by Professor Emily M. Hunt
Recent advancements in the field of nano-technology focused attention on developing
materials with new and useful characteristics. In particular, there is interest
in designing nanocomposite thermites for self-propagating high-temperature synthesis
(SHS) applications. The composite material consists of nano-scale particles
that are in nearly atomic scale proximity but constrained from reaction until
triggered. Once initiated, the reaction will become self-sustaining and a new
intermetallic alloy product will be produced1.
These materials consist of a mixture containing nano-scale metals, metal oxides,
and/or organic and inorganic polymer binders and are being used for overlapping
technologies ranging from materials synthesis1 to
local energy generation applications2. For example,
when a reactant mixture is ignited to produce a new material, the combustion
synthesized product can be useful as a biomaterial3
or the reactants can be tailored to synthesize corrosion resistant high temperature
metallic alloys4. When the reaction is significantly
exothermic and results in rapid flame propagation, the reactant mixture can
be tailored to generate energy for industrial, civil or military applications5.
This short article is focused on examples of future opportunities within nanostructured
metallic alloys, but there is a relatively large literature base reporting on
observations of unique behaviors of nano versus micron (traditional size) particles1-15.
While some of the fundamentally unique observations specific to nanoparticles
are just now being realized, these developments can be exploited to help researchers
address current critical issues such as energy generation applications, biomaterials
for bone implants and skeletal repair, and the spread and transfer of infectious
bacteria and molds.
When individual fuel and oxidizer particles approach nanometer dimensions,
nanoparticle thermal and combustion behaviors are unique in that the energy
required to initiate a reaction can actually be stored and build-up within the
particles. These energy storage formulations consist of fuel metal nanoparticles
combined with metals or metallic oxide nanoparticles and are referred to as
The Nanocharger stores energy as the particles are held in an inert state,
and when triggered, the slow gasless reaction that occurs is controlled by mass
transport and energy required to diffuse reactants toward each other. Ignition
leads to a slow controlled, self propagating, high temperature conversion of
chemical energy to thermal energy. The unique thermal, mechanochemical and combustion
properties for nanoparticles are the foundation for this innovative concept:
use nanoparticle fuel and oxidizer composites as reactants in a Nanocharger
that will store and deliver energy on demand.
Figure 1 shows time stamped still-frame images from the IR camera indicating
the nanometric sample stored energy 46 seconds longer than the micron-composite
material. Nanochargers could be described as a cross between a battery and molten
salts. The reactants achieve similar thermal inertia properties of molten salts
(enabling storage), but energy delivery is based on a chemical reaction, more
similar to a battery. Storing thermal energy for extended periods may be useful
for some forms of renewable energy, such as solar thermal. In this way, heat
energy stored within the Nanocharger during the day could be easily delivered
during evening hours, when the sun does not shine16.
Infrared thermal images time stamped for nanometric particulate mixtures
and micrometer particulate mixtures composed of Al/Mn. (adapted from
In the field of combustion synthesis, much work has been done to generate porosity
by adding blowing agents to the reactant matrix and Moore et al. provide a review
of much of this literature17-18. A foam-type
product can be created when a mildly energetic composite includes a modest amount
of gasifying agent (GA). During a reaction the gasifying agent generates nucleation
sites that promote the formation of bubbles. As the reaction wave passes, the
gas pockets within the bubbles escape leaving a porous structure.
In previous combustion synthesis studies, a gasifying agent may be added as
a separate reactant and usually in the form of a powder or granular material19.
This strategy was shown to be highly successful for synthesizing ceramic materials
for biological applications15. However, incorporating
blowing agents to synthesis metal alloys has not been widely pursued yet has
recently been shown to be feasible using nanoparticles15.
Control over properties of the final product, such as porosity, can be achieved
by tailoring reactant composition. In 2006, combustion synthesis was used to
form porous nickel aluminide and showed that the porosity of the final product
is a function of the percentage of gasifying agent present in the reactant matrix15.
Today, this work is extended to understand the mechanics of synthesizing a
porous titanium aluminide (AlTi) alloy in order to create an axially graded
porosity distribution. Metallic foams are synthesized by means of a self propagating
high-temperature reaction producing a highly porous solid metal alloy with customizable
material properties. Nano-scale aluminum and nano-scale titanium particles are
mixed with either nano-scale aluminum passivated with a gasifying agent such
as perfluoroalkyl carboxylic acid (C13F27COOH) or polytetrafluoroethylene
(Teflon) (C2F4) particles. When pressed into pellets and
ignited with a laser, they produce a reaction product composed of an AlTi alloy
that has a highly porous structure.
In this way, combustion synthesis can be used to create a functionally graded
porous AlTi alloy and identify correlations between the product microstructure
and parameters such as type and amount of gasifying agent present in the reactants.
Photographic data allow interpretation of the reaction propagation while characterization
of the final product indicated porosity and morphology. These nanostructured
metallic alloys may have applications in biomaterial development by tailoring
porosity throughout the matrix. Figure 2 show a scanning electron micrograph
(SEM) of this AlTi alloy.
SEM of AlTi nanostructured metallic alloy
Bacterial contamination in hospitals, food industries, and public environments
create a major public health issue. Despite considerable research and development
efforts, the problem of contaminations related to biomedical devices and food
preparation persists. Traditional cleaning methods, such as aerosolized disinfectant
sprays or wipes have a limited effectiveness. There is a strong need to mitigate
bacterial colonization by engendering materials with properties that include
surface chemistry20-22 and surface roughness23-25
which are unfavorable for bacterial attachment and growth. Silver has been used
for years in many bactericidal applications because of its strong toxicity to
a wide range of micro-organisms20-27.
Research has shown that the bactericidal properties of silver are size dependent,
and only nanoparticles present a direct interaction with the bacteria22.
Titanium dioxide (TiO2) has also become a popular agent for bacterial
neutralization. Several commercial products have been developed that incorporate
nanoparticles of TiO2 for antibacterial applications28.
Highly porous, antibacterial solid metallic alloys (or foams) can be created
through combustion synthesis. By combining nano-scale Silver Oxide (Ag2O)
or TiO2 particles with Aluminum (Al) nano-scale particles, the reaction
can produce a self-propagating heat wave that will synthesize metallic foams
made of pores only nanometers wide that inherently exhibit antibacterial properties.
The extraordinarily high surface areas these foams possess serve as an excellent
platform for the neutralization of bacteria. These newly synthesized alloys
present a novel approach to bacterial neutralization.
Figure 3 shows nanostructured metallic alloys undergoing bacterial growth tests
at 24 and 48 hours. . The bacterial growth is highlighted with a white circle.
Figure 3D shows a control sample before and after exposure.
Al-based metallic foams after exposure A. Ag2O; B. nano TiO2;
C. micron Ag2O; D. control
Five important conclusions can be drawn from these results.
- Combustion synthesis can be used to create nanostructured metallic alloys
that have antibacterial properties.
- Bacteria growth kinetics are a function of reactant particle size.
- Nanoscale reactants are more effective in neutralizing bacteria.
- TiO2 particles can delay, but not prevent bacterial growth, and;
- metallic nanofoams composed of nano-scale Al and Ag2O prevent
growth of bacteria.
These nanostructured metallic alloys can be easily created as a structural
material or a metallic coating through combustion synthesis and have far-reaching
applications in the renewable energy, food service and medical industry. Nanostructured
metallic alloys truly are the material of the future.
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High-temperature Synthesis (SHS) of Nano-particle Reactants," Journal
of Materials Research 19(10), 3028-3036 (2004).
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Reactants in the Self-Propagating High-Temperature Synthesis of Nickel Aluminide,"
Acta Materialia 52(11), 3183-3191 (2004).
14. E. M. Hunt and M. L. Pantoya, "Ignition
Dynamics and Activation Energies of Metallic Thermites: From Nano- to Micron-scale
Particulate Composites," Journal of Applied Physics 98(3), 034909 (2005).
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Texas A&M University)