A Nano-thermite or "super-thermite" is a metastable intermolecular composite (MICs) characterized by a particle size of its main constituents, a metal and a metal oxide, under 100 nanometers. This allows for high and customizable reaction rates. 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 for general applications involving propellants, explosives, and pyrotechnics.
What distinguishes MICs from traditional thermites is that the oxidizer and a reducing agent, normally iron oxide and aluminium, are in the form of extremely fine powders (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 proceed much more quickly.
Historically, pyrotechnic or explosive applications for traditional thermites have been limited due to their relatively slow energy release rates. Because nanothermites are created from reactant particles with proximities approaching the atomic scale, energy release rates are far greater.
MICs or Super-thermites are generally developed for military use, propellants, explosives, and pyrotechnics. Research into military applications of nano-sized materials began in the early 1990s. Because of their highly increased reaction rate, nanosized thermitic materials are being studied by the U.S. military with the aim of developing new types of bombs several times more powerful than conventional explosives. Nanoenergetic materials can store more energy than conventional energetic materials and can be used in innovative ways to tailor the release of this energy. Thermobaric weapons are one potential application of nanoenergetic materials.
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, that with potassium permanganate has the highest pressurization rate.
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 are nano-laminated pyrotechnic compositions, or energetic nanocomposites, in which fuel and oxidizer are deposited alternately in thin layers. The materials and the thickness of the layers control the detonating properties.
A method for producing nanoscale, or ultra fine grain (UFG) aluminium 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. Another production method for nanoaluminium powder is the pulsed plasma process developed by NovaCentrix (formerly Nanotechnologies). The powders made by both processes are indistinguishable. 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 aluminium particles required considerable effort, and commercial sources for the material were limited. Current production levels are now beyond 100 kg/month.
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.
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) in 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.
Like conventional thermite, super thermite reacts at very high temperature and is difficult to extinguish. The reaction produces 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.
The reactivity of a nanolaminate can vary, possibly making it more sensitive than thermite. In addition, super thermites are very sensitive to electrostatic discharge (ESD). Surrounding the metal oxide particles with carbon nanofibers may make nanothermites safer to handle.
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