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1.
AbstractA study has been made of the order-disorder phase transformations in the homogeneity region of the VCy 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 VCy carbide below 1450 K, depending on the actual composition. The effect of off-stoichiometry and structural vacancy ordering
on the specific heat of the VCy carbide has been investigated. The temperatures and heats of the reversible order-disorder equilibrium transitions have been
determined. The ordering in the VCy 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) 相似文献
2.
A. I. Gusev 《JETP Letters》2009,90(3):191-196
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
. 相似文献
3.
A nanocrystalline powder of nonstoichiometric vanadium carbide VC0.875 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 VC0.875 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. 相似文献
4.
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. 相似文献
5.
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. 相似文献
6.
G. E. Abrosimova A. S. Aronin I. I. Zver’kova A. F. Gurov Yu. V. Kir’yanov 《Physics of the Solid State》1998,40(1):8-13
Radiography, differential scanning calorimetry, luminescence and high-resolution electron microscopy are used to study the
production, nanocrystalline structure, stability, and microhardness of alloys from the Ni-Mo-B system containing from 27 at.
% to 31.5 at. % Mo and 10 at. % B. All studies of these alloys indicated that annealing at 600 °C leads to the creation of
a granular phase consisting of FCC nanocrystallites with average grain sizes of 15–25 nm, depending on the chemical composition
of the alloy. Annealing these nanocrystalline samples isothermally at a temperature of 600 °C has no appreciable effect on
the grain size. Structurally, the nanocrystalline phase consists of grains of an FCC solid solution of Mo and B in Ni, dispersed
in an amorphous matrix that isolates them from one another. The lattice parameters of the FCC nanocrystallites depend on the
alloy composition and the duration of their isothermal anneal. Within this latter time, molybdenum and boron atoms diffuse
from the FCC solid-solution lattice into the surrounding amorphous matrix. The stability of the nanocrystalline structure
is determined by the thermal stability of the amorphous matrix, whose crystallization temperature increases with the isothermal
annealing time due to enrichment by boron and molybdenum. As the structure forms, the alloy becomes harder as the nanocrystalline
grains grow in size. This relation between hardness and grain size, which is opposite to the Hall-Petch law, is explained
by hardening of the amorphous matrix due to changes in its chemical composition.
Fiz. Tverd. Tela (St. Petersburg) 40, 10–16 (January 1998) 相似文献
7.
The experimental results on the change in the crystal structure and specific heat of nonstoichiometric titanium carbide TiCy (0.5<y<0.65) near disorder-order phase transitions are reported. It is established that at temperatures below 1000 K the ordered
phases Ti2C with cubic and trigonal symmetry and an orthorhombic ordered phase Ti3C2 form in titanium carbide by a close-to-first-order phase transition mechanism. The temperatures and heats of order-disorder
phase transformations are determined.
Pis’ma Zh. éksp. Teor. Fiz. 69, No. 9, 631–637 (10 May 1999) 相似文献
8.
Phase transformations of the disorder-order type in the homogeneity region of nonstoichiometric titanium carbide TiCy (0.5<y<1.0) have been studied. It has been established that, depending on the actual composition of TiCy, there may form in it for T<980–1000 K a cubic or a trigonal ordered Ti2C phase (space groups Fd3m and
, respectively) and a rhombic ordered Ti3C2 phase (space group C2221). The effect of ordering on the electrical resistivity of the nonstoichiometric carbide TiCy was studied, and the temperatures of the reversible disorder-order equilibrium transitions determined. The ordering in titanium
carbide is shown to be a first-order phase transition.
Fiz. Tverd. Tela (St. Petersburg) 40, 1332–1340 (July 1998) 相似文献
9.
The influence of temperature and of the density and distribution of structural vacancies in the carbon sublattice on the resistivity
of nonstoichiometric titanium carbide TiCy(0.5⩽y⩽0.98) is studied. It is shown that in titanium carbide TiCy with y<0.7 reversible disorder-order structural phase transitions occur at temperatures below 1000 K. The temperatures of order-disorder
phase transformations are determined. The dependence of the residual resistivity on the composition of the disordered titanium
carbide is explained by the change in the carrier density in the region of homogeneity of the carbide TiCy, on the one hand, and the atom-vacancy interaction, on the other.
Pis’ma Zh. éksp. Teor. Fiz. 70, No. 4, 284–289 (25 August 1999) 相似文献
10.
A study of the changes in the structure of melt-quenched Fe90Zr10 amorphous alloys by x-ray diffraction, Auger spectroscopy, and transmission electron microscopy is reported. The samples
were subjected to isochronous (for 1 h) and isothermal anneals at 100–650 °C. It is shown that an amorphous alloy annealed
for one hour at 300–500 °C crystallizes with formation of a supersaturated solid solution of Zr in α Fe and the intermetallic compound Fe3Zr. Isothermal anneal at 100 °C for up to 7000 h produces nanocrystallites 110–30 nm in size, with fuzzy interfaces between
the grains. An alloy subjected to such an anneal contains two solid solutions of Zr in Fe, having a cubic and a weakly tetragonal
lattice. Crystallization taking place during low-temperature anneals is preceded by phase segregation of the alloy within
the amorphous state. The lattice periods of the solid solutions have been determined. The possibility of the alloy crystallizing
by spinodal decomposition during prolonged annealing is discussed.
Fiz. Tverd. Tela (St. Petersburg) 40, 1769–1772 (October 1998) 相似文献
11.
L. K. Orlov E. A. Shteinman N. L. Ivina V. I. Vdovin 《Physics of the Solid State》2011,53(9):1798-1805
The light-emitting properties of cubic silicon carbide films grown by vacuum vapor phase epitaxy on Si(100) and Si(111) substrates
under conditions of decreased growth temperatures (T
gr ∼ 900–700°C) have been discussed. Structural investigations have revealed a nanocrystalline structure and, simultaneously,
a homogeneity of the phase composition of the grown 3C-SiC films. Photoluminescence spectra of these structures under excitation of the electronic subsystem by a helium-cadmium
laser (λexcit = 325 nm) are characterized by a rather intense luminescence band with the maximum shifted toward the ultraviolet (∼3 eV)
region of the spectral range. It has been found that the integral curve of photoluminescence at low temperatures of measurements
is split into a set of Lorentzian components. The correlation between these components and the specific features of the crystal
structure of the grown silicon carbide layers has been analyzed. 相似文献
12.
N. G. Galkin V. M. Astashynski E. A. Chusovitin K. N. Galkin T. A. Dergacheva A. M. Kuzmitski E. A. Kostyukevich 《Journal of Applied Spectroscopy》2009,76(6):840-846
Consecutive plasma-epitaxial synthesis on silicon wafers is used for the first time to fabricate monolithic nanoheterostructures
with embedded nanocrystals (NC) of chromium disilicide (Si–NC CrSi2–Si). It is found that, initially, nanoislands form on the surface and within a coating layer of silicon, followed by the
formation of small (10–15 nm) nanocrystals of semiconducting chromium disilicide (CrSi2) at a high occupation density ((2–3)⋅1011 cm–2). During formation of silicon-silicide-silicon heterostructures, CrSi2 nanocrystallites “float up” into the near surface area of the covering silicon layer. 相似文献
13.
N. L. Arabadzhyan V. I. Serdobintsev V. M. Tavkhelidze 《Physics of the Solid State》1998,40(4):679-681
Low-frequency acoustics is used to study the β-δ phase transition in the vanadium hydride VH0.73. It is established that the nature of the transition depends significantly on the degree of perfection of the crystal structure
of the hydride. It is shown that such behavior can be attributed to long-period ordering in the investigated system.
Fiz. Tverd. Tela (St. Petersburg) 40, 738–740 (April 1998) 相似文献
14.
A. V. Voitsekhovskii S. N. Nesmelov S. M. Dzyadukh V. S. Varavin S. A. Dvoretskii N. N. Mikhailov Yu. G. Sidorov 《Russian Physics Journal》2011,54(3):263-270
The effect of regions with periodic sharp 48–54 nm thick inhomogeneities in composition on the electrophysical characteristics
of MIS structures based on graded-gap n-Hg1–x
Cd
x
Te (x = 0.29–0.31) grown by molecular-beam epitaxy is studied. It is found that major electro-physical and photo-electrical
characteristics are qualitatively similar for MIS structures based on n-Hg1–x
Cd
x
Te with sharp inhomogeneities in composition (barriers) and without “barriers”. It is shown that the electrical characteristics
are mostly affected by the “barrier regions” located close to the insulator – semiconductor interface. This effect is manifested
in an increase of the effective dielectric thickness, which can be due to the fact that the regions of enhanced composition
form potential barriers for electrons, and in a decrease in the relaxation time of non-equilibrium carriers due to recombination
at the boundaries of the regions with sharp changes in composition. 相似文献
15.
The monoclinic (space group C2/m) superstructure of V14O6, which is formed in the atom-vacancy ordering of the tetragonal solid solution of oxygen in vanadium, is studied by the methods
of x-ray diffraction and symmetry analysis. It has been found that the channel of the order-disorder phase transition attributed
to the formation of the monoclinic suboxide V14O6 includes six superstructure vectors belonging to three non-Lifshitz stars {k
1−1}, {k
1−2}, and {k
1–3} of one type {k
1}. The distribution function of the O atoms in the V14O6 monoclinic superstructure has been calculated. It has been shown that the displacements of V atoms distort the body-centered
tetragonal metal sublattice, thus preparing the formation of the fcc sublattice and the transition from the suboxide V14O6 to the cubic vanadium monoxide with the B1 structure.
Original Russian Text ? A.I. Gusev, D.A. Davydov, 2007, published in Pis’ma v Zhurnal éksperimental’noĭ i Teoreticheskoĭ Fiziki,
2007, Vol. 86, No. 10, pp. 746–751. 相似文献
16.
P. Yang M. Lü D. Xu D. Yuan C. Song G. Zhou 《Applied Physics A: Materials Science & Processing》2002,74(4):525-528
Direct synthesis of ZnS nanocrystallites doped with Ti3+ or Ti4+ by precipitation has led to novel photoluminescence properties. Detailed X-ray diffraction (XRD), fluorescence spectrophotometry,
UV–vis spectrophotometry and X-ray photoelectron spectroscopy (XPS) analysis reveal the crystal lattice structure, average
size, emission spectra, absorption spectra and composition. The average crystallite size doped with different mole ratios,
estimated from the Debye–Scherrer formula, is about 2.6±0.2 nm. The nanoparticles can be doped with Ti3+ and Ti4+ during the synthesis without the X-ray diffraction pattern being altered. The strong and stable visible-light emission has
been observed from ZnS nanocrystallites doped with Ti3+ (its maximum fluorescence intensity is about twice that of undoped ZnS nanoparticles). However, the fluorescence intensity
of the ZnS nanocrystallites doped with Ti4+ is almost the same as that of the undoped ZnS nanoparticles. The emission peak of the undoped sample is at 440–450 nm. The
emission spectrum of the doped sample consists of two emission peaks, one at 420–430 nm and the other at 510 nm.
Received: 27 April 2001 / Accepted: 16 August 2001 / Published online: 17 October 2001 相似文献
17.
B. P. Gorshunov E. S. Zhukova E. G. Maksimov A. S. Prokhorov V. I. Torgashev T. Zhang D. Wu M. Dressel 《JETP Letters》2010,91(7):336-340
The nature of the low-energy excitations of polycrystalline and nanostructured La0.25Ca0.75MnO3 samples has been analyzed in order to investigate the mechanisms of charge ordering in manganites. It has been found that
the electrodynamic response spectra of La0.25Ca0.75MnO3 in the energy range of 0.5 to 90 meV and the temperature range of 5 to 300 K have no resonance features that could be attributed
to the collective excitations of the charge-ordered phase. It has been shown that the absorption lines observed at frequencies
of 20–40 and 80–100 cm–1 are attributed to usual acoustic phonons becoming optically active owing to the structure phase transition and the appearance
of a fourfold superstructure with a quadruple period along the crystallographic a axis. The suppression of the superstructure has been revealed in samples with nanocrystallites (≤40 nm). This suppression
indicates a relatively weak coupling of the charge and magnetic order parameters with the phonon subsystem. 相似文献
18.
We have examined the influence of nonstoichiometry and order-disorder phase transformations on the basis period (of type B1) of the structure of titanium carbide TiCy (0.5<y<1.0). We found that ordering of titanium carbide TiCy with formation of superstructures of the type Ti Ti2C and Ti3C2 leads to growth of the period of the basis crystal lattice in comparison with the disordered carbide. Taking the change in
the lattice period into account, we discuss the question of the directions of the static displacements of atoms near a vacancy.
Fiz. Tverd. Tela (St. Petersburg) 41, 1134–1141 (July 1999) 相似文献
19.
W. Kulisch P. Colpo P.N. Gibson G. Ceccone D.V. Shtansky E.A. Levashov M. Jelinek P.J.M. Philip F. Rossi 《Applied Physics A: Materials Science & Processing》2006,82(3):503-507
TiC/a-C nanocomposite films doped with CaO have been deposited by means of a hybrid PVD/PACVD technique, which combines dc
magnetron sputtering of a TiC0.5+10% CaO target, with a subsequent high density inductively coupled plasma (ICP) in order to excite and ionize the sputtered
species to a high degree. The films were characterized according to their morphology, structure and thickness by scanning
electron microscopy, their composition and bonding structure by X-ray photoelectron spectroscopy, and their crystalline properties
by X-ray diffraction. The films consist of fcc titanium carbide nanocrystallites with grain sizes of 5–15 nm and an amorphous
carbon phase. The average composition is Ti0.43C0.35Ca0.02O0.15N0.05. The influence of the ICP plasma power and the bias voltage applied to the substrate on the major film properties has been
investigated. A number of such TiC/a-C/CaO nanocomposite films on silicon substrates have been subjected to a 60 day cell
test with human osteoblastic cells in order to investigate their suitability for the coating of prostetic implants. The results
of these cell tests, some of which turned out to be rather promising, are discussed in terms of film properties such as surface
roughness and biaxial stress.
PACS 68.55.-a; 81.07.-b; 87.68.+z 相似文献
20.
Molecular structures of the isolated tetrahedral oxovanadium(IV) and bridged μ-oxo-divanadium(IV) complexes hosted by the
clusters mimicking surfaces of amorphous silica-based materials were investigated using density functional theory (DFT) calculations.
Principal values of the g and A tensors for the monomer vanadyl species were obtained using the coupled-perturbed DFT level of theory and the spin–orbit
mean-field approximation (SOMF). Magnetic exchange interaction for the μ-oxo bridged vanadium(IV) dimer was investigated within
the broken symmetry approach. An antiferromagnetic coupling of the individual magnetic moments of the vanadium(IV) centers
in the [VO–O–VO]2+ bridges was revealed and discussed in detail. The coupling explains pronounced decrease of the electron paramagnetic resonance
signal (EPR) intensity, observed for the reduced VO
x
/SiO2 samples with the increasing coverage of vanadia, in terms of transformation of the paramagnetic monomer species into the
dimers with S = 0 ground state. 相似文献