Preparation of alkali metal hydrides by mechanical alloying

Article abstract of DOI:10.1016/S0925-8388(01)01615-2

Rubidium and cesium hydrides are not commercialized and we have set up, a few years ago, a method of synthesis at the laboratory scale. It is based on the reaction of alkali metal with hydrogen obtained by thermal decomposition of uranium hydride UH₃ at a temperature of 450°C, which gives a pressure of hydrogen close to 3 bars. This synthesis leads to a very pure alkali metal hydride MH, but the rate of the reaction remains quite small: a few hundreds of milligrams in 24 h. A new method, based on mechanical alloying, consists in milling the alkali metal, at room temperature, under a pressure of hydrogen close to 5 bars. The reaction proceeds in 16 h and gives 3-15 g of very pure MH (from sodium to cesium, respectively) at once.

Full text of DOI:10.1016/S0925-8388(01)01615-2

Journal of Alloys and Compounds 329 (2001) L5–L8
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www.elsevier.com/locate/jallcom
Letter
Preparation of alkali metal hydrides by mechanical alloying
1
*
´
L. Elansari , L. Antoine, R. Janot, J.C. Gachon, J.J. Kuntz, D. Guerard
´
`
Laboratoire de Chimie du Solide Mineral (UMR 7555), UHP Nancy I, BP 239, 54506 Vandoeuvre-les-Nancy Cedex, France
Received 6 April 2001; accepted 14 May 2001
Abstract
Rubidium and cesium hydrides are not commercialized and we have set up, a few years ago, a method of synthesis at the laboratory
scale. It is based on the reaction of alkali metal with hydrogen obtained by thermal decomposition of uranium hydride UH3 at a
temperature of 4508C, which gives a pressure of hydrogen close to 3 bars. This synthesis leads to a very pure alkali metal hydride MH,
but the rate of the reaction remains quite small: a few hundreds of milligrams in 24 h. A new method, based on mechanical alloying,
consists in milling the alkali metal, at room temperature, under a pressure of hydrogen close to 5 bars. The reaction proceeds in 16 h and
gives 3–15 g of very pure MH (from sodium to cesium, respectively) at once.
2001 Elsevier Science B.V. All rights reserved.
Keywords: Gas–solid reactions; Mechanical alloying; X-ray diffraction
1. Introduction
is either a flow of gas in an open circuit at atmospheric
pressure, with the risk of explosion [2] or the gas coming
from the thermal decomposition of uranium hydride heated
at 4508C, which provides a pressure of 3 bars in a sealed
tube, with the problems connected to the radioactivity and
toxicity of uranium [3]. It is necessary to set up a new
synthesis method for quantities larger than 1 g.
The industrial preparation of the alkali metal hydrides is
based on the reaction of hydrogen on the molten metal
dispersed in a mineral oil stirred, (with generally addition
of dispersing agents, which leads to the presence of
impurities [4–6]), in order to renew the surface of the
metal. The mechanical alloying is precisely known to clean
continuously the surface of the reactants [7] and, then,
constitutes a chosen tool for such a synthesis. The me-
chanical alloying is also used for synthesis of several
hydrides with, in particular Mg, Ti, Zr [8,9].
The formation of the alkali metal hydrides is exothermic
(between 50 and 180 kJ/mole) [1], however the direct
synthesis by reaction of hydrogen on the molten metal is
very slow due to the protection role played by the hydride
layer, formed readily on the surface of the metal and which
slows down the diffusion of the metal vapor as well as
hydrogen. The rubidium and cesium hydrides are not
commercially available and should be synthesized at the
laboratory scale. There are two methods, based on the
reaction of hydrogen on the molten metal in the presence
of a temperature gradient: the metal is heated at around
2508C, temperature high enough to decompose partially
the hydride formed on its surface. The metal vapor
(pressure around 5 Torr), in equilibrium with hydrogen and
the free metal distillates towards the cold part of the
apparatus, whose temperature is 1508C. Close to the cold
wall, it reacts with hydrogen and the formed hydride is
deposited on that cold part. At that temperature, the
decomposition pressure of the hydride is negligible: around
1022 Torr. The reaction rate is limited due to the low metal
vapor pressure and the yield is about 400 mg per day for 1
cm2 of surface of the molten metal. The hydrogen source
2. Experimental
The balls and alkali metal are introduced in the vial
under the purified argon atmosphere of a glove box. The
desired hydrogen pressure is adjusted, after removal of the
argon, at a maximal value of 30 bars. Taking into account
the volume of the vial and that of balls and alkali metal,
the hydrogen volume is around 7 l (NTP). Practically, the
initial amounts are generally as follows: 2 g of alkali
*
Corresponding author. Tel.: 133-383-91-21-07; fax: 133-383-91-24-
47.
1
´
Permanent address: Laboratoire de Chimie du Solide Mineral, Uni-
´
er
versite Mohamed 1 Oujda, Maroc.
0925-8388/01/$ – see front matter
PII: S0925-8388(01)01615-2
2001 Elsevier Science B.V. All rights reserved.

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L. Elansari et al. / Journal of Alloys and Compounds 329 (2001) L5–L8
Fig. 1. XRD pattern of alkali metal hydride powders.

L. Elansari et al. / Journal of Alloys and Compounds 329 (2001) L5–L8
L7
Table 1
X-ray data on the alkali metal hydrides (l MoKa50.70926 A)
˚
hkl
NaH
KH
RbH
CsH
dhkl
(A)
Ihkl /I111
exp.(%)
Ihkl /I111
calc.(%)
dhkl
(A)
Ihkl /I111
exp.(%)
Ihkl /I111
calc.(%)
dhkl
(A)
Ihkl /I111
exp.(%)
Ihkl /I111
calc.(%)
dhkl
(A)
Ihkl /I111
exp.(%)
Ihkl /I111
calc.(%)
˚
˚
˚
˚
111
200
220
311
222
400
331
420
2.822
2.445
1.727
1.470
1.409
1.220
1.117
1.089
0.994
0.938
100.0
79.3
35.0
24.4
9.8
2.4
7.3
3.7
,1
100.0
76.9
36.1
23.5
25.2
13.2
9.8
8.9
5.1
5.1
3.30
2.86
2.02
1.72
1.65
1.426
1.31
1.276
100.0
78.8
35.0
25.0
7.5
2.5
7.5
5.0
100.0
76.6
36.4
24.6
22.0
15.0
12.0
8.7
3.48
3.02
2.13
1.82
100.0
96.0
375.0
9.3
100.0
76.6
36.4
24.6
3.65
3.19
2.27
1.92
1.82
1.59
1.46
1.42
1.30
1.23
1.13
1.08
100.0
54.0
49.2
45.4
13.1
62.0
18.5
15.4
9.2
1
,1
,1
˚
100.0
79.7
43.2
31.6
28.9
21.4
17.7
13.6
11.9
11.9
9.7
422
511–333
440
,1
531
8.7
]
˚
˚
˚
a
4.8860.01 A
a54.883 A
5.7160.01 A
a55.714 A
6.0360.01 A
a56.049 A
6.3660.06 A
˚
a56.388 A
˚
˚
˚
After [10]
metal, 10 steel balls (diameter 10 mm), hydrogen pressure
between 5 and 7 bars.
Table 1. First of all, the a parameters calculated from the
dhkl distances are in good agreement with those given in
the literature [10], the only observed reflections are those
for which all Miller indices are of the same parity,
characteristic of FCC symmetry.
The vial is then placed on a planetary mill (Retsch PM
400). An initial low rotation rate (around 50 rpm) is kept
for 15 min in order to avoid a too-large spread-out of the
alkali metal on the wall and cover of the vial. During that
period, a small quantity of hydride is formed, which
presents two interesting features: firstly there is a modi-
fication of the surface tension of the metal and secondly
the hydride acts as a catalyst for further hydridation. After
that initial period, the rotation is progressively increased
(in 2 h) up to 250 rpm and that rate is kept for 12 h. At this
step of the synthesis, the hydride is a greyish powder
which contains small quantities of free metal finely
divided. It is thus necessary, before the opening of the vial,
to place it in an oven at 1208C for 1 h. The dispersed metal
is rapidly transformed in a pure MH hydride with a M/H
ratio equal to 160.02 as determined by chemical analysis
(reaction with water and acidimetry coupled with the
hydrogen evolved volume). One has to note that the XRD
performed on the greyish powder exhibits a pure MH
diagram, which means that the free metal is actually very
finely dispersed and, in that case, the eye is a better tool
than X-rays.
The experimental intensities fit fairly well with that
]
calculated on the basis of a Fm3m unit cell with M in
position 4a and H in position 4b, except for RbH which
presents a large discrepancy between calculated and ex-
perimental intensity values. This fact can be easily ex-
plained by the experimental conditions: the wavelength
(MoKa) provokes an intense fluorescence with rubidium.
On the other hand, with the copper wavelength, the
absorption is too high even with a small diameter capillary.
However, the interplanar distances are correct even under
that condition.
4. Conclusion
Ball milling under hydrogen pressure constitutes an easy
and reproducible method for the preparation of large
quantities of pure alkali metal hydrides. The moderate
rotation rate and the quite low hardness of the reactants are
such that the risk of contamination of the hydride by
metallic impurities coming from the shocks between balls
and vial wall remains very small. This process can easily
be adapted for industrial applications since we are already
able to synthesize at once in 16 h up to 16 g of NaH or 89
g of CsH at the laboratory scale.
3. Results
The X-rays patterns of the hydrides obtained by this
method are shown in Fig. 1. The peaks are sharp,
indicating a good crystalline organization of the powder.
On the other hand, the peaks correspond to a FCC cubic
unit cell with the Miller indices of the same parity and the
intensities fit well with that calculated for the cubic unit
References
¯
cells, space group Fm3m, (see Table 1).
´
[1] A. Herold, C.R. Acad. Sci. Paris 228 (1949) 686.
The results concerning the X-ray reflections relative to
the hydrides NaH, KH, RbH and CsH are represented in
´
`
[2] A. Herold, These, Paris, (1951);
´
A. Herold, Ann. Chim. 6 (1951) 536.

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L. Elansari et al. / Journal of Alloys and Compounds 329 (2001) L5–L8
´
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