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Shock Compression of Liquid Carbon Monoxide and Methane to 90 GPa (900 kbar)

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Dynamic equation-of-state data for liquid CO and CHâ were measured in the shock pressure range 5--92 GPa (50--920 kbar) using a two-stage light-gas gun. The liquids were shocked from initial states near their saturation curves at 77 and 111 K for CO and CHâ, respectively. The experimental technique used to double-shock CHâ is described. The CO data were examined by using three theoretical models: (1) a chemically nonreactive model, (2) a quasi-chemical-equilibrium model that allows CO to dissociate into gaseous species and graphite, and (3) a chemical-equilibrium model that also includes a dense carbon phase which exists at higher pressures and temperatures than graphite. This dense phase is assumed to be diamond. Our analysis shows that at low pressure chemical equilibrium takes much longer than a typical shock passage time. As a consequence, the experimental data initially follow the nonreactive Hugoniot to pressures well beyond the chemical dissociation limit. Both the experimental data and the Hugoniot computed with case (3) agree satisfactorily at high pressure. Further consequences of these observations to high-explosive studies are discussed. The theoretical analysis for the CHâ data was presented in an earlier paper.
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... More recently, Leonhardi performed AIMD simulations to characterise CO fluid to 140 GPa and 5000 K and obtained an EOS (Leonhardi-2017 EOS) (Leonhardi and Militzer, 2017). Several shockwave experiments exist on CO and CO 2 , potentially allowing an assessment of the various EOSs, however little accurate temperature determination exists (Nellis et al., 1991;Nellis et al., 1981). Notably, in the shock experiments, CO and CO 2 molecules decomposed at high P and condensed into the diamond phase. ...
... For CO fluids, we found one set of EOS data from classical MD simulations (BS-1991) (Belonoshko and Saxena, 1991b), and one set of EOS data based on AIMD simulations (Leonhardi-2017) (Leonhardi and Militzer, 2017). We plot them in Fig. 6 together with our results and the shock-wave data from Nellis et al. (1981). It was shown by Nellis's experiment (Nellis et al., 1981) that the CO fluids with molar volume less than 13 cm 3 /mol are all deeply polymerised or solidified. ...
... We plot them in Fig. 6 together with our results and the shock-wave data from Nellis et al. (1981). It was shown by Nellis's experiment (Nellis et al., 1981) that the CO fluids with molar volume less than 13 cm 3 /mol are all deeply polymerised or solidified. Nellis also suggested a dense solid carbon phase at these conditions. ...
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There is significant interest in establishing a capability for tailored synthesis of next-generation carbon-based nanomaterials due to their broad range of applications and high degree of tunability. High pressure (e.g., shockwave-driven) synthesis holds promise as an effective discovery method, but experimental challenges preclude elucidating the processes governing nanocarbon production from carbon-rich precursors that could otherwise guide efforts through the prohibitively expansive design space. Here we report findings from large scale atomistically-resolved simulations of carbon condensation from C/O mixtures subjected to extreme pressures and temperatures, made possible by machine-learned reactive interatomic potentials. We find that liquid nanocarbon formation follows classical growth kinetics driven by Ostwald ripening (i.e., growth of large clusters at the expense of shrinking small ones) and obeys dynamical scaling in a process mediated by carbon chemistry in the surrounding reactive fluid. The results provide direct insight into carbon condensation in a representative system and pave the way for its exploration in higher complexity organic materials. They also suggest that simulations using machine-learned interatomic potentials could eventually be employed as in-silico design tools for new nanomaterials.
... For example, carbon monoxide (CO) has been used for carbon nanotube production under ambient to slightly elevated pressures, and explored as a precursor for other nanocarbon materials, [16][17][18] while shock compression of liquid CO to 10s of GPa has yielded convincing evidence of nanocarbon formation. 6,19 Furthermore, C/O systems serve as a logical stepping stone to the CNO (and CHNO) organic compounds traditionally employed for high-pressure nanodiamond synthesis (e.g. benzotrifuroxan, C 6 N 6 O 6 ), 8 or those recently shown to yield carbon nano-onions (e.g. ...
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Full-text available
There is significant interest in establishing a capability for tailored synthesis of next-generation carbon-based nanomaterials due to their broad range of applications and high degree of tunability. High pressure (e.g. shockwave-driven) synthesis holds promise as an effective discovery method, but experimental challenges preclude elucidating the processes governing nanocarbon production from carbon-rich precursors that could otherwise guide efforts through the prohibitively expansive design space. Here we report findings from large scale atomistically-resolved simulations of carbon condensation from C/O mixtures subjected to extreme pressures and temperatures, made possible by machine-learned reactive interatomic potentials. We find that liquid nanocarbon formation follows classical growth kinetics driven by Ostwald ripening (i.e. growth of large clusters at the expense of shrinking small ones) and obeys dynamical scaling in a process mediated by carbon chemistry in the surrounding reactive fluid. The results provide direct insight into carbon condensation in a representative system and pave the way for its exploration in higher complexity organic materials. They also suggest that simulations using machine-learned interatomic potentials could eventually be employed as in-silico design tools for new nanomaterials.
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... For water, we use the polynomial approximation of the EOS determined by Ree (1976), as well as the ab initio EOS of Mazevet et al. (2019). Due to their smaller expected abundances, the accuracy for CH 4 and NH 3 is less crucial, and again, we use the polynomial approximations given by Hubbard et al. (1995) for the shockwave CH 4 data determined by Nellis et al. (1981) as well as the approximation given by Hubbard et al. (1995) for the zero-temperature CH 4 equation of state. Finally, to model the density of rock within the planets, we employ the EOS from used by Hubbard & Macfarlane (1980) for the mixture of 38% SiO 2 , 25% MgO, 25% FeS, and 12% FeO which we similarly take to approximately constitute "rock." ...
Preprint
Interior models of Uranus and Neptune often assume discrete layers, but sharp interfaces are expected only if major constituents are immiscible. Diffuse interfaces could arise if accretion favored a central concentration of the least volatile constituents (also incidentally the most dense); compositional gradients arising in such a structure would likely inhibit convection. Currently, two lines of evidence suggest possible hydrogen-water immiscibility in ice giant interiors. The first arises from crude extrapolation of the experimental hydrogen-water critical curve to $\sim 3$ GPa (Bali et al. 2013). The data are obtained for an impure system containing silicates, though Uranus and Neptune could also be "dirty." Current ab initio models disagree (Soubiran & Militzer 2015), though hydrogen and water are difficult to model from first-principles quantum mechanics with the necessary precision. The second argument for hydrogen-water immiscibility in ice giants, outlined herein, invokes reasoning about the gravitational and magnetic fields. While consensus remains lacking, here we examine the immiscible case. Applying the resulting thermodynamic constraints, we find that Neptune models with envelopes containing a substantial water mole fraction, as much as $\chi \gtrsim 0.1$ relative to hydrogen, can satisfy observations. In contrast, Uranus models appear to require $\chi \lesssim 0.01$, potentially suggestive of fully demixed hydrogen and water. Enough gravitational potential energy would be available from gradual hydrogen-water demixing, to supply Neptune's present-day heatflow for roughly ten solar system lifetimes. Hydrogen-water demixing could slow Neptune's cooling rate by an order of magnitude; different hydrogen-water demixing states could account for the different heatflows of Uranus and Neptune.
... Nellis et al. (11)(12)(13)(14) measured the rates of dissociation of N2, CO2, and CO plus ionization of H2O under high shock pressures. However, the shock temperatures reached for the individual diatomic or triatomic molecules are considerably higher than those of reaction product mixtures at the same shock pressures. ...
Conference Paper
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