J. Sharma

Nanofractography of shocked RDX explosive crystals with atomic force microscopy

Examination with atomic force microscopy has revealed apparent shear-type cleavage steps with heights as small as 0.05 nm, smaller than the size of cyclotrimethylenetrintramine (RDX) molecules, on the fracture surfaces of crystals that were subjected to aquarium shocks of 61.6 or 129 kbar, both greater than the pressure (38 kbar) required for the alpha-to-gamma phase transformation. The shocked centimeter size, originally transparent crystals became opaque and white from prolific fractures and internal cracks that are associated with their breakup into nanocrystallites of sizes extending from 500 down to 20 nm. The submolecular steps are related geometrically to the macroscale (K∥) fracture mechanics mode of shear fracturing that has obvious consequences at the nanoscale level for nonregistry between molecules across the crack surfaces. The results are of interest in relation to lattice trapping of crack fronts and as a contribution to the possibility of deformation-induced chemical decomposition/detonations.

Nanosecond and picosecond laser-induced cracking and ignition of single crystals of ammonium perchlorate

Nanosecond and picosecond laser irradiations have been used to study the decomposition of ammonium perchlorate (NH4ClO4) crystals, a main component of propellants. Chlorate (NH4ClO3) decomposition product was detected via X-ray photoelectron spectroscopy. The decomposition is initiated amid associated mechanical deformations and microcracking processes occurring on a time scale commensurate with actual frequencies of energetic crystal decompositions pertinent to propellant combustion. Optical, scanning electron and atomic force microscopy methods have been applied to characterization of the laser-damage zones. Individual initiation or residual “hot spot” sites have been detected in the electron and atomic force microscope images, and are related to the cracking behaviour of the perchlorate allotropic phases. Evidence of the 240°C orthorhombic to rock-salt type cubic transformation was obtained in nanosecond laser irradiations through a remnant microstructure of ultrafine cracks whose intersection points marked an array of decomposition sites. A dislocation model description is given for the connected cracking and decomposition site observations.

Structure of crystal defects in damaged RDX as revealed by an AFM

An atomic force microscope (AFM) was employed to reveal the structure of defects produced in single crystals of cyclotrimethylenetrinitramine (RDX), damaged either by indentation, heat or underwater shock. In general, all of these stimuli produced dislocation pits, cracks, fissures and mosaics, however, the details were different. Indentation generated a large number of triangular dislocation pits, which in their turn produced fissures, cracks and holes by coalescing. Heat produced fine parallel cracks. Slivers as thin as sixty molecules across were observed. Shock caused the crystal to become a three-dimensional mosaic structure, 100–500 nm in size, produced by intensive cleavage and delamination. In all cases very fine particles, 20–500 nm in size, were ejected onto the surface as debris from the formation of defects. The AFM has revealed for the first time un-etched dislocation pits in their pristine condition, so that their internal structure could be investigated. A dislocation density of 106 cm−2 ha...

Determination of radiation exposure history of common materials and computer hardware by using atomic (and magnetic force) microscopy

Defects produced by ionizing radiation are smaller than a micrometer and are unobservable in an optical microscope. An atomic force microscope was utilized to reveal their counts and structure in common materials like mica, silicon, organic solids, polymers, sugar, quartz, and calcite. A magnetic force microscope has shown the damage of radiation on computer hard disks. The present work shows that exposure to radioactive material leaves a permanent record, which can be read for dosimetric or forensic purposes by using atomic force microcopy on common objects or a magnetic force microscope on magnetic media.

DISLOCATION DENSITY VARIATION IN SHOCKED SINGLE CRYSTAL AMMONIUM PERCHLORATE

Single crystal AP has been shocked at 24.4 kbar while immersed in mineral oil. The crystal remained intact, however, was nonuniformly “cloudy” in appearance due to the generation of defects (dislocations). X-ray photoelectron spectroscopy (XPS) Cl(2p3/2) linewidths with respect to position in the shocked ammonium perchlorate (AP) crystal have been found to correlate with the extent of damage induced by shock loading. A width of 1.58 eV was obtained from the Cl(2p3/2) photopeak in the area of greatest damage, compared to 1.22 eV for control AP. A Vickers hardness impression made prior to shocking was found to concentrate the formation of dislocations. Production of up to 9.5% chlorate [Cl, (+5)], as a partial decomposition product, was detected in the vicinity of the impression.

Sub‐molecular Fracture Steps in Shock‐Shattered RDX Crystals and Follow‐On Nano‐Indentation Evaluation of Early Stage Plasticity

Nano‐crystallites of RDX produced by aquarium shock, were examined using atomic force microscopy and found to contain sharply defined, apparent shear‐type (Mode II) fracture steps having heights less than the size of an RDX molecule. The sub‐molecular steps run for substantial distances along crystallite surfaces, thus indicating concerted disregistry in depth between juxtaposed molecules across the crack surfaces. The observation of locally jumbled regions of displaced/misoriented molecules suggests an obstacle barrier, not previously considered, to either shear crack propagation or dislocation pile‐up release with sufficient stress intensity to cause hot spot initiation. Recent follow‐on nanostructural results are reported for separate cleaved RDX crystals containing nano‐indentations showing only plastic deformation with evidence of cracking.

Nanostructure of porosity (and entrapped solvent effects) in laboratory-grown crystals of RDX as revealed by an AFM

Internal porosity within laboratory-grown crystals of RDX has been investigated with an atomic force microscope (AFM) in extension of previous work [1]]. The crystals were cleaved along {001} planes so as to provide cross-sections of the defects. The pores showed up as nano-caverns, ranging in size from 10 nm to a few microns, with very complicated shapes and tentacle-like arms. They were only 5–200 nm deep, and were seen in various orientations. The complex shapes seem to arise from anisotropic local crystal growth and from thermal influences with associated redistribution of solvent and solute. RDX crystals, grown in an ultracentrifuge were also investigated. They did not show any bubbles or solvent inclusions. Both kinds of crystals studied, frequently showed one molecule high terraces running over distances of microns. These seem to have been left by edge dislocations which have egressed out of the crystals altogether.

Nanofractography of Composition B Fracture Surfaces With AFM

Abstract : The characteristics of TNT crystals in the fracture surface of Composition B (a melt-cast composition of RDX, TNT and wax) have been studied using atomic force microscopy (AFM). The sub-microscopic size of the TNT crystal component is revealed by the surface structure that is exhibited after mechanical failure. The failure surfaces are produced by subjecting the material to high acceleration in an ultracentrifuge under conditions in which the shear or tensile strength is exceeded. AFM examination of the TNT component fracture surface topography reveals that essentially brittle cleavage has occurred across the very finely-spaced columnar grains. The width of the grains varies narrowly in size between ^ 1 m and ^ 2 m. The height elevations of the inclined and stepped surfaces range in size between ^ 50 nm to ^ 300 nm. Shear-type deformation that has occurred prior to cleavage fracture is evidenced in the lowest magnification images, that is, for the largest AFM scans (8 m and 13.5 m). Specularly cleaved column surfaces alternate with ones containing profuse river patterns identifying the directions of crack growth. The river pattern markings are observed to originate at the boundaries between adjacent columns whereas the average profile angles between adjacent columnar surfaces are within the theoretical limit for dislocations. The smallest, individual, nm-size step heights at the river pattern ledges are smaller than the unit cell dimensions of either allotropic crystal phase of TNT. Such steps, smaller than unit cell dimensions, may relate to the important issue of breaking intramolecular bonds.

AFM Studies of Fracture Surfaces of Composition B Energetic Materials

The characteristics of TNT (trinitrotoluene) crystals in Composition B have been studied using atomic force microscopy (AFM). The size of TNT crystals has been examined by analyzing the surface structure that is exhibited after mechanical failure of the Composition B. The mechanical failure occurs when the material is subjected to high acceleration (high g )inan ultracentrifuge and. the shear or tensile strength is exceeded. AFM examination of the topography of the Composition B fracture surface reveals fracture across columnar grains of the TNT. The width of the columnar TNT grains ranges in size from ~ 1 μm to~2 μm. Their height ranges in size from ~ 50 nm to ~ 300 nm.