CA2427733A1 - Magnesium based alloys for hydrogen storage - Google Patents

Description

lvIyEI:r ALLOS~S FOR HYI~R.OGE~RA~
F1~LD C)F T~ INVENTI~N
This inventiart relates to Mg-based alloys, and particulaarly hydrides of such Mg_ based alloys.
BACI~CicRCUND GF TIDE 1.NVENTIQN
Metal hydrides have been the t°ocus of recent intensive research activities on hydrogen storage. Significant progress has been made on sodium alanate, advanced BCC
alloys, Mg based alloys arid others [L. Schlapbach and A. Zuttel, Nature, 414 (2001) 353). Although suffering from a high thermadyxtamic stability, magnesiuul hydrids~ is still atAra.ctive as a potential hydrogen storage xttaterial because of its hagh storage capacity and low cost [P. Selvam, B. Viswauathan, C.S. Swaaxty attd V.
Srinivasan, ant.
J. Hydrogen Energy, 11 (198 169; N. Gexard and S. Cno, apt ccFlydrogert fn anlermetallic Compounds LIs>, ed. by L. Schlapbach, Springer-V~erlag, (1992), ppl7$-182]. The improvement of hydrogen sarpCion kinetics has been achieve$ by various treatments [IvLH. Mintx, S. Malkiely, Z. Gavara, Z. Hadari. :1. Inarg. Nucl.
Cheat., ~.0 (1978) 1951; J. Renner, H.J. C".rtablce, Z. Nletall., 63(1972) 289; J.J.
Really and R Y~.
Wiswall, Jr, Inorg. Chern., 7 (I9~68) 2254; E. Ivanov, I. Konstanchul~ A.
Stepauov and ~l. Baldyrev, ,F. Less-Common ,~I~letalr, 131 (19$'7)] especially 1>y intensive milling of MgHz with sotxae irax~sition metal additives. For example, the MgHz-5ai.%V
composite desorbs at 473K (tinder vacuum) and re-absorbs hydrogen rapidly even. at mom temperature [G. Liaug, J. Huot, S. »oily, A. Van lVeste and ~.. Schulz, 7.
Alloy Comp., 291 (1999) 29$; G. Lung, .I. gIuot, S. Boily, A. Vary Neste arid R. Schulz, 3.
Alloy Comp., 292 (1999) 247.x. Current activities are directed to low-cost synthesis using reactive milling [J. I3uot, M. L. Trernbly and R. Schttlz, J. Alloy Comp., ita press; J. L.
Bobet, S. I~esmoulius-Krawiec, E. Grigorova, F. Cansell and 13. Chevalier, J.
~Iloy Comp., 351 (2003) 217. and oxide catalysts [W. t3elerich, T. kalssen a~ad R.
Horrnann, J. ~llAys Comp., 322 (2001) LS-1:.91.
The high thermodynamic stability of MgHz remains the major obstacle for applications. Research conducted in the last few decades showed that the plateau.

tsac~surc of ~agnes'auvza hyc~~°idc does nit chnryc--ls~~- 1'a~iz~~-.~::~sitsr, '~,~ e~a,~~zra treatment, by adding c-ats~s;..o~ .~y -goz~.~gw j~°~cy.-~s~.tr,K~ m..~~
ew:".~
atlt~ys such as Mg2~Ti, Mg2C'u, Mgx.f~l.~ are Formed [I'. Selvaxn, H.
Viswallathan, C.S.
Swarxiy and V. Srizxw2~san, Int. .~ .Hydrogen Energy, 11 (1986) 1699 N.
G'rerard and S.
(Jtto, in <cHydrogeta in intermetallic Compounds II>r, ed. by 1.....
chlapbach, Springer-'Verlag, (1992), pp178-182; J.J. Reilly and R, ~. Wiswall, Jr, Inorg. Chem., 7 {1968) 2254; A. Zaluska, 1G. Zaluski, J. C~. Scrom-~lsen, J. ~111oy Comp., X89 (1999) 197; CJ.
Lung, J. Huot, S. Hoily, A. Van Neste and R. Schulx, J. Alloy Comp., 292 (1999) 247), Some reports claimed that the plateau pressure of MgbI2 cant be changed by forznittg Ice,' fly'n_a.~1?~~h.fl~F.!e~e,~~?g.~,;.~,~~'~ ~-.~1; Ni-ZJr,~FY ~achman and l7.aA..
April 19$1 (T.N. Veziroglu ed.) p557, Pergatnon Press, NY, 1982], Mg-FeTi(Mfn) [P.
lVlandal and O.N. Srivastava, ~: Alloy Comp., 20S {1994) 111.], and Mg-Zn-Y
[A.
Zaluska, L. zaluski, J. c~. Strorn-C~lsezt, J. Alloy, Comp 288 (1999} 217]
systems. The changes in plateau pressures were explained by the comple~c multiphase nature of the composites [.1~. Zalvtska, L. Zaluski, J. ~. S~om-~lsen, d. Alloy, Corctp X88 (1999) 217;
P. Mandal and ~.r1'. Srivastava, ,d Alloy Comp., 20~ (199A~) 111], however, The real physical reasons behind have not been understood or reported.
Magnesium forms solid solution and compounds swath #'ew elements under equilibrium states. Non-equilibxiutn processing xxtetltods, such as mechanical alloying, rapid quenching caz~ produce flew magnesium alloys atxd new structures, such as amorphous phases, extended solid solutions, and non-stoichiometrie iztt~rixletallic compounds which may alter the thermodynamic properties. However, naffs of these new alloys axtd structures is stable at elevated temperature e. ~ 3d30°C, which is needed for activation of most air exposed Mg-based allays. 'The mufti~component sixtgle phase (usually amorphous phase) transforms to the conventional equilibrium mufti-phase composites after activation, and the end result is: improved kinetics and no destabilization of the magnesium. hydride ~K.~. Hong axld K Sapxu, Int. .T.
plydrogen Energy, 12 (1987) x.11; R. L. ~c~ltz and IVLA.. Imam, J. Mater. Sez., 32 (1997) 2267; T.
Spassov and U. Koster, J. Alloy G'omp., 279 (1998) 279; K, Tanaka, Y. Kattda, ~I.
F~uuhashi, K. Saito, K. Kuroda axed I~. Sake, J Alloy C'omp., 293-295( 1999) 52I; S.

~rimo, K. Ikeda, ~lL. p'ujii, Y. Fujikawa, Y. I~itaxio and K. Yamamoto, Acro M,~~er., ~t5 (1997) 2271; K. Ikeda, S. C3aamo, E~.. Zuttel, ~.. Schlapbach, I3. Fujii, ,l Adloy C'omp., 280 (1998) 279.]. If the as-prepared alloy can be hydrogenated in situ without exposition to air (this was the case for MglPd sputtered film[I~. F~iguchi, H. Kajiolsa, K.
Toiyama, .
Fujii, S. C7rimo, Y. Kikuchi, .7. Alloy Comp., 293 (1999) A~84j), novel properties may be obtained.
BRTFF L~FSCFtIPTION DF I~RAWINCYS
fiig. 1 XRI7~ spectra of the Mg-Cd alloys alder uxecb.ataical alloying and hydsiding/delaydridixlg (HD). (a), as-milled pure Mg: (b) Mg-IOCd-V-C, after HIS; (c) Mg..2~Cd-~r-C, after HD; (d) as-milled Mg-l OCd> (a) as-milled Mg-~OCd lFig. 2 Lattice parameters ofthe T~C~' Mg(Cd) phase as a function ofCd content.
Fig. 3 'The ~-ray spec~a of the hydrogenated Mg(Cd) alloys Fig. 4 Hydrogexl absorption curves of the Mg(Cd)-V-C Composites at 573K under a pressure of 1.QMPa.
Fig. 5 Hydrogen desorption, curves of the Mg(Cd)-V-C composites at 573K
against a pressure of O.UI~MPa.
Fig. 5 PCT curves of Mg(Cd)-V-C cottrposites at 573K.
F'ig. 7 L7esor~tian plateau pressure versus unit cell volume of Mg(Cd) alloys.
Fig. 8 PCT curves pf~tg-1~Cd-V-C coxuposite at various temperatures.
Fig. ~ Variations of plateau pressures (at the midpoint of plateaus) with inverse temperatures f~r Mg'f~z and Mg(Cd)-V-C alloys.
Fig. 10 The unit cell vtelume of various Mg solid solutions as a function of solubility fiig. 11 The grain size of various Mg alloys as a function of anxyealizy eemperat~ure.
P'ig. 12 The microstrain of various Mg alloys as a ~xction of axtnealing temperature.
Fig. 13 The lattice parameters of various Mg alloys as a function of annealing temperature.

Fig. 14 Hydrogen absoxptic~xt curves ~f various Mg alloys at 623 under 1.35MPa.
Fig. 15 Hydrogen desorption kinetics of various Mg alloys at 623K. under 4.ISMPa.
Fig. 16 PCT curves (the desorprion part) of vatinus Mg alloys at 623I4.
hig. 17 Van't Hoff plot of various Mg alloys.
Fig. 18 Hydrogen absorption/desorptioxa hysteresis of various Mg alloys at vazious temperatures.
DETA,1LED DESCRIPTI(3N
The present invention provides a Mg-Cd solid solutiaat, wherein Cd is dissolved in an HCl' Mg phase. A. solid solution of Cd in Mg is indicated lay the absence of a diffraction peak characteristic of Cd in a X-ray diffraction spectrum of the Mg-Cd alloy.
In one embodiment, the Mg-Cd solid solution is a supersaturated solution of Cd in an I~CP Ivlg phase.
Itt one embodiment, the Mg-Cd alloy is formed by mechanical allaying Mg wrath Cd. Preferably, the Mg-Cd alloy is forzrxed by way Qf mechanical grinding by high energy ball milling of a mixture of Mg and Cd.. I'xeferably, the Cd content in the Mg-Cd solid solution is less than about ZO at %, as additional Cd eauteatt derogates from the hydrogen storage capacity of the alloy.
The Mg-Cd allay of the preserve invention is partiCU,larly useful for hydrogen storage applications, In this respect, to improve the Itiraetics of hydrogen absotptionldesorption by the Mg Cd alloy, vanadium is added to the Mg-Cd allt~y to form a Mg-Cd-V composite after ball milling of the mixture, whereiax the Cd is dissolved in an HCP Mg phase, axtci the V is not dissolved in the HCP Mg phase. ~ this respect, when present in the cotuposite, V fia~actions as a catalyst for hydrogext absorptionldasorption by the Mg-Cd alloy. Yn Qne e~badimerat, the Mg-Cd-V coanposite is a ~ncfcoxtxposite.
preferably, graphite is added to the Mg-Cd-V composite and futxctions as an anti-stickixig agent and an activation agevt, sa that a Mg,Cd-V-C composite forms upon high energy ball milling of the precursor mixture. In the case of the Mg-Cd-V-C composite, the Cd is dissolved in an HCP Mg phase, and neither the V nor the C is dissolved in.
the HCP Mg LZ~~c~~st~a~t~ ~s.t~.W.s~uaa~~~sts ~a~~!.~i~~3~i+~a~ua~ v~ ~~i-.c.~~r~'r v''d'~°ar~.'wi apy ov long as sufficient amounts ofV oz C were added to the mixture to permit detection by the diffxactoxueter.
As mentioned above, the Mg-Cd alloy of the present invention is particularly useful as a hydrogen storage material. In this respect, the present izxvention also provides a hydrided Mg-Cd alloy. The hydrided Mg-~d alloy can be a component of any of the above-mentioned composites. Ita this respect, the hydride forms by reacting the above-mentioued Mg-~d alloy or arty of the above-mentioned composites with hydrogen gas.
The present invention also provides a Mg-In solid solution wherein Ia is dissolved in an PTCP Mg phase. A solid solution of gn in Mg is indicated by the absence of a diffraction peak characteristic of In in a .~-ray difhraction spectrum of the Mg-In alloy. In one embodiment, the Mg-In solid solution is a supersaturated solution of In in an I~C~' Mg phase.
Ir3. one etnbodimeut, the Mg-In alloy is formed, by mechanical alloying Mg with In. Preferably, the Mg-Zn alloy is formed by way of mechanical grinding by high energy ball milling of a mixture ofMg and ~.
The Mg-rn alloy of the present invention is particularly useful for hydrogen storage applications. In this respect, to improve the kinetics of hydrogen absorptionldesorption by the Mg-ha alloy, vanadium is added to thQ Mg-.lit xxiixture to form art Mg-In-V composite a#fier ball milling of the zui~ctuxe, wherein the In is dissolved in an HCP Mg phase, and the V is not dissokved in tlae I-ICP Mg phase. In this respect, when present in the composite, V functions as a catalyst for hydrogen absorptionldesorption by the Mg-In alloy. In one erttbadixrtent, the Mg-In-V
coxrtposite is a nanocomposite. Preferabky, graphite is added to the Mg-In V composite and functions as an anti-sticking agent and an activation agent, so that a Mg-Izl-V-C
composite forms upon high energy ball mikling of the precursor mixture. In the case of the Mg-In-V-C
composite, the In is dissolved in au flGl' Mg phase, arid neither the V oar the C is dissolved in the HCI' Mg phase. The fact that the V and C are not dissolved an the I-ICP
Mg phase is indicated by the presence of separate diffraction peaks in a X-xay diffra~ctiot~

spectrum of the allay, so long as sufficient amounts of V or C were added to the mixture to permit detection by the di~fractometer.
As mentioned above, the lVlg-In alloy of the present invexltiol~ is particularly useful as a hydrogen starage material. xn this respect, the present invention also provides $ hydrided lVlg-In alloy. The hydrided Mg-Yn alloy can be a coanpo~aent of any of the above-mentioned camposites. 3tt this respect, the hydride forms by reacting the abave-described Mg-Itt alloy or arty of the above-xnentio~ed composites with hydrogen gas.
Ex es Mg based alloys were prepared by mechanical alloying of l~Ig (99.9~o pure) with other elements (99,9°I° pure) in a Spex 8000 ball mill under the lsrotectian of argozt. ,A
hardened steel . crucible anti three steel balls of I2.7 ttxm ire diameter were used for xrtihing. The ball to powder weight retie was 10:1. Mg was milled with the subject element for Z0 hours to form 11~g solid solution. Vanadium catalyst (2.Sat.%) was then added, and tnilled with the formed IVIg solid solution for additional 20 ltouxs in order to distribute the catalyst_ Crraphite (2wt_%) was then added as anti-sticlang agent and for activatian purposes, and the mixture was milled far 0,5 bents. The alloy compositions indicated axe ir. atomic percent. The V and ~ content are oitratted from the chemical faruxula far siznpliiacation.
X-ray diffraction analysis of the mechanically allayed powders was perrforme~, on a Siemens 17-S00 ~-ray diffractometer with CuKa radiation. 2'he lattice parametez$ were determined fraxra the x-ray peak positions by least square method [F~LP. Klug and L.
Alexander, X ray d~action procedures f'or polycrystalline and amorphous materials, 2"~ eon., John Wiley & Sans, 1VY, 1974j. The hydrogen storage properties of the mech.anicalTy milled powders were evaluated by using an antaxnated SieverG's apparatus.
Tlte mechanically alloyed powders were inserted into the reacta~, evacuated for 2~
minutes, then heated to 5738 to do the first absorption under a hydrogen pressure of 1.OMPa. Hydrogen desarption was performed under vacuum witlh a large volume, sa that the pressure buildup during desorption is small (0.007Mpa).
(aj Mg-Cd Alloys Fig.l shawl the X ray di~'raction spectra of the Mg-iOCd and Mg-ZOCd (in atomic percent) powders after 2~ hours ofmechanical alloying. The spectx°u~x of pure Mg is also given far comparison. It can be seen that the X-ray disfraction peaks of the Mg phase shift to high angles, and the di~raCtion peaks of cadmitun disappear in the rriechattieally alloyed Mg-Cd alloys. This indicates that the cadmium has been dissolved in the HCP magnesium phase. Since the atomic size ol' Cd is stualler than that of Mg, the dissolution of cadmium atoxxts in the 1"rTCP Mg phase causes shrinking of the lattice spacing. The very broad diffraction peaks indicate that the grain size is small for the as-milled powders. The czystallite size bas been determined to be 39th and 3716 ntta for Mg-IOCd and Mg-2iDCd respectively frorra peak broadening using the Williarnsota-Hall method jH.P. Klug and I-, Alexander, X-ray diffraction procedures for polycrystalline axtd amorphous rttaterials, 2na edn., John Wiley & Sons, ~1'Y, 1g74~_ The vaxraditura axtd graphite additives do not react with Mg r~r Cd upon 2t3 h.al~rs. ....__- ..__ __ ___ _ of mechanical alloying, The XItD spectra of the as-milled Mg-Cd-V-C powders only show the shifted Mg peaks and a very broad small vanadium. peak. Since the carbon contsnt is very low, na diffraction peak fxa..m.. nh;tP ,',c nr~:cerv~r3.
'~'s,,e arirliri~n of vanadium and graphite do not change the lattice parameter of Mg, as confirmed by ball milling Mg-V, lVlg-C attd IVIg-V-C separately. There is no solid salutiara of V axtd C irt Mg obtained under the present mechanical alloying conditions.
At equilibrium, an ordered MgCd compound exists below S26K iu the composition range of DLO-6Qat.%Mg. At lower temperatures, an additional order phase Mg~Cd forms on the Mg rich side rA.A. Nayeb-Hasherai and d.~. Clark, Phase Dia~gratns ~~eT~ t~~it~ ~ d4 ."~~CSCal~9oYl~~AC lvJ,~~rVl~..'tl ~' p1C tll~ilXlIn temperature tinder equilibrium conditions. I3owever, no such phase was formed during mechanical allaying.
After several cycles of hydrogen absarptiorvdesoxptiou at 350°G during ~4 hears, grain.
growth of the Mg(Cd~-V-C composite is obsea ved. Fig.l shows the XRD spectra of the Mg{Cd)-V-C composites {the V and C contents are specified in the experimental) aver hydrogen desorption at 350°C and quenched at a cooling rate of 2°C/min down to room tez~.perrature. The diffraatian peal~.s are sharper ix2 comparison to that of the as-milled powders. The grain size increases to 8416 and 80t20nm far Mg-lDCd-V-C and Mg-20Cd-V-C powders respectively. The peak position of Mg(Cd) does not change, indicating that the cadmium retrains in solution in the HCP Mg.
°3°his is not setrprisia~g because magnesium and cadmium show continuous mutual solubility in the IiCP
[Mg,Cd) phase at elevated temperatures. Rapid cooling after hydrogen desorption suppresses the phase separation (precipitation of the Mg3Cd ghase from the supersaturated HCP Mg(Cd) solid solution). After hydrogen desorption, the diffraction peak of vanadium at 20=42.17° can be clearly seen. No other phase is present ou the XRD spectra. Since the rxlixing heats of V-Cd and V-Mg are positive (F.R. de Boer, R, Boom, W.C.M. Matters, A.R. Miedema and A.rC. Niessen, Cohesion in metals, transition metal alloys, North-lIolland lPhysios :Publishing. P'.O. Box 103, Amsterdam, The Netherland, 1988], no reactions between V and Cd or V and Mg are expected. Although vanadium has a strong tendeancy to react with graphite to foxcn vaxtadiurn carbide VC, no such phase was observed after 20 hydrogen absorptiortldesorption cycles.
The lattice parameters ''a'° and "c" of th.e MCP Mg phase are r~aown ire Fig.2. The values from literature [A.A. Nayeb-Hashemi and J.B. Clark, Phase Diagrams of Binary magnesium alloys, ASM international, 1988, Metals Park, Chin 44Q?3] are also givea~. It can be seen that the lattice spacings of M,g do shrink when Cd dissolves in the hcp-Mg lattice. Based on the trend of the lattice parameters versus Cc1 content izt Fig.2, and without wishing to be hound by theory, it is believed that cadmium is completely dissolved in the Mg lattice for IVIg-IOCd and Mg-20Cd samples after mechanical alloying. After hydrogen desorption of the hydrogenated saunple, the lattice parameters of the hcp Mg(Cd) phase are basically the same as those of the as-mi.Lled alloys.
The roam temperature X-ray diffraction spectra of the hydroge~at~d samples are shown irt ~'ig.3. ~ the case of Mg-IOCd, beside the Mgliz phase, a Mg3Cd phase is present, In the case of Mg-2DCd, a MgCd intermetallic compound forms. It is not clear wheg the phase separation occurred. It could. be during hydrogenation at high temperatures or during cooling a.~er hydrogenation. xlte lattice parameters of lVIgT~2 in the hydrogenated samples at room temperature are exactly the same as that of pure IVIgAz given in the literature CA.A. Nayeb-Hashemi and J.B. Clark, Phase piagrams of Binary s magnesium alloys, ASM internatienal, 19$8, Metals Park, Ohio 44073) indicating that the MgHz is very stoichiometric, no Mg{Cd)ZTIz phase is obtained at xoQtx~
ternpexature.
7Che pure Mg(Gd) alloys without 'V and ~C additives could not be activated at b23IG under a hydx~gen pressure of I.SMPa in 24 hours. ~'he IVIg{Cd)-V-C
composite absorbs hydrogen rapidly even in the first cycle upotl exposure to 1.aMPa of hydrogen at 573K. No incubation pexi.od is observed on the first hydrogen absntption curve as shown in Fig.4. The ~xs~t hydrogen absorption of Mg-20Cd-V-C is faster than that of Mg-SCd-~l-C. It can be fully hydrided in 1000seconds in the farst cycle. The absorptiou/desotption kinetics improves wifih cycling in the ~itial a cycles, and theax remains stable. After this activation period, the composite can be fully hydrided iu 500seconds. 'Zhe variation in Cd content does xtot lead to significant changes of the hydrogen absorption kinetics.
The hydrogen desorptiun kinetics are very fast also for the vanadium catalyzed Mg(Cd) solid solution alloys. The hydaogen desorption curves are shown in Fig.S. The desorpCioz~ kinetics improves with increasing Cd content. For the case of Mg-20Cd-V'-C, the hydrogen desorption is completed in 100seconds. This is almost ~ rimes faster thaxt that of MgHz-SY reported previously ~O. Liang, J. Fiuot, S. »aily, A.. Yon Neste and 1~..
Schulz, J. Alloy Comp., 291 (1999) 295.
The theranodynamic properties cef the composites changes with Cd co~.tent.
Fig.b shows the pressure--coxt~positzon isotherms (7PCI) of Mg-Cd alloys with various Cd contents. The hydrogen absorption ldesotption plateau pressures increase, the plateau length becomes shorter, and the slope becomes bigger with increasing Cd content. P'or the case of Mg-20Cd-Y-C, a very sloping fCI cure is observed. As well tloctxmeuted irr the kiteratures [K 2eng, T. K.lassera, ~, Oelexich and ~. ~r~naa~n, Int. J.
Hydrogen Energy, 24 {1998) ; J.F. Starnpfer, C.E. IJolley and J.~'. Suttle, J. Arra.
them. Soc., 82 (1960) 3504; lB. Bogdanovic, K.. $ohmhannniel, B, Christ, A. ~eiser, ~K.
Schlichte, I~..
~7ehlen, i3. Wolf, J. .Alloy Comp., 28B (1999) 84~, pure Mg shows very flat plateau ota hydrogen absorption and desorgtiorn. Saz'neties, a sloping absorption plateau arises, due to the slow absorption kinetics, however, the desorption plateau is fairly flat ~K.
~~sC:lb~-T:-~~Y'., y; I: ~3e#erie~ ~.nd-R: ~e~a~xs;-T.~.t. ~. ~~w-~ba : ~..-~a:b~,~,'.'.~. ~lnnv j J

The Mg(Cd),r sold solution may decompose upon hydrogenation accordixtg to the equation:
6al.V 1.~K~~'0~~1~11f~1vLV6,1.~~~3.itda~~YJEh(~./a3~.lYa:Z1'CI~~.~..~.a.~~~...
.~~ ., 9 ~.#lV$.V..$~s, 1'y... . .
the pressure in eq,uilibxiuzlx with pure Mg and MgHz. AG~~ is the relative partial xx8olar free energy of Mg an the M,gCdx alloy. Since ~GMg is negative for MgCdx alloy, the hydrogenn absorptioze/desoeption plateau pressure of MgCdx alloys is higher that that of M~2.
The empitscal rule of hydride stability for intermetallic cozzxpounds, such as .P~s and AF32, shows that the hydride stability is lower where the matt cell volume of the compound is smaller [M. H. Mendelsohn, D.M. Crruexi and A.E. Dwight: Nature (1977) 46; li. l~akano, S. Waicao and T. Shimizu, J. Alloy Comp., 25~~254 (1997) 609.
This rule is valid for the interstixial hydrides in which the metallic bonding plays an important rule [Y. Tsushio and E. Ak~'ba, J. Alloy Cflmp., 267 (1998) 246]. Yn the case of magnesium hydride, a high proportion of ionic and covalent bonding is expected [J.
Felsteiner, M. I3eilper, g. Gertuer, A.C. Tanxte~r and R. t~pher, Phys, li.ev.
8, 23 (181) $156]. The empirical rule in. this case may not be valid. I-lowever, plots of the desorprion plateau pressures aE the midpoint of the PCl cuxves versus the unit cell volumes of the Mg(Cd) solid solutions show approximately straight lines in Pig.7; the smaller the unit call volume is, the higher is the desorption plateau pressure. The plateau pressure is very sensitive to changes in. the unit cell volume. This relationship indicates that the magnesium hydrides can be destabilized by reducing the emit cell volume of Mg.
1~ig.8 shows the PCi (pressure-composition-isotherm) curves at various temperatures for Mg-lOCd-V-C sample. The hystexesis of hydrogen absorptioWdeso~ption becomes smaller with increasing temperature. The relationship of plateau (at the midpoint of the plateau) pressures with tetuperature are shown in ~g.9.
Since a bigger hysteresis is observed at lower temperatures, the calculation of the thezxttodyuanlic parameters treaction enthalpy and entropy) is not accurate. A
roengh calculation. ixidicateess that the ea~thalpy of hydxogen desorption is reduced to 7lkJ/mol fnF
l0 Mg~SCd-V-C from 75kTlmol for pure magnesium hydride. The rea.etian enthalpy varies with the hydrogetx content aczoss the plateau, as reported in the ZrMn2,.x system [W. Luo, S. Majorowski, J.D. Clewlsy and T.:~. Flanagan, Z. Phys. Cheat. N.F., ~d 1.63 {1989) S81-86], the details will be discussed in an upcomix~.g report.
(b) Clther Mg-based Alloys Magnesium forms solid solutions r~ritb earns elements under equilibrium states.
The unit cell volume of Mg can be altered by foaming solid solution. Fig.i shows the unit cell volume and the maximum terminal sc~lui~ality of some lVlg solid solutions under equilibrium conditions. The unit cell volume was derived from the lattice parameters ~Ven in [A.A. Nayeb-Hashemi and J.~. Clark, Fhase ,Dia~r-ams ~r~''Finary rnagrresaa~m alloys, ASM international, 1988, Metals lPark, Ohio 44073]. Clearly some elements such as Cd, In, Li, Al, Zn, Ag, Ga and Sn reduce the Mg lattice parameters by forming solid solution, while Sc and Pb expand tile Mg lattice.
The salubilities of Al, Zn, Ga, Ag in Mg are quite small at roam temperature although the maximum terminal solubilities at elevated temperature are high in some cases. For example, the solubility is less than 1 at.~/o A1 at room temperature far Mg(Al).
The terminal solubility is roars than 10 at.°fo Al at eutectic temperature. The txraximutn solubility of A1 atoms izl Mg phase decreases with temperature.
The nanostructure and extended solubility in supersaturated Mg solid solution may lead to novel hydrogem storage properties. The maximum solubilities of various alloying elerltents in Mg solid solution can be eactended lay mechanical alloying. The lvlg_ Ti system is a good example. The solubility of Ti in Mg is negligible under equilibtiutn state. lVlecha~°tcal alloying of Mg and Ti elemental powder mixture lead to a nanocrystalline Ivlg('Ti) solid solution. The maximum solubility of Ti in Mg can reach 12.5at.% [[~'r. Liang arid lZ. Schul~, J. Mater. Sci,, 38{2003)1179-11$4].
Figs.2 and 3 show the variations of the crystallite sloe and microstrain with annealing temperature for various alloys. The crystallite size and tnicrostxain were measured using the Williauison-~a11 method [see G. l.iang and 1~.. Schultz, J.
Mater. Sci., 3$ (20Q3) 1179.] The grain size of Mg-SZn and the muliicoruponent Mg-2?n-2A1-Ag_ Ga allays increases rapidly with annealing texr~perature. There xs not much grain groavth la belaw 500°C for Mg-20Ti. Hydrogen absorption/desarptac~u does result in grain reftnemeut. This is the reasota that smaller grain size is observed far the samples after hydrogen absotptionldesorptioza (P~CI measureaxent) than those after simple annealing treamcent.
The strain release is very fast even at lawer temperatures far Mg-SZn and Mg-2Zn-2A1-Ag-Ga as shown in Fig.3_ I~owever9 the strain release as slow below 300°C for Mg 20Ti. The high thermal stability of the nattocrystalline Mg-20Ti might result from high fracrion of one Ti particles and solute atoms which pits grain boundaries and dislocations. Cotttparing the results on. Mg-52n and Mg-20Ti, and without wishing to be bound by theory, it is believed that second phase particles and solute atoms in Mg phase have great effect on the grain refinercent and strain accumulation during mechanical alloyixtg and on the thermal stability of the Mg-based alloys.
Fig.4 shawl the variation of the lattice parameters with annealing teanperature for Mg SZn and lVlp-22xt 2A1-Ag-Ga.1'hc results for Mg-20'fi ~G_ ~,iang and ~.
Schultw, !~.
Mater. Sci., 38 (2003) 1179 are also plotted for cotnpatison.. The lattice parajneters of Mg-SZn increase with temperature to 100C, and then remain unchanged at higher terxlperatures. This indicates that some Zn atoms came out of the Mg lattice at temperature as low as 1400 leading to a reduced solubility. dome ~n atoms still remain is Mg solid solution. The lattice parattaeters of the ?vfg; ??n-2A1-Ag-(tea alloy increases with annealing temperatcre and reaches a maximum at 2000 due to phase separation of the supersaturated solid solution, and then decreases at higher temperatures owing to the fact that grecipitates are dissolved back to the ~Cp Mg phase. This f s because the solubilities of Ag, Ga, Al in IVIg phase are higher at elevated temperatures.
only slight ~L..~~.. _s ~L.. t..ae_-.. s.......~.. . .~. .-. .,...... -...z -~ .._ ~ _.. _ . , _a _~ nrn°~ r_ ~ r! nnrn~
laxastic increase begins at chant 3000 and stags at SOOC where the lattice parameters reach values close to those of pure Mg at 500°C.
Various mechanically alloyed Mg-based alloys show very different hydra,~en absotption/desotption kinetics although their particle size and specific surface area are about the same. Fig.S shows the ltydragen absorption curves of various Mg-based alloys with 2.Sat.°!°V and 2wt.°!o of C additives. For comparison, the kinetics curve of the MgHz-5V pxepared by ball trilling of MgT~2 and 1f far 20 hours ~G. ~.iang, ~.
Huot, S.
zz Boily, A. Van Neste and R. Schulz, ~. Alloy Comp., 291 (1999) 295 is also shown.
Evidently, the 1VI2-SV gives the best hydrogen absorption kinetics, but not much better than that of Mg-SZn, Mg l0i.i and Mg-SIu, which have only 2.5ax.~/° V
additive. IVIg-9At gives even. slower kinetics in. spite of the same amount of 2.5%V being present. 'The ~auhi-component Mg-2Zn-2Al-Ag-taa alloy gives the slowest absorption kinetics.
'phe beneficial catalytic effect of V on hydrogen absorption is coaxtterbalanced try adding A,l.
Without wishing to be bound 6y theory, this might be because hydrogenation of Mg(Ak) solid solution needs phase separation and long range diffusion of Ak, which is atl inherent slow process. Additionally, V tttay react with 1i,1 fc~rmvg intetznetallic coanpounds and therefore the catalytic effect of V disappears.
Fig.6 shows the hydrogen desorption kinetics of various Mg-based akloys with 2.Sat.% V and 2 wt.% C additives. The Mg-SZn and the MgHx-SV give the fastest desoxptian kinetics, followed by Mg-SIn, Mg 9A1, Mg-IDLi, Tha slowest one is the cotxtglex Mg 2Zn-2A1-Ag-Ga. Again, the mulricoxnpcmeut nature does harm to desorprion >unetics.
The hydmgen desorption part of the pressure-composition isotherms (PC:~ of various alloys with 2.5 ax.°l° V and 2wt.% C additives are shown itx Fig.7. It is seen that Mg, lVlg-V and Mg-TOX.i have basicakly the same desarptiott plateau pressure.
Since ~
and V do not form alloys wish lvig, adding V arid tr does not cltatxge the thercnodynarrfic properties of magnesium hydrides. Li is dissolved in Mg forming Mg(Li) solid solution after mechanical alloying. 1-lydrogenation of Mg(Ifii) solid solution leads t4 phase separation, namely, a mixture of MgHz and lriH forms. Since LiH as very stable, it does not decompose in the present hydrogen desorption conditions. Therefore, adding Li to Mg C does not affect the plateau pressure.
Mg-SAg, Mg-9A1 and Mg-SZn shows sliglxtly higher plateau pressure than that c~f Mg. However, the hydrogen storage capacity is reduced signi~.cat7tly. In the care of Mg_ 5Ag, a second desorption plateau at high pressure appears, which as related to Mg3Ag.
The Mg-SCd and lvig SIn give even higher plateau pressures. Since Cd and In have higher atomic weight than that of Zn and 2~g, the hydrogen storage capacity is reduced further. The PCI curve of the MgzNi-24wt.% Mg composite is also givc~a ax1 Fig.B. The MgzNi phase is clearly of higher plateau pressure but lower storage capacity than that of Mg-SIn. The theoretical capacity of Mg~N phase is 3.6wt,°J°. The lower plateau shown in the curve comes from the proporCion of iVtg in the composite which has basicahy the same value as that ofpure Mg, Mg-~ and Mg-Li alloys.
Fig. ~ shows the Van't Floff plat of various Mg-based alloys at certain temperature range. It caa be seen that TiC3~, V, ~.i does not change the thermadynart2ic properties. Zn, A1 and I~Ti-Zn improve tltce plateau pressure slightly. In fact, the Mg-Ni-Zn and Mp Zn give the same g~lateau pressure. This is probably because hTi reacts with Mg forming MgzNi as confirmed by PCI and Xl~ al~alysea It is the fact that Zn atoms remained in solid solution at this temperature that give the effect on plateau pressure. The multicomponent Mg-Al-2n-Ag-Ga does not give k~igher plateau pressure than Mg-Zn, Mg A1 or Mg-Ag does. No syteergy effect of nculticomponeut t~u the thermodynamic properties is observed. Considering the slow hydrogen absorptianldesorption kinetics, it does not appear that multicomponent Mg-based system have advantages than the siulple system for hydrogen storage.
Mg-5In and Mg-SCd alloys have plateaus very close to that of Mg2N'i at higher temperatures while the discrepaxtcy becomes bigger at lower temperatures.
Mg2Al~ has even higher plateau pressure bat lower storage capacity (2.$wt.%} than that of MgzNi. A
rough catculation of the reaction enthalpy of the Mg-Sxu alloy gives 70kJlmol, which is in between those of Mg and MgxNi. 'fhi.s result indicates that MgH2 can be destabilized to same extent by forming solid solution withoe~t causing too mush reductiop of storage capacity.
Fig.9 shows the hysteresis ofhydrogen abst~rpuonldesolption. The general trend is that the hysteresis increases with decreasing teaaperature. Especially for the cases of multicaznponent Mg-2A1-2Zn-Ag-2In and Mg-2Al-2Zn-Ag-Ca alloys. The high hysteresis at low temperature is probably caused by the slow hydrogen absorptionldesorption kinetics in the complex systems. The structural nat~ce might be also imgartant hecause the solubility changes with temperature, and the plateau pressure is affected by the changes of solubility or partial molar free energy of Mg in the solid solution.

Claims (2)

1. A hydrogen storage material comprising an Mg-X solid solution, wherein X is dissolved in an HCP Mg phase, and wherein X is Cd or In.

2. A method of forming an Mg-X solid solution, wherein X is dissolved in an HC
Mg phase, comprising:

mixing Mg with X; and grinding the Mg and X mixture to form the Mg-Cd solid solution~