Atomic order and cluster energetics of a 17 wt% Si-based glass versus the liquid phase

Abstract

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 ...

Bona-fide method for the determination of short range order and transport properties in a ferro-aluminosilicate slag

“…The quantities A , ρ and C are constants which are meant to be modified, such that the structure in the atomic supercell satisfies experimental datasets (e.g. the structure factor and/or the pair distribution function arising from total scattering of X ray or neutron sources)123. Here, parameters A , ρ and C for Cr and Cu were determined via force field optimization, as no relevant data exist in the literature (see section ‘Force field optimization’ below).…”

“…The consistency of the SRO characteristics present in the amorphous slag has been highlighted in our precursor work, by Reverse Monte Carlo (RMC) fitting of experimental structure factors attained from high-energy X ray and neutron diffraction123. Here we utilize the SRO established in that work as a starting structure for MD calculations aimed at extracting the transport properties of the slag in the high temperature regime relevant to EAF operation.…”

“…Ferronickel is produced by reductive smelting (RS) of nickel laterite ores in electric submerged arc furnaces (EAF)123. RS involves industrial infrastructures of grand physical proportions with correspondingly high requirements for electric energy, in excess of 40 MWatts per EAF.…”

“…It is of course expected that, on a fundamental level, such a ratio of phases must precipitate from the atomic coordination requirements of both Al and Ca. The coordination of Al is known to depend on neighboring cations, albeit Al is believed to be prominently four-fold coordinated12319, as flagged by the most frequently observed value for the polyhedron O-Al-O bond angle (approximately equal to 109° 1823). It has been suggested that the tendency of other network-modifying cations to charge-balance [AlO 4 ] 5− polyhedra decreases with increasing ionic field strength, according to the sequence K +  >Na +  >Ca 2+  >Mg 2+  19.…”

A CFD analysis of slag properties, electrode shape and immersion depth effects on electric submerged arc furnace heating in ferronickel processing

“…Ferronickel is produced in submerged-arc electric furnaces by reductive smelting (RS) of nickel laterite ores [1][2][3] . RS involves the formation of a slag bath (consisting of silicon, iron, aluminum and calcium oxides), floating over the ferronickel bulk, the latter accumulating in the EAF lower part.…”