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Supercritical water is a green solvent used in many technological applications including materials synthesis, nuclear engineering, bioenergy, or waste treatment and it occurs in nature. Despite its relevance in natural systems and technical applications, the supercritical state of water is still not well understood. Recent theories predict that liquid-like (LL) and gas-like (GL) supercritical water are metastable phases, and that the so-called Widom line zone is marking the crossover between LL and GL behavior of water. With neutron imaging techniques, we succeed to monitor density fluctuations of supercritical water while the system evolves rapidly from LL to GL as the Widom line is crossed during isobaric heating. Our observations show that the Widom line of water can be identified experimentally and they are in agreement with the current theory of supercritical fluid pseudo-boiling. This fundamental understanding allows optimizing and developing new technologies using supercritical water as a solvent.

The thermodynamics, structural and transport properties (density, melting point, heat capacity, thermal expansion coefficient, viscosity and electrical conductivity) of a ferro-aluminosilicate slag have been studied in the solid and liquid state (1273–2273 K) using molecular dynamics. The simulations were based on a Buckingham-type potential, which was extended here, to account for the presence of Cr and Cu. The potential was optimized by fitting pair distribution function partials to values determined by Reverse Monte Carlo modelling of X-ray and neutron diffraction experiments. The resulting short range order features and ring statistics were in tight agreement with experimental data and created consensus for the accurate prediction of transport properties. Accordingly, calculations yielded rational values both for the average heat capacity, equal to 1668.58 J/(kg·K), and for the viscosity, in the range of 4.09–87.64 cP. The potential was consistent in predicting accurate values for mass density (i.e. 2961.50 kg/m3 vs. an experimental value of 2940 kg/m3) and for electrical conductivity (5.3–233 S/m within a temperature range of 1273.15–2273.15 K).

Aerodynamic levitation of a multi component 17 w.t.% Si glass formed by rapid quenching of the melt phase was studied by high resolution X-ray diffraction (XRD) and Reverse Monte Carlo (RMC) modeling. The main local atomic order features comprised of interactions between Si, Fe and Mg polyhedra, the stereochemistry of which was on a par with literature. Both the glass and the liquid state appeared to consist of the same fundamental Si-O, Fe-O and Mg-O clusters, with only the relative number of each varying between the two. Transition from liquid to the glass involved a 3-fold decrease in uncoordinated O (to within the first minimum of the total g(r)) and a marked increase of Fe-Si-Mg polyhedra bridging O. Octahedral Fe coordination was not suggested by the RMC data. All-electron open-shell Density Functional Theory (DFT) calculations of the most prominent clusters suggested independence between the Fe oxidation state and its polyhedra O-coordination. Of secondary thermodynamic importance were indications of network-forming Fe 2+ and Fe 3+ distorted trigonal and tetrahedral polyhedra. In all occasions, the Fe ferrous and ferric states involved comparable binding energies within similar clusters which indicate a dynamic equilibrium between the two. IntroductionMulticomponent aluminosilicate oxides in the amorphous state are of ubiquitous engineering importance, largely as much as they are intractable in terms of their atomic structure. Applications related to these oxides are invariably dependent on their mesoscale properties, such as viscosity and thermal conductivity, which, in turn, have been correlated to the number and movement of cations through the silicate network and to the availability of free (not fully coordinated) oxygen atoms. In alkaline earth oxides, the basicity of the melt has been empirically related to the mass ratio of (CaO+MgO)/SiO 2 [1] and appears to be inversely proportional to viscosity [2, 3]. Certainly, complementary effects between network formers and modifiers are dependent on ionic radii, valence and electron orbitals [4]. Of particular interest, the melt oxide conductivity has been linked to the concentration of (ferric) Fe 3+ ions while the existence of the latter appears to depend on the Fe 3+ /Fe 2+ redox equilibrium [5][6][7] the kinetics of which are greatly affected by network-modifying cations [6, 8, 9]. Perhaps counterintuitively, Fe was reported to assume either in a network modifying or network forming role depending on its oxygen coordination rather than the oxide elemental concentration [6]. According to experimental observations, Fe tetrahedral (oxygen) coordination is achieved for all ferric iron when the Fe 3+ :Fe 2+ ratio exceeds 1:1 [10]. Octahedral Fe 2+ is widely assumed to act as a network modifier, whereas there is broad consensus, albeit not based on atomic studies, that tetrahedral Fe 2+ acts as a network former [7, 9]. To address the issue of atomic structure in complex, high temperature oxide systems, we have reported on the short-range order of a ...

In a nuclear waste repository, the corrosion of metals and the degradation of the organic material in the waste matrix can generate significant amounts of gases. These gases should be able to migrate through the multibarrier system to prevent a potential pressure build-up that could lead to a loss of barrier integrity. Smectite mineral particles form a tortuous pore network consisting of larger interparticle pores and narrow interlayer pores between the platelets of the smectite minerals. These pores are normally saturated with water, so one of the most important mechanisms for the transport of gases is diffusion. The diffusion of gases through the interparticle porosity depends on the distribution of gas molecules in the water-rich phase, their self-diffusion coefficients, and the tortuosity of the pore space. Classical molecular dynamics simulations were applied to study the mobility of gases (CO2, H2, CH4, He, and Ar) in Na-montmorillonite (Na-MMT) under saturated conditions. The simulations were used to estimate the gas diffusion coefficient (D) in saturated Na-MMT as a function of nanopore size and temperature. The temperature dependence of the diffusion coefficient was expressed by the Arrhenius equation for the activation energy (E a). The predicted D values of gases were found to be sensitive to the pore size as the D values gradually increase with increasing pore size and asymptotically converge to the gas diffusion coefficient in bulk water. This behavior is also observed in the self-diffusion coefficients of water in Na-MMT. In general, H2 and He exhibit higher D values than Ar, CO2, and CH4. The predicted E a values indicate that the confinement affects the activation energy. This effect is due to the structuring of the water molecules near the clay surface, which is more pronounced in the first two layers of water near the surface and decreases thereafter. Atomic density profiles and radial distribution functions obtained from the simulations show that the interaction of the gas with the liquid and the clay surface influences mobility. The obtained diffusion coefficient for different gases and slit pore size were parameterized with a single empirical relationship, which can be applied to macroscopic simulations of gas transport.

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Supercritical water is used in a variety of chemical and industrial applications. As a consequence, a detailed knowledge of the structure-properties correlations is of uttermost importance. Although supercritical water was considered as a homogeneous fluid, recent studies revealed an anomalous behaviour due to nanoscale density fluctuations (inhomogeneity). The inhomogeneity is clearly demarked through the Widom line (maxima in response factions) and drastically affect the properties. In the current study the physical properties of supercritical water have been determined by classical molecular dynamics simulations using a variety of polarized and polarizable interatomic potentials. Their validity which was not available at supercritical conditions has been assessed based on the ability to reproduce experimental data. Overall, the polarized TIP4P/2005 model accurately predicted the properties of water in both liquid-like and gas-like regions. All interatomic potentials captured the anomalous behaviour providing a direct evidence of molecular-scale inhomogeneity.

a b s t r a c tA 2D, steady-state computational fluid dynamics (CFD) analysis of an industrial electric arc furnace (EAF) is presented. The analysis accounts for the electrode shape and immersion depth, as well as for the dependence of Joule heating on the properties of the slag. The equations for the electric potential, momentum and heat transfer were solved across four distinct regions (i.e. air, slag, ferronickel and firebricks) and the final profile of the slag/metal interface was calculated as a function of the operating parameters of the furnace. The results indicate that the amount of Joule heat produced by the Söderberg electrodes increased with increasing applied voltage and electrical conductivity of the slag; the Joule heat peaked for a value of slag electrical conductivity equal to 3 S/m. A highly conductive slag along with a greater electrode immersion depth was found to facilitate the melting process. The maximum slag velocity values computed were of the order of 0.8 m/s in the vicinity of the electrode tips.