3 resu 2 - 東海大学 paper by h.uchida.pdf · 2 pj < s 2 hp

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Takeshi Harakib, Keigo Oishia, Hirohisa Uchidaa, Yasuo Miyamotob, Masatake Abec,Takayuki Kokajic, Satoshi Uchidad

a Tokai University, Course of Applied Science, Graduate School of Engineering, Kanagawa, Japanb Tokai University, Technical Service Coordination Office, Kanagawa, Japanc NASU DENKI TEKKO Co. Ltd., R&D Department, Tokyo, Japand Tokyo Metropolitan Industrial Research Institute, Tokyo, Japan

Properties of hydrogen absorptionby nano-structured FeTi alloysDedicated to Professor Dr. Reiner Kirchheim on the occasion of his 65th birthday

In this study, two different nano-structured samples of theFeTi compound were prepared by mechanical alloying andmechanical grinding. For these samples, kinetics of the in-itial rate of hydrogen absorption, and the equilibrium hy-drogen pressure as a function of hydrogen concentrationwere measured. Mechanical alloying of Fe and Ti atomsproduced the FeTi compound powder samples with micro-structures of a mixture of nano-structured FeTi grains andamorphous phases. This sample exhibited a high initial rateof hydrogen absorption even at 298 K, however, a stronglyreduced hydrogen storage capacity. Mechanical grindingof the FeTi produced samples of particles with a particularmicrostructure: surface layers with a mixture of nano-struc-tured FeTi grains and amorphous phases, and a single crys-talline phase of FeTi below the surface layers for each parti-cle. This sample exhibited a high initial rate of hydrogenabsorption without a significant reduction of the hydrogenstorage capacity compared with that of the standard FeTisample. This mechanical grinding treatment was found tobe an effective method of surface modification to improvethe initial activation of the FeTi hydrogen storage alloy.

Keywords: FeTi; Nano-structure; Hydrogen storage; Ki-netics of hydrogen absorption

1. Introduction

The FeTi intermetallic compound is one of the most con-ventional hydrogen storage alloys, and much work has beendone since the first finding of this interesting feature ofFeTi by Reilly and Wiswall [1]. FeTi is an attractive candi-date for practical use as a hydrogen storage material be-cause of its relatively inexpensive material costs and a hy-drogen storage capacity around H/FeTi = 1.9.

As is well known, the surface of FeTi exhibits very inac-tive behaviour on exposure to hydrogen gas. Cyclic hydro-genation– dehydrogenation is needed for the initial activa-tion of FeTi at high temperatures and hydrogen pressures.Attempts to enhance the rate of the initial activation havebeen made by various methods: the mechanical milling ofMg into FeTi alloys [2]; the partial substitution of the con-stituent elements by Ni, Mo, Cr [3], Zr [4], Mm [5] or V[6]. Sandrock and Goodell found that the addition of smallamounts of oxygen to FeTi enhances the initial rate of hy-drogen uptake [7]. Khatamian et al. [8], and Hirata [9, 10]reported that particular oxides at the surface contribute toincreasing the initial rate of hydrogen absorption.

Recent studies on the synthesis of intermetallic com-pounds by mechanical alloying (MA) have been reported[11 – 13]. The intermetallic hydrogen storage compounds(AB5, AB, AB2) prepared by MA showed a significant en-hancement in the initial rates of hydrogen absorption [11].The FeTi alloy forming through MA also tends to exhibit a

T. Haraki et al.: Properties of hydrogen absorption by nano-structured FeTi alloys

Int. J. Mat. Res. (formerly Z. Metallkd.) 99 (2008) 5 507

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higher initial rate of hydrogen absorption than FeTi pre-pared by melting processes [14]. However, the MA processpronouncedly reduces the hydrogen storage capacity. Asimilar marked reduction in the maximum hydrogen stor-age capacity was also found for milled Pd powder [15].The maximum hydrogen storage capacity does not dramati-cally recover for those alloys even after annealing treatment[11].

In this study, we prepared two different types of nano-structured FeTi samples prepared by MA and mechanicalgrinding (MG), and investigated the effects of these treat-ments on the initial rate of hydrogen absorption and the hy-drogen storage capacity of these samples.

2. Experimental procedure

2.1. Sample preparations by MA and MG processes

FeTi was prepared from the constituent elements (Fe, Ti)with a purity grade higher than 99.9 %. In the MA process,a stoichiometric mixture of the sieved powder samples(< 150 lm) and stainless steel balls were loaded togetherinto a stainless steel pot under an Ar gas atmosphere. Thevolume of the pot and the diameter of the ball were250 cm3 and 25 mm, respectively. The ball-to-sample ratioin weight was 5 : 1. The MA was carried out for a millingtime from 5 h to 100 h at a speed of 200 rpm by a planetaryball milling system (Fritsch P5). In the MG process, theFeTi intermetallic was prepared by radio frequency melt-ing, and then mechanically milled for up to 5 h using theplanetary ball milling system. In order to keep the millingtemperature below 323 K, the MA and MG were stoppedevery one hour. Samples of the milled powders were char-acterised every 5 h for MA, and every 15 min for MG. Thestructural analysis and compositional analysis were madeby powder X-ray diffraction (XRD) and energy dispersiveX-ray spectroscopy (EDX), respectively. Scanning electronmicroscopy (SEM) and transmission electron microscopy(TEM) were used for observation of the sample microstruc-ture.

2.2. Measurements of hydrogen absorption rate andhydrogen equilibrium pressures

The initial rate of hydrogen absorption and the pressure(P) – composition (C) – temperature (T) isotherms for theprepared samples were measured volumetrically using aSieverts’ type apparatus [16, 17]. In order to measure the ki-netic properties reproducibly, several factors should bestrictly taken into account: effects of heat transfer in a pow-der or bulk sample, and between a sample and the wall of areaction cell used, vacuum leak rate of the system used, andthe purity of hydrogen gas used [17, 18]. The attained va-cuum of the system was less than 7 · 10 – 4 Pa, the vacuumleak rate of the system was 1.1 · 10 – 9 Pa m3 sec – 1, andthe purity of hydrogen gas was 99.99999 %. The samplewas heated up to 573 K for 2 h and cooled down in a highvacuum before each hydrogen gas exposure. TEM observa-tions revealed that this heating– cooling process had no in-fluence on microstructural phases, the coexistence of amor-phous and nano-crystalline phases or the single crystallinephase, induced by the mechanical treatments.

3. Results and discussion

3.1. FeTi samples prepared by mechanical alloying ( MA)

Figure 1 shows changes in the XRD patterns of Fe –Ti alloysamples prepared by MA as a function of MA time from 0 hto 100 h. In the beginning, the XRD peaks of Fe and Ti werebroadened. The peak close to FeTi appears after 10 h, and itshifts to the lower angles as the period of MA was pro-longed. Finally, the formation of the FeTi intermetalliccompound was confirmed with a trace amount of Ti after90 h. From this result, further investigation was made usingsamples milled by MA for 90 h.

The sample prepared by MA for 90 h exhibits powderswith particle diameters of around a few mm as shown inFig. 2. The microstructure of the powder sample was char-acterised by TEM and electron diffraction in the surfacearea and inside of a particle. Figure 3 shows a TEM image


508 Int. J. Mat. Res. (formerly Z. Metallkd.) 99 (2008) 5

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Fig. 1. Changes in the X-ray diffraction patterns of FeTi alloy samplesprepared by mechanical alloying (MA) as a function of MA time from0 h to 100 h.

Fig. 2. Secondary electron SEM observations of FeTi samples pre-pared by mechanical alloying for 90 h.

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and electron diffraction patterns of the surface area of a par-ticle sample. The nano-crystalline and amorphous phasesFeTi were observed. Figure 4 shows a TEM image and dif-fraction patterns of the inside of this particle sample. Theelectron scattering contribution from nano-crystallinity inthe inner region seems to be higher than that in the surfacearea. However, the image shows a mixture of nano-crystal-line and amorphous phases, which is a similar trend to thatof the surface area. These observations are identical to theresults of electron diffraction patterns indicating a mixtureof crystalline and amorphous phases. From these observa-tions, the alloy sample prepared by MA for 90 h was foundto be composed of a mixture of nano-crystalline FeTi andamorphous phases.

This nano-structured FeTi (n-FeTi) sample was exposedto hydrogen gas at 298 K and at a hydrogen pressure of0.5 MPa. As shown in Fig. 5, the n-FeTi by MA exhibits adramatic enhancement of initial activation behaviour withhigh rates of hydrogen absorption compared to the arcmelted sample.

Figure 6 presents the P – C isotherms at 298 K for hydro-gen absorption of an n-FeTi sample prepared by MA for90 h, and of an arc melted polycrystalline FeTi sample.The n-FeTi sample exhibited lower plateau pressures, anda lower hydrogen storage capacity (H/FeTi = 1.3) than the

polycrystalline sample. Inui et al. [19] reported that the lat-tice defects lead to a decrease in plateau pressure with thea(H solid solution) ? b(FeTiH1.0) transformation, whilethe b ? c(FeTiH2.0) transformation occurs in the directionof higher pressure. The MA obviously induces a large num-ber of lattice defects, and we found the fact that the MA-sample showed the occurrence of the b? c transformationabove 20 MPa in [20]. In the present study, the pressuretransduces was limited to below 10 MPa. This leads to theconclusion that the b ? c transformation takes place at apressure higher than the pressure transducer could monitor,



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Fig. 3. A transmission electron microscope image and electron dif-fraction patterns of the surface area of a particle of an FeTi sample pre-pared by mechanical alloying.

Fig. 4. A transmission electron microscope image and electron dif-fraction patterns of the inside area of a particle of an FeTi sample pre-pared by mechanical alloying.

Fig. 5. Curves of hydrogen absorption at 298 K for FeTi samples: (a)for the 1st and (b) for the 2nd hydrogen absorptions by a sample pre-pared by mechanical alloying, and (c) for the 1st hydrogen absorptionfor an arc melted sample.

Fig. 6. Pressure –composition isotherms at 298 K of a nano-structuredFeTi sample prepared by mechanical alloying for 90 h, and for an arcmelted FeTi sample.

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and the P – C isotherms show only the formation of b-hy-dride as the maximum hydrogen content. The region of hy-drogen solid solution of n-FeTi was found significantlybroadened compared with that of the standard FeTi sampleprepared by arc melting. This may be associated with a so-lution of hydrogen in the grain boundary of the polycrystal-line phase. A recent neutron scattering study concluded thatthe considerable amounts of hydrogen in n-FeTi are ratherpopulated at the grain boundary surrounding 4 Ti or3 Ti + 1 Fe tetrahedral coordination [21]. It is suggestedthat an H atom preferentially occupies the tetrahedral Ti4or Ti3Fe at the grain boundary because of the strong affinitybetween Ti and H, which, in turn, increases the solid solu-tion limit of the FeTi-H phase.

3.2. FeTi samples prepared by mechanical grinding (MG)

In order to enhance the initial rate of hydrogen absorptionwithout a pronounced reduction in the hydrogen storage ca-

pacity, nano-structured surface layers were intentionallyformed in the surface area by MG. The inside of the surfacelayers of each alloy particle was maintained as a singlecrystalline phase.

Figure 7 shows changes in the XRD patterns of FeTi al-loy samples treated by MG as a function of grinding timefrom 0 h to 5 h. As the time for MG treatment increased,the XRD peaks broadened. The SEM images show thechanges in the particle size of the FeTi by MG for 0 h, 1 h,2 h and 3 h (see Fig. 8). After 2 h MG, no marked changewas observed in the particle size, with a diameter of about2 lm. From this result, the powder samples treated by MGfor 2 h were used for further investigation.

Figures 9 and 10 show TEM images and electron diffrac-tion patterns for the surface and inside of a particle of thesample, respectively. On the surface, nano-crystallinephases were observed together with amorphous phases,which were similar to the microstructure of a sample pre-



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Fig. 7. Changes in X-ray diffraction patterns of FeTi samples preparedby mechanical grinding as a function of grinding time from 0 h to 5 h.

Fig. 8. SEM micrographs of an FeTi sampleprepared by mechanical grinding for (a) 0 h,(b) 1 h, (c) 2 h, and (d) 3 h.

(a) (b)

(c) (d)

Fig. 9. A transmission electron microscope image and electron dif-fraction patterns of the surface of a particle of FeTi prepared by me-chanical grinding for 2 h.

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pared by MA. However the ratio of nano-crystalline toamorphous phases is higher than that of a sample preparedby MA. On the other hand, the array of the crystal planecan clearly be seen for the inside of the particle sample pre-pared by MG. The electron diffraction pattern indicates thepresence of a single crystal structure inside the particle byMG for 2 h (Fig. 10). These facts suggest that MG treat-ment effectively induces layers of nano-structured FeTi inthe surface by maintaining a single crystal phase insideeach FeTi particle.

The sample produced by MG for 2 h was exposed to hy-drogen gas at a pressure of 0.5 MPa and at 298 K. As shownin Fig. 11, the sample exhibited high initial rates of hydro-gen absorption in the first and second hydrogen gas expo-sures. On the other hand, an arc melted FeTi sample didnot react with the hydrogen gas over 1 h. The P – C iso-therms for a sample prepared by MG for 2 h and an arcmelted FeTi are displayed in Fig. 12. The sample preparedby MG shows a slightly lower plateau pressure, but, a simi-

lar hydrogen storage capacity (H/FeTi = 1.7) to that of astandard sample prepared by arc melting. In MG, the for-mation of the amorphous phase as well as lattice deforma-tions were mainly induced in surface regions of a FeTi par-ticle. This may contribute to avoiding significant reductionin the plateau pressure and the hydrogen storage capacityat pressure below 10 MPa.

4. Conclusions

A nano-structured FeTi alloy was formed by MA. The mi-crostructure was found to consist of a mixture of nano-crys-tal grains and an amorphous phase. This nano-structuredFeTi exhibited a high initial rate of hydrogen absorption at298 K, however, with a markedly reduced hydrogen storagecapacity. The reduction in hydrogen storage capacity maybe attributed mainly to the presence of an amorphous phase,and lattice deformations induced by MA. Using MG, a FeTipowder sample with a particular structure of nano-struc-tured surface layers and a single crystal FeTi below thelayers was successfully prepared. This FeTi sample exhib-ited a high rate of the initial activation without a significantreduction in hydrogen storage capacity. This MG can beconsidered as an effective surface modification treatmentto enhance the reactivity of the FeTi with the hydrogen gas.

This study was supported by a Grant-in-Aid from the Japan Society forthe Promotion of Science (JSPS) KAKENHI 17560623. The authorsare very grateful to Prof. Dr. Masashi Sato, Department of AppliedChemistry, School of Engineering, Tokai University, for his kind dis-cussions for this study.

References

[1] J.J. Reilly, R.H. Wiswall, Jr.: Inorg. Chem. 13 (1974) 218.[2] T. Kondo, K. Shindo, M. Arakawa, Y. Sakurai: J. Alloys Compd.

375 (2004) 283.[3] A. Szajek, M. Jurczyk, E. Jankowska: J. Alloys Compd. 348

(2003) 285.



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Fig. 10. A transmission electron microscope image and electron dif-fraction patterns of the inside of a particle of FeTi prepared by mechan-ical grinding for 2 h.

Fig. 11. Hydrogen absorption curves at 298 K for FeTi samples: (a)for the 1st, and (b) for the 2nd hydrogen absorptions by a sample pre-pared by mechanical grinding for 2 h, and (c) for an arc melted FeTisample.

Fig. 12. Pressure –composition isotherms at 298 K of an FeTi sampleprepared by mechanical grinding for 2 h, and of an arc melted FeTisample.

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[4] B.K. Singh, A.K. Singh, C.S. Pandey, O.N. Srivastava: Int. J. Hy-drogen Energy 24 (1999) 1077.

[5] B.K. Singh, A.K. Singh, O.N. Srivastava: Int. J. Hydrogen Energy21 (1996) 111.

[6] S.V. Mitrokhin, V.N. Verbetsky, H. Cunmao, Z. Yufen: Z. Phys.Chem. 181 (1993) 283.

[7] G.D. Sandrock, P.D. Goodell: J. Less-Common Met. 73 (1980)161.

[8] D. Khatamian, G.C. Weatherly, F.D. Manchester, C.B. Alcock:J. Less-Common Met. 89 (1983) 71.

[9] T. Hirata: J. Less-Common Met. 107 (1985) 23.[10] T. Hirata: Z. Phys. Chem. N. F. 164 (1989) 1331.[11] G. Liang, R. Schulz: Mater. Trans. 42 (2001) 1593.[12] L. Zaluski, S. Hosatte, P. Tessier, D.H. Ryan, J.O. Strom-Olsen,

M.L. Trudeau, R. Schulz: Z. Phys. Chem. 183 (1994) 45.[13] L. Zaluski, A. Zaluska, J.O. Strom-Olsen: J. Alloys Compd. 253–

254 (1997) 70.[14] K. Aoki, H. Aoyagi, A. Memezawa, T. Matsumoto: J. Alloys

Compd. 203 (1994) L7.[15] T. Kuji, Y. Matsumura, H. Uchida, T. Aizawa: J. Alloys Compd.

330–332 (1997) 718.[16] H. Uchida, A. Hisano, K. Terao, N. Sato, A. Nagashima: J. Less-

Common Met. 172 –174 (1991) 1018.[17] H. Uchida, M. Ozawa: Z. Phys. Chem. N.F. 147 (1986) 77.[18] H. Uchida, Y. Ohtani, M. Ozawa, T. Kawahata, T. Suzuki:

J. Less-Common Met. 172 –174 (1991) 983.[19] H. Inui, T. Yamamoto, M. Hirota, M. Yamaguchi: J. Alloys and

Compd. 330 –332 (2002) 117.[20] H. Hotta, M. Abe, T. Kuji, H. Uchida: J. Alloys and Compd. 439

(2007) 221.[21] K. Ito, H. Sasaki, H.T. Takeshita, K. Mori, T. Fukunaga: J. Alloys

Compd. 404 –406 (2005) 95.

(Received November 14, 2007; accepted February 29, 2008)

Bibliography

DOI 10.3139/146.101669Int. J. Mat. Res. (formerly Z. Metallkd.)99 (2008) 5; page 507 –512# Carl Hanser Verlag GmbH & Co. KGISSN 1862-5282

Correspondence address

Professor Dr. Hirohisa UchidaDepartment of Energy Science and EngineeringSchool of EngineeringTokai University,1117 Kita-Kaname, Hiratsuka-City, Kanagawa 259-1292, JapanTel.: +810463 581211 (ext. 4000)Fax: +81 0463 593581E-mail: [email protected]

You will find the article and additional material by enter-ing the document number MK101669 on our website atwww.ijmr.de



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