IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 30, NO. 4, JUNE 2020

6800406

Effects of Carbon Doping on Trapped Magnetic Fieldof MgB2 Bulk Prepared by in-situ Hot Isostatic
Pressing Method
Tomoyuki Naito , Arata Ogino , Hiroyuki Fujishiro , and Satoshi Awaji

Abstract—We report the effects of carbon doping on the trappedfield properties of MgB2 bulks doped with graphite (C) and B4 C,
which were prepared by an in-situ hot isostatic pressing method.
The trapped fields, BT ’s were degraded by the graphite and B4 C
doping above 15 K, which was predominantly due to the lowered Tc
by the C substitution. However, the slope of BT (T ) and the irreversibility line, Birr (T ), determined from the resistive transitions
indicated that the BT of the graphite-doped bulk was expected
to exceed that of the pristine bulk below 15 K. Contrary to theliterature by other groups, B4 C did not enhance the vortex pinning
properties.
Index Terms—MgB2 , trapped field, carbon doping.

I. INTRODUCTION
HE MgB2 bulk magnets have been intensively studied
since the discovery of superconductivity in MgB2 [1],especially in the past decade [2]–[8]. A trapped field, BT , of 5 Tclass can be obtained by the combination of various processing
methods, impurity doping, and so on, to date [5], [7]. Since
the filling factor of MgB2 fabricated via an in-situ route underambient pressure is only about 50%, high critical current density,
Jc , has been generally realized for dense MgB2 prepared by ahigh pressure synthesis [9]–[13] or by a diffusion (an infiltration)
method [8], [14], [15]. The titanium group elements [11], [16]–[19] and carbon [8], [20]–[26] are well known to be effective
dopants and additives to enhance the vortex pinning propertiesof MgB2 . The titanium created thin TiB2 lamellae at the surfaceof the MgB2 grain, which prevented the grain growth of MgB2 ,and then enhanced the grain boundary pinning [16], [17]. We
obtained a BT of 4.6 T at 14.1 K in center of the stacked Ti-doped
MgB2 bulks fabricated by an in-situ hot isostatic pressing (HIP)
method [7]. On the other hand, cost-effective carbon doping
has been studied mainly for wire production. Carbon substitutes
on the boron site, and shortens the coherence length, that is,T

Manuscript received September 25, 2019; accepted March 30, 2020. Dateof publication April 6, 2020; date of current version May 18, 2020. This work
was supported by JSPS KAKENHI under Grants JP15K04718, JP15K04646,
JP18K04920. (Corresponding author: Tomoyuki Naito.)
Tomoyuki Naito, Arata Ogino, and Hiroyuki Fujishiro are with the Faculty of
Science and Engineering, Iwate University, Morioka 020-8551, Japan (e-mail:tnaito@iwate-u.ac.jp; cloud3254@gmail.com; fujishiro@iwate-u.ac.jp).
Satoshi Awaji is with the High Field Laboratory for Superconducting Materials, Institute for Materials Research, Tohoku University, Sendai 980-8577,
Japan (e-mail: awaji@imr.tohoku.ac.jp).
Color versions of one or more of the figures in this article are available online
at https://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/TASC.2020.2985355

enhances the upper critical field Bc2 . Since simple forms ofcarbon such as graphite were found to be difficult to substitute on
the B-site [21], various carbon-containing compounds have beenused as carbon sources. SiC, which was used in the pioneering
work [20], offered enhanced vortex pinning properties, however
concomitantly created an impurity phase of Mg2 Si preventing
the flow of supercurrent. Among various carbides, B4 C is apromising dopant, because it has only the constituent elementsof C-doped MgB2 and a high reactivity, that is, the carbon was
efficiently introduced into MgB2 . The carbon coating to theboron powder by various aromatic compounds such as coronene
is quite effective to enhance both Jc and the irreversibility field,
Birr , of wires [24]. However, this attractive method is difficult
to apply to the fabrication of a centimeter-scale bulk, because it
requires quite a large amount of starting materials, comparedto the short wires for the feasibility study. Ball milling wasreported to promote the impurity diffusion at the surface of
MgB2 grains, in addition to the increase of grain boundaries [27].
Therefore, carbon substitution should also be encouraged byball milling. In this paper, we have measured the trapped field,
electrical resistivity and critical current density of pristine andcarbon-doped MgB2 bulks, in which graphite (for reference) andB4 C were used as the carbon sources. The bulks were preparedfrom the as hand-ground and as ball-milled precursor powders.
Both the effects of carbon doping and grain refining on thetrapped field properties are discussed.
II. EXPERIMENTAL
The pristine and carbon-doped MgB2 bulks were prepared by
the in-situ hot isostatic pressing (HIP) method, where graphite
and B4 C were used as the carbon source. The precursors wereprepared in the following procedure. Raw powders of Mg (99%in purity, ≤180 μm in particle size, Kojundo Chemical Laboratory Co., Ltd), amorphous B (99% in purity, ≤46 μm in
particle size, Furuuchi Chemical Corp.), C (graphite) (99.9%
in purity, ∼50 μm in particle size, Kojundo Chemical Laboratory Co., Ltd), and B4 C (99% in purity, ≤46 μm in particle
size, Furuuchi Chemical Corp.) were mixed in the molar ratios
Mg : B : C = 1.0 : 1.8 : 0.2 and Mg : B : B4 C = 1.0 : 1.0 : 0.2,
respectively, for the C- and B4 C-doped bulks. These were groundusing an agate mortar and pestle, additionally pulverized by aplanetary ball milling (BM) device (PM100, Retsch GmbH) witha rotating speed of 250 rpm for 12 h using an agate container

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6800406

IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 30, NO. 4, JUNE 2020

Fig. 1. (a) X-ray diffraction patterns of the pristine and the C10%- and B4 C10%-doped MgB2 bulks. BM means the ball milling. The data of the pristine MgB2
without BM are from Ref. [7]. (b) The 2θ value of the MgB2 (110) plane and the lattice constant a for the indicated MgB2 bulks. (c) The critical temperature,
Tc , and the lattice constant a as a function of the carbon content for the C10%- and B4 C10%-doped MgB2 bulks and C-doped single crystalline MgB2 [28]. The
dashed lines are guide to the eyes.

and balls. The mixed powder was pelletized to a disc 40 mm indiameter and 20 mm in thickness under a uniaxial pressure ofabout 12 MPa, and subsequently the pellet was further densified
by a cold isostatic pressing method under a pressure of 196 MPa.
The precursor, which was enclosed in a stainless steel container
under vacuum by electron beam welding, was sintered at 900 ◦ C
for 3 h under an Ar gas pressure of 98 MPa in the HIP furnaceand cooled down to room temperature by furnace cooling.
Disc-shaped MgB2 bulks of about 38 mm diameter and 7 mm
thickness were prepared by machining under dry conditions.
The constituent phases and structure were examined by an X-ray
diffraction pattern taken at the flat surface of each bulk using a
Cu-Kα radiation (wave length was 0.15406 nm).
The MgB2 bulks were magnetized by field cooling (FC)in a magnetic field, parallel to the thickness direction using a commercial 10-T cryogen-free superconducting magnet(JMTD-10T100, JASTEC, Inc.). Trapped field was measured
by a cryogenic Hall sensor (BHT-921, F.W. Bell Inc.) mounted
on the center of the bulk surface. The temperature of the bulk
was monitored by a Cernox thermometer which was adhered
beside the Hall sensor on the bulk surface. The resistivity andmagnetization were measured using a small piece cut from thebulk. The resistivity was measured by a conventional dc four
probe method with a typical current density of 10 A cm−2
under magnetic fields up to 16 T using the 18-T superconducting magnet (JASTEC Inc.) at the High Field Laboratory
for Superconducting Materials, Institute for Materials Research,
Tohoku University. The magnetic field dependent magnetization
was measured using a vibrating sample magnetometer (VSM)

coupled with the 15-T cryogen-free superconducting magnet(JASTEC Inc.) at the HFL-SM, IMR, Tohoku Univ. The critical current density, Jc (B), was estimated from the magnetic
hysteresis using the extended Bean model [29], [30].
III. RESULTS AND DISCUSSION
Fig. 1(a) shows the X-ray diffraction patterns of the indicated
MgB2 bulks. The observed major peaks were almost confirmed
as the MgB2 phase for all the present MgB2 bulks. As shownin the left axis of Fig. 1(b), the peak of the MgB2 (110) planeat around 2θ = 60◦ was shifted toward higher angles for both
C10%- and B4 C10%-doped bulks, which suggests the shrinkageof the a-axis of MgB2 due to the carbon substituted for B.
The ball-milled precursors offered a further peak shift for both
C10%- and B4 C10%-doped bulks. The amount of peak shift for
the B4 C10%-doped bulk was somewhat larger than that for a
C10%-doped one, regardless of ball milling. Note that a similar
peak shift was confirmed for the a-axis related crystallographic
planes such as (100) and (101) planes. As shown in the rightaxis of Fig. 1(b), the a-axis was shortened by the carbon doping.
The lattice constants of the a-axis were estimated to be 0.3080,
0.3066 and 0.3059 nm for the pristine, C10%- and B4 C10%doped MgB2 bulks respectively after the ball milling process.
Note that ball milling also shortened the a-axis for both C10%and B4 C10%-doped MgB2 bulks. Fig. 1(c) shows the criticaltemperature, Tc , (left axis) and the lattice constant a (right axis)
as a function of the C content. The actual amounts of carbon
substitution to the boron site were estimated to be about 3–4.5%

NAITO et al.: EFFECTS OF CARBON DOPING ON TRAPPED MAGNETIC FIELD OF MgB2 BULK PREPARED

6800406(FCM) from the initial applied field of 5 T at the initial bulk
temperature of 16.2 K. The trapped field, BT , decreased moderately with withdrawing the applied magnetic field, Bex , at a rateof −0.022 T/min and finally reached 2.98 T at the end of ramp
(Bex = 0 T). After that, the BT continued to decrease slightly,
which originated from vortex creep. The obtained BT value of
2.98 T at 16.2 K is about 20% higher than the value of 2.5 T at
16.2 K reported for the HIP-processed MgB2 bulk (38 mm indiameter and 6.9 mm in thickness) without ball milling [7]. Thisis believed to originate from the increase of grain boundaries,
which are well known as the dominant pinning center of MgB2 ,by ball milling [5], [27], [31], [32]. Thus, we estimated the
crystallite size, τ , of both the pristine bulks with and withoutball milling by the following Scherrer’s relation [33],τ=Fig. 2. (a) Time dependences of the trapped field, applied field and temperatureof the pristine MgB2 bulk with ball milling during FCM from the applied fieldof 5 T at the initial bulk temperature of 16.2 K. The left- and right-hand axes are
the magnetic field and temperature, respectively. (b) Temperature dependenceof the trapped field, BT (T ), of the pristine and the C10%- and B4 C10%-doped
MgB2 bulks. The data of the pristine without BM are from Ref. [7]. BM meansball milling.

and 4.5–6% for the C10%- and B4 C10%-doping, respectively,
considering the relationship among the nominal C content, thelength of a-axis and the critical temperature for C-substituted
single crystalline MgB2 [28]; the Tc ’s of the present MgB2
bulks were determined by the temperature dependences of thetrapped field and the resistivity, as will be shown later. The C
contents estimated from the lattice constant a were about 1.5%
larger than those from the Tc for both carbon-doped MgB2
bulks. This difference corresponded to a 2θ of about 0.15◦ ,which might originate from the shift of the machine origin of
the XRD device. The MgO impurity was observed for all thebulks, regardless of ball milling. The MgB2 C2 impurity phasealso appeared for both C10%- and B4 C10%-doped bulks, which
originated from the excess carbon. The peak intensity of the
MgB2 C2 phase was reduced by the ball milling for both C10%and B4 C10%-doped bulks, while on the contrary that of the
MgO phase was increased. These results also suggest that theball milling promoted carbon substitution on the B site.
Fig. 2(a) shows the typical magnetizing process of the pristine
MgB2 bulk with ball milling during field-cooled magnetizationKλ,β cos θ(1)

where, K is the shape factor, generally ∼0.9, λ the wave lengthof the Cu-Kα radiation, β the full width at the half maximum(FWHM) of the diffraction peak, and θ the Bragg angle. Using
the main peak of MgB2 (101) plane, the τ value of 32.8 nm
estimated for the pristine bulk without ball milling reduced
slightly to 31.6 nm by ball milling; note that this reduction
corresponded to the difference in β of only 0.01◦ , which was
smaller than the interval of angle of 0.02◦ . Therefore, the increase of grain boundaries by ball milling should be confirmedby an additional microstructural analysis such as an electronbackscattered diffraction [32], [34]. We found almost the same
XRD patterns between the pristine bulks with and without ball
milling, although the peak of the MgO impurity of the bulk withball milling was slightly larger than that of without ball milling.
This suggested that the quality of both bulks were almost thesame and that the increase of grain boundaries was the mostplausible origin for the enhancement of BT .
During FCM, the temperature of the bulk increased quite moderately from 16.2 to 16.6 K, which was caused by heat generation
due to the vortex dynamics. The magnetizing processes of otherthe MgB2 bulks (not shown here) were similar to that of thepristine one.
After the magnetizing process, each MgB2 bulk was heated up
beyond the critical temperature. Fig. 2(b) shows the temperaturedependence of the trapped field, BT (T ), of the indicated MgB2
bulks. The BT (T ) decreased monotonically with increasing
temperature, because of the decrease in Jc with temperature.
The BT (T ) relationships for the pristine MgB2 bulks withand without BM were just as the trapped fields at the end of
ramp. The Tc , defined as the temperature at BT = 0 T, wasabout 39 K for both pristine bulks. For the C10%-doped MgB2
bulks, the BT (T ) of the bulk with BM, which was somewhat
higher than that without BM at the magnetizing temperature,
decreased steeply with increasing temperature, went below theBT (T ) of the bulk without BM around 20 K, and finally reachedzero at Tc = 35 K. On the other hand, the C10%-doped bulk
without BM kept a non-zero BT up to Tc = 38 K. The BT (T )was strongly suppressed, and simultaneously the curvature of
the BT (T ) line was changed from convex to concave by the
C10%-doping. A rather small BT (T ) with concave curvature6800406

IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 30, NO. 4, JUNE 2020

Fig. 4. The temperature dependence of the irreversibility field, Birr (T ), of thethe pristine and the C10%- and B4 C10%-doped MgB2 bulks with ball milling(BM).

Fig. 3. Temperature dependence of resistivity of (a) the pristine and (b) the
C10%- and (c) B4 C10%-doped MgB2 bulks with ball milling (BM) undermagnetic fields up to 16 T.

was observed for the B4 C10%-doped MgB2 bulks, comparedwith the pristine bulks. Ball milling resulted in the further
degradation of BT (T ) also for the B4 C10%-doped MgB2 bulks.
The Tc ’s at BT = 0 T were 32 and 33 K, respectively, for
the B4 C10%-doped bulks with and without BM. The low
BT (T ) values observed for the present carbon-doped bulks
originated from the suppression of Tc , which is well known as adisadvantage of the carbon doping. For the present carbon-doped

MgB2 bulks, the promotion of the reaction of C with B isbelieved to exceed the effect of grain refinement by ball milling.
However, it is noteworthy that extrapolating all the BT (T )’s
toward low temperatures suggested that the BT of the C10%doped bulk with BM is expected to exceed the BT of the pristinebulk with BM below 15 K.
Fig. 3 shows the temperature dependence of the resistivity,
ρ(T ), under the magnetic fields up to 16 T for the pristine and the
C10%- and B4 C10%-doped MgB2 bulks with ball milling process. The Tc , defined as the temperature at which the resistivity
went to zero, was 38.0, 33.8, and 30.8 K for the pristine, C10%and B4 C10%-doped bulks, respectively. Although these resistive
Tc values were about 1 K lower than those estimated from the
BT (T ) lines for the present three bulks, the magnitude of Tc
degradation by carbon doping was almost the same as for themagnetic Tc . The irreversibility temperature, Tirr , was definedas the temperature at which the resistivity was 0.1 μΩcm. The
estimated Tirr was plotted on the magnetic field vs. temperature
diagram, as shown in Fig. 4. The irreversibility line, Birr (T ), of
the C10%-doped bulk exceeded that of the pristine bulk below
approximately 13 K. On the other hand, the Birr (T ) of theB4 C10%-doped MgB2 bulk was smaller than that of the pristine
one in the whole measured temperature range. The behaviors
of Birr (T ) lines support strongly the expectation that a higher
trapped field can be obtained for the C10%-doped bulk with BM
at low temperatures below 15 K.
Fig. 5 shows the magnetic field dependence of the criticalcurrent density, Jc (B), at temperatures of 10 and 20 K for thepristine and C10%-doped MgB2 bulks with ball milling. The
Jc (B) at 20 K of the C10%-doped bulk was smaller than that ofthe pristine one, and furthermore the irrevesibility field of 3.8 T,
defined as the magnetic field at which the Jc was 100 A cm−2 ,was also low compared to Birr = 4.7 T of the pristine one. At
10 K, a comparable magnitude of Jc (B) above about 5 T andalmost the same Birr = 8.6 T were found for both pristine andC10%-doped bulks. The somewhat smaller Birr compared to the

NAITO et al.: EFFECTS OF CARBON DOPING ON TRAPPED MAGNETIC FIELD OF MgB2 BULK PREPARED

6800406

University for her assistance in the magnetization measurements
using VSM at the HFL-SM, IMR, Tohoku University.

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Fig. 5. Magnetic field dependence of critical current density, Jc , of the pristineand C10%-doped MgB2 bulks with ball milling (BM) at 10 and 20 K.

resistive Birr is due to the difference in the criterion of Jc , asis well known. These results also suggest that carbon doping iseffective at low temperatures, that is, the operation of the MgB2
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IV. SUMMARY
We have studied carbon doping effects on the trapped field
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which were prepared by the in-situ HIP method using as handground and as ball-milled starting mixtures. Although the nominal content of carbon was 10% with respect to boron for both
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