ARTICLE IN PRESS
2144
P.A. Berseth et al. / Journal of Physics and Chemistry of Solids 69 (2008) 2141–2145
Note that the LiAlH
4
seen is a high temperature-pressure phase for
4. Conclusion
which the structure has not been solved, so the weight percent
could not be found with least squares—this is an estimate of the
upper limit based on the relative peak intensities of this phase.
A wide variety of hydride mixtures, with and without light
transition metal additives, were investigated. This work provides
insight into the relative chemical stabilities of chemical reactions
that take place between four hydride materials that are candi-
dates for transportation applications, if the systems can be
modified to overcome either temperature, kinetic or weight
percent limitations.
When the transition metal additive contains vanadium, LiAlH
remains in the product mixture, while it does not for either Cr or
Mn. This suggests that the V 1.1 is catalyzing the decomposition
4
2
H
of the alanates to a lesser extent than the Cr and Mn reactants,
which are in the elemental state. However, the product phase
Na
2
LiAlH
6
does show the highest weight percent for the V
2
H
1.1
The addition of transition metal elements to NaAlH4 does
not create mixed metal alanates with ball milling, and MSP
mixture relative to Cr and Mn, indicating that the vanadium
catalyzes the decomposition to a greater extent than the Cr and
Mn reactants. Given that only the crystalline phases appear in the
XRD patterns, and sample pyrophoricity prevents examination by
complementary techniques on our electron microscopes, we note
that there is a difference in the reaction of the vanadium versus
the chromium and manganese quaternary samples and further
investigation is needed to better characterize this subtle variance.
generally produces
3 6
a decomposition product, Na AlH . The
addition of elements or other hydrides destabilizes LiAlH —when
4
mixed with Cr or Mn, Al is observed, while mixtures with binary
hydrides produce the hexahydride, Li AlH (or K AlH in the case
3
6
3
6
of KH).
NaAlH4 in combination with Mg materials often produces
NaMgH . When the Mg compound used is Mg NiH , NaMgH
3
2
4
3
2
forms with just ball milling. Mixtures with MgH do not form the
mixed metal hydride unless heated above 60 1C in the MSP step.
Two explanations are likely, first that the presence of Ni catalyzes
2 4
3.2. Mixtures of alanates and Mg NiH
the decomposition of NaAlH
kinetically stable, and therefore more reactive, than MgH
temperature effect is seen in the hexahydride product as well, but
for a different reason. Mixtures containing LiAlH and NaAlH
form mixed metal Na LiAlH with MSP of 170 1C or higher, while
Li AlH is formed with MSP at 60 1C rather than the mixed metal
product. The difference in products is likely due to the fact that at
0 1C, all of the materials are solids, and diffusion in the solid state
is very slow. When the reaction temperature is increased above
the melting point of LiAlH (125 1C), molten LiAlH can mix with
the other ingredients much more easily, and the mixed metal
hexahydride Na LiAlH forms.
Quaternary mixtures lead to known mixed metal products
LiAlH and NaMgH . When the transition metal additive is Cr
or Mn, no residual LiAlH
does, while the transition metal additive V
4
, and second that Mg
2
NiH
4
is less
The Mg
and (3) NaAlH
component on the alanate materials. The mixture with LiAlH
results in the starting Mg compound completely decomposing
with ball milling to form four products: MgH , Mg NiH, Mg
and Al1.1Ni0.9. The Mg NiH partially disproportionated to MgH
and Ni. No crystalline Li species are present, and it is likely
that the LiAlH has decomposed to some extent, providing the Al
2
NiH
4
was ball milled with: (1) LiAlH
4 4
, (2) NaAlH ,
2
. The
4
with 4 mol% TiCl to explore the affect of the Ni
3
4
4
4
2
6
2
2
2
NiH0.3
3
6
2
4
2
6
4
metal which then combines with the Ni to form the aluminum
rich AlNi phase.
4
4
The ball milled mixture with NaAlH
common with the LiAlH mixture, and that is Mg
also starting materials present after ball milling, as well as the
previously known mixed metal hydride NaMgH . When 4 mol%
TiCl is added in addition to NaAlH , Mg NiH0.26 is formed along
with NaMgH and Al, and starting materials remain. If the mixture
with TiCl is hand ground in a mortar and pestle (much lower
energy than a ball mill) the mixed metal hydride NaMgH does
4
has only one product in
2
6
4
2
NiH0.3. There are
Na
2
6
3
3
4
reactant remains but residual MgH
1.1 leaves LiAlH
2
3
4
2
2
H
4
3
reactant remaining in the product mixture, while simultaneously
3
producing a greater weight percent of crystalline Na
2 6
LiAlH
3
relative to Cr and Mn reactions.
not form, which suggests that the energy and/or heat associated
with ball milling is required for this reaction to take place.
Acknowledgments
4
3.3. Mixtures of LiAlH and binary hydrides
The authors would like to acknowledge the assistance and
helpful discussions of Dr. Art Jurgensen in the matter of XRD data
collection and interpretation and of Mr. Martin Scott for
laboratory support. Drs. Susanne Opalka and Xia Tang provided
extensive technical discussions, which are greatly appreciated.
Funding was provided by DOE Contract # EB4202000 under the
program management of Dr. Carol Read.
Mixtures of LiAlH
2) MgH and (3) CaH
with KH produced two ion exchange compounds, KAlH
AlH , and left unreacted LiAlH which continued the ion
exchange with MSP. Ball milling with MgH causes the LiAlH to
completely decompose to form Li AlH which is an indicator that
the MgH has a catalytic effect on LiAlH decomposition. The
combination containing CaH produced starting materials,
Li AlH , and Al with ball milling, indicating that the LiAlH
partially decomposes while the CaH
with MSP, this mixture forms CaH
expected products from the decomposition of LiAlH
of the chloride from TiCl with Li decomposition products.
4
, 4 mol% Ti, and the binary hydrides: (1) KH,
were investigated. Ball milling the mixture
and
(
2
2
4
K
3
6
4
2
4
3
6
2
4
2
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3
6
4
2
does not react. When heated
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Berseth, Polly A.
