Yu. M. Solonin

Electronic structure of hexagonal tungsten trioxide: XPS, XES, and XAS studies

Abstract X-Ray photoelectron (XPS), emission (XES) and absorption (XAS) spectroscopy methods were used to study the electronic structure of hexagonal tungsten trioxide, h-WO3. Its precursor, hexagonal hydrogen tungsten bronze, HxWO3, and the monoclinic form of tungsten trioxide, m-WO3, were also studied. For the mentioned compounds, both the XPS valence-band and core-level spectra, as well as the O Kα emission bands and the W LIII absorption edges were derived. It was established that, binding energies of both the W 4f and O 1s core-level electrons do not change when going from the tungsten trioxides to HxWO3. A high-energy shift of the inflection point of the XAS W LIII spectrum of the HxWO3 bronze, with respect to its position on the spectrum of pure metallic tungsten, was found to be close to those of the spectra of the two WO3 forms studied. Half-widths of both the XPS valence-band spectra and the O Kα bands increase somewhat in the sequence m-WO3→h-WO3→HxWO3. The formation of a near-Fermi sub-band, which is absent for both the modifications of WO3, was observed on the XPS valence-band spectrum of hexagonal HxWO3. The energy positions of the centers of gravity of the O Kα band remain constant for all the compounds studied.

XES, XPS and NEXAFS studies of the electronic structure of cubic MoO1.9 and H1.63MoO3 thick films

Abstract X-ray emission spectroscopy (XES), X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS) methods were used to study the electronic structure of cubic MoO1.9 and H1.63MoO3 thick films. For comparison, the orthorhombic form of molybdenum trioxide, MoO3, was also studied. XP valence-band and Mo 3d and O 1s core-level spectra, O Kα emission bands and NEXAFS O 1s spectra were measured. Half-widths of both the XP valence-bands and O Kα emission bands decreases in the sequence cubic H 1.63 MoO 3 → orthorhombic MoO 3 → cubic MoO1.9. The XPS and NEXAFS data reveal that the charge states of oxygen atoms do not change in the above sequence of compounds. The formation of an additional near-Fermi sub-band is characteristic for the XP valence-band spectra of cubic MoO1.9 and H1.63MoO3. The XES measurements indicate that the energy positions of maxima and of the centers of gravity of the O Kα bands remain constant within accuracy of the experiment for all the compounds studied.

Electronic structure of the monoclinic and hexagonal trioxides of tungsten and hexagonal hydrogen tungsten bronze H0.24WO3

The methods of x-ray photoelectron spectroscopy (XPS) and x-ray emission spectroscopy (XES) were used to study the electronic structure of the monoclinic and hexagonal modifications of WO3, as well as hexagonal hydrogen tungsten bronze H0.24WO3. The OKα emission bands and the XPS spectra of the valence and core electrons were obtained for all of the compounds. It is shown that the half-width of the OKα bands and the XPS spectra of the valence electrons increase in the modification sequence WO3 → hexagonal WO3 → H0.24WO3. The near-Fermi region of the XPS valence-electron spectrum of the compound H0.24WO3 was found to contain an additional subband that is absent from both modifications of WO3. It was established that, within the experimental error, the binding energies of the W4f and O1s core electrons remain constant for all of the compounds that were studied.

Gas atomized powders of hydride-forming alloys and their application in rechargeable batteries

Abstract Gas atomized powders of LaNi 4.5 Al 0.5 and LaNi 2.5 Co 2.4 Al 0.1 alloys were prepared and separated into fractions with different particle sizes. The dependence of morphology, oxygen content and crystal structure of the powders on particle size have been analyzed by means of XRD, SEM and EDS. The hydriding and electrochemical properties of different fractions were also evaluated. All fractions had a similar particle morphology and X-ray lattice parameters. EDS of the gas atomized alloys indicated that the surface of the smaller particles was less contaminated by oxygen than course particles. At the same time the fractions with particle sizes below 50 μm had poor activity both for gas hydrogenation and electrochemical charging. On the DTA curve of the small particles fraction an additional exothermal peak was observed, which may be attributed to a thermal-induced crystallization of the more developed amorphous component of the small particles structure. Gas atomized powders having large particle sizes display hydrogen and electrochemical capacity similar to those of arc-melted alloys.

Solid-phase sintering of ultrafine W(Mo)-Cu composite powders

ConclusionsThe solid-phase sintering of W-Cu composites can be markedly intensified by using ultrafine starting powder charges, the degree of intensification being a maximum with charges produced by reduction of oxide mixtures subjected to prior annealing in air. In this way it is possible to obtain virtually nonporous W-Cu composites with copper contents of 10–35%. The composition dependence of shrinkage in the solid-phase sintering of ultrafine W(Mo)-Cu powder composites is nonmonotonic in character. An anomaly is observed which would appear to be linked with the phenomenon of zonal isolation.

Electronic Structure of h-WO3 and CuWO4 Nanocrystals, Harvesting Materials for Renewable Energy Systems and Functional Devices

— The electronic structure of hexagonal WO3 and triclinic CuWO4 nanocrystals, prospective materials for renewable energy production and functional devices, has been studied using the X-ray photoelectron spectroscopy (XPS) and X-ray emission spectroscopy (XES) methods. The present XPS and XES results render that the W 5d-and O 2p-like states contribute throughout the whole valence-band region of the h-WO3 and CuWO4 nanocrystalline materialls, however maximum contributions of the O 2p-like states occur in the upper, whilst the W 5d-like states in the lower portions of the valence band, respectively.

XPS and transmission electron microscopy of themulticomponent hydride-forming alloys

Abstract The photoelectron spectrograms of the Ni2p, Ni3p, Co3p, Mn2p, Al2p,Ols and Cls in LaNi4.6Mn0.4, LaNi4.5Al0.5, MmNi3.5Co0.7Al0.8, and Zr3d, Ti2p, V2p, Ni2p, Co2p, Ols, and Cls inZr0.9VCo0.55Ni0.55 and Zr0.59Ti0.41V0.53Cr0.22 Fe0.20Co0.27Ni0.78 have been obtained.In a surface layer about 2–2.5 nm of both alloys, the main part of nickel atoms are in metallic state(Ni0) and only a third of them in oxidized state (Ni+2, Ni+3). At the same time the concentration of nickel in surface layer of Zr-containing alloy is muchlower as compared with Mm-containing alloy. The chemical state of Co and V atoms is similar toNi atoms state, but their concentrations, especially of V, in surface layer is lower than that ofnickel. The results of XPS have been compared with those of transmission electron microscopy ofthe thin foils of La- and Zr- based alloys. Dark field microphotographs of the oxidized alloysindicate presence of two different phases, which, according to electron microdiffraction, arenickel-based alloy and oxide. On the first stage of oxidation on the surface of foil the fine nickelparticles (about 10 nm) and amorphous oxide phase appear. Complete oxidation of the foil leadsto formation of eutectic-like structure of mixture of metallic and oxide phase. Some difference ofthe oxidized layer structure of investigated alloys have been observed.

Sintering of tungsten-copper composites of various origins

ConclusionsIn the presence of a liquid phase the most active sintering was exhibited by W-Cu composites produced from copper tungstate (43% Cu) and from an oxide mixture annealed in air (20% Cu). The reason for this was that air-annealing preceding reduction completely (composite 2) or partially (composite 4) transformed the mechanical mixtures of oxides into homogeneous compounds. Under these conditions the tungsten and copper were “mixed together” at an almost atomic level, which imparted a high degree of homogeneity and dispersion to the W-Cu charges after reduction. In compacts from such a charge a disperse structure with a copper matrix readily forms in the initial stage of sintering, and it is this structure that ensures a high rate of densification. The dependence of the degree of homogeneity of a W-Cu charge on method of preparation was found to be reflected in an increase in the mean pore size in the initial stage of sintering. As expected, the mean pore size grew only negligibly in the initial stage of sintering of compacts from the most homogeneous charges (composites 2 and 4), while in composites 5 and 6 the increase in pore size constituted a serious obstacle to macroscopic shrinkage.

XPS and transmission electron microscopy of the multicomponent hydride-forming alloys for electrochemical applications

Abstract Photoelectron spectra of the Ce4d, Ni2p, Ni3p, Co3p, Al2p, O1s, C1s in Mm(Ni,Co,Al) 5 and Zr3d, Ti2p, V2p, Ni2p, O1s, C1s in Zr 1− X Ti X (Ni,V,Co,Cr) 2 alloys have been obtained. In the surface layer, about 2–2.5 nm of both alloys, the main part of the nickel atoms are in the metallic state (Ni 0 ) and only a third of them in oxidized states (Ni +2 , Ni +3 ). At the same time the concentration of nickel in surface layer of Zr-content alloy is much smaller when compared with Mm-content alloy. The chemical state of the Co and V atoms is similar to the Ni atoms state, but their concentrations, especially of V, in the surface layer is smaller than that of nickel. The results of XPS have been compared with the data of transmission electron microscopy of the thin foils of La- and Zr- based alloys. Dark field microphotographs of the oxidized alloys indicate the presence of two different phases, which, according to electron microdiffraction, are nickel-based alloy and oxide. In the first stage of oxidation on the surface of the foil, fine nickel particles (about 10 nm) and an amorphous oxide phase appear. Complete oxidation of the foil leads to the formation of a eutectic like structure consisting of a mixture of metallic and oxide phases. Some difference in the oxidized layer structure of the investigated alloys has been observed.

Direct nitridation synthesis and characterization of Si3N4 nanofibers

Silicon nitride nanofibers were prepared by direct nitridation of silicon powder, with an iron catalyst, under nitrogen flow. The as-prepared nanofibers have the α-Si3N4 structure. Micromorphology and phase composition of the as-prepared α-Si3N4 nanofibers were characterized by scanning and transmission electron microscopy and by X-ray diffraction analysis, respectively. The electronic structure of silicon nitride nanofibers was compared with that of Si3N4 powder by X-ray photoelectron spectroscopy and X-ray emission spectroscopy.