APPLIED PHYSICS LETTERS
VOLUME 84, NUMBER 23
7 JUNE 2004
Ignition studies of AlÕFe O energetic nanocomposites
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a)
L. Menon, S. Patibandla, K. Bhargava Ram, S. I. Shkuratov, D. Aurongzeb, and M. Holtz
Department of Physics and Nanotech Center, Texas Tech University, Lubbock, Texas 79409
J. Berg
Department of Mechanical Engineering and Nanotech Center, Texas Tech University, Lubbock, Texas 79409
J. Yun and H. Temkin
Department of Electrical Engineering and Nanotech Center, Texas Tech University, Lubbock, Texas 79409
͑
Received 26 January 2004; accepted 12 April 2004; published online 20 May 2004͒
We prepare energetic nanocomposites, which undergo an exothermic reaction when ignited at
moderate temperature. The nanocomposites are a mixture of Al fuel and Fe O oxidizer where
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Fe O is in the form of an array of nanowires embedded in the thin Al film. We achieve a very high
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packing density of the nanocomposites, precise control of oxidizer–fuel sizes at the nanoscale level,
and direct contact between oxidizer and fuel. We find that the flame temperature does not depend on
ignition temperature. © 2004 American Institute of Physics. ͓DOI: 10.1063/1.1759387͔
Energetic materials1,2 are a class of substances that store
energy chemically and, when ignited, undergo an exothermic
reaction without the need for an external substance such as
oxygen. Traditionally, fabrication of such materials has in-
volved processes such as grinding and mixing oxidizer and
fuel components ͑as in black powder͒ or the introduction of
oxidizer and fuel into one molecule ͑as in trinitrotoluene͒.
Nanoscaled energetic composites, due to increased surface to
volume ratio of the reactants, can result in higher-energy
release in comparison with traditional materials. In this re-
gard, two processes to prepare such nanocomposites are be-
ing developed, namely, sol–gel processing and multilayered
foils. Sol–gel chemistry produces nanometer-size particles
alumina membranes.7–9 Fabrication of the membranes in-
volves dc electrochemical anodization of an aluminum foil in
an acid. During anodization, aluminum is converted to nan-
oporous aluminum oxide. The diameter of the pores is con-
trolled by the anodization voltage and the acid used. Typical
pore diameters range from 8 to 200 nm. Figure 1͑a͒ shows
the top view image of a nanoporous alumina template with
pore diameter 50 nm. The image demonstrates the high level
of ordering obtained for these templates. The pores do not
reach the aluminum surface ͓see schematic diagram in Fig.
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͑b͔͒ due to the presence of a 30 to 40 nm thick barrier layer
of aluminum oxide at the bottom of the pores. Presence of
this barrier layer is sufficient to inhibit interdiffusional reac-
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tion between the aluminum and an oxidizer (Fe O ).
immersed in a solid network. Such structures are macro-
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Once the templates are synthesized, nanowire arrays can be
scopically uniform due to the small particle size and the
small interparticle separations. However, the particle distri-
bution is random which can inhibit self-sustaining processes
by locally separating the fuel and oxidizer. In addition, sol–
gel reactants often have organic impurities that make up
prepared by ac electrodeposition1
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inside the pores ͓see
Fig. 1͑b͔͒. Length of the wires can be controlled by adjusting
the time of deposition. Figure 1͑c͒ shows a typical cross-
section scanning electron microscopy ͑SEM͒ image demon-
strating nanowires arranged in a parallel manner inside nan-
oporous alumina templates.
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about 10% of the sample mass. These factors result in re-
duced energy release. Multilayered foils consisting of alter-
nating layers of oxidizer and fuel material provide large,
A schematic illustration of the approach developed here
for fabricating such nanocomposites is shown in Fig. 2. Fe
nanowires are electrodeposited into nanoporous alumina
templates ͑steps I–III͒. The electrodeposited sample is then
coated with a thin organic layer ͑step IV͒ followed by soak-
ing in 3% mercuric chloride solution ͑step V͒ to remove the
Al layer at the bottom. The organic layer protects the Fe
wires inside the pores from being etched away by mercuric
chloride solution during this process. In the next step, the
organic layer is removed in ethyl alcohol solution ͑step VI͒
and the sample is dried. The sample is soaked in a mixture of
chromic-phosphoric acid at 60 °C to partially etch the pores
from the top ͑step VII͒, revealing Fe wires. At this stage, the
exposed part of the Fe nanowires, are oxidized. The sample
is rinsed and dried and a thin film of Al ͑50 nm͒ is coated on
top by means of thermal evaporation ͑step VIII͒. Al film is
regular planar interfaces and very close physical contact be-
tween oxidizer and fuel reactants.5,6 They are nanoscaled in
one dimension and the energy release proceeds through in-
terdiffusion at the interface.
We demonstrate a fabrication approach to prepare ener-
getic nanocomposites based on advanced nanoengineering
and processing methods. Our composites are highly struc-
tured consisting of an array of nanowires partially embedded
in a thin film. The wires are perpendicular to the film allow-
ing for the maximum possible density of the nanowires. Our
approach allows precise control of the oxidizer and fuel
structure for achieving excellent physical contact.
Oxidizer nanowires are formed in a regular array by self-
assembly, and embedded in the fuel by means of thin-film
deposition. The nanowires are prepared using nanoporous
now in contact with Fe/Fe O3 nanowires. The sample is
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briefly annealed at 100 °C to further improve the interface
contact between Al and the nanowires. It is then soaked in
a͒Electronic mail: latika.menon@ttu.edu
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