Papers by Author: Y.H. Sun

Abstract: The electrical properties of Dy-bismuth titanate (Bi_4-xDy_xTi₃O₁₂) prepared by a conventional ceramic technique have been investigated. At applied d.c. field below 200V/mm, the current-voltage curve of Dy-doped sample exhibits a simple ohmic behavior. The impedance spectrum of Dy-doped sample indicates that consist of semiconducting grain and moderately insulating grain boundary regions. XRD, SEM and EPMA analyses reveal crystalline phase characterized by a Bi-layered perovskite structure of Bi₄Ti₃O₁₂ and the distribution of every element is uniform. Dy-doped sample exhibit randomly oriented and plate-like morphology.

Abstract: La-doped bismuth titanate (Bi_4-xLa_xTi₃O₁₂: BLT) and pure Bi₄Ti₃O₁₂ (BIT) thin films with random orientation were fabricated on Pt/Ti/SiO₂/Si substrates by rf magnetron sputtering technique. These samples had polycrystalline Bi-layered perovskite structure without preferred orientation, and consisted of well developed rod-like grains with random orientation. La-doping into BIT caused a large shift of the Curie temperature ( T_C ) from 675°C to lower temperature and a improvement in dielectric property. The experimental results indicated that La doping into BIT also result in a remarkable improvement in ferroelectric property. The P_r and the E_c values of the BLT film with x=0.75 were 20 μC/cm² and 82kV/cm, respectively.

Abstract: The electrical properties of Gd-doped bismuth titanates Bi_4-xGd_xTi₃O₁₂) prepared by a conventional ceramic technique have been investigated. At applied d.c. field below 200V/mm, the current-voltage curve of Gd-doped sample exhibits a negative differential resistance behavior. The conducting filamentary model has been used to explain the negative differential resistance phenomenon in Gd-doped bismuth titanates. The impedance spectrum of Gd sample indicates that both consist of semiconducting grain and moderately insulating grain boundary regions. XRD, SEM and EPMA analyses reveal crystalline phase characterized by a Bi-layered perovskite structure of Bi₄Ti₃O₁₂ and the distribution of every element is uniform. Gd-doped sample exhibites randomly oriented and plate-like morphology.

Abstract: Bi_3.25La_0.75Ti_2.97V_0.03O₁₂ (BLTV) ceramic were prepared by solid state reaction. These samples had polycrystalline Bi-layered perovskite structure without preferred orientation, and consisted of well developed plate-like grains with random orientation. BLTV caused a large shift of the Curie temperature ( T_C ) of Bi₄Ti₃O₁₂ (BIT) from 675°C to 395°C. The remanent polarization and the coercive field of the BLTV were 31μC/cm² and 54kV/cm at an electric field of 90kV/cm, respectively. Furthermore, the dielectric permittivity and dissipation factor were 300 and 3×10^-3 at 1MHz, at 1V and at room temperature, respectively. These ferroelectric properties of BLTV are superior to V-doped Bi₄Ti₃O₁₂ (~20μC/cm² and 80kV/cm) and (Sr, Ta)-doped Bi₄Ti₃O₁₂ (~12μC/cm² and 71kV/cm) ceramics. In addition, the dense ceramics of BLTV could be obtained by sintering at temperatures 100─200°C lower than those of the SrBi₂Ta₂O₉ system.

Abstract: The ferroelectricity of Bi_3.25Dy_0.75Ti₃O₁₂ (BDT), and Bi_3.25Dy_0.75Ti_2.97V_0.03O₁₂ (BDTV) ceramics prepared at 1100°C by a conventional ceramic technique was investigated. These ceramics possess random-oriented polycrystalline structure. The remanent polarization (P_r) and coercive field (E_c) of the BDT ceramics are 15 µC/cm² and 64kV/cm, respectively. Furthermore, V substitution improves the P_r value of the BDT ceramics up to 23 μC/cm², which is much larger than that of the BDT ceramics. Therefore, co-sustitution of D and V in Bi₄Ti₃O₁₂ (BIT) ceramic is effective for the improvement of its ferroelectricity.

Abstract: The electrical properties of La-doped bismuth titanate, Bi_4-xLa_xTi₃O₁₂ (BLT) ceramics prepared by a conventional electroceramic technique have been investigated. XRD analyses revealed Bi-layered perovskite structure in all samples, and indicted that Bi ions were only substituted near the Ti-O octahedron layers by La ions. SEM micrographs show randomly oriented and plate-like morphology. The remanent polarization ( P_r ) and coercive field ( E_c ) of the BLT ceramic with x=0.75 were above 19μC/cm² and 60KV/cm, respectively. The large value of remanent polarization and low coercive field of La-doped bismuth titanate ceramics promote these materials to potential applications.

Abstract: Bi₂WTi₃O₁₂ ceramics are fabricated by conventional solid-state reaction process. XRD analysis reveals that Bi₂WO₆ is the main phase and Bi₄Ti₃O₁₂ is the second phase. With increasing temperature the sample first appears metallic behavior, then strong electrical fluctuations above 100°C, and finally exhibits stable nonlinear properties characterized by semiconductivity above 300°C at low field (E ≤ 100V/mm). The Arrhenius law for electrical conductivity by thermal activation is not suitable to explain the anomalous results. Based on the phase transition of tungsten trioxide from room temperature to about 300°C, the electrical properties of Bi₂WTi₃O₁₂ ceramics can be explained.

Abstract: Ferroelectric Bi₄Ti₃O₁₂ ceramics are fabricated by conventional solid-state reaction process. The current-voltage characteristic of Bi₄Ti₃O₁₂ sample exhibits a voltage-controlled negative differential resistance behavior at low field (E≤100V/mm), and an obvious PTC effect appears at around 100°C on the resistance-temperature curve. Based on conducting filament model about electrical transport, instead of Heywang-Jonker model, the experimental results of Bi₄Ti₃O₁₂ ceramics are suitably explained.

Abstract: The ferroelectricity of Bi_3.25Eu_0.75Ti₃O₁₂ (BET), and Bi_3.25Eu_0.75Ti_2.97V_0.03O₁₂ (BETV) ceramics prepared at 1100°C by a conventional ceramic technique was investigated. These ceramics possess random-oriented polycrystalline structure. The remanent polarization (P_r) and coercive field (E_c) of the BET ceramics are 16 µC/cm² and 62kV/cm, respectively. Furthermore, V substitution improves the P_r value of the BET ceramics up to 25 μC/cm² which is larger than that of the BET ceramics. Therefore, co-sustitution of Eu and V in Bi₄Ti₃O12 (BIT) ceramic is effective for the improvement of its ferroelectricity.

Abstract: The electrical properties of Dy-doped bismuth titanate, Bi_4-xDy_xTi₃O₁₂ (BDT) ceramics prepared by a conventional electroceramic technique have been investigated. XRD analyses revealed Bi-layered perovskite structure in all samples, and indicted that Bi ions were only substituted near the Ti-O octahedron layers by Dy ions. SEM micrographs show randomly oriented and plate-like morphology. The remanent polarization (P_r) and coercive field (E_c) of the BDT ceramic with x = 0.75 were above 19μC/cm² and 50KV/cm, respectively. The large value of remanent polarization and low coercive field of Dy-doped bismuth titanate ceramics promote these materials to potential applications.