Dislocation Pile-Up Mechanism for Initiation of Energetic Crystals
Advances in Chemical Reaction Dynamics
The role of dislocations in assisting initiation of (explosive) chemical decomposition of energetic materials has connection with the known influences for crystals and polycrystals of dislocations facilitating permanent deformations and phase transformations. X-ray topographic observation of relatively few dislocations in solution-grown crystals relates to the influence of large Burgers (displacement) vectors that are characteristic of molecular crystal bonding. Both model evaluations of the
... aluations of the load dependence of cracking at hardness indentations and the derived hardness stress-strain behaviors show that dislocation movement is difficult whether in the indentation strain fields or at the tips of indentation-induced cracks. Thus, energetic crystals are elastically compliant, plastically hard, and relatively brittle . Nevertheless, cracking is shown to be facilitated by the shear stress driven, normally limited, dislocation flow that, on molecular dynamics and dislocation pile-up model bases, is shown to be especially prone to producing localized hot spot heating for explosive initiations. Such model consideration is in agreement with greater drop-weight heights being required to initiate smaller crystals. The crystal size effect carries over to more difficult combustion occurring for compaction of smaller crystals. The total results relate to dual advantages of greater strength and reduced mechanical sensitivity accruing for the development of nanocrystal formulations. In consequence, also, several levels of dislocation-assisted modeling are described for initiation mechanisms under shock wave loading conditions. Dislocation origins in energetic crystals The crystallographically imperfect structures of conventional, solution grown, energetic crystals, for example, of RDX (cyclotrimethylenetrinitramine), show a segmented growth sector structure in agreement with dislocation-based crystal growth predictions , as shown in Figure 1a -d. In 1a, first, a cross-sectional sketch is shown of the overall growth sector and boundary structure that is normally developed during progressive growth of a polygonally-shaped crystal and, in 1b, is shown separately the threading dislocation "bundle" structure emanating from a crystal seed, S. The dislocations produce crystal growth more easily than any other mechanism. As indicated in 1b, the dislocation bundles may emanate from inclusion particles. Profuse nucleation of such dislocation bundles was shown to occur at interrupted sapphire crystal growth surfaces via x-ray diffraction topographic imaging of crystal sections  . Here, a tabular RDX crystal sectioned parallel to its planar (001) top/bottom surface  is shown in Figure 1c .