JP2000265233A - Hydrogen storage alloy - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a hydrogen storage alloy having high hydrogenation velocity and high dehydrogenation velocity even if activation treatment is omitted. SOLUTION: The hydrogen storage alloy is manufactured by mechanical alloying and is constituted of a matrix which is composed of an aggregate of plural Mg crystalline grains and in which the average grain diameter D of the Mg crystalline grains is regulated to <=500 nm, plural fine particles of intermetallic compound which are dispersed in the matrix and whose particle diameter d satisfies 50 nm<=d1<=500 nm, and plural fine particles which are dispersed in the plural Mg crystalline grains and whose particle diameter d2 satisfies 5 nm<=d2<=20 nm.

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, particularly to a Mg-based hydrogen storage alloy.

[0002]

2. Description of the Related Art Conventionally, as a hydrogen storage alloy of this kind, M
What consists of a g-Ni alloy is known (for example,
See Japanese Patent Publication No. 7-84636.

[0003]

However, conventional alloys require activation after smelting, which leads to an increase in man-hours and costs, and despite the activation, There was a problem that the hydrogenation rate and the dehydrogenation rate were still slow.

[0004]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a hydrogen storage alloy having a high hydrogenation rate and a high dehydrogenation rate without performing an activation treatment.

[0005] To achieve the above object, according to the present invention,
A matrix comprising an aggregate of a plurality of Mg crystal grains and having an average particle size D of D ≦ 500 nm, and a matrix dispersed in the matrix and having a particle size d ₁ of 50 nm
A plurality of fine particles of an intermetallic compound satisfying ≦ d ₁ ≦ 500 nm,
Dispersed in the plurality of Mg crystal grains and having a particle size d ₂ of 5 nm
There is provided a hydrogen storage alloy comprising a plurality of ultrafine particles satisfying ≦ d ₂ ≦ 20 nm.

[0006] The hydrogen storage process in a hydrogen storage alloy can be roughly divided into the following first and second stages. First stage: adsorption of hydrogen molecules on the surface of the hydrogen storage alloy and dissociation of the hydrogen molecules into hydrogen atoms, second stage: diffusion of hydrogen atoms into the hydrogen storage alloy.

[0007] Among the three constituent requirements of the hydrogen storage alloy, the intermetallic compound fine particles activate the matrix,
Contribute to the promotion of the first stage. The ultrafine particles contained in the matrix and its crystal grains contribute to the latter stage of the first stage, that is, the dissociation of hydrogen molecules into hydrogen atoms and the promotion of the second stage.

However, if the particle diameter d ₁ of the intermetallic compound fine particles is outside the above range, the effect of activating the matrix and promoting the first stage cannot be obtained. Also the average particle diameter D and the ultrafine particle size d ₂ of the matrix deviates from the above range, no effect of dissociation and the second stage acceleration of the hydrogen atoms of the hydrogen molecules is obtained.

On the other hand, the hydrogen release process in the hydrogen storage alloy can be roughly divided into the following first and second stages. First step: assembly of hydrogen atoms on the surface of the hydrogen storage alloy by diffusion, second step: generation of hydrogen molecules by bonding of hydrogen atoms and detachment of hydrogen molecules from the surface of the hydrogen storage alloy.

Of the three constituent elements of the hydrogen storage alloy, the ultrafine particles contained in the matrix and its crystal grains are used for promoting the first stage and for generating hydrogen molecules by bonding hydrogen atoms before the second stage. Contribute. The intermetallic compound fine particles activate the matrix and contribute to the promotion of the second stage.

However, if the average crystal grain size D of the matrix and the grain size d _{2 of the} ultrafine particles deviate from the above ranges, the first
The effect of the generation of hydrogen molecules by the step promotion and the bonding of hydrogen atoms cannot be obtained. Also, the particle diameter d ₁ of the intermetallic compound fine particles
Is out of the above range, the effect of activating the matrix and promoting the second stage cannot be obtained.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS As shown in FIG. 1, a hydrogen storage alloy comprises an aggregate of a plurality of Mg crystal grains.
a matrix in which the average particle size D of the g crystal grains is D ≦ 500 nm;
₁ and a plurality of intermetallic compound fine particles is 50 nm ≦ d ₁ ≦ 500 nm, dispersed in a plurality of Mg crystal grains, and the particle diameter d ₂
Is composed of a plurality of ultrafine particles satisfying 5 nm ≦ d ₂ ≦ 20 nm. The particle size refers to the length of the longest part such as Mg crystal grains in a microstructure diagram (or a micrograph showing a metal structure).

For example, the hydrogen storage alloy is composed of a Mg—Ni alloy, in which the intermetallic compound fine particles are Mg ₂ Ni fine particles and the ultrafine particles are Mg—Ni fine particles.
Ni-based ultrafine particles. In this case, the Mg content is 66.
7 atomic% ≦ Mg ≦ 99 atomic%, and the Ni content is set to 1 atomic% ≦ Ni ≦ 33.3 atomic%. N
If the i content is Ni <1 atomic%, Mg ₂ Ni fine particles and Mg—Ni-based ultrafine particles cannot be formed.
At Ni> 33.3 atomic%, the entire matrix is Mg ₂ N
i, it is not possible to obtain the above-mentioned microscopic structure. In order to realize a high hydrogen storage amount of 4 wt% or more, the upper limit of Ni is set to 27 atomic%.

Hereinafter, a specific example will be described. (A) was 99.9% Manufacturing purity each of the hydrogen storage alloy by mechanical alloying, and the Mg powder and Ni powder is smaller than 100 mesh particle size, respectively, the composition of the hydrogen storage alloy is Mg ₉₅ Ni _It was weighed so as to be ₅ (the unit of the numerical value was atomic%) to obtain a total of 3 g of the mixed powder. A planetary ball mill (Furitsch,
P-5) Pot with 80 ml capacity (JISSUS316)
10mm diameter ball (JIS SUS316)
Eighteen were placed together, and the pot was evacuated to 10 ^-3 Torr. After evacuation, hydrogen pressure of 2 MPa was applied in the pot, and ball milling was performed under the conditions of a pot rotation speed of 780 rpm, a disk rotation speed of 360 rpm, and a processing time of 10 hours. After ball milling, the hydrogen storage alloy powder was collected in a glove box. This is an example. (B) Production of hydrogen storage alloy by smelting method Mg powder and Ni powder each having a purity of 99.9% were weighed so that the composition of the hydrogen storage alloy was Mg ₉₅ Ni ₅ (the unit of the numerical value is atomic%). Then, the weighed material was subjected to high frequency melting and then cast to obtain an ingot. This ingot is crushed and classified in a glove box,
A hydrogen storage alloy powder having a particle size smaller than 10 μm was obtained. Further, this powder was subjected to an activation treatment. In the activation treatment, the powder was placed in a container, and the inside of the container was evacuated to 10 ⁻⁴ Torr at 350 ° C.
The hydrogen pressurization of a was performed, and this was repeated for 5 cycles as one cycle. The hydrogen storage alloy powder thus obtained is used as a comparative example. (C) Observation of metal structure For the examples, transmission electron microscope and attached EDX
The metal structure was observed using (energy dispersive X-ray diffraction). FIG. 2 shows the microstructure of the example, in which a matrix composed of an aggregate of Mg crystal grains,
Mg ₂ Ni fine particles dispersed in the matrix,
Mg-Ni-based ultrafine particles dispersed in the crystal grains are observed. FIG. 3 shows a microscopic structure corresponding to the case where FIG. 2 is partially enlarged. In this drawing, Mg crystal grains and Mg-Ni-based ultrafine particles dispersed therein are observed.

The metal structure of the comparative example is coarser than that of the example due to the fact that the metal structure is obtained by the melting method, and the average particle size of the Mg crystal grains and the particle size of the Mg ₂ Ni particles are respectively About 1 μm, and Mg-
No Ni-based ultrafine particles were found. (D) Hydrogen absorption / desorption characteristics and PCT line For the examples and comparative examples, 300 ° C. and 300 ° C. were determined in accordance with the vacuum origin method specified in the pressure-composition isotherm (PCT line) measurement method (JISH7201) by the volumetric method. A hydrogenation rate test was performed at 350 ° C., and a dehydrogenation rate test was performed at 300 ° C.

FIGS. 4 and 5 show the results of hydrogenation rate tests at measurement temperatures of 300 ° C. and 350 ° C., respectively. In this test, a high-pressure hydrogen pressurization of 3.2 MPa was performed from a vacuum state. The Example and the Comparative Example have the same composition (Mg ₉₅ N
i ₅₎ a is in spite, and in between them occurs a large difference relates hydrogenation rate, example, 60 sec after the hydrogen introduction
It has an excellent hydrogenation property of absorbing 5 wt% or more of hydrogen between them. Further, the examples finally have a high hydrogen storage amount of 6 wt% or more.

FIG. 6 shows the results of a dehydrogenation rate test at a measurement temperature of 300 ° C. In this case, the initial set hydrogen pressure was 0.03 MPa due to the plateau pressure at 300 ° C. of the example and the comparative example and restrictions on the specifications of the apparatus. As is clear from FIG. 6, in the example, the slope of the hydrogen release curve after the start of the release is extremely steep as compared with the comparative example, and therefore, the example has an excellent dehydrogenation rate. I understand. Note that the amount of released hydrogen in the example was about 5.1.
The reason why the concentration is constant at wt% is that the hydrogen pressure in the sample container increases with the release of hydrogen, and reaches the equilibrium dissociation pressure when 5.1 wt% is released.

FIG. 7 shows a hydrogen release curve as a result of the measurement for the example and the comparative example. The measurement conditions were as follows: measurement temperature: 300 ° C .; pressure stabilization time: 20 sec; convergence time: 5 min; convergence judgment time: 1 min;
01 MPa; plateau judgment: 0.3 Log (P) / (w
t%). In this measurement, the convergence time was set to 5 min, so that the plateau pressure was extremely low in the comparative example because the reaction speed was slow. The time required for the measurement of the hydrogen release curve was 2 hours and 50 minutes in the case of the example, while
In the case of the comparative example, it took 20 hours. Therefore, it can be seen that under the measurement conditions that can be applied industrially, a large difference in plateau pressure appears between the example and the comparative example, and a difference also occurs in the hydrogen storage amount.

[0019]

According to the present invention, with the above-described structure, a high hydrogenation rate and a high hydrogen storage amount can be obtained without performing the activation treatment, and the dehydrogenation rate is also high. Thus, it is possible to provide a hydrogen storage alloy having excellent practicality and a wide range of industrial applications.

[Brief description of the drawings]

FIG. 1 is a schematic view of a microstructure of a hydrogen storage alloy.

FIG. 2 is a microscopic organization diagram of an example.

FIG. 3 is a microscopic organization diagram corresponding to a case where FIG. 2 is partially enlarged.

FIG. 4 is a graph showing a relationship between an elapsed time and a hydrogenation amount in a hydrogenation rate test in which a measurement temperature is set to 300 ° C.

FIG. 5 is a graph showing a relationship between an elapsed time and a hydrogenation amount in a hydrogenation rate test in which a measurement temperature is set to 350 ° C.

FIG. 6 is a graph showing a relationship between an elapsed time and a hydrogenation amount in a dehydrogenation rate test in which a measurement temperature is set to 300 ° C.

FIG. 7 shows a hydrogen release curve at a measurement temperature of 300 ° C.

Claims (3)

[Claims] 1. A matrix comprising an aggregate of a plurality of Mg crystal grains and having an average particle size D of D ≦ 500 nm, and a matrix dispersed in the matrix and having a particle size d ₁ of 50 nm ≦ d. ₁ ≦ 500 nm and a plurality of ultrafine particles dispersed in the Mg crystal grains and having a particle diameter d ₂ of 5 nm ≦ d ₂ ≦ 20 nm. Characteristic hydrogen storage alloy. 2. The hydrogen storage alloy according to claim 1, wherein the intermetallic compound fine particles are Mg ₂ Ni fine particles. 3. The hydrogen storage alloy according to claim 1, wherein the ultrafine particles are Mg—Ni-based ultrafine particles.