A study on martensitic transformation in Ti50−x/2Ni50−x/2Cu x alloys with X≤10 at%

The effect of Cu additions on the martensitic transformation sequence and temperature in Ti_50–x/2Ni_50–x/2Cu _x alloys with x: 1–10 at% are investigated by ER, DSC, X-ray and IF measurements. Experimental results show that the transformation sequence of Ti_50–x/2Ni_50–x/2Cu _x alloys with x: 1–4 at% proceeding as two-stage B2RB19 transformation on cooling and Ti_50–x/2Ni_50–x/2Cu _x alloys with x=5, 10 at% have no martensitic transformation. The addition of Cu in Ti_50–x/2Ni_50–x/2Cu _x alloys assists the formation of R-phase, a behaviour which is quite different from that in Ti₅₀Ni_50–x Cu _x alloys. Both the Ms and T _R temperatures decrease rapidly with increasing Cu addition in Ti_50–x/2Ni_50–x/2Cu _x alloys with x: 1–4 at%. It is proposed that the Cu+Ni effects on the Ms temperature in Ti_50–x/2Ni_50–x/2Cu _x alloys is similar as Cu +Ni effects in Ti₅₀Ni_50–x Cu _x alloys and as Ni effects in as-quenched Ni-rich TiNi alloys.     

Superconductivity of La2−x Sr x CuO4 with high strontium concentration by sulphur doping

The effects of sulphur substitution for oxygen on the structure and properties of nonsuperconducting La_2–x Sr _x CuO₄, where x=0.26 and 0.30, have been studied. In La_2–x Sr _x CuO _4–y -S _y , the sulphur-doped samples exhibit superconducting transition at 37–39 K and a larger Meissner effect than undoped samples. It was found that the superconductivity of La_2–x Sr _x CuO_4–y S _y is apparently determined by the value of x–y and the strontium concentration region in which La_2–x Sr _x CuO₄ shows superconductivity shifts to higher strontium concentrations with sulphur doping.     

p–T–x Phase Equilibria in the Cd–Zn–Te System

The p–T–x–y phase equilibria in the Cd–Zn–Te system are analyzed. The p–T and T–x–yprojections of the p–T–x–y phase diagram and a T–x–y isobar (for pressures at which Cd_1–x Zn _x Te_{1 ±} solid solutions sublime congruently in terms of Te) are mapped out. The key features of the sublimation behavior of the solid solution are examined. The p–T projection is studied by static vapor pressure measurements at temperatures from 700 to 1300 K and pressures of up to 101.3 kPa. The p–T sections of the phase diagram are constructed for x = 0.05, 0.10, 0.15, 0.25, 0.50, 0.75, 0.90, and 1. The solid solution containing 35 mol % ZnTe is found to phase-separate at 473 K.     

Ferromagnetic Properties of Mn-Doped III–V Semiconductor Quantum Wells

We investigate magnetic properties of In_1–xMn_xP and Ga_1–xMn_xN quantum wells in the mean-field approximation and show the difference between them. In the case of the In_1–xMn_xP, the dependence of the Curie temperature (T_c) on the hole density exhibits a step-like behavior, reflecting the effect of a two-dimensional Fermi (hole) gas, when the hole–hole exchange interaction is ignored. When we take into account the hole–hole exchange interaction, however, this behavior is broken by the appearance of peaks at the specific two-dimensional carrier densities, and T_c is substantially enhanced in this region. In the case of Ga_1–xMn_xN, the step-like behavior is obscure, and it appears that T_c increases rather continuously with the increasing two-dimensional (2D) carrier density. This shows very weak step-like behavior compared to other III-Mn-V DMS quantum wells, because the hole effective mass of Ga_1–xMn_xN is very large, and the large hole effective mass reduces the energy splitting due to the confinement effect. In a multi-heavy-valence-subband model, the Curie temperature of the In_1–xMn_xP quantum well is about 68 K with 6.5×10¹² holes per cm² and the Mn mole fraction x = 0.05 and the exchange constant J_pd = 0.15 eV nm³. The Curie temperature of the p-type Ga_1–xMn_xN quantum well can be above room temperature, unless the spin-exchange interaction integral is abnormally small.     

High-temperature thermoelectric properties of the sintered Bi2Sr2Ca1 − xYxCu2Oy (x = 0 – 1)

Electrical conductivity and Seebeck coefficient were measured in a temperature range of 320–1073 K for sintered samples of Bi₂Sr₂Ca_{1 – x} Y _x Cu₂O _y (x = 0, 0.2, 0.4, 0.6, 0.8, 1.0). It has been found that the conduction behavior changes from n-type metallic to p-type semiconducting with increasing yttrium concentration. The power factors were in a range of 1.7–3.0 × 10^–5 Wm^–1 K^–2 for the sample with x = 0.8, being maximized by the optimization of the yttrium concentration. The thermal conductivity for the sample with x = 0.8 was 0.73 Wm^–1 K^–1 at 310 K, and decreased with increasing temperature. The values of thermoelectric figure of merit were estimated to be in a range of 3.4–4.8 × 10^–5 K^–1 at temperatures of 320–673 K for the sample with x = 0.8.     

Ab Initio Study of Magnetism in III-V- and II-VI-Based Diluted Magnetic Semiconductors

Electronic structure and magnetic properties of Ga_1–x Mn _x As, Ga_1–x Mn _x N, Zn_1–x M _x O, and Zn_1–x M _x Te (M=V, Cr, Mn, Fe, and Co) diluted magnetic semiconductors (DMS) are calculated by the tight-binding LMTO method in the 64-atom supercell. Calculations are made at several x with varied spatial distribution of dopant atoms and codoping of DMSs. The results show that stability of the ferro- and antiferromagnetic (FM and AFM) states in DMSs strongly correlates with the occupation and energy position of 3d-dopant bands. Adequacy of the double exchange and superexchange mechanisms for explanation of the FM vs. AFM competition is discussed.     

Phase Composition of Heat-Treatment Products in the ZrO(OH)2–Y(OH)3–FeOOH System

The phase composition of the heat-treatment products in the ZrO(OH)₂–Y(OH)₃–FeOOH system is determined as a function of the precipitation procedure and calcination temperature (620–1570 K) for the compositions 0.97ZrO₂· xY₂O₃· yFe₂O₃(x+ y= 0.03; x= 0, 0.01, 0.015, 0.02, 0.03) and (1 – x– y)ZrO₂· xY₂O₃· yFe₂O₃(x= y= 0.02, 0.025, 0.03, 0.04). At a given ZrO₂: stabilizer ratio, partial substitution of Fe³⁺for Y³⁺increases the degree of ZrO₂stabilization and retards the low-temperature degradation of the material.     

Fabrication of YBa2Cu3O7−x bulk superconductor from the powder without reoxygenation

The fabrication of YBa₂Cu₃O_7–x sintered ceramic and YBa₂Cu₃0_7–x melt product from the powder without additional reoxygenation is reported. The YBa₂Cu3O_7–x is fabricated inside a commercial silicon carbide tube. The SiC tube effectively does not react with the YBa₂Cu3O_7–x and acts to prevent oxygen from diffusing out of the YBa₂Cu3O_7–x . The fabrication procedure can, in principle, be used with any tube enclosure which neither reacts with YBa₂Cu3O_7–x nor allows diffusion of oxygen to the outside atmosphere.     

Machinable and mechanical properties of sintered Al2O3-Ti3SiC2 composites

Two-phase composites consisting of (1 – x) Al₂O₃ and xTi₃SiC₂ (x = 0–1) were prepared by spark plasma sintering (SPS). Sintered densities larger than 98% of theoretical density were achieved when the specimens were sintered at 1300°C for 5 min (in vacuum, at pressure 30 MPa). When content of Ti₃SiC₂ increased up to 30 wt%, composites were found to be machinable—they could be drilled easily using conventional Fe-Mo-W drills or gravers. The mechanical properties of the (1 – x) Al₂O₃–xTi₃SiC₂ composites were evaluated. The bending strength, Vickers hardness of the specimens had the following ranges: 428 ± 10.2 (x = 0) to 673 ± 15.4 Mpa (x = 1) (bending strength at room temperature); 19.9 (x = 0) to 4.0 GPa (x = 1) (Vickers hardness).     

X-ray photoelectron spectroscopy study of perovskite-type mixed oxides (La1−x A x ′ CoO3)

The surface states of the perovskite-type cerium-, thorium- and strontium-doped lanthanum cobalt oxides (La_1–x Ce _x CoO₃), La_1–x Th _x CoO₃, La_1–x Sr _x CoO₃ x = 0 to 0.03) have been investigated using X-ray photoelectron spectroscopy (XPS). The binding energy differences (BE) between Co2p_3/2 and 01s of lattice oxygen have a maximum value atx = 0.02 in both La_1–x Ce _x CoO₃ and La_1–x Th _x CoO₃. From this result, it is expected that the bonding state between the surface cobalt and lattice oxygen is mostly ionic at this point. The valence band (VB) spectra change greatly atx = 0.02 in both cerium- and thorium-doped samples. We speculate that the high spin trivalent cobalt ion is dominant atx = 0.02 on the surface. On the other hand, the ABEs between Co2p_3/2 and 01s of lattice oxygen decrease with increasingx in La_1–x Sr _x CoO₃. It is expected that the bonding states become covalent in proportion tox. The change of the VB spectra for La_1–x Sr _x CoO₃ near the Fermi level (E _F) can be explained by the formation of the impurity level.     

Incorporation of bismuth into Ba6− x R8+2/3x Ti18O54(R = Nd, Gd)

Microstructural investigations and microanalyses of a series of Ba_6–x R _8+2/3x Ti₁₈O₅₄ ceramics (R = Nd, Gd) revealed that the solid-solubility limit for the isovalent substitution of R ³⁺ by Bi³⁺ depends on the composition of the Ba_6–x R _8+2/3x Ti₁₈O₅₄ phase. For the Nd analogue the solid-solubility limit (y in Ba_6–x (Nd_1–y Bi _y )_8+2/3x Ti₁₈O₅₄) decreases with a decrease in x from y = 0.16 for x = 2.0 to y = 0.10 for x = 0.8. An even lower solid-solubility limit (y = 0.06) was found for the Ba_4.5(Gd_1–y Bi _y )₉Ti₁₈O₅₄ compound (x = 1.5). All substituted Nd compositions exhibit higher permittivities (93–99), lower temperature coefficients of permittivity (11–15 ppm/K) and higher dielectric losses (Q · f = 1300–5500 GHz) than the parent compositions. By exceeding the solid-solubility limit, abrupt changes in the microstructural and dielectric characteristics are induced.     

Microhardness study of some novel compounds and alloys

Vickers microhardness has been measured at room temperature on a numerous compounds and alloys containing Ce (4f) and/or U (5f) metals (all together on 43 samples with different compositions represented by the parameter x) belonging to the following series of systems: Y_{1 – x} U _x Ru₂Si₂, Ce_{1 – x} Y _x Ru₂Si₂, Ce_{1 – x} Y _x Cu_2.05Si₂, Ce_{1 – x} La _x Cu_2.05Si₂, Ce_{1 – x} U _x Al₂, Ce_{1 – x} U _x NiSn. The significant changes were observed for all examined series of the systems with the discernible maximum or minimum in the recorded hardness curves. Whereas the maximum could be easily explained by the available theories, explanation of the minimum needs additional studies.     

Study of Amorphous Compounds Obtained by Ball Milling in the Si–C–O System

Generally, the Si–C–O system is composed of SiO₂, SiC, and pure C as crystalline phases. In the present study, we focus on the preparation of ternary silicon oxycarbide compound. For this purpose, different mixtures of quartz, silicon, and graphite were ball milled to cover the following range of composition: (1) SiC _x O_{2(1 – x)} + excess of C; (2) SiC _x O_{2(1 – x)} stoichiometric; (3) SiC _x O_{2(1 – x)} + excess of Si. The course of the reaction is followed by X-ray diffraction, by measuring the relative intensity change of the Bragg's peaks as a function of the mechanical treatment. The Rietveld method is applied to the patterns for quantitative analysis and determination of crystallite size and microstrain. Finally, the behavior of each phase is reported across the three starting compositions examined here and the presence of silicon oxycarbide compounds induced by ball milling is assessed by [²⁹Si-MAS]NMR.     

Manufacture, microstructure and mechanical properties of Mo–W–N nanostructured hard films

Mo_1−xW_xN_y (x = 0–0.67) hard films were fabricated on wafers of silicon and high speed steel by dc magnetron sputtering technique. The effect of tungsten concentration on the phase composition, microstructure, surface morphology, hardness, adhesion, and corrosion resistance of the films was studied by X-ray diffraction, scanning electron microscopy, nano-indentation, and scratch test. It was found that if the W concentration (x) in the film is in the range of 0–0.52, the films exhibit fcc (Mo,W)N_y single phase where larger W atoms substituted Mo atoms in fcc MoN_y. At higher x values (x > 0.52) the films exhibit a two-phase structure consisting of fcc (Mo,W)N_y and pure bcc tungsten phase. The hardness of the Mo_1−xW_xN_y films increases at first with increasing x, and then decreases after passing a maximum. The maximum hardness of 47 GPa is obtained at x = 0.37 corresponding to an adhesion strength of 60 N. The Mo–W–N coated high speed steel has a lower corrosion current density and higher corrosion potential than the bare high speed steel substrates.     

Thermal stability of amorphous phases in vapour-deposited binary alloy thin films of palladium with vanadium,niobium and tantalum

Binary alloys of the systems V-Pd, Nb-Pd and Ta-Pd were vapour deposited and investigated by transmission electron microscopy. The atomic fraction,x, was varied in steps of 0.1 from one pure element to the other. The range over which an amorphous phase is observed is found to increase in width going from 3d to 5d alloying element in palladium: the compositions where amorphous phases are found are V_1–x Pd _x (x = 0.5), Nb_1–x Pd _x (x = 0.4 to 0.6) and Ta_1–x Pd _x (x = 0.2 to 0.6). The composition range over which a crystalline phase is found correlates well with the single-phase solid solution region close to the melting temperature in the phase diagram. Crystallization of the V_0.5Pd_0.5 alloy takes place at 550 K. The amorphous Nb_1–x Pd _x (x = 0.4, 0.5) films crystallize at 850 K, whereas the amorphous Ta-Pd films crystallize between 850 and 1050K, depending on the composition. For Nb_1–x Pd _x (x = 0.4 to 0.5) and Ta_1–tx Pd _x (x = 0.2 to 0.6) primary crystallization takes place into an f c c phase. The second crystallization step leads to a phase with a complex structure. The result is a two-phase system. V_0.5Pd_0.5 and Nb_0.4Pd_0.6 crystallize polymorphically to an f c c solid solution. The crystallization temperatures for the compositions which display primary crystallization are higher than for the compositions which crystallize by a polymorphic reaction.     

Preparation and Properties of Bi2Sr2CaCu2O y Coatings on Sr–Ca–Cu–O Substrates

Bi-2212 coatings on Sr_{1 – x} Ca _x CuO₂ (x = 0, 0.5) and Sr_{14 – x} Ca _x Cu₂₄O₄₁ (x = 0, 5) substrates were prepared by partial melting at 890 and 950°C. The effects of the partial-melting temperature and the temperature and duration of subsequent heat treatment in air on the phase composition of the coatings and substrates and the electrical resistance and superconducting transition temperature of Bi-2212 were studied. After partial melting and heat treatment, the T _c(R = 0) of Bi-2212 attained 85–86 K. Sr_{1 – x} Ca _x CuO₂ and Sr_{14 – x} Ca _x Cu₂₄O₄₁ substrates caused no degradation of the superconducting properties of Bi-2212, in contrast to Al₂O₃, and ensured a higher thermal shock resistance of Bi-2212.     

Molecular Dynamics Simulations of Ba1 – x Gd x F2 + x Solid Solutions over a Wide Temperature Range: II. Structural Characteristics and Fluoride Ion Diffusion

The atomic-scale structure (pair correlation functions and anion-density distribution) and fluoride-ion diffusion in Ba_{1 – x} Gd _x F_{2 + x} solid solutions are investigated over a wide temperature range, including the superionic transition, by molecular dynamics simulations with Born–Mayer–Huggins pair potentials. According to the simulation results for x > 0.1, both the 4b(cubic site symmetry) and 48i positions accommodate fluorine interstitials (F^– _i). Below the superionic transition, the migration of F^– ions at low doping levels is dominated by noncollinear jumps between lattice and interstitial sites. Computer simulations confirm that trigonal Gd³⁺–F^– _i dipolar complexes prevail over tetragonal complexes. It is shown using elements of Monte Carlo simulation that the rare-earth ions in the solid solutions have a tendency to aggregate.     

Zinc Doping Profile in AlGaAs/GaAs Heteroepitaxial Structures Grown by Metalorganic Chemical Vapor Deposition

The Zn profile in Al _x Ga_{1 – x} As/GaAs (x = 0.2–0.4) quantum-well heteroepitaxial structures doped during growth by metalorganic chemical vapor deposition is modeled with allowance made for the diffusional broadening of the nominal doping profile. Experimentally determined carrier distributions in the heterostructures are used to refine the diffusion coefficient of Zn at a growth temperature of 770°C. The average value of D _Zn is determined to be 6.0 × 10^–14 cm²/s. The position of the p–n junction in Al _x Ga_{1 – x} As/GaAs heterostructures is assessed as a function of the nominal Zn profile and growth rate. The ways of optimizing the doping profile are outlined.     

Synthesis of new apatite phases by spray pyrolysis and their characterization

This paper describes the synthesis of some apatite compounds, called britholites, by an aerosol pyrolysis technique, which compared to conventional methods of solid-state chemistry, presents the advantages of reducing noticeably the temperature and the duration of the treatment. Britholites are minerals based on Ca_{10 – x} Ln _x (PO₄)_{6 – x} (SiO₄) _x X ₂ formula (with X = F^–, OH^–, X ₂ = O^2–); they are promising materials for nuclear waste storage. In a second part we have synthesized and characterized a new apatite phase where the substitution of calcium by barium in britholite leads to a monophased solid solution Ca_{9 – x} Ba _x Nd(PO₄)_{6 – x} (SiO₄)F₂ for 6 < x < 9. The possible interest of this new material is discussed.