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.     

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.     

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     

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.     

Magnetoresistivity and microstructure of YBa2Cu3Oy prepared using planetary ball milling

We have studied the microstructure and the magnetoresistivity of polycrystalline YBa₂Cu₃O_y (YBCO or Y-123 for brevity) embedded with nanoparticles of Y-deficient YBCO, generated by the planetary ball milling technique. Bulk samples were synthesized from a precursor YBCO powder, which was prepared from commercial high purity Y₂O₃, Ba₂CO₃ and CuO via a one-step annealing process in air at 950 °C. After planetary ball milling of the precursor, the powder was uniaxially pressed and subsequently annealed at 950 °C in air. Phase analysis by X-ray diffraction (XRD), granular structure examination by scanning electron microscopy (SEM), microstructure investigation by transmission electron microscopy (TEM) coupled with energy dispersive X-ray spectroscopy (EDXS) were carried out. TEM analyses show that nanoparticles of Y-deficient YBCO, generated by ball milling, are embedded in the superconducting matrix. Electrical resistance as a function of temperature, ρ(T), revealed that the zero resistance temperature, T_co, is 84.5 and 90 K for the milled and unmilled samples respectively. The milled ceramics exhibit a large magnetoresistance in weak magnetic fields at liquid nitrogen temperature. This attractive effect is of high significance as it makes these materials promising candidates for practical application in magnetic field sensor devices.     

A low temperature internal friction study of Y0.85Ca0.15Ba2Cu3O7−δ with different oxygen content

The ceramic sample of Y_0.85Ca_0.15Ba₂Cu₃O_7−δ was prepared by standard solid-state reaction method, and samples with different oxygen concentration were obtained by quenching from high temperature. The internal friction was measured using the vibrating reed method from liquid-nitrogen temperature to room temperature at kilohertz frequency. An internal friction peak was observed around 250 K in Y_0.85Ca_0.15Ba₂Cu₃O_7−δ quenched from 1023 K. The peak is related to the one observed around 220 K (labeled as P3 peak) in undoped YBa₂Cu₃O_7−δ (Y123). This result shows the dependence of P3 peak on carriers density and P3 peak has a strong correlation to the abnormal behavior of Y123 in the underdoped range. The variation of two low temperature thermal activated relaxation peaks (P1 and P2) on oxygen content were also investigated. And consistent explanations were given based on all recent researches.     

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.     

On the existence of a homologous series of BamCum+nOy oxides with the cubic structure of the BaCuO2 oxide

Phase relations have been studied in the BaO–CuO_x system in the range of 42.0–83.0 mol.% CuO at P(O₂) = 21 kPa (air) by visual polythermal analysis (VPA), powder X-ray diffraction (XRD), differential thermal analysis (DTA), thermogravimetric analysis (TGA), chemical analysis (CA), and electron diffraction (ED) with simultaneous elemental analysis (EA) in a transmission electron microscope (TEM). The existence of discrete crystallization fields of barium–copper oxides of cation compositions Ba₄Cu₅O_y, Ba₅Cu₆O_y, Ba₇Cu₈O_y, Ba₁₂Cu₁₃O_y, and Ba₂₄Cu₂₅O_y, which have the cubic structure of the BaCuO₂ oxide, is revealed in the studied region of the system. The oxides may be represented as members of a Ba_mCu_m+nO_y homologous series. The BaCuO₂ oxide does not exist in the subsolidus region and does not have its own crystallization field. The oxygen-deficient oxide BaCuO_1.78 of the cation composition (Ba:Cu) 1:1 with the BaCuO₂ cubic structure is found in melted samples of the 50.0 mol.% CuO composition quenched at 1020–1060 °С.     

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.     

Si doped Cu0.5Tl0.5Ba2Ca2Cu3O10−δ superconductors

The Si doped (Cu_0.5Tl_0.5)Ba₂Ca₂Cu_3−ySi_yO_10−δ (y=0, 0.25, 0.5, 0.75, 1.0, 1.25) superconductor samples have been synthesized to investigate the effect of mobile carriers on the critical temperature and magnitude of diamagnetism of this family of cuprates. The Si doped samples have tetragonal structure as observed from the X-ray diffraction spectra. The c-axis length of the unit cell of (Cu_0.5Tl_0.5)Ba₂Ca₂Cu_3−ySi_yO_10−δ was increased after the increase of Si concentration, whereas the critical temperature and the magnitude of diamagnetism have been decreased. The decrease in T_c(0) and magnitude of diamagnetism is possibly due to the deficiency of carriers in CuO₂/SiO₂ planes caused by the +4 state of Si atoms. However, the post-annealing of these samples in oxygen atmosphere has increased the critical temperature and the magnitude of diamagnetism. The FTIR absorption measurements of (Cu_0.5Tl_0.5)Ba₂Ca₂Cu_3−ySi_yO_10−δ samples have shown a softening of the apical oxygen mode of the type Cu(1)O_ACu(2) and TlO_ACu(2) with increased concentration of Si in the unit cell; the softening of this mode is directly linked with the increase in the c-axis lattice parameter of the (Cu_0.5Tl_0.5)Ba₂Ca₂Cu_3−ySi_yO_10−δ superconductor.     

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.     

Analysis of the possible reasons for the suppression of superconductivity in the Y1−x PrxBa2Cu3Oy system on the basis of thermoelectric power data

The temperature dependence of the thermoelectric power in samples of Y_1−x Pr_xBa₂Cu₃O_y (x=0−0.6) is investigated. The principal parameters of the system of charge carriers are determined on the basis of an analysis of this dependence within a narrow-conduction-band model, and their variation with increasing praseodymium content is analyzed. It is found that an increase in x leads to a very weak increase in the electron filling and considerable broadening of the conduction band accompanied by strong localization of the charge carriers. It is concluded on the basis of a comparative analysis of the results obtained and data for the YBa₂Cu₃O_y system in the case of aliovalent substitutions in the barium and copper sites that the valence of praseodymium in Y_1−x Pr_xBa₂Cu₃O_y is very close to 3+. It is shown that there is a universal correlation between the effective width of the conduction band and the critical temperature following various cation substitutions in the Y-Ba-Cu-O system. It is concluded that the main reason for suppression of the superconducting properties of Y_1−x Pr_xBa₂Cu₃O_y is the strong modification of the band spectrum caused by praseodymium. Fiz. Tverd. Tela (St. Petersburg) 39, 1520–1525 (September 1997)     

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.     

Calcium and oxygen doping in Y Ba2Cu3Oy

Both oxygen and calcium play important roles in inducing superconductivity in Y Ba₂Cu₃O_y (YBCO), which is an antiferromagnetic insulator at low O and Ca content. O induces superconductivity in Ca-free YBCO, while Ca does similarly in oxygen-deficient YBCO. For doping oxygen HgO was used as it decomposes at 476 ^°C into Hg, which escapes from the matrix leaving the crystal unaltered, and O, which provide a way to dope O in YBCO. Considering these facts, polycrystalline samples of Y _1−xCa_xBa₂Cu₃O_y with x=0, 0.1 and 0.2 with and without HgO addition were prepared through a solid-state reaction method. The samples were sintered at 950 ^°C in open atmosphere. These synthesized samples were characterized through using the X-ray diffraction technique (XRD) for phase evaluation, scanning electron microscopy (SEM) for grain morphology, energy dispersive X-ray analysis (EDX) for compositional analysis and the four-contact measurement technique for determining the superconducting transition temperature.     

Superconductivity in the LnBaCuO system (Ln = Y,Ho, Er)

Superconductivity around 90K is observed in mixed and single phase system of LnBaCuO (Ln = Y, Ho, Er). Resistivity, X-ray powder diffraction and AC susceptibility measurements were carried out on samples of the mixed phase systems Ln_2−xBa_xCuO_4−y (Ln = Ho, Er), Y_1.2Ba_0.8Cu_1+xO_4+x−y(0<x<5) and single phase system Ba₂LnCu₃O₇ (Ln = Y, Ho, Er). Ln_2−xBa_xCuO_4−y consists of Ba₂LnCu₃O₇, BaCuLn₂O₅, Cu₂Ln₂O₅ and starting materials and shows a small diamagnetic signal below T_c in the temperature dependent AC susceptibility. Y_1.2Ba_0.8Cu_1+xO_4+x−y is a mixture of Ba₂YCu₃O₇, BaCuY₂O₅ and CuO and also indicates a small diamagnetic signal at 92K. Interesting resistivity anomalies occur in the Y_1.2Ba_0.8Cu₄O_7−y and Ho_0.8Ba_1.2CuO_4−y composites including large drops in the resistivity at ∼220 and ∼120K, respectively. The pure compounds, Ba₂LnCu₃O₇ (Ln = Y, Ho, Er) show sharp resistive superconducting transitions and bulk Meissner effects of 90%.     

Crystal structure of the superconducting infinite-chain cuprate Ba2Cu2.89O6−y

The orthorhombic structure of Ba₂Cu_2.89O_6−y (sp. gr. Pccm, a=13.065(15), b=20.654(21) and c=11.431(8) Å) contains two nonintersecting symmetrically equivalent sets of parallel CuO₂-chains running along [110] and [1̄10] directions. Along the c-axis the Cu–O sheets are separated by identical layers of Ba-atoms. The Cu–O chains are distorted within the xy plane producing zigzag. Three of 14 positions of copper atoms were found to be partly vacant that caused the significant shifts of the neighbouring Ba atoms in the plane. The determined structure of Ba₂Cu_2.89O_6−y is compared to that of the monoclinic Ba₂Cu₃O₆ modification, of another infinite-chain compound Sr_0.73CuO₂, and to one of the substructures of ladder compounds (Sr,Ca)₁₄Cu₂₄O_41+y. The superconductivity in Ba₂Cu_2.89O_6−y is associated with the CuO₂-chains consisting of CuO₄-squares sharing edges. The attention is paid to the coincidence of the temperatures of the superconducting transitions in Ba₂Cu_2.89O_6−y and in ladder compound with the temperature of the sharp magnetic transition in another infinite-chain compound Sr_0.73CuO₂.     

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.     

Low concentration of the superconducting phase and thermoelectric power in 90 K superconductors

The thermoelectrical power (TEP) and the electrical resistivity behaviour of three 90 K superconductors viz, Y₁Ba₂Cu₃O_7−x , Sm₁Ba₂Cu₃O_7−x and Gd₁Ba₂Cu₃O_7−x , after the specimens were quenched from the sintering temperature (920°C) to 77 K, are reported. Interestingly the Y₁₂₃ specimen, which has the presence of trace amount of the orthorhombic phase in an otherwise tetragonal phase and does not show a superconducting transition down to 77 K shows zero TEP around 82 K, theT _c for the well oxygenated specimen. The Sm and Gd specimens on the other hand show completely tetragonal structure, semiconducting behaviour in resistivity and no zero TEP up to 77 K. It is argued that the critical concentration of the superconducting phase necessary to make the TEP zero is much smaller than that required for zero resistivity.     

The Nernst-Ettingshausen coefficient in the normal phase of doped HTSCs of the YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>y</Subscript> system

The temperature dependence of the Nernst-Ettingshausen coefficient Q(T) in the normal phase of doped HTSCs of the yttrium system was studied. The main features characterizing the behavior of this coefficient were revealed, and the character and mechanism of the effect that various nonisovalent substituents exert on the Q(T) dependence were analyzed. It is shown that the narrow-band model permits one not only to describe all the specific features observed in the Q(T) curves but also to perform a simultaneous quantitative analysis of the temperature dependences of four kinetic coefficients (the electrical resistivity and the Seebeck, Hall, and Nernst-Ettingshausen coefficients) with the use of a common set of model parameters characterizing the band structure and carrier system in the normal phase of an HTSC. This approach was employed to determine the carrier mobilities and the asymmetry of the dispersion curve in the systems studied (YBa₂Cu₃O_y, y = 6.37–6.91; YBa₂Cu_3?xCo_xO_y, x = 0–0.3; Y_1?xCa_xBa₂Cu₃O_y, x = 0–0.25; Y_1?xCa_xBa_2?xLa_xCu₃O_y, x = 0–0.5) and to analyze the effect of the substitutions involved on the variation of these parameters.     

Mechanism of a strong rise of Tc due to the calcium doping in Y1−xCaxBa2Cu2.8Zn0.2Oy

The normal-state transport properties and the critical temperature, T_c, for three series of Y_1?xCa_xBa₂Cu_2.8Zn_0.2O_y (x = 0–0.2) samples with different oxygen content have been investigated and discussed in comparison with results for other calcium-doped YBa₂Cu₃O_y. We have observed the calcium doping to restore the superconductivity in the YBa₂Cu₃O_y system even in case of the preliminary T_c suppression by the direct destruction of the CuO₂ planes due to a preliminary doping by zinc. The T_c value rises strongly with increasing calcium content and this rise becomes faster for the oxygen-reduced series. Based on a narrow-band model, the parameters of the band spectrum structure and the charge-carrier system in the normal state were determined from the analysis of the thermopower temperature dependences in order to reveal the mechanism of the T_c rise under the Ca influence. It is shown that the T_c increase in Y_1?xCa_xBa₂Cu_2.8Zn_0.2O_y is related to the direct Ca ions effect. The mechanism of the calcium influence on the T_c value is discussed with respect to the Ca-induced conduction band modification. The calcium doping in Y_1?xCa_xBa₂Cu_2.8Zn_0.2O_y is shown to give a noticeable contribution to the density-of-states function leading to a strong rise in its value at the Fermi level that results in a restoration of the superconducting properties.