JP3263605B2 - Hydrogen storage alloy - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hydrogen storage alloy for storing and releasing hydrogen.
The present invention relates to a hydrogen storage alloy having a small hysteresis, a low plateau, and excellent reversibility of hydrogen absorption / desorption.

[0002]

2. Description of the Related Art The hydrogen absorption / release characteristics of a hydrogen storage alloy are as follows.
It is used in various devices such as fuel cells, heat pumps, and heat storage devices. The characteristics of the hydrogen storage alloy can be evaluated by a hydrogen pressure-hydrogen absorption amount isothermal curve (PCT characteristic curve). The PCT characteristic curve is represented by a hydrogen solid solution region and a metal hydride compound region, and a plateau region sandwiched between these regions. The hydrogen storage alloy has a hysteresis between the time of hydrogen absorption and the time of hydrogen release, and the plateau is not flat but inclined. These tendencies are found in almost all hydrogen storage alloys, but these properties greatly affect the performance of the device. For example, if the hysteresis is large, it is difficult to obtain a hydrogen pressure difference, and the operating temperature range is likely to be restricted. In addition, when the inclination of the plateau is large, the amount of moving hydrogen is greatly affected, so that the efficiency is deteriorated. Therefore, it is desired to develop a hydrogen storage alloy having a small hysteresis and a flat plateau, that is, a small inclination of the plateau, and capable of performing hydrogen absorption / desorption with high reversibility.

In order to reversibly absorb and release hydrogen, it is important to ensure the homogeneity of the alloy structure. As a method for producing a hydrogen storage alloy, there is a method of melting a powder of a hydrogen storage alloy in an arc melting furnace or a high-frequency induction furnace. The hydrogen storage alloy produced by this method may have a heating temperature of 10 if necessary.
After heating at 00 to 1400 K for 2 to 24 hours, a heat treatment for slow cooling is performed. However, since the structure of the obtained hydrogen storage alloy is not sufficiently homogenized, hydrogen absorption / release is highly reversible. Could not be done.

Further, as a method for producing a hydrogen storage alloy, a method of rapid cooling such as a roll quenching method in which a molten hydrogen storage alloy is injected onto a roll rotating at high speed is known. However, the production of alloys by quenching is complicated in the production process.Also, since the alloy is cooled from the molten state, the homogeneity of the structure is improved, but the cooling rate is too high and the crystal lattice is distorted. In some cases, reversibility was impaired.

[0005]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a hydrogen storage alloy which has a small hysteresis, a small plateau gradient, and which can reversibly absorb and release hydrogen.

[0006]

Means for Solving the Problems In order to solve the above-mentioned problems, the present invention provides a composition represented by the formula A _1-x Q _x B _5-y R _y (where A is La
Or Mm (Misch metal), Q is Y, B is Ni, R is A
l and / or Mn, x and y are each 0.1 ≦ x ≦ 0.3,
The ingot of the alloy represented by 0 ≦ y ≦ 1) is heated for a predetermined time at a temperature lower than the melting point, that is, at a temperature at which the alloy maintains a solid state, and then immersed in a liquid refrigerant and rapidly cooled. By performing the quenching treatment , Y is evenly distributed in the matrix.
This is for producing a dispersed hydrogen storage alloy. The solid phase quenching treatment is desirably performed at a temperature that is at least 2/3 of the absolute temperature of the melting point of the alloy and lower than the melting point. Further, the solid phase quenching treatment is desirably performed in an atmosphere of a high-purity inert gas of 99.9999% or more.

[0007]

The ingot of a hydrogen storage alloy is heated at a temperature lower than the melting point and rapidly cooled with a liquid refrigerant, whereby the homogeneity of the alloy structure can be improved. Therefore, a hydrogen storage alloy having small hysteresis, small plateau slope, and high reversibility can be obtained. The reason that the solid phase quenching treatment is performed at a temperature lower than the melting point of the alloy is to perform the solid phase quenching treatment in a solid state where the alloy is not melted. This is because when heated at a temperature equal to or higher than the melting point of the alloy, the alloy is melted, and the crystal lattice may be distorted during rapid cooling. The reason that the solid phase quenching treatment is performed at a temperature equal to or more than 2/3 of the absolute temperature of the melting point is that if the temperature is lower than this, it may be difficult to sufficiently homogenize the alloy. The solid-phase quenching treatment is performed in a high-purity inert gas atmosphere of 99.9999% or more because the hydrogen storage alloy is easily oxidized, and thus it is necessary to prevent the oxidation.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS First, a heat treatment apparatus for producing a hydrogen storage alloy according to the present invention will be described with reference to examples. The heat treatment equipment consists of a heating furnace (1) and a quenching chamber, as shown in Figs.
(2) is connected by a transport passage (3), and a shutter (31) for airtightly partitioning the passage is provided in the transport passage (3). The heating furnace (1) is provided with a pair of heaters (11) and (11) arranged opposite to each other and reflectors (12) and (12) provided behind each heater. The space between) is the effective space of the furnace. One set of heater (11) and reflector (12) is attached to the furnace door (10). Closing the door (10) keeps the furnace airtight.

The heating furnace (1) has a transfer means for transferring the hydrogen storage alloy from the furnace to the quenching chamber (2) through an airtight transfer passage (3).
(5) is provided. The transfer means (5) is provided in the transfer passage.
On the extension of (3), a rod-shaped pusher (51) penetrating the side surface of the heating furnace (1) so as to be slidable in an airtight manner, and the base end of the pusher is provided outside the heating furnace (1). Connected to the slide drive device (52). The slide drive device (52) is a mechanism such as a combination of a rack and a pinion, a cylinder device, etc., and causes the pusher (51) to reciprocate linearly with a stroke whose tip reaches the quenching chamber (2) from the heating furnace (1). be able to.

The quenching chamber (2) has a table (2) on which the hydrogen storage alloy fed by the pusher (51) is placed.
1), a tank (2) containing a liquid refrigerant disposed at a position below the table (21) in a direction orthogonal to the pusher (51).
2), a drop-off rod (6) that penetrates the side wall of the quenching chamber (2) in a slidable manner and drops the hydrogen storage alloy on the table (21) into the tank (22) is provided. The base end of 6) is connected to a slide drive device (61) provided outside the quenching chamber (2). Slide drive (61) of push-down rod (6)
Has a mechanism similar to that of the slide drive device (52) of the pusher (51), and stores the hydrogen storage alloy of the table (21) in a tank.
(22). Quenching oil, water, liquid nitrogen, and the like can be given as the liquid refrigerant stored in the tank (22). A door (20) is provided on the side of the quenching chamber (2), and when the door (20) is closed, the airtightness of the quenching chamber (2) is maintained.

Next, an apparatus for replacing the atmosphere in the heating furnace (1) and the quenching chamber (2) with an inert gas will be described. The heating furnace (1) and the quenching chamber (2) are connected to a common suction device (4). The suction device (4) is a mechanical booster pump (43)
To the upstream side, a pipe (40) equipped with a rotary pump (44) on the downstream side is bifurcated to the upstream side, and each branch end is connected to the heating furnace (1) and the quenching chamber (2). I have. An angle valve (41) for switching the flow path is provided at the branch of the pipe (40).
(41) and a first valve (42) (4) between the heating furnace (1) and the quenching chamber (2).
2a) is provided. A Pirani vacuum gauge (45) is provided between the angle valve (41) and the mechanical booster pump (43).

A gas cylinder (9) filled with an inert gas such as Ar gas is connected to the heating furnace (1) and the quenching chamber (2) through supply paths (91) and (91a). Gas cylinder (9) in supply path (91) (91a)
A second valve (92) (92a) is provided on the side close to the inert gas purifying section (7), the heating furnace (1) and the quenching chamber (2).

A mechanical booster pump (43), a rotary pump (44), an angle valve (41),
First valve (42) (42a), Pirani vacuum gauge (45), slide drive device (52) on heating furnace (1) side, slide drive device (61) on quenching chamber (2) side, transfer passage (3) The shutter (31) and the inert gas supply path (91) are electrically connected to the controller (8) and are automatically controlled. On the controller (8) panel, a start button (81), a heating furnace gas replacement button (82), a quench chamber gas replacement button (83), and a simultaneous gas replacement button for the heating furnace and quench chamber
(84), various operation buttons such as a replacement frequency selection button group (85) are provided.

The following operation is automatically performed by the controller (8). After replacing the heating furnace (1) with an inert gas, the hydrogen storage alloy is heated. Simultaneously with or after the replacement of the gas in the heating furnace (1), the quenching chamber (2) is replaced with an inert gas, the shutter (31) of the transfer passage (3) is opened, and the pusher is pressed.
By (51), the heated hydrogen storage alloy is projected onto the table (21) of the quenching chamber (2). Close the shutter (31) to maintain the gas atmosphere in the quench chamber (2). The hydrogen storage alloy is dropped into the tank (22) with the plunging rod (6) and rapidly cooled. As shown in the flow charts of FIGS. 3 and 4, the controller (8) performs a suction device when replacing the gas in the heating furnace (1) and the quenching chamber (2).
(4) and the second valves (92) and (92a) on the gas supply path (91) side are controlled.

The gas replacement process will be described with reference to a flowchart. The first valve (42) (42a) and the second valve (92) (92a) are closed. First, when replacing only the heating furnace (1) with gas (shown on the left side of the flowchart), the heating furnace gas replacement button (82) is pressed. The number of times of gas replacement is selected with the number of replacement times selection button group (85).

Press the start button (81). Angle valve (4
1) is opened to allow communication between the first valve (42) on the heating furnace (1) side and the mechanical booster pump (43), and the first valve (42a) on the quenching chamber (2) side and the mechanical booster pump. Cut off between (43). The rotary pump (44) operates. 1st valve (4
2) Open the heating furnace (1) and mechanical booster pump (4
3) and the inside of the heating furnace (1) becomes negative pressure. If the Pirani gauge (45) senses that it is below 10 Torr,
The mechanical booster pump (43) operates. If the Pirani gauge senses less than 1 × 10 ^-2 Torr,
The first valve (42) closes, and the mechanical booster pump (43) stops.

When the second valve (92) on the heating furnace (1) side is opened, the inert gas supply passage (91) communicates with the heating furnace (1), and the dry column
After passing through (71), Ar gas flows into the heating furnace (1),
The atmosphere in the heating furnace (1) is replaced with Ar gas. When the above operation is repeated for the set number of replacements, the rotary pump (4
4) stops, and gas replacement ends.

If only the quenching chamber (2) is replaced with gas (shown on the right side of the flow chart), the angle valve (4)
1) connects the first valve (42a) on the quenching chamber (2) side with the mechanical booster pump (43), and the first valve on the heating furnace (1) side.
(42) and the mechanical booster pump (43), and only the difference between the supply path (91) from the gas cylinder (9) and the second valve (92a) is that the operation of the heating furnace (1) is different. ) Is the same as in the case of gas replacement.

According to the heat treatment apparatus, the hydrogen storage alloy can be subjected to heat treatment at a desired temperature and for a desired time in a high-purity inert gas atmosphere. In the present invention, the composition formula A _1-x Q _x B _5-y R _y (where A is La
Or Mm (Misch metal), Q is Y, B is Ni, R is A
l and / or Mn, x and y are respectively 0 ≦ x ≦ 0.3, 0
≦ y ≦ 1).
Under an atmosphere of an inert gas of 9999% or more, at a temperature not less than 2/3 of the absolute temperature of the melting point of the alloy and a temperature lower than the melting point, 2 to 1
After heating for 6 hours, a quenching treatment was performed to produce a hydrogen storage alloy having a small hysteresis, a small plateau gradient, and capable of highly reversibly absorbing / desorbing hydrogen. In addition, as long as the apparatus can perform the same processing, it is needless to say that the present invention can be implemented without being limited to the heat treatment apparatus.

[0020]

Example 1 In Example 1, a solid phase quenching treatment of the present invention was performed on an ingot of a hydrogen storage alloy represented by the composition formula La _0.8 Y _0.2 Ni _4.4 Al _0.6 by changing the heating temperature to obtain a hysteresis and The plateau slope is measured and compared. The raw material powder was La: Y: Ni: Al = 0.8: 0.2: 4.4: 0.6.
(Atomic ratio), and after mixing, pressing was performed, and a plurality of alloy ingots (composition formula La _0.8 Y _0.2 Ni _4.4 Al _0.6 ) melted in an arc melting furnace and allowed to cool naturally were prepared. .

The obtained alloy ingot is subjected to 473K, 573K, 823K, 1073K, 1
Heating was performed at 323 K and 1423 K, respectively, for 8 hours. The atmosphere in the heat treatment equipment is A with a purity of 99.9999% or more.
r gas. After heating, the mixture was dropped into a quenching oil at a temperature of 298 K and rapidly cooled. In the alloy that had been quenched, the alloy surface that had reacted with the oil was filed off with a file to form a match gold No. 1 to No. 6 (see Table 1).

◎

[Table 1]

In order to make a comparison, the prepared alloy ingot was subjected to an alloy without any heat treatment.
o.7) and an alloy by normal heat treatment after heating at a temperature of 1323 K for 8 hours in an Ar gas atmosphere having a purity of 99.9999% or more (available gold No. 8) and an alloy ingot. The alloy was melted and sprayed on a roll in an Ar atmosphere having a purity of 99.9999% to rapidly cool the molten metal to obtain an alloy (available gold No. 9) (see Table 1).

The prepared match Nos. 1 to 9 were pulverized to a particle size of about 100 μm, 5 g of which was sealed in a stainless steel reaction vessel (internal volume: 10 cc), and activated (vacuum exhaust temperature). 353K, applied hydrogen pressure 18atm, activation time 60
Min). Thereafter, using a known Siebeltz apparatus, the pressure-hydrogen absorption amount isothermal characteristic (PC-
T curve) was measured at a temperature of 403K. In addition, the hysteresis and plateau slope of each match were measured. The results are shown in Table 1, FIG. 5 and FIG.

Referring to Table 1, FIG. 5 and FIG. 6, it can be seen that, in the match money Nos. 1 to 6, the hysteresis decreases and the plateau slope decreases as the heating temperature increases.

Comparing No. 1 to No. 6 with No. 7 which does not perform any heat treatment, No. 1 has almost the same hysteresis and plateau slope as No. 7. However, in No.2, the inclination of the plateau was No.1 and No.7.
It can be seen that it is smaller by about 20%. In addition, No. 3 to No. 6 both have smaller hysteresis and plateau slope than No. 7, indicating that they are superior to No. 7.

Comparing the match money No. 1 to No. 6 with the match money No. 8, both No. 1 to No. 3 have larger hysteresis and plateau slope, and are inferior to No. 8. . However, it can be seen that No. 4 to No. 6 have smaller hysteresis and plateau slope, and are superior to No. 8.

Comparing the match money No. 1 to No. 6 with the match money No. 9, No. 1 to No. 3 both have a large hysteresis and a plateau slope, and are inferior to No. 9. . Also, N
In Nos. 4 and 5, the hysteresis was slightly inferior to No. 9, but the slope of the plateau was reduced by 40% or more, indicating that the No. 9 was superior to No. 9. In addition, No. 6
Although the hysteresis is almost the same as that of No. 9, the slope of the plateau is reduced by about 60%, which indicates that the plateau is superior to No. 9.

Comprehensively evaluating from the above measurement results and comparison results, it can be seen that the match money Nos. 4 to 6 can simultaneously reduce the hysteresis and the plateau slope. That is, by performing the hydrogen storage alloy at a temperature that is at least 2/3 of the melting point of the alloy (the alloy of this embodiment is about 1600K) and lower than the melting point, the hysteresis is small, the plateau slope is small, and the high reversibility is obtained. Can be produced.

Match No. 5 prepared in Example 1 above
7 and 8 show X-ray images (1000 times) obtained by electron beam probe microanalysis (EPMA) for No. 8 and Match No. 8. Referring to FIG. 8, a match-treated gold No. 8 which has been subjected to a normal heat treatment has a boundary between a phase in which Y is distributed (a portion that looks white) and a phase in which Y is not distributed (a black band-shaped portion). Is clear. On the other hand, the match money N of the present invention
In the case of o.5, a condensed portion of Y (four white spots appearing in the figure) is observed, but in the parent phase, Y is relatively uniformly distributed, and the difference between the boundaries shown in FIG. Not clear. It is considered that the match money No. 5 of the present invention has high reversibility as described above because the distribution of Y is more uniform.

In the following examples, the components of the alloy of the present invention and the reasons for limiting the replacement amount will be described. Example 2 Match No. 5 of the present invention prepared in Example 1 above,
Match No. 8 with normal heat treatment (both with composition formula La)
_0.8 Y _0.2 Ni _4.4 Al _0.6 ) was measured using a Sibeltz apparatus. Also, for comparison, Ti represented by the composition formula Ti _0.85 Zr _0.15 MnV _0.4 Ni _0.6
The same solid-phase quenching treatment as the above-mentioned match No. 5 and the same heat-treated alloy as the match no. The CT characteristic curve was measured with a Siebeltz apparatus. The results are shown in FIGS. As can be seen from FIG. 9, the solid-phase quenched No.
5 shows that the hysteresis is smaller and the plateau slope is smaller than that of No. 8 subjected to the ordinary heat treatment. Also, it can be seen that the effective hydrogen transfer amount indicated by the plateau width (see FIG. 9) is large. On the other hand, as shown in FIG. 10, when an alloy obtained by subjecting a Ti-based alloy to a solid phase quenching treatment and an alloy subjected to a normal heat treatment are compared, the alloy subjected to the solid phase quenching treatment has a normal hysteresis. It turns out that it is almost the same as the heat-treated alloy, and that the inclination of the plateau is larger than that of the normal heat-treated alloy. In other words, the solid-phase quenching treatment exerts an excellent effect when applied to a hydrogen storage alloy represented by the composition formula La _0.8 Y _0.2 Ni _4.4 Al _0.6 , but when applied to a Ti alloy, the hydrogen absorption / desorption of the Ti alloy can be improved. It turns out that the characteristics deteriorate rather. Therefore, the solid-phase quenching treatment of the present invention is suitable for a La—Ni-based hydrogen storage alloy,
It was found that it was not suitable for i-based alloys. A hydrogen storage alloy represented by the composition formula A _1-x Q _x B _5-y R _y (where A is La or Mm (misch metal), Q is Y, B is Ni, and R is Al and / or Mn) It is presumed that when the solid phase quenching treatment is performed, the hysteresis and the slope of the plateau can be reduced, and the hydrogen absorption / desorption can be performed highly reversibly.

[0032] Example 3 Example 3, the hydrogen storage alloy whose composition formula is represented by _{La 1-x Y x Ni 5} -y Al y, effective hydrogen when changing the amount x of Y to be substituted for La It measures the amount of movement. L
Part of a is substituted with Y because the substitution can increase the equilibrium pressure of the hydrogen storage alloy.

An alloy in which the amount x of Y to replace La was varied from 0 to 0.3 as shown in Table 2 and the amount y of Al to replace Ni was 0.6 (in the order of No. 11 To No. 14), heating temperature: 1323 K, heating time: 8 hours, processing atmosphere: Ar gas (purity 99.9999%), quenching medium: quenching oil, medium temperature: 298 K, solid phase quenching The treatment was performed, and the effective hydrogen transfer amount of each alloy was measured using a Siebeltz apparatus. The results are shown in Table 2 and FIG.

◎

[Table 2]

Referring to Table 2 and FIG. 11, it can be seen that the effective hydrogen transfer amount gradually decreases as the Y substitution amount increases. In other words, from the viewpoint of the effective hydrogen transfer amount, the smaller the substitution amount of Y, the better. However, as described above, when a part of La is replaced with Y, the equilibrium pressure of the hydrogen storage alloy can be increased. Therefore, it is desirable that the substitution amount of Y is adjusted so that the effective hydrogen transfer amount (about 0.6% by weight) of the normally required hydrogen storage alloy can be ensured, that is, x is 0.3 or less. .

Example 4 In Example 4, the hydrogen storage alloy represented by the composition formula La _1-x Y _x Ni _{5-y A y} was used to determine the amount y of Al substituted with Ni.
This is to measure the effective hydrogen transfer amount when the value is changed.
The reason why part of Ni is replaced by Al is that the substitution can improve the durability of the hydrogen storage alloy.

The amount y of Al to be replaced with Ni is changed from 0 to 1.0 as shown in Table 3, and the amount x of Y to be replaced with La is x.
Was prepared as an alloy (in the order of No. 20 to No. 25) and subjected to a solid phase quenching treatment under the same conditions as in Example 3.
The effective hydrogen transfer amount of each alloy was measured using a Siebelz apparatus. The results are shown in Table 3 and FIG.

◎

[Table 3]

Referring to Table 3 and FIG. 12, it can be seen that the effective hydrogen transfer amount gradually decreases as the Al substitution amount increases. That is, from the viewpoint of the effective hydrogen transfer amount, the smaller the substitution amount of Al, the better. However, as described above, when part of Ni is replaced with Al, the durability of the hydrogen storage alloy can be improved.
Therefore, it is desirable to adjust the replacement amount of Al so that the effective hydrogen transfer amount (about 0.6% by weight) of the normally required hydrogen storage alloy can be ensured, that is, y is 1.0 or less. . The same effect can be obtained by substituting a part of Ni for Mn instead of Al or for Al and Mn.

In Examples 1 to 4, the composition formula A
_1-x Q _x B _5-y R _y (where 0 ≦ x ≦ 0.3, 0 ≦ y ≦ 1) Only the example where A of the hydrogen storage alloy is La has been described, but A is Mm ( The same effect can be obtained as (mish metal).

In the above embodiment, the hydrogen storage alloy ingot was manufactured by the arc melting furnace. However, the alloy ingot is not limited to the arc melting furnace, but may be manufactured by a known melting method such as a high frequency induction furnace. it can.

[0042]

According to the hydrogen storage alloy of the present invention, since the alloy structure is homogenized, the hysteresis is small, the slope of the plateau is small, and the absorption / release of hydrogen can be performed with high reversibility.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing a configuration of a heat treatment apparatus.

FIG. 2 is an enlarged plan view showing a heating furnace and a quenching chamber portion of the heat treatment apparatus.

FIG. 3 is a flowchart showing a control flow of the heat treatment apparatus.

FIG. 4 is a flowchart showing a control flow of the heat treatment apparatus.

FIG. 5 is a graph showing a relationship between hysteresis and processing temperature.

FIG. 6 is a graph showing a relationship between a plateau inclination and a processing temperature.

FIG. 7 is a drawing substitute photograph (× 1000) showing the structure of match No. 5 subjected to solid phase quenching treatment by EPMA.

FIG. 8 shows the structure of No. 8 of the match gold which has been subjected to the normal heat treatment.
It is a drawing substitute photograph (x1000) shown by PMA.

FIG. 9 is a P-C-T characteristic curve of match money No. 5 and No. 8;

FIG. 10 is a PCT characteristic curve of a Ti-based alloy.

FIG. 11 is a graph showing the relationship between the amount of La replaced with Y and the amount of effective hydrogen transfer.

FIG. 12 is a graph showing the relationship between the amount of partial replacement of Ni with Al and the effective amount of hydrogen transfer.

[Explanation of symbols]

 (1) Heating furnace (2) Quench room

Continuing from the front page (72) Inventor Ikuo Yonezu 2-5-5 Keihanhondori, Moriguchi-shi, Osaka Sanyo Electric Co., Ltd. (72) Inventor Koji Nishio 2-5-5 Keihanhondori, Moriguchi-shi, Osaka Sanyo (56) References JP-B-59-28624 (JP, B1) JP-B-59-28625 (JP, B1) JP-B-59-28626 (JP, B1) (58) Fields surveyed (Int .Cl. ⁷ , DB name) C22C 19/00 C22F 1/10

Claims (3)

(57) [Claims] 1. A composition formula A _1-x Q _x B _5-y R _y (where A is La
Or Mm, Q is Y, B is Ni, R is Al and / or M
n, x and y are respectively 0.1 ≤ x ≤ 0.3, 0 ≤ y ≤ 1) An ingot of the alloy represented by the formula is heated at a temperature lower than the melting point for a predetermined time and then immersed in a liquid refrigerant. Made by solid-phase quenching process in which quenching is performed , and Y is uniformly dispersed in the matrix.
A hydrogen storage alloy, which is characterized in that: 2. The hydrogen storage alloy according to claim 1, wherein the solid phase quenching treatment is performed at a temperature that is at least two-thirds of the melting point of the alloy and lower than the melting point. 3. The hydrogen storage alloy according to claim 1, wherein the solid phase quenching treatment is performed in a high purity inert gas atmosphere of 99.9999% or more.