Joseph T. Mang

As high explosive production capabilities modernize and performance models become more sophisticated, there is an increasing need for detailed microstructural information such as void size and morphology within pressed parts. Little is known of the relationship between the initial high explosive particle size distribution and the void size and morphology within a compact. Small-angle neutron scattering techniques were used to study the evolution of this relationship in TATB samples pressed from three distinct particle size distributions over the density range from 1.55 – 1.89 g/cc. In all samples studied the void volume is well described as ramified networks of voids. Details of the void network, including size and connectivity, will be discussed in terms of the pressed density and the initial TATB particle size distribution.

Ultra‐small‐ and small‐angle neutron scattering techniques were employed to quantify thermally driven changes to the microstructure of the TATB‐based composite high explosive, PBX 9502. Samples of PBX 9502 were studied in‐situ at temperatures ranging from 25 to 220 °C and after re‐equilibration at ambient temperature. Significant changes to the void size and morphology within the PBX 9502 microstructure were observed upon heating and are consistent with the known increase in the shock sensitivity of thermally‐treated PBX 9502. More extensive microstructural changes were found after the thermally treated samples were re‐equilibrated at ambient temperature. Utilizing a model of ramified porosity, the size and the volume and surface dimensions of voids in the PBX 9502 microstructure were quantified as a function of temperature. The temperature dependence of the void radius of gyration is well correlated with the known increase in the shock sensitivity of PBX 9502 with increasing temperature, providing a microstructural origin for the increased shock sensitivity of PBX 9502 at elevated temperatures.

Ultra‐ and small‐angle neutron scattering techniques were used to study the morphology of voids in directions parallel and perpendicular to the compaction plane of a uniaxially pressed cylinder of PBX 9502. Two‐dimensional scattering patterns from samples cut parallel to the compaction plane had circular equal‐intensity contours, implying that along this view the voids are circular or randomly oriented. Identical measurements of samples cut perpendicular to the compaction plane produced elliptical equal‐intensity contours, indicating preferred orientation of voids. Analysis of the two dimensional scattering patterns confirmed that the voids are non‐spherical with an average aspect ratio of 1.20, flattened so that the larger dimensions of the voids lie within the compaction plane. Model analysis of the combined ultra‐ and small‐angle neutron scattering curves revealed that the PBX 9502 porosity is interconnected, in the form of aggregates comprised of non‐spherical, primary voids. The volume dimension was found to be the same in both directions, indicating that the aggregate is randomly oriented throughout the sample, and that it is the primary voids that are preferentially oriented. Void interfaces with the surrounding matrix were found to be rough or polydisperse, with a common roughness dimension or polydispersity in all directions.

There are often situations where estimates are analyzed as measurements. This article considers in one application whether the raw data underlying these estimates should be analyzed directly.

Small‐angle neutron scattering techniques were used to study the evolution of void morphology with pressed density of the insensitive high explosive, TATB. Samples were studied as a loose powder and as pressed pellets, ranging in density from approx. 1 to 1.804 g cm⁻³. Inter‐granular voids in the loose powder were randomly arranged (non‐fractal) and had a surface defined mean size of 0.66 μm. Pressing was found to induce a fractal network of voids with fractally rough interfaces. The surface‐defined mean void size of the pressed samples was between 0.21–0.33 μm over the range of densities studied and was found to increase with pressed density up to 1.720 g cm⁻³, decreasing thereafter. The volume fractal dimension, indicative of the void arrangement, mirrored the changes in the mean void size. No systematic change in the surface fractal dimension was found. Surface area analysis allowed the average TATB grain size within the pressed samples to be quantified. An initial decrease of the mean grain size followed by an increase with pressed density suggests that the TATB grains behave in a brittle fashion at low densities and ductile at higher pressed densities.

Small-angle neutron scattering measurements were performed on the high explosive diaminoazoxy-furazan (DAAF) in order to quantify the void size and morphology as a function of the pressed density and the pre-pressed grain size. DAAF synthesized by different methods, resulting in volume-weighted mean grain sizes (radii) of 45,18, and 4 µm, was studied at pressed densities ranging from 1.583 to 1.682 g/cm3. The void volume is well described as a fractal network of voids with fractally rough interfaces. The mean void size, volume, and surface fractal dimensions were found to be dependent upon the pressed density. Mean void sizes ranged from 0.059 to 0.195 µm, but were correlated over lengths ranging from 2.1 to >9 µm. Volume fractal dimensions ranged between 2.13 and 2.78 and surface fractal dimensions were between 2.061 and 2.589. Methods are discussed by which the mean particle size and crystal density within a pressed sample can be calculated from the small-angle neutron scattering data.

Small‐angle neutron scattering with contrast variation was used to measure the interfacial surface area in a composite high explosive formulated with a deuterated binder. Continuing our work on the effect of varying the pressing intensity on void and binder size distribution, the effect of pressing intensity on the three interfaces (HMX‐binder, HMX‐voids and binder‐voids) of the PBX 9501 microstructure was studied. Formulation of PBX 9501 with a deuterated binder allowed the neutron scattering length density contrast to be varied and thus allowed differentiation of the three interfaces. Porod analysis was used to measure the surface area. The surface area at the interfaces of HMX and binder was found to increase with increasing pressing intensity, while the surface area between HMX and voids may have decreased slightly. No evidence was found for voids within the binder at any pressing intensity.

We describe organic aerogels derived from multifunctional isocyanates through reaction with water (polyureas), acid andydrides (polyimides) and carboxylic acids (polyamides). All processes are invariably single-step, one-pot and take place at room or slightly elevated temperatures. The resulting materials are robust, their density may vary over a very wide range and their nanomorphology can be either particulate or fibrous, but in all cases they all consist of similarly sized primary particles.