Cary B. Skidmore

Characterization of HMX particles in PBX 9501

The particle size distribution and morphology of HMX (cyclotetramethylene-tetranitramine) in the plastic-bonded explosive, PBX 9501 (95% HMX and 5% polymeric binder, by weight), are important to understanding the micromechanical behavior of this material. This paper shows that the size distribution of the “as-received” HMX powder, as measured by light scattering, is not preserved through the processing operations of formulation into molding powder and subsequent consolidation through hydrostatic pressing. Morphological features such as cracking and twinning are examined using reflected light microscopy. This technique helps confirm and interpret the results of the particle size analysis. These results suggest that use of the particle size distribution of the “as-received” powder could potentially yield significant errors in detailed simulations of formulated materials.

Application of small-angle neutron scattering to the study of porosity in energetic materials

Small-angle neutron scattering (SANS) and the method of contrast variation were used to measure porosity and crystallite surface area in the energetic system octahydro-1, 3, 5, 7- tetranitro-1, 3, 5, 7-tetrazocine (HMX) and to gauge the effects of mechanical deformation on the pore-size distribution and crystallite surface area. The crystallite surface area and the presence of voids (pores) in a high explosive system are known to affect its behavior and overall performance. Measures of these two quantities after an insult, resulting from various process and accident scenarios, can be used to predict the performance of an explosive system after process- and accident-related mechanical deformation. The contrast variation technique allows us to discriminate between internal pores and features that are on or contiguous with the crystallite surface. Measurements were conducted on loose powders of HMX (261 and 10 {mu}m, volume averaged mean particle diameters) and pellets made by uniaxial consolidation to 7 and 10 vol% porosity, respectively. Analysis of the SANS data indicates significant alteration of the intragranular pore structure and systematic shifts in the surface area that are dependent upon mechanical deformation. (c) 2000 Materials Research Society.

Microstructural effects in PBX 9501 damaged by shear impact

Various microstructural mechanisms have been suggested for ignition in explosives subjected to impact by low-velocity projectiles. In this study, the effects of shear on the microstructure of PBX 9501 are described. The pseudo two-dimensional, shear-impact experiment, previously employed by Asay, et al. to dynamically study strain localization, is engaged to create shear damage. Impact is achieved by utilizing a gas gun projectile to drive a plunger, which is in contact with the explosive. Post-test microstructural analysis corroborates the observations of other researchers using different diagnostics. Observed features include evidence of shear displacement, the formation of a wedge structure, and reaction in open cracks emanating from the high shear region of the sample. This study also contributes insights concerning the behavior of HMX particles subjected to shear stress.

Second-harmonic generation and the shock sensitivity of TATB

The recent discovery of significant differences in second-harmonic generation (SHG) from various grades of 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) prompted an investigation into the sensitivity of TATB based upon the crystal structure and morphology as indicated by SHG intensity. The Floret test was used as a means for rank ordering the shock sensitivity properties of fine TATB samples. Two types of TATB, which showed low SHG and performed poorly, were heated to obtain a much higher level of SHG, and then tested to compare sensitivities. One of the samples was significantly desensitized, and the sensitivity of the other was unchanged. Interpretations are presented which attempt to explain the relationship of SHG to the shock sensitivity of TATB. Although particle size and pressing density appear to be the main drivers for shock sensitivity, SHG intensity evaluations may help explain departures from those trends.

Particle characterization of pressed granular HMX

It is widely accepted that particle size and morphology in granular beds of HE plays a large role in combustion and detonation events. This work reports the characteristics of coarse granular HMX (Class A) at a range of densities from stock density to 95% TMD. We report measurements of the particle size distribution of original granular HMX, as well as the size distribution of pressed (higher density) samples. Scanning electron microscope (SEM) pictures are presented and are found to be useful in interpreting the size distribution measurements of the granular HMX, as well as in helping to more fully characterize the state of the particles. We find that the particle size distribution changes significantly with pressing. Particles are observed to be highly fractured and damaged, especially at higher pressed densities. Also, we have found that sample preparation can significantly affect size distribution measurements. In particular, even short duration ultra-sonic or “sonication” treatment can have a signifi...

Structural characterization of energetic materials by small angle scattering

Microstructural aspects (particle size, defect structures, etc.) of energetic materials can affect their response to certain stimuli and are thus of great interest from both safety and performance perspectives. Small angle scattering (SAS) is well suited for microstructural characterization of energetic materials, allowing for quantitative measurement of particle/pore (open and closed) morphology and size distribution, as well as surface area. Here, we present small angle neutron (SANS) and x-ray (SAXS) scattering measurements of loose powders and pressed pellets of the energetic materials HMX, PBX 9501 and TATB. Analysis of the SANS and SAXS data reveals number averaged particle size distributions in good agreement with light scattering techniques and significant alteration of the intragranular pore structure and pressing-dependent shifts in the surface area.