­

Materials Science and Engineering B 273 (2021) 115390

Contents lists available at ScienceDirect

Materials Science & Engineering B
journal homepage: www.elsevier.com/locate/mseb

Complex pulse magnetization process and mechanical properties of spark
plasma sintered bulk MgB2
Muralidhar Miryala a, *, Sai Srikanth Arvapalli a, Naomichi Sakai a, Masato Murakami a,
Hidehiko Mochizuki b, Tomoyuki Naito b, Hiroyuki Fujshiro b, Milos Jirsa c, Akira Murakami d,
Jacques Noudem eaMaterials for Energy and Environmental Laboratory, Superconducting Materials Group, Shibaura Institute of Technology, 3-7-5 Toyosu, Koto-Ku, Tokyo 135-8548,
Japan
Faculty of Science and Engineering, Iwate University, Morioka, 4-3-5 Ueda, Japanc
Institute of Physics ASCR, Na Slovance 2, Praha 8 18221, Czech RepublicdNational Institute of Technology, Ichinoseki College, Ichinoseki, Iwate 021-8511, Japane
CRISMAT-CNRS, ENSICAEN, Normandy University, Caen, Franceb

A R T I C L E I N F O

A B S T R A C T

Keywords:
MgB2
Spark plasma sintering
Pulse field magnetization
Critical current density
Mechanical properties

High-density MgB2 bulks with superior mechanical and superconducting properties were fabricated using spark
plasma sintering (SPS). The sharp superconducting transition at 37.5 K proved high quality of the supercon­
ductors. Critical current density significantly exceeded that of conventionally sintered bulks. Flux pinning dia­grams indicated dominance of grain boundary pinning, with peak position at 0.2. SPS bulks showed improved
mechanical properties with 8-times higher bending strength compared to dense hot isostatic pressed bulks.
Trapped field was measured at 14 K and 20 K, 1 mm above the bulk’s surface, with applied pulse field up to 2 T. A
local overheating together with a complex trapped field formation in the MgB2 bulks during pulse-fieldmagnetization were studied to elucidate limitations of Pulse Field Magnetization.

1. Introduction
Superconductivity in MgB2 was discovered in 2001 [1]. It soon
gained a huge attention because it brought a superconducting state
different both from conventional metallic superconductors and the highTc ones, cuprates. It presented new challenges for superconductivity
theory. Interesting was also its simple and cheap chemical structure.
Although the intermediate critical temperature (around 37 K) is notcompetitive with high-Tc compounds, it is still much better than inmetallic superconductors. For potential applications, one can work withcheaper alternative coolants to liquid He such as liquid H2 (~20 K),
liquid Ne (~27 K) or recently developed advanced cryocoolers. The
conventional superconductors like Nb3Sn (Tc ~ 18 K) and Nb-Ti(Tc ~ 10 K) used in most applications till now somewhat lose their ap­
peal due to need to be operated at or below 4.2 K, boing point of liquid
helium. Hence, superconducting materials such as MgB2 and REBa2
Cu3Oy “REBCO” (RE: Y, Nd, Sm, Gd, Eu) have become a new point ofinterest. Apart from the magnet applications such as NMR, MRI, watercleaning etc., MgB2 superconductor has potential uses in super­
conducting transformers, rotors, magnetic drug delivery, space appli­
cations, transmission cables etc. [2–6]. The competing REBCO has ahuge advantage over MgB2 in Tc, however it has severe issues like weaklinks at grain boundaries, a long processing time, expensive precursors,and inability to be used as powder-in-tube technology (PIT). In order totackle these issues, research suggested solutions such as single-grain
growth using melt growth (MG) and/or infiltration growth (IG), postoxygenation etc., which all consume huge amount of time. On theother hand, the absence of weak links at grain boundaries of bulk MgB2
superconductors ensures that they do not need to be single-grain and canbe fabricated by just sintering [7]. Due to the low Tc, the operating
temperatures of Nb3Sn and Nb-Ti superconductors are extremely low
and flux jumps have been observed several times [8]. While the criticalcurrent densities, Jc, at respective operating temperatures of these su­
perconductors are comparable, MgB2, due to the low weight, possesses aclear advantage e.g. in space applications as well as in various motors for
all-electric cars or planes [2]. Another class of superconductors are the* Corresponding author.
E-mail address: miryala1@shibaura-it.ac.jp (M. Miryala).
https://doi.org/10.1016/j.mseb.2021.115390
Received 29 January 2021; Received in revised form 11 June 2021; Accepted 5 August 2021
0921-5107/© 2021 The Authors.
Published by Elsevier B.V. This is an(http://creativecommons.org/licenses/by-nc-nd/4.0/).

open

accessarticle

under

the

CC

BY-NC-ND

licenseM. Miryala et al.

Materials Science & Engineering B 273 (2021) 115390

Fig. 1. Spark Plasma Sintering MgB2 bulk with dimensions (left) and the bulk with Hastelloy support ring (right).

Fig. 3. DC susceptibility vs temperature of SPS processed MgB2 bulk.

Fig. 2. Normalized X-ray diffraction pattern of SPS processed MgB2 bulk.

magnetic shielding cups for circular proton colliders etc. [14]. Only, themechanical properties of bulk MgB2 superconductors fabricated bysintering under the ambient pressure are not as good as in bulk REBCO
superconductors [13,14]. In addition, there are some other issues suchas high porosity (low packing ratio), low Hc2 and weak flux pinning.
Moreover, it is known from prior research that bulk superconductors
experience thermal stresses when subjected to electromagnetic force[15,16]. Improved mechanical properties of the bulk can thus enhance
trapped magnetic field. Fabrication techniques such as hot isostaticpressing (HIP) and spark plasma sintering (SPS) result in high packing
ratio in bulk MgB2 and thus they are good technology candidates for
improving trapped field properties [15–19]. SPS is robust and fast
technique capable of producing high-density (high packing ratio) MgB2
bulks, just what required in this system. With SPS, it is possible to obtainvery high densities, up to 99 %. This technique enables control overpressure and temperature in various time frames. Complicated heating
patterns can be executed. In the case of MgB2, Mg reacts with boron
leaving pores behind; it requires high pressure to close these pores as thereaction proceeds. Such technique needs optimization for obtaining best
performance bulk MgB2. In the present work we display the enhanced
properties of SPS prepared bulk MgB2 specimens compared to othertechniques, especially hot-isostatic pressing (HIP) by pressures of 98 and
196 MPa at 900 ◦ C for 3 h resulting in packing fractions of 63 and 92 %[15].

iron based superconductors, pnictides, that are very intriguing due totheir non-BCS superconductivity as well as tunable Tc with chemical
substitution. Their Tc ranges from 25 to 55 K, it is thus comparable to
MgB2, also Jc is decent. However, majority of these superconducting
compounds involves Arsenic, which is volatile and highly toxic andmakes them hard to deal with [9]. There have been studies of arsenicfree iron-based superconductors, their properties were, however,rather bad [10]. On the other hand, MgB2 doesn’t pose such problems
and is easy to handle and fabricate.
Sintering of bulk MgB2 is commonly carried out at ambient pressure.
The bulks exhibit a good trapped magnetic field with a simple peak. In
previous trials, excellent trapped field (TF) values of 1.5–3 T were re­ported in sintered bulk MgB2 superconductors (diameter 20–30 mm)[11–13]. Recent research showed that the MgB2 bulk magnet due to its
uniform TF as well as low decay is a promising candidate for the mag­netic pole in NMR [13]. The authors demonstrated the use of MgB2
magnet to detect the 1H nuclear magnetic resonance. Moreover, it is wellknown that trapped magnetic field is directly proportional to bulk
diameter. While fabrication of large single-grain REBCO bulks is difficult
to due to the tedious multi-nucleation, this task is not difficult forpolycrystalline MgB2 bulks as the sintering process is simple andtunable. Due to this advantage, MgB2 is commercially very attractive
and intricate shapes and sizes can be easily fabricated, for example as2

M. Miryala et al.

Materials Science & Engineering B 273 (2021) 115390

Fig. 4. Critical current densities of SPS MgB2 bulk at various temperatures (5–35 K) and fields (0–5 T).

Fig. 6. Stress – strain curves of the current SPS MgB2 specimen in comparison
with previous highly dense bulks.

Fig. 5. Flux pinning diagrams of SPS MgB2 bulk at various temperatures. At all
temperatures, the peak position is 0.2.

2) Mechanical properties:
Mechanical properties of bulk MgB2 superconductors processed by
SPS were evaluated via bending tests. Three cuboidal specimens(2.8 × 2.1 × 2.1 mm3) were cut from various locations of the bulk MgB2
samples by a wire electric-discharge cutting machine. These specimens
were subjected to 3-point bending test at room temperature. The force
was applied along thickness in 0.2 mm/min speed by a crosshead using
the SHIMADZU AG-50KNE testing machine. To record the strain, a strain
gauge (0.2 mm in length) was attached to the surface experiencing
tensile force. The 3-point bending stress σ was calculated using relation)((1)
σ = (M/I) × (t/2) = 3PL/ 2wt2

2. Experimental
1) Bulk fabrication:
The commercial MgB2 powder from ABCR GmbH (Karlsruhe, Ger­many) was used as a precursor. SPS was applied at 1150 ◦ C underpressure of 50 MPa for 20 min and alternatively in vacuum of 10-2 Pa.
The pulsed electric current of 2 kA was used to heat the sample. A
graphite die of 40 mm diameter was used to prepare bulk samples about
10.5 mm thick. Soon after sintering, the bulk samples were polished toremove the graphite coating (see Fig. 1, left). Details of the SPS process
and superconducting properties of a spark plasma sintered bulk MgB2
piece were reported elsewhere [19]. The bulks were later characterizedusing X-ray diffraction (XRD- Smart lab/Rigaku) to confirm the singlephase state and atomic force microscopy (AFM) to see the microstruc­tural modifications upon employing SPS technique.

where M is bending moment, I is moment of inertia, P is applied load, L isouter supporting span (21 mm), w and t are width (2.8 mm) and thick­ness (2.1 mm) of the specimen, respectively.
3) Superconducting properties:3M. Miryala et al.

Materials Science & Engineering B 273 (2021) 115390

this system. The superconducting critical transition temperature (Tc)
was measured using SQUID magnetometer in magnetic field of 10 Oe.
Fig. 3 depicts the normalized susceptibility vs. temperature. Tc,onset wasaround 37.5 K, while Tc,zero was close to 36.5 K. The critical transition
width was around 1 K, a sharp transition telling us that the bulk was ofhigh quality. In general, this value might be affected by a scarce MgOformation and by contaminations of commercial Mg and B powders.
These values are similar to conventionally sintered MgB2, indicating thatSPS does not deteriorate sample quality [20]. The critical current den­sity (Jc) was calculated from M− H curves obtained by SQIUD mea­
surements using the extended Bean critical state model [21], which
reads
[
)
Jc = 20Δm/ a2 c (b − a/3 ](2)where a, b are cross sectional dimensions, b > a, and c is thickness of thespecimen (a, b, c usually around 1, 1, 0.7 mm, respectively) and Δm (inemu units, 1 emu = 10− 3 Am2) is the difference of magnetic moments
during decreasing and increasing field in the M-H loop. Superconducting
performance was tested at a series of temperatures covering the entire
superconducting range of the material, 5, 10, 15, 20, 25, 30, and 35 K
(see Fig. 4 left). In addition, Jc as a function of magnetic field is dis­cussed, in particular to analyze the high field performance. Self-field Jc
values of 320, 300, and 260 kA/cm2 were observed at 5, 10, and 15 K
respectively. These values are slightly higher compared to conventional
regularly sintered MgB2. Fig. 4 right, displays Jc of the SPS bulk sample
at various temperatures and fields. It shows that the bulks can be used inapplications at 5, 10, 15, and 20 K up to 2, 1.5, 1, and 0.5 T, respectively.
Flux pinning diagrams in general are used to indicate the prevailing
pinning mechanism. The results were evaluated in terms of Dew-Hughes
general expression [22]Fig. 7. Bending strength plotted against packing ratio of current SPS MgB2
specimens in comparison with previous highly dense bulks.

fp = A(h)p (1 − h)q(3)

where A is a constant, fp is normalized flux pinning force, fp = Fp/ Fp,max,and h is reduced magnetic field, h = H/Hirr, where the irreversibility
field, Hirr, was determined as the field, where Jc in the Jc(H) dependence
fell down to 100 A/cm2, a standard practice in our works. The fp(h)dependence was analysed at 20 K. Dew-Hughes model correlates thepeak position of the normalized pinning force density vs. magnetic fieldnormalized to Bc2 (or Birr) with different types of pinning regimes in thematerial. For example, peak position at 0.2 implies grain boundary
pinning, at 0.33 implies δTc pinning and 0.5 indicates point pinning. The
peak shifts to the right if the Jc at high magnetic fields is improved, i.e. if
there are effective high-field pinning centers. MgB2 superconductor
studied here is a bulk granular system. Microstructure studies show atypical grain size of a few hundred nm to a few micrometers. In cupratebulks grain boundaries are weak links preventing flow of large criticalcurrents over the sample. Thus, cuprate bulks have to be single-crystals
to exhibit high critical currents. Grain boundaries in MgB2 are strong
links allowing for a free flow of currents over the entire bulk. Taking the
low anisotropy of the MgB2 into consideration, the polycrystalline. Fromelectromagnetic point of view, the polycrystalline MgB2 bulk behaves
like a single-crystal, which is unique. From structural point of view,grains in the MgB2 bulk still exist and play role of effective large defects[23]. It is consistent with the pinning diagram indicating peak position
around 0.2 in all MgB2 samples. Additives of various kinds trying to
induce some sort of nm-sized point-like defects in MgB2 samples [24]change the pinning diagram only a little, proving that the principal
pinning mechanism in MgB2 is boundary pinning as in Fig. 5.
Fig. 5 shows that in our case hmax ~ 0.2, consistently for various
temperatures, 5, 10, 15, 20, 25, 30, 35 K. This indicates that the domi­
nant pinning mechanism is associated with grain boundaries, as in ourprior bulk MgB2 systems [25–27]. A slight shift of hmax towards higher
fields can be seen with increasing temperature, which is due to tem­
perature dependence of inherent effective mass anisotropy, γ (which
decreases) [28]. In addition, a shoulder on the right part of the curve

Fig. 8. Microstructure of SPS MgB2 bulk studied by AFM. Almost zero porosity
can be seen.

To reduce temperature rise and to reinforce mechanically the bulk,
SUS304 ring was fixed on the 40 mm in diameter bulk MgB2 before thetrapped field experiment (see Fig. 1, right). The bulks were magnetizedusing pulse field magnetization (PFM) technique. The bulk was placedon a soft iron yoke with dimensions of 40 mm diameter and 20 mm
thickness, which was tightly attached to the cold stage of a Gif­
ford–McMahon (GM) cycle helium refrigerator. The sample space waslater evacuated using an oil diffusion pump. Totally, three Hall probes
were attached on the sample surface for a careful measurement, one at
the center, one close to the edge, and one outside the surface. During thePFM, the time dependence of the local magnetic field at the center of thebulk surface was measured by Hall sensor connected to an oscilloscope.
The total trapped magnetic flux was measured using an axial Hall sensor,
which scanned stepwise on the vacuum chamber 5 mm above the bulk
surface with a pitch of 1.2 mm. Temperature on the bulk surface wasmeasured using a fine thermocouple.
3. Results and discussion
Fig. 2 depicts the diffraction pattern of the bulk using XRD, all the
peaks belong to the MgB2 phase, with only a few low peaks belonging to
MgO phase. This confirms the formation of MgB2 single phase along withscarce MgO impurities that are unavoidable and generally observed in4M. Miryala et al.

Materials Science & Engineering B 273 (2021) 115390

Fig. 9. Time evolution of the applied field (Bex) and local field (BLC) at the center of bulk surface at 14 K. a, b, c, d, e, f, and g correspond to Bex = 1, 1.2, 1.4, 1.6, 1.8,
2, and 2.2 T, respectively.

appears, indicating an improved Jc at high fields. This shape implies thatgrain boundaries are the principal pinning mechanism but there is also a
weak effect of point-like pins, effective at higher magnetic fields. This
point requires a further study.
In the present work, we used the sintering temperature of 1150 ◦ C for
SPS bulk MgB2, significantly higher than 800 ◦ C found as optimum re­action temperature for conventional sintering. It is well know that ahigher reaction temperature favors grain growth and agglomeration that
result in growth of large grains associated with a lower grain boundary
density [29]. We believe that the heat cycle of SPS process can be
optimized to obtain highly dense bulks with small grains and a higher
grain boundary density.
As we deal with a brittle granular system, we need in addition togood superconducting properties also decent mechanical properties.
Especially, when targeting high TF superconducting magnets, we needsufficient mechanical strength to sustain the considerable Lorentz forces
during magnetization up to certain fields. For trapping magnetic fields
higher than 7–8 T, a special stainless-steel ring reinforcement is needed[30]. Fig. 6 depicts the stress–strain curves of various specimens cut
from the SPS bulk. The curves show a linear stress–strain dependence
until the fracture point in bulks with both high and low packing fraction(99, 92 and 63 %). However, sometimes a non-linear behavior could beobserved in low packing fraction bulks, which could occur due to theslow propagation speed of cracks while loading [15]. The fracture stressof the present SPS bulks with 99% packing fraction is around
330–480 MPa, while those of the dense bulks prepared by A. Murakami
et al. [15], with 92% and 63% packing fraction (via hot isostatic pressing“HIP”), were around 220 and 50 MPa on average, respectively. This
points out to the brittleness and ceramic nature of the high-density MgB2
bulk specimens. In addition, the slopes of the slope of stress-strain curves
i.e.Young’s modulus, is high for high density/packing fraction bulks. As
seen in Fig. 7, the less dense samples with a lower packing fraction have
low bending strengths. The average bending strength of the present SPS
bulks is around 410 MPa, while that of prior HIP high dense bulk with
92% packing fraction was around 200 MPa. On the other hand, thebending strengths of HIP low dense sample with 63% packing fraction
and reference bulk with 50% packing fraction were around 50 and

10 MPa, respectively. The bending strengths of current bulk is almost 8
times that of HIP processed bulk. The bending strength increases withincreasing packing ratio, which rises with the SPS temperature. Thisbending strength improvement is due to the increase of the net crosssectional area and material density of the bulk caused by the increaseof the packing ratio compared to regular sintering where the net crosssectional area is very low due to 50% porosity. In addition, reductionof stress concentration centers (especially defects, voids etc.) in thematrix together with the increase of packing ratio also contributes to theimprovement of the bending strength. The bending strength results did
not vary with the loading direction of the specimen (parallel orperpendicular to thickness), which points out that matrix of the SPS
specimens is isotropic and uniform. The isotopic bending strength re­lates to Young’s modulus, which also does not change with the loading
direction [31]. As regards the isotropic behavior of the anisotropic
material, we have to bear in mind that the tested bulk is polycrystalline.
In such a sample, we can expect that mechanically weakest is theintergranular space. If the used technology enables preparation of adenser material (with respect to the reference, which possesses hugeporosity and only 50% of the theoretical density), then the increased
density appears naturally in the inter-grain space, which is isotropic onmacroscopic scale. No wonder that bending strength is isotropic, too.
The mechanical strength testing is not about individual MgB2 grains,
however about a macroscopic system of arbitrarily oriented MgB2
grains. While MgB2 compound is slightly anisotropic, its polycrystalline
bulk system as a whole is isotropic. One important point to note is thathigh SPS sintering temperature enables high density. However, if wewant to reduce the sintering or fabricating temperature (to maintain finegrain structure by avoiding grain coalescence), we have to increasepressure. It demands use of expensive dies capable of withstanding thehigh pressure, which reduces commercial viability. Therefore, one needs
to compromise the parameters to get an optimized final product. To findthe optimum, we studied the microstructure of the SPS bulk specimens
using AFM. Fig. 8, left shows AFM images taken after polishing. One cansee a highly dense MgB2 material without pores. In higher magnification(Fig. 8, right), we tried to see the grain size distribution playing a crucialrole in improving the critical current density. Note that the size of the5M. Miryala et al.

Materials Science & Engineering B 273 (2021) 115390

Fig. 10. Trapped field profiles of the SPS MgB2 bulk mapped 1 mm above the surface at 14 K for various applied fields. a, b, c, d, e, f, and g correspond to Bex = 1, 1.2,
1.4, 1.6, 1.8, 2, and 2.2 T, respectively.

MgB2 grains is around 50–100 nm. In bulk MgB2 material the grain size
plays a crucial role in improving critical current density since the grainboundary pinning is dominant and the grain boundary density increases
with grain size reduction. The smaller the grain size, the higher is criticalcurrent density and trapped field [32]. The suppression of porosity

resulted in increase in Jc, as the superconducting volume per unit vol­ume was now higher than before.
As high critical current density enhances trapped field (TF), we
studied TF performance of the SPS bulks. Pulse field magnetization(PFM) was used to magnetize the bulk. The bulk’s dimensions were6

M. Miryala et al.

Materials Science & Engineering B 273 (2021) 115390

Fig. 11. Time evolution of the applied field (Bex) and local field (BLC) at the center of bulk surface at 20 K. a, b, c, d, e, and f correspond to Bex = 1, 1.2, 1.4, 1.6, 1.8,
and 2 T, respectively.

around 40 mm diameter and 9.5 mm thickness. The packing fraction
measured by Archimedes method was around 99.8%. The TF measure­ment was done after a sequence of increasing external magnetic fields(Bex) from 1 T to 2.2 T in steps of 0.2 T at 14 K and later 1 T to 2 T in stepsof 0.2 T at 20 K. All the TF data presented were measured at 5 mm above
the sample surface.
The pulsed external field Bex(T) was applied as shown in the Fig. 9(a–g), while the local trapped field BLC(T) was measured at 14 K and theresults are following.
i) With Bex = 1 and 1.2 T, there was no significant local trapped field(Fig. 9 (a, b)) compared to the applied field. However, trapped field
linearly increased up to the peak Bex, and then remained constant ataround 0.2 T.
ii) When Bex = 1.4 T (Fig. 9 (c)), local TF raised up to 0.5 T and stood
constant there. In fact, there was a slight drop in trapped field,
commonly observed in MgB2 bulk system. The reason is not yet clear,but it has been observed in many other works [33].
iii) When Bex > 1.6 T, the bulk started to exhibit flux jumps, andhence we plotted estimated TF (Bestimated), calculated as Bestimated = a/t + b, where t - time; a,b – constants. At Bex = 1.6 T, local TF was initiallyaround 0.7 T, until a sudden flux drop occurring after a minute (Fig. 9(d)). The local TF then stabilized at 0.5 T. Since there is sudden drop in
the TF, we added the estimated curves to simulate the TF behavior
without flux drop, via extrapolation. In this case, the extrapolation
resulted in the relation Bestimated = 5/t + 0.55 and can be used as areference.
iv) With Bex = 1.8 and 2 T, the initial local TF was around 0.9 and1.3 T, respectively. In Fig. 9 (e) we can see flux drops as well as irregularfluctuations and in Fig. 9 (f) there are two flux drops (initially small one,then a larger one). In both cases, the trapped field finally reached aconstant value 0.5 T. The estimated TF curves were Bestimated = 7.5/
t + 0.59 and Bestimated = 7.5/t + 0.94 for Bex = 1.8 and 2 T, respectively.
v) When Bex = 2.2 T (Fig. 9 (g)), there was an interference of the flux
flow due to the large temperature rise with the dynamic motion of themagnetic flux (flux jump) at t = 25 ms. In the end, the TF value reached
0.7 T with a slow decrease resembling characteristic TF decay in MgB2
7

M. Miryala et al.

Materials Science & Engineering B 273 (2021) 115390

Fig. 12. Trapped field profiles of the SPS MgB2 bulk mapped 1 mm above the surface at 20 K for various applied fields. a, b, c, d, e, f, and g correspond to Bex = 1, 1.2,
1.4, 1.6, 1.8, and 2.2 T, respectively.

(t = 30 to 60 sec).
To conclude this part, the maximum local TF in the bulk magnetizedat 14 K saturated at 0.7 T. TF profiles mapped 1 mm above the bulk
surface (Fig. 10 (a-g)) give more consistent results. At low applied fields(Bex = 1, 1.2, and 1.4 T) the profiles showed a M− shaped TF distribution
due to a partially magnetized state of the bulk [34–36]. In the appliedfields 1.6 and 1.8 T, the TF profiles showed perfect uniform cone with TF
increasing with applied field increment. In Bex = 2 T, TF dropped
maintaining still a uniform cone shape. It resembled an over-magnetizedstate of bulk. In Bex = 2.2 T, the bulk showed a high TF with a uniform
cone shape resembling the fully magnetized state of bulk.
The same experiments as at 14 K was repeated at 20 K (the common
operating temperature for MgB2 superconductors).
i) With Bex = 1 and 1.2 T, there was a small local trapped field,
increasing with Bex up to pulse peak, and TF reached 0.1 and 0.3 T,
respectively, and remained constant as shown in Fig. 11 (a,b).
ii) With Bex = 1.4 T (Fig. 11 (c)), local TF first grew up to 0.6 T andthen dropped slowly from t = 20 ms up to t = 70 ms, resembling thus thecharacteristic TF decay observed in most MgB2. The latter consisted of aslight sudden flux loss at t = 70 ms followed by stabilization on 0.5 T.
iii) With Bex = 1.6 and 1.8 T, the bulk showed a characteristic decay(Fig. 11 (d, e)) followed by flux drops at t = 70 ms and t = 100 ms. The

local TF of the bulk increased upto around 0.7 and 1.2 T followed bydrop and reached saturation on 0.3 T for both Bex = 1.6 and 1.8 T. The
extrapolated reference curves for Bex = 1.6 and 1.8 T are Bestimated = 4/
t + 0.46 and Bestimated = 3/(t-23) + 0.7 respectively. Figures show thatthe flux flow out or flux losses are very high, probably due to a large
temperature increase caused by a rapid movement of the magnetic flux.
On the other hand, at 14 K such large local TF drops were absent because
at low operating temperature flux pinning was high and did not allow fora fast flux movement.
iv) With Bex = 2 T (Fig. 11 (f)), a similar behavior as with Bex = 1.6
and 1.8 T but much more violent and resulted in the very low final TF
value saturated at 0.2 T as compared to the extrapolation, Besti­
mated = 20/t + 0.5. However, a peak TF of 1.4 T was observed until theflux flow.
The TF profiles were then also mapped 1 mm above the bulk surface
at 20 K (Fig. 12 (a - f)). Similarly as in 14 K, at low applied fields (Bex = 1
and 1.2 T) the TF profiles showed a uniform M− shaped TF distribution
corresponding to a partially magnetized state. In moderate applied fields(Bex = 1.4 and 1.6 T), the TF profiles showed a perfect cone shape. Only,
the TF value for Bex = 1.6 T was lower than for Bex = 1.4 T. This showedthat the sample was in over-magnetized and in fully magnetized state,
respectively. In Bex = 1.8 and 2 T, the TF profile showed distorted single
8M. Miryala et al.

Materials Science & Engineering B 273 (2021) 115390

Fig. 13. Local trapped field at the center of the bulk surface and the modeltrapped field as a function of the applied pulse field at 14 and 20 K.

Fig. 15. Temperature dependence on the applied pulse field at 14 and 20 K.

done on Gd-Ba-Cu–O bulks, where they have observed that the trapped
flux distribution may vary with increasing may vary with applied field.
The trapped field had a tendency to reach a multi-peak profile due topartial flux motion when multiple pulses were used to increase theapplied field. This led to TF saturation or decrease with increasing theapplied peak field [36,37]. The same can be expected with the MgB2
bulks when subjected to pulsed-field magnetization (PFM). Fig. 14
shows the total trapped flux (ɸT) dependence on Bex, 1 mm above bulk
surface. In both temperatures, 14 and 20 K, high total trapped flux was
observed when Bex = 0.8 T. In both temperatures, the trapped flux was
not proportional to applied field, instead, there were dips and rises.
Evidently, magnetic flux does not enter the bulk at every extra pulse
uniformly. The magnetization history in an incompletely magnetizedsample and the local overheating in the sample volume and the related
flux loss play an important role. This behavior is not unique and has
been observed also elsewhere[36]. The curves of BT vs Bex and ɸT vs Bex
for 14 and 20 K differed in the trapped field distortion in high external
magnetic fields. We measured the temperature of the bulk during field
application by means of a Cernox thermometer on the side of the SUS
ring. The rise in temperature (ΔT) was calibrated. Fig. 15 shows thetemperature during pulse field application for both 14 and 20 K. Tem­
perature rises due to a local pinning failure, flux motion, the associatedrise in electrical resistivity, local overheating and so on. The rise inelectrical resistivity is a consequence of flux motion that leads to a local
loss of superconductivity. The rise in resistivity is microscopic and local.
The local temperature keeps rising with increase in applied pulse field.
Tatsuya et al. [33] showed an increase of temperature with increasing
applied field similar to the present work. It implies that thermal con­ductivity is an important issue when considering production of hightrapped field MgB2 magnets. MgB2 does not have good thermal con­ductivity and it would be desirable to reinforce it by a compound highthermally conductive.

Fig. 14. Dependence of the total trapped flux (ɸT) 1 mm above bulk’s surface
on applied field at 14 and 20 K.

cones, probably due to flux jumps. When applying a large external
magnetic field, moreover too fast, the material sometime cannot
accommodate the critical state, the critical current density locally ex­
ceeds its maximum, and the material at that point loses superconducting
nature locally, nonzero local resistivity causes a local overheating andcritical current reduction appears in the form of so-called flux jumps.
Fig. 13 shows plot of the experimental and model local trapped fieldat the center of bulk surface as a function of Bex for 14 K and 20 K. The
highest local trapped field at 14 K was BT = 0.64 T (for Bex ~ 2.2 T), at
20 K BT = 0.42 T (for Bex ~ 1.4 T). The highest model trapped field was
Bestimated = 0.97 T (for Bex ~ 2 T) at 14 K and Bestimated = 0.71 T (forBex ~ 1.8 T) at 20 K. The lower operation temperature showed better
performance and a shift of the BT peak to higher applied fields. This wasa consequence of increase in Jc, in pinning force, and in magnetic flux
shielding. The TF values lowered or dropped before than estimated at
Bex > 1.2 T. In case of 14 K, the TF reached dip at Bex = 2 T, mainly due
to flux flow out and over magnetization. While in case of 20 K, the bulk
reached saturation magnetic TF value of 0.5 T approximately at
Bex = 1.4 T. This decrease can also be supported with the TF analysis

4. Conclusion
We have successfully demonstrated good trapped field properties of a
SPS fabricated MgB2 bulk. XRD measurements showed single phase
MgB2 formation with a fractional amount of MgO impurities. The M-Tloops showed a sharp superconducting transition at 37.5 K. The Jc values
are also high compared to regular sintered bulks. Flux pinning diagrams
confirmed dominance of grain boundary pinning, with peak position at
0.2. Bending tests showed that SPS MgB2 bulks can exhibit high bending9M. Miryala et al.

Materials Science & Engineering B 273 (2021) 115390

strengths but are highly brittle. These mechanical properties correlatewith high packing ratio of SPS MgB2 bulk, close to 99.8%. Magnetization
process was studied at 14 and 20 K, the stray field originating from thetrapped field was measured 1 mm above the bulk’s surface. At 14 K, Mshaped trapped field distribution (partial magnetized state) was
observed at low applied pulse fields, while at higher applied fields(>1.6 T) a cone shaped trapped field distribution was observed (fully
and over magnetized states). At 20 K, moderate applied fields(1.4–1.6 T) led to a cone shaped TF distribution and higher applied fields(>1.6 T) resulted in distorted TF profiles. At the higher applied fields,
flux jumps were observed, which caused a sudden drop in local TF
values. In addition, characteristic MgB2 TF decay with time was also
observed. Local trapped field BT and model trapped field Bestimated of SPS
MgB2 bulk were compared for two temperatures, 14 and 20 K. The rise intemperature during flux jump was proportional to the applied pulse
field. To reduce unwanted flux jumps, the MgB2 blocks need a goodthermal conductive reinforcement around or soaked into. All in all, PFM
is an efficient and relatively cheap method of the bulk magnetization but
due to flux jumps trapped field is not simply proportional to the applied
pulse and the optimum pulse value is specific for each sample size,
material, and operating temperature.

[13] T. Naito, T. Sasaki, H. Fujishiro, Trapped magnetic field and vortex pinning
properties of MgB 2 superconducting bulk fabricated by a capsule method,
Supercond. Sci. Technol. 25 (9) (2012) 095012, https://doi.org/10.1088/09532048/25/9/095012.
[14] L. Gozzelino, R. Gerbaldo, G. Ghigo, D. Torsello, V. Bonino, M. Truccato, M.
A. Grigoroscuta, M. Burdusel, G.V. Aldica, V. Sandu, I. Pasuk, P. Badica, Highmagnetic shielding properties of an MgB 2 cup obtained by machining a sparkplasma-sintered bulk cylinder, Supercond. Sci. Technol. 33 (4) (2020) 044018,
https://doi.org/10.1088/1361-6668/ab7846.
[15] A. Murakami, H. Teshima, T. Naito, H. Fujishiro, T. Kudo, H. Fujishiro, Mechanicalproperties of MgB2 bulks, Phys. Procedia. 58 (2014) 98–101, https://doi.org/
10.1016/j.phpro.2014.09.034.
[16] S. Safran, Critical current density and mechanical performance of MgB2
superconductors prepared with different magnesium sources, Ceram. Int. 45 (8)(2019) 10243–10249, https://doi.org/10.1016/j.ceramint.2019.02.077.
[17] T.C. Shields, K. Kawano, D. Holdom, J.S. Abell, Microstructure andsuperconducting properties of hot isostatically pressed MgB 2, Supercond. Sci.
Technol. 15 (2) (2002) 202–205, https://doi.org/10.1088/0953-2048/15/2/303.
[18] S.Y. Lee, S.I. Yoo, Y.W. Kim, N.M. Hwang, D.Y. Kim, Preparation of Dense MgB2
Bulk Superconductors by Spark Plasma Sintering, J. Am. Ceram. Soc. 86 (2003)
1800–1802. doi:10.1111/j.1151-2916.2003.tb03559.x.
[19] J.G. Noudem, M. Aburras, P. Bernstein, X. Chaud, M. Muralidhar, M. Murakami,
Development in processing of MgB 2 cryo-magnet superconductors, J. Appl. Phys.
116 (16) (2014) 163916, https://doi.org/10.1063/1.4900725.
[20] M. Muralidhar, K. Nozaki, H. Kobayashi, X.L. Zeng, A. Koblischka-Veneva, M.
R. Koblischka, K. Inoue, M. Murakami, Optimization of sintering conditions in bulk
MgB2 material for improvement of critical current density, J. Alloy. Compd. 649(2015) 833–842, https://doi.org/10.1016/j.jallcom.2015.07.191.
[21] C.P. BEAN, Magnetization of High-Field Superconductors, Rev. Mod. Phys. 36 (1)(1964) 31–39.
[22] D. Dew-Hughes, Flux pinning mechanisms in type II superconductors, Phil. Mag. 30(2) (1974) 293–305.
[23] V. Zablotskii, M. Jirsa, P. Petrenko, Phys. Rev. B 65 (2002) 2245081-4. doi:
10.1103/PhysRevB.65.224508.
[24] M. Jirsa M. Rameš, M. Miryala, P. Svora, J. Duchon, O. Molnarova, S.S. Arvapalli,
M. Murakami, Supercond. Sci. Technol. 33 (2020) 094007 (1–8) doi:10.1088/
1361-6668/aba01c.
[25] S.S. Arvapalli, M. Miryala, M. Jirsa, M. Murakami, A. Sai Srikanth, M. Muralidhar,
J. Milos, M. Murakami, Size reduction of boron particles by high-power ultrasoundfor optimization of bulk MgB2, Supercond. Sci. Technol. 33 (2020), 115009,
https://doi.org/10.1088/1361-6668/abb63e.
[26] M. Miryala, S.S. Arvapalli, P. Diko, M. Jirsa, M. Murakami, Flux pinning andsuperconducting properties of bulk MgB 2 with MgB 4 addition, Adv. Eng. Mater.
22 (3) (2020) 1900750, https://doi.org/10.1002/adem.201900750.
[27] S.S. Arvapalli, M. Muralidhar, M. Murakami, High-performance bulk MgB2
superconductor using amorphous nano-boron, J Supercond Nov Magn 32 (7)(2019) 1891–1895, https://doi.org/10.1007/s10948-018-4919-x.
[28] M. Jirsa, M. Rames, M.R. Koblischka, A. Koblischka-Veneva, K. Berger, B. Douine,
Relaxation and pinning in spark-plasma sintered MgB 2 superconductor,
Supercond. Sci. Technol. 29 (2) (2016) 025006, https://doi.org/10.1088/09532048/29/2/025006.
[29] M.P. Anderson, D.J. Srolovitz, G.S. Grest, P.S. Sahni, Computer simulation of grain
growth—I. Kinetics, Acta Metall. 32 (5) (1984) 783–791, https://doi.org/10.1016/
0001-6160(84)90151-2.
[30] J.H. Durrell, A.R. Dennis, J. Jaroszynski, M.D. Ainslie, K.G.B. Palmer, Y.-H. Shi, A.
M. Campbell, J. Hull, M. Strasik, E.E. Hellstrom, D.A. Cardwell, A trapped field of
17.6 T in melt-processed, bulk Gd-Ba-Cu-O reinforced with shrink-fit steel,Supercond. Sci. Technol. 27 (8) (2014) 082001, https://doi.org/10.1088/09532048/27/8/082001.
[31] A. Murakami, A. Iwamoto, J.G. Noudem, Mechanical properties of bulk MgB2
superconductors processed by spark plasma sintering at various temperatures, IEEE
Trans. Appl. Supercond. 28 (3) (2018) 1–4.
[32] M. Muralidhar, K. Inoue, M.R. Koblischka, M. Tomita, M. Murakami, Optimzationof processing condiations towards high trapped fields in MgB2 bulk, J. Alloy.
Compd. 608 (2014) 102–109.
[33] T. Hirano, Y. Takahash, S. Namba, T. Naito, H. Fujishiro, A record-high trappedfield of 1.61 T in MgB2 bulk comprised of copper plates and soft iron yoke cylinderusing pulsed-field magnetization, Supercond. Wires Basics Appl. 33 (2020) 10.
[34] H. Fujishiro, T. Hiyama, T. Tateiwa, Y. Yanagi, T. Oka, Importance of initial “Mshaped” trapped field profile in a two-stage pulse field magnetization (MMPSC)
method, Physica C (Amsterdam, Neth.) 463-465 (2007) 394–397, https://doi.org/
10.1016/j.physc.2007.03.440.
[35] S. Zou, V.M.R. Zermeño, A. Baskys, A. Patel, F. Grilli, B.A. Glowacki, Simulationand experiments of stacks of high temperature superconducting coated conductors
magnetized by pulsed field magnetization with multi-pulse technique, Supercond.
Sci. Technol. 30 (1) (2017) 014010, https://doi.org/10.1088/0953-2048/30/1/
014010.
[36] M.D. Ainslie, J. Srpcic, D. Zhou, H. Fujishiro, K. Takahashi, D.A. Cardwell, J.
H. Durrell, Toward optimization of multi-pulse, pulsed field magnetization of bulk
high-temperature superconductors, IEEE Trans. Appl. Supercond. 28 (4) (2018)
1–7, https://doi.org/10.1109/TASC.2017.2788924.
[37] T. Ida, H. Matsuzaki, Y. Akita, M. Izumi, H. Sugimoto, Y. Hondou, Y. Kimura,N. Sakai, S. Nariki, I. Hirabayashi, M. Miki, M. Murakami, M. Kitano,
Magnetization properties for Gd–Ba–Cu–O bulk superconductors with a couple ofpulsed-field vortex-type coils, Physica C 412-414 (2004) 638–645, https://doi.org/
10.1016/j.physc.2003.12.082.

Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
Acknowledgements
This work was partly supported by Shibaura Institute of Technology(SIT) International Research Center for Green Electronics and Grant-inAid FD research budget code: 721MA56383. One of the authors (SSA)
acknowledges support from SIT for his post-doctoral fellowship.
References[1] J. Nagamatsu, N. Nakagawa, T. Muranaka, Y. Zenitani, J. Akimitsu,
Superconductivity at 39K in magnesium diboride, Nature 410 (2001) 63–64.
[2] S.I. Schlachter, W. Goldacker, A. Frank, B. Ringsdorf, H. Orschulko, Properties of
MgB2 superconductors with regard to space applications, Cryogenics 46 (2-3)(2006) 201–207, https://doi.org/10.1016/j.cryogenics.2005.11.003.
[3] M. Tomsic, M. Rindfleisch, J. Yue, K. McFadden, J. Phillips, M.D. Sumption,
M. Bhatia, S. Bohnenstiehl, E.W. Collings, Overview of MgB 2 superconductor
applications, Int. J. Appl. Ceramic Technol. 4 (3) (2007) 250–259, https://doi.org/
10.1111/j.1744-7402.2007.02138.x.
[4] W. Yao, J. Bascuñán, S. Hahn, Y. Iwasa, MgB2 coils for MRI applications, IEEE
Trans. Appl. Supercond. 20 (2010) 756–759, https://doi.org/10.1109/
TASC.2010.2044035.
[5] B.A. Glowacki, M. Majoros, M. Eisterer, S. Toenies, H.W. Weber, M. Fukutomi,
K. Komori, K. Togano, MgB2 superconductors for applications, Physica C 387 (1-2)(2003) 153–161, https://doi.org/10.1016/S0921-4534(03)00662-2.
[6] K. Vinod, R.G.A. Kumar, U. Syamaprasad, Prospects for MgB 2 superconductors formagnet application, Supercond. Sci. Technol. 20 (1) (2007) R1–R13, https://doi.
org/10.1088/0953-2048/20/1/R01.
[7] N. Rogado, M.A. Hayward, K.A. Regan, Y. Wang, N.P. Ong, H.W. Zandbergen, J.
M. Rowell, R.J. Cava, Low temperature synthesis of MgB2, J. Appl. Phys. 91 (2002)
274–277, https://doi.org/10.1063/1.1420771.
[8] A.K. Ghosh, L.D. Cooley, A.R. Moodenbaugh, Investigation of instability in high Jc
Nb3Sn strands, IEEE Trans. Appl. Supercond. 15 (2005) 3360–3363, https://doi.
org/10.1109/TASC.2005.848904.
[9] P.M. Aswathy, J.B. Anooja, P.M. Sarun, U. Syamaprasad, An overview on ironbased superconductors, Supercond. Sci. Technol. 23 (7) (2010) 073001, https://doi.org/10.1088/0953-2048/23/7/073001.
[10] Y. Kamihara, H. Hiramatsu, M. Hirano, R. Kawamura, H. Yanagi, T. Kamiya,
H. Hosono, Iron-based layered superconductor: LaOFeP, J. Am. Chem. Soc. 128(2006) 10012–10013, https://doi.org/10.1021/ja063355c.
[11] A.G. Bhagurkar, A. Yamamoto, L. Anguilano, A.R. Dennis, J.H. Durrell, N. Hari
Babu, D.A. Cardwell, A trapped magnetic field of 3 T in homogeneous, bulk MgB 2
superconductors fabricated by a modified precursor infiltration and growth
process, Supercond. Sci. Technol. 29 (3) (2016) 035008, https://doi.org/10.1088/
0953-2048/29/3/035008.
[12] K. Nagashima, T. Higuchi, J. Sok, S.I. Yoo, H. Fujimoto, M. Murakami, The trappedfield of YBCO bulk superconducting magnets, Cryogenics 37 (10) (1997) 577–581.

10