CBET Award Achievements
Notable Accomplishments from CBET Awards

A Nanoparticle with a Big Bang

Michelle Pantoya  -  Texas Tech University

Background:  Experiments on spherical nano-particles of aluminum have shown that when triggered by an external ignition source, nano-particles will explode into atomic size clusters that are spewed in all directions.  The particle transforms from a single extremely small, dense and very hot particle into an expansion of atomic size clusters of highly reactive molten aluminum that are propelled in all directions at high velocities.

One use for nano-aluminum particles is as a fuel ingredient in a thermite reaction.  Thermites represent a mixture of metal fuel and oxidizers which produce high temperatures and a self propagating reaction.  Research at Texas Tech has shown that nanoparticle thermites are highly reactive compared to their micron-scale counterparts.  Nano-aluminum containing thermites are highly sensitive to ignition, reduce ignition times by up to three orders of magnitude and can increase reaction speeds from a sluggish crawl to Mach speeds.  These superlatives have been hard to explain until the development of a new theory at Texas Tech tailored for the unique properties of nanoparticles.

Results:  The theory applies for the fast oxidation of aluminum nanoparticles that are covered by a thin oxide shell.  Once a particle is exposed to an ignition source, the fast heating creates huge internal thermal stresses due to thermal expansion and volumetric strain during aluminum melting.  In a nanoparticle, the volume change associated with melting induces pressures of 0.1 to 4 GPa.  This huge pressure build-up causes spallation of the oxide shell (See Figure below).  The unbalanced pressure forces between the exposed molten aluminum surface and core generate an unloading wave that creates huge tensile pressures resulting in dispersion of atomic size liquid aluminum clusters which fly with high velocity.  The clusters may react with oxygen (or nitrogen) in the air or other gaseous oxidizer, or they may hit a solid oxidizer, partially penetrating in it.  In either scenario aluminum oxidation is not limited by diffusion.  Traditional thermite reactions are controlled by diffusion of molecules through an aluminum particle's growing oxide shell (Figure below).  However, when the external radius of the fuel particle reduces to 10-60 nm the ignition times and reaction speeds are too fast for a diffusion controlled mechanism to be possible.  Thus, finding the physical mechanism of material transport and reaction for nanoparticles is one of the most important problems in combustion physics today.  Our new theory is called the melt-dispersion-mechanism and explains nanoparticle reactions and how they can be much faster than diffusion will allow.

Michelle Pantoya Image
Micron scale particles react by diffusion of aluminum through an oxide shell, which grows to a critical thickness and is followed by tensile features within the shell exposing the aluminum core.  Nano scale particles react by a melt-dispersion-mechanism where the oxide shell spallates exposing the molten core and creates an unloading pressure wave which disperses atomic size aluminum clusters in all directions.

Credit:  Michelle Pantoya, Texas Tech University

This work is notable because in the field of combustion there are two mechanisms by which reactions are characterized:
(1) diffusion or mass transport controlled; and,
(2) chemistry or kinetically controlled.  Nano-particle reactions have shown not to be described by either.  This work will broaden the depth of combustion science by introducing a new mechanism of reaction.  This melt-dispersion-mechanism describes reactions involving nano-particles and answers the question:  "Why are nano-particles so highly reactive?"

This work involves multidisciplinary research by drawing upon the expertise of mechanics, chemistry, material science and combustion.

Program Officer:   Phillip Westmoreland
NSF Award Number:   0210141
Award Title:   NIRT: Nanocomposite Reactions in the Self-propagating High Temperature Synthesis of Materials
PI Names:   Michelle Pantoya
Institution Name:   Texas Tech University
Program Element:   1407
CBET Nugget:   FY 2006

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This Nugget was Updated on 25 September 2008.