ROLE OF GAS-PHASE AND CONDENSED-PHASE KINETICS IN BURNING RATE CONTROL OF ENERGETIC SOLIDS

A simplified two-step kinetics model for the combustion of energetic s olids has been used to investigate the effect of gas-phase activation energy on flame structure and burning rate and the role of gas- versus condensed-phase kinetics in determining burning rate. The following a ssumptions are made: a single-step, unimolecular, high activation ener gy decomposition process which is overall relatively energetically neu tral, is followed by a highly exothermic single-step, bimolecular, gas -phase reaction with arbitrary activation energy, (E) over tilde(g). T he results show that at extremely low (<10(4) Pa) or high (>10(12) Pa) pressures the burning rate is controlled by the condensed-phase react ion kinetics for any (E) over tilde g. At intermediate pressures (10(5 )-10(10) Pa) gas reaction kinetics contribute strongly to the burning rate. In this pressure range the value of (E) over tilde(g) plays an i mportant function in determining the role of gas- and condensed-phase reactions: for high (E) over tilde(g) a gas-phase kinetically controll ed regime exists; for low (E) over tilde(g) both condensed and gas-pha se kinetics are important. The limiting behaviour of asymptotically la rge (E) over tilde(g) (gas kinetically controlled burning rate) occurs at about (E) over tilde(g) = 20 kcal mol(-1) for parameters represent ative of HMX, while the vanishingly small (E) over tilde(g) behaviour occurs near (E) over tilde(g) = 1 kcal mol(-1). Previous comparison wi th burning rate and temperature profile data has suggested that the sm all-(E) over tilde(g) limit is the more accurate of the two extremes. This may imply that the important (burning rate influencing) primary g as reaction zone near the surface has more the character of, a chain r eaction mechanism than the classical high activation energy thermal de composition mechanism. To the degree that the low-(E) over tilde(g) ch ain reaction model is a better approximation than the high-(E) over ti lde g thermal decomposition model, the possibility exists that the che mistry of either reaction zone, including the molecular structure of t he material, might be exploited for favourable tailoring of burning ra te. The low-(E) over tilde(g) model also provides a rational mechanist ic explanation of observed trends in burning rate temperature sensitiv ity with pressure and temperature for materials like HMX in terms of a gradual transition from mixed gas- and condensed-phase kinetic contro l to condensed-phase only kinetic control as the pressure decreases.