Influence of specific surface area of aluminium powder on the reactivity of aluminium/ammonium perchlorate composition

Aluminium/ammonium perchlorate (40/60 wt.%) is a pyrotechnic composition widely used as an igniter charge far small calibre rocket motors. Five different types of aluminium powders with specific surface area varying from 0.52 m(2)/g to 6.00 m(2)/g have been studied to evaluate the influence of specific surface area on the ignition characteristics of the charge. While the peak pressure varies from 35.5 ksc to 54.5 ksc (0.30 g in 48.0 cm(3) vol.). the corresponding time to P-max varies from 11.2 ms to 1.6 ms showing higher reactivity of the mix with increase in aluminium surface area. This order of reactivity agrees with the order of reactivity determined from the DTG curves of the metal: oxidation by comparing the peak height to half width ratio. However, the total gas generated by all the charges is approximately the same and the difference in peak pressure is caused by the difference in the rate of reaction and after about 30 ms all the compositions exhibit the same order of temperature and pressure. The charge has got an ignition temperature of about 640 K and the ignition is preceded by the exothermic decomposition of ammonium perchlorate. The activation energy varies from 92.2 kJ/mol to 127.6 kJ/mol and the corresponding, pre-exponential factor from 9.7 x 10(6) min(-1) to 7.1 x 10(9) min(-1) and they exhibit the kinetic compensation effect with an isokinetic temperature of 647 K.