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Nanoparticle AlxMoyOz/Al Thermites

These thermites are relatively insensitive, but burn relatively rapidly once ignited.

Research on the microstructures, chemical compositions, and reactivities of thermites that consist of or contain mixtures of MoO3 and Al particles has led to development of a process for making thermites that consist of or contain mixtures of AlxMoyOz and Al nanoparticles. The reactivities of the AlxMoyOz/Al thermites can be tailored through choices of ingredients at critical process steps. The findings of this and related research and development efforts may lead to the use of AlxMoyOz/Al thermites as components of insensitive weapon ignition systems.

In previous research, it had been found that MoO3/Al thermites in which Al is structured at the nanoscale have thermal sensitivities two orders of magnitude greater and combustion rates much higher than those of MoO3/Al thermites in which Al is structured at the microscale. Taking a different approach, the research reported here was oriented toward tailoring reactivity through modification of the compositions and structures of the molybdenum trioxide particles.

The term "AlxMoyOz" as used here signifies, more specically, a mixture of nanometer-sized phases that contain alumina (Al2O3) associated with molybdenum oxides (MoOu where 2<u<3). The process for making the AlxMoyOz includes a sol-gel subprocess in which agar is used as an agent for structuring aluminum paramolybdate (APM), which, as explained below, serves as a molybdenum oxide precursor. The product of this subprocess is an agar/APM composite gel (see figure).

These Scanning Electron Micrographs depict representative structures of two composite gels: one containing 40 weight percent and one containing 68 weight percent of agar.
The agar is removed and the APM converted to molybdenum oxide by subjecting the composite gel to a thermal oxidative treatment (calcination). Direct calcination of the composite gel leads to the formation of micron-sized MoO3 particles but if the composite gel is impregnated with a solution comprising anhydrous aluminum trichloride (AlCl3) in diethyl ether prior to calcination, then the product of the calcination consists of nanometer- sized AlxMoyOz phases. The structures of these phases are correlated with chemical composition. The AlxMoyOz product is mixed with aluminum nanoparticles to obtain an AlxMoyOz/Al thermite. The energetic properties of the thermite can be set through selection of the proportions of composite gel and AlCl3 prior to calcination and/or through selection of the proportions of AlxMoyOz and Al in the final mixture.

AlxMoyOz/Al thermites have been found to be very insensitive to mechanical and thermal stresses. On the other hand, they can be easily ignited by focused laser beams. Their ignition delay times are no more than a few milliseconds. The rates of combustion of these thermites lie between 12 and 60 times those of thermites comprising Al nanoparticles mixed with micron-sized MoO3 particles.

This work was done by M. Comet and D. Spitzer of the Institut Franco-Allemand de Recherches de Saint-Louis. For more information, download the Technical Support Package (free white paper) at www.defensetechbriefs.com/tsp  under the Materials category. ISL-0001

Topics:
Aluminum Chemicals Combustion and combustion processes Composite materials Emissions Engines Exhaust emissions Materials Metals Particulate matter (PM) Powertrains Research and development
This Brief includes a Technical Support Package (TSP).
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Nanoparticle AlxMoyOz/Al Thermites

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    Defense Tech Briefs Magazine

    This article first appeared in the April, 2007 issue of Defense Tech Briefs Magazine (Vol. 1 No. 2).

    Read more articles from the archives here.

    Overview

    The document titled "Elaboration and Characterization of Nano-Sized AlxMoyOz/Al Thermites," authored by M. Comet and D. Spitzer, presents research conducted at the Institut franco-allemand de recherches de Saint Louis (ISL). The paper was presented at the 33rd International Pyrotechnics Seminar in July 2006 and focuses on a new method for controlling the reactivity of thermites composed of molybdenum trioxide (MoO3) and aluminum (Al).

    The authors describe a novel sol-gel process that allows for the synthesis of nano-sized mixed AlxMoyOz phases. This method enables the adjustment of the energetic properties of the resulting thermites by physically mixing these phases with nano-sized aluminum (Al 50-P). The resulting nanothermites exhibit significant advantages, including insensitivity to mechanical and thermal stresses, while still being easily ignitable with a laser beam. The nano-structuring of the oxidative phase is shown to decrease sensitivity, shorten ignition delay times, and dramatically increase combustion rates.

    The research highlights the potential of these metastable interstitial composites (MICs) as components for insensitive weapon ignition systems. The ignition delay times for the nanothermites do not exceed a few milliseconds, and their combustion rates are reported to be twelve to sixty times faster than those of traditional thermites composed of micron-sized MoO3 particles.

    The synthesis process involves using agar as a structuring agent for a molybdenum oxide precursor, leading to the formation of composite gels. After the removal of agar and subsequent thermal oxidative treatment, the gels convert into nano-sized molybdenum oxide phases. The authors emphasize that the energetic characteristics of the nanothermites can be fine-tuned by adjusting the synthesis parameters, particularly the ratio of the components used.

    In conclusion, the document provides a comprehensive overview of the innovative approaches to synthesizing and characterizing nano-sized thermites, emphasizing their promising applications in military and pyrotechnic fields. The findings suggest that these materials could significantly enhance the safety and performance of ignition systems, paving the way for further research and development in the area of energetic materials.

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