Session T24: Matter at Extreme Conditions: Carbon and Related Materials

In situ X-ray Diffraction of Shock-Compressed Nanopolycrystalline Diamond

Over 50 years of shock experiments have demonstrated that planar shock waves deform crystals first along one dimension before the crystal rapidly relaxes along three dimensions. However, previous experiments have lacked the necessary spatiotemporal resolution to resolve this ultrafast lattice dynamics in detail. By performing in-situ x-ray diffraction measurements on shock-compressed full-density nanopolycrystalline diamond (NPD) with an average grain size of 10-20 nm, we accurately measured the evolution of strain in continuous elastic-plastic deformations of NPD. The experiments were performed at EH5 of SPring-8 Angstrom Compact Free Electron Laser (SACLA) where a high-energy drive laser is synchronized to the femtosecond pulsed X-ray Free Electron Laser (XFEL). Our results show that some of the anisotropy of the preceding elastic deformation remains in the subsequent plastic deformation, indicating that the strength of NPD persists even under stresses exceeding its Hugoniot elastic limit. The obtained structural data also show that the diamond structure is stable (or metastable) up to a shock pressure of at least 700 GPa. The detailed view of how ultrahard materials like NPD yield under high strain-rate shock compression gives key insights into the fundamental understandings of material disturbances in Inertial Confinement Fusion, hypervelocity planetary impact, material processing, and high-energy-density experiments that occasionally use a diamond as an ablator.   

Inelastic Response of Diamond to Shock Compression

Recent shock experiments aimed at exploring diamonds response in the sub-melting pressure region have demonstrated significant orientational dependence of the diamond strength. However, the nature and atomic scale mechanisms of inelastic deformations is currently unknown. Extreme-scale molecular dynamics simulations using machine-learning Spectral Neighbor Analysis Potential (SNAP)  have been performed to uncover the nature and atomic scale mechanisms of inelastic deformations of shock-compressed diamond. The systematic exploration of intelastic response included several crystallographic directions and a wide range of shock intensities.   

Multiphase Equation of State for Solid, Liquid, and Plasma Carbon

The multiphase equation of state (EOS) and thermodynamic properties of carbon (C) in a wide range of temperature and density are required to perform the principal shock Hugoniot calculations and hydrodynamic simulations. A parametric EOS model based on the DFT-QMD data is developed for solid (diamond, BC8, simple-cubic, simple-hexagonal) and liquid phases of C and implemented in the Radiative Emissivity and Opacity of Dense Plasmas (REODP) code [Miloshevsky et al. PRE 92 (2015) 033109]. In this model the Helmholtz free energy of each phase is decomposed into the zero-Kelvin, ion-thermal, and electron-thermal contributions that are parametrized separately for each of C phases. The REODP results on the EOS and other thermodynamic functions (enthalpy, entropy, heat capacity, isothermal compressibility, etc.) of solid and liquid phases of C are validated against the available computational and experimental data. The EOS of warm and hot dense C plasma are calculated using the first principle Hartree-Fock-Slater - Collisional-Radiative Steady-State model implemented in the REODP code. The plasma EOS is found to be in good agreement with published computational and experimental data.   

Proposing a new model for ramp compression from ab initio calculations

High-pressure states of matter can be reached through static compression, using diamond

anvil cells, or by dynamic shock and ramp compression experiments. While shock

compression is relatively well understood and described by the formalism of the Hugoniot

equations, no such formalism exists for ramp compression. The compression is thought to

be quasi-isentropic, leading to lower temperatures than shock compression, but there are

currently no measurements of temperature during ramp compression or a theoretical

framework to derive how much heating occurs. In this work, we present a model of ramp

compression that is based on thermodynamics and ab initio calculations where we

approximate ramp loading as a series of compression and relaxation steps. We apply our

model to diamond and we compare our predictions with the measured stress-density

relations reported from experiments [1-3]. We find good agreement between our model and

a multishock Hugoniot scheme, as well as with a recently proposed strength model for

diamond based on plastic work [4].

References:

[1] D. Bradley et al., PRL 102, 075503 (2009).

[2] R. Smith et al., Nature 511, 330 (2014).

[3] A. Lazicki et al., Nature 589, 532 (2021).

[4] D.C. Swift et al., arXiv 2004.03071 (2020).   

Resolving Nanodiamond Diffraction during Detonation of High Explosives

Carbon condensation during high-explosive detonations occurs under extreme conditions and on very short time scales.  The nanoscale crystalline phases formed under these conditions provide key insight into the condensation pathways. While previous time-resolved measurements have confirmed a morphology, characteristic of detonation nanodiamond, observing the diffraction from nanodiamond, in-situ, remains a challenge. Using ultra-fast (sub-μs) X-ray diffraction with a narrow-band of polychromatic X-rays, we show that a broad peak corresponding to the diamond 111 can be observed at early-times, but is adjacent to a strong fluid-like peak that shifts to shallower angles as the detonation progresses. Challenges associated with the certainty and amount of diamond are discussed in light of results from a range of explosives that produce a broad range of carbon phases.   

Kinetics of Carbon Condensation in Detonation of High Explosives: First-Order Phase Transition Theory Perspective

The kinetics of carbon condensation, or carbon clustering, in detonation of carbon-rich high explosives is modeled by solving a system of rate equations for concentrations of carbon particles [1]. Unlike previous efforts, the adopted rate equations account not only for the aggregation of particles, but also for their fragmentation in a thermodynamically consistent manner. Numerical simulations are performed, yielding the distribution of particle concentrations as a function of time. In addition to that, analytical expressions are obtained for all the distinct steps and regimes of the condensation kinetics, which facilitates the analysis of the numerical results and allows one to study the sensitivity of the kinetic behavior to the variation of system parameters. The latter is important because the numerical values of many parameters are not reliably known at present. In this presentation, we will discuss such physical phenomena and regimes of carbon condensation as the coagulation, nucleation, growth, and Ostwald ripening, and their dependence on various parameters of detonation.   

The Role of Soot Formation in TATB Detonation

The detonation of carbon-rich CHNO-based insensitive high explosives (HEs) yields carbonaceous precipitates, or soots, the formation of which may increase the reaction zone to the microsecond regime, compared to tens of nanoseconds for conventional HEs. In particular, triaminotrinitrobenzene (TATB)-based HEs produce soots with a relatively large nitrogen content, which may further affect explosive performance by attenuating energy release during detonation. The exact chemical composition and thermodynamic properties of TATB-based soots are still under investigation. Therefore, in this work, we use recent measurements of soot composition, experimental detonation data, and the thermochemical code Cheetah, to predict the equation of state of TATB soot by numerical optimization. These results will inform future experimental and theoretical efforts towards understanding TATB detonation.

This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344

LLNL-ABS-816282   

Kinetics of Diamond Formation from Temperature Extrapolatable Kinetics Model built using Molecular Dynamics Simulations

In the atmosphere of icy giant planets, methane is pyrolyzed under extreme conditions and is thought to convert into diamond. However, the exact conditions of diamond formation are poorly understood: depending on the experimental setup, static compression or shockwave experiments, diamond have been observed in conditions that vary by tens of gigapascals and thousands of Kelvins. Rationalizing these differences is complicated by the lack of understanding of the mechanisms of diamond formation from liquid hydrocarbons.

In this work, we present a detailed kinetics model of this mechanism built using molecular dynamics simulations. To correctly describe the reactions between small molecules, but also the growth of long carbon particles, microscopic reactions were defined in terms of atoms and their local environment. This description also allowed us to build a kinetics model that can be accurately extrapolated in time and temperature, so predictions are made on a wide range of temperature and are not limited by the computational cost of molecular dynamics simulations.

We found that additional demixing or dihydrogen absorption mechanisms are necessary to explain the observation of diamond in static compression experiments, and shockwave experiments may be too short to form diamond.   

Metalization of Warm Dense Polystyrene Along the Principal Hugoniot

Using recently developed thermal meta-GGA exchange-correlation (XC) density functional, T-SCAN-L, we perform fully consistent density functional theory simulations to investigate equation of state up to 7 Mbar and the insulator-metal transition (IMT) of shocked polystyrene (CH) along the principal Hugoniot. CH is often used as an ablator material in inertial confinement fusion (ICF) experiments; therefore, its properties in warm dense conditions are crucial for ICF target designs. Our results are compared with an OMEGA experiment as well as the results obtained from the zero-temperature PBE XC. We show that the inclusion of thermal and inhomogeneity effects at the meta-GGA level via T-SCAN-L XC significantly improves the agreement with the IMT dynamics in terms of the behavior of optical reflectivity observed in the OMEGA experiment. Analysis of the pair correlation functions suggests that the IMT is driven by the molecular dissociation of CH that occurs in the pressure range of 1 to 2 Mbar and apparently coincides with the turn-on of optical reflectivity and dc conductivity that is accompanied by the respective band-gap closure.   

Detonation Synthesis of Boron Nitride via 1, 3, 5-trimethylborazine Precursor

Detonation synthesis of boron nitride was studied through detonation of an RDX based explosive composition. Boron nitride has found use in a wide variety of applications, from high temperature lubricants to automotive oxygen sensors. While previous research into detonation synthesis utilized the excess carbon in explosives with a negative oxygen balance, the addition of precursor materials into the explosive charge prior to detonation allows a wider variety of non-carbon-based molecules to be formed, and a more neutral oxygen balance to be used. In this study, liquid 1,3,5 Trimethylborazine was introduced as an aromatic boron nitride precursor at the center of an RDX based explosive charge with multi point initiation. This configuration produced converging shock conditions from the detonation on the liquid precursor inclusion to pressures and temperatures in excess of 20 Gigapascals and 2000 Kelvin, respectively, reaching the wurtzite region of the boron nitride phase diagram. Isolation of the boron nitride from the detonation products was conducted using nitric and sulfuric acid purification. X-ray diffraction analysis, X-ray photoelectron spectroscopy, and transmission electron microscopy were used to examine the soots for the presence of boron nitride.   

Pressure dependence of sound velocity in Viton and its carbon-filled composite

Viton A (a dipolymer composed of hexafluoropropylene and vinylidene fluoride) is used as a binder for high explosives, and its carbon-filled composite is widely used in high vacuum sealing O-rings over a broad temperature range. However, Viton shows variations in mechanical strength from lot to lot, which has caused concern about its performance in extreme conditions. Binder crystallinity has been recognized as a potential factor which may influence the mechanical strength of Viton, but this has not yet been fully explored. We carried out ultrasonic and x-ray diffraction measurements of Viton under pressure to 7 GPa in the Paris-Edinburgh cell to investigate the elastic and crystalline properties of Viton and its carbon filled composite. We observed the first broad diffraction peak in addition to crystalline diffraction peaks from both Viton and the composite. For comparison, we also investigated the acoustic properties and first diffraction peak of Sylgard polymer (another explosive binder), which shows a broad amorphous peak but does not exhibit any crystalline diffraction.   

Boron carbide under torsional stress at 5 GPa combined to density functional theory (DFT) calculations: evidence of the formation of chain vacancies in the plastic regime driven with the rotating tomography Paris Edinburgh cell (RoToPEC)

The behavior of boron carbide under stress/deformation has been a long-standing puzzle with, on the one hand, outstanding static mechanical properties, and on the other one, a shear strength in the shocked state that rapidly decreases beyond the Hugoniot elastic limit, resulting in premature failure of the material as the shock stress reaches a threshold value of 20 GPa. Several explanations have been put forward: phase transition, occurrence of shear bands containing amorphous solid, formation of chain vacancies followed by carbon-carbon bond reformation at high pressure. 

In the present work, damage and point defects generated with the RoToPEC have been identified at ambient pressure by energy dispersive X-ray microdiffraction, Raman spectroscopy, and DFT calculations of atomic structures and phonon frequencies.

We show that apart from the signals due to B4C, new peaks appear in both characterisation methods, and that most of them can be attributed to boron atom vacancies in the intericosahedral chains of boron carbide, as predicted in Ref. [1]. Some of the Raman spectra also show peaks that have been attributed to amorphous boron carbide in the literature. Deformed boron carbide thus shows small inclusions of clusters of B4C, with chain vacancies, and/or small zones interpreted as amorphous zones [2].