Resolving Detonation Nanodiamond Size Evolution and Morphology at Sub-Microsecond Timescales during High-Explosive Detonations

Abstract

Characterization of the initial morphology of detonation nanodiamond (DND) has been the focus of many research studies that aim to develop a fundamental understanding of carbon condensation under extreme conditions. Identifying the pathways of DND formation has the potential for significant impact on many of the controlled synthesis of nanoscale carbon with a tailored functionality; currently, a wide range of possible (and conflicting) mechanisms of nucleation and growth have been proposed, and further research is essential. Building a comprehensive understanding of DND formation is challenging because it requires in situ characterization on the sub-microsecond (sub-μs) timescale during a high-explosive detonation. In this study, time-resolved small-angle X-ray scattering (TR-SAXS) is used to reveal the early-stage DND morphology from <0.1 to 6 μs after the detonation front passes through the X-ray beam path. We address the ambiguity of models previously reported for the analysis of small-angle scattering from DND by comparing (i) in situ, TR-SAXS recorded during early-stage particulate formation and (ii) ex situ SAXS and transmission electron microscopy (TEM) measurements of products recovered from detonation of the same high explosive within a carefully designed ice chamber. The SAXS from both late-time (>1 μs) in situ and recovered DND exhibits consistent features in the I(q) curve. Such a close similarity allows a high-fidelity SAXS model derived from the ex situ SAXS and TEM measurements to be applied to the in situ data, which yields new insight into the early-stage (<1 μs) morphology of DND. Our analysis indicates that the size distribution of DND particles is observed within 0.1 μs postdetonation, which indicates that carbon is condensed within the reaction zone. Between 0.1 and 0.3 μs, the mean size of the diamond particles increases slightly toward 4 nm, and evidence of surface texture is observed. Based on TEM imaging, this surface texture consists of hemispherical protrusions that extend ∼1 nm from the surface. Beyond 0.3 μs, neither the mean size of the diamond core nor the surface texture changes significantly for several microseconds after the detonation. Combined with thermochemical simulations, these results indicate that during detonation of composition B, carbon is condensed into nanoscale diamond much faster than that previously reported in other studies. Furthermore, the surface texture of the DND is shown to arise during condensation rather than via subsequent graphitization.

Observation of Variations in Condensed Carbon Morphology Dependent on Composition B Detonation Conditions

“…Copious detonation nanodiamond appears in TEM imaging of recovered soots from both the conventional and the colliding detonation, and determination of the most prevalent morphology in the products facilitates the SAXS modeling. Collected recovered detonation products from colliding detonations contain an abundance of detonation nanodiamond (Figure ) and some graphitic fibers, all similar in size and morphology to what has been observed during conventional Composition B detonations . Electron diffraction confirms a mixture of graphite and diamond, with extensive regions of high diamond content.…”

“…The evolution of the temperature and pressure, obtained by thermochemical modeling, superimposed on the carbon phase diagram, appears in Figure b. The conventional detonation C−J is deeply within pressure and temperature conditions for diamond as previously demonstrated . In the colliding case, the pressure‐temperature evolution is highly position‐dependent because the x‐ray scattering measurement samples all points along a diameter through the part with a beam that is ∼100 μm wide and 50 μm tall, while the angle between curved detonation fronts locally continuously changes with position from planar at the central position to angled at the cylinder edge.…”

“…Many models exist for small angle scattering from detonation nanodiamond and each has its advantages and limitations. In most cases, detonation nanodiamond is considered to be aggregated spherical particles with a surface that is not necessarily smooth nor well‐defined, as the high‐q scattering does not follow a perfect Porod decay . Here, the model accounts for the departure from Porod scattering by including surface texture, as it mimics the morphology observed by TEM imaging and has been proven to reproduce an accurate size distribution from the SAXS data .…”

“…The kinetics of particulate growth, measured by SAXS, requires that the data be fit to a model that can account for the unique morphology of detonation nanodiamond, namely a diamond or diamond-like core surrounded by heterogeneous surface layers that both contribute to the scattered intensity [24]. Briefly, the model assumes a spherically symmetric particulate, whose scattering is calculated by the equation:…”

“…Generally, Equation 1 can be used to model the scattering from any spherically symmetric particulate as long as the function, 1(r), is known or parameterized; the scattering length density is proportional to the electron density [29]. In the case of detonation nanodiamond, 1(r), contains a radial segment that is associated with diamond, as well as some surface texture modeled by three segments [24]. Carbon onions, produced by, and observed in the soots of hydrogen-free high pressure and temperature explosives [30], can also be modeled with Equation 1 by parameterizing 1(r) in such a way that it is a Heaviside func-tion that mimics the graphitic layers.…”