Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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HIGH-TEMPERATURE SUPERCONDUCTOR ARRANGEMENT
The present invention relates to the field of
high-temperature superconductors. It concerns a high-
s temperature superconductor arrangement according to the
precharacterizing clause of patent claim 1.
European Patent Application EP-A 0 911 889
discloses a high-temperature superconductor arrangement
for use in a. current limiter. . The arrangement
comprises a superconducting layer and a perforated
steel plate, designed as an electrical bypass, which
forms a composite conductor with the superconducting
layer. The bypass is adhesively bonded onto the
superconductor by means of a conductive epoxy resin.
This conducting transitional or intermediate layer,
provided for improving the contact resistance between
the superconductor and the bypass, makes it possible
for the current to change from one layer into the other
layer.
The transitional layer referred to consists for
example of a conducting polymer composite material
which comprises an electrically insulating polymeric-
resin base material and an electrically conducting
filler. Metal powders, fibers or flakes as well as
metalized small parts or particles come into
consideration as the filler. The filler must be used
in an adequate proportionate amount by volume, in order
that the individual particles or fibers have electrical
contact with one another and the composite material
produced is conductive.
For cooling to operating temperature, high-
temperature superconductors are brought into thermal
contact with a cooling medium, preferably with liquid
nitrogen LN2. On account of the different coefficients
of thermal expansion of the superconductor and bypass
on the one hand and the polymer composite material on
the other hand, stresses occur during this cooling.
The layer of polymer composite material in particular,
which has a great coefficient of thermal expansion, is
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subjected to tensile stress by the adjacent layers,
contracting to a lesser extent, and forms cracks.
The Patent EP-B 0 257 466 discloses a laminate
comprising a metal layer and a layer of a cured
polymer-matrix composite material which serves for
example for the cooling of electronic components
mounted on a printed-circuit board. A reinforcing,
heat-conducting material of low thermal expansion in
the form ,of particles, fibers or fabrics. is
incorporated into the layer of composite material.
This reinforcing layer serves for the heat conduction
between the heat source (the electronic components) and
the heat sink (metal layer) as well as to balance the
coefficients of thermal expansion of the printed-
circuit board and the metal layer. The composite layer
is electrically nonconducting and designed such that
the components which heat up during operation can be
cooled well.
The object of the present invention is to
reduce the thermomechanical stresses produced during
cooling to operating temperature in an electrically
conducting polymer-composite based transitional layer
provided between a superconductor and an electrical
bypass and prevent the formation of cracks in the
transitional layer in a high-temperature superconductor
arrangement of the type stated at the beginning. This
object is achieved by a high-temperature superconductor
arrangement having the features of patent claim 1.
The essence of the invention is to reduce the
coefficient of thermal expansion of the transitional
layer by the use of suitable means. As a result, said
coefficient approaches the unchanged coefficient of
thermal expansion of the superconductor and electrical
bypass and the tensile stresses occurring in the
transitional layer during cooling of the arrangement
are reduced.
In a first preferred embodiment, the conducting
polymer composite material has a second, electrically
nonconducting filler mixed with it. This filler
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preferably has a coefficient of thermal expansion which
is lower than that of the polymer matrix.
In a further embodiment, fibers are
additionally added to the polymer composite material to
improve the mechanical stability.
Further advantageous embodiments emerge from
the dependent patent claims.
The invention is explained in more detail below
on the, basis of an exemplary embodiment in conjunction
with the drawing, in which:
Figure 1 shows a high-temperature
superconductor arrangement with a polymer-composite
transitional layer modified according to the invention.
The designations used in the drawing are
summarized in the list of designations.
Represented in Figure 1 is a detail from a
cross section through a high-temperature superconductor
arrangement, as is used for example in current
limiters. A high-temperature superconductor 1 is
connected in an areal manner over a main area to an
electrical bypass layer 2 and forms a composite
conductor with the latter. Between these two layers l,
2 there is a transitional layer 3 based on a polymer
composite. The latter comprises a polymer matrix 30, a
conducting first filling material 31 and a
nonconducting second filling material 32.
For the high-temperature superconductor
arrangement represented in the drawing and the
following description, relating to the latter, a planar
geometry has been chosen. This can be symmetrically
supplemented, for example by a second bypass layer
being provided on a side of the superconductor 1 lying
opposite the first bypass layer 2. The invention can
be used equally well in the case of cables or wires
with a superconducting core which is surrounded by an
electrical bypass. In the case of the last-mentioned,
axially symmetrical arrangement and in the case of the
aforementioned planar configurations with a plane of
symmetry, it is ensured that the thermally induced
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change in length runs in a direction parallel to the -
axis or plane of symmetry and does not lead to any
bimetal-like distortion of the arrangement.
The transitional layer 3 comprises a polymer
composite material with a matrix 30 and at least one
conducting filling material 31. The matrix systems are
preferably three-dimensionally crosslinking
thermosetting materials and are based for example on
epoxy, silicon or polyester resins. The coefficient of
thermal expansion of such a polymer matrix 30 lies in
the range of 60-10010-6/K, in comparison with which the
coefficient of thermal expansion of the ceramic
superconductor 1 is typically approximately 10-10-6/K,
and that of a bypass 2 of steel is around 15-10-6/K.
During the cooling of the arrangement from room
temperature or the curing temperature of the polymer
matrix 30 to operating temperature (77K), the polymer
composite would consequently contract to a much greater
extent than the adjacent layers. Since the actually
resulting change in length is the same for all the
layers of the arrangement, stresses correspondingly
build up and, in particular in the case of a number of
thermal cycles, i.e. repeated heating up and cooling
down, may lead to cracks in the transitional layer 3.
To reduce said stresses, it is proposed
according to the invention to reduce the coefficient of
thermal expansion of the transitional layer 3,
preferably to a value which is possibly only 2-3 times
greater than that of the steel bypass 2.
The conducting filler 31 comprises for example
silver particles and often has itself a low coefficient
of thermal expansion. An increase of the particle
density of the conducting filler, i.e. the number of
silver particles per volume, consequently already
brings about a reduction in the coefficient of thermal
expansion of the polymer composite. For reasons of
cost and so as not to influence the electrical
properties of the transitional layer 3 in an
uncontrolled way, instead a second, nonconducting
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filler 32 may be added. This comprises small particles
or fibers with a diameter of 1-100 ~.m, of a material
with a small coefficient of thermal expansion of
preferably less than 1010-6/K. Quartz (Si02) , aluminum
oxide (A1203) or aluminum nitride (A1N) come into
consideration, the two last-mentioned additionally
being distinguished by their heat-conducting and heat-
storing properties.
. The mechanical properties. of a. polymer
composite-based transitional layer of this type can be
improved by adding glass, carbon or aramid fibers to it
as a reinforcing carrier material in a way similar to a
fiber composite layer. A layer modified in this way
has, for example, a higher bending strength and
consequently serves at the same time for the mechanical
stabilization of the composite conductor.
A polymer composite according to the invention
is prepared by mixing between 10 and 40o by volume of
silver powder and approximately the same amount of a
second, nonconducting filling material with an epoxy
resin. The polymer composite is then applied over a
large area and uniformly to a first layer of the
composite conductor. The layer to become the
transitional layer is then brought into congruent
alignment with the second layer of the composite
conductor and cured under a vacuum. Suitable for
applying a viscous layer with a controlled thickness
are screen-printing methods or, with the assistance of
a solvent, also spray-coating methods.
LIST OF DESIGNATIONS
1 superconductor
2 electrical bypass
3 transitional layer, polymer composite material
30 polymer matrix
31 first filling material, for example silver
particles
32 second filling material
