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Molecular beam epitaxy of K-doped Ba122 (Ba$_{1-x}$K$_x$Fe$_\text{2}$As$_\text{2}$) superconductor was realized on a MgO substrate. Microstructural observation revealed that the undoped Ba122 served as a perfect buffer layer for epitaxial growth of the K-doped Ba122. The film exhibited a high critical temperature of 39.8 K and a high critical current density of 3.9 MA/cm$^\text{2}$ at 4 K. The successful growth of epitaxial thin film will enable artificial single grain boundary on oxide bicrystal substrates and reveal the grain boundary transport nature of K-doped Ba122., 5 pages, 4 figures, accepted manuscript Supercond. Sci. Technol 2022

In order to improve the electrical transport properties of REBa2Cu3O7−δ nanocomposite films, SmBa2Cu3O7−δ films with and without BaHfO3 nanoparticles were grown by chemical solution deposition, and their microstructural and transport properties were investigated in a detailed study using transmission electron microscopy and transport measurements in magnetic fields up to 24 T. The optimization process of the crystallization step (temperature and oxygen partial pressure) as well as an aging effect, which is due to the release of trapped fluorine, are described. Critical temperature and critical current densities surprisingly improve initially during the aging. Due to the complex microstructure, the additional BaHfO3 nanoparticles have only a positive effect at low magnetic fields for our samples.

We report on the growth of NdFeAs(O,H) epitaxial thin films on MgO (001) substrates and their electrical transport properties. The maximum carrier density of the NdFeAs(O,H) films was more than twice as large as that of our NdFeAs(O,F) films. This enabled us to investigate the physical properties of heavily electron doped NdFeAsO, which could not be achieved with NdFeAs(O,F). The irreversibility field Hirr of NdFeAs(O,H) was larger than that of NdFeAs(O,F) for H || c due to the decrease in the anisotropy of the upper critical field. A very high critical current density of 17 MA cm^−2 was recorded at 4 K, which corresponds to about 13% of the depairing current density of NdFeAs(O,F)., ファイル公開：2021-09-01

Submicron sized strip lines of NdFeAs(O,F) were fabricated and the transport properties were studied. By using laser lithography and dry etching, strip lines with a width of down to 0.84 μm were fabricated. The superconducting critical temperature (T c) was not seriously affected by the fabrication process, but a significant decrease of the critical current density (J c) was observed for strip lines narrower than 2 μm when the sample was patterned by isotropic etching. By employing an anisotropic etching process, on the other hand, the deterioration of J c was largely prevented, and the yield of obtaining a strip line with J c higher than 5 MA/cm2 rose considerably.

Understanding the nature of grain boundary (GB) characteristics in combination with creating low-energy GBs by modifying the processing conditions, so-called GB engineering, is of great importance for controlling and reducing the defect density, leading to improved functionalities of polycrystalline metals and ceramics. For superconductors particularly, including both low- and high-temperature superconductors, GB engineering has been developed to improve especially the critical current densities, Jc, across these GBs. The intrinsic physical properties of a given superconductor such as the coherence length, the order parameter symmetry, and their anisotropies would determine the strategy of GB engineering. In this topical review, we present an overview of the GB characteristics and GB engineering of Fe-based superconductors (FBS) in the form of polycrystalline bulks and wires, and thin films with application potential, e.g. for high-field magnet wires. Prior to the FBS, GB engineering of the cuprates and MgB2 are also briefly covered., Invited review

We have systematically investigated the grain boundary (GB) angle dependence of transport properties for NdFeAs(O,F) fabricated on [001]-tilt symmetric MgO bicrystal substrates. In our previous study, NdFeAs(O,F) bicrystal films showed a weak-link behaviour even at a GB angle of 6{\deg}. However, this was caused by an extrinsic effect originating from the damage to both NdFeAs(O,F) and MgO substrate by excess F-diffusion along the grain boundary. To investigate the intrinsic nature of grain boundaries, we minimized the damage to NdFeAs(O,F) and MgO by reducing the deposition temperature of NdOF over-layer needed for F-doping. The resultant NdFeAs(O,F) bicrystal films have a critical angle of 8.5{\deg}, above which Jc starts to decrease exponentially. This critical angle is almost the same as those of other Fe-based superconductors., Comment: Accepted in Superconductor Science and Technology

The effect of $\alpha$-particle irradiation on a NdFeAs(O,F) thin film has been investigated to determine how the introduction of defects affects basic superconducting properties, including the critical temperature $T_c$ and the upper critical field $H_{c2}$, and properties more of interest for applications, like the critical current density $J_c$ and the related pinning landscape. The irradiation-induced suppression of the film $T_c$ is significantly smaller than on a similarly damaged single crystal. Moreover $H_{c2}$ behaves differently, depending on the field orientation: for H//c the $H_{c2}$ slope monotonically increases with increasing disorder, whereas for H//ab it remains constant at low dose and it increases only when the sample is highly disordered. This suggests that a much higher damage level is necessary to drive the NdFeAs(O,F) thin film into the dirty limit. Despite the increase in the low temperature $H_{c2}$, the effects on the $J_c$(H//c) performances are moderate in the measured temperature and field ranges, with a shifting of the pinning force maximum from 4.5 T to 6 T after an irradiation of $2\times10^{15} cm^{-2}$. On the contrary, $J_c$(H//ab) is always suppressed. The analysis demonstrates that irradiation does introduce point defects acting as pinning centres proportionally to the irradiation fluence but also suppresses the effectiveness of c-axis correlated pinning present in the pristine sample. We estimate that significant performance improvements may be possible at high field or at temperatures below 10 K. The suppression of the $J_c$(H//ab) performance is not related to a decrease of the $J_c$ anisotropy as found in other superconductors. Instead it is due to the presence of point defects that decrease the efficiency of the ab-plane intrinsic pinning typical of materials with a layered structure., Comment: 12 pages, 9 figures

Silver-doped Y?Ba?Cu?O (YBCO) bulk superconductors have been fabricated successfully in the form of large, single grains by a seeded infiltration and growth (IG) technique. A uniform distribution of silver in the superconducting YBa2Cu3O7?? (Y-123) phase matrix was achieved by adding Ag2O to both the Y2BaCuO5 (Y-211) pre-form and a liquid source precursor pellet. Although Ag can also be dispersed well in Ag-doped YBCO fabricated by the top-seeded-melt-growth (TSMG) technique, bulk samples prepared by the IG technique typically exhibit significantly lower porosity due to the absence of the formation of oxygen during the peritectic decomposition process within the Y-211 pre-form. This yields samples with potentially enhanced current-carrying ability. The density of voids in bulk samples can be decreased further by adding metallic Ag, rather than Ag2O, to the Y-211 pre-form in the IG process. The distribution of Y-211 in the Y-123 matrix in samples fabricated by both IG and TSMG is rather inhomogeneous. As a result, the superconducting properties of individual small samples cut from large parent grains are similar for samples fabricated by both IG and TSMG.

Insulating RE2BaCuO5 (RE-211) second-phase inclusions embedded within the REBa2Cu3O7−δ (RE-123) superconducting phase matrix are known to contribute significantly to flux pinning in melt-processed superconductors. The critical current density in these materials has been observed to increase significantly in the presence of low external magnetic fields, in particular due to the presence of the RE-211 phase. Jc at higher applied magnetic field, however, is unaffected by these inclusions. Recently we have introduced successfully nanoscale (RE)2Ba4CuMOy (RE-2411, where M = W,Nb,Ag and Bi) phase inclusions into the RE-123 phase matrix using a top-seeded melt growth (TSMG) technique. Here we report the flux pinning behaviour of various REBa2Cu3O7−δ/(RE)2Ba4CuMOy nanocomposites and compare their properties with those of REBa2Cu3O7−δ/RE2BaCuO5 single-grain composites. A direct correlation between increased Jc and increased concentration of nanoscale inclusions is observed. Finally, the presence of RE-2411 nanoscale inclusions is observed generally to improve both Jc and the irreversibility field of bulk RE–Ba–Cu–O superconductors processed by TSMG.

A Mg-doped Nd–Ba–Cu–O (NdBCO) generic seed crystal has been developed recently for the fabrication of any type of rare earth (RE) based (RE)-Ba–Cu–O single grain bulk superconductor. The new generic seed significantly simplifies the top seeded melt growth (TSMG) process for light RE based (Nd, Sm, Gd or mixed rare earth elements) bulk superconductors in particular. In this paper we report that single domain, bulk GdBCO samples have been fabricated successfully in air using the new seed in a cold seeding process. In this study, precursor powders were enriched with different amounts of BaO2 to investigate the extent of substitution of Gd for Ba in the Gd1+xBa2−xCu3O7−δ (Gd-123) solid solution phase. The particle size of the Gd2BaCuO5 (Gd-211) phase inclusions in the Gd-123 phase matrix and the spatial variation of Tc and ΔTc along the a/b and c-axes of single grains fabricated with varying amounts of externally added BaO2 have been investigated as a function of BaO2 content. Jc values as high as 7.2 × 104 A cm−2 at 77 K and 0 T have been observed in large GdBCO grains processed under air.

Large, single grains of Y?Ba?Cu?O (YBCO) bulk superconductor composed of a YBa2Cu3O7?? (Y-123) matrix with Y2BaCuO5 (Y-211) inclusions have been fabricated using a seeded infiltration and growth (IG) process. Y-211 precursor pellets prepared from powders containing different Y-211 particle sizes were processed by the seeded IG technique to investigate the effect of Y-211 size on the single grain microstructure. The superconducting properties of the single grains fabricated in this study are observed to correlate well with the variation in Y-211 particle size in the fully textured Y-123 phase matrix. The critical current density, Jc, in particular, has been observed to increase from 68?000?A?cm?2 for a reference sample to 94?000?A?cm?2 at 77?K in self-field for a sample prepared using refined Y-211 precursor powder with added platinum oxide.

Single-grain Gd–Ba–Cu–O (GdBCO) bulk superconductors have been grown by a seeded infiltration and growth (SIG) technique under a 1% O2+N2 atmosphere using a generic MgO-doped Nd–Ba–Cu–O (MgO–NdBCO) seed placed on the sample surface at room temperature (the so-called the cold-seeding method). Partial melting of the MgO–NdBCO seeds fabricated in air under notionally identical thermal processing conditions, however, limited the reliability of this bulk GdBCO single-grain process. The observed seed decomposition is attributed to the dependence of the peritectic temperature Tp of MgO-doped Nd1+xBa2−xCu3Oy solid solution (MgO-doped Nd-123ss, where ss indicates solid solution) compounds on both oxygen partial pressure during the melt process and the level of solid solution (x). The peritectic decomposition temperature of MgO-doped Nd-123ss, with x ranging from 0 to 0.5 under p(O2) = 1.00 atm, was observed to remain constant at 1120 °C. Tp was observed to decrease linearly as a function of solid solution level, on the other hand, under oxygen partial pressures of both p(O2) = 0.21 and 0.01 atm. Based on these results, MgO-doped NdBCO seed crystals should be grown under reduced oxygen partial pressure in order to obtain a stable MgO-doped NdBCO seed crystal suitable for cold-seeding processes of large-grain (RE)BCO bulk superconductors (where RE is a rare earth element).

We report a practical processing method for the fabrication in air of large, single grain LRE-Ba–Cu–O [(LRE)BCO, where LRE = light rare earth ] superconductors in a simple box-type furnace that exhibit high Tc and Jc. The technique is potentially economical, offers a significant degree of freedom in terms of both processing parameters and reproducibility in the growth of oriented single grains and yields bulk samples with significantly improved superconducting and field-trapping properties. The process is based initially on the development of a new type of generic seed crystal that can promote effectively the epitaxial nucleation of any (RE)-Ba–Cu–O system, and secondly on the suppression of the formation of (LRE)/Ba solid solution in a controlled manner within large (LRE)BCO grains processed in air. The characteristics of the generic seed are discussed in detail. In addition, the degree of homogeneity of large grain Nd–Ba–Cu–O, Sm–Ba–Cu–O and Gd–Ba–Cu–O superconductors fabricated in air by this novel processing technique has been investigated from the spatial distribution of the superconducting transition temperature and the critical current density along both the a and c growth sectors of each single grain. As a result, the optimum processing conditions for the fabrication of single grains with improved homogeneity and with high Jc have been established. Finally, Jc of 105 A cm−2 has been measured for large grains of Gd–Ba–Cu–O bulk superconductor at 77 K.

We report the seeded infiltration and growth (IG) of Gd–Ba–Cu–O (GdBCO) bulk superconductors for varying molten Ba–Cu–O liquid compositions using an Mg-doped Nd–Ba–Cu–O (Mg-NdBCO) melt-textured seed crystal developed recently at the IRC. All the samples were grown in the form of single grains, although the size of the grain was observed to decrease for an increasingly Ba-rich initial composition. The size of Gd2BaCuO5 (Gd-211) particles in the GdBa2Cu3Oy (Gd-123) phase matrix is almost constant for samples with different BaO2 content in the liquid source pellet. The volume fraction of Gd-211 in the Gd-123 matrix (Vf211), however, decreases significantly with increasing BaO2 content. The superconducting transition temperature, Tc, and transition width within the entire seeded IG bulk GdBCO grain are also extremely sensitive to BaO2 concentration. A maximum critical current density, Jc, of 88 000 A cm−2 at 77.3 K in self-field is observed in this study in the sample containing the greatest concentration of BaO2 in the liquid source pellet grown in air and without further Ar and/or low oxygen heat treatment (which would give further improvements in the superconducting properties of the samples). The highest value of Jc observed in this study of 115 000 A cm−2 at 77.3 K in self-field was obtained in the sample grown in a 1% O2+N2 atmosphere. These results indicate that the seeded infiltration and growth technique produces effective pinning centres in the bulk microstructure, at least in the self-field limit.

Nanocomposite Y?Ba?Cu?O superconductors containing a wide range of Y2Ba4CuMOy (Y-2411), where M = Nb, Ta, Mo, W, Zr, Hf, Ru, Ag, Sb, Sn?and?Bi, embedded in the YBa2Cu3O7?? matrix, have been fabricated successfully. The Y-2411 phase typically has dimensions in the range 20?50?nm and it is observed to form effective flux pinning sites even at higher magnetic fields. A direct correlation between the Y2Ba4CuMOy phase inclusion content in the YBa2Cu3O7?? matrix and increased magnetic flux pinning has been observed. The critical current density increases by an order of magnitude for a nanocomposite compared to that of Y-123 without inclusions.

Single domain Y?Ba?Cu?O (YBCO) composed of a YBa2Cu3Oy (Y-123) superconducting bulk matrix with discrete, non-superconducting Y2BaCuO5 (Y-211) phase inclusions has been fabricated by a seeded infiltration and growth (IG) technique in the form of cylindrical pellets up to 32?mm in diameter. Sample shrinkage in the radial direction for single domains prepared by this technique is relatively low at 5% and independent of sample size, in contrast to the shrinkage observed in samples grown by conventional melt processing, which increases significantly with increasing sample diameter. Furthermore, samples grown by the IG technique exhibit low porosity of typically 0.9% of the bulk volume fraction, compared with a corresponding value of around 4.9% observed in samples fabricated by conventional melt processing. Fine Y-211 particles were observed to be embedded within the Y-123 superconducting matrix for the IG processed samples, leading to a high critical current density, Jc, of over 100?000?A?cm?2 at 77.3?K in self-field. The distribution of Y-211 particles in the IG sample microstructure, however, was inhomogeneous (unlike in previous reports), which leads to a variation in the spatial distribution of Jc. The volume fraction of Y-211 in the vicinity of the seed crystal (i.e.?corresponding to the initial c-sector growth stage), in particular, is typically around 5%, compared with a value of up to 30% in the a growth sectors more distant from the seed crystal (which corresponds well to the theoretical value for the sample composition studied here). The volume fraction of Y-211 inclusions in the c growth sector more distant from the seed was around 22%. Finally, a trend of the variation in the distribution of Y-211 particles in the Y-123 matrix grown by the IG technique was similar to that in sample grown by conventional melt processing.

The effect of the addition of BaZrO3 and Y Ba2ZrO5.5 zirconium-containing compounds on the microstructure and superconducting properties of single-domain Y–Ba–Cu–O (YBCO) bulk superconductors has been investigated for various levels of Zr content. The distribution of Y2BaCuO5 (Y-211) particles along both the crystallographic a and c axes of the Y Ba2Cu3Oy (Y-123) superconducting matrix is observed to be rather discontinuous in Zr-containing samples, even with a small amount of secondary phase addition. The YBCO microstructure is observed to separate into regions of high and low concentrations of Y-211 particles when Zr-containing compounds are added to the precursor powder. Furthermore, Y-211 particles are observed to accumulate along the growth sector boundaries in the Y-211 free region of the Y-123 matrix. The extent of the Y-211 free region increases with increasing zirconium content. Increased concentrations of zirconium-containing compounds in the precursor powder trigger an undesirable segregation within the melt-processed microstructure of a band consisting of a liquid channel and a high concentration of Y-211 particles in both a and c axis growth sectors. The addition of BaZrO3 and Y Ba2ZrO5.5 to the precursor powders does not affect the superconducting transition temperature, Tc, of the bulk material. Critical current densities, Jc, of around 60 000 and 47 000 A cm−2 in self-field at 77.3 K were recorded for the samples containing 0.329 mol% BaZrO3 and 0.658 mol% Y Ba2ZrO5.5, respectively.

The effect of nano-size ZrO2 powder addition on the microstructure and superconducting properties of Y–Ba–Cu–O single grain bulk superconductors has been investigated. Significant pushing phenomena of Y2BaCuO5 (Y-211) particles, particularly in the c growth sector, were observed even with a small amount of ZrO2 added to the precursor powder. An increase in Y-211 particle pushing was observed with increasing ZrO2 content, leading to an inhomogeneous bulk microstructure. In addition, a growth cycle consisting of a Y-211 free layer–porous narrow layer–Y-211 high concentration layer was observed for samples prepared with 0.25 wt% ZrO2 in both the a and c growth sectors. The extent of the growth of single grain Y–Ba–Cu–O (YBCO) decreased with increasing ZrO2 content due to increased pushing of Y-211 particles towards molten liquid. The superconducting transition temperature, Tc, of the ZrO2 containing YBCO samples was sharp but depressed slightly (by up to 1 K) compared with an undoped YBCO grain, indicating a relative insensitivity of Tc to ZrO2 content. Finally, the sensitivity of critical current density, Jc, to applied magnetic field in large grain bulk YBCO containing ZrO2 was observed to depend critically on position due to the inhomogeneous sample microstructure.

Irreversible fields Birr of melt-processed (Sm,Eu,Gd)Ba2Cu3O7−δ bulk superconductors with varying (Sm,Eu,Gd)2BaCuO5 concentrations have been measured with pulsed magnetic fields up to 28 T in a temperature range from 55 to 85 K. The temperature dependence of Birr was studied by taking account of the effect of heat generation. In comparison with that including 5 mol% (Sm,Eu,Gd)2BaCuO5 phase the samples with 40 mol% (Sm,Eu,Gd)2BaCuO5 inclusion displayed a lower Birr value for temperatures above 75 K, while Birr was enhanced below 75 K. TEM observation unveiled a sharp contrast between a uniform nano-scaled compositional modulation in compositions with 40% (Sm,Eu,Gd)2BaCuO5 inclusion and an inhomogeneous distribution with very low (Sm,Eu,Gd)2BaCuO5 concentration. The present result could thus be understood in the context of different defect states induced by locally compositional fluctuation.

The effect of size, morphology and crystallinity of seed crystals on the nucleation and growth of melt processed Y–Ba–Cu–O (YBCO) during top seeded melt growth (TSMG) has been investigated. Conventional seeding of bulk YBCO with a small seed crystal of square planar or irregular geometry leads to point nucleation of the YBa2Cu3O7−δ (Y-123) phase and a subsequent square planar c-sector growth habit. The use of seeds of triangular and round geometries, however, results in a deviation from this growth habit symmetry. The use of large surface area seed crystals of dimensions 20 mm × 10 mm, on the other hand, influences the growth of bulk YBCO in two significant ways. Firstly, rapid epitaxial nucleation of the Y-123 phase occurs beneath the seed crystal, followed by grain growth via the usual peritectic solidification in the incongruent melt. Secondly, a large area seed reduces the processing time of large single grains compared to that achieved by TSMG with small area seeds. The present study is particularly relevant to the fabrication of single-grain samples with controlled growth sectors, reduced processing times and the multiple seeding of large area bulk materials containing good grain boundaries.

Ag-added Y–Ba–Cu–O bulk superconductors were joined using Ag-added Er–Ba–Cu–O solder and different Er211 contents. Microstructural analysis revealed that the volume fraction of Er211 at the joint with a composition of Er123:Er211 = 4:1 was the same as that for the expected volume fraction of the initial composition. That is, this composition seems to be optimum, since the mass balance was maintained during the crystal growth from the initial growth stage to the final stage. The trapped-field distribution for the joint using this composition was uniform and the only single peak indicated that strong coupling was achieved.

The growth rate of Er–Ba–Cu–O bulk superconductor as a function of undercooling was investigated using an isothermal solidification technique. The behaviour of the growth rate of Er–Ba–Cu–O under various undercooling was similar to that of Y–Ba–Cu–O. Furthermore, macro-segregation of Er211 was also observed in the a and c growth regions. However, macro-segregation of Er211 was not observed in the Er123 crystal fabricated by a continual slow cooling method far from the seed crystal. The optimum concentration of Er211 was determined to be Er123:Er211 = 3:1 on the basis of microstructure analysis and SQUID measurements. Furthermore, the trapped field capability as well as the critical current density of Er–Ba–Cu–O bulk superconductor with an optimum composition fabricated by a continual slow cooling method employing very small particles of Er211 was dramatically improved.

We have fabricated the stator of a superconducting bearing module for a 10 kWh class flywheel system. The bearing is of radial type in that YBCO bulks are installed in the inner stator and ring-shaped permanent magnets in the outer rotor. YBCO bulk superconductors have a Japanese roof tile shape with epoxy resin impregnation for reinforcement and surface protection. The inner stator is a cylinder (OD = 123 mm, ID = 93 mm, height = 60 mm) that consists of eight YBCO blocks. The c-axis of the YBCO blocks was parallel to the radial direction of the cylinder. The cylindrical set of the YBCO bulks was fixed in a cryostat with epoxy resin. Before installment, we measured the trapped magnetic field distribution of the individual YBCO bulks by using Hall probes at 77 K after field-cooled in the presence of 1 T. The peak field values ranged from 0.4 to 0.7 T. For some samples, the field distribution was not symmetrical, suggesting the presence of some cracks or defects. We also measured the repulsive force between the YBCO bulk at 77 K and a permanent magnet using a tensile machine with a permanent magnet 25 mm in diameter and 5 mm in thickness having surface induction of 0.3 T. Unlike the trapped field, the repulsive force was almost constant for all the samples and was about 50 N at a 0.5 mm gap.

We have succeeded in joining Y–Ba–Cu–O bulk superconductors using sintered Yb–Ba–Cu–O and Er–Ba–Cu–O bars as welding solders. The key to the fabrication of strongly coupled joints was the control of the welded surfaces such that both of the joined surfaces are parallel to the (110) plane. Another key was the use of highly dense sintered Yb–Ba–Cu–O and Er–Ba–Cu–O bars as welding solders. Microstructural observations have shown that welded regions were free from voids, residual liquid phase, and the segregation of second phase particles. The joint made with the Er–Ba–Cu–O solder was superior to that with Yb–Ba–Cu–O, presumably due to the inherently superior superconducting properties of Er–Ba–Cu–O.

We report on the growth of epitaxial Fe1+δ Se0.5Te0.5 thin films on 0°, 5°, 10°, 15° and 20° vicinal cut CaF2 single crystals by pulsed laser deposition. In situ electron and ex situ x-ray diffraction studies reveal a tilted growth of the Fe1+δ Se0.5Te0.5 films, whereby under optimized deposition conditions the c-axis alignment coincides with the substrate [001] tilted axis up to a vicinal angle of 10°. Atomic force microscopy shows a flat island growth for all films. From resistivity measurements in longitudinal and transversal directions, the ab- and c-axis components of resistivity are derived and the mass anisotropy parameter is determined. Analysis of the critical current density indicates that no effective c-axis correlated defects are generated by vicinal growth, and pinning by normal point core defects dominates. However, for Hab the effective pinning centers change from surface defects to point core defects near the superconducting transition due to the vicinal cut. Furthermore, we show in angular-dependent critical current density data a shift of the ab-planes maxima position with the magnetic field strength.

We report the NdBa2Cu3Oy (Nd123)-Ba3Cu5Oz(035) pseudo-binary section in the Nd2O3-BaO-CuO system under the oxygen partial pressure pO2 = 0.01 atm. We performed thermogravitational and differential thermal analysis (TG-DTA) and powder x-ray diffraction (XRD) measurements for quenched samples. We also performed high-temperature XRD for the determination of equilibrium solid phases at elevated temperatures. In situ observation of the primary solid phases with laser scanning microscopy was also performed for an accurate determination of the liquidus line. In comparison with the Nd123-035 pseudo-binary phase diagram in air, the liquidus line under pO2 = 0.01 atm shifts to a lower temperature.

We have studied the microstructure and the superconducting properties of NdxEu1-xBa2Cu3Oy compounds synthesized by the solid-state reaction and the melt process. NdxEu1-xBa2Cu3Oy formed a complete solid solution in a full range (0

We fabricated large grain (Sm, Gd)-Ba-Cu-O bulk superconductors with the top-seeded, melt-growth process in air and 1%O2-Ar. Ag addition was effective in suppressing the amount of cracking. Unlike Sm-Ba-Cu-O, the (Sm, Gd)-Ba-Cu-O bulk showed relatively high Tc and large Jc values even when melt-processed in air. The trapped field value of 0.83 T was recorded at 77 K for the sample melt processed in air.

We have succeeded in fabricating a (Nd, Sm)-Ba–Cu–O bulk superconductor with high Tc and sharp superconducting transition using Ba-rich (Nd0.8, Sm0.2)3.6Ba2.4Cu1.8O9.6 (NS422ss) powder in air. The magnetic property of this melt-textured bulk was similar in Tc and superconducting transition to the bulk with the same nominal composition melt-textured under low oxygen partial pressure (pO2). The samples also showed relatively high Jc values with the secondary peak effect, although Jc values were slightly lower than the samples melt-textured in low pO2. It was also found that CeO2 and Pt doping was very effective in improving Jc values even for Nd-Sm123 bulk melt textured in air.

The effect of Gd2Ba6Al4O15 powder addition on microstructure, field trapping capability and critical transition temperature, Tc, of GdBCO‐Ag superconductors grown in air is reported. Single-grain GdBCO‐Ag bulk superconductors with different amounts of Gd2Ba6Al4O15 powder were fabricated by the top seeded melt growth process. Addition of 0.3 wt% Gd2Ba6Al4O15 powder increased the maximum trapped field value, BZ max. Microstructure analysis confirmed that, up to 1.0 wt% addition of this powder, Al-based phase is not formed and Al is supposed to substitute Cu in the GdBa2Cu3O7 matrix. Addition of 2.0 wt% Gd2Ba6Al4O15 powder leads to formation of Pt, Al-based phase, which indicates that the solubility limit of Al in the studied system was exceeded. The critical transition temperature, Tc, decreases monotonically with increasing Gd2Ba6Al4O15 powder addition. (Some figures may appear in colour only in the online journal)

Optimized, biaxially textured BaFe1.8Co0.2As2 thin films with an in-plane alignment of 1.7° have been realized on high-quality IBAD-textured MgO-coated technical substrates utilizing additional Fe buffer layers. High critical current densities (Jc) were achieved, comparable to films on single crystalline MgO (Jc ≥ 1 MA cm−2 at 4 K, self-field). Transmission electron microscopy investigations reveal a small number of c-axis correlated defects introduced by the MgO template. The effect of these defects on the Jc anisotropy was determined in angular-dependent electronic transport measurements.

In this work we present preparation details and measurement results for an edge-type hybrid Josephson junction based on Co-doped BaFe2As2. The base electrode was formed by ion beam etching of a Ba-122 thin film, while the counter-electrode was patterned by evaporating lead. Finally, an indium protection layer was evaporated. The junction shows asymmetric I–V-characteristics with a total IC RN -product of about 12 μV . The characteristics can be fitted within a resistively shunted junction model assuming different fitting parameters for the positive and negative branches. There is a high excess current of unknown origin. The magnetic field dependence of the critical current indicates a non-homogeneous junction network and effects by flux trapping. It shows a variation of IC in the positive as well as in the negative bias branch, but does not suppress it completely. Also the influence of microwave irradiation on the junctions is shown. Thereby IC as well as the excess current can be suppressed, while first and higher order Shapiro steps can be observed.

We investigated the irreversibility field of GdBa2Cu3O7 − δ thin films grown epitaxially by pulsed laser deposition on single crystalline LaAlO3(100) substrates in pulsed magnetic fields up to 42 T and at temperatures between 50 and 100 K. The critical temperature Tc reached values of 93.5 K with a ΔTc of 0.6 K and an inductively measured self-field critical current density Jc at 77 K of 3.7 MA cm − 2. The pulsed magnetic field measurements yield a strong dependence of Hirr on the deposition rate, but the film thickness has a small influence on the resistive transition in high magnetic fields. A maximum irreversibility field Hirr of 40.1 T was measured at a temperature of 50 K for a 300 nm thick film.

Recycling is fundamental to the sustainable and economic use of natural resources. Bulk RE–Ba–Cu–O ((RE)BCO, where RE is a rare earth element or Y) can be used for variety of sustainable engineering and power sector applications. However, current costs of production and relatively high process failure rates of up to 30% severely limit the manufacture of these materials for industrial use. It is necessary, therefore, to develop an effective way of recycling failed (RE)BCO bulk samples to reduce both production costs and the consumption of the constituent rare earth elements. Failed, melt processed samples have a different chemical composition to that used initially in the precursor powder and, as a result, are generally scrapped, rather than re-processed. We report a novel method for re-processing failed (RE)BCO samples to restore their chemical composition. Failed bulk samples recycled using this process exhibit superconducting properties that are comparable to those of single-grain samples fabricated by conventional top seeded melt growth.

Top seeded melt growth (TSMG) has been used extensively to fabricate large, single grain Y?Ba?Cu?O (YBCO) bulk superconductors that can trap large magnetic fields. The TSMG method is relatively economical and has enabled the development of batch processes for the fabrication of a large number of bulk single grain superconductors in a single furnace. In addition, the technique allows the fabrication of complex-shaped bulk samples with controlled and strongly connected grains by using a novel, multi-seeding process. A practical processing route for processing of LRE?Ba?Cu?O (where LRE represents a light rare earth element) single grain superconductors (which have superior properties to YBCO) has been developed at Cambridge over the past three years, based on the development of a generic seed of melt textured Mg-doped Nd-123 and suppression of solid solution phase formation in air by enriching the precursor composition with excess Ba. In this paper we report the successful application of a practical TSMG process in the fabrication of high performance Gd?Ba?Cu?O (GdBCO) single grain superconductor. This method has enabled the development of a batch process for GdBCO and we demonstrate for the first time the fabrication of a large number of high performance single grains of this material in a single process. Finally, we report the processing of bulk GdBCO in the form of complex geometries with controlled grain orientation for bespoke engineering applications.

Superconducting and epitaxially grown LaFeAsO1?xFx thin films were successfully prepared on (001)-oriented LaAlO3 substrates using pulsed laser deposition. The prepared thin films show exclusively a single in-plane orientation with the epitaxial relation and a full width at half-maximum value of 1?. Furthermore, resistive measurement of the superconducting transition temperature revealed a Tc,90% of 25?K with a high residual resistive ratio of 6.8. The preparation technique applied, standard thin film pulsed laser deposition at room temperature in combination with a subsequent post-annealing process, is suitable for fabrication of high quality LaFeAsO1?xFx thin films. A high upper critical field of 76.2?T was evaluated for magnetic fields applied perpendicular to the c-axis and the anisotropy was calculated to be 3.3 assuming single band superconductivity.

Power applications based on YBa2Cu3O7?? coated conductors demand an enhancement of the critical current density Jc in magnetic fields to be achieved using artificial pinning centres. A?well-known approach to studying pinning by second phase nanoparticles is based on quasi-multilayer deposition of YBa2Cu3O7?? (Y123) and a dopant material. In this study we use the Y2Ba4CuMOy (M = Nb, Zr; M2411) phase as a dopant material, which proved to be very successful in increasing Jc for bulk samples. Although stability of the M2411 phase in bulk Y123 has been reported, experimental evidence of its stability in thin films is still lacking. We found that during the quasi-multilayer deposition, yttrium doped Ba(M1?xYx)O3 forms instead of the M2411 secondary phase, and this causes an increase of Jc at high magnetic fields as well as an increase in the irreversibility field below 80?K as compared against pure Y123 thin films.

Large Ag2O doped Gd?Ba?Cu?O (GdBCO) single grains containing Gd2Ba4CuNbOy (Gd-2411(Nb), a niobium compound) have been fabricated successfully in air by a cold-seeding melt-processing technique using a generic Mg?Nd?Ba?Cu?O seed crystal. An optimum amount of BaO2 addition to the precursor composition is found to suppress Gd/Ba solid-state substitution in the superconducting phase composition. A study of the microstructures and superconducting properties of these GdBCO bulk superconductors containing such Gd-2411(Nb) phases is presented. A trapped field of over 0.9?T has been recorded finally at 77?K for the GdBCO/Ag contained Gd-2411(Nb) with 26?mm diameter.

The trapped field of undoped, bulk, single grain Y–Ba–Cu–O (YBCO) fabricated by top seeded melt growth (TSMG) has been measured to investigate the field generating potential of this material. Bulk samples were prepared from precursor powders that incorporated Y2O3 of particle size 20–50 nm, rather than Y2BaCuO5 (Y-211), which is used more commonly, in order to introduce nano-Y-211 inclusions in the superconducting YBa2Cu3Oy (Y-123) matrix. Relatively small bulk samples of diameter 20 mm processed from this precursor are observed to trap a peak magnetic flux density of nearly 0.8 T at 77 K. This is the first time that such a trapped field of this magnitude has been observed in undoped YBCO (i.e. in the absence of chemical additions). The increase in trapped field is accompanied by an associated enhancement in Jc by up to a factor of five over the applied magnetic field up to 4 T at 77 K compared to standard single grain YBCO fabricated from precursor powders that contain the Y-211 phase.

We have measured the temperature dependence of the in-plane thermal conductivity, κab(T), for both Er–Ba–Cu–O and Ho–Ba–Cu–O superconducting bulks. The absolute values of κab(T) for both bulks are smaller than that of Y–Ba–Cu–O bulk, but are larger than that of Dy–Ba–Cu–O bulk, which is known to show an anomalous small κab. κab(T) for Ho–Ba–Cu–O shows a positive slope (dκab/dT>0) in the normal state, similarly to that for Dy–Ba–Cu–O. On the other hand, a slightly negative slope (dκab/dT