Y123 Crystals Research Articles

A study of microstructure and ductility of directionally solidified Ni50Al20Fe30

The particle segregation mode of two different inclusion phases of Y2BaCuO5 (Y211: a dissolving phase in a Ba–Cu–O liquid phase) and BaCeO3 (a nondissolving phase) was investigated in the melt-textured YBa2Cu3O7−y (Y123) with BaCeO3 addition (0–20 wt. %), and with 30 wt.% Y211 plus BaCeO3 (0–20 wt. %). The segregation mode of the inclusion phases is dependent not only on the type of the inclusion phases but also their amounts. When the trapped amount of the Y211 is small, they make an X-like pattern on the diagonal planes of the Y123 crystal. When the amount of the Y211 is large, meanwhile, the Y211 particles are trapped within four tetrahedral spaces (normal to the c-axis) bounded by the diagonal planes of the Y123 crystal, with no Y211 trapping within two tetrahedral spaces parallel. On the other hand, the nondissolving BaCeO3 particles make linear tracks normal to the {100} growth fronts of the Y123 crystal.

Crystal growth and superconductivity of Y1−xNdxBa2Cu3O7−δ solid solutions

Abstract Single crystals of Y1−xNdxBa2Cu3O7−δ [Y(Nd)BCO] solid solutions, in which the Nd content x ranged from 0.098 to 0.410, were grown by the crystal pulling method. The Nd substitution content x in the crystal can be controlled with the processing temperature. With an increase of the processing temperature, the Nd concentration in the liquid increased, and accordingly, resulted in the higher Nd substitution content in single crystals. When x=0.410, the ratio of Nd+Y to Ba in single crystals became high and the Tc was depressed, indicating evidence of the Nd substitution at Ba sites. The critical temperature Tc of 89±2 K was obtained from oxygenated Y1−xNdx Ba2Cu3O7−δ single crystals at 340°C for 200 h and could be enhanced up to 91 K by annealing at 500°C for 3 days. For all Y(Nd)BCO solid solutions, even a small amount of Nd content (x=0.098), the second peaks appear in the range of 1.2–1.7 T for the applied field H parallel to the c-axis of samples in the M–H loops, which may be due to a double substitution of both Y3+ and Ba2+ by Nd3+. X-ray measurements of the lattice parameter proved the above double substitution.

Phase relation in Y211–Y123–Ag system and morphology of silver in Y123 crystal

Phase relation in the Y 2BaCuO 5 (Y211)– YBa 2Cu 3O 7− x (Y123)–Ag system, which is important for the fabrication of Y123/silver composite, and the morphology of silver particles dispersed in the Y123 matrix were investigated. The formation temperature of Y123 monotonously decreased to about 970°C as increasing Ag content up to a pseudo-monotectic composition and kept constant at the temperature with further Ag addition. The Y123/silver composite could be obtained at the temperatures lower than 970°C. These results suggest that Y123 phase is solidified by monotecto-peritectic reaction; Y211+L1 (Ba–Cu rich liquid)=Y123+L2 (Ag rich liquid). The morphology of silver particles in Y123 was altered by the addition of excess Y211. In the samples without excess Y211, silver particles dispersed in Y123 were disk-like shaped. On the other hand, those became spherical in the samples with excess Y211. The capturing mechanism of L2 liquid is considered to be the Uhlmann, Chalmers, and Jackson (UCJ) type pushing one. The amount of residual Y211 particles at the growth interface is considered to cause the morphological difference of captured silver between samples with and without excess Y211.

Magnetic studies of flux pinning in YBa 2Cu 3O 7− x, YBa 2Cu 4O 8 and Hg-based single crystals

The flux-pinning mechanism in YBa 2Cu 3O 7− x (Y-123), YBa 2Cu 4O 8 (Y-124), HgBa 2CuO 4+ x (Hg-1201) and HgBa 2Ca 2Cu 3O 8+ x (Hg-1223) is compared. Single crystals of Y-123, Y-124 and Hg-1201 show a very pronounced fishtail effect, observed in a wide temperature range. For each compound the irreversibility field and the field at which the peak in the fishtail occurs are well described by a power-law dependence H irr/peak(T) = H irr/peak(0)(1 − T/T c) α with very similar exponents. In Hg-1223 the pinning has a more three-dimensional character at higher temperatures, but becomes rather two dimensional at low temperatures.

Preferred orientation of Y 2BaCuO 5 particles entrapped by melt-textured YBa 2Cu 3O 7− δ crystals

This study is an attempt to understand the orientation relationships between Y 2BaCuO 5 (Y211) inclusions and YBa 2Cu 3O 7− δ (Y123) matrix in a melt growth sample. The Y123 crystals were grown by the constant undercooling method with top seeded growth using a nominal composition of Y123 + 0.4Y211 + 0.5wt%Pt. It was revealed that the Y211 particles tended to be oriented in the Y123 crystals and that the preferred orientation depended on the degree of undercooling (i.e. growth rate of Y123), the growth direction of Y123 and the shape of Y211 particles. The tendency was significantly observed in the Y123 crystal grown under the growth condition which caused pushing phenomenon of Y211 particles. The orientation relationship could be qualitatively explained by the prevalent trapping/pushing theory considering the peritectic reaction as well as the anisotropy of interfacial energy and geometry of Y211 particles.

低温化液相エピタキシャル法によるYBa<SUB>2</SUB>Cu<SUB>3</SUB>O<SUB>7−δ</SUB>コート導体の作製

Liquid phase epitaxy (LPE), which is performed under ambient pressure with a simple growth apparatus, allows us to fabricate high-JcYBa2Cu3O7−δ (Y123) crystalline thick films at high growth rate. We propose the LPE growth of Y123 films on single crystalline oxide fibers as a possible way to synthesize Y123-coated conductors for power application. In the conventional case of using a BaO-CuO mixture as the solvent, with which the Y123 crystals grow at as high as about 1000°C, Y123 coating on YSZ or SrTiO3 single crystalline fibers has not been successfully achieved because of a reaction between the solvent and the substrates. Recently we have discovered that the BaF2-added solution decreases the crystallization temperature of Y123 down to 900°C and suppresses the reactivity with no degradation of superconductivity of the Y123 crystal. The Y123-coated fibers are successfully prepared by using this solution. The characteristic of the critical currents under magnetic fields showed that this Y123 coated fibers have a potential to use under a high magnetic field at the liquid nitrogen temperature.

Entrapment of Y 2BaCuO 5 particles in melt-textured YBa 2Cu 3O 7−δ crystals and its effect on J

In order to investigate the entrapment phenomenon of Y 2BaCuO 5 (Y211) particles and its effect on the properties of the critical current density ( J c) in YBa 2Cu 3O 7−δ (Y123) crystals, Pt-added Y123 crystals were grown by two different methods; the constant undercooling method and the continuous cooling method, with top-seeded growth. It was revealed that the amounts and size distribution of trapped Y211 particles in solidified Y123 crystals depended on the growth direction (〈001〉, 〈100〉) and the growth rate ( R) as a function of undercooling ( ΔT). These phenomena could be at least qualitatively explained by the prevalent trapping/pushing theory. The J c values of the Y123 crystals were found to vary with their Y211 distributions, which in turn depended on growth conditions such as growth direction and/or growth rate even when the nominal composition was kept constant. Considering the effect of R or ΔT on the Y211 particle entrapment/pushing and J c properties, it was shown that the conventional continuous cooling methods (i.e. MPMG/QMG methods) were favorable for Y211 particle entrapment, leading to high J c values.

Effect of temperature gradient in the solution on spiral growth of YBa2Cu3O7−x bulk single crystals

Bulk single crystals of Y123 are required to clarify the superconductivity phenomena and develop electronic devices using unique superconductive properties. Only the Solute-Rich Liquid–Crystal Pulling (SRL-CP) method has succeeded in continuous growth of the Y123 single crystal. In this paper, we investigated the growth of Y123 single crystals under different temperature gradients in the solution in order to understand the growth mechanism of Y123. It was revealed that Y123 single crystals grow with a spiral growth mode, which is in good agreement with the BCF theory.

Effect of precursor processing on refinement of Y 2BaCuO 5 in melt-processed YBa 2Cu 3O 6+ x by unidirectional solidification

The effect of precursor processing on refinement of Y 2BaCuO 5 (Y211) in the unidirectional solidification method for Y211 rich YBa 2Cu 3O 6+ x (Y123) superconductors with Pt addition was investigated. It was found that Y211 particle sizes in the precursors showed significant dependence of the calcination process. Further, the difference of these Y211 particle sizes in each precursor affects Y211 particle size during melt-growth processing and finally the Y211 particle size in the grown Y123 crystals as well.

Effect of temperature gradient in the solution on the growth rate of YBa 2Cu 3O 7− x bulk single crystals

The bulk single crystals of Y123 are required to clarify the superconductivity phenomenon and to develop electronic devices using unique superconductive properties. Only the solute rich liquid crystal pulling method developed by Yamada et al. has succeeded in continuous growth of the Y123 single crystal. In this paper, we investigated the growth rate of Y123 single crystals under different temperature gradients in the solution in order to understand the growth mechanism of Y123. It was revealed that Y123 single crystals grow with a spiral growth mode, which is in good agreement with the BCF theory.

Relationship between growth rate and undercooling in Pt-added Y1Ba2Cu3O7−x

Y1Ba2Cu307−x (Y123) crystals were grown by two different methods, the constant undercooling solidification and the continual cooling method, with top seeding by Sm123 seed crystals in order to investigate a relationship between undercooling (ΔT) and a growth rate (R). The crystals of Y123 with a sharp faceted interface, which consisted of {100} and {001} faces, grew epitaxially from the seed. It was found that the growth rates of {100} face (Ra) and that of {001} face (Rc) showed an increasing trend with increasing ΔT, and Rc was faster than Ra within these experimental conditions, ΔT < 20 K. The relation between R and ΔT follows the parabolic equation, viz. Ra ∝ ΔT1.9 and Rc ∝ ΔT1.3 for {100} and {001} faces, respectively. The simulated crystal size using the R and ΔT relations obtained from the constant undercooling method showed good agreement with experimental data by the continual cooling.

Macrosegregation of Y2Ba1Cu1O5 particles in Y1Ba2Cu3O7−δ crystals grown by an undercooling method

Macrosegregation of Y2Ba1Cu1O5 (Y211) particles was observed in Pt-added Y1Ba2Cu3O7−δ (Y123) crystals grown by an undercooling method. It was found that the macrosegregation of Y211 particles depended on the growth direction and the growth rate (R) as a function of undercooling (ΔT). The amount of Y211 particles in Y123 crystals grown at large R was larger than at small R. Also, the amount of Y211 in Y123 growing along the a-direction was larger than that along the c-direction. Further, it was noted that the smaller Y211 particles in size were distributed in Y123 grown at large R. These phenomena could be at least qualitatively explained by the prevalent trapping/pushing theory. In the direct observation of magnetic flux with the Faraday effect of iron garnet film, the flux pinning force was found to be in good agreement with the macrosegregation of Y211 particles.

The growth mechanism of a La-doped Y1Ba2Cu3O7-y crystal prepared by a melt process

The growth behaviour of a Y123 crystal was investigated in a melt-textured Y0.75La0.25Ba2Cu3O7-y specimen in which La had been substituted for Y. A chi -shaped pattern, which is likely to be related to the crystal orientation, was observed. The concentration of Y and La atoms varied gradually from the centre to the surface of the grains. The growth pattern of a La-doped Y123 crystal, which is very similar to the annual rings of a tree, was explained by a solubility difference between Y and La atoms in a melt that in turn led to the different formation temperature of a La-doped Y123 crystal.

Transport and thermodynamic studies of Y1?xTbxBa2Cu3O7 single crystals

The electrical resistivity ρ(T,x) and magnetic susceptibility χ(T,H), and specific heat C(T,x) have been measured on single crystals of Y1−x TbxBa2Cu3O7. The results strongly suggest the appearance of strong 4f electron hybridization. It becomes apparent that the Tb doped system possesses properties that largely deviate from the well-known behavior of the other high Tc and doped high Tc systems.

Normal-state electronic Raman-scattering efficiencies of YBa2Cu3O7- delta, Bi2Sr2CaCu2O8, and Tl2Ba2Ca2Cu3O10: Effects of local-density-approximation Fermi-surface mass fluctuations.

Absolute electronic Raman-continuum cross sections have been measured for orthorhombic single-domain Y-123 and Bi-2212 crystals and a tetragonal Tl-2223 film on a MgO substrate (${\mathit{T}}_{\mathit{c}}$=120 K) in different polarization geometries. They agree well with effective mass fluctuations obtained for Y-123 from Fermi-surface averages of the dispersion calculated from the full-potential linear muffin-tin orbital band structure of Andersen et al. The results suggest that anisotropic mass density fluctuations involving various sheets and regions of the Fermi surface are indeed responsible for the electronic Raman continuum and predictable with realistic local-density-approximation band-structure calculations. Decomposition into contributions from the four sheets of the Fermi surface permits separation of interband and intraband mass fluctuations. Anisotropies corresponding to the orthorhombicity and the purely tetragonal ${\mathit{A}}_{1\mathit{g}}$, ${\mathit{B}}_{1\mathit{g}}$, and ${\mathit{B}}_{2\mathit{g}}$ symmetries are determined for the three superconductors.

Electronic Raman scattering in single domain YBa 2Cu 3O 7−δ, k-space anisotropy and comparison to LDA Fermi surface masses

Measurements of the absolute electronic Raman cross sections are presented for a single domain Y-123 crystal for different polarization geometries. They are compared to the corresponding inverse effective mass fluctuations evaluated from the LMTO full-potential band structure of O.K. Andersen et al. The agreement of the relative intensities for the xx- yy anisotropy and between the crossed polarizations parallel and at 45° to the crystal ab axes, i.e. B 2 g and B 1 g symmetries, with the LDA single particle masses is good and suggests that isotropic and anisotropic density fluctuations involving various sheets and regions of the Fermi surface are responsible for the electronic Raman continuum. Resonance effects and screening are taken into account.

The origin of texture in bulk superconductors fabricated by the melt texture growth process

Melt texture growth YBCO bulk superconductors have been fabricated by making use of the nonzero spatial temperature gradients (∇T) at certain locations and with temporal variation (∂T/∂t) inside a common tube furnace. The texture of the YBa 2Cu 3O y (Y123) single crystal layers and Y 2BaCuO 5 (Y211) grains of the specimen are investigated by TEM/EDX, SEM/EDX, XDP, χ-T and R-T measurements. The phase distribution results of Y123 and Y211 using EDX covering an area of single crystal layer and Y211 grain indicate that the crystal layers are not “true layers” but “bulk crystal with gap lines and planes”. The formation of gap lines and planes depends on how the Y123 phase grows from the Y211 grain. If the Y211 grain can act as a nucleus of Y123 phase, “gap plane” can be formed when two Y123 crystal blocks grow to touch each other. However, if the Y211 grain cannot take the role of a nucleus, it can only act as a perturbation and generate a gap line as the crystal growth front passes through the grain. Moreover, since the orientation of the crystal layers relative to the top and bottom surfaces of the bulk specimen depends on ∇T and ∂T/∂t, we can control the crystal layer orientation by varying the above two parameters using a simple tube furnace in order to produce high quality melt texture YBCO material.

Continuous crystal growth of YBa 2Cu 3O 7− x by the modified top-seeded crystal pulling method

Continuous growth of YBa 2Cu 3O 7− x (Y123) crystals was achieved by a pulling method using a BaO-CuO solution with the solid Y 2BaCuO 5 (Y211) as a solute. A suitable crucible was also investigated, and an yttria crucible was found to be the best selection for this method. The growth was performed in an ambient atmosphere at about 1000°C. When a textured Sm- Ba 2Cu 3O 7− x (Sm123) rod was used as a seed crystal, the Y123 crystal grew epitaxially from the seed. The grown crystal contained no Y211 particles; however, it had a twin structure near the surface, which had been formed during the in situ cooling process. The temperature dependence of the resistance was metallic in both directions, along the c-axis and in the ab-plane, after appropriate oxygen annealing.

Scanning tunneling microscopic studies of growth spirals in bulk single crystals of superconducting YBa2Cu3O7 and flux pinning

Growth spirals have been observed in the basal plane of bulk single crystals of Y-123 high Tc cuprate, using scanning tunneling microscope with the unit cell resolution. The density of screw dislocations present is of the order of 107–108/cm2, close to the recently reported values for the sputtered epitaxial thin films of Y-123. The growth steps or terraces which are generally shown to possess a step height equal to the c parameter of the unit cell, have been further resolved into substeps corresponding to each of the triple perovskite block. Further, in the limit of ultimate resolution we have been able to demonstrate the substep heights corresponding to the individual atomic layers of the unit cell and thus provide a persuasive evidence that single crystals of Y-123 grow via layer-by-layer growth on the atomic scale within the purview of the spiral growth mechanism. Growth features are briefly discussed and it is argued that the observed screw dislocation density is still too low and the orientation is not favorable for realizing optimum flux pinning in these HTSC systems.

STM studies of growth spirals in bulk single crystals of superconducting Y-123 and Bi-2122

The STM observations of growth spirals in bulk single crystals of Y-123 and Bi-2122 high Tc cuprates are reported for the first time. The density of screw dislocations present is of the same order as recently reported for sputtered epitaxial thin films of Y-123. Growth features are briefly discussed and it is argued that the observed dislocation density is still too low for realizing optimum flux pinning in these HTSC systems.