JPH10121180A - Hydrogen storage alloy and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a hydrogen storage alloy requiring no particular use of expensive alloying elements such as V, having a hydrogen absorbing and releasing capacity equal to that of the conventional BCC type hydrogen storage alloy, and advantageous in respect of manufacturing costs, and its production. SOLUTION: The hydrogen storage alloy has a composition represented by formula Ti100-a-b Cra Xb [where X is at least either of Mo and W and the symbols (a) and (b) satisfy, by atomic %, 40<=a<=70 and 0<b<=20, respectively]and also has a crystalline cubic structure composed of body-centered structure (BCC type). As the method of manufacture of this hydrogen storage alloy, the aforesaid alloy is melted and cast, and the resultant ingot is held at 1200-1400 deg.C (excluding TiCr single phase region) for 1-5hr and then subjected to rapid cooling treatment, by which the crystalline structure is provided with body-centered cubic structure (BCC type) at ordinary temp.

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, and in particular, has the same level of hydrogen storage / release capability as conventional BCC type hydrogen storage alloys without using expensive alloying elements such as V. The present invention relates to a hydrogen storage alloy which is advantageous in terms of production cost and a production method thereof.

[0002]

2. Description of the Related Art As a means for storing and transporting hydrogen, a hydrogen storage alloy can store and store hydrogen gas of about 1000 times or more the volume of the alloy itself.
It is almost equal to or higher than liquid or solid hydrogen. As this hydrogen storage material, V, Nb, Ta or Ti
VMn based metal having a body-centered cubic structure, such as TiVCr alloy (hereinafter referred to as BCC) is already AB such AB ₅ type alloys and TiMn _2, such as LaNi ₅ has been put to practical use
It has been known for a long time that it absorbs a larger amount of hydrogen than type ₂ alloys. This is because in the BCC structure, there are many hydrogen storage sites in the crystal lattice, and the calculated hydrogen storage amount is H
/M=2.0 (about 4.0 wt% for alloys such as Ti and V having an atomic weight of about 50), which is extremely large.

[0003] A pure vanadium alloy absorbs about 4.0 wt%, which is almost the same as the value calculated from the crystal structure, and releases about half of it under normal temperature and normal pressure. It is known that Nb and Ta of group 5A elements of the same periodic table also show a large amount of hydrogen storage and good hydrogen release characteristics. However, pure metals such as V, Nb, and Ta have very high costs. Therefore, in an industrial application requiring a certain amount of alloy such as a hydrogen tank or a Ni-MH battery, Ti-
In an alloy in a component range having a BCC structure such as V,
Its properties have been studied. On the other hand, such a BCC type hydrogen storage alloy containing Ti has a high capacity but contains expensive V. Therefore, a hydrogen storage alloy having no V and having the same level of capacity, In applications requiring a high-capacity hydrogen storage alloy, such as a hydrogen tank for an electric vehicle (EV), a breakthrough cost advantage can be expected.

[0004] In addition, V is eluted in the electrolytic solution in the Ni-MH battery, but an alloy containing no V can be expected to be applied to the negative electrode material of the Ni-MH battery. As a known technique in this field, Japanese Patent Application Laid-Open No. Hei 4-210446 discloses a TiCrMo-based and TiCrMoF with the aim of increasing the hydrogen storage amount at a relatively low material cost and further increasing the reaction rate.
It is disclosed that in the e system, arc melting can be performed in a high-purity Ar gas atmosphere to improve the amount of hydrogen storage / release at -40 ° C, the efficiency of storage / release, and the reaction rate. JP-A-61-176067 discloses that Ti
Disclosed is a hydrogen storage alloy for providing a hydrogen storage electrode having a long cycle life by charge and discharge by adding a kind of element selected from alkaline earth metals and the like to a -Cr alloy.

However, in these BCC alloys, in addition to the fact that the reaction rate, which is a problem in the V system, is slow and the activation is difficult, in addition, at a practical temperature and pressure, only the occlusion and the release amount are small. A new problem has arisen. As a result, an alloy having a BCC phase as a main constituent phase has not yet been put to practical use. Further, development of an alloy which does not contain an expensive alloy element such as V and has excellent hydrogen absorption / desorption characteristics is desired.

[0006]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a conventional BCC type hydrogen storage alloy of Ti-V-Mn type, Ti-V
-Consider replacement of V in a Cr-based alloy with Mo and / or W to provide a high-capacity and inexpensive hydrogen storage alloy. Another object of the present invention is to provide a hydrogen storage / release characteristic that is applicable to a hydrogen tank or a Ni-MH battery on an industrial scale by using the alloy that is advantageous in terms of manufacturing cost and has excellent hydrogen storage / release properties. Provide a hydrogen storage alloy. Another object of the present invention is to achieve, by the above-mentioned novel BCC alloy, a heat treatment method, which is an optimum production process for enabling production on an industrial scale at low cost.

[0007]

SUMMARY OF THE INVENTION The object of the present invention is to provide a composition comprising:
General formula Ti _100-ab C _a X _b , wherein X is at least one of Mo and W, wherein a and b are represented by atomic% and 40 ≦
This is achieved by a hydrogen storage alloy represented by a ≦ 70, 0 <b ≦ 20, and characterized in that the crystal structure is a body-centered cubic structure (BCC type). In addition, the above-mentioned object is to melt and cast the alloy, and then put the ingot in the range of 1200 to 14
The crystal structure has a body-centered cubic structure (BCC type) at room temperature by holding at a temperature of 00 ° C. (excluding the TiCr ₂ single phase region) for 1 to 5 hours and then performing a quenching treatment. This is also achieved by a method for producing a hydrogen storage alloy characterized by the following.

[0008]

DETAILED DESCRIPTION OF THE INVENTION As an example of the alloy of the present invention, Ti-
FIG. 1 shows the range of the Cr-Mo alloy. In the ternary phase diagram, there is a single phase region of the Laves phase, which is Ti—Cr-based TiCr ₂ , and the present invention avoids this range and sets the crystal structure to a range consisting of BCC. That is,
Points A (Ti ₃₀ Cr ₇₀ ), B (Ti ₁₀ Cr ₇₀ X ₂₀ ), C (Ti ₄₀ Cr
₄₀ X ₂₀ ) and D (Ti ₆₀ Cr ₄₀ ) within a range surrounded by a line segment, provided that the composition includes a line segment other than AD.

On the other hand, according to the findings of the present inventors,
Among the BCC alloys, those that regularly decompose into two fine phases on the order of nanometers due to spinodal decomposition therein have significantly improved hydrogen release characteristics. This is a binary phase diagram that is the basis of the ternary alloy.
It can be seen that the -Mo system and the Cr-W system have a region of two-phase separation. As an example, FIG. 6 shows a binary phase diagram of a Ti—Cr system. In this figure, 1370 ° C. of TiCr ₂
And a solid phase line of two-phase separation connecting the eutectoid point to the solid phase. In such a phase diagram, two phases having different lattice constants formed by spinodal decomposition and grown in a specific crystal orientation are from 1.0 nm to 10 nm.
It will have a periodic structure at 0 nm intervals.

With this regular nano-order periodic structure,
BCC alloy releases a large amount of hydrogen stored structurally in a practical temperature and pressure range, relaxes the activation conditions,
The reaction rate can be improved. On the other hand, the existence region of the BCC homogeneous phase in the alloy system of the present invention is more specifically shown in the pseudo-binary phase diagram in FIG. 5 where X is TiCr ₂ and Mo and / or W as the third element. It is. In this figure, the heat treatment temperature of the present invention is set to a range in which the BCC can be uniformly brought to room temperature, excluding the portion indicated by oblique lines, that is, the TiCr ₂ single phase region.

As described above, the present invention uses an expensive V
Of the same level as conventional BCC type hydrogen storage alloy
It is possible to provide an alloy having an ability to absorb and release hydrogen. further,
Cost reduction by optimizing the manufacturing process is also possible. Next
Now, the alloy composition and heat treatment conditions of the present invention will be described.
You. The alloy composition of the present invention comprises Ti, Cr, Mo and / or
Or W as a component, so hydrogen using conventional V etc.
Compared to occlusion alloys, it reduces costs and reduces V
And / or W-substituted components in the phase diagram
The range of solution treatment is expanded, so that sufficient phase separation occurs.
Alloy with excellent hydrogen absorption / desorption characteristics in the two-phase state.
You. Hereinafter, the reasons for limiting the components of the present invention will be described. one
General formula Ti_100-abCr_aX _bWhere X is Mo and / or
Or W, wherein a and b are represented by atomic%, and 40 ≦ a ≦ 7
0,0 <b ≦ 20. The composition range is BCC phase
Uniformity in the alloy and the formation of two-phase separated
Optimizes the crystal structure distortion and makes hydrogen as a hydrogen storage alloy
Optimum composition for microstructure enabling mobility enhancement
It can be said that.

That is, when the Cr content is less than 40 at%, the hydrogen storage / release characteristics of the hydrogen storage alloy (pressure composition isotherm: PCT
(Diagram), the equilibrium pressure is low, and it becomes difficult to remove the stored hydrogen at room temperature again. In addition, Cr is 70 at
When the equilibrium pressure is higher than 11%, the hydrogen storage amount at normal temperature is small. Further, when Mo and / or W is 0 at%, the alloy is not converted to BCC even if heat treatment is performed. Also, 2
If it exceeds 0 at%, the amount of hydrogen storage decreases, which is not practical. For this reason, it was limited to the above composition range. The preferred range of Mo and / or W is 5% or more and 20% or less. The preferable range of Cr is 50% or more and 70% or less.

[0013] In the present invention, the heat treatment conditions as a manufacturing method are specified so that the BCC phase appears uniformly in the alloy and the amount of hydrogen absorbed and released is maximized. That is, after melting and casting the master alloy, the ingot is heated to 1200 to 1400 ° C.
(Excluding the TiCr ₂ single phase region) at a temperature of 1 to 5
It is characterized in that the alloy is converted to BCC by holding for a time and then quenching in oil or ice water.
That is, as the heat treatment conditions of the present invention, in the alloy having the above composition range, a high-capacity hydrogen-absorbing BCC phase is stably present only at 1200 ° C. or higher. On the other hand, an alloy melted by an induction heating method, an arc melting method, or the like transforms to a more stable C14 Laves phase at 1200 ° C. or less during normal cooling. For this reason, in order to form a BCC phase with the above composition, it is necessary to freeze a high-temperature stable BCC phase to room temperature.

[0014]

EXAMPLES As an example of the present invention, a sample of a hydrogen storage alloy was prepared as follows. The composition is within the scope of the present invention, that is, the composition range in the ABCD of FIG. 1A, which is Ti ₂₇ Cr ₆₆ Mo ₇ , Ti ₃₀ Cr ₆₃ Mo ₇ , and Ti ₃₃ Cr ₆₀ M shown in FIG.
The components were adjusted to o ₇ , Ti ₃₆ Cr ₅₇ Mo ₇ , and Ti ₃₉ Cr ₅₄ Mo ₇ . All samples were arc-melted in argon using a water-cooled copper hearth in about 20 g ingots. All the data of the present embodiment are obtained by performing a heat treatment of heating at 1400 ° C. for 2 hours after casting, followed by water cooling to make the structure BCC, pulverizing the ingot in air, and activating the ingot at 60 ° C. and 10 ^−4.
torr evacuation + 50atm After repeating hydrogen pressurization for 4 cycles, the hydrogen storage capacity and hydrogen absorption / desorption characteristics of the alloy were determined by the vacuum origin method specified in the pressure composition isotherm measurement method (JIS H7201) by the volumetric method. It is a thing.

The structural analysis of the alloy was performed using a transmission electron microscope and an attached EDX (energy dispersive X-ray diffraction). Further, a crystal structure model was created based on information obtained by a transmission electron microscope, and Rietveld analysis of powder X-ray diffraction data was performed. In the Rietveld analysis, unlike the ordinary X-ray diffraction method, the crystal structure parameters can be refined by using the diffraction intensity, and the weight fraction of each phase can be obtained by calculation. For the Rietveld analysis, the analysis software RIETA N9 developed by Dr. Izumi, Inorganic Materials Research Laboratory
4 was used.

FIG. 2 shows the above-mentioned samples of this embodiment.
FIG. 5 is a diagram showing the hydrogen storage and release processes at 0 ° C. in FIG.
In this figure, Ti₂₇Cr₆₆Mo₇And Ti₃₀Cr₆₃Mo₇Is hydrogen
The storage amount is small and it hardly releases. Ti₃₃Cr₆₀Mo₇and
Ti₃₆Cr₅₇Mo₇In, the hydrogen storage capacity is improved and Ti₃₃Cr
₆₀Mo₇Plateau equilibrium pressure is almost 1MPa and Ti₃₆Cr ₅₇
Mo₇Shows the maximum value of both occlusion and release below 1MPa
However, it also shows a good value in the plateau flatness. Ma
Ta₃₉Cr₅₄Mo₇About, the hydrogen storage capacity is small,
It shows a tendency to hardly release. FIG.₃₉Cr
₅₄Mo₇About the hydrogen storage properties at 0 ° C and 40 ° C
Show. At 40 ° C, this component has a hydrogen storage capacity and
Improved release and release and flat at plateau pressure
The properties are quite good, and this component system is promising for practical use.
It turned out to be.

The pressure in the flat region of the pressure composition isotherm (plato equilibrium pressure) is Ti / Cr as shown in FIGS.
Varies depending on the composition ratio. In this figure, the amount of Ti and Cr is changed while fixing Ti-Cr-Mo-based Mo at 7 at%. In the present invention, a temperature of -40 ° C. is considered as a use environment for applications such as a hydrogen tank and a heat pump.
Alloy composition showing an occlusion pressure of 0 MPa or less, and 100 ° C.
An alloy composition exhibiting a discharge pressure of 0.01 MPa or more in the present invention was defined as a claim.

FIG. 4 shows the hydrogen absorption and release processes (PCT characteristics) of Ti ₄₁ Cr ₅₆ W _{3 in} the same manner as described above, after arc melting, holding at 1400 ° C. for 2 hours, and performing heat treatment of water quenching. ). In this figure, the maximum hydrogen storage amount at 40 ° C. shows a good value of about 2.3 Wt%, and that at 0 ° C. shows a good value of about 2.4 Wt%. Results are shown. FIG.
4 and FIG. 4 show substantially the same hydrogen storage characteristics. From this, it is found that the atomic weight of W is about twice as large as that of Mo, so that a sufficient BCC conversion effect can be obtained even with a small amount of addition. It is thought to be due to. Table 1 shows a comparison of the hydrogen absorbing and releasing ability between the material of the present invention and the comparative material.

[0019]

[Table 1]

From this table, it can be seen that Ti ₃₃ Cr ₆₀ Mo _{7 of the} present invention
Hydrogen storage amount is 598cc / g and release amount is 598cc / g
g, Ti ₃₆ Cr ₅₇ Mo ₇ has a hydrogen storage capacity of 674 cc / g
And the release amount is as good as 502 cc / g.
On the other hand, in the comparative material Ti ₂₀ Cr ₇₃ Mo ₇ , the hydrogen storage amount was 146.
In terms of cc / g, the release amount was 40 cc / g, which is considerably smaller than that of the material of the present invention. The material of the present invention clearly has a higher effective hydrogen transfer amount than the Laves alloy, and is excellent in hydrogen absorption / desorption characteristics, and the characteristic values are at the same level as the conventional BCC alloy containing V. Table 2 shows a comparison between the mother alloy and the phase fraction of BCC and Laves phase by XRD after heat treatment for the composition Ti ₃₆ Cr ₅₇ Mo ₇ .

FIGS. 7A and 7B are charts of XRD (Cu electrode, output 48 kv) of X-ray relative intensity and diffraction angle of Ti ₄₁ Cr ₅₆ W ₃ . (A) is a sample immediately after arc melting, and (b) is a heat-treated material (140).
0 ° C. → water cooling). Immediately after the arc melting, the Laves phase and other phases were present, but it can be seen that the BCC became a uniform single phase by the heat treatment.

[0022]

[Table 2]

From this, it can be seen that while the master alloy is a Laves single phase, the heat-treated alloy is significantly converted to BCC.

[0024]

According to the present invention, it is possible to manufacture a BCC type hydrogen storage alloy which does not contain expensive V or the like and has a hydrogen absorption / desorption characteristic comparable to that of a conventional alloy containing V or the like. Also, the cost of alloy raw materials can be significantly reduced. Therefore, according to the present invention, a high-capacity BCC type hydrogen storage alloy can be manufactured at extremely low cost, and practical application to various uses becomes possible.

[Brief description of the drawings]

FIG. 1 TiCrX (Mo and / or W) of the present invention
Figure 3 shows the composition according to the diagram shown on the ternary phase diagram of the system,
It is a figure which shows a composition range and (b) Example composition of a TiCrMo type | system | group.

FIG. 2 is a PCT diagram showing hydrogen absorption / desorption characteristics according to a TiCrMo-based composition according to an example of the present invention.

FIG. 3 is a PCT diagram showing hydrogen absorption / desorption characteristics at 0 ° C. and 40 ° C. of a TiCrMo-based material according to an example of the present invention.

FIG. 4 shows 0 ° C. and 4 ° C. of TiCrW-based material according to the embodiment of the present invention.
FIG. 4 is a PCT diagram showing hydrogen absorption / desorption characteristics at 0 ° C.

FIG. 5 is a TiCr ₂ -X pseudo-binary phase diagram showing the heat treatment range of the present invention.

FIG. 6 is a Ti-Cr-based binary phase diagram related to the present invention.

FIG. 7 is a TiCrW-based XRD chart according to an example of the present invention, in which (a) immediately after arc melting and (b) after heat treatment.

FIG. 8 shows the relationship between the plateau equilibrium pressure and the amount of Cr according to the example of the present invention.
It is a figure which shows the discharge pressure of ° C.

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

Claims (2)

[Claims] 1. The composition according to claim 1, wherein said composition is of the general formula Ti _100-ab Cr
_{a Xb} , wherein X is at least one of Mo and W, wherein a and b are represented by atomic%, and 40 ≦ a ≦ 70, 0 <b ≦ 20.
And a crystal structure having a body-centered cubic structure (BCC type). 2. After melting and casting the alloy of claim 1, the ingot is heated to 1200 to 1400 ° C. (however, TiCr ₂
(Excluding the single-phase region) for 1 to 5 hours, and then subjected to a quenching treatment so that the crystal structure has a body-centered cubic structure (BCC type) at room temperature. Manufacturing method.