Harold Sandusky

Preparation and reactivity analysis of novel perfluoroalkyl coated aluminium nanocomposites

Abstract Passivation of unpassivated aluminium nanoparticles using C13F27COOH is reported with materials containing as much as 32·95%Al. Characterisation data, including scanning electron microscopy (SEM), thermogravimetric analysis (TGA) and attenuated total reflectance Fourier transform infrared (ATR FTIR) spectrometry, indicate that the C13F27COOH molecule binds to the surface of the Al particle, protecting the surface from oxidation in ambient air. The fast reaction capability of the Al–C13F27COOH material was investigated using two different means. Response of the material to strong shock was analysed using the small scale shock reactivity test (SSRT). SSRT results show that the Al–C13F27COOH material formulated with cyclotetramethylenetetranitramine (HMX) and hydroxyl terminated polybutadiene (HTPB) binder produced a significantly larger dent than comparable conventional Al specimen. Additionally, laser ablation/ignition studies of the prepared Al–C13F27COOH material demonstrate the fast reaction capability of the material.

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.

Relating deformation to hot spots in shock-loaded crystals of ammonium perchlorate

The purpose of this work is to perform a microscopic-scale study of the role that crystal defects have in forming hot spots during shock loading of large, optical quality, pure single crystals of ammonium perchlorate (AP). The crystals were immersed in mineral oil at various distances from a detonator that provided the shock. The small explosive donor permitted recovery of the crystals for quantitative chemical analysis of decomposition and microindentation hardness testing. Hardness testing was also performed on an unshocked crystal to determine 1) the slip systems associated with primary and secondary deformation in accommodating the indenter and 2) the crack propagation directions at the surface as well as into the crystal. High-speed photographs of the shock-loaded crystals showed slip and cracking systems identified by hardness testing. Some of the systems were luminous. In addition, when a crystal with a large indentation was shocked near its reaction threshold, significant light appeared in the vicinity of the identation following shock passage. As such, preferred chemical reactivity in AP has been associated with its deformation systems and the presence of large strain centers.

Microwave interferometry of shock waves. I. Unreacting porous media

Microwave interferometry appears to be a promising method for the study of wave and mass velocity in shocked dielectric materials. This paper discusses the mathematics concerning the frequencies and amplitudes in the microwave reflections from the shock wave and from an impacting piston which drives the shock into a nonreacting porous solid. Methods for the determination of the state variables in the compressed region between the shock wave and the piston are given. In this paper, these methods have been confirmed by experimental measurements in nonenergetic materials in which there is no ionization from chemical reaction. The microwave reflection from the shock wave traveling through an inert media is shown to result from a dielectric discontinuity at the wave front. Studies on energetic materials (explosives and propellants) are considered in a companion paper where ionization associated with the chemical reactions is required to explain the increased reflection from the shock front.

Microwave interferometry of shock waves. II. Reacting porous media

Shocked porous ball powders have been investigated by means of microwave interferometric techniques. Equations developed for interferometric measurements on inert materials apply for energetic materials until reaction at the shock front begins. As reaction begins, the microwave interferometer (MI) output exhibits characteristic changes in both the absorption and reflection of the microwave signal. These changes have been related to hot spot development at the shock front. The hot spots in a reacting bed have been experimentally approximated to a first order by including metallic particles in unreacting beds and measuring their effects on the propagation of microwaves. With the aid of dielectric measurements of the metallized beds, hot spot concentrations as a function of time were predicted from the MI output of reacting beds.

Microstructural basis for enhanced shock-induced chemistry in single crystal ammonium perchlorate

The effect of concentrated lattice defects (dislocations) on shock reactivity was investigated for an optical quality, single crystal of high-purity ammonium perchlorate. Prior to shock loading, localized regions of increased lattice defects and strain were created by placing diamond pyramid (Vickers) hardness impressions into exterior cleavage surfaces. The crystal was immersed in mineral oil with the (210) surface 6.0 mm from a detonator. When fired, the detonator delivered a 24.4-kbar shock, corresponding to the reaction threshold for that crystal orientation. High-speed photographs showed luminosity near some of the hardness impressions. The photographs also revealed the occurrence of the same slip deformation identified previously from hardness testing. The shocked crystal was recovered intact and cleaved twice through Vickers impressions on the (001) and shockentry (210) surfaces, allowing spatial chemical analysis of the interior regions of the crystal using x-ray photoelectron spectroscopy (XPS). Along these freshly cleaved surfaces, the XPS results showed enhanced perchlorate decomposition as a result of the impressions. The greatest decomposition was not immediately adjacent to the impressions, but near the tips of cracks and along slip planes that extended, at least several millimeters, from these impressions.

DEVELOPMENT OF THE SMALL‐SCALE SHOCK SENSITIVITY TEST (SSST)

A small‐scale test to measure shock sensitivity with less than a half gram of sample per test and six tests at most was developed. The goal is to screen new energetic compositions before the need for costly scale‐up. The concept merged aspects of the Small‐Scale Shock Reactivity Test (SSRT) developed at IHDIV, NSWC with those of standard gap tests. The SSRT measures the shock reactivity (explosiveness) of potentially energetic materials, often well‐below critical diameter, without requiring a transition to detonation. Gap tests are used to gage shock sensitivity of explosives, but require a sample size large enough for steady detonation. The new test arrangement combined the shock‐attenuating gap before the sample and the air gap after the sample found in gap tests with the small sample size and high confinement of the SSRT. The results for a plastic‐bonded explosive formulated with either a regular or insensitive RDX confirmed the difference in sensitivities observed in traditional gap tests. Also, the r...

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...

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.

Defect density measurements in shocked single crystal ammonium perchlorate by x-ray photoelectron spectroscopy

The linewidths of x-ray photoelectron spectra have been correlated with dislocation densities in a shock-damaged crystal of ammonium perchlorate (AP). A centimeter-size AP crystal was loaded at several sites with a diamond pyramid (Vickers) indenter, creating localized strain centers. The crystal was nonuniformly damaged by a rapidly decaying shock (peak pressure of 24.4 kbar at the entry surface), recovered intact, and cleaved through the indentations. The cleaved planes permitted interior analysis of the crystal by x-ray photoelectron spectroscopy (XPS) over a pattern of 1 mm by 1 mm areas. The linewidth of the Cl(2 p 3/2 ) spectra ranged from 1.70 eV for the region of greatest visible damage to 1.22 eV for the region of no visible damage, the same linewidth as that obtained for unshocked AP (control). The observed damage was compared to photographs in the literature of gamma-ray irradiated AP crystals, for which dislocation densities were reported. This provided an approximate correlation of dislocation density versus XPS linewidth. The correlation was refined by chemically etching and determining densities on another cleaved plane in the recovered crystal. By this technique, a ∼100X increase in dislocation density was determined for the region of greatest shock damage relative to an unshocked crystal. The strain fields associated with the impressions were found to enhance perchlorate decomposition when driven by shock. Distortion of the molecular lattice in the vicinity of a dislocation is the physical mechanism responsible for the broadening of the photoelectron lines. Ab initio calculations of the Cl(2 p ) energy level in the perchlorate anion predicted variations of 0.1 to 0.46 eV. Variations of this magnitude are sufficient to produce the observed linewidth broadening.