Joseph T. Mang

Combustion velocities and propagation mechanisms of metastable interstitial composites

Combustion velocities were experimentally determined for nanocomposite thermite powders composed of aluminum (Al) fuel and molybdenum trioxide (MoO3) oxidizer under well-confined conditions. Pressures were also measured to provide detailed information about the reaction mechanism. Samples of three different fuel particle sizes (44, 80, and 121nm) were analyzed to determine the influence of particle size on combustion velocity. Bulk powder density was varied from approximately 5% to 10% of the theoretical maximum density (TMD). The combustion velocities ranged from approximately 600 to 1000m∕s. Results indicate that combustion velocities increase with decreasing particle size. Pressure measurements indicate that strong convective mechanisms are integral in flame propagation.

One-step room-temperature synthesis of fibrous polyimide aerogels from anhydrides and isocyanates and conversion to isomorphic carbons

Monolithic polyimide aerogels (PI-ISOs) have been prepared by drying wet-gels synthesized via a rather underutilized room-temperature reaction of pyromellitic dianhydride (PMDA) with 4,4′-methylene diphenyl diisocyanate (MDI). The reaction is followed up to the gelation point by liquid 13C-NMR in DMSO-d6 and it proceeds through a seven-member ring intermediate that collapses to the imide by expelling CO2. PI-ISOs are characterized comparatively with aerogels referred to as PI-AMNs, obtained via the classic reaction of PMDA and 4,4′-methylenedianiline (MDA). The two materials are chemically identical, they show similar degrees of crystallinity (30–45%, by XRD) and they both consist of similarly sized primary particles (6.1–7.5 nm, by SANS). By N2-sorption porosimetry they contain both meso- and macroporosity and they have similar BET surface areas (300–400 m2 g−1). Their major difference, however, is that PI-AMNs are particulate while PI-ISOs are fibrous. The different morphology has been attributed to the rigidity of the seven-member ring intermediate of PI-ISOs. PI-AMNs shrink significantly during processing (up to 40% in linear dimensions), but mechanically are much stronger materials than PI-ISOs of the same density. Upon pyrolysis at 800 °C both PI-ISO and PI-AMN are converted to porous carbons; PI-AMNs loose their nanomorphology and more than 2/3 of their surface area, as opposed to PI-ISOs, which retain both. Etching with CO2 at 1000 °C increases the BET surface area of both PI-AMN (to 417 m2 g−1) and PI-ISO (to 1010 m2 g−1), and improves the electrical conductivity of the latter by a factor of 70.

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.

Quantitative analysis of damage in an octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazonic-based composite explosive subjected to a linear thermal gradient

The microstructure within a slowly heated, consolidated explosive will be influenced by both physical changes and chemical reactions prior to thermal ignition. Thermal expansion, exothermic decomposition, endothermic phase change, and increased binder viscosity play significant roles in the cook-off to detonation. To further explore the details of this intricate cook-off process, we have conducted a series of experiments in which a carefully controlled temperature gradient has been applied along a cylinder of PBX 9501 [94.9/2.5/2.5/0.1-wt % octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX)/Estane 5703/a eutectic mixture of bis(2,2 dinitropropyl) acetal and bis(2,2-dinitropropyl) formal [abbreviated BDNPA-F]/Irganox] and maintained for a specified amount of time. After heating and subsequent cooling of the PBX 9501, the sample morphology has been probed with polarized light microscopy and small-angle x-ray scattering. Using these techniques we have quantitatively characterized the particle morphology,...

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.

Small-angle neutron scattering study of a thermally aged, segmented poly(ester urethane) binder

Abstract Small-angle neutron scattering (SANS) measurements have been performed on a thermally aged polymeric binder to understand the effects of aging on its microstructure. The binder is a 50%–50% (by weight) mixture of a segmented poly(ester urethane), known as EstaneB 5703, and a nitro-plasticizer (NP). This compound is of interest because it is used in the high-explosive (HE) PBX9501. The addition of the polymeric binder to the crystalline HE imparts both structural integrity and plasticity to the HE, allowing it to be readily machined and pressed to specific densities. In addition, the binder significantly affects the performance and the sensitivity of the HE, due in part to its influence on the propagation of microstresses between crystalline grains under shock or loading conditions. So changes in the binder over time can influence the behavior of the HE system as a whole.

Lyotropic liquid crystals under simple couette and oscillatory shear

Abstract Small angle neutron scattering (SANS) has been utilized to investigate the flow properties of the discotic micellar isotropic (I), nematic (N) and lamellar (L α) phases in aqueous solutions of cesium perfluoro-octanate. Under simple shear, no preferred orientation was observed in the I phase. The N phase aligned with the nematic director along the shear gradient velocity, providing the first evidence for a positive value of the viscosity parameter, α2. The L α phase oriented with lamellae parallel to the shear plane. A reorientation of the director near the N-L α transition was witnessed upon cooling from the N phase at a constant shear rate and is attributed, primarily to a change in the value of α2 as a result of increasing lamellar correlations in the nematic phase. The application of oscillatory shear induced significant shifts in the phase boundaries, including nematic-like order some 7 K into the quiescent I phase.

Characterization of detonation soot produced during steady and overdriven conditions for three high explosive formulations

The detonation of high explosives (HE) produces a dense fluid of molecular gases and solid carbon. The solid detonation carbon contains various carbon allotropes such as detonation nanodiamonds, onion-like carbon, graphite and amorphous carbon, with the formation of the different forms dependent upon pressure, temperature and the environmental conditions of the detonation. We have collected solid carbon residues from controlled detonations of three HE formulations (Composition B-3, PBX 9501, and PBX 9502). Soot was collected from experiments designed to produce both steady and overdriven conditions, and from detonations in both an ambient (air) atmosphere and in an inert Ar atmosphere. Differences in solid carbon residues were quantified using X-ray photoelectron spectroscopy and carbon isotope measurements. Environmental conditions, HE formulation, and peak pressures influenced the amount of and isotopic composition of the carbon in the soot. Detonations in an Ar atmosphere produced greater amounts of ca...

Oedometric Small-Angle Neutron Scattering: In Situ Observation of Nanopore Structure During Bentonite Consolidation and Swelling in Dry and Hydrous CO2 Environments

Results of oedometric consolidation experiments linked with small-angle neutron scattering (SANS) measurements are presented, using SWy-2 Wyoming bentonite clay in dry and water-bearing N2 and CO2 atmospheres. Oedometric SANS involves deforming a porous sample under uniaxial strain conditions with applied axial force and internal pore pressure control, and combines with SANS for in situ observation of pore structure evolution and interaction. Scattering from both interlayer (clay intra-aggregate) and free (interaggregate) pores is observed, showing decreasing pore size with dry consolidation and interactions between interlayer and free pore types with swelling and consolidation. Introduction of dry liquid CO2 at zero effective stress (axial stress minus pore pressure) produces large shifts in interlayer scatterers, but is reversible back to pre-CO2 levels upon decreasing pore pressure and increasing effective stress. Introduction of wet liquid CO2, conversely, produces large but irreversible changes in interlayer scatterers, which are interpreted to be the combined result of CO2 and H2O intercalation under hydrostatic conditions, but which diminish with application of effective pressure and consolidation to higher bentonite dry densities. Consideration of CO2 intercalation in smectite-bearing CO2 caprocks needs to include effects of both water and nonhydrostatic stress.

CHARACTERIZING THE EFFECTS OF RATCHET GROWTH ON PBX 9502

Pressed composites of TATB (2,4,6‐trinintro‐l,3,5‐benzenetriamine) undergo irreversible volume change when subjected to thermal cycling. Using micro x‐ray computed tomography and ultra‐small angle neutron scattering, we have characterized the micro‐structure of as‐pressed and ratchet grown specimens of PBX 9502, a TATB‐based composite, thereby distinguishing the effects of ratchet growth from the effects of density alone. Porosity differences are shown to effect mechanical properties, presented here, with ongoing efforts to evaluate sensitivity and/or performance effects.