Effect of nickel alloying by using ball milling on the hydrogen absorption properties of TiFe

\ PERGAMON

International Journal of Hydrogen Energy 13 "0888# 774Ð789

E}ect of nickel alloying by using ball milling on the hydrogen absorption properties of TiFe M[ Bououdina\ D[ Fruchart\ S[ Jacquet\ L[ Pontonnier\ J[L[ Soubeyroux Laboratoire de Cristallographie du CNRS\ BP 055\ 27931 Grenoble cedex 8\ France

Abstract After ball!milling\ TiFe under a pure argon atmosphere\ with a small amount of pure nickel\ can readily absorb large amounts of hydrogen without any activation process[ The H:M system reaches equilibrium after 7 h under 09 bars of hydrogen pressure "H:M  9[51#[ Even after 09 days exposure to the air\ the same sample reacts with hydrogen\ but with slower kinetics[ The impact of the ball!milling time on the hydrogen absorption kinetics and the maximum hydrogen content is discussed[ Þ 0888 International Association for Hydrogen Energy[ Published by Elsevier Science Ltd[ All rights reserved[ Keywords] Metal hydride^ Ball!milling^ Activation process

0[ Introduction The intermetallic compounds absorbing large amounts of hydrogen "LaNi4\ TiFe\ Mg1Ni\ Laves phases\ [ [ [# pre! sent a great interest for integrated or massive energy storage applications[ One of the most important steps for the practical use of these ternary hydrides is the activation process[ Some of the most promising materials in terms of storage at a large scale\ e[g[ TiFe or Mg1Ni\ are known to need severe activation procedures prior to exhibiting fully reversible hydrogenation:dehydrogenation cycles[ Usually\ the activation process consists in performing thermal cycles "high and low temperature exposures namely for Mg1Ni and TiFe# under relatively high hydro! gen gas pressure in order to have complete absorptionÐ desorption isotherms in short times[ It has been shown that adding small amounts of extra elements during the melt could induce intergranular active precipitates in the alloys "e[g[ Cu\ Pd additions in Mg1Ni ð0Ð2Ł\ 2d metal and other metal substitution in LaNi4 ð2\ 3Ł#[ In the case of TiFe\ an alloy particularly di.cult to activate\ more reactive samples were obtained after substitution of 4Ð 09) of iron by manganese ð4Ł[ Nevertheless the life time

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"¼number of cycles# remains limited^ either it is not obvious to simultaneously optimise the formula and the size of the precipitates\ or after a rest time\ corrosion is still induced via some extra phases and the properties are degraded again[ Recently\ the use of the ball milling technique has provided new alternatives to avoid an activation process that degrades the alloy when it must be applied either long periods or several times i[e[ over!temperature or over!pressure can initiate disproportionation of the star! ting phases[ Up to now\ the ball milling technique has been developed through three di}erent ways[ The _rst one consists in the synthesis of the intermetallic com! pound directly from adjusted proportions of the pure elements[ It is especially appropriate when the range of melting temperatures of the elements is particularly wide "e[g[ Mg1Ni related compounds# ð0\ 5Ł[ The second pro! cedure is based on the use of ball milling under controlled atmosphere on alloys initially prepared by using con! ventional melting techniques[ Both methods give rela! tively good results\ amorphous or nanocrystalline parts of the alloys actively absorb hydrogen and progressively activate the whole sample ð0\ 2\ 5Ł[ However\ due to further disproportionate processes\ or after exposure to air\ the hydrogenation performances of these nano! structured "often metastable# alloys are found to drop readily[ Thirdly\ when a small amount of palladium "less

9259!2088:88:,19[99 Þ 0888 International Association for Hydrogen Energy[ Published by Elsevier Science Ltd[ All rights reserved PII] S 9 2 5 9 ! 2 0 8 8 " 8 7 # 9 9 0 5 2 ! 2

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than 0 wt)# was added to the powder in the course of the ball!milling process\ Zaluski et al[ ð1\ 3Ł found a considerable enhancement of the hydrogenation kinetics\ e[g[ for TiFe[ The nanocrystalline material containing palladium clusters at the surface of the grains\ readily absorbs hydrogen at room temperature without any acti! vation[ A signi_cant enhancement of the bulk hydro! genation properties was observed\ the powder absorbs hydrogen without any activation process after handling and long period storage[ The role of palladium as a cata! lyst element has been explained in terms of a {spillover| e}ect on the surface of the material[ The work of Aoki et al[ ð2Ł shows that pure TiFe ball milled in an inert gas atmosphere quickly absorbs hydrogen without any activation[ In contrast to the procedure involving palladium\ when the undoped material is exposed to the air\ even for a short time\ hydrogenation becomes de_! nitely impossible[

1[ Experimental details As mentioned in the introduction\ alloying a few per! cent of manganese made the activation procedure on millimeter scale particles e}ectively complete after several tens of hours of activation^ it also needs up to 1 or 2 weeks of treatment when using a pure TiFe sample[ Such a type of sample\ namely\ of composition TiFe9[89Mn9[09 delivered by the Billings Co[ "U[S[A[# was already used for in situ crystal structure analysis ð4Ł[ In the present case we used a pure TiFe as the starting alloy\ prepared by the cold crucible induction technique from 2N4 and 3N purity elements\ respectively[ A mass of about 3 g of the starting TiFe material was crushed in a stainless steel mortar and then 199 mm mesh sieve[ After that\ 4) weight of pure nickel powder "88[88# of 09 mm particle size was added to the TiFe powder placed in the vial of the ball mill[ In this Fritsch P4 planetary mill\ we used both tungsten carbide balls and vial[ Five balls of 09 mm in diameter were used most of the time[ Before the experiment\ the vial was closed in a glove box under pure argon gas "purity 88[8884# after several vacuum argon rinsing cycles[ The milling pro! cedure was applied for periods of 29 min\ and because this is a very energetic process\ the machine was stopped for 04 min in order to cool down the vial and the sample to room temperature[ In order to assess the crystal quality of the samples\ X!ray di}raction patterns were collected using a UÐ1U Philips di}ractometer "lCu−Ka  0[4307 _#\ equipped with a backscattering graphite monochromator[ A computer monitored thermogravimetric apparatus was used in order to determine the hydrogenation charac! teristics ð6Ł[ These experiments were carried out using a mass of 0 g of powder at T  187 K under a constant hydrogen gas pressure of 0 MPa[

2[ Results and discussion In a _rst series of experiments\ the TiFe alloy was ball milled free of nickel addition[ In a second series\ about 4 at) of nickel powder was added to the TiFe powder[ Both these experiments were undertaken for di}erent milling times from 09 to 54 h "as the sum of the 29!min periods of e}ective milling#[ After each experiment\ the vial was opened under argon atmosphere and a small amount of powder was used for characterisation by X! ray di}raction[ Since iron and nickel have close atomic radii\ formation of a pseudo!binary alloy is not excluded via substitution of Fe by Ni[ In Fig[ 0\ the X!ray di}raction patterns of the starting pure TiFe and the milled TiFe¦Ni mixture are plotted[ The crystalline TiFe samples "ainitial  1[879 _# where traces of Ti1Fe or Ti1FeOx impurity are present\ pro! gressively transform to less crystallised materials[ A posi! tive e}ect of such ternary n!carbide like oxides on the H! reactivity on TiFe has already been mentioned ð4\ 7Ł[ However\ the impact of these phases is far less marked than that of nickel addition at the surface\ as seen here! after[ From the data analysis\ no signi_cant change of the lattice parameter was noticed[ However\ the X!rays pat! terns reveal a progressive broadening of the di}raction lines[ For example\ the half width at middle height "hwmh# of "099#\ the strongest di}raction line\ was esti! mated along the successive steps of the milling treatment[ The measured values are ¼9[014 for the starting TiFe "in agreement with the instrumental resolution#\ ¼9[24 after 11 h of milling time "hmt#\ ¼9[07 after 26 hmt and ¼9[49 after 54 hmt\ respectively[ The various broadening can be correlated to di}erent states of the microstructure of the sample along the milling procedure[ After 11 hmt\ the broadening of the X!ray!lines should be attributed to internal strains\ stacking faults [ [ [ \ resulting from the many shocks on the large "¼199 mm size# starting particles[ Then for 26 hmt\ the hwmh is reduced by half compared to the previous one\ but the "099# line pro_le is markedly of a lorentzian type[ This corresponds to almost well crystallised and _ne particles with mean size "¼499 _ as roughly estimated by using the Scherrer formula# under the limit of di}raction coherency[ After 54 hmt\ a marked broadening of the lines is observed as the result of the crystallite size reduction "to less than 199 _# and probably some amorphisation e}ects[ At _rst sight\ the width of the "000# line of Ni which is close to the "099# TiFe varies similarly to the largest one\ but the Ni line seems to persist up to 54 hmt[ Traces of tungsten carbide were observed only after 54 hmt "1UB ¼ 25>#[ Amorphisation e}ects\ reduction of the grain size "both correlated with oxygen pollution# and traces of tungsten carbides were already detected in some long!time milled TiFe samples by Tessier ð8Ł[ Microstructure analyses of the samples have been

M[ Bououdina et al[ : International Journal of Hydrogen Energy 13 "0888# 774Ð789

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Fig[ 0[ X!rays di}raction patterns of TiFe samples "a# Starting TiFe samples\ some extra weak lines of Ti1Fe1Ox are visible in the range of 1u of 27Ð33> "apart from the largest of TiFe "009# line#^ "b# TiFe¦4) Ni after 11 hmt^ "c# TiFe¦4) Ni after 26 hmt where Ti1Fe1Ox is still visible^ "d# TiFe¦4) Ni after 54 hmt where tunsgten carbide traces are visible at 1u ¼ 25Ð26>[

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M[ Bououdina et al[ : International Journal of Hydrogen Energy 13 "0888# 774Ð789

achieved by means of S[E[M[\ E[D[X[\ and T[E[M[ Com! parison of the 26 and 54 hmt samples starting from pure and FeTi¦Ni compounds reveals that in both cases the overall grain size is close to 0Ð2 mm\ in full agreement with Ref[ ð2Ł[ No reduction of particle size was obtained when increasing the milling time "from 26 to 54 hmt#[ However\ a slight tendency for smoother shapes can be observed with the materials milled for a long time "Fig[ 1aÐd#[ In the case of Ni containing powders\ E[D[X[ analysis shows an homogenous distribution of this metal within the sample[ From the T[E[M[ analysis\ no clear result has been obtained] on some parts of the TiFe grains\ a few nickel rich clusters have been found\ but it is not possible to tell whether these are pure Ni grains\ or if the added nickel is combined "alloyed# at the surface of the TiFe grains[ Further investigations are needed "i[e[ using HREM# to clarify this point[ No oxygen pollution "down to 1)# has been detected in any sample[ Then\ the di}erent samples were submitted to 0 MPa of hydrogen gas pressure in the thermogravimetric appar! atus ð6Ł for about 01 h\ without applying any initial acti! vation procedure[ First we noticed that the ball milled pure TiFe does signi_cantly react with hydrogen gas within the time experimented here[ The TiFe"Ni# ball! milled sample "brie~y exposed to air during transfer# readily absorbs hydrogen and reaches its maximum absorption capacity after only a few hours[ The uptake

is 0[2 H atom per formula unit as normally found under this equilibrium pressure for a perfectly activated sample ð09Ł[ In Fig[ 2\ the absorption curves measured at room temperature are reported[ We notice that 11 hmt are su.cient to get a very reactive powder\ while further milling seems to have a negative e}ect[ The sample ball milled for 26 h still reacts with hydrogen but the maximum uptake is reduced by 04)[ For the longest milling time\ the reactivity is dramatically reduced] in the sample ball milled 54 h\ very little hydrogen is absorbed[ After a 09!day exposure to air\ the reactivity of the sam! ples "11 and 26 hmt# remains rather large "¼69) of the initial rate#[ After one month exposure to air the best sample is still reactive even if the maximum hydrogen uptake is divided by 2 "H:M ¼ 9[14# although this is an exposure period 5 times longer "79 h#\ compared with the _rst experiment "Fig[ 3#[ All the curves recorded exhibit the typical shape of a nucleation and growth controlled kinetic process[ In order to check for a possible poisoning by oxygen at the surface\ E[D[X[ microanalysis was per! formed again on the latter sample\ without any evidence of a signi_cant oxidation[ 3[ Conclusion The ball milling technique was applied under neutral atmosphere on the TiFe compound to modify its hydro!

Fig[ 1[ "a# up and left\ pure TiFe after 26 hmt^ "b# up and right\ pure TiFe after 54 hmt^ "c# down and left\ TiFe¦Ni after 26 hmt^ "d# down and right TiFe¦Ni after 54 hmt[

M[ Bououdina et al[ : International Journal of Hydrogen Energy 13 "0888# 774Ð789

778

Fig[ 2[ First absorption kinetics at 14>C under 1 MPa H1 gas pressure of "0# the 11 hmt TiFe¦Ni sample^ "1# the 26 hmt TiFe¦Ni sample^ "2# the 54 hmt TiFe¦Ni sample[ Note that in the same conditions\ the kinetics of absorption of a pure TiFe sample is lower than for sample "2#[

Fig[ 3[ Kinetics of hydrogen absorption for the sample 0 "11 hmt# after 29 days of rest time in air[

genation properties[ The addition of nickel gave an important enhancement of kinetic properties to a ball! milled TiFe powder for 19Ð29 h[ The improved material is also corrosion!resistant to air for a much longer time[ It is suggested that during the _rst period of milling\ fresh surfaces of mm!scale TiFe particles are created\ immedi! ately covered by very _ne nickel grains acting as catalytic centres for hydrogen molecule dissociation[ Further! more\ the TiFe surfaces appear locally protected against corrosion[ However\ after a longer milling time\ the reac! tivity with hydrogen is considerably reduced[ Hence\ decreased nickel is probably dissolved or alloyed into the binary alloy where kinetics properties are dropped down[ In parallel\ the size of the particles is markedly reduced\

even amorphised\ which makes the particles very fragile to air[

References ð0Ł Zaluski L\ Zaluski A\ Strom!Olsen JO[ J of Alloys + Com! pounds 0884^106]134Ð8[ ð1Ł Zaluski L\ Zaluski A\ Tessier P\ Strom!Olsen JO\ Schulz R[ J of Alloys + Compounds 0884^106]184Ð299[ ð2Ł Aoki K\ Aoyagi H\ Memezawa A\ Masumoto T[ J of Alloys + Compounds 0883^192]L6Ð8[ ð3Ł Zaluski L\ Zaluski A\ Tessier P\ Strom!Olsen JO\ Schulz R[ J of Materials Science 0885^20]584Ð7[

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ð4Ł Fruchart D\ Commandre M\ Sauvage D\ Rouault A\ Tellgren R[ J of the Less!Comm Metals 0879^63]44Ð52[ ð5Ł Orimo S\ Fujii H\ Yoshimo T[ J of Alloys + Compounds 0884^106]176[ ð6Ł Bououdina M\ Soubeyroux JL\ Juen P\ Mouget C\ Argoud R\ Fruchart D[ J of the Less!Comm Metals 0884^120]311[

ð7Ł Stioui C\ Fruchart D\ Rouault A\ Fruchart R\ Roudaut E\ Rebiere J[ Mat Res Bull 0870^05]758Ð65[ ð8Ł Tessier P[ PhD thesis\ McGill University\ Montreal\ 0884[ ð09Ł Thompson P\ Pick MA\ Reidinger F\ Corliss JM\ Hastings JM\ Reilly JJ[ J Phys F 0867^7"3#]L64[