Investigation of Voids In Nanostructured RDX-Based Compositions Using Ultra-Small-Angle X-Ray Scattering

Investigation of Voids in Nanostructured RDX-based Compositions using Ultra-Small-Angle X-ray Scattering

 

Victor Stepanov¹, Trevor M. Willey² and Jan Ilavsky³

¹US Army, RDECOM-ARDEC, Munitions Engineering Technology Center

 Picatinny Arsenal, NJ 07806, USA

²Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, CA 94550, USA

³Advanced Photon Source, Argonne National Laboratory, Argonne, IL 60439, USA

 

Introduction

The structure of voids within novel cyclotrimethylene trinitramine (RDX)-based nanocomposites was investigated. Voids are an important structural element due to their strong influence on properties such as sensitivity and performance. In order to probe the “sealed” voids, synchrotron-based Ultra-Small-Angle X-ray Scattering (USAXS) with Bonse-Hart configuration was employed. This powerful technique enabled probing voids with sizes ranging from ca. 1 nm to 3 mm with statistically meaningful sample sizes. Scattering data on three compositions with varying RDX crystal size, including 200 nm, 500 nm, and 2 mm, was measured. Modeling of the scattering intensity profiles revealed bimodal, log-normal void size distributions for all samples. The mean void size was found to increase with the RDX crystal size and was found to be of similar dimensions to RDX crystals.   In this work, Ultra-Small-Angle X-ray Scattering (USAXS) was employed to characterize the voids within cyclotrimethylene trinitramine (RDX)-based nanocomposites. Nanostructured explosive compositions are gaining interest in regard to lowering the initiation sensitivity and improving performance. Recent work with nano-RDX-based compositions revealed that the shock and impact sensitivities were significantly lower compared to compositions with micron- scale HE crystals [Stepanov, Qiu]. Moreover, an important attribute of such nanocomposites was low shock sensitivity even at high porosities, ca. 10 %. It is expected that such behavior is a consequence of a very small mean voids size and the absence of large voids. Characterization of voids in these compositions is critical in explaining the observed behavior.   Compositions with 88 wt. % RDX and 12 wt. % binder were prepared by slurry coating of 200 nm, 500 nm, and fluid energy milled (FEM) RDX specimens. Formulated RDX specimens were pressed into nominally 5.08 mm x 0.8 mm pellets for USAXS analysis. The materials were pressed at 110 MPa and room temperature.     Results   Experimental data reduction and analysis were performed using the Indra[website] and Irena[Ilavsky 2009] codes respectively, which are subroutines created within IGOR Pro software specifically for treating scattering data. Representative desmeared scattering intensity profiles for the three compositions are shown in Figure 1.         Figure 1. USAXS intensity as a function of  for 200 nm, 500 nm, and FEM RDX-based compositions. A reference line with a slope of -4 is included.        The Maximum Entropy algorithm within the Irena package was used to evaluate the void size distributions. This inversion method converges to a unique size distribution for a given form factor by maximizing the configurational entropy of the calculated size. Among the required input parameters were shape and aspect ratio, range of diameters, and the scattering length density contrast. The calculated void size distributions for the three specimens are shown in Figure 2. All materials exhibit bimodal size distributions with log-normal peaks.
 
Figure 2. Void size distributions of 200 nm, 500 nm, and FEM RDX-based compositions calculated from USAXS data.     Based on the calculated size distributions it can be concluded that the mean void size is proportional to the crystal size.