Interface and grain boundary structures in YBa2Cu3O7-x and YBa2Cu4O8 materials

Y₂BaCuO₅YBa₂Cu₃O_7-x (Y211/Y123) interfaces in melt-processed YBa₂Cu₃O_7-x were studied by high-resolution transmission electron microscopy and energy dispersive X-ray spectroscopy. Yttrium enrichment and barium depletion were observed locally at the Y211/Y123 interfaces where Y123 (001) facets were present. This effect may be interpreted as the result of lattice substitution of Ba by Y near these interfaces. Cation nonstoichiometry was found near Y211/Y123 interfaces where liquid phases (Cu-Ba-O) were present. This chemical disorder introduces numerous point defects in the Y123, and these defects may act as additional pinning sites alongwith stacking faults. A comparison of grain boundary (GB) chemical composition in polycrystalline YBa₂Cu₃O_7-x and YBa₂Cu₄O₈(Y124), studied using nanoprobe parallel-detection electron energy-loss spectroscopy (EELS), is presented. The studies of Y124 show that stoichiometric grain boundaries can also form weak links between superconducting grains. It is suggested that weak-link behavior is determined largely by misorientation at grain boundaries.     

Effect of vapors of the simplest alcohols on the structure and properties of YBa2Cu3Oy compounds

With low-temperature treatments (200–300°C), the effect of alcohol vapors on the structural and electrophysical properties of YBa₂Cu₃O_y (123) having different oxygen content has been studied. It has been established that CuO and YBa₂Cu₃O_y are catalysts for the dehydration of alcohols to form water and aldehydes. Similar to the action of water vapors, the treatment in vapors of the simplest alcohols leads to hydration of 123 and its transformation into the pseudo-124 phase. In this case, planar stacking faults are formed to be centres of pinning and can improve the critical characteristics of high-temperature superconductors (HTSС) in magnetic fields. The influence of the alcohol vapors is more ‘mild’ as compared to the treatment in water vapors. At t?=?300°C, the alcohols are reducing agents of copper, followed by the decomposition of 123 into simple compounds.     

在工频外磁场下高温超导体的交流损耗

在工频外磁场条件下,研究了不同工艺的Bi-2223/Ag带和Cu位元素替代的YBa₂Cu_3-xSn_xO_y超导体的交流损耗.实验结果表明：在单芯Bi-2223/Ag带中,热处理时间、温度及超导芯厚度的增加都将增加Bi-2223相的含量,因而导致磁滞损耗增加.将单芯带细分成19芯Bi-2223/Ag带能减少磁滞损耗,但另一方面,超导芯间的耦合增加了银基体的涡流损耗,因此19芯带的损耗只比单芯带减少20%左右.在YBa关键词：     

Effects of binder addition on the mechanical properties of bulk Y–Ba–Cu–O superconductors

We studied the effects of binder addition on the mechanical properties of bulk Y–Ba–Cu–O superconductors. We prepared YBa₂Cu₃O_y, Y₂BaCuO₅ powders and polyvinyl alcohol mixed with water as a binder. These raw materials were mixed, and the binder-added powders were pressed into pellets. The hardness of the green compacts with binder is higher than that without the binder. However, the hardness of green compacts with 8% binder is the same as that with 4% binder. The maximum compression strength of the precursor with binder is higher than that without binder. Equally, the maximum strength of the green compacts with 8% binder is higher than that with 4% binder. The differential thermal analysis measurements showed that the exothermic reaction due to the decomposition of the organic binder started at 550 °C and gradually proceeded with further heating. After de-binder treatment, BaCO₃ powders were produced on the green compacts. The green compacts were subjected to melt-processing. We also measured trapped magnetic fields of binder-added bulk Y–Ba–Cu–O superconductors with a Hall probe scanning device. Trapped magnetic field of the bulk added with 4% is higher than that of the binder-free bulk. Hence, Y–Ba–Cu–O bulk with suitable amount of binder shows good influence for mechanical strength and trapped magnetic field.     

Transport studies of YBa2Cu3Oy/YxPr1−xBa2Cu3Oy and YBa2Cu3Oy/R0.7M0–3MnO3 superlattices,in which R=La or Nd,and M=Ca or Sr

We report the transport studies of YBa₂Cu₃O_y/Y_xPr_1−xBa₂Cu₃O_y and YBa₂Cu₃Oy/R_1−xM_xMnO₃ superlattices in magnetic fields in which R=La or Nd, and M=Ca or Sr. The X-ray diffraction of samples shows superlattice structure. The resistive transition in a magnetic field shows thermal activated behavior. The flux pinning is reduced when the coupling strength between YBCO layers is decreased. The angular dependence of the critical current of YBa₂Cu₃O_y/PrBa₂Cu₃O_y superlattices reveals the dimensionality of superlattices. The magnetoresistance ratio (MR), |Δρ(H=7 T)−Δρ(H=0)|/Δρ(H=7 T), of YBa₂Cu₃O_y/R_1−xM_xMnO₃ superlattices is affected by the layer coupling of R_1−xM_xMnO₃ layers. The enhancement of the MR ratio in the tri-layer YBa₂Cu₃O_y/La_0.7M_0.3MnO₃/YBa₂Cu₃O_y film in the low temperature regime is significant and has a value of 33650% at T=75 K. We attribute this enhancement of the MR to the ordering of magnetic moment in ferromagnetic layers in magnetic fields. The results are discussed in terms of existing theories.     

Top-seeded infiltration growth of Y–Ba–Cu–O bulk superconductors

A top-seeded melt-growth (TSMG) process is widely used to fabricate single domain YBa₂Cu₃O_y (Y–Ba–Cu–O) bulk superconductors. Pores are often found in the TSMG-processed Y–Ba–Cu–O samples due to the oxygen gas evolution during the molten stage. Recently developed liquid infiltration growth (LIG) process is known to be effective in suppressing the pore evolution and in refining the size of Y₂BaCuO₅ (Y211) particles dispersed in YBa₂Cu₃O_y matrix. The LIG process utilizes the liquid (Ba₃Cu₅O₈) infiltration into a pre- sintered Y211 contact and slow cooling through a peritectic temperature. In this study, we fabricated bulk Y–Ba–Cu–O superconductors by the LIG process combined with top-seeding with SmBa₂Cu₃O_y seed and confirmed that a single-domain bulk can be produced. Trapped field measurements however showed that some distortion in the field distribution was observed in the region near the seed crystal, which was attributed to Y211 density and its relatively large size.     

Magnetic tri-axial orientation in (Y1−xErx)2Ba4Cu7O15−y superconductors

We report the tri-axial grain-orientation effects under a modulated rotation magnetic field for (Y_1?xEr_x)₂Ba₄Cu₇O_y [(Y, Er)247]. The magnetic easy axis at room temperature was drastically changed around x ～ 0.1; however, the Er-doping levels for the conversion of magnetic easy axes from the c-axis to the ab-direction and from the a- to b-axes were quite different. Tri-axial single-ion magnetic anisotropy of Er³⁺ was roughly 10 times greater than tri-axial magnetic anisotropy generated by both the superconducting CuO₂ plane and the blocking Cu–O chain layer. An appropriate choice of rare-earth (RE) ions in RE-based cuprate superconductors enables the reduction of the required magnetic field for the production of bulks and thick films based on the magnetic orientation technique.     

Effects of Y2O3 additions on the oxygen diffusion in top-seeded melt growth processed YBa2Cu3O7−y superconductors

To understand the effect of Y₂BaCuO₅ (Y211)/YBa₂Cu₃O_7?y (Y123) interfaces on the oxygen diffusion in single grain YBa₂Cu₃O_7?y superconductors, single grain Y123 superconductors with 0.05 and 0.3 moles of Y₂O₃ additions were fabricated by a top-seeded melt growth (TSMG) process. Y123 compacts with Y₂O₃ additions were subjected to melt growth heating cycles with a cooling rate of 1 °C/h through a peritectic temperature (1015 °C) and then annealed at 450 °C for 200 h in flowing oxygen. The superconducting temperature (T_c) and critical current density (J_c) were estimated for the three different regions (top surface (s), intermediate (i) and center (c)) of samples. The amount of Y211/Y123 interface area in single grain Y123 superconductors was successfully controlled by Y₂O₃ additions. The T_c values of s regions were higher than those of i and c regions, which indicates the presence of more oxygen at the sample surfaces. In addition, the T_c values of i and c regions of the Y123 sample with 0.3 mole Y₂O₃ addition were higher than those of the same regions of the Y123 sample with 0.05 mole Y₂O₃ addition due to the promoted oxygen diffusion through Y211/Y123 interfaces and other related defects. In spite of the promoted oxygen diffusion by Y₂O₃ addition, the large T_c difference among the regions still existed, which suggests sluggish oxygen diffusion into single Y123 grains.     

Effects of oxide coating on the growth of single grain YBCO bulk superconductors

Surface oxide coating and bottom inserting of oxide plates have been conducted to top seeded melt growth (TSMG) processed YBa₂Cu₃O_7−y (Y123) bulk superconductors with an aim of controlling the Y123 nucleation and growth. The coating medium for surfaces was Yb₂O₃ solution and the bottom inserts were Yb₂O₃/Y₂O₃ powder compact. Many vertical cracks were found to develop at the compact/insert interfaces when an Yb₂O₃ insert was used, but the crack evolution was greatly reduced when a (Yb₂O₃ + Y₂O₃) insert was used. The formation of the vertical cracks is ascribed to the difference in thermal expansion between the YBCO compact and bottom insert. Presence of vertical cracks was found to be crucial to the trapped magnetic field and levitation forces of single grain YBCO bulk superconductors. The Y123 nucleation and growth in TSMG-processed YBCO bulk superconductors were successfully controlled by conducting surface coating and bottom plating using a (Yb₂O₃ + Y₂O₃) insert and as a result, the levitation properties were much enhanced.     

NQR investigations of high-T c superconducting oxides

Nuclear quadrupole resonance (NQR) of⁶³Cu has been performed to study the electronic and magnetic properties of YBa₂Cu₃O_6.1 ceramics, treated in iodine or bromine vapours. An analysis of the NQR shows undistorted CuO₂ planes and the absence of an increase of the oxygen content in chains of Cu(1)-O in halogen treated Y-Ba-Cu-O ceramics.¹³⁹La NQR spin-lattice relaxation measurements in powdered La_2?x CuO_4?y compounds are also reported. The present results indicate that the¹³⁹La nuclear relaxation is dominated by the Cu⁺⁺-Cu⁺⁺ magnetic correlations.     

Study of magnetic flux pinning in granular YBa2Cu3O7 − y /nanoZrO2 composites

In this work, the effect of ZrO₂ nanoparticles prepared in a low-pressure arc discharge plasma on magnetic flux pinning of granular YBa₂Cu₃O_{7 ? y} /nanoZrO₂ composites has been studied. It has been shown that the ZrO₂ nanoparticles do not change the superconducting transition and the microstructure of superconductors. At a temperature of 5 K, the addition of 0.5 and 1 wt % of ZrO₂ nanoparticles may lead to the additional effect of magnetic flux pinning and the increase in the critical current density J _c. The J _c value for composites with 1 wt % is two times larger than that for the reference sample. The fishtail effect is observed for YBa₂Cu₃O_{7 ? y} /nanoZrO₂ composites at the temperatures of 20 and 50 K. The problems associated with the additional effect of magnetic flux pinning of granular YBa₂Cu₃O_{7 ? y} /nanoZrO₂ composites and the appearance of the fishtail effect have been discussed.     

Novel configuration of processing bulk textured YB2Cu3O7−x superconductor by seeded infiltration growth method

Mono-domain YBa₂Cu₃O_7?x (Y123) bulk superconductors have been processed using seeded infiltration growth technique (SIG). The combination of melt infiltrated liquid source (Ba₃Cu₅O₈) into the Y₂BaCuO₅ (Y211) pre-form and the nucleation of Y123 domain from SmBa₂Cu₃O₇ crystal seed has been investigated. The different configurations of SIG process were compared in this study. In addition, the effect of the starting Y211 particles size has been studied. The results reveal that, the Y211 particle size and different configurations strongly influence the properties of the final bulk superconductor sample.     

Comparative analysis of the effect of La and Co on the superconductivity and energyband spectrum of YBa2Cu3Oy for different oxygen contents

Temperature dependences of the resistivity and Seebeck coefficient of Y(Ba_1−x La_x)₂Cu₃O_y and YBa₂Cu_3−x Co_xO_y samples (x=0–0.25) have been measured under maximum sample saturation with oxygen, as well as following their anneal in an oxygen-deficient atmosphere. The T _c (x) dependences for as-prepared samples were found to pass through a maximum at x=0.05, which persists after annealing for Y(Ba_1−xLa_x)₂Cu₃O_y and disappears for YBa₂Cu_3−x Co_xO_y. A phenomenological model of the band spectrum in normal phase has been used to determine the parameters of the conduction band and of the carrier system, and to analyze their variation with the dopant type and content, as well as with annealing. Despite the differences observed in the T _c (x) dependence, the critical temperatures for all the sample series studied were found to correlate with the conduction-band effective width. The mechanism of the effect of impurities on the band-structure parameters and the reasons for the different influence of annealing on the properties of Y(Ba_1−x La_x)₂Cu₃O_y and YBa₂Cu_3−x Co_xO_y are discussed. Fiz. Tverd. Tela (St. Petersburg) 41, 389–394 (March 1997)     

Hole doping by Li substitution and antiferromagnetism in YBa Cu O studied by neutron powder diffraction measurements

The magnetic structure of tetragonal insulating YBa₂Cu_3-xLi_xO_y has been studied as a function of x and y. The Néel temperature and the mean ordered magnetic moment on the Cu2 sites were determined by neutron powder diffraction measurements. The decrease of these two parameters as compared to YBa₂Cu₃O₆ is much stronger for lithium than for zinc substitution. The difference is quantitatively explained by the presence of holes created in the CuO₂ planes. These holes arise from the substitution of plane Cu²⁺ by Li⁺. We suggest an explanation why such holes are not seen for the same substitution of plane Cu²⁺ by Li⁺ in orthorhombic superconducting YBa₂Cu_3-xLi_xO _{7 - δ} . Received 31 October 2001 and Received in final form 6 March 2002 Published online 25 June 2002     

Suppression of samarium diffusion in top seeded melt growth processed Y123 superconductors using a Sm123 seed

To suppress the samarium diffusion from a SmBa₂Cu₃O_7?y (Sm123) seed into an YBa₂Cu₃O_7?y (Y123) compact during a top seeded melt growth (TSMG) process, Y211 buffer disks of various thicknesses (t) were inserted at the seed/compact interface. The effectiveness of the Y211 buffer insert on the suppression of samarium diffusion was estimated from the susceptibility curve and composition analysis for the three top surface regions (region just below the seed/buffer, intermediate region between the buffer and compact edge and compact edge). The difference in the superconducting temperature (T_c) and the superconducting transition width (ΔT) in the three regions was large, when no buffer or thin buffers (t is smaller than 1 mm) were used. As a buffer disk of an appropriate thickness was inserted into the seed/compact interface, the difference in T_c and ΔT in the three regions was much reduced. The composition analysis across the seed/buffer/compact region clearly revealed that the T_c decrease was caused by the samarium diffusion from a seed, which led to the formation of Y_1?xSm_xBa₂Cu₃O_7?y in the Y123 compact region. The buffer insert technique was demonstrated to insure the uniform superconducting properties of the top surface of TSMG processed Y123 superconductors.     

Mechanisms of interaction of YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript> <Emphasis Type="Italic">y</Emphasis> </Subscript> with water vapor at low-temperature annealing

Abstract—The interaction of YBa₂Cu₃O_y (123) with water vapor at temperatures T ≤ 150° has been studied. It has been shown that, with an increase in temperature, the mechanism of its interaction with water changes. Near room temperature, the main process is hydrolytic decomposition. At T ~ 100°C, the absorption of water is significantly reduced, because the role of hydrolysis becomes less important and water penetrates the structure weakly and is incorporated into oxygen vacancies mainly in the form of OH–-groups, which leads to the transition of YBa₂Cu₃O_y from the tetragonal to orthorhombic phase. With an increase in temperature to 150°C, the absorption of water increases again. In this case, the main mechanism is the penetration of water to the 123 structure, which leads to splitting of Cu–O chains and a phase transition from the 123 to pseudo-124 structure. The role of different mechanisms of interaction with water essentially depends on the oxygen content in the 123 structure. At a low oxygen index (y = 6.3), the role of hydrolysis is more important, and, at y ≥ 6.5, the incorporation of water into the structure prevails. It has been revealed that, at T = 150°C, after absorption of water, YBa₂Cu₃O_6.96 becomes a proton conductor.     

Disordering effects in the atomic structure of fine-crystalline HTSC YBa2Cu3O y

The atomic structure of YBa₂Cu₃O _y fine-crystalline HTSC samples with various average crystallite sizes ??D?? ranging from 0.4 to 2 ??m and an oxygen concentration y close to the optimal value for superconductivity (y ?? 6.93) is investigated by the neutron diffraction technique. We have found some effects associated with the redistribution of cations and oxygen atoms and with variations in the positions of atomic layers in the unit cell, which are not observed in macrocrystalline samples. In all probability, these effects appear due to nonequilibrium conditions of synthesis required for obtaining this compound in the fine-crystalline state. The results have made it possible to explain the peculiar physical properties of fine-crystalline YBa₂Cu₃O _y samples (in particular, the coexistence of high superconducting transition temperatures T _c and noticeably lower values of magnetization in strong magnetic fields for T < T _c ). It is shown that a nanoscale structural inhomogeneity exists in fine-crystalline YBa₂Cu₃O _y samples with the optimal oxygen content and changes the fundamental superconducting parameters, viz., the magnetic field penetration depth and the coherence length.     

A comparative study of magnetic relaxation in YBa2Cu3−x Fe x O7−y single crystals

The zero-field-cooled magnetization relaxation has been studied over wide temperature and field ranges on iron-doped YBa₂Cu₃O_7–y single crystals. The relationship between the twin boundary density, which depends on the iron composition, and the pinning strength of the flux lines is determined systematically. For magnetic fields up to 1 T the inductive critical current density decreases as the iron composition increases. The Bean critical state model and the Anderson-Kim thermally activated flux-creep model are utilized to analyze and interpret the experimental results.     

Optimization of the properties of codoped single-domain Y–Ba–Cu–O bulk superconductors

The intrinsic effects of nanoscopic MnO₂ powders addition combined with Fe cation substitutions for copper sites on the microstructure and superconductive properties of YBa₂Cu₃O_7?δ (Y123) melt-solidified bulks have been investigated. On the one hand, an increase in Y₂BaCuO₅ (Y211) particle pushing, leading to an inhomogeneous bulk microstructure, is caused by increasing MnO₂ content due to increased net interfacial energy, Δσ₀; and, on the other hand, an addition of MnO₂ powders is effective in enhancing both the δT_c-type and δl-type pinning. It also shows that the Fe addition helps to optimize the high magnetic field performance and Y211 particle distribution in textured pellets. Further, this experiment suggests that a combination of the element substitution and the nanoscopic particle is a beneficial way to optimize the microstructure and superconductive properties of single-domain bulk superconductors.     

Features of the superconducting state and structural disorder in ultrafine particles of the HTSC YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript>

We study the reasons for which the compound YBa₂Cu₃O _y begins to lose its superconducting ability with a decrease in particle sizes to values of ∼0.1 μm. Our analysis of the results of structural and magnetic studies has allowed us to reveal changes in the parameters of the crystal structure and of the superconducting state in small particles of an HTSC and to show that the main cause of these changes is a special kind of a structural disorder peculiar only to small particles of YBa₂Cu₃O _y and is realized as a consequence of the need for nonequilibrium conditions during their synthesis.