Ignition of a Mixture of Tungsten with Teflon and Aluminum Additives

Abstract

A thermodynamic analysis is performed and the dependence of ignition and product structure formation in powder mixtures of tungsten with Teflon (Tf) and aluminum additives is investigated. The use of tungsten as one mixture component is associated with the necessity to fabricate condensed products with a high density, while aluminum is applied as the energy additive, decreasing the ignition temperature and increasing the combustion temperature of a mixture. Compositions with a fixed ratio of tungsten and Teflon are used for the investigation, and the aluminum concentration is varied according to formula (1–x)(0.8W + 0.2Tf) + xAl = const. Mixtures are prepared in an AGO-2 planetary mill in the hexane medium. Samples with a weight of 0.01–0.02 g are pressed from the mixtures and heated in a boron nitride crucible in argon under atmospheric pressure. The crucible heating rate is variable. An abrupt jump in the sample temperature is observed in the thermogram upon reaching the ignition temperature. It is shown that an increase in the heating rate increases the ignition temperature of systems, which can be associated with the transition from the thermal explosion mode to the ignition mode. Compositions with small aluminum additives evolved a large volume of gaseous products during ignition and combustion. These products either completely fly away or form a highly porous structure. An analysis of products for systems with a high aluminum content shows that the main product is WAl4. The results of experiments and thermodynamic calculations substantially differ at a high aluminum concentration, which is explained by the data absence for tungsten aluminides in the Thermo program and by the fact that actual reaction conditions are far from equilibrium and adiabatic conditions. The calculated and experimental data show that, in order to form molten high-density products $$({{\\rho }_{{{{{\\text{W}}}_{2}}{\\text{C}}}}}$$ = 17.2 g/cm3), the optimal aluminum concentration should be about 10 wt %. If this value is exceeded, the main product, WAl4, has a much lower density $$({{\\rho }_{{{\\text{WA}}{{{\\text{l}}}_{4}}}}}$$ = 6.6 g/cm3), which is insufficient for the practical application.