTiO₃-based PTCceramics with high TC. However, it was quite difficult to form semiconductingmaterials with the increase of NBT content.Compounding NBT into the BaTiO₃-based PTC composites increased the roomtemperature resistivity of samples. In order to decrease the room temperatureresistance, an amount of Ni powder was added into BaTiO₃-based PTC compositeswith 1.0mol% NBT. The composites with low room temperature resistivity(15.56Ω·cm) and resistance jump (169.82) were obtained by adding 12wt% Nisintered in reducing atmosphere, then heat-treated at 900°C in weak reducingatmosphere with some NBT for 50min.It was found that adding copper powder into the pre-synthesized BaTiO₃ powdercould impactfully reduce the room temperature resistance of composites. Theoptimize content of copper powder was from 2mol% to 4mol% which was about tenthof the content of Ni powder. The Cu/BaTiO₃ PTC composites with low roomtemperature resistivity (101.12Ω·cm ) and resistance jump (102.58 ) were obtained when the amount of added Cu was 4mol%, and the samples were sintered at 1275°C for20min in reducing atmosphere, then heat-treated at 800°C for 60min in weak reducingatmosphere.The results showed that adding CuO and SiO2 into the BaTiO3-based PTCceramics could increase TC, It illustrated that dopping on B site could also strengthenTi-O bond and enhance the phase-transition temperature, but the quantity of thedopant was limited.The influences of different donor dopants at A site on the properties of PTCceramics were investigated. Compared with Sb2O3 and Y2O3, Bi2O3 as A site donormade the materials have the maximum temperature coefficient 46.42%oC-1. When theheating rate was 200oC/h which was slow, the room temperature resistance of samplesincreased in some short, but the PTC effects were improved obviously. At the sametime, the TC of samples shifted to the high temperature to some certain extent.