Direct synthesis of Mg-Ti-H FCC hydrides from MgH2 and Ti by means of ball milling

Article abstract of DOI:10.1016/j.jallcom.2009.02.152

MgH₂ and Ti were milled by means of ball milling and Mg-Ti-H hydride phases with a face centered cubic (FCC) structure were successfully synthesized. The Mg-Ti-H FCC hydride phases synthesized had chemical formulae of Mg₄₀Ti₆₀

Full text of DOI:10.1016/j.jallcom.2009.02.152

Journal of Alloys and Compounds 481 (2009) L8–L11
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Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Letter
Direct synthesis of Mg–Ti–H FCC hydrides from MgH and Ti by means of
2
ball milling
∗
Kohta Asano , Etsuo Akiba
National Institute of Advanced Industrial Science and Technology (AIST), AIST Central-5, 1-1-1, Higashi, Tsukuba, Ibaraki 305-8565, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
MgH2 and Ti were milled by means of ball milling and Mg–Ti–H hydride phases with a face centered
cubic (FCC) structure were successfully synthesized. The Mg–Ti–H FCC hydride phases synthesized had
chemical formulae of Mg40Ti60H113 and Mg29Ti71H57. The hydrogen contents of the Mg40Ti60H113 and
Mg29Ti71H57 FCC hydride phases corresponded to 2.9 and 1.4 mass%, respectively.
Received 5 February 2009
Received in revised form 20 February 2009
Accepted 24 February 2009
Available online 17 March 2009
©
2009 Elsevier B.V. All rights reserved.
Keywords:
Magnesium–titanium hydrides
Crystal structure
Mechanical alloying
Ball milling
Hydrogen storage
1. Introduction
covered with Pd absorbed hydrogen by means of both electrochem-
ical method and gas–solid reaction [6–11]. They reported that the
Formation of Mg–Ti–H hydrides has been reported lately
1–16] and high hydrogen storage capacity on a weight basis has
crystal structure of MgxTi1
–H hydrides resembled to the TiH2
00−x
[
FCC phase in the region of 55 ≤ x ≤ 87 and the MgH phase with a
2
been expected for them because Mg and Ti have the density of
body centered tetragonal (BCT) structure in the region of 90 ≤ x ≤ 95
−
3
−3
1
.74 Mg m and 4.54 Mg m [17] and form hydrides of MgH2 and
[9,10]. The pressure-optical-transmission isotherms of MgxTi
100−x
TiH , respectively. Mg and Ti have a hexagonal close packed (HCP)
(60 ≤ x ≤ 89) thin films were measured and MgxTi
–H hydrides
2
100−x
structure at ambient temperatures and Ti has a body centered cubic
were transparent [8] and similar in stability to MgH2 thin film [19].
Liang et al. [20] have synthesized MgxTi1 alloys at x = 80 and
(
BCC) structure above 1155 K [18]. Solubility of Mg to Ti and Ti to Mg
00−x
is less than 2 at.% and no Mg–Ti intermetallic compound is found in
the reported binary phase diagram of the Mg–Ti system [18]. There-
fore, synthesis of a Mg–Ti alloy seemed to be difficult especially
using the melting method.
90 by means of ball milling. These alloys consisted of the Mg–Ti
HCP phase and the Ti phase. It was reported that the HCP phase was
formed by solution of Ti into Mg. Mg80Ti20 alloy absorbed hydrogen
at 563 K and decomposed to MgH2 and TiH . On the other hand,
2
We have successfully synthesized MgxTi1
alloys with BCC
Kalisvaart et al. [14,15] have synthesized MgxTi
alloys in the
00−x
100−x
structure in the region of 25 ≤ x ≤ 65 [1–5], face centered cubic (FCC)
structure in the region of 35 ≤ x ≤ 80 [4] and HCP structure in the
region of 65 ≤ x ≤ 80 [4] by means of ball milling. We have reported
that the Mg–Ti BCC phase is metastable because it decomposed to
Mg and Ti above 673 K [2]. Hydrogen absorption by Mg–Ti BCC, FCC
and HCP alloys has been also observed and the Mg–Ti–H hydride
phases with a FCC structure were formed from these alloys and
gaseous hydrogen [3,4].
region of 65 ≤ x ≤ 85 by means of ball milling which consisted of
the FCC and HCP phases. Hydrogen absorption by these alloys was
observed using the electrochemical method. The hydrogen content
of the HCP phase was 1.7 times higher than that of the FCC phase
at x = 65 [15].
Kyoi et al. [16] have synthesized Mg7Ti–H (Mg87.5Ti12.5–H) FCC
hydride using a high pressure anvil cell by reacting a mixture
of MgH2 and TiH1.9 . This FCC hydride phase had the Ca7Ge type
superstructure which consisted of doubled FCC substructure. It
was reported that the temperature of hydrogen desorption from
Mg7Ti–H FCC hydride was lower than that of MgH2 and TiH1.9
hydrides.
Notten and co-workers [6–11] and Bao et al. [12,13] have syn-
thesized Mg–Ti thin films by the sputtering method. MgxTi1
00−x
(
50 ≤ x ≤ 95) thin films had a HCP structure [6–11]. These thin films
As far as we know, direct synthesis of Mg–Ti–H hydrides by
means of ball milling has never been reported. Choi et al. [21] have
milled MgH2 and TiH2 but no hydride was found in the milled
∗ Corresponding author. Tel.: +81 29 861 4410; fax: +81 29 861 4410.
E-mail address: k.asano@aist.go.jp (K. Asano).
0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2009.02.152

K. Asano, E. Akiba / Journal of Alloys and Compounds 481 (2009) L8–L11
L9
products. On the other hand, Sun et al. [22] have reported that
Mg dissolved in Ti during ball milling of Mg and Ti powders and
MgxTi1
HCP alloys were formed in the region of 4 ≤ x ≤ 24. We
00−x
have demonstrated that Mg dissolved in Ti and the crystal struc-
ture of Ti where Mg solved transformed from the HCP structure to
the BCC structure in the ball milling process of Mg50Ti50 BCC alloy
[
4,5]. It seems that Mg–Ti–H hydrides are not formed by milling of
hydrides but there must be possibility to synthesize from MgH2 as
a solute and Ti as a solvent for Mg and hydrogen.
In the present work, we have tried to synthesize Mg–Ti–H
hydrides by making solution of both Mg and hydrogen into Ti using
the ball milling method. Our concept is that Mg–Ti–H hydrides are
possibly synthesized by milling of MgH2 and Ti. Shang et al. [23]
have milled MgH and Ti in a molar ratio of 92:8. They observed the
2
Fig. 2. X-ray diffraction pattern of Mg50Ti50 BCC alloy hydrogenated under a hydro-
MgH , Mg and TiH phases by X-ray diffraction (XRD) measurement
gen pressure of 8 MPa at 423 K (modified from the previous work [3]).
2
2
after ball milling. Ti slightly dissolved in MgH2 in their ball milling
process because the unit cell volume of MgH2 reduced by solution
of Ti which has smaller atomic radius compared to Mg [17]. Liang
et al. [24] have milled MgH2 in addition to 5 at.% of Ti as a catalyst
3
. Results and discussion
Fig. 1 shows the XRD patterns of Mg50Ti50 BCC [3–5], Mg50Ti50
for hydrogen absorption and desorption of MgH . The hydrogen
2
FCC [4] and Mg80Ti20 HCP [4] alloys synthesized by means of
ball milling. The lattice parameters of those three alloys were
determined to be a = 0.342(1) nm for Mg50Ti50 BCC alloy [3–5],
a = 0.431(7) nm for Mg50Ti50 FCC alloy [4] and a = 0.317(7) nm and
c = 0.513(9) nm for Mg80Ti20 HCP alloy [4] from the observed peak
positions, respectively. Mg50Ti50 BCC alloy was hydrogenated under
a hydrogen pressure of 8 MPa at 423 K in a stainless steel auto-
clave. The hydrogen content of the product was determined to be
absorption and desorption kinetics of MgH2 was enhanced by Ti
addition and Mg–Ti–H hydride phase was not found in the XRD
pattern of the milled product. In order to dissolve Mg and hydrogen
in Ti the Ti content was not sufficient in their experiments.
In the present work, the powders of MgH and Ti in molar ratios
2
of 50:50 and 80:20 have been ball milled. The crystal structure and
the chemical composition of the milled products will be presented.
5
.0 mass% using the volumetric method [3]. Fig. 2 shows the XRD
2.
Experimental
pattern of Mg50Ti50 BCC alloy hydrogenated at 423 K [3]. Diffrac-
tion peaks of the FCC and MgH hydride phases were observed. The
The powders of MgH2 (98 mass%) purchased from Johnson Matthey and Ti
2
(
99.9 mass%) purchased from Wako Pure Chemical were weighed in a globe box
metallic molar ratio of Mg and Ti was fixed at 50:50 and the phase
abundance of the FCC and MgH2 hydride phases was refined by the
Rietveld method [3]. The chemical formula and lattice parameter
of the FCC hydride phase were determined to be Mg42Ti58H177 and
a = 0.449(7) nm, respectively [3]. Fig. 3 shows the XRD patterns of
Mg50Ti50 BCC, Mg50Ti50 FCC and Mg80Ti20 HCP alloys hydrogenated
filled with purified argon. The ball milling of MgH2 and Ti powders in molar ratios
of 50:50 and 80:20 was performed using a Fritsch P5 planetary ball mill for 200 h.
The rotation speed was fixed at 200 rpm. The product obtained from the mixture of
MgH2 and Ti in a molar ratio of 50:50 was designated as 50–50 and that from the
mixture in a molar ratio of 80:20 as 80–20, respectively.
Hydrogen content of the products was measured by a LECO RH-402 hydrogen
determinator. The powder X-ray diffraction (XRD) was measured using a Rigaku
2
500 V diffractometer with Cu K␣ radiation. The diffraction patterns were analyzed
by the Rietveld refinement program RIETAN-2000 [25,26].
Fig. 3. X-ray diffraction patterns of Mg50Ti50 BCC, Mg50Ti50 FCC and Mg80Ti20 HCP
alloys hydrogenated under a hydrogen pressure of 4.5 MPa at 773 K (modified from
the previous work [4]).
Fig. 1. X-ray diffraction patterns of Mg50Ti50 BCC [3–5], Mg50Ti50 FCC [4] and
Mg80Ti20 HCP [4] alloys (modified from the previous work [3–5]).

L10
K. Asano, E. Akiba / Journal of Alloys and Compounds 481 (2009) L8–L11
the present work, the synthesis process of the 50–50 and 80–20
products has been discussed on the assumption that the products
consist of the observed phases by XRD because the amorphization
induced by ball milling of MgH2 and Ti was not observed [23,24].
The crystal structure of the metal lattice of the FCC and Mg
phases in the 50–50 product and that of the FCC and MgH2 phases
in the 80–20 product were refined by the Rietveld method. A cubic
structural model of the space group Fm3m was employed for the
FCC phase, a hexagonal structural model of P6 /mmc for the Mg
3
phase and a tetragonal structural model of P4 /mnm for the MgH2
2
phase. In case of the 50–50, the metallic molar ratio of Mg:Ti = 50:50
was fixed and the phase abundance of the FCC and Mg phases was
refined. The metal composition of the FCC phase was determined to
be Mg33Ti50 (Mg:Ti = 40:60). The mass fraction of the FCC phase and
the Mg phase was 89:11 which corresponded to the mole fraction
of 1:17. The refined XRD pattern of the 50–50 product is shown in
Fig. 5(a). On the assumption that the FCC phase contained hydro-
gen which was observed from the 50–50 product using a hydrogen
determinator, the synthesis process of the FCC hydride phase by
means of ball milling was expressed as follows according to the
results of Rietveld analysis.
Fig. 4. X-ray diffraction patterns of 50–50 and 80–20 products.
5
0MgH + 50Ti → Mg33Ti50H94 (FCC) + 17Mg
(1)
2
Hydrogen in the Mg33Ti50H94 phase was provided from MgH2
which is a hydrogen source. The hydrogen content of the 50–50
product was lower than that of MgH2 powder before milling
because a small amount of hydrogen in MgH2 was released from
the product during milling in an argon atmosphere. Mg33Ti50H94 is
rewritten to be Mg40Ti60H113 (H/M = 1.13) and the hydrogen content
is 2.9 mass%.
In the same way, the metal composition of the FCC phase in the
8
0–20 product was determined to be Mg Ti (Mg:Ti = 29:71) and
8 20
the mass fraction of the FCC phase and the MgH2 phase was 39:61.
The mole fraction of those phases was 1:72. The refined XRD pat-
tern of the 80–20 product is shown in Fig. 5(b). Hydrogen observed
from the 80–20 product using a hydrogen determinator should be
contained in the FCC and MgH2 phases. The synthesis process of
the FCC hydride phase was expressed as follows.
Fig. 5. Result of Rietveld refinement of X-ray diffraction patterns for (a) 50–50 and
b) 80–20 products.
8
0MgH + 20Ti → Mg Ti20H16 (FCC) + 72MgH2
(2)
2
8
(
Mg Ti20H16 is rewritten to be Mg29Ti71H57 (H/M = 0.57) and the
8
under a hydrogen pressure of 4.5 MPa at 773 K [4]. Diffraction peaks
hydrogen content is 1.4 mass%. The crystal structural data of the
metal lattice in the 50–50 and 80–20 products was summarized
in Table 1 in comparison with that of hydrogenated Mg50Ti50
BCC alloy [3]. The lattice parameters of the Mg40Ti60H113 FCC and
Mg29Ti71H57 FCC phases were a = 0.445(3) nm and 0.445(0) nm,
respectively. These values are larger than a = 0.440 ± 0.0005 nm
which is the lattice parameter of Ti hydride with a FCC structure
in the region of H/M ≤ 1.56 [27]. This indicates that the FCC phases
synthesized in the present work contain Mg because Mg has larger
atomic radius compared to Ti [17]. We have successfully synthe-
sized Mg–Ti–H FCC hydrides by means of direct milling of MgH2
and Ti powders as well as hydrogenation of Mg50Ti50 BCC, Mg50Ti50
FCC and Mg80Ti20 HCP alloys [3,4].
of the FCC and MgH hydride phases were also found in the diffrac-
2
tion patterns of FCC and HCP alloys after hydrogenation.
The milled products 50–50 and 80–20 which were prepared
from MgH2 and Ti powders contained hydrogen and the hydrogen
contents of them were determined to be 2.6 mass% (H/M = 0.94) and
5
.0 mass% (H/M = 1.43) using a hydrogen determinator, respectively.
Fig. 4 shows the XRD patterns of the 50–50 and 80–20 products. In
the diffraction pattern of the 50–50 product, diffraction peaks of the
FCC phase were observed in addition to small peaks of Mg. It is clear
that a part of MgH2 was dehydrogenated by milling. In the diffrac-
tion pattern of the 80–20 product, diffraction peaks of the FCC and
MgH2 phases were observed and the Mg phase was not found. In
Table 1
Crystal structures and lattice parameters of Mg–Ti–H hydrides synthesized by direct milling of MgH2 and Ti and by hydrogenation of Mg50Ti50 BCC alloy [3].
Phase
Composition of metal
Structure
Space group
Lattice parameter (nm)
Metal mass fraction
Composition of hydride
a
c
5
(
8
(
0–50 product
50MgH2 + 50Ti)
0–20 product
80MgH2 + 20Ti)
FCC
Mg
FCC
MgH2
FCC
MgH2
Mg40Ti60
Mg
Mg29Ti71
Mg
Mg42Ti58
Mg
Cubic
Hexagonal
Cubic
Tetragonal
Cubic
Fm3m (No. 225)
P63/mmc (No. 194)
Fm3m (No. 225)
P42/mnm (No. 136)
Fm3m (No. 225)
P42/mnm (No. 136)
0.445(3)
0.321(2)
0.445(0)
0.451(8)
0.449(7)
0.451(5)
–
89
11
39
61
91
9
Mg40Ti60H113
Mg
Mg29Ti71 H57
MgH2
0.521(9)
–
0.300(5)
–
0.301(8)
Hydrogenated
Mg50Ti50 BCC [3]
Mg42Ti58H177
MgH2
Tetragonal

K. Asano, E. Akiba / Journal of Alloys and Compounds 481 (2009) L8–L11
L11
FCC hydride phase was also synthesized by milling in a molar ratio
of MgH :Ti = 80:20 and the metal composition was determined
2
to be Mg:Ti = 29:71. The hydrogen contents of the Mg40Ti60 and
Mg29Ti71 FCC hydride phases were H/M = 1.13 (2.9 mass%) and 0.57
(1.4 mass%), respectively, and the chemical formulae of those two
FCC hydride phases were Mg40Ti60H113 and Mg29Ti71H57.
Acknowledgements
The authors would like to thank Dr. Y. Nakamura of National
Institute of Advanced Industrial Science and Technology (AIST)
for her valuable discussion. This work is supported by The
New Energy and Industrial Technology Development Organization
(NEDO) under “Development for Safe Utilization and Infrastructure
of Hydrogen”.
References
Fig. 6. Lattice parameters of Mg–Ti–H FCC hydrides.
[
[
1] Y. Tsushio, S.J. Choi, H. Enoki, E. Akiba, Japanese Patent No. P2003-253360A,
003.
2] Y. Tsushio, S.J. Choi, H. Enoki, E. Akiba, Collected Abstracts of the 2002 Spring
Meeting of the Japan Inst. Metals, 2002, p. 499.
2
Fig. 6 shows lattice parameters of Mg–Ti–H FCC hydrides. The
lattice parameter increases with increase of the Mg content, x,
because of the difference in atomic radii between Mg and Ti [17]. In
addition, absorbed hydrogen atoms occupy interstitial sites in metal
crystal lattice and the lattice size increases by hydrogen absorption
[3] K. Asano, H. Enoki, E. Akiba, J. Alloys Compd 478 (2009) 117.
[
[
4] K. Asano, H. Enoki, E. Akiba, J. Alloys Compd., doi:10.1016/j.jallcom.2009.01.086,
in press.
5] K. Asano, H. Enoki, E. Akiba, Acta Mater., submitted for publication.
[
28–30]. The lattice parameters of Mg40Ti60H113 , Mg29Ti71H57 and
[6] R.H.A. Niessen, P.H.L. Notten, Electrochem. Solid-State Lett. 8 (2005) A534.
[
[
7] P. Vermeulen, R.A.H. Niessen, P.H.L. Notten, Electrochem. Commun. 8 (2006) 27.
8] R. Gremaud, C.P. Broedersz, D.M. Borsa, A. Borgschulte, P. Mauron, H. Schreuders,
J.H. Rector, B. Dam, R. Griessen, Adv. Mater. 19 (2007) 2813.
Mg42Ti58H177 [3] FCC hydrides are larger than that of MgxTi
100−x
(
x = 35 and 50) FCC alloys [4] but smaller than that of TiH reported
2
as a = 0.45345 nm [31]. The hydrogen contents of the Mg40Ti60H113
Mg29Ti71H57 and Mg42Ti58H177 [3] phases correspond to H/M = 1.13,
.57 and 1.77. It was reported that the hydrogen contents of the
–H (x = 70 and 80) [7,10] and Mg7Ti–H [16] phases were
,
[9] D.M. Borsa, R. Gremaud, A. Baldi, H. Schreuders, J.H. Rector, B. Kooi, P. Ver-
meulen, P.H.L. Notten, B. Dam, R. Griessen, Phys. Rev. B 75 (2007) 205408.
10] P. Vermeulen, P.C.J. Graat, H.J. Wondergem, P.H.L. Notten, Int. J. Hydrogen Energy
[
0
33 (2008) 5646.
MgxTi1
00−x
[11] P. Vermeulen, H.J. Wondergem, P.C.J. Graat, D.M. Bosa, H. Schreuders, B. Dam,
R. Griessen, P.H.L. Notten, J. Mater. Chem. 18 (2008) 3680.
12] S. Bao, K. Tajima, Y. Yamada, M. Okada, K. Yoshimura, Appl. Phys. A 87 (2007)
21.
[13] S. Bao, K. Tajima, Y. Yamada, M. Okada, K. Yoshimura, Sol. Energy Mater. Sol.
Cells 92 (2008) 224.
14] W.P. Kalisvaart, H.J. Wondergem, F. Bakker, P.H.L. Notten, J. Mater. Res. 22 (2007)
640.
15] W.P. Kalisvaart, P.H.L. Notten, J. Mater. Res. 23 (2008) 2179.
[16] D. Kyoi, T. Sato, E. Rönnebro, N. Kitamura, A. Ueda, M. Ito, S. Katsuyama, S. Hara,
D. Noréus, T. Sakai, J. Alloys Compd. 372 (2004) 213.
17] Metal Databook Ver. 3, Japan Institute of Metals, 1993 (in Japanese).
18] A.A. Nayeb-Hashemi, J.B. Clark, Phase Diagrams of Binary Magnesium Alloys,
1988, p. 324.
19] A. Krozer, B. Kasemo, J. Less-Common Met. 160 (1990) 323.
20] G. Liang, R. Schulz, J. Mater. Sci. 38 (2003) 1179.
21] Y.J. Choi, J.L. Lu, H.Y. Sohn, Z.Z. Fang, J. Power Sources 180 (2008) 491.
H/M = 1.5, 1.8 and 1.6, respectively. The Mg–Ti–H hydride phases
possibly absorb much more hydrogen because the hydrogen con-
[
6
tents of them are lower than H/M = 2 of TiH . We tried hydrogen
2
absorption by the 50–50 product which contained the Mg40Ti60H113
phase under a hydrogen pressure of 8 MPa at 423 K. However, obvi-
ous hydrogen absorption could not be observed. Hydrogen atoms in
TiH2 occupy the tetrahedral site in the FCC lattice [32]. The hydro-
gen occupation in the Mg–Ti–H hydride phases is speculated to be
[
1
[
[
[
the tetrahedral site in the same FCC structure to TiH . Vermeulen
2
et al. [11] have proposed that hydrogen atoms occupy the tetrahe-
dral site in the Mg70Ti30–H FCC lattice because the thermal stability
of the Mg70Ti30–H hydride is attributed to that of hydrogen atoms
in the tetrahedral site. In order to refine hydrogen positions in the
metal sublattice, neutron diffraction studies on the Mg–Ti–H FCC
hydrides are underway.
[
[
[
[22] F. Sun, F.H. (Sam) Froes, J. Alloys Compd. 340 (2002) 220.
[23] C.X. Shang, M. Bououdina, Y. Song, Z.X. Guo, Int. J. Hydrogen Energy 29 (2004)
7
3.
24] G. Liang, J. Huot, S. Boily, A. Van Neste, R. Schulz, J. Alloys Compd. 292 (1999)
47.
[25] F. Izumi, Rigaku J. 17 (1) (2000) 34.
26] http://homepage.mac.com/fujioizumi/.
27] P. Millenbach, M. Givon, J. Less-Common Met. 87 (1982) 179.
[28] Y. Fukai, The Metal–Hydrogen System, Springer-Verlag, 1995, p.21.
[29] E. David, J. Mater. Process. Technol. 162–163 (2005) 169.
30] M. Latroche, J. Phys. Chem. Solids 65 (2004) 517.
31] http://crystdb.nims.go.jp/.
32] K. Kai, S. Ikeda, T. Fukunaga, N. Watanabe, K. Suzuki, Physica 120B (1983) 342.
[
2
4
. Conclusions
[
[
Mg–Ti–H hydrides were directly synthesized from MgH2 and
Ti by means of ball milling. The FCC hydride and Mg phases were
[
[
[
synthesized by milling in a molar ratio of MgH :Ti = 50:50 and the
2
metal composition of the FCC hydride phase was determined to
be Mg:Ti = 40:60 by Rietveld refinement of the XRD pattern. The