Abstract
This paper describes a kinetic model dedicated to thermite nanopowder combustion, in which core equations are based on condensed phase mechanisms only. We explore all combinations of fuels/oxidizers, namely Al, Zr, B/CuO, Fe2O3, WO3, and Pb3O4, with 60 % of the theoretical maximum density packing, at which condensed phase mechanisms govern the reaction. Aluminothermites offer the best performances, with initiation delays in the range of a few tens of microseconds, and faster burn rates (60 cm s−1 for CuO). B and Zr based thermites are primarily limited by diffusion characteristics in their oxides that are more stringent than the common Al2O3 barrier layer. Combination of a poor thermal conductivity and efficient oxygen diffusion towards the fuel allows rapid initiation, while thermal conductivity is essential to increase the burn rate, as evidenced from iron oxide giving the fastest burn rates of all B- and Zr-based thermites (16 and 32 cm·s−1, respectively) despite poor mass transport properties in the condensed phase; almost at the level of Al/CuO (41 versus 61 cm·s−1). Finally, formulations of the effective thermal conduction coefficient are provided, from pure bulk, to nanoparticular structured material, giving light to the effects of the microstructure and its size distribution on thermite performances.
Highlights
For all considered thermite couples, the initiation delay, temperature and burn rate are reported in Table 1, with some highlighted materials properties
This paper reports on the modelling of the initiation characteristics and the burn rate of a series of thermites combining Al, B, and Zr fuels with CuO, Fe2O3, WO3, and Pb3O4 oxidizers
Stoichiometric mixtures with 60% compaction are systematically compared and discussed in light of their thermal conductivity, ability to transport oxygen species, reaction enthalpies, and the more complex disruption temperature, which assumes that the combustion is stopped at the destruction of the initial thermite structure, causing the combustion to continue in the gas phase through molecules or thermite fragments, away from their original position
Summary
Thermites in the form of mixed particles composed of a fuel, most commonly aluminum, and an oxidizer, such as iron, copper oxide, or molybdenum oxide [1,2,3,4,5,6,7,8,9,10], have attracted considerable attention due to their controllable versatility and high reaction enthalpies, making them good candidates for a number of applications including actuation [11,12], micropropulsion [13,14,15,16,17], heat sources for welding or joining [18,19], and, more recently, micro-initiation and environmentally clean primers [20,21,22,23]. A debate in the community of thermite materials was focused on whether condensed phase rather than gas phase mechanisms were responsible for the different combustion regimes, e.g., initiation and combustion Both mechanisms are expected to be present when considering the level of heating and temperatures reached upon initiation and subsequent reaction propagation; yet, the balance between the two mechanisms, and the potential domination of one over the other in either stage of combustion is complex and difficult to ascertain. This balance is shown to depend heavily on the overall nanostructure (dense vs. low compaction, open vs. closed architecture), as well as the method of initiation (very high vs. low heating rate, laser heating, ESD, etc.), and obviously, the chemical nature of the thermite ingredients [38,39]
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