Oxidation behavior of Mo≤5Si3C≤1 and its composites

The oxidation behavior of Mo₅Si₃C₁ and its composites was studied in air over the temperature range of 500°C–1600°C. Experiments revealed poor oxidation resistance of monolithic Mo₅Si₃C₁ at high temperature. The oxidation was quite rapid at 1200°C and above, resulting in complete oxidation of specimens in a short time. The addition of 2.0 wt% boron was found to produce a Mo₅Si₃C₁ composite with three other phases of MoB, MoSi₂, and SiC, and showed remarkable improvement in oxidation resistance. The mechanism for the improvement was attributed to the viscous sintering of the scale to close the pores formed during the initial oxidation period. Oxidation tests were also conducted on SiC-Mo₅Si₃C₁ composite at 800°C, 1300°C and 1600°C for more than 100 hours. The oxidation resistance of the composite was found to be very good. The results demonstrate that, though oxidation resistance of monolithic Mo₅Si₃C₁ is far insufficient for high-temperature applications, boron-modification and/or composites with SiC are viable methods to improve oxidation resistance to a practically acceptable level.     

Phase diagram of the CrxVSe2 system (0 ≤ x ≤ 1.0)

The phase diagram of the pseudo-binary CrVSe₂ (i.e. Cr_xVSe₂) system was determined by means of X-ray and DTA measurements, in the composition range 0≤x≤1.0. As the results, the following 6 phases have been confirmed to be existent in the temperature range 300≤T≤1100 : CdI₂,M₅X₈,M₃X₄,α,NiAs and Cr. Among them, M₅X₈,M₃X₄ and α phases undergo a phase transition to the CdI₂- or NiAs-type structures at high temperatures. The α phase (0.825≤x≤0.875 at 500°C) is a new phase firstly confirmed by this study.     

Specific Heat of LaMnO3 + δ at 50 K ≤ T ≤ 160 K

We have developed a shell model, which includes the long-range coulomb, van der Waals interaction, and the short-range Hafemeister–Flygare repulsive interaction operative up to second neighbor atom to study the cohesive and thermal properties of LaMnO_{3 + δ}. The results on cohesive energy obtained by us are in good agreement with that of calculated value by DeSouza et. al. (R. A. DeSouza, M. S. Islam, and E. I. Tiffee, J. Mater. Chem. 9, 1621 (1999)). In addition, we have also calculated molecular force constant (f), compressibility (β), restrahlen frequency (ν_o), Debye temperature (Θ_D), and the low temperature specific heat at 50 K ≤ T ≤ 160 K. Our results on Debye temperature and specific heat for the temperature range 50 K ≤ T ≤ 160 K are closer to the recently measured data with an automated quasi-adiabatic pulse technique.     

Diffusion of mercury into for HgxCd(1−x)Te 0≤x≤0.03

Experiments carried out to fabricate Hg_xCd_(1-x)Te by diffusing mercury into CdTe slices from the vapour, resulted in obtaining slices with a maximum value of x=0.004. Measurements on the diffusion of mercury into such slices, where 0≤x≤0.03, under saturated vapour pressure conditions, resulted in two component profiles, each profile giving two values of D. These results were similar to ones obtained earlier, indicating that the diffusion was rate-limiting. In addition, the diffusivities were found to be independent of x. This revised version was published online in July 2006 with corrections to the Cover Date.     

Phase Formation in the Course of Oxidation of In x Se1 − x Alloys (0.47 ≤ x ≤ 0.53)

We have determined the characteristic temperature ranges of heating of In _x Se_{1 – x} (0.50 x 0.52) alloys in air. We have established that the annealing of InSe powder in selenium vapors leads to the stabilization of the rhombohedral structure.     

Neutron investigation of Nd2-x-yCexLayCuO4 (0≤x≤0.2; y=0.5,1)

Samples of the electron-doped superconductor precursor Nd_2-x-yCe_xLa_yCuO₄ with 0x0.2 and y=0.5, I have been synthesized with a sol-gel method. While the as-prepared samples are in the normal state, oxygen reduction results in superconducting samples. The dependence of T_c upon Ce-doping x and oxygen content (4-) was determined in detail. It turned out that the internal pressure exerted by the La ions results in an extended superconducting range in x and compared to Nd_2-xCe_xCuO_4- Elastic neutron scattering was used to investigate the dependence of structural features upon doping. Upon oxygen reduction a non-isotropic shrinking of the unit cell (lattice parameter a increases, c decreases) and indications for interstitial oxygen were found. The local electronic surrounding of the Nd ions was observed via the crystal-field splitting by inelastic neutron scattering and turned out to be inhomogeneous. Such electronic inhomogeneities were previously measured for other high-T_c superconductors.     

Specific Heat of LaMnO3 + δ at 50 K ≤ T ≤ 160 K

We have developed a shell model, which includes the long-range coulomb, van der Waals interaction, and the short-range Hafemeister–Flygare repulsive interaction operative up to second neighbor atom to study the cohesive and thermal properties of LaMnO_{3 +} . The results on cohesive energy obtained by us are in good agreement with that of calculated value by DeSouza et. al. (R. A. DeSouza, M. S. Islam, and E. I. Tiffee, J. Mater. Chem. 9, 1621 (1999)). In addition, we have also calculated molecular force constant (f), compressibility (), restrahlen frequency (_o), Debye temperature (_D), and the low temperature specific heat at 50 K T 160 K. Our results on Debye temperature and specific heat for the temperature range 50 K T 160 K are closer to the recently measured data with an automated quasi-adiabatic pulse technique.     

Hydrothermal Synthesis and Transport Properties of FeS1-xTex (0 ≤ x ≤ 0.15) Single Crystals

Journal of Superconductivity and Novel Magnetism - In this work, a series of FeS1-xTex (0 ≤ x ≤ 0.15) single crystals were successfully synthesized by a hydrothermal method for the...     

Comment on the magnetic properties of the system Sr2−xCaxFeMoO6, 0≤x≤2

The structural and magnetic properties of the n-type-metallic double-perovskite system Sr_2−xCa_xFeMoO₆ are reported. At 5 K, the saturation magnetization remains M_s(5 K)=3.5 μ_B/formula unit compared to a theoretical spin-only moment of 4.0 μ_B/formula unit for a perfectly ordered compound. From the shape of the M–H hysteresis loops below T_c, it is suggested that ferromagnetic long-range ordered domains are coupled antiferromagnetically across antiphase boundaries; random disorder within domains may be small. An unprecedented dependence of the paramagnetic susceptibility on the applied magnetic field is reported and attributed to an overlapping Fe³⁺/Fe²⁺ and Mo⁶⁺/Mo⁵⁺ redox couple, first suggested by Sleight and Weiher [Sleight AW, Weiher JF. J. Phys Chem Solids 1972; 33: 679.], which results in localized S=5/2 spins on the iron atoms coexisting with itinerant minority-spin electrons in a narrow π* band.     

Thermoelectric Properties of TlIn1 – x Yb x Te2 (0.01 ≤ x ≤ 0.09) Crystals

The thermoelectric properties of TlIn_{1 – x} Yb _x Te₂ (0.01 x 0.09) crystals are studied. The results indicate that the crystals possess high thermoelectric figures of merit in the range 500–700 K.     

Preparation and Structure of Ga2 – x Sc x O3 (0.42 ≤ x ≤ 0.52)

The structure of a scandium gallium oxide single crystal grown by the Czochralski process from a charge of composition Ga_1.5Sc_0.5O₃ is determined by x-ray diffraction. The structure is shown to be a superstructure to -Ga₂O₃. The superstructure parameters are found to vary both along the axis of the crystal and radially, which is due to variations in Ga : Sc ratio. A structural model of the Ga_1.5Sc_0.5O₃ crystal is proposed.     

Effect of reducing atmosphere on the magnetism of Zn(1-x)Co(x)O (0≤x≤0.10) nanoparticles

We report the crystal structure and magnetic properties of Zn(1-x)Co(x)O?(0≤x≤0.10) nanoparticles synthesized by heating metal acetates in organic solvent. The nanoparticles were crystallized in the wurtzite ZnO structure after annealing in air and in a forming gas (Ar95%?+?H5%). The x-ray diffraction and x-ray photoemission spectroscopy (XPS) data for different Co content show clear evidence for the Co(2+) ions in tetrahedral symmetry, indicating the substitution of Co(2+) in the ZnO lattice. However, samples with x = 0.08 and higher cobalt content also indicate the presence of Co metal clusters. Only those samples annealed in the reducing atmosphere of the forming gas, that showed the presence of oxygen vacancies, exhibited ferromagnetism at room temperature. The air annealed samples remained non-magnetic down to 77?K. The essential ingredient in achieving room temperature ferromagnetism in these Zn(1-x)Co(x)O nanoparticles was found to be the presence of additional carriers generated by the presence of the oxygen vacancies.     

The penetration depth of HgBa2CuO4+δ with 0.07 ≤ δ ≤ 0.35

The magnetic penetration depth (T) of three HgBa₂CuO₄₊ samples with 0.16 < 0.27 has been determined from the reversible magnetization. The obtained follows a BCS-like correlation of 1/² 1–(T/T_c)² over whole measured temperature range in an underdoped sample with T_c 90 K, but deviates significantly from similar fits in an overdoped sample with the same T_c and an optimum doped sample, whose 1/² 's depends on T nearly linearly below T_c/2. This asymmetry between the underdoped and overdoped samples suggests that the T-dependence of 1/² is affected by doping in a complicated way.     

Effect of Doping and High-Temperature Annealing on the Structural and Electrical Properties of Zn_(1-X)Ni_XO(0≤X≤0.15) Powders

This paper reported the synthesis,crystal structure and electrical conductivity properties of Ni-doped ZnO powders(i.e.Zn 1 X Ni X O binary system,X=0,0.0025,0.005,0.0075 and in the range 0.01≤X≤0.15).Iphase samples,which were indexed as single phase with a hexagonal(wurtzite) structure in the Zn 1 X Ni X O binary system,were determined by X-ray diffraction(XRD).The widest range of the I-phase was determined as 0≤X≤0.03 at 1200 C;above this range the mixed phase was observed.The impurity phase was determined as NiO when compared with standard XRD data,using the PDF program.We focused on single I-phase ZnO samples which were synthesized at 1200 C because of the widest range of solubility limit at this temperature.It was observed that the lattice parameters a and c of the I-phase decreased with Ni doping concentration.The morphology of the I-phase samples was analyzed with a scanning electron microscope.The electrical conductivity of the pure ZnO and single I-phase samples were studied by using the four-probe dc method at temperatures between 100 and 950 C in air atmosphere.The electrical conductivity values of pure ZnO and 3 mol% Ni-doped ZnO samples at 100 C were 2×10 6 and 4.8×10 6 1 ·cm 1,and at 950 C they were 1.8 and 3.6 1 ·cm 1,respectively.In other words,electrical conductivity increased with Ni doping concentration.     

Appearance of Griffiths Phase in La0.7−x Pr x Ba0.3MnO3 (0 ≤x≤ 0.2)

Polycrystalline La_0.7?x Pr _x Ba_0.3MnO₃ (0 ≤x≤ 0.2) samples were prepared using solid-state reaction and checked by X-ray diffraction. Magnetization measurements versus temperature and applied magnetic field were used to investigate their magnetic properties. For samples with x= 0.2, the Griffiths phase is observed when the inverse of susceptibility (1/ χ vs. T) is analyzed.     

Synthesis of Cr x Ti1−2x Nb x O2O20≤x≤0.5 rutile solid solutions from alkoxides

In this study, Cr _x Ti_1–2x Nb _x O₂ (0 x 0.5) rutile solid solutions have been synthesized from gels built from hydrolysis-condensation of Cr (III) acetylacetonate, NbCl₅ and Ti (IV) isopropoxide mixture (polymeric gel). Characterization of these solid solutions was carried out by X-ray diffraction, ultraviolet-visible and infrared spectroscopy, differential thermal and thermogravimetric analysis and CIELAB (Commission Internationale del'Eclairage L^*a^*b^*) parameter measurements. The results obtained by the polymeric gel method were compared with those obtained by traditional ceramic synthesis. This comparison reveals some differences with regard to synthesis temperatures and reaction mechanisms. The formation of Cr _x Ti_1–2x Nb _x O₂ (0 x 0.5) rutile solid solutions by the ceramic method requires temperatures of about 1200°C and soaking times of several days. These solid solutions are synthesized at 1000°C in 24 h by the polymeric gel method. In ceramic synthesis, the CrNbO₄ compound with rutile structure appears as an intermediate compound in the formation of rutile solid solutions. In polymeric gel synthesis, however, the CrNbO₄ rutile compound was not detected in the samples.     

Ellipsometric study of dielectric functions of Mg(1-y)Ca(y)H(x) thin films (0.03≤y≤0.17)

Mg(1-y)Ca(y)H(x) (0.03≤y≤0.17) alloy thin films covered with thin Pd were hydrogenated using 4% H(2) in Ar under atmospheric pressure at room temperature. The complex dielectric functions (ε=ε(1) + iε(2)) in the range of 0.75 to 5.0 eV of these hydrides were evaluated with spectroscopic ellipsometry. The imaginary part of the dielectric function ε(2) was composed of one Drude and three Lorentz oscillator terms whose peak positions were about 1.2 eV (L(1)), 2.8 eV (L(2)), and 5.2 eV (L(3)). The L(3) term observed in all hydrides is attributed to optical absorption of MgH(2) with E(g)=5.16 eV. The Drude term, which may be due to unhydrogenated Mg-metal, was observed in the hydrides with a low Ca composition y of less than 0.04. The L(1) term was observed in the hydrides with a Ca composition of more than 0.05, while the L(2) term was observed for more than 0.11. These L(1) and L(2) terms were estimated from a variation with hydrogenation of ellipsometric angle Ψ which is due to optical absorption of CaH(2-α) (L(1)) and MgCaH(3.72) (L(2)).     

Absence of chromatic effect: crystal relaxation around CeIII in Y1−xCexPS4 (0≤x≤1)

Ce-containing chalcogenides may exhibit interesting chromatic properties due to the presence of a Ce^III-5f/Ce^III-4d gap. In such insulating materials, color varies from red to yellow depending on the covalent character of the Ce–S bond. Thanks to this correlation, a continuous modification of the Ce–S bond characteristics may be induced to control the hue. Attempts in modifying these bonds have been undertaken by synthesizing the solid solution Y_1−xCe_xPS₄ (x=0, 0.005, 0.01, 0.05 and 0.1), YPS₄ being colorless. The substitution of Y³⁺ cations (rY³⁺(CN8)=1.019 Å) for Ce³⁺ cations (rCe³⁺(CN8)=1.143 Å) let expect a regular shift of the physical parameters deemed to influence the chromatic properties. Nevertheless, diffuse reflectance measurements did not evidence any absorption threshold shift vs. x in the Y_1−xCe_xPS₄ series. The lack of sizeable influence of the Y/Ce substitution on the color is correlated not only to the intrasite, band to band nature of the Ce-5f→Ce-4d transition, but also to the occurrence of local relaxations around Ce in the YPS₄:Ce solid solution insuring a chemical environment of the rare earth very similar to that observed in CePS₄. EXAFS measurements performed at the Y–K and the Ce–L_III edges evidence the maintaining of the average Y–S and Ce–S distances in the Y_1−xCe_xPS₄ solution from x=0 to x=1 as they are in the non-substituted end compounds YPS₄ and CePS₄.     

Effect of diffusion-annealing time (0.5 h ≤ t ≤ 2 h) on the mechanical and superconducting properties of Cu-diffused bulk MgB2 superconductors by use of experimental and different theoretical models

This study reports the effect of different annealing time (0.5 h ≤ t ≤ 2 h) on the electrical, physical, microstructural, mechanical and superconducting properties of Cu-diffused bulk magnesium diboride (MgB₂) system by means of dc resistivity, X-ray diffraction analysis (XRD), scanning electron microscopy (SEM) and microhardness measurements (H _v ). The room temperature resistivity (at 300 K), critical transition (T _c ^offset and T _c ^onset ) temperature, variation of transition temperature, grain size, phase purity, lattice parameter, texturing, surface morphology, crystallinity and Vickers microhardness values of the samples are evaluated and compared with each other. The resistivity results obtained reveal that the (T _c ^offset and T _c ^onset ) values of the samples produced ascend with the enhancement in the annealing time up to 1 h beyond which these values start to reduce systematically and in fact the smallest T _c ^onset of 38.1 K and T _c ^offset of 36.2 K are observed for the sample annealed for 2 h. Similarly, the SEM micrographs display that the surface morphology, crystallinity and grain connectivity improve until a certain diffusion-annealing time (1 h), and after this point, all the properties obtained start to degrade with the increase of the annealing time. Furthermore, the peak intensities, grain sizes and lattice parameters deduced from the XRD measurements illustrate that a systematic elongation in the a and c axis lengths is detected with the annealing time until 1 h beyond which a regular contraction in the lattice parameters is observed for the samples. Likewise, the peak intensities belonging to MgB₂ phase enhance with the increment of the annealing time up to 1 h after which they reduce slowly; however, a new peak belonging to Mg₂Cu phase appears in the sample annealed for the duration of 2 h, confirming both the reduction of the grain size and degradation of the critical temperature. Additionally, we have focused on the microhardness measurements for the examination of the mechanical properties of the samples studied. Experimental results of microhardness measurements are estimated using the various models such as Meyer’s law, proportional sample resistance model, modified proportional sample resistance model and Hays-Kendall (HK) approach. Based on the simulation results obtained, the Hays-Kendall (HK) approach is determined as the most suitable model describing the mechanical properties of samples prepared.     

Thermal Expansion and Isothermal Compressibility of TlIn1 – xNdxSe2 (0 ≤ x≤ 0.08) Crystals

The thermal expansion coefficient () and isothermal compressibility ( _T ) of TlIn_{1 – x} Nd _x Se₂(0 x 0.08) crystals were measured between 77 and 400 K. In the range 77–160 K, both and _T increase with temperature, the increase in being much steeper. At higher temperatures, and _T change very little. The observed composition dependences of and _T are interpreted in terms of energy-band structure.