Melt-textured Growth Research Articles

Effects of thermomechanical treatments on the critical current densityof TSMTG-processed YBa2Cu3O7-δ ceramics

TSMTG (top seeding melt texture growth)-processedYBa2Cu3O7-δ ceramics have reached such critical currentdensities (Jc) that it is now possible to envisage technical applicationssuch as magnetic bearings or fault current limiters. Most authors believe thatin this material, defects (such as Y-211 inclusions, oxygen vacancies,dislocations, stacking faults, etc) can act as pinning centres and henceincrease critical current densities. The effects on the critical currentdensity of a uniaxial pressure, applied along a direction perpendicular to thec-axis of Y-123, at 400 °C under an air atmosphere are investigated.These conditions were chosen to avoid oxygen loss in the samples, becauseoxygen content is the most critical parameter for superconducting properties.Thus this allows a sample with the same oxygen content to be compared directlybefore and after experiment. TSMTG-processed Y-123 ceramics exposed tothermomechanical treatment show a complex behaviour. For low pressures (lowerthan 2 MPa) a significant increase in the critical current density (up to 1.37times at H = 0, for a pressure of 1.38 MPa applied for 22 h at 400 °Cunder an air atmosphere) is observed and attributed to an increase in thedislocation density. For higher pressures, the critical current densitiesunder low magnetic fields are always lower but sometimes slightly higher atmoderate magnetic fields (between 1-3 T).

Texturation of NdBa 2Cu 3O y bars under air in a vertical Bridgman furnace

Nd123 bars were textured using the melt texturing growth process in a vertical furnace by the floating zone method. Different applied growth rates were tested, from 1 to 4 mm/h, combined with different secondary phases additions (Nd 4Ba 2Cu 2O 5 and Nd 3.6Ba 2.2Cu 1.9O y ). Nd123 single-domains were obtained independently of the tested conditions and correlation between process conditions (composition, growth rate), microstructure and superconducting properties were investigated. An optimized T c (96 K) with a sharp transition is obtained without Ar annealing at high temperature generally necessary to recover the optimal T c. This attractive result is linked with the process used. J c curves were measured and results were discussed in relation with process conditions.

Investigation of microstructure of textured YBCO with addition of nanopowder SnO 2

By using top seeding melt-textured growth (TSMTG) process, large single domain YBa 2Cu 3O 7− δ /Y 2BaCuO 5 (YBCO) samples were prepared with addition of nanometric SnO 2 and normal CeO 2. Polarized light microscope studies at each step of the TSMTG process showed that before YBa 2Cu 3O 7− δ (123) melting, the Sn-based reacted products were uniformly distributed in micrometer scale; after solidification, the trapped Y 2BaCuO 5 (211) and Sn-based inclusions were more homogeneously distributed and with mean sizes of about one micrometer. Composition images of scanning electronic microscope for the textured samples showed that, on the whole, the Sn-based inclusions were smaller than the 211. They must be on average below one micrometer in size. Although evidence was found that very fine Sn-based inclusions were uniformly trapped in 123 matrix, the fraction of them was small in comparison to that of 211. No significant enhancement of superconducting critical current density ( J c) was evidenced for the nanometric SnO 2-doped samples at a low magnetic field ( B<1 T ). However, values of J c were decreased at intermediate magnetic field ( 1<B<3 T ).

The peak effect observed in the Na substituted melt textured growth YBCO

The effect of Na substitution in melt textured growth (MTG) YB(Na)CO (YBa 2− x Na x Cu 3O y + 40 mol% Y 2BaCuO 5) system with x=0.0, 0.1, 0.2 on the critical current characteristics has been investigated. It was observed that with certain Na substitution, MTG YB(Na)CO showed an increased critical current density and a second peak around 0.8–1.0 T at 77.3 K in the critical current density versus applied field curve. The post-annealing at 900°C in air reduced the critical current density and suppressed the second peak. However, further oxygenation at 400°C raised critical current density and recovered the second peak partly, but could not completely recover to those before the high temperature annealing. The possible source of such a behavior was supposed to be connected with inhomogeneous distribution of Na (different x) in MTG YB(Na)CO, which leads to a distribution of different T c clusters.

Microstructure, magnetization and dc transport properties of MTG-YBa1.8Na0.2Cu3Oy crystal

The Na-doping MTG-YBCO crystal with the nominal compositionYBa1.8Na0.2Cu3Oy + 40 mol% Y211 has been successfully synthesizedby the melted-textured-growth (MTG) method with top seed. The microstructurephotographs show that the crystal has a good layered structure with a numberof twins. There is a second peak in the magnetization curve for the magneticfield parallel to the c-axis and at 77 K. The field of zero resistancetemperature Tc0 shows a power law, H = H0 (Tc0(0)-Tc0(H))nwith n = 1.52 for H∥ab and n = 1.41 for H∥c, respectively. The thermallyactivated flux creep model can scale the dissipative resistivity below thepeak temperature Tp. The temperature and field dependences of theeffective activation energy can be described by the formula U = U0(1-T/Tp)mH-α, wherem = 1.75, α = 0.75 for H∥ab and m = 2.5,α = 1.52 for H∥c, respectively. The effect of Na-doping induceddisorder on flux pinning has been discussed.

Influence of a magnetic field on melt-growth process of YBa 2Cu 3O x

Differential temperature analysis (DTA) in a magnetic field was carried out for growth of YBa 2Cu 3O 7, in order to investigate the influence of magnetic fields up to 14 T on the melt-textured growth process. The DTA peak for the heating process, which corresponds to melt transition, was insensitive to the magnetic field. For the cooling process, however, the peak corresponding to crystal formation was shifted to a lower temperature under application of a magnetic field of 14 T. The shift of the DTA peak by a magnetic field is due to the lowering crystal growth rate by the magnetic field.

Top seeding melt texture growth of NdBaCuO pellets in air

Most of NdBaCuO synthesis were realized under oxygen controlled melt growth (OCMG) which allows to avoid the solid solution formation. However, for industrial applications air synthesis is preferable due to its simplicity and its cost. In this work, different seeds and thermal cycles have been investigated in order to optimize the nucleation conditions for the obtention of a single domain. Several criteria have been defined for the choice of the seed. The solid solution Nd1+xBa2−xCu3Oy formed in these air synthesised samples appears in the range of x = 0.10 to 0.15, corresponding to between 60 K and 80 K. The nominal composition and the synthesis conditions (substrate, thermal treatments.) have to be accurately defined in order to avoid neodymium-rich solid solution formation.

Determination of U( x,t) and j in MTG YBCO bulk by magneto-optic imaging

We use the magneto-optic imaging to determine the spatial distribution and the time dependence of the magnetic induction B z ( x,t) on the broad surface of the melt texture growth YBCO bulk. The model-independent effective activation energy U and the current density j could be calculated from B z ( x,t), ∂ B z ( x,t)/∂ t and ∂ B z ( x,t)/∂ x. The good fit shows that the thermally activated flux creep in the melt texture growth bulk is consistent with the logarithmic model in the temperature range of liquid nitrogen.

Flux-pinning in Y 0.5/Nd 0.5(123) melt-textured sample

Flux-pinning in Y 0.5/Nd 0.5 (123) melt-textured growth sample that had been doped with excess 20wt.% Y 1/Nd 1(211) and 0.5 wt.% PtO 2 has been analyzed by means of a scaling approach. The different parameters of scaling, irreversibility field ( B .irr ), magnetic field ( B jp ) and current density( J p ) of the fishtail peak effect, and the field ( B fp ) corresponding to the maximum pinning force density ( F p,max ), were determined using the measurements of magnetically induced current density as a function of field and temperature. For the commonly used scaling of normalized volume pinning force, ( F p = J cxB a )/ F p,max , versus reduced magnetic fields h=B a/B irr , (where B a stands for applied magnetic field), the curves collapse into a universal curve with a peak corresponding to h=0.44. Based on the approach of Dew-Hughes [1], we interpret the position of this peak as a clear indication of a δT c-pinning type. Similar scaling was also obtained with the use of B pf instead of B irr, but poor scaling was obtained with the use of J p . Our results are compared with those for other Re (123) systems.

Processing of large Y1Ba2Cu3O7-x single domains forcurrent-limiting applications

Bulk-textured YBa2Cu3O7-x single domains could be usedfor current-limiting applications by cutting and assembling long meanders,which would be submitted to network electric fields before using a breaker tointerrupt a fault current. For that purpose, large YBaCuO single domains up to93 mm in diameter can be isothermally grown by using a standard melt texturinggrowth (MTG) process with a SmBaCuO seed. The essential parameters that haveto be controlled in order to reach this size are the temperature growthwindow, the substrate reactivity and the temperature homogeneity in thesample. Standard 45 mm diameter single domains show excellent superconductingproperties, such as Jc above 105 A cm-2 and a homogeneoussuperconducting-to-normal transition at 91.8 K for 20 cm long conductors cutin these pellets. These measurements demonstrate the long -range homogeneityof single domains regarding Tc and Jc. Nevertheless the high Jcvalues lead to a too large a dissipation in the normal state at T = 77 K.Different methods to reduce the critical current density are described inorder to fulfill the conditions for a safe recovery of the material afterundergoing a magnetothermal transition.

Effects of Precursors with Fine BaZrO3 Inclusions on the Growth and Microstructure of Textured YBCO

We prepared precursors with fine BaZrO3 inclusions for melt-texture growth of high-temperature superconductors with nominal composition of Y1.5Ba2Cu3Oy + x BaZrO3 (x = 0, 0.04, 0.08, and 0.1), by the process of liquid-phase powder melt-texture growth (LPMG). We then investigated the effects of the precursors on the texture growth and the microstructure of composite YBa2Cu3O7-δ/Y2BaCuO5 (YBCO). The BaZrO3 particles were about 100 nm in size in the textured YBa2Cu3O7-δ (Y123), observed by the scanning electron microscopy (SEM). Owing to the pushing effects of growing fronts, fine particles were pushed out of and thus gathered in front of the growing fronts or between Y123 grains. The gathered particles in turn blocked the further growth of the Y123 grains. However, BaZrO3 particles seeded Y123 grains, giving rise to multigrain growths. These negative effects of BaZrO3 fine particles exclude themselves as effective flux pinning centers in the textured YBCO.

Fabrication and characterization of large Nd–Ba–Cu–O grains prepared under low oxygen pressure

A large, single-grain Nd–Ba–Cu–O (NdBCO) composite consisting of superconducting NdBa2Cu3O7-δ containing nonsuperconducting Nd4Ba2Cu2O10 phase inclusions was fabricated up to 2 cm in diameter using a top-seeded melt-textured growth technique. A MgO single-crystal seed was used to provide a heterogeneous nucleation site at the center of a presintered pellet heated above its peritectic temperature and cooled continuously in a conventional tube furnace in reduced oxygen partial pressure. This process produces individual grains with the c axis oriented at ≈10° to the seed surface which, from vibrating-sample magnetization measurements, exhibit a pronounced peak effect in their magnetic moment over a wide temperature range (50–90 K) when the supercurrent flows in the a-b planes. A very high irreversibility field (&gt;9 T at 77 K) is also observed in these grains for field applied both perpendicular and parallel to the crystallographic c axis which is significantly greater than that observed in good-quality melt-processed Y–Ba–Cu–O. These results underline the potential of NdBCO for high-field engineering applications.

Effects of target microstructure on pulsed laser deposited YBa 2Cu 3O 7− δ thin films

Superconducting YBa 2Cu 3O 7− δ thin films were prepared by pulsed laser deposition using two targets; one was fabricated by solid-state sintering (SSS) and the other was fabricated by modified melt-textured growth (MTG). Microstructures of the targets and the films were investigated using X-ray diffraction and scanning electron microscopy. Also, superconducting properties of the films were examined by electrical measurements. When compared with SSS targets, MTG targets showed a denser microstructure consisting of larger grains, and the surfaces of MTG targets were less modified with the cumulative laser incidence. The films deposited from MTG targets showed better-aligned c-axis orientation, a higher density microstructure, and exhibited enhanced superconducting properties as compared to the films prepared from SSS targets. These results were ascribed to higher kinetic energies and an improved uniformity of the vapor species ejected from MTG targets.

Comparison of microstructures of pulsed laser deposited YBa 2Cu 3O 7−δ thin films using solid-state sintered and modified melt-textured grown targets

YBa 2Cu 3O 7−δ (YBCO) thin films were deposited by pulsed laser deposition (PLD) using differently prepared targets. The targets were fabricated using both solid-state sintering (SSS) and modified melt-textured growth (MTG) methods. The films deposited from MTG targets had better superconducting properties, such as T c, Δ T and J c, than the films deposited from SSS targets. In order to understand the enhanced superconducting properties, the film microstructures were investigated using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). As the laser energy density was increased, the films showed an increase of a-axis outgrowth. However, the observed a-axis outgrowth formation indicated that the deposition using MTG targets would result in a more homogeneous and stable film growth as compared to SSS targets. Also, TEM analysis implied that the film deposited from MTG targets had an enhanced grain connectivity that would reduce weak-link effects in the film. Thus, these microstructural differences of films deposited from MTG targets were considered to result in higher J c than the films deposited from SSS targets.

DyBa 2Cu 3O 7− x growth on different polycrystalline Dy 2O 3 interacting layers

We have considered two types of interacting substrates for testing the melt-textured growth of DyBa 2Cu 3O 7− x , i.e., a sintered Dy 2O 3 or a compacted Dy 2O 3 powder. Resulting microstructures are compared. Compacted powders are found to provide the most developed 123 grains. Physical and chemical arguments are given for such findings.

The effect of the grain alignment on the levitation force in single domain YBa 2Cu 3O y bulk superconductors

Single-domain YBa 2Cu 3O y (YBCO) bulk superconductors ( Φ 30×15 mm 3) with high levitation force of 70 N have been fabricated by modified melt-textured growth process. The sample was cut into a group of small rectangular samples with the same dimension of 3.5×3.5×5 mm 3 along different grain-orientation. The levitation force of the small samples has been measured at liquid nitrogen temperature. It is found that the levitation force decreases monotonously from θ=0° to 90°, where θ is the angle between the c-axis and the normal to the polished surfaces. A simple model has been proposed to interpret the experimental result, which is in good agreement with the experimental data.

Microstructure and transport properties of YBCO zone melted samples processed in a microwave cavity and infra-red furnace

The unidirectional solidification method was used to prepare well textured YBa 2Cu 3O 7− δ (Y123) bars for current lead applications using two different processes. The first one consists in using the Melt Textured Growth method (MTG) in a classical furnace, applying an axial thermal gradient ( G) of about 60°C/cm. In the second process, a microwave cavity working at 2.45 GHz is used, allowing to obtain larger G (about 260°C/cm). The different values of G corresponding to the two processes were considered to study the effect of G on the Y123 growth rate, microstructure, crystal orientation and transport properties. Thus, for both processes, the maximum pulling rate which permits a continuous growth of Y123 was determined. Microstructure of samples was studied, using polarised optical microscopy for each process. Moreover, the microstructure was correlated with the transport properties of textured bars, using pulse current with the four-point technique. Although the crystal orientation was not perfect ( a– b planes are not aligned with the bar axis), high values of self field J c (higher than 20 kA/cm 2) along several centimetres were measured with a high reproducibility for samples presenting single-grain features independently on the texturing process. However, when the pulling rate increases ( R>2 mm/h), the measured J c values were higher for samples obtained using the microwave melt process than for samples processed under lower thermal gradient. The difference in the term of J c is well correlated with the different microstructure.

Substrate Effects On The Surface Morphology Of High - Temperature Superconductor Layers

Layers of superconductor TNa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} materials synthesized from oxides have been sedimented on ceramics and single crystal substrates and fired in accordance with melt textured growth (MTG) technology. Results of microscopic studies of surfaces of the layers are reported. The effect of substrate material on the structure formation of the layers is discussed.

Localized superconducting to normal transition at the grain boundary for transport critical current in YBa2Cu3O7−δ

Local measurements of the critical current (Ic) at 77 K have been made on YBa2Cu3O7−δ consisting of mm sized textured domains denoted “grains”. The samples were cut and polished into thin slices less than 500μm thick in order to isolate various grain boundaries. In one case a current path across the grain boundary was observed to strongly reduce Ic, compared to the values obtained for current paths in the neighboring grains. The superconducting to normal transition at Ic was also shown to be localized at the grain boundary. In two other cases, the grain boundary was observed to modify only slightly the superconducting characteristics of the material. In a polycrystalline form of the ceramic superconductor YBa2Cu3O7−δ the presence of grain boundaries has been deduced to be one of the major factors limiting the critical current (Ic). This conclusion is essentially based on the three following observations. First, the critical current at 77 K, Ic, decreases strongly on application of an external magnetic field which is the characteristic of a Josephson junction. The grain boundaries are thus considered to be at the origin of such weak link behavior [1]. Second, experiments carried out on thin film bicrystal show strong attenuation in transport Ic when the superconducting current path crosses the grain boundary [2]. Third, forms of YBa2Cu3O7−δ containing a reduced number of grain boundaries, such as textured material or thin films, yield measured values of critical current density (Jc) which are higher than the values obtained for small grain ceramics by one order of magnitude or more [3, 4]. This letter discusses the local electrical response around the grain boundaries in ‘bicrystals’ cut out of large grain ceramics fabricated by a method close to melt textured growth. A particular attention is paid to the local nature of the transition to the normal state at the boundary when the transport critical current is just exceeded. With this type of behavior the sample can be considered to be consti-

A microwave melt texture growth process of YBa 2Cu 3O 7- δ

Microwave heating in a single mode cavity can produce large thermal gradients ( G>150°C cm −1), so this process was used to grow large YBa 2Cu 3O 7- δ (Y123) textured domain bars by using the classical MTG process operating in a TE102 cavity. Thus, this paper is devoted to presenting this new technology involving a material-conducting microwave interaction and describe a device allowing microwave heating of Y123. In this paper, a model has been developed concerning microwave–matter interactions for conducting materials and a study of the relevant parameters which control the process has been performed. Thus, a temperature regulation algorithm has been presented. In terms of material processing, we applied our results to a Y 2BaCuO 5 rich Y123 composition doped with cerium dioxide, CeO 2, and the growth of a large quasi-single domain at a 2 mm h −1 growth rate in a microwave cavity was obtained. Owing to the particular heating mechanism, there is no thermal inertia leading to a faster process than the classical. Besides, the good superconducting properties obtained demonstrates that this original process can be competitive with the classical MTG process operating in a vertical furnace.