Laser ignition of pyrotechnic mixtures: Ignition mechanisms

Abstract

A laser method for studying the ignition processes of a pyrotechnic mixture is presented. The experimental equipment consists mainly of a 250 W CO2 laser, an explosive chamber, a power meter, and a mechanical shutter. Using the laser ignition apparatus a parametrical study of the ignitability of a pyrotechnic mixture (Mg/NaNO3) was conducted. The influence of the following parameters on the ignition energy was studied: pulse width, beam radius, particle size, different sorts of Mg, and gas pressure. These measurements are complemented by some ignition measurements on a Mg/BaO2 mixture where the influence of catalysis is studied. Furthermore some high-speed photography of the ignition process is presented. The thermal explosion theory is then applied to these measurements for a calculation of some of the important parameters followed by a discussion of the ignition mechanisms. The activation energy (Ea=148 kJ/mol), frequency factor (K0=7.1×1017 s−1), and absorption coefficient (α=3.1104 m−1) were calculated from the measurements. Using the experimentally determined parameters a 2D finite-element computer modeling of the ignition was carried out, showing that the ignition energy can be calculated with a fair degree of accuracy but that the time to ignition cannot be modeled with a pure heat transfer model. The study is concluded with a discussion of ignition mechanisms for pyrotechnic mixtures, which especially points out that the ignition mechanism for Mg/NaNO3 is a melting of the oxidizer followed by a decomposition of the oxidizer, diffusion of the produced oxygen through the oxide layer on the magnesium particles with a consecutive reaction of oxygen and magnesium.