Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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The present invention relates to 6elid partR, composed of
the oxides of bismuth, 6trontium, calcium and copper and
of the sulfates of strontium and/or barium, and made of
high-temperature superconductor material with an enhanced
current-carrying capacity.
Solid parts made of high-temperature superconductor
~ material are of interest for applications such as, e.g.,
¦ current limiters in power engineering, and currant leads
j in magnet technology. Since all the high-temperature
¦ 10 superconductors known hitherto with a transition tempera-
ture above the boiling point of liquid nitrogen (77 R)
are ceramic substances, solid parts made therefrom are
often fabricated via ceramic shaping processes. To this
end, the substances are first prepared in the form of
powders or as a granular material. In a further step, a
ceramic molding is produced which then has to be sub-
~ected to a final heat treatment.
~von if no organic ad~uvants such as binders etc. are
used in the ceramic shaping, the electrical and magnetic
proporties of the solid parts thus produced are inade~
quate for industrial purposes, the critical current
densities of theee solid parts generally being distinctly
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less than 1 kA/cm2. Moreover, in such samples a drastic
drop of the critical current densities is observed even
at field strengths below 100 Gauss. In contrast, the
critical current densities in superconducting grains and
in thin film~ grown in an orientated manner are in the
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range of 1000 kA/cm2. The reason for the poorer quality
of the ceramic ~amples made from high-temperature super-
conducting material is that the superconducting current
cannot pass the grain boundaries unimpeded, which has to
S do, on the one hand, with the low connectivity of the
cuperconducting grains and, on the other hand, with their
random relative orie~tation.
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¦ DB-A-4124823 disclose8 solid high-temperature ~upercon-
¦ ductors which are composed of the oxides of bismuth,
strontium, calcium and copper and optionally lead and of
sulfates of strontium and/or barium. In order to produce
the high-temperature superconductors, said oxides and
sulfates are heated to temperatures of from 870 to
1300C, with the formation of a homogeneous melt. The
homogeneous melt is poured into permanent molds and
allowed to solidify slowly therein. Finally, the moldings
taken from the permanent molds are annealed at tempera-
tures of from 700 to 900~ in an oxygen-containing
atmosphero. Thls production procedure for solid parts
20 avoids detrimental material properties of the solid parts -
obtained by ceramic shaping of granular material: the
part~ produced according to the melt-casting method do
not exhibit the drastic drop in the critical current
density in magnetic fields owing to weak links, but are -
distinguished by strong links between the grains of the
microstructuro (cf. Supercond. Scl. Technol. 6, (1993),
pages 413 to 420, especially Figure 1). This is caused by
the special aspect of the melt-casting method: during the
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tempering proce~s there arise6 in the solid part~ a
- partially liquid phase, as a result of which the
connectivity of the grain~ i8 improved.
Various methods are deæcribed in the literature for
improving the mutual orientation of the superconducting
grains in ~olid specimens. Thi~ is achieved by a gra-
dient, during the growth proces~ of the superconducting
phaso, acting on a eolid specimen somehow den~ified or
sintered. This can be offected by zone melting, i.e. by
applying a steep temperature gradient, for example
100 R/cm, at the location of the sample. Note ~hould
always be taken of the fact that the cuprates in the
high-temperature superconductor material decompose
peritectically when fusing, giving off oxygen, i.e.
conversely it $e neceseary for oxygen to diffuse into the
eample during the growth process of the crystallites.
Coneequently, the growth processes are slow, the growth
rato~, e.g. for emall rode which are pulled through a
t~mporaturo gradient are in the range of a few mm/day.
Other processos deecribe the use of strong magnetic
fielde during the growth proce~s at from 900 to 1000C.
~hus it is poesible to achieve a certain orientation of
the cryetallitee for yttrium barium cuprate (cf.
D. Bourgault et al in: Physica C, 194 (1992~, pagee 171
to 176).
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The ob~ect of the preeent invention is to speclfy solid
parte, compoeed of the oxidee of bismuth, strontium,
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calcium and copper and of the sulfate~ of 6trontium
and/or barium, and made of high-temperature ~uperconduc-
tor material with an enhanced current-carrying capacity,
which have a high connectivity and at the ~ame time an
improved mutual orientation of the superconducting
grains. The solid part~ according to the invention are
therefore distinguished by a preferential orientation of
the high-temperature superconducting BiSrCaCuO crystal-
lites, the crystallographic c axis being perpendicular to
the temperature gradient during their solidification from
the melt.
The invention further relates to a process for producing
the solid parts made of high-temperature superconductor
material by casting a homogeneous melt of the oxides of
bismuth, strontium, calcium and copper and of the
sulfates of strontium and/or barium having temperatures
of from 900 to 1300C in permanent molds, ~low
solidification thorein and subsequent annealing of the
solid part~ taken ~rom the permanent molds in an oxygen-
20 containing atmosphere at from 700 to 900C, and is
distinguished by the homogeneous melt being cast in molds
having at least two parallel walls, as a result of which
there is produced in the solid parts, both after their
solidification and after they have been annealed, a crys-
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tallographic preferential orientation of the
crystallites, the crystallographic c axis of the high~
temperature suporconducting BiSrCaCuO phase in the ~; ;
annealed solid parts being perpendicular to the
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temperature gradient during the ~olidification of the
melt.
This process can also be modified in that the casting is
carried out in permanent mold having a round profile and
in that corresponding segments are cut out from the ~olid
parts taken from the permanent molds.
~inally, the modified process may also be additionally
refined in that
a) the cutting out of the segments is done with a
diamond saw; ~ ~ -
b) there are cut out, from round bars, bars having a
rectangular profile;
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c) tubes are cut up in their longitudinal direction
into approx~matoly ogual-size segment8;
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d) the segments aro arranged in the shape of a star,
the external diameter of this arrangement
corresponding roughly to the external diameter of
the o-iginal tube.
¦ The critical current of high-t~perature superconducting
bars having a rectangular profile i8 distinctly higher
under the influence of an external field - with a favor-
ablo orientation with respect to the magnetic field
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vector - than that of bars having a cro~s section which
is of comparable ~ize but round.
The rectangular segment6 cut out from high-temperature
superconducting round bars behave anisotropically, in an
external magnetic field, with respect to their critical
current.
Tho segments which are obtalned by cutt~ng up a tube,
made of high-tompsrature superconductor material, in its
longitudinal direction and which are arranged in the
shape of a star, attain twice as high a critical current
under self-field influence as the original tube.
Surprisingly, the proferential orientation of the crys-
tallite~ in the solidified melt also induces a preferen-
tial orientation of the high-tomperature suporconductor
phase which i8 only formed during tempering. This is true
evon though, during tempor~ng, the system passes through
a partially lton state and even though in this case the --
high-temperaturo superconductor phase is only formed
completely afresh. Moroover, owing to the melting process
involved during temporing, better connectivity between
the grains prevails. ~ ~,
By using beneficlal permanent-mold shapes (rectangular - ;
profile) or by cutting out corresponding segment~ from
shapes having a round profile (round bar or tube), there
are obtained in a direct manner, in tho process according
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to the invention, superconducting ~olid part~ having a
cry~talline preferential orientation. Parts thus produced
have, owir,g to said preferential orientation, either a
higher current-carrying capacity in the self-field or,
with an appropriate alignment with re~pect to the field
vector, a higher current-carrying capacity under the
influence of an external magnetic field.
In the accompanying drawing, Figure 1 depicts variou~
permanent molds which can be used in the process
according to the invention, whereas Figure 2a ~hows a
tube, cut up into segments, made of high-temperature
supercon~luctor material. These segments are arranged, in
Figure 2b, in a manner according to the invention in the
shape Oc a star to form a high-temperature superconductor
having anisotropic properties, its external diameter
being approximately equal to the external diameter of the
original tube.
~xample 1 (comparative example)
In accordance with the process according to DE-A-4124823,
20 a series of more than 100 round bars (0 8 mm, lengths
varying between 50 and 200 mm) was produced by casting
the melt in quartz tubes sealed on one side and having a
round cro~s section. In accordance with the process
according to DE-A-4118988, sheets of silver were mounted
in tho quartz mold~ prior to casting, and the bars thus
at the samo time provided with integrated current
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contacts via which a direct current wa~ fed in. The
average critical current density achieved was 930 A/cm2
(at 77 X, voltage criterion 1 ~V/cm). The 6tandard
deviation over the entire ~eries was only 50 A/cm2.
On a number of round bars thus produced (0 8 mm), the
I critical current was measured under the influence of a
i magnetic field up to 1 kGauss. This always resulted in a
! comparatively flat linear drop in the magnetic field. In
order to obtain a possible effect of the direction o~ the
magnetic field on the specimen, the transport-current
~ measurement was repeated on a bar after the bar had been
¦ rotated about its longitudinal axis from its original
position by 10. The measurements were repeated after
further rotations in 10 steps until the bar had been
rotated by 180 from its original position. In all the
measurements, identical current densities in the zero
field and a completely identical variation of the criti-
cal cusrent in the magneti¢ field wero observed, i.e. the
solid round bar behaves completely isotropically, over
its entire circumference, with respect to the magnetic
field applied.
Example 2 (comparative example)
From a round bar produced according to Example 1, there
was cut out, centrally in its longitudinal direction, a
flat disk (dimensions: 0.7 x 8 x 40 mm3) which was
divided once more longitudinally and a small rod was
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prepared therefrom (dimensions: 1 x 0.7 x 40 mm3) . The
small rod had been cut out from the round bar in such a
way that the crystallographic c axis of the cry~tal
plateletæ and the longitudinal axis of the small rod were
in the same plane. Thi~ small rod was u6ed to determine,
with the aid of direct-current and pulse measurement6,
the variation of the critical current in the magnetic
field up to 4.5 kGauss, the small rod - analogously to
Example 1 - being rotated in the field to study the
influence of the orientation of the magnetic field
vector. Depending on the alignment with respect to the
magnetic field, this resulted, for the mo6t favorable
alig lent, in a critical current approximately eighty
times higher than from alignment perpendicular thereto,
thus providing the proof for anisotropic behavior of the
small rod (cf Figure 3).
Example 3 (according to the invention)
Melts anPlogous to Example 1 were cast in quartz tubes
having a rectangular cro~s section (8 x 22 mm2 internal
dimension), prof$1ed bar~ being obtained, a~ a result,
after solidification. Silver foil had been mounted in the
quartz mold~, and tempering was carried out in two
stage~. In analogy to Example 1, the bar wa~ rotated
about it~ longitudinal axi~, and the critical current wa~
determined in various relative positions. Ani~otropic
behavior wa~ ob~orved in the proces~.
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Example 4 (according to the invention~
A profiled bar produced according to Example 3 had its
borders trimmed away in the region of the short edges.
When the critical current was measured as a function of
the rotation of the profiled bar, this resulted, for a
complete rotation, in marked extreme values without
intermediate maxima.
~or a field of 1500 Gauss, the critical current density
in the profiled bar in its most favorable position (field
perpendicular to the broad side of the profile) was twice
as high as in the round bar according to Example 1. ~-
Example 5 (according to the invention)
In a Cu book mold having the dimensions ;~
6 x 150 x 200 mm3, a melt according to Example 1 was ~;
ca~t. The casting was removed after cooling of the melt
and tempered in an oxygon-¢ontaining atmosphere. Then the
ingot was cUt longitudinally into strip~ having a cros~
section of 6 x 2 mm2, the strips originating from the
lateral border zone being rejected. The remaining strip~, -
when the critical current was measured a~ a function of
the orientation of an external magnetic field, exhibited
distinct anisotropic behavior. ~;
Example 6 (comparative example)
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A high-temperature ~uperconductor tube (0 35 mm,
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L 200 mm; A 4.8 cm2) was produced by centrifugal casting
and provided on both ~ides, to feed in current, with
sealed-in 6ilver contactæ. The measurement of the criti-
cal current without an external magnetic ~ield gave
2000 A (77 R, 1 ~V/cm). The mea~urement when an external
magnetic field was applied gave isotropic beha~ior with
respect to the rotation of the tube about its longitudi-
nal axls.
Example 7 (accordlng to the invention)
~10 The tube prepared accord$ng to Example 6 wa~ cut up
Ilongitudinally, with the aid of a diamond saw, into
eight egual-size segments. The mea~urement of the
critical current on individual segments gave, on average,
910 A/cm2 without an external field, which was twice as
high a value a~ the value of 420 A/cm2 determined for the
complete tube according to Example 5. This is evidently
cau~ed by the ~maller ~elf-field effect in the ca~e of
the segment~ havlng a smaller cro~c ~ectlon.
Example 8 (according to the invention)
The eight ~egments according to Example 7 were ~oldered,
at their top and bottom ends, lnto copper holders 80 as
to produce a star shape. The external circumference of
thls arrangement corre~ponde to the external diameter of
the original tube. The mea~urement of the critical
25 current gave 3830 A (77 R, 1 ~V/cm), almost twlce the
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value for the case where no additional field was acting
apart from the self-field of the conductor.
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In this arrangement, the original radial direction of the
tube is parallel to the circumference and vice versa. The
rotationally symmetric self-field is thus parallel to the
platelet planes of the crystallites (and thus to the
crystallographic a-b plane containing the Cu-O layers).
Thus, owing to the lnfluence of the self-field, a
considerably smaller drop of the critical current density
0 i8 observed, compared with the original tube.
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