Deposition and super liquid repellency of fluorinated ZnO nanoparticles on carbon fabrics

The liquid-repellent behavior of fluorinated zinc oxide (ZnO) nanoparticles deposited onto carbon fabric (CF) by a pulse microwave-assisted (MA) method followed by surface fluorination treatment was investigated. The MA process is performed at 80 °C within 10 min with different pH values of 5.5, 8 and 12. The hexagonal ZnO nanoparticles with an average size of 100 nm exhibit a well-defined wurtzite crystal structure without any heat treatment. The ZnO nanoparticles produced by MA synthesis at pH = 8 display the maximal density over CF substrate. The fluorination coating effectively imparts super water and oil repellencies on the ZnO–CF surface; i.e., the contact angles are 163° (water) and 153° (ethylene glycol, EG). The liquid repellencies toward water and EG droplets show an increasing function of surface density of ZnO nanoparticles. This result can be attributed to the fact that an air layer is confined in the nanoparticles, thereby inducing a rougher gas–vapor–solid contact line, referred to as the Cassie state. Based on the Young–Duprè equation incorporated with the Cassie parameter, the lowest work of adhesion (W_ad) values of the ZnO–CF surface for water and EG repellencies are estimated to be 3.16 and 4.93 mJ/m², respectively. Accordingly, this work sheds some light on the creation of a two-tier texture by an efficient MA route and on how the surface density of ZnO nanoparticles strongly affects the repellent behavior of the resultant ZnO–CF composites.     

One-pot hydrothermal synthesis of ruthenium oxide nanodots on reduced graphene oxide sheets for supercapacitors

Ruthenium oxide nanodots have been deposited on reduced graphene oxide (RGO) sheets homogeneously by hydrothermal and annealing methods. Adding NaOH solution in GO colloids prevents the restack and agglomeration of GO sheets when mixed with ruthenium chloride solution. Local crystallization of RuO₂ in the composites is revealed by X-ray diffraction and transmission electron microscopy. The element mapping image demonstrates the uniform distribution of Ru on RGO sheets. Unlike the pure crystalline RuO₂ exhibiting poor electrochemical performance, the composites present superior capacitive properties. The hydrothermal time is optimized and a maximum of 471 F g⁻¹ is measured in the composites at 0.5 A g⁻¹ when loaded with 45 wt% of RuO₂. After 3000 cycles, its specific capacitance remains 92% of the maximum capacitance. Our results suggest potential application of the reduced graphene oxide/ruthenium oxide composites to supercapacitors.     

In situ synthesis of Co3O4/graphene nanocomposite material for lithium-ion batteries and supercapacitors with high capacity and supercapacitance

Co₃O₄/graphene nanocomposite material was prepared by an in situ solution-based method under reflux conditions. In this reaction progress, Co²⁺ salts were converted to Co₃O₄ nanoparticles which were simultaneously inserted into the graphene layers, upon the reduction of graphite oxide to graphene. The prepared material consists of uniform Co₃O₄ nanoparticles (15-25 nm), which are well dispersed on the surfaces of graphene nanosheets. This has been confirmed through observations by field emission scanning electron microscopy, transmission electron microscopy and atomic force microscopy. The prepared composite material exhibits an initial reversible lithium storage capacity of 722 mAh g⁻¹ in lithium-ion cells and a specific supercapacitance of 478 F g⁻¹ in 2 M KOH electrolyte for supercapacitors, which were higher than that of the previously reported pure graphene nanosheets and Co₃O₄ nanoparticles. Co₃O₄/graphene nanocomposite material demonstrated an excellent electrochemical performance as an anode material for reversible lithium storage in lithium ion cells and as an electrode material in supercapacitors.     

Facile decoration of polypyrrole nanoparticles onto graphene nanosheets for supercapacitors

A facile approach was developed to prepare the graphene nanosheets (GNS) supported polypyrrole (PPy) nanoparticles via the in situ chemical oxidative polymerization of pyrrole onto the surfaces of the GNS modified with sodium dodecyl sulfonate (SDS) as surfactant for GNS and dopant for PPy simultaneously. The morphologies of the graphene nanosheets supported polypyrrole nanoparticles (GNS/PPy nanocomposites) with different feeding ratios were characterized with transition electron microscopy (TEM). It indicated that the PPy nanoparticles had been successfully decorated onto the GNS surfaces. The electrochemical performances of the GNS/PPy nanocomposites were investigated with cyclic voltammetry (CV), constant current charge–discharge and electrochemical impedance spectroscopy (EIS) techniques. The nanocomposite exhibited specific capacitance of 294 F g⁻¹ at the charge–discharge current density of 10 mA cm⁻² in 1.0 M NaNO₃ electrolyte. It showed that the GNS/PPy nanocomposites might be promising electrode materials for supercapacitors.     

Sol-gel synthesis and electrochemical performance of Li4Ti5O12/graphene composite anode for lithium-ion batteries

Li₄Ti₅O₁₂/graphene composite was prepared by a facile sol-gel method. The lattice structure and morphology of the composite were investigated by X-ray diffraction (XRD) and scanning electronic microscopy (SEM). The electrochemical performances of the electrodes have been investigated compared with the pristine Li₄Ti₅O₁₂ synthesized by a similar route. The Li₄Ti₅O₁₂/graphene composite presents a higher capacity and better cycling performance than Li₄Ti₅O₁₂ at the cutoff of 2.5-1.0 V, especially at high current rate. The excellent electrochemical performance of Li₄Ti₅O₁₂/graphene electrode could be attributed to the improvement of electronic conductivity from the graphene sheets. When discharged to 0 V, the Li₄Ti₅O₁₂/graphene composite exhibited a quite high capacity over 274 mAh g⁻¹ below 1.0 V, which was quite beneficial for not only the high energy density but also the safety characteristic of lithium-ion batteries.     

Preparation of Fe2O3/graphene composite and its electrochemical performance as an anode material for lithium ion batteries

The micro-sized sphere Fe₂O₃ particles doped with graphene nanosheets were prepared by a facile hydrothermal method. The obtained Fe₂O₃/graphene composite as the anode material for lithium ion batteries showed a high discharge capacity of 660 mAh g⁻¹ during up to 100 cycles at the current density of 160 mA g⁻¹ and good rate capability. The excellent electrochemical performance of the composite can be attributed to that graphene served as dispersing medium to prevent Fe₂O₃ microparticles from agglomeration and provide an excellent electronic conduction pathway.     

Friction and wear characteristics of multi-layer graphene films investigated by atomic force microscopy

Friction and wear characteristics of multi-layer graphene films deposited on a Si substrate by mechanical exfoliation were investigated by atomic force microscopy (AFM). The graphene films consisted of a few layers of carbon basal plane. The number of graphene layers was determined by AFM and Raman spectroscopy. For the AFM friction measurement, loads in the range of − 5 to 30 nN were applied on the Si tip that slid against the graphene specimen. It was found that graphene films exhibited much lower friction (from 0.36 to 0.62 nN) than bare Si surface (from 1.1 to 4.3 nN) when the applied loads ranged from 3 to 30 nN. The wear characteristics were also assessed using the AFM. Detectable wear of graphene was generated when sliding was performed for 100 cycles under 5 μN applied load. The wear mechanism of graphene was proposed to be due to breakage of in-plane bonds between carbon atoms and shearing at the interface of graphene layers.     

Synthesis, vacuum sintering and dielectric characterization of zirconia (t-ZrO2) nanopowder

Phase pure zirconium oxide powders have been synthesized using the single step auto-ignition combustion method, the particles were nanometer sized (20 nm) and the size distribution was very narrow (3.4 nm). Systematic structural characterization revealed the t-ZrO₂ and indexed for its tetragonal structure (a = 3.5975 Å and c = 5.1649 Å). Calculated microstrain in most of the plane indicated the presence of compressive stress (65-288 MPa) along various planes of the particles. Observed space group (P4₂/nmc) revealed the presence of cations in the 8e positions (0.75, 0.25, 0.75) and the anions in the 16 h positions (0.25, 0.25, 0.4534). The metal-oxide (Zr-O) band observed at the low wavenumber region further confirmed the phase purity of the as-prepared ZrO₂ nanopowders. Peaks at the binding energy positions 2.042 and 0.525 keV in the energy dispersive X-ray spectrum revealed oxygen deficient zirconia. The particle size estimated by TEM was in good agreement with the results obtained through X-ray line broadening (20.81 nm) measurements. The nanopowders were sintered to above 98% of the theoretical density by using vacuum sintering technique at a relatively low temperature of 1300 °C. Stable tetragonal ZrO₂ experimentally yield the permittivity value of about 28 at 10 MHz.     

Electron-irradiation-induced reinforcement of reduced graphene oxide papers

Here we show that the graphene sheets in reduced graphene oxide papers (rGOPs) can be cross-linked by electron-irradiation-induced sp³ carbon atoms, resulting in significant enhancements in both the mechanical and electrical properties compared to unirradiated rGOPs. We find that the residual oxygen-containing functional groups in rGOPs are removed during irradiation. We also observe that the Wigner energy release temperature in rGOPs is 102–188 °C, which is lower than that (～200 °C) in pure graphite. The mechanisms to form sp³ cross-links and the evolutions of sp³ carbon atoms under irradiation are revealed through molecular dynamics simulations.     

Nucleation and growth of selective oxide particles on ferritic steel

In the continuous annealing process, steel sheets are annealed at 800 °C in an atmosphere of nitrogen and hydrogen (5 vol.%) containing low partial water pressure (20-50 Pa). Under these conditions, the most oxidizable alloying elements in the steel segregate towards the surface where they form oxide particles. The nucleation and growth of those oxides were examined. Oxide nucleation mainly occurs between 650 and 750 °C. During their growth, the oxides take the form of a spherical cap and are composed of MnO, Mn₂SiO₄ (or MnSiO₃), MnAl₂O₄, SiO₂, Al₂O₃ and B₂O₃. Particle nucleation and growth are favored on grain boundaries.     

Enhanced electromagnetic characteristics of carbon nanotubes/carbonyl iron powders complex absorbers in 2-18 GHz ranges

The electromagnetic (EM) characteristics of the carbon nanotubes/carbonyl iron powders (CNTs/CIPs) complex absorbers synthesized by mixing CNTs with CIPs were studied at 2-18 GHz, for the aim of the absorbing coating with thinness, lightness, width, and strength. Compared with CIPs, the CNTs/CIPs composites had higher electrical conductivity, permittivity, and dielectric loss, which gradually increased with the increasing CNTs content (W_CNTs). Among them, with W_CNTs = 2.2 %, a reflection loss (R_L) exceeding −20 dB was obtained in the frequency range of 6.4-14.8 GHz for a coating thickness of 1.2-2.5 mm. Particularly, a minimum R_L of −33.3 dB was found at 11.2 GHz corresponding to a matching thickness of 1.5 mm. The excellent EM-wave absorption properties are a consequence of a proper EM matching and enhanced absorption abilities resulting from the addition of a small quantity of CNTs with high electrical conductivity, permittivity, and dielectric loss. Thus, CNTs/CIPs complex absorbers may be promising candidates for EM-wave-absorption materials with strong-absorption, thin-thickness, light-weight, and low-cost.     

HIP synthesis of η-carbide-type nitrides Fe3W3N and Fe6W6N and their magnetic properties

We have succeeded in synthesising iron-tungsten nitrides using the hot isostatic pressing (HIP) method and have measured their magnetic properties. Two η-carbide-type iron-tungsten nitrides with lattice constants a = 11.043(1) and 10.937(2) Å were synthesised directly from metal elements under high-pressure nitrogen gas. Their compositions are expected to be Fe₃W₃N and Fe₆W₆N in analogy with other η-carbide-type compounds. Fe₃W₃N is a ferromagnet with a Curie temperature T_C = 110 K and a saturation moment P_S = 0.78μ_B/Fe, whereas Fe₆W₆N is an antiferromagnet with a Néel temperature T_N = 75 K and shows a metamagnetic transition at around 25 T.     

Large magnetocaloric effect in re-entrant ferromagnet PrMn1.4Fe0.6Ge2

Magnetocaloric effects (MCE) at multiple magnetic phase transition temperatures in PrMn_1.4Fe_0.6Ge₂ were investigated by heat capacity and magnetization measurements. PrMn_1.4Fe_0.6Ge₂ is of a re-entrant ferromagnet and performs multiple magnetic phase transitions in the temperature range from 5 to 340 K. A large magnetic entropy change (−ΔS_M) 8.2 J/kg K and adiabatic temperature change (ΔT_ad) 4.8 K are observed for a field change of 0-1.5 T around 25.5 K, associated with the field-induced first order magnetic phase transition (FOMT) from the antiferromagnetic to the ferromagnetic state with an additional Pr magnetic contribution. These results suggest that a re-entrant ferromagnet is probably promising candidate as working material in the hydrogen and nature gas liquefaction temperature range magnetic refrigeration technology.     

Morphology influence of the oxidation kinetics of carbon nanofibers

This paper reports the stability and oxidation rate of five types of carbon nanofiber (CNF) with distinctly different orientation of their graphite sheets based on conversion to CO₂ when heated in the presence of oxygen. A non-isothermal technique was used to determine the oxidation kinetic parameters including the activation energy (E_a). Graphite shows a similar activation energy (E_a = 158 kJ/mol⁻¹) to CNF with longitudinal alignment (E_a = 156 kJ/mol⁻¹). CNF type herringbone (E_a = 126 kJ/mol⁻¹) and platelet (E_a = 145 kJ/mol⁻¹) show the lowest oxidation resistance which improved dramatically after a heat treatment at 3023 K of the herringbone (E_a = 216 kJ/mol⁻¹) and platelet (E_a = 174 kJ/mol⁻¹) structures.     

Nanoscale synthesis and optical features of metallic nickel nanoparticles by wet chemical approaches

Nickel nanoparticles (Ni-NPs) were successfully synthesized and attached on indium tin oxide (ITO) substrate by two different methods: from solution reduction process by using sodium borohydride (NaBH₄) as reducing agent in the presence of poly(N-vinilpyrrolidone) (PVP) as protective and stabilizing agents and by polyol process under ethylene glycol EG as a solvent. The results indicated that the samples prepared in aqueous solution show the occurrence of face-centered cubic metallic nickel nanoparticles with a medium diameter of ∼31 nm and good size dispersion compared to the preparation in EG that revealed large size ∼150 nm. The dynamics of the nanoparticle's growth in the solvents and comparison with optical absorption is presented.     

Synthesis, structure, and high temperature Mössbauer and Raman spectroscopy studies of Ba1.6Sr1.4Fe2WO9 double perovskite

Ba_1.6Sr_1.4Fe₂WO₉ has been prepared in polycrystalline form by solid-state reaction method in air, and has been studied by X-ray powder diffraction method (XRPD), and high temperature Mössbauer and Raman spectroscopies. The crystal structure was resolved at room temperature by the Rietveld refinement method, and revealed that Ba_1.6Sr_1.4Fe₂WO₉ crystallizes in a tetragonal system, space group I4/m, with a = b = 5.6489(10)Å, c = 7.9833(2)Å and adopts a double perovskite-type A₃B′₂B″O₉ (A = Ba, Sr; B′ = Fe/W, and B″ = Fe/W) structure described by the crystallographic formula (Ba_1.07Sr_0.93)_4d(Fe_0.744W_0.256)_2a(Fe_0.585W_0.415)_2bO₆. The structure contains alternating [(Fe/W)_2aO₆] and [(Fe/W)_2bO₆] octahedra. Mössbauer studies reveal the presence of iron in the 3+ oxidation state. The high temperature Mössbauer measurements showed a magnetic to paramagnetic transition around 405 ± 10 K. The transition is gradual over the temperature interval. The decrease in isomer shift is in line with the general temperature dependence. While the isomer shift is rather linear over the whole temperature range, the quadratic dipolar ΔE temperature dependence shows an abrupt change at 405 K. The latter results allow concluding that a temperature-induced phase transition had occurred. The high temperature Raman study confirms the Mössbauer results on the magnetic to paramagnetic transition.     

Ellipsometry of passive oxide films on nickel in acidic sulfate solution

Nickel passive film has been studied in acidic sulfate solutions at pH 2.3 and 3.3 by ellipsometry. During anodic passivation followed by cathodic reduction, the roughness increases with dissolution of nickel, being indicated by gradual decrease of reflectance. However, the ellipsometric parameters, Ψ (arctan of relative amplitude ratio) and Δ (relative retardation of phase), are relatively insensitive to the roughness increase. From the change of Ψ and Δ, δΨ and δΔ, during the anodic passivation and reduction, thickness of the passive oxide film was estimated with assumption of refractive index of n_f = 2.3 of the film. The thickness estimated is a range between 1.4 and 1.7 nm in the passive potential region from 0.8 to 1.4 V vs. RHE, having a tendency of thickening with increase of potential. Cathodic reduction at constant potential induces a change of the oxide film to an oxide film with lower refractive index of n_f = 1.7, accompanied by thickening of the film about 30% more in the initial stage of reduction for 30 s. The gradual decrease of thickness takes place for the oxide with the lower refractive index in the latter stage. The potential change from the passive region to cathodic hydrogen evolution region may initially cause hydration of the passive oxide of NiO, i.e., NiO + H₂O = Ni(OH)₂, and during the latter stage of reduction, the hydrated nickel oxide gradually dissolves.     

Synthesis of tungsten oxide (W18O49) nanosheets utilizing EDTA salt by microwave irradiation method

We report the synthesis of crystalline W₁₈O₄₉ with nanosheet like morphology by low cost microwave irradiation method without employing hydrothermal process for the first time. Initially, WO₃·H₂O was synthesized using ethylenediaminetetraacetic acid (EDTA) as surface modulator. The product was annealed at 600 °C for 6 h in ambient atmosphere in order to obtain anhydrous tungsten oxide W₁₈O₄₉. Powder X-ray diffraction results confirmed the as prepared WO₃·H₂O to be orthorhombic and W₁₈O₄₉ to be monoclinic phase, respectively. Transmission electron micrographs (TEM) revealed that the W₁₈O₄₉ nanosheets have the average dimensions of the order of 250 nm in length and around 150 nm in width. UV-visible diffusion reflectance spectroscopic (DRS) studies revealed the band gap energies to be 3.28 and 3.47 eV for WO₃·H₂O and W₁₈O₄₉ samples, respectively. The growth mechanism of two dimensional W₁₈O₄₉ nanosheets is discussed.     

Transmission electron microscopy observations on the phase composition and microstructure of the oxidation scale grown on as-polished and yttrium-implanted β-NiAl

Phase transformations and microstructural evolution of thermally grown oxide scale on polycrystalline β-NiAl at 1100 °C up to 6 h, with and without (e.g., as-polished) yttrium implantation, were examined by glancing angle X-ray diffraction, photostimulated luminescence, scanning and transmission electron microscopy. Site-specific TEM specimens were prepared by using focused ion beam in-situ lift-out technique. The oxide scale developed on as-polished β-NiAl consisted of the islands of 390 nm-thick flat regions (e.g., patches) in 916 nm-thick scales. Regardless of microstructure, the oxide scale consisted of α-Al₂O₃ with very little trace of θ-Al₂O₃, and had uniform compressive residual stress. The oxide scale on Y-implanted β-NiAl had a two-layer microstructure: the outer layer was mainly α-Al₂O₃ and the inner layer was made up of α-, δ-, and θ-Al₂O₃ phases. Clearly, the Y addition retarded the θ-to-α Al₂O₃ phase transformation. The oxide scale on Y-implanted β-NiAl, in general, consisted of a 722 nm-thick layer with islands of 470 nm-thick patched regions, some of which contained Y-rich nodules that protruded with thickness up to 1200 nm. Except for islands of patch-regions, the oxide scale developed on Y-implanted β-NiAl was thinner (722 nm) than that on as-polished β-NiAl (916 nm).     

Analysis of the electrochemical reaction behavior of alloy AZ91 by EIS technique in H3PO4/KOH buffered K2SO4 solutions

The EIS technique was used to analyze the electrochemical reaction behavior of Alloy AZ91 in H₃PO₄/KOH buffered K₂SO₄ solution at pH 7. The corrosion resistance of Alloy AZ91 was directly related with the stability of Al₂O₃ · xH₂O rich part of the composite oxide/hydroxide layer on the alloy surface. The break down of the oxide layer was estimated to occur mainly on the matrix solid solution phase in Alloy AZ91. The mf capacitive loop arose from the relaxation of mass transport in the solid oxide phase in the presence of Al₂O₃ · xH₂O rich part and from Mg⁺ ion concentration within the broken area in the absence of Al₂O₃ · xH₂O rich part in the composite oxide structure on the alloy surface. The lf inductive loop had tendency of disappear when the dissolution rate of the alloy decreased as a result of the formation of the protective oxide layer.