Supercritical Density Research Articles

The role of local structure on formic acid decomposition in supercritical water: A hybrid quantum mechanics/Monte Carlo study

We explored water-assisted decompositions of formic acid in supercritical water in terms of local structure near reactant. A hybrid quantum mechanics/molecular mechanics (QM/MM) simulation used in this paper includes QM part as first solvation shell members around the reactant. A present QM/MM approach can simulate supercritical water solution with a reasonable computational load while keeping the simulation preciseness because a density functional theory of B3LYP/6-31+G(d) level was iterated at every 1000 Monte Carlo solute moves. The formic acid converts mainly decarboxylation by water-assisted mechanism, and the coordinated water molecules play an important role for understanding supercritical water density dependence of the reaction. We analyzed a contour map based on the solute–solvent interaction energy along with the reaction pathway. Coordinated water molecule restricted the dehydration pathway by means of hydrogen bond with formic acid, however, the coordinated water promotes the decarboxylation pathway by means of stabilization of the transition state structure with one catalytic water molecule. The contour map of the pair interaction energy along the reaction path elucidates the role of local structure on reactions in supercritical water.

Remediation of flare pit soils using supercritical fluid extraction

Supercritical fluid extraction (SFE) is a promising remediation technology for contaminated soils. This work investigates the use of SFE to remove petroleum hydrocarbons (PHCs) from flare pit soils. Extractions were performed on two flare pit soils at pressures ranging from 11.0 to 24.1 MPa and at temperatures ranging from 40 to 80 °C in an attempt to identify the best extraction conditions and to understand the effects of pressure, temperature, supercritical fluid flow rate, soil type, and extraction time on the extraction efficiency. For the conditions studied, the efficiency of the SFE process appeared to be solvent-density dependent. Conditions of 24.1 MPa and 40 °C (highest supercritical fluid density) yielded the highest extraction efficiency for both soils (89% for the sand and 80% for the loam). An increase in temperature at a fixed pressure led to a decrease in the extraction efficiency while an increase in pressure at a fixed temperature led to an increase in the extraction efficiency. The treated soils appeared to be drier, grainy, and lighter coloured than the soil prior to extraction.Key words: supercritical fluid extraction (SFE), supercritical carbon dioxide, flare pit soils, contaminated soil, soil remediation, biorecalcitrant petroleum hydrocarbons.

Supercritical density relaxation as a new approach of droplet vaporization

The isobaric density relaxation of a cold, slightly supercritical, fluid package in a hotter atmosphere of this same fluid at a near critical pressure is studied in this paper. We solve the Navier–Stokes equations describing the evolution of an initial density inhomogeneity in a supercritical van der Waals fluid. These numerical simulations allow exploring the whole range of surrounding conditions, including the slightly supercritical ones. Strong discrepancies are found from the classical decay of the squared diameter of the droplet observed in clearly subcritical conditions. We present a quasisteady analysis that allows neglecting unsteady terms in the equations that describe the problem, provided that the moving reference frame is well defined. The strong density gradient that follows the “droplet” as well as the time dependant mass flow rate location are considered with a particular attention. The obtained solutions of the equations match with the numerical results and generalize to near critical conditions the classical subcritical quasisteady analysis of droplet vaporization performed in the combustion science field.

Effect of supercritical water density on the rate constant of aliphatic nitrocompounds decomposition

It was shown experimentally that the rate of aliphatic nitrocompounds decomposition in supercritical water (SCW) depends on pressure/density at constant temperature. The SCW density dependence of the apparent rate constant of decomposition for each of three compounds under consideration (nitromethane, nitroethane and 1-nitropropane) was found to correlate with the density dependence of the H 3O + concentration in dissociated SCW. The rate constant of RNO 2 decomposition in SCW decreased with increasing the length of hydrocarbon radical R.

Quantitative modelling of nucleation kinetics in experiments for poly-Si growth on SiO2 by hot wire chemical vapor deposition

We apply a rate-equation pair binding model of nucleation kinetics to the nucleation of Si islands grown by hot wire chemical vapor deposition on SiO 2 substrates. The grain size of poly-Si films increases with H 2 dilution, which is attributed to atomic H etching of Si monomers rather than stable Si clusters during the early stages of nucleation. The nucleation density increases sublinearly with time at low coverage, implying a fast nucleation rate until a critical density is reached, after which grain growth begins. The nucleation density decreases with increasing H 2 dilution (H 2 :SiH 4 ) due to etching, and with increasing temperature, due to enhanced Si monomer diffusivity on SiO 2 . From temperature-dependent measurements, we estimate the activation energy for surface diffusion of Si monomers on SiO 2 to be 0.47±0.09 eV. Simulations of the temperature-dependent supercritical cluster density lead to an estimated activation energy of 0.42±0.01 eV and an estimated surface diffusion coefficient prefactor of 0.1±0.03 cm 2 /s.

Gases of exploding laser-heated cluster nanoplasmas as a nonlinear optical medium

The manner in which strongly heated nanoclusters explode in the presence of intense laser fields influences all applications of this interaction. By measuring, with femtosecond time resolution, the ensemble average polarizability in a gas of intense laser-heated clusters, we have inferred the cluster explosion dynamics. The time evolution of the polarizability is characteristic of competition in the optical response between supercritical and subcritical density regions of the exploding cluster. These results are consistent with complementary time-resolved Rayleigh scattering measurements and with the predictions of a near-field plasma hydrodynamic model of the laser–cluster interaction. A significant implication of this cluster evolution appears in its macroscopic effect on a laser beam: a gas of exploding cluster plasmas causes nonlinear beam propagation owing to the space and time dependence of the ensemble polarizability. A strong self-focusing effect is observed experimentally that strongly contrasts with the ionization-induced refraction and beam spreading usually observed in the interaction of intense femtosecond laser pulses with nonclustered gases. These results illustrate how high energy density femtosecond plasma physics at the nanoscale determines what at first appears to be a gas-phase interaction.

Quantitative modelling of nucleation kinetics in experiments for poly-Si growth on SiO 2 by hot wire chemical vapor deposition

We apply a rate-equation pair binding model of nucleation kinetics to the nucleation of Si islands grown by hot wire chemical vapor deposition on SiO 2 substrates. The grain size of poly-Si films increases with H 2 dilution, which is attributed to atomic H etching of Si monomers rather than stable Si clusters during the early stages of nucleation. The nucleation density increases sublinearly with time at low coverage, implying a fast nucleation rate until a critical density is reached, after which grain growth begins. The nucleation density decreases with increasing H 2 dilution (H 2:SiH 4) due to etching, and with increasing temperature, due to enhanced Si monomer diffusivity on SiO 2. From temperature-dependent measurements, we estimate the activation energy for surface diffusion of Si monomers on SiO 2 to be 0.47±0.09 eV. Simulations of the temperature-dependent supercritical cluster density lead to an estimated activation energy of 0.42±0.01 eV and an estimated surface diffusion coefficient prefactor of 0.1±0.03 cm 2/s.

Time-resolved explosion of intense-laser-heated clusters.

We investigate the femtosecond explosive dynamics of intense laser-heated argon clusters by measuring the cluster complex transient polarizability. The time evolution of the polarizability is characteristic of competition in the optical response between supercritical and subcritical density regions of the expanding cluster. The results are consistent with time-resolved Rayleigh scattering measurements, and bear out the predictions of a recent laser-cluster interaction model [H. M. Milchberg, S. J. McNaught, and E. Parra, Phys. Rev. E 64, 056402 (2001)]].

Numerical Study of Cryogenic Jets Under Supercritical Conditions

The present work is devoted to the study of cryogenic nitrogen jets in sub- and supercritical conditions. A general trend to operate under increasingly higher combustor pressures is observed in rockets, gas turbines, and diesel engines, primarily as a result of enhanced effects on thrust, power, or efficiency. In these conditions the injected fluid(s) can experience ambient pressures exceeding the critical pressure(s) of the propellants, and recent experimental work of Chehroudi et al. showed a quantitative similarity to gas-jet-like behavior (Chehroudi, B., Cohn, R., and Talley, D., Spray/Gas Behaviour of Cryogenic Fluids under Sub- and Supercritical Conditions, Paper, July 2000; Chehroudi, B., Cohn, R., and Talley, D., Initial Growth Rate and Visual Characteristics of a Round Jet into a Sub- to Supercritical Environment of Relevance to Rocket, Gas Turbine, and Diesel Engines, AIAA Paper 99-0206, Jan. 1999). This conclusion suggested that it would be expected that the mathematical models and numerical methods used for gaseous flows could also be used for supercritical flows. This paper reports an investigation, exploring this hypothesis, and aims to evaluate the capabilities and limitations of a computational method developed for incompressible but variable density flows when applied to supercritical conditions. The predicted initial jet growth rate was compared with available experimental data for liquid/gaseous jets and mixing layers and showed a good agreement for different supercritical density ratios. For subcritical conditions, when the flow deviates from the gaseous-like behavior, and approaches a more spray-like behavior the incompressible gaseous flow formulation was found inadequate.

Planetesimal Formation by Gravitational Instability

We investigate the formation of planetesimals via the gravitational instability of solids that have settled to the midplane of a circumstellar disk. Vertical shear between the gas and a subdisk of solids induces turbulent mixing that inhibits gravitational instability. Working in the limit of small, well-coupled particles, we find that the mixing becomes ineffective when the surface density ratio of solids to gas exceeds a critical value. Solids in excess of this precipitation limit can undergo midplane gravitational instability and form planetesimals. However, this saturation effect typically requires increasing the local ratio of solid to gaseous surface density by factors of 2-10 times cosmic abundances, depending on the exact properties of the gas disk. We discuss existing astrophysical mechanisms for augmenting the ratio of solids to gas in protoplanetary disks by such factors and investigate a particular process that depends on the radial variations of orbital drift speeds induced by gas drag. This mechanism can concentrate millimeter-sized chondrules to the supercritical surface density in ≤ few × 106 yr, a suggestive timescale for the disappearance of dusty disks around T Tauri stars. We discuss the relevance of our results to some outstanding puzzles in planet formation theory—the size of the observed solar system and the rapid type I migration of Earth-mass bodies.

Evidence of supercritical density in 45-fs-laser-irradiated Ar-cluster plasmas

Plasmas have been created by irradiating atomic clusters of argon with high-contrast, ultrashort $(l100\mathrm{fs})$ laser pulses and recording the corresponding K-shell emission spectra. The high-resolution Ar K-shell spectra were analyzed using a theoretical two-temperature collisional-radiative model of irradiated atomic clusters incorporating the effects of highly energetic electrons $(\ensuremath{\sim}5\mathrm{keV}).$ Comparison of the measured spectrum with the calculations indicates that a supercritical density near ${N}_{e}=2\ifmmode\times\else\texttimes\fi{}{10}^{22}{\mathrm{cm}}^{\ensuremath{-}3}$ was attained in the shortest-pulse and highest-contrast shots. Results from additional experiments using pulses on the order of 300--400 fs are also consistent with a hydrodynamic model of laser-cluster interactions.

Scaling in the lattice gas model

A good quality scaling of the cluster size distributions is obtained for the Lattice Gas Model using the Fisher's ansatz for the scaling function. This scaling identifies a pseudo-critical line in the phase diagram of the model that spans the whole (subcritical to supercritical) density range. The independent cluster hypothesis of the Fisher approach is shown to describe correctly the thermodynamics of the lattice only far away from the critical point.

Stability of Ionic and Radical Molecular Dissociation Pathways for Reaction in Supercritical Water

Molecular dynamics simulations have been used to examine the effect of supercritical water solvent density on the competition between reaction pathways for the dissociation step of a model SN1 reac...

Quantitative Modelling of Nucleation Kinetics in Experiments for Poly-Si Growth on Sio2 by Hot-Wire Chemical Vapor Deposition

ABSTRACTWe apply a rate-equation pair binding model of nucleation kinetics [1] to the nucleation of Si islands grown by hot-wire chemical vapor deposition on SiO2 substrates. Previously, we had demonstrated an increase in grain size of polycrystalline Si films with H2 dilution from 40 nm using 100 mTorr of 1% SiH4 in He to 85 nm with the addition of 20 mTorr H2. [2] This increase in grain size is attributed to atomic H etching of Si monomers rather than stable Si clusters during the early stages of nucleation, decreasing the nucleation density. Atomic force microscopy (AFM) measurements show that the nucleation density increases sublinearly with time at low coverage, implying a fast nucleation rate until a critical density is reached, after which grain growth begins. The nucleation density decreases with increasing H2 dilution (H2:SiH4), which is an effect of the etching mechanism, and with increasing temperature, due to enhanced Si monomer diffusivity on SiO2. From temperature-dependent measurements, we estimate the activation energy for surface diffusion of Si monomers on SiO2 to be 0.47 ± 0.09 eV. Simulations of the temperature-dependent supercritical cluster density lead to an estimated activation energy of 0.42 eV ± 0.01 eV and a surface diffusion coefficient prefactor of 0.1 ± 0.03 cm2/s. H2-dilution-dependent simulations of the supercritical cluster density show an approximately linear relationship between the H2 dilution and the etch rate of clusters.

Stochastic gene expression: density of defects frozen into permanent Turing patterns.

We estimate density of defects frozen into a biological Turing pattern which was turned on at a finite rate. Self-locking of gene expression in individual cells, which makes the Turing transition discontinuous, stabilizes the pattern together with its defects. Defects frozen into the pattern are a permanent record of the transition-they give an animal its own characteristic lifelong "fingerprints" or, as for vital organ formation, they can be fatal. Density of defects scales like the fourth root of the transition rate. This dependence is so weak that there is not enough time during morphogenesis to get rid of defects simply by slowing down the rate. A defect-free pattern can be obtained by spatially inhomogeneous activation of the genes. If the supercritical density of activator spreads slower than certain threshold velocity, then the Turing pattern is expressed without any defects.

Physical processes in a laser-greenhouse target: Experimental results, theoretical models, and numerical calculations

The paper is devoted to recent results concerning investigation of physical processes occurring in a “laser greenhouse” target. Results of experimental and theoretical studies of laser-pulse interaction with a low-density absorber of the target, namely, with a porous substance having density close to the plasma critical density, are presented. On the basis of a vast cycle of experiments carried out in a number of laboratories, it is shown that the absorption of the laser radiation in porous media, including those with a density exceeding the critical one by at least a factor of 4 to 6, has a bulk nature and is distributed over the target depth. In particular, the laser-radiation absorption region in a porous substance with density 10−3–10−2 g/cm3 is extended into the target 400–100 μm, respectively. The coefficient of absorption of laser radiation with intensity 1014–1015 W/cm2 in porous substances, including those of the supercritical density, is 70–90%. Experiments have not shown enhanced (compared to a solid-state target) radiation intensity associated with a possible development of parametric instabilities in an extended laser plasma of low-density porous media, as well as noticeable contribution of fast electrons to the energy balance and their effect on the energy transfer. In this paper, theoretical models are developed explaining features of the laser-radiation absorption and energy transfer in porous media. These models are based on the phenomenon of laser-radiation interaction with solid components of a porous substance and plasma production inside pores and cells of the medium. The efficiency of energy conversion in the vicinity of the ignition threshold for the laser-greenhouse target is investigated in the case of an absorber having the above properties. Numerical calculations have shown that a thermonuclear-gain coefficient of 1 to 2 (with respect to the energy absorbed) is attained for a laser-radiation energy of 100 kJ.

A non-destructive method to determine the safranal content of saffron ( Crocus sativus L.) by supercritical carbon dioxide extraction combined with high-performance liquid chromatography and gas chromatography

A supercritical carbon dioxide extraction method to obtain selectively volatile compounds of saffron without sample destruction has been developed. The influence of both pressure and temperature was studied, 20 MPa and 100°C being the best conditions to extract the total safranal content. A decrease in supercritical fluid density was shown to be a critical parameter for enhancing the extraction power of carbon dioxide. For all the assay conditions, the extracts mainly contained safranal and HTCC, as demonstrated by gas chromatography and high-performance liquid chromatography analyses. Both chromatographic methods were suitable for safranal quantification and showed excellent agreement. Supercritical extracts from five different saffron types were studied by high-performance liquid chromatography and their safranal contents were determined.

Plasma harmonic emission from laser interactions with dense plasma

A nonlinear procedure is used to account for the mechanism of laser harmonic generation to high orders and plasma emission in laser-dense plasma interactions. A Lagrangian model is used to describe harmonic generation in terms of the oscillatory behavior of an electron plasma. It is shown that the effect of pulse shaping constitutes an important issue for the generation and enhancement of harmonics. It is found that in the presence of electron density perturbations further harmonics beyond the order that corresponds to the upper shelf plasma density can be excited. Computations were performed for mildly relativistic electrodynamics and time scales where wave breaking effects do not take place. Using particle-in-cell (PIC) simulations, plasma emission was observed for the interaction of laser light with super-critical density plasmas with spectral characteristics resembling those found from the fluid model.

Long-range magnetic order and the darwin lagrangian

We simulate a finite system of N confined electrons with inclusion of the Darwin magnetic interaction in two and three dimensions. The lowest-energy states are located using the steepest descent quenching adapted for velocity dependent potentials. Below a critical density the ground state is a static Wigner lattice. For supercritical density the ground state has a nonzero kinetic energy. The critical density decreases with N for exponential confinement but not for harmonic confinement. The lowest-energy state also depends on the confinement and dimension: an antiferromagnetic cluster forms for harmonic confinement in two dimensions.

Nonequilibrium laser-produced plasma of volume-structured media and ICF applications

This paper is devoted to the investigation of powerful laser pulse interaction with regularly and statistically volume-structured media with near critical average density and properties of laser-produced plasma of such a media. The results of the latest experiments on laser pulse interaction with plane foam targets performed on Nd-laser facilities “ABC” in the ENEA-EURATOM Association (Frascati, Italy) and “MISHEN” in the Troitsk Institute of Innovation Thermonuclear Investigations (TRINITI, Troitsk Russia), and J-laser “ISKRA-4” in the Russian Federal Nuclear Center, All-Russian Scientific Research Institute of Experimental Physics (RFNC-VNIIEF, Sarov, Russia) are presented and analyzed. High efficiency of the internal volume absorption of laser radiation in the foams of supercritical density was observed, and the dynamics of absorbing region formation and velocity of energy transfer process versus the parameters of porous matter are found. Some inertial confinement fusion (ICF) applications based on nonequilibrium properties of laser-produced plasma of a foam and regularly structured media such as the powerful neutron source with yield of 109–1011 DT-neutrons per 1 J of laser energy, laser-produced X-ray generation in high temperature supercritical plasma, and the compact ICF target absorbers providing effective smoothing and ablation are proposed.