Influence of TiB2 addition upon thermal stability and decomposition temperature of the MgH2 hydride of a Mg-based mechanical alloy

Article abstract of DOI:10.1016/j.jallcom.2007.10.125

Intensive mechanical milling was used to synthesize a MgH₂ (50 wt.%) + TiB₂ (50 wt.%) composite. Thermal stability of the composite versus H-desorption was studied employing the thermodesorption spectroscopy (TDS) method. The TDS data have revealed that TiB₂ addition decreases the dissociation temperature of the MgH₂ hydride by about 50 °C. An X-ray photoelectron spectroscopy analysis has shown that TiB₂ addition does not alter the surface chemical state of particles of the MgH₂ hydride, a component of the composite under consideration. The effect of decreasing decomposition temperature of MgH₂ due to addition of TiB₂ has been attributed to catalytic influence of a TiB₂ particle surface on processes of associative hydrogen desorption taking place on the surface of MgH₂ particles, as well as to a higher degree of dispersion of magnesium dihydride provided by the presence of TiB₂.

Full text of DOI:10.1016/j.jallcom.2007.10.125

Journal of Alloys and Compounds 465 (2008) 177–182
Influence of TiB2 addition upon thermal stability and decomposition
temperature of the MgH2 hydride of a Mg-based mechanical alloy
V.D. Dobrovolskya,∗, O.G. Ershovaa, Yu.M. Solonina, O.Yu. Khyzhuna, V. Paul-Boncourb
a Frantsevych Institute for Problems of Materials Science, National Academy of Sciences of Ukraine, 3 Krzhyzhanivsky str., UA-03142 Kyiv, Ukraine
b Chimie Me´tallurgique des Terres Rares, Institut de Chimie et des Mate´riaux de Paris-Est, Centre National de la Recherche Scientifique,
2/8 rue Henri-Dunant, F-94320 Thiais Cedex, France
Received 10 September 2007; received in revised form 24 October 2007; accepted 25 October 2007
Available online 4 November 2007
Abstract
Intensive mechanical milling was used to synthesize a MgH2 (50 wt.%) + TiB2 (50 wt.%) composite. Thermal stability of the composite versus
H-desorption was studied employing the thermodesorption spectroscopy (TDS) method. The TDS data have revealed that TiB2 addition decreases
the dissociation temperature of the MgH2 hydride by about 50 ◦C. An X-ray photoelectron spectroscopy analysis has shown that TiB2 addition does
not alter the surface chemical state of particles of the MgH2 hydride, a component of the composite under consideration. The effect of decreasing
decomposition temperature of MgH2 due to addition of TiB2 has been attributed to catalytic influence of a TiB2 particle surface on processes of
associative hydrogen desorption taking place on the surface of MgH2 particles, as well as to a higher degree of dispersion of magnesium dihydride
provided by the presence of TiB2.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Metal hydride; Thermal stability; Mechanical milling; Thermodesorption spectroscopy; X-ray photoelectron spectroscopy; Scanning electron microscopy
1. Introduction
milling does not change the thermodynamic properties of mag-
nesium hydride.
Analysis of recent publications shows that, a number of
researches have improved significantly hydrogen absorption–
desorption kinetics of Mg-based alloys. At the same time,
thermodynamic stability and decomposition temperature of Mg-
based hydrides are still too high for their practical utilization
as hydrogen accumulators. Therefore, scientists still continue
their attempts towards decreasing thermal stability of the MgH2
hydride due to its mechanical dispersion (i.e. decreasing parti-
cle sizes) [1–3] and due to addition of transition metals [4–6]
and their compounds with the VI and VII Group elements of the
Periodic Table [7–14]. The authors [1–3] have revealed that the
mechanical alloying of MgH2 (without addition of any transition
metals and their compounds) leads to improvement of hydro-
gen sorption/desorption kinetics because of the introduction of
the internal structure defects. However, pressure concentration
isotherms presented in Refs. [1–3] indicate that high-energy
The combination of the mechanically induced disordered
MgH2 structure with transition metal catalysts increases syner-
getic effects and excellent absorption/desorption properties [4].
The 3d-metal additions could reduce significantly the activa-
tion energy of hydrogen desorption [4]. Deledda et al. [7] have
showed that fluorine additions with Fe (and Ni) catalyst decrease
effectively the desorption temperature of MgH2 to about 227 ◦C.
In a series of works [8–14] the catalyst effect of cheap metal
oxides on hydrogen desorption properties of MgH2 has been
reported. Aguey-Zinsou et al. [15] have revealed that co-milling
MgH2 and MgO does not modify structural or thermodynamic
properties of the MgH2 hydride, but leads to a decrease of its
particle size and therefore improved hydrogen kinetics. How-
ever, the role of metal oxide catalyst on hydrogen desorption
properties has not been established so far.
The influence of milling with TiB2 has already been observed
in Ref. [16] for titanium dihydride and the maximum decreasing
temperature of hydride decomposition was found to be 230 ◦C
due to addition of 50 wt.% titanium diboride to TiH1.9. In the
present work, thermal stability of the mechanical alloy MgH2
∗
Corresponding author. Tel.: +380 44 424 33 64; fax: +380 44 424 21 31.
E-mail address: dobersh@ipms.kiev.ua (V.D. Dobrovolsky).
0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2007.10.125

178
V.D. Dobrovolsky et al. / Journal of Alloys and Compounds 465 (2008) 177–182
(50 wt.%) + TiB2 (50 wt.%) was studied employing the method
of thermodesorption spectroscopy (TDS) and the surface state
of the alloy was investigated using the X-ray photoelectron
spectroscopy (XPS) method. The influence of TiB2 upon decom-
position temperature of the MgH2 hydride of this alloy was
studied as well. Microstructures of the initial powder mixture
and the composite derived were studied by the scanning electron
microscopy (SEM) method.
2. Experimental details
In order to test the influence of TiB2 upon thermal stability of MgH2, a pow-
der mixture containing MgH2 (50 wt.%) and TiB2 (50 wt.%) was undergone by
high-energy ball milling in a spherical planetary mill (20 min, argon atmosphere,
rotation speed 1630 rpm). The ratio of steal balls weigh to initial burden’s weight
was equal to 20:1. The MgH2 hydride, a component of the mixture treated, was
derived by direct hydriding a magnesium powder (purity greater than 99.9%; an
average particle size was found to be 4 ␮m) in gaseous medium. The dihydride
MgH2 used in our investigation was synthesized from magnesium powder at
pressure 3.0 MPa and temperature 400 ◦C in hydrogen atmosphere. In order to
avoid air exposure of the MgH2 hydride used in the present experiments, after its
synthesis we reloaded MgH2 into special vials in a glove-box filled with argon.
Microstructures of the initial powder mixture and the composite MgH2
(50 wt.%) + TiB2 (50 wt.%) derived were studied using a Super-Probe 733 scan-
ning electron microscope. Chemical states of the above powder mixture and the
composite were studied by recording XPS O 1s, Mg 2p and Mg 2s core-level
spectra. The XPS measurements were carried out using an ES-2401 spectrom-
eter equipped with an ion-pumped chamber having a base pressure less than
5 × 10−8 Pa. The XPS spectra were excited by Mg K␣ radiation (E = 1253.6 eV).
The energy scale of the ES-2401 spectrometer was calibrated by setting the
measured Au 4f7/2 binding energy of pure gold to 84.0 eV with respect to the
Fermi energy of a spectrometer energy analyzer. Energy drift due to charging
effects was corrected by taking as a reference the XPS C 1s line of hydrocarbons
(285.0 eV).
Fig. 1. X-ray diffraction pattern of the mechanical alloy MgH2 (50 wt.%) + TiB2
(50 wt.%) prepared by mechanical milling for 20 min.
and TiB2 (50 wt.%) and of the mechanical alloy obtained after
mechanical alloying the mixture for 20 min in argon atmosphere
are presented in Fig. 2. From the SEM patterns presented in
Fig. 2(a) it is obvious that particles of magnesium dihydride
derived by direct hydriding of magnesium in gaseous medium
possess nonspherical shapes with friable surfaces and sharp
angles while smoothed surface and round-off shapes are char-
An X-ray diffraction (XRD) analysis of the MgH2 (50 wt.%) and TiB2
(50 wt.%) powder mixture treated for 20 min in argon atmosphere was carried
out with a DRON-3M programming diffractometer employing X-ray Cu K␣
radiation and a graphite monochromator. The present XRD measurements were
made with a step of 0.1o and accumulative time was 10 s in every point.
Thermal stability of the MgH2 phase, a component of the composite under
consideration, was studied using the TDS method employing a digital device
allowing to evaluate quantities of hydrogen desorbing from a specimen heated
either in hydrogen or in argon atmosphere at normal pressure. For comparison,
the method was employed to investigate thermal stability of MgH2 (without the
presence of TiB2) treated at the same conditions. The TDS spectra were mea-
sured using Siverts’ method as follows. An autoclave with the sample (m = 0.2 g)
was attached to a volumetric device equipped with a piston mechanism; displace-
ments of the latter were measured using a sensor calibrated on changes of volume
of gas in the system. The autoclave with the sample was placed into a quartz
reactor equipped with a K-type thermocouple and heated by an electric furnace
connected to a controller and providing uniform heating the samples with speed
5 ◦C/min in the present experiments. The thermocouple and the displacement
sensor were connected to a data acquisition system. After loading the sample,
the system was evacuated by a rotary pump. The system was filled with hydro-
gen or argon prior to starting desorption experiments during the heating. After
the maximum temperature of TDS experiments was reached, the furnace was
switched off, and the reactor was cooled to room temperature.
3. Results and discussion
Fig. 1 shows that the XRD analysis of the mechanical alloy
MgH2 (50 wt.%) + TiB2 (50 wt.%) reveals the presence of three
phases, namely MgH2, TiB2 and MgO. Microstructures and
morphologies of particles of the initial mixture MgH2 (50 wt.%)
Fig. 2. SEM images of the composite MgH2 (50 wt.%) + TiB2 (50 wt.%) mate-
rial: (a) before (100×) and (b) after mechanical dispertion (10,000×).

V.D. Dobrovolsky et al. / Journal of Alloys and Compounds 465 (2008) 177–182
179
Fig. 3. Curves (panels a and c) and spectra (panels b and d) of thermodesorption of hydrogen from the MgH2 hydride of the MgH2 (50 wt.%) + TiB2 (50 wt.%)
composite (panels a and b) and mechanically treated MgH2 hydride (panels c and d).
acteristic for TiB2 particles. A morphology of MgH2 particles
was undergone to dramatic changes due to intensive dispersion
in the high-speed planetary mill: there was a significant quan-
tity of nano-size MgH2 particles, with almost spherical shapes,
covering TiB2 particles (the latter particles did not change their
morphology due to such a dispersion in the mill; see Fig. 2(b)).
High-energy influence on the powder mixture containing MgH2
(50 wt.%) and TiB2 (50 wt.%) leads to increasing its specific sur-
face from 0.7 up to 4.87 m2/g. Taking into account this value of
the specific surface, an average size of powder particles for the
composite synthesized was calculated to be 0.4 ␮m. The aver-
age particle size was calculated from the data of specific surface
area (SSA) by following equation SSA = 6/ρdp, where ρ is the
density of the material and dp is the average diameter of the
particles, as suggested in Ref. [17].
the MgH2 (50 wt.%) + TiB2 (50 wt.%) composite and at 445 ◦C
for the as-milled MgH2. Several weak peaks on the desorption
spectra presented in Fig. 3(b) and (d), in the temperature range
from 50 to 300 ◦C are artefact of the measurements. Comparison
of the desorption curves on the same temperature scale (Fig. 4)
allows to conclude that addition of TiB2 to MgH2 during milling
leads to decreasing the decomposition temperature of MgH2 by
50 ◦C.
An analogous effect of decreasing the decomposition tem-
perature of titanium dihydride was detected previously when
milling the TiH1.9 hydride for 20 min in argon atmosphere [16].
From comparison of the TDS curve of the mechanically treated
TiH1.9 (50 wt.%) + TiB2 (50 wt.%) mixture with the curve of
Thermal stability of the MgH2 phase in the composite MgH2
(50 wt.%) + TiB2 (50 wt.%) was studied using the TDS method.
For comparison, the method was also employed to investigate
thermal stability of MgH2 treated at the same conditions and
without alloying with TiB2. Results of these studies are pre-
sented in Fig. 3 as desorption curves (i.e. dependencies of
the volume of hydrogen releasing from the specimen upon
its temperature; panels a and c) and as desorption spectra
(dV/dT dependencies; panels b and d) that were derived by
heating the samples with the rate 5 ◦C/min in hydrogen atmo-
sphere. The desorption spectra presented in Fig. 3 reveal that
an intensive release of hydrogen begins in the composite MgH2
(50 wt.%) + TiB2 (50 wt.%) and in the as-milled MgH2 at tem-
peratures 350 and 410 ◦C, respectively. Additionally, from the
desorption spectra shown in Fig. 3(b) and (d), it is apparent that a
maximum release of hydrogen occurs at temperatures 395 ◦C for
Fig. 4. Curves of hydrogen desorption from the MgH2 (50 wt.%) + TiB2
(50 wt.%) composite and the MgH2 hydride matched on the same temperature
scale.

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V.D. Dobrovolsky et al. / Journal of Alloys and Compounds 465 (2008) 177–182
Fig. 5. Curve (panel a) and spectrum (panel b) of thermodesorption of hydrogen from the MgH2 hydride of the MgH2 (50 wt.%) + TiB2 (50 wt.%) composite during
its heating in Ar atmosphere.
thermal desorption of the non-treated TiH1.9 hydride presented
in Ref. [16], one could see that mechanical alloying TiH1.9 with
TiB2 decreased additionally the decomposition temperature of
dispersedTiH1.9 hydrideby120 ◦C. Themaximumtotaleffectof
decreasing temperature of hydride decomposition corresponds
to 230 ◦C if we conclude about decomposition temperature of
TiH1.9 taking into account peaks of speed of a hydrogen release
on the desorption spectra [16].
Fig. 5 displays the curve (panel a) and spectrum (panel b) of
thermal desorption of hydrogen from the MgH2 hydride of the
MgH2 (50 wt.%) + TiB2 (50 wt.%) composite during its heat-
ing in argon atmosphere. On the above spectrum of hydrogen
thermodesorption, as in the case of desorption in hydrogen
atmosphere (Fig. 3), one can observe a strong peak at the
same temperature (395 ◦C). An XRD pattern of the MgH2
(50 wt.%) + TiB2 (50 wt.%) composite after hydrogen thermod-
esorption (the composite was heated in hydrogen atmosphere)
reveals the presence of Mg, MgO and TiB2 (Fig. 6).
The mechanism of influence of TiB2 upon the tempera-
ture decomposition of the MgH2 phase is not quite clear. The
high-energy milling of the MgH2 (50 wt.%) + TiB2 (50 wt.%)
composite does not lead to formation of a new hydride phase.
The decreasing thermal stability of MgH2 cannot be associated
with the influence of TiB2 upon surface chemical states of par-
ticles of the MgH2 hydride. The XPS Mg 2s, Mg 2p and O 1s
core-level spectra of the MgH2 (50 wt.%) + TiB2 (50 wt.%) com-
posite resemble those of mechanically treated MgH2 (Fig. 7).
Therefore, in the above two samples under consideration, there
are no differences in the chemical states of the MgH2 parti-
cles. It is noteworthy to mention that, XPS results reveal almost
complete absence of the Ti 2p and B 1s lines in the MgH2
(50 wt.%) + TiB2 (50 wt.%) composite under study. The above
observations can be explained by the fact that the MgH2 hydride,
which is more brittle and more dispersive as compared with
TiB2, will possess higher dispersion during milling. Therefore,
almost every TiB2 particle is covered by very thin layer consist-
ing of MgH2 nanoparticles. Another explanation of the absence
of Ti 2p and B 1s peaks in the XPS spectrum for TiB2 milled
with MgH2 is the insertion of titanium diboride into magne-
sium dihydride, the effect similar to that observed earlier by
Friedrichs et al. [18] for Nb2O5 milled with MgH2. Taking into
account the fact that TiB2 is dispersed in higher degree as com-
pared with MgH2, therefore TiB2 will promote higher degree of
dispersion of MgH2 in the composite decreasing its decomposi-
tion temperature [6,19]. In addition, decreasing decomposition
temperature of MgH2 can be attributed to catalytic properties
of surfaces of TiB2 particles with respect to processes of asso-
ciative hydrogen desorption on surfaces of MgH2 particles in
the MgH2 (50 wt.%) + TiB2 (50 wt.%) composite (as mentioned
above, such MgH2 particles come into contact with TiB2 parti-
cles). In this case, both titanium and boron atoms, components
of TiB2, can influence significantly on the mentioned process of
associative hydrogen desorption, taking part in the near-surface
layer, as the following. We assume that, high thermal stability of
MgH2 hydride is caused with the ionic component of the Mg–H
bonds. It is well known (see e.g., Refs. [20,21]) that, the com-
pounds formed by boron with magnesium or titanium posses
mainly covalent component of the chemical bonding and boron
can decrease the positive charge of magnesium atoms (and, con-
sequently, the ionic component of the Mg–H bonds). This leads
to decreasing thermal stability of the magnesium dihydride. The
above mechanism of boron influence on thermal stability of the
hydride phase follows the correlation between ionicity degree of
the metal–hydrogen bonds in a hydride and its thermal stability
observed earlier in Ref. [22].
For a number of hydride-forming alloys based on transition
and rare-earth metals it has been established [23–25] that their
XPS O 1s core-level spectra can be decomposed on two fea-
tures (lines) positioned at about 530 and 532 eV. The feature
Fig. 6. X-ray diffraction pattern of the MgH2 (50 wt.%) + TiB2 (50 wt.%)
mechanical alloy after desorption in H2.

V.D. Dobrovolsky et al. / Journal of Alloys and Compounds 465 (2008) 177–182
181
venting dissociative hydrogen chemisorption, adsorbed on the
particle surfaces. It is noteworthy to mention that Friedrichs et
al. [26] have shown that the XPS O 1s core-level spectrum of
MgH2 prepared by high-energy ball milling can be deconvoluted
into two peaks at ca. 531.4 and 533.2 eV for oxide and hydrox-
ide/water adsorbed species, respectively. Taking into account
energy drifts due to charging effects in the present work and in
Ref. [26], the maximum at ca. 532 eV of the XPS O 1s spectrum
in Fig. 7(a) can be attributed partly to amorphous hydroxide pas-
sivation layer on MgH2 particles. From Fig. 7(a) it is obvious
that the addition of 50 wt.% TiB2 to MgH2 decreases signif-
icantly the quantity of the amorphous hydroxide layer on the
magnesium dihydride particles.
4. Conclusion
Ball milling techniques were used to produce a composite
containing MgH2 (50 wt.%) and TiB2 (50 wt.%). The thermal
stability and decomposition temperature of powders of the com-
positewereevaluatedbytheTDSmethod. Ithasbeenestablished
that, high-energy treatment of the mixture of MgH2 synthesized
by directly hydriding magnesium in gaseous medium and TiB2
powders decreases the thermal stability of the MgH2 hydride.
The decomposition temperature of MgH2 decreases by 50 ◦C in
the case of the treatment of magnesium with TiB2 addition as
compared with the decomposition temperature of magnesium
treated at the same condition but without TiB2 addition. It has
been made a supposition about the origin of influence of TiB2
addition upon thermal stability of magnesium dihydride taking
into account results of the XPS analysis of surface chemical state
of dihydride particles.
Acknowledgement
Financial support of INTAS (project # 05-1000005-7671) is
gratefully acknowledged.
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