Confinement in the diode laser ignition of energetic materials

Abstract

The diode laser is increasingly used as an ignition device for pyrotechnic mixtures or propellants and for explosives. The ignition properties of different energetic materials are important for understanding the ignition mechanism or choosing the best or suitable material for the current laser ignition application. One of the most important variables is the ignition energy. Thus it is reasonable to study the minimum ignition energy of many energetic materials and choose the best material among them. Other criteria may be, for example, the detonation properties of the current material as a booster or an igniter and the ageing properties of the current material.

A strong dependence between the ignition energy and the ambient pressure was observed in the results. For example, in the case of RDX98/1/1+1% carbon black the measured energy in an ambient pressure of 10 bar was 180 mJ and in an ambient pressure of 50 bar it was 32.6 mJ. Mean ignition energy densities were 29,9 J/cm2 and 5,4 J/cm2, respectively. The carbon black content’s effect on the ignition energy is clear between 1% and 3%, the ignition energy at 2% carbon black content being 27% lower than at 1% carbon black content, and 31% lower than at 3% carbon black content. According to the experiments, the mechanical properties of the RDX pellet are fragile at 3% or higher carbon black content. Thus the optimum carbon black content may be 1,5 to 2,5%.

According to the diode laser ignition experiments with synthetic air and argon the ignition energies are essentially the same in the same confinement. These results suggest that oxygen (in synthetic air) has no remarkable reactions in the laser illuminated point of the RDX pellet or more generally in the laser illuminated point of the explosive.

For nitrogen the ignition energies are slightly higher compared with air (and also with argon) in the same pressure. This is analogous compared with the CO2 laser ignition results at lower pressures by other authors, but at higher pressures of air iv and nitrogen the experimental results of this work and of the reference are in reverse order.

Evaporated RDX and gaseous decomposition products expand and will displace synthetic air, nitrogen or argon. Initial decomposition takes place in the vapour phase of RDX and on the surface of melted RDX. Highly exothermic reactions begin in the vapour phase and are followed by more rapid decomposition in the vapour phase and in the liquid phase and ignition of RDX. The rate of the reactions is deflagration, but it accelerates to the steady state detonation in the environment of high confinement. The conclusion that can be drawn is that the ignition process is not only a solid phase reaction but a complex process where gas, liquid, and/or solid phase reactions are involved.

According to this work, the degree of confinement has a strong role in the deflagration to detonation transition ignition mechanism. Using a degree of confinement that is heavy enough, a closed and tight mechanical structure, a high enough pressure inside the device, and good absorbance, for example using carbon black in the energetic material, the ignition energy would be low enough for economically viable applications of compact diode laser igniters.