Nano-thermite

Nano-thermite, also called "super-thermite", is the common name for a subset of metastable intermolecular composites (MICs) characterized by a highly exothermic reaction after ignition. Nano-thermites contain an oxidizer and a reducing agent, which are intimately mixed on the nanometer scale. MICs, including nano-thermitic materials, 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.

Uses

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.

MICs or Super-thermites are generally developed for military use, propellants, explosives, and pyrotechnics. Because of their highly increased reaction rate, nanosized thermitic materials are being researched by the U.S. military with the aim of developing new types of bombs that are several times more powerful than conventional explosives. Nanoenergetic materials can store higher amounts of energy than conventional energetic materials and can be used in innovative ways to tailor the release of this energy. Thermobaric weapons are considered to be a promising application of nanoenergetic materials. Research into military applications of nano-sized materials began in the early 1990s.

Types

There are many possible thermodynamically stable fuel-oxidizer combinations. Some of them are:

• Aluminium-molybdenum(VI) oxide
• Aluminium-copper(II) oxide
• Aluminium-iron(II,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)

In military research, aluminium-molybdenum oxide, aluminium-Teflon and aluminium-copper(II) oxide have received considerable attention. 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.

Production

A method for producing nanoscale, or ultra fine grain (UFG) aluminum powders, a key component of most nano-thermitic materials, is the dynamic gas-phase condensation method, pioneered by Wayne Danen and Steve Son at Los Alamos National Laboratory. A variant of the method is being used at the Indian Head Division of the Naval Surface Warfare Center. A critical aspect of the production is the ability to produce particles of sizes in the tens of nanometer range, as well as with a limited distribution of particle sizes. In 2002, the production of nano-sized aluminum particles required considerable effort, and commercial sources for the material were limited. An application of the sol-gel method, developed by Randall Simpson, Alexander Gash and others at the Lawrence Livermore National Laboratory, can be used to make the actual mixtures of nanostructured composite energetic materials. Depending on the process, MICs of different density can be produced. Highly porous and uniform products can be achieved by supercritical extraction.

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. Los Alamos National Laboratory (LANL) is developing super-thermite electric matches that use comparatively low ignition currents and resist friction, impact, heat and static discharge.

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.

The products of a thermite reaction, resulting from ignition of the thermitic mixture, are usually metal oxides and elemental metals. At the temperatures prevailing during the reaction, the products can be solid, liquid or gaseous, depending on the components of the mixture. Super-thermite electric matches developed by LANL can create simple sparks, hot slag, droplet, or flames as thermal-initiating outputs to ignite other incendiaries or explosives.

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.

Alleged application in September 11th Attacks

In 2009, the use of nano-thermite was posited by independent researchers studying dust from the collapse of the World Trade Center. Informed of the findings, NIST, the government agency charged with investigating the collapses, countered that there was no "clear chain of custody" proving that the dust indeed came from the WTC site. Dr. Steven E. Jones, one of the researchers involved in the study, invited NIST to conduct its own studies with dust under custody by NIST itself.

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
• John J. Granier (2005). Combustion Characteristics of Al Nanoparticles and Nanocomposite Al+MoO₃ Thermites (PDF). Retrieved May 3, 2009 (PhD thesis). {{cite book}}: Unknown parameter |month= ignored (help); Unknown parameter |university= ignored (help)CS1 maint: postscript (link)