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Semiconductive properties of anodic niobium oxides
,A.I. de; Rangel,C.M.; Skeldon,P.; Thompson,G.E.;
Portugaliae Electrochimica Acta , 2006,
Abstract: the semiconductive properties of anodic niobium oxides formed at constant potential and constant current density to different final voltages have been examined by mott-schottky analysis. thin anodic oxides were formed on sputtered niobium specimens at constant potential in the range of 2.5 to 10 vag/agcl in a borate buffer solution. thicker oxides were formed, also on sputtered niobium specimens, at a constant current density of 5 ma cm-2 in 0.1 m ammonium pentaborate solution to final voltages of 10, 50 and 100 v. capacitance measurements were performed in a borate buffer solution of ph 8.8, at a frequency range of 200 to 2000 hz, at a sweep rate of 5 mv s-1 from +2.5 to -1 vag/agcl. the results obtained show n-type semiconductor behaviour with a carrier density in the range of 8 ′ 1018 - 6 ′ 1019 cm-3 on films formed to 10 v. thicker films showed lower carrier densities in the range of 1 ′ 1018 - 2 ′ 1018 cm-3 with a calculated charge depletion layer of 33-36 nm.
Semiconductive properties of anodic niobium oxides  [cached]
A.I. de Sá,C.M. Rangel,P. Skeldon,G.E. Thompson
Portugaliae Electrochimica Acta , 2006,
Abstract: The semiconductive properties of anodic niobium oxides formed at constant potential and constant current density to different final voltages have been examined by Mott-Schottky analysis. Thin anodic oxides were formed on sputtered niobium specimens at constant potential in the range of 2.5 to 10 V Ag/AgCl in a borate buffer solution. Thicker oxides were formed, also on sputtered niobium specimens, at a constant current density of 5 mA cm-2 in 0.1 M ammonium pentaborate solution to final voltages of 10, 50 and 100 V. Capacitance measurements were performed in a borate buffer solution of pH 8.8, at a frequency range of 200 to 2000 Hz, at a sweep rate of 5 mV s-1 from +2.5 to -1 V Ag/AgCl. The results obtained show n-type semiconductor behaviour with a carrier density in the range of 8 ′ 10(18) - 6 ′ 10(19) cm-3 on films formed to 10 V. Thicker films showed lower carrier densities in the range of 1 ′ 10(18) - 2 ′ 10(18) cm-3 with a calculated charge depletion layer of 33-36 nm.
Effect of small additions of vanadium and niobium on structure and mechanical properties of ductile iron  [PDF]
Fra? E.,Górny M.,Kawalec M.
Archives of Foundry Engineering , 2007,
Abstract: Results of investigations of influence of small additions of vanadium (up to 0,15 % V) and niobium (up to 0,04% Nb) on structure of ductile iron is presented in this work. Effect of these additions on distribution of graphite nodule diameter, nodule count, fraction and carbide count have been determined. Investigations of effect of small additions of vanadium and niobium on mechanical properties taking into account tensile strength, yield strength and elongation have also been made.
Hydrothermal Synthesis of Nanostructured Vanadium Oxides  [PDF]
Jacques Livage
Materials , 2010, DOI: 10.3390/ma3084175
Abstract: A wide range of vanadium oxides have been obtained via the hydrothermal treatment of aqueous V(V) solutions. They exhibit a large variety of nanostructures ranging from molecular clusters to 1D and 2D layered compounds. Nanotubes are obtained via a self-rolling process while amazing morphologies such as nano-spheres, nano-flowers and even nano-urchins are formed via the self-assembling of nano-particles. This paper provides some correlation between the molecular structure of precursors in the solution and the nanostructure of the solid phases obtained by hydrothermal treatment.
Correlation and surface effects in Vanadium Oxides  [PDF]
S. Schwieger,M. Potthoff,W. Nolting
Physics , 2003, DOI: 10.1103/PhysRevB.67.165408
Abstract: Recent photoemission experiments have shown strong surface modifications in the spectra from vanadium oxides as (V,Cr)_2O_3 or (Sr,Ca)VO_3. The effective mass is enhanced at the surface and the coherent part of the surface spectrum is narrowed as compared to the bulk. The quasiparticle weight is more sensitive at the surface than in the bulk against bandwidth variations. We investigate these effects theoretically considering the single-band Hubbard model for a film geometry. A simplified dynamical mean-field scheme is used to calculate the main features of the interacting layer-dependent spectral function. It turns out that the experimentally confirmed effects are inherent properties of a system of strongly correlated electrons. The reduction of the weight and the variance of the coherent part of the surface spectrum can be traced back to the reduced surface coordination number. Surface correlation effects can be strongly amplified by changes of the hopping integrals at the surface.
On the Problem of Metal-Insulator Transitions in Vanadium Oxides  [PDF]
A. L. Pergament,G. B. Stefanovich,N. A. Kuldin,A. A. Velichko
ISRN Condensed Matter Physics , 2013, DOI: 10.1155/2013/960627
Abstract: The problem of metal-insulator transition is considered. It is shown that the Mott criterion is applicable not only to heavily doped semiconductors but also to many other materials, including some transition-metal compounds, such as vanadium oxides (particularly, VO2 and V2O3). The low-temperature transition (“paramagnetic metal—antiferromagnetic insulator”) in vanadium sesquioxide is described on the basis of this concept in terms of an intervening phase, between metal and insulator states, with divergent dielectric constant and effective charge carrier mass. Recent communications concerning a possible “metal-insulator transition” in vanadium pentoxide are also discussed. 1. Introduction Strongly correlated transition metal oxides exhibiting metal-insulator transitions (MIT) are currently considered as basic functional materials of oxide electronics [1]. That is why the theoretical models describing MITs are of such importance. Vanadium oxide Magneli phases, forming the homologous series ( ), may be considered as intermediate structures between the end members VO2 ( ) and V2O3 ( ). With one exception (V7O13), the vanadium Magneli phases exhibit MITs [2] as functions of temperature—see Figure 1 and [1–10]. Figure 1: Resistivity versus reciprocal temperature for vanadium oxides. In recent years, a number of papers [4–6] have appeared concerning a “metal-insulator transition” in V2O5 [3]. Here, we do not intend to dispute scientific aspects of these works, and the object of our discussion will be different. The fact is that the term “metal-insulator transition”, particularly with respect to vanadium oxides (and not only to these—generally to a variety of transition metal oxides and related materials [7]), has long been used in the scientific literature to describe quite a certain class of phenomena and experimental facts. Thus, in this paper we will comment on the aforecited references [4–6], with a more detailed discussion than that in our brief note [3], in view of the problem of MITs in vanadium oxides. Also, the MITs in V2O3 are discussed, and an attempt is made to describe them in terms of a model based on the Mott criterion concept, developing some ideas of our previous work [7]. However, first, in Section 2.1, we have to acquaint a reader with the main general points of discussion and recall some results obtained earlier for vanadium dioxide and other related materials. 2. Mott Metal-Insulator Transitions in Vanadium Oxides 2.1. Strongly Correlated Systems and Mott Transition At present, there is no unified approach to interpret the electronic
Surface characterization of alumina reinforced with niobium carbide obtained by polymer precursor
Acchar, Wilson;Silva, José Roberto Bezerra da;
Materials Research , 2006, DOI: 10.1590/S1516-14392006000300005
Abstract: active filler controlled pyrolysis of polymers (afcop) is a recent method for obtaining near-net shaped ceramic bodies. alumina based composites have been developed for use as cutting tools, so knowledge of the surface composition is extremely important because it is directly related to the hardness and wear resistance samples containing a fixed concentration of 60 wt. (%) of polysiloxane and a mixture of metallic niobium and alumina powder were homogenized, uniaxially warm pressed at 80 °c and subsequently pyrolyzed in flowing argon at 1200, 1400 and 1500 °c. analysis of the surface composition was carried out by x ray photoelectron spectroscopy, infrared spectroscopy, x ray diffraction and scanning electron microscopy. the results have indicated that the formation of the phases on the surface depends strongly on the niobium/carbon ratio in the raw materials.
Effect of nitrogen on structure and mechanical properties of ductile iron with small additions vanadium and niobium  [PDF]
Fra? E.,Górny M.,Kawalec M.
Archives of Foundry Engineering , 2007,
Abstract: Results of investigations of influence of small additions of vanadium (about 0,08 and 0,12 % V) and niobium (about 0,05 and 0,16% Nb) as well as nitrogen (32 - 58 ppm.) on mechanical properties and structure of ductile iron is presented. Effect of these additions on graphite diameter distribution, nodule count, and ferrite fraction is determined. It has been also shown that vanadium and niobium lead to formation of their complex carbides, while nitrogen – complex carbide-nitrides containing magnesium and silicon.
Influence of the C/O ratio on titanium and vanadium oxides in protoplanetary disks  [PDF]
M. Ali-Dib,O. Mousis,G. S. Pekmezci,J. I. Lunine,N. Madhusudhan,J. -M. Petit
Physics , 2013, DOI: 10.1051/0004-6361/201321780
Abstract: Context. The observation of carbon-rich disks have motivated several studies questioning the influence of the C/O ratio on their gas phase composition in order to establish the connection between the metallicity of hot-Jupiters and that of their parent stars. Aims. We to propose a method that allows the characterization of the adopted C/O ratio in protoplanetary disks independently from the determination of the host star composition. Titanium and vanadium chemistries are investigated because they are strong optical absorbers and also because their oxides are known to be sensitive to the C/O ratio in some exoplanet atmospheres. Methods. We use a commercial package based on the Gibbs energy minimization technique to compute the titanium and vanadium equilibrium chemistries in protoplanetary disks for C/O ratios ranging from 0.05 to 10. Our calculations are performed for pressures ranging from 1e-6 to 1e-2 bar, and for temperatures ranging from 50 to 2000 K. Results. We find that the vanadium nitride/vanadium oxide and titanium hydride/titanium oxide gas phase ratios strongly depend on the C/O ratio in the hot parts of disks (T > 1000 K). Our calculations suggest that, in these regions, these ratios can be used as tracers of the C/O value in protoplanetary disks.
Synthesis of nanoparticeles in ductile iron with small additions of vanadium and niobium and its mechanical properties
E. Fra?,M. Górny,M. Blicharski,S. Dymek
Archives of Foundry Engineering , 2007,
Abstract: It has been shown that the heat treatment of 1095oC/640 oC type of ferritic ductile iron with small addition of 0.08% vanadium permits to obtain of the rounded VC nanoparticles with an average size of 50 nm and 0.13 volume fraction. Results of investigations of influence of small vanadium up to 0.3%, niobium up to 0.16% and nitrogen up to 58 ppm additions and heat treatment of 1080oC-24h/640 oC and 1080oC-24h/600 oC type on structure and mechanical properties (tensile strength, yields strength and elongation) of ductile iron are also presented in this work. It has been demonstrated that heat treatment and small additions of vanadium, and niobium as well as nitrogen enable to obtain material, which can be classified a EN-GJS-450-18 to EN-GJS-700-2 grade ductile iron.
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