Microstructure and superconductivity of MgB2 synthesized by using Mg-based compound powders

Article abstract of DOI:10.1016/j.physc.2003.11.080

MgB₂ can be synthesized through a new route using Mg ₂Cu instead of pure Mg. When the temperature is elevated to around the melting point of Mg₂Cu (568 °C), the T_c for Mg₂Cu/B Fe-sheathed wires increases rapidly to about 37.5 K. Mg ₂Cu with its low melting point promotes the diffusion reaction to form a high T_c MgB₂ at low temperatures. A lot of second phases forms with small amounts of MgB₂ in the core of the wires. However, the transport J_c value, which is greater than 10⁵ A/cm² at 4.2 K in ambient fields, is rather high. Since the J _c value is estimated from a net-cross-sectional area of MgB ₂ it is expectedly high. Moreover, Mg₂ Cu/B/metal-substrate layered composites were also prepared in this study. After a heat treatment at 600 °C for 10 h in a vacuum, a 50 μm thick MgB ₂ diffusion layer forms with a high-density microstructure. Cu is released from Mg₂Cu, and collects on the surface of the diffusion layer. This behavior of Cu is similar to that of the bronzed process used for Nb₃ Sn practical conductors.

Full text of DOI:10.1016/j.physc.2003.11.080

Physica C 412–414 (2004) 1174–1178
www.elsevier.com/locate/physc
Microstructure and superconductivity of MgB synthesized
2
by using Mg-based compound powders
*
A. Kikuchi , Y. Iijima, Y. Yoshida, N. Banno, T. Takeuchi, K. Inoue
Superconducting Materials Center, National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan
Received 29 October 2003; accepted 25 November 2003
Available online 1 June 2004
Abstract
MgB
around the melting point of Mg
Mg Cu with its low melting point promotes the diffusion reaction to form a high T
second phases forms with small amounts of MgB in the core of the wires. However, the transport J
greater than 10 A/cm at 4.2 K in ambient fields, is rather high. Since the J value is estimated from a net-cross-
sectional area of MgB it is expectedly high. Moreover, Mg Cu/B/metal-substrate layered composites were also pre-
pared in this study. After a heat treatment at 600 °C for 10 h in a vacuum, a 50 lm thick MgB diffusion layer forms
with a high-density microstructure. Cu is released from Mg Cu, and collects on the surface of the diffusion layer. This
2
can be synthesized through a new route using Mg
2
Cu instead of pure Mg. When the temperature is elevated to
Cu/B Fe-sheathed wires increases rapidly to about 37.5 K.
MgB at low temperatures. A lot of
value, which is
2
Cu (568 °C), the T for Mg
c
2
2
c
2
2
c
5
2
c
2
2
2
2
3
behavior of Cu is similar to that of the bronzed process used for Nb Sn practical conductors.
Ó 2004 Elsevier B.V. All rights reserved.
PACS: 74.72.Yg; 74.62.)c
Keywords: MgB compound; Superconducting wire; Diffusion process; Thick layer
2
1
. Introduction
The discovery that MgB
starting powder characteristics such as particle size
and surface condition should be important. It is
best to start with powders that have a fine particle
size and are resistive to surface oxidization. In
addition, it is desirable that the heat treatment be
carried out at low temperatures because of the
2
exhibits supercon-
ductivity [1] sparked new interests into the prac-
tical conductor research because of its high critical
c
temperature T of 39 K, simple binary chemical
composition and relatively low material costs.
In order to achieve a reaction between Mg and
2
formation of fine MgB grains. We investigated
the synthesis of in situ MgB wires and thick-film
2
B powders during the fabrication of MgB , the
2
coated layers through the liquid–solid diffusion
reaction between Mg Cu and amorphous B.
2
Mg
easy to fabricate Mg
mechanical grinding. Moreover, Mg
lower melting point (568 °C) than that of pure Mg
2
Cu is an intermetallic brittle compound so it is
Cu fine particle powders by
Cu has a
*
2
Corresponding author. Tel.: +81-298-59-2353; fax: +81-
98-59-2301.
E-mail address: kikuchi.akihiro@nims.go.jp (A. Kikuchi).
2
2
0
921-4534/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.physc.2003.11.080

A. Kikuchi et al. / Physica C 412–414 (2004) 1174–1178
1175
(
diffusion reaction at low temperatures.
650 °C), thus Mg
2
Cu is expected to promote a
SQUID magnetometer (Quantum Design: MPMS-
5). X-ray diffraction (XRD) analysis for the phase
identification was made for both specimens, and
microstructures were observed using a field-emis-
sion scanning electron microscope (FE-SEM).
Elemental mapping and quantitative composition
analysis were carried out with an energy dispersive
X-ray (EDX) spectrometer.
2
. Experimental
Mg Cu ingot was made in an Ar atmosphere
2
with a Tammann furnace. The ingot was then
crushed and mechanically ground into a coarse
powder of about 50 lm in average diameter. Phase
identification was carried out with X-ray diffrac-
tion (XRD) analysis using Ni-filtered CuKa radi-
3. Results and discussion
ation for the coarse Mg
2
Cu powder. Thermo
Fig. 1 shows the TGA–DTA results for Mg
and pure Mg. The Mg
is lower than that of pure Mg (650 °C). Both
2
Cu
Cu melting point (568 °C)
gravimetric analysis (TGA) and differential ther-
mal analysis (DTA) were also carried out for the
coarse Mg Cu powder. Both TGA and DTA were
2
weight ratios, Mg Cu and pure Mg, increase with
increasing temperature, which indicates that both
powders oxidize at high temperatures. However,
2
2
carried out with an auto simultaneous instrument
(
2
Shimadzu: TGA-60H) in open air. The Mg Cu
powder was mixed with amorphous B powder
using an atomic ratio of Mg:B ¼ 1:2. The average
size of the amorphous B powder was a few tens of
the weight ratio for Mg
pure Mg, therefore, Mg
tive to oxidation compared to pure Mg.
2
Cu is smaller than that of
Cu should be more resis-
2
microns in diameter. The Mg
2
Cu and B mixture
c
Fig. 2 shows the relationship between T versus
was tightly packed into Fe tube (99.99% purity)
with an outer diameter and inner diameter of 10
and 6 mm, respectively. Wire drawing was carried
out using a grooved-roller and cassette-roller-dies.
Precursor wires had a diameter of 0.72 mm. Nor-
mal precursor wires using pure Mg and amor-
phous B powders were also made for comparison.
temperature for Mg Cu/B/Fe and Mg/B/Fe wires.
When the temperature is elevated to around the
2
melting point of Mg
Mg Cu/B Fe-sheathed wire specimens rapidly in-
creased and reached about 37.5 K due to the for-
mation of the MgB phase through the liquid
(Mg Cu)–solid (B) diffusion reaction. The T val-
ues are comparable to the reported values for
2
Cu (568 °C), the T
c
for
2
2
2
c
2
For the other study, a thick MgB layer on a metal
substrate was fabricated. Amorphous B layer (200
lm thick) was formed on the pure Nb substrate-
tape by using a plasma spraying technique. Then,
Mg
coated on the surface of the plasma-sprayed
amorphous B layer creating Mg Cu/B/metal-sub-
2
Cu slurry using ethyl alcohol as a solvent was
2
strate layered composites. Both the PIT Fe-
sheathed wires and layered composites were heated
to between 550 and 650 °C in a dynamic vacuum
ꢀ
3
under a pressure of less than 10 Pa.
After heat treatment, T and transport critical
current I (4.2 K) were measured by a DC four-
probe method. I (4.2 K) measurements were made
under magnetic fields up to 10 T using a super-
conducting magnet. J for the PIT Fe-sheathed
wires is defined as the I divided by a total cross-
c
c
c
c
Fig. 1. Comparison of TGA–DTA results for Mg
2
Cu and pure
c
Mg. Solid circles and triangles represent increasing weight ra-
sectional area of the powder filled core. Magnetic
transition curves were also measured using a
tios calculated from TGA data for the Mg
respectively.
2
Cu and pure Mg,

1176
A. Kikuchi et al. / Physica C 412–414 (2004) 1174–1178
c 2
Fig. 2. T versus heating temperature for Mg Cu/B (open circle)
and Mg/B (solid circle) Fe-sheathed wires. Heating time is fixed
for 10 h.
2
Fig. 3. XRD patterns of the Mg Cu/B Fe-sheathed wire re-
acted at 600 °C as a function of heating time. (a), (b) and (c) are
for wires reacted at 10 min, 30 min and 1 h, respectively.
MgB single crystal [2]. This result indicates that
2
the MgB
sized through a new route using Mg
pure Mg. At the same temperatures, MgB
formed in the Mg/B Fe-sheathed wires. In this
case, MgB was synthesized by a solid (Mg)–solid
B) diffusion reaction through rather slower reac-
tion.
Fig. 3(a), (b) and (c) show XRD patterns for
Mg
Cu/B Fe-sheathed wires reacted at 600 °C for
0 min, 30 min and 1 h, respectively. As the
reactions progressed the Mg Cu decomposed and
released Mg, thus changing the phase composition
into MgCu . The Mg that was released reacted
with amorphous B and formed the MgB super-
2
superconducting phase can be synthe-
Cu instead of
also
must flow like a percolation due to the weak
linkage between MgB
Fe-sheathed wire that reacted at 600 °C for 1 h
shows a high transport J which was greater than
10 A/cm at 4.2 K in a ambient field as shown in
Fig. 4. The J (4.2 K)-B curve for the Mg Cu/B Fe-
sheathed wire is comparable with a result reported
by Goldacker et al. [3]. As expected, the J of the
present Mg Cu/B wire should be very high since it
is estimated from the net-cross-sectional area of
MgB . An improvement in J for the PIT in situ
wires using Mg Cu is promising if the volume
2 2
grains. However, Mg Cu/B
2
2
c
5
2
2
(
c
2
c
2
2
1
2
2
c
2
fraction of second phases can be reduced by
optimizing wire-fabrication conditions, such as the
size of starting powders and the powder mixture
ratio, etc. When fine second phase particles are
dispersed in the wire core, it can also be expected
that they will behave as effective flux pinning
centers.
2
2
conducting phases. These chemical reactions oc-
curred within 10 min at 600 °C. No diffraction
peaks for MgO were observed in Fig. 3. The
intensity of diffraction peaks for MgB
stronger with increasing heating time, however the
volume fraction of MgB was still small and a lot
of second phases such as MgCu remained in the
2
became
Fig. 5 shows the SEM image and EDX element
mappings of the cross-section of Mg Cu/B/Nb
2
2
2
core even though the specimens were heat treat-
ment for 10 h. According to SEM observation, the
second phases are as large as a few tens of microns
in diameter and may become obstacles to the su-
perconducting current flow. The transport current
tape-substrate composite reacted at 600 °C for 10
h in a dynamic vacuum. The diffusion layer that
formed near the surface of the specimen was about
50 lm thick and showed a high-density micro-
structure. On the other hand, the plasma-sprayed

A. Kikuchi et al. / Physica C 412–414 (2004) 1174–1178
1177
diffusion layer with heat treatment. This behavior
is similar to that of the bronze process used for
3
Nb Sn practical conductors. Its magnetic super-
conducting transition measured by SQUID ap-
peared at about 38 K. The thick, high density
2
MgB layer may be promising for applications
involving magnetic-shielding tubes or in the super-
conducting cavity.
4. Conclusions
Mg Cu exhibits a stronger resistivity to oxida-
2
tion than that of pure Mg resulting in a reduced
amount of MgO contamination. Mg Cu, which
2
has a low melting point of 568 °C promoted the
2 2 2
formation of MgB . Mg Cu changed into MgCu
Fig. 4. J
00 °C for 1 h. Data reported by Goldacker et al. is attached
for a comparison.
c 2
(4.2 K)-B curves for Mg Cu/B/Fe wire reacted at
with the decomposition and release of pure Mg.
The Mg that was released reacted with B and
6
formed MgB
quid(Mg Cu)–solid(B) reaction at low tempera-
tures. Although a lot of second phases such as
Mg Cu formed in the core of the Mg Cu/B Fe-
sheathed wires, its transport J exhibited a rather
2
superconducting phase by a li-
2
B layer shows porous and granular microstruc-
tures. When the Mg Cu on the plasma-splayed B
layer melted, liquid Mg Cu easily infiltrated into
spaces among amorphous B particles. However,
most of the Cu collected on the surface of the
2
2
2
2
c
5
2
high value, which was over 10 A/cm at 4.2 K
in ambient fields. J estimated from the MgB
c
2
Fig. 5. SEM image and EDX element mappings of the cross-section of Mg
0 h in a dynamic vacuum.
2
Cu/B/Nb tape-substrate composite reacted at 600 °C for
1

1178
A. Kikuchi et al. / Physica C 412–414 (2004) 1174–1178
net-cross-sectional area should be very high. For
the other study, the Mg Cu/amorphous B/metal-
substrate layered composite that was prepared
c
superconducting magnet during J -B measure-
2
ments. A part of this study was financially sup-
ported by the Industrial Technology Research
Grant Program from New Energy and Industrial
Technology Organization (NEDO) of Japan, and
by the Budget for Nuclear Research of the Min-
istry of Education, Culture, Sports, Science and
Technology.
after a heat treatment at 600 °C for 10 h in a
vacuum, formed a 50 l m thick MgB diffusion
2
layer with a high density microstructure. Cu was
released from Mg Cu, and collected on the out-
ermost surface. This characteristic is similar to that
2
of bronze processing for Nb
ductors. A thick, dense MgB
3
Sn practical con-
layer may be
2
promising for magnetic shielding tube or super-
conducting cavity applications.
References
[
[
1] N. Nagamatsu, N. Nakagawa, T. Muranaka, Y. Zenitani,
J. Akimitsu, Nature 410 (2001) 63.
2] M. Xu, H. Kitazawa, Y. Takano, J. Ye, K. Nishida, H. Abe,
A. Matsushita, N. Tsuji, G. Kido, Appl. Phys. Lett. 79
Acknowledgements
(2001) 2779.
3] W. Goldacker, S.I. Schlachter, S. Zimmer, H. Reiner,
Supercond. Sci. Technol. 14 (2001) 787.
The authors would like to thank Dr. A. Mat-
sumoto of NIMS for his assistance with the 13 T
[