JPH10245653A - Hydrogen storage alloy excellent in initial activity and reaction rate - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a hydrogen storage alloy having alloy phases relatively easy to activate in particular and alloy phases relatively hard to activate as mixed phases and excellent in initial activation and the reaction rate as to a hydrogen storage alloy composed of many phases. SOLUTION: This hydrogen storage alloy is composed of alloy phases relatively easy to activate the occlusion of hydrogen and alloy phases relatively hard of the activation for improving the initial activation and reaction rate after the pulverization of the hydrogen storage alloy in the air. Furthermore, the alloy phases relatively easy of the activation are composed of Laves phases, and the alloy phases relatively hard of the activation are composed of BCC phases. Moreover, the hydrogen storage alloy is composed of a Ti-Mn-V alloy, the Laves phases are composed of C14 phases, and the phase fractional rate of the C14 phases is regulated to 3 to 80wt.%.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a multi-phase hydrogen storage alloy, and more particularly to a multi-phase hydrogen storage alloy having an alloy phase which is relatively easy to activate and an alloy phase which is relatively difficult to activate. The present invention relates to a hydrogen storage alloy having excellent activation and reaction rates.

[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. . When using a hydrogen storage alloy, an activation treatment for increasing the rate of hydrogen storage and release is first required. Normal,
The activation process is completed by evacuating the container containing the alloy powder while evacuating it and repeating hydrogen absorption and desorption several times.The conditions and frequency of this treatment depend on the type of hydrogen storage alloy and the poisoning of the alloy surface. It depends on the situation.

[0003] There have been many reports that a hydrogen storage alloy is made into a powder or a multiphase to improve the initial activity and the reaction rate. However, the alloys indicated as the main phases in these reports are all AB ₅ type alloys (LaNi ₅ , Mm
Ni ₅ etc.), AB ₂ type alloys (TiMn ₂ , ZrNi
₂ , Zr (FeV)) and AB type alloy (TiFe).

However, these alloys have an occlusion amount of 1 to 2 wt%, and the occlusion / release amount does not increase even if the initial activity, the reaction rate or the durability is improved. Also mixed 2nd
Regarding the effect of the phase, there are many descriptions of catalytic action,
The explanation of the actual mechanism and effect was ambiguous. Many of the mixed materials did not have a hydrogen storage effect, and the hydrogen storage amount as a whole did not increase. As a well-known technique, for example, Japanese Patent Application Laid-Open No. Sho 60-135538 discloses a hydrogen storage having a mixed structure of a first phase having a hydrogen storage action and a second phase having a catalytic action, and having a eutectic structure in part. A method for making an alloy is disclosed.

Japanese Patent Application Laid-Open No. Sho 56-17901 discloses a hydrogen storage alloy in which Fe-Ti oxide is dispersed in Fe-Ti and no activation treatment is required. However, these have an AB type alloy phase as a main phase. On the other hand, according to the findings of the present inventors, in an alloy in which the Laves phase and the BCC phase coexist in two phases,
The amount of hydrogen storage is described by the compound rule between its constituent phases.In the properties of these alloys, characteristics other than equilibrium characteristics such as hydrogen storage amount and equilibrium pressure, such as ease of activation and reaction rate, should be evaluated. Turned out to be important. BCC
It has been considered that high-capacity alloys such as molds often have problems with the reaction speed and the easiness of activation, and this has been an obstacle to practical application. Therefore, it is desired to improve the activation characteristics by examining the easiness of activation and the magnitude of the reaction rate by examining the interaction between multiple phases, particularly the effect of the Laves phase dispersed in the BCC phase. I was

[0006]

SUMMARY OF THE INVENTION It is an object of the present invention to study the improvement of the activation characteristics of a hydrogen storage alloy comprising a BCC phase and a Laves phase as described above. An object of the present invention is to provide a hydrogen storage alloy having an improved reaction rate. Another object of the present invention is to reduce the incubation period by activating the Laves phase as the second phase upon activation after pulverization in air to form a fresh BCC phase surface. It is an object of the present invention to provide a hydrogen storage alloy which enables the following. Still another object of the present invention is to provide a hydrogen storage alloy in which the ratio of two phases is optimized so that powdering proceeds sequentially from the surface of the fresh BCC phase to promote activation.

[0007]

SUMMARY OF THE INVENTION The object of the present invention is to improve the initial activation and the reaction rate of a hydrogen-absorbing alloy in the air when the hydrogen-absorbing alloy is pulverized. This is achieved by a hydrogen storage alloy characterized by being composed of a mixed phase of an alloy phase which is relatively easy and an alloy phase which is relatively difficult to activate. The above object is also achieved by a hydrogen storage alloy in which the alloy phase that is relatively easy to activate is the Laves phase and the alloy phase that is relatively difficult to activate is the BCC phase. Further, the object is that the hydrogen storage alloy is a Ti—Mn—V alloy, the Laves phase is a C14 phase, and the phase fraction of the C14 phase is 3 to 80% by weight.
This is also achieved by a hydrogen storage alloy.

[0008]

DETAILED DESCRIPTION OF THE INVENTION In the present invention, the easiness of activation and the magnitude of the reaction rate are determined based on the interaction between multiple phases, especially BCC.
Based on the ternary phase diagram of the Ti-Mn-V system, the fraction of the two phases of the BCC phase and the Laves phase was determined in order to clarify whether the effect was due to the Laves phase dispersed in the phase. These alloys were selected and their characteristics were examined. Conventionally, the BCC phase has a large amount of occlusion, but it has been considered that activation is difficult due to the presence of a strong oxide film. However, in the present invention, first, by using a mixed phase with the Laves phase that is easy to activate, The phase hydrogenates and pulverizes. At this time, the fracture also propagates to the BCC phase portion, and a fresh BCC phase surface not poisoned by air is formed starting from the fracture surface. It is considered that as the hydrogenation proceeds from here to the inlet, the pulverization of the BCC phase portion progresses, and the activation accelerates.

[0009] That is, as a mechanism for exhibiting the effects of the present invention, a) a BCC alloy pulverized in the air has a strong oxide film and requires a considerable incubation period. b) When the Laves phase is mixed with the second phase as the second phase, the Laves phase is first hydrogenated and powdered. c) This creates a fresh BCC phase surface that is not poisoned by air. d) It is considered that the hydrogenation proceeds from here to the entrance, whereby the BCC phase is also pulverized, and the activation accelerates. Therefore, as a combination of two phases, a combination of an alloy that is easily activated and an alloy that is difficult to activate even when pulverized in the air can be basically considered.

[0010] Further, by such a mechanism, the produced powder is brought into an optimal pulverized form (= particle size distribution) at a certain mixing ratio of two phases, and as a result, a large reaction rate is obtained. Due to the mechanism (estimation) as described above, a hydrogen storage alloy mainly composed of a BCC phase, which has conventionally been considered to have a poor initial activity and reaction rate, has a large storage amount of, for example, 3.0 wt% or more. Speed is AB5 type alloy or AB
The BCC type alloy, which is said to be inferior to the type 2 alloy, has a short activation latency and a high reaction rate. Hereinafter, the present invention will be described in more detail with reference to the accompanying drawings.

[0011]

EXAMPLE An alloy design and a method for evaluating initial activity and reaction rate in this example will be described. As shown in FIG.
The Ti _1.0 Mn _1.0 V _1.0 alloy, the alloy C and the alloy N all have a composition on Ti _1.0 MnxV _{1-X in the} ternary phase diagram, and are a mixed phase of a Laves phase and a BCC phase. In this example, the component range was further expanded on this line, and the Laves phase single phase alloy C2, the BCC + (C14 / BCC) phase boundary composition alloy B4 and the BCC phase single phase alloy B10 were selected. Table 1 shows the phase fractions of the alloys dissolved in these alloy components.

[0012]

[Table 1]

The easiness of activation and the reaction rate can be measured by measuring the time change of the pressure drop when a constant initial pressure of hydrogen is applied by using a pressure-composition isotherm measuring apparatus. It was evaluated by calculating the amount-time curve.
The weight of the sample was about 5 g, and the pressure was increased so that the initial hydrogen pressure was 3 MPa, and the measurement was performed until the occluded amount reached equilibrium. This occlusion amount-time curve measurement was repeated immediately after pulverization,
Ease of activation was evaluated by the number of repetitions until the amount of occlusion determined from the incubation period, which is the time required until the occlusion started, and the equilibrium characteristics was obtained. The reaction rate was calculated from the slope of the storage amount-time curve after the start of storage. That is, the reaction rate constant k was calculated from the _{dx / dt = k (P 0} -P eq) becomes equation.

Hereinafter, the results of the relationship between the hydrogen storage alloy and time in the present embodiment will be described. Figure 2 (a) shows TiVMn
(BCC phase + C14 alloy), Fig. 2 (b) shows C (BCC phase + C14 alloy).
FIG. 2 (c) shows the occlusion amount-time curves of three alloys in which the Laves phase and the BCC phase are mixed in two phases, as shown in N (BCC phase alloy). In any case, at the time of the first storage, there is an increase in the incubation period (time until hydrogen storage starts) of 1 to 2 minutes, but the storage is performed up to 100% of the equilibrium storage amount within about 5 minutes. On the other hand, the second and subsequent occlusions show almost no incubation period.

On the other hand, FIG. 3A shows B10 (BCC phase 10).
0%), as shown in FIG. 3 (b) for B4 (BCC phase 96.3%) and FIG. 3 (c) for C2 (C14 phase 100%), almost BCC single phase alloy B4 or completely BCC single phase Phase Alloy B10
In, the incubation period increases from several minutes to about one hour. However, even in these alloys, there is almost no incubation period after the second occlusion, and the reaction rate after the start of occlusion is sufficiently high. Conventionally, BCC single phase pure vanadium or T
It has been said that i-V alloys are difficult to activate and have a low reaction rate. On the other hand, in the present example, all of the series of BCC alloys were activated by one hydrogen absorption, and the reaction rate was equal to or higher than that of the Laves phase alloy, which is said to have a high reaction rate.

Therefore, the above-mentioned phenomenon will be described with reference to the results of study of the relationship between the incubation period and phase separation. FIG. 4 shows Ti _1.0 MnxV obtained from the storage amount-time curve measured in the previous section.
The relationship between the incubation period and the phase fraction in each composition of the _1-X alloy is shown. The incubation period decreases with increasing fraction of the C14 phase. The incubation period required for activation is more influenced by the surface state of the alloy, especially by the surface oxidation by air, than the internal structure or crystal structure of the original alloy. In the case of this embodiment, since the alloy is pulverized in the air, the surface is oxidized during the first activation after the pulverization, but after the second activation, hydrogen is applied from the fresh surface pulverized by hydrogenation. It is considered that there is no incubation period and the reaction rate is high because ozone is stored. Furthermore, the latent period is reduced by two-phase mixing because of the Laves phase, which is brittle in crystal structure and easily pulverized by hydrogenation.
This is probably because it became easier to make a fresh surface.

Next, in order to verify the above mechanism,
The effect of the Laves phase on the incubation period and the reaction rate when the shape was changed by heat treatment was investigated. FIG.
i _1.0 Mn _0.9 V _1.1 tissue-cast alloy (N), Fig. 5 (b) Ti _1.0 was 2h heat-treated at 1200 ° C.
1 shows an optical microscope structure of a Mn _0.9 V _1.1 alloy (N). The needle-like precipitate as cast is granulated by heat treatment. Further, FIG. 6A shows that Ti _1.0 Mn _0.9 V
_As shown in FIG. 6B, 1200 alloy (N) was cast as it was.
Ti _1.0 Mn _0.9 V _1.1 alloy (N) heat treated at 2 ° C. for 2 hours) Storage amount of Ti _1.0 Mn _0.9 V _1.1 alloy (N) −
The change of the time curve was shown. The heat treatment significantly increased the incubation period. This is presumably because the shape of the precipitate changed due to the heat treatment, and it became difficult to become a starting point of crushing for forming a fresh surface of the BCC phase.
Here, the relationship between the incubation period and the tissue in the present embodiment is based on the precondition that the BCC phase is easily affected by air poisoning. Then, in order to investigate the influence on air poisoning, the activated BCC alloy was left in the air, and then the hydrogen storage / release characteristics were examined.

FIG. 7 shows a Ti-Cr-V of a BCC single phase alloy.
3 shows the change in pressure-composition isotherm before and after the alloy is left in air. In this figure, only the rough pulverization in the air slightly reduces the hydrogen storage capacity, and when this is activated again at 500 ° C., it recovers. Further, it is understood that the occlusion amount almost disappears due to air poisoning when left in the air for 24 hours.
Therefore, it is understood that the BCC phase is easily affected by air poisoning.

Hereinafter, the reaction rate in this embodiment will be described. FIG. 8 shows the relationship between the reaction rate calculated from the storage amount-time curves after activation in FIGS. 2 and 3 and the fraction of the C14 phase contained in the BCC phase. First, the reaction rate of the BCC single phase alloy was higher than that of the C14 single phase alloy. Conventionally, it has been said that BCC alloys have a low reaction rate, but this is because the apparent reaction time is long because the BCC single phase has a large incubation period for activation. Conceivable. The results of this example show that the reaction rate of the activated BCC alloy is not necessarily low. Further, from this figure, it can be seen that when the C14 phase of the Laves phase is less than 3% or more than 80%, the initial target reaction rate cannot be improved. Further, the reaction rate of the two-phase mixed alloy was higher than any of the single-phase alloys. This indicates that the interaction between the two phases, which was not observed in the equilibrium characteristics such as the storage amount and the equilibrium pressure, is observed in the reaction rate.
It is considered that such an interaction between the two phases is a result of the interface between the two phases acting during the hydrogen pulverization of the alloy, resulting in a particle size distribution and a powder shape different from the powder of each single-phase alloy. .

[0020]

According to the present invention, the reaction rate and the initial activation can be improved by the interaction between the BCC alloy and the Laves phase. Further, in the activation, the time until the start of occlusion (latent period) can be reduced by mixing with the Laves phase until almost no longer exists. Further, in the alloy in which the Laves phase was mixed at about 20%, a higher reaction rate was obtained than in any single-phase alloy, and interaction due to the two-phase mixing was confirmed. Furthermore, mixing the Laves phase that is crystallographically brittle and easily pulverized during mechanical pulverization has the effect of facilitating pulverization as compared with a BCC single-phase alloy that is ductile and difficult to pulverize.

[Brief description of the drawings]

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

FIG. 2 shows activation according to an embodiment of the present invention, wherein (a) T
iVMn (BCC + C14) alloy, (b) C (BCC +
It is a figure which shows C14) alloy and (c) N (BCC) alloy.

FIG. 3 shows activation according to an embodiment of the present invention, wherein (a) B
10 alloy (BCC), (b) B4 alloy (BCC + C1
4) and (c) are views showing a C2 alloy (C14).

FIG. 4 shows activation latency and C14 according to an embodiment of the present invention.
It is a figure which shows the relationship with a phase fraction.

FIG. 5 shows Ti _1.0 Mn _0.9 V _{1.1 according to an} example of the present invention.
It is a 400-fold optical microscope structure photograph of the alloy, (a) as-cast, (b) heat-treated at 1200 ° C for 2 hours.

FIG. 6 shows Ti _1.0 Mn _0.9 V _{1.1 according to an} example of the present invention.
Showing activation of the alloy, (a) as cast, (b) 1200
It is a figure which shows what was heat-processed at 2 degreeC for 2 hours.

FIG. 7 is a diagram showing a pressure-composition isotherm of a Ti—Cr—V alloy according to an example of the present invention after being left in air.

FIG. 8 is a diagram showing a relationship between a reaction constant and a C14 phase fraction according to an example of the present invention.

Claims (3)

[Claims] When the hydrogen storage alloy is pulverized in the air, the structure of the alloy is relatively easy to activate hydrogen storage in order to improve the initial activation and reaction rate. A hydrogen storage alloy comprising a mixed phase with an alloy phase, which is relatively difficult to form. 2. The alloy phase which is relatively easy to activate is a Laves phase, and the alloy phase which is relatively difficult to activate is BC.
The hydrogen storage alloy according to claim 1, which is a C phase. 3. The hydrogen storage alloy according to claim 2, wherein the hydrogen storage alloy is a Ti—Mn—V alloy, the Laves phase is a C14 phase, and a phase fraction of the C14 phase is 3 to 80% by weight. .