GB2490423A - Thermal conductor for superconductor made from high purity aluminium alloy - Google Patents

Abstract

A thermal conductor made from an aluminium alloy which has a purity of at least 99.999 % by weight and an iron content of 1 ppm by weight or less. This thermal conductor can be used for cooling a superconducting magnet.

Description

SPECIFICATION

TITLE OF THE INVEt'TION

LOW TEMPERkIURE ThERNkL COIWUCTOR 3ACKGROU4D OF THE IN1NTION

Field of the Invention

[0001] The present invention relates to a thermal conductor which exhibits excellent conductivity at low temperature of, for example, 77 IC or lower, especially at cryogenic to 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.

Description of the Related Art

[0002] A superconducting magnet has been used in various fields, for example, MRIs (manetic 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 liid helium, and high-temperature superconducting coils cooled to about 20 K by a refrigerator.

There is a. need to use a thermal conductor which exhibits high thermal conductivity at low temperature of a

-

boiling point of lixid nitrogen (77) or lower, especially cryogenic temperatures of 20 K or lower, so as to cool these superconducting coils efficiently and uniformly.

[0003J S JP 2007-063671A discloses cold-worked aluminum, as a thermal conductor which exhibits high thermal conductivity at low temperature.

[00041 OP 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 "SN" (five nines) and, in the mass percentage is 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 "6W' (six nines)), which exhibits high heat transfer properties at cryogenic temperatures, or aluminum having a. purity of 99.99% by mass or more (4N).

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.

[0005] 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.

[0006J 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.

It is known that copper shows a remarkable nagnetoresistance effect and the electrical resistivity at low temperature remarkably increases with iDcreasing 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.

10007] In lots of metals, including copper, aluminum and alloys thereof, the electrical resistivity has a close relation with the thennal conductivity, and the thexmtal conductivity decreases when the electrical resistivity increases (conductivity decreases) -.

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-

StJtv1ARY OF THE flflJENT ION [0008] 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 liid nitrogen temperature (77 K) or lower, especially cryogenic temperatures of 20!( or lower in a strong magnetic field of a magnetic flux density of 1 T or more.

[0009] The present invention provides, in an aspect 1, a is thermal conductor to be used at low temperature(s) of 77 K or lower in the magnstic 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.

10010] The present inventors have found that even aluminum (Al) can remarkably suppress the magnetoresistance effect by controlling the purity to 99999 by mass or more and also controlling the content of iron to 1 ppm by mass or less.

The thermal conductor made of such aluminmn has high thermal conductivity and exhibits excellent heat transfer properties even when used at cryogenic temperatures of, for example, 77 IC or lower in a strong-magnetic field of a magnetic flux density of 1. T or more.

S [D01lJ 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.

EDO 12] 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.

[0013) The present invention provides, in an aspect 4, the thermal conductor according to any one of the aspects I to 3, wherein the aluminum contains an inter-metallic cornpoundAl3Fe.

[0014] The present invention provides, in an aspect S the thermal conductor for cooling a superconducting magnet, using the thermal conductor according to any one of the aspects 2. tO 4.

[00153 According to the present invention, it is possible to rovide 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 I T or more.

S BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Fig. 2. is a graph showing a relation between the conductivity index and the applied magnetic field (magnetic fluc density) -Fig. 2 is a graph showing a relation between the thermal conductivity and the applied magnet field (magnetic flux density).

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 0? THE IIVENTION

[0017] 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.metic_fietdofa magnetic flux density of 1 T or more.

The present inventors have fotrnd, 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 completeth [0018] As disclosed, for example, in JP 2309-242865A and JP 2009-242856A, it has beers known that the electrical resistivity at cryogenic temperatures1 for example, liquid helium temperatures decreases as the purity of aluminum increases, like SN (purity of 99.999% by mass or more) and GN (purity of 99.9999% by mass or more).

£0019] As disclosed, for example1 in JP 20l0l06329A, aluminum having a purity of 99.999% by mass or more azfl also having * 15 the content of iron of 1 ppm by mass or less has also been known.

[0020J * . It has been known that although aluminum enables an improvement in electrical conductivity at. cryogenic 20. teaperatures in a State where the magnetic field is not applied by increasing the purity to about 4W, 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 SN o EN purity, similarly to the aluminum of 4W purity.

Therefore, it is considered that aluminum having a purity of 92.299% by mass or more and also having the content of iron of 1 ppm by mass or less was not used in a thermal conductcr which is used in the magnetic field of a magnetic flax density of 1 T or more.

[0021] 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 conventicnally been conceived, does not occur in high purity aluminum of SN or higher level and also having the content of iron of 1 ppm by mass or less- 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 SN, EN 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.

[0022] 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.

As will be described below for details, the reason is considered as follows: the magnetoresistance effect is surely suppressed by controlling the amount *o-f 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.

[0023] The thermal conductor according to the present invention remarkably exerts the effect by use in a state where the temperature is 77 K (-196°C) or lower, and more preferably 20 K (-253°C) or lower, and also the magnetic field of a magnetic flux density of 1 T or more is applied.

[0024] 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.

(0025] -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 exthánced 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 tan-be obtar&wha1rae. alactrons are easily movable.

£0025] Wiedemann-Franz (NP') law has been known as a. relation between the thermal conductivity and the electrical donductivity of common metals. It has also been knotm that the thermal conductivity of about 40 K or lower of high purity aluminum can be determined from the following eiation (1) as a more accurate relational emiation of high purity * metals, and the thermal conductivity of about 40 K or lower * of high purity coper can be determined from the following e.iation (2) (both equations are cited from TEION KOGAPUJ, vol. 39 (2004) * -No. l,pp2522) [0027J c= 1/(1.83 x x + l.09/RRR/T.) (1) K = 11(6.41 x l0 x T24 + 0.685/RRR/T) (2) where K: Thermal conductivity (N/rn/K) T: Temperature (K) *RRR: Residual resistivity ratio (0028) The residual resistivity ratio RRR is represented by the following equation (3).

RRR P297x/PT (3)

-IC-where

P297:<: Resistivity at temperature of 297 K (nQcrn) PT: Resistivity at temperature T (K) (nQcm) [00293 S Herein) it has been kno that p297KO copper and P27K of aluminum are scarcely influenced by the purity and the magnetic field to be applied from the outside, and are almost constant (for example, P297K of aluminum is about 2,700 and P:ei of copper is about 1,500).

Therefore, as is apparent from the equations (2) to (3) * the thermal conductivity of copper and aluminum increases as * the Electrical conductivity is improved (as the electrical resistivity decreases).

* [0030] Details of the thermal conductor according to the present invention will be described below.

(1) Level of impurities 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 I ppm by mass or less. The purity is preferably 99.9999% by mass or more, and more preferably 99*. 99992% by mass or more (hereinafter sometimes referred to as t'6N8") for the following reasons. That is., the higher the *1, purity, the less the decrease in electrical conductivity under a strong magnetic fieldS Furthentore, in case of the purity of 99.9999% by mass or more, the electrical resistivity may sometimes decrease in a strong magnetic field of I T or more as compared with the case where the magnetic

field is not applied. toon]

The content of iron is preferably 0.1 ppm by mass or less.

The reason is that a decrease in condtctivity in a strong magnetic field can be suppressed more surely.

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 oredictale mechanism does not limit* the scope of the present invention.

[0032J -. 12 -Ni and Co are known as ferromagnetic elements other than iron lowever, since these elements are easily removed in a known process or highly purification of aluminum, the numerical value of the content is out of the question.

S However, the contents of these Ni and Co are also preferably 1 ppm or less, andmore preferably 0.1 ppm or less.

[0033] 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.

[00341 :erein, 33 elements contained as impurities are lithium (Li), berllium (Be), boron (3), sodium Na), magnesium (Mg), silicon (Si), potassium (K), calcium (Ca), titanium. (Ti), vanadium (V)1 chromium (Cr]1 manganese (Mn), iron (Fe), nickel (Ni), cobalt (Cc), copper (Cu), zinc (Zn), gallium (Ga), arsenic (As), zirconium (Zr), molybdenum (Mo), silver (Ag), cadmium (Cd), indium (In), tin (Sn), antimony (Sb), barium (3a), lantern (La), cerium (Ce), platinum (Pt), mercury (Hg), lead (Pb) and bismuth (Bi).

-13 -The contents of these elements can be determined, for example, by glow discharge mass spectrometry.

[0035) (2) Purification Method 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-100361 Three-layer Electrolysis Process It is possible to use, as one of methods of obtaining high purity aluminum1 a three-layer electrolysis process in which commercially available aluminum having comparatively is low purity (for example with special grade 1 of 99.9k 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 bariurn * fluoride therein is arranged thereon, and thus high purity aluminum is produced on a cathode.

In the three-layer electrolysis process, aluminum having a purity of 99.99fl by mass or more can be mainly obtained. It is possible to suppress the content of iron in aluminum to I ppm by mass or less, comparatively easily.

L00371 -14 -Unidirectional Solidification Process For example1 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.

The content of Fe and the respective contents of Ti, V, Cr and Zr can be selectively decreased by the unidirectional solidification process-03 8) It has been known that theunidirectional solidification process Is, for example, a method in which aluminum is melted in a. furnace tube using a furnace body moving tpe 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 maybe determined, which specific portion of the ingot obtained by the -15 -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 ccntents of Fe, and Ti, V, Cr and Zr S is sufficiently decreased, is allowed to remain.

[0039] H 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. Purificaticn 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.

In such way, aluminum having a purity of 99.999flby mass or more can be obtained by using the three-layer electrolysis process in combination with the unidirectional solidification process. It is alsopossible to suppress the content of iron in aluminum to 1 ppm by mass or less, and -16 -more preferably 01 ppm by mass or less in a comparatively easy manner.

[0040] Zone Melting Process Furthermore, a zone melting process can be used so as to obtain aluminum having high purity, for example, a purity of 92.9999fl 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 1ess and more preferably 0.1 ppm by mass or less, more surely.

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-.

[0041] 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 x iC Pa or less, and more preferably from 3 x l0 Pa to 2 10s Pa, so as to surely separate impurities from molten aluminum.

[0042] it is preferred to carry out a pretreatment1 in which a -17 -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 S treatments used in the relevant technical field can be used so as to remove the surface layer of the aluminum raw material -Examples of the pretreatment include an acid treatment, an electrolytic poflshing treatment and the like.

* 10 (0043] 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-[0044] The width of the melting section where aluminum is melted during the zone melting purification is preferably adjusted. to WA1 x.5 or mere and vi x.5 less based on a cross sectional size w1 of the aluminum raw material.

(00453 An aluminum raw material to be used in the purification is obtained by using the three-layer electrolysis process in corrthination with the unidirectional solidification process and, for example, high purity aluminum having a purity of 99.9999% by mass or more is preferably used.

-12 -[C 046] In the zone melting process, for example, the melting section is moved from one end of a raw aluminum toward the other end by rn&ring a. high fre.xency coil for high frequency S heating, and tinS 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 tote 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 -[0047] After moving the melting.section within a predetermined distance, like a. distance from oneS 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 thefl solidification, an aluminum material is cut out (for example, both ends are cut of f) to obtain a purified high purity aluminum materiafl [0048] When a plurality of aluminum raw materials are arranged in a longitudinal direction (in a movement direction of the -19 -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.

S moved from one end (i.e., one of two ends where adjacent aluminum raw materials are not present ma longitudinal direction among ends of the plurality of aluminum raw materials) to the other end (Le., 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).

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.

[0049] As mentioned above, after zone, melting (zone melting turification) 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 restriqtiye.

[0050] In order to effectively remove the peritectic 7 elements, the number of passes is preferably 3 or more, and -20 -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.

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 nuther 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 i less likely to be obtained.

[0051] (3) Forming Method The ingot of the high purity aluminum obtained by the above-mentioned purification method is formed into a desired shape using various methods.

The forming method will be shom. below. However, the forming method is not limited thereto..

[0032) Roiling When a thermal conductor to be obtained is a p1.te or a wire, rolling is an effective forming method.

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 -21 -these rolls while applying a pressure. There is no particular limitation on specific techniques and conditions (temperature of materials and rolls, treatment timer 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.

[0053] 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 0.1mm to 3 mm in case of the plate, or the diameter is from 0.1 mm to 3 mm in case of the wire..

When the thickness is less than 01 mm, sufficient conduction characteristics required as the thermal conductor is 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.

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.

[0054] -22 -The rolling may be hot rolling or warm rolling in which an ingot is heated in advance and then roiling 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.

[0055] 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 8009C, and heating and melting is usually performed it-i vacuum or an inert gas (nitrogen gas, argon gas, etc.) atmosphere in a crucible such as a graphite crucible.

tOOSG] Forming Method other than Rolling 7ire rawing 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 -23 -cross section.

[0057] A desired wire shape may be obtained by rolling before drawing to obtain a rolled wire (rolled wire rod) and.then s drawing the rolled wire.

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.

[00583 The desired shape may also be obtained by cutting the ingot, except for drawing or extrusion.

(4) nealing Furthermore, the formed article of the present invention obtained by the above forming method such as rolling may be ottionally 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 -There is no particular limitation on the conditions of the annealing treatment, and a method of maintaining t 400 to 600°C for one or more hours. is preferable..

When the temperature is lower than 400C, strain (dislocation) included in the ingot is not sufficiently decreased for the following reason. Since strain -24 - (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°C1 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.

[0 05 9) More preferably, the temperature is maintaIned at 430 to 550°C for one or more hours for the following reason.

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.

[0050] The following reasons are also exemplified.

* As an intennetallic compctud of iron and aluminum, for example, a plurality of kinds such as A15Fe, Al3Fe and AlmFe (m 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 intennetallic compound of iron. F arid, aluminum, which exists in. a high purity aluminum material * obtained after annealing within a temperature. range (430 to 550C), is A13Fe. . F -25 -This Al3 Fe has such an advantage that it scarcely exerts an adverse influence on the conductivity even in case of existing as a precipitate.

Existence of Al3 Fe and the volume ratio thereof can be S 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 TEN) and further analysis.

[OOG1] The thermal conductor according to the present invent ion 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.

While a thermal conductor for cooling a superconducting magnet is illustrated as.specific applications of the thermal conductor according to the present invention, the stecific 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 NM?..

-26 -

Examples

[0062] Example. 1 (purity o± 99. 999% by mass or more, SN-Al), Example 2 (purity of 999999% by mass or more, EN-Al) and Example 3 (purity of 9?.9999fl by mass or more, ENS-Al), details of which are shown below, were produced as example samples, and then. resistivity (specific electrical resistivity) was measured.

Comparative Example I (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.

[0063] For comparison with copper a sample of capper having a purity of 5N level was prepared and then the resistivity was measured as Comparative Example 3.

[0064] 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 SN level, and Comparative Example 6 is copper sample having a purity of EN level.

[00651 (1) Production of High Purity Aluminum -27 -First, the method for producing a high purity aluminum used in Examples.1. to.3..ts_sbobaLow._

Example 1

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.

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%A1F3- 35%BaF2 -14%CaF2 -10%NaF. Electricity was supplied at 760°C and a high purity aluminum deposited at a cathode side was collected.

* 15 The contents of the respective elements in this high * purity aluminum were analyzed by glow discharge mass spectrometry (using "VG9000", manufactured by THERMO EL3CTRON Co., Ltd) to obtain the results shotc in Table l [0066]

Example 2

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 apurity99.9999% by mass or more and an iron content of 1 ppm by mass or less.

-28 -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 x 400 nra in length mm in height) and the crucible, was accommodated inside a S furnace tube (made of quartz, 100 mm in inside diameter 1,000 mm in length) of a furnace body transfer type tubular furnace. The high purity aluminum was melted by controlling a furnace body (oncible) to 700°C in a vacuum atmosphere of 1 x 102 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 x 100 mm in length.x 30 mm in thickness was obtained.

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 shovm in Table 1.

[0067d Zxample 3 A high purity aluminum having a purity of 99.9999fl by mass or more and the iron content of 0.1 ppm or less was 23 obtained by the following zone melting process.

-29 -After cutting into a quadran9ular prism measuring about lB mm x 3 mm 100 mm or a similar shape from the EN aluminum ingot obtained by the above-mentioned unidirectional * solidification process, and further acid pickling with an * 5 aqueous 20 hydrochloric acid solution prepared by diluting with pure water for 3 hours, an aluminum raw material was obtained -[0068] Using this aluminum raw material, a zone melting process was carried out by the following method * A graphite boat was placed inside a vacuum chamber (a * quartz tiie 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 alumjna powder AK? Series (purity: 999fl) 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. 0069]

* 20 The graphite boat was baked by high frequency heating under vacuum.

The baking was carried out by heating' in vacuum of 10 to 10' Pa using a high frequencyheating 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 eid of the boat at a speed of 100 mm/hour thereby sequentially heating the entire graphite boat.

[0070) The above-mentioned 9 aluminum raw materials in total weight of about 780 g were arranged on the portion (measuring mm x 20 mm 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 j,0 materials = 18 mm, length L 900 mm, i.e. L = w x 50) £0071] After sealing inside a chamber, evacuation was carried out by a turbo-molecular pump and an oil sealed rotary tump until the pressure reaches 1 io Pa or less. Then, one lS end of the aluminum raw material in a. longitudinal direction was heated and melted using a high frequency heating coil (high frequency coil) to fat-rn a melting secti9n.

* The output of the hifl frequency power source (frequency: 100)cHz, maximum output: 5 kW) was adjusted so that the melting width of the melting section becomes about mm. Then, the high frequency coil was moved at a speed of mm per hour thereby moving the melting section by about 900 mm. At this time, the pressure in the chamber was from 5 x io to 9 x l0 Pa. The temperature of the melting section was measured by a radiation thermometer. As a result, -31 -it was from 0°C to 800°C.

10072] Then, high frequency output was gradually decreased thereby solidifying the melting section.

S 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).

[0073] 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 x 2.8 to w. x 3.9 based on a cross sectional size w of the aluminum raw material to be purified.

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.

The obtained aluminum was out out and glow discharge mass spectrometry component analysis was carried out in the s same manner as described above. The results are shown in

Table 1.

-33 -10075)

Table I

_____ -Unit:ppmbyrnass Comparative Comparative Example 1 Example r Example 3 ________ Example I Exarp_ ______________________ Li. 0.016. <0.001 <0.001 <(1001 Be 0.042 --<0.001 <0.001 <0.001 -B 1.5 2.8 0.019 0.007._0.00I Na 1.4 JTo.o12 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 21 21 0007 0004 0006 - ________ 230 12 0.60 0.089 0.001 Ni 0.19 ____________ 0.018 0.004 0.001 Co -13 03 <0001 <0001 <0001 Cu 0.72 1 1.1 0.14 -0.016 Zn. 13 2 0.22 0.002 0.001 Ga 93 12 0:006 0.00 0.001 As. . 0.023 -, 0.029 0.001 I (1001 Zr -4.8 ______________. 0.023 0.030 J 0.036 Mo 0.35.. _____________ <0.00 1 <0.003 j <0.004 ________ Li _____________ <0.001 <0.001 <0.001 Cd -0.001 ______________ 0.002 0.002 1 0,002 In 0009 ___________ <0001 <0001 <0001 -Sn 11 -0001 0001 -0002 Sb. <0.001. -<0.001 cO.001 <0.001 Ba 0.004 ______________ Ro.ooi <0.001.<0.00*I La 0.038 _____________ 0.045 0.001 0.001 Ce 0095 _____________ 0.17 0.001. 0.001 Pt. <0.001 _____________ 0.002 -0.001 -0.001 Hg <0001 __________ 0001 0003 0002 ____.9. ________ 0P94 0.001 0.001 Bi <0.001.J -. 0.001 0.001 0.0 1 Total 669 67 <5.4 <8.3 1 <0.18 [0 076] Then, the thus obtained high purity aluminum of Examples I to 3 were respectively cut to obtain materials for wire drawing each measuring 5 mm in width x 6 mm in thickness 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.

[0077] The obtained material for wire drawing was dratm to a diameter of 0.5 mm by rolling using grooved rolls and wire drawing. The specimen obtained by wire drawing was fixed to IS a artz jig, maintained in vacuum at 500°C for 3 hours and then furnace-cooled to obtain a sample for the resistivity measurement - * Furthermore, a commercially available high purity * copper having a purity of 5N level (manufactured by NewNet Koch, 99.99fl Cu, 0_s mm in diameter) as the sample of Comparative Example 3 was fixed to a quattz jig, washed with an organic solvent, maintained in vacuum at 500°C for 3 hours and then furnace-cooled to obtain a sample for the resistivity measurement.

[0078] (2) Derivation of Resistivity Measurement of Resistivity With respect to the samples of Examples 1 to 3 and Comparative Example 3, the resistivity was actually measured.

After immersing the obtained sample in liquid helium (&2 K), the resistivity was measured by varing 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.

The magnetic field was apDlied in a direction parallel to a longitudinal direction of the sample.

(00 79 Calculation of Resistivity With respect to Comparative xample I 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. Corrucc5.ni, NBS Technical Note, 218 (1954). in the equation (4), apt. is an amount of an increase in resistivity in the magnetic field. PRT is resistivity at room temperature when the magnetic field is not applied, and was set to 2,753 n«=cm since it can be treated as a nearly H given value in high purity. aluminum having a purity of 3M or more. p is resistivity at 4.2 IC when the magnetic field is not applied and largely varied depending on the purity.

Therefore, the following experimental values were used; 9.42

--

n.Qcn (PSR = 265) in 4N-Al and 117 nQcn (RtR = 23) in 3N-Al.

These equations are obtained in case the ma9fletic field is perpendicular to a longitudinal direction of the sample.

However, since similar equations in case the magnetic field s 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 oi resistivity at 297 K to resistivity at a helium temperature (4.2 IC).

[0080) ___ -H(1+0.0O177H1) 4 p (l.8÷I.6H+0.53H$) ( where It. = H/lao PRT/P

H = Intensity of applied magrtetic.field (Tesla)

PAT = Resistivity at room temperature when magnetic

field is not applied

p = Resistivity when magnetic field is not applied

[0081] Citation from Literatures relating to esistivity cith respect to Comparative xamples 4 to 6, the resistivity was obtained from the literature: Fujiwara S. at.

al., tnt. Conf. Process. Mater-Prop., 1st (l993) 909-912.

In these literature data, a relation between the application -37 -direction of the magnetic field and the longitudinal direction of the sample is not described.

The thus derived values of resistivity of Examples 1 to 3 and Comparative Examples 1 to 6 are sho.m in Table 2.

t0082J

Table 2

_____ _____ _____ Resisthtyp(nflcm) -_____ _____ _______ UT iT 21 ST 41 6T ST J lOT 12T 1ST Example 3 0.333 0.260 0.261 0.246 0.253 0.260 0.249 0.254 0.268 0.286 Example 2 0.294 0.292 0.298 0.297 0.303 0.318 0.328 0.384 Example 0.72 1.04 1 LOS 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 26.2 27.3 Example 2 _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ Comparative 117 120 127 135 144 163 179 194 206 221 Exp1ei _____ _____..,.... -Comparative 1.57 3.58 4.73 5.4 5.8 6.4 6.7 7.0 72 7.4 Example3 ____ ____ ____ ____ ____ -____ ____ ____ Comparative 3 6.1 10 13 17 22 28 35 41 53 Example6 ____. ____ ____ ____ ____ ____ Comparative 3.3 7.3 14 18 24 303 35 41 53 Example _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ Comparative 4.6 9 13 17. 21 28 34 40 46 56.5 Example 4 _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ [0083J 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..

-38 -10084) 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.

In Example 1 (SN 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.

In. Example 2 (EN level), the resistivity slightly increases (within iDE even at IS T as compared with the case where the magnetic field is absent-When the magnetic flux deneity is within a. range from I 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.

* As for Example 3 (ENS level), the resistivitydecreases * 20 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.

[0085] rig. I is a graph showing a relation between the electrical conductivity index and the applied magnetic field -39 - (magnetic flux density). The electrical conductivity index is an index which indicates the magnitude of the electrical condtctivity 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 deteined 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.

The electrical conductivity index of the ordinate was indicated by logarithm since samples of Examples exhibit extremely remarkable effect.

[0086J 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 tithes (Exau1e 3) higher at I T, and the conductivity further increases since it is 26 times (Example 1) tO 96 times (Example 1) higher at 15 T. [0087] -40 -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 S Comparative Example 2. Namely, it is found that, in case of copper, a decrease in conductivity due to magnetoresistance cannot be sutpressed even if the.purity is increased to SN level (as is avparent from Table 1, in samples of Comparative Examples 3 to 6, the resistivity at 15 T increases by S to 13 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 ?9.99fl by mass or more, found by the present inventors! is peculiar to aluminum.

[0088] The reason why, the magnetoresistance suppression effect by highly purification is not exhibited in copper but is exhibited in aluminum, is unclear. flowever, 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 -41 -factor of the high purity aluminum is the scattering of conduction -el-ectrons by impurity -azoms,-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 SN or more. However, this predictable mechanism does not restrict the scope of the present invention.

[0039] Then, the thermal conductivity of each sample was calcUlated from the results of Table 2 The results of Table 2 and the results the residual resistivity ratio P.RR calculated from the above-mentioned equation (3) are shown in Table 3-The value (i.e-, IS resistivity at 4-2 K) in Table 2 was used as Pr of the equation (3). As mentioned above, P297K is scarcely influenced by the purity and the magnetic field applied from the outside in copper and aluminum, and is almost constant nd can be treated as a given value in the high purity metals.

Therefore, 2,153 nOcm was used as P297K of aluminum and 1,500 nflcm was used as P297K of copper.

[00903

Table 3

_______RRR________

-.OT -IT 2tLji_ 4T 6T ST I.P2_LJT 1ST 8256 10582 10558 111208 10861 10606 11057 [10828 f_10281 9531 Exainpie2 7795 9358 9439. 9238 9272 9099 8980 8553 8385 7165 Example I 3829 2642 2707 2630 2697 2688.2606 2609 2627 2592 Comparafive 292 123 117 111 107.105 103 101 Example 2 -_____ _____ _____ _____ _____ _____ _____ ______ _____ Comparative 24 23 22 20_ 19 17 15 14 13 12 Comparatiw 957 419 317 279 257 235 223 215 209 202 Example i _____ ______ ______ ______ ______ ______ ______ ______ ______ _____ Comparative 500 246 150 115 88 68 -54 43 37 28 Comparative 455 205 136 107 83 63 49 43 37 28 Example 5 ____ _____ _____ _____ __________ _____ _____ _____ _____ Comparative 326 167 115 88 71 54 * 38 33 [0091] Then, thermal conductivity was calculated using the talue of RRR in Table 3, and the equations (1) and (2) Fig. 2 is a graph showing a relation between the.

thermal conductivity and the applied magnetic field (magnetic flux density).

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 condtctor made of a conventional copper (6W level), increases thermal IS conductivity decreases. At the magnetic flux density of 15 1', -43 -the thermal conductivity is only at 1,238 WIn/K even in case of Comparative Example 3 which exhibits the highest thermal conductivity among Comparative Examples.

-[0092] S Tc the contrary, in Examples 1 to 3, a decrease in thermal conductivity is suppressed even if the intensity of

the magnetic field increases.

in Example 1, the thermal conductivity is stable until T after decreasing at 1 T, and high thermal conductivity (about 9,500 WIn/K) is exhibited even at 15 T. In Example 2, thermal conductivity increases in a range from 3. 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. 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 applied2 and very high thermal conductivity * (about 33,000 W/m/IC) is exhibited even at 15 T. [0093] Using the thus obtained thermal conductivity, a * temperatlfl'e difference1 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.

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 coflnected 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.

S The temperature difference AT between both ends was determined by the euat ion (5) f 0094] AT = Q x (L/1,000)J(w/l,000)/(t/l,000)/A (5) where Q: Heat input (N) L: Length of sheet-shaped sample (turn) W: Width of sheet-shaped sample (mm) t: Thickness of sheet-shaped sample (mm) A: Thermal conductivity (W/m/K) [0095] 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.

A temperature difference is scarcely recognized in Examples 1 to 3. AT 1.7 K even at 15 T in Example 1, AT = 0.6 K in. Example 2 and AT = 0.5 K in Example 3.

(0096] -45 -To the contrary, in any of Comparative Examples, as the intensity of the magnetic field increases, AT also increases.

Also in Comparative Example 3 in which AT at 15 T is the smallest among Comparative Examples, AT is 13 K. AT of S Comparative Example 2 corresvonding to a thermal conductor made of a conventional aluminum (4N level) is 42 K. Moreover1 these values are values obtained without taking a temperature dependence of the thermal conductivity A into consideration, and AT further increased in case of taking the temperature dependence into consideration.

C0097] 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. C09S]

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 IC -4E-or lower in a strong maetic field of a. nagnetic flux density of 1 T o more...

Claims (7)

1. CLAIMS: 1. A thermal conductor to be used at a temperature of 77 K or lower in a 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. 2. The thermal conductor according to claim 1, wherein the aluminum has a purity of 99.9999% by mass or more.
3. 3. The thermal conductor according to claim 1, wherein the aluminum has a purity of 99.99998% by mass or more.
4. 4. The thermal conductor according to any one of claims 1 to 3, wherein the aluminum contains an intermetallic compound Al3 Fe.
5. 5. A thermal conductor according to claim 1 which is substantially as hereinbefore described with reference to any one of Examples 1 to 3.
6. 6. Apparatus for cooling a superconducting magnet, comprising the thermal conductor according to any one of claims 1 to 5.-48 -
7. 7. Use of aluminum having a purity of 99.999% by mass or more and having a content of iron of lppm by mass or less in a thermal conductor for cooling a superconducting magnet.-49 -