JPH10245663A - Production of hydrogen storage alloy - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing hydrogen storage alloy capable of controlling its hydrogen occluding characteristics to those in accordance with use, as to the production of a hydrogen storage alloy, by allowing, particularly, a plateau region flattened by homogenizing treatment to have a tilt again by the subsequent aging treatment and suitably selecting this aging treatment. SOLUTION: As primary heat treatment after melting and casting, homogenizing treatment of executing heating at 850 to 1400 deg.C for 10min to 30hr is performed, and after that, as secondary heat treatment, aging treatment of executing heating at 400 to 800 deg.C for 10min to 100hr is performed. The alloy having hydrogen occluding characteristics is essentially consisting of BCC phases, and furthermore, the aging treatment precipitates matching grains having the same crystal structure by fine two phase separation at the inside of the BCC phases.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the production of a hydrogen storage alloy, and more particularly, to a method in which a plateau region flattened by a homogenization treatment has a slope again by a subsequent aging treatment, and the aging treatment is appropriately performed. And a method for producing a hydrogen storage alloy capable of controlling the hydrogen storage characteristics according to the application.

[0002]

2. Description of the Related Art Since hydrogen storage alloys can reversibly store and release hydrogen, they are used in applications such as energy storage tanks and nickel-metal hydride batteries, as well as heat pumps that utilize an endothermic reaction when releasing hydrogen. . As a prior art, for example, Japanese Patent Application Laid-Open No. 57-63669 discloses a method for producing a hydrogen absorbing alloy of a misch metal-nickel ternary alloy which is heated to 500 to 1000 ° C. and has a flat plateau region. And JP-A-7-65834.
Japanese Patent Application Laid-Open Publication No. H11-157, discloses a method in which heat treatment is performed at a temperature of 700 ° C. or higher and a melting point or lower for homogenization, and hydrogen is occluded and pulverized in a pressurized hydrogen atmosphere. These AB ₅ type alloys and AB ₂
It has been known that, in a hydrogen storage alloy of a type alloy, the inclination of the plateau region of the pressure-composition isotherm can be flattened by a high-temperature heat treatment. Originally, the plateau region of the pressure-composition isotherm is flat in principle for a single-phase alloy, but has a slope when cast as it is due to segregation during production, variation in composition, and precipitation of the second phase. Often. This can be flattened by heat treatment at a high temperature to form a single phase. Such a heat treatment process is generally called homogenization.

On the other hand, when a hydrogen storage tank is designed, in the case of an alloy having a completely flat plate region, the absorption and desorption can be easily controlled by controlling the temperature, but the hydrogen moves rapidly due to a small change in temperature or pressure. there's a possibility that. Therefore, a technique for controlling the plateau pressure at an appropriate inclination is required. For the storage tank, a method of calculating the remaining amount of the tank based on the released flow rate can be considered, but if the plate has a certain inclination within the practical operation range, the remaining amount can be obtained from the internal pressure of the tank. Therefore, technology development to control the inclination of the plateau is desired.

[0004]

SUMMARY OF THE INVENTION The object of the present invention is to provide a BC
It is an object of the present invention to examine a heat treatment for homogenizing a continuous concentration variation in a C solid solution type alloy and to provide a method for producing a hydrogen storage alloy capable of controlling a plateau equilibrium pressure at an appropriate slope. It is another object of the present invention to provide a method for producing a hydrogen storage alloy which can consider heat treatment for obtaining a flat plateau equilibrium pressure and realize the optimization of the alloy composition and alloy structure. Further, another object of the present invention is to
By providing a relatively simple heat treatment from a flat plateau equilibrium pressure state to provide a constant inclination within the practical operation range again, and to provide a method of manufacturing a hydrogen storage alloy capable of controlling the inclination. It is in.

[0005]

The object of the present invention is to provide a first stage heat treatment after melting and casting, which is performed at a temperature of 850 ° C. or higher and 1400 ° C.
Perform the homogenization process to heat for 10 minutes to 30 hours below,
Thereafter, as a second stage heat treatment, 400 to 800
This is achieved by a method for producing a hydrogen storage alloy, which comprises performing an aging treatment by heating at a temperature of 10 minutes to 100 hours. Further, the above object is also achieved by a method for producing a hydrogen storage alloy in which the alloy having hydrogen storage properties is mainly composed of a BCC phase. In addition, the above objectives are
The aging treatment can also be achieved by a method for producing a hydrogen storage alloy in which consistent particles having the same crystal structure are precipitated by fine two-phase separation inside the BCC phase.

[0006]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the present invention, the plateau region is flattened by eliminating the concentration variation by the homogenizing annealing. Further, by performing the aging treatment, fine particles are separated by two-phase separation from the BCC phase having no concentration variation, whereby the plateau region can be inclined again.
Further, by appropriately selecting the aging treatment conditions, it is possible to control the inclination according to the use. As an example of this application, for example, there is a hydrogen tank. In this case, it is not preferable that rapid movement of hydrogen is caused by a small change in temperature and pressure, and the present invention can be applied to this.

The present invention has been achieved by the following findings of the present inventors. When the aging treatment is performed after the homogenization performed on the BCC solid solution type alloy, a variation due to a continuous change in the composition is likely to occur. Furthermore, fine two-phase separation of β → β1 + β2 due to concentration fluctuation on the order of nanometers has also been confirmed. As a method for controlling the flatness of the plate region of the BCC type alloy, the following combination which is a feature of the present invention is optimal. First, for the purpose of homogenizing the continuous concentration variation, at 850 ° C. to 1400 ° C., 10 min to 30
h). Next, after the heat treatment, the homogenized alloy is further subjected to an aging heat treatment at a constant temperature of 400 ° C. to 800 ° C. for 10 minutes to 100 hours. The purpose of this heat treatment is to grow fine two-phase separation inside the BCC phase. By such growth, the inclination of the platen region can be increased again. In the first stage heat treatment, at a homogenization temperature lower than 850 ° C., the effect of homogenizing the concentration cannot be expected. On the other hand, a temperature exceeding 1400 ° C. deviates from the industrial heat treatment, and causes a problem in equipment and the like.
The homogenization time is related to the homogenization temperature,
If it is less than min, it is not sufficient for homogenization, and if it exceeds 30 h, its effect tends to be saturated.

[0008] As the second stage heat treatment, aging treatment is 400
If the aging temperature is lower than ℃, the above-mentioned effect of precipitation cannot be expected.
On the other hand, when the temperature exceeds 800 ° C., the effect is saturated. The aging time is related to the aging temperature. However, if the aging time is less than 10 minutes, the precipitation is insufficient, and if the aging time exceeds 100 hours, the effect tends to be saturated. Hereinafter, the present invention will be described in more detail with reference to the accompanying drawings.

[0009]

EXAMPLE The reason for setting the alloy composition in this example will be described. As shown in the ternary phase diagram of FIG. 1, in a Ti—Cr—V system as an alloy system, there is a region where a Laves (C14) phase and a BCC phase coexist, as in the Ti—Mn—V system. Therefore, in order to examine the effect of microstructure of nano-order in the present embodiment, V-TiC _r2 in FIG. 2
Referring to the pseudo-binary phase diagram of the line, the composition was set to Ti ₂₅ Cr ₃₅ V ₄₀ where no C14 phase appeared even when the alloy composition was heat-treated at a low temperature.

FIG. 3 shows pressure-composition isotherms of the Ti ₂₅ Cr ₃₅ V ₄₀ alloy as cast and after each heat treatment. In the alloy as cast by arc melting, the equilibrium pressure of the pressure-composition isotherm is greatly inclined. 400 to flatten this
The homogenization heat treatment was performed at a temperature of from 1200C to 1200C.
At temperatures up to 1000 ° C., little change was observed in the flatness of the plate, but in the heat treatment at 1200 ° C.,
The plate was largely flattened. As a study on such a flattening effect, FIG. 4 shows the result of composition analysis of the concentration distribution of each alloy element by EPMA between the as-cast Ti ₂₅ Cr ₃₅ V ₄₀ alloy and the heat-treated product at 1200 ° C. FIG.
(A) As-cast, FIG. 4 (b) shows a heat treatment at 1200 ° C. for 2 hours. Table 1 shows the results of analysis using the EDX attached to the TEM.

[0011]

[Table 1]

In the as-cast state, α-Ti dendrites are crystallized, and there is a positive concentration gradient of Ti toward the dendrites. There is a concentration difference of about 7 at% in the Ti-enriched and depleted portions of the matrix of the BCC phase, which is considered to increase the inclination of the plateau. On the other hand, in the heat-treated product, the α-Ti dendrites remain in the form of islands, but the matrix of the BCC phase is sufficiently homogenized, and the concentration difference is within 1 at%. It is considered that the slope of the equilibrium pressure plate of the line is due to the concentration distribution of Ti and Cr.

Next, the result of confirming the effect of the two-step heat treatment as the structure change by the heat treatment of the present embodiment will be described. FIG. 5 shows an as-cast Ti ₂₅ Cr ₃₅ V ₄₀ alloy (FIG. 5).
(A)) A transmission electron microscope structure of a sample homogenized by a heat treatment at 1200 ° C. for 5 hours (FIG. 5 (b)).
Inside the BCC phase of these samples, Zr-Ti-VM
No fine layered structure normally observed in n-type and Ti-Cr-V alloys is not found. Based on this state, aging treatment was further performed at a lower temperature to grow the internal structure of the BCC phase. FIG. 6 shows the results of pressure-composition isotherm measurement of the alloy heat-treated at different aging temperatures. The aging heat treatment clearly increases the inclination of the plate. Further, FIG. 7 shows the result of pressure-composition isotherm measurement of the alloy heat-treated at 700 ° C. for various times. Again, it was confirmed that the inclination of the plateau increased with the aging time.

By the way, as shown in FIG. 10 (a), when the plateau pressure is completely flat, even if the occlusion amount increases from X1 to X2 or decreases from X2 to X1, the pressure always becomes P0.
Is constant. On the other hand, as shown in FIG.
When the inclination is appropriate, the equilibrium pressure increases or decreases according to the amount of occlusion, so that the amount of occlusion can be known from the pressure. In the case of an application such as a hydrogen tank, it is not preferable to occlude or release the gas outside the platen region, but it is possible to design a system for detecting the limit point of the platen. Further, although the plateau pressure increases and decreases due to a temperature change, FIG.
As shown in FIG. 11A, when hydrogen is completely flat, the entire amount of hydrogen moves. However, as shown in FIG. It becomes possible to control the amount of occlusion and release due to a change in temperature by the characteristics of the alloy itself.

It is considered that such a plate inclination is caused by the growth of the modulation structure. For heat-treated samples at 700 ° C. × 20 h and 100 h in which the plate inclination is increased,
FIG. 8 (a) 700 shows the result of transmission electron microscope observation.
8C and 20 hours, and FIG. In the bright-field image of the heat-treated sample at 700 ° C for 20 hours, a granular precipitate different from the striped modulation structure is observed. In the more aged 100h sample,
It was confirmed that this precipitate was coarse. This precipitate has the same BCC structure as the matrix,
The composition analysis by X was almost the same as that of the matrix. From this result, it is considered that the granular precipitate is a result of the growth of the modulated structure of the matrix. Originally, a growth process is known in which the growth of a modulation structure formed by spinodal decomposition increases its concentration wavelength and gradually loses its consistency to show interfacial dislocations. On the other hand, in the present embodiment, 1200 ° C.
At × 5 h, the modulated structure is once dissolved, but it is considered that the modulated structure formed at an arbitrary temperature during the cooling process became the nucleus of the aging precipitation at 700 ° C., resulting in such a heterogeneous structure. .

From the above, it is conceivable that the cooling after the first-stage heat treatment has some influence on the precipitates. FIG.
Are the results of examining various solution treatment conditions for this examination, but in any case, no further flattening effect was obtained. This indicates that the effect of the cooling rate between the first-stage heat treatment and the second-stage heat treatment is small, and that it is possible to appropriately select cooling. In some cases, it is possible to directly lower the temperature from the first heat treatment temperature to the second heat treatment temperature.

[0017]

According to the present invention, the concentration gradient of the solute element existing on the order of microns is homogenized by the two-step heat treatment, whereby the flatness of the plate can be greatly improved. After growing, the inclination of the plateau flatness can be increased again. Further, by adjusting the heat treatment, the inclination of the platen region can be appropriately controlled.

[Brief description of the drawings]

FIG. 1 is a diagram showing the composition and lattice constant of an example in a TiCrV-based ternary alloy phase diagram according to the present invention.

FIG. 2 is a diagram showing a phase diagram of a TiCr ₂ -V pseudo binary alloy according to the present invention.

FIG. 3 is a diagram showing pressure composition isotherms as-cast and after homogenization according to an example of the present invention.

FIG. 4 is a diagram showing the results of analyzing the concentration distribution of alloy elements after (a) as-casting and (b) after homogenization according to the example of the present invention.

FIG. 5 is a transmission electron micrograph of a metal structure of a Ti ₂₅ Cr ₃₅ V ₄₀ alloy according to an example of the present invention after homogenization.
(A) As cast, (b) 1200 ° C. × 2 h.

FIG. 6 is a graph showing a pressure composition isotherm after aging treatment of a Ti ₂₅ Cr ₃₅ V ₄₀ alloy according to an example of the present invention.

FIG. 7 is a diagram showing a pressure composition isotherm according to the aging treatment time of the Ti ₂₅ Cr ₃₅ V ₄₀ alloy according to the example of the present invention.

FIG. 8 is a transmission electron micrograph of a metal structure after aging treatment of a Ti ₂₅ Cr ₃₅ V ₄₀ alloy according to an example of the present invention.
700 ° C. × 20 h, (b) 700 ° C. × 100 h.

FIG. 9 is a diagram showing a pressure composition isotherm according to a cooling rate after a homogenization treatment of a Ti ₂₅ Cr ₃₅ V ₄₀ alloy according to an example of the present invention.

FIGS. 10A and 10B are explanatory diagrams showing plateau flatness according to the present invention, showing (a) a flat plate and (b) a plate having a slope.

FIGS. 11A and 11B are explanatory diagrams showing plateau flatness according to the present invention, in which (a) completely transfers hydrogen and (b) partially transfers hydrogen.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁶ Identification code FI C22F 1/00 691 C22F 1/00 691B 691C

Claims (3)

[Claims] 1. After melting and casting, as a first stage heat treatment,
A homogenization treatment of heating at 850 ° C. or more and 1400 ° C. or less for 10 minutes to 30 hours is performed, and then, as a second stage heat treatment, an aging treatment of heating at 400 to 800 ° C. for 10 minutes to 100 hours is performed. Manufacturing method of hydrogen storage alloy. 2. The alloy having hydrogen storage properties is BCC.
The method for producing a hydrogen storage alloy according to claim 1, wherein the method mainly comprises a phase. 3. The method for producing a hydrogen storage alloy according to claim 2, wherein the aging treatment precipitates consistent particles having the same crystal structure by fine two-phase separation inside the BCC phase.