Condensation Effects in the Detonation of Tetramethylsilane

Detonation limits have been studied for binary mixtures of tetramethylsilane (TMS) with oxygen and also for ternary mixtures including up to 92% of helium by volume. The limits for the binary system are 1⋅8% and 48 to 50% TMS by volume. The object of these researches was to study factors controlling transition from detonation to deflagration, particularly when a time lag is introduced in the liberation of chemical energy by the need to condense and crystallize or to polymerize non-volatile reaction products before the large heat changes involved can contribute to the propagation of detonation. Solids formed in the detonating mixtures were examined to determine their composition, density, structure, surface area and electrical conductivity. As the composition of the mixture is progressively changed, a systematic trend is observed in properties of the solid products. This appears to correspond with different modes of propagation of detonation. Starting with the oxygen rich limit where carbon-free silica is formed, the solids change colour rapidly from white to black in a transition region between 28 and 32% by volume of TMS, remaining black for richer mixtures. Black solids incorporate appreciable proportions of carbon. Two peaks in ‘detonation strength’ characterize different regions, in which the trends of fluctuations in the velocity of propagation of detonation correspond with changes in the solid products. Calculations make it clear that the heat of formation of various solids contributes towards the hydrodynamic propagation of the detonation wave. However, the condensation of silica appears to act in a manner different from the formation of carbon. Extremely dilute mixtures of TMS will still detonate. For binary mixtures, the lower limit is found at 1⋅8% TMS by volume. For ternary mixtures, detonation is still found with 0⋅9% TMS, 7⋅8% oxygen and 91⋅2% helium. Some implications of this finding are discussed.