Brian C. Glancy

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.

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.

MICROWAVE INTERFEROMETRIC HOT SPOT DENSITY MEASUREMENTS IN ENERGETIC MATERIALS

Frequency analysis of microwave interferometric signals from impacted and shocked energetic materials has been used to measure front and particle motion. More recently, amplitude analysis of the signals was used to estimate hot spot density behind the compaction front in impact experiments on TS 3659 ball propellant. This report presents interferometric measurements from experiments on the high explosives tetryl and HMX, and contrasts these data with those from the propellant. Data presented indicate some early reaction near the piston face in HMX that was not evident in the ball propellant experiments. Also, the reflected microwave signal from the HMX detonation front is unlike that seen in tetryl or the propellant, possibly indicating a difference in its mechanism of detonation.