Effect of metal and carbon vacancies on the band structure of hexagonal tungsten carbide

The band structure of hexagonal tungsten carbide (β-WC) containing vacancies in metal and carbon sublattices is investigated within the first-principles full-potential linear muffin-tin orbital (LMTO) approach for a model of 16-atom supercells. Specific features of the formation of “vacancy” states are discussed, and the formation energies of defects and their charge states are estimated. The results obtained are compared with previous calculations and available experimental data.     

Shell-model studies of the 90Y, 91Zr, 92Nb and 93Mo nuclei

A shell-model calculation of the N = 51, 39 ≦ Z ≦ 42 nuclei is presented. The ⁸⁸Sr nucleus is assumed to be an inert closed core. The extra-core protons are restricted to the ($\text{2}\text{p}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$, $\text{1}\text{g}{}_{\mathrm{<mtext>9</mtext><mtext>2</mtext>}}$) configurations, and the active neutron is allowed to occupy the $\text{2}\text{d}{}_{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}$, $\text{3}\text{s}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$, $\text{2}\text{d}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ and $\text{1}\text{g}{}_{\mathrm{<mtext>7</mtext><mtext>2</mtext>}}$ orbits. The proton-proton effective interaction is directly taken from the previous analysis on the energy levels for N = 50 isotones by Ball et al. The proton-neutron effective interaction is assumed to be of the form of the surface δ-interaction. The energy spectra are calculated from a least-squares fit to the experimental data, varying the T = 0 and T = 1 strengths of the surface δ-interaction. Spectroscopic factors, E2 transition rates and two-body matrix elements are also calculated and compared with the observed values and the previous theoretical results.     

Electronic structure of H and He in metal vacancies

The electronic structure of H and He impurities in metals is calculated using the Gunnarsson-Hjelmberg method of solving the Kohn-Sham equation. The jellium model is used for the metal. The differences between interstitial and substitutional impurity sites are emphasized. For normal metallic densities it is found that both impurities favour sites of lower than average conduction electron density because here the impurity electrons can relax to a situation of lower kinetic energy.     

Mechanism of formation of the columnar structure in films of transition metal diborides

The formation of film coatings based on transition metal diborides was investigated using x-ray diffractometry and electron microscopy. A scheme of the transformation of the crystal lattice was proposed, and the corresponding interplanar distances were calculated. An analysis of the results obtained made it possible to describe qualitatively the mechanism of formation of a columnar structure of the films prepared from transition metal diborides by using tantalum diboride TaB₂ as an example. It was demonstrated that, upon the synthesis of the films of the superstoichiometric composition, metal and nonmetal atoms have the octahedral coordination and form strong covalent bonds along the 〈00.1〉 direction, which favors texturing and the formation of a columnar structure. The results obtained were confirmed by the investigation of the physicomechanical characteristics of the films synthesized.     

Hyperfine Interaction Studies on Y, Zr, Nb, Mo, Rh, In and Xe in Co

Nuclear magnetic resonance on oriented nuclei and modulated adiabatic fast passage on oriented nuclei measurements were performed on several 4d and 5sp impurities in polycrystalline Co(fcc) foils and Co(hcp) single crystals. The hyperfine fields of Y and Zr in Co(fcc), the hyperfine fields of Y, Zr, Nb, Mo, Rh, In and Xe in Co(hcp), the electric field gradients of Zr, Nb and In in Co(hcp), and the nuclear spin-lattice relaxations of Zr, Nb, Rh and In in Co(hcp) were determined. The dependence of the hyperfine fields and electric field gradients in Co(hcp) on the angle between the magnetization and the c axis was investigated in most cases. The magnetic-field dependence of the spin-lattice relaxation was studied for Nb, Rh and In in Co(hcp), applying the magnetic field perpendicular to the c axis. The known hyperfine interaction parameters of the 4d and 5sp impurities in Co(fcc) and Co(hcp) are summarized. The new results provide a more detailed picture of the hyperfine interaction in Co.     

Decay of Ru, Zr and Nb

The decay of ⁹⁷Ru, ⁹⁷zr and ⁹⁷Nb to levels in ⁹⁷Tc, ⁹⁷Nb and ⁹⁷Mo has been studied with Ge(Li), NaI(Tl), plastic scintillation, and Si (Li) detectors in singles and coincidence experiments. Level schemes were constructed with states in ⁹⁷Te at 96.5, 215.2, 324.4, (560), 785.5, 856 and 971 keV; in ⁹⁷Nb at 743.5, 1148.6, 1251.4, 1276.6, 1549.2, 1751.2, 1764.0, 1852.0, 2106.9 and 2247 keV; and in ⁹⁷Mo at 656.0, 1022.0, 1274.5 and 1514 keV. From β-endpoint energies, disintegration energies of ⁹⁷Zr and ⁹⁷Nb were determined to be 2580 ± 50 keV and 1830 ± 50 keV, respectively. Decay scheme considerations and an upper limit to the β⁺/ec ratio established the disintegration energy of ⁹⁷Ru to be about 1100 keV. Neither the effective-interaction method nor the pairing-correlation model accounts for the experimentally determined levels in mass-97 nuclei.     

Effect of Nb doping on morphology,crystal structure,optical band gap energy of TiO2 thin films

Nb–TiO₂ nanofibers and thin films were prepared using a sol–gel derived electrospinning and spin coating, respectively, by varying the Nb/Ti molar ratios from 0 to 0.59 to investigate the effect of Nb doping on morphology, crystal structure, and optical band gap energy of Nb–TiO₂. XRD results indicated that Nb–TiO₂ is composed of anatase and rutile phases as a function of Nb/Ti molar ratio. As the Nb/Ti molar ratio rose, the anatase to rutile phase transformation and the reduction in crystallite size occurred. The band gap energy of Nb–TiO₂ was changed from 3.25 eV to 2.87 eV when the anatase phase was transformed to rutile phase with increasing the Nb doping. Experimental results indicated that the Nb doping was mainly attributed to the morphology, the crystal structure, the optical band gap energy of Nb–TiO₂, and the photocatalytic degradation of methylene blue.     

The Zr(d,n)Nb and Zr(d,n)Nb reactions

The (d, n) reaction on ⁹⁰Zr and ⁹⁶Zr has been studied at 12 MeV deuteron bombarding energy using the neutron time-of-flight technique with an overall neutron time resolution of 1.9 ns. Angular distributions of neutron groups leading to states in ⁹¹Nb and ⁹⁷Nb were measured in the angular range between 15° and 60°. The measured cross sections were analyzed in the framework of the distorted-wave theory of stripping reactions to deduce l-values and proton spectroscopic factors of states in the residual nuclei. The results are compared with the corresponding data available from (³He, d) studies. The fractionation of the single-particle proton states and their centroid energies are determined.     

Generation of oxygen vacancies in the surface of ferroelectric Pb(Nb,Zr,Ti)O3

Controlled generation of oxygen vacancies in the surface of ferroelectric thin films is crucial to study how surface reduction affects molecular adsorption and catalysis of gas-surface phenomena. We demonstrate the effective reduction in the surface of 4% niobium doped 20/80 PZT (PNZT) thin films. The sample was characterized by X-ray diffraction (XRD), Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM), and heated at 200, 250 and 300 °C in a high vacuum system at 10⁻⁵ T of H₂. Auger peak-to-peak intensities was used to study the elemental concentrations during the reduction experiment. High-resolution XPS spectra were acquired before and after reduction process for detecting the changes of the oxygen signal. Vacancies production rates as slow as 0.21% per minute were achieved and the temperature was not a key parameter in the process. Experiments at higher hydrogen pressures and lower temperatures might improve the control of the vacancies production.     

The geometrical structure, electronic structure and magnetism of bimetallic Au_nM_2 (n=1, 2; M=Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd) clusters

The geometrical structure, stability, magnetism, and electronic structure of bimetallic clusters AuM2 and Au2M2, where M are 4d transition metal elements, are investigated systematically by using the first-principles method based on density functional theory. The calculation results show that there is a large amount of low-energy isomers with the very similar structure. AuM2 and Au2M2 clusters display dramatic magnetism. The magnetic moment of the 4d element is either enhanced or weakened with respect to th...     

Study of the influence of iron and lanthanum oxides on the band structure of Pb(Zr,Ti) O3

The measurements of diffuse reflectance spectra in the range of 400; 2500 nm and of photoacoustic spectra in the range of 350; 800 nm were used for the study of the influence of iron (III) oxide and lathanum oxide additions on the band structure of polycrystalline lead zirconate — lead titanate solid solutions. The molar ratio of zirconium and titanium in the studied solid solutions was 60 40, and the concentrations of the dopants werex FeO_1.5, andy LaO_1.5, werex 0; 0·05 mol, andy 0; 0·10 mol, respectively. The diffuse reflectance spectra were expressed in terms of the Kubelka-Munk function values versus incident electromagnetic radiation energy. These dependences were decomposed into partial absorption bands using a computer. Transitions of charged carriers between energy levels or bands which differ mutually by the energy of 0·9, 1·2–1·3, 1·6, 1·65–1·7, 2·4–2·5, 2·75–2·8, 2·9–3·2, 3·4–3·8 eV, were identified. Their intensities depend on the concentration of Fe and/or on La in the Pb(Zr_0.60 Ti_0.40) solid solutions.The authors are very grateful to Mrs. Iva Havlasová, Vra Andrsová, Ing. Jan Netuka, and Mrs. Liana Karlíková from the first authors' institute, for their technical assistance.     

XPS valence bands of Ti,Zr, Nb,Mo and Hf

The XPS valence band spectra of polycrystalline Ti, Zr, Nb, Mo and Hf have been measured with a resolution of 0.6 eV. The spectra were compared with theoretical densities of states. Good agreement is achieved if the partial densities of states are considered and a many body line shape function and life time broadening are taken into account. The sigularity index determined at various core lines as well as the experimentally determined relative valence band cross sections are reported.     

Quasimolecular KX-ray excitation by bombarding As,Zr, Nb,Mo and Rh target with Nb ions

By bombarding various targets with 65 MeV and 40 MeV Nb ions continuous X-ray distributions have been obtained, which range up to the KX-ray energies of quasiatoms woth Z=Z₁+Z₂. The hogh-energy parts of these continua are interpreted as KX-radiation of quasimolecules transiently formed during the adiabatic heavy-ion atomic collisions.     

Effect of oxygen vacancies on adhesion at the Nb/Al2O3 and Ni/ZrO2 interfaces

The atomic and electronic structures of the Nb/Al₂O₃(0001) and Ni/ZrO₂(001) interfaces are calculated using density-functional theory. The formation energy of oxygen vacancies is estimated in bulk materials and in surface layers and interfaces for different uppermost atomic layers of oxide surfaces. The work of separation of metal films from oxide surfaces is determined. The effect of oxygen vacancies on the bonding of transition metals to atoms of a substrate determining adhesion at the metal-oxide interfaces is discussed. It is shown that the Nb(Ni)-O interaction at the interfaces weakens in the presence of surface oxygen vacancies.     

Rare earth rhodium borides RRh3B (R = Y, Zr, and Nb): mechanical, thermal and optical properties

Conversations allow the quick transfer of short bits of information and it is reasonable to expect that changes in communication medium affect how we converse. Using conversations in works of fiction and in an online social networking platform, we show that the utterance length of conversations is slowly shortening with time but adapts more strongly to the constraints of the communication medium. This indicates that the introduction of any new medium of communication can affect the way natural language evolves.     

ICP－AES法测定高温合金中铌、钽、锆、铪

本文研究了高温合金中Ｎｂ、Ｔａ、Ｚｒ和Ｈｆ的分析方法，同时探讨了ＩＣＰ－原子发射光谱法中的某些影响因素及其校正。通过精密度与准确度试验，表明该分析方法准确、可靠，并且用于合金样品分析取得了满意的效果。