Superconductivity of Y–Ba–Cu–O and substituted systems

Superconductivity of Y-Ba-Cu-O system is studied in the composition 2:2:3 and 1:2:3 of Y:Ba:Cu. The effect of replacement of Y or Ba by divalent Sr and Ca, trivalent Ce and tetravalent Zr is studied. X-ray diffraction, SEM and TEM techniques are used for materials characterization. Superconducting transition temperatures are measured resistively. Rapid resistance drop observed above 230 K in Y-Ba-Sr-Cu-O and Y-Ba-Ca-Cu-O systems indicate the possible existence of superconductivity above 230 K. Substitution of Ce in place of Y is found to reduce the onset Tc from 95 K to 80 K. For the first time, replacement of Cu by Zr in Y-Ba-Cu-O has yielded the onset Tc of about 105 K.     

High-Temperature superconductivity in the Y–Ba–Cu–O system

High-temperature superconductivity in the Y-Ba-Cu-O system has been discussed with special reference to the identification and characterization of the pure monophasic compound responsible for the superconductivity. The crucial role of oxygen has been examined in the light of the structure and thermogravimetric analysis.     

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.     

Approaches to the synthesis of High-TC superconducting oxides in La–Ba–Cu–O and Y–Ba–Cu–O systems

Abstract

High-T_C superconducting oxides of nominal La_1.85Ba_0.15 CuO₄ and YBa₂ Cu₃ O₇ have been prepared by using nitrate, carbonate, oxalate/malonate and citrate precursors. While the samples in the Y-system are generally monophasic YBa₂Cu₃O_7?δ with T_C around 90K, the preparations in the La-system are biphasic containing K₂NiF₄-like La_1.85Ba_0.15 CuO₄ (T_C = 30K) and a perovskite-like phase with' a much higher T_C (200–300K). Effect of Ca, Zr, Ce as well as S substitution in YBa₂Cu₃O_7?δ has also been investigated     

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.     

Fast joining of melt textured Y–Ba–Cu–O bulks with high quality

Y–Ba–Cu–O bulk superconductors were fast joined using Ag-doped Y–Ba–Cu–O solder (metallic Ag powder additive). The joining process was relatively shorter (around 24 h) comparing with the traditional joining method. The microstructures and the superconducting properties of the joints were evaluated carefully. Microstructure analysis revealed that the crystal pattern in the joint was almost the same as that of the base material. This indicated that the bonding zone grew along the growth direction of the base material. The trapped-field distribution of the joined bulk was almost uniform and only single peak was found. This demonstrated that strong bonding was achieved in the joining process. The ratio of joined bulk’s levitation force to that of original base material was up to 93.5%.     

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.     

Enhancement of mechanical strength in Y–Ba–Cu–O bulk superconductor through liquid binder addition

We have studied the effects of the liquid binder (polyvinyl alcohol) addition (0–10 wt%) on the mechanical properties of the green compacts and also on the superconducting properties of bulk Y–Ba–Cu–O superconductors of 20 mm diameter produced with the top-seeded melt growth (TSMG) process. The mechanical properties of the green compacts with binder addition were characterized with the compression tests, which revealed that mechanical strength increased dramatically with increasing the amount of the binder addition. The binder-added green compacts were then subjected to the TSMG process and oxygen annealing. The trapped field measurements showed that we could produce single-grain bulk Y–Ba–Cu–O samples with binder additions up to 8 wt% without any deterioration in the superconducting properties.     

Vortex pinning in bulk-processed Y–Ba–Cu–O with ZrO2 nano-particles: Optimum pinning center size

Crystalline defects on the nano-scale were successfully introduced into YBCO high-temperature superconductors (HTS) by ZrO₂ nanometer particles addition in order to strongly pin the quantized vortices. Three batches of ZrO₂ nano-particles with different particle size distributions were used. The corresponding mean nano-particle diameters are respectively, 287, 536 and 764 nm. Serving as artificial pinning centers (APC), non-superconducting nano-particles cause a remarkable enhancement of critical current density (J_c) at T = 77 K. This improvement has been shown to depend on the size of APC. The pinning strength of nano-particles inclusions has been found to be greater with wide size dispersed nano-particles. Our results indicate that pinning properties and vortex dynamics depend on the size of APCs. The introduction of APCs with controlled size is indispensable to achieve a high J_c.     

The improvement of the superconducting Y–Ba–Cu–O magnet characteristics through shape recovery strain of Fe–Mn–Si alloys

Since bulk Y–Ba–Cu–O superconductors are brittle ceramics, reinforcement of mechanical properties is important for practical applications. It has been reported that bulk Y–Ba–Cu–O can be reinforced with Al or Fe based alloy ring, in that compression force acts on bulk Y–Ba–Cu–O due to a difference in thermal expansion coefficients. However, the shrinkage of the metal ring was not so large, and therefore careful adjustment of the circumference of the bulk and the metal rings was necessary. In this study, we employed Fe–Mn–Si shape memory alloy rings to reinforce bulk Y–Ba–Cu–O. The advantage of the shape memory alloy is that the shrinkage can take place on heating, and furthermore, the alloy shrinks and compresses the bulk body on cooling. Bulk Y–Ba–Cu–O superconductor 22.8 mm in diameter was inserted in a Fe–Mn–Si ring 23.0 mm in inner diameter at room temperature. Beforehand, the Fe–Mn–Si ring was expanded by 12% strain at room temperature. Then the composite was heated to 673 K. At room temperature, the Fe–Mn–Si ring firmly gripped the bulk superconductor. We then measured trapped fields before and after the ring reinforcement, and found that the trapped field was improved through the treatment.     

Alumina nano-inclusions as effective flux pinning centers in Y–Ba–Cu–O superconductor fabricated by seeded infiltration and growth

The YBa₂Cu₃O_7?x (Y123) textured bulk superconductors with various amounts of nanometer alumina particles were fabricated by a seeded infiltration and growth process. The addition of nanometer alumina was found to be effective for an improvement of the superconducting properties. The critical current density (J_c) values were increased twice in self field with a slight addition amount of nanometer alumina particles (maximum J_c at 0.01 wt.% alumina addition). The present work suggests that the use of insulating inclusions in the nanometer sub-scale can stabilize the flux-line lattice and greatly enhance the pinning capabilities of the infiltrated samples. No refinement of Y211 particles was observed with alumina addition. The J_c improvement by nanometer alumina inclusions is likely rendered to the insulating nano-pinning centers that have been successfully embedded into superconducting Y123 matrix. On the other hand, we examined the effect of the pinning centers size on the superconducting properties of infiltrated YBCO bulk samples. To this effect insulating nano-pinning centers with two different size distributions has been successfully incorporated within YBCO matrix of bulk superconductor by slightly doping with nano-particle alumina dispersions. Two alumina nano-particle dispersions with mean size diameters of about 20 nm and 130 nm were used. It was shown that the size of the pinning centers can affect considerably the J_c performances and the pinning mechanism.     

Y—Ba—Cu—Sn—O高温超导体

用Sn分别部分替代Y-Ba-Cu-O系超导体中的Y或Cu,制备了Y-Ba-Cu-Sn-O系超导体,实验结果表明,在相当大的替代范围(30%的 Y,40%的 Cu)内,超导体仍保持正交结构,其零电阻转变温度基本上稳定在90K左右.     

Effect of small magnetic field on the absorption property of superconducting Y–Ba–Cu–O (the magnetic memory effect)

A magnetic memory effect is observed in the absorption of electromagnetic waves of 20–70 MHz in YBa₂Cu₃O₇at 77–300 K.     

Laser Melting Solidification Process of Y BaCu O Superconductors

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Magnetic resonance in a gallium-doped Cu–Cr–S structure

A layered Cu–Cr–S structure doped with Ga ions and consisting of single-crystal CuCrS₂ layers, embedded with thin plates of spinel phases CuCr₂S₄ and CuGa_xCr_2–xS₄, has been studied using the magnetic resonance and magnetic susceptibility methods. The Curie temperature and the saturation magnetization of the spinel phases of the samples have been determined. The spinel phase layer thickness has been estimated.     

Hume-Rothery electron concentration rule across a whole solid solution range in a series of gamma-brasses in Cu–Zn,Cu–Cd,Cu–Al,Cu–Ga,Ni–Zn and Co–Zn alloy systems

The aim of the present work is to examine if the Hume-Rothery stabilisation mechanism holds across whole solid solution ranges in a series of gamma-brasses with especial attention to the role of vacancies introduced into the large unit cell. The concentration dependence of the number of atoms in the unit cell, N, for gamma-brasses in the Cu–Zn, Cu–Cd, Cu–Al, Cu–Ga, Ni–Zn and Co–Zn alloy systems was determined by measuring the density and lattice constants at room temperature. The number of itinerant electrons in the unit cell, e/uc, is evaluated by taking a product of N and the number of itinerant electrons per atom, e/a, for the transition metal element deduced earlier from the full-potential linearised augmented plane wave (FLAPW)-Fourier analysis. The results are discussed within the rigid-band model using as a host the density of states (DOS) derived earlier from the FLAPW band calculations for the stoichiometric gamma-brasses Cu₅Zn₈, Cu₉Al₄ and TM₂Zn₁₁ (TM = Co and Ni). A solid solution range of gamma-brasses in Cu–Zn, Cu–Cd, Cu–Al, Cu–Ga and Ni–Zn alloy systems is found to fall inside the existing pseudogap at the Fermi level. This is taken as confirmation of the validity of the Hume-Rothery stability mechanism for a whole solute concentration range of these gamma-brasses. An exception to this behaviour was found in the Co–Zn gamma-brasses, where orbital hybridisation effects are claimed to play a crucial role in stabilisation.     

Superconductivity in Pb-1212–Cu1−xPbxSr2Y0.6Ca0.4Cu2O7 (x = 0.5–0.9)

We report synthesis and superconductivity of Pb-1212–Cu_1?xPb_xSr₂Y_0.6Ca_0.4Cu₂O₇ (x = 0.5–0.9) compounds. These compounds were synthesized through solid-state reaction route with optimized sintering temperatures and conditions. In particular, one needs to employ reducing atmosphere conditions to achieve superconductivity in higher Pb content samples. The X-ray diffraction (XRD) patterns reveals that all the compounds are crystallized in space group P4/mmm RE-123 structure. Superconductivity at 56 K (onset) is achieved for Pb content as high as 90%. Our study reveals that superconductivity and structure stabilization in Pb-based are more critical to synthesizing conditions than other cuprates. It is concluded that superconductivity can be introduced in Pb-1212 compounds by synthesizing the same in reducing atmosphere and thus the Pb in lower (<+4) valence state. This ensures the replacement of Cu–O_x chains of RE-123 by Pb–O_x sheets acting as charge reservoir carrier donating blocks.     

Structural analysis of rapidly solidified Mg–Cu–Y glasses during room-temperature embrittlement

The structural changes of rapidly-quenched amorphous Mg–Cu–Y alloys during time-dependent room-temperature embrittlement were investigated. A continuous reduction in enthalpic content with time, corresponding to a loss in free volume, and radial atomic distances obtained from synchrotron X-ray diffraction experiments imply an overall reduction in volume. Two distinct lifetimes were found by positron annihilation lifetime spectra, with the longer-lifetime component (attributed to areas of larger free volume) vanishing after an ageing time corresponding roughly to the time of embrittlement. Coincidence Doppler broadening suggests that the immediate environment of the annihilation sites is not altered during ageing. The mechanical behaviour correlates with the structural changes in the alloy. The densification observed is assumed to influence the sharp ductile-to-brittle transition in these Mg-based glasses, and the abrupt change in bending ductility is found to result from disappearing large free volume.