Advanced Materials Research Vol. 624

Abstract: The perovskite La_1-xSr_xMnO₃ (LSMO) nanocrystalline compounds with the doping concentration x in the range of 0.2~0.5 have been synthesized by combustion method using the pair of citric acid and ethylene glycol as a burning fuel. It was found that the sintering temperatures at which the compounds began forming the single-phase of LSMO increased from 900 °C up to 1200 °C corresponding to the increasing of Sr-doping content. X-ray pattern investigations presented the crystalline sizes of all samples sintered at 1200 °C evidently reduced from 77.7 nm to 18.9 nm as the doping concentration x increases. The effects of Sr-doping levels on Mn⁴⁺/Mn³⁺ ratio as well as on their electrical resistivity are also studied by X-ray photoelectron spectroscopy and four-point probes method, respectively. By using a resistive approximation model, the role of the Sr-doping with respect to the phase-separation between ferromagnetic and paramagnetic phases with respect to colossal magnetoresistance effect of these compounds is discussed.

Abstract: In this paper, β-eucryptite glass ceramics were synthesized by using solid reaction method. Phase constitution, structure and properties of the material were studied by X-ray diffraction (XRD) and differential thermal analysis (DTA). Furthermore, the effects of heat treatment temperature and preservation time on the thermal expansion coefficient were also analyzed. The results showed that the crystallization temperature of β-eucryptite glass ceramics was in the range of 810-860 °C and the content was more than 90%. With the increase of heat treatment temperature, the material expansion coefficient decreased.

Abstract: The electrocaloric effect (ECE) of barium titanate ceramics with hydrothermal synthesized nano-sized starting powders was investigated. The Curie point and latent heat of the phase transition were characterized by a differential scanning calorimeter (DSC). The P-E hysteresis loops were measured in the temperature range from 30-160oC and ECE adiabatic temperature change was obtained according to the Maxwell relation. A double hysteresis loop is observed at the temperature slightly above the Curie point, which indicates a good consistency of phase transition similar to single crystal. It is also proved by the sharp endothermic peak of phase transition in DSC curves. It exhibits a high ECE of ΔTmax=1.3K under a low applied field of 10kV/cm, which is much higher than that of the sample using micron-sized starting powders.

Abstract: The possibility of improving microwave absorption ability for magnetic slabs by adding conductive grains was analyzed within the theory of effective medium. It was found that pure magnetic slabs cannot mediate the contradiction between high absorption ability and high reflection property. Gradient magnetic structure cannot improve the absorption either. Adding conductive grains, however, can simultaneously improve the matching effect and absorption ability of slabs. This result means some methods for fabricating high absorption slabs.

Abstract: In this paper, firstly, the BIT powder was synthesized by ball grinding and the sequent calcinations, starting from TiO2 and Bi2O3. Typically, the grinding technique was facile to form the smaller particles, and the sintering process was utilized to obtain the pure BIT phase. The influence of heated temperature, time and the content of doped rare earth metal (La, Y) on BIT matrix have been investigated in details. The procedure consisted of being pressed, sintered, silver coated to obtain the BIT ferroelectric blocks. Lastly, the BIT density and ferroelectric properties of obtained blocks have been studied. The results showed that via the milling process and the addition of La and Y elements, BIT could be shaped at the lower temperature. Meanwhile, the ferroelectric properties have been improved.

Abstract: The Ferroelectric properties and Microstructures of Bi_3.3Tb_0.6Ti₃O₁₂ (BTT) and Bi_3.3Tb_0.6Ti_2.97V_0.03O₁₂ (BTTV) ceramics prepared at 1100°C by a conventional ceramic technique were investigated. These ceramics possess random-oriented polycrystalline structure. The remanent polarization (Pr) and coercive field (Ec) of the BTT ceramics are 23µC/cm2 and 80kV/cm, respectively. Furthermore, V substitution improves the Pr value of the BTTV ceramics up to 35μC/cm², which is larger than that of the BTT ceramics. Therefore, co-sustitution of Tb and V in Bi₄Ti₃O₁₂ (BIT) ceramic is effective for the improvement of its ferroelectricity.

Abstract: The electrical properties of Sm-doped bismuth titanate，Bi_4-xSm_xTi₃O₁₂ (BST) ceramics prepared by a conventional electroceramic technique were investigated. XRD analyses revealed Bi-layered perovskite structure in all samples. The P-V hysteresis loops of samples with x=0.4 and 1.2 were characterized by large leakage current, whereas for samples with x=0.6 and 0.8 the P-E hysteresis loops were the saturated and undistorted hysteresis loops. The remanent polarization ( Pr ) and coercive field (Ec) of the BST ceramic with x=0.8 were above 20μC/cm² and 60KV/cm , respectively.

Abstract: The ferroelectricity of Bi_3.2Er_0.8Ti₃O₁₂ (BET), and Bi_3.2Er_0.8Ti_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 (Pr) and coercive field (Ec) of the BET ceramics are 18 µC/cm² and 64kV/cm, respectively. Furthermore, V substitution improves the Pr value of the BETV ceramics up to 30 μC/cm², which is much larger than that of the BET ceramics. Therefore, co-sustitution of Er and V in Bi₄Ti₃O₁₂ (BIT) ceramic is effective for the improvement of its ferroelectricity.

Abstract: The electrical properties of Er₂O₃-doped bismuth titanate，Bi_4-xEr_xTi₃O₁₂ (BET) ceramics prepared by a conventional electroceramic technique were investigated. XRD analyses revealed Bi-layered perovskite structure in all samples. SEM micrographs showed randomly oriented and plate-like morphology. For the samples with x=0.4 and 1.0 the current-voltage characteristics exhibited negative differential resistance behaviors and their P-E hysteresis loops were characterized by large leakage current, whereas for the samples with x=0.6 and 0.8 the current-voltage characteristics showed simple ohmic behaviors and their P-E hysteresis loops were the saturated and undistorted hysteresis loops. The remanent polarization ( Pr ) and coercive field (Ec) of the BET ceramic with x=0.8 were above 20μC/cm² and 65KV/cm , respectively.

Abstract: Bi_2.9Pr_0.9Ti_2.97V_0.03O₁₂ (BPTV) ceramics 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. BPTV caused a large shift of the Curie temperature ( TC ) of Bi₄Ti₃O₁₂ (BIT) from 675°C to 385°C. The remanent polarization and the coercive field of the BLTV were 31μC/cm² and 60kV/cm at an electric field of 100kV/cm, respectively. Furthermore, the dielectric permittivity and dissipation factor were 300 and 3.3×10-3 at 1MHz, at 1V and at room temperature, respectively. These ferroelectric properties of BPTV 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 BPTV could be obtained by sintering at temperatures 100─200°C lower than those of the SrBi₂Ta₂O₉ system.