Chang Ma

Preparation and electrochemical performance of heteroatom-enriched electrospun carbon nanofibers from melamine formaldehyde resin

Melamine formaldehyde resin was used to prepare heteroatom-enriched carbon nanofibers by electrospinning for the first time. The melamine formaldehyde resin-based carbon fibers without any activation treatment showed a moderate specific surface area ranging from 130 to 479 m2/g and rich surface functionalities (2.56-5.34 wt.% nitrogen and 10.39-11.2 9 wt.% oxygen). Both the specific surface area and surface functionality greatly depended on the carbonization temperature. The capacitive performance was evaluated in 6M KOH aqueous solution. The electrochemically active surface functionalities played an important role in improving the surface capacitance of the electrodes. The sample carbonized at 600°C showed the highest specific surface capacitance of 1.4 F/m2, which was attributed to the most active functionalities (10.69 wt.% of N and O). In addition, the sample carbonized at 750°C exhibited the highest specific capacitance of 206 F/g.

Nitrogen-doped hierarchical porous carbon with high surface area derived from graphene oxide/pitch oxide composite for supercapacitors

A nitrogen-doped hierarchical porous carbon has been prepared through one-step KOH activation of pitch oxide/graphene oxide composite. At a low weight ratio of KOH/composite (1:1), the as-prepared carbon possesses high specific surface area, rich nitrogen and oxygen, appropriate mesopore/micropore ratio and considerable small-sized mesopores. The addition of graphene oxide plays a key role in forming 4 nm mesopores. The sample PO-GO-16 presents the characteristics of large surface area (2196 m(2) g(-1)), high mesoporosity (47.6%), as well as rich nitrogen (1.52 at.%) and oxygen (6.9 at.%). As a result, PO-GO-16 electrode shows an outstanding capacitive behavior: high capacitance (296 F g(-1)) and ultrahigh-rate performance (192 F g(-1) at 10 A g(-1)) in 6 M KOH aqueous electrolyte. The balanced structure characteristic, low-cost and high performance, make the porous carbon a promising electrode material for supercapacitors.

Microstructure and tetragonal-to-orthorhombic phase transition of AFe(2)As(2) (A=Sr,Ca) as seen via transmission electron microscopy

The structural properties of the SrFe2As2 and CaFe2As2 compounds have been extensively analyzed by transmission electron microscopy (TEM) from room temperature down to 20K. The experimental results demonstrate that the SrFe2As2 crystal, in consistence with previous x-ray data, has a tetragonal structure at room temperature and undergoes a tetragonal (T)-orthorhombic (O) phase transition at about 210K. Moreover, twinning lamella arising from T-O transition evidently appears in the orthorhombic phase. On the other hand, TEM observations of CaFe2As2 reveal the presence of a pseudo-periodic structural modulation with the periodicity of around 40nm at room temperature. This modulation is likely in connection with the local structural distortions within the Ca layer. In-situ cooling TEM observations of CaFe2As2 reveal the presence of complex domain structures in the low-temperature orthorhombic phase.

The effect of Mg doping on the structural and physical properties of LuFe2O4 and Lu2Fe3O7

The structural and physical properties of the recently discovered electronic ferroelectric materials LuFe(2)O(4) and Lu(2)Fe(3)O(7) have been investigated for Mg substitution of Fe. X-ray diffraction data demonstrate that the lattice parameters in both systems change progressively with increasing Mg content, with a smaller unit cell volume on replacing Fe(2+) by Mg(2+). X-ray absorption near-edge spectroscopy experiments at the Fe K-edge show that the average Fe oxidation state is slightly increased along with Mg doping in Lu(2)Fe(3)O(7) materials, consistent with isomorphous replacement of Fe(2+) by Mg(2+). Measurements of dielectric properties demonstrate that Mg doping could have an effect on the electron hopping energy between Fe(2+) and Fe(3+) ions. Transmission electron microscopy and magnetization analysis reveal that Mg doping in LuFe(2)O(4) has a much greater influence than in Lu(2)Fe(3)O(7) on both the charge ordering and the low-temperature magnetic properties.

Effects of layered structural features on charge/orbital ordering in (La, Sr)n+1MnnO3n+1 (n = 1 and 2)

The charge/orbital ordering (COO) of the layered mixed-valence manganites (La,Sr)(n+1)Mn(n)O(3n+1) (n = 1 and 2) is examined by first-principles calculations and discussed in comparison with the La(0.5)Ca(0.5)MnO(3) perovskite phase ([Formula: see text]). The results demonstrated that the layered structural features could yield not only visibly weak coupling between Mn-O layers but also various features in the orbital ordering associated with different types of local structural distortions. In both La(0.5)Sr(1.5)MnO(4) (n = 1) and LaSr(2)Mn(2)O(7) (n = 2) phases, the orbital ordering can be chiefly assigned to the d(x(2)-y(2)) orbital, in contrast with the zigzag-type d(z(2)) orbital ordering in the [Formula: see text] perovskite phase. Our theoretical analysis shows that a variety of essential factors, including the local structural distortions of the MnO(6) octahedra, the on-site Coulomb interaction, and magnetic interaction, have to be properly considered in order to achieve acceptable COO ground states for the layered variants in (La,Sr)(n+1)Mn(n)O(3n+1).

Superconductivity in (La1-xCex)(O0.9F0.1)FeAs and (La1-xPbx)OFeAs

Here we report the synthesis and superconductivity of (La1-xCex)(O0.9F0.1)FeAs and (La1-xPbx)OFeAs, which were prepared via partially substituting the trivalent La with either Ce in LaO0.9F0.1FeAs or Pb in LaOFeAs, respectively. Influence of the doping level x on the superconducting transition temperatures (Tc) is investigated in correlation with the dopant fraction values in these two sets of superconductors. The accurate existence of the doped Ce or Pb cations inside the typical samples is confirmed by means of electron energy loss spectroscopy (EELS) and energy dispersive X-ray spectroscopy (EDX) measurements. Structural analysis reveals that the increase of Ce content in the samples leads to a systematic shrinkage of the crystal lattice parameters. The observation of superconductivity in (La1- xPbx)OFeAs without F doping indicates a new method for the introduction of charge carriers in this system.

Correlations among superconductivity, structural instability, and band filling in Nb1- xB2 at the critical point x≈ 0.2

We performed an extensive investigation on the correlations among superconductivity, structural instability and band filling in Nb1-xB2 materials. Structural measurements reveal that a notable phase transformation occurs at x=0.2, corresponding to the Fermi level (EF) in the pseudogap with the minimum total density of states (DOS) as demonstrated by the first-principles calculations. Superconductivity in Nb1-xB2 generally becomes visible in the Nb-deficient materials with x=0.2. Electron energy-loss spectroscopy (EELS) measurements on B K-edge directly demonstrated the presence of a chemical shift arising from the structural transformation. Our systematical experimental results in combination with theoretical analysis suggest that the emergence of hole states in the sigma-bands plays an important role for understanding the superconductivity and structural transition in Nb1-xB2.

Preparation of diameter-controlled multi-wall carbon nanotubes by an improved floating-catalyst chemical vapor deposition method

Abstract Multi-wall carbon nanotubes (MWCNTs) with controlled diameters were synthesized by an improved-floating catalyst chemical vapor deposition method, using toluene and ferrocene as a carbon source and catalyst precursor, respectively. Ferrocene was sublimed in a heater and carried as a gas mixed with toluene vapor into a reactor where MWCNTs were formed. The effects of the sublimation temperature, hydrogen content in the gas mixture and gas flow rate on the diameter and diameter distribution of the CNTs formed were investigated. Results indicated that the CNT diameter distributions could be controlled by changing the sublimation temperature. The higher the sublimation temperature, the narrower the distribution of CNT diameters. The average CNT diameter decreased and levelled off with increasing hydrogen content in the gas mixture from 0 to 40 vol%. The CNT diameter decreased with increasing gas flow rate.

Preparation and Comparative Study of Microporous and Mesoporous Carbon Nanofibers as Supercapacitor Electrodes

Microporous carbon nanofibers (Mi-CNFs) and mesoporous carbon nanofibers (Me-CNFs) with high surface area were prepared by electrospinning resol resin/PVP/TEOS/F127 ethanol solution, followed by curing, carbonization and pickling process. TEOS was responsible for structural stability and producing micropores, while F127 for forming mesopores. Mi-CNFs showed high specific surface area of 1841 m2 g-1, while Me-CNFs possessed both high specific surface area of 1674 m2 g-1 and mesoporosity of 64%. The electrochemical test revealed that Mi-CNFs had higher capacitance (276 F g-1 at 0.5 A g-1) and Me-CNFs possessed higher capacitance retention (71%, 150 F g-1 at 30 A g-1).

Direct imaging of incommensurate wave pockets in the charge-density-wave state of LaTe2

We herein report the study of the atomic structure for a fully incommensurate CDW in LaTe2 using Cs-corrected scanning transmission electron microscopy (STEM). It is directly revealed for the first time that the atomic displacements adopt an incommensurate wave-pocket structure along each Te chain. This pocket structure has a long periodicity determined by the CDW incommensurability. We can use the sinusoidal waves as the first-order approximation to characterize the atomic motions within the pocket pattern, which can yield atomic displacements in good agreement with the theoretical model commonly used for studying CDW. These facts demonstrate that the incommensurate pocket patterns should be an essential structural nature in the CDW states and play a critical role for developing the mechanism of the CDW transitions.