Study of the ratio of fuel to oxidant on the kinetic of ignition reaction of Mg/Ba(NO3)2 and Mg/Sr(NO3)2 pyrotechnics by non-isothermal TG/DSC technique

Abstract

The effect of mass ratio of fuel to oxidant in pyrotechnic compositions of Mg/Ba(NO3)2 and Mg/Sr(NO3)2 was studied using the non-isothermal thermogravimetry (TG) and differential scanning calorimetry (DSC) techniques. The mass ratios 10:90, 25:75 and 50:50 of Mg powder to oxidant (nitrate salt) were used for preparation of pyrotechnic compositions. The kinetic parameters of thermal ignition reactions of pyrotechnics were obtained by using TG/DSC curves under nitrogen atmosphere at heating rates of 20, 25 and 30 °C min−1. The DSC curves showed that the 25:75 weight ratio of fuel to oxidant resulted in a complete ignition reaction and showed the highest enthalpy of the reaction. Also, this mass ratio was not indicated a mass gain after the combustion step in the TG curves. The model-free methods of Kissinger, Ozawa–Flynn–Wall (OFW) and Kissinger–Akahira–Sunose (KAS) were used for calculation of activation energy (Ea) of ignition reactions of pyrotechnics. The Ea values 130.5 and 146.3 by Kissinger, 138.3 and 153.2 by OFW and 132.2 ± 2.4 and 146.3 ± 1.7 kJ mol−1 by KAS methods, respectively, were obtained for Mg/Ba(NO3)2 and Mg/Sr(NO3)2 pyrotechnics. Also, the nonlinear model-fitting method was used to determine the pre-exponential factor (A) and kinetic model function. The sigmoidal shapes were resulted from the curves of conversion factor (α) versus T. The model of A3/2 with the functions of g(α) = [− ln(1 − α)]2/3 and f(α) = 3/2(1 − α)[− ln(1 − α)]1/3 as a nucleation reaction model was proved by using the model-fitting method for the ignition reaction of both pyrotechnic compositions. The ln A values 8.1 and 7.6 min−1 were obtained, respectively, for the ignition reaction of magnesium powder with Ba(NO3)2 and Sr(NO3)2.