JP2002105511A - Hydrogen storage alloy having excellent durability and its production method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a hydrogen storage alloy having high hydrogen occuluding performance and a long-term repeated hydrogen absorbing and discharging life suitable for the storage and transport of hydrogen, usable in a temperature in the vicinity of room temperature, maintaining excellent oxidation resistance over a long period and easily treatable in the air. SOLUTION: The surface of rapidly solidified hydrogen storage alloy powder having a composition expressed by TiaV1-a-b-c-dCrbAcBd (wherein; A is one or more kinds selected from Mn, Fe, Co, Cu, Nb, Zn, Zr, Mo, Ag, Hf, Ta, W, Al, Si, C, N, P and B; B is one or more kinds selected from Ln (lanthanoid metals) and Y; a is 0.2 to 0.5, b is 0.1 to 0.4, c is 0.01 to 0.2, and d is 0.001 to 0.03) is formed with a Cu coated layer and an Ni coated layer in this order by plating, mechanical alloying or the like, thereafter, heat treatment is performed at 400 to 1,000 deg.C to form a Ti-Ni compound layer on the surface of the powder, and also, a Cu concentrated region is formed in the vicinity of the boundary between the Ti-Ni compound layer and the base metal alloy.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention has a high hydrogen storage capacity (hydrogen storage capacity), is excellent in durability and oxidation resistance, has little characteristic deterioration due to repeated hydrogen absorption and release, and can be used at a temperature near room temperature. The present invention relates to a hydrogen storage alloy having a feature of being relatively inexpensive and a method for producing the same. The hydrogen storage alloy of the present invention or the hydrogen storage alloy produced by the method of the present invention,
Particularly, it is most suitable for hydrogen gas storage / transport, hydrogen gas separation / purification, heat transport system and cooling system, static compressor, fuel cell using hydrogen gas as fuel, etc.

[0002]

2. Description of the Related Art Hydrogen gas is converted into water when burned, and does not form carbon dioxide gas or sulfur oxides unlike fossil fuels. Therefore, hydrogen gas is a clean energy source.

[0003] The storage and transportation of hydrogen gas is generally performed as high-pressure gas by compression. Despite the need for heavy and bulky pressure vessels for high pressure hydrogen gas, the volume is 2
This is inefficient, about 1/00, and has a problem in safety. Therefore, it has been studied to use a hydrogen storage alloy capable of reversibly absorbing and releasing hydrogen gas by cooling and heating for storing and transporting hydrogen gas. The hydrogen storage alloy has a higher storage density of hydrogen gas per unit volume than a high-pressure hydrogen gas container, becomes a lighter and smaller volume hydrogen gas storage container, and facilitates the transport of hydrogen gas. In addition, since the pressure is low, the safety is high, and it is resistant to mechanical shock during transportation.

[0004] Hydrogen storage alloys for the purpose of storing and transporting hydrogen have been developed in the past, and are already used for storing hydrogen on a small scale. Research on low-pollution hydrogen vehicles that use hydrogen gas as an alternative fuel to gasoline is also in progress, and various hydrogen storage alloys such as FeTi alloys are being studied as hydrogen storage devices. Furthermore, hydrogen is also used as fuel in fuel cells that are being put into practical use.

Other applications in which the hydrogen storage alloy is expected to be put to practical use include a heat energy storage / transport system utilizing its function of converting heat and chemical energy, a cooling system as a chemical heat pump, and a heat pump. There are thermal-driven static hydrogen compressors and actuators using mechanical energy conversion functions, as well as hydrogen gas purification and hydrogen isotope separation.

[0006] As described above, the hydrogen storage alloy has a wide range of applications, but the hydrogen storage amount is the most important characteristic for any application. In addition, since all of the above applications require a relatively large amount of hydrogen storage alloy, even if the hydrogen storage alloy is repeatedly used, the functions are not reduced much, the durability is excellent, and the price of the alloy is relatively low. It is also important that it be inexpensive. For some applications, relatively low temperatures around room temperature (e.g., 15
(0 ° C or less) is required to absorb and release hydrogen.

[0007] For example, LaNi ₅ or Mm
Since AB ₅ type hydrogen storage alloy represented by Ni ₅ it is expensive, also be used for small secondary battery such as to utilize less Ni- MH batteries of the hydrogen storage alloy, such as for hydrogen gas storage For applications requiring a large amount of hydrogen storage alloy, it is difficult to use in terms of price. Also, the hydrogen storage amount is not so large.

The absorption and release of hydrogen gas by the hydrogen storage alloy are as follows:
Each is a chemical reaction involving volume expansion and contraction. To obtain a practical reaction rate, it is necessary to increase the surface area by using a hydrogen storage alloy in powder form. However, if the volume expansion and contraction of the alloy powder are repeated during use, the powder is cracked due to internal strain, and eventually breaks into fine particles to become fine powder. As the pulverization proceeds, the hydrogen gas does not easily flow due to blockage, or the fine powder moves into the gas pipe while mixing with the flow of the hydrogen gas. Therefore, this pulverization is the long-term repetitive hydrogen absorption / release life of the hydrogen storage alloy.
(I.e., durability) is a major cause.

As hydrogen storage alloys which are relatively inexpensive and have a large amount of hydrogen storage, Japanese Patent Publication No. 59-38293 and Japanese Patent Laid-Open No. 7-252560.
Japanese Patent Application Laid-Open No. 7-268513 and Japanese Patent Application Laid-Open No. 7-268514 describe a Ti-Cr-V-based alloy, respectively. These have been developed as alloys having a large hydrogen storage capacity, but in practice, often do not reach a predetermined hydrogen storage capacity. In addition, these hydrogen storage alloys do not solve the problem of durability due to pulverization described above.

[0010] The oxidation resistance of the hydrogen storage alloy is also an important characteristic. When the hydrogen storage alloy is left in the atmosphere, its surface is oxidized and an oxide film is formed. In particular, a V-containing alloy tends to form an oxide film. This oxide film becomes a hindrance to hydrogen absorption, and cannot exhibit a predetermined hydrogen storage capacity. Therefore, the hydrogen storage alloy powder often requires an activation treatment to remove an oxide film before use. This activation treatment is performed by putting the alloy powder in a pressure-resistant container and applying high-pressure hydrogen gas of several tens of kg / cm ^{2 at} a high temperature for one day to several days, and both the container and the treatment are expensive. . Therefore, a hydrogen storage alloy powder that is hardly oxidized even when left in the air is required so that the activation treatment is not required.

Japanese Patent Application Laid-Open No. 60-190570 discloses that by coating copper and / or nickel metal on a hydrogen storage alloy powder by wet electroless plating, the influence of contamination by impurity gas in the atmosphere can be reduced. It is described that activation is unnecessary or can be reduced. This surface coating is effective in improving the oxidation resistance of the hydrogen storage alloy powder, but since the coating metal is a Cu or Ni metal itself having no hydrogen storage ability, the hydrogen storage amount is reduced by the amount of the coating metal. Decrease.

[0012]

SUMMARY OF THE INVENTION The present invention provides a high hydrogen storage capacity and durability useful for applications such as storage and transportation of hydrogen gas, purification and separation of hydrogen gas, heat transportation and cooling systems, and hydrogen compressors. Relatively low around room temperature (150
(° C or lower), and has excellent oxidation resistance, and its excellent oxidation resistance is maintained even after repeated absorption and release of hydrogen. It is an object to provide an occlusion alloy and a method for producing the occlusion alloy.

The present inventors have previously obtained a Ti—Cr—V-based hydrogen storage alloy powder having a small crystal grain size and a specific composition produced by a rapid solidification method, having a high hydrogen storage capacity and an excellent repetitive hydrogen absorption / absorption property. It has a release life (durability) and can be used at a relatively low temperature near room temperature.This hydrogen-absorbing alloy powder is coated with Ni by plating or mechanical alloying, and then heat-treated to alloy the Ni in the coating. By reacting with Ti in the powder to form a Ni-added layer mainly composed of a Ni-Ti compound layer on the powder surface, it is possible to significantly improve the oxidation resistance of the hydrogen-absorbing alloy powder without impairing the hydrogen-absorbing ability. (See JP-A-11-80865).

However, the above-mentioned hydrogen-absorbing alloy in which the Ni-added layer is formed on the surface to increase the oxidation resistance, causes cracks on the powder surface or local exfoliation when hydrogen absorption and desorption are repeated. As a result, it was found that the oxidation resistance gradually decreased, and the hydrogen storage amount after standing in the air decreased. The present invention is intended to solve the above-mentioned problems by manufacturing a hydrogen storage alloy capable of preventing the reduction in oxidation resistance.

[0015]

Means for Solving the Problems The present inventors disclosed in Japanese Patent Application Laid-Open
-Addition of Ni layer on the surface as proposed in -80865
The means for preventing the oxidation resistance of Ti-Cr-V-based hydrogen storage alloy powder from decreasing due to repeated hydrogen absorption and release were investigated.

As a result, the deformability of the Ti—Ni compound (ie, intermetallic compound such as Ti ₂ Ni, TiNi, and Ni ₃ Ti), which is the main component of the Ni addition layer, is smaller than that of the hydrogen storage alloy, Since the Ti-Ni compound cannot follow the deformation of the alloy such as alloy expansion and alloy shrinkage during hydrogen release, cracking and local peeling occur on the powder surface.This cracking and local peeling are caused by the Ni-Ti compound. It has been found that it can be effectively prevented by interposing a Cu-enriched region between the layer and the hydrogen storage alloy.

Here, the present invention relates to a compound represented by the formula: Ti _a V _1-abcd
Cr _b A _c B _d ‥‥ (1) A hydrogen storage alloy powder having a main phase having an average crystal grain size of 40 μm or less has a Ti-Ni compound layer on the surface thereof and has a hydrogen storage capacity. Alloy and Ti-Ni
A hydrogen storage alloy having a Cu-enriched region near a boundary with a compound layer and having excellent oxidation resistance after repeated hydrogen absorption and release.

In the above formula, A is Mn, Fe, Co, Cu, Nb, Z
n, Zr, Mo, Ag, Hf, Ta, W, Al, Si, C, N, P, and B means one or more elements selected from B, and B is Ln (lanthanoid metal) And one or more elements selected from Y and Y, the value of a is 0.2 or more and 0.5 or less, the value of b is 0.1 or more and 0.4 or less, and the value of c is
0.01 or more and 0.2 or less, and the value of d is 0.001 or more and 0.03 or less.

The hydrogen storage alloy of the present invention has the above formula (1)
(A, B, a, b, c, and d have the same meanings as described above), and a Cu coating layer and a Ni coating are formed on the surface of the hydrogen storage alloy powder having an average crystal grain size of the main phase of 40 μm or less. It is manufactured by a method characterized by forming a coating layer in this order and then performing a heat treatment at a temperature of 400 to 1000 ° C.

Before the heat treatment, the total adhesion amount of the Cu coating layer and the Ni coating layer is 1 to 20% by mass of the mass of the hydrogen storage alloy powder, and the amount of Cu in the Cu coating layer is 50% of the total adhesion amount. It is preferable that the content is not more than mass%.

[0021]

BEST MODE FOR CARRYING OUT THE INVENTION Hydrogen storage alloy powder The hydrogen storage alloy according to the present invention comprises (1) Ti _a V _1-abcd Cr _b A
characterized _c B _d becomes formula (A in the formula, B, to d are as defined above) having a composition represented by, (2) an average grain size of the main phase is less and fine 40μm Having.

The main phase of this hydrogen storage alloy is body-centered cubic, and its crystal lattice is a solid solution composed of three elements, Ti, V, and Cr, a part of which is replaced by element A. The average crystal grain size of the fine main phase of the above (2) can be obtained by manufacturing a hydrogen storage alloy by a rapid solidification method such as a roll quenching method or a gas atomizing method. Therefore, it can be said that the hydrogen storage alloy of the present invention is "a rapidly solidified alloy having a chemical composition represented by the above formula (1)". For example, when the cooling rate at the time of solidification is low as in the arc melting method, crystal grains grow during the solidification and become coarse, and the average crystal grain size of the main phase exceeds 40 μm.

The reason why the hydrogen storage alloy of the present invention has a high hydrogen storage capacity, is hardly pulverized, has excellent durability, and can be used at a relatively low temperature (150 ° C. or lower) near room temperature is as follows. Is done.

The rapidly solidified body-centered cubic Ti-V-Cr alloy has a hydrogen equilibrium pressure (hydrogen absorption / pressure) of 0.1 MPa close to the atmospheric pressure.
(Equilibrium gas pressure for the release reaction) is as low as 150 ° C or less, so even in a temperature range of 150 ° C or less, a large amount of hydrogen can be absorbed and it is difficult to pulverize. I have.

However, such a high hydrogen storage amount and excellent durability cannot be obtained with an alloy manufactured by a conventional method in which solidification after melting is slow as in the arc melting method. This is because when the cooling rate at the time of solidification is reduced, the second phase mainly composed of TiCr ₂ having a small hydrogen storage amount is precipitated at a considerable rate during solidification. The precipitates of the second phase not only reduce the hydrogen storage capacity but also lower the hydrogen equilibrium pressure, making it impossible to reversibly release the absorbed hydrogen. Makes it easier for them to occur. In the hydrogen storage alloy of the present invention, the precipitation amount of the second phase is very small.
It is possible to avoid a decrease in the hydrogen storage amount and pulverization due to this phase.

According to the present invention, the rapidly solidified Ti-V-
By adding elements represented by A and B in the above formula (1) to the Cr-based alloy, the characteristics of absorbing and releasing hydrogen gas at almost the same temperature and pressure as the original alloy are maintained, and the formation of the second phase is performed. The hydrogen storage amount can be further increased while suppressing the above. Although the reason has not been completely elucidated, it is considered as follows.

Element A (Mn, Fe, Co, Cu, Nb, Zn, Zr, M
o, Ag, Hf, Ta, W, Al, Si, C, N, P, and B) are replaced with Ti, V, and Cr constituting the body-centered cubic crystal of the main phase to enlarge the lattice size. It is expected that the hydrogen storage capacity of the alloy itself is increased. If these elements also have a low cooling rate after melting, they tend to form intermetallic compounds, carbides and borides with Ti or Cr, and the hydrogen storage capacity decreases. Therefore, in order to obtain a high hydrogen storage amount, it is necessary to rapidly solidify the compound in order to suppress crystallization or precipitation of such a compound. Thereby, pulverization starting from this compound is also suppressed.

Element B (lanthanoid metal <Ln> or Y) is hardly present in the body-centered cubic crystal of the main phase, and is considered to be present by forming oxides with impurity oxygen contained in the alloy. The impurity oxygen exists in a state of invading between the body-centered cubic metal atoms of the main phase, and the position at which these elements enter is also the position at which hydrogen atoms enter when hydrogen is occluded. Accordingly, the impurity oxygen blocks the hydrogen intrusion site, which causes a reduction in the amount of hydrogen occlusion. It is presumed that when Ln or Y, which easily combines with oxygen, is added, impurity oxygen is driven out of the main phase, and the hydrogen storage amount increases.

The atomic ratio of each element of the hydrogen storage alloy according to the present invention was determined as described above in consideration of obtaining a high hydrogen storage amount at a low temperature of 150 ° C. or lower and near the atmospheric pressure. Next, the reason will be described. As can be seen from the above equation (1), the amounts of the respective elements are all atomic ratios, and the total is 1
It is.

Titanium (Ti) When the amount of Ti increases, the lattice size of the body-centered cubic crystal, which is the main alloy phase, increases, and the hydrogen storage amount increases. To obtain a high hydrogen storage capacity, 0.2 or more Ti is required. As the amount of titanium increases, the amount of hydrogen storage increases, but the hydrogen equilibrium pressure decreases with it, making it unusable at room temperature and near atmospheric pressure.In addition, the lifetime against repeated hydrogen absorption and release due to pulverization (durability) Sex) is reduced. From the viewpoint of the balance between hydrogen storage capacity and durability, the Ti content is 0.2 or more and 0.5 or less, preferably 0.3 or more, 0.45 or less, more preferably 0.3 or more, and 0.
4 or less.

Chromium (Cr) As the amount of Cr increases, the amount of hydrogen occlusion increases.
Since it is not so large, the main purpose of adding Cr is to control the hydrogen equilibrium pressure. Therefore, the amount of Cr changes depending on the amount of Ti, the intended use temperature, and the hydrogen equilibrium pressure. However, when the Cr content is 0.1
If the Ti content is less than 0.2, the hydrogen equilibrium pressure at room temperature becomes significantly lower than the atmospheric pressure when the Ti content is 0.2, and it becomes impossible to reversibly absorb and release hydrogen near room temperature. On the other hand, when Cr exceeds 0.4, the amount of the TiCr ₂ phase precipitated as the second phase increases, and not only the hydrogen storage amount decreases, but also the pulverization tends to occur and the life for repeated hydrogen absorption and release decreases. . From the viewpoint of the balance between hydrogen storage capacity and durability, the Cr content is 0.1 or more, 0.4
Or less, preferably 0.2 or more and 0.4 or less, more preferably 0.2 or more and 0.35 or less.

In the binary system of vanadium (V) Ti—Cr, a large amount of TiCr ₂ is formed as a second phase, the hydrogen storage capacity and the life for repeated hydrogen absorption / desorption are reduced, and the hydrogen equilibrium pressure is too low and the room temperature is too low. Use in the vicinity is also difficult. Therefore, V is added together. By adding V, a large body-centered cubic phase of the main phase is obtained, and the hydrogen storage amount increases. The amount of V is automatically determined by the amounts of Ti, Cr, A element, and B element.

Element A (Mn, Fe, Co, Cu, Nb, Zn, Zr, M
o, Ag, Hf, Ta, W, Al, Si, C, N, P, B) These additional elements are any of Ti, Cr, and V, which are metals constituting the body-centered cubic crystal of the main phase. It is an element effective for substituting and expanding the lattice size to increase the amount of hydrogen occlusion. Since these elements easily form intermetallic compounds, carbides and borides with Ti or Cr, they cannot be added in large amounts. When the amount of element A is more than 0.2, the hydrogen storage amount of the entire alloy is reduced instead. On the other hand, the amount of element A is 0.
If it is less than 01, no increase in hydrogen storage due to the addition is observed. From the balance between the formation amount of intermetallic compounds and the like and the hydrogen storage amount, the amount of element A is 0.01 or more and 0.2 or less, preferably 0.03 or more, 0.15 or less, more preferably 0.05 or more,
0.15 or less.

Element B [Ln (lanthanoid metal element),
Y] These additional elements are added to form a compound with impurity oxygen present at the hydrogen invasion site of the main phase of the alloy. Therefore, the amount of B element is affected by the amount of impurity oxygen in the alloy. If an inexpensive raw material containing a large amount of impurities is used during the production of an alloy, it is necessary to add a large amount of the raw material.

Even when a raw material having a large amount of impurities is used, the amount of B element is sufficient up to 0.03. When the amount of the B element is 0.001 or less, impurity oxygen cannot be removed, and the hydrogen storage amount does not increase. From the above, the amount of the B element is set to 0.001 or more and 0.03 or less. Since the addition amount of the expensive B element is very small, the cost increase by the addition is small. Further, since the oxide of the element B has an effect of suppressing the coarsening of the crystal grain size during the heat treatment, the upper limit of the heat treatment temperature at which a fine crystal structure can be obtained by adding the element B is Ti-V- It can be higher than that of the Cr alloy, and the heat treatment time after surface coating can be shortened.

The lanthanoid metal may be an element such as La or Ce purified as a pure metal alone, but it is an alloy of rare earth metals and is an inexpensive misch metal containing many lanthanoid metals. The use of an alloy referred to as, further reduces the production cost of the hydrogen storage alloy of the present invention.

[0037] As described above average crystal grain size of the main phase, the hydrogen storage alloy of the present invention, Ti-Cr- to originally main phase more body-centered cubic of hydrogen storage capacity
By adding two types of elements A and B to the V-based alloy, the hydrogen storage amount was successfully increased.

However, even with this Ti-Cr-VAB system chemical composition, the hydrogen storage capacity of this alloy varies depending on the production method and the average crystal grain size of the main phase. It was found that when the solidification rate (cooling rate) after dissolution was reduced and the average crystal grain size of the main phase exceeded 40 μm, the hydrogen storage amount was reduced even with the same composition. In addition, the average crystal grain size of the main phase is 40 μm due to the slow solidification rate during alloy production.
It was found that, when the hydrogen absorption and desorption tests were repeated, pulverization (which can be determined by a decrease in the average particle diameter of the powder) became remarkable, and the life of the alloy (durability) also decreased remarkably. Therefore, in the hydrogen storage alloy of the present invention, the average crystal grain size of the main phase (body-centered cubic) is set to 40 μm or less.

In order to further improve these characteristics of the hydrogen storage alloy of the present invention, the average crystal grain size of the alloy main phase is 20 μm or less, particularly 15 μm, before forming a Ni coating layer described later.
m or less. Further, when the average crystal grain size of the precipitate formed as the second phase, such as TiCr ₂ or an intermetallic compound with the element A, is 5 μm or less, pulverization is difficult to occur, and when it is 2 μm or less, pulverization hardly occurs. It has been found.

The hydrogen storage alloy of the present invention having a fine main phase having an average crystal grain size of 40 μm or less can be produced by the rapid solidification method as described above. The specific method of rapid solidification is not limited as long as an alloy having the above average crystal grain size is obtained. Possible rapid solidification methods include a rotating electrode method, a method of pouring a molten alloy onto a rotating drum or roll (eg, a single-roll or twin-roll quenching method), a method of thin casting on a water-cooled copper plate, and a gas atomizing method. Is mentioned.

Of these methods, the rotary electrode method and the atomizing method can produce spherical powder of a hydrogen storage alloy, which eliminates the need for a pulverizing step for pulverization and makes the powder shape substantially spherical. This is advantageous in that the density is increased. In the case of another method, the obtained hydrogen storage alloy is pulverized into powder as required. Hydrogen crushing,
Any of mechanical pulverization can be adopted, and both may be used in combination.

The hydrogen storage alloy of the present invention has an average particle size of 10 to
It is appropriate to use a powder form of about 50 μm. As a result, the surface area increases, and the hydrogen absorption / desorption reaction is promoted. If necessary, adjust the average particle size by classification.

The hydrogen storage alloy produced by the rapid solidification method generally has a minute rapid cooling strain. In the present invention,
Since the hydrogen storage alloy powder is heat-treated after forming the Cu coating layer and the Ni coating layer later, the quenching strain is removed at that time. Therefore, it is not necessary to perform a heat treatment for removing the quenching strain, but it is also possible to perform a heat treatment before the coating.

Surface Coating The hydrogen storage alloy having the composition of the above formula (1) and having the average crystal grain size of the main phase defined in the above (2), as it is, when left in the air, is close to room temperature. The hydrogen storage capacity measured at low temperatures (eg, 80 ° C) decreases. It is considered that the surface of the alloy is oxidized when the alloy is left in the air, and the oxide film acts as an obstacle to decrease the amount of hydrogen absorbed at low temperatures. The hydrogen storage alloy whose hydrogen storage capacity has been reduced by being left in the air as described above is activated by heating it to 500 ° C in high-pressure hydrogen gas (for example, 20 atm) to increase the hydrogen storage capacity, and the absorption capacity before storage To recover. However, as mentioned above, this activation process is expensive.

In a device using a hydrogen storage alloy, contact with the atmosphere cannot be completely avoided during the manufacturing process.
In order to avoid the above activation treatment, it is desired to improve the oxidation resistance of the hydrogen storage alloy of the present invention so as not to be oxidized even when it comes into contact with the atmosphere.

The oxidation resistance of the hydrogen storage alloy is described in JP-A-60-19.
As described in Japanese Patent No. 0570, improvement is achieved by coating the alloy surface with Ni. However, although this method is effective for improving oxidation resistance, Ni itself coated on the alloy surface has almost no hydrogen storage capacity, so that the hydrogen storage amount per unit weight of the alloy is considerably reduced.

As proposed in Japanese Patent Application Laid-Open No. 11-80865, heat treatment is performed after Ni coating to cause the Ni coating layer to react with the base material alloy to form a Ti—Ni compound layer on the powder surface. Since the -Ti compound layer has a significantly greater hydrogen storage capacity than pure Ni, the oxidation resistance can be imparted to the hydrogen storage alloy without substantially reducing the hydrogen storage amount. However, since the Ti—Ni compound has a very low deformability as compared with the base alloy, the Ti—Ni compound cannot follow the deformation of the alloy due to hydrogen absorption and desorption, so that cracks and local peeling occur in this portion. Therefore, when the absorption and release of hydrogen are repeated, the effect of improving the oxidation resistance is gradually lost, and the amount of hydrogen absorbed after leaving in the atmosphere is reduced.

According to the present invention, the powder of the hydrogen storage alloy having the above composition and the average crystal grain size of the main crystal is first coated with Cu, then coated with Ni, and heat-treated. That is, a coating layer having a two-layer structure of a Cu coating layer as a first layer and a Ni coating layer as a second layer is first formed on the powder surface. During the subsequent heat treatment,
As a result of diffusion of Ti from the alloy powder to the Ni coating layer on the surface through the Cu coating layer, a Ti—Ni compound layer is formed on the surface of the hydrogen storage alloy powder. During this heat treatment, the Cu
Dissolves in the hydrogen storage alloy and the Ti—Ni compound layer of the base material, and forms a Cu-enriched region in each. As a result, a Cu-enriched region having a higher Cu concentration than other regions is generated near the boundary between the Ti-Ni compound layer and the base material alloy on the powder surface. Ti
-The presence of a Cu-enriched region in the vicinity of the boundary between the Ni compound layer and the base metal alloy allows Ti
The stress applied to the -Ni compound layer is reduced, and the generation of cracks in the Ti-Ni compound layer is prevented.

In the present invention, the Cu coating layer and the Ni coating layer
Neither need to be composed of pure metal of Cu or Ni,
It may be a layer containing Cu or Ni. Preferably, the Cu coating layer and the Ni coating layer each have a Cu or Ni content of 50% by mass or more, and more preferably are layers substantially composed of Cu or Ni.

Since the Ti—Ni compound layer formed on the powder surface by the heat treatment contains Ni at a higher concentration than the base metal hydrogen storage alloy, it exhibits oxidation resistance for protecting the internal hydrogen storage alloy powder from oxidation. In addition, since the Ti—Ni compound layer often incorporates Cr from the base metal alloy, the Ti—Ni compound layer is more excellent in oxidation resistance than the binary intermetallic compound of Ti—Ni. The Cu-enriched region, which is located near the boundary between the Ti-Ni compound layer on the surface and the base metal alloy, has a smaller deformability than the hydrogen-absorbing alloy and is larger than the Ti-Ni compound. It exerts a cushioning effect of alleviating the stress applied to the Ti-Ni compound layer due to the deformation, and effectively prevents cracking and local peeling of the Ti-Ni compound layer. As a result, even if hydrogen absorption / desorption is repeated, Ti-Ni
The effect of improving the oxidation resistance of the compound layer is maintained, and a high hydrogen storage amount can be maintained even if the hydrogen storage alloy is exposed to the atmosphere while repeatedly used.

In order to sufficiently achieve this effect, the adhesion amount of the oxidation-resistant Cu coating layer and the Ni coating layer described above is such that the total adhesion amount of the two coating layers is 1 to 20 mass of the mass of the hydrogen storage alloy powder. %, And the amount of Cu in the Cu coating layer relative to the total
It is preferable that the content be 50% by mass or less. More preferably, the total adhesion amount is 5 to 20% by mass of the hydrogen storage alloy powder, and the Cu amount is 15 to 45% by mass of the total adhesion amount.

Each of the Cu coating layer and the Ni coating layer can be formed by electrolytic plating, electroless plating, mechanical alloying, mechanochemical alloying, or gas phase reaction. Mechanical alloying is a process in which a hydrogen absorbing alloy powder and a fine powder of Cu or Ni are subjected to plastic working with a high-speed ball mill. Mechanochemical alloying is similar in operation to mechanical alloying, but can form an adhesion layer on a surface with less energy. As the gas phase reaction, a coating method utilizing gas phase thermal decomposition of a compound such as Cu carbonyl or Ni carbonyl is exemplified.

The metal coating having a two-layer structure in which a Ni coating layer is provided on a Cu coating layer is formed by first forming a Cu coating layer and then forming a Ni coating layer by using any of the above coating methods. It can be formed by two treatments. As another method, the plating solution used for electrolytic Ni plating or electroless Ni plating contains a certain amount of Cu ions, and one plating treatment is performed using the property that Cu precipitates preferentially over Ni. It is also possible to form a coating layer having the above-mentioned two-layer structure.

Before the coating treatment, if necessary, the hydrogen storage alloy powder may be pickled with a non-oxidizing acid such as hydrofluoric acid or hydrochloric acid to remove an oxide layer on the surface of the alloy. . After the coating layer having the two-layer structure is formed on the surface of the hydrogen storage alloy powder, heat treatment is performed at a temperature in the range of 400 to 1000 ° C, preferably 450 to 900 ° C. During this heat treatment, Ti diffuses from the base metal alloy into the coating layer, and the Ni coating layer forms a Ti-Ni compound (for example, Ni ₃
Ti, TiNi, changes to a layer of Ti ₂ Ni), TiNi compound layer is produced on the powder surface. At the same time, Ni on the surface and elements of the internal hydrogen storage alloy diffuse into the Cu coating layer underneath the Ni coating layer, and the portion where the Cu coating layer was originally had a higher Cu concentration than the other regions. Area. As a result, a Cu-enriched region is generated near the boundary between the Ti—Ni compound layer and the base metal alloy. This Cu-enriched region may be a Ti-Ni compound layer containing a high concentration of Cu or a hydrogen storage alloy, and does not substantially contain a metallic Cu phase. Ti−
If a layer made of Cu metal remains near the boundary between the Ni compound layer and the base metal alloy, the absorption and release (movement) of hydrogen is hindered, so the heat treatment changes the Ni coating layer to a Ti-Ni compound layer. ,
The process is performed until the Cu coating layer changes to a Cu-enriched region substantially not containing a metal Cu phase. If the heat treatment temperature is too high, the crystal grain size of the hydrogen storage alloy becomes coarse, and the durability decreases. If the heat treatment temperature is too low, the reaction between Ti and Ni and the above change in the Cu coating layer do not easily proceed.

The heat treatment is preferably performed in a vacuum or in an inert gas atmosphere in order to prevent oxidation of the hydrogen storage alloy. Heat treatment time is Ti-Ni compound layer and Cu
A concentrated region can be formed, and the average crystal grain size of the main crystal is 40
Set so as not to become coarser than μm. Since the hydrogen storage alloy contains the B element, the upper limit of the heat treatment temperature is increased, and the heat treatment can be completed in a short time. The formation of the Ti—Ni compound layer and the formation of the Cu-enriched region substantially containing no metallic Cu can be confirmed by X-ray diffraction or the like.

[0056]

The following examples illustrate the invention. In Examples,% is% by mass unless otherwise specified.

A hydrogen storage alloy powder was prepared by an argon gas atomizing method (10 kg / ch). The raw material used for melting was misch metal (abbreviated as Ln) which is an alloy of 99% pure titanium sponge, 98% pure vanadium, 99% pure chromium, and a lanthanoid rare earth metal (La = 46%, Ce
= 5%, Nd = 37%, Pr = 10%, total rare earth content 99.5%)
, 99% pure Fe, Mn, Co, Nb, Y, Zr, 99.9% pure A
l, Ta. Light elements (Si, C, B) are Ti or Cr
(TiC, TiB ₂ etc.).

The prepared hydrogen storage alloy had Ti = 0.30 and V = 0.
25, Cr = 0.30, A = 0.14, B = 0.01
(However, the types of the elements A and B were changed). From the resulting gas atomized powder, a powder having a size of more than 100 μm was removed by sieving. The average particle size of each powder thus obtained was about 50 μm. The average crystal grain size of the main phase of this hydrogen storage alloy was about 5 to 20 μm, which was sufficiently smaller than 40 μm.

First, a Cu coating layer was formed as a first coating layer on the powder surface of the obtained hydrogen storage alloy. Cu coating was performed by electroless Cu plating or mechanical alloying using Cu fine powder. Thereafter, a Ni coating layer was formed as a second coating layer by electroless Ni plating, mechanical alloying using Ni fine powder, or gas phase reaction. Mechanical alloying uses Cu fine powder or Ni fine powder with a particle size of about 1 μm,
Performed by time treatment. A commercially available electroless Ni plating solution was used.

The formation of the Ni coating layer by the gas phase reaction is performed by using Ni (CO) ₄
This was performed using gas. Diameter with gas injection and exhaust holes
In a quartz cylindrical container of 150 mm x 300 mm length, width 100 mm x length
A 150 mm quartz boat was placed, and the hydrogen storage alloy powder was stored in the boat. This cylindrical container was placed in an electric resistance heating furnace controlled at a temperature of about 150 ° C, and after the whole powder reached about 150 ° C, 80% by volume of Ni (CO) ₄ gas and 20% by volume of CO gas
Was injected into the cylindrical container from the tube connected to the injection hole, and the mixture was treated until the desired amount of Ni was attached to the powder.

After forming the Cu coating layer and the Ni coating layer, a heat treatment is performed in an argon gas atmosphere to obtain a hydrogen having a Ti-Ni compound layer on the powder surface and a Cu-enriched region near the boundary between this layer and the base metal alloy. An occlusion alloy was obtained. The presence or absence of the formation of the Ti-Ni compound layer on the powder surface was confirmed by an X-ray diffraction method. In addition, this Ti-
It was also confirmed by X-ray diffraction that in the hydrogen storage alloy on which the Ni compound layer was formed, as a result of the formation of the above-mentioned Cu-enriched region, the metallic Cu phase had disappeared.

FIG. 1 shows that the vicinity of the boundary between the Ti—Ni compound layer and the base metal alloy in the hydrogen-absorbing alloy heat-treated in this manner (No. 1 alloy in Table 1 shown later) is a transmission type electron with an analyzer. The result of having analyzed about Cu concentration with a microscope is shown. As shown in FIG. 1, the Cu concentration became very high near this boundary.
There was a Cu enriched region.

Measurement of Initial Hydrogen Storage Amount The initial hydrogen storage of the hydrogen-absorbing alloy powder coated and heat-treated as described above was determined by the activation origin method using a Sibeltz-type hydrogen absorption / desorption test apparatus as follows. Measured.

The gold was placed in a container, evacuated to determine the origin, and then heated to 300 to 500 ° C. under a hydrogen pressure of 0.5 MPa for activation. 5% by volume of the test alloy before the activation treatment to eliminate the effect of oxidation of the alloy powder surface before the test
The product was pickled with a hydrofluoric acid aqueous solution. The hydrogen gas release-absorption cycle used in the test consisted of hydrogen gas absorption in which the hydrogen pressure was increased from 0.1 MPa to 3.0 MPa at an alloy temperature of 20 ° C, and an alloy temperature of 60 ° C.
And release of hydrogen gas to reduce the hydrogen pressure from 3.0 MPa to 0.1 MPa.

The hydrogen storage amount was determined by preparing a hydrogen release curve at the time of releasing hydrogen gas after one cycle, obtaining the value of the hydrogen storage amount at a pressure of 1 MPa, and determining the hydrogen amount with respect to the number of metal atoms constituting the alloy. H / M which is the ratio of the number of hydrogen atoms
And evaluated.

Evaluation of Oxidation Resistance After repeating the above hydrogen gas absorption / desorption cycle for 100 cycles , the resulting gold was subjected to a dehydrogenation treatment at 400 ° C. for 30 minutes. After that, the match gold was left in a constant temperature and humidity atmosphere at a temperature of 25 ° C. and a humidity of 65% for one week, and then was heated to 80 ° C. without activation using a Sibeltz-type hydrogen absorption / release test apparatus.
, A hydrogen gas absorption test of 2.5 MPa was performed to measure the hydrogen storage capacity.

Separately, the hydrogen storage amount was measured for the gold subjected to one cycle of hydrogen gas absorption / desorption cycles, after dehydrogenation treatment and leaving it in the air atmosphere for one week as described above. did. The reduction rate of the hydrogen storage capacity after 100 cycles with respect to the hydrogen storage capacity after one cycle was calculated by the following formula, and the oxidation resistance was evaluated. If the reduction rate of the hydrogen storage amount is 10% or less, the oxidation resistance is acceptable.

Hydrogen storage rate decrease rate (%) = [(AB) / A] × 100 A = 1 hydrogen absorption / release after one cycle of absorption and release of hydrogen B = 100 cycles of hydrogen absorption / release Hydrogen storage amount measured after leaving in the air after release The results of these tests were determined based on the composition of the hydrogen storage alloy, the method of forming each of the first (Cu) and second (Ni) coating layers, and the adhesion amount (% by mass based on the alloy powder). Table 1, together with the heat treatment conditions and the presence or absence of the formation of the Ti-Ni compound layer, are shown in Table 1 below.

[0069]

[Table 1] As can be seen from Table 1, after forming the first Cu coating layer and the second Ni coating layer according to the present invention, a heat treatment was performed to form a Ti-Ni compound layer on the powder surface and a Cu-enriched region therein. The hydrogen-absorbing alloy powder formed with a high initial hydrogen-absorbing capacity, and has excellent oxidation resistance even after 100 cycles of hydrogen gas absorption / desorption cycles. The rate of decrease of the amount was suppressed to 10% or less.
In other words, the hydrogen-absorbing alloy powder can be handled in the air while the absorption and release of hydrogen gas is repeated without impairing the hydrogen-absorbing ability, so that the handling becomes very easy and costly activation treatment is unnecessary. Or be reduced. It is to be noted that this hydrogen storage alloy powder is hard to be finely divided even when hydrogen gas is repeatedly absorbed and released, and has excellent durability.
As demonstrated in JP-A-11-80865.

In the comparative example, when the surface coating layer was not formed at all, the decrease in the hydrogen storage alloy after leaving in the air was as large as 37%. Even when the heat treatment was performed by forming only the Ni coating layer without forming the Cu coating layer as the first layer, the reduction rate of the hydrogen storage amount was still as large as 20%. Further, the first Cu coating layer and the second
Even if a two-layer metal coating of the Ni coating layer was applied, without heat treatment, no Ti-Ni compound layer was formed on the surface, and the hydrogen storage amount reduction rate was as large as 17%.

[0071]

The hydrogen storage alloy of the present invention has a high hydrogen storage capacity and absorbs and releases hydrogen at a relatively low temperature near room temperature, so that it is easy to use for various applications, and can absorb and release hydrogen for a long period of time. It is hard to pulverize even if repeated, and relatively inexpensive. Furthermore, the surface of this hydrogen storage alloy
A Ti—Ni compound layer is generated with the thickened region interposed.
The oxidation resistance of the alloy is improved by the Ti-Ni compound layer on the surface,
In addition, since the stress on the Ti-Ni compound layer is reduced by the Cu-enriched region, the effect of improving the oxidation resistance is maintained even when hydrogen is repeatedly absorbed and released. As a result, even if the gas is released to the air on the way, a high amount of hydrogen storage can be obtained by repeating the absorption and release of the hydrogen gas without performing an expensive activation treatment, and the usability is improved.

The hydrogen storage alloy produced by the method of the present invention is used for hydrogen gas storage / transport, hydrogen gas separation / purification, heat transport system and cooling system, static compressor, fuel cell using hydrogen gas as fuel, etc. Ideal for use.

[Brief description of the drawings]

FIG. 1 shows a Cu concentration distribution near a boundary between a Ti—Ni compound layer on a powder surface of a hydrogen storage alloy according to the present invention manufactured in an example and an internal base metal alloy.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) C22F 1/00 661 C22F 1/00 661C 682 682 691 691B 1/18 1/18 H H01M 8/04 H01M 8 / 04 J

Claims (4)

[Claims] 1. A _{formula: Ti a V 1-abcd Cr} b A c B has a _{composition d} ‥‥ represented by (1), on the surface of the hydrogen-absorbing alloy powder having an average grain size of the main phase is 40μm or less Characterized by having a Ti-Ni compound layer, and having a Cu-enriched region near the boundary between the hydrogen storage alloy and the Ti-Ni compound layer, and having excellent oxidation resistance after repeated hydrogen absorption and release. Storage alloy. In the above formula, A is Mn, Fe, Co, Cu, Nb, Zn, Zr, Mo, Ag, Hf, Ta,
Means one or more elements selected from W, Al, Si, C, N, P, and B; and B is one selected from Ln (lanthanoid metal) and Y
A or two or more elements, a value of 0.2 or more and 0.5 or less, b value of 0.1 or more and 0.4 or less, c value of 0.01 or more and 0.2 or less, d value of 0.001 or more and 0.03 or less . 2. The formula (1) according to claim 1, wherein A, B,
a, b, c, d are the same as in claim 1), and a Cu coating layer and a Ni coating layer are formed on the surface of a hydrogen storage alloy powder having a main phase having an average crystal grain size of 40 μm or less. 2. The method for producing a hydrogen storage alloy according to claim 1, wherein the heat treatment is performed at a temperature of 400 to 1000 [deg.] C. after the formation. 3. The method according to claim 2, wherein the total amount of the Cu coating layer and the Ni coating layer is 1 to 20% by mass of the mass of the hydrogen storage alloy powder. 4. The method according to claim 1, wherein the amount of Cu in the Cu coating layer is
The method according to claim 2 or 3, wherein the amount of is less than or equal to 50% by weight.