Single-grain Bulk Research Articles

Novel approach for manufacture of single‐grain EuBCO/Ag bulk superconductors via modified single‐direction melt growth

AbstractDespite the success of top‐seeded melt growth (TSMG) and TSIG, some key aspects of their manufacture need to be addressed including the presence of microstructural defects, inhomogeneities in trapped field and the difficulty of growing large‐diameter bulk monoliths. Recently, a promising new method single‐direction melt growth (SDMG) appears to address the majority of the shortcomings of TSMG and TSIG. Single‐grain EuBCO/Ag bulk with the highest peritectic temperature to date was grown by modified SDMG. This remarkable achievement was made possible by lowering the peritectic temperature of the precursor composition by the addition of silver and by using single‐grain EuBCO fabricated by TSMG as a seed. The EuBCO/Ag bulk grown by SDMG has demonstrated significant improvements in comparison to a reference TSMG sample. A reduction of approximately 30% in defect area (associated primarily with porosity), as well as a significantly enhanced homogeneity of trapped field and critical temperature, was observed. Furthermore, the critical current density curves exhibit a considerable improvement, particularly in the 2–5 T field range. The SDMG technique has considerable potential for scale‐up for the fabrication of large‐diameter bulk single grains, with only growth along the c‐axis involved during the thermal process.

Statistical evaluation of the mechanical and flux trapping properties of standard and thin‐wall EuBCO(Ag) bulk superconductors

AbstractThe mechanical and flux trapping properties of 19 EuBCO(Ag) and 20 thin‐wall EuBCO(Ag) bulk superconductors were investigated statistically and compared to those observed for YBCO using Brazilian testing and high‐field magnetization. The average trapped fields of EuBCO(Ag) and thin‐wall EuBCO(Ag) bulk superconductors were significantly higher than those of YBCO bulk samples at 77 K measured under similar experimental conditions. However, due to the wide range of trapped field data for both materials, a statistical overlap was observed between the highest trapped fields observed for YBCO and the lowest values observed for EuBCO(Ag) bulk superconductors. All EuBCO(Ag) bulk materials measured in this study exhibit a superior tensile strength compared to YBCO bulk single grains. Unlike the case of YBCO, however, the introduction of artificial holes to EuBCO(Ag) bulk superconductors prior to melt processing was observed to decrease the average tensile strength of the single grain. Surprisingly, despite their superior mechanical and flux pinning properties, EuBCO(Ag) bulk superconductors do not exhibit enhanced performance when exposed to high fields at low temperatures.

Microstructure of DyBCO bulk superconductors prepared using single-direction melt-growth (SDMG) method

The microstructure of DyBCO bulk single grain superconductors (BSS) with a nominal composition of 7 mol DyBa2Cu3Oy + 3 mol Dy2BaCuO5 (Dy211) + 10 wt % Ag2O + 0.5 wt % CeO2 prepared by single-direction melt-growth (SDMG) was studied using electron and optical microscopy. Microstructural parameters such as subgrain structure, porosity, size and macroscopic distribution of Dy211 particles, interdiffusion of the Eu from the EuBCO seed, as well as the position of the remaining tetragonal non-oxygenated regions in the sample were characterized. The volume fraction of Dy211 particles increases with distance from the seed, which induces thermal mechanical stresses that are compressive at the seed side and tensile at the growth end with an estimated maximum of 110 MPa. Multinucleation on the large seed leads to the appearance of steps at the growth front and the formation of a-growth subsectors with a higher concentration of Dy211. A high concentration of Eu is diffused from the EuBCO seed in the first part of the samples. The diffusion depth is similar to that of YBCO BSS from a Yb-containing substrate.

Statistical evaluation of the mechanical properties of partially oxygenated YBCO, oxygenated YBCO and oxygenated YBCO(Ag) single grains

It is of great significance to enhance the mechanical properties in single grain YBCO bulk superconductors, as mechanical failure limits the suitability of these materials for high-field applications. The influence of the microstructure on the mechanical properties of YBCO bulk materials was studied in 11 partially oxygenated YBCO, 11 oxygenated YBCO, and 10 oxygenated YBCO(Ag) melt-processed bulk single grains in a statistically significant way using Brazilian testing. Surprisingly, the oxygenation cracks evolving during the tetragonal to orthorhombic phase transition do not influence the average tensile strength of YBCO bulk single grains, with compaction cracks and pores as the main microstructural flaws responsible for mechanical failure. The mechanical properties of bulk YBCO single grains even improve following oxygenation, as indicated by an increase in Weibull modulus. An addition of 10 wt% Ag2O to YBCO increases the average tensile strength and the Weibull modulus further, making the material mechanically more robust. However, an overlap in the measured tensile strengths of oxygenated YBCO and oxygenated YBCO(Ag) bulk materials reveals that not all silver-containing samples have better mechanical properties compared to oxygenated YBCO bulk samples.

The influence of porosity on the superconducting properties of Y–Ba–Cu–O single grains

The trapped field in Y–Ba–Cu–O (YBCO) bulk single grain superconductors correlates directly with the critical current density J c, which, in turn, is dependent on the microstructure of the bulk material. It has been shown recently and indirectly that porosity influences J c of these technologically important materials, in addition to the presence of well-researched Y2BaCuO5 (Y-211) particles in the bulk sample. In this work, we report the direct impact of porosity on the critical current density of a single grain YBCO bulk superconductor using 3D x-ray computer tomography scans and superconducting quantum interference device measurements. It is concluded that porosity has a considerably more substantial impact on the measured critical current density than Y-211 on the micrometre scale with, predictably, a decrease in porosity leading to an increase in J c. J c is directly proportional to the trapped field, so any method that can reduce porosity, therefore, improves J c, and, subsequently, the trapped field of these technologically important bulk superconductors.

Pulsed field magnetization of a rectangular Y–Ba–Cu–O bulk, single grain superconductor assembly

The practical magnetization of arrays of multiple single grain, bulk high temperature superconductors is essential for practical applications, such as trapped flux rotating machines, magnetic resonance imaging and nuclear magnetic resonance. We report a systematic investigation of the pulsed field magnetization (PFM) of a bulk assembly consisting of two rectangular Y–Ba–Cu–O bulk single grains, in close proximity, at various temperatures. The measurements of the dynamic variation of the magnetic flux density, supported by numerical analysis, reveal that the induced screening currents during the rise of a pulsed field may greatly enhance the flux density in the region of the junction leading to uneven flux penetration and to an increased likelihood of flux jumps in this region. Such coupling between field and current promotes magnetic flux penetration and improves the peak trapped field from 3.01 T for a bulk single grain to 3.11 T for the bulk assembly at 30 K, improving the magnetization efficiency from 80% to 90%. The peak trapped field was further enhanced to 3.39 T and 3.31 T for the single bulk single grain and the bulk assembly, respectively, by employing a two-step multi-pulse PFM process.

Exploring the effects of liquid source on superconducting properties and microstructure in ternary bulk (Gd, Y, Er)−123 by infiltration growth process

Superconductors are used in a wide range of applications, each requiring specific magnetic field strengths to operate at their best. The optimal magnetic strength is determined by the superconducting properties, such as critical current density (Jc). In this study, we propose that by using different compositions of liquid source, the superconducting properties and microstructure of bulk superconductors can be optimized according to their intended application. Ba3Cu5O8, Yb2O3:BaCuO2:CuO (1:10:6) and Er123 + Ba3Cu5O8 (1:1) were tested as liquid sources, and ternary single-grain bulks (Gd, Y, Er) Ba2Cu3Oy were produced using the top-seeded infiltration growth process (TSIG) in the air. Microstructural analysis with a scanning electron microscope revealed that using Er123 +Ba3Cu5O8(1:1) as the liquid source resulted in a significant reduction in the 211 secondary phase particle size, with a mean particle size reduction of 43.13% compared to the conventional Ba3Cu5O8. Each composition of the liquid source had a significant impact on the superconducting properties, and the Er123 + Ba3Cu5O8 liquid source yielded the highest Jc of 81.91 kA/cm2 in self-field and 28.29 kA/cm2 at 1 T for (Gd, Y, Er)−123 ternary system. These results emphasize the importance of liquid sources in controlling the final microstructure, which ultimately determines the performance of the bulk ternary (Gd, Y, Er)−123 system produced by the IG process.

Complex microstructural analysis of YBCO single-grain bulks with artificial holes: Effect on superconducting properties

This work presents an investigation of the microstructural and superconducting properties of standard single-grain bulk superconductor Y–Ba–Cu–O (YBCO) and samples fabricated with artificial holes. It was shown that the presence of artificial holes reduces the absolute porosity of the sample. The presence of artificial holes did not improve the homogeneity of oxygen distribution in the samples after oxygenation. The distribution of Y211 particles in the samples has a negligible effect on the macroscopic superconducting properties. In the process of crystal growth, the ellipticity of the introduced holes and their displacement in the direction of crystal growth in the growth sectors arise. Macroscopic superconducting properties, such as levitation force and trapped magnetic field at 77 K, were supplemented by magnetization measurements of superconducting transition temperature and critical current density. The levitation force and the trapped magnetic field were higher for the sample with holes. For the measured trapped magnetic field, the decrease of the porosity in the a-growth sectors, which have a higher volume fraction of Y2BaCuO5 (Y211) particles, and consequently stronger pinning, is significant.

Transport measurements in single-grain GdBCO+Ag bulk superconductors processed by infiltration growth

Transport measurements performed on a cross-sectional slice prepared from a bulk GdBa2Cu3O7-x-Ag single-grain superconductor of 25 mm in diameter are described and the results presented. The sample, which was fabricated via the buffer-assisted top-seeded infiltration growth process, was capable of trapping a maximum magnetic field of ~ 1 T at 77 K. Transport measurements on superconducting, bulk single-grain RE-Ba-Cu-O [(RE)BCO] samples are generally very challenging due to their large critical current densities and poor mechanical properties. We present a straightforward and reliable approach to prepare reproducibly specimens from the parent single grain and results of transport property measurements on these samples in a commercial Physical Property Measurement System (PPMS). Critical current densities determined via magnetic and transport measurements are compared and discussed.

Tensile Properties of (Gd,Y,Er)BaCuO Superconducting Bulk Material Fabricated by the Infiltration Growth Technique

In order to evaluate the mechanical properties of a (Gd,Y,Er)BaCuO low porosity single-grain bulk sample fabricated by the infiltration growth technique, tensile tests of specimens cut from the bulk sample were carried out. Relationship between the tensile strength and porosity was investigated for the specimens. Tensile strength increases with decreasing the porosity, which is due to the increase of net cross-sectional area and reduction of defects where the stress concentration occurs. Tensile strength of the (Gd,Y,Er)BaCuO bulk sample is comparable to those of other low porosity single-grain bulk materials, (Gd,Dy)BaCuO fabricated by the infiltration growth technique and DyBaCuO fabricated by the melt-processing in O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> atmosphere. Solidified liquid phase regions and segregated secondary phase particles are observed for the low tensile strength specimens.

The effect of facet lines on critical current density and trapped field in bulk RE–Ba–Cu–O single grains

Bulk, single grain RE–Ba–Cu–O (where RE = rare earth or yttrium) [(RE)BCO] high temperature superconductors could potentially be used to generate stable magnetic fields for magnetic resonance imaging (MRI) and nuclear magnetic resonance (NMR). In these applications, however, the homogeneity of the magnetic field is of critical importance. As a result, the spatial distribution of critical current density, J c, within the bulk single grain and the effects of the magnetisation process, which are primary drivers of the uniformity of the achievable trapped magnetic field, are fundamental to assessing the performance of these technologically important materials. This paper reports the systematic measurement of the distribution of J c–B at 77 K over a vertical cross-section of a single grain along a facet line and through the seed crystal [(110)-F] at 20 positions within a 20 mm diameter Gd–Ba–Cu–O sample in an attempt to understand and assess the distribution of J c along this microstructural feature. A comparison of the data within the whole vertical plane across the seed measured along the a or b direction within the [(100)-a] plane shows that J c–B at 77 K at the facet line is more than 10% higher for applied fields between 0.2 T and 2.5 T. The effect of the J c–B relationship of the facet line on the overall trapped field measured in an individual bulk sample was investigated by measuring the magnitudes of trapped fields and their contour maps for sections cut from four single grain samples of GdBCO–Ag at different sizes and shapes parallel to the ab-plane from the top to the bottom of the bulk sample. Based on the results reported here, we demonstrate a method to achieve more uniform trapped fields through an optimal arrangement of an assembly of sections of individual GdBCO single grains.

Microstructure and superconducting properties of bulk EuBCO-Ag with and without holes

Bulk single-grain EuBCO-Ag superconductors with holes and without holes grown using a top-seeded melt-grow process are analysed. It turns out that the levitation force and the trapped magnetic field at 77 K are higher for the sample with holes (0,85 N and 1 T for the sample with a diameter of 28 mm and a height of 10 mm). Careful complex microstructural analyses undertaken by polarised light optical microscopy and scanning electron microscopy show that the reason for this behaviour is mainly the lower porosity of the sample with holes and the lower density of the of a/c-oxygenation cracks in non-porous areas along the holes. It also appears that other microstructural features, such as higher volume fraction of Eu2BaCuO5 particles in the a-growth sector than in the c-growth sector, uniform distribution of Ag particles in samples, presence of tetragonal EuBa2Cu3O7−x regions are similar on both types of samples and cannot significantly affect macroscopic superconducting properties. The influence of porosity on the local critical current density and transition temperature is also illustrated.

Dynamic guidance performance of GdBaCuO and YBaCuO bulk single grain superconductors under a varying external magnetic field

High temperature superconducting (HTS) maglev technology has attracted considerable attention from researchers around the world. It provides a new direction of development for urban rail and high-speed rail transit due to its considerable advantages of frictionless transportation, low noise, and environmental protection. HTS bulk materials are able to provide a significant performance advantage to maglev technology and are a key component to achieving both levitation and vehicle guidance. HTS bulk samples with enhanced flux pinning characteristics can enable improved curve negotiating ability and lateral stability for the maglev vehicle. This paper reports the results of experiments on the dynamic guidance force of single grain Y–Ba–Cu–O (YBCO) and Gd–Ba–Cu–O (GdBCO) bulk HTS single grains in order to explore their potential for enhanced maglev guidance performance. A customized HTS maglev dynamic measurement system (SCML-03) with a rotating, circular permanent magnet guideway (PMG) was employed to simulate the dynamic operation of the HTS vehicle above the PMG at different translational speeds. It was observed from the experimental results that the GdBCO bulk superconductor is more able to resist the attenuation of the dynamic guidance force compared to YBCO under the same operational conditions. In addition, the GdBCO bulk single grain is more able to return to the original equilibrium position following exposure to an external displacement. It is concluded that the GdBCO bulk single grain offers greater potential than YBCO for practical application in HTS maglev given its enhanced guidance performance. The results of these tests provide an important benchmark for the future design of HTS maglev transportation systems.

Flux Pinning Properties of Single-Grain Bulk GdBCO Superconductors Processed by Different Thicknesses of Y123 Liquid Source.

The performance of critical current density of GdBa2Cu3O7−δ (GdBCO or Gd123) superconductor bulk has an important influence on its practical applications. In this work, four single-domain GdBCO superconductor bulks were successfully processed by the modified top-seeded melt-texture growth method. The addition of a YBa2Cu3O7−δ (Y123) liquid source with different thicknesses, 0 mm (S0), 3 mm (S3), 5 mm (S5), 7 mm (S7), was introduced to study the influence on the superconducting properties. GdBCO bulk with the addition of the Y123 liquid source with a 3-mm thickness shows the best superconducting properties. The addition of the Y123 liquid source results in a decrease in the Gd3+ ion concentration required for Gd123 growth; thus, Gd2BaCuO5 (Gd211) particles in the liquid source need a larger self-decomposition to diffuse Gd3+ ions to Gd123 growth front, which refines the size and leads to a homogenous distribution of the Gd211 particles in the bulks. Thus, the more pinning centers of fined Gd211 particles improve the superconducting properties of GdBCO bulk. With increases in the thickness of Y123 liquid source to 5 mm and 7 mm, high RE3+ (Gd3+ and Y3+) concentration can coarsen Gd211 particles and fuse with Gd211 liquid source. The superconducting properties apparently drop. Therefore, the addition of a Y123 liquid source with a suitable thickness is a positive modification to obtain high-performance GdBCO bulk.

Dynamic Guidance Performance of Gdbacuo and Ybacuo Bulk Single Grain Superconductors Under a Varying External Magnetic Field

Dynamic Guidance Performance of Gdbacuo and Ybacuo Bulk Single Grain Superconductors Under a Varying External Magnetic Field

A novel preform compaction method for fabricating high-performance Y–Ba–Cu–O single-grain bulk superconductor

A novel preform compaction method based on one new type of divisible mould was employed for fabricating single-grain YBCO bulk superconductor, which can complete the preform demoulding process through opening of the mould, rather than pushing the preform out in the regular mould. Thus it has natural superiority on eliminating macro-cracks, which has been proved by the sample surface morphology and the trapped field characterization. In addition, the divisible mould pressed sample exhibits higher levitation force and trapped field properties than the regular mould pressed samples, verifying the potentials of the divisible mould on improving the bulk performance. The optical micrograph results prove the superiority of the divisible mould on eliminating tiny cracks on sample surface. The processing facility and sample reliability brought by the divisible mould should also be emphasized, because the crushing and re-pressing of preform when it presents visible cracks can be omitted and the failed samples with surface cracks will no longer appear. Consequently, the experimental efficiency and stability are both enhanced.

The magnetic and levitation characteristics of single-grain YBaCuO and GdBaCuO-Ag bulk superconductors in high magnetic fields

In principle, high-temperature superconducting (HTS) flux-pinning maglev has the advantage of self-stable levitation and is therefore a good development prospect as a new rail transit technology. The magnetic field intensity is one of the key factors affecting the levitation performance of single-grain bulk superconductors in the HTS maglev system. To date, however, most researchers have focused on the levitation performance of HTS bulk superconductors in magnetic fields within a permanent magnet guideway (PMG) configuration. The magnitude of the external working magnetic field provided by a PMG is limited to about 0.5 T at 77 K, given that the superconductor usually levitates at a typical working height of 10 mm above the PMG. Therefore, the weak magnetic fields generated by the PMG significantly limits the levitation performance of HTS maglev systems to that of the permanent magnet (PM). In this article, a superconducting magnet test bench was utilized to provide a 0–3 T external magnetic field (B) environment for the study of the levitation force in HTS single grains. The difference in levitation performance between YBaCuO (YBCO) and GdBaCuO-Ag (GdBCO-Ag) bulk superconductors under large, changing magnetic fields was studied systematically. The relationships between levitation force and magnetic field characteristics, including vertical magnetic field component (Bz), transverse magnetic field component (Br), and vertical magnetic field variation (ΔBz) were investigated. The result of variation curves about YBCO and GdBCO bulks' levitation forces under different ΔBz shows that the levitation force of the bulk, single-grain superconductors will reach saturation under a varying ΔBz environment (0.72 and 1.5 T). The values of ΔBz corresponding to the saturation positions are found to be independent of the sizes of the HTS samples. At the same time, GdBCO-Ag exhibits a levitation force that is 112.8% greater than YBCO in B = 3 T magnetic environment. As a result, it is demonstrated that the levitation potential of single-grain HTS bulks can be better exploited by optimizing the distribution of the magnetic field.

The influence of CeO2 addition on microstructure and superconducting properties of GdBCO-Ag single grain bulk superconductors

Bulk single-grain GdBCO superconductors (nominal composition 1 mol GdBa2Cu3O7-δ + ½ mol Gd2BaCuO5 + 20 wt% Ag2O + Z wt% CeO2 (Z = 0.4, 1.0, 1.6) grown using the top-seeded melt-grow process are studied by differential thermal analysis, X-ray diffraction, scanning electron microscopy and magnetic measurement of local superconducting properties. The amount of CeO2 added is shown to have a significant effect on thermal behaviour, phase composition, microstructure and local superconducting properties. The observed behaviour is explained by the dissolution of Ce in the melt L' formed by monotectic melting of the samples. Ce dissolved in melt L' reduces the substitution of Ba by Gd in the Gd123 crystal lattice. This effect causes an increase in the superconducting transition temperature Tc with added CeO2, an increase in Tc with distance from the seed and influences the critical current density by chemical pinning. The obtained results open a new effective way to improve the superconducting properties of LREBCO bulk single-grain superconductors grown in air.

(Gd,Y,Er)Ba2Cu3Oy bulk superconducting system processed via infiltration growth

Ternary bulk REBa2Cu3Oy superconductors processed via oxygen-controlled melt growth (OCMG) have exhibited superior superconducting properties with respect to binary and single systems. However, the OCMG process is rather complicated, expensive, and difficult to scale-up into industrial level. To overcome these issues, we have studied the yet unexplored ternary bulk (Gd,Y,Er)Ba2Cu3Oy “(Gd,Y,Er)-123″ system, fabricated via trendy infiltration growth (IG) process. We studied the effect of varying Y-211 and Er-211 ratio (Gd: Y: Er = 0.33: 0.33-x: 0.33+x, where x = 0, 0.05, 0.1, 0.15 and 0.2) in the preform on the final superconducting properties and microstructure. We succeeded to fabricate (Gd,Y,Er)-123 single-grain bulks (diameter – 20 mm). The critical current density (Jc) and trapped field increased proportionally with the increased amount of Er-211 in the preform. Hall-probe measurements showed that the best bulk (x = 0.2) exhibited trapped field of 0.53 T at 77 K (applied field of 1 T) and self-field Jc values of 238 and 63 kA/cm2 at 50 and 77 K, respectively. FE-SEM micrographs revealed formation of two different types of secondary phases in the matrix (small and large). EDX analysis showed that the smaller 211 particles were Er-rich, while the larger ones were an irregular mixture of three RE-211 phases.

Employment of NdBCO/YBCO/MgO film seed in the infiltration growth technique for fabricating single-grain YBCO bulk superconductors

The quality of the seed is a key factor for growing well c-axis oriented RE–Ba–Cu–O (REBCO) single-grain superconductors. In this study, the commercial NdBCO/YBCO/MgO film (300 nm NdBCO with 20 nm YBCO buffer layer on MgO substrate) is employed as seed in the infiltration growth (IG) method for growing YBCO single grains, including the conventional 211-IG and the novel 011-IG techniques. And an embedded seed mode was used for preventing the movement of the thin film during heat treatment process in the 011-IG technique. The results indicate that, YBCO single grains have been successfully grown via the seeding of NdBCO/YBCO/MgO film in the two IG processes, which proves the feasibility of the film seed in the IG method. Four strictly perpendicular growth sector boundaries on the top surface of the samples and clear (0 0 l) diffraction peaks in the X-ray diffraction (XRD) results both indicate the high c-axis orientation and well growth quality of the samples, which proves the excellent performance of the NdBCO/YBCO/MgO film seed in the IG method. Superconducting property measurement showed that, the 011-IG YBCO sample exhibited enhanced levitation force and Jc properties than the 211-IG sample.