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Zhang Y.,Wuhan University | Zhang Y.,Hubei Key Laboratory on Organic and Polymeric Opto electronic Materials | Yang Z.,Wuhan University | Yin D.,Wuhan University | And 7 more authors.
Journal of Magnetism and Magnetic Materials | Year: 2010

Cobalt ferrite nano-particles were prepared using the co-precipitation method followed by annealing treatment. The formation of nano-particles with different composition, microstructure and sizes were confirmed by X-ray diffraction, Raman, thermogravimetric-differential thermal analysis and transmission electron microscope. The magnetic hysteresis loops measured at room temperature revealed smaller effective magnetic anisotropy constant, coercivity and remanence ratio for the samples prepared by adding the NaOH solutions into the mixed solutions of Co2 and Fe3 ions due to the formation of Co3 ions. A small saturation magnetization and an enhanced coercivity were observed for the nano-particles prepared by adding the mixed solutions of Co2 and Fe3 ions into the NaOH solutions, which was related to the formation of outer layers with poor crystallization on the surfaces of the cobalt ferrite nano-crystals. Furthermore, the existence of these outer layers induced the oxidation of Co2 ions in cobalt ferrite nano-crystals at 200 and 300 °C, and led to a large change on the composition and magnetic properties. © 2010 Elsevier B.V. All rights reserved.


Lv H.Y.,Wuhan University | Liu H.J.,Wuhan University | Pan L.,Wuhan University | Wen Y.W.,Wuhan University | And 4 more authors.
Applied Physics Letters | Year: 2010

The electronic properties of (Sb0.75 Bi0.25) 2 Te3 compound are examined by using the full-potential linearized augmented plane-wave method. The transport coefficients are then calculated within the semiclassical Boltzmann theory, and further evaluated as a function of chemical potential assuming a rigid band picture and constant relaxation time. The ZT value is thus estimated by inserting an averaged thermal conductivity. Our theoretical calculations give a valuable insight on how to enhance the thermoelectric performance of this compound, and many potential doping elements and their optimal concentrations are suggested. © 2010 American Institute of Physics.


Yang Z.,Wuhan University | Zhang Y.,Huazhong University of Science and Technology | Zhang K.,Wuhan University | Yin D.,Wuhan University | And 5 more authors.
Journal of Materials Science: Materials in Electronics | Year: 2013

Ca1-x Rbx Cu3Ti4O12 (x = 0, 0.03, 0.05, 0.07 and 0.09) ceramics have been synthesized by the sol-gel method. The results of X-ray diffraction, scanning electron microscopy and energy dispersive X-ray analysis indicate that a small amount of CuO and the grain boundary layers with Cu-rich and Ti-poor compositions are formed in the Rb-doped ceramic samples. The curves of dc electrical current density versus electric field strength have been collected at different temperatures, and the electrical conduction behaviors at grain boundary have been studied in detail based on the grain-boundary Schottky model. The results show that the formation of the Cu-rich and Ti-poor grain boundaries greatly affects the properties of grain-boundary Schottky potential barrier. For example, the barrier width increases with the increase on doping concentration x, and the barrier height (ΦB) is also greatly enhanced as the concentration of dopants is very small, but as x is larger than 0.05, the ΦB values reduce. These results should be ascribed to the second phases at grain boundary and the change of electronic structure. © 2012 Springer Science+Business Media, LLC.


Fei C.,Wuhan University | Zhang Y.,Wuhan University | Zhang Y.,Hubei University of Education | Yang Z.,Wuhan University | And 6 more authors.
Journal of Magnetism and Magnetic Materials | Year: 2011

CoFe2O4/Fe3O4 nano-composite ceramics were synthesized by Spark Plasma Sintering. The X-ray diffraction patterns show that all samples are composed of CoFe2O4 and Fe3O4 phases when the sintering temperature is below 900 °C. It is found that the magnetic properties strongly depend on the sintering temperature. The two-step hysteresis loops for samples sintered below 500 °C are observed, but when sintering temperature reaches 500 °C, the step disappears, which indicates that the CoFe2O4 and Fe3O4 are well exchange coupled. As the sintering temperature increases from 500 to 800 °C, the results of X-ray diffractometer indicate the constriction of crystalline regions due to the ion diffusion at the interfaces of CoFe2O4/Fe 3O4 phases, which have great impact on the magnetic properties. © 2011 Elsevier B.V. All rights reserved.


Yang Z.,Wuhan University | Zhang Y.,Huazhong University of Science and Technology | Xiong R.,Wuhan University | Xiong R.,Hubei University | And 3 more authors.
Materials Research Bulletin | Year: 2013

CaCu3Ti4O12 (CCTO) ceramics were sintered in air and in oxygen, and the oxygen pressures are 1 atm and 3 atm, respectively. The results of dc electrical conduction and dielectric measurement show that for the CCTO ceramics sintered in oxygen, leakage, grain conductivity and the low-frequency dielectric constant are greatly reduced; these changes of electrical properties may be attributed to the enhancement of activation energies for electrical conduction in grain. In addition, for the CCTO ceramics sintered in oxygen, in the spectra of dielectric loss the strong peaks appear in the low-frequency range, which may be attributed to the contact-electrode effect on ceramic surface. © 2012 Elsevier Ltd.


Yang Z.,Wuhan University | Zhang Y.,Huazhong University of Science and Technology | Lu Z.,Wuhan University of Science and Technology | Zhang K.,Wuhan University | And 5 more authors.
Journal of the American Ceramic Society | Year: 2013

Ca1-xRbxCu3Ti4O12 (x = 0, 0.03, and 0.05) ceramics were synthesized by the sol-gel method. Their microstructure and electrical properties were investigated. In the Rb-doped samples, the Cu-rich and Ti-poor grain-boundary layers are formed, and electrical properties are also changed by doping: With the increase in doping concentration, the grain resistivity and the grain-boundary Schottky potential barrier are changed, the grain-boundary resistivity is enhanced, and the low-frequency dielectric constants and loss are reduced. These results were discussed in terms of the internal barrier layer capacitor (IBLC) mechanism, particularly focusing on the electrical properties in grains and the cationic nonstoichiometry at grain boundaries. © 2012 The American Ceramic Society.


Liu Y.,Wuhan University | Li C.,Wuhan University | Wang J.,Wuhan University | Yin D.,Wuhan University | And 3 more authors.
Physica B: Condensed Matter | Year: 2010

The thermoelectric power (TEP), electrical resistivity and X-ray photoemission spectrum (XPS) of the W-doped rubidium blue bronzes Rb 0.3Mo1-xWxO3 (x=0, 0.001, 0.003, 0.005) were investigated. It is shown that the Peierls transition is suppressed with the increase in the concentrations of the doped W ions. The sign change of the TEP was observed for all the samples. At low temperature below the Peierls transition, the W-doping makes the obvious decrease in the absolute values of TEP, especially for Rb0.3Mo1-xWxO 3(x=0.003, 0.005). Furthermore, as compared with the pure blue bronzes, the enlarged difference between Peierls transition temperature (T p) and the sign changed temperature (Ts) in Rb 0.3Mo0.999W0.001O3 bronzes was observed and discussed in detail. © 2010 Elsevier B.V. All rights reserved.

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