Session G03: Materials in Extremes: Carbon and Related Materials at Extreme Conditions

Detonation-induced transformation of graphite to hexagonal diamond.

The structural evolution of graphite under elevated thermodynamic conditions has been the subject of intense research interest. Early studies, back in 90s, clearly indicate the shock-induced transformation of graphite to a phase with much higher density, presumably a sp³ allotrope, above 20 GPa. However, only recently the capability of in-situ X-ray diffraction under shock conditions allowed the structural characterization of the relevant phases. Here we explore the structural evolution of highly oriented pyrolytic graphite (HOPG) under detonation-induced shock conditions using in situ synchrotron X-ray diffraction in the ns time scale. We observe the formation of hexagonal diamond (lonsdaleite) at pressures above 50 GPa, in qualitative agreement with recent gas gun experiments. Moreover, we observe an extended pressure stability of the initial HOPG crystal structure up to ~50 GPa, in contrast with previous shock and static compression results.


  

Kinetics of carbon clustering in detonation of high explosives: Particle growth along hydrodynamic streamlines

Chemical reactions in detonation of carbon-rich high explosives yield solid carbon as a major constituent of the products. Efforts to theoretically describe the kinetics of carbon clustering go back to the seminal paper by Shaw and Johnson, where it was modeled as a diffusion-limited irreversible coagulation of smaller clusters into larger ones. In this talk, we will discuss recent direct experimental observations of carbon cluster growth in detonation of high explosives, based on time-resolved small-angle X-ray scattering (TR-SAXS). We will focus on comparison of these results with calculations based on (i) realistic hydrodynamic modeling of detonation of PBX 9502 high explosive, (ii) the Shaw-Johnson model of carbon particulate growth along the streamline obtained from the hydrodynamic simulations, and (iii) accurate simulations of the TR-SAXS signal. We demonstrate that this three-step simulation approach agrees well with the recent experimental results. The implications of this agreement on our present understanding of in-detonation carbon clustering processes will be discussed.   

Morphological evaluation of nanophase carbons recovered from deflagrated vs. detonated PBX 9502

Understanding the correlation between detonation conditions and nanophase carbon formation is important for achieving greater accuracy in predicting high explosive performance. To advance our understanding, and ultimately improve our ability to accurately predict the nanophase carbon products, requires knowledge of the operable carbon framework reactions and nascent particle assembly mechanisms ensuing behind the shock front. The detonation chemical reaction zone (CRZ) is temporally short (ns) and spatially small (100’s microns), making it difficult to directly probe. Further complicating direct interrogation is the high optical opacity. Deflagration, however, is less extreme characterized by a larger dimensional reaction zone (mm) that is temporally longer (ms). Furthermore, deflagration produces lower opacity at pressures in the MPa range. Thus, deflagration offers a means for carrying out direct spectroscopic monitoring of the generated and evolving carbon products as a function of HE chemical composition. In this presentation, the morphology of nanophase carbons recovered from the detonation and deflagration soot derived from PBX 9502 is characterized. The data is interpreted in the context of emission spectra recorded in the deflagrating flame.   

Probing metastability of carbon at high pressures by predictive first-principles simulations

In spite of extensive experimental and theoretical efforts, the behavior of carbon at extreme conditions is not completely understood. Previous hydrostatic studies demonstrate the stability of diamond and the absence of any other phase of carbon at pressures up to 1 TPa. However, new metastable phases might appear upon uniaxial compression generated by shock waves. We explore unknown metastable phases of carbon under both hydrostatic and uniaxial compressions up to 5 TPa using first-principles crystal structure prediction methods. It is predicted that several new metastable phases appear to survive upon decompression to ambient conditions. The experimental validation of these predictions will open up a new avenue in synthesizing new metastable carbon materials.
  

Bottom up molecular doping approach to synthesizing HPHT color centers in nanostructured diamond

Diamond nanocrystals are advantageous for a myriad of biological and technological applications because its optical color centers are tunable between the ultraviolet and near-infrared (NIR) spectral regions. Our work describes a molecular approach to dope diamond nanocrystals with silicon heteroatoms at extreme temperature (>1800K) and pressure (>15 GPa) conditions using argon as a near-hydrostatic pressure medium. Tetraethylorthosilicate (TEOS) molecules chemically dope an amorphous carbon aerogel during nanodiamond synthesis within a laser-heated diamond anvil cell. Pressure-dependent photoluminescence is used in tandem with ab initio quantum cluster calculations to measure the pressure dependence of the SiV- centers’ zero phonon line (λ ~ 738 nm, 0.9 meV / GPa). Aberration-corrected scanning transmission electron microscopy images heteroatoms on the surfaces and also the interiors of diamond nanocrystals. Scanning transmission x-ray absorption microscopy (STXM) measurements, along with electron energy loss spectroscopy and ab initio quantum cluster calculations, suggest a partial graphitic surface reconstruction of the diamond nanocrystals.   

Facile diamond synthesis from lower diamondoids

Carbon-based nanomaterials have exceptional properties that make them attractive for a variety of technological applications. In this talk, I discuss the use of diamondoids (diamond-like hydrocarbons) as promising precursors for laser-induced high-pressure, high-temperature diamond synthesis. The lowest pressure and temperature (P-T) conditions that yielded diamond were 12 GPa (at ~2000 K) and 900 K (at ~20 GPa), respectively. This represents a significantly reduced transformation barrier compared with diamond synthesis from conventional (hydro)carbon allotropes. At 20 GPa, diamondoid-to-diamond conversion occurs rapidly within < 19 us, forming diamond in a range of sizes ~tens of nm to ~4 um crystals, depending on the synthesis conditions. Molecular dynamics simulations indicate that once dehydrogenated, the remaining diamondoid carbon cages reconstruct themselves into diamond-like structures at high P-T. The surprisingly low P-T regime necessary to grow diamond from diamondoids is attributed to the similarities in the structure and bonding of diamondoids and bulk diamond. This study is the first successful mapping of the P-T conditions and onset timing of the diamondoid-to-diamond conversion and elucidates the physical and chemical factors that facilitate diamond synthesis.   

First-principles phase diagram of carbon at extreme conditions

The accurate phase diagram of carbon at extreme temperatures and pressures is of paramount importance for design of inertial confinement fusion capsules where diamond is used as an ablator material. In spite of intensive experimental and theoretical efforts, there still exist outstanding problems including disagreement between theoretical predictions and experiment. We present results of first-principles molecular dynamics simulations of thermodynamic properties of carbon at high temperatures and pressures, which are performed with the goal of constructing an accurate phase diagram of carbon. To address the issue of accuracy and reliability, a relatively large number of atoms is used for calculation of melting transitions (melt curve) as a function of pressure using two-phase method. We specifically focus on important region of high pressure, high temperature phase diagram where melt line of diamond exhibits a negative slope, while intersecting the melt line of high-pressure bc8 phase at diamond-bc8-liquid triple point.   

Color center qubit formation by local electronic excitation

Color centers e. g. in diamond are promising qubit candidates. Efforts to find color centers with desired properties, e. g. emission in the telecom bands and long coherence times, are limited by the parameter space in which materials can be processed. Recently, color center formation with fs-laser pulses and using swift heavy ions have been reported. Intense pulses of local excitation can drive materials very far from equilibrium follwed by rapid quenching. This extends the parameter range where novel color centers can be formed. We report on mechanisms underlying NV-center formation with swift heavy ions, our search for rare-earth color centers in diamond and new capabilities that we are developing for color center formation far from equilibrium using intense, local pulses of ions and lasers [1-3].
References
[1] K. Nakamura et al., IEEE J. Quant. Electr. 53, 1200121 (2017)
[2] J. J. Barnard, T. Schenkel, J. Appl. Phys. 122, 195901 (2017)
[3] J. Bin et al., Rev. Sci. Instr. 90, 053301 (2019)   

Probing the strain field and structural variation in shock-produced amorphous silicon by scanning nano-probe electron diffraction

Shock-induced amorphization can occur when crystalline solids are uniaxially strained in an extremely short time scale. This phenomenon also opens a new door for prototyping the exotic amorphous structures when the conventional methods failed. Taking silicon as an example, we have recovered its amorphous phase from laser shock compression experiments and illustrated the formation mechanisms by both experiments and simulations. However, one missing information is whether the amorphous silicon produced by shock wave is fully relaxed, i.e. are they structurally different than the amorphous structure produced by other techniques such as chemical vapor deposition. We use a newly developed scanning nano-probe electron diffraction technique to tackle this question. A nanometer-sized electron probe is focused onto and rastering over the electron-transparent sample and simultaneously, convergent beam electron diffraction patterns at each pixel are collected, which can be used to compute strain distribution of the region of interest. The strain field in the vicinity of the amorphous band and the radial distribution function within the amorphous domain will be discussed. A tremendous amount of structural variation can be found even within the amorphous domain.   

Computer Simulations and Experimental Studies on Transition Metal Borides to 390 GPa

The attainment of near TPa static pressure in the laboratory has led to a renewed interest in phase transformations and shear strength measurements of incompressible transition metals rhenium (Re), osmium (Os), and their superhard diborides (ReB₂ and OsB₂) We have compressed hexagonal ReB₂, OsB_2, and Os₂B₃ to multi-megabar pressures using Focused Ion Beam machined toroidal diamond anvils. The phase transformations and equation of state were studied using micro-beam angle-dispersive x-ray diffraction at HPCAT beamline 16-BM-D. The platinum pressure marker was employed to the highest pressure of 390 GPa. First-principles simulations based on density functional theory (DFT) were utilized to model ReB₂ and OsB₂ under external pressure. The ambient-pressure lattice parameters computed by the projector augmented-wave method with generalized gradient approximation agree well with the experiment within 1% of error. We will present a comparison of experimental shear strength and equation of state with the elastic constants from DFT calculations up to the highest pressure under study.   

Thermal stability of silicon carbide

A large volume cubic anvil press integrated with synchrotron energy-dispersive X-ray diffraction was employed to study the yielding behavior of powdered beta SiC under high pressure and high-temperature conditions up to 7.4 GPa and 1400^oC. During the compression and thereafter heating, x-ray pattern was collected at each pressure-temperature point, and then via assessing the peak width of the X-ray diffraction pattern, the strains/stresses developed inside the sample under varied pressure-temperature conditions was calculated. From the constitutive response of the sample as a function of pressure and temperature, we did not observe the yielding occurrence in SiC at cold compression. In contrast, high temperature induces a yielding at 1100^oC with a constant loading pressure ~7.4 GPa. By the comparison, we found that this material is the most stable material, among the other three strong materials (diamond, moissanite, and alfa silicon nitride), in terms of the yielding under high pressure and temperature conditions. Along with its much higher pressure and temperature requirements for phase transition and decomposition, SiC is a competent material to be used in the harshly extreme working environment, such as deep drilling, high-speed cutting, and aerospace engineering.   

On the strain-rate dependence of dynamic tensile strength in single and nanocrystallie SiC

The strain-rate dependence of dynamic tensile strength in single and nanocrystalline SiC is investigated via large scale molecular dynamic simulations. A quasi-isentropic loading method is used to evaluate the strain rate to over six-order from 10⁷ to 10¹² s^-1. SiC with [001] orientation exhibits a perfectly reversible deformation twinning mechanism that enables a high tensile strength, while [110] and [111] crystals contain irreversible defects after unloading that results in a significant decrease in strength. Octahedral cleavage along {111} family planes is found to occur only in [001] SiC within 10⁹ s^-1 of the strain rate due to its covalent bond. A power model can be fit basing on the tensile strengths at extremely high strain rate regime which yields a good prediction of strengths at plate-impact experimental strain rates.   

High pressure forms of aqueous carbon

The chemistry of carbon in aqueous fluids at extreme pressure and temperature conditions is of great importance to Earth's deep carbon cycle. A major obstacle to understanding deep carbon transport is the lack of knowledge of carbon reactions in water at the extreme conditions found in Earth's deep interior. Here, by applying first-principles molecular dynamics simulations, we predicted a few interesting and important carbon reactions in supercritical water, which are dramatically different from what we know at close to ambient conditions. We found that carbonic acid can be the most abundant carbon species in aqueous CO₂ solutions at ∼10 GPa and 1000 K. In CO₂-rich solutions, significant proton transfer between carbonic acid molecules and bicarbonate ions may enhance the conductivity of solutions. In less oxidized solutions, the reactions of carbon monoxide may participate in the diamond formation.
N. Stolte and D. Pan, J. Phys. Chem. Lett. 10, 5135 (2019)