Melt-textured YBCO Research Articles

Trapped Fields of Hot-Pressed MgB2 for Applications in Liquid Hydrogen

The development of hydrogen energy and, in particular, high-performance submersible liquid hydrogen (LH) pumps requires superconducting bearings which can trap magnetic fields up to 1 T at 20 K. MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> is hence a promising candidate for this application. The superconducting properties and microstructure of differently prepared MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> were compared and the ability of different composite materials to trap magnetic fields was studied. Hollow cylinders of the same geometry were manufactured from hot pressed (under 30 MPa) blocks prepared from Mg:2B with Ti, TiC and Ti-O additives as well as from melt-textured YBCO ceramics. The high critical current densities and critical magnetic fields should ensure high trapped fields in all these materials. Indeed all materials demonstrated the required performance; however, flux jumps are a serious issue in MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> even in crack free cylinders and impeded higher trapped fields.

Influence of Oxygen Concentration and Distribution on Microstructure and Superconducting Characteristics of MgB2-Based Materials and Melt-Textured YBCO

The oxygen concentration and distribution in the microstructure of MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> - and YBa <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7-δ</sub> -based materials affect the formation of nanostructural defects and thus influence the critical current density, upper critical magnetic field and irreversibility field. For MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> oxygen containing additions (Dy-O, Ti-O) in the form of nanograins occurred not to be very effective for an increase of critical current density, <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">j</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> . Sn-O additions to MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> can provoke even a significant decrease of <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">j</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> due to a chemical interaction of Sn with MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> . The processing pressure and temperature influence the defect density of the relevant pinning centers and the regularity of their distribution in doped and undoped MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> as well as in MT-YBCO.

Development of a superconducting joint between a GdBa2Cu3O7-δ-coated conductor and YBa2Cu3O7−δ bulk: towards a superconducting joint between RE (Rare Earth) Ba2Cu3O7−δ-coated conductors

We have started to develop a superconducting bridge joint between two GdBa2Cu3O7−δ (Gd123)-coated conductors, where both conductors are placed in an end-to-end arrangement on the surface of a melt-textured YBCO (including Y2BaCuO5 and YBa2Cu3O7−δ) bulk, which acts as a superconducting medium between the coated conductors. As a first step in the development, one half of the bridge joint assembly was modeled and investigated. Experimental results achieved are as follows: (a) the higher-melting-temperature textured Gd123-coated conductor acts as a seed for the melt texture of the YBa2Cu3O7−δ (Y123) bulk, and (b) the superconducting phase continues across the Y123/Gd123 boundary. The critical current of the joint model is 10 A, which is about 10% of the original Gd123-coated conductor, at 77 K in a self-magnetic field. These results are considered to be extensible to the superconducting bridge joint between the Gd123-coated conductors.

Production run of 2 cm diameter YBCO trapped field magnets with surface field of 2 T at 77 K

A production run of 60 melt-textured YBCO trapped field magnets (TFMs), 2 cm in diameter, is reported. The TFMs have pinning centers of Y211 and (Pt0.4U0.6)YBa3O6 deposits and also damage tracks of ions from uranium fission. The resulting average trapped field at the center of the seed-side surface is 〈BT(r = 0)〉 = 2.04 T at 77 K. The rms spread in BT is 5.9%. TFM samples rejected due to problems with the SmBCO seeds totaled ∼10%, and an additional 2% were lost due to cracking under magnetic pressure. Methods to significantly reduce these losses are discussed. The ratio of BT on the TFM bottom to that on the seed side is ∼67%. Evidence of a method to raise this ratio to 1.0 is presented. The magnitude and time dependence of the radioactivity resulting from the uranium fission are reported.

Flux pinning effect in a melt textured YBCO bulk evaluated by using tension measurements

Research on the flux pinning effect in type-II superconductors has usually been focused on microor nanosize pinning centers, and mm-sized pinning centers have been relatively less studied. In order to investigate the flux pinning effect caused by mm-sized pinning centers, we introduce a tension measurement method in this research. A cm-sized melt-textured YBCO bulk, in which holes with a 2 mm diameter are made, is prepared. The YBCO bulk is field-cooled by using a strong magnet in liquid nitrogen, and the bulk and the magnet are separated from each other. Then, an attractive force (fa) between them is generated, and fa is detected by using a tension measuring device. As the distance (d) between them is increased, fa increases at short distance and decreases at long distance, showing a maximum value, fam, at a specific distance. The measurement of fa is stopped when d reaches a value defined as the breaking distance (dbk), as if a ‘string’ between the magnet and the YBCO bulk is broken. As the number of holes (n) made in the YBCO bulk increases from 1 to 6, fam and dbk increase, in spite of the superconducting volume loss. fam and dbk for n ≥ 7 converge to nearly constant values, which are smaller than the values for n = 6. This means that the critical current density can be calculated by using fam or dbk for a sufficient number of holes.

Interaction mediated by size differences between Y 2BaCuO 5 and CeO 2 particles in melt-textured YBCO superconductors

The Refining of Y 2BaCuO 5 (Y211) is crucial in the fabrication of YBa 2Cu 3O x (Y123) bulk superconductors with a high critical current density ( J c ) for power engineering applications. In this work, the influence of an addition of Y211 and CeO 2 of different sizes on the Y211 refinement and superconducting properties of melt-textured Y123 superconductors is investigated. Nano-size Y211 and nano-size CeO 2 were prepared by attrition-milling (250 rpm and 2 h). Using mixed powders consisting of 85 wt.% Y123, 15 wt.% Y211, and 1 wt.% CeO 2, Y123 quasi-single crystals were fabricated via a top seed melt-textured growth process. The interaction between Y211 and CeO 2 of different particle sizes was examined in terms of the size, uniformity and density of the Y211 particles that existed in the melt-textured samples. The reaction between the nano-size Y211 and the nano-size CeO 2 was more violent due to the more frequent collisions compared to the other reactions (micro-size Y211/nano-size CeO 2 or micro-size Y211/micro-size CeO 2), resulting in the most effective for refinement of Y211 particles and the best J c value of 4.2 × 10 4 (A/cm 2) (at 77 K and a zero-field).

Nano-mechanical properties of silver-welded YBCO bulks

The mechanical properties of melt-textured YBCO bulk boundaries are a topic of special interest especially in the manufacturing of superconductor devices for engineering applications. Instrumented indentation technique is a powerful tool widely employed to characterize the mechanical properties of small volumes. We presented results of the nanohardness (H) and Young's modulus (E) across [001]-welds fabricated by silver welding. The residual imprints performed by a sharp tip indenter were observed by Field Emission Scanning Electron Microscope (FE-SEM) to investigate the different fracture mechanisms nucleated during the indentation process. Results show a decrease of the nanohardness across the grain boundary related to the appearance of a "welding path" free of Y-211 particles. The FE-SEM images show chipping and sink-in effects in that region. Remarkably, for zero-angle welds the mechanical properties are unaltered across the joint, thus probing the high quality of silver welds for large-scale applications.

Enhanced Growth Rate of a YBCO Bulk in the Melt-Textured Process Under 1 atm Oxygen Pressure

By the employment of 1 atm oxygen pressure as the growth atmosphere, we report a success in the enhanced growth rate of the melt-textured (MT) YBCO bulk. A well-oriented c -axis single-domain bulk with a size of 42 mm in diameter was fully grown at a high average growth rate of about 0.32 mm/h, which is 1.5 times larger than that in air. It was found that the increased oxygen partial pressure has a profound influence on the MT growth (MTG) for YBCO. The enhanced growth rate can be interpreted by both increased solubility and diffusion coefficient of yttrium in the Ba-Cu-O liquid under high oxygen partial pressure. After oxygenation, single-domain samples were determined to have a sharp superconducting transition temperature at 91.7 K, indicating that the superconductivity of crystals grown under high oxygen pressure does not degrade. Additionally, due to the rapid growth, the trapping effect of Y211 particles becomes predominant. As a result, a relatively more homogeneous distribution of Y211 particles was observed in the Y123 matrix. Finally, the most important advantage of this technique is that the enhanced growth rate of Y123 gives less opportunity from self-nucleation in the liquid, which takes place beyond the growth window and the embryonic period. Thus, this oxygen-increased MTG process is very promising for producing large-sized YBCO bulks with good superconducting properties.

Effects on Jc of pinning center morphology for multiple-in-line-damage in coated conductor and bulk, melt-textured HTS

The properties of discontinuous aligned pinning centers (PCs) created by high-energy heavy-ions are compared for bulk melt-textured and coated conductor HTS. Properties of PCs, which increase J c (pinning potential and entanglement), and negative properties which decrease J c (e.g., decreased T c and percolation paths) are evaluated. Mechanisms are proposed to explain the very large increases in J c resulting from multiple-in-line-damage (MILD) compared to continuous columnar pinning centers (CCPC). In particular, a mechanism which results in fluxoid entanglement, even for parallel (unsplayed) PCs, is discussed. The same mechanism is found to also account for restoration of much of the pinning potential expected to be lost due to the gaps in MILD PCs. It also accounts for the fact that at high fluence, J c increases as fluence is increased, instead of decreasing as expected. The very low self-field in coated conductor permits separation of the negative and positive effects of PCs. It is found that parameters developed to quantify the negative effects in bulk melt-textured YBCO, by 63 GeV U 238 ions, successfully describe damage to 2.1 μm thick coated conductor by 1 GeV Ru 44 ions. Coated conductor at 77 K and self-field is generally known to have J c about 100 times that of melt-textured YBCO. However, at 77 K and applied field of 1 T, when both forms of HTS are processed with comparable numbers of near-optimum MILD PCs, the difference in J c is reduced to a factor of 1.3–2. Whereas J c for melt-textured YBCO increased sharply, by a factor of up to 16.8 for high-fluence MILD PCs, J c in coated conductor increased by a smaller factor of 2.5–3.0. Nevertheless, 2.1 μm thick coated conductor, with near-optimum MILD PCs, exhibits J c = 543 kA/cm 2 at 77 K and applied field of 1.0 T, and I c = 114 A/cm-width of conductor. This is the highest value we find in the literature. The phenomenology developed indicates that for optimum MILD PCs in coated conductor, J c ∼ 700 ± 70 kA/cm 2 should be achievable at 77 K, 1.0 T.

Electron Microprobe Characterization of Melt-Textured YBCO Created Using Hybrid Powder Metallurgy

An YBCO monolith was created by conventional top-seeded melt-texture processing using a mixture of Y and YBa2Cu3O7-delta (Y123) powders. The composition of the bulk monolith thus lies in the Y-Ba-Cu oxide ternary system rather than on the binary join between Y123 and Y2BaCuO5 (Y211) and the liquid line of descent, therefore, differs from the peritectic one exhibited by Y211-Y123 mixtures. In particular, Y123 crystallizes over a temperature interval that may be as large as 50degC and from a liquid with a considerable range in Cu content. Y123 in the Y-Y123 monolith exhibits a range in composition, particularly in Cu content that we ascribe to the change in crystallization conditions. This allows the spatial pattern of crystallization of the monolith to be mapped. Crystallization commenced at the seed crystal but then appears to have moved to the outer parts of the monolith before ending in the interior of the sample.

The Numerical Modeling and Measurement of Demagnetization Effect in Bulk YBCO Superconductors Subjected to Transverse Field

In this paper, the crossed field demagnetization effect of the bulk type-II superconducting materials in a variety of external magnetic field condition has been investigated both numerically and experimentally. A melt-textured YBCO bulk sample was magnetized along the axial direction and after that a transverse field perpendicular to the central axis applied. The collapse of the original trapped field was measured using a quantum design physical property measurement system. A numerical solver based on critical state model is proposed to simulate the measured results.

Coaxial magnetic brakes using single-domain YBCO

In coaxial magnetic brakes, the changing field produced by movement of a solenoidal magnet induces a current in the wall of a conductive tube. The interaction of the field and current leads to a repulsive force that slows the motion of the magnet. For brake applications that require high force density, melt-textured YBCO is a clear choice of material for the magnet because it can carry high currents at a given field and temperature, and is inherently capable of operating in persistent current mode. We present calculations of the performance of this type of brake as a function of magnet current density for catch tubes composed of aluminum and titanium. These results are validated with low speed (20 m/s) tests. Calculations indicate that melt-textured magnets can decelerate projectiles with a mass of 1 kg from 2000 m/s to rest in distances on the order of 10 m. This suggests that this type of brake is suitable for use in hypervelocity experiments, which sometimes requires nondestructive deceleration of projectiles for diagnostic purposes.

Li and Ce doping of melt-textured YBCO: Improved jc at medium fields

The top seeded melt textured growth method was applied to a Ce doped YBCO material with a partial substitution of Cu with Li. The amount of Li in the samples varied from 0.25 to 0.05 at% relative to the copper atoms. The change of T c and j c compared to a reference sample was measured using SQUID magnetometer. Field induced pinning was indicated by elevated j c in the j c( B) curve. The peak effect at 77 K appeared in the moderate field region around 0.5 T. Optimal lithium content was determined to be at 0.05 at%. Final Li content in textured bulk material was verified with atomic absorption.

EBSD analysis of melt-textured YBCO with embedded Ag-2411 nanoparticles

Nanoparticles of the type M-2411 (with M = U, Nb, Zr, Ag, etc.) can be embedded within the superconducting YBCO matrix to provide flux pinning sites. Among all these types of nanoparticles, the Ag-2411 particles are special in the sense that they may provide also metallic Ag particles to be formed within the superconductor. The addition of metallic Ag (or silver oxide) was shown to be a means to create a more stable system by filling cracks and voids. Using the advanced polishing method of the YBCO surfaces, we achieve a high EBSD image quality of the recorded Kikuchi patterns. In this contribution, we perform a four-phase EBSD analysis including YBa 2Cu 3O x , Y 2BaCuO 5, Ag-2411 and metallic Ag. As a result, we find nanometer-sized Ag particles being distributed within the YBCO matrix, and a reduced amount of Ag-2411 particles as compared to the nominal composition. The Ag-2411 particles are well oriented with the surrounding YBCO matrix as found earlier, but the metallic silver particles as detected by EBSD do not exhibit a preferred orientation.

Chemical characterization of melt-textured YBCO produced by hybrid powder metallurgy

Monolithic YBCO samples were made by traditional top-seeded melt-texturing processes from cold-milled mixtures of Y123 (YBa 2Cu 3O 7− δ ) and elemental Y. The bulk composition does not lie on the Y211 (Y 2BaCuO 5)–Y123 join so, formation of Y123 from liquid and Y211 is not an essentially isothermal process on cooling. The reaction liquid + Y211 = Y123 is a ternary reaction and occurs over several tens of degrees. The Y123 thus produced has a range in compositions – particularly in Cu:(Y + Ba) – which may reflect crystallization over the thermal interval. The liquid migrates to an invariant point at which CuO also crystallizes with complete loss of liquid. The presence of trains of CuO grains in the YBCO indicates the locations of the last liquids to be preserved in the sample. These trains are dominantly in an annulus 1–3 mm from the edge of the 20-mm diameter sample. Mapping the compositional variation in Y123 may allow mapping the path of crystallization of these monolithic YBCO samples.

Bulk YBCO with discontinuous irradiation defects: Bose-glass behaviour and very high critical current densities

Columnar defects were produced in melt-textured YBCO by irradiation with high-energy U238 ions at a constant matching field of Bϕ = 10 T and for several energy losses between Se = 1.67 and 2.4 keV/Å. The influence of discontinuous or multiple-in-line-damage (MILD) columnar defects in melt-textured YBCO on flux pinning and the vortex matter phase diagram was investigated. The critical current density jc(H,T) was found to strongly increase with Se due to the increasing length of the MILD pins. Simultaneously, the irreversibility field Birr(T) for fields along the c-axis progressively shifts upwards reaching 9 T at 77 K. For Se= 2.4 keV/Å, a pronounced kink is observed in Birr(T) at 8 T which is a strong indication for Bose glass behaviour. The data for the irradiated YBCO are compared with data for bulk YBCO in which a periodic array of nanoscale twin boundaries was obtained by RuO2 additions.

Suppression of magnetic levitation force in melt-textured YBa2Cu3O7-x superconductors by a transverse AC magnetic field

We have studied experimentally the influence of transverse ac magnetic fields on the levitation force arising between a permanent NdFeB magnet and a bulk melt-textured HTSC YBCO superconducting sample. The axes of superconducting disc and cylindrical magnet were coinciding while the transverse ac magnetic field generated by resistive coil was directed parallel to surface of a disc i.e., perpendicular to the disc axis. We found that application of both impulse and alternative transverse magnetic fields results in suppression of the value of levitation force and its relaxation rate. Namely, the variable magnetic field with amplitude 12 mT, that approximately in 20 times is less than field of a constant magnet, causes suppression of force more than twice. Monotonous behavior of value of levitation force reduction with the increase in transverse magnetic field amplitude was observed. The possible origin of observed phenomenon is discussed.

Strongly enhanced irreversibility fields and Bose-glass behaviour in bulk YBCO withdiscontinuous columnar irradiation defects

Flux pinning properties and irreversibility fieldsBirr(T) of melt-textured YBCO with discontinuous or multiple-in-line-damage(MILD) columnar defects produced by irradiation with high-energyU238 ions were studied at a constant matching field ofBϕ = 10 T and for severalenergy losses between Se = 1.67 and 2.4 keV Å−1. Withincreasing Se and increasing length of the MILD pins, the critical current densityjc(H,T) stronglyincreases and Birr(T) forfields along the c axis progressively shifts upwards reaching 9 T at 77 K. ForSe = 2.4 keV Å−1, a pronouncedkink is observed in Birr(T) at 8 T which is a strong indication of Bose-glass behaviour. Thejc(H) dependence of this sample shows a peak at a low applied fieldBp. This peak effect is explained by the entanglement of vortices. It is argued that for MILD pinssingle vortices interact simultaneously with many short columnar defects in neighbouring iontrails resulting in an automatic splay in the vortex orientation. The observed decrease ofBp with increasing temperature is estimated taking the increasing thermal fluctuations intoaccount.

Contactless method for measuring the parameters of the current-voltage characteristic of YBCO superconductors in a perpendicular magnetic field

A contactless method for determination of the parameters of the current-voltage (I—V) characteristic of hard superconductors by measuring the magnetization relaxation of the superconductor in a perpendicular magnetic field has been developed. The relaxation curves obtained for melt-textured YBCO samples are well approximated by the curves calculated within the electrodynamic model based on power-law I—V characteristics. This procedure uses only two fitting parameters: the critical current density J c and the exponent m of the I—V characteristic.

Fabrication of HTS Bearings With Ton Load Performance

The considerable development effort of HTS magnetic bearings with respect to heavy loads is demonstrated. Radial type 200 mm bearing in an optimized design was developed for implementation in a new 5 kWh superconducting flywheel energy storage system. A prototype bearing, consisting of a modular assembled melt textured YBCO bulk 200 mm hollow cylinder with permanent magnet rotor is presented. Axial rotor displacement causes pinning forces of 8 and 10 kN axially at a temperature of 78 K and 71 K, respectively. Maximum radial forces are 4.7 kN at sub- cooled LN2. Fabrication technology incorporates the construction and engineering of a low-weight, nonmagnetic G-10 bearing cryostat. Ultra-low heat transfer structural elements were designed and constructed to reduce the thermal loss through the support to less than 2 Watt under 1 ton load.