IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 26, NO. 4, JUNE 2016

6800104

Pulsed Field Magnetization of Single-Grain Bulk
YBCO Processed From Graded Precursor Powders
Mark D. Ainslie, Member, IEEE, Hidehiko Mochizuki, Hiroyuki Fujishiro, Wei Zhai, Devendra K. Namburi,
Yun-Hua Shi, Jin Zou, Anthony R. Dennis, and David A. Cardwell

Abstract—Large single-grain bulk high-temperature superconducting materials can trap high magnetic fields in comparison withconventional permanent magnets, making them ideal candidates
to develop more compact and efficient devices, such as actuators,
magnetic levitation systems, flywheel energy storage systems, andelectric machines. However, macrosegregation of Y-211 inclusions
in melt-processed Y–Ba–Cu–O (YBCO) limits the macroscopic
critical current density of such bulk superconductors, and hence,the potential trapped field. A new fabrication technique using
graded precursor powders has recently been developed by our
research group, which results in a more uniform distribution of
Y-211 particles, in order to further improve the superconducting
properties and trapped field capability of such materials. In thispaper, experimental results on the pulsed field magnetization ofa graded single-grain YBCO bulk sample are presented. Pulsed
fields of magnitude up to 5.5 T were applied to the sample, at temperatures of 40 and 65 K. The trapped field profiles indicate thatmagnetic flux enters the sample in a more uniform manner thanstandard YBCO samples, resulting in a higher overall trappedfield and indicating the success of this processing technique to
produce a more homogeneous sample.
Index Terms—Trapped field magnets, bulk superconductors, graded YBCO, pulsed field magnetization, magnetic flux
penetration.

I. I NTRODUCTION

L

ARGE, single-grain bulk high-temperature superconducting materials can trap large magnetic fields over 17 T attemperatures below 30 K [1], [2] and up to 3 T at the technologically important temperature of 77 K [3]. This makes them attractive as permanent magnet equivalents (as so-called trappedfield magnets) in a number of practical applications, such as rotating machines [4]–[7] and magnetic separation [8]. However,in standard melt-processed Y–Ba–Cu–O (YBCO) samples, ma-

Manuscript received October 18, 2015; accepted December 4, 2015. Dateof publication December 17, 2015; date of current version January 7, 2016.
This work was supported by the Royal Society International Exchanges Scheme
under Grant IE131084. The work of M. D. Ainslie was supported by a
Royal Academy of Engineering Research Fellowship. The work of J. Zou was
supported by Churchill College; by the China Scholarship Council; and by the
Cambridge Commonwealth, European and International Trust.
M. D. Ainslie, W. Zhai, D. K. Namburi, Y.-H. Shi, J. Zou, A. R. Dennis,
and D. A. Cardwell are with the Bulk Superconductivity Group, Departmentof Engineering, University of Cambridge, Cambridge CB2 1PZ, U.K. (e-mail:mark.ainslie@eng.cam.ac.uk).
H. Mochizuki and H. Fujishiro are with the Department of Materials Scienceand Engineering, Faculty of Engineering, Iwate University, Morioka 020-8551,
Japan (e-mail: fujishiro@iwate-u.ac.jp).
Color versions of one or more of the figures in this paper are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/TASC.2015.2509167

Fig. 1. Graded bulk YBCO sample fabricated by varying the weight of CeO2 in
the precursor powders (top = 2 wt%; bottom = 1 wt%) arranged in a nested
U-shaped cylindrical stack. Precursor powders containing 70 wt% Y-123 +30 wt% Y-211 were used for both stacks. A buffer pellet is used to improve
processing reliability and the sample’s superconducting properties.

crosegregation of Y-211 inclusions can limit the macroscopic
critical current density, Jc , and can also result in a non-uniform
Jc distribution throughout the bulk [9]. In order to improve thesuperconducting material properties of such bulks, our grouphas been developing a fabrication technique using graded precursor powders in order to achieve a more uniform distributionof Y-211 particles throughout the sample to improve the uniformity of Jc [10]. In this paper, we examine the trapped field performance of a graded YBCO sample and compare the results to
those of standard YBCO samples, using field cooling (FC) magnetization and pulsed field magnetization (PFM) techniques.
II. S AMPLE I NFORMATION
For this study, a graded, single-grain YBCO sample was fabricated using a buffer-aided, top-seeded melt-growth (TSMG)
process [11]–[13]. Two layers of precursor powders were arranged in U-shaped stacks, as shown in Fig. 1, with varying
CeO2 content in the top and bottom regions. CeO2 powders areusually added in the TSMG process to reduce the size of Y-211
inclusions and CeO2 particles can react with Y-211 particles
prior to melting to produce BaCeO3 particles that can act aseffective pinning centers, both of which act to increase Jc [11].
Precursor powders containing 70 wt% Y-123 + 30 wt% Y-211
were used for both stacks, with 2 wt% CeO2 added in the top
region (close to the seed/buffer) and 1 wt% CeO2 in the bottom
region [11]. A buffer pellet is used improve the success rateof the melt-growth process and to aid the growth of the Y-123
phase for improved superconducting properties [12], [13]. The
as-grown, pre-polished sample and the sample after polishingand mounting for magnetization experiments are shown in
Fig. 2. The final sample diameter is 31.2 mm and its thickness
is 17.7 mm.

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IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 26, NO. 4, JUNE 2016

Fig. 2. Photograph of the top surface of the as-grown prepolished graded
YBCO bulk sample (left) and the sample after polishing and mounting in thestainless steel ring for the PFM experiments (right).

Fig. 4. FC magnetization results from Fig. 1, normalized using the Beanmodel/Biot–Savart law as described in [14], to account for different sample
geometries.

Fig. 3. FC magnetization results for the graded YBCO sample as the temperature is increased from 40 to 100 K after application of a 7-T magnetic field. For
comparison, the results for two standard YBCO samples are also shown: one isa new sample made for this study and the other was presented in [16].

III. F IELD C OOLING (FC) M AGNETIZATIONThe FC magnetization technique gives the best indication of
the trapped field capability of a bulk superconductor sample
and is useful for comparing the maximum potential of different
samples [14], [15]. Fig. 3 shows the FC results for the graded
YBCO sample, measured by a Hall sensor placed at the centerof the top surface of the sample, as the temperature is increased
from 40 to 100 K after application of a 7 T magnetic field.
This graded YBCO sample achieved trapped field values of
Bt = 5.95 T at 40 K, Bt = 2.38 T at 65 K, and Bt = 0.92 T
at 77 K. For comparison, the results for two standard YBCO
samples are also shown: one is a new sample made for this study(diameter 26 mm, thickness 11.6 mm) and the other (diameter
32 mm, thickness 15 mm) was presented in [16]. Both standard
samples were fabricated by the TSMG process, similar to that
described in previous research [17].
In order to make a fair and adequate comparison between all of the samples, which are of different dimensions,Fig. 4 presents the FC magnetization results normalized using
a correction factor, as described in [14]. Using the Bean model/
Biot–Savart law, the original, measured trapped field is divided
by the factor kμ0 R to consider the average, in-field criticalcurrent density for each sample, averaged over the sample
volume. Here k is a geometric constant to account for different
sample radii and thicknesses and R is the radius of the sample.
From Figs. 3 and 4, it is clear that large, single-grain bulk
YBCO can be fabricated using this graded technique withtrapped field capabilities equal to or better than standard samples.

Fig. 5. Trapped fields for the three samples at an operating temperature of
65 K, measured at the center of the top bulk surface (z = 0 mm).

IV. PFMFC and zero field cooling (ZFC) magnetization techniques
invariably require large magnetizing coils, which is impractical for most applications of bulk superconductors. The PFM
technique, where a large magnetic field is applied via a pulseon the order of milliseconds, currently shows the most promise
as a compact, mobile and relatively inexpensive magnetization
technique for bulks acting as trapped field magnets (TFMs) inpractical applications [18].
An overview of the PFM experimental setup is described in[16]. The magnetizing coil can provide pulsed external fields
up to Bex = 6.4 T, applied parallel to the c-axis of the sample,
with a rise time of tr = 12 ms and duration of approximately
td = 120 ms. The trapped field close to the center of the samples was measured dynamically during the application of eachpulsed field using an axial-type Hall sensor located on the top
surface of the samples, and two-dimensional (2D) trapped field
distributions were measured inside the vacuum chamber using
an x–y stage controller and the same Hall sensor positioned
approximately 1 mm above the top surface.
A. Trapped Field Performance Using PFM
Figs. 5 and 6 compare the PFM trapped field results forthe graded and standard YBCO samples at 65 K and 40 K,

AINSLIE et al.: PFM OF SINGLE-GRAIN BULK YBCO PROCESSED FROM GRADED PRECURSOR POWDERS

Fig. 6. Trapped fields for the three samples at an operating temperature of
40 K, measured at the center of the top bulk surface (z = 0 mm).

Fig. 7. Two-dimensional trapped field profile for the graded YBCO sample
after PFM at 65 K for an applied pulsed magnetic field Bex = 2.45 T, measured
at 1 mm above the top surface.

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Fig. 8. Two-dimensional trapped field profile for the graded YBCO sample
after PFM at 65 K for an applied pulsed magnetic field Bex = 3.03 T, measured
1 mm above the top surface.

Fig. 9. Two-dimensional trapped field profile for the new standard YBCO
sample after PFM at 65 K for an applied pulsed magnetic field Bex = 2.45 T,
measured 1 mm above the top surface.

respectively, measured close to the center of the top surfaceof the samples. It is clear from these results that the gradedsample traps a larger magnetic field than the standard samples,
which correlates well with the FC magnetization results in
Fig. 3. Furthermore, the full activation field, defined as theexternal field required to fully magnetize the sample, is lower
at both temperatures for the graded sample, suggesting the flux
penetrates into this sample in a more uniform manner thanfor the standard samples. The steep slope of the line at 65 K
also suggests a more uniform penetration of magnetic flux as
the trapped field increases suddenly around the full activation
field. In order to provide further evidence for these hypotheses,
we examine the uniformity of the flux penetration in the next
section, using the 2D trapped field distributions.
B. Two-Dimensional Trapped Field Profiles
Figs. 7 and 8 show the 2D trapped field profiles for the graded
YBCO sample after PFM at 65 K for an applied pulsed magnetic field lower than the full activation field (Bex = 2.45 T)
and after fully magnetizing the sample (Bex = 3.03 T), respectively. For comparison, Figs. 9 and 10 show the 2D trapped field
profiles for the standard YBCO sample for similar applied fields(Bex = 2.45 T, below the full activation field, in Fig. 9 and
Bex = 3.01 T, close to the full activation field, in Fig. 10). It is

Fig. 10. Two-dimensional trapped field profile for the new standard YBCO
sample after PFM at 65 K for an applied pulsed magnetic field Bex = 3.01 T,
measured 1 mm above the top surface.

clear from these observations that the flux penetration into thegraded sample is more uniform than for the standard sample,
which behaves similarly to the standard sample presented in[16]. This suggests a more homogeneous Jc distribution has
been achieved in the graded sample, which is extremely important in order to achieve enhanced, homogeneous trapped

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IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 26, NO. 4, JUNE 2016

field performance of a graded YBCO sample, fabricated using
a buffer-aided, top-seeded melt-growth (TSMG) process, andcompare the results to those of standard YBCO samples, using
field cooling (FC) magnetization and PFM techniques.
The FC magnetization results show that large, single-grainbulk YBCO can be fabricated using this graded technique withtrapped field capabilities equal to or better than standard samples. The PFM results show a more uniform flux penetration
for the graded sample, suggesting a more homogeneous Jc
distribution has been achieved, which is extremely importantin order to achieve enhanced, homogeneous trapped magnetic
fields from bulk superconductors. This resulted in a lower full
activation field and a higher trapped field.
Fig. 11. Two-dimensional trapped field profile for the graded YBCO sample
after PFM at 40 K for an applied pulsed magnetic field Bex = 3.25 T, measured
at 1 mm above the top surface.

Fig. 12. Two-dimensional trapped field profile for the graded YBCO sample
after PFM at 40 K for an applied pulsed magnetic field Bex = 4.09 T, measured
at 1 mm above the top surface.

fields from bulk superconductors [9]. An inhomogeneous Jc
distribution affects dynamics of the flux entering the sample andcan reduce the overall trapped field because of localized heating
within the sample and preferential movement and pinning offlux lines in regions of low and high Jc , respectively [16].
Figs. 11 and 12 show the 2D trapped field profiles forthe graded YBCO sample after PFM at 40 K for an applied pulsed magnetic field lower than the full activation field(Bex = 3.25 T) and after fully magnetizing the sample (Bex =4.09 T), respectively. The lower operating temperature resultsin stronger flux pinning in the sample, and hence has a larger
full activation field. However, the flux penetration shows a similar uniformity to that at 65 K, which is particularly noticeable
when compared directly with results for the standard sample(for example, see [16]).
V. C ONCLUSION
A new fabrication technique for melt-processed Y–Ba–Cu–O(YBCO) using graded precursor powders has recently beendeveloped by our research group, which results in a more
uniform distribution of Y-211 particles, in order to further
improve the superconducting properties and trapped field capability of such materials. In this paper, we examine the trapped

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