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Pulsed-field magnetisation of Y-Ba-Cu-O bulk superconductors
fabricated by the infiltration growth technique
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Superconductor Science and Technology
Supercond. Sci. Technol. 33 (2020) 115012 (12pp)

https://doi.org/10.1088/1361-6668/abb590

Pulsed-field magnetisation of Y-Ba-Cu-O
bulk superconductors fabricated by the
infiltration growth technique
Devendra K Namburi1, K Takahashi2, T Hirano2, T Kamada2, H Fujishiro2,Y-H Shi1, D A Cardwell1, J H Durrell1 and M D Ainslie1
1

Department of Engineering, University of Cambridge, Cambridge CB2 1PZ, United Kingdom
Department of Physical Science and Materials Engineering, Faculty of Science and Engineering, Iwate
University, 4-3-5 Ueda, Morioka 020-8551, Japan2

E-mail: dkn23@cam.ac.uk and ndevendra@gmail.com
Received 19 June 2020, revised 28 July 2020
Accepted for publication 6 September 2020
Published 30 September 2020
Abstract

Bulk high temperature superconductors based on the rare-earth copper oxides can be usedeffectively as trapped field magnets capable of generating large magnetic fields. The top-seeded
infiltration growth (TSIG) processing technique can provide a more homogeneous
microstructure and therefore more uniform superconducting properties than samples grownusing conventional melt growth processes. In the present investigation, the properties of bulk,
single grain superconductors processed by TSIG and magnetised by the pulsed-fieldmagnetisation technique using a copper-wound solenoid have been studied. A trapped field of~3 T has been achieved in a 2-step buffer-assisted TSIG-processed Y-Ba-Cu-O (YBCO) sample
at 40 K by magnetising the bulk superconductor completely via a single-pulse magnetisation
process. Samples were also subjected to pulsed-field magnetisation at 65 K and by conventional
field-cooled magnetisation at 77 K for comparison. Good correlation was observed between the
microstructures, critical current densities and trapped field performance of bulk samples
fabricated by TSIG and magnetised by pulsed-field and field-cooled magnetisation. The
homogeneous distribution of Y2 BaCuO5 inclusions within the microstructure of bulk YBCO
samples fabricated by the 2-step buffer-assisted TSIG process reduces inhomogeneous flux
penetration into the interior of the sample. This, in turn, results in a lower temperature rise of thebulk superconductor during the pulsed-field magnetisation process and a more effective andreliable magnetisation process.
Keywords: pulsed-field magnetisation, bulk YBCO, infiltration and growth, trapped field,
homogeneous and dense microstructure, flux-pinning strength(Some figures may appear in colour only in the online journal)

1. Introduction
Bulk, (RE)-Ba-Cu-O ((RE)BCO, where RE = rare earth orY) high temperature superconductors fabricated in the formof large, single grains can be used to form strong trappedfield magnets and hence have considerable potential for awide variety of engineering and technological applications,
including, but not limited to, magnetic resonance imaging,

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D K Namburi et al

magnetic separation, rotating machines and non-contact mixers [1–4]. Bulk superconductors are unique in that theirmagnetisation scales with the size (diameter) of the single
grain, unlike conventional Sm-Co or Nd-Fe-B permanent
magnets whose magnetisation is generally independent ofsample volume. Hence, the larger the size of the (RE)BCO
bulk single grain and the higher its current-carrying ability, the larger its trapped magnetic field. Examples of thetrapped field potential of bulk (RE)BCO superconductors
include 14.3 T in conventional undoped YBCO [5], 16 T
in YBCO processed with Ag and Zn [6], 17.24 T at 29 K
in YBCO reinforced with resin impregnation and Wood’s
metal [7] and 17.6 T at 26 K in GdBCO with Ag, reinforced with shrink-fit stainless steel [8] in a two-sample stackconfiguration.
The substantial effort world-wide made over the last
30 years to refine various melt-process fabrication techniques
has established effective pathways and processes for growing large (RE)BCO single grains with engineered microstructures that exhibit superior superconducting and field generating properties [6–12]. However, inherent limitations to a number of these processes, such as considerable sample shrinkage during processing [13, 14] and non-uniform superconducting properties across the sample volume [15], have motivatedresearchers to seek alternative approaches to sample fabrication. In this context, the infiltration and growth technique[13, 14, 16–22] has emerged as a competitive alternative totop seeded melt growth (TSMG), in particular. The infiltration and growth technique has, however, two significant drawbacks: (i) the success rate of single grain growth has, todate, been very poor, due primarily to the presence of a relatively large component of aggressive liquid phase during themelt process, which makes the crystallisation and growth steps
particularly challenging, and (ii) the superconducting properties of samples fabricated by this approach have been characteristically inferior compared to those fabricated by conventional melt-growth processes. Only recently have significant
improvements been made towards developing a more effective
infiltration and growth process, which have led to a significant
enhancement in the reliability of (RE)BCO single grain growthand improved superconducting properties [23–26]. Three particularly significant developments made in the area of infiltration and growth processing are: (i) a suitable liquid-phasereservoir pellet (of appropriate composition, size and weight),(ii) integration of a buffer technique to aid the seeding process,and (iii) tuning and control of the RE2 BaCuO5 (RE-211) distribution within the microstructure to facilitate improved flux
pinning. The combination of these three enhancements has led
to the development of the so-called 2-step buffer-assisted topseeded infiltration and growth processing methodology, which
has enabled the fabrication of (RE)BCO bulk superconductors
with superior and uniform superconducting properties, withcritical current densities (Jc s) exceeding 50 000 A cm−2 at
77 K [27].
Magnetisation is an important process in developing effective engineering applications of (RE)BCO bulk
superconductors, of which field cooled magnetisation (FCM)

and pulsed-field magnetisation (PFM) are two effective
approaches for this purpose. Although FCM allows higher
trapped fields to be generated, it requires access to a large
magnetising field, provided usually by a large and expensive superconducting magnet. This is not feasible for many
practical applications, however, where in-situ charging is anecessity (for example, for rotating machines). It is, however,an extremely useful technique to characterise the maximum
trapped field capability of a given bulk sample or system. The
alternative PFM approach, on the other hand, is quick, compact and relatively inexpensive and is therefore considered
generally to be the most practical magnetisation method forengineering applications. PFM is performed by applying amagnetic field, typically of duration of the order of 100s of
milliseconds, using either a solenoid, vortex or split coil configuration [28, 29]. One significant disadvantage of PFM is theconsiderable temperature rise during the process associatedwith the rapid dynamic movement of magnetic flux within
the superconductor, although even this, if addressed appropriately, can facilitate the trapping of significant magnetic
fields for engineering applications of bulk single grains [28].
Recently, several modifications and significant improvements
have been made to the PFM approach by optimising the pulse
rise time, pulse duration, efficient cooling and heat dissipationand so on, which have improved the efficiency and reliabilityof PFM [30–32]. Homogeneity of J c plays a critical role inthe magnetic flux dynamics during PFM and any inhomogeneities within the bulk can result in localised heating, a larger
temperature rise and a reduced and/or distorted trapped field
profile [30]. We are not aware of any PFM studies to date on
bulk samples fabricated by infiltration and growth processingthat are motivated by the positive impact anticipated froma more homogeneous microstructure, which is the focus of
the current investigation. Furthermore, this work examines
pathways to improve the trapped field ability of single grainsusing PFM based specifically on the crystal growth technique,rather than more generally on obtaining higher and uniform
Jc characteristics.
In this work, detailed PFM studies have been carried out
on two bulk single grain YBCO samples fabricated by infiltration growth techniques. One of these samples was fabricatedvia conventional top-seeded infiltration growth (TSIG) processing, but using a liquid phase reservoir of optimised composition, and the other by the recently developed, so-called,‘2-step BA-TSIG’ approach. Both these fabrication techniques
exploit the use of a buffer pellet. PFM was carried out at
65 K and 40 K using a solenoid coil with applied fields of
up to 6 T for both samples. Additionally, FCM measurements
were used to assess the trapped field capability of the samples
at 77 K, and Jc and Jc (B) of small sub-specimens cut fromeach bulk single grain were measured at several temperatures to assess the strength and uniformity of the intrinsic flux
pinning. Finally, microstructural studies were performed tounderstand the correlation between the Y2 BaCuO5 (Y-211)
content present in the continuous YBa2 Cu3 O7-x (Y-123) phasematrix and the observed superconducting properties of thesingle grain.

2Supercond. Sci. Technol. 33 (2020) 115012

D K Namburi et al

Table 1. Dimensions of the TSIG-processed YBCO samples investigated in this study.

SampleYBCOYBCOSample code
Fabrication route
Diameter of the preform compact
Diameter of the sample, as grown
Diameter of the sample, after machining
Thickness of the sample, after machining
316-Stainless-steel ring: Inner diameter
316-Stainless-steel ring: Outer diameter#700
1-step Buffer-assisted TSIG
31.3 mm30.8 mm29.5 mm
16.4 mm
29.52 mm
56 mm

#911
2-step Buffer-assisted TSIG
32.0 mm32.5 mm29.1 mm
14.1 mm29.12 mm
56 mm

Figure 1. YBCO samples fabricated via (a) 1-step BA-TSIG and (b) 2-step BA-TSIG techniques. The heat treatments for each fabrication

approach are shown in the figure. Tmax , Tp , Tg start and Tg final represent the maximum processing temperature, peritectic temperature,
growth-starting temperature and growth-final temperatures of the respective processes. The fully processed, YBCO single grains obtained
after heat treatment (labelled as #700 and #911) are also shown, including both the machined and polished surfaces of the samples.

2. Sample fabrication and characterisation

The precursor sample assembly for fabricating YBCO
single grains by the infiltration growth-based techniques
investigated here consisted of a primary pre-form compact
comprised of Y-211 + 1 wt.% CeO2 placed in direct contact
with a liquid phase reservoir pellet, comprised of a mixtureof Yb2 O3 , BaCuO2 , and CuO in the ratio 1: 10: 6 [33, 34],
with each assembly supported by a thin disc of Yb2 O3 . A buffer pellet of composition 75 wt.% Y-123 + 25 wt.% Y-211 and5 mm in diameter was placed on the pre-form and capped with
an NdBCO seed crystal to promote heterogeneous nucleationand subsequent single grain growth. The buffer-aided growthtechnique was used both to overcome diffusion of the seed
crystal element into the bulk sample at elevated temperature
and, simultaneously, to minimise liquid phase segregation in

2.1. TSIG-processed YBCOTwo YBCO samples were fabricated for this study; onevia conventional buffer-assisted top-seeded infiltration andgrowth (hereafter referred to as ‘1-step BA-TSIG’) and theother via the 2-step BA-TSIG technique developed recently.
The dimensions of both samples were ~30 mm in diameterand ~15 mm in height. Additional details on the sample
dimensions are provided in table 1. Schematic diagrams illustrating the detail of the heat treatments for these two fabrication methodologies and with photographs of the samples produced, are shown in figures 1(a) and (b), respectively.

3Supercond. Sci. Technol. 33 (2020) 115012

D K Namburi et al

Figure 2. (a) and (b) show trapped field profiles measured at 77 K after field-cooled magnetisation using a 1.4 T electromagnet for the

TSIG-processed YBCO samples #700 and #911 at heights of 1.5 mm and 2 mm, respectively, above the top surface of the single grain.

Figure 3. (a) Location within the parent single grain (central region, highlighted by the red box) of the sub-specimens extracted forcomparative magnetic measurements. Field dependence of the critical current density, Jc (B), obtained at different temperatures between
30–85 K for (b) 1-step BA-TSIG and (c) 2-step BA-TSIG processing techniques.

the vicinity of the seed crystal, thereby minimising the probability of seed failure during processing [35]. Further detailsof each of these fabrication methodologies can be found elsewhere [23, 27]. The single-grain YBCO samples fabricated by
the 1-step and 2-step BA-TSIG techniques were subsequently
oxygenated at a temperature of 450 ◦ C for 200 h in flowing
oxygen (with a flow rate of 100 ml min−1 ) to transform thenon-superconducting tetragonal phase into the orthorhombic
superconducting phase. The trapped field capability of thesamples at 77 K was then measured following field-cooledmagnetisation.
2.2. Trapped field measured via field-cooled magnetisation

The surfaces of each single grain were polished flat prior tofield-cooling in liquid nitrogen in an applied magnetic field,
generated by an electromagnet, of 1.4 T. The external field wasremoved once the field-cooled sample had reached thermal4Supercond. Sci. Technol. 33 (2020) 115012

D K Namburi et al

Figure 4. (a) Locations within the single grain YBCO sample fabricated by the 2-step BA-TSIG process (sample #911) where thesub-specimens were extracted for magnetisation measurements. (b) Normalised magnetisation (M) as a function of temperature showing theonset of critical temperature Tc for sub-specimens obtained from the c-growth region of the sample to be ~92 K. (c) and (d) Field
dependence of the critical current density, Jc (B), obtained within both the a- and c-growth regions of the sample, respectively.

Figure 5. Optical micrographs obtained under a magnification of 1000x showing the presence of Y-211 inclusions in the Y-123 matrix for asingle grain sample processed by (a) 1-step (#700) and (b) 2-step BA-TSIG (#911).

equilibrium. The trapped magnetic field was then measured
using a rotating scanning system equipped with a linear arrayof 19 Hall probes. The three-dimensional trapped field profiles measured at 77 K at heights above the sample surfaceof 1.5 mm (#700) and 2 mm (#911) are shown in figure 2.
The difference of 0.5 mm in the measurement heights was

unavoidable due to an essential modification made to the setup
to add a 0.5 mm protective Stycast™ layer to avoid damage
to the Hall probes during their rotation in close proximity to
the sample. This modification was made (in the period of several months) after full characterisation of the 1-step BA-TSIG
sample and before the 2-step BA-TSIG sample was fabricated.
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D K Namburi et al

Figure 6. Optical micrographs obtained from the 2-step BA-TSIG processed YBCO sample (#911) under both low (50x) and high (500x)

magnification, showing the porosity and Y-211 inclusions and their distribution within the Y-123 matrix. These features observed along thea-growth and c-growth regions of the sample are shown in (a) and (b), respectively. The labels indicate the same positions corresponding tothose shown in figure 4(a).

The single peak observed in each of these measurements indicates that each sample constitutes a single grain. It can be seenthat both samples exhibited a trapped field of ~0.86 T at 77 K
at a height of 1.5–2 mm above the surface of the single grain.

2.3. Microstructure and critical current density

After the pulsed-field magnetisation experiments were completed (see section 3), the bulk samples were sliced using arotary diamond saw for examination of their microstructures6

Supercond. Sci. Technol. 33 (2020) 115012

D K Namburi et al

Figure 7. Quantitative analysis of the microstructural features of the 2-step BA-TSIG processed YBCO sample (#911). Variation of porosityand average pore-size analysed along the a-growth and c-growth regions of the sample are shown in (a) and (b), respectively. Y-211 area
fraction and average size of Y-211 particles along both the a-growth and c-growth regions of the sample are shown in (c) and (d),respectively. Lines joining the data points are included as a guide for the eye.

and measurement of their local superconducting properties,
including critical temperature Tc and Jc (B). Sub-specimens
were extracted from the centre of the parent single grain formagnetometry studies, as shown schematically in figure 3(a).
Magnetisation as a function of applied field (M-H) was measured for the sub-specimens using a SQUID magnetometer(Quantum Design). The M-H loops were measured systematically over a temperature range 30–85 K to quantify the flux
pinning strength. The in-field, temperature-dependent criticalcurrent density, Jc (B, T), was then calculated from each MH loop using the extended Bean critical state model [36], asshown in figures 3(b) and (c). It is clear that the overall superconducting performance of the 2-step BA-TSIG sample issuperior to that of the 1-step BA-TSIG sample, in good agreement with previous studies [25, 27]. The 2-step BA-TSIG processing resulted in current densities ⩾40 kA cm−2 in a field of
5 T at 65 K and ⩾100 kA cm−2 in a field of 5 T in the temperature range: 30–50 K. The uniformity of the superconducting
properties in the 2-step BA-TSIG processed YBCO sample
was assessed by measuring Jc (B) at 77 K for sub-specimens
extracted from both the a-growth and c-growth regions of thesample, as shown in figure 4. A uniform Tc with onset ~92 K
and a narrow superconducting transition width ∆Tc < 1 K is
observed in the samples, as shown in figure 4(b). It can beseen in figures 4(c) and (d) that the field dependence of criticalcurrent density at 77 K was nearly uniform with Jc (0) ~ 50–
60 kA cm−2 and specimens were found to maintain Jc of~1 kA cm−2 even at 5 T external field. This unique uniformity in local superconducting properties is achieved through theadvancement in the processing of (RE)BCO bulk superconducting materials.
The second half of each of the sliced single grains waspolished sequentially with SiC paper and a colloidal suspension containing diamond particles down to the 1-micron leveland observed under an optical microscope equipped with apolariser. The optical micrographs obtained under a magnification of 1000x revealing the presence of the Y-211 inclusions in the Y-123 phase matrix, obtained for the samples
processed by 1-step and 2-step BA-TSIG, are shown in figures 5(a) and (b), respectively. These micrographs were analysed for Y-211 content using ImageJ software. The analysis
showed that the 1-step BA-TSIG processing technique resulted in Y-211 particles with an average size of ~2.2 µm
and comprising ~40% of the sample volume, whereas 2-step
BA-TSIG processing reduced the average Y-211 particle size
to ~1.5 µm with an associated volumetric content of ~24%,
which is promising for achieving improved superconducting
properties through optimised flux pinning. The control and7Supercond. Sci. Technol. 33 (2020) 115012

D K Namburi et al

optimisation of Y-211 content in the Y-123 matrix has been
known to enhance the critical current density via the creationof interfacial defects, which are known to aid significantly
magnetic flux pinning.
A detailed microstructural study carried out on the 2-step
BA-TSIG YBCO sample, along both the a-growth and cgrowth regions, is presented in figure 6. Microstructural features, such as porosity and typical Y-211 size and distribution
within the Y-123 matrix, can be seen from the optical micrographs recorded under both low (50x) and high (500x) magnification. These microstructural features were analysed quantitatively using ImageJ software, as shown in figure 7. It can beseen that the porosity is significantly smaller (<5 %) in comparison to the conventional TSMG-processed samples (where
the porosity is ~15%), confirming the physically dense natureof the sample processed by infiltration growth. Furthermore, it
can be observed from the optical micrographs that the Y-211
content is within the range of 18%–25% in the areas analysed.
This relatively smaller variation in Y-211 secondary phase
content is a key figure of merit for 2-step BA-TSIG processing,which leads to a more homogeneous microstructure and is theprincipal reason for observation of uniform superconducting
properties [see figures 4(c) and (d)]. Analysis of the microstructure has clearly demonstrated a more homogeneous Y211 distribution with an overall more optimum Y-211 content
for the sample fabricated by the 2-step BA-TSIG technique, in
addition to further refinement of these flux pinning sites. The
combination of these characteristics of the sample has together
produced a considerable improvement in both the superconducting performance of YBCO and in obtaining a uniform
J c (B, T) across the sample volume. The results obtained forthese samples with respect to J c (B, T) and microstructure arealso in good agreement with those reported previously for the
2-step BA-TSIG processing technique [25, 27].

applied to each sample, with a typical rise time of the excitation pulse of 13 ms and a duration of ~120 ms. After applying
the pulsed field, the resultant trapped field in the sample wasrecorded after a 180 ms time delay, i.e. at t = 300 ms, using anaxial-type Hall sensor on a controlled x-y stage. The magnitudeof the pulsed field applied to each sample was calculated bymeasuring the current flowing through a shunt resistor in thecircuit.
PFM experiments were carried out at temperatures of 65 K
and 40 K with applied, pulsed magnetic fields up to 6 T. Figure
9 shows the trapped field (Bt ) measured as a function of appliedfield (Bex ), obtained independently in both the samples. Similar PFM experiments were also carried out on a YBCO sample(#747; 29.5 mm diameter, 14.3 mm height) fabricated via the
traditional TSMG approach to enable comparison between thetwo processing techniques (details of the TSMG sample usedin this comparison can be found elsewhere [37]) and the results are included in figure 9 for reference. It should be notedthat the measurement was carried out at the sample surface(0 mm) for the TSMG and 1-step BA-TSIG samples, whereas
there was 1 mm separation between the surface of the sample
and the Hall probe in the experiments carried out on the single
grain fabricated by 2-step BA-TSIG (1 mm). The sample fabricated by 2-step BA-TSIG performed similarly to the othersamples at 65 K, but clearly demonstrated superior performance at 40 K, trapping a maximum field of 2.19 T. It is to be
noted that the trapped field measurements on the samples werecarried out at different surface-Hall probe separations (0 mm
in the 1-step BA-TSIG sample and + 1 mm in the 2-step BATSIG sample), which was unavoidable due to modifications
made to the experimental arrangement between the sets ofmeasurements (a more meaningful comparison between thesamples is made later in figure 11), but this does not detractfrom the significance of the results of this investigation. Twodimensional trapped field profiles measured at 2 mm/3 mmabove the sample surfaces at both 65 K and 40 K are also
shown in the figure. Trapped field as a function of time at 40 K,
which represents a comparative performance of all the threesamples, is shown in figure 10. The trapped fields measured ineach of the samples at t = 300 ms, a temperature of 40 K and
an applied field of 5 T are provided in the table in figure 10.
The time dependence of the measured magnetic field can
assist in elucidating the trapped field improvement in the bulk
superconductor fabricated by 2-step BA-TSIG. There are two
conditions that should be considered in the PFM process totrap fields closer to the sample capability as indicated by FCM,
i.e. Bt ≈ Bex :3. Pulsed-field magnetisation experiments
The machined and polished TSIG-processed YBCO samples
for the PFM experiments were mounted tightly in nonmagnetic 316-stainless steel ring-shaped sample holders using
Stycast™, as shown in figure 8(a). The dimensions of thesesample assemblies were as provided in table 1. The outer diameter of the stainless-steel sample holder was machined to
56 mm so that it matched the diameter of the PFM cold stage
for effective and efficient cooling. Further details of the PFM
facility employing the copper-wound solenoid coil can befound elsewhere [30], and only a brief description is provided
here. A schematic illustration of the PFM assembly is shown infigure 8(b). The system comprises a Gifford-McMahon (GM)closed-cycle helium refrigerator and a copper-wound solenoid
coil, 99 mm in inner diameter, 121 mm in outer diameter and
50 mm in height. This solenoidal magnetising coil is cooledusing liquid nitrogen outside of the vacuum chamber used tocontain the sample during magnetisation. The sample holder
is fixed to the cold stage such that the applied field is parallelto the c-axis of the sample. Applied fields (Bex ) up to 6 T were

• During the field ascending stage (i.e. below the peak of Bex ,which is achieved at t = 13 ms) Bin is equivalent to Bex , andthe flux penetration is homogeneous within the interior of
the bulk single grain. In other words, Bt generated by PFM
cannot exceed Bin during this stage of the magnetisation
process. In this case Bt should be <3.60 T after PFM under
the specified conditions for the sample fabricated by the 2step BA-TSIG process.
• During the field descending stage (i.e. after 13 ms) there isno significant flux flow without heat generation, so as small8Supercond. Sci. Technol. 33 (2020) 115012

D K Namburi et al

Figure 8. (a) A YBCO sample mounted tightly in 316-stainless steel using Stycast™. (b) Schematic illustration of the pulsed-fieldmagnetisation system using a copper-wound solenoid coil. A cylindrical iron yoke (20 mm in diameter and 20 mm in height) is locatedunderneath the sample.

Figure 9. Trapped field (Bt ) obtained via PFM experiments carried out at 65 K and 40 K for the TSIG-processed YBCO samples (1-step

BA-TSIG #700 and 2-step BA-TSIG #911). For reference, similar experimental data obtained from a traditional TSMG-processed YBCO
sample (#747) are also provided. It should be noted that the trapped field was measured at the sample surface for the TSMG and 1-step
BA-TSIG samples, whereas the Hall sensor was positioned 1 mm away from the sample surface for the 2-step BA-TSIG sample. 2D trappedfield profiles measured at 2 or 3 mm above the sample surface are also provided. Here, Ts is the sample temperature, Bex is the applied field
and h is the height above the sample at which the trapped field is measured.

9Supercond. Sci. Technol. 33 (2020) 115012

D K Namburi et al

Figure 10. Results from the PFM experiments carried out on the YBCO samples (2-step BA-TSIG, 1-step BA-TSIG and TSMG) at 40 K

for an applied pulsed field of 5 T, which is close to the full magnetisation field (see figure 9). The applied pulsed field (curve in black color)
and trapped magnetic field as a function of time are shown. The actual applied field ‘Bex ’, the induced field ‘Bin ’ (the field measured at thepeak of the pulse, i.e. t = 13 ms) and the trapped field ‘Bt ’ (after the pulse, at t = 300 ms) are provided in the table.

Figure 11. (a) Applied field (Bex )-dependence of the maximum temperature rise (dT) for the bulk YBCO samples (2-step BA-TSIG, 1-step

BA-TSIG and TSMG) under investigation at 65 K and 40 K. The location of the Cernox temperature sensor mounted on the outer-side of
the stainless steel ring and used to measure the temperature rise, dT is shown in the inset of the figure. (b) The recalibrated trapped fields forthe 2-step BA-TSIG sample are provided in (b) to enable a meaningful comparison to be made. A trapped field as high as 3 T has beenachieved at 40 K on the surface of the YBCO sample processed by the 2-step BA-TSIG technique.

a temperature rise as possible is desired, and the presenceof the iron yoke below the bulk sample can assist in reduced
flux dispersion within the x-y plane.

focused on understanding the microstructural features of thesample could improve significantly the trapped field performance by PFM (i.e. a homogeneous distribution of Y-211
inclusions and subsequent uniform Jc properties, can resultin more efficient pulsed-field magnetisation and the generation of higher trapped fields under the same magnetisingconditions).
As mentioned earlier, the trapped fields shown in figure
9 were measured at different heights above the sample surface: 0 mm in the 1-step BA-TSIG sample and + 1 mm
in the 2-step BA-TSIG sample. To enable a fair comparison, the trapped fields for the 2-step BA-TSIG sample arerecalculated using equation (1), based on the Biot–Savart
law, to estimate their value at the surface of the single
grain [38].

The reason for the improvement in trapped field illustrated in figure 10 is likely to be associated with the morehomogeneous Y-211 distribution in the sample microstructure as observed in figures 6 and 7. This results in morehomogeneous flux penetration and thereby reduces heat generation during PFM, which is evidenced in the maximum
temperature rise, dT, shown in figure 11(a), measured bya Cernox temperature sensor mounted on the outer-side of
the stainless steel ring. The sample processed by 2-step BATSIG exhibits the lowest temperature rise at both 65 K
and 40 K. These results indicate that a specific approach10Supercond. Sci. Technol. 33 (2020) 115012



D K Namburi et al

√( )
D 2


2
+
(z
+
t)
µ 0 Jc 2Bt (z) =(z + t) log
2
z+t
√( )


D
D 2
2
+
+
z
2
2
 .
−z log 
(1)
z
D
+
2

ORCID iDs
Devendra K Namburi  https://orcid.org/0000-0003-32192708
K Takahashi  https://orcid.org/0000-0002-8278-2688
T Hirano  https://orcid.org/0000-0003-1658-914X
H Fujishiro  https://orcid.org/0000-0003-1483-835X
Y-H Shi  https://orcid.org/0000-0003-4240-5543
D A Cardwell  https://orcid.org/0000-0002-2020-2131
J H Durrell  https://orcid.org/0000-0003-0712-3102
M D Ainslie  https://orcid.org/0000-0003-0466-3680

z is the distance above the centre of the top surface of thesample, and D and t are the diameter and thickness of the discshaped bulk superconductor.
As a result, a more meaningful comparison of the values of trapped field is provided in figure 11(b). It is promising to observe in the present study that a surface trappedfield of ~3 T has been achieved in a basic YBCO sample
processed by infiltration growth processing via a relatively
simple single-pulse magnetisation process using a copperwound solenoid coil. This value of trapped field is comparable
with, or superior to, the results of many previous PFM studies
on YBCO and GdBCO bulk single grains of similar dimensions and at similar temperatures [37–42]. There is, of course,
further scope to improve this performance either via multipulse magnetisation with step-wise cooling and/or through theuse of a split coil magnetisation geometry. The fact that higher
pulsed-field magnetisation efficiency is achievable by employing (RE)BCO bulk single grains with greater homogeneity isgreatly encouraging from an applications perspective.

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4. Conclusion
Pulsed-field magnetisation using a copper-wound solenoid has
been used to investigate the magnetic field trapping properties of bulk, single grain YBCO superconductors fabricatedby TSIG process at 65 K and 40 K. The YBCO sample fabricated by the 2-step buffer-assisted TSIG technique trapped asignificantly larger magnetic field from a single pulse magnetisation process (~3 T at 40 K) than is obtained typically in traditional melt grown samples. This improvement is attributed
to the lower temperature rise in samples fabricated by TSIG
during pulsed-field magnetisation due to the more homogeneous microstructure and uniform superconducting propertiesof these samples. This observation suggests that the use of a2-step BA-TSIG process for the fabrication of (RE)BCO systems both with and without added Ag could lead to a substantial improvement in pulsed-field magnetisation performanceand to more competitive engineering applications.
Acknowledgments
This research was supported by the following projects: JSPS
KAKENHI Grant No. 15K04646; King Abdualaziz City for
Science and Technology (KACST); and the Engineering andPhysical Sciences Research Council (EPSRC) with grantcode: EP/P00962X/1. M D Ainslie would like to acknowledge financial support from an EPSRC Early Career Fellowship, EP/P020313/1. Data supporting this publication areavailable at the University of Cambridge data repository
https://doi.org/10.17863/CAM.53945
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