Synthesis and Characterization of Ni<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Cu<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Bi<Subscript>3</Subscript> Superconductor

In this paper, we characterized the microstructure and superconducting properties of Cu-doped NiBi₃ samples. The polycrystalline Ni_1?x Cu _x Bi₃ (0 ≤ x ≤ 0.10) samples were prepared using a solid-state reaction method. The crystal structure and unit cell parameters were determined by Rietveld refinement of powder X-ray diffraction. The data showed that the main phase present corresponded to NiBi₃ without dependence on the Cu concentration, but with small quantities of Ni and Bi. The SEM and AFM measurements revealed that the main phase was inhomogeneous at microscopic level, with Bi richer regions in comparison to other regions. However, Raman spectroscopy results did not show significant changes in the spectra with Cu doping and in different regions of the samples. Another finding was that regardless of Cu doping, the superconducting transition temperature was 4.05–4.06 K.     

Synthesis and properties of SrFe<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>M<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>O<Subscript>3 − <Emphasis Type="Italic">z</Emphasis></Subscript> (M = Mo,W) perovskites

We have synthesized SrFe_{1 ? x} M _x O_{3 ? z} (M = Mo, W; 0 < x ≤ 0.5) solid solutions. Our results indicate that the introduction of stable MO₆ octahedra narrows the range of oxygen stoichiometries of the material and suppresses the perovskite-brownmillerite structural phase transition at low temperatures and oxygen partial pressures. We have studied the thermal stability of the synthesized materials in a reducing atmosphere and the effect of oxygen stoichiometry on their electronic and oxygen-ionic conductivity and phase transformations.     

Angular-Dependent Vortex Pinning Properties of YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7−<Emphasis Type="Italic">δ</Emphasis></Subscript>/Y<Subscript>2</Subscript>O<Subscript>3</Subscript> Quasi-Multilayers

A series of quasi-multilayers of YBa₂Cu₃O_7?δ (YBCO)/Y₂O₃ specifically 70 × (m YBCO/n Y₂O₃) were prepared on SrTiO₃ single crystal using pulsed-laser deposition (PLD) with a controlled deposition pulses of m = 40 and n = 2, 5, and 10 for YBCO and Y₂O₃, respectively. The x-ray diffraction patterns indicate that all the present quasi-multilayers exhibit good c-axis orientation. The angular dependence of critical current density (J _c ) on applied magnetic field directions are systemically measured to study the anisotropic vortex pinning performances for those quasi-multilayers. It is revealed that compared with the pure YBCO films, the quasi-multilayers with n = 2, i.e., a proper constituent pulse of Y₂O₃, exhibits the enhanced vortex pinning abilities in all angles between c-axis orientation and the applied magnetic field direction. As well, such a quasi-multilayer film (n = 2) shows the higher lift factor J _c (Θ)/ J _c (90°) and much better vortex pinning properties at high fields and high temperatures, showing promising potential for coated conductor application.     

Microhardness of the YbAg<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>In<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Cu<Subscript>4</Subscript> alloy system

We report Vickers microhardness measurements on flux grown single crystals of the YbAg_xIn_{1 − x}Cu₄ alloy system. Although sample dependent, the microhardness exhibits a clear concentration dependence: in general, it decreases with x. The lattice parameter as a function of x exhibits a similar behavior. For x < 0.5, where the lattice parameter is almost constant, the microhardness exhibits a weak enhancement. Similar concentration dependence of the lattice parameter, resistivity and microhardness allows us to conclude that the microhardness reflects the evolution of the YbAg_xIn_{1 − x}Cu₄ alloy system towards more metallic character with increasing x.     

Effect of Ba/Y Molar Ratio on Superconducting Properties of BaTiO<Subscript>3</Subscript>-Doped YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript><Emphasis Type="Italic">O</Emphasis><Subscript>7−<Emphasis Type="Italic">δ</Emphasis></Subscript> Films

YBa₂Cu₃ O _7?δ (YBCO) films with BaTiO₃ (BTO) nanostructures were prepared by using the precursor solutions with different cationic molar ratios of Y:Ba:Cu = 1.0:1.6–2.0:3.0 in the TFA-MOD process. These YBCO films were deposited on (00 l)-oriented LaAlO₃ single-crystal substrates using a spin coater. The high superconducting critical current density (J _C) (77 K, self-filed) of more than 10 MA/cm² for the final BTO-doped YBCO film was obtained. Moreover, the effect of different Ba/Y molar ratios in the precursor solution on superconducting properties of BTO-doped YBCO films was investigated. Compared with the BTO-doped YBCO film deposited by using the precursor solutions with Ba/Y molar ratio of 2.0, an enhancement of J _C in a magnetic field for the film from the solution with Ba/Y molar ratio of 1.9 was achieved. For Ba/Y molar ratios of 1.6 and 1.7, a reduction of J _C in a magnetic field occurred. The J _C enhancement may be mainly ascribed to the enhanced flux pinning by the Y₂Cu₂ O ₅ nanostructures with the optimal number dispersing in YBCO matrix.     

Fabrication and Analysis of Tri-layer YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript><Emphasis Type="Italic">O</Emphasis><Subscript>7−<Emphasis Type="Italic">δ</Emphasis></Subscript> Thick Films by Low-Fluorine MOD Method

Improving the thickness of superconducting layer in coated conductors is an effective way to enhance its critical current. In this work, tri-layer YBCO/YBCO/YDyBCO films were successfully deposited on buffered Hastelloy substrate using the multi-coating lowfluorine metal-organic decomposition (LF-MOD) method and the thickness of the films can be up to 2.4 μ m. The effects of high-temperature annealing time on microstructures and superconducting properties of the films were systematically studied. Energy dispersive X-ray spectroscopy (EDS) results reveal that there remains a large amount of F element in the upper layer of the film when the annealing time is too short. With increasing the annealing time, the fluoride-containing precursor converts to YBCO grains completely. But the coarsening of grains appeared, and the critical current density (J _c) of the film dropped slightly when the annealing time is too long. The cross-sectional scanning electron microscope (SEM) image and EDS plane analysis were applied to investigate the microstructure and element distribution of the final triple-layer YBCO films, respectively. The critical current of the final YBCO superconducting film could reach 316 A (77 k, self-field) for 1.2-cm-wide tapes with the optimal annealing conditions.     

Low-temperature heat capacity of Li<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Ni<Subscript>2 − <Emphasis Type="Italic">x</Emphasis></Subscript>O<Subscript>2</Subscript> solid solutions

The heat capacity of Li _x Ni_{2 − x} O₂ (x = 0.40–0.76) oxides has been measured using an adiabatic calorimeter, and their thermodynamic functions have been determined. The results indicate that the lithium nickelate solid-solution series contains a two-phase region and that near-stoichiometric LiNiO₂ has a layered structure, in accordance with earlier results.     

Fluorine doping in dilute magnetic semiconductor Sn<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Fe<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>O<Subscript>2</Subscript>

Recent studies have reported room-temperature ferromagnetism (FM) in Fe doped SnO₂. The FM in semiconductors due to transition metal doping has been argued to be carrier mediated. Fluorine (F) doping in pure SnO₂ has been reported to significantly increase the carrier concentration. In this work, we investigated the role of F doping in the range from 0% to 0.79% on the FM of chemically synthesized single phase Sn_1?x Fe _x O₂ using X-ray diffraction, UV–Vis spectrophotometry, particle-induced X-ray emission, particle-induced gamma ray emission and magnetometry. The saturation magnetization M _s (0.03 emu/g) increased by a factor of 2.5 and the lattice volume and band gap energy decreased by 0.35 Å³ and 0.2 eV, respectively, with 0.67% F doping (F/Sn atom %) compared to the sample without any fluorine.     

Synthesis and Characterization of LaBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7−<Emphasis Type="Italic">δ</Emphasis></Subscript> System by Combustion Technique

This paper reports the synthesis and characterization of a lanthanum–barium–copper oxide, based on a LaBa ₂Cu ₃ O ₇ system, using a wet chemical route that enables the combustion–polymerization of citrate species, in order to generate materials with enhanced surface and textural and morphological properties for potential applications. The synthesized precursor in a form of a coordination complex was characterized by Fourier transform infrared spectroscopy (FTIR) analysis in order to evaluate the formation of homogeneous and soluble citrate species as intermediates of reaction. The morphological and structural characterizations were performed over calcined material with X-ray diffraction (XRD) and electron microscopy (scanning electron microscopy (SEM)-transmission electron microscopy (TEM)) analyses, confirming the obtention of an orthorhombic crystalline phase type Pmmm (47) in the nanometric range ≈8.9 nm. Analyses of the ceramic oxide by Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) allowed to perform qualitative and quantitative assessments of the material composition, showing that the final oxide is closely related to the desired composition, discarding the presence of carbonaceous residues of the synthesis process.     

Enhanced Superconductivity in Double-Doping Cu<Subscript>0.15</Subscript>TaSe<Subscript>2−<Emphasis Type="Italic">x</Emphasis></Subscript><Emphasis Type="Italic">S</Emphasis><Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>

Superconducting Cu _x TaSe₂(x=0.05, 0.15) and Cu_0.15TaSe_2?x S _x (x=0, 0.5, 1, 1.5) single crystals have been systematically fabricated by a chemical vapor transport method. It is found that the double doping in TaSe₂, i.e., the simultaneous intercalation of Cu and substitution of Se by S, can substantially enhance the superconducting transition temperature. Transport property measurements give evidence of the coexistence and competition of charge density wave state and superconductivity in Cu _x TaSe₂ which provide meaningful information to understand the complex electronic states in this system. The parallel shift and the fan-shape broadening behaviors are observed in the superconducting transition curves under magnetic fields of Cu_0.15TaSeS and TaSeS, respectively, indicating an increase of coherence length and suppression of superconducting fluctuation induced by copper intercalation.     

Properties of YBaCuFe<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Ni<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>O<Subscript>5</Subscript> (0 < <Emphasis Type="Italic">x</Emphasis> ≤ 0.3) solid solutions

YBaCuFe_{1 – x}Ni_xO₅ solid solutions are shown to exist for x 0.3. Data are presented on the lattice parameters, thermal stability, thermal expansion, electrical conductivity, thermoelectric power, magnetic susceptibility, and dielectric properties of the solid solutions. Ni substitution for Fe notably increases the electrical conductivity of the solid solutions, reduces their thermoelectric power and thermal expansion, and shifts the antiferromagnetic—paramagnetic phase transition and dielectric anomalies in YBaCuFe_{1 – x}Ni_xO₅ to lower temperatures.Translated from Neorganicheskie Materialy, Vol. 40, No. 12, 2004, pp. 1515–1519.Original Russian Text Copyright © 2004 by Chizhova, Klyndyuk, Bashkirov, Petrov, Makhnach.     

Sintering mechanisms in YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7−<Emphasis Type="Italic">x</Emphasis></Subscript> superconducting ceramics

The densification of ceramic compacts of YBa₂Cu₃O_7−x (123) was studied with a vertical dilatometer. The runs effected under isothermal conditions (ISO) covered the 920–970^∘C range and were performed under static air atmosphere. Also, controlled heating rate (CHR) runs, from about 800 to 1050^∘C, were conducted at 5^∘C/min under either flowing oxygen or static air. The ISO data could be satisfactorily fitted by the solution-precipitation (SP) model giving an activation enthalpy of 221 kJ/mol. Furthermore, the CHR data for 920–970^∘C was also fitted with the same model giving 207 kJ/mol as the activation energy. From analysis of CHR data, the initial stage sintering is driven by solid state sintering between 827–894^∘C (823–908^∘C in O₂). Then, in the interval 902–920^∘C (914–934^∘C in O₂) the intermediate stage driven by grain growth (GG), competes with the rearrangement process associated to the presence of a liquid phase. This last process applied because the next sintering stage in the range 922–970^∘C (938–990^∘C in O₂ flow) could be fitted by the SP model with an activation enthalpy of 207 kJ/mol (229 kJ/mol in O₂). In the range 972–995^∘C (990–1014^∘C in O₂), the solid state (GG) intermediate stage mechanism and/or viscous flow competes with the SP process.     

Study of YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7−<Emphasis Type="Italic">δ</Emphasis></Subscript> Superconductor/Graphene Oxide Composite

The aim of this research was to fabricate YBa₂Cu₃O_7?δ (YBCO) superconductor composite with graphene oxide nanosheets and to study the effect of the graphene oxide nanosheets on YBCO superconductor properties. For this purpose, the samples of pure superconductor and superconductor composite with 0.001, 0.01, and 0.1 wt.% graphene oxide were synthesized. First, graphite oxide was made by Hummer’s chemical method; after that, graphene oxide nanosheets were produced by bath-keeper ultrasonic. Then, different amounts of graphene oxide were added to the process of superconductor fabrication, which was made by solid-state reaction method. The samples were characterized and studied by Meissner effect test, XRD analysis, FESEM imaging, EDX measurement, and ac magnetic susceptibility. The critical current density (J_c) of samples was measured by four probes method. The results showed that by increasing the weight ratio of graphene oxide, J_c and T_c decrease.     

Superconducting and Magnetic Properties of Zn-doped YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7−<Emphasis Type="Italic">δ</Emphasis></Subscript>

The superconducting properties and AC/DC magnetic properties of YBa₂Cu_3−x Zn _x O_7−δ , (x=0.0, 0.01, 0.03, 0.05, 0.10 and 0.15) compounds were investigated. Samples were synthesized through solid-state reaction route. X-ray diffraction data confirms the single-phase orthorhombic/tetragonal crystallization for the studied samples. Thermo-Gravimetric Analysis (TGA) was done for pure and Zn-doped YBa₂Cu₃O_7−δ (YBCO) samples. Results reveal that at higher dopant levels the doped samples are more oxygen-deficient than the undoped ones. The superconducting properties of YBa₂Cu_3−x Zn _x O_7−δ system always enhance with addition of oxygen to Cu–O chains, which causes enhancement of superconducting carrier density together with orthorhombic structure. Zn substitution causes an overall reduction of superconducting condensation energy with systematic degradation of superconducting properties like T _c and J _c. This occurs via induced non-homogeneity of hole carrier density created by extended nature of perturbation on electronic structure in CuO₂ planes and its weak-link type behavior. AC susceptibility measurement reveals that Zn doping reduces the inter-granular couplings. Scanning Electron Microscope (SEM) images indicate that increment of average grain size with increasing of Zn concentration.     

Electrical conductivity and thermoelectric power of LaCo<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Ga<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>O<Subscript>3</Subscript> solid solutions

Increasing the degree of Ga³⁺ substitution for Co³⁺ ions in LaCo_{1 − x} Ga _x O₃ solid solutions (x = 0–1) considerably reduces their electrical conductivity: at T= 850 K, from 190.5 S/cm in LaCoO₃ to 1.32 × 10⁻⁵ S/cm in the x = 0.95 solid solution. The anomaly in the temperature-dependent conductivity of the solid solutions, due to the broad semiconductor-metal transition, decreases with increasing x. For x ≥ 0.8, there is a very weak or no anomaly. The activation energies for conduction in the samples with x = 0.90 and 0.95 are 0.89 and 0.92 eV, respectively. At room temperature, the materials with 0 ≤ x ≤ 0.3 have a negative thermoelectric power. With increasing temperature, it increases, crosses zero between 435 and 530 K, reaches a maximum in the range 500–650 K, and decreases at higher temperatures.     

Sol-gel synthesis and photoluminescence of CaTi<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Zr<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>O<Subscript>3</Subscript>: Pr<Superscript>3+</Superscript> phosphors

CaTi_1−x Zr _x O₃: Pr³⁺ phosphors have been synthesized by sol-gel and solid state methods, with x = 1/300, 2/300, 3/300, 4/300, 5/300, 6/300, 7/300, respectively. Powder X-ray diffraction (XRD), UV-visible absorption spectra, photoluminescent spectra (PL), and scanning electron microscopy (SEM) images are used to characterize the powder samples. The inverse absorption at 610 nm appearing in the UV-visible absorption spectra is due to the ¹ D ₂ → ³ H ₄ characteristic emission of Pr³⁺. Changes in the emission spectra at 610 nm were agreed with those in UV-visible absorption spectra. The strongest red excitation obtained from CaTi_1−x Zr _x O₃: Pr³⁺ (x = 4/300) and CaTi_1−x Zr _x O₃: Pr³⁺ (x = 5/300) possesses the strongest emission at 610 nm, similar to the intensities of Ca(Ti_1−x Zr _x )O₃: Pr³⁺ (x = 3/300, 4/300, 6/300), which may be corresponded to the cell parameters of CaTi_1−x ZrxO₃: Pr³⁺.     

Polycrystalline YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7−<Emphasis Type="Italic">δ</Emphasis></Subscript> with Nano-sized Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Inclusions

Series of (YBa₂Cu₃O_7−δ )_1−x (Al₂O₃) _x samples have been prepared using solid state reaction method. Various amount of nano-sized Al₂O₃ particles (∼50 nm) were added with (x=0, 0.005, 0.01, 0.02, and 0.05). The microstructure and the morphology of the polycrystalline samples have been characterized by X-ray diffraction (XRD) and atomic force microscopy (AFM). The magneto-transport properties of these samples were investigated using resistance–temperature (R–T) and current–voltage (I–V) characteristics. All samples showed an orthorhombic structure with a tendency to transformation to tetragonal phase at higher levels of nanoparticles addition. The morphology of the surface of pure samples reveals a considerable number of weak-links, randomly oriented and clean grain boundaries. While in samples with nano inclusions, grain boundaries were filled with nanosized particle and have less number of weak-links. Significant enhancement of the superconducting critical current density J _c in applied magnetic field was observed due to nano Al₂O₃ inclusions. However, further increase in the value of x decreases the transition temperature T _c and the critical current density J _c . These results were interpreted in terms of the flux pinning mechanisms in granular superconducting networks which leads to a better basic understanding of the performance of YBCO system in high applied magnetic fields.     

Certification of YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7−<Emphasis Type="Italic">x</Emphasis></Subscript> Single Crystals

A method for evaluation of the critical temperature T _c and the width of the superconducting transition ΔT _c in HTSC single crystals has been developed. By this method, the first derivative of the temperature dependence of the resistivity, \(\frac{\partial \rho (T)}{\partial T}\), is constructed. A technique for synthesis of YBa₂Cu₃O_7?x single crystals with highly reproducible physical-mechanical properties has been described. A standard sample with T _c=94 K and ΔT _c=0.25 K has been synthesized and certified.     

Influence of Oxygen on the Tribomechanical Properties of Single Crystals of YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7−<Emphasis Type="Italic">δ</Emphasis></Subscript>

This paper reports a study on the mechanical and tribological properties of ab- and a (b) c?planes of YBa₂Cu₃O_7?δ single crystals. The single crystals were grown using a CuO-BaO self-flux method. The oxygenation effect on the mechanical and tribological properties of ab- and a (b) c?planes is reported. For the ab- plane, the hardness and elastic modulus were around 6 and 50 GPa, respectively. In this case, significant differences were not observed among the hardness and elastic modulus at different oxygenation states. However, the hardness and elastic modulus for as-grown and oxygenated YBa₂Cu₃O_7?δ single crystals were different from that of the a (b) c?plane, and were observed to be slightly higher for the as-grown than for the oxygenated samples. For as-grown and oxygenated samples, we observed hardness values around 4.7 and 2.0 GPa, respectively. Regarding the elastic modulus, the values were 75 and 40 GPa, respectively. The indentation fracture toughness values on the ab- plane for the as-grown and oxygenated YBa₂Cu₃O_7?δ single crystal were 3.7 ± 1.2 and 2.9 ± 1.2 MPa m^1/2, respectively. For the ab- plane, the scratch resistance of the as-grown sample was higher than that of the oxygenated sample and the scratches under load were deeper for the oxygenated sample. As regards the a (b) c?plane, the scar features were seemingly constant through all the scratch lengths and the scratches under load were deeper and larger for the oxygenated than that for the as-grown sample.     

Hydrochemical synthesis of AgCl<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Br<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript> solid solutions

A phase-component model is proposed for the hydrochemical conversion of silver chloride to AgCl _x Br_{1 − x} solid solutions, and the corresponding quantitative relations are derived. Experimental data are used to obtain a relation between the equilibrium composition of solid solutions and that of the liquid phase at different temperatures. The possible mechanisms of solid-solution formation are considered.