Study On Preparation, Properties And Mechanism Of BaTiO₃/BaBiO₃ Thermal Sensitive Composite Materials

According to the latest development and shortcoming of thermosensitive materials, the effects of the BaBiO₃ content in BaTiO₃/BaBiO₃ composites on the phase composition, microstructure, electrical properties and conducting mechanisms of composites were investigated by using the differential thermal analysis (DTA), X-ray diffraction (XRD) , scanning electron microscope (SEM) ,energy spectrum analysis and a electrical property test apparatus. The semiconducting mechanisms of BaTiO₃-based ceramics with low BaBiO₃ content and conducting mechanisms of BaTiO₃/BaBiO₃ composites with high BaBiO₃ content were put forward respectively in this paper. The microstructure and electrical properties of BaTiO₃-based PTCR ceramics with donor dopant BaBiO₃, acceptor dopant Mn(NO₃)₂ and sintering aids AST phase were systemically investigated. And the microstructure and electrical properties of BaTiO₃/BaBiO₃ NTC composites doped with rare earth oxide were systemically investigated. The results showed as the following:The effect of the BaBiO₃ content on the electrical properties of BaTiO₃/BaBiO₃ composites was systemically investigated for the first time. BaTiO₃-based ceramics showed good semiconducting effect doped with low BaBiO₃ content after sintering at 1300?. With an increase in BaBiO₃ content the room temperature resistivity for BaTiO₃-based ceramics decreased to a minimal value and then began to increase, and the minimal value was about 5?·cm. BaTiO₃-based ceramics doped with BaBiO₃ showed PTC effect. However BaTiO₃/BaBiO₃ composites only showed negative temperature coefficient (NTC) effect with high BaBiO₃ content sintering at high temperature. With an increase in BaBiO₃ content the room temperature resistivity for BaTiO₃/BaBiO₃ composites decreased and thermal sensitivity coefficient for that remained constant.The semiconducting mechanisms of BaTiO₃-based ceramics with low BaBiO₃ content were put forward for the first time. BaBiO₃ contains bismuth ions of different oxidation states (+3 and +5). As the elements for achieving semiconducting grains, Bi³⁺ substitute for Ba ions in the lattices and Bi⁵⁺ substituted for Ti ions. With an increase in BaBiO₃ content the room temperature resistivity for BaTiO₃-based ceramics decreased and an electric compensation had in effect in this process. However with an further increase in BaBiO₃ content the room temperature resistivity for BaTiO₃-based ceramics began to increased and an vacancy compensation had in effect in this process.The NTC effect mechanisms of BaTiO₃/BaBiO₃ composites with high BaBiO₃ content were put forward for the first time. And the percolation model for BaTiO₃/BaBiO₃ composites was built. BaTiO₃/BaBiO₃ composites showing NTC effect is attributed to the fact that BaBiO₃ is semiconducting materials. BaTiO₃/BaBiO₃ composites were mixtures of BaTiO₃ and BaBiO₃ in extreme disorder. BaBiO₃ is dispersed network phase in BaTiO₃/BaBiO₃ composites with low BaBiO₃ content. The resistivity for BaTiO₃/BaBiO₃ composites approached to the value of insulator. With an increase in BaBiO₃ content BaBiO₃ phases connect to each other to lead to low resistivity for BaTiO₃/BaBiO₃ composites.One kind of dopant method was put forward for semiconducting BaTiO₃-based PTCR ceramics for the first time, namely donor dopants can supply two kinds of different valence state (+3 and +5) ions whose radius are equal to the radius of Ba²⁺ and Ti⁴⁺ respectively. In the same technological condition semiconducting degree for BaTiO₃-based ceramics was checked on base of same Bi content in Bi₂O₃ and BaBiO₃ dopants. And the results showed that the room temperature resistivity for BaTiO₃-based ceramics doped with BaBiO₃ was about half as big as that doped with Bi₂O₃.The effect of different donor dopants, acceptor dopants, sintering aids and sintering technology on the microstructure and electrical properties of BaTiO₃-based PTCR ceramics was systemically investigated. As a donor dopant, BaBiO₃, Ba₂LaBiO₆ and BaNb₂Bi₂O₉ can supply two different kinds of cations in valence which substituted for Ba²⁺ and Ti⁴⁺ respectively. And doped- ceramics showed good electrical properties. The effect of content of them on the microstructure and electrical properties of ceramics were similar. And with an increase in donor dopants mean grains of ceramics decreased in size, while the room temperature of ceramics decreased to minimal value and then began to increase.The matter between grains consisted of acceptor and AST phase. Given the content of BaBiO₃ and AST phase in ceramics and the sintering technology, with an increase in Mn(NO₃)₂ content coarse and fine grains are even distribution and the area of fine grains trended to larger. While the lift-drag ratio of ceramics quickly increase and then remained constant with an increase in Mn(NO₃)₂ content. Given the content of BaBiO₃ and Mn(NO₃)₂ in ceramics and the sintering technology, with an increase in AST phase the mean grains of ceramics increased in size and the room temperature resistivity of ceramics monotonously increased.The electrical properties of BaTiO₃-based PTCR ceramics are sensitive to sintering technology. Given the technological conditions, with an increase in sintering temperature the room temperature resistivity of ceramics decreased to minimal value and then began to increase and the mean grains of ceramics increased in size. With an increase in soaking time and cooling rate the room temperature resistivity of ceramics decreased to minimal value and then began to increase and the microstructure of ceramics was not sensitive to the soaking time and cooling rate.The effect of rare earth oxides on the phase composition, microstructure and electrical properties for BaTiO₃/BaBiO₃ composites was investigated for the first time. The mechanism of action for rare earth oxides in BaTiO₃/BaBiO₃ composites was studied. BaTiO₃/BaBiO₃ composites doped with Y₂O₃, CeO₂ and La₂O₃ respectively was sintered at high temperature, rare earth cations were soluted into BaBiO₃ phases. When sintering temperature was very high, the mean grains of composites decreased to minimal value and then began to increase with an increase in Y₂O₃ content. However when sintering temperature was low, the microstructure of composites consisted of particles which remained unchanged in size with an increase in CeO₂ or La₂O₃ content. Y₂O₃ and La₂O₃ are trivalent oxides and the effect of them on the electrical properties of composites is similar that the room temperature resistivity of composites decreased to minimal value and began to increase with an increase in rare earth oxides content. CeO₂ is tetravalent oxide and the room temperature resistivity of composites increased with an increase in CeO₂ content.The effect of Nb⁵⁺, La³⁺ and K⁺ ions on the crystal structure and electrical properties for BaBiO₃ compounds was systemically investigated for the first time respectively. The mechanism of action for dopant ions in BaBiO₃ compounds was studied. With an increase in dopant ions the lattice parameters and unit cell volume of BaBiO₃ solid solution doped with Nb⁵⁺, La³⁺ and K⁺ ions regularly changed. The effect of Nb⁵⁺, La³⁺ and K⁺ ions on the electrical properties of BaBiO₃ solid solution was not similar respectively. With an increase in Nb⁵⁺ content the room temperature resistivity of BaBiO₃ solid solution increased. However With an increase in La³⁺ or K⁺ content the room temperature resistivity of BaBiO₃ solid solution decreased to minimal value and then began to increase.