A. Teresiak et al. / Journal of Alloys and Compounds 481 (2009) 144–151
145
hedral sites. This is a new hydride type with mixed ionic–covalent
character, which represents an interesting new target for hydrogen
storage studies.
Moreover, it is known for several years that La–Mg–Ni is a glass-
forming system in the concentration ranges of 10–40 at.% Ni and
The algorithm of this analysis is given by the PANalytical X’Pert HighScore plus
software package 2.2 with
ꢀ
ꢂ
ꢁ
ꢂ
ꢃ
Di =
ꢂ
(1)
(2)
180
Wi − Wstd
and εi
5–30 at.% La with maximum glass-forming ability for La20Mg50Ni30
[
18,19]. Therefore, rapid quenching or ball milling may be also in this
(
Ui − Ustd) − (Wi − Wstd)
case suitable processing routes for amorphization and formation of
εi =
√
1
/100(180/ꢂ)4 2 ln 2
nanostructured LaMg Ni, which may exhibit enhanced hydrogen
2
The peak broadening FWHM (full width at half maximum) is defined as Hk with
sorption reactivity. But until now, the crystallisation behaviour of
La–Mg–Ni glasses has only been scarcely investigated.
ꢂ
Hk =
U tan2 ꢁ + V tan ꢁ + W
(3)
First studies regarding nanostructuring of LaMg Ni and its
2
hydrogenation behaviour have been conducted by Di Chio et al.
U, V and W are parameters, which are refined stepwise.
Ui contains the information about the strain broadening and the Wi parameter
contains a possible grain size broadening [23].
[
20,21,22]. They found that intensive milling of inductively molten
stoichiometric pre-alloys with LaMg Ni main phase and minor
2
Formation enthalpies and transformation enthalpies of the Mg-hydrides were
determined using the thermal desorption method coupled with a mass spectrometer
(Leybold) at heating rates of 5, 10 and 20 K/min. Heating rates of 5, 10, 20, 30, 40,
phases leads to strong grain refinement. Spex milling can enhance
the subsequent hydrogen absorption, but significantly reduces the
fraction of LaMgNiH7 due to preferential formation of impurity
phases. Spex milling under hydrogen was found to lead always to
60 and 100 K/min were used for the Kissinger analysis [24]. The pre-activation of
the ribbon surface was performed in 0.1 M HF solution for 30 s to eliminate passive
films.
LaH and an amorphous phase. Amorphous ribbons were obtained
2
High-energy ball milling was carried out for achieving a fine grained alloy
microstructures. A RETSCH PM4000 planetary ball mill and stainless steel vials were
used. The crushed master alloy powders were milled under Ar atmosphere up to 30 h
with 200 rpm using a ball to powder ratio of 10:1. High-energy ball milling under
hydrogen atmosphere was performed using similar milling parameters for achiev-
ing the complex hydride. A hydrogen pressure of 0.5 MPa was used. After 30, 60,
90, 120, 180, 300 and 450 min milling time the process was interrupted and a small
amount of the sample powder was taken for XRD measurements to follow the phase
reactions.
by rapid quenching, which crystallise under formation of various
phases – among them nanocrystalline LaMg Ni. However, limited
2
microstructural effects on the hydrogen absorption were detected.
In the present paper we report on our first studies on the
Mg50Ni25La25 and Mg50Ni30La20 alloys, which aim at preparing
the nanostructured LaMg Ni phase by following the rapid quench-
2
ing plus annealing route and the intensive milling route with a
planetary ball mill, respectively. Hydrogenation experiments were
conducted to obtain the LaMg NiH7 phase and the subsequent
3. Results and discussion
2
desorption behaviour was observed. We demonstrate that the for-
mation of these phases of interest depends very sensitively on the
3.1. Nanostructured LaMg2Ni by rapid quenching and annealing
alloy composition and processing conditions. Single phase LaMg Ni
2
was obtained in a one-step crystallisation of rapidly quenched
glassy La20Mg50Ni30 resulting in a mean grain size of 27 nm and
also by intensive milling of La25Mg50Ni25 leading to a mean grain
The XRD patterns of the master alloys reveal only the
equilibrium phase LaMg2Ni for the stoichiometric composition
La25Mg50Ni25 (Fig. 1a) and a phase mixture of LaMg2Ni, LaMg3
and La2Mg17 for the Ni-rich alloy La20Mg50Ni30 (Fig. 1b). Small
fractions of La2O3 were also detected in some cases. The average
size of 15 nm. Complete formation of LaMg NiH7 was achieved by
2
reactive milling of both alloys under hydrogen.
lattice constants and the unit cell volume of the LaMg Ni phase
2
2
.
Experimental details
were determined for samples of both alloy compositions (Table 1).
These estimated values are in a good agreement to the reported lat-
tice constants of Di Chio et al. [22]. However, the results differ from
the single crystal values, which were determined by Renaudin et
al. [16] and which are also given in Table 1. A deviation of the La
concentration in the unit cell is noticed, which might be due to the
additional consumption of La by the formation of La O . Thus, a
Master alloys with the nominal composition La25Mg50Ni25 and La20Mg50Ni30
were prepared by induction melting of the pure elements Ni and La and subse-
quent cold crucible casting of the re-molten binary alloys under addition of Mg.
This way losses of Mg due to evaporation could be minimized. Ribbons with a
width of about 4 mm and a thickness of 0.03 mm were prepared by single-roller
melt-spinning (BUEHLER melt spinner) under highly purified argon atmosphere.
The chemical compositions of the master alloys and of the melt-spun ribbons were
determined by inductively coupled plasma-optical emission spectroscopy (ICP-OES).
The methodical deviation in the composition is about 0.5 wt.%. The IRIS Advantage
High resolution device (Thermo Electron GmbH) was used.
2
3
range of homogeneity of LaMg Ni is assumed. The mean crystallite
2
DSC measurements were carried out using a NETZSCH and a Perkin Elmer DSC7
device under Ar atmosphere at a heating rate of 10 K/min.
The microstructure of the as-quenched samples was studied using transmission
and high resolution electron microscopy (HRTEM, Tecnai F30). EDX studies were
used to obtain the elemental composition of the investigated sample region.
The thermal behaviour was observed by using an in situ high temperature X-ray
attachment XRK900 (PAAR) and an X’Celerator semiconductor detector. The accu-
racy of the temperature was controlled by the calibration with various substances up
◦
to 900 C. The temperature stability was found to be ± 1.5 K. The temperature treat-
◦
ment was carried out with a heating rate of 10 K/min up to 100 C followed by steps
of 10 K to the next temperature with the same heating rate. The X-ray diffraction
measurements, using Cu-K␣ radiation, were started after a holding time of 2 min at
◦
the chosen temperature in the 2ꢁ region 20–100 and in a time window of only
about 12 min. Before heating, the ribbon samples were reduced to small pieces by
using a mortar. The crystallisation behaviour of the melt-spun samples was investi-
gated using 0.05 MPa helium atmosphere and the gas–solid reaction with hydrogen
was observed under 0.5 MPa hydrogen atmosphere at elevated temperatures. After
the heat treatment XRD patterns were recorded for the determination of the lattice
constants by using a X’Pert MPD system with a PW3040 diffractometer and Co-K␣-
radiation. For data analysis the PANalytical X’Pert HighScore Plus software package
2
.2 was used. The grain sizes were estimated from the XRD profile using the Pseudo
Fig. 1. XRD patterns of the master alloys La25Mg50Ni25 (a) and La20Mg50Ni30 (b)
Voigt function by means of a LaB6 standard.
obtained by inductive melting and subsequent cold crucible casting.