JP2000087173A - Hydrogen storage alloy - Google Patents

Abstract

PROBLEM TO BE SOLVED: To increase an amount of occlusion/release of hydrogen and to improve plateau characteristic by providing a structure in which a main phase of body-centered cubic structure containing V, Zr, Ti, and Ni and a network grain boundary phase composed of two or more phases are present and the grain boundary phase contains a C14 Laves phase and a phase containing specific amounts of Zr and also regulating the volume percentage of the grain boundary phase to a value in a specific range. SOLUTION: In this hydrogen storage alloy, the volume percentage of the grain boundary phase is regulated to 25-50%. This grain boundary phase contains a C14 Laves phase and a phase containing >=35 atomic% Zr. Accordingly, the total composition of this alloy contains, by atom, >75.0-85.0% V, 2.0-15.0% Zr, 2.0-15.0% Ti, and 2.0-15.0% Ni. By casting a molten alloy of this composition and then carrying out annealing, plateau characteristic can be improved to a greater extent. As to annealing conditions, it is preferable to perform annealing at 800-1,300 deg.C for 0.5-100 hr.

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 used for a hydrogen storage / transport energy system, a negative electrode alloy for a nickel-metal hydride battery, and the like.

[0002]

2. Description of the Related Art In recent years, as electronic devices have become smaller and more sophisticated and more portable, demands for higher capacity and longer life of small secondary batteries, which are power supplies for electric devices, have been increasing. Mist metal nickel alloy (AB
Nickel-metal hydride batteries using (system 5) can achieve higher capacities than conventional nickel-cadmium batteries, and are currently widely used. In recent years, development of ecological cars has been promoted due to environmental problems, and electric vehicles, hybrid vehicles, fuel cell vehicles, hydrogen vehicles, and the like have been developed and some of them have begun to be put into practical use. In these, a hydrogen storage alloy is used as a negative electrode of a nickel-metal hydride battery or a hydrogen storage tank. Similarly, from the viewpoint of environmental issues, it is necessary to establish alternative energy systems other than fossil fuels.
As one of them, the development of an energy system for producing hydrogen using natural energy, storing and transporting hydrogen by a hydrogen storage alloy, and converting it to various energies by a hydrogen engine% fuel cell or the like is being promoted.

[0003] As described above, the applications of hydrogen storage alloys are rapidly expanding, and the characteristics required for each application differ. However, in any case, further increasing the amount of hydrogen storage / release is described as a top priority. In the case of a hydrogen storage system, the flatness (plateau property) of the equilibrium hydrogen pressure in a curve (hereinafter referred to as a PCT curve) showing the relationship between the amount of hydrogen in the hydrogen storage alloy and the equilibrium hydrogen gas pressure is particularly important. Has been requested.

As the hydrogen storage alloy, a vanadium solid solution type having a body-centered cubic lattice, such as an AB5 type alloy such as a misch metal nickel alloy, an AB2 type alloy such as a titanium manganese alloy, an A2B type alloy such as a magnesium nickel alloy, and a titanium vanadium alloy. Alloys (hereinafter referred to as VBCC alloys) and the like are known. The vanadium solid solution type alloy (VBC
Research and development have been actively conducted on C alloy) as a material that can be expected to have a high capacity.

The amount of hydrogen released from a vanadium solid solution type single phase alloy (VBCC alloy) is about 1.9 wt%, which is larger than that of AB5 alloy and AB2 alloy, but has extremely poor activation characteristics. It has been considered difficult to apply secondary batteries to negative electrode materials and the like. By the way, a second phase of a TiNi-based Laves phase is introduced into a V-Ti-Ni-based alloy as a three-dimensional network-like microstructure, and the second phase has a catalyst and a micro current collecting function. A multi-phase alloy-based hydrogen storage alloy for batteries has been proposed (JP-A-6-228699). JP-A-6-228699 discloses the composition range and addition of Zr and the like in a V-Ti-Ni system, but the hydrogen capacity is not yet at a satisfactory level, and the plateau property is still insufficient. On the other hand, in JP-A-6-228699, Ti-V-N
In an i-M-based solid solution alloy (where M is an element selected from Co, Cu, and Nb), a phase mainly composed of Ti-Ni forms a three-dimensional network skeleton, and a composition range of the phase. Information is disclosed. Similarly, the properties of the alloy are not at satisfactory levels in terms of hydrogen capacity and PC flatness.

[0006]

The present invention relates to a vanadium solid solution type multi-phase hydrogen storage alloy which has solved the above-mentioned problems. That is, by controlling the overall composition of the alloy and the manufacturing conditions, particularly the heat treatment conditions, which are important factors in the V-Zr-Ti-Ni system, by optimizing the alloy structure, the type and composition of the appearing phase, and the volume ratio of the phase. Another object of the present invention is to provide a hydrogen storage alloy having a high hydrogen storage / release amount and a good plateau property.

[0007]

In order to develop a VBCC multiphase alloy having a high hydrogen capacity and an excellent plateau property, the present inventors have determined the composition, combination and volume ratio of the VBCC phase and the grain boundary phase. Focusing on these factors, the effects of these factors on the hydrogen absorption / desorption characteristics were examined in detail. As a result, the following findings were obtained. The VBCC phase and the grain boundary phase are not used alone, and excellent characteristics can be obtained only by combining them. The role and characteristics of each phase are as follows: the VBCC phase mainly has a role as a hydrogen storage phase, that is, has a high hydrogen storage amount as a feature, while the grain boundary phase mainly has an activation property and a catalyst property. Function as an excellent hydrogen diffusion path. It is also necessary to control such that the grain boundary phase surrounds the periphery of the VBCC phase without a break. VBCC with high hydrogen capacity and good activation and catalytic properties
These are the conditions for a multi-phase hydrogen storage alloy. Further, in order to flatten the plateau, the composition in each phase needs to be uniform. Usually, heat treatment is performed to make the composition in each phase uniform, but it is necessary that the network of the grain boundary phase is maintained without breaking even after the heat treatment. VBCC that satisfies the above conditions
In order to develop multi-phase alloys, Z is used as a constituent element in various hydrogen storage alloys and is a key element
Focusing on r, based on V and Zr, an alloy that satisfies the above object by multiplying the elements was studied. As a result of intensive research, by limiting the content of V to a specific range, combining specific elements, and optimizing the alloy structure, the appearance phase, the composition of the appearance phase, the volume ratio of the phase, and the heat treatment conditions, etc. ,
It has been found that a vanadium solid solution type multi-phase hydrogen storage alloy can be obtained which has a high hydrogen capacity and a good plateau property.

That is, the hydrogen storage alloy of the present invention has an atomic percentage of 75.0% <V ≦ 85.0%, 2.0% ≦ Zr ≦ 1
5.0%, 2.0% ≦ Ti ≦ 15.0%, 2.0% ≦ N
A V-Zr-Ti-Ni-based alloy having a composition consisting of unavoidable impurities with i ≦ 15.0%, wherein V, Zr, T
There is a main phase of a body-centered cubic structure containing i and Ni, and a network-like grain boundary phase composed of two or more phases, and this grain boundary phase contains at least C14 Laves phase and 35 atomic% or more of Zr. A hydrogen storage alloy comprising a phase and a volume ratio of a grain boundary phase being 25% or more and 50% or less. The vanadium solid solution type alloy (VBCC) of the present invention is obtained by dissolving Zr atoms in a conventionally known V-Ti-Ni-based hydrogen storage alloy. In this case, the ratio of Zr: Ti: Ni is 1: 1: 1 and the ratio of V to (Zr, Ti, Ni) is 3.0 to 5.67, preferably 3.2 to 4.0. Has the highest hydrogen storage capacity. That is, the preferred composition of the whole alloy is Vx Zr _1/3 Ti _1/3 Ni _1/3 ,
x = 3.0 to 5.67, preferably x = 3.2 to 4.67.
It is in the range of 0. Next, C, one of the grain boundary phases of the present invention,
The 14 Laves phase is an MgZn2-type alloy and exhibits a hexagonal system. Here, V and Ti enter the Mg site, and Zn
It is considered that V, Ti, and Ni atoms enter the site.

[0009]

DETAILED DESCRIPTION OF THE INVENTION The present invention will be described in more detail. FIG. 1 shows the composition as V77% -Zr6% -Ti9%-
The X-ray diffraction result when Ni is 8% is shown. The peaks that appeared were three phases: a BCC phase, a C14 Laves phase, and a Zr-rich phase. FIG. 2 is a secondary electron beam image of this alloy structure. It was observed that the white grain boundary phase was present in a network around the black massive VBCC main phase. The volume fraction of the grain boundary phase was about 29%. In addition, at least C14
A Laves phase and a Zr-rich phase exist, and as a result of analysis by EDX, the composition of the C14 Laves phase is Zr: 30.
at%, Ti: 18 at%, Ni: 27 at%, V: 25 at%
Met. Therefore, (Zr, Ti) :( Ni, V, T
i) = (30 + 3) :( 27 + 25 + 15) = 1: 2. On the other hand, the composition of the Zr rich phase is Z
r: 42 at%, Ti: 14 at%, Ni: 27 at%, V:
17 at%. As described above, a high hydrogen capacity alloy was obtained in an alloy in which the main phase of VBCC and the network-like grain boundary phase were present.

FIG. 3 shows V77% -Zr6% -Ti9% -N
PC at 40 ° C. of the alloy of the present invention having a composition of 8%
3 shows a T curve. You can see that it has a very flat plateau. FIG. 4 shows the hydrogen absorption / desorption characteristics when the V composition is changed while maintaining the Zr: Ti: Ni ratio at 1: 1: 1 in this alloy. V composition is 70at% <V ≦ 85at
%, Preferably 75 at% ≦ V ≦ 80 at%, it can be seen that the alloy has a high hydrogen capacity. V composition is 70at%
In the following, since the volume ratio of the VBCC phase, which is the main hydrogen storage / release phase, decreases, the amount of hydrogen that can be stored in the alloy as a whole decreases. On the other hand, if the V composition exceeds 85 at%, the grain boundary phase, which is responsible for activation and catalysis, cannot sufficiently wrap the VBCC phase, and its action is weakened, resulting in a decrease in the amount of hydrogen absorbed and released by the entire alloy. . The reason that the composition range of Zr, Ti, and Ni is 2 at% or more and 15 at% or less, respectively, is as follows. Is weakened, and the hydrogen absorption / desorption amount is reduced. When the amount exceeds this range, the volume ratio of the VBCC phase is reduced, so that the hydrogen absorption / desorption amount is reduced. Further, in the field of hydrogen storage alloys, usually, a method of adding a small amount of elements to improve the adjustment of various properties, such as improvement of corrosion resistance and control of equilibrium hydrogen pressure, is employed. It may be contained in a range that does not adversely affect the effect.

The present invention is characterized in that a main phase VBCC phase and a network-like grain boundary phase are present, and the volume ratio of the grain boundary phase is 25% or more and 50% or less. As described above, the grain boundary phase is an important phase for establishing the properties of the VBCC multiphase alloy. If the volume ratio is less than 25%,
It is difficult to surround the main phase in a network without a break, and it is difficult to maintain good activation characteristics and catalytic performance of the entire alloy. If it exceeds 50%, the volume of the VBCC phase of the main phase having a large amount of hydrogen storage decreases, and the amount of hydrogen storage of the entire alloy decreases, which is not preferable. Under such an abundance ratio, since V, Ti, and Ni are constituent elements of the main phase and the grain boundary phase, the total composition of the alloy of the present invention is atomic%, that is, 75.0% <V ≦ 85. 0%, 2.0% ≦
1. Zr ≦ 15.0%, 2.0% ≦ Ti ≦ 15.0%,
0% ≦ Ni ≦ 15.0%.

In order to improve the plateau property,
It is necessary that the composition in each phase be uniform. Usually, heat treatment is performed to make the composition uniform,
It is necessary that the network-like grain boundary phase is maintained without breaking even after the heat treatment. The alloy of the present invention can sufficiently exhibit properties in an as-cast state, regardless of the casting method used, but can further improve the plateau property by annealing. By setting the annealing conditions at a temperature of 800 ° C. or more and 1300 ° C. or less and 0.5 hours or more and 100 hours or less, an alloy with particularly excellent plateau flatness can be obtained. If the temperature is lower than 800 ° C., diffusion of atoms is slow. If the temperature is higher than 1300 ° C., a sufficient annealing effect cannot be obtained because of a problem of oxidation or a required structure cannot be formed due to appearance of a molten phase. If the time is less than 0.5 hour, the diffusion time is not sufficient, and if the time is more than 100 hours, it causes an oxidation problem and an increase in cost. Therefore, the optimal heat treatment condition is a temperature of 800 ° C. or more and 1300 ° C. or less and 0.5 hours or more and 100 hours or less.

Further, as a method for producing the alloy, a method conventionally used can be used. Melting is performed in a vacuum induction melting furnace, an arc melting furnace, a plasma arc furnace, or the like. Casting is performed by ordinary casting into a box mold or various rapid cooling methods, such as a turntable method, a centrifugal casting method, a strip casting method, or various ultra-rapid cooling methods, such as a copper roll method or an atomizing method. There is no particular limitation on the melting method or casting method.

[0014]

The present invention will be described in more detail with reference to the following examples. (Examples 1 to 4) Each component was weighed in the ratio of atomic% shown in Table 1 and arc-melted in a water-cooled copper hearth in an argon atmosphere. In arc melting, the process of inverting and melting after solidification is repeated three times to increase the homogeneity of the sample, and the diameter is about 50 mm.
Was prepared. After that, the ingot was inserted into the opaque quartz tube, and the
After holding at 0 ° C. for 18 hours, the mixture was rapidly cooled. The phases were identified by X-ray diffraction. CuKα rays were used for the measurement, the filament voltage was 40 KV, and the current was 100 mA. Tissue observation
The sample piece cut out from the ingot was embedded in resin, polished to a mirror surface with a buff, and subjected to an optical microscope and a scanning electron microscope.
The volume ratio of the grain boundary phase was measured from the secondary electron beam image. Also,
The composition of the phase was also analyzed by EDX. The measurement of the hydrogen absorption / desorption characteristics was carried out at 40 ° C. for all the samples by the capacitance method using a Siebeltz apparatus. The sample was coarsely pulverized into small particles having a diameter of about 2 mm, charged into a reaction tube, evacuated by an oil rotary pump for one hour, and then the measurement was started. In the measurement, no special initial activation treatment was performed, and the hydrogen absorption / desorption amount and the flatness of the plateau were calculated from the measurement result in the third cycle. The electrode characteristics were measured by crushing the alloy ingot to obtain a powder of 75 μm or less, making it 85 wt%, and adding 5 wt% of PTFE (polytetrafluoroethylene) and nickel powder to 1 wt%.
0 wt% was added and kneaded, filled in a foamed nickel plate, and press-molded to obtain a negative electrode plate. An open nickel-metal hydride battery was obtained using a nickel hydroxide electrode as the positive electrode, Hg / HgO as the reference electrode, and a 6N KOH aqueous solution as the electrolyte. The charge and discharge conditions were a charge and discharge current of 50 mA / g and a cutoff voltage of -0.75 V, and the discharge capacity was calculated from the measurement at the tenth cycle.

[0015]

[Table 1]

The results of Examples 1 to 4 are also shown in Table 1. Each sample had a large hydrogen absorption / desorption amount of 2 wt% or more, and the flatness of the plateau was excellent. By scanning electron microscope, it was observed that the grain boundary phase surrounded the main phase VBCC phase in a network. From the secondary electron beam image, the volume ratio of the grain boundary phase was 26 to 31 Vol%. From X-ray diffraction, any of the samples showed a VBCC phase, a C14 Laves phase,
A Zr-rich phase was identified. The composition of the Laves phase from EDX was (Zr, Ti) :( Ni, V, Ti) = 1: 2. The electrode characteristics were measured for Examples 1 and 2, and showed extremely large discharge capacities of 505 mAh / g and 475 mAh / g, respectively.

Table 1 shows the alloy compositions and results of Comparative Examples 1 to 4. In each case, the hydrogen absorption and desorption amount is as small as less than 2 wt%,
The slope of the PCT curve is very large. Electrode characteristics were performed only in Comparative Examples 1 and 2, but 291 mAh / g,
The discharge capacity is as small as 360 mAh / g.

[0018]

According to the present invention, the alloy composition, the appearance phase, the composition of the appearance phase, and the volume ratio of the phases can be optimized by selecting the overall composition of the alloy and optimizing the heat treatment conditions. It is possible to obtain a hydrogen storage alloy having an extremely large amount of hydrogen storage and desorption, good plateau properties, and excellent electrode characteristics.

[Brief description of the drawings]

FIG. 1 is a view showing an X-ray diffraction result of an alloy of the present invention.

FIG. 2 is a secondary electron beam image showing the alloy structure of the present invention.
Magnification 1000 times.

FIG. 3 is a PCT curve at 40 ° C. of the alloy of the present invention.

FIG. 4 is a diagram showing the relationship between V composition and hydrogen absorption / desorption characteristics.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme court ゛ (Reference) C22F 1/00 641 C22F 1/00 641A 682 682 691 691B 691C (72) Inventor Takahiro Kuriiwa Aoba-ku, Sendai-shi Yonegabukuro 3-6-17 (72) Inventor Yoichi Hirose 1505, Shimokagemori, Ochi, Chichibu City, Saitama Prefecture Showa Denko Corporation Chichibu Plant (72) Inventor Masahide Utsunomiya 1505, Shimokagemori, Chihirobu City, Saitama Prefecture Engineering Co., Ltd. Chichibu Factory

Claims (4)

[Claims] 1. Atomic%: 75.0% <V ≦ 85.0
%, 2.0% ≦ Zr ≦ 15.0%, 2.0% ≦ Ti ≦ 1
A V-Zr-Ti-Ni-based alloy having a composition of 5.0%, 2.0% ≦ Ni ≦ 15.0% and inevitable impurities, and a body-centered cubic containing V, Zr, Ti, and Ni There is a network-like grain boundary phase composed of a main phase of the structure and two or more phases,
This grain boundary phase is at least 35 atomic% with the C14 Laves phase.
A hydrogen storage alloy comprising the above Zr-containing phase, and wherein the volume ratio of the grain boundary phase is 25% or more and 50% or less. 2. The method according to claim 1, wherein (Zr,
Ti) and (Ni, V, Ti) composition ratio is 1: 1.8 ≦
(Zr, Ti): A hydrogen storage alloy characterized by being in the range of (Ni, V, Ti) ≦ 1: 2.2. 3. Atomic%: 75.0% <V ≦ 85.0
%, 2.0% ≦ Zr ≦ 15.0%, 2.0% ≦ Ti ≦ 1
After casting a molten alloy having a composition of 5.0%, 2.0% ≦ Ni ≦ 15.0% and inevitable impurities, heat treatment at a temperature of 800 ° C. to 1300 ° C. for 0.5 hour to 100 hours. A method for producing a hydrogen storage alloy. 4. An electrode for a nickel-metal hydride battery using the hydrogen storage alloy according to claim 1 or 3.