Relationships between ignition and product structure formation in W–Teflon (Tf)–Al powder mixtures was explored by thermodynamic and structural analyses. The use of tungsten as one of mixture components was dictated by the need to obtain high-density condensation products. Aluminum was used as a heat-generating agent to reduce ignition temperature and increase mixture combustion temperature. Combustion experiments used compositions with a fixed tungsten-to-Teflon ratio, while aluminum content varied according to the formula: (1 – x)(0,8W + 0,2Tf) + xAl = const. After intermixing in the AGO-2 planetary mill in hexane environment, the powders were compressed into 0,01–0,02 g samples and then heated in a BN crucible in argon environment under atmospheric pressure at a variable crucible heating rate. The sample temperature increased sharply on the thermogram once the ignition temperature was reached. It is shown that as the heating rate increases, the ignition temperature of systems grows, and this may be due to transfer from thermal explosion mode to ignition mode. Low-aluminum mixtures yielded large amounts of gaseous products during ignition and combustion, and this results either in defragmentation of combustion product or in formation of porous cakes. The analysis of products obtained with high-aluminum systems indicated WAl4 as a main product. For higher aluminum content results of thermodynamic calculations strongly differed from experimental ones owing to the lack of thermodynamic data for tungsten aluminides in the Thermo software and to the strong mismatch between the actual reaction conditions and adiabatic equilibrium ones. Calculated and experimental results suggest that the formation of fused high-density products (ρW2C = = 17,2 g/cm3) is possible at an optimal aluminum content ≈10 wt.%. When this value is exceeded, the main product, WAl4, has a much lower density (ρWAl4 = 6,6 g/cm3), which is inadequate for practical implementation.
Mechanism of ignition of single coal particle: effect of heating rate on particle-size dependence of ignition temperature.
