S. H. Xie

Comparison of the effective conductivity between composites reinforced by graphene nanosheets and carbon nanotubes

Both graphene nanosheets and carbon nanotubes have exceptional electric and thermal conductivities, rendering them excellent candidates as second-phase fillers in composite materials to substantially enhance their effective conductivities. Their markedly different geometries, however, can have significant effect on the effective conductivities of composites, which we investigate using an effective medium approximation. It is demonstrated that graphene nanosheet is more effective in conductivity enhancement than carbon nantubes, and both fillers lead to substantially higher conductivity and much reduced percolation threshold in composites. The effects of conductivity anisotropy and interfacial resistance are also discussed.

Multiferroic nanoparticulate Bi3.15Nd0.85Ti3O12-CoFe2O4 composite thin films prepared by a chemical solution deposition technique

Multiferroic xBi3.15Nd0.85Ti3O12–(1−x)CoFe2O4 composite thin films with x=0.5, 0.6, and 0.7 were fabricated on Pt∕Ti∕SiO2∕Si(100) substrates by a chemical solution deposition technique. X-ray diffraction shows that there are no other phases but bismuth-layered perovskite Bi3.15Nd0.85Ti3O12 and spinel CoFe2O4 phases in the films. Scanning electron microscopy reveals that CoFe2O4 aggregates locally into nanoparticles and embeds in the Bi3.15Nd0.85Ti3O12 matrix. The composite films exhibit both good ferroelectric and magnetic properties at room temperature, as well as distinct magnetoelectric coupling behaviors, which are comparable with those of multiferroic composite films with conventional Pb-based ferroelectric as a ferroelectric component.

Multiferroic CoFe2O4–Pb(Zr0.52Ti0.48)O3 nanofibers by electrospinning

Multiferroic materials possess two or more types of orders simultaneously that couple the electric and magnetic fields, and composite multiferroics have been widely explored for their excellent magnetoelectric coupling. In this letter, we report a strategy to hybrid multiferroicity at nanoscale. Multiferroic CoFe2O4–Pb(Zr0.52Ti0.48)O3 nanofibers are synthesized by sol-gel process and electrospinning, the spinel structure of CoFe2O4 (CFO) and perovskite structure of Pb(Zr0.52Ti0.48)O3 (PZT) are verified by x-ray diffraction and high resolution transmission electron microscopy, and the multiferroicity of the nanofibers are confirmed by piezoresponse force microscopy and magnetic hysteresis. Excellent ferroelectric and ferromagnetic properties have been observed, which could enable multiferroic devices at nanoscale.

Electronic structures and thermoelectric properties of layered BiCuOCh oxychalcogenides (Ch ¼ S, Se and Te): first-principles calculations†

The p-type BiCuOCh (Ch = S, Se and Te) compounds exhibit very low lattice thermal conductivities and moderate power factors in the medium temperature range, resulting in high thermoelectric figures of merit. In this paper, we investigated their electronic structures using density functional theory, and discovered that a mixture of heavy and light bands near the valence band maximum is beneficial for good thermoelectric performance, and the Cu 3d–Ch np antibonding state near the valence band edge determines the transport properties of BiCuOCh. Semi-classic Boltzmann transport theory was then used to calculate the Seebeck coefficients, electrical conductivities and power factors of BiCuOCh, and the optimal doping concentrations were estimated based on the predicted maximum power factors. The temperature dependence of the thermoelectric transport properties of BiCuOSe were also estimated and compared with experimental data, with good agreement observed.

Electrospinning and multiferroic properties of NiFe2O4–Pb(Zr0.52Ti0.48)O3 composite nanofibers

In this paper, we report a strategy for hybrid multiferroicity at nanoscale. Multiferroic NiFe2O4–Pb(Zr0.52Ti0.48)O3 composite nanofibers are synthesized by a sol-gel process and electrospinning, with fiber diameters ranging from 100 to 400 nm. Energy dispersive spectroscopy and transmission electron microscopy indicate that nanocrystalline Pb(Zr0.52Ti0.48)O3 and NiFe2O4 grains are randomly distributed in the composite nanofibers, with grain size around 30 nm. The spinel structure of NiFe2O4 and the perovskite structure of Pb(Zr0.52Ti0.48O3) are verified by x-ray diffraction, and multiferroicity of the nanofibers is confirmed by piezoresponse force microscopy and magnetic hysteresis. The structures and properties of the composite nanofibers are also compared with single-phase Pb(Zr0.52Ti0.48)O3 and NiFe2O4 nanofibers. These composite nanofibers could lead to unconventional multiferroic structures and devices.

On the effective thermoelectric properties of layered heterogeneous medium

The effective thermoelectric behavior of layered heterogeneous medium is studied, with the distribution of temperature, electric potential, and heat flux solved rigorously from the governing equations, and the effective thermoelectric properties defined through an equivalency principle. It is discovered that the effective thermoelectric figure of merit of a composite medium can be higher than all of its constituents even in the absence of size and interface effects, in contrast to previous studies. This points toward a new route for high figure of merit thermoelectric materials.

Electronic structure and thermoelectric properties of half-Heusler Zr0.5Hf0.5NiSn by first-principles calculations

The electronic structures of Zr0.5Hf0.5NiSn and the parent compounds ZrNiSn and HfNiSn are investigated by using first-principles calculations, and the thermoelectric properties are calculated on the base of the semi-classical Boltzmann transport theory and the empirical thermal conductivity model. The temperature dependence of thermoelectric transport properties of these three compounds is discussed and compared with experimental data, and good agreements are observed. To further optimize the thermoelectric performance of the Zr0.5Hf0.5NiSn compound, the chemical potential dependence of electrical transport properties at three different temperatures is investigated, and the maximum power factors and corresponding optimal p- or n-type doping levels are evaluated, suggesting that the compound has better thermoelectric performance when it is p-type doped.

Energetics of charged domain walls in ferroelectric crystals

We calculated the energy variation due to the emergence of a new domain in barium titanate single crystal using equivalent inclusion method, and energy minimizing domain morphology and orientations have been identified. It is found that the energy difference between the charged and uncharged domain walls is relatively small, and the charged domain wall is metastable. It is also concluded that the compatibility of transformation strain plays a more important role in ferroelectric domain configurations than polarization compatibility, and the incompatibility of piezoelectric strain is insignificant.

Structure and electrical properties of Bi3.15Nd0.85Ti3O12 nanofibers synthesized by electrospinning and sol-gel method

Bi3.15Nd0.85Ti3O12 (BNT) nanofibers with radii in the range of 30–200 nm have been prepared by the electrospinning and sol-gel method, and the structure and morphology of the nanofibers were characterize by x-ray diffraction, scanning electron microscopy, transmission electron microscopy, and Raman spectroscopy. The phase transition and piezoelectric characteristics of the BNT nanofibers were performed by thermal analysis and scanning probe microscopy, respectively. The BNT nanofiber exhibits an effective piezoelectric coefficient of 89 pm/V and a Curie temperature of 500 °C, which are higher than that of the BNT bulk.

The effective medium approximation for annealed magnetoelectric polycrystals

Magnetoelectric polycrystals usually possess antiferromagnetic domains with oppositive magnetoelectric coefficients, and have to be annealed under the appropriate electric and magnetic fields to make the polycrystals macroscopically magnetoelectric. In this paper, we developed an effective medium approximation to calculate the macroscopic coefficients of magnetoelectric polycrystals annealed through Neel’s temperature, and studied the effects of temperature as well as shape and orientation distribution of grains on the macroscopic magnetoelectric coefficients of polycrystalline Cr2O3. It is observed that the effective magnetoelectric coefficient of polycrystal is higher than single-crystalline a11 but lower than single-crystalline a33, and that calculated from the effective medium approximation is higher than simple volume averaging and agrees with experimental data better. It is also noted that polycrystals with randomly oriented grains are optimal for a11∗, while those with fiber texture are optimal for...