Steven Dean
Plan for Research Project
In conventional explosives, those with an oxidizer incorporated into the explosive
molecule, the breakup and formation of chemical bonds is the primary means of
combustion. Reactions involving thermites, mixtures of aluminum and an oxidizer, are
also governed to a large degree by chemistry. However another crucial factor in
determining the behavior of a thermite reaction is an understanding of the mechanical
phenomena that occur within the thermite. Fracturing of the aluminum oxide shell before
detonation can have a significant impact on the thermites combustion properties.
Aluminum naturally oxidizes when exposed to air, much like iron rusts. In
aluminum however, unlike iron, the oxide layer does not continue to penetrate the surface
of the material. This is often a desirable quality of aluminum, as the oxide shell that
forms is extremely non-reactive, making non-treated aluminum suitable for use in many
applications that untreated steel would be unsuitable for. This oxide shell also plays a
large role in determining the physics behind an aluminum thermite reaction. The oxide
shell has a higher melting point than the aluminum core. This allows the pure aluminum
in the core to reach the liquid phase before being exposed to the oxidizing agent when the
aluminum oxide shell melts or is ruptured by internal forces created by the molten
aluminum core. Liquid aluminum reacts much more violently than solid aluminum,
which greatly increases its combustion properties, such as burn rate.
If the oxide shell has cracks or scratches in it, however, then it will break apart
much more quickly than if it was uniform throughout. This is caused by a stress
concentration that develops at the crack when it is subjected to the pressure caused by the
expansion of the aluminum core. If the oxide shell breaks before the aluminum core has
had time to melt, then the reaction will occur much less violently.
It is believed that a major source of crack formation is the pressing process of
pellet formation. Thermite itself is a powder, although a binding agent is often used to
keep the powder from being disrupted. Reactions that are of concern to the Combustion
Laboratory, however, take place without a binding agent. In order to keep the thermite
powder together it is pressed into pellets about the size of the tip of the “pinky” finger. It
is thought that the stresses applied to the particles of the thermite during this pressing
process can lead to cracks in the oxide shell. The quantity used to describe how much a
pellet has been pressed is as a percentage of theoretical maximum density (TMD). One
hundred percent TMD is defined as a mixture with no voids and a density equal to that of
the densities of its component parts multiplied by their respective mass ratios, it is a
purely theoretical number and impossible to reach in practice. A very high TMD is 80%,
while a low TMD that is still stable is approximately 20%.
It has been observed that the higher the TMD, the lower the flame propagation
rate in the thermite. This is the source of the theory that cracks caused in the pellet
formation process lead to early oxide shell failure. The purpose of this project would be
to try to identify cracks that have been formed by pressing at higher TMDs, and also
whether crack formation is influenced by the oxide used (PTFE as opposed to
molybdenum trioxide, for example) through the use of the SEM. The aluminum particles
used have a mean diameter of 50 nm.