Xianjie Wang

Enhanced high temperature thermoelectric characteristics of transition metals doped Ca3Co4O9+δ by cold high-pressure fabrication

A series of Fe, Mn, and Cu doped Ca(3)Co(4)O(9+delta) samples, Ca(3)(Co,M)(4)O(9+delta) (M=Fe, Mn, and Cu), were fabricated by cold high-pressure compacting technique. Their thermoelectric properties were investigated from room temperature up to 1000 K. The cold high-pressure compacting method is advantageous to increasing density and texture, in favor of the improvement of thermoelectric performance. The electrical transport measurements indicate that Fe/Mn substitutes for Co mainly in [CoO(2)] layers whereas the substitution of Cu for Co takes place in [Ca(2)CoO(3)] layers. The thermoelectric properties as well as electronic correlations depend not only on the substitution ion but also the Co site that is replaced. Thermopower can be well calculated by the carrier effective mass according to Boltzmann transport model, indicating that the electronic correlation plays a crucial role in the unusual thermoelectric characteristics of this system. From the changes in thermopower, resistivity, and thermal conductivity, thermoelectric performance of Ca(3)Co(4)O(9+delta) is efficiently improved by these transition metals doping. Fe doped samples possess the highest ZT values. Combining cold high-pressure technique, ZT of Ca(3)Co(3.9)Fe(0.1)O(9+delta) can reach similar to 0.4 at 1000 K, which is quite large among ceramic oxides, suggesting that Fe doped Ca(3)Co(4)O(9+delta) could be a promising candidate for thermoelectric applications at elevated temperatures.

Temperature- and magnetic-field-induced magnetization reversal in perovskite YFe0.5Cr0.5O3

Perovskite YFe0.5Cr0.5O3 exhibits magnetization reversal at low applied fields due to the competition between the single ion magnetic anisotropy and the antisymmetric Dzyaloshinsky–Moriya interaction. Below a compensation temperature (Tcomp), a tunable bipolar switching of magnetization is demonstrated by changing the magnitude of the field while keeping it in the same direction. The present compound also displays both normal and inverse magnetocaloric effects above and below 260 K, respectively. These phenomena coexisting in a single magnetic system can be tuned in a predictable manner and have potential applications in electromagnetic devices.

Direct Transformation from Graphitic C3N4 to Nitrogen-Doped Graphene: An Efficient Metal-Free Electrocatalyst for Oxygen Reduction Reaction

Carbon-based nanomaterials provide an attractive perspective to replace precious Pt-based electrocatalysts for oxygen reduction reaction (ORR) to enhance the practical applications of fuel cells. Herein, we demonstrate a one-pot direct transformation from graphitic-phase C3N4 (g-C3N4) to nitrogen-doped graphene. g-C3N4, containing only C and N elements, acts as a self-sacrificing template to construct the framework of nitrogen-doped graphene. The relative contents of graphitic and pyridinic-N can be well-tuned by the controlled annealing process. The resulting nitrogen-doped graphene materials show excellent electrocatalytic activity toward ORR, and much enhanced durability and tolerance to methanol in contrast to the conventional Pt/C electrocatalyst in alkaline medium. It is determined that a higher content of N does not necessarily lead to enhanced electrocatalytic activity; rather, at a relatively low N content and a high ratio of graphitic-N/pyridinic-N, the nitrogen-doped graphene obtained by annealing at 900 °C (NGA900) provides the most promising activity for ORR. This study may provide further useful insights on the nature of ORR catalysis of carbon-based materials.

Correlation between the Structural Distortions and Thermoelectric Characteristics in La(1-x)A(x)CoO(3) (A = Ca and Sr)

Detailed structures and thermoelectric (TE) properties are investigated for the perovskite La(1-x)Ca(x)CoO(3) and La(1-x)Sr(x)CoO(3) with 0 <or= x <or= 0.3. The monoclinic crystal structures for all samples are refined from powder X-ray diffraction, which reveals that the La(1-x)Sr(x)CoO(3) series has smaller global distortions but larger local distortions. The different structural distortions strongly influence the transport and TE properties in terms of resistivity, thermopower, thermal conductivity, bandwidth, and effective mass as well as electronic correlation. These results unambiguously demonstrate a close correlation between the structural distortions and TE characteristics in this family. Furthermore, a new approach is suggested to improve the TE performance by controlling the structure factors in such strongly correlated oxide systems. The effects of the spin-state transition of Co ions on the TE properties are also discussed.

Enhanced electron correlation in rare-earth doped Ca3Co4O9

Thermoelectric and thermodynamic properties of a series of Ca-site rare-earth R3+ doped Ca3Co4O9 were investigated. For a fixed doping level, with the reduction in R3+ radius, thermopower is found to gradually increase despite the carrier concentration remains unchanged. The scaling behavior between thermopower and electron specific heat coefficient reveals that the additional increase in thermopower originates from enhanced electron correlation which may be attributed to doping-induced chemical pressure.

Origin of colossal dielectric permittivity of rutile Ti0.9In0.05Nb0.05O2: single crystal and polycrystalline

In this paper, we investigated the dielectric properties of (In + Nb) co-doped rutile TiO2 single crystal and polycrystalline ceramics. Both of them showed colossal, up to 104, dielectric permittivity at room temperature. The single crystal sample showed one dielectric relaxation process with a large dielectric loss. The voltage-dependence of dielectric permittivity and the impedance spectrum suggest that the high dielectric permittivity of single crystal originated from the surface barrier layer capacitor (SBLC). The impedance spectroscopy at different temperature confirmed that the (In + Nb) co-doped rutile TiO2 polycrystalline ceramic had semiconductor grains and insulating grain boundaries, and that the activation energies were calculated to be 0.052 eV and 0.35 eV for grain and grain boundary, respectively. The dielectric behavior and impedance spectrum of the polycrystalline ceramic sample indicated that the internal barrier layer capacitor (IBLC) mode made a major contribution to the high ceramic dielectric permittivity, instead of the electron-pinned defect-dipoles.

Effects of substituting La3+, Y3+ and Ce4+ for Ca2+ on the high temperature transport and thermoelectric properties of CaMnO3

We report the high temperature transport and thermoelectric properties of Ca(1-x)R(x)MnO(3) (R = La, Y and Ce) perovskites in the electron-doped range. The substitution of different valence ions has a similar effect on the thermoelectric properties of this system. Resistivity and thermopower change regularly as the enhancement of doping level; they are both determined mainly by electron concentration. Thermal conductivity is dependent mainly on the weight of R ions, and it decreases monotonically with increasing doping content. The optimal electron concentration is around 0.1-0.12 for thermoelectric performance; in this range the ZT value of Ca(0.9)La(0.1)MnO(3) exceeds 0.12 at 1000 K, exhibiting a good high temperature thermoelectric applied potential. Besides, the hopping nature of the charge carriers is well explained in the framework of polaron theory. The high temperature thermopower values which are inconsistent with nominal Mn(3+) ion t(2g)(3)e(g)(1) configuration are also discussed.

The contribution of doped-Al to the colossal permittivity properties of AlxNb0.03Ti0.97−xO2 rutile ceramics

The search for colossal permittivity (CP) materials continues to attract considerable interest motivated by not only academic research but also potential applications. Very recently, CP with low dielectric loss was reported in In + Nb co-doped rutile TiO2 polycrystalline ceramics. However, the mechanism of CP in this material system and the effect of doping ions are still unclear. Here, we investigated the dielectric properties of AlxNb0.03Ti0.97−xO2 (x = 0, 0.01, 0.03 and 0.05) ceramics. CP with low dielectric loss was found in AlxNb0.03Ti0.97−xO2 samples (x ≤ 0.03). Once the amount of Al-doping exceeds Nb, the CP disappears. The change in dielectric response with varying Al-doping concentration and Ti3+ concentration together with the conductive activation energy in AlxNb0.03Ti0.97−xO2 ceramics clearly suggest that the CP mechanism in co-doped TiO2 ceramics could be attributed to the internal barrier layer capacitor effect. We believe that our research provides comprehensive guidance for the development of CP materials.

Large area orientation films based on graphene oxide self-assembly and low-temperature thermal reduction

Graphene oxide (GO) and reduced graphene oxide (RGO) have many outstanding physical and mechanical properties. Uniform and thickness controllable RGO films with large area were prepared by evaporation-induced self-assembly at a liquid/air interface on glass substrates in combination with low temperature thermal reduction at 200 °C. This process has the advantage of good compatibility with flexible and non-flexible substrates. The films are of centimeter scale and their thickness can be controlled. The structural evolution was characterized. The obtained thermal RGO films exhibit excellent optical properties, a high elastic modulus of 76.18 GPa, and a hardness of 6.89 GPa.

The thermal-transport properties of the Ca3−xAgxCo4O9 system (0≤x≤0.3)

Polycrystalline Ca3−xAgxCo4O9 (0≤x≤0.3) samples were prepared by solid-state reaction and their thermo-transport properties were studied from 5 K to room temperature. With the substitution of Ag+ for Ca2+, the internal chemical pressure induced by Ag+ doping has a strong effect on the transport properties of such a strongly correlated Fermi liquid system. The electrical conductivity and the thermoelectric power increase simultaneously because of the enhancement of carrier concentration and the change of carrier mobility. The thermal conductivity decreases monotonically up to x = 0.3 due to the Ag ion acting as a rattler in the system. These results showed that the thermoelectric performance of the Ca3Co4O9 system can be improved by doping with Ag.