Nano-thermite

Metastable intermolecular composites (MIC), also called super-thermites, or superthermites, are pyrotechnic compositions containing an oxidizer and a reducing agent which undergoes a very powerful exothermic reaction when heated to a critical temperature. Superthermites are variants of thermite compositions but are intimately mixed on the nanometer scale. MICs are a type of reactive materials investigated for military use, as well as in applications in propellants, explosives, and pyrotechnics.

What separates MICs from traditional thermites is that the oxidizer and a reducing agent, normally iron oxide and aluminium are not a fine powder, but rather nanoparticles. This dramatically increases the reactivity relative to micrometre-sized powder thermite. As the mass transport mechanisms that slow down the burning rates of traditional thermites are not so important at these scales, the reactions become kinetically controlled and much faster.

History

Historically, pyrotechnic or explosive applications for traditional thermites have been limited due to their relatively slow energy release rates. But because nanothermites are created from reactant particles with proximities approaching the atomic scale, energy release rates are far improved.

Types

There are many possible thermodynamically stable fuel-oxidizer combinations. However, only a handful have been investigated. Some of them are:

• Aluminium-molybdenum(VI) oxide
• Aluminium-copper(II) oxide
• Aluminium-iron(III) oxide
• Antimony-potassium permanganate
• Aluminium-potassium permanganate
• Aluminium-bismuth(III) oxide
• Aluminium-tungsten(VI) oxide hydrate
• Aluminium-fluoropolymer (typically Viton)
• Titanium-boron (burns to titanium diboride)

Other compositions tested were based on nanosized RDX and with thermoplastic elastomers. PTFE or other fluoropolymer can be used as a binder for the composition. Its reaction with the aluminium, similar to magnesium/teflon/viton thermite, adds energy to the reaction. Of the listed compositions, the Al-KMnO₄ one shows the highest pressurization rates, followed by orders of magnitude slower Al-MoO₃ and Al-CuO, followed by yet slower Al-Fe₂O₃.

Nanoparticles can be prepared by spray drying from a solution, or in case of insoluble oxides, spray pyrolysis of solutions of suitable precursors. The composite materials can be prepared by sol-gel techniques or by conventional wet mixing and pressing.

Similar but not identical systems are nano-laminated pyrotechnic compositions, or energetic nanocomposites. In these systems, the fuel and oxidizer is not mixed as small particles, but deposited as alternating thin layers. For example, an energetic multilayer structure may be coated with an energetic booster material. Through selection of materials (the range of which includes virtually all metals) and size scale of the layers, functional properties of the multilayer structures can be controlled, such as the reaction front velocity, the reaction initiation temperature, and the amount of energy delivered by a reaction of alternating unreacted layers of the multilayer structure.

Ignition

Nanoscale composites are easier to ignite than traditional thermites. A nichrome bridgewire can be used in some cases. Other means of ignition can include flame or laser pulse.

MICs have been investigated as a possible replacement for lead (e.g. lead styphnate, lead azide) containing percussion caps and electric matches. Compositions based on Al-Bi₂O₃ tend to be used. PETN may be optionally added. MICs can be also added to high explosives to modify their properties. Aluminium is typically added to explosives to increase their energy yield. Addition of small amount of MIC to aluminium powder increases overall combustion rate, acting as a burn rate modifier.

Uses

MICs or Super-thermites, are generally developed for military use, propellants, explosives, and pyrotechnics.

Hazards

Like conventional thermite, super thermite usage is hazardous due to the extremely high temperatures produced and the extreme difficulty in smothering a reaction once initiated. Additionally, with nanothermites, composition and morphology are important variables for safety. For example, the variation of layer thickness in energetic nanolaminates can allow control of the reactivity of it.

The thermite reaction releases dangerous ultra-violet (UV) light requiring that the reaction not be viewed directly, or that special eye protection (for example, a welder's mask) be worn.

See also

• Thermate
• Thermite
• Pyrotechnic composition

External links

• Synthesis and Reactivity of a Super-Reactive Metastable Intermolecular Composite Formulation of Al/KMnO4
• Metastable Intermolecular Composites for Small Caliber Cartridges and Cartridge Actuated Devices
• Performance of Nanocomposite Energetic Materials Al-MoO3
• Active Thermitic Material Discovered in Dust from the 9/11 World Trade Center Catastrophe