Mg-based alloy for hydrogen storage

Abstract

A range of alloys of Mg and at least one of Cu, Si, Ni and Na alloys that is particularly suitable for hydrogen storage applications. The alloys of the invention are formed into binary and ternary systems. The alloys are essentially hypoeutectic with respect to their Cu and Ni contents, where one or both of these elements are present, but range from hypoeutectic through to hypereutectic with respect to their Si content when that element is also present. The terms hypoeutectic and hypereutectic do not apply to Na if it is added to the alloy. The alloy compositions disclosed provide high performance alloys with regard to their hydrogen storage and kinetic characteristics. They are also able to be formed using conventional casting techniques which are far cheaper and more amenable to commercial use than the alternative ball-milling and rapid solidification techniques which are much more expensive and complex. Each of the individual binary Mg-E systems, where E=Cu, Ni or Si, forms a eutectic comprising of Mg metal and a corresponding Mg.sub.xE.sub.y intermetallic phase.

Claims

1. A Mg—Cu alloy that is able to be formed by casting methods and which is suitable for hydrogen storage applications and which consists of: i. an amount of Cu such that the Mg—Cu alloy is hypoeutectic with respect to Cu content, that is greater than zero and less than 32 mass % Cu; and an amount of Ni such that the Mg—Cu alloy is hypoeutectic with respect to Ni content, that is between 0.1 mass % and 9 mass % Ni; ii. an amount of Na that is greater than zero and up to 20,000 ppm by mass; iii. incidental impurities totaling less than 0.5 mass %; and iv. a balance of the Mg—Cu alloy mass being Mg, wherein the Mg—Cu alloy has a primary crystallized Mg phase and a subsequently crystallized mixture of Mg—Cu based eutectic and Mg—Ni based eutectic and/or Mg—Cu—Ni based eutectic.

2. The Mg—Cu alloy of claim 1, wherein the amount of Na is between 200 and 4000 ppm by mass.

3. The Mg—Cu alloy of claim 1, wherein the amount of Na is between 800 and 2000 ppm by mass.

4. The Mg—Cu alloy of claim 1, wherein the amount of Ni is between 0.1 mass % and 5.5 mass %.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a Mg—Cu binary phase diagram;

(2) FIG. 2 is a Mg—Ni binary phase diagram;

(3) FIG. 3 is a Mg—Si binary phase diagram;

(4) FIG. 4 is a graph of hydrogen absorption curves at 4.sup.th test cycle for some of the alloys described in examples 2, 6 and 7 (see Table 3 also);

(5) FIG. 5 is a graph of hydrogen desorption curves at 4.sup.th test cycle for some of the alloys described in examples 2, 6 and 7 (see Table 3 also); and,

(6) FIG. 6 is a micrograph (SEM) showing the microstructure of a Mg-5.3% Ni-0.34% Si-0.093% Na alloy showing the presence of two distinct eutectic structures.

DETAILED DESCRIPTION OF THE EMBODIMENTS

(7) The alloy compositions disclosed provide high performance alloys with regard to their hydrogen storage and kinetic characteristics. They are also able to be formed using conventional casting techniques which are far cheaper and more amenable to commercial use than the alternative ball-milling and rapid solidification techniques which are much more expensive and complex.

(8) Each of the individual binary Mg-E systems, where E=Cu, Ni or Si, forms a eutectic comprising of Mg metal and a corresponding Mg.sub.xE.sub.y intermetallic phase (FIGS. 1-3). In addition to the ability of Mg metal to absorb hydrogen to form the hydride MgH.sub.2, each of the Mg.sub.xE.sub.y intermetallic phases is also able to absorb hydrogen to various degrees to form corresponding hydrides of the form Mg.sub.aE.sub.bH.sub.c, or similar.

(9) The alloy structures and compositions described above (and illustrated in Table 2) are capable of providing alloy hydrogen storage characteristics that are comparable to or better than those obtained using ball-milled Mg-alloy material. In particular, in terms of specific test values such as effective peak hydrogen (EPH), expressed in mass % H.sub.2. This value is measured after repeated absorption and desorption cycles, typically four cycles, carried out at approximately 350° C./10-15 bar and 350° C./1-2 bar, respectively.

(10) (Note: other important test values include the initial activation time, that is, the delay or incubation period before hydrogen absorption starts to occur during the first test cycle, and the saturation hydrogen level achieved after the initial 20 hour period.) These excellent hydrogen storage characteristics are achieved without the use of expensive and exotic elemental additions such as Pd, without the need for very high addition levels of Ni and Cu into the hypereutectic range, and without the need for complex and expensive alloy forming methods. Rather, each of the claimed alloys compositions is capable of being made using conventional casting techniques.

(11) The alloy compositions described and claimed are also capable of providing hydrogen absorption and desorption rates that differ from those of Mg—Ni—Na alloy, thus allowing the compositions of the alloys to be tailored to performance levels suitable for specific applications, and which are superior on a weight-for-weight basis, or a cost-for-cost basis cost, or to some other similar customer-based or production-based comparative parameter.

(12) Certain Mg alloys that contain eutectic forming elements, such as Ni, have far superior hydrogen storage characteristics compared to pure Mg metal. It has consequently been found that it is the presence of the eutectic intermetallic phases, for example Mg.sub.2Ni, that provide this benefit by: acting as catalysts for hydrogen storage in Mg, providing alternative hydriding phases, and/or providing easy physical pathways for the transport of hydrogen atoms.

(13) Given this understanding it is not surprising that one skilled in the art might predict that any of the eutectic forming elements (that is, Al, Ag, Au, Ba, Bi, Ca, Ce, Cu, Eu, Ga, Gd, Ge, Hg, La, Ni, Pb, Sb, Si, Sm, Sn, Sr, Th, Tl, Y, Yb and Zn) or combinations thereof should substantially improve the hydrogen storage characteristics of Mg.

(14) However, this is not the case as is demonstrated by the example of a Mg—Al—Ca alloy given below. It is noted that Al and Ca are both eutectic-forming with Mg, and form these eutectics at 33 mass % and 16.2 mass % of the element, respectively. These values should be compared with the eutectic value of 23.5 mass % for Ni, which is midway between these two.

(15) Note that in all the examples presented below, alloy compositions and EPH values are expressed in mass % values, unless otherwise stated. The data described in the examples is also tabulated in Table 3 for ease of comparison.

Example 1

(16) Mg-5 mass % Al-3 mass % Ca alloy (without Na and with a nominal addition of 2000 parts per million by mass [that is, 2000 ppm or 0.2 mass %] of Na) was melted and cast into a metallic mould and then tested for hydrogen storage properties. This alloy contains a comparable total mass % of eutectic elements (in this case, 8 mass %) as a commercial Mg—Ni alloy, and based on the eutectic compositions should also form a similar amount of eutectic and intermetallic compounds as the commercial Mg—Ni alloy.

(17) The alloy without Na addition performed extremely poorly with an activation time of 15 hours and a cyclic EPH value of 1.1 mass % H.sub.2. This was improved with the nominal addition of 2000 ppm Na yielding a shortened activation time of 8 hours and an increased cyclic EPH value of 3.7 mass %. Although this demonstrates a clear positive benefit for a Na addition to an otherwise poor alloy, the improved values still fall well short of a high performance alloy with a zero activation time and an EPH value of approximately 6.5 mass % H.sub.2.

(18) It is evident from this example that it is not simply a matter of arbitrarily picking a eutectic element, or combinations of eutectic elements, and then randomly choosing an addition amount(s) to obtain improved hydrogen storage characteristics in an Mg-based alloy. Rather, a substantial degree of inventiveness is required in order to identify and optimise superior alloy compositions. The inventiveness flows from determining and understanding the features that individual elements and combinations of particular elements contribute to hydrogen storage performance and other process-related and application-related parameters.

(19) This patent application discloses an improved set of Mg-based hydrogen storage alloys that are carefully prepared using selected amounts of subsets of the aforementioned eutectic forming elements, sometimes in conjunction with Na added as a performance-enhancing element. These alloys are described and illustrated in examples in more detail below.

(20) Cu Added to Mg

(21) As noted in an earlier section, the main effect of Cu in Mg is to improve the kinetics of hydrogen activation, hydrogen absorption rate and hydrogen desorption rate over that of pure Mg. It is believed that Cu does this by acting as a catalyst that facilitates the breakdown of hydrogen gas into hydrogen atoms and facilitates the transport of those atoms into the Mg where they react and form as solid hydrides, for example, MgH.sub.2. In addition, Cu provides a layered eutectic structure of Mg and Mg.sub.2Cu intermetallic that surrounds and interpenetrates the primary Mg grains and which provides easy transport paths for hydrogen atoms to reach their reaction sites, which may be with primary Mg grains or with the Mg.sub.2Cu intermetallic constituent in the eutectic.

(22) We have observed that Cu is similar in effect to Ni (a well-known addition to Mg for hydrogen storage purposes) but that its performance is asymmetric with respect to Ni. In particular, Cu provides an improved overall rate of hydrogen absorption after the first hour compared to a similar mass % addition of Ni, even though Ni displays a more accelerated initial stage of absorption (FIG. 4). However, Cu shows a slower rate of hydrogen desorption compared to a similar mass % addition of Ni, but it does this with no loss of EPH storage capacity. Since Cu is a lower cost element than Ni, it is clear that Cu may be beneficially used rather than Ni for applications in which the desorption rate is less critical than the absorption rate.

(23) We have further found that the addition of Na to Mg—Cu alloys (Mg-5.2% Cu, Mg-14% Cu, Mg-20% Cu) enhances the hydrogen storage characteristics, in particular, providing consistent improvements in effective peak hydrogen levels (to values as high as 6.3%). It also improves cycling kinetics, and in general reduces the activation time for hydrogen absorption to commence. It is specifically found that Na substantially improves hydrogen desorption rates during cycling (thus ameliorating the slower desorption rate noted above for binary Mg—Cu alloys), compared to Mg—Ni alloys in which Na additions tend to improve the hydrogen adsorption rate instead.

Example 2

(24) Mg-5.2% Cu alloy has been tested and found to have an EPH value of 4.3% under the previously noted conditions. The nominal addition of 2000 ppm of Na to the alloy significantly raises the EPH of the alloy to 6.4%, a highly acceptable and commercially-useful level.

(25) Si Added to Mg

(26) Although Cu, Ni and Si are each eutectic-forming elements with Mg, silicon differs from Cu and Ni in that it is both lightweight and cheap, and also exhibits its eutectic point at a very low addition level, 1.3% Si. This means that a high proportion of eutectic structure can be achieved with relatively small additions of silicon. Such a microstructure has large amounts of interfacial area between the phases through which hydrogen transport can easily take place.

(27) In addition, by exceeding 1.3% Si, the formation of primary Mg.sub.2Si intermetallic phase particles can be facilitated should this be considered a desirable structural feature to further increase options for hydrogen transport pathways. This latter device is not possible in Mg—Cu or Mg—Ni alloys without adding significantly larger amounts (greater than 32% and greater than 23.5%, respectively) of these heavy elements. We have found that the addition of even relatively small amounts of Si to the alloy, for example, approximately 0.2%, also improve the chip formation process achieved during mechanical processing of the alloy, which is a significant processing benefit.

(28) As a sole addition, Si is not as effective in as Cu or Ni in shortening activation time or in increasing absorption or desorption rates. However, the addition of Na to Mg—Si binary alloy has significant positive impacts on certain performance aspects, as shown in the following examples.

Example 3

(29) In a hypoeutectic Mg-0.5% Si alloy, a nominal Na addition of 2000 ppm did not change activation time (which remained at 5 hrs), but did improve the hydrogen cycling kinetics resulting in an improved multiple cycle EPH value of 5.2% H.sub.2 (with Na) compared to 4.0 (without Na). This improvement was achieved despite the Na addition not providing any significant microstructural refinement to the eutectic. This is a surprising finding given that microstructural refinement has been generally accepted as the key mechanism behind improved performance in eutectic-based alloys. Clearly, Na is able to provide benefits for hydrogen storage via more than this one mechanism.

Example 4

(30) There are also measurable benefits of adding Na to Mg alloys with hypereutectic Si levels, i.e. above 1.34% Si. For example, for a Mg-1.6% Si alloy, although the activation times were still long with a nominal 2000 ppm Na addition (18 hrs compared to >20 hrs without Na) the multiple cycle EPH value was dramatically increased from 0 to 5.1% H.sub.2 with the addition of Na.

Example 5

(31) Furthermore, we have found that increasing the Si content of the alloy to 2.1% resulted in a decreased activation time of 6 hours and an increased cyclic EPH of 2.6% even without addition of Na. When Na is added at nominal 2000 ppm level to the Mg-2.1% Si alloy, the activation time dropped to 3 hours and the cyclic EPH rose to 5.5%.

(32) Cu and Ni Added to Mg

(33) We have already compared Mg—Cu alloy with the Mg—Ni alloy and shown that Cu and Ni exhibit differing responses to their hydrogen absorption and desorption kinetics. This surprising observation suggested to us the beneficial use of Mg—Cu as a replacement for Mg—Ni in applications where desorption rates were not critical but where cost was the more important driver.

(34) From this insight, we have hypothesised, investigated and determined that Ni can be added to a Mg—Cu alloy (or vice versa) such that at particular mass % combinations of the elements not only are the counter-effects offset, but improved hydrogen storage capacity during cycling may be achieved. Our claimed preferred composition ranges reflect these findings. In fact, the best hydrogen capacity is achieved at relatively low overall alloy addition levels (for example, Mg-4% Cu-1% Ni, compared with existing higher levels of Ni used commercially). We can therefore tailor Mg—Cu—Ni alloy compositions to provide improved properties at potentially reduced alloy weight and alloy cost.

(35) We have further found that Na improves the overall performance of Mg—Cu—Ni alloys, presumably among other things, by Na acting to improve the reduced hydrogen desorption rates caused by Cu (thus ameliorating the rate loss noted above) compared with Mg—Ni alloys in which Na additions tend to improve the hydrogen adsorption rate. We have found that the benefit of Na is conferred especially at particular mass % combinations of Cu and Ni additions.

Example 6

(36) It has been determined that a Mg-5.2% Cu alloy (without Na) has a multiple cycle EPH value of 4.3% H.sub.2. When some of the Cu is substituted by Ni, as in the Mg-4% Cu-1% Ni alloy (without Na), the EPH value is improved to 5.1%. A further nominal addition of 2000 ppm Na to the alloys raises the EPH of the nominal Mg-4% Cu-1% Ni alloy to 6.5%, which is higher than the EPH values achieved for either the Mg-5.2% Cu alloy with Na addition or a Mg-4.7% Ni alloy with Na, which were measured at 6.4% and 6.3% respectively.

Example 7

(37) Surprisingly, a nominal addition of 2000 ppm Na to the nominal Mg-4% Cu-1% Ni alloy (approximately 5% total alloy content) yields a greater improvement to EPH value (5.1 to 6.5%) than the same Na addition does to a nominal Mg-3.5% Cu-3.5% Ni alloy (approximately 7% total alloy content) where EPH is improved from 5.1 to 6.0%. Not only does this demonstrate that a leaner composition alloy can have as good a hydrogen capacity as a richer composition alloy, but that Na clearly has other positive effects on EPH that are independent of any beneficial effects that it has on the Mg—Cu and Mg—Ni based eutectics present in the structure.

(38) TABLE-US-00003 TABLE 3 Composition and hydrogen performance data for various Cu—Ni Compositions Cu—Ni Related Compositions with no Na Cu—Ni Related Compositions with Na Added Cu Ni Si Act'vtion EPH4 Cu Ni Si Act'vtion EPH4 3.6 0.9 4 5.1 3.9 1 0 6.5 5.2 0 7 4.3 5.2 0 3 6.4 0 4.7 0 6.3 2.6 2.6 5 4.6 2.4 2.5 4 6.2 2.45 0.98 0 6.2 10 0 5 3.4 14 0 4 6.0 3.5 3.7 5 5.1 3.2 3.3 2 6.0 9 5 2 5.5 9 5 5 5.9 6.8 6.5 1 5.1 6.8 6.5 1 5.8 1 2.5 4 5.8 2.62 0.99 0.36 10 2.1 2.56 0.98 0.36 3 5.8 3.5 0 12 2.7 3.5 0 4 5.7 5 9 6 4.9 5 9 2 5.7 0 14 2 5.5 1 1 0 5.3 11 0 2 5.3 24 0 2 4.7 0 14 6 5.1 0 2.58 2 4.7 0 4.7 6 5.1 0 1.3 3.5 3.5
Cu and Si Added to Mg

(39) The addition of Cu and Si together to Mg does not appear to provide any substantial improvement to hydrogen storage capacity compared to that achieved in the simple binary Mg—Cu or Mg—Si alloys. However, the combination of adding Cu and Si together in Mg improves the responsiveness of the alloy to a Na addition and achieves greater increase in the EPH values compared to the effect of Na added to either of the simple binary alloys, even though it may not achieve the highest EPH value in the ternary alloy. This is a surprising result that could not be predicted a priori.

(40) Nevertheless, when the addition of Si to Mg—Cu alloys (particularly where the amount of Si added is a direct substitute for some of the Cu content) results in hydrogen storage characteristics that are similar to the binary Mg—Cu alloy then a commercially attractive alloy has been formulated. This is because silicon is much cheaper and is much less dense than Cu, and is also cheaper than Mg, resulting in a potentially cost-effective and weight-effective alloy for less demanding hydrogen storage applications.

(41) This is believed to occur because Si, even at relatively small addition levels, dramatically increases the amount of eutectic intermetallic particles present in the microstructure thus increasing the interfacial area available for transport of hydrogen atoms. It is believed that Na tends to segregate to these interfaces and that its presence facilitates the transport of hydrogen to the key reaction and absorption sites provided by the primary Mg phase and the Mg—Cu eutectic, rather than the less responsive Mg—Si eutectic.

Example 8

(42) As noted above, it has been determined that a Mg-5.2% Cu alloy (without Na) has a multiple cycle EPH value of 4.3% H.sub.2. When some of the Cu is substituted by Si, as in the Mg-4% Cu-1% Si alloy (without Na), the EPH value is decreased to 2.6%. However, a nominal addition of 2000 ppm Na to the alloys raises the EPH of the nominal Mg-4% Cu-1% Si alloy to 6.2%, which is a substantially greater increase (+3.6%) in EPH value, compared to that achieved in Mg-5.2% Cu alloy, that only achieves a +2.1% increase to an EPH of 6.4%.

(43) Review of the data for Mg—Cu—Si alloys (Na-added) shows that for copper-rich alloys with reducing silicon contents, the EPH reduces significantly as the copper concentration drops, and this reduction is not arrested even with the addition of Ni. Surprisingly, the addition of silicon to form the alloy of this example (Mg-4% Cu-1% Si alloy with Na addition) reverses this trend to produce an alloy showing an EPH of 6.2 wt % H.sub.2. This may be due to the embrittling effect of the silicon or its ability to disproportionately expand the volume of eutectic phase. This alloy has attractive performance at a favourable cost, making it commercially attractive.

(44) TABLE-US-00004 TABLE 4 Composition and hydrogen performance data for various Cu—Si Compositions Cu—Si Related Compositions with no Na Cu—Si Related Compositions with Na Added Cu Si Ni Act'vtion EPH4 Cu Si Ni Act'vtion EPH4 5.2 0 7 4.3 5.2 0 3 6.4 3.6 0.75 11 2.6 3.6 0.7 3 6.2 10 0 5 3.4 14 0 4 6.0 4 3 5 5.9 2.62 0.36 0.99 10 2.1 2.56 0.36 0.98 3 5.8 2 0.2 5 5.8 0 4.5 7 5.8 3.5 0 12 2.7 3.5 0 4 5.7 2 2 5 5.7 0 0.12 3 4.0 0 0.28 4.5 5.5 0 2.1 6 2.6 0 2.06 3 5.5 0 0.52 5 4.0 0 0.52 5 5.2 1 1.2 2 5.2 0 1.6 >20 0.0 0 1.6 18 5.1 11 0 2 5.3 24 0 2 4.7
Ni and Si Added to Mg

(45) The addition of Ni and Si together to Mg differs from the combined effect of Cu and Si additions noted above. There appears to be little gain in adding Si at small amounts to alloys with Ni contents in the higher range, for example, about 5%; however, there are distinct gains when Si is added to alloys with lower Ni contents in the 1% range. We believe that this is due to fact that Mg—Ni eutectics, even in the absence of Na, are more potent than Mg—Cu eutectics in terms of their catalytic capability, therefore when the Ni content is high the addition of Si adds little to the alloy's hydrogen storage capacity. We further contend that when the amount of Mg—Ni eutectic is reduced as in a low Ni content alloy, a Si addition, even at low levels, is able to exert its very positive influence by forming large amounts of Mg—Si eutectic that open up pathways for hydrogen atom transport within the Mg grain structure (FIG. 6).

(46) It appears that the direct substitution of Si for Ni will be possible in Mg—Ni alloys without substantial loss of hydrogen storage performance, and also that there may opportunities to reduce the total Ni plus Si alloy content as well, while maintaining adequate hydrogen storage characteristics for certain applications. As above, Si is much cheaper and lighter than Ni, and is also cheaper than Mg; therefore both of the above options will lead to a cheaper and lighter alloy that suits particular hydrogen storage or transportation applications that would otherwise be unavailable to higher Ni content alloys. This is true even for Si additions that are in the hypereutectic range, that is, greater than 1.3%.

(47) As for the Mg—Cu—Si alloys described above, the addition of Na appears to exert greater beneficial effects in particular regions of the composition window for Mg—Ni—Si alloys. For example, the addition of Na appears to yield greater improvements to lower Ni content alloys than to higher Ni content alloys. This is believed to be because of the inherently greater catalytic effect of Ni than Si thus partially negating the benefit of Na addition.

Example 9

(48) The addition of approximately 0.8% Si (a hypoeutectic amount) to a Mg-1.3% Ni alloy containing a further nominal addition of 2000 ppm Na increases the effective peak hydrogen storage level after cycling from 3.5 to 5.3%, a useful though modest level. This should be compared with a cyclic EPH value of 5.9% for a Na-containing Mg-5.3% Ni-0.4% Si alloy.

Example 10

(49) We have found that a nominal Mg-1.6% Si-1% Ni alloy, that is containing a hypereutectic amount of Si, has an activation time of 6 hours and multiple cycle EPH value of only 1.6 wt %, a low level. However, the nominal addition of 2000 ppm Na to the alloy reduces activation time to 3 hours and raises the multiple cycle EPH value to 5.8 wt %, a commercially attractive level for certain applications. By comparison, a simple binary hypereutectic Mg-1.6% Si alloy displays an EPH of 0% before Na addition, and an EPH of 5.1% after Na addition, but with very long activation time of greater than 20 hours and 18 hours, respectively.

(50) TABLE-US-00005 TABLE 5 Composition and hydrogen performance data for various Cu—Ni Compositions Ni—Si Related Compositions with no Na Ni—Si Related Compositions with Na Added Ni wt % Si wt % Cu wt % Act'vtion EPH4 Ni wt % Si wt % Cu wt % Act'vtion EPH4 4.7 0 6 5.1 4.7 0 0 6.3 6.4 0.2 5 5.2 5.3 0.34 3 5.9 1 1.6 6 1.6 1 1.5 3 5.8 0.99 0.36 2.62 10 2.1 0.98 0.36 2.56 3 5.8 0 4.5 7 5.8 5.2 0.15 3 5.7 14 0 6 5.1 14 0 2 5.5 0 0.12 3 4.0 0 0.28 4.5 5.5 0 2.1 6 2.6 0 2.06 3 5.5 1.1 0.84 8 5.3 1 0.2 12 2.6 1 0.2 2 5.3 0 0.52 5 4.0 0 0.52 5 5.2 0 1.6 >20 0.0 0 1.6 18 5.1 4 3 3 4.9 2.58 0 2 4.7 4 1 3 4.3 1.3 0 3.5 3.5
Cu, Ni and Si Added to Mg

(51) The effects of a combined addition of Cu, Ni and Si into Mg (a quaternary alloy) are complex and result in interesting and useful changes to activation kinetics. In addition, there are clear benefits of adding Na to the quaternary alloys with strong improvements in both activation kinetics and overall storage performance observed.

Example 11

(52) A nominal Mg-2.5% Cu-1% Ni-0.4% Si alloy without Na addition shows an activation time of 10 hours and a cyclic EPH value of 2.1% H.sub.2. When a nominal addition of 2000 ppm Na is made, the alloy shows an improved activation time of 3 hours and an improved EPH of 5.8%.

(53) If the same Na-containing quaternary alloy is compared with the Na-containing Mg-1.3% Ni-0.8% Si alloy from Example 9, it can be seen that there is an improvement in cyclic EPH value in the quaternary alloy compared to the ternary alloy, that is 5.8% compared to 5.3%.

(54) If the same Na-containing quaternary alloy is compared with a Na-containing Mg-2.5% Cu-1% Ni ternary alloy, there are improved activation kinetics despite an increased activation period (3 hours activation, and saturation hydrogen value at 20 hour of 5.4%) compared to the Si-free ternary alloy (0 hours activation, saturation hydrogen value of 5.0%). Note: this data is not shown in Table 3. As such, the Mg-2.5% Cu-1% Ni alloy with Na is attractive for its favourable EPH performance, but the addition of the 0.4% Si can make the alloy cheaper and improve the activation performance.

(55) As noted previously, Si is much cheaper and lighter than either Cu or Ni, and is also cheaper than Mg. Cu is also cheaper than Ni. Therefore any direct substitution of Si or Cu for Ni that is possible, or any reduction in total alloy content, will lead to a cheaper and lighter alloy.

(56) TABLE-US-00006 TABLE 6 Composition and hydrogen performance data for various Cu—Ni—Si Compositions Cu—Ni—Si Related Compositions with no Na Cu—Ni—Si Related Compositions with Na Added Cu Ni Si Act'vtion EPH4 Cu Ni Si Act'vtion EPH4 2.6 1 0.4 10 2.1 2.6 1 0.4 3 5.8 1 1 0.2 3 3.8 1 1 0.2 2 4.9 1 1 2 Not 3.4 1 1 2 2 5.0 measured

Further Examples

Example 12

(57) Review of the data for Mg—Ni—Si alloys (Na-added) shows that for alloys consisting of only Mg and Si with Na, the EPH values decrease with increasing Si. However, surprisingly, the increase is reversed at Mg-4.5% Si with Na addition; this alloy shows 5.8 wt % H.sub.2. The light weight of the silicon combined with an extensive network of primary Mg.sub.2Si may be combining to create this effect. As noted, Si is cheaper than Mg, so the cost of the alloy is actually decreased at this relatively high addition, making it a commercially attractive.

SUMMARY

(58) It will be understood that the invention disclosed and defined in this specification extends to all alternative combinations of two or more of the individual features mentioned or evident from the text or drawings. All of these different combinations constitute various alternative aspects of the invention.

(59) TABLE-US-00007 TABLE 3 Composition and hydrogen performance data for examples 1-11 above Nominal alloy composition (mass % of Nominal Activation Example elements; Na time Cyclic EPH No. balance Mg) (ppm) (hr) (mass %) 1 5 Al, 3 Ca 0 15 1.1 2000 8 3.7 2 5.2 Cu 0 7 4.3 2000 3 6.4 3 0.5 Si 0 5 4.0 2000 5 5.2 4 1.6 Si 0 >20 0 2000 18 5.1 5 2.1 Si 0 6 2.6 2000 3 5.5 6 4 Cu, 1 Ni 0 4 5.1 2000 0 6.5 7 3.5 Cu, 3.5 Ni 0 5 5.1 2000 2 6.0 8 4 Cu, 1 Si 0 11 2.6 2000 3 6.2 9 1.3 Ni, 0.8 Si 0 >20 3.5 2000 8 5.3 5.3 Ni, 0.4 Si 2000 3 5.9 10 1 Ni, 1.6 Si 0 6 1.6 2000 3 5.8 11 2.5 Cu, 1 Ni, 0 10 2.1 0.4 Si 2000 3 5.8 12 4.5 Si 0 Not Not Tested Tested 2000 7 5.8