Nanoparticle AlxMoyOz/Al Thermites

April 1, 2007

Institut Franco-Allemand de Recherches de Saint-Louis, St. Louis, France

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 Al_xMo_yO_z and Al nanoparticles. The reactivities of the Al_xMo_yO_z/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 Al_xMo_yO_z/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 "Al_xMo_yO_z" as used here signifies, more specically, a mixture of nanometer-sized phases that contain alumina (Al₂O₃) associated with molybdenum oxides (MoO_u where 2<u<3). The process for making the Al_xMo_yO_z 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 Al_xMo_yO_z phases. The structures of these phases are correlated with chemical composition. The Al_xMo_yO_z product is mixed with aluminum nanoparticles to obtain an Al_xMo_yO_z/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 Al_xMo_yO_z and Al in the final mixture.

Al_xMo_yO_z/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