US20120273181A1 - Low temperature thermal conductor - Google Patents

Low temperature thermal conductor Download PDF

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US20120273181A1
US20120273181A1 US13/458,080 US201213458080A US2012273181A1 US 20120273181 A1 US20120273181 A1 US 20120273181A1 US 201213458080 A US201213458080 A US 201213458080A US 2012273181 A1 US2012273181 A1 US 2012273181A1
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aluminum
purity
magnetic field
mass
thermal conductor
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Takayuki Tomaru
Kenichi Sasaki
Hiroaki Hoshikawa
Hiroshi Tabuchi
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Sumitomo Chemical Co Ltd
Inter University Research Institute Corp High Energy Accelerator Research Organization
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Sumitomo Chemical Co Ltd
Inter University Research Institute Corp High Energy Accelerator Research Organization
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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22F—CHANGING THE PHYSICAL STRUCTURE OF NON-FERROUS METALS AND NON-FERROUS ALLOYS
    • C22F1/00—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working
    • C22F1/04—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working of aluminium or alloys based thereon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C1/00—Making non-ferrous alloys
    • C22C1/02—Making non-ferrous alloys by melting
    • C22C1/026—Alloys based on aluminium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C21/00—Alloys based on aluminium
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01F—MAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
    • H01F6/00—Superconducting magnets; Superconducting coils
    • H01F6/04—Cooling

Definitions

  • 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.
  • 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.
  • MRIs magnetic resonance imaging
  • NMRs nuclear magnetic resonance
  • 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.
  • 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.
  • JP 2007-063671A discloses cold-worked aluminum, as a thermal conductor which exhibits high thermal conductivity at low temperature.
  • 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).
  • a thermal conductor thermal bus bar
  • thermal conductor using copper such as oxygen-free copper having a purity of 99.99% by mass or more (4N), in addition to aluminum.
  • 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.
  • the electrical resistivity has a close relation with the thermal conductivity, and the thermal conductivity decreases when the electrical resistivity increases (conductivity decreases).
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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 3 Fe.
  • 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.
  • thermo 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.
  • FIG. 1 is a graph showing a relation between the conductivity index and the applied magnetic field (magnetic flux 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).
  • 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.
  • 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.
  • 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.
  • the amount of iron contained in aluminum is controlled to 1 ppm by mass or less.
  • 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.
  • 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.
  • thermal conductor using a material having excellent electrical conductivity has high thermal conductivity.
  • 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).
  • the residual resistivity ratio RRR is represented by the following equation (3).
  • ⁇ 297 K of copper and ⁇ 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, ⁇ 297 K of aluminum is about 2,700 and ⁇ 297 K of copper is about 1,500).
  • 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.
  • 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.
  • the content of iron is preferably 0.1 ppm by mass or less.
  • the reason is that a decrease in conductivity in a strong magnetic field can be suppressed more surely.
  • 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.
  • 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.
  • 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).
  • the contents of these elements can be determined, for example, by glow discharge mass spectrometry.
  • 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.
  • 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.
  • 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.
  • purification by the three-layer electrolysis process and purification by the unidirectional solidification process 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.
  • 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.
  • 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.
  • 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.
  • 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 ⁇ 10 ⁇ 5 Pa or less, and more preferably from 3 ⁇ 10 ⁇ 6 Pa to 2 ⁇ 10 ⁇ 5 Pa, so as to surely separate impurities from molten aluminum.
  • 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.
  • a pretreatment method 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.
  • Examples of the pretreatment include an acid treatment, an electrolytic polishing treatment and the like.
  • 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.
  • the width of the melting section where aluminum is melted during the zone melting purification is preferably adjusted to w Al ⁇ 1.5 or more and w Al ⁇ 6 or less based on a cross sectional size w Al of the aluminum raw material.
  • 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.
  • 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.
  • 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.
  • 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).
  • zone melting zone melting purification
  • zone melting can be repeated again from one end to the other end.
  • the number of repeat times 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.
  • the number of passes is preferably 3 or more, and more preferably 5 or more.
  • 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.
  • 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.
  • 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 shown below. However, the forming method is not limited thereto.
  • a thermal conductor to be obtained is a plate 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 these rolls while applying a pressure.
  • 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.
  • specific techniques and conditions temperature of materials and rolls, treatment time, reduction ratio, etc.
  • these specific techniques and conditions may be appropriately set unless the effects of the present invention are impaired.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • hot rolling or warm rolling may be used in combination with cold rolling.
  • 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.
  • the heating temperature is usually from 700 to 800° C.
  • 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.
  • 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.
  • 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.
  • 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.
  • the desired shape may also be obtained by cutting the ingot, except for drawing or extrusion.
  • 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.
  • 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.
  • the heat treatment temperature is higher than 600° 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.
  • the temperature is maintained at 430 to 550° C. for one or more hours for the following reason.
  • strain can be sufficiently removed and also iron exists as an intermetallic compound with aluminum without being solid-soluted into the matrix.
  • an intermetallic compound of iron and aluminum for example, a plurality of kinds such as Al 6 Fe, Al 3 Fe and Al m Fe (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 intermetallic compound of iron and aluminum, which exists in a high purity aluminum material obtained after annealing within a temperature range (430 to 550° C.), is Al 3 Fe.
  • This Al 3 Fe has such an advantage that it scarcely exerts an adverse influence on the conductivity even in case of existing as a precipitate.
  • 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.
  • 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.
  • 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.
  • Comparative Example 1 (4N—Al) as aluminum having a purity of 4N level
  • 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.
  • Comparative Example 4 is copper sample having a purity of 4N level
  • Comparative Example 5 is copper sample having a purity of 5N level
  • Comparative Example 6 is copper sample having a purity of 6N level.
  • 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.
  • 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.
  • 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.
  • 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.
  • the graphite boat was baked by high frequency heating under vacuum.
  • the baking was carried out by heating in vacuum of 10 ⁇ 5 to 10 ⁇ 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.
  • a high frequency heating coil heating coil winding number: 3, 70 mm in inside diameter, frequency of about 100 kHz
  • the above-mentioned 9 aluminum raw materials in total weight of about 780 g were arranged on the portion (measuring 20 mm ⁇ 20 mm ⁇ 1,000 mm), where the raw materials are placed, provided in the graphite boat.
  • 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 ⁇ 10 ⁇ 6 to 9 ⁇ 10 ⁇ 6 Pa. The temperature of the melting section was measured by a radiation thermometer. As a result, it was from 660° C. to 800° C.
  • 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.
  • 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).
  • 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 ⁇ 2.8 to w ⁇ 3.9 based on a cross sectional size w of the aluminum raw material to be purified.
  • the chamber was opened to atmospheric air and then aluminum was removed to obtain a purified aluminum of about 950 mm in length.
  • 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
  • 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° C. for 3 hours and then furnace-cooled to obtain a sample for the resistivity measurement.
  • 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.
  • the magnetic field was applied in a direction parallel to a longitudinal direction of the sample.
  • ⁇ H is an amount of an increase in resistivity in the magnetic field.
  • ⁇ RT 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 given value in high purity aluminum having a purity of 3N or more.
  • ⁇ is resistivity at 4.2 K when the magnetic field is not applied and largely varied depending on the purity.
  • 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).
  • ⁇ RT Resistivity at room temperature when magnetic field is not applied
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • any of copper samples 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.
  • Table 3 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 ⁇ T of the equation (3).
  • ⁇ 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 ⁇ cm was used as ⁇ 297 K of aluminum and 1,500 n ⁇ cm was used as ⁇ 297 K of copper.
  • thermal conductivity was calculated using the value 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).
  • 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.
  • 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.
  • 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.
  • the temperature difference ⁇ T between both ends was determined by the equation (5).
  • ⁇ T Q ⁇ ( L/ 1,000)/( w/ 1,000)/( t/ 1,000)/ ⁇ (5)
  • 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.
  • these values are values obtained without taking a temperature dependence of the thermal conductivity ⁇ into consideration, and ⁇ T further increased in case of taking the temperature dependence into consideration.
  • 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.
  • thermo 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.

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JP7209492B2 (ja) * 2018-08-17 2023-01-20 住友化学株式会社 アルミニウムクラッド材及びその製造方法

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