Microinhomogeneity of the Structure of Nanocrystalline Niobium and Vanadium Carbides

The evolution of the microstructure of nonstoichiometric niobium carbides NbC_y (y = 0.77, 0.84, 0.96) and vanadium carbide V₈C₇ subjected to high-energy milling has been studied by the time-of-flight neutron diffraction method. It has been found that milled nanocrystalline powders have microinhomogeneous structure: two fractions with different sizes of particles have been identified in them. The content of the nanofraction is more than 90 wt %; the size of particles of this fraction varies from 90 to 250 Å, depending on the composition of the initial carbide and the duration of milling. The size of particles of the second fraction is more than 2000 Å. Anisotropic deformation distortions have been revealed. The mean size of coherent scattering regions and microstrains in nanocrystallites have been estimated.     

Microstructure of nanocrystalline powders of nonstoichiometric vanadium VC<Subscript>0.875</Subscript> and niobium NbC<Subscript>0.93</Subscript> carbides

The evolution of the microstructure of nonstoichiometric vanadium VC_0.875 and niobium NbC_0.93 carbide powders subjected to high-energy ball milling is investigated by neutron diffraction. It is established that milling produces non uniform powders and two distinct fractions with differing microstructure can be identified in them. It is shown that the time-of-flight neutron-diffraction technique is promising for studying the microstructure of highly deformed nonstoichiometric carbides and for quantitative determination of the anisotropy of microstrains.     

Sequence of phase transformations in the formation of superstructures of the M6C5 type in nonstoichiometric carbides

A symmetry analysis has been made of monoclinic and trigonal superstructures of the M₆C₅ type, which are formed in nonstoichiometric cubic carbides MC _y with a B1 structure. Channels of disorder-order transitions MC _y → M₆C₅ have been revealed, and the distribution functions of carbon atoms in the M₆C₅ superstructures have been calculated. The atomic-vacancy ordering in nonstoichiometric cubic carbides of vanadium VC _y and niobium NbC _y has been investigated using neutron diffraction and X-ray diffraction analyses. It has been shown that, as the temperature decreases, the Group V transition metal carbides MC _y can undergo two physically admissible sequences of transformations associated with the formation of M₆C₅ phases.     

Microstructure of nanocrystalline nonstoichiometric vanadium carbide VC0.875

A nanocrystalline powder of nonstoichiometric vanadium carbide VC_0.875 has been prepared by the high-energy ball milling method. The crystal structure, microstructure, morphology, and size distribution of particles of the initial and milled powders have been investigated using X-ray diffraction, laser diffraction, and scanning electron microscopy. For vanadium carbide, the model calculation of the particle size of a VC_0.875 nanopowder as a function of the milling duration has been performed for the first time. A comparison of the experimental and theoretical results has demonstrated that a nanopowder with an average particle size of 40–80 nm can be obtained by a 10-h high-energy ball milling of the initial vanadium carbide powder with an average particle size of ～6 μm.     

Effect of small particle sizes on the measured density of nanocrystalline powders of nonstoichiometric tantalum carbide TaC<Subscript> <Emphasis Type="Italic">y</Emphasis> </Subscript>

Nanocrystalline powders of the nonstoichiometric tantalum carbide TaC_y (0.81 ≤ y ≤ 0.96) with an average particle size in the range from 45 to 20 nm have been prepared using high-energy ball milling of coarse-grained powders. The density of the initial coarse-grained and prepared nanocrystalline powders of TaC_y has been measured by helium pycnometry. The sizes of particles in tantalum carbide powders have been estimated using the X-ray diffraction analysis and the Brunauer–Emmett–Teller (BET) method. The density of TaC_y nanopowders measured by helium pycnometry is underestimated as compared to the true density due to the adsorption of helium by the highly developed surface of the nanocrystalline powders. It has been shown that the difference between the true and measured densities is proportional to the specific surface area or is inversely proportional to the average particle size of the nanopowders. The large difference between the true and measured pycnometric densities indicates a superhydrophobicity of the tantalum carbide nanopowders.     

Investigation of the influence of nonstoichiometry and doping with carbon and nitrogen on the electronic spectrum of rutile by the coherent potential method

The electronic spectra and magnetic properties of rutile (TiO₂), nonstoichiometric rutile (TiO_{2 ? δ}), and stoichiometric and nonstoichiometric rutiles doped with carbon or nitrogen (TiO_{2 ? y ? δ}C _y and TiO_{2 ? y ? δ}N _y , y(δ) = 0, 0.03, 0.06) have been calculated using the coherent potential method. The used method of investigation has made it possible for the first time to calculate the disordered arrangement of impurity atoms and vacancies in the oxygen sublattice with their arbitrary concentration. The changes in the electronic spectrum and magnetic properties with variations in the concentration of dopants have been investigated. The possible photocatalytic activity of considered compositions has been analyzed.     

Angle-resolved photoemission study of NbC(100): Theory and experiment

The electronic band structure of NbC_0.83 has been investigated with angle-resolved photoelectron spectroscopy using HeI, NeI and ArI radiation. Experiments were carried out on the (100) surface with the emission azimuth lying in the (001) and the (0 1) symmetry planes. The experimental data are compared with theoretical spectra for NbC_1.0(100). The results show that most of the structures observed in the experimental spectra can be well explained by the bulk energy band structure of NbC_1.0. The large differences observed when comparing the spectra obtained in the two emission planes are discussed.     

Disintegration of a coarse-grained vanadium monoxide VO<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript> powder

A coarse-grained powder of nonstoichiometric cubic vanadium monoxide VO _y is disintegrated in a Retch PM 200 planetary ball mill. Milling of the coarse-grained vanadium monoxide powder VO _y at a rate of rotation of 500 rpm for 2 h significantly broadens diffraction lines, and the crystal structure of vanadium monoxide VO_1.00 after milling remains the same. High-resolution scanning electron microscopy and X-ray diffraction studies of the microstructure of vanadium monoxide demonstrate that high-energy milling can produce vanadium monoxide powders with an average crystallite size of 23 ± 10 nm. The vanadium monoxide produced by milling has a crystallite size that is half the crystallite size in the titanium monoxide produced by severe plastic deformation.     

Electronic band structures of the NbC—NbN alloy system from the X-ray photoelectron spectroscopic measurement

The energy distribution curves (EDC's) of valence band of NbC—NbN system have been measured by X-ray photoelectron spectroscopy. The characteristic features of the EDC's are well interpreted by the super-position of the densities of states of NbC and NbN in proportion to their concentration. Each band of NbC and NbN remains rigid and self-asserting in the band structures of NbC_xN_1-x alloy system.     

Magnetic relaxation near a fishtail peak of isothermal magnetization in B1 NbCy encapsulated in multiwall carbon nanocages

Isothermal magnetization near a fishtail peak in nanocrystalline B1 NbC_y encapsulated in multiwall carbon nanocages is studied within the time window of 100 < t < 4000 s. The current density J exhibits a linear logarithmic time decay. The effective activation energy U_eff increases linearly with temperature T and is independent of applied magnetic field H. The results of J(t) and U_eff (T, H) are consistent with the Anderson–Kim flux–creep model for thermally activated motion of uncorrelated vortices or vortex bundles over a net potential barrier U_eff. U_eff at a fishtail peak field H_fp evolves quickly above a fishtail peak temperature T_fp, but slowly below that temperature. The result suggests that a decrease of flux viscosity coefficient above T_fp at H_fp is the origin of the fishtail peak in nanocrystalline B1 NbC_y encapsulated in multiwall carbon nanocages.     

Elastic diffuse neutron scattering study of the defect structure of TiC0.76 and NbC0.73

The elastic diffuse neutron scattering of the non-stoichiometric carbides TiC_0.76 and NbC_0.73 has been measured at room temperature, in the (110) reciprocal lattice plane, for $\text{0.2?q = 4π sin O/λ?4}\text{A}\text{?}{}^{-1}$. The Cowley-Waren short-range order parameters were obtained for the eight first shells of neighbours of the metalloid f.c.c. sublattice. The degree of short-range ordering was found much larger in NbC_0.76. Strong displacements of the metal atoms towards their first neighbour carbons occur in both cases.     

Cluster probabilities in ordered titanium monoxide TiO y as functions of the long-range order parameters

The thermodynamic method is being developed for calculating the titanium-oxygen phase diagram in the domain of existence of the nonstoichiometric titanium monoxide TiO _y with vacancies in the metallic and nonmetallic sublattices. To develop the adequate thermodynamic model, the ordering in TiO _y was studied in detail by the x-ray diffraction method. The study showed that the low-temperature state of titanium monoxide is characterized by a nearly ideal atomic ordering in both metallic and oxygen sublattices. The structural analysis of the ordered Ti₅O₅ phase (space group C2/m) in titanium monoxide showed that one should use the multicluster approximation in modeling the free energy of the nonstoichiometric monoxide.     

Small-polaron mobility in nonstoichiometric cerium dioxide

The high temperature drift mobility (μ_d) of charge carriers in nonstoichiometric cerium dioxide (CeO_2?x) has been calculated by combining the electrical conductivity and nonstoichiometry data on the basis of the oxygen vacancy model with correct ionization state. The electrical conductivity was measured by a four-probe d.c. technique and the nonstoichiometry by thermogravimetric analysis. The dilute solution model of the point defects is valid up to x = 0.03. From the magnitude of μ_d and its temperature dependence, the charge carriers in CeO_2?x, are proposed to be small-polarons formed by localization of electrons at cerium sites and the charge transport process is proposed to occur by a hopping mechanism. The observed temperature dependence of μ_d is in accord with that derived by Holstein and Friedman for small-polaron transport by the hopping mechanism. The activation energy of mobility is found to increase with increasing x as expected for the hopping model.     

Elastic and thermal properties of Zr z Nb1 − z C x N y solid solutions

The temperature and concentration dependences of the elastic moduli and the thermal linear expansion coefficient of Zr _z Nb_{1 ? z} C _x N _y solid solutions containing from 3 to 8 at % of structural vacancies in a nonmetallic sublattice have been found. The temperature dependences of the Debye temperature Θ_D(T) have been calculated using the elastic data and the data on the heat capacity. It has been shown, using carbide NbC_0.97 as an example, that the Θ_D(T) dependences found from the elastic properties and the heat capacity coincide in the temperature range ～220–300 K. By analogy with the niobium carbide, the heat capacity C _p (300) of Zr _z Nb_{1 ? z} C _x N _y solid solutions of various compositions is calculated based on the values of Θ_D(300) determined from the elastic properties.     

Crystal structure and physical properties of magnetic shape memory alloys Ni50 − x Cu x Mn29Ga21

The phase composition, crystal structure, and physical properties (magnetization, electrical resistivity, thermoelectric power, relative elongation, and thermal expansion coefficient) of the stoichiometric alloy Ni₅₀Mn₂₅Ga₂₅ and nonstoichiometric alloys Ni_{50 ? x} Cu _x Mn₂₉Ga₂₁ (x = 0, 1, 2) with the thermoelastic martensitic transformation have been investigated. The influence of the chemical composition on the transformations and physical properties of the alloys has been determined.     

Correlation between microstructure and first-order magnetization reversal in the SmCo5/α-Fe nanocomposite magnets

SmCo₅/α-Fe nanocomposite magnets with different morphology have been fabricated by ball milling of the micrometer sized SmCo₅ and α-Fe powders. The α-Fe grains vary from elongated nano-strips to spherical nanoparticles with increasing milling time. The inter-phase exchange coupling is enhanced with increasing milling time due to reduced grain size. The first-order reversal curves (FORCs) are taken to identify optimal conditions for exchange coupling. It has been found that the stripped morphology results in weak inter-phase exchange coupling, while enhanced exchange coupling is observed with further reduction of the soft-phase grain size. Compared with the measurement of demagnetization curves, FORC analysis provides more information on the magnetostatic as well as the exchange interactions.     

Self-diffusion of 95Nb in single crystals of NbCx

The self-diffusion of ⁹⁵Nb in single crystals of NbC_0.868, NbC_0.834 and NbC_0.766 has been studied in the range of 2370–2660K. The diffusion coefficients are composition independent and can be described by the expression:

$\text{D}{}^1{}_{\mathrm{Nb}}\text{= (4.54}{}^{\mathrm{+\; 2.85}}{}_{\mathrm{?1.75}}\text{) exp(?}\text{(140.0 ±2.4}\text{kcal}\text{mole}\text{)}\text{RT)}\text{cm}{}^2\text{sec}$

An analysis of the results indicates that Nb diffuses by an (0-0) mechanism, just as a pure metal diffuses in a f.c.c. lattice, wherein the atom migrates from its lattice position directly to an analogous vacant site. As this process involves the energies of both migration and Nb vacancy formation, the slower diffusion rates of this species, relative to that of C whose mechanisms of transport involve only the energy of migration, are thus explained.     

Mixing of iron with various metals by high-energy ball milling of elemental powder mixtures

The alloying of Fe with T=V, Cr and Mn by high-energy ball milling of elemental powder mixtures has been studied from the scale of a powder particle down to the atomic scale using X-ray and neutron diffraction, Mössbauer spectrometry and magnetic measurements for Fe_1?x T _x alloys with x=0.50, 0.65 for T=V, x=0.50, 0.70 for T=Cr and x=0.72 for T=Mn. Different alloying behaviours are observed according to T once powder particles have the final composition. The rather fast mechanical alloying of Fe with Mn reflects the statistical nature of the milling process in contrast to the slow mixing of Fe with V and of Fe with Cr. Hyperfine magnetic field distributions remain stationary in shape in the last milling stage at room temperature both for T=V and T=Cr. Magnetic measurements evidence the persistence with milling time of a large population of nanometer-sized Fe-Cr zones that are superparamagnetic at room temperature and at 400 K. By contrast, room-temperature Mössbauer spectra show only a single line for long milling times. The unmixed stationary state of milled p-Fe_0.7Cr_0.3 is discussed in the light of a recent model of systems driven by competing dynamics.     

Ab initio calculations of the CaTiO3 (111) polar surfaces

The stoichiometric terminations and nonstoichiometric terminations have been constructed for CaTiO₃ (111) surface. The cleavage and surface energies, surface grand potential and surface electronic structure have been calculated for the two main classes of terminations using ab initio plane wave ultrasoft pseudopotential method based on Local Density Approximation (LDA). The results show that the stoichiometric terminations are unstable compared with the nonstoichiometric terminations and the polarity compensation achieved through the modification of the surface stoichiometry is more effective than that by the anomalous filling of the surface states. In the O and Ca chemical environments, only CaO₂ and TiO terminations can be formed; the CaO₃ and Ti terminations cannot be stabilized, even in very O-rich chemical environment.     

Electronic structure and stability of nonstoichiometric titanium monoxide TiO y with structural vacancies in one of the sublattices

The electronic structure of nonstoichiometric titanium monoxide TiO _y with different compositions y, which contains structural vacancies either in the metallic sublattice or in the nonmetallic sublattice, has been investigated using the supercell method within the DFT-GGA approximation with pseudopotentials. The cases of ordered and disordered arrangements of vacancies have been considered. It has been found that the complete removal of vacancies from the sublattice is energetically unfavorable, and the ordering of oxygen vacancies according to the type of the Ti₆O₅□₁ superstructure, as well as titanium vacancies according to the Ti₅?₁O₆ type, does not lead to the stabilization of the B1 basic structure of titanium monoxide.