Using percolation theory to interpret microstructure’s role in hotspot formation and growth.*

To understand the formation and growth of hotspots in a polymer bound explosive (PBX), documenting microstructure features to interpret respective shock-induced reactivity is imperative. However, acquiring images of well resolved microstructure features and developing meaningful descriptors has been a challenge. In this work, we cross-section PBX samples to probe microstructure. We analyze the micrographs of sectioned faces to present descriptors such as the percentage of a particle perimeter in contact with neighbors, size of polymer islands between crystal clusters, number of clusters and their size distributions, among others.  We apply the concepts of percolation theory to identify ‘phase changes’ where packing dynamics of crystals change markedly as more explosive is added to the PBX. The concentrations at which these microstructural changes are observed are also expected to show notable changes in shock-driven reactivity. Additional examples will be discussed to elaborate on the importance of chosen microstructural features.