Population analysis solution to hardness enhancement in TiCxN1−x

We investigated the hardness enhancement in titanium carbonitrides (TiC_xN_1−x) by the population analysis method based on first-principles calculations. Populations for bonds TiC and TiN in TiC_xN_1−x (0.25<x<0.75) are all positive. The enhanced hardness for titanium carbonitrides is well explained by overlap population analysis. Intrinsic hardness of TiC_xN_1−x has been calculated based on the obtained overlap populations. The calculated results are in good agreement with the available experimental data.     

Epitaxial growth of titanium oxycarbide on MgO (0 0 1) substrates by pulsed laser deposition

Epitaxial TiC_xO_y thin films were grown on MgO (0 0 1) substrates by using pulsed laser deposition method. High-resolution X-ray diffraction and transmission electron microscopy were used to examine crystallinity and microstructure of epitaxial TiC_xO_y film on MgO. The chemical composition of the film is determined to be x ∼ 0.47 and y ∼ 0.69 by X-ray photoelectron spectroscopy. Atomic force microscopy revealed that the surface of TiC_xO_y film is very smooth with roughness of 0.18 nm. The resistivity of the TiC_xO_y film measured by four-point-probe method was about 137 μ Ω cm.     

Thermodynamic properties of titanium dioxide,niobium dioxide and their solid solutions at high temperature

By means of thermogravimetric measurements, it has been possible to obtain information on the nature of the intrinsic and extrinsic defects of TiO₂, NbO₂ and solid solutions Nb_yTi_1?yO_2±x Pure TiO₂ is an oxygen-deficient oxide The main defects are oxygen vacancies, doubly ionized V^.._o or singly ionized V^._o, and interstitial titanium Ti³_i NbO₂ is a metal-deficient oxide The main defects are neutral niobium vacancies. The solid solutions Nb_yTi_1?yO_2±x may be divided into two groups If y > 0 04, the behavior is analogous to that of NbO₂; with the same defects, but the width of the homogeneity range decreases with the titanium content and Nb_{0 04}Ti_{0 96}O₂ is a stoichiometnc oxide If y < 004, the oxides are both metal deficient and oxygen deficient according to the oxygen partial pressure. We have particularly studied the solution Ti_{0 995}Nb_{0 005}O₂ In the oxygen-deficient domain, the main defects are assumed to be neutral or singly ionized oxygen vacancies In the metal-deficient domain, the main defects are metal vacancies V⁴_Ti From these results we have deduced the nature of intrinsic defects in TiO₂ to be Schottky defects: 2V^.._O + V⁴_T1.     

Interpretation of ESCA spectra for non-stoichiometric titanium carbides on the basis of MO—LCAO calculations

The results of electron structure calculations for the single clusters TiC₆^20?, TiC₅□^16?, TiC₄□₂^12?, TiC3□3^8?, TiC₅□^16.64?, TiC₄□₂^13.32?; and double clusters Ti₂C₁₀^32?, Ti₂C₈□₂^24?, Ti₂C₁₀□^32.72? and Ti₂C₈□₂^24.80? by the semi-empirical Mulliken—Wolfsberg—Helmholtz method with self-consistency of charges and configurations are given. The data obtained are compared with measurements of the Ti 2p-level binding energies and X-ray photoelectron valence band spectra for TiC_x(x = 0.86–0.97). The variation of the metal atom charge in the limits of the homogeneity region for TiC and the appearance of this effect as a chemical shift of the Ti 2p-level are discussed. A special set of cluster calculations was performed to check the possibility of Ti valency variation in the homogeneity region of TiC_x.     

Synthesis of nanocrystalline multiphase titanium oxycarbide (TiC<Subscript>x</Subscript>O<Subscript>y</Subscript>) thin films by UNU/ICTP and NX2 plasma focus devices

Nanocrystalline multiphase titanium oxycarbide (TiC_xO_y) thin films composed of TiC₂, TiO_0.325, Ti₂O₃ and graphitic carbon have been deposited on titanium substrates, using energetic carbon ions delivered by the UNU/ICTP and the NX2 plasma focus devices operated at different repetition rates. X-ray diffraction (XRD) patterns of the nanocomposite films reveal the relative transformation of various oxide and carbide phases accompanied by the suppression of the TiC₂ phase when the energy flux of the ion beam and the repetition rate are increased. A field emission scanning electron microscope (FESEM) with an energy dispersive X-ray spectroscopy (EDS) attachment reveals a non-porous microcrack-free nanocrystalline granular surface morphology of the composite films with uniform carbon distribution. X-ray photoelectron spectroscopy (XPS) confirms the formation of oxycarbides (TiC_xO_y) along with significant carbon adsorbate. Raman studies of the films also verify the relative phase transformation in the TiC_xO_y nanocomposite by tuning the deposition parameters. The Vickers microhardness of the sample surface is improved more than 400%. PACS 52.59.Hq; 52.77.Dq; 68.55.Jk; 81.15.-z; 81.65.Lp     

Concentration-phase transitions in solid solutions Li x Na1 − x Ta0.1Nb0.9O3, Li0.12Na0.88Ta y Nb1 − y O3, and NaTa y Nb1 − y O3 and their manifestations in Raman spectra

Using the Raman-spectroscopy method, we have studied concentration-phase transitions in the solid solutions Li _x Na_{1 ? x} Ta_0.1Nb_0.9O₃, Li_0.12Na_0.88Ta _y Nb_{1 ? y} O₃, and NaTa _y Nb_{1 ? y} O₃ (x = 0?0.16, y = 0–1). It has been revealed that, for the solid solutions Li _x Na_{1 ? x} Ta_0.1Nb_0.9O₃ and Li_0.12Na_0.88Ta _y Nb_{1 ? y} O₃, the concentration-phase transition is a transition between the ferroelectric and antiferroelectric states. It is accompanied by the disappearance from the spectrum of a line that corresponds to stretching bridge vibrations of oxygen atoms along the polar axis, which is forbidden by selection rules in the centrosymmetric phase, and by splitting into two components of a line that corresponds to librational vibrations of oxygen octahedra as a whole, which can be caused by doubling of the unit cell in the antiferroelectric phase. Manifestation and variation of intensities of lines with frequencies 860–873 and 900–905 cm^?1 upon variation of the concentration of tantalum for solid solutions Li_0.12Na_0.88Ta _y Nb_{1 ? y} O₃, and NaTa _y Nb_{1 ? y} O₃ is caused by the formation of polar clusters in the medium, which is centrosymmetric in general due to disordering in the sublattice of niobium and tantalum.     

Surface segregation of ZrC from a carbide solid solution

The surface segregation of zirconium carbide from carbide solid solutions is investigated. The spontaneous surface segregation of ZrC grains from solid solutions in the pseudobinary ZrC-NbC system is revealed for the first time. It is demonstrated that the ZrC precipitation is associated with the decomposition of the car-bide homogeneous solid solutions Zr_1?x Nb_xC ≡ (ZrC)_1?x (NbC)_x. The boundaries of the latent solid-phase decomposition region formed at T<1200 K are determined for the solid solutions formed by ZrC_y and NbC _y′ carbides with different nonstoichiometry. The experimental and theoretical estimates obtained for the segregation energy of ZrC are equal to ?50 and ?31 kJ mol^?1, respectively.     

Investigation of TiC-C coatings by X-ray photoelectron spectroscopy

Titanium carbide-based coatings deposited by arc-technology in C₂H₂/Ar atmosphere were studied by X-ray photoelectron spectroscopy. It was found that, in addition to the cubic phase of TiC _x O _y oxycarbide, the films contain carbon in the amorphous, presumably graphite-like state. In carbon C1s spectra, bands at 282.0, 284.4, and 286.0 eV correspond to the TiC _x state, amorphous carbon, and C-C bonds, respectively. The maximum at 283.0 eV was interpreted as the C state in titanium carbide nanoforms, i.e., Ti₁₄C₁₃ clusters or Ti₈C₁₄ carbohedra. The phase ratio was varied by coating deposition conditions, i.e., TiC/a-C deposition by Ti cathode sputtering in C₂H₂/Ar, and composite Ti/C target sputtering in Ar and C₂H₂/Ar. When using the Ti cathode and C₂H₂/Ar gas mixture, the ratio of carbide and amorphous a-C phases was estimated as 1: 1; the surface layer ～15 nm thick was enriched with amorphous carbon. It was assumed that TiC/a-C composite coatings with an additional a-C film on the surface would have an increased stability in reaction media and good biocompatibility.     

Features of the local structure of hydrated fluorine-substituted solid solutions based on Ba2In2O5

It is found for hydrated solid solutions of Ba_2?0.5x In₂O_5?x F _x · nH₂O, Ba₂In₂O_5?0.5y F _y · nH₂O (0 ≤ x ≤ 0.30; 0 ≤ y ≤ 0.24) that increasing the concentration of fluorine lowers the degree of hydration. The composition of proton-containing groups and the places of their localization in the structure of hydrated oxyfluorides are determined. The presence of fluorine in the complex oxide’s structure helps increase the mobility of protons.     

Ellipsometrical determination of barrier thicknesses of metal-insulator-metal tunnel junctions

The thickness of the tunneling barrier of AlAl_xO_yAl and AlAl_xO_yPb junctions was determined using an automated ellipsometer. It is shown that the tunneling resistance depends exponentially on the barrier thickness. A practical consequence is that any wanted tunneling resistance can be realized within the accuracy of half an order of magnitude.     

Effect of cation substitution on the lattice dynamics and ferroelectric instability of cubic BaTiO<Subscript>3</Subscript> and BaZrO<Subscript>3</Subscript> doped with Bi and La ions

The spectra of lattice vibration frequencies of solid solutions Ba_{1 ? x} Bi_2x/3? _x/3Ti(Zr)O₃ and Ba_{1 ? x} La _x Ti(Zr)_{1 ? x/4}? _x/4O₃ are calculated in terms of a generalized Gordon-Kim model with inclusion of the dipole and quadrupole polarizabilities. Over the entire concentration range, the calculated phonon spectra contain a ferroelectric soft mode. The effect of various interactions on the ferroelectric instability of these solid solutions is studied. It is shown that the character of ferroelectric instability is largely determined by the mechanism of charge compensation.     

Mixed conducting perovskite-like ceramics on the base of lanthanum gallate

Ceramic perovskite solid solutions (La_0.9Sr_0.1)[(Ga_1−xM_x)_0.8Mg_0.2]O_3−y, 0 ≤ x ≤ 0.5, M = Fe, Ni, Cr (systems I–III) and brownmillerite solid solutions (La_0.2Sr_1.8)[Ga(Fe_1−xMg_x)]O_5−z, 0 ≤ x ≤ 0.5, (system IV) have been prepared. The samples have been studied by X-ray diffraction and electron microscopy methods, dielectric spectroscopy and permeability measurements. The correlation between the composition, unit cell parameter changes, electrical transport and oxygen permeation properties has been revealed. Introduction of transition metals (Fe, Ni, or Cr), substituting for gallium, ensures the enhancement of the electronic constituent of the conductivity in the perovskite systems I–III. Stabilization of the transition metal high valence states 4+ or 5+ has been suggested for compositions I and III. This leads to a unit cell volume contraction and provides a decrease in the concentration of oxygen vacancies. The oxygen permeability reaches its maximum values in compositions I–III with x ∼ 0.3. On the contrary, increasing concentration of the doping element with lower valence state (magnesium), substituting for iron, determines the expansion of the brownmillerite unit cell volume and provides an increase of the oxygen vacancy concentration, which in turn, favors the enhancement of oxygen permeability of composition IV.     

Effect of nonstoichiometry and ordering on the period of the basis structure of cubic titanium carbide

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 TiC_y (0.5<y<1.0). We found that ordering of titanium carbide TiC_y 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)     

Synthesis and physical properties of negative thermal expansion materials Zr1−xMxW2O8−y (M=Sc, In, Y) substituted for Zr(IV) sites by M(III) ions

Zr_1−xM_xW₂O_8−y (M=Sc, In and Y) solid solutions substituted up to x=0.04 for Zr(IV) sites by M(III) ions were synthesized by a solid-state reaction. X-ray diffraction experiments from 90 to 560 K revealed that all solid solutions had a cubic crystal structure and showed negative thermal expansion coefficients. The lattice parameters of Zr_1−xM_xW₂O_8−y were smaller than that of ZrW₂O₈ probably due to oxygen defects, though the ionic radii of substituted M³⁺ ions were larger than that of Zr⁴⁺. Order-disorder phase transition temperatures of the substituted samples drastically decreased in the order of Y, In and Sc compared to the percolation theory, and decreased with increasing M content.     

An Ab Initio Study of the Structural and Electronic Properties of the Low-Defect TiC(110) Surface Simulating Oxygen Adsorption after Exposure to Laser Plasma

An ab initio simulation of the adsorption of atomic oxygen on the low-defect titanium carbide (110) surface reconstructed by laser radiation was performed. The relaxed atomic structures of the (110) surface of the O/Ti _x C _y system with Ti and C vacancies observed during the thermal treatment were studied in terms of the density functional theory. DFT calculations of their structural, thermodynamic, and electronic properties were performed. The bond lengths and adsorption energies were determined for various reconstructions of the atomic structure of the O/Ti _x C _y (110) surface. The effects of the oxygen adatom on the band and electronic spectra of the O/Ti _x C _y (110) surface were studied. The effective charges on the titanium and carbon atoms surrounding the oxygen atom in various reconstructions were determined. The charge transfer from titanium to oxygen and carbon atoms was found, which is determined by the reconstruction of the local atomic and electronic structures and correlates with chemisorption processes. The potential mechanisms of laser nanostructuring of the titanium carbide surface were suggested.     

Heat transport in the insulating phases of Sm1+x Ba2-x Cu3Oy high temperature superconductors

Thermal conductivity (κ) of two series of high-temperature superconductors with general formula Sm_1+x Ba_2-x Cu₃O_y have been measured. Both series begin with the same sample of T _c = 90.4 K and extend to nonsuperconducting phases. The first series is of 123 cation stoichiometry and variable oxygen content y, the second is a series of solid solutions with variable x. The temperature dependences of κ are very similar for superconducting partner samples from both series (i.e. with the same T _c), whereas the nonsuperconducting samples reveal dramatic differences. We propose to attribute the huge increase of κ and the change of its temperature characteristics of insulating oxygen deficient sample to some additional heat carriers, supposedly of magnetic origin. Absence of this additional heat transport mechanism for insulating solid solution allows to treat it as a proper reference for estimation of phonon contribution in the superconducting 123 compounds.     

Dependence of the resistivity of nonstoichiometric titanium carbide TiCy on the density and distribution of carbon vacancies

The influence of temperature and of the density and distribution of structural vacancies in the carbon sublattice on the resistivity of nonstoichiometric titanium carbide TiC_y(0.5⩽y⩽0.98) is studied. It is shown that in titanium carbide TiC_y 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 TiC_y, 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)     

Synthesis,structure, and properties of BIFENBVOX

Solid solutions Bi₄V_2-x Fe _x/2Nb _x/2O_11-δ (х?=?0.05–1.0) and Bi₄V_2-х-y Fe _x Nb _y O_11–δ (with fixed x or y?=?0.2 and variable х or y?=?0.2–0.5 with step 0.1) were synthesized by the standard ceramic technology in the temperature range 773–1113 K and by mechanochemical activation method using Bi₂O₃, V₂O₅ Fe₂O₃, and Nb₂O₅ oxides as initial compounds. The formation of solid solutions was studied. Ranges of stability and temperature values of phase transitions for different polymorphous modifications were defined using dylatometric and thermo gravimetric studies. The morphology and the local chemical composition of the ceramic samples were studied. Samples with concentration of dopants x?>?0.3 contain two phases; both major and impurity phases are solid solutions of the BIFENBVOX type although the dopants atoms distribution between them is random. The thermal expansion coefficients (TEC) were measured. The electrical conductivity of ceramic samples was investigated in a wide range of temperatures. The highest conductivity values among the studied solid solutions are observed for the sample with a small amount of dopants x?=?0.25.     

Concentration phase transitions in Li x Na1 − x Ta y Nb1 − y O3 solid solutions

The ferroelectric-antiferroelectric concentration phase transitions in solid solutions of Li _x Na_{1 ? x} Ta _y Nb_{1 ? y} O₃, which are promising environmentally friendly (lead-free) ferropiezoelectric materials, are studied by means of Raman spectroscopy.     

Effect of N content on phase configuration, nanostructure and mechanical behaviors in Ti-Cx-Ny thin films

Ti-C_x-N_y thin films with different nitrogen contents were deposited by way of incorporation of different amounts of nitrogen into TiC_1.02 using unbalanced reactive unbalanced dc magnetron sputtering method. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), high-resolution transmission electron microscopy (HRTEM) and microindentation methods were used to investigate their phase configurations, nanostructures and mechanical behaviors in order to investigate their dependences on nitrogen content. The result indicated that the nitrogen content had a significant effect on phase configuration, nanostructure and mechanical behaviors of Ti-C_x-N_y thin films. The nitrogen-free TiC_1.02 films exhibited a polycrystallite with nano-grains. On one hand, incorporated nitrogen substituted C in TiC_1.02, producing Ti(C,N), and subsequently linked to the substituted C, forming C-N. On the other hand, the substituted C lined to each other, forming C-C. As a result, nanocomposite thin films consisting of nanocrystalline Ti(C,N) and amorphous (C, C-N) were produced. With further incorporation of nitrogen more C was substituted, accompanying with formation of more amorphous matrices and decrease of size of nanocrystalline Ti(C,N). The trend was enhanced with further increase of nitrogen content. A microhardness maximum of ∼58 GPa was obtained in nitrogen-free TiC_1.02 thin films. This value was linearly decreased with incorporation of N or increase of N content, and finally a hardness value of about 28 GPa was followed at a N content of ∼25 at.%. Both elastic modulus and residual compressive stress values exhibited similar trends.