Tomoyuki Naito

RE-Ba-Cu-O (REBCO, RE: rare earth element) superconducting bulk is a compact source of tesla-class magnetic field and has potential applications in compact NMR spectrometers, motors, drug delivery, etc. To operate the REBCO bulk magnets at high temperatures around 40–50 K, the enhancement of the trapped field is necessary, for which fine RE $_{{\text{2}}}$ BaCuO $_{{\text{5}}}$ (RE211) is quite effective. Fine RE211 was obtained by ball-milling using ZrO $_{{\text{2}}}$ media which has excellent wear resistance and low contamination. However, a small amount of impurities is known to act as pinning centers; therefore, we prepared the fine Gd211 powder by ball-milling using a stainless steel container and balls. Single grain GdBCO bulks were fabricated by top-seeded melt growth method from a mixture of GdBa $_{{\text{2}}}$ Cu $_{{\text{3}}}$ O $_{{\text{7}}-x}$ (Gd123), ball-milled Gd211, and CeO $_{{\text{2}}}$ powders; we also used non-ball-milled Gd211 for a reference. A trapped field at 40 K increased from 3.96 T of the bulk using non-ball-milled Gd211 to 4.29 T of that using the ball-milled one, although a decrease in the $T_\mathrm{c}$ was observed for the latter bulk. The obtained results can be explained by dilute doping of iron derived from the stainless steel media.

Single-grain RE-Ba-Cu-O (REBCO) bulks are tesla-class superconducting magnets known to capture magnetic fields of over 10 T by field-cooled magnetization. However, electromagnetic forces often cause mechanical fractures during and after the magnetization process. As pores in the REBCO bulk act as a starting point for fractures, reducing the porosity of the REBCO bulk is critical. Single-grain GdBCO bulks were fabricated from precursors via a top-seeded melt growth method. The precursors were manufactured using spark plasma sintering (SPS) from GdBa $_{2}$ Cu $_{3}$ O $_{7-x}$ and Gd $_{2}$ BaCuO $_{5}$ powders in a 7:3 molar ratio. For comparison, a conventional precursor was prepared using only uniaxial pressure. The density of the GdBCO bulk increased with that of the SPSed precursor. An SPS temperature of 900 °C produced the densest precursor and GdBCO bulk, respectively, at approximately 93 and 94% of the ideal density of the mixed GdBa $_{2}$ Cu $_{3}$ O $_{7-x}$ and Gd $_{2}$ BaCuO $_{5}$ powders. At 77 K, the trapped magnetic flux density $B_\mathrm{T}$ in SPSed bulks was comparable to that in conventional bulks, but the critical current density was higher. Using an SPS-processed precursor might enhance the vortex pinning properties of single-grain REBCO bulks. To improve the $B_\mathrm{T}$ a GdBCO bulk with a highly homogeneous texture must be prepared by optimizing the fabrication conditions.

An in-situ infiltration and reaction (IR) method is one of the most promising methods for the mass production of dense MgB $_{2}$ with a filling factor of over 90%. This is because it does not require an apparatus that applies physical pressure during synthesis, in contrast to the hot isostatic pressing, hot pressing, and spark plasma sintering methods. To promote the infiltration of Mg melt into the B preform, a low-reactivity crystalline B has usually been used to create MgB $_{2}$ bulks with centimetric dimensions. In this paper, we attempted to fabricate large MgB $_{2}$ bulks by the IR method using highly reactive amorphous B, which should result in a high critical current, as reported in the literature. To promote the intrusion of Mg melt, we mixed a small amount of Mg into the amorphous B preform, which is expected to give a similar effect to the so-called premix method. We successfully produced IR-processed MgB $_{2}$ disk bulks, of 20 mm diameter and 5 mm thickness, at a reaction temperature of 700 °C with a holding time of 9 or 24 h. A trapped field $B_\mathrm{T}$ of 1.85 T at 20 K was obtained at the surface centers of the MgB $_{2}$ bulks, which is comparable with the $B_\mathrm{T}$ values of dense MgB $_{2}$ bulks prepared by the hot isostatic pressing, hot pressing, and spark plasma sintering methods. Therefore, we can conclude that the Mg premixing method is an effective way of preparing dense MgB $_{2}$ bulks with a high $B_\mathrm{T}$ by the IR process using amorphous B.

An infiltration-reaction (IR) method can fabricate a dense MgB $_{2}$ bulk under ambient pressure, thus being a promising method for mass production. However, the IR-processed MgB $_{2}$ bulks are known to contain the unreacted Mg and B and the intermediate product, Mg $_{2}$ B $_{25}$ , which degrades the trapped field, $B_\mathrm{T}$ , and critical current density, $J_\mathrm{c}$ , of the bulks. To expedite the creation of the MgB $_{2}$ phase during the IR process, we prepared the refined B powder by ball-milling and fabricated the MgB $_{2}$ bulks by the IR method. We achieved both improved $B_\mathrm{T}$ and $J_\mathrm{c}$ for the MgB $_{2}$ bulks made from the ball-milled B powders with sub-micron size. The microstructural analysis suggested that these results originated from the refinement of MgB $_{2}$ grains and the increase of the volume fraction of MgB $_{2}$ phase. The refinement of the starting B powder is quite effective to enhance the magnetic properties for the IR-processed MgB $_{2}$ bulks.

We measured ¹ H nuclear magnetic resonance (NMR) spectra from silicon rubber in a magnetic field trapped by a compact tubular MgB ₂ superconductor bulk magnet (60 mm outer diameter, 40 mm inner diameter, and 60 mm height) which was magnetized by field-cooled magnetization from an applied field, B a p p , of 0.4778 T at 20 K. The sharpest ¹ H NMR spectrum of the resonant frequency, f R , of 20.314 MHz, which corresponded to a field strength of 0.4771 T, was observed at 1 mm below the center of the bore of the MgB ₂ tube. The homogeneity of the trapped field was within 100 ppm between z = −2 and +2 mm along the central z -axis, which was evaluated by the position-dependent f R . No change in the trapped field was observed for about 25 d, which corresponded to a decay rate of less than 0.1 ppb h ⁻¹ . The obtained results suggest that the MgB ₂ bulk magnet is a promising candidate for a magnetic pole in an NMR apparatus.

We have studied the trapped field properties of triple and double stacking of M g B 2 / T i B 2 composite superconductor bulks magnetized by field-cooled magnetization. The triple-stacked M g B 2 / T i B 2 bulks trapped a magnetic field of 5.58 T successfully at 11.3 K, which was only 0.18 T but evidently higher than the world record trapped field of 5.4 T among M g B 2 bulk magnets. We also found a potential ability of the present M g B 2 / T i B 2 bulks trapping a 7-T class magnetic field at low temperatures below 4.2 K, although the FCM experiments at both 2.2 and 4.2 K were disturbed by the flux jumps.

A trapped field of B _T = 1.61 T was experimentally achieved at the central surface of an MgB ₂ bulk composite (60 mm in diameter and 20 mm in height) at 20 K by double pulsed-field magnetization (PFM) using a split-type coil. The composite bulks consisted of two MgB ₂ ring bulks sandwiched by thin copper ring plates, which were then stacked, and a soft iron yoke cylinder was inserted in the central bore of the rings. The copper ring plates delayed the rise time and duration of the magnetic pulse due to eddy currents. The inserted soft iron yoke attracted the magnetic flux and enhanced the trapped field strength mainly due to its large permeability. As a result, the trapped field was enhanced from B _T = 0.34 T for the single MgB ₂ ring bulk without the copper plates and soft iron yoke to B _T = 1.00 T for the composite with both copper plates and the soft iron yoke. The inserted soft iron yoke can be exploited to enhance the trapped field because the intrinsic B _T of the single MgB ₂ ring bulk was smaller than the saturation field of the yoke (∼2 T). Using an optimized second pulse application after suitable flux trapping from the first pulse application, the trapped field was enhanced considerably to B _T (2nd) = 1.61 T, which is a record-high trapped field for an MgB ₂ bulk by PFM to date. The combination of the longer magnetic pulse application by the copper plates, the enhancement of the effective applied field by the inserted soft iron yoke, and the double pulse application using split-type coil is an effective technique to enhance the trapped field in the MgB ₂ bulk using PFM.