Disorder-order phase transformations and electrical resistivity of nonstoichiometric titanium carbide

Phase transformations of the disorder-order type in the homogeneity region of nonstoichiometric titanium carbide TiC_y (0.5<y<1.0) have been studied. It has been established that, depending on the actual composition of TiC_y, 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 TiC_y 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)     

Effect of ordering on the structure and specific heat of nonstoichiometric titanium carbide

The experimental results on the change in the crystal structure and specific heat of nonstoichiometric titanium carbide TiC_y (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)     

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 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     

Identification of structural vacancies in carbides,oxides, and sulfides by Doppler broadening of the gamma-ray line

A method has been developed to identify the sublattices of a crystal structure in which there are atomic vacancies. This method is based on determining the chemical environment of vacancies and is implemented using the method of the positron annihilation by measuring the momentum distribution of core electrons. To determine the characteristic momentum distribution of electrons, a special two-detector spectroscopy is used, which ensures measurements of Doppler broadening of the annihilation gamma-ray line with a high (up to 10⁶) signal-to-noise ratio. To test the method, vacancies in irradiated silicon carbide (6H-SiC), sintered nonstoichiometric titanium carbides (TiC _y ) and titanium monoxides (TiO _y ), and chemically deposited lead and cadmium sulfides (PbS and CdS) have been identified. Vacancies in the carbon and silicon sublattices have been identified in silicon carbide after irradiation by low- and high-energy electrons, respectively. Vacancies in the nonmetal sublattice have been identified in TiC _y . Vacancies in the metal sublattice have been identified in TiC _y , as well as in PbS and CdS.     

Effect of Ti on the galvanomagnetic properties of single-crystal Sb2−x TixTe3

A study is reported of the lattice parameters, Hall effect, Shubnikov-de Haas effect, and thermopower of single-crystal Sb_2−x Ti_xTe₃ as functions of titanium content within the range 0<x<0.04. An increase in titanium content is shown to decrease the initial hole concentration in a sample without a noticeable change of the energy spectrum. Fiz. Tverd. Tela (St. Petersburg) 40, 441–444 (March 1998)     

Concerning the structural mechanism of oxygen inclusion into the lattice of titanium carbide

Variations in the unit cell occupancy of TiC_xO_y phase are considered in terms of successive oxygen inclusion into the lattice of defect carbides TiC_0.5 ? TiC_0.9. A classification of TiC_xO_y solid solutions formed and the concentration limits of their existence are presented. Three types of cubic ternary solid solutions are shown to occur in the system TiCO: solid solutions of interstitial oxygen inclusion into the initial carbide; solid solutions of oxygen inclusion—carbon and metal exclusion; and solid solutions of oxygen inclusion—oxygen substitution for carbon-carbon and metal exclusion.Specific points (inflection and breaks) on the plots where n_i is the number of atoms per unit cell, n_i = f(y) for TiC_xO_y, are accounted for by both variations in the type of solid solution and the possibility of atomic arrangement in the vicinity of these points. On the basis of the above scheme some experimental peculiarities of Ti carbide phase oxydation are explained.     

The influence of a metalloid on ordered structure in titanium carbide

X-ray and neutron diffraction were used to investigate the effect of oxygen and nitrogen atoms on the formation of the intermediate δ′-cubic (sp. gr. Fd3m) and δ″-tetragonal (sp. gr. P3₁21) ordered structures in titanium carbide. In TiC_0.545O_0.08, TiC_0.545N_0.09 samples that were investigated, the metal atoms were shifted from ideal positions in the direction from vacancies to metalloid atoms. In the intermediate cubic phase (δ′), the free-parameter values of titanium atoms in the two samples are the same, but are different from their analogous value in titanium carbide. Fiz. Tverd. Tela (St. Petersburg) 39, 2207–2209 (December 1997)     

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)     

Relationship between short- and long-range orders in nonstoichiometric titanium monoxide (TiO<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript>)

The relationship between the short- and long-range orders in various phases of nonstoichiometric titanium monoxide (TiO _y ) has been analyzed for the first time. The types of the local environment of lattice sites in the metal and nonmetal (oxygen) sublattices of Ti₅O₅, Ti₃O₂, Ti₂O₃, and Ti₄O₅ superstructures are described. It is established that, in phases where ordering takes place simultaneously in both sublattices, all parameters of the superstructural short-range order determining the positions of atoms and vacancies in the first three coordination spheres can be uniquely expressed via the long-range order parameters. If the ordering takes place only in one sublattice, then five of the six short-range order parameters vanish. It is shown that, using data on the maximum absolute values of six short-range order parameters and on the fractions of occupied atomic positions in titanium and oxygen sublattices, it is possible to predict the type of ordered phase expected to form in the nonstoichiometric titanium monoxide TiO _y .     

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.     

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.     

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.     

Two-sublattice ordering in titanium monoxide

The disordered and ordered structures of nonstoichiometric titanium monoxide Ti_xO_z≡TiO_y(y=z/x) containing structural vacancies simultaneously in the nonmetallic and metallic sublattices were studied. In the stoichiometry range from TiO_0.9 to TiO_1.1, an ordered monoclinic phase [space group C2/m (A12m/1)] of the Ti₅O₅ type is formed in the TiO_y monoxide at temperatures below 1300 K. The disorder-order TiO_y?Ti₅O₅ phase-transition channel involves Lifshitz {k₁₀} and non-Lifshitz {k₄} and {k₁₁} star rays. The ordering proceeds as a first-order phase transition with a decrease in the volume of the basal cubic lattice. The titanium and oxygen distribution functions in the metallic and nonmetallic sublattices of titanium monoxide are calculated. The domain of allowed values is determined for the long-range order parameter.     

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.     

Atomic-vacancy ordering and magnetic susceptibility of nonstoichiometric hafnium carbide

Experimental data on the magnetic susceptibility of nonstoichiometric hafnium carbide HfC_y(0.6< y<1.0) are presented. Anomalies are found in the temperature dependences of the magnetic susceptibility of the carbides HfC_0.71, HfC_0.78, and HfC_0.83 in the temperature range 870–930 K. These anomalies are due to the formation of a superstructural short-range order in their nonmetallic sublattice. It is shown that the short-range order in the carbides HfC_0.71 and HfC_0.78 corresponds to the ordered phase Hf₃C₂, while in the carbide HfC_0.83 it corresponds to the ordered phase Hf₆C₅. It is found that the magnetic susceptibility of the carbide HfC_0.78 in the temperature interval 910–980 K is zero. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 4, 296–301 (25 February 1999)     

Influence of the electron concentration on the superconducting properties of the spinel system Li1-yTi2O4

In the spinel system Li_1-y Ti₂O₄ the Li content was reduced by oxidative extraction by means of a solution of I₂ as well as Br₂ in CH₃CN or by oxygen. The extraction is associated with a transition of Ti³⁺ to Ti⁴⁺: the system is Li_1-y Ti³⁺_1-yTi⁴⁺_1+y O₄. Consequently, with increasing y the charge carrier concentration decreases from the starting value of 0.5 electrons/Ti ion for LiTi₂O₄. The extraction proceeds in two steps. At the beginning, the Ti₂O₄ framework remains intact and the transition to superconductivity increases with decreasing charge carrier concentration to a maximal value of 13.2 K. Separated by a two-phase region for medium extraction levels for higher y values, a Li depleted material of approximate composition Li_0.3Ti₂O₄ is obtained. The crystal structure of Li_0.3Ti₂O₄ can be deduced from the original spinel lattice by a partial Ti migration from 16d to 16c and 8 a sites (space group Fd3m). Consequently, the Ti₂O₄ framework of Li_0.3Ti₂O₄ is disconnected and the material is no longer superconducting.     

Formation and photodecomposition of cationic titanium oxide clusters

In this paper we report on the titanium oxide cluster cations Ti _x O _y ⁺, generated by laser ablation of a titanium target in the region of the nozzle expansion of oxygen. The mass distribution of the clusters produced is recorded with a time-of-flight mass spectrometer. Three different series, namely TiO(TiO₂) _n ⁺ , TiO(TiO₂) _n O₂⁺, and (TiO₂) _n ⁺, appear in the spectra. Two different ablation wavelengths (infrared at 1064 nm and ultraviolet at 308 nm) are used to generate the titanium oxide clusters. At the shorter wavelength the maximum size of the clusters formed decreases. The interaction of the UV photons with the Ti _x O _y ⁺ clusters is further investigated in a separate two-laser arrangement with an IR laser for ablation and after some mm downstream with an UV system for the cluster beam irradiation. These studies indicate that the intensity of the T _x O _y ⁺ clusters with x≥4, y≥7 is strongly influenced by the absorption of UV photons. This is attributed mainly to dissociation into smaller ones.     

MICROSTRUCTURE AT THE INTERFACE OF TITANIUM CARBIDE AND NICKEL ALUMINIDES

Microstructure at the interface of titanium carbide and nickel aluminides in the samples obtained by infiltration of molten Ni₃Al alloy has studied by a scanning electron microscopy (SEM) and an analytical transmission electron microscopy (ATEM) with an energy dispersive spectrometer (EDS). It is found that the morphology at the interfaces between hard phase skeleton of TiC_0.7 and metallic phases depends on the ratio of Ti/C in carbide. Some periodic zigzag fringes are observed at a smooth interface between metallic phase and carbides in the sample of Ni₃Al/TiC_0.7. The results of analysis using EDS show that Ti in TiC_0.7 carbide is easier than that in TiC_0.7 to dissolve into the molten alloy during solid-liquid reaction. The formation of this periodic zigzag fringe,which may be a growth zone of a new Ti-Ni-Al phase,in the interface of TiC_0.7/Ni₃Al would occur during the initial stage of solidification.