Cryogenics 43 (2003) 477–481
www.elsevier.com/locate/cryogenics

Additional eﬀect of Ag and 211 phase on thermal expansion
of REBaCuO bulk superconductor (RE ¼ Sm, Y)
H. Fujishiro a,*, M. Ikebe a, S. Kohayashi b, K. Noto a,c, K. Yokoyama caFaculty of Engineering, Iwate University, 4-3-5 Ueda, Morioka 020-8551, Japan
Central Research Laboratory, Dowa Mining Co., Ltd., Honjo 367-0002, Japanc
Iwate Industrial Promotion Center, 3-35-2 Iioka-shinden, Morioka 020-0852, Japanb

Received 13 March 2003; accepted 28 May 2003

Abstract
The anisotropic thermal expansion DL=L was examined for melt-processed REBaCuO(Ag) bulk composites (RE ¼ Sm, Y) as
functions of not only the contents of the RE2 BaCuO5 second phase but also Ag metal added to the highly oriented REBa2 Cu3 O7
matrix. The DL=Ljc values in the c-direction of REBaCuO(Ag) bulks are about 1.5–1.8 times as large as the DL=Ljab values in the abplane. DL=L increases with increasing Ag content in both directions but DL=Ljc is by far more sensitive to the Ag content. The
resultant compressive strain due to Ag particles in the c-direction through the melt-processing was estimated and its possible
contribution for the improvement of the superconducting characteristics of the matrix was discussed.
 2003 Elsevier Ltd. All rights reserved.
Keywords: Thermal expansion; REBaCuO bulk superconductors; RE2 BaCuO5 ; Silver

1. Introduction
Because of the high critical current density Jc and thehigh magnetic ﬂux trapping ability, the melt-processed
REBaCuO bulk superconductors (RE: rare earth ions)have widely been exploited for the applicational use,such as the high-ﬁeld bulk magnets, power current leads
for liquid-He-free superconducting magnets and so on[1–3]. Through the melt-processing, RE2 BaCuO5 (RE211)
particles precipitate in the REBa2 Cu3 O7 (RE123) superconducting main phase and act as eﬃcient pinning centers in the bulk materials [1]. The amount of the RE211
particles can be controlled by adjusting the compositionof the starting materials.
Another standard technique to improve the superconducting characteristics of the melt-processed bulks isthe addition of 10–20 wt.% silver (Ag) metal, which isbelieved to be eﬀective in improving the crystalinity of
the RE123 matrix, reinforcing the mechanical strength*

Corresponding author. Tel.: +81-19-621-6363; fax: +81-19-6216363/6373.
E-mail address: fujishiro@iwate-u.ac.jp (H. Fujishiro).
0011-2275/$ - see front matter  2003 Elsevier Ltd. All rights reserved.
doi:10.1016/S0011-2275(03)00124-3

at the same time. A very small amount (<0.5 wt.%) of
platinum (Pt) metal is also added to disperse the RE211
particles more ﬁnely and uniformly in the matrix. The
Ag-doped melt-processed REBaCuO(Ag) bulk can be
regarded as a composite superconducting material,consisting of RE123, RE211 and Ag.
We measured the thermal expansion DL=L of the
SmBaCuO and YBaCuO composites. The amounts ofAg and the RE211 phase were systematically changed.
Our main attention was placed on the SmBaCuO composites because REBaCuO(Ag) bulks with right rareearth element (RE ¼ Nd, Sm) were reported to achieve
higher Jc values than REBaCuO bulks with heavy RE or
YBaCuO, especially in high magnetic ﬁelds. Therefore,
the SmBaCuO(Ag) bulk is a very useful material for theapplications. The YBaCuO composites have also beenmeasured for comparison. We have already reported thethermal conductivity jðT Þ, the thermal diﬀusivity aðT Þand the thermoelectric power SðT Þ on the SmBaCuO(Ag) and YBaCuO(Ag) systems [4,5]. The abundant and methodical database of the thermal properties,
however, has not yet been completed for these bulk
superconductors. In this paper, the eﬀect of the induced
strain due to Ag addition was elucidated from theviewpoint of the thermal expansion.

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H. Fujishiro et al. / Cryogenics 43 (2003) 477–481

2. Experimental

3. Results and discussion

The SmBaCuO(Ag) bulk samples were prepared at
the laboratory of DOWA Mining Co., Ltd. through thefollowing procedure [6,7]. A mixture of BaCO3 and CuO
powders was calcined at 880 C for 30 h to obtain the
BaCuO2 and CuO mixture. The calcined powder was
mixed with Sm2 O3 , 10–19.7 wt.% Ag and 0.42 wt.% Pt,
which were sintered at 900 C for 20 h. The resultant
sintered polycrystal consisted of the Sm123 matrix, additional X mol Sm211 phase, y wt.% Ag and 0.42 wt.%
Pt. We express this composite by the formula, Sm1þ2X Ba2þX Cu3þX O7þ5X (Ag:y). The sintered polycrystals werepulverized and pressed into disks (53£ · 18 mm). The
disks were partially melted at 1100 C in a 2-zone furnace
for 30 min and cooled at a speed of 10 C/min down to1020 C, keeping temperature gradient of 5 C/cm between the top (low temperature side) and bottom of the
disk. A seed SmBaCuO (Ag:0) crystal was attached tothe top-center of the disks to realize highly oriented bulkdisks. The bulk-disks were cooled down at a speed of 1
C/h from 1020 to 870 C and at a speed of 40 C/h from870 C to the room temperature. The process essentiallyfollows the one for high-Jc commercial base products of
Dowa Mining Co., Ltd. The sizes of the Sm211 phaseand the Ag particles in the bulks were 1–5 and 20–30 lm,
respectively, both of which were dispersed uniformly as
conﬁrmed by the optical microscope observation. The Xray diﬀraction analyses conﬁrmed that the Sm123 matrix
was crystallographically highly oriented with the surfaceof the grown bulks parallel to the ab-plane. In the abplane, however, the bulk crystals had highly disordereda- and b-axis alignment. Y1þ2X Ba2þX Cu3þX O7þ5X (Ag:y)bulks were fabricated through almost the same process.
On the other hand, we could not observe any preferredorientations of Ag particles and 211 phases in the RE123
matrix. The grown bulks were cut into pillars with edges
parallel to the ab-plane and to the c-axis for the measurements. The typical size of the pillar samples was
4.0 · 4.0 · 25.0 mm3 . The density of the samples was
about 95% of the ideal ones, which was determined bymeasuring the size and the weight. The Sm211 ceramic
was also fabricated by the solid-state reaction at 1150 C
in air in order to compare the DLðT Þ=L value with thoseof the melt-processed bulks. The density of the Sm211
ceramic was 6.18 g/cm3 , which was 94% of the ideal one.
The thermal expansion DLðT Þ=L was automaticallymeasured between 20 and 300 K using a strain-gage
method. The strain-gages (KFL-1-120-C1-16: Kyowa
Electronic Instruments Co., Ltd.) with the size of 2 mm
were adhered along the ab-plane and along the c-axis of
the samples using the gage cement (PT-6). We observed
no orientation dependence of DLðT Þ=L in the ab-plane
probably because of the highly disordered a- and b-axisalignment. The accuracy of the DLðT Þ=L measurement
was better than 106 .

Fig. 1 shows the temperature dependence of the abplane and c-axis thermal expansion DLðT Þ=L of theSm1þ2X Ba2þX Cu3þX O7þ5X (Ag:10) bulk composites for aﬁxed Ag content of 10 wt.%. In this study, the thermal
expansion is normalized with respect to the sample
length at 20 K and is deﬁned by DLðT Þ=L ¼
ðLðT Þ  Lð20 KÞÞ=Lð20 KÞ, LðT Þ being the sample
length at the temperature T . As shown in Fig. 1, DL=L
increases with increasing temperature for both directions. The temperature dependence of the c-axis thermal
expansion DLðT Þ=Ljc is roughly 1.5 times larger thanthat in the ab-plane. The DLðT Þ=L values of the Sm211
polycrystal, also given in Fig. 1, are almost the same as
those of DLðT Þ=Ljab and pretty smaller than those ofDLðT Þ=Ljc of the Sm-bulk composites. The DL=Ljc values and the anisotropy ratio, DL=Ljc =DL=Ljab , of thebulk composites decrease with increasing the Sm211
content X .
Fig. 2 shows DL=L of Sm1þ2X Ba2þX Cu3þX O7þ5X(Ag:10) at 200 and 300 K as a function of the volume
fraction f211 of the Sm211 phase. Very roughly, thelinear extrapolation of data points of DL=Ljc from theregion of f211 < 0:25 to f211 ¼ 1:0 falls on the DL=Lj211
data point of the Sm211 polycrystal. Thus, a kind of theaddition theorem holds for the case of Sm211 addition,i.e.,
DL=Ljc ¼ DL=Ljc1230 f1230 þ DL=Lj211 f211 ;ð1Þcwhere DL=Lj1230 is the c-axis thermal expansion of theSm123 main phase containing 10 wt.% Ag, and f1230 is
the total volume fraction of the Sm123 and Ag(f1230 ¼ f123 þ fAg ). It should be noticed that the addition

Fig. 1. The temperature dependence of the ab-plane and c-axis thermal
expansion DLðT Þ=L of Sm1þ2X Ba2þX Cu3þX O7þ5X (Ag:10) superconducting bulk composites with various Sm211 second phase contents X .
The Ag content is ﬁxed to y ¼ 10 wt.%. DLðT Þ=L of the Sm211 polycrystal is also presented.

H. Fujishiro et al. / Cryogenics 43 (2003) 477–481

Fig. 2. DL=L at 300 and 200 K as a function of Sm211 volume fraction
f211 . The linear lines correspond to the addition theorem (Eq. (1) in thetext).

theorem based on Eq. (1) implicitly assumes independent expansion (or contraction) of each component,
exerting negligible additional strains one another. Thus
the results in Fig. 2 suggest that the induced strain by
Sm211 is not serious even in the c-direction of the matrix
lattice. The DL=Ljab values show only a very slight f211
dependence, which is reasonable from the viewpoint ofabthe addition theorem because DL=Lj1230 and DL=Lj211
values are almost in agreement one another. The induced strain due to Sm211 addition should be negligibly
small in this direction.
Fig. 3 shows the temperature dependence of the abplane and c-axis DL=L of Sm1þ2X Ba2þX Cu3þX O7þ5X(Ag:y) as a function of the Ag content y for a ﬁxed X(¼0.2). The measured expansion of Ag metal (DL=LjAg )is also presented for comparison. DL=L is the largest for
Ag and DL=L of the SmBaCuO bulks increases withincreasing y in both directions. What is a novel andcharacteristic feature is the behavior of DL=Ljc ; the ydependence of DL=Ljc is far stronger than that ofFig. 3. The temperature dependence of the ab-plane and c-axis thermal
expansion DLðT Þ=L of Sm1þ2X Ba2þX Cu3þX O7þ5X (Ag:y) for X ¼ 0:2(ﬁxed) and for various Ag wt.% y.

479

Fig. 4. DL=L of the SmBaCuO(Ag) bulks at 20 K as a function of Ag
volume fraction fAg . The Sm211 content is ﬁxed at X ¼ 0:2. The inset
shows the c-axis DL=L in a magniﬁed scale for fAg < 0:2. The verticalarrows indicate the sizes of the forced strains. The dotted linear lines
correspond to the addition theorem.

DL=Ljab in spite of the far smaller diﬀerence between
DL=Ljc and DL=LjAg in comparison with that between
DL=Ljab and DL=LjAg . The DL=Ljc values are signiﬁcantly
enhanced and the anisotropy ratio DL=Ljc =DL=Ljab alsorather increases with increasing y, which makes amarked contrast with the X dependence. Fig. 4 shows
DL=L of the Sm-based bulk composites at 300 K as afunction of the volume fraction fAg of Ag metal. As canbe seen, DL=Ljc increases rapidly and nonlinearly withincreasing fAg while the fAg dependence of DL=Ljab isroughly in accord with the addition theorem. The
characteristic layered crystal structure of the Sm123
main phase should be the origin of this anisotropic
behavior of DL=L against the Ag addition.
Now let us brieﬂy discuss the strain in the Sm123
main phase induced by Ag particles. Anisotropic
YoungÕs moduli E of melt-processed YBaCuO bulks(X ¼ 0:2, y ¼ 0) were measured by Katagiri [8] asEab  125 GPa and Ec  40 GPa for in the ab-plane andin the c-direction, respectively. Although we have not
the E value data for SmBaCuO bulks, it may be reasonable to assume that the E values are not much different between YBaCuO and SmBaCuO bulks preparedby the same melt process. The YoungÕs modulus of Ag is
EAg  100 GPa [9], which is intermediate between Eaband Ec of the present REBaCuO bulks. Because of thesmaller Ec of REBaCuO than EAg , the RE123 phase inbulk systems should be subjective to the signiﬁcant cdirection strains introduced by the added Ag particles.
In contrast, Eab is larger than EAg and induced in-planestrain in the bulk composites should not be signiﬁcant.
In Fig. 4, we can notice that DL=Ljc nonlinearly increases as a function of Ag volume fraction fAg deviating
heavily from the addition theorem line. As we have already mentioned, the addition theorem corresponds to480

H. Fujishiro et al. / Cryogenics 43 (2003) 477–481

the case of the independent expansion of the constituents of the bulk composites. Accordingly, the deviationfrom the addition theorem line may roughly correspondto the net induced c-direction strain in the Sm123 (+20
mol.% Sm211) phase due to added Ag particles. The
inset of Fig. 4 shows the c-axis DL=L at 300 K in amagniﬁed scale for fAg < 0:2. The smooth interpolationof the c-axis DL=L to y ¼ 0 and the deviation of the data
points from the addition theorem line suggest that
Sm123 main phase is under the inﬂuence of forced
compressive strain of about 0.9 · 104 , 1.5 · 104 and
3.7 · 104 in the c-direction for y ¼ 10 wt.% (fAg ¼
0:058), 15 wt.% (fAg ¼ 0:088) and 19.7 wt.% (fAg ¼
0:112), respectively. We performed the thermal expansion
measurement only below 300 K but the melt-process to
prepare the SmBaCuO(Ag) bulk starts above the AgAgmelting point (Tmelt
 1230 K). By linearly extrapolating
the DL=L data to 1230 K, the total induced c-direction
strain may be roughly 4 times as large as the above given
values. Actually, this strain results in a forced compressive stress for the Sm123 superconducting phaseduring the cooling process after the sample melting. The
resultant strain may aﬀect the crystal quality and thesuperconducting properties of the Sm123 main phase as
well as possible other factors such as a small amount ofAg solution into the matrix phase or oxygen content
variation. Fig. 5 shows resistive superconducting transition for the same samples as in Fig. 4. The superconducting transition temperature Tc of the ab-plane
samples is enhanced from y ¼ 10:0 to 15.0 wt.% anddegraded from y ¼ 15:0 to 19.7 wt.%. This result suggests that the total c-axis compressive strain of4 · 1.5 · 104 corresponding to y ¼ 15:0 wt.% is suitable to realize the best superconducting characteristicsof the Sm123 phase and that too heavy strain exceeding
103 , which corresponds to y ¼ 19:7 wt.%, degrades thesuperconducting characteristics. Measurements in a

Fig. 6. The temperature dependence of the ab-plane and c-axisDLðT Þ=L of the Y1þ2X Ba2þX Cu3þX O7þ5X (X ¼ 0:4) superconducting
bulks for the Ag content y ¼ 0 and 15 wt.%.

previous paper on the critical current densities and residual magnetic ﬂux densities of the SmBaCuO(Ag) bulk
composites supported that the added Ag content of 10–
15 wt.% corresponded to the best condition to improve
the superconducting characteristics [6].
Finally, Fig. 6 presents the temperature dependenceof the thermal expansion of the Y1þ2X Ba2þX Cu3þX O7þ5X(X ¼ 0:4) bulk composites with the Ag content of y ¼ 0
and 15 wt.%. The ab-plane and the c-axis DL=L show ananisotropic behavior. DL=L of an YBa2 Cu3 O7 single
crystal has been investigated by Maingast et al. [10], andthe reported DL=L values from 300 to 10 K are 0.0015 in
the ab-plane and 0.0036 along the c-axis. Comparing
with these values, DL=Ljc of the present Ag-nondoped
Y1þ2X Ba2þX Cu3þX O7þ5X (X ¼ 0:4) bulk crystals issomewhat smaller, possibly because of the presence of
the Y211 particles (see Fig. 1). Both DL=Ljc and DL=Ljab
increase by addition of Ag, but the increase in DL=Ljc ismore distinct, showing the similar tendency to the
SmBaCuO(Ag) bulk composites. The large compressive
stress eﬀect in the c-direction caused by addition of Ag
metal is a peculiar but a common feature in the high-Tc
cuprates with the layered crystal structures.

4. Summary
The ab-plane and c-direction thermal expansion DL=L
were measured for Sm1þ2X Ba2þX Cu3þX O7þ5X (Ag:y)bulk composites, which consisted of the superconducting SmBa2 Cu3 O7 (Sm123) main phase, X mol
Sm2 Ba1 Cu1 O5 (Sm211) second phase and added y wt.%
silver (Ag) metal. Main results obtained in this study aresummarized in the following.
Fig. 5. The temperature dependence of the ab-plane electrical resistivity qab ðT Þ for the same samples as in Fig. 4.

(i) The thermal expansion measured along the abplane and the c-axis of the Sm123 phase showed an-

H. Fujishiro et al. / Cryogenics 43 (2003) 477–481

isotropy, reﬂecting the layered lattice structure of
the main Sm123 phase. The DL=Ljc values in thec-direction were 1.5–1.8 times larger than those of
DL=Ljab in the ab-plane.
(ii) The DL=Ljc values and the anisotropy ratio
DL=Ljc =DL=Ljab of SmBaCuO(Ag) composites decreased with increasing content X of the Sm211 second phase. The addition theorem, which assumes
that the thermal expansion of the composite is given by the sum of the independent contributions
proportional to the volume fractions of the constituents, roughly explained the X dependence ofDLðT Þ=L.
(iii) In contrast, the addition of Ag metal particles drastically enhanced the DL=Ljc values of the composites, far deviating from the addition theorem,
while the addition theorem remained roughly valid
for DL=Ljab . The diﬀerent behavior of DL=Ljab and
DL=Ljc on Ag addition can be understood on thebasis of the anisotropic YoungÕs modulus of the
RE123 main phase with the layered lattice structure.
Through the cooling process in the melt-processing
procedure, the Ag addition forces a considerable
compressive stress in the c-direction of the Sm123
phase. The resultant strain seems to make an important factor in improving the superconducting characteristics of the bulk composite for Ag content range,y 6 15 wt.%.
(iv) DL=L of YBaCuO(Ag) bulk composites also
showed a similar behavior for Ag metal addition.
The forced compressive strain due to Ag particles

481

in the c-direction of the main phase is a common
feature of the REBaCuO bulk composites.
Acknowledgements
This work is partially supported by Japan Science andTechnology Corporation under the Joint-research Project for Regional Intensive in the Iwate Prefecture on‘‘Development of practical applications of magnetic ﬁeld
technology for use in the region and in everyday living’’.
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