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.     

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.     

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.     

Nanostructure and atomic ordering in vanadium carbide

Nanostructured nonstoichiometric vanadium carbide VC_0.87 was obtained in powdered form using the ordering effect. The composition, structure, and properties of the carbide were studied by chemical and thermogravimetric analysis, gas chromatography, x-ray diffraction, optical and electronic microscopy, electron-positron annihilation, magnetic susceptibility, and microhardness methods. Nanostructured vanadium carbide VC_0.87 possesses the crystal structure of the cubic ordered phase V₈C₇ with space group P4₃32. Vanadium carbide nanocrystallites are shaped in the form of 400–600 nm in diameter and 15–20 nm thick curved petals. The surface layer of the nanocrystallites contains defects of the vacancy agglomerate type. The microhardness of vanadium carbide, obtained by vacuum sintering of VC_0.87 nanopowder was 60–80 GPa, which is 3–4 times greater than the microhardness of coarse-grained vanadium carbide with the same composition and close to the hardness of diamond. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 6, 436–442 (25 March 1999)     

Effect of carbon vacancies on the electric resistivity of nonstoichiometric VC<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript> vanadium carbide

The influence of the temperature, concentration, and distribution of structure vacancies of the carbon sublattice on the electric resistivity ρ of nonstoichiometric VC _y vanadium carbide (0.66 ≤ y ≤ 0.875) has been studied in the temperature range of 300–1200 K. The symmetry and structure characteristics of the ordered V₆C₅ and V₈C₇ phases formed owing to low-temperature annealing on various sections of the homogeneity region of the VC _y carbide. The dependence of the residual electric resistivity on the content of the disordered vanadium carbide is explained by the atom-vacancy interaction and the change in the carrier concentration in the homogeneity region of VC _y .     

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.     

Effect of nonstoichiometry on the lattice constant of cubic vanadium carbide VC<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript>

The effect of nonstoichiometry on the lattice constant of cubic vanadium carbide VC _y (0.65 < y < 0.875) is studied. It is found that the ordering of vanadium carbide VC _y with the formation of superstructures V₆C₅ and V₈C₇ leads to an increase in the base lattice constant in comparison with disordered carbide. Taking into account the change in the lattice constant, the direction of the static displacements of atoms near the vacancy is discussed.     

Effects of nonstoichiometry and ordering on the basic lattice constant of vanadium carbide VC<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript>

The effect of nonstoichiometry and ordering on the lattice constant a _B1 of the basic lattice of vanadium carbide VC _y (0.65 < y < 0.875) is studied. A change in the lattice constant of disordered carbide VC _y at the reduction of the carbon content is considered using the direction of static displacements of atoms near a vacancy. A model for the calculation of the basic lattice constant a _B1 of vanadium carbide is proposed taking into account nonstoichiometry and ordering. It is shown that the ordering of vanadium carbide VC _y with the formation of V₆C₅ and V₈C₇ superstructures results in an increase in the basic lattice constant as compared to disordered carbide.     

Order-disorder phase transformations and specific heat of nonstoichiometric vanadium carbide

AbstractA study has been made of the order-disorder phase transformations in the homogeneity region of the VC_y nonstoichiometric cubic vanadium carbide (0.66<y<0.88). It has been established that an ordered V₆C₅ phase with monoclinic (space group C2/m) or trigonal (P3₁) symmetry, and a cubic (space group P4₃32) ordered V₈C₇ phase can form in the VC_y carbide below 1450 K, depending on the actual composition. The effect of off-stoichiometry and structural vacancy ordering on the specific heat of the VC_y carbide has been investigated. The temperatures and heats of the reversible order-disorder equilibrium transitions have been determined. The ordering in the VC_y carbide is shown to be a first-order phase transition. An equilibrium diagram of the V-C system taking into account ordering in the nonstoichiometric vanadium carbide has been constructed. Fiz. Tverd. Tela (St. Petersburg) 41, 529–536 (March 1999)     

Study of nanocrystalline and amorphous powders prepared by mechanical alloying

The process of mechanical alloying consists of intimate mixing and mechanical working of elemental powders in a high-energy ball mill. It has been well established that this process is able to produce nanocrystalline and amorphous material. In this study, the structural effects of mechanical alloying of pure Fe, Fe₅₀W₅₀ and Fe₅₀Mo₅₀ powders were investigated by X-ray diffraction and Mössbauer spectroscopy. For all cases, nanocrystalline and/or amorphous fractions were found after milling. The resulting particle size was determined by X-ray diffraction. Pure Fe does not amorphize even after prolonged milling times. For the nanocrystalline powder, significant changes in the linewidth and the hyperfine field are found. Powder mixtures of Fe₅₀Mo₅₀ and Fe₅₀W₅₀ are completely amorphous after milling times of 10 h, as seen by Mössbauer spectroscopy, but nanocrystalline fractions of the non-iron part are still found in X-ray diffraction. Also in the amorphous state, further changes in the hyperfine parameters are found with increasing milling time.     

Model for milling of powders

A model is proposed for mechanical milling of powders that relates the applied energy to average particle size D in the powders. It is shown that the milling energy is consumed for the rupture of interatomic bonds in crystalline particles and for the creation of an additional surface during powder fragmentation. The appearance of microstrains ɛ retards powder fragmentation. Average particle size D after milling decreases with increasing milling time t and decreasing particle size in the initial powder or its mass M. The calculated results are compared to the experimental data obtained on a tungsten carbide WC powder.     

Accounting for nonstoichiometry of niobium carbide NbC y upon milling to a nanocrystalline state

The dependence of the size of particles in the prepared nanocrystalline powders on the composition of nonstoichiometric compounds within their homogeneity intervals has been considered in terms of the high-energy ball milling model. It has been shown that the effect of nonstoichiometry on the milling manifests itself in the concentration dependences of the main characteristics (parameters of the crystal structure, energy of interatomic bonds, elastic properties) of the milled nonstoichiometric compound. The results of model calculations performed for nonstoichiometric cubic niobium carbides NbC _y have been compared with the experimental data on milling of the NbC_0.93 carbide.     

Preparation of α-Fe2O3 nanoparticles by high-energy ball milling

α-Fe₂O₃ nanoparticles were prepared by high-energy ball milling using α-FeOOH as raw materials. The prepared samples were characterized by transmission electron microscopy (TEM), Mössbauer spectroscopy, X-ray diffraction (XRD) and differential thermal analysis–thermogravimetric analysis (DTA–TGA). The results showed that after 90 h milling α-Fe₂O₃ nanoparticles were obtained, and the particle size is about 20 nm. The mechanism of reaction during milling is supposed that the initial α-FeOOH powder turned smaller and smaller by the high-speed collision during ball milling, later these particles turned to be superparamagnetic, at last these superparamagnetic α-FeOOH particles were dehydrated and transformed into α-Fe₂O₃.     

Microstructure characterization of mechanosynthesized nanocrystalline NiFe2O4 by Rietveld's analysis

Nanocrystalline nickel ferrite (NiFe₂O₄) is synthesized at room temperature by high-energy ball milling the stoichiometric mixture of (1:1 mol%) of NiO and α-Fe₂O₃ powders. The structural and microstructural evolution of NiFe₂O₄ caused by milling is investigated by X-ray powder diffraction. The relative phase abundance, particle size, r.m.s. strain, lattice parameter changes of different phases have been estimated employing Rietveld structure refinement analysis of X-ray powder diffraction data. Particle size and content (wt%) of both NiO and α-Fe₂O₃ phases reduce rapidly with increasing milling time and a significant amount of nanocrystalline NiFe₂O₄ is formed within 1 h of ball milling. Particle sizes of all the phases reduce to ∼10 nm within 5 h of milling and remain almost unchanged with increasing milling time up to 20 h. Lattice parameter of cubic NiO decreases linearly with increasing milling time, following the Vegard's law of solid-solution alloy. A continuous decrease in lattice parameter of cubic NiFe₂O₄ phase clearly suggests that smaller Ni atoms have occupied some of the vacant oxygen sites of ferrite lattice. Cation distribution both in octahedral and tetrahedral sites changes continuously with milling time and the normal spinel lattice formed at early stage of milling, transforms to inverse spinel lattice in the course of milling. High-resolution transmission electron microscope (HRTEM) micrographs of 11 h milled sample corroborates the findings of X-ray profile analysis.     

Kinetics of cementite mechanosynthesis

The influence of ball to powder ratio and discontinous milling during ball milling of Fe_0.75C_0.25 elemental powder mixture is presented. The formation of Fe₃C cementite, Hägg and ε carbides has been follwed by X-ray diffraction and Mössbauer Spectroscopy. Higher ball to powder ratios induce faster kinetics. Synthesized cementite has crystallite sizes 9 to 11 nm.     

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.     

Evolution of structure, microstructure and hyperfine properties of nanocrystalline Fe50Co50 powders prepared by mechanical alloying

Nanostructured Fe₅₀Co₅₀ powders were prepared by mechanical alloying of Fe and Co elements in a vario-planetary high-energy ball mill. The structural properties, morphology changes and local iron environment variations were investigated as a function of milling time (in the 0-200 h range) by means of X-ray diffraction, scanning electron microscopy (SEM), energy dispersive X-ray analysis and ⁵⁷Fe Mössbauer spectroscopy. The complete formation of bcc Fe₅₀Co₅₀ solid solution is observed after 100 h milling. As the milling time increases from 0 to 200 h, the lattice parameter decreases from 0.28655 nm for pure Fe to 0.28523 nm, the grain size decreases from 150 to 14 nm, while the meal level of strain increases from 0.0069% to 1.36%. The powder particle morphology at different stages of formation was observed by SEM. The parameters derived from the Mössbauer spectra confirm the beginning of the formation of Fe₅₀Co₅₀ phase at 43 h of milling. After 200 h of milling the average hyperfine magnetic field of 35 T suggests that a disordered bcc Fe-Co solid solution is formed.     

Analysis of ball-milled Mo powder using X-ray diffraction

X-ray diffraction measurements and analysis were carried out on ball-milled Mo powder. During the ball milling of Mo powder, several stages of deformation could be identified. After short durations of ball milling, still undeformed starting powder was present and the volume fraction of this was determined. The initial aggregates of deformed powder particles exhibited a deformation texture. On prolonged ball milling, the particle size decreased, the deformation texture disappeared and internal strains built up. By simulation and matching of the corresponding line profiles using a Monte Carlo type of line-profile simulation based on a simple three-dimensional model of the distribution of straight dislocations, an estimate of the dislocation density in the ball-milled particles was obtained.     

Investigation of phase transition of γ-alumina to α-alumina via mechanical milling method

In this research, the results obtained from studying the phase transition of γ-alumina (γ-Al₂O₃) to α-alumina (α-Al₂O₃) during intense mechanical activation in high-energy ball milling are presented. The powder samples were characterized by scanning electron microscope (SEM), X-ray diffraction (XRD), thermogravimetric and Differential thermal analyses (TG-DTA). With respect to the results achieved from above analyses, the transition of γ → α-alumina through δ- and θ-phases, can be initiated. Also, it was found that the pure γ-alumina phase showed a great stability during high-energy ball milling and there was no transformation to any other phase after a long milling time (30 h). On the other hand, γ-alumina containing a small amount of the α-alumina seed, showed a gradual phase transition from γ-alumina to α-alumina in milling. The phase transition mechanism during milling is nucleation and growth, which is promoted by the α-alumina seed.     

Combustion synthesis of nano-sized tungsten carbide powder and effects of sodium halides

The synthesis of nano-size tungsten carbide powder has been investigated with a WO₃ + Mg + C + carbonate system using alkali halides. The effects of different types of alkali halides on combustion temperature and tungsten carbide formation were discussed. Sodium fluoride had a notable effect on the particle size of the product and the degree of transformation from the initial mixture. A small amount of ammonium carbonate activated the carburization of tungsten carbide by the gas phase carbon transportation. X-ray diffraction data and particle analysis showed that the final product synthesized from a WO₃–Mg–C–(NH₄)₂CO₃–NaF system contains pure-phase tungsten carbide with a particle size of 50–100 nm.