Aluminum Nanoparticles as the Energy Source for Multi-Sample Mars Return Mission (2002)

An approach is discussed for manufacturing of composite Al nanoparticulate fuel for Martian multi-sample mission (with atmospheric CO2 as oxidizer). An approach implies the production of Al nanoparticles in an original technique and chemical modification of their surfaces to provide them required properties. Peculiarities of Al nanoparticles reaction with CO2 are considered. Possible combination “nanoparticles surface treatment substance – composite matrix” are proposed. Introduction. An idea of usage of carbon dioxide (most abundant component in Mars atmosphere) as an oxidizer for metal (aluminum, magnesium etc.) particulate fuel is known for a long time. Such fuel is a candidate for application as energy source for engines of Mars atmosphere planes and, may be, for power plants. The usage of “domestic” oxidizer gives possibility to transport on the Mars surface a fuel only. But the use of commercial aluminum particle in such engines is not efficient enough because of two obstacle: 1. 2. Rough aluminum particles in combustion chamber burn incompletely and do not yield the expected amount of energy (there possible coalescence in collisions and accumulation of combustion products on their surface still more aggravate this disadvantage); Usual commercial powder has strong oxide film that suppresses ignition of the particles preventing ignition and decelerates combustion. It is expected that the usage of submicronic and nanosize particles of aluminum instead of usually used particles of the micron size will allow to increase the efficiency of combustion process since will change the mechanism of burning. The realization of following criteria may ensure it: • time of burning of a single particle should be less, than time between collisions of two particles in which they may stick together with formation of a single particle of greater mass; • time of burning of a separate particle should be less, than time of formation of a stable surface film of solid combustion products suppressing the process of burning. The development of means decreasing agglomeration of nanosize particles during their manufacturing, storage, transportation to Mars surface and usage before burning in combustion chamber is necessary to guarantee the maximal effect from reduction of used particles size. The separate problem is to prevent the formation of Al2O3 inert film on the surface of aluminum particles within the term between their manufacturing and the beginning of usage in burning process. The presence of such film will reduce the content of metal aluminum in a fuel increasing cost of deliverable energy and hinder the ignition of Al particles. The purpose of the present work is the creation of a technique to modify the surface of aluminum particles: 1. to prevent nanoparticles from agglomeration during their manufacturing, storage, transportation to Mars surface and usage before burning in combustion chamber; 2. to prevent the surface of aluminum particles from formation of inert Al2O3 film before beginning of their use in combustion process; 3. to realize a storage and transportation of metal fuel in a compact form enlarging the bulk density of Al powder. We propose a technique for synthesis of mixed (composite) metallic fuel of nanosized nonagglomerated aluminum particles. It includes the following stages: • during production of aluminum nanoparticles, fresh-generated (clean) surface of particles will be coated with a barrier coatings, one up to several monolayers thick. This coating will prevent the agglomeration of particles at contacts of their surfaces but will not hinder the ignition of metal particles. It is expected also, that such coating will prevent hard the formation of metal oxide film on the surface of particles at their contact with air; • aluminum powder with the modified surface will be dispersed in a specially chosen liquid matrix (to increase essentially its bulk density) and in such kind will be transported to Mars surface. The substance of matrix may be both reacting at high temperatures with carbon dioxide or aluminum particles with heat release, or inert volatile substance. Specified two-phase fuel might be supplied directly into atomizers of propulsion engine (using Mars atmosphere CO2 as oxidizer), using external pump or excess pressure of evaporating volatile matrix. As candidate solution, the mixture of metal nanocomposite with liquid carbon dioxide will be considered. 1 NanoTech 2002 At the Edge of Revolution 9-12 September 2002, Houston, Texas AIAA 2002-5734 Copyright © 2002 by the author(s). Published by the American Institute of Aeronautics and Astronautics, Inc., with permission. Experimental. We are using of levitation method of metal vapor condensation-in-flow (Gen-Miller technique) for Al nanoparticles production. Fig.1 The scheme of experimental apparatus: WSU – wire supply uni; GCU – gas control unit; SH – sampler head. Inductor – counter-current HF inductor with levitating molten metal drop. Fig.1 demonstrate the scheme of apparatus. In more details, the design and operation of the apparatus are presented in paper. Metal particles with narrow size distribution with average particle size preset within the range 51000 nm using this apparatus. Fig.2 shows the TEM image of Al particles with average diameter 93 nm, used in the present study. Gas phase treatment of “fresh” surface of Al particles with vapors of specially selected compounds allows the chemical modification of metal surface. Our study had shown real opportunity to synthesize quasi-monomolecular and polymer chemisorbed organic, organo-silicon and inorganic coatings, that prevents particles from agglomeration, that provides the possibility to make the suspension of individual nano-particles of aluminum (using a pre-set solvent). It was found, that the particles treatment with all compounds used decrease the particles agglomeration in comparison with oxidized in air ones (see Fig. 3). 0 100 200 300 400 500 0 20 40 60