U.S. patent application number 13/458080 was filed with the patent office on 2012-11-01 for low temperature thermal conductor.
This patent application is currently assigned to SUMITOMO CHEMICAL COMPANY, LIMITED. Invention is credited to Hiroaki HOSHIKAWA, Kenichi SASAKI, Hiroshi TABUCHI, Takayuki TOMARU.
Application Number | 20120273181 13/458080 |
Document ID | / |
Family ID | 46330431 |
Filed Date | 2012-11-01 |
United States Patent
Application |
20120273181 |
Kind Code |
A1 |
TOMARU; Takayuki ; et
al. |
November 1, 2012 |
LOW TEMPERATURE THERMAL CONDUCTOR
Abstract
A thermal conductor material having excellent heat transfer
properties by obtaining high thermal conductivity even at low
temperature of, for example, a liquid nitrogen temperature (77 K)
or lower is to provide. A thermal conductor to be used at low
temperature of 77 K or lower in the magnetic field of a magnetic
flux density of 1 T or more, includes aluminum which has a purity
of 99.999% by mass or more and also has the content of iron of 1
ppm by mass or less.
Inventors: |
TOMARU; Takayuki;
(Tsukuba-shi, JP) ; SASAKI; Kenichi; (Tsukuba-shi,
JP) ; HOSHIKAWA; Hiroaki; (Niihama-shi, JP) ;
TABUCHI; Hiroshi; (Niihama-shi, JP) |
Assignee: |
SUMITOMO CHEMICAL COMPANY,
LIMITED
Tokyo
JP
INTER-UNIVERSITY RESEARCH INSTITUTE CORPORATION HIGH ENERGY
ACCELERATOR RESEARCH ORGANIZATION
Tsukuba-shi
JP
|
Family ID: |
46330431 |
Appl. No.: |
13/458080 |
Filed: |
April 27, 2012 |
Current U.S.
Class: |
165/185 |
Current CPC
Class: |
C22F 1/04 20130101; C22C
1/026 20130101; C22C 21/00 20130101 |
Class at
Publication: |
165/185 |
International
Class: |
F28F 7/00 20060101
F28F007/00 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 28, 2011 |
JP |
2011-101767 |
Claims
1. A thermal conductor to be used at low temperature of 77 K or
lower in the magnetic field of a magnetic flux density of 1 T or
more, comprising aluminum having a purity of 99.999% by mass or
more and having the content of iron of 1 ppm by mass or less.
2. The thermal conductor according to claim 1, wherein the aluminum
has a purity of 99.9999% by mass or more.
3. The thermal conductor according to claim 1, wherein the aluminum
has a purity of 99.99998% by mass or more.
4. The thermal conductor according to claim 1, wherein the aluminum
contains an intermetallic compound Al.sub.3 Fe.
5. A thermal conductor for cooling a superconducting magnet, using
the thermal conductor according to claim 1.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a thermal conductor which
exhibits excellent conductivity at low temperature of, for example,
77 K or lower, especially at cryogenic temperatures of 20 K or
lower; and more particularly to a thermal conductor which exhibits
excellent conductivity even when used in a strong magnetic field
of, for example, 1 T or more.
[0003] 2. Description of the Related Art
[0004] A superconducting magnet has been used in various fields,
for example, MRIs (magnetic resonance imaging) for diagnosis, NMRs
(nuclear magnetic resonance) for analytical use or maglev trains.
There have been used, as a superconducting magnet, low-temperature
superconducting coils cooled to helium's boiling point of 4.2 K
(Kelvin) using liquid helium, and high-temperature superconducting
coils cooled to about 20 K by a refrigerator.
[0005] There is a need to use a thermal conductor which exhibits
high thermal conductivity at low temperature of a boiling point of
liquid nitrogen (77 K) or lower, especially cryogenic temperatures
of 20 K or lower, so as to cool these superconducting coils
efficiently and uniformly.
[0006] JP 2007-063671A discloses cold-worked aluminum, as a thermal
conductor which exhibits high thermal conductivity at low
temperature.
[0007] JP 2004-283580A discloses a structure of a magnetic
resonance assembly, and also describes that it is possible to use,
as a thermal conductor (thermal bus bar) located between a
refrigerator and a freezing container, aluminum having high purity
of 99.999% by mass or more (hereinafter sometimes referred to as
"5N" (five nines) and, in the mass percentage notation which
indicates a purity, notation is sometimes performed by placing "N"
in the rear of the number of "9" which is continuous from the head,
for example, purity of 99.9999% by mass or more is sometimes
referred to as "6N" (six nines)), which exhibits high heat transfer
properties at cryogenic temperatures, or aluminum having a purity
of 99.99% by mass or more (4N).
[0008] There is also known a thermal conductor using copper such as
oxygen-free copper having a purity of 99.99% by mass or more (4N),
in addition to aluminum.
[0009] However, these materials having high heat transfer
properties at low temperature also have a problem that the thermal
conductivity decreases in the vicinity of a superconducting coil
(superconducting magnet), for example, under a strong magnetic
field where the magnetic field produced by the superconducting coil
is 1 T or more, and thus high heat transfer properties cannot be
obtained.
[0010] This problem is caused by the magnetoresistance effect. This
effect is known as a phenomenon in which electrical resistivity
varies depending on the external magnetic field.
[0011] It is known that copper shows a remarkable magnetoresistance
effect and the electrical resistivity at low temperature remarkably
increases with increasing magnetic field. It is known that aluminum
also shows the magnetoresistance effect, although not comparable to
copper, and that causes a remarkable increase in electrical
resistivity at low temperature in the magnetic field.
[0012] In lots of metals including copper, aluminum and alloys
thereof, the electrical resistivity has a close relation with the
thermal conductivity, and the thermal conductivity decreases when
the electrical resistivity increases (conductivity decreases).
[0013] As a result, there was a problem that cooling efficiency of
a superconducting coil decreases as heat transfer properties of a
thermal conductor to be used under a strong magnetic field
deteriorate.
SUMMARY OF THE INVENTION
[0014] Thus, an object of the present invention is to provide a
thermal conductor having excellent heat transfer properties by
obtaining high thermal conductivity even at low temperature of, for
example, a liquid nitrogen temperature (77 K) or lower, especially
cryogenic temperatures of 20 K or lower in a strong magnetic field
of a magnetic flux density of 1 T or more.
[0015] The present invention provides, in an aspect 1, a thermal
conductor to be used at low temperature(s) of 77 K or lower in the
magnetic field of a magnetic flux density of 1 T or more, including
aluminum which has a purity of 99.999% by mass or more and also has
the content of iron of 1 ppm by mass or less.
[0016] The present inventors have found that even aluminum (Al) can
remarkably suppress the magnetoresistance effect by controlling the
purity to 99.999% by mass or more and also controlling the content
of iron to 1 ppm by mass or less. The thermal conductor made of
such aluminum has high thermal conductivity and exhibits excellent
heat transfer properties even when used at cryogenic temperatures
of, for example, 77 K or lower in a strong magnetic field of a
magnetic flux density of 1 T or more.
[0017] The present invention provides, in an aspect 2, the thermal
conductor according to the aspect 1, wherein the aluminum has a
purity of 99.9999% by mass or more.
[0018] The present invention provides, in an aspect 3, the thermal
conductor according to the aspect 1, wherein the aluminum has a
purity of 99.99998% by mass or more.
[0019] The present invention provides, in an aspect 4, the thermal
conductor according to any one of the aspects 1 to 3, wherein the
aluminum contains an intermetallic compound Al.sub.3Fe.
[0020] The present invention provides, in an aspect 5, the thermal
conductor for cooling a superconducting magnet, using the thermal
conductor according to any one of the aspects 1 to 4.
[0021] According to the present invention, it is possible to
provide a thermal conductor having excellent heat transfer
properties by having high thermal conductivity even at low
temperature of, for example, a liquid nitrogen temperature (77 K)
or lower, especially cryogenic temperatures of 20 K or lower in a
strong magnetic field of a magnetic flux density of 1 T or
more.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] FIG. 1 is a graph showing a relation between the
conductivity index and the applied magnetic field (magnetic flux
density).
[0023] FIG. 2 is a graph showing a relation between the thermal
conductivity and the applied magnet field (magnetic flux
density).
[0024] FIG. 3 is a graph showing a relation between the temperature
difference of both ends of a sheet-shaped sample and the magnetic
field (magnetic flux density).
DETAILED DESCRIPTION OF THE INVENTION
[0025] The thermal conductor according to the present invention
includes aluminum which has a purity of 99.999% by mass or more and
also has the content of iron of 1 ppm by mass, so as to be used
even in the magnetic field of a magnetic flux density of 1 T or
more.
[0026] The present inventors have found, first, that aluminum,
which has a purity of 99.999% by mass or more and also has the
content of iron of 1 ppm by mass, does not remarkably exert the
magnetoresistance effect even when the magnetic field of a magnetic
flux density of 1 T or more is applied, and thus suppressing a
decrease in thermal conductivity. Consequently, the present
invention has been completed.
[0027] As disclosed, for example, in JP 2009-242865A and JP
2009-242866A, it has been known that the electrical resistivity at
cryogenic temperatures, for example, liquid helium temperatures
decreases as the purity of aluminum increases, like 5N (purity of
99.999% by mass or more) and 6N (purity of 99.9999% by mass or
more).
[0028] As disclosed, for example, in JP 2010-106329A, aluminum
having a purity of 99.999% by mass or more and also having the
content of iron of 1 ppm by mass or less has also been known.
[0029] It has been known that although aluminum enables an
improvement in electrical conductivity at cryogenic temperatures in
a state where the magnetic field is not applied by increasing the
purity to about 4N, remarkable magnetoresistance effect appears
when a strong magnetic field of a magnetic flux density of 1 T or
more is applied, and thus causing a decrease in conductivity It has
been considered that high conductivity cannot be obtained under a
strong magnetic field also in high purity aluminum of 5N or 6N
purity, similarly to the aluminum of 4N purity.
[0030] Therefore, it is considered that aluminum having a purity of
99.999% by mass or more and also having the content of iron of 1
ppm by mass or less was not used in a thermal conductor which is
used in the magnetic field of a magnetic flux density of 1 T or
more.
[0031] It is as mentioned above that the present inventors have
found, first, that an increase in electrical resistivity (i.e., a
decrease in thermal conductivity) under a strong magnetic field,
which has conventionally been conceived, does not occur in high
purity aluminum of 5N or higher level and also having the content
of iron of 1 ppm by mass or less.
[0032] Although details will be described in the below-mentioned
examples, a drastic decrease in conductivity is recognized in a
strong magnetic field even in a high purity copper of 5N, 6N or
higher level purity, although this material is commonly used as a
thermal conductor. Therefore, a phenomenon in which high
conductivity is maintained even in a strong magnetic field by
achieving high purity, found by the present inventors, is peculiar
to aluminum.
[0033] In the thermal conductor according to the present invention,
as mentioned above, the amount of iron contained in aluminum is
controlled to 1 ppm by mass or less.
[0034] As will be described below for details, the reason is
considered as follows: the magnetoresistance effect is surely
suppressed by controlling the amount of iron as a ferromagnetic
element, and thus making it possible to surely suppress a decrease
in thermal conductivity caused by the applied strong magnetic
field.
[0035] The thermal conductor according to the present invention
remarkably exerts the effect by use in a state where the
temperature is 77 K (-196.degree. C.) or lower, and more preferably
20 K (-253.degree. C.) or lower, and also the magnetic field of a
magnetic flux density of 1 T or more is applied.
[0036] Before making a description of details of the thermal
conductor according to the present invention, a description is made
why a thermal conductor using a material having excellent
electrical conductivity has high thermal conductivity.
[0037] In lots of metals including copper, aluminum and alloys
thereof, transfer of free electrons is the main mechanism of
electric conduction and the electrical conductivity can be enhanced
by making free electrons to easily transfer. On the other hand,
free electrons significantly contribute to thermal conduction of
these metals, and high thermal conductivity can be obtained when
free electrons are easily movable.
[0038] Wiedemann-Franz (WF) law has been known as a relation
between the thermal conductivity and the electrical conductivity of
common metals. It has also been known that the thermal conductivity
of about 40 K or lower of high purity aluminum can be determined
from the following equation (1) as a more accurate relational
equation of high purity metals, and the thermal conductivity of
about 40 K or lower of high purity copper can be determined from
the following equation (2) (both equations are cited from TEION
KOGAKU, Vol. 39 (2004), No. 1, pp. 25-32).
.kappa.=1/(1.83.times.10.sup.-7.times.T.sup.2+1.09/RRR/T) (1)
.kappa.=1/(6.41.times.10.sup.-8.times.T.sup.2.4+0.685/RRR/T)
(2)
where
[0039] .kappa.: Thermal conductivity (W/m/K)
[0040] T: Temperature (K)
[0041] RRR: Residual resistivity ratio
[0042] The residual resistivity ratio RRR is represented by the
following equation (3).
RRR=.rho..sub.297 K/.rho..sub.T (3)
where
[0043] .rho..sub.297 K: Resistivity at temperature of 297 K
(n.OMEGA.cm)
[0044] .rho..sub.T: Resistivity at temperature T (K)
(n.OMEGA.cm)
[0045] Herein, it has been known that .rho..sub.297 K of copper and
.rho..sub.297 K of aluminum are scarcely influenced by the purity
and the magnetic field to be applied from the outside, and are
almost constant (for example, .rho..sub.297 K of aluminum is about
2,700 and .rho..sub.297 K of copper is about 1,500).
[0046] Therefore, as is apparent from the equations (1) to (3), the
thermal conductivity of copper and aluminum increases as the
electrical conductivity is improved (as the electrical resistivity
decreases).
[0047] Details of the thermal conductor according to the present
invention will be described below.
(1) Level of Impurities
[0048] As mentioned above, the thermal conductor according to the
present invention is characterized by including aluminum which has
a purity of 99.999% by mass or more and also has the content of
iron of 1 ppm by mass or less. The purity is preferably 99.9999% by
mass or more, and more preferably 99.99998% by mass or more
(hereinafter sometimes referred to as "6N8") for the following
reasons. That is, the higher the purity, the less the decrease in
electrical conductivity under a strong magnetic field. Furthermore,
in case of the purity of 99.9999% by mass or more, the electrical
resistivity may sometimes decrease in a strong magnetic field of 1
T or more as compared with the case where the magnetic field is not
applied.
[0049] The content of iron is preferably 0.1 ppm by mass or
less.
[0050] The reason is that a decrease in conductivity in a strong
magnetic field can be suppressed more surely.
[0051] There are still many unclear points in the mechanism in
which a decrease in electrical conductivity in a strong magnetic
field can be suppressed by controlling the content of iron to 1 ppm
by mass or less. However, predictable mechanism at the moment is
considered as follows. That is, iron is likely to be influenced by
a strong magnetic field since it is a ferromagnetic element and, as
a result, when iron exists in the content of more than 1 ppm by
mass, an influence exerted on the conductivity increases, and thus
the conductivity under a strong magnetic field may decrease. When
the content of iron is 0.1 ppm, an influence due to the
ferromagnetic material can be excluded almost completely. However,
this predictable mechanism does not limit the scope of the present
invention.
[0052] Ni and Co are known as ferromagnetic elements other than
iron. However, since these elements are easily removed in a known
process for highly purification of aluminum, the numerical value of
the content is out of the question. However, the contents of these
Ni and Co are also preferably 1 ppm or less, and more preferably
0.1 ppm or less.
[0053] The purity of aluminum can be defined in some methods. For
example, it may be determined by the measurement of the content of
aluminum. However, it is preferred that the purity of aluminum is
determined by measuring the content (% by mass) of the following 33
elements contained as impurities in aluminum and subtracting the
total of these contents from 100%, so as to determine the purity of
aluminum with high accuracy in a comparatively simple manner.
[0054] Herein, 33 elements contained as impurities are lithium
(Li), beryllium (Be), boron (B), sodium (Na), magnesium (Mg),
silicon (Si), potassium (K), calcium (Ca), titanium (Ti), vanadium
(V), chromium (Cr), manganese (Mn), iron (Fe), nickel (Ni), cobalt
(Co), copper (Cu), zinc (Zn), gallium (Ga), arsenic (As), zirconium
(Zr), molybdenum (Mo), silver (Ag), cadmium (Cd), indium (In), tin
(Sn), antimony (Sb), barium (Ba), lantern (La), cerium (Ce),
platinum (Pt), mercury (Hg), lead (Pb) and bismuth (Bi).
[0055] The contents of these elements can be determined, for
example, by glow discharge mass spectrometry.
(2) Purification Method
[0056] Such high purity aluminum may be obtained by using any
purification (refinement) method. Some purification methods for
obtaining high purity aluminum according to the present invention
are exemplified below. However, the purification method is not
limited to these methods as a matter of course.
Three-Layer Electrolysis Process
[0057] It is possible to use, as one of methods of obtaining high
purity aluminum, a three-layer electrolysis process in which
commercially available aluminum having comparatively low purity
(for example with special grade 1 of 99.9% purity as specified in
JIS-H2102) is charged in an Al--Cu alloy layer and is used as an
anode in a molten state, and an electrolytic bath containing
aluminum fluoride and barium fluoride therein is arranged thereon,
and thus high purity aluminum is produced on a cathode.
[0058] In the three-layer electrolysis process, aluminum having a
purity of 99.999% by mass or more can be mainly obtained. It is
possible to suppress the content of iron in aluminum to 1 ppm by
mass or less, comparatively easily.
Unidirectional Solidification Process
[0059] For example, a unidirectional solidification process can be
used so as to further increase a purity of the high purity aluminum
obtained by the three-layer electrolysis process.
[0060] The content of Fe and the respective contents of Ti, V, Cr
and Zr can be selectively decreased by the unidirectional
solidification process.
[0061] It has been known that the unidirectional solidification
process is, for example, a method in which aluminum is melted in a
furnace tube using a furnace body moving type tubular furnace and
then unidirectionally solidified from the end by pulling out a
furnace body from a furnace tube, and that the contents of the
respective elements of Ti, V, Cr and Zr selectively increase at the
side of the solidification initiation end, and also the content of
Fe selectively increases at the side of the solidification
completion end (opposite side of the solidification initiation
end). Therefore, it becomes possible to surely decrease the
contents of the respective elements of Fe, and Ti, V, Cr and Zr by
cutting off the both sides of solidification initiation end and the
solidification completion end of the obtained ingot. It may be
determined, which specific portion of the ingot obtained by the
unidirectional solidification process must be cut, by analyzing the
contents of elements at appropriate intervals along a
solidification direction so that only portion, where the total
content of the contents of Fe, and Ti, V, Cr and Zr is sufficiently
decreased, is allowed to remain.
[0062] There is no particular limitation on the order of
implementation of purification by the three-layer electrolysis
process and purification by the unidirectional solidification
process. Usually, purification is implemented by the three-layer
electrolysis process, and then purification is implemented by the
unidirectional solidification process. Purification by the
three-layer electrolysis process and purification by the
unidirectional solidification process may be implemented, for
example, alternately and repeatedly, or any one of or both
purifications may be repeatedly implemented, respectively. It is
particularly preferred that purification by the unidirectional
solidification process is repeatedly implemented.
[0063] In such way, aluminum having a purity of 99.9999% by mass or
more can be obtained by using the three-layer electrolysis process
in combination with the unidirectional solidification process. It
is also possible to suppress the content of iron in aluminum to 1
ppm by mass or less, and more preferably 0.1 ppm by mass or less in
a comparatively easy manner.
Zone Melting Process
[0064] Furthermore, a zone melting process can be used so as to
obtain aluminum having high purity, for example, a purity of
99.99998% by mass or more. When the zone melting process is
appropriately used, the content of iron in aluminum can be
suppressed to 1 ppm by mass or less, and more preferably 0.1 ppm by
mass or less, more surely.
[0065] In particular, it is effective to use a purification method
of aluminum through the zone melting process invented by the
present inventors (method described in Japanese Patent Application
No. 2010-064544. The disclosure of Japanese Patent Application No.
2010-064544 is incorporated by reference herein.).
[0066] In order to prevent impurities from diffusing into heated
aluminum when removing impurities in aluminum through zone melting
purification process, it is preferred that an alumina layer is
formed in advance on a surface of a boat in which aluminum is
placed, and also zone melting purification is performed in vacuum
under a pressure of 3.times.10.sup.-5 Pa or less, and more
preferably from 3.times.10.sup.-6 Pa to 2.times.10.sup.-5 Pa, so as
to surely separate impurities from molten aluminum.
[0067] It is preferred to carry out a pretreatment, in which a
surface layer of an aluminum raw material to be subjected to zone
melting purification is dissolved and removed in advance, before
zone melting purification is performed. There is no particular
limitation of the pretreatment method, and various treatments used
in the relevant technical field can be used so as to remove the
surface layer of the aluminum raw material.
[0068] Examples of the pretreatment include an acid treatment, an
electrolytic polishing treatment and the like.
[0069] The above-mentioned boat to be used in the zone melting
purification process is preferably a graphite boat, and is
preferably baked in an inert gas or vacuum after formation of the
above-mentioned alumina layer.
[0070] The width of the melting section where aluminum is melted
during the zone melting purification is preferably adjusted to
w.sub.Al.times.1.5 or more and w.sub.Al.times.6 or less based on a
cross sectional size w.sub.Al of the aluminum raw material.
[0071] An aluminum raw material to be used in the purification is
obtained by using the three-layer electrolysis process in
combination with the unidirectional solidification process and, for
example, high purity aluminum having a purity of 99.9999% by mass
or more is preferably used.
[0072] In the zone melting process, for example, the melting
section is moved from one end of a raw aluminum toward the other
end by moving a high frequency coil for high frequency heating, and
thus the entire raw aluminum can be subjected to zone melting
purification. Among impurity metal element components, peritectic
components (Ti, V, Cr, As, Se, Zr and Mo) tend to be concentrated
to the melting initiation section and eutectic components (26
elements as a result of removal of peritectic 7 elements from the
above-mentioned 33 impurity elements) tend to be concentrated to
the melting completion section, and thus a high purity aluminum can
be obtained in the region where both ends of the aluminum raw
material are removed.
[0073] After moving the melting section within a predetermined
distance, like a distance from one end to the other end in a
longitudinal direction of an aluminum raw material, high frequency
heating is completed and the melting section is solidified. After
the solidification, an aluminum material is cut out (for example,
both ends are cut off) to obtain a purified high purity aluminum
material.
[0074] When a plurality of aluminum raw materials are arranged in a
longitudinal direction (in a movement direction of the melting
section), it is preferred that the aluminum raw materials in a
longitudinal direction are brought into contact with each other to
treat as one aluminum raw material in a longitudinal direction, and
then the melting section is moved from one end (i.e., one of two
ends where adjacent aluminum raw materials are not present in a
longitudinal direction among ends of the plurality of aluminum raw
materials) to the other end (i.e., the other one of two ends where
adjacent aluminum raw materials are not present in a longitudinal
direction among ends of the plurality of aluminum raw
materials).
[0075] The reason is that ends of the aluminum raw material
contacted with each other are united during zone melting, and thus
a long aluminum material can be obtained.
[0076] As mentioned above, after zone melting (zone melting
purification) from one end to the other end of the aluminum raw
material, zone melting can be repeated again from one end to the
other end. The number of repeat times (number of passes) is usually
1 or more and 20 or less. Even if the number of passes is more than
the above range, an improvement in the purification effect is
restrictive.
[0077] In order to effectively remove the peritectic 7 elements,
the number of passes is preferably 3 or more, and more preferably 5
or more. When the number of passes is less than the above range,
peritectic 7 elements are less likely to moved, and thus sufficient
purification effect is not obtained.
[0078] The reason is as follows. When a plurality of aluminum raw
materials are arranged in contact with each other in a longitudinal
direction, when the number of passes is less than 3, a shape
(especially, height size) of the purified aluminum after uniting
becomes un-uniform, and thus the melting width may sometimes vary
during purification and uniform purification is less likely to be
obtained.
(3) Forming Method
[0079] The ingot of the high purity aluminum obtained by the
above-mentioned purification method is formed into a desired shape
using various methods.
[0080] The forming method will be shown below. However, the forming
method is not limited thereto.
Rolling
[0081] When a thermal conductor to be obtained is a plate or a
wire, rolling is an effective forming method.
[0082] The rolling may be performed using a conventional method,
for example, a method in which an ingot is passed through a pair of
rolls by interposing into the space between these rolls while
applying a pressure. There is no particular limitation on specific
techniques and conditions (temperature of materials and rolls,
treatment time, reduction ratio, etc.) in case of rolling, and
these specific techniques and conditions may be appropriately set
unless the effects of the present invention are impaired.
[0083] There is no particular limitation on the size of the plate
and wire to be finally obtained by rolling. As for preferable size,
the thickness is from 0.1 mm to 3 mm in case of the plate, or the
diameter is from 0.1 mm to 3 mm in case of the wire.
[0084] When the thickness is less than 0.1 mm, sufficient
conduction characteristics required as the thermal conductor may be
sometimes less likely to be obtained since a cross section
decreases. In contrast, when the thickness is more than 3 mm, it
may sometimes become difficult to deform utilizing flexibility.
When the thickness is from 0.1 mm to 3 mm, there is an advantage
such as easy handling, for example, and the material can be
arranged on a side surface of a curved container utilizing
flexibility.
[0085] As a matter of course, the shape obtainable by rolling is
not limited to the plate or wire and, for example, a pipe shape and
an H-shape can be obtained by rolling.
[0086] The rolling may be hot rolling or warm rolling in which an
ingot is heated in advance and then rolling is performed in a state
of being set at a temperature higher than room temperature, or may
be cold rolling in which the ingot is not heated in advance.
Alternatively, hot rolling or warm rolling may be used in
combination with cold rolling.
[0087] In case of rolling, it is also possible to cast or cut the
material into a desired shape in advance. In case of casting, a
conventional method may be employed, but is not limited to, for
example, a method in which high purity aluminum is heated and
melted to form a molten metal and the obtained high purity aluminum
molten metal is solidified by cooling in a mold. Also, there is no
particular limitation on the conditions or the like in case of
casting. The heating temperature is usually from 700 to 800.degree.
C., and heating and melting is usually performed in vacuum or an
inert gas (nitrogen gas, argon gas, etc.) atmosphere in a crucible
such as a graphite crucible.
Forming Method Other than Rolling
[0088] Wire Drawing or extrusion may be performed as a forming
method other than rolling. There is no limitation on the shape
obtained by drawing or extrusion. For example, drawing or extrusion
is suited to obtain a wire having a circular cross section.
[0089] A desired wire shape may be obtained by rolling before
drawing to obtain a rolled wire (rolled wire rod) and then drawing
the rolled wire.
[0090] The cross section of the obtained wire is not limited to a
circle and the wire may have a noncircular cross section, for
example, an oval or square cross section.
[0091] The desired shape may also be obtained by cutting the ingot,
except for drawing or extrusion.
(4) Annealing
[0092] Furthermore, the formed article of the present invention
obtained by the above forming method such as rolling may be
optionally subjected to an annealing treatment. It is possible to
remove strain, which may be usually sometimes generated in case of
cutting out a material to be formed from the ingot, or forming, by
subjecting to an annealing treatment.
[0093] There is no particular limitation on the conditions of the
annealing treatment, and a method of maintaining at 400 to
600.degree. C. for one or more hours is preferable.
[0094] When the temperature is lower than 400.degree. C., strain
(dislocation) included in the ingot is not sufficiently decreased
for the following reason. Since strain (dislocation) serves as a
factor for enhancing electrical resistivity, excellent conduction
characteristics may not be sometimes exhibited. When the heat
treatment temperature is higher than 600.degree. C., solution of
impurities in solid, especially solution of iron into a matrix
proceeds. Since solid-soluted iron has large effect of enhancing
electrical resistivity, conduction characteristics may sometimes
deteriorate.
[0095] More preferably, the temperature is maintained at 430 to
550.degree. C. for one or more hours for the following reason.
[0096] When the temperature is within the above range, strain can
be sufficiently removed and also iron exists as an intermetallic
compound with aluminum without being solid-soluted into the
matrix.
[0097] The following reasons are also exemplified.
[0098] As an intermetallic compound of iron and aluminum, for
example, a plurality of kinds such as Al.sub.6Fe, Al.sub.3Fe and
Al.sub.mFe (m.apprxeq.4.5) are known. It is considered that the
majority of (for example, 50% or more, and preferably 70% or more
in terms of volume ratio) of an intermetallic compound of iron and
aluminum, which exists in a high purity aluminum material obtained
after annealing within a temperature range (430 to 550.degree. C.),
is Al.sub.3Fe.
[0099] This Al.sub.3Fe has such an advantage that it scarcely
exerts an adverse influence on the conductivity even in case of
existing as a precipitate.
[0100] Existence of Al.sub.3Fe and the volume ratio thereof can be
confirmed and measured by dissolution of a matrix (base material)
using a chemical solvent, and collection by filtration, followed by
observation of the residue collected by filtration using an
analytical electron microscope (analytical TEM) and further
analysis.
[0101] The thermal conductor according to the present invention may
be composed only of the above-mentioned high purity aluminum having
a purity of 99.999% by mass or more and may contain the portion
other than the high purity aluminum, for example, protective
coating so as to impart various functions.
[0102] While a thermal conductor for cooling a superconducting
magnet is illustrated as specific applications of the thermal
conductor according to the present invention, the specific
application is not limited thereto and the thermal conductor
according to the present invention can be used as thermal
conductors for various applications used at low temperature (77 K
or lower) under a strong magnetic field (1 T or more), for example,
thermal conductors used for cooling specimens to be measured in
NMR.
EXAMPLES
[0103] Example 1 (purity of 99.999% by mass or more, 5N--Al),
Example 2 (purity of 99.9999% by mass or more, 6N--Al) and Example
3 (purity of 99.99998% by mass or more, 6N8-Al), details of which
are shown below, were produced as example samples, and then
resistivity (specific electrical resistivity) was measured.
[0104] Comparative Example 1 (4N--Al) as aluminum having a purity
of 4N level, and Comparative Example 2 (3N--Al) as aluminum having
a purity of 3N level are shown below as Comparative Examples. The
resistivity of Comparative Examples 1 and 2 was determined by
calculation.
[0105] For comparison with copper, a sample of copper having a
purity of 5N level was prepared and then the resistivity was
measured as Comparative Example 3.
[0106] As for copper, literature data were used as Comparative
Example. Comparative Example 4 is copper sample having a purity of
4N level, Comparative Example 5 is copper sample having a purity of
5N level, and Comparative Example 6 is copper sample having a
purity of 6N level.
(1) Production of High Purity Aluminum
[0107] First, the method for producing a high purity aluminum used
in Examples 1 to 3 is shown below.
Example 1
[0108] A commercially available aluminum having a purity of 99.92%
by mass was purified by the three-layer electrolysis process to
obtain a high purity aluminum having a purity 99.999% by mass or
more and an iron content of 1 ppm by mass or less.
[0109] Specifically, a commercially available aluminum (99.92% by
mass) was charged in an Al--Cu alloy layer and the composition of
an electrolytic bath was adjusted to 41% AlF.sub.3-35%
BaF.sub.2-14% CaF.sub.2-10% NaF. Electricity was supplied at
760.degree. C. and a high purity aluminum deposited at a cathode
side was collected.
[0110] The contents of the respective elements in this high purity
aluminum were analyzed by glow discharge mass spectrometry (using
"VG9000", manufactured by THERMO ELECTRON Co., Ltd) to obtain the
results shown in Table 1.
Example 2
[0111] The high purity aluminum obtained by the above-mentioned
three-layer electrolysis process was purified by the unidirectional
solidification to obtain a high purity aluminum having a purity
99.9999% by mass or more and an iron content of 1 ppm by mass or
less.
[0112] Specifically, 2 kg of the high purity aluminum obtained by
the three-layer electrolysis process was placed in a crucible
(inside dimension: 65 mm in with.times.400 mm in length.times.35 mm
in height) and the crucible was accommodated inside a furnace tube
(made of quartz, 100 mm in inside diameter.times.1,000 mm in
length) of a furnace body transfer type tubular furnace. The high
purity aluminum was melted by controlling a furnace body (crucible)
to 700.degree. C. in a vacuum atmosphere of 1.times.10.sup.-2 Pa,
and then unidirectionally solidified from the end by pulling out
the furnace body from the furnace tube at a speed of 30 mm/hour.
After cutting out from the position which is 50 mm from the
solidification initiation end in a length direction to the position
which is 150 mm from the solidification initiation end, a massive
high purity aluminum measuring 65 mm in width.times.100 mm in
length.times.30 mm in thickness was obtained.
[0113] The contents of the respective elements in this high purity
aluminum were analyzed by glow discharge mass spectrometry in the
same manner as described above to obtain the results as shown in
Table 1.
Example 3
[0114] A high purity aluminum having a purity of 99.99998% by mass
or more and the iron content of 0.1 ppm or less was obtained by the
following zone melting process.
[0115] After cutting into a quadrangular prism measuring about 18
mm.times.18 mm.times.100 mm or a similar shape from the 6N aluminum
ingot obtained by the above-mentioned unidirectional solidification
process, and further acid pickling with an aqueous 20% hydrochloric
acid solution prepared by diluting with pure water for 3 hours, an
aluminum raw material was obtained.
[0116] Using this aluminum raw material, a zone melting process was
carried out by the following method.
[0117] A graphite boat was placed inside a vacuum chamber (a quartz
tube measuring 50 mm in outside diameter, 46 mm in inside diameter,
1,400 mm in length) of a zone melting purification apparatus. A
high purity alumina powder AKP Series (purity: 99.99%) manufactured
by Sumitomo Chemical Company, Limited was applied to the portion,
where the raw material is placed, of the graphite boat while
pressing to form an alumina layer.
[0118] The graphite boat was baked by high frequency heating under
vacuum.
[0119] The baking was carried out by heating in vacuum of 10.sup.-5
to 10.sup.-7 Pa using a high frequency heating coil (heating coil
winding number: 3, 70 mm in inside diameter, frequency of about 100
kHz) used in zone melting, and moving from one end to the other end
of the boat at a speed of 100 mm/hour thereby sequentially heating
the entire graphite boat.
[0120] The above-mentioned 9 aluminum raw materials in total weight
of about 780 g were arranged on the portion (measuring 20
mm.times.20 mm.times.1,000 mm), where the raw materials are placed,
provided in the graphite boat. The aluminum raw materials were
arranged in the form of a quadrangular prism consisting of 9 raw
materials (cross sectional size w of aluminum raw materials=18 mm,
length L=900 mm, i.e. L=w.times.50).
[0121] After sealing inside a chamber, evacuation was carried out
by a turbo-molecular pump and an oil sealed rotary pump until the
pressure reaches 1.times.10.sup.-5 Pa or less. Then, one end of the
aluminum raw material in a longitudinal direction was heated and
melted using a high frequency heating coil (high frequency coil) to
form a melting section.
[0122] The output of the high frequency power source (frequency:
100 kHz, maximum output: 5 kW) was adjusted so that the melting
width of the melting section becomes about 70 mm. Then, the high
frequency coil was moved at a speed of 100 mm per hour thereby
moving the melting section by about 900 mm. At this time, the
pressure in the chamber was from 5.times.10.sup.-6 to
9.times.10.sup.-6 Pa. The temperature of the melting section was
measured by a radiation thermometer. As a result, it was from
660.degree. C. to 800.degree. C.
[0123] Then, high frequency output was gradually decreased thereby
solidifying the melting section.
[0124] The high frequency coil was moved to the melting initiation
position (position where the melting section was formed first) and
the aluminum raw material was heated and melted again at the
melting initiation position to form a melting section while
maintaining vacuum inside the chamber. Zone melting purification
was repeated by moving this melting section. At the moment when
zone melting purification was carried out three times (3 passes) in
total at a melting width of about 70 mm and a traveling speed of
100 mm/hour of the melting section, the shape from the melting
initiation section to the completion section became almost uniform,
and uniform shape was maintained from then on (during 7 passes
mentioned below).
[0125] Then, zone melting purification was carried out 7 passes at
a melting width of about 50 mm and a traveling speed of 60 mm/hour
of the melting section. The melting width was from w.times.2.8 to
w.times.3.9 based on a cross sectional size w of the aluminum raw
material to be purified.
[0126] After completion of 10 passes in total, the chamber was
opened to atmospheric air and then aluminum was removed to obtain a
purified aluminum of about 950 mm in length.
[0127] The obtained aluminum was cut out and glow discharge mass
spectrometry component analysis was carried out in the same manner
as described above. The results are shown in Table 1.
TABLE-US-00001 TABLE 1 Unit: ppm by mass Comparative Comparative
Example 1 Example 2 Example 1 Example 1 Example 3 Li 0.016
<0.001 <0.001 <0.001 Be 0.042 <0.001 <0.001
<0.001 B 1.5 2.8 0.019 0.007 0.001 Na 1.4 0.012 0.001 0.001 Mg
5.2 0.1 0.48 0.10 0.001 Si 200 25 2.3 0.34 0.003 K <0.001 0.013
0.008 0.008 Ca 1.3 0.002 0.002 0.003 Ti 29 0.7 0.060 0.027 0.031 V
53 2.2 0.023 0.027 0.023 Cr 3.9 2.1 0.025 0.026 0.022 Mn 2.1 2.1
0.007 0.004 0.006 Fe 230 12 0.60 0.089 0.001 Ni 0.19 0.018 0.004
0.001 Co 13 0.3 <0.001 <0.001 <0.001 Cu 0.72 1 1.1 0.14
0.016 Zn 13 7 0.22 0.002 0.001 Ga 93 12 0.006 0.001 0.001 As 0.023
0.029 0.001 0.001 Zr 4.8 0.023 0.030 0.036 Mo 0.35 <0.001
<0.003 <0.004 Ag 1.1 <0.001 <0.001 <0.001 Cd
<0.001 0.002 0.002 0.002 In 0.009 <0.001 <0.001 <0.001
Sn 1.1 0.001 0.001 0.002 Sb 0.001 <0.001 <0.001 <0.001 Ba
0.004 <0.001 <0.001 <0.001 La 0.038 0.045 0.001 0.001 Ce
0.095 0.17 0.001 0.001 Pt <0.001 0.002 0.001 0.001 Hg <0.001
0.001 0.003 0.002 Pb 1.9 0.004 0.001 0.001 Bi <0.001 0.001 0.001
0.001 Total 669 67 <5.4 <8.3 <0.18
[0128] Then, the thus obtained high purity aluminum of Examples 1
to 3 were respectively cut to obtain materials for wire drawing
each measuring 6 mm in width.times.6 mm in thickness.times.100 mm
in length. In order to remove contamination elements due to cutting
of a surface of the material for wire drawing, acid pickling was
performed using an acid prepared at a ratio (hydrochloric acid:pure
water=1:1) for 1 hour, followed by washed with running water for
more than 30 minutes.
[0129] The obtained material for wire drawing was drawn to a
diameter of 0.5 mm by rolling using grooved rolls and wire drawing.
The specimen obtained by wire drawing was fixed to a quartz jig,
maintained in vacuum at 500.degree. C. for 3 hours and then
furnace-cooled to obtain a sample for the resistivity
measurement.
[0130] Furthermore, a commercially available high purity copper
having a purity of 5N level (manufactured by NewMet Koch, 99.999%
Cu, 0.5 mm in diameter) as the sample of Comparative Example 3 was
fixed to a quartz jig, washed with an organic solvent, maintained
in vacuum at 500.degree. C. for 3 hours and then furnace-cooled to
obtain a sample for the resistivity measurement.
(2) Derivation of Resistivity
Measurement of Resistivity
[0131] With respect to the samples of Examples 1 to 3 and
Comparative Example 3, the resistivity was actually measured.
[0132] After immersing the obtained sample in liquid helium (4.2
K), the resistivity was measured by varying the magnetic field to
be applied to the sample from a magnetic flux density 0 T (magnetic
field was not applied) to 15 T, using the four wire method.
[0133] The magnetic field was applied in a direction parallel to a
longitudinal direction of the sample.
Calculation of Resistivity
[0134] With respect to Comparative Example 1 and Comparative
Example 2 with the composition shown in Table 1, calculation was
performed using the following equation (4) disclosed in the
literature: R. J. Corruccini, NBS Technical Note, 218 (1964). In
the equation (4), .DELTA..rho..sub.H is an amount of an increase in
resistivity in the magnetic field. .rho..sub.RT is resistivity at
room temperature when the magnetic field is not applied, and was
set to 2,753 n.OMEGA.cm since it can be treated as a nearly given
value in high purity aluminum having a purity of 3N or more. .rho.
is resistivity at 4.2 K when the magnetic field is not applied and
largely varied depending on the purity. Therefore, the following
experimental values were used: 9.42 n.OMEGA.cm (RRR=285) in 4N--Al
and 117 n.OMEGA.cm (RRR=23) in 3N--Al. These equations are obtained
in case the magnetic field is perpendicular to a longitudinal
direction of the sample. However, since similar equations in case
the magnetic field is parallel to a longitudinal direction of the
sample are not obtained, these equations were used for comparison.
RRR is also called a residual resistivity ratio and is a ratio of
resistivity at 297 K to resistivity at a helium temperature (4.2
K).
.DELTA..rho. H .rho. = H * 2 ( 1 + 0.00177 H * ) ( 1.8 + 1.6 H * +
0.53 H * 2 ) ( 4 ) ##EQU00001##
where
H*=H/100.rho..sub.RT/.rho..sub.R
[0135] H=Intensity of applied magnetic field (Tesla)
[0136] .rho..sub.RT=Resistivity at room temperature when magnetic
field is not applied
[0137] .rho.=Resistivity when magnetic field is not applied
Citation from Literatures relating to Resistivity
[0138] With respect to Comparative Examples 4 to 6, the resistivity
was obtained from the literature: Fujiwara S. et. al., Int. Conf.
Process. Mater. Prop., 1st (1993), 909-912. In these literature
data, a relation between the application direction of the magnetic
field and the longitudinal direction of the sample is not
described.
[0139] The thus derived values of resistivity of Examples 1 to 3
and Comparative Examples 1 to 6 are shown in Table 2.
TABLE-US-00002 TABLE 2 Resistivity .rho. (n.OMEGA.cm) 0 T 1 T 2 T 3
T 4 T 6 T 8 T 10 T 12 T 15 T Example 3 0.333 0.260 0.261 0.246
0.253 0.260 0.249 0.254 0.268 0.286 Example 2 0.353 0.294 0.292
0.298 0.297 0.303 0.307 0.318 0.328 0.384 Example 1 0.72 1.04 1.02
1.05 1.02 1.02 1.06 1.06 1.05 1.06 Comparative 9.42 16.8 20.5 22.4
23.6 24.9 25.7 26.3 26.8 27.3 Example 2 Comparative 117 120 127 135
144 163 179 194 206 221 Example 1 Comparative 1.57 3.58 4.73 5.4
5.8 6.4 6.7 7.0 7.2 7.4 Example 3 Comparative 3 6.1 10 13 17 22 28
35 41 53 Example 6 Comparative 3.3 7.3 11 14 18 24 30.5 35 41 53
Example 5 Comparative 4.6 9 13 17 21 28 34 40 46 56.5 Example 4
[0140] As is apparent from Table 2, in the sample of Comparative
Example 2 corresponding to a thermal conductor made of a
conventional aluminum (4N level), the resistivity increases as the
intensity of the magnetic field (magnetic flux density) increases
as compared with the case where the magnetic field is absent (0 T),
and the resistivity increases by about 3 times at 15 T.
[0141] To the contrary, in Examples 1 to 3, the resistivity is
small such as a tenth or less as compared with Comparative Example
2 in a state where the magnetic field is absent, and also the
resistivity increase is slight even if the magnetic field
increases.
[0142] In Example 1 (5N level), the resistivity at 15 T slightly
increases (about 1.5 times) as compared with the case where the
magnetic field is absent, and it is apparent that the increase of
the resistivity caused by magnetic field is small compared with
Comparative Example 2.
[0143] In Example 2 (6N level), the resistivity slightly increases
(within 10%) even at 15 T as compared with the case where the
magnetic field is absent. When the magnetic flux density is within
a range from 1 to 12 T, the value of the resistivity decreased as
compared with the case where the magnetic field is not applied, and
thus remarkable magnetoresistance suppression effect is
exhibited.
[0144] As for Example 3 (6N8 level), the resistivity decreases as
compared with the case where the magnetic field is absent even at
any magnetic flux density of 1 to 15 T, and thus remarkable
magnetoresistance suppression effect is exhibited.
[0145] FIG. 1 is a graph showing a relation between the electrical
conductivity index and the applied magnetic field (magnetic flux
density). The electrical conductivity index is an index which
indicates the magnitude of the electrical conductivity of the
respective samples based on Comparative Example 2 which exhibits
the resistivity in a strong magnetic field of aluminum having a
purity of 4N. Namely, in each magnetic flux density the electrical
conductivity index is determined by dividing the value of the
resistivity of Comparative Example 2 with the value of the
resistivity of each sample. The larger the value of this index, the
superior the conductive properties under the magnetic flux density
is compared with the sample of Comparative Example 2.
[0146] The electrical conductivity index of the ordinate was
indicated by logarithm since samples of Examples exhibit extremely
remarkable effect.
[0147] As is apparent from FIG. 1, samples of Examples show the
conductivity is about 13 to 28 times higher than that of
Comparative Example 2 even in the case where the magnetic field is
absent. As the magnetic field is applied, the conductivity compared
with Comparative Example 2 increases. The conductivity is 16 times
(Example 1) to 65 times (Example 3) higher at 1 T, and the
conductivity further increases since it is 26 times (Example 1) to
96 times (Example 1) higher at 15 T.
[0148] As is apparent from FIG. 1, any of copper samples
(Comparative Examples 3 to 6) shows a right downward curve and, as
the intensity of the magnetic field increases, the
magnetoresistance effect increases as compared with Comparative
Example 2. Namely, it is found that, in case of copper, a decrease
in conductivity due to magnetoresistance cannot be suppressed even
if the purity is increased to 6N level (as is apparent from Table
1, in samples of Comparative Examples 3 to 6, the resistivity at 15
T increases by 5 to 18 times as compared with the resistivity in
case where the magnetic field is absent), and that the effect
capable of suppressing a decrease in conductivity in the magnetic
field by increasing the purity to 99.999% by mass or more, found by
the present inventors, is peculiar to aluminum.
[0149] The reason why, the magnetoresistance suppression effect by
highly purification is not exhibited in copper but is exhibited in
aluminum, is unclear. However, it is deduced that it is caused by a
difference in electrical resistivity factor. Namely, it is
considered that a main electrical resistivity of the high purity
copper is the scattering of conduction electrons due to grain
boundaries or dislocations, and the electrical resistivity factor
slightly varies even by highly purification, and thus
magnetoresistance also slightly varies. On the other hand, a main
electrical resistivity factor of the high purity aluminum is the
scattering of conduction electrons by impurity atoms, and the
electrical resistivity factor is decreased by highly purification.
Therefore, it is considered that excellent characteristics such as
little increase in electrical resistivity in the magnetic field may
be exhibited in aluminum having a purity of 5N or more. However,
this predictable mechanism does not restrict the scope of the
present invention.
[0150] Then, the thermal conductivity of each sample was calculated
from the results of Table 2.
[0151] The results of Table 2 and the results the residual
resistivity ratio RRR calculated from the above-mentioned equation
(3) are shown in Table 3. The value (i.e., resistivity at 4.2 K) in
Table 2 was used as .rho..sub.T of the equation (3). As mentioned
above, .rho..sub.297 K is scarcely influenced by the purity and the
magnetic field applied from the outside in copper and aluminum, and
is almost constant and can be treated as a given value in the high
purity metals. Therefore, 2,753 n.OMEGA.cm was used as
.rho..sub.297 K of aluminum and 1,500 n.OMEGA.cm was used as
.rho..sub.297 K of copper.
TABLE-US-00003 TABLE 3 RRR 0 T 1 T 2 T 3 T 4 T 6 T 8 T 10 T 12 T 15
T Example 3 8256 10582 10558 11208 10861 10606 11057 10828 10281
9631 Example 2 7795 9358 9439 9238 9272 9099 8980 8653 8386 7165
Example 1 3829 2642 2707 2630 2697 2688 2606 2609 2627 2592
Comparative 292 164 134 123 117 111 107 105 103 101 Example 2
Comparative 24 23 22 20 19 17 15 14 13 12 Example 1 Comparative 957
419 317 279 257 235 223 215 209 202 Example 3 Comparative 500 246
150 115 88 68 54 43 37 28 Example 6 Comparative 455 205 136 107 83
63 49 43 37 28 Example 5 Comparative 326 167 115 88 71 54 44 38 33
27 Example 4
[0152] Then, thermal conductivity was calculated using the value of
RRR in Table 3, and the equations (1) and (2).
[0153] FIG. 2 is a graph showing a relation between the thermal
conductivity and the applied magnetic field (magnetic flux
density).
[0154] As is apparent from FIG. 2, when the intensity of the strong
magnetic field in all Comparative Examples, including Comparative
Example 2 corresponding to a thermal conductor made of a
conventional aluminum (4N level) and Comparative Example 6
corresponding to a thermal conductor made of a conventional copper
(6N level), increases, thermal conductivity decreases. At the
magnetic flux density of 15 T, the thermal conductivity is only at
1,238 W/m/K even in case of Comparative Example 3 which exhibits
the highest thermal conductivity among Comparative Examples.
[0155] To the contrary, in Examples 1 to 3, a decrease in thermal
conductivity is suppressed even if the intensity of the magnetic
field increases.
[0156] In Example 1, the thermal conductivity is stable until 15 T
after decreasing at 1 T, and high thermal conductivity (about 9,500
W/m/K) is exhibited even at 15 T.
[0157] In Example 2, thermal conductivity increases in a range from
1 T to 12 T as compared with the case where the magnetic field is
not applied, and high thermal conductivity (about 25,000 W/m/K) is
exhibited even at 15 T.
[0158] In Example 3, thermal conductivity increases in a range from
1 T to 15 T as compared with the case where the magnetic field is
not applied, and very high thermal conductivity (about 33,000
W/m/K) is exhibited even at 15 T.
[0159] Using the thus obtained thermal conductivity, a temperature
difference, generated at both ends of the sample when one end of
the sample is connected to a refrigerator and a heat input is
applied to the other end, was calculated.
[0160] More specifically, a temperature difference, generated
between both ends when one end of a sheet-shaped thermal conductor
measuring 100 mm in width w, 400 mm in length L and 0.5 mm in
thickness is connected to a cooling stage of a refrigerator cooled
to about 4 K and a heat input Q of 2 W is applied to the other end
separated by 400 mm, was calculated.
[0161] The temperature difference .DELTA.T between both ends was
determined by the equation (5).
.DELTA.T=Q.times.(L/1,000)/(w/1,000)/(t/1,000)/.lamda. (5)
where
[0162] Q: Heat input (W)
[0163] L: Length of sheet-shaped sample (mm)
[0164] w: Width of sheet-shaped sample (mm)
[0165] t: Thickness of sheet-shaped sample (mm)
[0166] .lamda.: Thermal conductivity (W/m/K)
[0167] FIG. 3 is a graph showing a relation between the temperature
difference of both ends and the magnetic field (magnetic flux
density) of the thus obtained sheet-shaped sample. The temperature
difference of the ordinate was indicated by logarithm because of a
large difference between samples of Examples and sample of
Comparative Examples.
[0168] A temperature difference is scarcely recognized in Examples
1 to 3. .DELTA.T=1.7 K even at 15 T in Example 1, .DELTA.T=0.6 K in
Example 2, and .DELTA.T=0.5 K in Example 3.
[0169] To the contrary, in any of Comparative Examples, as the
intensity of the magnetic field increases, .DELTA.T also increases.
Also in Comparative Example 3 in which .DELTA.T at 15 T is the
smallest among Comparative Examples, .DELTA.T is 13 K. .DELTA.T of
Comparative Example 2 corresponding to a thermal conductor made of
a conventional aluminum (4N level) is 42 K.
[0170] Moreover, these values are values obtained without taking a
temperature dependence of the thermal conductivity .lamda. into
consideration, and .DELTA.T further increased in case of taking the
temperature dependence into consideration.
[0171] In such way, when using the thermal conductor according to
the present invention, which has high thermal conductivity even at
cryogenic temperature under a strong magnetic field and exhibits
excellent heat transfer properties, the cross section can be
decreased as compared with a conventional thermal conductor.
Therefore, miniaturization and weight saving of an apparatus
including a superconducting magnet can be performed.
[0172] According to the present invention, it is possible to
provide a thermal conductor having excellent heat transfer
properties by high thermal conductivity even at low temperature of,
for example, a liquid nitrogen temperature (77 K) or lower,
especially a cryogenic temperature of 20 K or lower in a strong
magnetic field of a magnetic flux density of 1 T or more.
[0173] The present application claims priority based on Japanese
Patent Application No. 2011-101767. The disclosure of Japanese
Patent Application No. 2011-101767 is incorporated by reference
herein.
* * * * *