Physica C 468 (2008) 1469–1472

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Physica C
journal homepage: www.elsevier.com/locate/physc

Trapped ﬁeld enhancement of ﬁve-aligned superconducting bulk magnetizedby pulse ﬁeld for magnetic separation
T. Hiyama a, H. Fujishiro a,*, T. Tateiwa a, T. Naito a, H. Hayashi b, K. Tone bab

Faculty of Engineering, Iwate University, 4-3-5 Ueda, Morioka 020-8551, Japan
Kyushu Electric Power Co. Inc., 2-1-47 Shiobaru, Fukuoka 815-8520, Japana r t i c l ei n f oArticle history:
Available online 28 May 2008
PACS:
74.25.Bt
74.80.Bj
74.60.Ge
Keywords:
Superconducting bulk magnets
Multi-bulk
Pulse ﬁeld magnetization
Trapped ﬁeld
Magnetic separationa b s t r a c t
Five-aligned superconducting bulk magnet system has been improved and the trapped ﬁeld characteristics have been investigated by the pulse ﬁeld magnetization (PFM). The trapped ﬁeld BT(z = 0 mm) isenhanced to 2.7 T at the bulk surface because of the lowering temperature Ts and the enhancement of
the applied ﬁeld Bex, compared with those for the proto-type system [Physica C 445–448 (2006) 399].
The operating ﬁelds, BT(2 mm) at the vacuum sheath surface and BT(9 mm) at the surface of the rotating
membrane in water-treatment system, are also enhanced because of some structural improvement, andthe multi-bulk magnet system available for the magnetic separation is realized.
Ó 2008 Elsevier B.V. All rights reserved.

1. Introduction
The practical applications using a high-Tc superconducting(HTSC) bulk have been proposed and intensively developed. Thesuperconducting bulk magnet using a strong pinning force is oneof the promising applications, on which the trapped ﬁeld BT is ashigh as a few Tesla and is about one order of magnitude higher
than that of strong Nd–Fe–B permanent magnet. Although, a ﬁeld
cooling magnetizing method (FCM) using a solenoid-type superconducting magnet (SM) is suitable to magnetize HTSC bulks, it
is impossible to magnetize the multi-HTSC bulks in a line using
it. Saho et al. of Hitachi Ltd., Japan, developed a water-treatment
system consisting of a membrane separator using seven-alignedbulks, which were magnetized by the modiﬁed FCM; the bulks
were cooled below superconducting transition temperature Tc,
and during cooling, the bulks were inserted into and withdrawn
from the split-type SM repeatedly [1]. This system was used to remove phytoplankton multiplying in highly eutrophic lakes anddams [2]. However, since the magnetizing system for the modiﬁed
FCM is large, heavy and expensive, the magnetizing process must
be operated at a speciﬁed institute or laboratory. On the otherhand, a pulse ﬁeld magnetization (PFM) method, which has been
* Corresponding author. Tel./fax: +81 19 621 6363.
E-mail address: fujishiro@iwate-u.ac.jp (H. Fujishiro).
0921-4534/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.physc.2008.05.141

intensively investigated and developed, is a compact, inexpensive
and mobile experimental setup to magnetize HTSC bulks. We havedeveloped a new HTSC bulk magnet system to purify waste-waterusing PFM [3]. The system consists of ﬁve-aligned GdBaCuO bulks
cryocooled to 50 K from the side face and the bulks were magnetized in turn by PFM using a split-type copper coil. The highest
trapped ﬁeld was BT = 2.0 T on the bulk surface and the ﬁeldBT(6 mm), 6 mm distant above the bulk surface at the vacuum
sheath surface, was 1.0–1.2 T for each bulk. However, the operatingﬁeld BT(OP) on a membrane separator at d = 18 mm from the bulk
surface, was as low as 0.3 T, which was not strong enough for thepractical separation. In this paper, we report the enhancements
both of BT and BT(OP) of ﬁve-aligned superconducting bulk magnet
system by several improvements.
2. Experimental procedures
The schematic views of the ﬁve-aligned superconducting bulk
magnet and the structure of membrane–magnetic separator forwater treatment are shown in Fig. 1. Table 1 shows the typicaldimensions, the characteristic values and the results of the presentbulk magnet system, compared with those of the proto-type system[3]. In this study, several improvements were performed in the system to enhance the BT and BT(OP). c-axis oriented ﬁve rectangularshaped GdBaCuO bulks (bulk #1–bulk #5; 34  34  15 mm3,

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Fig. 1. (a) The top and side views of the ﬁve-aligned superconducting bulk magnet. (b) The structure of membrane separator for water treatment. Magnetic ﬂocks depositedon the membrane are removed by the magnetic force generated using HTSC bulks.

Table 1
The typical dimensions, the characteristic values and the trapped ﬁelds of the presentsystem, compared with those of the proto-type [3]Ts (K)Maximum applied ﬁeld Bex (T)Thickness of bulk D (mm)G (mm)T (mm)L (mm)
Zop (mm)
BTOP (T)BTmax (T)Improved-type (this work)

Proto-type (Ref. [3])

30
6.0
15
1
1.0
49
90.7
2.6

50
5.5
20
4
1.5
70
18
0.3
1.6

sheath width L, G and T. The maximum applied ﬁeld Bex was enhanced to 6.0 T because of the reduction of the gap of the split-type
pulse coil by the reduction of the bulk thickness D. The trappedmagnetic ﬁeld BT were monitored at the center of the bulk of #1
and #3 using the Hall sensor (F.W.Bell, Model BHT921). The pulse
ﬁeld of Bex = 3.0–6.0 T with a rise time of 12 ms was applied tothe zero-ﬁeld cooled bulk. After the completion of magnetizing abulk, the split-type magnetizing coil was moved in parallel and another bulk was magnetized in turn in the same manner. The successive pulse applications with identical strength (SPA) wereperformed to enhance BT. The magnetic ﬁeld BT(z) along the z direction away from the sheath surface (z = 2 mm) was measured using
an axial-type Hall sensor.
3. Results and discussion

Nippon Steel Co., Ltd.) were tightly fastened with the copper holder
from the side face (along the ab-plane) and the holder was attachedto the cold stage of a double stage Gifford McMahon (GM) cycle helium refrigerator. The temperature of the #1 bulk was controlled as
low as Ts = 30 K. The vacuum gap G between the bulk surface andthe inner surface of the vacuum sheath, and the thickness T of thevacuum sheath made of stainless steel were reduced to 1.0 mm
and 1.0 mm, respectively. As a result, the distance between the bulksurface and the outer surface of the vacuum sheath became as shortas z = G + T = 2 mm. The operating distance Zop, which was the distance from the bulk surface to the surface of the membrane ﬁlter,
was reduced to 9.0 mm because of the reduction of the vacuum

Fig. 2a and b show the trapped ﬁelds BT(z) at the bulk surface(z = 0 mm) and at the vacuum sheath surface (z = 2 mm) as a function of the applied ﬁeld Bex for the #1 and the #3 bulk, respectively.
The right-hand side ordinate shows the Bex dependence of the total
trapped ﬂuxes UT integrated 0.5 mm above the sheath surface. Forthe #1 bulk, the BT values are pretty low, slightly increases withincreasing Bex up to 5 T and show high values at Bex = 5.8 T. Forlower Bex region (Bex < 5 T), however, UT shows a ﬁnite value andslightly increases with increasing Bex, even if the BT(z = 0 mm) value is nearly zero at the bulk center. This result suggests that themagnetic ﬂuxes are mainly trapped at the bulk periphery. On theT. Hiyama et al. / Physica C 468 (2008) 1469–1472

1471Fig. 4. The line scan proﬁle of the trapped ﬁeld BT(z) in open space, along thedirection through the bulk center after the magnetization of the ﬁve bulks.

Fig. 2. The trapped ﬁelds BT(z) at the bulk surface (z = 0 mm) and at the vacuum
sheath surface (z = 2 mm) as a function of the applied ﬁeld Bex for (a) the #1 and (b)the #3 bulk, respectively. The right ordinate shows the Bex dependence of the total
trapped ﬂuxes UT integrated 0.5 mm above the sheath surface.

other hand, for the #3 bulk, the magnetic ﬂuxes start to intrude
into the bulk center from relatively low Bex value (Bex > 4 T) andthe BT value took a maximum at Bex = 5.2 T and then decreased.
Since the ﬁeld trapping ability for the #1 and #3 bulk is nearly
the same (BT = 1.26–1.30 T at 77 K by FCM), the difference in theBT vs. Bex curve between #1 and #3 bulk results from the differencein the starting temperature. The temperature of the copper support
near the #1 bulk was controlled at 30 K, but we did not measure
the temperature at the bulk surface. The temperature would behigher for the bulks far from the cold stage, namely, the temperature of the #3 bulk would be higher than that of the #1 bulk because of the heat intrusion and/or the low cooling power. Then,the appropriate applied ﬁeld Bex was independently decided foreach bulk.
Fig. 3a and b shows the trapped ﬁeld BT(z = 2 mm) on the vacuum sheath and the total trapped ﬂux UT for each bulk after applying single pulse ﬁeld Bex and the SPA procedure, respectively. InSPA, the pulse ﬁelds with identical strength were applied twiceor three times after returning to the initial temperature. In Fig.
3a, the applied ﬁeld Bex is indicated for each bulk, which wasslightly decreased with increasing bulk number as discussed
above. The BT(z = 2 mm) value of the #1, #3 and #4 bulks slightly
increases by SPA, but that of the #2 and #5 bulks remain constant.
However, the UT value is enhanced by SPA for all the bulks. The BTand UT values are enhanced by the SPA process for a single bulk,
but it should be noticed that these values sometimes might be reduced by SPA. Because the generated heat by PFM propagates tothe next bulk and the already trapped ﬂuxes might escape due toFig. 3. (a) The trapped ﬁeld BT(z = 2 mm) and (b) the total trapped ﬂux UT for each bulk after applying single pulse ﬁeld Bex and the successive pulse application with identicalstrength (SPA).

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experimentally obtained data. As shown in Fig. 1b, the operatingdistance on a rotating membrane separator is ZOP = 9.0 mm in thewater-treatment system, at which the magnetic ﬁeld is estimatedto be BTOP = 0.7 T. The value is about two times larger than thatin the proto-type separator [3]. But the magnetic strength is still
weak to attract the magnetic ﬂock sufﬁciently. The further
improvement to enhance the trapped ﬁeld is in progress.
4. SummaryFig. 5. The trapped ﬁeld BT(z) at the center of the #1 and #3 bulks as a function ofthe distance z. The estimated BT(z) is also shown, which is calculated using a Biot–Savart’s law.

the temperature rise. An inverse magnetic ﬁeld due to the leak of
the applied ﬁeld might be applied to the next bulk, which resultsin the decrease of BT on the next bulk.
Fig. 4 shows the line scan proﬁle of the trapped ﬁeld BT(z) inopen space, along the direction through the bulk center after themagnetization by SPA of the ﬁve bulks. The BT(z) proﬁle changes
periodically and the BT(z) value decreases with increasing z, i.e.,the maximum BT(z) value is 1.8–2.2 T at z = 2 mm, but that at
z = 5 mm decreases to 1.0–1.2 T.
Fig. 5 shows the trapped ﬁeld BT(z) at the center of the #1 and#3 bulks as a function of the distance z from the bulk surface. TheBT(z) value is sharply reduced with increasing z. The estimatedBT(z) line is also shown in ﬁgure, which was calculated using aBiot–Savart’s law. The calculated BT(z) proﬁle can reproduce theWe have improved the ﬁve-aligned superconducting bulk magnet system and investigated the trapped ﬁeld characteristics by thepulse ﬁeld magnetization. The trapped ﬁeld BT(z = 0 mm) is enhanced to 2.7 T at the bulk surface by lowering the temperature
Ts and the enhancement of the applied ﬁeld Bex, relative to thosefor the proto-type system. The operating ﬁeld BT(z = 9 mm) at thesurface of the rotating membrane in water-treatment system wasalso enhanced to 0.7 T because of some structural improvements.

Acknowledgments
The authors thank Professor T. Oka of Niigata University for thevaluable discussions. This work is supported in part by a Grant-inAid for Scientiﬁc Research from the Ministry of Education, Culture,Sports, Science and Technology, Japan (No. 19560003).
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