Nanothermit ist der gebräuchliche Name für die Gruppe der metastabilen intermolekularen Gemische (metastable intermolecular composites (MICs)), die sich durch eine äußerst exotherme Reaktion nach ihrer Entzündung auszeichnet. Nanothermite enthalten einen Oxidator und ein Reduktionsmittel, welche auf nanoskaliger Ebene fein vermischt wurden. Metastabile intermolekulare Gemische (inklusive nanothermitischer Materialien) werden zu den reaktiven Materialien gezählt und auf eine mögliche Verwendung in Militär-, Sprengstoff- und Pyrotechnik, in Form von Treib- und Explosivstoffen, hin untersucht[1].
Was Nanothermit von traditionellem Thermit unterscheidet ist die Partikelgröße von Oxidator und Reduktionmittel - normalerweise Eisenoxid und Aluminium: Sie liegen nicht bloß als feines Pulver, sondern in Form von Nanopartikeln vor. Der Einfluss der Massetransportmechanismen auf die Brennrate ist auf nanoskopischer Ebene nicht so groß wie auf mikroskopischer Ebene. Während er die Brennrate traditioneller Thermite begrenzt, wird die Brennrate von Nanothermiten im wesentlichen nur durch die kinetischen Energien der Reaktionspartner bestimmt. Die Reaktionsfreudigkeit von Nanothermit ist daher wesentlich größer als die von traditionellem - durch Mischung von mikrometergroßen Komponenten hergestelltem - Thermit.
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)
In military research, Aluminum-molybdenum oxide, Aluminum-Teflon and Aluminum-copper oxide have received considerable attention.[2] 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. [3] 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. [4]
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.[5]
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.[2] 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.[2]
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. [6][7] MICs can be also added to high explosives to modify their properties. [8] 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. [9]
The power of MICs can be tuned from about 10 KW/cc to 10 GW/cc, with reaction front velocities varying from 0.1 m/s to 1500 m/s. Reaction zone temperatures can exceed 2700 degrees Celsius.[2]
Uses
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.[10] 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.[2]
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.[11]
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
References
- ↑ Effect of Al particle size on the thermal degradation of Al/teflon mixtures
- ↑ a b c d e James S. Murday: The Coming Revolution: Science and Technology of Nanoscale Structures. In: AMPTIAC Quarterly. 6. Jahrgang, Nr. 1, 2002 (p2pays.org [PDF; abgerufen am 8. Juli 2009]).
- ↑ 2002 Assessment of the Office of Naval Research's Air and Surface Weapons Technology Program, Naval Studies Board (NSB)
- ↑ Reaction Kinetics and Thermodynamics of Nanothermite Propellants
- ↑ (WO/2005/016850) Nano-laminate-based Ignitors
- ↑ Metastable Intermolecular Composites (MIC) for Small Caliber Cartridges and Cartridge Actuated Devices (PDF)
- ↑ Pyrotechnic Literature Series - Kosanke - Part 7
- ↑ Los Alamos National Lab -- Chemistry Division Capabilities
- ↑ Aluminum Burn Rate Modifiers Based on Reactive Nanocomposite Powders (PDF)
- ↑ John Gartner: Military Reloads with Nanotech. In: MIT Technology Review. 21. Januar 2005 (technologyreview.com [abgerufen am 3. Mai 2009]).
- ↑ (WO/2005/016850) Nano-laminate-based Ignitors
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: Combustion Characteristics of Al Nanoparticles and Nanocomposite Al+MoO3 Thermites. 2005 (ttu.edu [PDF; abgerufen am 3. Mai 2009]).
Category:Pyrotechnic compositions Category:Incendiary weapons Category:Explosives