Interaction of YBa2Cu3O6.8 with atmospheric moisture during low-temperature annealing

The interaction of YBa₂Cu₃O_6.8 (123) with water vapors at T = 200°C and the water influence on the structure and electrophysical properties of the compound have been studied using Raman spectroscopy, magnetometry, and X-ray diffraction. It has been found that the penetration of water into the 123 structure leads to its transition to a hydride-oxyhydroxide H_{2x ? z} YBa₂Cu₃O _{y + x ? z} (OH) _z containing fragments of the 124-type structure and exhibiting the spectrum of two-magnon scattering characteristic of antiferromagnetic 123 á compositions; in this case, the superconducting properties of the material as a whole are conserved. After short-time recovery annealing and subsequent oxidation, the water is removed from the compound structure, which leads to the disappearance of the spectrum of spin fluctuations. A possible mechanism of change in the 123 structure upon hydrogen and water intercalation has been discussed.     

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.     

Analyzing anode and cathode products obtained by the electrolysis of Y-Ba-Cu-O and Y-Ba-Cu-K-O systems

Energy dispersive X-ray spectroscopy (EDX) and electron diffraction and high-resolution electron microscopy are used to study the composition and structure of anode and cathode deposits formed during the high temperature (950°C) electrolysis of the Y-Ba-Cu-O system and low temperature (450°C) electrolysis of the Y-Ba-Cu-K-O system. It is found for the first time that an oxide with the YBa₂Cu₃O _y structure (123 phase) is synthesized during the high temperature electrolysis of Y_0.02Ba_0.30Cu_0.70O _y and Y_0.02Ba_0.25Cu_0.75O _y melts. The 123 phase is not synthesized during low temperature electrolysis, where melts are formed from Y₂O₃, BaO₂, CuO, and KOH. Two new Pt-containing oxides with hexagonal structure are found in the products of high temperature electrolysis.     

Effect of low-temperature treatment and subsequent high-temperature annealing on the critical current density of YBa2Cu3O y

The field dependences of the critical current density of the HTSC compound YBa₂Cu₃O _y recovered at T = 920?C950°C after the low-temperature treatment have been investigated. At T = 200°C, structural defects are formed in a wet environment, which are capable of initiating pinning of magnetic vortices. A short-term (1?C3 h) recovery annealing performed at T = 930?C950°C leaves in the samples a fairly large amount of structural defects formed during the low-temperature treatment, which results in a substantial increase in the critical current density in magnetic fields of ??2 T as compared to the ceramics not subjected to double annealing. A longer high-temperature treatment removes the structural defects formed and brings the electrophysical properties of YBa₂Cu₃O _y to the level characteristic of the ceramics produced by standard technology.     

Cation nonstoichiometry and its contribution to the nanostructured inhomogeneity of tetra and ortho modifications of YBa2Cu3O7 − δ oxide

The cation composition and structure of YBa₂Cu₃O_{7 ? δ} (YBCO, the 123 phase) oxide samples synthesized at 990°C in air and annealed in oxygen at 450°C are studied via elemental analysis in a transmission electron microscope and high-resolution electron microscopy. The occurrence of cation nonstoichiometry and nanostructured inhomogeneity of the 123 phase in tetragonal and orthorhombic (superconducting) structures is confirmed. The nanostructured inhomogeneity of the 123 phase is attributed to the presence of nanocrystallites (2–5 nm) of oxides with different cation compositions belonging to the Y _n Ba _m Cu _{m + n} O _y series.     

Structure and superconducting properties of nonstoichiometric YBa2Cu3Oy annealed at 300°C in a dry argon atmosphere

Using the full-profile X-ray analysis of diffraction lines, the behavior of YBa₂Cu₃O _y nonstoichiometric ceramics upon annealing at 300°C in a dry inert atmosphere has been investigated. It has been established that, in the YBa₂Cu₃O_6.3 sample with the initial tetragonal structure, this treatment leads to nucleation of an orthophase with a low degree of rhombicity, which becomes prevailing after annealing for 56 h. In this case, superconductivity with an initial temperature of the transition of ∼50 K occurs in the sample. In the YBa₂Cu₃O_6.7 sample with the initial orthorhombic structure, the orthophase with a somewhat enhanced rhombicity arises during annealing and becomes prevailing after annealing for 56 h; in this case, T _c of the system grows from 57 to 63 K. Data of the magnetic measurements for both cases can be interpreted as the presence of two orthorhombic phases with different temperatures of transition to the superconducting state in the samples after annealing.     

Influence of chemical composition on the stability of YBa2Cu3O y in a humid atmosphere

The influence of replacements of yttrium and barium in YBa₂Cu₃O _y and doping of this compound with CeO₂, ZrO₂, and Pr₂O₃ on the capability of absorbing water from the annealing atmosphere at T = 200°C has been studied. The complication of the chemical composition of the 123-type compounds leads to an enhancement of their stability to aggressive components of the gaseous phase (H₂O, CO₂). These compounds have a variable chemical composition both in oxygen and water and can exist in three modifications: the oxygen-ordered ortho-phase, the oxygen-disordered tetra-phase, and the pseudocubic phase with a variable water content, which at T = 200°C separates into an oxygen-deficient water-enriched phase and an oxygen-rich anhydrous phase.     

Obtaining YBa2Cu3O y crystals via YO1.5-BaO-CuO x melt electrolysis

Mass transfer during the melt electrolysis of Y_0.02Ba_0.30Cu_0.70O _y and Y_0.02Ba_0.25Cu_0.75O _y samples is investigated at a temperature of 950°C (0.5 h) and a current of 5–10⁵⁰ A in a cell. Crystal deposits of YBa₂CVu₃O_{6 + δ} tetragonal oxide (123) are grown, and their cation composition and structure are investigated by means of X-ray phase analysis, electron diffraction, elemental analysis, and high resolution on a transmission electron microscope. Deviation of the cation composition of oxide (123) from the stoichiometric ratio and its nanostructured state at nanocrystallite sizes of 2–5 nm are observed. The temperature dependence of magnetic susceptibility after oxygen annealing (450°C, 1 h) has four curve bends, indicating there are four superconducting phases with T _s = 45, 52, 75, and 86 K.     

Effect of low-temperature annealing on the critical parameters of highly textured YBa2Cu3O y

The effect of treatment at a temperature of 200°C and the natural aging on the critical parameters of a highly textured yttrium barium cuprate YBa₂Cu₃O_6.9 has been investigated. It has been shown that non-superconducting (at T = 77 K) particles precipitated during phase decomposition of this compound are effective pinning centers. At 200°C, the YBa₂Cu₃O _y compound interacts with atmospheric moisture. This inter-action results in the formation of stacking faults, which also provide pinning of magnetic vortices. The structural changes occurring during low-temperature annealing and natural aging of the compound lead to an increase in the critical current density and the first critical field. The presence of pinning centers of different nature in the structure causes a synergistic effect, which significantly increases the current-carrying capacity of materials, including those in strong magnetic fields.     

Effect of hydration on the structure of perovskite-like cuprates

The effect of hydration at T = 150 and 200°C on the structures of YBa₂Cu₃O_y (123) and a number of binary cuprates has been studied. It has been shown that the compounds containing oxygen vacancies in their structures interact with hydrogen significantly more strongly than cuprates without vacancies. Depending on the cuprate structure, hydrogen can be embedded in interstitial sites with the formation of hydrides and be attached to oxygen with the formation of hydroxides. The phase transition of the 123 phase to a defect tetragonal 124-type phase occurs only for the compounds with a high oxygen content. All the cuprates under study are more stable to reduction as compared to CuO.     

Effect of the percentage of yttrium oxide on the phase composition of annealed samples of YzBa5Cu7Oy and YzBa3Cu5Oy composition

The phase composition of Y _z Ba₅Cu₇O _y (1) and Y _z Ba₃Cu₅O _y (2) samples with a variable percentage of yttrium up to the stoichiometric composition of the Y _n Ba _m Cu _{m + n} O _y series is investigated by X-ray phase and elemental analyses, electron diffraction, and high resolution imaging in a transmission electron microscope at a temperature of 930°C in the crystallization field of a matrix oxide (Ba: Cu) of 5: 6 composition on the phase diagram of the BaO-CuO _x system at P(O₂) = 21 kPa. The substantial effect of yttrium oxide’s presence on the phase composition of both objects is found, providing evidence of a complex ionic equilibrium within the melt. The fine-domain oxide structure of the YBa₂Cu₃O₆ tetragonal form, which is due to the coexistence of oxides of an Y _n Ba _m Cu _m+n O _y homologous series of (Y: Ba: Cu) 235, 123, and 257 composition is revealed. The domain size for these phases is 20–50 ?. The domains are joined coherently along axis c.     

Magnetic uni- and tri-axial grain-orientation in superconductors with layered structures

We report the grain-orientation effects under a modulated rotation magnetic field for Y-based cuprate superconductors and LaFeAsO (La1111). Tri-axial orientation has been successfully achieved only for orthorhombic Y₂Ba₄Cu₇O_y and YBa₂Cu₄O₈ powders without a twin microstructure, while separation of three crystallographic axes could not be observed in twinned YBa₂Cu₃O_y (Y123) and tetragonal La1111 powders. The morphology of grains, in addition to the symmetry of crystal structures, seriously affects the degrees of tri-axial orientation, which means that the control of twin microstructures is required for the tri-axial magnetic orientation in Y123.     

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：     

Electrophysical properties and the structure of the YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript> compound thermally restored after low-temperature decomposition

The electrophysical properties and structure of the nonstoichiometric high-temperature superconductor YBa₂Cu₃O _y restored at T = 930–950°C after low-temperature decomposition (T = 200°C) into phases different in the oxygen content have been studied. It has been shown that, unlike heat treatments at T ≤ 900°C, the superconducting properties are almost completely restored for 3–5 h during grain recrystallization, which is impossible at lower temperatures. After short-term annealing at T = 930–950°C (for 1–2 h), the ceramic material still contains a significant number of structural defects, most likely, in cation sublattices. These defects can contribute to the pinning of magnetic vortices, which substantially increases the critical current density in magnetic fields up to 2 T as compared to ceramic materials produced by the conventional technology.     

Low-temperature growth of high-Tc superconducting films by plasma-enhanced metal-organic chemical vapor deposition

Plasma-enhanced MOCVD in which metal-organic compounds are sublimated directly into the growth chamber is studied for the first time as a new low-temperature process for growing superconducting YBa₂Cu₃O-_-x thin films. Y(THD)₃, Ba(THD)₂, Cu(THD)₂ and oxygen are used as metal sources and oxydizing agent. Emission spectroscopy reveals that activated metal-organic compounds and activated oxygen species are present during film growth. Superconducting YBa₂Cu₃O_7-x films whose zero-resistivity temperature are 50 K and 82 K are grown at 550°C and 600°C.     

Primary crystallization field of YnBamCum + nOy oxides with YBa2Cu3O6 tetragonal structure

The phase composition of specimens in the primary crystallization field of the Y _n Ba _m Cu _{m + n} O _y oxide series with compositions of (Y:Ba:Cu) 235, 123, and 257 was investigated by means of X-ray powder diffraction, X-ray microprobe analysis, electron diffraction, and high resolution imaging in a transmission electron microscope. Y _n Ba _m Cu _{m + n} O _y oxides have the YaBa₂Cu₃O₆ tetragonal structure and can be represented in the general form as 123 oxides. It was found that the crystallization field of 123 oxides corresponds to the crystallization fields of the Ba _m Cu _{m + n} O _y oxide series with the BaCuO₂ cubic structure. These crystallization fields are located in the matrix phase diagram of the BaO-CuO _x system at P(O₂) = 21 kPa in the range of 50–90 mol % CuO and 910–1000°C. The 123 oxides are formed according to an intercalation mechanism with the participation of barium-copper oxide matrices with the composition (Ba: Cu) 3: 5, 2: 3, and 5: 7     

Single crystal growth of YBa2Cu3O7−δ through a Pt compound intermediate phase

We report the synthesis of YBa₂Cu₃O_7−δ (Y-123 phase)-single crystals, through a different path to the traditional one – which involves Y₂BaCuO₅ (Y-211 phase) as a precursor –. In our case, at 970°C a Pt compound appears: Y₂Ba₃Cu₂PtO₁₀ which dissolves in the melt when heated at 1100°C allowing the growth of Y-123 phase. Large single crystals of Y-123 phase obtained through the route developed in this study showed good superconducting properties with a transition temperature onset at about 85 K; this transition temperature is shifted towards higher temperatures through oxygenation processes that are favoured by their dimensions.     

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.     

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.     

Influence of a presintering treatment on melt-textured Y−Ba−Cu−O growth

Summary The influence of a presintering treatment on Y-Ba-Cu-O prepared by a modified MPMG process was investigated. The sinterization, reducing the material porosity, was found to enhance the homogeneity of the Y₂BaCuO₅ (Y-211) particles distribution during the partial melting at 1100°C. As a consequence, the melttextured YBa₂Cu₃O_7−x (Y-123) grows more uniformly producing oriented domain up to 3cm in size. Microstructural and diffractometrical analyses were performed on samples quenched at different stages during the thermal treatment. Paper presented at the ?VII Congresso SATT? Torino, 4–7 October 1994. Under a Fondazione Confalonieri grant.