Sample characteristics and maximal trapped field values in liquid nitrogen.
The present chapter describes an overview of flux trapping with enhancement of the critical current density (
The top-seeded melt-growth (TSMG) method has been widely used to fabricate large, single-grain RE-Ba-Cu-O superconducting bulks that show a considerable ability in magnetic flux trapping and great potential for large-scale applications . Hot seeding and cold seeding procedures have been studied. For hot-seeding processes, Nd-Ba-Cu-O or Sm-Ba-Cu-O crystals with a high decomposition temperature are put on the matrix during the growth of the bulk, around a peritectic temperature (
An idea for the novel cold-seeding of a top-seeded melt-growth with a RE-Ba-Cu-O bulk has been worked on by employing an MgO crystal seed and a buffer pellet . The growth process is composed of two stages. The MgO seed was for the texture-growth of the small RE-Ba-Cu-O pellet with a high melting point (
Detailed information for the preparation of the samples is described elsewhere . GdBa2Cu3O7-
According to the results of the differential thermal analysis (DTA) measurements , we used the heat treatment profile shown in Fig. 1. The sample was heated within 10 hours to
Fig. 2 shows the appearance of the bulk samples prepared by conventional hot-seeding (a), cold-seeding using a Nd123 thin film (b), and cold-seeding in association with a MgO-buffer pellet (c). The
The present cold-seeding method can be used for growth with a high
The growth can be transferred from a high-
Fig. 3 shows the microstructure of the portion at the buffer/matrix interface. The boundary is denoted by the broken line. A different contrast of Gd211 density was observed below and above the boundary. Because of the push effect of Gd211, a high Gd211 density area is formed at the interface. The composition of the matrix was measured by EPMA for points indicated by green closed circles in Fig. 3 (a). There is Gd1+0.02Ba2-0.02Cu2.72 in the composition of the buffer side and it is approximately close to Gd1+0.09Ba2-0.09Cu2.64 in the matrix side. We suspect that because of the large undercooling and growth rate, the Gd/Ba substitution is inhibited at the buffer side.
Secondly, we emphasize how to launch additional pinning centres into the RE123/Ag matrix. There are several strategies which are partly analogue to the implantation of pinning centres in thin film forms. Partial atomic substitutions of the Ba2+ site with RE3+ in RE123 induce a so-called “peak effect” around 1.5-2.0 T in the
matrix with Gd211. Gd211 tends to form domains of a large size inside Gd123. Various kinds of oxides and RE2Ba4
Cardwell et al.  and Muralidhar et al.  have developed general process routes to grow batches of RE-Ba-Cu-O single domain superconductors with good pinning performance. The flux pinning and
Xu Yan et al
SEM observations were also carried out to confirm the information of the Fe-rich region obtained from TEM. The representative back scattered electron image is shown in Fig. 5 (a), where the larger particles represent silver, and the homogeneous distributed small particles are Gd-211 embedded in the Gd-123 matrix, according to the EDX analysis. Consistent with the results from TEM, the Fe element was only found in the vicinity of silver, as shown in Fig. 5(b). This may be attributed to the following three reasons:
First, silver and Fe3O4 possess a cubic structure with a lattice mismatch:
Besides this, in the growth process, the added Pt may exist around the boundary between Gd123 and Ag, for example. Fe is known to be with Ag. The addition of magnetic oxide, such as Fe2O3 or other kinds of Fe alloys, has been investigated from the viewpoint of the magnetic pinning effect. Tsuzuki et al. have reported that Fe2O3 was introduced into the Gd123 matrix . The maximum trapped flux increased by over 30 %. In the case of Fe-B particles addition,
Separately, the optimal addition of these magnetic particles induces an increase of the number of Gd211 particles while decreasing the size. We emphasize the current issues concerning the homogeneity of the distribution of these particles together with TEM observations .
Another unique aspect concerning flux trapping is to distribute holes drilled within the bulk pack. The recently reported [41-45] hole-patterned YBa2Cu3Oy (Y123) bulks with improved superconducting properties are highly interesting from the points of view of material quality and their variety of application. It is well known that the core of plain bulk superconductors needs to be fully oxygenated, and some defects like cracks, pores and voids [46, 47] must be suppressed in order that the material can trap a high magnetic field or else carry a high current density. Some previous studies [48-51] demonstrated that, by filling the cracks, enhancing thermal conductivity or by reinforcing the YBCO bulk material, the properties can be improved and a trap field of up to 17 T at 29 K can be reached. One of the interests of this new sample geometry is in increasing specific areas for thermal exchange, shortening the oxygen diffusion path, and offering the possibility of reinforcing the superconductor materials. To minimize the above defects, we propose the improvement of the superconducting material with an innovative approach - “material by design” based on the concept of a YBa2Cu3Oy (Y123) bulk with multiple holes.
The details of the multiple holes process of YBa2Cu3Oy (Y123) are reported elsewhere [41, 42]. Basically, the holes in the pre-sintered bulk were realized by drilling cylindrical cavities with different diameters, 0.5-2 mm through the circular or square shaped sample. The holes are arranged in a regular network on the plane of the samples. A SmBa2Cu3Ox (Sm123) seed used as a nucleation centre was placed (between the holes close to the centre) on the top so as to obtain the single domain of the samples. The seed orientation was chosen to induce a growth with the c-axis parallel to the pellet axis. The elaboration of single domains through the drilled pellets is then conducted in a manner similar to the plain pellets. But how to claim a single domain on the drilled sample? The demonstration of the growth of single domains from the perforated structure is shown by Fig. 7(a). The growth lines of faceted growth on the surface of the drilled single domain half are not clearly observed, but they exist when compared to the plain half. This shows that the pre-formed holes do not seem to disturb the growth of the single domain, which is confirmed by the top seed melt growth process of other perforated samples prepared by Chaud et al. . Basically, the ability of a growth front to proceed through an array of holes or a complex geometry is not evident
The various square or circular-shaped Y123 were grown into a single domain including an interconnected structure. Optical macrographs of as-grown samples with holes are shown in Fig. 8. Fig. 9 (a and b) illustrate the cross sections of plain and perforated samples. The porosity is drastically reduced for the drilled sample. For the plain sample, a large porosity and crack zones are noticeable. Scanning Electron Microscopy between two holes shows (i) the compact, crack–free microstructure and (ii) a uniform distribution of fine Y211 particles into the Y123 matrix .
Fig. 10 presents the flux trapping obtained on plain and perforated samples (36 mm in diameter and 15 mm in height) after conventional oxygenation at 450 C for 150 hours. The samples (Fig. 4c) were previously magnetized at 1 T, 77 K, using an Oxford Inc. superconducting coil. The 3D representation of the magnetic flux shows the single dome in the both cases corresponding to the signature of a single-domain. The network of the holes has not affected the current loops at the large scale. This result was confirmed by the neutron diffraction measurements (D1B line at ILL, France) showing  only one single domain bulk orientation with mean c-axes parallel to the pellet axis. The trapped field value is higher in the perforated pellet (583 mT) than in the plain one (443 mT). This represents an increase of 32% for the drilled sample compared with the plain one, in agreement with our previous report . This increasing of the trapped field value is probably due to: (i) better oxygenation and/or less cracks and porosities for the drilled pellet, as illustrated by Fig. 3b, (ii) strong pinning, because the hole could be favourable to the vortices’ penetration, (iii) enhancement of the cooling, because the sample with holes offers a large and favourable surface exchange into the liquid nitrogen bath.
On the other hand, pulse magnetization was used on the drilled and plain pellets. Both samples (16 mm diameter samples, 8 mm thick) were tested with a series of pulse magnetization experiments. A Helmholtz coil was used to generate a homogeneous magnetic field. The maximum amplitude of the magnetic field is 1 T and the raising time of the pulse was 1 ms while the decay time was 10 ms. After the pulse, the trapped field was mapped with a hall sensor probe at 0.5 mm above the sample. The result shows that for the application of a 1 T pulse the trapped field increases by up to 60% for a drilled pellet  as compared with to the plain one. This is an interesting result for such a form of new geometry, demonstrating the ability of the textured Y123 with multiple holes to trap a high field.
According to their thin wall geometry, the drilled bulk should be well oxygenated in comparison with the plain samples. The oxygen diffuses easily through the tube channels. The thermogravimetry technique was selected to compare the oxygenation quality of different pellets. The oxygen uptake was related to the increase of the sample weight. In this study, pellets of 16 and 24 mm diameter were used and a network of 30 holes was perforated. For each diameter, five drilled and five plain pellets were processed with the same heat treatment. All of the samples were weighted before and after the oxygenation, and the percentage of the weight gain was evaluated according to the following relation:
m (%) = 100 (mfinal-minitial)/minitial
The measurements were realized twice to check reproducibility. For that, the samples after the first measurement were de-oxygenated at 900 C, after half an hour, and followed by the quench step and then re-oxygenated. After the second measurement, the average values of the weight were estimated and plotted in Fig. 11. It was difficult to oxygenate the bulk sample with a big diameter and in this case the oxygen should diffuse into the core of the bulk. Generally, the big samples are annealed under oxygen at 400-450 C between 150 to 600 hours [42, 46, 53, 54]. These annealing dwell times are so long in order to allow for oxygen diffusion until the core of the monolith bulk materials. The drilled samples seem to offer an advantage (a saving of time) for annealing under oxygen of the superconductor bulk. This advantage is clearly shown in Fig. 11 where 25 hours is sufficient to obtain the full oxygenated sample; in the other word, maximum weight gain is quickly achieved. In addition, thin-wall geometry was introduced to reduce the diffusion paths and to enable a progressive oxygenation strategy . As a consequence, cracks are drastically reduced. In addition, the use of a high oxygen pressure (16 MPa) further speeds up the process by displacing the oxygen–temperature equilibrium towards the higher temperature of the phase diagram. The advantage of thin-wall geometry is that such an annealing can be applied directly to a much larger sample during a shorter time (72 hrs compared with 150 hrs for the plain sample). Remarkable results have been obtained by the combination of thin walls and high oxygen pressure. Fig. 13 shows the 3D distribution of the trapped flux mapping measured at 77 K on the perforated thin wall pellet. The maximum trapped field value of 0.8 T is almost twice that obtained on the plain sample (0.33 T).
On the other hand, the effect of the number of the holes has been investigated and reported . Table 1 summarizes the sample characteristics and the maximal trapped field values. We can clearly note that, for the samples having the same diameter and the same size of hole, the trapped field increases with the increase of the number of holes. An explanation could be that the better oxygenation is due to the large surface exchange with the density of the thin wall.
The Y123 domain with open holes could be reinforced, e.g. by infiltration with a low temperature melting alloy, so as to improve the mechanical properties that are useful for levitation applications or trapped field magnets. The perforated Y123 bulks with 1 or 2 mm diameter holes were dipped into the molten Sn/In alloy or an epoxy wax at 70 C for 30 minutes in a vessel after evacuating it with a rotary pump and venting air to enable the molten alloy or liquid resin to fill up the holes. After cooling, the impregnated bulk materials were polished. Some samples were impregnated with a BiPbSnCd-alloy using the process described elsewhere . Fig. 12 shows the top surface and the cross-sectional view of the impregnated Y123 bulk samples. We can see the dense and homogeneous infiltration of the wax epoxy and the Sn/In alloy. The magnetic flux mapping of the sample filled with a BiPbSnCd-alloy has been investigated. The same trapped field of 250 mT before and after impregnation has been measured. Presently, it is important to develop the specific shapes of bulk superconductors with mechanical reinforcement  for any practical application.
|sample thickness (mm)||7.6||7.6||7.8||7.5|
|number of holes||20||37||21||85|
Multiple holes or porous ceramic materials, such as alumina and zirconia, are established components in a number of industrial applications such as inkjet printers, fuel injection systems, filters, structures for catalysts, elements for thermal insulation and flame barriers. The combination of a high specific surface with the ability to be reinforced in order to improve mechanical and thermal properties makes the perforated YBCO superconductors interesting candidates both for a variety of novel applications and for fundamental studies. As an example, the artificial drilled Y123 in a desired structure [43, 57] is a good candidate for resistive elements in superconducting fault current limiters (FCL) [58, 59]. In this application, the thin wall between the holes allows more efficient heat transfer between a perforated superconductor and cryogenic coolant during an over-current fault compared with conventional bulk materials. The high surface area of the perforated materials, which may be adjusted by varying the hole diameter, makes them interesting candidates for studying fundamental aspects of flux pinning, since the extent of surface pinning, and hence Jc
Finally, we highlight the examples among recent progress of HTS bulk applications, flywheel, power devices as motors and generators, magnetic drug delivery systems and magnetic resonance devices as well.. As shown in Fig. 14, a variety of Gd123 bulks have been tested for the employment of field pole magnets as a way of intensifying flux trapping applications. The bulk magnets are cooled down to 30 K with step-by-step pulsed-field magnetization using a homemade large dc current source. A large pulsed current is momentary applied to armature copper windings by which a pulsed magnetic field is formed and applied to the bulk field poles [60-63].
In summary, for the application of bulk HTS rotating machines, the enhancement of the trapped flux is a crucial task for achieving practical applications with high torque density. The increase of critical current density using artificial pinning centres marks an efficient technique for the enhancement of the properties of flux trapping. We attempted to enhance both the Jc and the trapped flux in bulk HTS with the addition of magnetic/ferromagnetic particles. An Fe-B-Si-Nb-Cr-Cu amorphous alloy was introduced into the Gd123 matrix. The melt growth of single-domain bulks with different magnetic particles was performed in air. The enhancement of the critical current density Jc at 77 K was derived in those bulks with the addition of Fe-B-Si-Nb-Cr-Cu, while the superconducting transition temperature of 93 K was not degraded significantly. The experiment of magnetic flux trapping was then conducted under static magnetic field magnetization with liquid nitrogen cooling. In the bulk with 0.4 mol% of Fe-B-Si-Nb-Cr-Cu, the integrated trapped flux exceeds over 35% compared with the one without the addition of magnetic particles. On the other hand, the addition of CoO particles resulted in a reduction of both Jc and trapped magnetic flux. The recent results indicate that the introduction of magnetic particles gives significant effect to the flux pinning’s performance.
By inserting a buffer pellet with a higher
The single domain of Y123 bulks with multiple holes has been processed and characterized. SEM investigations have shown that the holes’ presence does not hinder the domain growth. The perforated samples exhibit a single domain character evidenced by a single dome trapped-field distribution and neutron diffraction studies. This new structure has great potential for many applications, with improved performances in place of Y123 hole free bulks, since it should be easier to maintain at liquid nitrogen temperature and/or to improve thermal conductivity during application, avoiding the appearance of hot spot. It is clear that the Y123 bulks with an artificial pattern of holes are useful for evacuating porosity from the bulk and assisting the uptake the oxygen. The ability of the Y123 material with multiple holes to trap a high field has been demonstrated. Using high pressure oxygenation, the trapped field increases up to 0.8 T at 77 K for the thin wall pellet, corresponding to 50% more than the bulk material without holes. Using pulse magnetization, the trapped fields increases by up to 60% for the drilled pellet with respect to the plain one. Superconducting bulks with an artificial array of holes can be filled with metal alloys or high strength resins to improve their thermal properties without any important decrease of the hardness , so as to overcome the built-in stresses in levitation and quasi-permanent magnet applications. The thin wall bulks superconducting on extruded shapes for portative permanent magnets are under development for the introduction at the large scale of this innovative approach of “material by design”.
The present work was supported by KAKENHI (21360425), Grant-in-Aid for Scientific Research (B) and the "Conseil Régional de Basse Normandie, France". This work was partly performed using the facilities of the Materials Design and Characterization Laboratory, Institute for Solid State Physics, University of Tokyo. The authors would like to thank Caixuan Xu, Yan Xu, Xu Kun, Keita Tsuzuki, Difan Zhou, Shogo Hara, Yufeng Zhang, Motohiro Miki, Brice Felder and Beizhan Li.