Femtosecond micromachining of internal voids in high explosive crystals for studies of hot spot initiation

Abstract

Femtosecond micromachining was used to produce controlled patterns of internal voids in high explosive single crystals of 1,3-dinitrato-2,2-bis(nitratomethyl) propane (PETN), 1,3,5-trinitroperhydro-1,3,5-triazine (RDX) and 1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX). The micromachined voids were characterized with optical microscopy and confocal Raman microscopy. Optical microscopy established that the voids generated near the threshold energy were localized to submicrometer diameters. Increasing the micromachining energy above threshold led to microcracking along preferred crystalline planes. Consolidation of hundreds to thousands of individual voids allowed creation of defined two- and three-dimensional structures. Production of three-dimensional consolidated structures led to extended crystal damage or residual strain over tens to hundreds of micrometers. Confocal Raman microscopy established that the defects generated were voids, with no chemical products observable and with diminished crystal spectral intensity. The results of this work suggest that large controlled arrays of internal voids can be produced in explosive crystals, with the exception that continuous three-dimensional defect structures are possible only if the extended damage is acceptable. These methods and materials are expected to be valuable for controlled studies of hot spot initiation in shocked explosives.