Physica C 297 Ž1998. 216–222
Origin of subgrain formation in melt-grown Y–Ba–Cu–O bulks
P. Diko
b
a,b,)
, S. Takebayashi b, M. Murakami
b
a
Institute of Experimental Physics, SloÕak Academy of Sciences, WatsonoÕa 47, 04353 Kosice, SloÕakia
SuperconductiÕity Research Laboratory, International SuperconductiÕity Technology Center, 1-16-25 Shibaura, Minatoku, Tokyo 105,
Japan
Received 7 October 1997; revised 17 November 1997; accepted 26 November 1997
Abstract
We have studied the origin of subgrain formation in a melt-grown Y–Ba–Cu–O bulk. Microstructural observations
suggest that the subgrains are associated with the dislocation walls formed by the amalgamation of dislocations which are
created at the growth front when Y2 BaCuO5 ŽY211. particles are incorporated into the YBa 2 Cu 3 O y ŽY123. matrix. The
presence of subgrain-free regions near the growth sector boundaries shows that the subgrain structure is not formed with
cellular growth. We suppose that the subgrain-free regions were formed during an incubation period of the growth, in which
the dislocation density at the growth front is not high enough to assemble dislocation walls. q 1998 Elsevier Science B.V.
Keywords: Subgrain formation; Y–Ba–Cu–O bulk; Dislocation wall
1. Introduction
Melt growth process has enabled us to fabricate a
large single-grain bulk Y–Ba–Cu–O superconductors, in which fine Y2 BaCuO5 ŽY211. particles are
dispersed in the YBa 2 Cu 3 O y ŽY123. matrix, having
Jc Žcritical current density. values on the order of
10 5 Arcm2 at 77 K w1–3x. Melt-grown single-grain
Y–Ba–Cu–O has no high angle grain boundaries but
contains various low-angle boundaries. Polarized optical microscope w4–13x and transmission electron
microscope observations w4,9,14x revealed that subgrains are formed in melt-grown Y–Ba–Cu–O. The
misorientation angle of the subgrains is 5–6 degrees
at maximum w5x. Low angle grain boundaries have
been believed not to act as weak links, however, it
)
Corresponding author.
has recently been shown that some low angle grain
boundaries act as weak links in a high field region
w15x, and therefore for high field applications it is
necessary to control low angle grain boundaries. For
this purpose, it is important to study the formation
mechanism of subgrains.
The origin of the subgrains has been studied by
many groups, and it is generally accepted that its
formation is related to the growth process w5–8,10–
13x. The subgrains in melt-grown Y–Ba–Cu–O have
a rectangular cross section with their boundaries
parallel to w100x, w010x and w001x directions. The type
of growth-related subgrains can be classified in four
according to the growth directions: a-subgrains; csubgrains; a–c subgrains; and a–a-subgrains w11,12x.
The subgrain formation in melt-grown Y–Ba–
Cu–O was once discussed with its relation to a
cellular growth w16x. However, there are several features in the subgrain structure, which contradict with
0921-4534r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved.
PII S 0 9 2 1 - 4 5 3 4 Ž 9 7 . 0 1 8 7 7 - 7
P. Diko et al.r Physica C 297 (1998) 216–222
217
the cellular growth mechanism. Among them the
most remarkable is the presence of subgrain-free
regions near the growth sector boundaries w11x ŽD.
Cardwell, private communication. and also inside the
growth sectors w7x.
In this paper, we present the results of detailed
analyses of the subgrain structures in melt-grown
Y–Ba–Cu–O single-grain samples and will discuss
the origin of subgrain formation on the basis of the
dislocation formation during the grain growth.
2. Experimental
The Y–Ba–Cu–O samples were prepared by the
top-seeded melt-growth ŽTSMG. process, the details
of which are described elsewhere w17x. The precursor
powders with chemical composition of Y1.8 Ba 2.4Cu 3.6 O x with 0.5 mass % Pt addition was calcined
and pressed into a pellet 40 mm in diameter. The
pellet was partially melted at 11508C in an electric
furnace without temperature gradient. A melt-grown
Sm–Ba–Cu–O seed crystal was placed on the center
top of the melted precursor at 10308C such that the
cleaved surface of the seed faces the top surface of
the pellet. The Y–Ba–Cu–O crystals were isothermally melt-grown in air at 9878C for 20 h. Microstructural observations were performed by optical
microscopy under polarized light.
3. Results
Fig. 1 shows an optical micrograph of the top
surface for TSMG processed Y–Ba–Cu–O. Here we
can see four regions grown from the seed in the
²100: directions. They are sectioned by boundaries
along ²110: directions. The c-axis is perpendicular
to the top surface. The growth front is planar and
perpendicular to the a-directions.
Fig. 2 shows optical micrographs of the top pellet
surface visualized under polarized light. Here the
subgrain structures can clearly be observed because
small misalignment between the subgrains yields
different contrast and color under polarized light. In
principle, two kinds of subgrains are formed. Most
of them have subgrain boundaries ŽSGBs. parallel to
the a-axis and called as a-subgrains w11,12x. Some
Fig. 1. Optical micrograph of TSMG-processed Y–Ba–Cu–O
under normal light. Single-grain has four growth sectors, which
are sectioned by the boundaries along the ²110: directions. The
growth front is perpendicular to the ²100: directions.
of them have SGBs tilted from the a-direction and
called as a–a subgrains. We can observe subgrainfree band developed near the seed and along the
boundary which subsection the a-growth regions.
The width of the subgrain-free bands in the a-growth
regions is almost constant and is around 500–600
m m. Subgrain-free regions were also developed along
the a–a subgrains Žsee Fig. 3a.. The size of subgrain-free regions is larger at the SGB with higher
misalignment Žsee Fig. 3b..
The relationship between subgrains and the growth
front was studied by observing the cross section both
parallel to the c-direction and the growth direction.
Generally SGBs were perpendicular to the growth
front when the growth front was planar without
steps. On the other hand, the presence of the steps in
the growth front leads to the formation of the SGBs
which were not perpendicular to the growth front. In
Fig. 4a, we present an example of a-subgrains with
straight SGBs perpendicular to the planar a-growth
front. The a–c subgrains developed at the stepped
planar c-growth front are shown in Fig. 4b. One can
see that the SGBs are connected with steps at inner
corners.
Dendritic platelets can occasionally be observed
between the a-growth front and the solidified liquid
ŽFig. 4a., however, such structure is absent in the
interlayer at the c-growth front Žsee Fig. 4b and c..
We suppose that the platelets grew on the way of
cooling from the growth temperature w11,18x and are
218
P. Diko et al.r Physica C 297 (1998) 216–222
Fig. 2. Polarized optical micrograph of TSMG-processed Y–Ba–Cu–O. Note that subgrain-free regions are observed near the growth sector
boundary ŽGSB. and the seed, indicating that some incubation period is necessary for the subgrain formation.
not associated with the steady grain growth of Y–
Ba–Cu–O crystal.
4. Discussion
According to crystal growth classification w19x,
the crystal exhibiting habit planes has different
growth sectors ŽGS., i.e., the regions grown on
different habit planes and thus having different
growth directions. The growth sectors are separated
by the growth sector boundaries ŽGSB. which represent the trajectory of the crystal edge between two
neighboring habits during the growth. The growth
sectors, growth sector boundaries and the habit planes
Žgrowth front. in the TSMG processed single-grain
P. Diko et al.r Physica C 297 (1998) 216–222
219
and Shiohara w16x proposed that elongated subgrains
are formed as a result of the cellular growth, which
takes place under certain conditions of GrR Ž G:
Fig. 3. Polarized optical micrographs of TSMG-processed Y–Ba–
Cu–O. Note that a-subgrains are not present near the a – a
subgrain boundaries Ža.. The width of the subgrain free region
along the a – a subgrain boundary Ž a – a SGB. is enlarged with
increasing subgrain misalignment Žb..
Y–Ba–Cu–O bulk are schematically illustrated in
Fig. 5. There are five growth sectors: four a-growth
sectors with habits perpendicular to the w100x, w100x,
w010x and w010x directions; and the c-growth sector
with habit perpendicular to the w001x direction. The
characteristic feature of the observed GS are subgrain boundaries predominantly perpendicular to the
growth front w11x.
It is known that the subgrains are typically formed
with the polygonization in deformed and annealed
metals w4,9x. However, it is clear through microstructural observations that the subgrains in melt-grown
Y–Ba–Cu–O grains are associated with the crystal
growth. The growth-related nature of these subgrains
was first proposed by Diko et al. w5–8,10x on the
basis of their shape and later confirmed by their
relation with the growth conditions w11,12x. Otshu
Fig. 4. Optical micrographs of the cross section both parallel to
the c-direction and the growth direction. Straight a-subgrains are
developed at the planar growth front without steps Žpolarized
light. Ža.. a – c subgrain boundaries Ž a – c SGB., which are connected with the inner corners of the steps forms on the stepped
growth front Žpolarized light. Žb.. The interlayer formed on the
way of cooling from the growth temperature Žnormal light. Žc..
220
P. Diko et al.r Physica C 297 (1998) 216–222
Fig. 5. Schematic illustration of the growth sectors ŽGS. formed in
the TSMG processed Y–Ba–Cu–O.
temperature gradient; R: growth rate.. The constitutional supercooling for the cellular growth is given
by the following condition w20,21x:
GrR - ymC0 Ž 1rk y 1 . rD,
Ž 1.
where m is the slope of the liquidus line, C0 is the
starting concentration of impurity Žor starting composition in the case of solidification of phase with
concentration homogeneity region., k is the distribution coefficient and D is the diffusion coefficient of
the impurity in the melt.
The solidification conditions Žundercooling, temperature gradient. were kept constant during the
melt-growth in the present experiment, and therefore
GrR is constant. Thus, if the subgrains are formed
as a result of the cellular growth, they must be
present in the grown crystal all through from the
seed to the growth front, however, it is not the case.
The presence of subgrain-free regions along the sector boundaries and at the a–a and a–c subgrain
boundaries shows that the cellular growth is not
responsible for the formation of subgrains.
In our opinion the growth-related subgrain structure observed in melt-grown RE–Ba–Cu–O is
formed by the dislocation arrangement into dislocation walls during the crystal growth according to the
mechanism described by Jackson w22x. During the
crystal growth, dislocations can amalgamate by the
process similar to polygonization and form subboundaries. These subboundaries intersect with the
growth front and can propagate primarily parallel to
the growth direction. The tilt angle of the SGBs
generally increases as the growth proceeds. The subgrain formation is assisted mainly by the edge dislocations with Burgers vector parallel to the growth
front, which was first suggested by Diko et al. w5x
and later observed by Sandiumenge et al. w9x. The
dislocations amalgamate by climbing, which will
take place more readily at the solid–liquid interface
than in the solid, since a dislocation which jogs at
the interface will have the displacement propagated
by the growth process. Thus, one vacancy at the
interface enables the displacement of a whole dislocation line, while the same displacement would require a row of vacancies in the bulk crystal. In the
climb process, the dislocation initially amalgamate to
form small-angle boundaries and some dislocations
will annihilate by others at the boundaries. The
subgrains can tolerate misorientation of up to several
degrees, usually, but not always, a rotation about the
growth direction w22x.
There are several mechanisms by which dislocations are formed during the crystal growth. All the
dislocations which are produced during the crystal
growth are the results of growth accidents w17,20x.
The types of growth accidents which produce dislocations are: misorientation due to dendritic growth;
incorporation of a small crystal into the growing
crystal with different orientation; incorporation of
foreign particles into the crystal; stresses in the
crystal resulting from mechanical constraints; inhomogeneous temperature distribution; and inhomogeneous impurity distributions in the crystal.
In the case of melt-grown Y–Ba–Cu–O grains,
the dislocation formation due to Y211 incorporation
seems to be the most plausible mechanism. There are
two processes by which foreign particles can produce dislocations during the growth process. If the
P. Diko et al.r Physica C 297 (1998) 216–222
particle forms a coherent interface with the matrix,
the dislocation must be formed to compensate the
lattice mismatch at the boundary. If the particlermatrix boundary is incoherent like the case of
Y211rY123, the mismatch energy is relaxed at the
boundary and a long range stress due to lattice
mismatch does not occur. In such a case, as the
particle is incorporated into the crystal, a mismatch
and therefore the dislocation can develop in different
parts of the matrix as it grows around the particle. As
the crystal grows, these dislocations will be accumulated through the length of the crystal, and after a
certain period their density will reach the level to
form dislocation walls and thus subgrain boundaries.
If our proposal that Y211 particles are the source of
dislocations is correct, their density should be proportional to the density of Y211, which is supported
by the fact that subgrain size is increased with
increasing the average interparticle distance of Y211
w7,8x. It is also true that the higher growth rate gives
rise to the more frequent growth accidents, which is
consistent with the observation that the subgrain size
is reduced with increasing the cooling rate w13x. The
fact that subgrains are not observed for single crystals may also support the idea that the incorporation
of Y211 into Y123 matrix is responsible for the
subgrain formation.
The subgrain-free regions are also formed along
the a–a and a–c subgrain boundaries. In such regions the grain growth is considered to take place
perpendicularly to the main growth front and thus a
step will be formed at the growth front, which is
schematically illustrated in Fig. 6. Here it should be
born in mind that though the present illustration is
two dimensional, the real crystal growth is three
dimensional so that the a–c and a–a subgrains are
pyramid. The pyramidal subgrains are intergrown as
the steps on the growth front meet or nucleate during
the domain growth. The dashed line ŽFig. 6. which is
one of two boundaries of the w100x a-growth subsector Žthe area B. is the place where the w100x a-growth
starts. The layer B is the barrier for subgrain boundaries to continue from the part A to the part C of the
w010x a-growth sector. The incubation period free of
subgrains consequently appears at the beginning of
the C part of the w010x a-growth sector. The growth-in
dislocations in the B subsector have a different Burgers vector from that of A and C parts, so that the
221
Fig. 6. Schematic view of the a – a subgrain boundary formed by
the growth in the w100x direction at the step on the growth front of
the w010x a-growth sector. The layer B grown by w100x a-growth is
a barrier for dislocation walls Ž a-subgrain boundaries. to travel
from the part A to the part C of the w010x a-growth sector.
dislocation density of B subsector is always small
and is difficult to create dislocation walls and thus
subgrain boundaries.
The appearance of a step at the growth front is
apparently caused by the growth accident and therefore its frequency will increase with the growth rate.
Inhomogeneity in Y211 particle size and concentration can also cause local hindering of Y123 grain
growth and lead to the step formation.
5. Conclusions
The subgrains observed in a melt-grown Y–Ba–
Cu–O bulk were studied by polarized optical microscopy. Single grain surface seeded Y–Ba–Cu–O
contains five growth sectors: four a-growth sectors
with habit planes perpendicular to the w100x, w100x,
w010x and w010x directions and one c-growth sector
with habit plane perpendicular to the w001x direction.
The subgrains were observed in all the growth sectors. Detailed microstructural observations suggest
that the subgrains are associated with the dislocation
walls formed by the climbing of the edge dislocations which are created at the growth front when
Y2 BaCuO5 ŽY211. particles are incorporated into the
YBa 2 Cu 3 O y ŽY123. matrix. The presence of subgrain-free regions near the growth sector boundaries
222
P. Diko et al.r Physica C 297 (1998) 216–222
shows that the subgrain structure is not formed with
cellular growth. We suppose that the subgrain-free
regions were formed during an incubation period of
the growth, in which the dislocation density at the
growth front is too small to form dislocation walls.
Acknowledgements
This work was supported by Grand Agency of
Slovak Academy of Sciences ŽProject No. 1323r94..
One of the authors ŽP.D.. is grateful to JSPS for the
financial support. A part of this research is also
supported by NEDO for the R & D of Industrial
Science and Technology Frontier Program.
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