The influence of inter-granular coupling on the magnetic susceptibility and critical current density of YBa2(Cu1−xAgx)3O7−δ superconductors prepared by ethylene glycol assisted solvothermal approach

Series of YBa₂(Cu_1−xAg_x)₃O_7−δ (0≤x≤0.09) powders were prepared by ethylene glycol assisted solvothermal approach. X-ray diffraction verified the formation of single phase crystal structure for all the investigated samples, implying the complete solubility of the silver ions into YBa₂Cu₃O₇. The partial substitution of the Ag atoms into the Cu sites caused a decrease of the lattice constant (c) and an increase of the lattice constants, a, and, b, to preserve the orthorhombic crystal structure of YBa₂Cu₃O₇. The scanning electron microscopy depicted that the incorporation of silver ions into copper sites adjusts the random orientations of the YBCO grains and suppresses the formation of grain boundary. The AC magnetic susceptibility measurement revealed that the superconducting performance of the YBa₂Cu₃O₇ is preserved at all Ag concentrations. The critical current density, J_c, measurement revealed that the addition of the Ag into the YBCO caused a strong pinning force is established due to the strong coupling among the grains instead of the weak Josephson junctions and in turn a remarkable increase of the zero field J_c from 2.7 MA/cm² to 9.1 MA/cm² was achieved.     

Catalytic behavior of YBa2Cu3O7-x in the partial oxidation of methanol to formaldehyde

The partial oxidation of methanol to formaldehyde was studied over YBa₂Cu₃O_7-x catalyst in a flow reactor. The structural change of YBa₂Cu₃O_7-x before and after the reaction was measured by means of XRD and iodometric titration method. The catalytic characteristics of YBa₂Cu₃O_7-x for the partial oxidation of methanol to formaldehyde was due to copper ions. It was found that Cu⁺² was responsible for the higher selectivity for formaldehyde.     

Magnetic doping effects on the superconductivity of Y1-xMxBa2Cu3O7-δ (M = Fe,Co, Ni)

The discovery of superconductivity in copper oxide compounds has attracted considerable attention over the past three decades. The high transition temperature (T_c) in these compounds, exhibiting proximity to an antiferromagnetic order in their phase diagrams, remains one of the main areas of research. It is believed that magnetic fluctuations provide substance for the exotic superconductivity observed in these compounds. The present study attempts to introduce Fe, Co and Ni magnetic impurities into the superconducting cuprate YBa₂Cu₃O_7-δ with the aim of exploring the T_c behavior. The solid-state synthesis method is exploited to prepare fully oxygenated Y_1-xM_xBa₂Cu₃O_7-δ (Y_1-xM_x-123) (M = Co, Fe, Ni) samples with low levels of doping (0.00000 ≤ x ≤ 0.03000). Systematic measurements are then employed to assess the synthesized samples using AC magnetic susceptibility, electrical resistivity and X-ray diffraction (XRD). The measurements revealed an increase in T_c as a result of magnetic substitution for Y. However, the study of non-magnetic dopings on the fully oxygenated Y_1-xM'_xBa₂Cu₃O_7-δ (Y_1-xM'_x-123) (M' = Ca, Sr) samples showed a decrease in T_c. Quantitative XRD analysis further suggested that the internal pressure could have minor effects on the increase in T_c. The normal state resistivity vs temperature showed a linear profile, confirming that the samples are at an optimal doping of the carrier concentration.     

A comparative study of (Ce) and (Gd) doping influence on the superconducting properties of YBCO ceramics

This work is devoted to investigate the structural and electrical properties of the Ce, Gd-doped YBCO superconductors bulk ceramics. YBa_2-xRE_xCu₃O_7−δ (x = 0, 0.01, 0.05, 0.1) (RE = Gd, Ce) samples were prepared by means of conventional solid-state reaction. X-ray diffraction analysis was carried out to identify the present phases in the as-prepared samples followed by the determination of their lattice parameters. Fourier Transform Infrared Spectroscopy (FTIR) was used to identify the functional groups. Furthermore, the morphology and the surface roughness of the studied samples were characterized using Scanning Electronic Microscopy (SEM) and Atomic Force Microscopy (AFM). Vickers Micro-hardness of the as-prepared samples was examined. Besides, the electrical resistivity measurements were achieved to determine the critical transition temperature T_C and the critical current density J_C.The effect of Ce and Gd additions is clearly noticed in the obtained results, where all the prepared samples are superconductors with the presence of Y123 as a major polycrystalline phase. From the XRD patterns, the intensities of the Y123 corresponding peaks decrease with further increasing the Ce and Gd contents. In addition, the variation of the cell parameters was significant after additions of both Ce and Gd, which affect the grain size and the oxygen content of the YBa_2-xRE_xCu₃O_7−δ system. An improvement of the structure and surface roughness is observed on SEM and AFM images. Likewise, Vickers micro-hardness has increased after the Ce and Gd additions. Although, the critical transition temperature T_C was not further increased upon Ce or Gd additions compared to the undoped YBCO samples. Nevertheless, an exception has been recorded with an increase of T_C for YBa_2-xRE_xCu₃O_7−δ with (RE = Gd, x=0.01) to reach 88 K. In contrary, an improvement of the deduced critical current density J_C was achieved for all Ce-doped YBCO samples unlike those of Gd-doped samples.     

Catalytic behavior of YBa2Cu3O7-x in the partial oxidation of ethanol to acetaldehyde

The partial oxidation of ethanol to acetaldehyde was performed over YBa₂Cu₃O_7-x in a flow reactor. The catalytic characteristics of YB₂Cu₃O_7-x were compared with those of an individual oxide comprising the YBa₂Cu₃O_7-x . The structural change of YBa₂Cu₃O_7-x and the other catalysts after the reaction was measured by means of XRD and XPS, and it was found that the high oxidation state of copper in YBa₂Cu₃O_7-x was responsible for the higher activity and the higher selectivity for acetaldehyde.     

Silver-substituted (Ag1-xCux)2ZnSnS4 solar cells from aprotic molecular inks

To battle the high open-circuit voltage deficit (V_OC,def) in kesterite (Cu₂ZnSnS₄ or CZTS) solar cells, a current field of research relates to point defect engineering by cation substitution. For example, by partly replacing Cu with an element of a larger ionic radius, such as Ag, the degree of Cu/Zn disorder decreases, and likewise does the associated band tailing. In this paper, solution-processed (Ag_1-xCu_x)₂ZnSnS₄ (ACZTS) samples are prepared through the aprotic molecular ink approach using DMSO as the solvent. The successful incorporation of silver into the CZTS lattice is demonstrated with relatively high silver concentrations, namely Ag/(Ag+Cu) ratios of 13% and 26%. The best device was made with 13% Ag/(Ag+Cu) and had an efficiency of 4.9%. The samples are compared to the pure CZTS sample in terms of microstructure, phase distribution, photoluminescence, and device performance. In the XRD patterns, a decrease in the lattice parameter c/a ratio is observed for ACZTS, as well as significant peak splitting with Ag addition for several of the characteristic kesterite XRD reflections. In addition to the improvement in efficiency, other advantageous effects of Ag-incorporation include enhanced grain growth and an increased band gap. A too high concentration of Ag leads to the formation of secondary phases such as SnS and Ag₂S as detected by XRD.     

Local structure and superconductivity in (Bi1.6Pb0.4Sr2Ca2Cu3O10+δ)1-x(Fe3O4)x compounds

In this work, a close correlation between variations of critical temperature (T_c) and the hole concentration of (Bi_1.6Pb_0.4Sr₂Ca₂Cu₃O_10+δ)_1-x(Fe₃O₄)_x systems was studied. The (Bi_1.6Pb_0.4Sr₂Ca₂Cu₃O_10+δ)_1-x(Fe₃O₄)_x samples were fabricated using the solid-state reaction method, where x ranged from 0 to 0.2. The T_c values of the samples deduced from magnetization versus temperature measurement gradually decreased with increasing the Fe₃O₄ content (x). To investigate a possible reason for the observed decrease in the values of T_c in the samples, the valence state of copper (V) and the hole concentration (p) in all samples were examined by analyzing the Cu K-edge and Cu L_2,3-edge X-ray absorption near edge structure (XANES) spectra. The values of V and p monotonously decreased with the increase in Fe₃O₄ doping content (x) and agreed with the behavior of T_c. The existence of Fe³⁺ was confirmed by analyzing Fe L_2,3-edge XANES. Hence, Fe³⁺ ions possibly entered the lattice structure of the samples and filled the holes in CuO₂ planes. The degradation of superconductivity in Fe₃O₄ doped samples was then explained.     

Plastic Forming of High‐Tc YBa2Cu3O7−x Bulk Superconductors

The large bulk high‐T_c YBa₂Cu₃O_7?x (YBCO) superconductor samples were prepared by plastic‐forming method. We examined the effects of the solvent and the crystallinity of YBCO powder on the composition, crystallinity, and superconducting properties of YBCO sheet samples (sample size: 50 mm × 50 mm× 3 mm). By changing the solvent from water to turpentine oil, the leaching of Ba²⁺ ions from YBa₂Cu₃(OH)_y multimetallic hydroxide particles used as an inorganic binder and the YBCO powder were reduced. This results in the composition of the grain boundaries of fired YBCO sheet samples to be the same as the composition of YBa₂Cu₃(OH)_y multimetallic hydroxide particles. Changing nondoped YBCO powder prepared by sintering to 5 wt% Pt‐doped YBCO powder prepared by melt texturing, J_c value of YBCO sheet samples changed from about 700 to 6,106 A/cm² at 77 K.     

Significant Improvement of Superconducting Properties in Nano‐NiFe2O4‐Doped Y–Ba–Cu–O Single‐Grain Superconductor

Introduction of refined second‐phase particles in superconducting YBa₂Cu₃O_7?x (Y‐123) matrix is known to be an effective route to improve the δl‐type pinning and the performance of Y–Ba–Cu–O (YBCO) single‐grain superconductors, while the δT_c‐type pinning induced by spatial fluctuations in matrix composition is also important and contributes to the in‐field J_c performance and high‐field applications of bulk superconductors. In this communication, chemical doping of nano‐sized NiFe₂O₄ (mean size 50 nm) in single‐grain YBCO superconductor is performed using a novel top‐seeded infiltration growth (TSIG) technique based on a solid source pellet (SSP) of nano‐Y₂O₃ + BaCuO₂. The results indicate that, significant improvement of bulk performances including levitation force (33.93 N) and trapped field (0.3628 T) has been observed in the 0.2 wt% nano‐NiFe₂O₄‐doped sample, which are much higher than the undoped sample (28.81 N and 0.2754 T). T_c measurement indicates that, a decreased onset T_c of about 87.5 K and a broadened transition width of about 5 K are observed in the NiFe₂O₄‐doped sample, indicating appearance of weak superconducting regions in superconducting matrix caused by Ni and Fe substitutions in Y‐123 crystal lattice. This study supplies a practical approach to increase the YBCO bulk performance significantly.     

Textured YBCO-Film Formation by Sol–Gel Process and Post Annealing

Thick YBCO-films were prepared on single crystalline SrTiO₃(001) and LaAlO₃(001) substrates by the dip-coating process starting with Y-, Ba- and Cu-acetylacetonates. When a characteristic temperature during transformation of the precursor into YBa₂Cu₃O_x is exceeded a strong biaxial texture with FWHM = 1·2° for 1 μm thick fims results. When the O₂ partial pressure of the annealing atmosphere is lowered the characteristic temperature decreases and films with increased current density (j_c) are obtained. The values of J_c achieved for films deposited onto SrTiO₃(001) at 77 K in self field are as high as 50·000 A cm⁻² with T_cs of 90K.     

Orthorhombic YBCO-123 ceramic oxide superconductor: Structural,resistive and thermal properties

Two YBa₂Cu₃O_7-x materials (YBCO-I sintered at 1000?°C, and YBCO-II sintered at 960?°C and following special cooling programs) belonging to the orthorhombic system were obtained. YBCO-I has a complete volume texture, while for YBCO-II it is only on sample surface. On heating in air (with 10?K?min^?1 in the temperature range 20–1000?°C), two endothermic processes are recorded on DSC curves (of YBCO-II) and they are due to oxygen diffusion from O1 to O5 sites, and then to the release into the environment; second endothermic effect is attributed to the decomposition reaction. Different behaviour is encountered on heating in argon: an endothermic process in two steps; continuous mass decrease (oxygen loss) during argon cooling is recorded on the thermogravimetric curve. In argon atmosphere, the obtained material shall correspond to the chemical composition: YBa₂Cu₃O_6.77, while in air atmosphere, the maximal composition than may be obtained is: YBa₂Cu₃O_6.88. The barrier energies from Arrhenius plots of resistivity vs. temperature (91–93?K) show 2D (YBCO-I) and 3D (YBCO-II) behaviour for the thermally activated flow of melted vortex lattice, below the critical temperature.     

Atomic charges in YBa2Cu3O7, YBa2Cu4O8, and Y2Ba4Cu7O15 ceramic samples

The atomic charges in the YBa₂Cu₃O₇, YBa₂Cu₄O₈, and Y₂Ba₄Cu₇O₁₅ lattices have been determined by comparing the experimental and calculated parameters of the crystal electric-field-gradient tensors. It has been demonstrated that the holes in these lattices are localized at oxygen ions around the Cu ions in the CuO₃ chains. These results are in agreement with the model according to which the interaction between electrons and two-atom two-electron centers with a negative correlation energy is considered as the mechanism responsible for high-temperature superconductivity.     

Wetting and spreading of Ca-Y-Ba-Cu-O solution on Y2O3 and CaSZ crucible in growing Y1-xCaxBa2Cu3O7-δ single crystal

Sizable and uniform Y_1-xCa_xBa₂Cu₃O_7-δ single crystals are of significant importance to study high-temperature superconductivity. However, the severe liquid loss resulting from intrinsic wetting property during top-seeded solution-growth (TSSG), makes it difficult to obtain such crystals. Here the reactive wetting performance of the Ca-Y-Ba-Cu-O solution on two types of crucibles was studied. It was identified that the spreading process on the Y₂O₃ crucible is characterized by forming double-layer of Y₂BaCuO₅ and YBa₂Cu₃O_7-δ, while that on the CaSZ crucible (Ca-stabilized ZrO₂) produces the BaZrO₃ layer with CuO phase. In the former case, the liquid has a low energy interface with the top layer of YBa₂Cu₃O_7-δ, leading to strong spreading and creeping behaviors. Conversely, due to a high interfacial energy between solution and BaZrO₃, the CaSZ crucible has a low wettability, particularly beneficial to solve the liquid loss problem. Consequently, with negligible liquid creeping out of CaSZ crucibles, we succeeded in growing a series of homogeneous Y_1-xCa_xBa₂Cu₃O_7-δ single crystals with an acceptable size up to a × b × c = 11.2 × 11 × 4.8 mm³. Moreover the wetting modes of solution on various kinds of crucibles for TSSG in growing doped YBa₂Cu₃O_7-δ single crystals were also elucidated. Most importantly, the understanding gained from this work is broadly applicable for producing other desirable doped-crystals.     

Steam Reforming of Methanol Over Cu/CeO2 Catalysts Studied in Comparison with Cu/ZnO and Cu/Zn(Al)O Catalysts

A series of Ce_1-xCu_xO_2- mixed oxides were synthesized using a co-precipitation method and tested as catalysts for the steam reforming of methanol. XRD patterns of the Ce_1-xCu_xO_2- mixed oxides indicated that Cu²⁺ ions were dissolved in CeO₂ lattices to form a solid solution by calcination at 773K when x < 0.2. A TPR (temperature-programmed reduction) investigation showed that the CeO₂ promotes the reduction of the Cu²⁺ species. Two reduction peaks were observed in the TPR profiles, which suggested that there were two different Cu²⁺ species in the Ce_1-xCu_xO_2- mixed oxides. The TPR peak at low temperature is attributed to the bulk Cu²⁺ species which dissolved into the CeO₂ lattices, and the peak at high temperature is due to the CuO species dispersed on the surface of CeO₂. The Ce_1-xCu_xO_2- mixed oxides were reduced to form Cu/CeO₂ catalysts for steam reforming of methanol, and were compared with Cu/ZnO, Cu/Zn(Al)O and Cu/AL₂O₃ catalysts. All the Cu-containing catalysts tested in this study showed high selectivities to CO₂ (over 97%) and H₂. A 3.8wt% Cu/CeO₂ catalyst showed a conversion of 53.9% for the steam reforming of methanol at 513K (W/F = 4.9 g h mol^-1), which was higher than that over Cu/ZnO (37.9%), Cu/Zn(Al)O (32.3%) and Cu/AL₂O₃ (11.2%) with the same Cu loading under the same reaction conditions. It is likely that the high activity of the Cu/CeO₂ catalysts may be due to the highly dispersed Cu metal particles and the strong metalsupport interaction between the Cu metal and CeO₂ support. Slow deactivations were observed over the 3.8wt% Cu/CeO₂ catalyst at 493 and 513K. The activity of the deactivated catalysts can be regenerated by calcination in air at 773K followed by reduction in H₂ at 673K, which indicated that a carbonaceous deposit on the catalyst surface caused the catalyst deactivation. Using the TPO (temperature-programmed oxidation) method, the amounts of coke on the 3.8wt% Cu/CeO₂ catalyst were 0.8wt% at 493K and 1.7wt% at 513K after 24h on stream.     

Structural and superconducting properties of Y(Ba1-xKx)2Cu3O7-δ ceramics

Y(Ba_1-xK_x)₂Cu₃O_7-δ ceramics (x = 0.00, 0.03, 0.05 and 0.08) were synthesized by thermal treatments of aqueous solution of metals nitrates and polyvinyl pyrrolidone (PVP) that acts as a capping agent. The effects of K-substitution on the crystal structure, microstructure and electrical resistance of samples were investigated. The X-ray diffractions results indicated an improvement of crystallinity and variation of lattice constant, a, b and c of YBa₂Cu₃O_7-δ (Y123) phase with K-substitution. The K-substitution resulted in increasing of orthorhombicity factor compared to pure Y123. Microstructural observation using scanning electron microscopy showed that K-substitution promotes the grain growth of Y123. The superconducting transitions (T_c) of the substituted-samples were higher than that of the pure Y123. The T_c (onset) were 93, 97, 95, 95 K for the samples with x = 0.00, 0.03, 0.05 and 0.08, respectively. Comparing with pure sample, the substituted-samples showed sharper superconducting transition (ΔT_c). The best superconducting properties was observed for sample with x = 0.03.     

Superheating nature coupled with Mg-doping effect of high thermal stable NdBa2Cu3O7-δ film

NdBa₂Cu₃O_7-δ thin film deposited on a MgO substrate has been verified to remain solid above its peritectic melting temperature (T_p). Such a superheating nature is attributed to its low energy surface and epitaxial interface with substrate. Here, combining superheating nature with doping effect, we report a novel structure, Mg-doped NdBa₂Cu₃O_7-δ film with YBa₂Cu₃O_7-δ buffer layer for the first time. Remarkably, this film presents a higher thermal stability level than heretofore possible when acting as a seed for preparing SmBa₂Cu₃O_7-δ bulks by melt growth, enduring a temperature of 1128°C, 43 K above its T_p for 1 hour. The utilization of such a high T_max in melt growth is beneficial to the fabrication of large-sized and high-performance bulk in terms of effectively broadening the growth window and suppressing the heterogeneous nucleation. More importantly, some high thermal stability required technological applications, such as batch growth and failed bulk recycling, are likely to be realized by this novel Mg-doped NdBa₂Cu₃O_7-δ film seed. Finally, we show how the observed metastable phase is linked to the distinctive film architecture.     

Ammoxidation of toluene over Y-Ba-Cu-Co-O perovskites

Ammoxidation of toluene over the perovskites YBa₂Cu₃O_6.1, YBa₂Cu₂CoO_6.7 and YBaCuCoO_4.9 was investigated at 400 °C. At low partial pressures of O₂ benzonitrile was selectively formed, while CO₂ was the main product at high pressures of O₂. Systematic differences in activity were observed for the three phases and are related to the crystal contents of Cu and Co. At low O₂ pressures, Cu-sites are active for nitrile formation, while Co-sites give CO₂. At high O₂ pressures, the activity for CO₂ of Cu-sites increases more than that of Co-sites due to filling of near-surface oxygen vacancies.     

Microstructural and electrical characterization of bulk YBa2Cu3O7−δ ceramics

The value of critical current density at 77 K in “zero” applied field (J_c) characterizing the superconducting state for YBa₂Cu₃O_7−δ ceramics is closely related to the microstructure.The interrelationships between the microstructural factors such as pore volume fraction, oxygen content, average grain size, are complex. However, these factors also influence the normal state resistivity measured at room temperature (ρ₃₀₀). We demonstrate how the current carrying cross section influences Jc and ρ₃₀₀ in a similar way. Data, reported for two classes of YBa₂Cu₃O_7−δ: small grain porous ceramics and larger-grain denser ceramics, reveal an approximate linear relation between ρ_300 K and J_c. Extrapolation of this relation to a fully dense small grain YBa₂Cu₃O_7−δ ceramic yields values of ρ₃₀₀ = 0.4 mΩ cm and J_c = 10³ A cm⁻².     

Correlation between local structure variations and critical temperature of (Bi1.6Pb0.4Sr2Ca2Cu3O10+δ)1-x(TiO2)x superconductor

TiO₂ doping content affects the critical temperature (T_c) and the variations of local structure on Bi_1.6Pb_0.4Sr₂Ca₂Cu₃O_10+δ. The (Bi_1.6Pb_0.4Sr₂Ca₂Cu₃O_10+δ)_1-x(TiO₂)_x samples were fabricated with the solid-state reaction process, where x = 0, 0.002, 0.004, 0.006, 0.008, and 0.01. The T_c values of the samples were obtained by measuring resistivity versus temperature, indicating that T_c gradually decreased with increasing doping content (x). To explain the obtained degradation of T_c, carrier properties, role of interlayer coupling, and local structure were systematically investigated using Azlamozov–Larkin theory and its Lawrence–Doniach modification for strong anisotropic superconductors. The calculation of excess conductivity at the mean field region showed that the c-axis coherence length (ξ_c) and the effective inter-layering spacing (d) increased with increasing doping content. X-ray diffraction patterns also showed that the bond distances increased with increasing TiO₂ content. The copper valence (V) and carrier concentration (p) of the samples were determined by analyzing the Cu L_2,3-edge X-ray absorption near edge structure spectra. The values of V and p showed the same trend of decreasing with increasing x. A close correlation between the changes in local structure parameters and degradation of T_c of (Bi_1.6Pb_0.4Sr₂Ca₂Cu₃O_10+δ)_1-x(TiO₂)_x was then probably concluded.     

Comparison of the catalysed decomposition of hydrogen peroxide over superconducting and insulating barium cuprates—kinetics of decomposition over Y2BaCuO5

Catalytic activities of two non-conducting cuprates (Ba₂Cu₃O₅, Y₂BaCuO₅) and a superconducting cuprate YBa₂Cu₃O_7–δ, prepared by two different methods, were compared using H₂O₂ solution. The two non-conducting barium cuprates were found to be about two orders of magnitude more active than the superconducting perovskite-like YBa₂Cu₃O_7–δ. The near coincidence of the kinetic curves for the catalysed decomposition of H₂O₂ over YBa₂Cu₃O_7–δ, prepared either from the perovskite-like Ba₂Cu₃O₅ and Y₂O₃ or from its components, proves that Ba₂Cu₃O₅ is essential in the formation of YBa₂Cu₃O_7–δ. First-order kinetics were observed for the catalysed decomposition of H₂O₂ solution over the non-perovskite Y₂BaCuO₅ at the initial stages of the reaction, similar to that of the superconducting perovskite catalyst. A tentative scheme of the possible catalytic reactions for the decomposition of H₂O₂ solution over the insulator Y₂BaCuO₅ is given.