(749a) Metal-Based Reactive Materials With Biocidal Reaction Products

Reactive materials with biocidal combustion products
capable of inactivating aerosolized microorganisms have been developed recently.  The focus was on additives to conventional energetic
formulations involving halogens, known to have strong biocidal properties.  Materials developed to date can be classified
into three main groups: metal-based fuels, oxidizers, and thermites (metal fuel
and oxidizer compositions) with at least one component producing biocidal combustion
products.  In addition to generating the
desired combustion products, reactive material components must be stable and
compatible with common binders to endure conventional processing routine.  Furthermore, insensitivity to spark, impact,
friction, and other common ignition stimuli is also desired for material
handling. 

Our work has focused on incorporating halogens or
halogen containing compounds in aluminum. 
Powders containing up to 20 wt % of iodine
with compositions of Al-I₂ [1],Al-B-I₂ [1], and
Al-CHI₃ have been developed and tested.  In this talk, efforts aimed at developing new
metal-based components with a substantially increased
halogen content will be discussed. 

The main challenge is to stabilize the halogen in a
reactive metal matrix, so that the material can be handled and formulated as a
regular metal fuel, e.g., aluminum powder, commonly added in energetic
formulations.   The approach taken in
this study is based on mechanical alloying (or ball milling) starting metal
powder and a halogen-containing material to produce a stable composite
powder.  Previous work showed that
cryo-milling was necessary to stabilize iodine in aluminum matrix.  Milling at room temperature could be used to
stabilize iodine in a binary Al-B composite. 
However, the amount of iodine that could be stabilized could not exceed
20 wt %.  Thus,
new material compositions need to be explored to produce materials with greater
halogen concentrations. 

To select most promising halogen-containing starting
components, properties of different metal halides were surveyed ranking them
based on the maximum enthalpy, DH,
required to convert the halide into the most stable metal oxide and oxidized
halogen.  This enthalpy is taken per mole
of halogen.  The results are shown in Fig.
1 where the estimated reaction enthalpy is plotted vs. weight % of the
respective halide that should be added to aluminum in order to prepare a
reactive material with 40 wt. % of halogen. 
The 40 wt % benchmark doubling the presently
achieved content of iodine in the Al-based powders serves as a
target halogen content for the new reactive materials.  In Fig. 1, the most desired compositions
would have the greatest negative reaction enthalpy and the lowest wt. % of
metal halide (allowing their encapsulation into aluminum matrix); thus, they
would be represented by symbols located at the lower left portions of the
plots, shown separately for chlorine, bromine and iodine.  It is clear that the iodine-containing
materials are most favorable energetically. 
Among all materials considered, only a few could be selected taking into
account their toxicity, stability, and cost. 
In particular, TiI₄ is selected for initial experiments.  Because SiI₄ is not readily
available commercially, ball-milling elemental iodine with Si and Al is also of
interest.  In addition, a
chlorine-containing starting material, NbCl₅ was selected to
evaluate the effect of chlorine as a biocidal fuel additive.  Due to the interest in stabilizing these
metal-halides in the aluminum matrix, composite particles may need to be coated
with polymers such as paraffin wax and/or Teflon.

Reference

[1] E.L. Dreizin et al., Int. J. of Energ. Mater. Chem. Propul., 10, p
297 (2011)
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