Tech Briefs

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

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Nanoparticle AlxMoyOz/Al Thermites (reference ISL-0001) is currently available for download from the TSP library.

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