The initiation and growth of explosion in the condensed phase

Abstract

The Slow Decomposition of Explosive Crystals: Electron microscope studies show that the thermal decomposition of some explosive crystals, e.g., silver cyanamide, occurs primarily on the surface. With others, e.g., silver azide, the surface reaction is again important but there is evidence also for some decomposition within the crystal which can cause it to break up into many small fragments. There is also evidence that, for many of these explosives, an actual melting must occur—it is only in the molten phase that the reacting species have sufficient mobility for a rapid acceleration to take place. The Effect of Fission Fragments and Nuclear Radiation: By choosing a crystal lattice which can be resolved in the electron microscope it has been possible to measure the extent of the damage with some precision. The damage depends upon the nature of the crystal and on other factors, but the track width may be approx. 100–120 . The disorder produced in the lattice and the holes and tunnels formed may be clearly seen. The damage may be interpreted on a thermal mechanism. The experiments at present suggest that even the intersection of two tracks would not produce explosion in an azide crystal. The intersection of three or more tracks within 10−11 sec is an unlikely event. The Effect of Discontinuities in Promoting Shock Initiation: Recent experiments with single crystals emphasize the importance of very tiny defects in the initiation of explosion by weak shock waves and the part they play in the growth process. A perfect crystal of silver azide is not initiated by a shock but a defect only a few microns in size will start reaction. This reaction in small crystals (0.01 to 1 mm) is a fast burning. With crystals greater than 2 mm it may grow to detonation. The defects may be present initially or may be introduced by a precursor stress wave moving at sonic velocity through the crystal. When a shock wave of appropriate intensity passes over a crystal containing multiple defects it can initiate a deflagration at each so that the forward movement of the deflagration is coupled to the shock wave. In this way a “pseudo-detonation” is set up. There is evidence that appropriately shaped cavities in liquids and solids can give rise to the formation of tiny Munro jets of high velocity. These might aid initiation by concentrating and increasing the velocity of impact and by breaking up the explosive.