HYDROGEN-RELEASING FILM

Abstract

The present invention provides a hydrogen-releasing film and a hydrogen-releasing laminated film that have high reliability as a safety valve since defects such as cracks do not occur before the internal pressure of an electrochemical element reaches a predetermined pressure. The hydrogen-releasing film contains an alloy having Pd as an essential metal, and the size of the crystal grains in the alloy is 0.028 m or more.

Claims

1. A hydrogen-releasing film containing an alloy having Pd as an essential metal, wherein the size of the crystal grains in the alloy is 0.028 m or more.

2. The hydrogen-releasing film according to claim 1, wherein the alloy contains a Group 11 element in an amount of 20 to 65 mol %.

3. The hydrogen-releasing film according to claim 2, wherein the Group 11 element is at least one kind selected from the group consisting of gold, silver, and copper.

4. The hydrogen-releasing film according to claim 2, wherein the hydrogen permeation coefficient at 50 C. is 1.010.sup.13 to 2.010.sup.9 (mol.Math.m.sup.1.Math.sec.sup.1.Math.Pa.sup.1/2), and the film thickness t and the film areas satisfy the following expression 1.
t/s<32.9m.sup.1<Expression 1>

5. A hydrogen-releasing laminated film, comprising a support on one surface or both surfaces of the hydrogen-releasing film according to claim 1.

6. The hydrogen-releasing laminated film according to claim 5, wherein the support is a porous body having an average pore diameter of 100 m or less.

7. The hydrogen-releasing laminated film according to claim 5, wherein a raw material of the support is at least one kind selected from the group consisting of polytetrafluoroethylene, polysulfone, polyimide, polyamide-imide, and aramid.

8. A safety valve for an electrochemical element, wherein the valve is provided with the hydrogen-releasing film according to claim 1.

9. An electrochemical element, wherein the element is provided with the safety valve according to claim 8.

10. The electrochemical element according to claim 9, wherein the electrochemical element is an aluminum electrolytic capacitor or a lithium ion battery.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] FIG. 1 is a schematic sectional view showing the structure of the hydrogen-releasing laminated film of the present invention.

[0026] FIG. 2 is a schematic sectional view showing the another structure of the hydrogen-releasing laminated film of the present invention.

MODE FOR CARRYING OUT THE INVENTION

[0027] Hereinafter, embodiments of the present invention will be described.

[0028] As the raw material of the hydrogen-releasing film of the present invention, an alloy having Pd as an essential metal is used. There is no particular limitation on other metals forming the alloy. However, it is preferable to use a Group 11 element in view of easily adjusting the crystal grain size of the alloy to 0.028 m or more, more preferable to use at least one kind selected from the group consisting of gold, silver, and copper, and even more preferable to use silver or copper. The alloy contains a Group 11 element preferably in an amount of 20 to 65 mol %, more preferably 30 to 65 mol %, even more preferably 30 to 60 mol %. By forming a hydrogen-releasing film with use of a PdAg alloy having an Ag content of 20 mol % or more, a PdCu alloy having a Cu content of 30 mol % or more, or a PdAu alloy having an Au content of 20 mol % or more, such a hydrogen-releasing film becomes less susceptible to embrittlement even at a low temperature range of about 50 to 60 C. or less. Further, the alloy may contain a Group IB metal and/or a Group IIIA metal as long as the effect of the present invention is not impaired.

[0029] The crystal grain size of the alloy is 0.028 m or more, preferably 0.04 m or more, more preferably 0.1 m or more, even more preferably 0.4 m or more. The larger the grain size is, the more hardly the defects such as cracks occur. Thus, there is no particular limitation on the upper limit value of the crystal grain size, but the crystal grain size is preferably 1000 m or less, more preferably 600 m or less, from the viewpoint that it is necessary to reduce the internal pressure by the self-destruction when the internal pressure of the electrochemical element is rapidly increased.

[0030] The hydrogen-releasing film of the present invention can be produced by, for example, a rolling method, a sputtering method, a vacuum deposition method, an ion plating method, and a plating method, but when producing a thick hydrogen-releasing film, it is preferable to use the rolling method and when producing a thin hydrogen-releasing film, it is preferable to use the sputtering method.

[0031] The crystal grain size can be adjusted to, for example, the desired size by adjusting the temperature in producing the hydrogen-releasing film. The temperature at which a hydrogen-releasing film having a crystal grain size of 0.028 m or more is produced is usually a temperature of from 50 C. to the melting temperature of the alloy, preferably from 50 C. to 500 C., more preferably from 100 C. to 400 C. By rolling at the above-mentioned temperature in the case of a rolling method, or by heating the substrate for forming a sputtered film to the above-mentioned temperature in the case of the sputtering method, a hydrogen-releasing film having a desired crystal grain size can be produced.

[0032] Even when producing a hydrogen-releasing film outside the temperature range, it is possible to adjust the crystal grain size by heating the hydrogen-releasing film again and then cooling the film. Further, when heating the hydrogen-releasing film and then cooling the film to an ambient temperature (usually about room temperature), it is possible to increase the crystal grain size by slow cooling instead of quenching. In addition, by slow cooling, the film surface of the hydrogen-releasing film becomes smooth because the crystal growth rate of the whole film is made uniform. As a result, the stress concentration to the interface between the crystal grains is suppressed, and thus cracks are further less likely to occur in the hydrogen-releasing film.

[0033] Further, by increasing the pressure of the press or rolling rolls when producing a hydrogen-releasing film, pressing the produced hydrogen-releasing film at a high pressure, or passing the hydrogen-releasing film through high pressure rolling rolls, it is possible to adjust the crystal grain size. Thus, the surface of the hydrogen-releasing film is smoothened by application of a higher pressure. As a result, the stress concentration to the interface between the crystal grains is suppressed, and thus cracks are further less likely to occur in the hydrogen-releasing film.

[0034] The rolling method may be a hot rolling method or a cold rolling method. The rolling method is a method comprising rotating a pair or pairs of rolls (rollers) and processing a raw material, Pd alloy into a film by passing it between the rolls under pressure.

[0035] The thickness of the hydrogen-releasing film obtained by the rolling method is preferably 5 to 50 m, more preferably 10 to 30 m. If the thickness of the film is less than 5 m, pinholes or cracks are likely to occur in the production of the film, and deformation of such a film easily occurs after absorbing hydrogen. On the other hand, when the thickness of the film is more than 50 m, such a film is not desirable because its hydrogen-releasing performance is reduced due to a long time required for the hydrogen permeation and because the film is inferior in terms of cost.

[0036] The sputtering method is not particularly limited, and can be carried out by using a sputtering apparatus such as a parallel flat plate type sputtering apparatus, a sheet type sputtering apparatus, a passing type sputtering apparatus, a DC sputtering apparatus, and an RF sputtering apparatus. For example, after having attached a substrate to a sputtering apparatus in which a PdAg alloy target is placed, the sputtering apparatus is evacuated, adjusted to a predetermined pressure value with an Ar gas, and a predetermined sputtering current is charged to the PdAg alloy target, thereby to form a PdAg alloy film on the substrate. Then, the PdAg alloy film is peeled off from the substrate to obtain a hydrogen-releasing film. It should be noted that it is possible to use, as the target, a single or multiple targets according to the hydrogen-releasing film to be produced.

[0037] As the substrate, it includes, for example, a glass plate, a ceramic plate, a silicon wafer, and a metal plate such as aluminum and stainless steel.

[0038] The thickness of the hydrogen-releasing film obtained by the sputtering method is preferably 0.01 to 5 m, more preferably 0.05 to 2 m. If the thickness of the film is less than 0.01 m, not only may pinholes be formed, but also it is difficult to obtain a required mechanical strength. Also, when the film is peeled off from the substrate, it is likely to be damaged and its handling after the peeling becomes difficult. On the other hand, when the thickness of the film is more than 5 m, it takes time to produce the hydrogen-releasing film and such a film is inferior in regards to cost, which is not desirable.

[0039] The film area of the hydrogen-releasing film can be appropriately adjusted in consideration of the hydrogen permeation amount and the film thickness, but when the hydrogen-releasing film is used as a component of a safety valve, the film area is about 0.01 to 100 mm.sup.2. It should be noted that the film area in the present invention is an area of actually releasing hydrogen in the hydrogen-releasing film and does not include a portion coated with a ring-shaped adhesive which will be described later.

[0040] The hydrogen-releasing laminated film may be formed by providing a support on one side or both sides of the hydrogen-releasing film. In particular, since the hydrogen-releasing film obtained by the sputtering method has a thin film thickness, it is preferable to laminate a support on one side or both sides of the hydrogen-releasing film in order to improve the mechanical strength.

[0041] FIG. 1 and FIG. 2 are each a schematic sectional view showing the structure of a hydrogen-releasing laminated film 1 of the present invention. As shown in FIG. 1 (a) or 1(b), a support 4 may be laminated on one side or both sides of a hydrogen-releasing film 2 using a ring-shaped adhesive 3, and as shown in FIG. 2 (a) or 2(b), the support 4 may be laminated on one side or both sides of the hydrogen-releasing film 2 using a jig 5.

[0042] The support 4 is hydrogen permeable and is not particularly limited as long as it can support the hydrogen-releasing film 2. The support may be a non-porous body or may be a porous body. Also, the support 4 may be a woven fabric or may be a non-woven fabric. As a material for forming the support 4, it includes, for example, polyolefin such as polyethylene and polypropylene, polyester such as polyethylene terephthalate and polyethylene naphthalate, polyarylethersulfone such as polysulfone and polyethersulfone, fluororesin such as polytetrafluoroethylene and polyvinylidene fluoride, epoxy resin, polyamide, polyimide, polyamide-imide, aramid and the like. Of these, at least one kind selected from the group consisting of polytetrafluoroethylene, polysulfone, polyimide, polyamide-imide, and aramid, which are chemically and thermally stable, is preferably used.

[0043] The thickness of the support 4 is not particularly limited, but is usually about 5 to 1000 m, preferably 10 to 300 m.

[0044] When producing the hydrogen-releasing film 2 by the sputtering method, such film can be directly formed on the support 4 which is used as a substrate and the hydrogen-releasing laminated film 2 can be produced without using the adhesive 3 or jig 5. Thus, this method is preferable from the viewpoint of physical properties and production efficiency of the hydrogen-releasing laminated film 1. In that case, it is preferable to use, as the support 4, a porous body having an average pore diameter of 100 m or less, more preferable to use a porous body having an average pore diameter of 5 m or less, and particularly preferable to use an ultrafiltration membrane (UF membrane).

[0045] The shape of the hydrogen-releasing film and the hydrogen-releasing laminated film of the present invention may be substantially circular or polygonal such as triangle, square, and pentagon. Any shape can be taken depending on the application to be described later.

[0046] The hydrogen-releasing film and the hydrogen-releasing laminated film of the present invention are particularly useful as a component of a safety valve for an aluminum electrolytic capacitor or a lithium ion battery. Furthermore, the hydrogen-releasing film and the hydrogen-releasing laminated film of the present invention may be provided on an electrochemical element as a hydrogen-releasing valve aside from the safety valve.

EXAMPLES

[0047] Description will be given of the invention with examples, while the invention is not limited to description in the examples.

Example 1

[Preparation of Hydrogen-Releasing Film by Rolling Method (Content of Ag: 20 mol %)]

[0048] The raw materials Pd and Ag were each weighed so that the content of Ag in an ingot became 20 mol %, charged into an arc melting furnace equipped with a water-cooled copper crucible and subjected to arc melting in an Ar gas atmosphere under atmospheric pressure. The obtained button ingot was cold-rolled to a thickness of 5 mm using a two-stage rolling mill having a diameter of 100 mm to obtain a rolled sheet material. Then the rolled sheet material was placed in a glass tube and the both ends of the glass tube were sealed. After reducing the inside pressure of the glass tube to 510.sup.4 Pa at room temperature, the temperature was then raised to 700 C. and the glass tube was allowed to stand for 24 hours, followed by cooling to room temperature. By this heat treatment, the segregation of Pd and Ag in the alloy was removed. Then, the sheet material was cold-rolled to 100 m using a two-stage rolling mill having a roll diameter of 100 mm and further cold-rolled to 25 m using a two-stage rolling mill having a roll diameter of 20 mm. Then the rolled sheet material was placed in a glass tube and the both ends of the glass tube were sealed. The inside pressure of the glass tube was reduced to 510.sup.4 Pa at room temperature, the temperature was then raised to 700 C., and the glass tube was allowed to stand for 1 hour, followed by cooling to room temperature. By this heat treatment, the internal strain in the PdAg alloy caused by rolling was removed, to prepare a hydrogen-releasing film containing PdAg and having a thickness t of 25 m and an Ag content of 20 mol %.

Example 2

[Preparation of Hydrogen-Releasing Film by Rolling Method (Content of Ag: 22 mol %)]

[0049] A hydrogen-releasing film containing PdAg and having a thickness t of 25 m and an Ag content of 22 mol % was prepared in the same manner as in Example 1, except that the raw materials Pd and Ag were respectively used so that the content of Ag in an ingot became 22 mol %.

Example 3

[Preparation of Hydrogen-Releasing Film by Rolling Method (Content of Ag: 60 mol %)]

[0050] A hydrogen-releasing film containing PdAg and having a thickness t of 25 m and an Ag content of 60 mol % was prepared in the same manner as in Example 1, except that the raw materials Pd and Ag were respectively used so that the content of Ag in an ingot became 60 mol %.

Example 4

[Preparation of Hydrogen-Releasing Film by Rolling Method (Content of Ag: 19.8 mol %)]

[0051] A hydrogen-releasing film containing PdAg and having a thickness t of 25 m and an Ag content of 19.8 mol % was prepared in the same manner as in Example 1, except that the raw materials Pd and Ag were respectively used so that the content of Ag in an ingot became 19.8 mol %.

Example 5

[Preparation of Hydrogen-Releasing Laminated Film by Sputtering Method (Content of Ag: 20 mol %)]

[0052] A polysulfone porous sheet (pore diameter: 0.001 to 0.02 m, manufactured by NITTO DENKO CORPORATION) as a support was attached to an RF magnetron sputtering apparatus (manufactured by Sanyu Electron Co., Ltd.) equipped with a PdAg alloy target in which the content of Ag is 20 mol %. Then, after evacuation of air in the sputtering apparatus to 110.sup.5 Pa or less, a sputtering current of 4.8 A was applied to the PdAg alloy target under 300 C. and an Ar gas pressure of 1.0 Pa to form a PdAg alloy film with 400 nm thickness t (Ag content: 20 mol %) on a polysulfone porous sheet to prepare a hydrogen-releasing laminated film.

Example 6

[Preparation of Hydrogen-Releasing Laminated Film by Sputtering Method (Content of Ag: 19.8 mol %)]

[0053] A PdAg alloy film (Ag content: 19.8 mol %) having a thickness t of 400 nm was formed in the same manner as in Example 5, except that a PdAg alloy target having an Ag content of 19.8 mol % was used, whereby a hydrogen-releasing laminated film was prepared.

Example 7

[Preparation of Hydrogen-Releasing Laminated Film by Sputtering Method (Content of Cu: 53 mol %)]

[0054] A PdCu alloy film (Cu content: 53 mol %) having a thickness t of 400 nm was formed in the same manner as in Example 5, except that a PdCu alloy target having an Cu content of 53 mol % was used, whereby a hydrogen-releasing laminated film was prepared.

Example 8

[Preparation of Hydrogen-Releasing Laminated Film by Sputtering Method (Content of Au: 20 mol %)]

[0055] A PdAu alloy film (Au content: 20 mol %) having a thickness t of 400 nm was formed in the same manner as in Example 5, except that a PdAu alloy target having an Au content of 20 mol % was used, whereby a hydrogen-releasing laminated film was prepared.

Example 9

[Preparation of Hydrogen-Releasing Film by Rolling Method (Content of Au: 30 mol %)]

[0056] A hydrogen-releasing film containing PdAu and having a thickness t of 25 m and an Au content of 30 mol % was prepared in the same manner as in Example 1, except that the raw materials Pd and Au were respectively used so that the content of Au in an ingot became 30 mol %.

Example 10

[Preparation of Hydrogen-Releasing Film by Rolling Method (Content of Au: 40 mol %)]

[0057] A hydrogen-releasing film containing PdAu and having a thickness t of 25 m and an Au content of 40 mol % was prepared in the same manner as in Example 1, except that the raw materials Pd and Au were respectively used so that the content of Au in an ingot became 40 mol %.

Example 11

[Preparation of Hydrogen-Releasing Laminated Film by Sputtering Method (Content of Au: 30 mol %)]

[0058] A PdAu alloy film (Au content: 30 mol %) having a thickness t of 400 nm was formed in the same manner as in Example 5, except that a PdAu alloy target having an Au content of 30 mol % was used, whereby a hydrogen-releasing laminated film was prepared.

Example 12

[Preparation of Hydrogen-Releasing Laminated Film by Sputtering Method (Content of Au: 40 mol %)]

[0059] A PdAu alloy film (Au content: 40 mol %) having a thickness t of 400 nm was formed in the same manner as in Example 5, except that a PdAu alloy target having an Au content of 40 mol % was used, whereby a hydrogen-releasing laminated film was prepared.

Comparative Example 1

[Preparation of Hydrogen-Releasing Laminated Film by Sputtering Method (Content of Ag: 19.8 mol %)]

[0060] A PdAg alloy film (Ag content: 19.8 mol %) having a thickness t of 400 nm was formed in the same manner as in Example 5, except that a PdAg alloy target having an Ag content of 19.8 mol % was used and the temperature at the time of sputtering was 25 C., whereby a hydrogen-releasing laminated film was prepared.

Comparative Example 2

[Preparation of Hydrogen-Releasing Laminated Film by Sputtering Method (Content of Ag: 20 mol %)]

[0061] A PdAg alloy film (Ag content: 20 mol %) having a thickness t of 400 nm was formed in the same manner as in Example 5, except that the temperature at the time of sputtering was 25 C., whereby a hydrogen-releasing laminated film was prepared.

[Measurement and Evaluation Methods]

(Measurement of Crystal Grain Size of Hydrogen-Releasing Film)

[0062] The surface of the produced hydrogen-releasing film was photographed using an optical microscope (ECLIPSE ME600, manufactured by Nikon Corporation) at a magnification of 50 times. Then, the photographed image was binarized using image analysis software (the United States National Institutes of Health [NIH], open source, Image J). In the binarization, the crystal grains were to be displayed in the bright part. Thereafter, the crystal grains were highlighted by correcting the brightness and contrast, and only the crystal grains were selected through setting of a threshold to obtain a binarized image. Then, the resulting binarized image was analyzed using image analysis software (A-ZO KUN, manufactured by Asahi Kasei Engineering Corporation). Incidentally, the bright part in the binarized image was taken as crystal grains, and the crystal grains overlapping the outer edge sides of the rectangular analysis range (3 mm2 mm) were excluded from the analysis. In addition, in the binarized image, when there were voids in the inside of the crystal grains gathered together, processing of filling such voids was not performed. Further, in the binarized image, processing of separating the crystal grains in contact with each other was not performed. The equivalent circle diameter determined by the above operation was taken as the crystal grain diameter (crystal grain size).

(Measurement of Crystal Grain Size of Hydrogen-Releasing Laminated Film)

[0063] The surface of the produced hydrogen-releasing laminated film was photographed using a scanning electron microscope (S-3000N, manufactured by Hitachi High-Technologies Corporation) at a magnification of 100000 times. Then, the photographed image was binarized using image analysis software (the United States National Institutes of Health [NIH], open source, Image J). In the binarization, the crystal grains were to be displayed in the bright part. Thereafter, the crystal grains were highlighted by correcting the brightness and contrast, and only the crystal grains were selected through setting of a threshold to obtain a binarized image. Then, the resulting binarized image was analyzed using image analysis software (A-ZO KUN, manufactured by Asahi Kasei Engineering Corporation). Incidentally, the bright part in the binarized image was taken as crystal grains, and the crystal grains overlapping the outer edge sides of the rectangular analysis range (1.5 m1 m) were excluded from the analysis. In addition, in the binarized image, when there were voids in the inside of the crystal grains gathered together, processing of filling such voids was not performed. Further, in the binarized image, processing of separating the crystal grains in contact with each other was not performed. The equivalent circle diameter determined by the above operation was taken as the crystal grain diameter (crystal grain size).

(Measurement of Hydrogen Permeation Coefficient)

[0064] The prepared hydrogen-releasing film or the prepared hydrogen-releasing laminated film was attached to a VCR connector manufactured by Swagelok Company, and an SUS tube was attached to one side of the connector. In this way, a sealed space (63.5 ml) was produced. After the pressure inside the tube was reduced by a vacuum pump, the pressure of the hydrogen gas was adjusted to 0.15 MPa, and a pressure change in an environment of 50 C. was monitored. Since the number of moles of hydrogen transmitted through the hydrogen-releasing film can be known by the pressure change, a hydrogen permeation coefficient was calculated by substituting the number of moles of hydrogen into the expression 2 below. In addition, the effective film area s of the hydrogen-releasing film used for the measurement is 3.8510.sup.5 m.sup.2, and the effective film area s of the hydrogen-releasing laminated film is 7.0710.sup.6 m.sup.2.

Hydrogen permeation coefficient=(Number of moles of hydrogenFilm thickness t)/(Film area stimesquare root of pressure)<Expression 2>

(Evaluation of Deterioration of Hydrogen-Releasing Film)

[0065] The produced hydrogen-releasing film or the produced hydrogen-releasing laminated film was attached and fixed to a hydrogen tank with a double-sided pressure-sensitive adhesive tape (No. 5615, manufactured by Nitto Denko Corporation). Thereafter, the hydrogen partial pressure in the hydrogen tank was adjusted to 0.05 MPa, and the film was allowed to stand in an environment of 50 C. for 12 hours. Then, the state of the hydrogen-releasing film was observed, and evaluated based on the following criteria.

: No change when observed with a microscope (magnification of 100 times)
: There were a few cracks when observed with a microscope (magnification of 100 times)
x: Broken to pieces

(Evaluation of Hydrogen Embrittlement of Hydrogen-Releasing Film Prepared by Rolling Method)

[0066] The prepared hydrogen-releasing film was placed in a glass tube and the both ends of the glass tube were sealed. The inside pressure of the glass tube was reduced to a pressure of 510.sup.3 Pa at 50 C., and the temperature was then raised to 400 C. Then hydrogen gas was introduced into the glass tube and allowed to stand for one hour under an atmosphere of 105 kPa. Thereafter, the glass tube was cooled to room temperature and the inside of the glass tube was evacuated to a pressure of 510.sup.3 Pa (30 minutes). Then, hydrogen gas was introduced into the glass tube again and allowed to stand for one hour under an atmosphere of 105 kPa. After repeating the above operation three times, the hydrogen-releasing film was removed from the glass tube and the appearance of the hydrogen-releasing film was visually observed and evaluated by the following criteria.

: No change in appearance such as distortion
x: Change in appearance such as distortion

(Evaluation of Hydrogen Embrittlement of Hydrogen-Releasing Laminated Film Prepared by Sputtering Method)

[0067] The prepared hydrogen-releasing laminated film was placed in a glass tube and the both ends of the glass tube were sealed. After the inside of the glass tube was reduced to a pressure of 510.sup.3 Pa at 50 C., hydrogen gas was introduced into the glass tube and the glass tube was allowed to stand for one hour under an atmosphere of 105 kPa. Thereafter, the hydrogen-releasing laminated film was removed from the glass tube and the surface of the film was observed by SEM, followed by evaluation using the following criteria.

): There were no cracks
x: There were cracks

TABLE-US-00001 TABLE 1 Example Example Example Example Example Example Example 1 2 3 4 5 6 7 Production Rolling Rolling Rolling Rolling Sputtering Sputtering Sputtering method method method method method method method method Composition PdAg PdAg PdAg PdAg PdAg PdAg PdCu of alloy (22%) (20%) (60%) (19.8%) (20%) (19.8%) (53%) Crystal grain 500 400 430 350 0.045 0.035 0.035 size (m) Hydrogen 2 10.sup.10 2 10.sup.10 1 10.sup.13 3 10.sup.9 2 10.sup.9 2 10.sup.9 4 10.sup.9 permeation coefficient (mol .Math. m.sup.1 .Math. sec.sup.1 .Math. Pa.sup.1/2) t/s (m.sup.1) 0.65 0.65 0.65 0.65 0.057 0.057 0.057 Evaluation of deterioration of hydrogen- releasing film Evaluation X X of hydrogen brittleness of hydrogen- releasing film Comparative Comparative Example Example Example Example Example Example Example 8 9 10 11 12 1 2 Production Sputtering Rolling Rolling Sputtering Sputtering Sputtering Sputtering method method method method method method method method Composition PdAu PdAu PdAu PdAu PdAu PdAg PdAg of alloy (20%) (30%) (40%) (30%) (40%) (19.8%) (20%) Crystal grain 0.055 530 520 0.060 0.065 0.027 0.025 size (m) Hydrogen 7 10.sup.10 3.8 10.sup.10 8.9 10.sup.11 4 10.sup.10 7 10.sup.11 1 10.sup.9 1 10.sup.9 permeation coefficient (mol .Math. m.sup.1 .Math. sec.sup.1 .Math. Pa.sup.1/2) t/s (m.sup.1) 0.057 0.65 0.65 0.057 0.057 0.057 0.057 Evaluation of X X deterioration of hydrogen- releasing film Evaluation X X of hydrogen brittleness of hydrogen- releasing film