1082
FOKIN et al.
The hydrogen-sorbing properties of the intermetal- using a VEGA TESCAN electron microscope at the
lic compound were noted to depend on the preparation accelerating voltage 20 kV. A polished section was pre-
method [17]: the highest hydrogen capacity (4 wt %) pared by grinding an alloy sample on abrasive cloths
was found in the hydride phase of an alloy prepared by followed by final polishing with a 1-μm diamond sus-
sintering, rapidly quenched, and then treated in a ball pension.
mill and hydrided at 320°C. The alloys prepared using
The thermal stability of reaction products was
studied on a STA 409 Luxx (Netzsch) simultaneous
TG–DTA/DSC thermal analyzer. Weight loss (TG)
curves and differential scanning calorimetry (DSC)
curves were recorded under programmed heating at
10 deg/min in flowing argon.
The specific surface areas Ssp of samples were
determined as the low-temperature krypton adsorp-
tion after volatiles were removed from the solid phase
in a vacuum of 1.3 × 10–3 Pa at 300°C for 5 h and cal-
culated by the Brunauer–Emmett–Teller method.
The determination error was 10%.
The compositions of the produced phases were
determined by volumetric and chemical analyses. The
hydrogen and nitrogen were determined on a Vario
Micro cube Elementar CHNS/O analyzer. The chlo-
rine was determined turbidimetrically.
a ball mill have nanometer particle sizes.
The p–c isotherms at 250°C in the Mg17Al12–H2
system feature a plateau corresponding to hydride for-
mation at 0.06 MPa (upon desorption, 0.04 MPa), and
this proves the ability of the alloy to absorb 3.2 wt %
hydrogen under 5.3 MPa and desorb it at 250°C [18].
When hydrided at 300°C [15, 16], the intermetallic
compound decomposes to yield the hydride MgH2
and the intermetallic compound β-Mg2Al3, the latter
not reacting with hydrogen at this temperature:
(1)
γ-Mg17Al12+ 9H2 → 4β-Mg2Al3 + 9MgH2.
When the temperature rises further to 350°C under
a hydrogen atmosphere, the newly formed intermetal-
lic compound β-Mg2Al3 decomposes by the reaction
(2)
β-Mg2Al3 + 2H2 → 2MgH2 + 3Al.
The hydrogen pressure was measured on an MO
reference gage, Class 0.4.
Thus, a mixture of aluminum (or the aluminum-
base solid solution Mg0.1Al0.9 as reported earlier [19])
with magnesium hydride is the final product of
hydriding γ-Mg17Al12 in the temperature range 250–
350°C [18, 20]; this fact makes the intermetallic com-
pound unsuitable for use in hydrogen accumulators.
One solution to this problem is to reduce the hydriding
temperature, i.e., to elucidate milder synthetic condi-
tions for the hydride phase. We showed [21–23] that,
in some cases, ammonia used, instead of hydrogen, for
hydrogenation of metals, alloys, and intermetallic
compounds provides milder conditions for the forma-
tion of hydride phases with the initial metal lattice
being conserved.
EXPERIMENTAL
Alloy powders to be treated with hydrogen and
ammonia were prepared by grinding beads in a metal
mortar followed by screening the 200 μm particle size
fraction. The specific surface area Ssp of these powders
was 0.08 m2/g.
The intermetallic compound was hydrided by
high-purity hydrogen (99.99%), which was evolved
under heating by a metal hydride accumulator based
on the intermetallic compound LaNi5.
Ammonium chloride (chemically pure grade) was
dried under vacuo for 9 h at 150°C. The NH3 dried
over metallic sodium was 99.99% pure.
The alloy powder was hydrided by hydrogen or
ammonia in a stainless steel container placed in the
autoclave reactor of a 60-mL laboratory high-pressure
setup.
The goal of this study was to determine the condi-
tions for hydriding the intermetallic compound
γ-Mg17Al12 with hydrogen and ammonia without
hydrogenolysis and disproportionation.
SUBJECTS AND METHODS
Intermetallic compound Mg17Al12 samples were
prepared by alloying the batch of constituent metals,
which were 99.95% (Mg) and 99.99% (Al) pure, in an
electric arc furnace with a non-consumable tungsten
electrode under a pressure of purified argon (0.2 MPa).
An X-ray powder diffraction analysis of the sam-
ples was carried out on an ADP-1 diffractometer
(CuKα-radiation). The error in unit cell parameters
did not exceed 0.005 Å. The Le Bail full-profile
refinement was performed in software PowderCell 3.3.
Before being hydrided by hydrogen, an alloy sam-
ple 2–3 g in size was degassed in vacuo (~1 Pa) at
room temperature or at 300–450°C for 1 h, and at the
same temperature the autoclave was filled with hydro-
gen under 3.0–4.5 MPa. After the hydriding was over,
the autoclave with the sample was kept for several
hours at room temperature to attain equilibrium.
The reactions of alloy powders with ammonia were
studied at the initial ammonia pressure 0.6–0.8 MPa
using NH4Cl (10 wt % of the amount of the interme-
The microstructure and local elemental composi- tallic compound) as the process activator in a stainless
tion of the alloy were studied by scanning electron steel container placed in the autoclave reactor of a 60-mL
microscopy (SEM) in secondary and back-scattered laboratory high-pressure setup. A weight of the pre-
electrons and energy-dispersive X-ray analysis (EDX) pared powder mixture (0.8–1.0 g) was evacuated to the
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY
Vol. 64
No. 9
2019