Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
2091~95
1 SUPERCONDUCTIVE CURRENT LEAD
FIE~D OF THE INVENTION
; This invention relates to a superconductive
current lead, and more particularly to, the improvement
of a superconductive current lead connecting a
superconductive machine placed in an atmosphere of an
; extreme low temperature to a power supply, etc. placed
in an atmosphere of a room temperature.
BACKGROUND OF THE INVENTION
- A current lead which is used for a
superconductive machine is designed to have a cross-
section, such that the sum of a heat generated by the
current flow through the current lead and a heat
transferred from a high temperature portion to the
current lead becomes minimum, and an evaporation amount
of coolant such as helium, etc. becomes also minimum.
In conventional cases, a current lead is
.;
composed of copper wires. Under this circumstance,
i a current lead using oxide system superconductive wires
-~-` each covered with Ag layer has been studied for the
.....
i; same purpose.
i According to a current lead composed of the Ag-
layered oxide system superconductive wires, however,
there is a disadvantage in that a heat transfer amount
- is large, because the thermal conductivity of Ag is
:,
2 2091595
1 large at an extreme low temperature region which is an
operation temperature region of a superconductive
machine as compared to other metals, although this
results in an advantage in that the stability of the
superconductive wires is increased.
In this current lead, there is a further
`disadvantage in that Joule heat generated by eddy
current and coupling current of the Ag layers is not
negligible, because an electric resistance of the Ag
~`10 layers becomes very small at an extreme low
~`
temperature, when AC current or transient current flows
` through the current lead.
: ~,
SUMMARY OF THE INVENTION
Accordingly, it is an object of the invention to
;1
~j provide a superconductive current lead, in which a heat
: i
~ -'!,, amount transferred from an external high temperature
.. .;
`;~ portion thereto is decreased.
It is a further object of the invention to
!20 provide a superconductive current lead, in which ~oule
heat generated by the flow of current is decreased.
It is a still further object of the invention to
. .
~ provide a superconductive current lead, by use of which
::............................................. .
an evaporation amount of coolant is decreased.
According to the invention, a superconductive
-~ current lead, comprises :
. .~
j A predetermined number of superconductive wires
, -., j
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,
'''
-
: . - - , , .
2 ~ 9 ~
: each of the superconductive wires, comprising :
a first member of an oxide system
: superconductive material ; and
a second member of a material representing a
predetermind thermal conductivity at a temperature
lower than a room temperature, the predetermined
thermal conductivity being lower than a thermal
conductivity at the room temperature.
In a material, lowering a thermal conductivity
i6 tend to lowering an electric resistance, so that the
decrease of eddy current loss or coupling current loss
is realized, when AC current or transient current flows
through the superconductive current lead.
~ ~ 15 In the invention, an oxide system
:~i superconductive material may be a material selected
:-
~ from, for instance, Y~Ba-Cu-O, Bi-Sr-Cu-O, Bi-Pb-Sr-Ca-
;
-. Cu-O, Tl-Ba-Cu-O, Tl-Ba-Ca-Cu-O, Tl-Sr-Ca-Cu-O, La-Sr-
Cu-O, La-Ba-Cu-O, and a material having a lower thermal
conductivity at a lower temperature than a room
temperature (equivalent to having a larger electric
resistance) may be a material selected from, for
` example, an alloy including Au or Ag as a main
component and at least one element of Pd, Pt, Mn, Mg,
. 25 Zr, and Au, a material including a transition metal
system alloy as a main component which will be oxide at
a thermal treatment of a superconductive material, and
.,
- . .. ~ . ~ ~ . . . . . . . . ... .
., .
2 ~
1 an oxide system material such as alumina, MgO, ~aAl03
. These latter materials may be used for a
reinforcing member in a superconductive current lead,
while a barrier layer of precious metal may be
interposed between the oxide system superconductive
material and the lower thermal conductivity material.
When Ag-Au alloy is used for a reinforcing
member, a content of Au is less than 15~, perferably
to 10%, by atomic ratio.
A predetermined number of superconductive wires
may ~e one or more superconductive wires.
BRIEF DESCRIPTION OF THE DR~WINGS
The invention will be explained in more detail
in conjunction with appended drawings, wherein :
Fig.1 is an explanatory diagram showing a
superconductive current lead connecting a
superconductive machine to a power supply ;
Fig.2 is a perspective view showing a
superconductive current lead of a first preferred
embodiment according to the invention ; and
`:
i Fig.3 is a perspective view showing a
- superconductive current lead of a second preferred
i embodiment according to the invention.
` DESCRIPTION OF THE PREFERRED EMBODIMENT
`'
Fig. 1 shows a superconductive current lead 4
,
209~595
l connected to a superconductive coil(machine) 3 and a
current lead 5 by connectors 41 and 42, and a power
supply 6 provided to apply a predetermined voltage
across the superconductive coil 3 via the current lead 5
and the superconductive current lead 4, wherein the
superconductive coil and current lead 3 and 4 are
immersed in liquid helium 2 contained in a container 1.
Fig.2 shows a superconductive current lead 4 of
a preferred embodiment according to the invention which
is used in the apparatus as shown in Fig.1. The
superconductive current lead 4 comprises a plurality of
tape-shaped wires 7 each comprising a core 8 of an
oxide system superconductive material and an alloy
covering layer 9.
In the first example of the first preferred
embodiment, the core 8 is of an oxide system
superconductive material(Tl-Pb-Sr-Ba-Ca-Cu-O) including
1223 phase of Tl and Pb single layer system as a main
~:- component, and the alloy covering layer 9 is of Au-5
atomic ~ Pb alloy, wherein each superconducting tape is
2.4 mm in width, 1.4 mm in thickness, and 3.3 mm2 in
,.
:~:. cross-sectional area. The superconductive current
:
~ lead 4 thus fabricated is immersed in the liquid helium
~ ,
2 having a temperature of 77k, wherein a
superconductive critical current of approximately lOOA
is obtained. When current flows through the leads 4
and 5, it is determined that a heat transferred to the
6 20915~
1 liquid helium 2 is approximately 0.05 mW per a current
of lA in accordance with the estimation of an
evaporation amount of the liquid helium 2. In
accordance with the result, it is confirmed that this
5 superconductive current lead 4 has a property having a
very small heat transfer. This is resulted from the
structure that the alloy covering layer 9 having a low
thermal conductivity at a iow temperature is used, so
-~ that a heat transferred through the alloy covering
10 layer 9 is decreased.
In case where AC current of 60 Hz flows through
the superconductive current lead 4, it is measured that
:
a heat of approximately lmN per a current of lA is
transferred to the liquid helium 2. This result
~ .1
'~ 15 confirms that the superconductive current lead 4 is a
conductor having a property of a very small thermal
Y loss. This is resulted from the structure that the
.i
alloy covering layer 9 has a large electric resistance,
-~ so that eddy current loss and coupling current loss of
2Q the conductor are decreased.
~; In the second example of the first preferred
embodiment, a superconductive current lead 4 having the
same size as that in the first example is fabricated to
include a core 8 of an oxide system superconductive
25 material (Bi-Sr-Ca-Cu-O) of Bil2212 system and an alloy
covering layer 9 of Ag-3 atomic ~ Au alloy.
A critical current of this superconductive
:
7 2~9~59~
1 current lead 4 immersed in the liquid helium 2 at a
temperature of 4.2K is 10~ A/cmZ which is the same
value as a value obtained in case where pure Ag is used
for a covering material. At the same time, a
resistivity which is as high as 0.7 to 1.2 ~ Q cm is
obtained to represent a significant change as compared
to the case where pure Ag is used at an extreme low
temperature region for a covering material.
In relation to the resistivity of the alloy
covering layer 9, a thermal conductivity of the
.i
`~superconductive current lead 4 become smaller than that
of common current lead of phosphorus-deoxidized copper.
Consequently, the superconductive current lead 4 can be
used with an eddy current loss of one percent as
~,15 compared to that in the case where pure Ag is used for
,,.~
~a covering layer.
'In regard to electric resistance of the
~lconnectors 42 and 41, the resistance value is not only
,~
`~decreased, but also stabilized in the elapse of time.
,~
; 20 In the first and second examples of the
`~
,~ preferred embodiment, the thermal conductivity (W/m k)
of the alloy covering layer 9 is shown in a below table
' relative to a temperature along with that of a pure Ag
covering layer.
.
:
.
1TEMPERATURE FIRST SECOND PURE Ag
; (k) EXAMPLE EXAMPLE
260 500 160 590
_ _
77 170 51 530
_ ~ ~
`` 20 75 1 10 25,000
: .
'-"` 10
r,~' '
~,`~;'
~.,.
~-~ Fig. 3 shows a superconductive current lead of a
i~, second preferred embodiment according to the invention.
`` 15 The superconductive current lead comprises a basic
-.~
member 10 of alumina, a thin layer 11 of Ag and an
. oxide system superconductive layer 12. The alumina
`~ basic member 10 may be replaced by an Ag-Au alloy basic
member.
20AS described above, a material representing a
lower thermal conductivity at a temperature lower than
~` a room temperature is used for a basic member in the
- invention, so that a heat transferring through the
~, -
basic member to coolant is not only decreased, but a
~`~ 25 heat of the lead generated by the flow of current is
~ also decreased, thereby decreasing the consumption of
;~ the coolant and a load of a refrigerator.
,":
~'' ' ,
.
9 2091595
1 Although the invention has been described with
respect to specific embodiment for complete and clear
disclosure, the appended claims are not to be thus
limited but are to be construed as embodying all
modification and alternative constructions that may
occur to one skilled in the art which fairly fall
within the basic teaching herein set forth.
~ 10
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