Key Engineering Materials Vol. 492

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.

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: 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: 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: Thermoelectric behavior and microstructure of carbon nanotubes/carbon fiber(CNTs/CF)- cement based composite have been measured in this study. An self-made experimental setup was applied to test the thermoelectric power (TEP) of the composites. The results show that the higher the CNTs content, the less positive the absolute thermoelectric power is. When CNTs addition incresed to 1.0% by weight of cement, the absolute thermoelectric power changed sign from positive to negative. Scanning electron microscopy (SEM) was used to characterize the morphology of CNTs, CF and the structure of Portland cement-CNTs-CF systems. SEM analysis of the results show that good interfacial adhesion between CNTs and cement matrix is seen with CNTs tightly wrapped by Calcium-Silicate-Hydrate (C-S-H). With the incorporation of CNTs/CF in cement based composite, the cement-CNTs-CF system exhibits a porous microstructure.

Abstract: Multi-walled carbon nanotubes (MWCNT) attached on ZnO quantum dots named ZnO/MWCNT composites were synthesized by a multi-steps wet chemical self-assembling technique. The morphology of ZnO/MWCNT was also studied in details. The as-prepared ZnO and ZnO/ MWCNT composites had been characterized by XRD, SEM and TEM. It was found that the co-existence of MWCNTs and ZnO in ZnO/MWCNT sample. Monodispersed ZnO nanoparticles and the MWNTs/ZnO conjunction structure can be vividly observed from TEM image. TEM image also indicated that the decoration of MWCNTs did not cause any damage to the tubular shape and the conjunction occurred at the outershell of MWCNTs. Moreover, to assess the optoelectronic properties, photoluminescence (PL) spectra were also compared between monodispersed ZnO QDs and the ZnO /MWCNT composites. The relationship between PL spectra and structure was discussed. It was shown that the particular PL behavior of ZnO in the ZnO/MWCNT had been quenched, which meant the electron transformation between the ZnO and MWCNT had happened.

Abstract: Grain growth in Ga₂O₃ and MnO co-doped ZnO was investigated for sintering from 950° to 1250°C in air. Microstructural observation revealed that the samples sintered at lower temperatures consist of uniform equiaxed grains while the samples sintered at higher temperatures consist of plat-like grains, implying that the grain growth mechanism for the examined ZnO ceramics changes when the sintering temperature increases above about 1150°C. The traditional kinetic grain growth equation was employed to analyze the variation of grain size with sintering temperature and sintering holding time. It was shown that the grain growth exponent, n, increases from 2.17 for samples with uniform equiaxed grain structure to 4.30 for samples with plate like grain structure, while the apparent activation energy, Q, increases from 237 kJ/mol for low-temperature-sintered sample to 405 kJ/mol for high-temperature-sintered samples. The increases in both n and Q were mainly attributed to the difference between the grain morphologies in low- and high-temperature ranges. The underestimation of the sizes of the plate-like grains was also considered to be another important origin for the higher values of n and Q for the high-temperature-sintered samples.

Abstract: In this paper, super fine Zn₂SnO₄ powder was synthesized by mechanochemical method starting from ZnO and SnO₂. The structure, size and morphology of Zn₂SnO₄ were explored with X-ray diffraction (XRD), scanning electron microscopy (SEM) and nitrogen sorption analysis with the Brunauer-Emmett-Teller (BET) method. The result showed that pure spinel Zn₂SnO₄ powder was prepared after 2h grinding with 3.0KW of power and the particle size was about 200 nm. The degradation experiment of organic dye was performed by the photocatalytic methods on the light of wavelength (= 365nm). The powder can degrade Alizarin red solution close to 90% under U-V light with a 9W high.

Abstract: The hollow ceramic microspheres were prepared by Self-reactive flame spraying method. The structural characteristics and morphology properties of the composite powders were obtained by XRD and SEM. The results show that the obtained particles are hollow ceramic microspheres. The average particle size is about 30μm. An analytical method for the determination of electromagnetic parameters (ε, μ) of materials under test is presented by the analysis of normalized general matrix of equivalent network of coaxial line filled with samples. The analysis of vector network analyzer show that reflectance of hollow composite microspheres could well absorb microwaves in 2-18 GHz. The reflection loss is less than -l0dB in the range of 12.4 to15.2GHz while the minimum reflection loss is -18.5dB at 13.6GHz.

Abstract: α-MnO₂ nanoparticles with the average size about 22-29 nm were synthesized by hydro- thermal method. The as-synthesized samples were characterized by X-ray diffraction (XRD), Fourier transform-Infrared Rediation (FT-IR) and thermogravimetric-differential thermal analysis (TG- DTA). We find that the average particle size of α-MnO₂ is increased with the reaction temperature increasing. The diluent reactant is beneficial for the growth of α-MnO₂. The smaller the molar ratio of K/Mn is, the more stable α-MnO₂ phase is. The Neel temperature (TN) of nano-α-MnO₂ is 5.0K obtained by the magnetic susceptibility determination, lower then the value of 24.5K for MnO2 single crystal phase. The magnetic hysteresis loop can be observed obviously at T = 5.0 K. It indicated that when the particles are at the level of nano-size level, the anti-ferromagnetic MnO₂ showed the ferromagnetic nature at the lower temperature. When the temperature is high up to 60.0K, the magnetic hysteresis loop can't be observed. While when the temperature is at 120.0K, a straight line of magnetic hysteresis loop is obtained, indicating the paramagnetic magnetic nature of our sample.