Preferred Void Orientation in Uniaxially Pressed PBX 9502

Abstract

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 equalintensity 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 nonspherical, 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.

Thermally‐Driven Changes to Porosity in TATB‐Based High Explosives

“…This projection produces isotropic scattering. However, previous measurements [19] made with neutrons incident perpendicular to the pressing direction revealed that the scattering intensity in this direction is dependent upon the direction of Q ( Q ), and that the Porod Invariant is two‐dimensional. Application of Eq.…”

“…A simple two‐phase approach can be used to describe the observed scattering [22]. The two‐phase approximation can also be used for PBX 9502 ( ρ =4.83×10 10 cm −2 ) samples, as the difference in scattering length density between TATB and Kel‐F 800 ( ρ =3.98×10 10 cm −2 ) is small, and contributes little to the scattering signal [19]. The observed scattering arises from the void phase, although in this case, it is contrasted against the TATB/Kel‐F 800 matrix.…”

“…Without phase information, it is occasionally possible to describe the observed scattering equally well by more than one physical model. Indeed, the small‐angle scattering from PBX 9502 has been equivalently modeled as arising from either highly polydisperse, uncorrelated spherical voids or correlated, ramified structures [10, 12, 18–19]. In a preliminary report of this work [23], the two approaches were shown to provide equivalent information.…”

“…Recent scattering studies have demonstrated that voids within uniaxially pressed PBX 9502 samples are non‐spherical and preferentially oriented [19]. For the current measurements, neutrons were incident parallel to the sample pressing direction.…”

“…The use of small‐angle scattering techniques to probe the total (inter‐ and intra‐granular) porosity of pressed samples of high explosives has increased in recent years due to their ability to provide unique and quantitative measurements of void size and morphology over length scales that are known to strongly affect shock initiation [12, 17–19]. For example, recent ultra‐ and small‐angle neutron scattering (USANS and SANS) studies have revealed that the voids within the microstructure of uniaxially pressed PBX 9502 are anisotropic, and preferentially oriented in a plane (the compaction plane) perpendicular to the pressing direction [19]. Such new microstructural details may be helpful in the development of microstructural‐aware models of high explosive performance [20].…”