Crystal structure, 57Fe Mössbauer spectroscopy and magnetization of UxFe6Sn6 (0≤x≤0.6)

The U_xFe₆Sn₆ (0≤x≤0.6) intermetallics were prepared as polycrystalline samples by arc melting followed by annealing. They crystallize in the P6/mmm space group as a lacunary variant of the HfFe₆Ge₆-type, with an ordered insertion of U within the Sn₈ voids of the FeSn framework, and are isostructural with the R_xFe₆Sn₆ compounds (R=Sc, Zr, Tm, Yb, Lu). ⁵⁷Fe Mössbauer spectroscopy shows that the Fe magnetic moments, μ_Fe, are already ordered at room temperature. Anisotropic contributions to the total hyperfine field at the Fe site indicates that μ_Fe are perpendicular to the crystallographic c axis in contrast to the Sc, Zr and rare-earth (except Yb) containing compounds, where μ_Fe are directed along the c axis. The values deduced for the angles between the main axes of the electric field gradient and μ_Fe further show that, within each (001) plane, μ_Fe have a ferromagnetic arrangement similar to that in FeSn and Yb_0.6Fe₆Sn₆. In U_xFe₆Sn₆, the hyperfine fields increase with U content. Magnetization data in the 2–300 K show small ferromagnetic components for the U_0.4Fe₆Sn₆ and U_0.6Fe₆Sn₆ compositions.     

Simultaneous growth of well-aligned diamond and graphitic carbon nanostructures through graphite etching

A hot filament chemical vapor deposition process based on hydrogen etching of graphite has been developed to synthesize diamond and graphitic carbon nanostructures. Well-aligned diamond and graphitic carbon nanostructured thin films have been synthesized simultaneously on differently pretreated silicon substrates in a pure hydrogen plasma. Graphitic nanocones, diamond nanocones and carbon nanotubes were selectively grown on uncoated, diamond and nickel pre-coated silicon substrates, respectively, in a single deposition process. The nanocones are solid cones with submicron scale roots and nanometer-size sharp tips. The nanotubes are hollow tubes with outer diameter of approximately 50 nm. The orientation of the well-aligned carbon nanostructures depends on the direction of the electric field at the samples surface. Nucleation and growth of diamond on the nanocones were further investigated under similar conditions without plasma. Diamond nanocomposite films have been obtained by depositing a nanocrystalline diamond film on the layer of diamond nanocones.     

Ambiguous temperature difference in aerodynamic levitation process: Modelling,solving and application

The aerodynamic levitation provides an efficient technique for the research on thermophysical properties and solidification behavior of refractory materials. However, there is a nonnegligible temperature differences across sample, causing unexpected uncertainty of measurement, such as, thermal expansivity and undercooling limit. We establish thermal filed model with properly simplified boundary condition, and derive quantitative expressions of this ambiguous temperature difference. Here we show that the temperature difference not only related to the average temperature, relative size and thermal conductivity of sample, but significantly influenced by the rotation pattern of sample. A huge temperature differences is almost inevitable when the sample with low thermal conductivity and high melting point is smelted in stationary suspension pattern, however, a drastically reduction of temperature difference can be fulfilled by simply making the sample rotation in up to down pattern. The thermal filed simulation was used to confirm the validity of these theoretical expressions. This work shed light on temperature difference in aerodynamic levitation. Based on this work, one can simply estimate the extent of temperature difference across the sample, and regulated that conveniently if needed, which benefit for novel material preparation and solidification mechanism study based on this technique.     

A new insight into promotion action of Co2+ in Ni-diamond composite electrodeposition

A new insight into the promotion action of Co²⁺ on both particle and metal deposition in Ni-diamond composite electrodeposition system was analyzed according to electrochemical measurements. The results showed that the addition of Co²⁺ made particles content in deposits increased remarkably. The change of particles content in deposits was related inversely to the change of cathodic zero potential with the increase of the concentration of cobalt sulfate. Zero charge potential of cathode was shifted to much more negative region. The negative shift of the zero potential, combining with positive shift of the zeta potential, increased the electrostatic force between the particle-adsorbed metallic cations and the cathode. It not only benefits to the transportation of particles in solution towards cathode, but also shortens their residence time on cathodic surface. Meanwhile, entry of particles is also promoted. For metals deposition, reduction resistance of metallic cations rises greatly and deposition current at cathodic potentiodynamic polarization decreases after cobalt sulfate has been added into electrolyte. These factors are favorable for increasing particles content in deposits. In addition, physical model of diamond particles deposition state before and after the addition of Co²⁺ has been discussed.     

Corrosion kinetics and patina evolution of galvanized steel in a simulated coastal-industrial atmosphere

The corrosion kinetics and patina (corrosion products) layer evolution of galvanized steel submitted to wet/dry cyclic corrosion test in a simulated coastal-industrial atmosphere was investigated. The results show that zinc coating has a greater corrosion rate during the initial period and a lower corrosion rate during the subsequent period, and the patina composition and structure can greatly affect the corrosion kinetics evolution of zinc coating. Moreover, Zn₅(OH)₆(CO₃)₂ and Zn₄(OH)₆SO₄ are identified as the main stable composition and exhibit an increasing relative amount; while Zn₁₂(OH)₁₅Cl₃(SO₄)₃ cannot stably exist and diminish in the patina layer as the corrosion develops.     

Low-valence ion addition induced more compact passive films on nickel-copper nano-coatings

Ni-Cu nano-coatings were prepared by pulsed electroplating technique in the baths containing various amount of boric acid. Their microstructure, morphologies and corrosion resistance were characterized in detail. The addition of boric acid strongly influences on the microstructure of the Ni-Cu coatings. The coating with a grain size of 130 nm, obtained from the bath containing 35 g L⁻¹ boric acid, shows the highest corrosion resistance. This is attributed to the low-valence Cu ion (Cu⁺) additions in nickel oxide, which could significantly decrease the oxygen ion vacancy density in the passive film to form a more compact passive film. The higher Cu⁺ additions and the lower diffusivity of point defects (D₀) are responsible for the formation of more compact passive film on the coating obtained from the bath with 35 g L⁻¹ boric acid.     

Effects of low temperature on the characteristics of tantalum thin films

In this paper the effects of substrate temperature (room temperature – 350 °C) on the phase composition and crystallization orientation of the tantalum thin film deposited by direct current magnetron sputtering in an extremely low power deposition regime are presented. In this experiment, heating the substrates to 350 °C resulted in the growth of the hard and brittle tetragonal crystalline structure (β-Ta). Deposited tantalum has a conical structure with large voided boundaries. Sheet resistance of samples is much larger than for the convenient conductors which decreased with increasing the substrate temperature.     

The effect of TiN inclusions and deformation-induced martensite on the corrosion properties of AISI 321 stainless steel

Stainless steel of type 321 is commonly used for the production of exhaust systems because of its temperature resistance and welding properties, which are better than those of AISI 304 or similar steels. AISI 321 is a titanium stabilized austenitic stainless steel, where this element is added to form carbides in order to avoid chromium impoverishment due to chromium carbide formation. Cold shaping can, in the case of austenitic stainless steel, cause the formation of deformation induced martensite, which can improve its mechanical properties, but unfortunately can also spoil its good resistance to corrosion. Titanium nitride inclusions are cathodic with respect to steels, and therefore cause their anodic dissolution. Martensite is, however, more susceptible to the corrosion than austenite in steels. The main aim of this study was to analyze the pitting corrosion and stress corrosion cracking which is initiated on prototype cold-formed outer exhaust sleeves during the testing of different cleaning procedures before chromium plating. Various microscopic methods were used to identify the initiation of corrosion and its propagation.     

Adoption of wide-bandgap microcrystalline silicon oxide and dual buffers for semitransparent solar cells in building-integrated photovoltaic window system

We focused on developing penetration-type semitransparent thin-film solar cells (STSCs) using hydrogenated amorphous Si (a-Si:H) for a building-integrated photovoltaic (BIPV) window system. Instead of conventional p-type a-Si:H, p-type hydrogenated microcrystalline Si oxide (p-μc-SiO_x:H) was introduced for a wide-bandgap and conductive window layer. For these purposes, we tuned the CO₂/SiH₄ flow ratio (R) during p-μc-SiO_x:H deposition. The film crystallinity decreased from 50% to 13% as R increased from 0.2 to 1.2. At the optimized R of 0.6, the quantum efficiency was improved under short wavelengths by the suppression of p-type layer parasitic absorption. The series resistance was well controlled to avoid fill factor loss at R = 0.6. Furthermore, we introduced dual buffers comprising p-a-SiO_x:H/i-a-Si:H at the p/i interface to alleviate interfacial energy-band mismatch. The a-Si:H STSCs with the suggested window and dual buffers showed improvements in transmittance and efficiency from 22.9% to 29.3% and from 4.62% to 6.41%, respectively, compared to the STSC using a pristine p-a-Si:H window.