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.     

AC Susceptibility and Superconducting Properties of Graphene Added YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7−<Emphasis Type="Italic">d</Emphasis></Subscript>

The effect of graphene (G) addition on YBa₂Cu₃O_7?δ(G)_x (x =?0 ? 0.03 wt%) has been studied using the X-ray diffraction method, scanning electron microscopy, electrical resistance versus temperature, transport critical current density J_c, and AC susceptibility measurements. XRD patterns showed single-phase YBa₂Cu₃O_7?δ (YBCO) for all samples. SEM micrographs showed filling of the voids between YBa₂Cu₃O_7?δ grains as graphene was added. The temperature-dependent electrical resistance curves showed metallic normal state behavior and onset transition temperature T_conset between 90 and 92 K for all samples. AC susceptibility measurement showed transition temperature \(T_{\mathrm {c} \chi ^{\prime }}\) between 90 and 93 K. \(T_{\mathrm {c} \chi ^{\prime }}\) was maintained or improved slightly as graphene was added. The x =?0.001 wt% showed the highest J_c, i.e., 2750 A cm^?2 at 77 K and 5570 mA cm^?2 at 30 K which was 13 and 40 times higher than that of the non-added YBCO, respectively. The peak temperature T_p of the imaginary part of the susceptibility χ^″ was around 78–82 K for all samples indicating grain coupling was not weakened as graphene was added.     

Effect of CdTe Addition on the Electrical Properties and AC Susceptibility of YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7−<Emphasis Type="Italic">δ</Emphasis></Subscript> Superconductor

The effect of CdTe addition on YBa₂Cu₃O_7?δ (CdTe) _x (x = 0–0.10) has been studied. XRD patterns showed the presence of impurities including CdTe for x ≥ 0.05 wt%. The resistance versus temperature curves showed metallic behavior for all samples. The onset temperature T _{c onset} for all samples is between 90 and 92 K. The superconducting transition width, ΔT _c = 4 K for all samples except for x = 0.1 wt% where ΔT _c = 6 K. The superconducting transition determined by AC susceptibility measurement also showed little change in \(T_{\mathrm {c}\chi ^{\prime }}\) (between 89 and 92 K). However, the peak temperature T _p of the imaginary part of the susceptibility χ″, showed a drastic decrease for samples with x > 0.05 wt%. This indicated that the superconducting grains were strongly decoupled for x > 0.05 wt% due to the impurities as observed in the XRD pattern. The intergrain critical current density, J _c at T _p for the x = 0 sample is J _c (82 K) = 17 A cm^?2. These results can be useful in the fabrication of semiconductor/YBCO superconductor hybrid systems.     

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.     

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.     

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.     

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.     

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.     

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.     

Electrical properties of YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7−<Emphasis Type="Italic">x</Emphasis></Subscript> films of various thicknesses

We have studied the electrical properties of high-T_c superconductor YBa₂Cu₃O_7?x films obtained by the ion-plasma deposition technique on sapphire substrates. Dependences of the surface resistance and the critical current density on the film thickness are determined.     

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.     

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.     

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.     

Improved Superconducting Properties of Thick YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7−<Emphasis Type="Italic">δ</Emphasis></Subscript> Film by Adding Amino-Rich Polyethyleneimine in Precursor Solution

Polyethyleneimine (PEI) with an amount of –NH₂ groups used in precursor solution could effectively reduce Cu²⁺ volatilization during the pyrolysis process. Thermogravimetric analysis shows that the temperature window of low-temperature pyrolysis for precursor solution with PEI (PEI-YBCO) is widened significantly. The slower pyrolysis process can enrich Cu²⁺ and improve critical current density (J_c) of PEI-YBCO films. The highest J_c is 3.03 MA/cm² at 77 K when the amount of PEI is 0.5 g/10 mL and the film thickness is 400 nm. Then the thickness increases from 0.4 to 2.0 μm by changing the coating times. The J_c values of PEI-YBCO films decrease gradually with the thickness increase. However, the critical current (I_c) can be up to 197 A/cm (at 77 K, self-field) and J_c can still keep 1.68 MA/cm² at 1.2 μm.     

The effect of pulsed magnetic fields on high-temperature superconductor YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7−<Emphasis Type="Italic">x</Emphasis></Subscript> ceramics

We have observed a change in the sign of the temperature coefficient of resistance of YBa₂Cu₃O_7?x ceramics in the region of the superconducting transition under the action of relatively weak (<1 T) pulsed magnetic fields. The effect is probably caused by the transition of the oxide based superconductor from the normal metallic state to a pseudogap regime as a result of the partial loss of oxygen atoms from Cu-O chains.     

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.     

Determination of the Glassy Exponent of Melt Textured YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7−<Emphasis Type="Italic">x</Emphasis></Subscript> by Magnetic Relaxation

By magnetic relaxation method we obtained the glassy exponent (T, H) of the melt textured Yba₂Cu₃O_7–x sample. The results have a similar behavior compared with ones using the method of magnetic field sweep, and are compatible with the model of collective pinning theory.     

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.     

The Effect of Sintering Temperature Variation on the Superconducting Properties of ErBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7−<Emphasis Type="Italic">δ</Emphasis></Subscript> Superconductor Prepared via Coprecipitation Method

The effect of heat treatment on the superconducting properties of ErBa₂Cu₃O_7−δ (ErBCO) ceramic materials has been studied. The nano-metal oxalate precursor was prepared using coprecipitation (COP) method. The prepared materials were subjected to calcination process at 900 °C for 12 h and then sintered under oxygen environment for 15 h at 920 °C, 930 °C, 940 °C, and 950 °C, respectively. All samples showed a metallic behavior and single-step transition in the R–T curves. The best zero critical current, T _C(R=0)=91.4 K, was for the sample sintered at 920 °C. XRD data showed single phase of an orthorhombic structure. As the sintering temperature increases, the formation of nonsuperconducting phases (impurities) was observed when the samples sintered above 920 °C. The formation of nano-oxalate powders via COP method is a very efficient procedure to produce high-quality superconductors with less processing temperature required.     

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.