Baoyi Wang

The helium star donor channel for the progenitors of Type Ia supernovae

Type Ia supernovae (SNe Ia) play an important role in astrophysics, especially in the study of cosmic evolution. Several progenitor models for SNe Ia have been proposed in the past. In this paper we carry out a detailed study of the He star donor channel, in which a carbon-oxygen white dwarf (CO WD) accretes material from a He main-sequence star or a He subgiant to increase its mass to the Chandrasekhar mass. Employing Eggletons stellar evolution code with an optically thick wind assumption, and adopting the prescription of Kato & Hachisu for the mass accumulation efficiency of the He-shell flashes on to the WDs, we performed binary evolution calculations for about 2600 close WD binary systems. According to these calculations, we mapped out the initial parameters for SNe Ia in the orbital period-secondary mass (log P(i)-M(2)(i)) plane for various WD masses from this channel. The study shows that the He star donor channel is noteworthy for producing SNe Ia (similar to 1.2 x 10(-3) yr(-1) in our Galaxy), and that the progenitors from this channel may appear as supersoft X-ray sources. Importantly, this channel can explain SNe Ia with short delay times (similar to 10(8) yr), which is consistent with the recent observational implications of young populations of SN Ia progenitors.

A superior catalyst with dual redox cycles for the selective reduction of NOx by ammonia

An environmentally benign Cu-Ce-Ti oxide catalyst exhibited excellent NH3-SCR activity, high N2 selectivity and strong resistance against H2O and SO2 with a broad operation temperature window. The dual redox cycles (Cu(2+) + Ce(3+) ↔ Cu(+) + Ce(4+), Cu(2+) + Ti(3+) ↔ Cu(+) + Ti(4+)) play key roles for the superior catalytic deNOx performance.

Oxygen vacancy–induced ferromagnetism in un-doped ZnO thin films

ZnO films became ferromagnetic when defects were introduced by thermal-annealing in flowing argon. This ferromagnetism, as shown by the photoluminescence measurement and positron annihilation analysis, was induced by the singly occupied oxygen vacancy with a saturated magnetization dependent positively on the amount of this vacancy. This study clarified the origin of the ferromagnetism of un-doped ZnO thin films and provides possibly an alternative way to prepare ferromagnetic ZnO films.

Highly Dispersed TiO6 Units in a Layered Double Hydroxide for Water Splitting

A family of photocatalysts for water splitting into hydrogen was prepared by distributing TiO(6) units in an MTi-layered double hydroxide matrix (M = Ni, Zn, Mg) that displays largely enhanced photocatalytic activity with an H(2)-production rate of 31.4 μmol  h(-1) as well as excellent recyclable performance. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) mapping and XPS measurement reveal that a high dispersion of TiO(6) octahedra in the layered doubled hydroxide (LDH) matrix was obtained by the formation of an M(2+)-O-Ti network, rather different from the aggregation state of TiO(6) in the inorganic layered material K(2)Ti(4)O(9). Both transient absorption and photoluminescence spectra demonstrate that the electron-hole recombination process was significantly depressed in the Ti-containing LDH materials relative to bulk Ti oxide, which is attributed to the abundant surface defects that serve as trapping sites for photogenerated electrons verified by positron annihilation and extended X-ray absorption fine structure (EXAFS) techniques. In addition, a theoretical study on the basis of DFT calculations demonstrates that the electronic structure of the TiO(6) units was modified by the adjacent MO(6) octahedron by means of covalent interactions, with a much decreased bandgap of 2.1 eV, which accounts for its superior water-splitting behavior. Therefore, the dispersion strategy for TiO(6) units within a 2D inorganic matrix can be extended to fabricate other oxide or hydroxide catalysts with greatly enhanced performance in photocatalysis and energy conversion.

Oxidation behaviour of NiCrAlY coatings on Ni-based superalloy

NiCrAlY coatings were prepared on cast Ni-based IN100 superalloy using are ion plating, leading to a dense structure. The surface became smoother after vacuum heat treatment. The oxidation kinetic curves of the IN100 alloy and the coating were obtained. The results indicate that NiCrAlY coating gains more weight after cyclic than after isothermal oxidation and no oxide spallation was evident. The oxidation resistance of IN100 superalloy when exposed to either isothermal or cyclic oxidation was markedly improved by coating with NiCrAlY. The oxide scales mainly consisted of alpha-Al2O3 and Cr2O3, at 900degreesC. In the initial oxidation stage at 1000degreesC, three kinds of oxides, NiO, Cr2O3 and alpha-Al2O3, formed simultaneously. With further oxidation, not only is the thickness of the oxide scale increased, but complex reactions also occurred

A hierarchical heterostructure based on Pd nanoparticles/layered double hydroxide nanowalls for enhanced ethanol electrooxidation

Finely dispersed Pd nanoparticles (PdNPs) anchored to CoAl layered double hydroxide nanowalls (LDH-NWs) have been fabricated via a facile in situ redox reaction between the LDH-NWs and the PdCl42− precursor. The integrated LDH-NWs play the roles of both a hierarchical support and a reductant without any external agent, ensuring the cleanness of the metal–support interface. Based on the effective exposure of the Pd active sites and the elaborate network architecture, the Pd/LDH-NW heterogenous material yields a largely improved catalytic activity as well as robust durability towards ethanol electrooxidation in comparison with the commercial Pd/C catalyst. Moreover, a density functional theory (DFT) calculation indicates that the enhancement in the electrocatalytic properties originates from the synergistic effect between the metal and support, in which the LDH support stabilizes the PdNPs via the formation of a Pd–HO bond which is accompanied by an electron transfer from the LDH to the PdNPs. This work provides a promising approach for the design and fabrication of highly efficient metal-supported nanocatalysts which can be used in fuel cells and other related catalytic reactions.

Origin of the defects-induced ferromagnetism in un-doped ZnO single crystals

We clarified, in this Letter, that in un-doped ZnO single crystals after thermal annealing in flowing argon, the defects-induced room-temperature ferromagnetism was originated from the surface defects and specifically, from singly occupied oxygen vacancies denoted as F+, by the optical and electrical properties measurements as well as positron annihilation analysis. In addition, a positive linear relationship was observed between the ferromagnetism and the F+ concentration, which is in support with the above clarification.

Exchange-coupling interaction, effective anisotropy and coercivity in nanocomposite permanent materials

The inter-grain exchange-coupling interactions, effective anisotropy, and coercivity in nanocomposite Nd2Fe14B/α−Fe magnets were investigated. The effective anisotropy of nanocomposite magnets has been calculated starting from the statistics of boundaries between magnetically hard-hard, hard-soft, and soft-soft grains. The result shows that the effective anisotropy decreases with reduction in grain size and/or increase in soft phase components. When grain sizes reduce to 4–5 nm, Keff decreases to 1/3−1/4 of the ordinary value of K. The coercivity in nanocomposite magnets demonstrates a similar behavior. The decrement of coercivity is mainly due to the reduction of effective anisotropy. Considering the opposite varying trend the remanence demonstrates with respect to the effective anisotropy and the coercivity, we conclude that the mean grain size should be in the range of 10–15 nm and the volume fraction of soft phase should be less than 50% in order to achieve high energy product magnets.

The role of oxygen vacancies in improving the performance of CoO as a bifunctional cathode catalyst for rechargeable Li–O2 batteries

The design and facile synthesis of noble metal-free efficient cathode catalysts to accelerate the sluggish kinetics of both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is still a big challenge for lithium–air batteries. In this study, oxygen vacancy-bearing CoO (CoO-A) has been successfully synthesized through a simple calcination of Co(Ac)2·4H2O in Ar, and the oxygen vacancies have been confirmed by Raman spectroscopy, HRTEM, X-ray photoelectron spectroscopy (XPS) and positron annihilation lifetime spectroscopy (PALS). In comparison with defect-free CoO-N, which is derived from the decomposition of Co(NO3)2·6H2O, an oxygen-deficient CoO-A based cathode shows much higher cycling stability, higher rate capability, higher coulombic efficiency, and lower charge–discharge overpotential. The enhanced performances of the CoO-A based cathode can be largely attributed to the synergetic effect of CoO itself and oxygen vacancies on the promotion of both ORR and OER. CoO provides catalytic activity for ORR, and at the same time the oxygen vacancies not only facilitate the electron and Li+ migration but also act as active sites binding to O2 and Li2O2, which accelerates the OER process. Furthermore, the formation and decomposition of Li2O2 during discharge–charge cycles have also been studied and the results indicate that CoO-A shows a high catalytic activity in the decomposition of Li2O2.

Effects of the crystal structure on electrical and optical properties of pyrite FeS2 films prepared by thermally sulfurizing iron films

Based on experimental results and theoretical analysis effects of the crystal structure on the optical and electrical properties of pyrite FeS2 films produced by thermally sulfurizing iron films at various temperatures have been systematically studied. The results indicate that the crystal structure and some related factors, such as the crystallization and the stoichiometry, remarkably influence the optical and electrical performances of the pyrite films. It is also shown that the preferred orientation of the crystal grain plays a major role in determining the crystal structure and the optical and electrical properties of the pyrite FeS2 films. Also we find that it is the crystal grains, rather than the particles that exercise a decisive influence on the electrical performance of pyrite films