2.4　HTS bulk materials

HTS bulk materials have a series of unique features in their processing techniques, their characteristic properties, and the areas of applications. HTS bulk materi-als are especially attractive for practical applications because of their features of large current transport capacity in the presence of strong magnetic fields and their ability to trap very high fields in a compact space, particularly essential features for self-stable levitation without active control systems.Although the critical cur rent density of bulk HTSC is about two orders of magnitude lower than that of thin films, the bulks offer a large effective critical current density（Jc ）since metal substrates are not required. Thus, HTS bulks are widely used in flywheel energy storage system, transportation vehicles, current leads, fault current limiters, wind power generators, ship motors, compact NMR/MRI etc62.In order to realize these applications, large size and excellent HTS bulks are required.It is necessary to improve the bulk properties further.Specific details about HTS bulk materials have been dissussed elsewhere63.This section only directly relates to the basic knowledge of HTS bulks for magnetic levitation.

2.4.1　Sintering HTS bulk materials

A single crystal is important for characterizing materials properties. However, sin-gle crystals do not have large transport current because the effective flux-pinning properties are only provided by defects and impurities, thus they have no value for engineering applications.

In the early stage of HTS bulk material development, HTS bulks were synthe sized by a sintering technique, which is a common ceramic processing route and generally easy to apply. The sintering techniques have a severe weak-link problem associated with grain boundaries, as a result, the techniques are most often em-ployed in the search for and development of new compounds.The sintering bulk process includes the precursor powder preparation, pressing and shaping, melt casting and recrystallization.HTS bulk materials can be achieved by a more con ventional sintering approach.In June 1987，scientists in Hefei Research Institute of Cryogenics and Electronics Technology, successfully prepared large bulk HTS sam ples using the sintering technique.The critical temperature of a sintered YBCO bulk sample with diameter 43 mm and thickness 2 mm was 86 K64.At the same time, the microwave surface resistance of this large YBCO bulk was measured.The results were not published, because the measured results were poor.Thereafter the other group published results are also poor, because the sintered bulks have no good surface of the conductive properties and have severe weak-link.

To compare with the conventional copper leads, HTS current leads can greatly reduced the heat leakage to superconducting magnets operating at low tempera tures. This is because the thermal conductivity of the HTS is two orders of magni tude smaller than that of copper.HTS bulks composed entirely of superconductingmaterials do not have metallic sheaths of high thermal conductivity as in HTS wires.Thus, the heat leakage of bulk HTS current leads is expected to be low.According to different application needs, the HTS bulk sintered materials can be processed into various shapes（bars, tubes, Plate, etc.）.Both bulk sintered BSCCO65and YBCO66can be made for current leads.The sintering technique is an important technology to produce BSCCO bars that can be used for current leads to supply power for superconducting magnets.A c-axis oriented sintered BSCCO bulk can achieved a Jc of 2.3×104A·cm−2@20 K，1 kOe）67.The Heat leakage of the commercial Bi-based HTS current leads is a tenth that of copper current leads.In comparison to conventional copper leads, the thermal load and the consumption of liquid helium were greatly reduced.Sintering techniques were also employed to fabricate tubes to shield external fields for the medical diagnosis of the human brain or heart68.Nishikubo et al.69have developed a RF shield using a HTS bulk.Rabbers et al.70have demonstrated the shielding of a DC magnetic field at 4.2 K, in fields up to 2 T, with a thick LHT MgB2 bulk cylinder，70 mm long and 18 mm bore.

A further improvement of HTS current leads is possible by replacing the Bi-based conductor with a Y-based one, since HTS YBCO bulk, especially the melt texture growth（MTG）YBCO, can increase the current capacity and the reliabil ity in magnetic fields. Thus, the MTG bulk HTS YBCO can be used for current leads.Large size single domain materials can shield higher fields.For instance, the shielding factor of a tube with 5 mm single domain YBCO exceeds the value for sintered YBCO by one order of magnitude in the low frequency range（1-200 Hz）71.Endoh et al.has prepared a 500 A class HTS current lead package using the YBCO rod whose size isφ3×30 mm with the heat leakage under typ-ical practical conditions, from 80 K hot end to 20 K cold end, was 163 mW66.Ohsemochi et al72has successfully achieved a HTS bulk current lead system with a rated current of 3000 A.

2.4.2　Melt process HTS bulk materials

The energy density of magnetized HTS bulks at 77 K is greater than that with HTS wires and tapes, since HTS bulks are capable of supporting a large persistent eddycurrent. However, for sintered YBCO HTS bulk, the Jc is low due to the existence of weak links at the large numbers of grain boundaries.The sintered materials cannot be used in HTS magnetic levitation engineering, thus a new preparation technique for bulk YBCO is required.Fabrication techniques for bulk（RE）BCO materials in the practical applications must overcome the weak-link problem associated with grain boundaries.

To overcome the Jc limitations, due to weak link properties of the grain bound-aries in sintered YBCO ceramics, Jin et al.73developed a melt process named melt texture growth（MTG）in 1988. The melt textured YBCO bulk can support a Jc at least three orders of magnitude higher than that of sintered samples.The value of the transport current density Jc of the MTG samples obtained by Jin et al.74had exceeded 104A/cm2in zero magnetic field at liquid nitrogen temperature of 77 K.This result indicates that grain alignment may have extensively reduced the weak links and lead to a great improvement of the material.In order to improve the Jc values in a magnetic field, effective pinning centers must be introduced into the materials.In the following years, this process technique was modified and improved by several researchers.The first improvement was achieved by Salama et al.75by introducing the liquid-phase processing technique.Murakami et al.76，77developed the melt powder melt growth（MPMG）method.REBCO prepared by MPMG has a high Jc at 77 K in high magnetic fields.MPMG can easily increase domain size to a cubic centimeter.Microstructural observations reveal that the size of the 211 phase is much finer and its distribution is much more uniform than that of the classical MTG samples.MPMG also improves the Jc values, especially under a magnetic field of 1 T.The Jc （B）variations which were calculated from the mag-netization measurements on MPMG processed samples exhibit 5∼6×104A/cm2in the self field and over 2×104A/cm2in fields under 1 T.

In order to synthesize products for engineering applications, numerous ap-proaches have been tried using a variety of synthesis technologies, such as melt texture growth（MTG），melt powder melt growth（MPMG），top seeded melt tex-tured growth（TSMG）and seeded infiltration and growth（SIG）78processes etc. Up to now, the melt processes of the HTS（RE）BCO bulk generally use both MPMG and SIG process methods.Recently, Li et al.79and Chen et al.80give detailedreviews of materials processing of single grain REBCO HTS bulks.The TSMG process for bulk（RE）BCO materials has been established as an effective method to fabricate large and single domain bulk REBCO with high performances.The TSMG is conductive to multi-block processing at the same time and controlled multi-domain bulk growth.However, the TSMG process technique has drawbacks, such as easy coarsening, large shrinkage and liquid outflow.Macroscopic cracks and pores are also easy to form in the melt growth.The chemical composition of the final product is inhomogeneous in general.In comparison to the TSMG process, the major advantage of the SIG process is the ability to provide fine-sized spherical RE-211 precipitates in the REBCO matrix, even without addition of compounds such as Pt and CeO2 .However, the SIG process has the disadvantage that the uniformity of microstructure and Jc is uncertain81.

2.4.3　Developments of HTS bulk materials

The rare-earth Ba-Cu-O（REBCO, RE=Y, La, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm and Yb etc.）HTS bulks have high critical current density Jc and high trapped magnetic flux. The maximum trapped magnetic field is approximately an order of magnitude higher than that for the presently used conventional PMs.Early studies of HTS bulk focused on YBCO.The critical current of HTS YBCO bulk materials had increased one thousand times in a few years from 1987 to 199182.Since then, HTS bulk materials and their applications have been considerable79.With a modified SIG process, the value of Jc is up to 2.3×105A/cm2under zero-field and 104 A/2 up to 7 T at 77 K in YBCO83.Muralidhar et al.84，85 in the Railway Technical Research Institute（RTRI）reported achievement in the（Nd, Eu, Gd）BaCuO（NEG-123）system of superior Jc values（9.25×105 A/2 at 65 K，6.40×105 A/2 at 77 K, and even 1×105A/cm2at 90 K）.A trapped field BT near 10 T may be possible at 77 K if proper chemical pinning centers are created86.

A breakthrough in the inexpensive growth of large amounts of high quality YBCO samples was made by top seeded melt growth（TSMG）method87. In this TSMG technique, RE123（Sm123 or Nd123）crystals with a higher melting temperature than Y123 were put on top of the precursor pellet.The processing conditionsfor the fabrication of NdBCO and SmBCO are considerably more complex than those for YBCO.A large number of single domain REBCO samples of various shapes were prepared with this method88，89，90，91，92，93.

The melt process for the RE（Nd, Sm, or Eu）BCO systems under controlled oxygen partial pressure（pO2 ）is known as the oxygen-controlled melt growth（OCMG）process94. The OCMG-processed REBCO superconductors exhibit a larger Jc in high magnetic fields and a considerably improved irreversible field Hirr at 77 K, implying that stronger flux pinning can be realized in a commercially feasible way.The GdBCO sample exhibited the highest zero-field critical current density of 8.8×104A/cm2at 77 K for H⊥c , while the highest secondary peak Jc of 6.5×104A/cm2was achieved in NdBCO at 1 T and 77 K for Hc 95.

A critical current density Jc value above 105A/cm2for melt processed YBCO superconductors has been obtained especially under high magnetic fields. For an external field parallel to both a-b planes and the direction of the current flow, critical current densities of 1.6×104A/cm2in self field and 1.1×104A/cm2under 1 T have been obtained96.

In the following years, many efforts have been made to increase the textured domain size and trapped flux.A single bulk GdBCO sample 50 mm in diameter exhibited a trapped field of 2.6 T at 77 K with the Hall sensor was in contact with the surface97.

A single bulk GdBCO sample 65 mm in diameter exhibited a trapped field of 3. 05 T at 77 K.One study measured the trapped field between two GdBCO bulks in order to minimize the demagnetizing effect and found that it reached 4.3 T at 77 K98.Enlargement of the single-grain bulk REBCO materials is effective in improving their ability to trap magnetic flux.However, it is difficult to obtain a single-grain bulk larger than about 80 mm because of undesirable nucleation at the positions away from the seed crystal99.

In order to develop manned Maglev trains, high quality MTG YBCO bulks100（Fig. 2.4.1（a），（b），（c））were prepared by the Beijing General Research Institute for Nonferrous Metals in 2000.The superconductors used in the first manned HTS Maglev test vehicle in the world were these YBCO bulk 30 mm diameter samples（Fig.2.4.1（c））.Single-and multi-seeded melt-textured HTS YBCO bulks101，102have been developed by the Adelwitz Technologiezentrum GmbH（ATZ），Germany.Some basic material parameters of ATZ melt-textured YBCO are summarized in Table 2.4.1.The 3-seed YBCO bulks have a size of 67 mm×35mm×15mm（Fig.2.4.2），and were used in the Maglev vehicle at the University of Rio de Janeiro in 2011 and the Applied Superconductivity Lab.（ASCLab），Southwest Jiaotong University, China, in 2013.

Table 2. 4.1 YBCO bulk material parameters101

A microgravity experiment for growing large GdBCO bulks，127 mm in diameter and 20 mm in thickness, was successfully performed on the spacecraft in 2003103. Superconductor research colleagues are looking forward to further data from this large GdBCO bulk.

In order to obtain a large single-grain bulk, Morita et al104，105 in Nippon Steel Corporation（NSC）succeeded in developing the RE compositional gradient tech nique and prepared large-grained YBCO bulk superconductors with high Jc val ues. Since then, the research and development of HTS bulks has made steady progress106.Nippon Steel Corporation reported fabrication of large single-grained REBCO bulk superconductors 150 mm in diameter using the RE compositional gradient technique.Fig.2.4.3 shows a single-grain，150 mm in diameter GdBCO bulk with a compositional gradient of Dy.The insert in Fig.2.4.3 is the trapped field distribution at 87 K, indicating that the sample is a single-grain bulk with no serious weak-links.

Another important superconducting characteristic of the melt textured YBCO samples for the application is trapping flux. The magnitude of the trapped flux of REBCO HTS bulks is proportional to its critical current density Jc and the vol ume.Krabbes et al.107prepared a cylindrical YBCO bulk with a modified melt crystallization process（MMCP）108.A maximum field of 16 T was trapped at 24 K in the gap of a mini-magnet made of two YBCO samples.B0 on top of a single sample was 12.5 T at 20 K and 9 T at 40 K.

A bulk YBCO superconductor as small as 2. 4 cm in diameter covered with carbon fiber fabric can trap an extremely high static field of 13. 55 T at 34 K, and a bulk SmBCO superconductor as small as 2.4 cm in diameter can trap an extremely high static field of 13.69 T at 47 K109.Tomita and Murakami prepared a YBCO bulk sample with high flux trapping after improving the mechanical stability and the thermal conductivity of the YBCO bulk.Their YBCO bulk was 2.65 cm diameter and trapped a magnetic field of 17.24 T at 29 K110.This is the highest trapped magnetic flux so far.

Using NdBCO/YBCO/MgO film seeds and the cold-seeding method in TSMG, Xu et al. 111 successfully reprocessed failed bulks, and demonstrated a novel, con venient, and effective process for recycling the failed REBCO bulks.Peng et al.112reported that a large size SmBCO，32 mm in diameter, was successfully grown in air by the cold-seeding method.In the conventional TSIG process, three types of powders, such as Gd2BaCuO5 ，GdBa2 Cu3 O7−x and Ba3 Cu5 O8 ，must be prepared.

Yang et al113has a new modified TSIG process technique, where only BaCuO2 powders are required during the fabrication of the single domain GdBCO bulk superconductors. Yang et al.114have observed in real time the growth process of single domain YBCO bulk superconductors using an in situ high temperature video camera, and obtained the growth rate of single domain YBCO bulk.The technique is helpful to find solutions for modifying preparation processes of YBCO bulk aswell as for improve batch product quality.

Yang et al.115 has reported a way to optimize the quality of the YBCO crystal by top-seeded melt-textured fabrication, with NdBCO thin film seeds.The optimal growth conditions of TSMG processing with NdBCO thin film seeds have been determined, and the optimal growth method is helpful for engineering fabrication.Wu et al.116reported a new approach for growing large size, single domains of YBCO by a top-seeded melt-textured growth（TSMG）process.Large YBCO single domains of 53 mm in diameter have been successfully produced using this method, and they predicted that samples with 75 mm in diameter can also be grown.

Shi et al.117have reported a successful multi-seeding technique for the fabri cation of fully aligned, artificial（0° misalignment）grain boundaries within large grain YBCO bulk superconductors using bridge-shaped seeds.Plechacek et al.118 have developed a process capable of simultaneously fabricating up to 64 pieces of HTS bulks.Zhou et al.119，120have reported that the SIG of GdBCO single grains using a YBCO pressed pellet as the liquid source can successfully settle the liquid source leakage problem, even at Tmax∼1100℃.The present method resulted in a significant enhancement of the trapped flux density of the GdBCO grains.

In order to improve the processing of single domains with such large dimen-sions, Noudem et al.121，122proposed drilling artificial holes in the sintered powder before the crystal growth. The single domain of YBCO bulk multiple holes can improve mechanical properties, thermal stability, and the process of oxidation, and increase interfacial flux pinning if the pores can be made sufficiently small.More efficient heat transfer, faster oxygenation and less micro-cracking, possibility of re inforcement and of interlocking connections, etc.may result from the development of processing methods of the perforated and textured Y123 with a high perfor mance which can open new pathways towards practical applications.Lousberg et al.123have reported the effects of filling the holes of drilled HTS samples with a soft ferromagnetic powder.The magnetic properties of the trapped field magnet were measured, and the experiments demonstrated an increase in flux trapping ability.

Sawh et al.124recently reported the results of studies on 53 melt-textured YBCO trapped field magnets（TFMs），2 cm in diameter. The average trapped field on the seed-side surface was 2.04 T at 77 K.