JP5478899B2 - High purity aluminum material - Google Patents

Description

  The present invention relates to a high-purity aluminum material that is useful as a material that requires low electrical resistivity at, for example, an extremely low temperature of 50K or less.

  For example, members such as superconducting magnets, cryopumps, and cryogenic refrigerators used in medical MRI (magnetic resonance imaging apparatus) and NMR (nuclear magnetic resonance analyzer) for analysis are usually 50K or less. Used at cryogenic temperatures. For this reason, the constituent materials of these members are required to have a characteristic of exhibiting excellent electrical conductivity in a cryogenic atmosphere. As such a material, conventionally, high-purity aluminum having a low electrical resistivity has been used (Patent Document 1).

JP 2007-066361 A

  However, even if the member is made of high-purity aluminum as in the past, when the shape thereof is a thin plate or foil material, the electrical conductivity inherent to the material (ie, aluminum) is sufficiently expressed. Therefore, there is a problem that the electrical resistivity at extremely low temperature does not reach a satisfactory level.

  Therefore, an object of the present invention is to provide a high-purity aluminum material that can exhibit a low electrical resistivity even at a very low temperature, even if it is a thin plate or a foil material.

  The present inventors have intensively studied to solve the above problems. As a result, when the shape of the member is a thin plate, a foil material, or the like, it has been found that the fine irregularities on the surface of the member act as electrical resistance, which causes a decrease in electrical conductivity. And based on this knowledge, by reducing the amount of protrusions on the surface of the member by surface polishing treatment, it was found that the original electrical conductivity of aluminum can be exhibited well, and improved electrical conductivity at cryogenic temperatures It has also been found that the higher the purity of aluminum, the better. By the way, in the above examination, it has been found that the above-mentioned adverse effects caused by unevenness on the surface of the member are more prominent as the purity of aluminum is higher and the thickness of the member is thinner. In view of this point, it has been found that it is very effective to subject the unevenness on the surface of the member to a surface polishing treatment to obtain a low electrical resistivity at an extremely low temperature. The present invention has been completed based on these findings.

That is, the present invention has the following configuration.
(1) A high-purity aluminum material comprising a rolled material of high-purity aluminum having a purity of 99.9999% by mass or more, wherein the surface of the rolled material is subjected to a surface polishing treatment.
(2) The surface of the rolled material has the following formula: surface roughness (μm) = volume of uneven portions on the surface of the measurement part (μm ³ ) / measurement area (μm ² )
The high-purity aluminum material according to the above (1), wherein the surface roughness required by 1 is less than 3 μm.
(3) The high-purity aluminum material according to (1) or (2), wherein the rolled material has a thickness of 1.5 mm or less.
(4) The high-purity aluminum material according to any one of (1) to (3), which is covered with an oxide film having a thickness of 0.2 to 2 μm.
(5) The high-purity aluminum material according to any one of (1) to (4), which is subjected to an annealing treatment that is held at 400 to 600 ° C. for 1 hour or more.

  According to the present invention, even when the shape of the member is a thin plate, a foil material, or the like, a low electrical resistivity can be exhibited at an extremely low temperature of, for example, 50K or less.

  The high-purity aluminum material of the present invention is a rolled material of high-purity aluminum having a purity of 99.9999% by mass or more. The upper limit of the purity of the high-purity aluminum is not particularly limited, but is usually less than 99.99999% by mass. The purity of such high-purity aluminum may be determined by measuring the Al content, but Fe, Ti, V, Cr, Zr, Si, Cu and Mg (hereinafter collectively referred to as “8 elements”) It is also possible to obtain the content of each element by subtracting the total amount from 100%. That is, generally, elements that can be contained as impurities in high-purity aluminum include Si, Cu, Mg, Fe, Ti, V, Cr, Zr, Li, Be, B, Na, K, Ca, Mn, Ni, Co Zn, Ga, As, Mo, Ag, Cd, In, Sn, Sb, Ba, La, Ce, Pt, Hg, Pb, Bi, Th, and U (hereinafter collectively referred to as “35 elements”) In addition, these elements may contain unavoidable impurities other than these 35 elements, but among these, elements other than the above 8 elements are usually very small even if contained. As described above, there is almost no error even if the purity is calculated assuming that the content of elements other than the above eight elements is zero.

  Among impurities that can be contained in the high-purity aluminum, the content of each element of Si, Cu and Mg is preferably 0.5 mass ppm or less, respectively, while each of Fe, Ti, V, Cr and Zr The element content is preferably 0.1 mass ppm or less. Thereby, a lower electrical resistivity can be obtained at an extremely low temperature. The high-purity aluminum preferably has a total content of 35 elements of 1 mass ppm or less for the same reason.

  Such high-purity aluminum can be obtained by refining ordinary aluminum having a relatively low purity (for example, a grade of about 1 type of JIS-H2102 having a purity of 99.9% by mass). The purification method is not particularly limited, but preferably, the high-purity aluminum is obtained through both purification by a three-layer electrolytic method and purification by a unidirectional solidification method. The order of performing the purification by the three-layer electrolysis method and the purification by the unidirectional solidification method is not particularly limited, but usually the purification is first performed by the three-layer electrolysis method and then purified by the unidirectional solidification method. Further, the purification by the three-layer electrolysis method and the purification by the unidirectional solidification method may be repeated alternately, for example, or either one or both may be repeated. Is preferably repeated. As specific methods and conditions for purification by the three-layer electrolysis method and purification by the unidirectional solidification method, methods, conditions, and the like usually used in this field may be adopted as appropriate.

The rolled material is obtained by rolling the high-purity aluminum ingot. The rolling process may be performed by adopting a normal method such as a method of passing the ingot between these rolls while applying pressure by sandwiching the ingot between a pair of rolls. Specific methods and conditions (processing temperature, processing time, processing rate, etc.) when performing the rolling process are not particularly limited, and may be appropriately set within a range not impairing the effects of the present invention.
The rolled material preferably has a thickness of 1.5 mm or less, more preferably 1.0 mm or less, in order to remarkably develop the effect of reducing electrical resistance by surface polishing treatment. That is, when the rolled material is a thin plate or foil material having such a small thickness, the effect of improving the electrical conductivity by the surface polishing treatment at a very low temperature becomes more remarkable. In addition, when a rolled material having a small thickness is used as a heat transfer material, there is an advantage that it is difficult to transmit vibration and has an anti-vibration effect. In addition, the thickness of the rolling material (namely, 1.5 mm or less) said here does not include the thickness of the oxide film mentioned later.

In addition, when rolling the high-purity aluminum, it is possible to perform a treatment such as casting into a desired shape and cutting in advance. For casting, for example, a normal method may be employed in which high-purity aluminum is heated and melted to form a molten metal, and the obtained high-purity aluminum molten metal is cooled and solidified in a mold, but is not limited thereto. is not. The conditions for casting are not particularly limited, but the heating temperature is usually 700 to 800 ° C., and the heating and melting is usually performed under reduced pressure or under an inert gas (nitrogen gas, argon gas, etc.) atmosphere made of graphite or the like. Performed in a crucible. When heat-melting is performed under reduced pressure, it is usually 1 × 10 ⁻² Pa to 1 × 10 ⁻⁴ Pa, preferably 1 × 10 ⁻³ Pa to 1 × 10 ⁻⁴ Pa. There is no problem in the performance of the high-purity aluminum material obtained even under a reduced pressure exceeding 1 × 10 ⁻⁴ Pa, but it is economically disadvantageous from the viewpoint of facilities and the like.

  The high-purity aluminum material of the present invention is obtained by subjecting the surface of the rolled material to a surface polishing treatment. By performing the surface polishing treatment in this way, the amount of convex portions on the surface can be reduced, and as a result, the electrical conductivity of the high-purity aluminum described above can be obtained even in the shape of a thin plate or foil material. It can be satisfactorily exhibited and the electrical resistivity at extremely low temperatures can be lowered.

  The surface polishing method and conditions are not particularly limited. For example, a known method such as electrolytic polishing or chemical polishing described in “Aluminum Handbook (7th edition)” p180 issued by the Japan Aluminum Association is used. Can be adopted. Preferably, electropolishing is preferable. In this case, specifically, a rolled material is used as an anode and a graphite electrode is used as a cathode, and usually in a mixed solution of perchloric acid and ethanol at a voltage of 15 to 25 V and 0.5. What is necessary is just to perform constant voltage electrolysis on the conditions for -5 minutes.

In the present invention, the surface of the rolled material that has been subjected to surface polishing treatment has the following formula: surface roughness (μm) = volume of uneven portions on the surface of the measurement part (μm ³ ) / measurement area (μm ² )
Is preferably less than 3 μm (hereinafter referred to as “surface roughness” in the present specification). In other words, the surface polishing treatment is preferably performed so that the surface roughness is less than 3 μm.
In addition, the said surface roughness can be calculated | required by measuring the volume of the uneven | corrugated | grooved part of a surface using the laser microscope which can measure the three-dimensional roughness information of the sample surface. At this time, the measurement area is desirably 10,000 μm ² or more in order to obtain a more accurate result.

The high-purity aluminum material of the present invention may be one in which the surface of the rolled material that has been subjected to the surface polishing treatment is covered with an oxide film. In this case, it is preferable to control the thickness of the oxide film to 0.2 to 2 μm in order to develop more excellent electrical conductivity. The method for forming the oxide film is not particularly limited, but it is preferable to form the oxide film by, for example, anodizing treatment because the thickness of the oxide film can be easily controlled by the treatment time.
In the case of forming an oxide film by the anodizing treatment, for example, using a rolled material as an anode and a graphite electrode as a cathode, a constant voltage electrolysis is usually performed at a current density of 0.01 to 0.03 A / cm ^2. Good. Examples of the electrolytic solution used for the anodizing treatment include a sulfuric acid aqueous solution and an oxalic acid aqueous solution.

  The high-purity aluminum material of the present invention may be annealed for the purpose of removing distortion that may occur during cutting or rolling. The annealing treatment can be performed by holding at 400 to 600 ° C. for 1 hour or more, for example. In order to exhibit excellent electrical conductivity at an extremely low temperature, the annealing treatment temperature is preferably 450 to 550 ° C., and the annealing treatment is preferably performed in a reduced-pressure atmosphere. In addition, this annealing treatment may be performed either before or after applying the surface polishing treatment to the rolled material, but in order to exhibit excellent electrical conductivity at an extremely low temperature, the surface polishing treatment is performed after the annealing treatment. Preferably it is.

  The high-purity aluminum material of the present invention as described above can exhibit a low electrical resistivity at an extremely low temperature regardless of the shape. Specifically, the high-purity aluminum material of the present invention exhibits a low electrical resistivity at an extremely low temperature of 50 K or less, for example, in the form of a thin plate or foil material of 1.5 mm or less. Such a high-purity aluminum material is suitably used as a member such as a superconducting magnet, a cryogenic refrigerator, or a cryopump.

EXAMPLES Hereinafter, although an Example demonstrates this invention in detail, this invention is not limited by this Example.
In the following Examples and Comparative Examples, the content of each element in aluminum (high purity aluminum) and the measurement of the surface roughness were performed by the following methods.

<Element content>
The content of each element in aluminum (high purity aluminum) was measured by glow discharge mass spectrometry (using “VG9000” manufactured by Thermo Electron).

<Surface roughness>
For the surface roughness, using a laser microscope ("VK-9700" manufactured by Keyence Corporation), the three-dimensional roughness information of the sample surface, the measurement area is 280 μm × 200 μm, 1024 × 768 pixels, height direction information Was measured every 1 μm to determine the volume of the uneven part on the surface of the measurement part, and calculated based on the following formula.
Surface roughness (μm) = Volume of uneven part on the surface of measurement part (μm ³ ) / Measurement area (μm ² )

Example 1
99.999 mass% aluminum obtained by refining ordinary aluminum with a purity of 99.92 mass% by a three-layer electrolytic method (Si = 3 mass ppm, Cu = 2 mass ppm, Mg = 1 mass ppm, Fe = 2 mass ppm, other 31 elements <1 ppm other than these, 35 impurities total <10 mass ppm) are used as raw materials and purified by a unidirectional solidification method to obtain high purity aluminum having a purity of 99.9999 mass%. It was.

Specifically, 1.8 kg of raw material aluminum (purity 99.999 mass%) is placed in a graphite crucible (inner dimensions: width 65 mm × length 400 mm × height 35 mm), and this is used as a furnace body-movable tubular furnace The furnace core tube (made of quartz, inner diameter 100 mm × length 1000 mm) was stored in the furnace body at 700 ° C. in a reduced pressure atmosphere of 1 × 10 ⁻² Pa to dissolve the raw material aluminum, The furnace body was solidified in one direction from the end by being pulled out of the core tube at a speed of 30 mm / hour. Then, in the length direction, a portion from 50 mm from the solidification start end to 150 mm from the solidification start end was cut out to obtain massive high-purity aluminum having a width of 65 mm × length of 100 mm × thickness of 26 mm.

  The content of each element in the high purity aluminum was as follows. That is, Si = 0.37 mass ppm, Cu = 0.11 mass ppm, Mg = 0.12 mass ppm; Fe = 0.064 mass ppm, Ti = 0.035 mass ppm, V = 0.033 mass ppm, Cr = 0.04 mass ppm, Zr = 0.02 mass ppm; the total content of the 35 elements ≦ 1 mass ppm.

  Next, the high-purity aluminum lump obtained above was cut into a size of width 50 mm × length 50 mm × thickness 20 mm, and this was cut at room temperature about 50 mm wide × about 2000 mm long × 0.5 mm thick To obtain a rolled sheet. In rolling, one pass is 2 mm from 20 mm to 8 mm thick, one pass is 1 mm from 8 mm to 3 mm, and one pass is 0.5 mm from 3 mm to 0.5 mm. It was.

  Next, three sheets of the obtained rolled sheet were cut into a size of width 10 mm × length 150 mm, and the surface was subjected to surface polishing treatment by electrolytic polishing. Specifically, the cut rolled plate is used as the anode, the graphite electrode is used as the cathode, and a mixed solution (20 ° C.) of perchloric acid: ethanol (volume ratio) = 1: 6 is used as the electropolishing liquid. Constant voltage electrolysis was performed under the conditions of seconds. The surface roughness of each rolled plate subjected to the surface polishing treatment was 1.4 μm.

Thereafter, each rolled plate subjected to the surface polishing treatment is annealed in a reduced pressure atmosphere of (a) a degree of vacuum of 10 ⁻³ Pa, (b) annealed in an air atmosphere, and (c) 30 ° C. at 100 ° C. in pure water. After performing the boehmite treatment which is boiled for a minute, it is air-dried in the air, and then subjected to three kinds of treatments, ie, annealing in the air atmosphere, and the high-purity aluminum material (1-a), (1- b) and (1-c) were obtained. The annealing conditions in each treatment were 500 ° C. and 3 hours.

  Each of the obtained high-purity aluminum materials was immersed in 4.2K liquid helium or 296K pure water, and the electrical resistance was measured by the four-terminal method, whereby the specific resistance value at 4.2K and the ratio at 296K were measured. The resistance value was determined. In the measurement, the distance between the voltage terminals was about 100 mm, and the measurement current was 3A for 4.2K (liquid helium) and 0.1A for 296K (pure water). The results are shown in Table 1.

(Comparative Example 1)
A high-purity aluminum material was produced in the same manner as in Example 1 except that the surface polishing treatment by the electrolytic polishing method was not performed.
That is, three rolled plates obtained in the same manner as in Example 1 were cut into a size of 10 mm wide × 150 mm long, and the same as in Example 1 without performing surface polishing treatment by electrolytic polishing. High-purity aluminum materials (C1-a), (C1-b), and (C1-c) for comparison were obtained by performing three types of treatments (a), (b), and (c). In addition, the surface roughness of each rolled sheet in a state where the surface polishing treatment was not performed was 4.1 μm.
About the obtained high purity aluminum material, it carried out similarly to Example 1, and calculated | required the specific resistance value in 4.2K, and the specific resistance value in 296K. The results are shown in Table 1.

(Comparative Examples 2 and 3)
A high-purity aluminum material was produced in the same manner as in Example 1 except that a surface roughening treatment was performed instead of the surface polishing treatment by the electrolytic polishing method.
That is, the rolled plates obtained in the same manner as in Example 1 were cut into three pieces each having a width of 10 mm and a length of 150 mm. In Comparative Example 2, coarse emery paper (# 320) was used. Using emery paper (# 1200), the surface was roughened by wet polishing. The surface roughness of each rolled plate subjected to the roughening treatment was 4.3 μm in Comparative Example 2 and 7.3 μm in Comparative Example 3.
Thereafter, each of the roughened rolling plates was subjected to the same three types of treatments (a), (b) and (c) as in Example 1, and in Comparative Example 2, high-purity aluminum for comparison. In Comparative Example 3, the materials (C2-a), (C2-b), and (C2-c) were obtained, and comparative high-purity aluminum materials (C3-a), (C3-b), and (C3-c) were obtained. It was.
About the obtained high purity aluminum material, it carried out similarly to Example 1, and calculated | required the specific resistance value in 4.2K, and the specific resistance value in 296K. The results are shown in Table 1.

(Example 2)
A rolled plate obtained in the same manner as in Example 1 was cut into a size of width 10 mm × length 150 mm and annealed at 600 ° C. for 3 hours in a reduced pressure atmosphere with a reduced pressure of 10 ⁻² Pa. Then, it cooled to room temperature in the furnace, and then surface-polished by the electrolytic polishing method, and the high purity aluminum material (2) of this invention was obtained. Specifically, the surface polishing treatment uses an annealed rolled plate as an anode, a graphite electrode as a cathode, and a perchloric acid: ethanol (volume ratio) = 1: 6 liquid mixture (20 ° C.) as an electrolytic polishing liquid. ), And constant voltage electrolysis was performed under the conditions of a voltage of 20 V and 60 seconds. The surface roughness of the rolled plate subjected to the surface polishing treatment (that is, the high-purity aluminum material (2)) was 1.4 μm.

About the obtained high purity aluminum material, it carried out similarly to Example 1, calculated | required the specific resistance value in 4.2K, and the specific resistance value in 296K, and also computed the residual resistance value (RRR) from the following formula. The results are shown in Table 2.
RRR = (specific resistance value at 296K) / (specific resistance value at 4.2K)

(Examples 3 to 5)
The high purity aluminum material (2) obtained in Example 2 is subjected to anodic oxidation treatment with varying treatment time for each Example, whereby the high purity aluminum of the present invention having oxide films with various thicknesses on the surface. Materials (3), (4) and (5) were obtained. That is, in Example 3, a high-purity aluminum material (2) was used as the anode, a graphite electrode was used as the cathode, a 15% sulfuric acid aqueous solution (20 ° C.) was used as the electrolyte, and the current density was 0.013 A / cm ^2. Constant voltage electrolysis was performed for 1 minute, 2 minutes in Example 4, and 20 minutes in Example 5. When the thickness of the oxide film on the surface of the obtained high-purity aluminum material was measured based on the SEM photograph of the cross section, the high-purity aluminum material (3) obtained in Example 3 was 0.4 μm and obtained in Example 4. The obtained high purity aluminum material (4) was 1 μm, and the high purity aluminum material (5) obtained in Example 5 was 7 μm.
About the obtained high purity aluminum material, it carried out similarly to Example 1, calculated | required the specific resistance value in 4.2K, and the specific resistance value in 296K, and also calculated the residual resistance value (RRR) similarly to Example 2. . The results are shown in Table 2.

(Examples 6 to 9)
A high-purity aluminum lump obtained in the same manner as in Example 1 was cut into a size of 50 mm wide × 50 mm long × 20 mm thick, and this was rolled at room temperature. Rolled plates with different thicknesses were obtained. That is, in Example 6, rolled to a size of about 50 mm wide × about 500 mm long × 2 mm thick, and in Example 7, rolled to a size of about 50 mm wide × about 1000 mm long × 1 mm thick. No. 8 was rolled to a size of about 50 mm wide × about 2000 mm long × 0.5 mm thick, and Example 9 was rolled to a size of about 50 mm wide × about 5000 mm long × 0.2 mm thick. In rolling, one pass is 2 mm from 20 mm to 8 mm thick, one pass is 1 mm from 8 mm to 3 mm, and one pass is 0.5 mm from 3 mm to 0.5 mm. From a thickness of 0.5 mm to a thickness of 0.2 mm, one pass was 0.1 mm.

  Next, in each example, the obtained rolled sheets were cut into two pieces each having a width of 10 mm and a length of 150 mm. Then, one of the cut outs was subjected to a surface polishing treatment by an electrolytic polishing method in the same manner as in Example 1 to set the surface roughness to 1.4 μm, and then in an air atmosphere at 500 ° C. for 3 hours. An annealing treatment was performed to obtain high-purity aluminum materials (6), (7), (8) and (9) of the present invention.

About the obtained high purity aluminum material, the improvement effect of the electrical conductivity by the surface polishing process at cryogenic temperature was evaluated.
That is, the same conditions as in Examples 6 to 9 above (under atmospheric conditions) in the state where the remaining one of the rolled plates cut out two by two is not subjected to surface polishing treatment and the surface roughness is 4.1 μm. , 500 ° C., 3 hours) High purity aluminum materials (6 ′), (7 ′), (8 ′) and (9 ′) without surface polishing treatment to be blanks for each example. Got.
About the high-purity aluminum materials (6) to (9) of the present invention and the high-purity aluminum materials (6 ′) to (9 ′) obtained above, the specific resistance value at 4.2 K was obtained in the same manner as in Example 1. For each example, the specific resistance value at 4.2K of the blank (without surface polishing treatment) is x, and the specific resistance value at 4.2K of the high-purity aluminum material of the present invention (with surface polishing treatment) is obtained. The specific resistance value reduction rate was calculated from the following formula as y.
Specific resistance value decrease rate (%) = [(xy) / x] × 100
For reference, the specific resistance value at 296K of each high-purity aluminum material was also obtained. These results are shown in Table 3.

Claims (4)

It consists of a rolled material of high-purity aluminum having a purity of 99.9999% by mass or more, and the surface of the rolled material is subjected to surface polishing treatment .
The surface of the rolled material is
Surface roughness (μm) = volume of uneven portion on the surface of the measurement part (μm ³ ) / measurement area (μm ² )
A high-purity aluminum material for use at an extremely low temperature of 50K or less, characterized in that the surface roughness required by 1 is less than 3 μm . The rolled material has a thickness of at 1.5mm or less claim 1 Symbol placement high purity aluminum material. The high-purity aluminum material according to claim 1 or 2 , which is covered with an oxide film having a thickness of 0.2 to 2 µm. The high-purity aluminum material according to any one of claims 1 to 3 , wherein an annealing treatment is performed at 400 to 600 ° C for 1 hour or more.