EP1478787A1

EP1478787A1 - Ca, mg and ni containing alloys, method for preparing the same and use thereof for gas phase hydrogen storage

Info

Publication number: EP1478787A1
Application number: EP03700734A
Authority: EP
Inventors: Guoxian Liang; Robert Schulz
Original assignee: Hera Hydrogen Storage Systems Inc
Current assignee: Hera Hydrogen Storage Systems Inc
Priority date: 2002-02-27
Filing date: 2003-01-17
Publication date: 2004-11-24
Also published as: AU2003202337A1; CN1639367A; WO2003072838A1; CA2373553A1

Abstract

Disclosed are new Ca, Mg and Ni-containing alloys of the general formula: ( Cac Mgd Me )b ( Ni1-z Tz )a where: M is at least one metal selected from the group consisting of Y, Ce, La, Pr, Nd, Th, Nd, Ti, V, Zr, Ta, Hf, Sr, Ba and Misch metals; T is at least one element selected from the group consisting of AI, Zn, Cu, Fe, Co, Mn, Cr, Mo, W, Si, Ga, Ge, In, Sn, Ag, C and B; a is an integer equal to 2 or 5; z is a number ranging from 0 to 0.5; when a is equal to 2, then 0.9 ≤b≤1.1; and when a is equal to 5, then either 1.75 ≤b≤2.25 or 0.75≤ b≤ 1.31. These alloys which are of the AB2, A2B5 or AB5 types, may be single phase or multiphase and are useful for reversibly absorbing hydrogen from a gas phase. Also disclosed is a method for preparing the above alloys.

Description

Ca, g AND Ni CONTAINING ALLOYS, METHOD FOR PREPARING THE SAME AND USE THEREOF FOR GAS PHASE HYDROGEN STORAGE

Field of the invention

The present invention is directed to Ca, Mg and Ni-containing alloys. It is also directed to a method for preparing these alloys and to their use for reversibly absorbing hydrogen from a gas phase.

Background of the Invention

The use of hydrogen gas as a fuel for PEM fuel cells has received considerable attention in recent years because PEM fuel cells using pure hydrogen can provide high efficiency and ultra clean power. Unfortunately, widespread use of hydrogen energy is not currently feasible because of .economic and technological barriers. One of the important barriers is the lack of cost effective, safe hydrogen storage method.

Hydrogen gas is very light. It can be compressed under high pressure and stored in pressurized vessels. It can also be liquefied and stored in liquid form. Hydrogen also reacts with metal or non-metals to form hydrides. Some metal hydrides called "low temperature metal hydrides" are reversible at ambient temperature and pressure. From a safety point of view, metal hydrides are intrinsically safe since the hydrogen must be released from the hydrides before it can burn or be oxidized. The volumetric density of hydrogen storage in metal hydrides is usually high.

The most serious shortcomings of the reversible metal hydrides and more particularly the low temperature metal hydrides are their low gravimetric storage density and the high cost. For stationary and some mobile applications, the weight of the hydrogen storage tank is not a problem. However, the high cost of conventional low temperature metal hydrides results in too expensive storage devices.

Amongst the known hydrogen storage materials, conventional lanthanum or mischmetal-based alloys of the ABs type like LaNis alloys are inherently expensive because they make use of a rare earth metal. CaNis is isomorphic to LaNis and has higher storage capacity than that of LaNis based hydrides. In the Ca-Ni system, there are four stable compounds, CaNi^'2, CaNiX Ca2Mi i7 and CaNis, but only the CaNis has been considered to be of practical interest since the plateau pressures of CaNis are adequate for applications. The other three compounds do form very stable hydrides. However, hydrogen can not be extracted at temperatures below 100°C under normal pressure.

In a paper published in 1980 (and identified A in the attached list of references), Osterreicher et al disclosed a Cao.5Mgo.5Ni2 alloy prepared by liquid sintering in a Ta tube. They also disclosed that such a Cao.5Mgo.5Ni2 alloy can absorb hydrogen to form Cao.5Mgo.5Ni₂H2.6 at 300 °K (corresponding to 1.7 wt.%). However, this hydride can only desorbs partially (half of the amount absorbed) at 400°K and 1 bar. The desorption isotherm is strongly sloped. According to these authors, the low reversibility of this alloy limits practical applications. However, this publication teaches that substitution of Ca by Mg in CaNΪ2 is possible. The alloys of the type CaxMgι-xNi2 are predominantly of CaNΪ2 structure (C15 type) when x is larger or equal to 0.5.

In a recent paper (B) Terashita et al. disclosed a Ca_xMgι-χNi2 system.

Moreover, they presented results of tests made on a Cao.33Mgo.67NΪ2 alloy prepared by high frequency induction melting. Once again, the plateau pressure of this alloy Cao.3₃Mgo.67Ni2 is still too low for most gas phase applications and it is impossible to raise the plateau pressure by substituting more Mg for Ca using the induction melt casting method because the MgNi∑ phase precipitates. That phase is known for not absorbing hydrogen and consequently the properties deteriorate.

Since Ca and Mg have very high evaporation rate, it is very hard to produce a stoichiometric Ca-Mg-Ni alloy in large quantity. For example, if use is made of a melt casting method, the Mg and Ca alloy in liquid form has to be protected by argon or SFβ because of the rapid oxidization and possible explosion. The composition has to be adjusted by trial and error due to evaporation loss of Mg and Ca. The so-cast alloys usually have very high macro-segregation and micro-segregation. Homogenization by long annealing treatments is required but adds costs to the alloys. In paper A, the Ca-Mg-Ni alloys were synthesized by liquid phase sintering in closed Ta tubes starting from elemental Mg, Ca and Ni at a temperature below 1250°C. Such a method also needs high temperature and long sintering time to reach complete reaction.

Mg-Ca-Ni alloys of the AB₃ type have also been synthesized by a powder sintering method. In a recent publication (C), Kadir et al disclosed a CaMg2Ni9 ternary alloy prepared by sintering fine powder mixtures of MgNi₂ and CaNis. This paper states that the sintering process involved raising the temperature stepwise to 600°C, 850°C and 990°C (in 0.6MPa Ar gas), with set temperature holds for 2-3h. Several attempts were necessary until an optimum composition was found. A slight excess of CaNis over the stoichiometric composition was necessary in order to compensate for the evaporative loss of Ca.

Mg and Ca are known to react with all type of ceramic crucible at elevated temperatures and Ni is known to react with refractive metals at high temperatures. Therefore, all the sintering of alloys containing Mg, Ca and Ni were performed so far in Mo or Ta crucible, especially when long sintering times were needed. In practice, this means that the use of this technology would be expensive.

Summary of the invention

The present invention as claimed hereinafter relates to new Ca, Mg and Ni-containing alloys of the general formula:

( Cac Mg Mβ )b ( NiVzTz )_a

where:

M is at least one metal selected from the group consisting of Y, Ce, La, Pr, Nd, Th, Nd, Ti, V, Zr, Ta, Hf, Sr, Ba and Misch metals;

T is at least one element selected from the group consisting of Al, 2n,

Cu, Fe, Co, Mn, Cr, Mo, W, Si, Ga, Ge, In, Sn, Ag, C and B; a is an integer equal to 2 or 5 z is a number ranging from 0 to 0.5, and when a is equal to 2, then b, c, d, e are numbers selected so that:

0.9 <b<1 .1 c = 0.4-x, d = 0.6-y, and e = x + y, with -0.4< y<0.5

-0.5<x<0.4, and x + y >0; when a is equal to 5, then b, c, d, e, are numbers selected so that either 1 .75 <b<2.25 c = 0.4-x, d = 0.6-y, and e = x + y, with -0.4<x<0.2, -0.2<y<0.4, and x + y>0 or 0.75<b<1 .31 , c = 0.6-y, d = x + y, and e = 0.4-x, with -0.6 <x < 0,4, -0.4<y<0.6, and 0 < x + y<0.5

When a is equal to 2, the alloys according to the invention can be said to be of the AB2 type.

When a is equal to 5 and b is ranging between 1 .75 and 2.25, the alloys according to the invention can be said to be of the A2B5 type.

When a is equal to 5 and b is ranging between 0.75 and 1 .31 , the alloys according to the invention can be said to be of the AB5 type.

In all cases, the alloys may be single phase or multiphase.

The invention also relates to a method for preparing the above mentioned alloys, comprising the steps of:

- milling a mixture of elemental powders and/or pre-alloyed combination of elemental powders of Ca, Mg, M, Ni and T in relative amounts corresponding to those found in the requested alloy; and

- annealing and/or sintering the so-milled mixture of powders at a temperature higher than 800°C to obtain the requested alloy. The invention further relates to the use of the above-mentioned alloys for reversibly absorbing hydrogen from a gas phase.

The invention and its advantages will be better understood upon reading the following non-restrictive description and examples made with reference to the accompanying drawings.

Brief description of the drawings

With respect to the alloys of the AB∑ type:

Fig. 1-1 shows the XRD (x-ray diffraction) spectra of a sample of composition Cao. Mgo.eNi₂ made by milling a powder mixture of Ca, Mg and Ni powders as a function of milling time.

Fig.1-2 shows the XRD spectra of a series of CaχMgι-xNi2 samples for x = 0,

0.2, 0.4, 0.5, 0.7and 1 obtained after 20h of milling.

Fig.l-3 shows the XRD spectra of a mechanically alloyed Cao.3Mgo.7Ni2 sample annealed for 1 hour at different temperature 600°C, 800°C and 1000°C.

Fig.l-4 shows the XRD spectra of mechanically alloyed CaχMgι-xNi2 for x = 0,

0.2, 0.3, 0.4 and 0,8 after 1 h annealing at 1000°C.

Fig.l-5 shows PCT (pressure-concentration isotherm) of CaχMgι-xNi2 alloys measured at 30°C. Fig.l-6 shows the XRD spectra of Zn (on the B site) and Mm (on the A site) substituted Cao. Mgo.sNi2 alloy after annealing at 1000°C for 1 h.

Fig.l-7 shows the XRD spectra of Cu and Fe (on the B site) and Mm (on the

A site) substituted Cao.3Mgo.7Ni2 alloy after annealing at 1000°C for 1 h.

Fig.l-8 shows the PCT of various (Cao.4-xMgo.6-_YMmx+_y)Ni2 substituted alloys. Fig.l-9 shows the position of the plateau pressures versus the lattice parameters for various CaxMgι-xNΪ2 alloys and (CaxMgo.7Mm_y)Ni₂. The substitution of Ca by Mm does not change much the lattice parameter but increases significantly the plateau pressures.

With respect to the alloys of the A2B5 type:

Fig.11-1 shows the XRD spectra of (Cao^Mgo.e^Nis after ball milling for various time using process A (i.e. starting with a mixture of intermetallics and elemental powders) .

Fig.ll-2 shows the XRD spectra of (Cao.₄Mgo.6)bNis for various A/B = b/5 ratios after annealing at 1 000°C.

Fig.ll-3 shows the XRD spectra of (Cao.37sMgo.625)2Nis made by process B

(i.e. by milling elemental powder of Ca, Mg and Ni) and annealed 1 h at various temperature.

Fig. H-4 shows the PCT curves measured at 30°C of (Cao.37sMgo.62s)2Nis annealed at various temperatures.

Fig. H-5 shows the PCT curves of (Cao.37sMgo.62s)₂Ni5 annealed at 1000°C and measured at various temperatures.

Fig.ll-6 shows the XRD spectra of various (CaxMgy Nis with various Mg/Ca ratios. Fig.ll-7 shows the PCT curves measured at 30°C of the various samples shown in Fig.ll-5.

Fig. II-8 shows the XRD spectra of various Mm substituted (Cao.375- Mgo.6₂5-yMm +y)2Ni5 - Substitution on the A site.

Fig.ll-9 shows the PCT curves of the various substituted alloys shown in Fig.ll-8.

Fig. H-10 shows the XRD spectra of (Cao.sMgo.37sMmo.i2s)2Ni5 and its hydride.

Fig.11-1 1 shows the XRD spectra of various A2B5 type alloys with substitution on the B site.

Fig.11-1 2 shows the PCT curves of various substituted alloys shown in Fig.ll-1 1 . With respect to the alloys of the ABs type:

Fig.lll-1 shows the XRD spectra of CaxMm-i-xNis alloys made by mechanical alloying followed by annealing Fig.lll-2 shows the lattice parameters of the CaxMnm-xNis alloys shown in Fig.lll-1 .

Fig. III-3 shows representative PCT curves of some of the ternary CaχMmι-χNi5 alloys shown in Fig.lll-1 . Fig.lll-4 a&b show XRD spectra of ball milled Ca-Mg-Ni ternary alloys before and after annealing treatment and demonstrate that the solubility of Mg in CaNis is very low.

Fig.lll-5 a&b show XRD spectra of ball milled Mm-Mg-Ni ternary alloys before and after annealing treatment and demonstrate that the solubility of Mg in MmNis is also very low. Fig.lll-6 shows XRD spectra of various mechanically alloyed Mm-Ca-Mg-Ni quaternary alloys before annealing treatment.

Fig.111-7 shows XRD spectra of various mechanically alloyed Mm-Ca-Mg-Ni quaternary alloys after annealing treatment. Fig.lll-8 shows the PCT curves of the quaternary alloys shown in Fig.lll-7 Fig.lll-9 a&b shows the XRD spectra of various Mm-rich quaternary Mm-Ca- Mg-Ni alloys (a) and the corresponding PCT curves (b). Fig.lll-10 a&b show the XRD spectra of various Mm-poor quaternary Mm- Ca-Mg-Ni alloys (a) and the corresponding PCT curves (b). Fig.lll-1 1 shows the relationship between the plateau pressure and the c/a parameter of various (Mm-Ca-Mg)bNis alloys. Detailed description of the invention

1. ALLOYS OF THE AB2 TYPE

As aforesaid, when a is equal to 2, then the alloys according to the invention are of the formula:

(Cao.4-xMgo.6-yMx + y)b(Nil-zTz)2

where M and T are defined as above and

0.9 <b<1 .1 ,

-0.5<x<0.4,

-0.4<y<0.5, x + y> 0; and O≤z≤O.5.

Preferably:

-0.1<x<0.25; and -0.1≤y<0.15.

These alloys are single or multiphase hydrogen storage compounds of the AB2 type, which are capable of absorbing and desorbing hydrogen from a gas phase at ambient temperature with a flat plateau pressure and a storage capacity larger than 1 .2 wt %.

These alloys may be prepared by a method comprising the following two steps.

The first one consists of preparing a powder by milling a mixture of elemental powders and/or pre-alloyed combination of the elemental powders

(ex.: Ca, Mg, Ni, Mm, CaN , MgNi₂, MmNis, when M = Mm) in respective amounts selected to obtain the requested alloy. The milling can be a conventional ball milling or a more intensive mechanical alloying that can be carried out at room temperature or at high temperatures with or without anti-sticking agents and in various kind of atmosphere. The second step consists of annealing and/or sintering the milled powder at elevated temperatures in a crucible made of, for example, stainless steel, for a short period of time in an inert or reactive atmosphere. This is an essential step to achieve a high reversible absorption capacity and a flat plateau. The annealing temperature should be higher than 800°C but not higher than 1050°C.

The invention is based on the discovery that the mechanical alloying of elemental powders (such as powders of Ca, Mg, Ni) and/or of mixtures of intermetallic compounds (such as powders of CaNis, MgNi₂ ) corresponding to the composition of the formula:

CaχMgι-xNi₂

leads to a nanocrystalline ternary intermetallic compound with the CaNi2 structure (C15-type) for x>0.3. This so-milled nanocrystalline alloy can reversibly absorb and desorb hydrogen at room temperature. However, the reversible absorption capacity is small and the slope of the PCT curves is very big like in the case of conventional large-grain polycrystalline CaxMgι-xNi2 alloys already reported in the art (see reference B).

The invention is also based on the discovery that, when a thermal treatment) of the present invention is applied to this mechanically alloyed powder, a substantial improvement in properties is achieved. Indeed, the hydrogen storage properties of the mechanically alloyed Mg-Ca-Ni are substantially improved by annealing the powder at temperatures higher than 800°C for short period of time, typically at 1000°C or slightly higher, for 0.5h to 1 h. Annealing at temperatures lower than 800°C does not improve the hydrogen storage properties very much.

The ball milling time can vary from a few minutes to several hours. When the milling time is of about 10 hours, then a true alloy is formed between the components (see Fig.1-1 ), i.e. a nanocrystalline ternary intermetallic compound with the CaNi₂ structure (C15-type)). Thus, in that case, the ball milling step is a mechanical alloying. Under these conditions, the post- thermal step becomes an annealing treatment which could be as short as 0.5 to 1 hour as indicated above.

On the other hand, when the ball milling step is only carried out for a few minutes, the powder particles are then only agglomerates of the various constituents. In that case, the post-thermal step is a sintering treatment which could take a few hours to produce the final alloy product.

Further improvements to these alloys may be achieved by adding other elements to Ca, Mg and Ni or their combinations. Such an addition may raise the plateau pressure and improve hydrogen storage properties such as plateau slope and the long-term stability while keeping the AB2 (C15-type) structure as the main phase.

Example 1-1 : Compound according to the prior art made by the method according to the invention

Cao.3Mgo.7Ni2 (b = 1 , x = 0.1 , y = -0.1 , z = 0) was synthesized in a SPEX^® high energy ball mill under the protection of argon. A Mg powder (> 99%, + l OOmesh), Ca granules (> 99.5, ~2mm in size) and a Ni powder ( <99.9%, -325mesh) were used as starting materials. Isothermal annealing was performed in a tubular furnace under the protection of argon flow. The mechanically alloyed powder was sealed in a stainless steel crucible before annealing. The powder was heated to 1000°C at a heating rate of 30C/min, and held at 1 000°C for 1 hour, then cooled down to room temperature in the furnace.

Hydrogen absorption/desorption properties were measured by using an automatic Sievert's type apparatus. The annealed powder normally needs mild activation treatment, such as heated to 200°C under vacuum and then cooled down. EDX analysis shows that the Fe content in the end product less than 0.2 at. %. The composition of the end product was close to the nominal composition. The activated alloy exhibited a relatively flat plateau and a high capacity.

Example 1-2: Compound according to the invention made by the method according to the invention

Cao.ιsMgo.7Mmo.i5Ni2 (x= 0.25, y = -0.1 , M = Mm, x + y = 0.1 5, b = 1 , z = 0) was synthesized by mechanical alloying of elemental powder blends. The alloy was annealed in the same manner as in Example 1 . This alloy had a hydrogen storage capacity of 1 .25wt.%. The plateau pressure was drastically raised, and the plateau slope was much less than that of Cao.3 Mgo.7Ni2 θf the example 1-1

Examples I-3 and subsequent

Other examples similar to example 1 -2 were carried out. The results of these other examples are reported in the accompanying drawings (see Figs. 1-1 to 9). 2. ALLOYS OF THE A2B5 TYPE

As aforesaid, when a is equal to 5, then the alloys according to the invention may be of the formula:

(Cao.₄-xMgo.6-yMx + y)b(Nil-zTz)5

where M and T are defined as above and

1 .75≤b<2.25, -0.4<x<0.2, -0.2<y<0.4, x + y>0, and O≤z≤O.5.

Preferably:

-0.1<x<0.1 , and -0.1 <y<0.2.

Preferably also, Mg and Ca are present in a Mg/Ca ratio ranging from 0.5 to 2 and more preferably from 1 .5 to 1.75.

As mentioned before, in a Ca-Ni system, there are four stable compounds CaNi2, CaNi3, Ca₂NΪ7 and CaNis. The CaNi₂ -type system (also called AB2) has been the subject of PART I hereinabove. This system has a cubic C1 5 structure and Mg can substitute Ca up to x = 0.3 in the CaχMgι-xNi2 alloys while maintaining a cubic structure. The unit cell volume decreases and the plateau pressure increases with increasing the Mg content.

In accordance with the invention, it has been found that the replacement of Ca and/or Mg by Mm in a quaternary AB2 type Ca-Mg-Mm-Ni system improves the hydrogen storage properties significantly

CaNi3 type alloys have been investigated in great details. The intermetallic compound CaNΪ3 can absorb hydrogen readily to form CaNi3H₄e but cannot release hydrogen under ambient conditions (see reference A). Oesterreicher et al. have investigated the pseudo-binary CaxMgι-xNi3 and found that Mg can substitute Ca up to x = 0.7(see again reference A). Recently, Kadir et al. reported a series of AB3 alloys of the type AB2C9 identified as RMg2Niθ where (R = La, Ce, Pr, Nd, Sm and Gd) (reference D) and a CaMg2 is alloy (reference C) synthesized by sintering a mixture of elemental powders or intermetallic compounds. They found that these AB2C9 type compounds have the PuNi3 structure (reference D) . The unit cell volume and hydrogen storage properties can be altered by different substitutions for the A, B and C elements.

In between CaNi₂ and CaNΪ3, a metastable Ca∑Nis compound with an hexagonal P63/m structure (space group 1 76) has previously been reported (see reference E). This material was prepared from the melt by a peritectic reaction at 1035 °C. The present inventors have not been able to synthesize this compound by using the method of preparation described in this document but have discovered that when one substitutes some Ca by Mg in the virtual Ca₂Nis compound, a new phase having a structure different from those of AB2 (MgCu∑ - cubic), AB3 (PuN^'13 - hexagonal) and Ca₂Nis (BaC6 - hexagonal) is formed when the compound is prepared by the method of the present invention. This new phase has a large hydrogen storage capacity and an adequate plateau pressure for reversibly storing hydrogen gas at room temperature.

Therefore, the second range of composition encompassed by the present invention is the range where the A/B ratio is between 0.45 and 0.35. As mentioned previously, A means (Cao.₄-xMgo.6-_vMx+y) and B means (N -zTz). This range corresponds to alloys of the A2B5 type.

As discussed hereinabove, a AB2 phase with a cubic C1 5 structure is formed when the A/B ratio is bigger than 0.45. When the A/B ratio is less than 0.35, an AB3 phase with the PuNi3 structure is formed. However, when the A/B ratio is in the range of 0.35-0.45, a new A∑Bs-type phase with a crystallographic structure which has not yet been identified, is obtained. This new alloy is made by the same method as the one discussed previously, which comprises first preparing a powder by milling a mixture of elemental powders and/or pre-alloyed combinations of elemental powders (ex.: Ca, Mg, Ni, CaNΪ2, MgNΪ2 ...) in amounts sufficient to achieve the requested composition, and then annealing and/or sintering the so milled mixture at an elevated temperature of about 1000°C to form the requested hydrogen storage alloy.

Thus, the invention is based on the discovery of new hydrogen storage alloys of the A2B5 type which contains Ca, Mg, Ni and other optional elements. These new alloys are capable of absorbing and desorbing hydrogen from the gas phase at ambient temperature with a flat plateau pressure and a storage capacity larger than 1 .5 wt%.

These alloys may be prepared by a method comprising two steps.

The first one consists of preparing a powder by milling a mixture of elemental powders and/or pre-alloyed combinations of the elemental powders (ex.: Ca, Mg, Ni, Mm, CaNi₂, MgNi₂, MmNis, when M = Mm) in respective amounts selected to obtain the requested alloy. The milling can be a conventional ball milling or a more intensive mechanical alloying. It can be carried out at room temperature or at high temperatures with or without anti-sticking agents and in various kinds of atmosphere.

The second step consists of annealing and/or sintering the milled powder at elevated temperatures in a crucible made of, for example, stainless steel, for a short period of time in an inert or reactive atmosphere. This is an essential step to achieve high reversible capacity and a flat plateau. The annealing temperature should be higher than 900°C but not higher than 1080°C.

Once again, the invention is based on the discovery that mechanical alloying of elemental powders (such as Ca, Mg, Ni) and/or mixtures of intermetallic compounds (such as CaNis, MgNi∑) corresponding to the requested composition (CaxMgi-x Nis leads to a highly disordered structure when the Mg/Ca ratio is between 0.33 and 1.67. The so-milled alloy can reversibly absorb and desorb hydrogen at room temperature. However, the reversible capacity is small and the slope of the PCT curves is very big.

The invention is also based on the discovery that a new A2B5 type phase is formed when the mechanically alloyed (CaχMgι-x)2Nis is annealed at temperature above 600 °C. The formation range of this new phase depends on the Mg/Ca ratio and the (Ca + Mg)/Ni ratio. For a stoichiometric A2B5 composition, the new phase no longer forms when the Mg/Ca ratio is lower than 0.3. If the Mg/Ca ratio is higher than 1 .75, a large amount of MgNi₂ phase is formed and, as a result, the storage capacity is reduced. When the Mg/Ca ratio is set at 1 .5, the new phase is formed when the (Mg + Ca)/Ni ratio is between 0.35 and 0.45.

The invention is further based on the discovery that when a thermal treatment is applied to this mechanically alloyed powder, substantial improvement in properties is achieved. Indeed, the hydrogen storage properties of the ball milled Ca-Mg-Ni are significantly improved by annealing the powder at temperatures higher than 900°C preferably around 1000°C or slightly higher for a short period of time, preferably 0.5 to 1 hour. Annealing at temperatures lower than 900 °C does not improve the hydrogen storage properties very much. It has also been discovered that further improvements are achieved by adding other elements to the basic elements used for preparing the alloys. Such an addition raises the plateau pressure and improves other hydrogen storage properties such as plateau slope and the long term stability .

Example 11-1

(Cao.375Mgo.625)2Ni5 (x = 0.025, y = -0.025, b = 2, z = 0) was synthesized in a SPEX high energy ball mill under an argon protective atmosphere. A Mg powder ( > 99%, + 100mesh), Ca granules (> 99.5%, 2mm in size) and a Ni powder ( > 99.9%, -325mesh) were used as starting materials. Isothermal annealing was performed in a tubular furnace under argon. The mechanically alloyed powder was sealed in a stainless steel crucible before annealing. The powder was heated to 1000°C at a heating rate of 30°C/min, held at 1000°C for 1 hour and then cooled down to room temperature in the furnace.

Hydrogen absorption/desorption properties were measured by using an automatic Sievert's type apparatus. The annealed powder normally needs a mild activation treatment, such as a heating treatment to 200 °C under vacuum followed by a cooling. EDX analysis showed that the Fe content in the end product is less than 0.2 at.%. The composition of the end product was close to the nominal composition. The activated alloy exhibited a relatively flat plateau and a high capacity

Example H-2

(Cao.3i2sMgo.625Mmo.o625)2Nis (x = 0.0875, y = -0.025, M = Mm, x + y =

0.0625, b = 2, z = 0) was synthesized by ball milling of elemental powder blends. The alloy was annealed in the same manner as in Example 11-1 . This alloy had a storage capacity of 1.65wt%. Its plateau pressure was flat and significantly raised as compared to that of Example 11-1 .

Example H-3 and subsequent

Other examples similar to examples 11-1 and 2 were carried out. The results of these other examples are reported in the accompanying drawings (see Figs. 11-1 to 12).

3. ALLOYS OF THE ABs TYPE

As aforesaid, when a is equal to 5, then the alloys according to the invention may also be of the formula:

(Cao.6-yMgx + yMθ.4-x)b(Nil-zTz)5

when M and T are defined as above and 0.75<b<1 .31 , -0.6 <x<0.4, -0.4<y < 0.6, 0 <x + y<0.5, and O≤z≤O.5.

Preferably: -0.2 <x<0.2, -0.2<y<0.2, and 0.1 <x + y<0.4

Alloys of the AB3 type have already been studied in great detail (see references C, D, F, G, H). Some of them have even been recently patented for battery applications (see references I and J). For example, the patent identified as reference I discloses alloys of the formula (Mgι-a-bR1 aM1 b)Niz where 3 < z < 3.8. Such corresponds to alloys AB where the ratio A/B is ranging between 0.263 and 0.33.

The alloys according to the invention as defined hereinabove are of the ABs type. LaNis- and MmNis- based ABs type alloys have been widely investigated as hydrogen storage materials (see reference 1 and 2). The La, Mm and Ni atoms can be substituted by many other elements to tailor the hydrogen storage properties for different applications (see reference 3). However, substitution of La by Mg in LaNis has not been successful. A second phase with an approximate composition of LaMg1.85Ni9.26 was found with a nickel-rich LaNis+x main phase in a melt casting Mgo.1Lao.9Ni5 sample (see reference 4). The hydrogen storage capacity was reduced due to the formation of this second phase.

CaNis intermetallic compound represents a category of low cost hydrogen storage materials with a storage capacity up to 1 .9wt. % (see references 5 to 7). However, little attention has been paid so far to such a system, probably due to its well-known bad cycling stability (see reference 8). Improvement of the hydrogen storage properties of CaNis by substitution of Ca or Ni with other elements has been tried (see references 5 to 8; A and K). Ternary CaxMrm-xNis alloys were produced by melt casting and were also investigated (see reference 5). Mm substitution for Ca can raise the plateau pressure of CaNis. However, the plateau slope is big for the as-cast ternary alloys due to segregation. Annealing at elevated temperatures ( > 1000°C) can reduce the slope to some extent. Previous works made by the present inventors have shown that CaNis and Mm or Zn-substituted CaNis type alloys with relatively flat plateau can be produced by mechanical alloying followed by an annealing treatment at low temperatures (640C) (see reference L) . Substitution of Mg for Ca in a CaNis structure has also been investigated. The reported results show that the Caι-xMg_xNis does not extend homogeneously beyond x = 0.06 (see reference A).

The invention is based on the discovery that slight substitution of Mg in the CaNis destroys the ABs structure, and a mixture of AB3 and free nickel is formed. The hydrogen storage capacity is also reduced. However, when the alloy of the ABs type containing Mg comprises more than 3 elements, the solubility limit can be extended and the plateau can be tailored by the Mg content. The plateau slope is also small and the reversible hydrogen storage capacity is bigger in comparison to ternary Ca-Mm-Ni AB5 type alloys.

Thus, the present invention provides new hydrogen storage alloys of the ABs type, which contains Ca, Mg, Ni and M. These alloys are capable of absorbing and desorbing hydrogen from a gas phase at ambient temperature with a flat plateau pressure and a storage capacity larger than 1 .3wt.%.

These new alloys may be prepared by a method similar to those already disclosed, which comprises two steps.

The first step consists of preparing a powder by milling a mixture of elemental powders and/or pre-alloyed combination of the elemental powders (ex.: Ca, Mg, Ni, Mm, CaNi₂, MgNi₂, MmNis, when M = Mm) in respective amounts selected to obtain the requested composition. The milling can be a conventional ball milling or a more intensive or mechanical alloying which can be carried out at room temperature or at high temperatures with or without anti-sticking agents and in various kinds of atmosphere.

The second step consists of annealing and/or sintering the mechanically alloyed powder at elevated temperatures in a crucible made of, for example, stainless steel for a short period of time in an inert or reactive atmosphere. This is an essential step to achieve high reversible capacity and a flat plateau. The annealing temperature should be higher than 800°C but not higher than 1 100°C.

Once again, the invention is based on the discovery that mechanical alloying of elemental powders (such as Ca, Mm, Ni) and/or mixtures of intermetallic compounds (such as CaNis, MmNis) corresponding to the requested composition CaxMmi-xNis leads to a nanocrystalline ABs type structure. The so-milled alloy can reversibly absorb and desorb hydrogen at room temperature. However, the reversible capacity is small and the slope of the PCT curves is very big.

The invention is also based on the discovery that when a thermal treatment is applied to this mechanically alloyed powder, substantial improvement in properties is achieved. Indeed, the hydrogen storage properties of the mechanically alloyed Mg-Ca-Ni are substantially improved when the powder is annealed at temperatures higher than 800°C typically at 1000°C or slightly higher, for short period of time, preferably 0.5h-1 h. Annealing at temperatures lower than 800°C does not improve the hydrogen storage properties very much.

It has also been discovered that further improvements are achieved by adding other elements (M) to the basic elements used for preparing the alloys. Such an addition raises the plateau pressure and improves other hydrogen storage properties such as plateau slope and the long-term stability while keeping predominantly the ABs-type structure.

Example III- 1 : Compound according to the prior art made by the method according to the invention.

Mmo.₄Cao.6Ni5 (x = 0, y = 0, b = 1 , z = 0) was synthesized in a SPEX high energy ball mill under an argon protective atmosphere. A Mm powder ( >99%, + l OOmesh), Ca granules ( > 99.5, ~ 2mm in size) and a Ni powder ( < 99.9%, -325mesh) were used as starting materials.

Isothermal annealing was performed in a tubular furnace under argon. The mechanically alloyed powder was sealed in a stainless steel crucible before annealing. The powder was heated to 1000°C at a heating rate of 30°C/min, held at 1000°C for 1 hour and then cooled down to room temperature in the furnace.

Hydrogen absorption/desorption properties were measured by using an automatic Sievert's type apparatus. The annealed powder normally needs mild activation treatment, such as a heating treatment to 200 °C under vacuum followed by a cooling. EDX analysis shows that the Fe content in the end product is less than 0.3 at.%. The composition of the end product was close to the nominal composition. The activated Mmo.₄Cao.6 Nis alloy exhibited a relatively flat plateau and a high capacity.

Example 111-2: Compound according to the invention made by the method according to the invention

(Mmo.333Cao.sMgo.i6₇)i.2Ni5 (M = Mm, x = 0.067, y = 0.1 , x + y = 0.167, b = 1.2 and b/5 = 0.24, z = 0) was synthesized by mechanical alloying of elemental powder blends. The alloy was annealed in the same manner as in Example III-1. This alloy had a hydrogen storage capacity of 1.45wt.%. The plateau pressure and reversible hydrogen storage capacity were significantly improved in comparison to those of the prior art alloy Cao.6 Mmo^Nis.

Example Hl-3: Compound according to the invention made by the method according to the invention: (Mmo.333Cao.4i7Mgo.2s)ι.₂Nis (M = Mm, x = 0.067, y = 0.183, x + y = 0.25, b = 1 .2 and b/5 =0.24, z = 0) was synthesized by mechanical alloying of elemental powder blends. The alloy was annealed in the same manner as in Example III-1 . This alloy had a hydrogen storage capacity of 1 .4wt.%. The plateau pressure was flat and even higher than the one of Example III-2.

Example HI-4 and subsequent

Other examples similar to those of Examples 111-2 and 3 were carried out. The results of these other examples are reported in the accompanying drawings (see Figs. 111-1 to 1 1 ).

REFERENCES

(A) "Hydriding Behavior of Ca-Mg-Ni-B" by H. Oesterreicher, K. Ensslen, A. Kerlin and E. Bucher, Mat. Res. Bull., 1 5 (1 980) 275

(B) "Structural and Hydriding Properties of (CaxMgι-x)NΪ2 Laves phases alloys" by N. Terashita, K. Kobayashi, T. Sasai, E. Akiba, J. of Alloys and Compounds 327 (2001 ) 275

(C) "Structural investigation and hydrogen capacity of CaMg2Ni9 : a new phase in the AB2C9 system isostructural with LaMg2Ni9" by K.

Kadir, N. Kuriyama, T. Sakai, I. Uehara, L. Ericksson, J. of Alloys and Compounds 284 (1 999) 145

(D) "Synthesis and structure determination of a series of hydrogen storage alloys; R g2Nig (R = La, Ce, Pr, Nd, Sm, and Gd) built from MgNi2 Laves-type layers alternating with ABs layers", by K. Kadir,

T. Sakai, I. Uehara, J. of Alloys and Compounds 257 (1 997) 1 1 5

(E) International Centre for Diffraction Data, JCPDS File # 1 9-0243 on

(F) "Structural investigation and hydrogen storage capacity of LaMg∑Niθ and (Lao.65Cao.35)(Mgι.32Cao..6s)Ni9 of the AB2C9 type structure" by

K. Kadir, T. Sakai and I. Uehara, J. of Alloys and Compounds 302 (2000) 1 12

(G) "Hydriding properties of LaNi3 and CaNi3 and their substitutes with PuNi3-type structure" by J. Chen, H.T. Takeshita, H. Tanaka, N. Kuriyama, T. Sakai, I. Uehara, M. Haruta, J. of Alloys and

Compounds 302 (2000) 304

(H) "Hydrogen storage properties of new ternary system alloys:

La2MgNig, LasMg2Ni23, LasMgNi " by T. Kohno, H. Yoshida, F.

Kawashima, T. Sakai, M. Yamamoto and M. Kanda, J. of Alloys and Compounds 31 1 (2000) L5

(I) "Hydrogen-Absorbing Alloy" US Patent No. 6, 1 30,006 (2000) (J) Japanese Patent JP 1 1 -217643 (1999)

(K) J.O. Jensen, N.J. Bjerrum, J. of Alloys and Compounds, 293 (1999)

185 (L) G. Liang, J. Huot and R. Schulz, J. of Alloys and Compounds 321 (2001 ) 146

(1 ) K.H. Buschow, in Handbook on the Physics and Chemistry of Rare

Earths, vol.6, edited by K.A. Gschneider, Jr. and L. Eyring, Elsevier

Science Publishers, B.V. (1984) chap 47 (2) T. Sakai, in Handbook on the Physics and Chemistry of Rare Earths, vol. 21 , edited by K.A. Gschneider, Jr. and L. Eyring, Elsevier

Science Publishers, B.V. (1995) chap 142 (3) G. D. Sandrock and G. Thomas, IEA technical report, compilation of

IEA/DOE/SNL hydride database, (1997) (4) P.D. Goodell, G. D. Sandrock and E.L. Huston, Pept. SAND 79-

7095, Sandia N.L. Albuquerque, NM 87185 (1980) pp 1 -176

(5) G.D. Sandrock, proc. 12^th intersociety energy conversion engineering conference, Am. Nuclear Soc, 1 (1977) 951

(6) G.D. Sandrock, in A.F. Andersen, A.J. Mealand (eds.) Proc. Int. Symp. on Hydrides for Energy Storage 9, Pergamon, Oxford (1979),

1 625

(7) G.D. Sandrock, J.J. Murray, M.L. Post and J.B. Taylor, Mat. Res. Bull., 17 (1982) 887

(8) P.D. Goodell, J. Less-Common Met., 99 (1984) 1

Claims

1 ) An alloy of the general formula:

( Cao Mgd Mβ )b ( Niι-zTz )_a

where:

T is at least one element selected from the group consisting of Al, Zn,

Cu, Fe, Co, Mn, Cr, Mo, W, Si, Ga, Ge, In, Sn, Ag, C and B; a is an integer equal to 2 or 5; z is a number ranging from 0 to 0.5, and when a is equal to 2, then b, c, d, e are numbers selected so that:

0.9 <b<1 .1 c = 0.4-x, d = 0.6-y, and e = x + y, with -0.5<x < 0.4

-0.4 < y<0.5 and x + y> 0; when a is equal to 5, then b, c, d, e are numbers selected so that either 1 .75 <b<2.25 c = 0.4-x, d = 0.6-y, and e = x + y, with

-0.4<x<0.2, -0.2<y<0.4, and x + y>0 or

0.75≤b≤1.31, c = 0.6-y, d = x + y, and e = 0.4-x, with -0.6 <x< 0,4, -0.4<y<0.6, and 0<x + y<0.5.

2) An alloy as claimed in claim 1 , wherein said alloy is of the formula:

(Cao. -xMgo.6-yMx + y)b(Nil-zTz)₂

where M and T are defined as in claim 1 and 0.9<b<1.1,

-0.5<x<0.4,

-0.4<y<0.5, x + y>0, and

O≤z≤O.5.

3) An alloy as claimed in claim 2, wherein:

-0.1<x<0.25 and

-0.1<y<0.15.

4) An alloy as claimed in claim 1, wherein said alloy is of the formula:

(Cao.₄-xMgo.6-yMx+y)b(Niι-zTz)s

where M and T are defined as claimed in claim 1 and 1.75<b<2.25, -0.4<x<0.2, -0.2<y<0.4, x + y>0, and O≤z≤O.5.

5) An alloy as claimed in claim 4, wherein:

-0.1<x<0.1, and -0.1<y<0.2.

6) An alloy as claimed in claim 4 or 5, wherein Mg and Ca are present in a Mg/Ca ratio ranging between 0.5 and 2.

7) An alloy as claimed in claim 6, wherein the Mg/Ca ratio is ranging between 1.5 and 1.75.

8) An alloy as claimed in claim 1, wherein said alloy is of the formula:

(Caθ.6-yMgx + yMθ.₄-x)b(Nil-zTz)5

where M and T are defined as in claim 1 and

0.75<b<1.31,

-0.6<x<0.4,

-0.4<y<0.6, o<x + y<0.5, and

O≤z≤O.5.

9) An alloy as claimed in claim 8, w herein: -0.2<x<0.2, -0.2<y<0.2, and

0.1 <x + y<0.4.

10) An alloy as claimed in any one of claims 1 to 9, wherein said alloy is a single phase alloy.

1 1 ) An alloy as claimed in any one of claims 1 to 9, wherein said alloy is a multiphase alloy.

12) A method for preparing an alloy as claimed in any one of claims 1 to 1 1 , comprising the step of:

- milling a mixture of elemental powders and/or pre-alloyed combination of elemental powders of Ca, Mg, M, Ni and T in relative amounts corresponding to those found in the requested alloy, and - annealing and/or sintering the so-milled mixture of powders at a temperature higher than 800°C to obtain the requested alloy.

13) Use of an alloy as claimed in any one of claims 1 to 1 1 or prepared as claimed in claim 12 for reversibly absorbing hydrogen from a gas phase.