SiO2 Melts Research Articles

Corrosion and depassivation of a chromia-forming Ni-based alloy in the 0.75Na2O-B2O3-2.75SiO2 melt

Corrosion resistance of nickel based alloys in molten silicates is linked to the temperature. Above a certain temperature, the protection of the alloy by a chromia layer is no more ensured: this depassivation phenomenon leads to active corrosion. Electrochemical methods were used for in situ characterization, and Scanning Electron Microscopy (SEM) and Scanning Electron Transmission Microscopy (STEM) were used in order to measure the oxide thickness after immersion. Corrosion tests with increasing temperature showed that above a certain temperature, the oxide thickness decreased homogeneously leading to the depassivation of the alloy. The different steps of this phenomenon are discussed here.

Melting behavior of SiO2 up to 120 GPa

The structure of liquid silicates is commonly described as a statistical mixture of various atomic entities with relative abundances that can vary with pressure, temperature and composition. Unfortunately, this view remains largely theoretical due to scarce experimental reports on the silicate melt structure, in particular under pressure. We performed X-ray diffraction of the SiO2 end-member to probe the melting curve up to ~120 GPa and 7000 K, and the melt structure up to ~80 GPa. We confirm the steep increase of the melting curve above ~14 GPa when stishovite becomes stable over coesite in subsolidus conditions, with a slope of about 80 K/GPa. Then, around 45 GPa and 5400 K, the melting curve flattens significantly, an effect most likely reflecting the densification of the SiO2 melt structure. The signal of diffuse X-ray scattering is compatible with a change of the Si coordination number from 4 to 6 along the melting curve, in agreement with previous works reporting a similar evolution during the cold compression of SiO2-glass. Because of the limited pressure range (within 10 to 20 GPa) in which the melting curve changes its slope, we speculate a difficult coexistence of tetrahedral SiO4 and octahedral SiO6 units in SiO2 melt at high pressures.

Notice of Retraction: Perspective and Prediction of the Rule of High Temperature Melting of SiO₂ via Visual Analysis

This paper focuses on how to see through the melting behavior of solid iron tailings in molten blast furnace slag and take a new non-contact visual analytical method to predict its melting law. The optimized convolution neural network (CNN) is used to track the moving target in charge coupled device (CCD) camera system efficiently and accurately, and the melting behavior of SiO2 is described by coordinate translation transformation theory. Hierarchical agglomerative clustering (HAC) and delaunay triangulation were used to extract the characteristic parameters of the melting process of SiO2. The prediction model of the melting rate of SiO2 at high temperature was established by least square fitting (LSF) and dimensional analysis, and compared with the actual melting rate of SiO2 obtained by experiments. The results show that the melting characteristics of SiO2 at high temperature are in accordance with certain function rule. The performance of optimized CNN in terms of processing time and the accuracy are significantly improved, and the fusion rate prediction model of SiO2 is verified by 100% accuracy. It provides theoretical support and model basis for the improvement of slag cotton preparation technology.

The viscosity of Bi2O3–B2O3–SiO2 glasses and melts

The viscosity of Bi2O3–B2O3–SiO2 glasses and melts

Pressure-induced structural change of CaO–Al2O3–SiO2 melt: Insight from molecular dynamics simulation

Short-range and intermediate-range order structures as well as network structure of multicomponent oxide system (CaO–Al2O3–SiO2) under compression are investigated by molecular dynamics simulation (MDS). The short-range order structure is clarified via investigation of pair-radial distribution function (PRDF), coordination number distribution and topology of TOx (T = Al, Si; x = 3, 4, 5, 6) units. The intermediate-range order structure and -T-O- network structure are clarified through investigation of distribution of OMy(M = Si, Al, Ca; y = 2, 3, 4, 5) linkages and T-O-T bond angles between adjacent TOx-units. The distribution of bridging-oxygens (BO), non-bridging oxygens (NBO), and TOx-cluster size is also investigated to clarify the degree of polymerization of -T-O- network under compression. The pressure-induced structural change, incorporation mechanism of Ca2+ cations into -T-O- network, aluminum avoidance as well as the compositional and structural heterogeneities (micro-phase separation) in CaO–Al2O3–SiO2 melt are also discussed in detail.

Thermal conductivity of Na2O–B2O3–SiO2 melts

ABSTRACT Borosilicate glasses are candidate materials for the immobilization of high-level radioactive waste. The values of thermal conductivity of different borosilicate melts are thus indispensable information when optimizing the temperature distribution in a glass melting furnace. In this study, the thermal effusivity of Na2O–B2O3–SiO2 melts was measured using a front heating–front detection laser-flash method. The thermal conductivity, which can be obtained by combining the measured thermal effusivity with the specific heat capacity and density, was calculated using the least-squares method; the values for the Na2O–B2O3–SiO2 melts either slightly decreased linearly with increasing temperature or remained almost constant over the investigated temperature range. The values of thermal conductivity of the Na2O–B2O3–SiO2 melts were higher than those of B2O3–SiO2 melts and lower than those of CaO–B2O3–SiO2 melts. Furthermore, the thermal conductivity of the Na2O–B2O3–SiO2 melts was compared with those of the B2O3–SiO2 and CaO–B2O3–SiO2 samples.

Electrochemical Behavior of Si(IV) on the Mo Electrode in the CaCl2–CaF2–CaO–SiO2 Melt

This work concerns a study on investigating the electrochemical behaviors of silicon using the molybdenum electrode in molten CaCl2–CaF2–CaO–SiO2 at 1023 K, by means of linear scan voltammetry, square wave voltammetry, chronoamperometry, open circuit chronopotentiometry, reversal chronopotentiometry and polarization curve. The results based on the linear scan voltammetry showed that reduction of Si(IV) in CaCl2–CaF2–CaO–SiO2 melt proceeds in a single step exchanging four electrons, which is a reversible process with diffusion-controlled mass transfer, and the diffusion coefficient for the reduction process of Si(IV) ions in CaCl2–CaF2–CaO (3.68 wt %)–SiO2 (4 wt %) is about 1.11 × 10−4 cm2 s−1, at 1023 K. The reversibility of the Si(IV)/Si redox couple on the molybdenum electrode is confirmed via linear scan voltammetry. Chronoamperometric measurements indicated that the I–t transients of Si(IV) follow instantaneous nucleation with varied the applied overpotential. Furthermore, the sample deposited on the molybdenum electrode using potentiostatic electrolysis was identified by X-ray diffraction (XRD). The XRD result indicates that the obtained deposits were Si and MoSi2.

Dehydration of melt inclusions in olivine and implications for the origin of silica-undersaturated island-arc melts

Primary subduction-related magmas build up modern continental crust and counterbalance massive recycling of crustal material into the deep mantle occurring at this tectonic setting. Melt inclusions in Mg-rich olivine are believed to be the best probes of primary subduction-related melts. However, unexpectedly, most of such inclusions are SiO2-undersaturated, in contrast to predominantly SiO2-saturated island-arc rocks. The origin of these melts has been explained by melting of amphibole-bearing pyroxenites in the lower crust or upper mantle. The current models fail, however, to explain the high abundance of SiO2-undersaturated melts as well as their compositional difference with host rocks for the major elements but not for incompatible trace elements. Here we report results from the investigation of rocks and melt inclusions in olivine from Klyuchevskoy volcano in Kamchatka. We show that experimental re-hydration of SiO2-undersaturated melt inclusions in olivine Fo85−90 at 300 MPa pressure and 1200°C causes a concomitant enrichment of melt in H2O and SiO2 so that re-hydrated inclusions (4–5 wt% H2O) become as silica-saturated as primitive Klyuchevskoy rocks. An experimental dehydration of previously re-hydrated inclusions also resulted in coupled depletion of melt in H2O and SiO2. The estimated stoichiometry of SiO2 and H2O gain/loss is consistent with dissolution/crystallization of metal-defect olivine on inclusion walls. The migration mechanism of water is controlled by hydrogen diffusion in the octahedral metal (Mg, Fe) vacancies through olivine structure as confirmed by FTIR spectroscopy. We conclude that the previously reported SiO2-undersaturated composition of many melt inclusions from hypersthene-normative island-arc rocks can be explained by the coupled loss of up to several weight percent of H2O and SiO2 from the initially trapped primitive SiO2-saturated hydrous melts. Thus, SiO2-undersaturated melt inclusions may not be representative of primitive island-arc magmas. The discovery of the coupled SiO2 and H2O loss from inclusions allowed us to propose a method for reconstruction of the initial water content even for completely dehydrated inclusions. The results of this study may indicate that the majority of primitive island-arc inclusions have not preserved their initial H2O content, and that primary arc melts contain on average ≥4 wt% H2O. The higher H2O concentration in primary arc melts implies the existence of a ‘crustal filter’ controlling the water content, which can be preserved in melt inclusions, and also the lower mantle melting temperatures and higher output H2O fluxes in subduction zones than previously estimated based on direct determination of H2O in potentially dehydrated melt inclusions.

Thermal conductivities of alkali–silicate and calcium–alkali–silicate melts

Thermal conductivities of alkali–silicate (R2O–SiO2 (R = Li, Na, K)) and calcium–alkali–silicate (CaO–R2O–SiO2 (R = Li, Na, K)) melts were evaluated by combining their thermal effusivities measured using the front-heating front-detection laser flash method with their density and specific heat capacity. Results of the evaluation revealed that the temperature-dependences of the thermal conductivities of the R2O–SiO2 and CaO–R2O–SiO2 (R = Li, Na, K) melts are small over the temperature range investigated. Moreover, the thermal conductivities of the CaO–R2O–SiO2 melts are almost equal to those of the R2O–SiO2 melts. Therefore, the network frame of the CaO–R2O–SiO2 melts can presumably become dense owing to the ion bonding between Ca2+ and the non-bridging oxygen.

Crystallization Studies of Poly(trimethylene terephthalate)/Silica Nanocomposites Prepared by Sol-Gel Technique.

Poly(trimethylene terephthalate) (PTT)/silica nanocomposite films were successfully fabricated using a novel sol–gel approach. The synthesis of these nanocomposites is being carried out by hydrolysis and condensation of tetraethoxysilane using trifluoroacetic acid with a small amount of water. The scanning electron microscopy and zetasizer result showed that the silica particles with a size range of 80–100 nm were homogeneously dispersed in the PTT matrix. The effect of different amounts of silica on crystallization of PTT was investigated using X-ray diffraction, differential scanning calorimetry (DSC), and optical microscopy. Polarized light microscopic results revealed that the spherulite size gradually decreased with increasing silica loading and increased with crystallization temperature for a given nanocomposite during isothermal melt crystallization. PTT with a small amount of SiO2 melt crystallized at low temperatures showed banded spherulites. DSC results revealed that nonisothermal cold crystallization temperature decreased with silica content, whereas no significant change in nonisothermal melt crystallization behavior was observed with silica content. The crystallinity of isothermally melt crystallized PTT increased with both crystallization temperature and silica loading.

Bulk properties and near-critical behaviour of SiO2 fluid

Rocky planets and satellites form through impact and accretion processes that often involve silicate fluids at extreme temperatures. First-principles molecular dynamics (FPMD) simulations have been used to investigate the bulk thermodynamic properties of SiO2 fluid at high temperatures (4000–6000 K) and low densities (500–2240 kg m−3), conditions which are relevant to protoplanetary disc condensation. Liquid SiO2 is highly networked at the upper end of this density range, but depolymerises with increasing temperature and volume, in a process characterised by the formation of oxygen–oxygen (OO) pairs. The onset of vaporisation is closely associated with the depolymerisation process, and is likely to be non-stoichiometric at high temperature, initiated via the exsolution of O2 molecules to leave a Si-enriched fluid. By 6000 K the simulated fluid is supercritical. A large anomaly in the constant-volume heat capacity occurs near the critical temperature. We present tabulated thermodynamic properties for silica fluid that reconcile observations from FPMD simulations with current knowledge of the SiO2 melting curve and experimental Hugoniot curves.

Monte Carlo simulations of bulk and nano amorphous silica (a-SiO2) melts

Monte Carlo simulations were carried on bulk and nano amorphous silica (SiO2) melts under different thermodynamic conditions and nano-sizes. The potential developed by Ersan, Tahir, Norman and William (ETNW) was used to model the interatomic interactions of SiO2 melts in both cases (bulk and nano systems). The bulk system was developed using periodic boundary conditions. The nano SiO2 melts were prepared by cutting out a sphere from the bulk amorphous SiO2 melt structure to the required size, and then simulated under free boundary conditions. The high-pressure, high-temperature and nano-size effects on the structure and properties of the melts were investigated and studied via radial distribution functions, radial density profiles, potential energy, density, ring size analysis, coordination number distributions, bond distances and angles. In addition, to understand the difference in properties of bulk and nano silica melts, a comparative study was done at the same thermodynamic conditions. Details of the modelling, simulation results and the study are presented in this paper.

Viscosity model for oxide melts relevant to fuel slags. Part 3: The iron oxide containing low order systems in the system SiO2–Al2O3–CaO–MgO–Na2O–K2O–FeO–Fe2O3

The viscosity model recently developed for the fully liquid system SiO2–Al2O3–CaO–MgO–Na2O–K2O is further extended to describe the viscosity of the iron oxide containing low order systems in the Newtonian range. The different structural roles of Fe2+ and Fe3+ to the viscosity are captured by the associate species. Using the monomeric associate species in combination with some specific larger structrual units, the model is capable of describing the viscosity of the melts FeO, FeO–SiO2, FeO–Al2O3, Fe2O3–CaO, Fe2O3–MgO, Fe2O3–Na2O, and Fe2O3–K2O over the whole range of compositions as well as a wide range of temperatures and oxygen partial pressures using only one set of model parameters. A new mechanism is proposed to describe the local viscosity maximum around the fayalite composition in the FeO–SiO2 melt. The model shows that the presence of the local viscosity maximum is dependent on the temperature and oxygen partial pressure. Moreover, the viscosity maximum caused by Al3+- or Fe3+-induced charge compensation is presented and a good agreement between the calculated viscosities and experimental data is demonstrated.

Archean‐Paleoproterozoic Lithospheric Mantle at the Northern Margin of the North China Craton Represented by Tectonically Exhumed Peridotites

AbstractTectonically emplaced peridotites from North Hebei Province, North China Craton, have retained an original harzburgite mineral assemblage of olivine (54%–58%) + orthopyroxene (40%–46%) +minor clinopyroxene (<1%)+spinel. Samples with boninite‐like chemical compositions also coexist with these peridotites. The spinels within the peridotites have high‐Al end‐members with Al2O3 content of 30 wt%–50 wt%, typical of mantle spinels. When compared with experimentally determined melt extraction trajectories, the harzburgites display a high degree of melting and enrichment of SiO2, which is typical of cratonic mantle peridotites. The peridotites display variably enriched light rare earth elements (REEs), relatively depleted middle REEs and weakly fractionated heavy REEs, which suggest a melt extraction of over 25% in the spinel stability field. The occurrence of arc‐ and SSZ‐type chromian spinels in the peridotites suggests that melt extraction and metasomatism occurred mostly in a subduction‐related setting. This is also supported by the geochemical data of the coexisting boninite‐like samples. The peridotites have 187Os/188Os ratios ranging from 0.113–0.122, which is typical of cratonic lithospheric mantle. These 187Os/188Os ratios yield model melt extraction ages (TRD) ranging from 981 Ma to 2054 Ma, which may represent the minimum estimation of the melt extraction age. The Al2O3–187Os/188Os‐proxy isochron ages of 2.4 Ga‐2.7 Ga suggest a mantle melt depletion age between the Late Achaean and Early Paleoproterozoic. Both the peridotites and boninite‐like rocks are therefore interpreted as tectonically exhumed continental lithospheric mantle of the North China Craton, which has experienced mantle melt depletion and subduction‐related mantle metasomatism during the Neoarchean‐ Paleoproterozoic.

Viscosity of CaO–MgO–Al2O3–SiO2 melts containing SiC particles

ABSTRACTThe influence of SiC particle on viscosity of CaO–MgO–Al2O3–SiO2 melts was investigated by the rotating-cylinder method. It was found that temperature dependence of viscosity could be described by the Arrhenius law for systems with or without SiC particle addition. The activation energies of liquid–solid mixtures were mainly determined by liquid phase. Temperature had little influence on the relative viscosity (defined as the viscosity ratio of solid–liquid mixture to pure liquid). Viscosity and relative viscosity increased as decreasing rotation speed and increasing volume fraction of SiC solid particle. For the same volume fraction of SiC particle, relative viscosity was affected by the liquid slag compositions. The relative viscosity was smaller when composition of liquid slag had a larger CaO/SiO2 ratio or MgO/Al2O3 ratio. Meanwhile, it was found that the smaller SiC particle will lead to a larger relative viscosity.

Application of IR and Raman spectroscopy for the determination of the role of oxygen fugacity in the formation of N–С–О–Н molecules and complexes in the iron-bearing silicate melts at high pressures

Large-scale melting of the Earth’s early mantle under the effect of global impact processes was accompanied by the generation of volatiles, which concentration was mainly controlled by the interaction of main N, C, O, and H gas-forming elements with silicate and metallic melts at low oxygen fugacity (fO2), which predominated during metallic segregation and self-oxidation of magma ocean. The paper considers the application of Raman and IR (infrared) Fourier spectroscopy for revealing the mechanisms of simultaneous dissolution and relative contents of N, C, O, and H in glasses, which represent the quench products of reduced model FeO–Na2O–Al2O3–SiO2 melts after experiments at 4 GPa, 1550°C, and fO2 1.5–3 orders of magnitude below the oxygen fugacity of the iron—wustite buffer equilibrium (fO2(IW)). Such fO2 values correspond to those inferred for the origin and evolution of magma ocean. It was established that the silicate melt contains complexes with N–H bonds (NH3, NH2+, NH2-), N2, H2, and CH4 molecules, as well as oxidized hydrogen species (OH– hydroxyl and molecular water H2O). Spectral characteristics of the glasses indicate significant influence of fO2 on the N–C–O–H proportion in the melt. They are expressed in a sharp decrease of NH2+, NH2-(O–NH2), OH–, H2O, and CH4 and simultaneous increase of NH2-(≡Si–NH2) and NH3 with decreasing fO2. As a result, NH3 molecules become the dominant nitrogen compounds among N–C–H components in the melt at fO2 two orders of magnitude below fO2(IW), whereas molecular СН4 prevails at higher fO2. The noteworthy feature of the redox reactions in the melt is stability of the ОН– groups and molecular water, in spite of the sufficiently low fO2. Our study shows that the composition of reduced magmatic gases transferred to the planet surface has been significantly modified under conditions of self-oxidation of mantle and magma ocean.

An unusual occurrence of coesite at the Lonar crater, India

AbstractCoesite has been identified within ejected blocks of shocked basalt at Lonar crater, India. This is the first report of coesite from the Lonar crater. Coesite occurs within SiO2 glass as distinct ~30 μm spherical aggregates of “granular coesite” identifiable both with optical petrography and with micro‐Raman spectroscopy. The coesite+glass occurs only within former silica amygdules, which is also the first report of high‐pressure polymorphs forming from a shocked secondary mineral. Detailed petrography and NMR spectroscopy suggest that the coesite crystallized directly from a localized SiO2 melt, as the result of complex interactions between the shock wave and these vesicle fillings.

Multicomponent diffusion in silicate melts: SiO2–TiO2–Al2O3–MgO–CaO–Na2O–K2O System

Nine successful diffusion couple experiments were carried out in a 7-component haplobasaltic silicate melt SiO2–TiO2–Al2O3–MgO–CaO–Na2O–K2O system to study multicomponent diffusion at ∼1500°C and 1GPa, typically with compositional gradients in only two components in each experiment. At least two concentration traverses were measured for each experiment. Effective binary diffusion coefficients (EBDC) for monotonic profiles were obtained by an error function fit, and the EBDC of a given component is dependent on its counter diffusing component, especially for SiO2. The EBDC’s of SiO2 vary from 15.7μm2/s when diffusing against Al2O3, to 102.9μm2/s when diffusing against K2O. Furthermore, the multicomponent diffusion matrix was obtained by simultaneously fitting profiles of all oxides in all experiments. Most features in the diffusion profiles, for example uphill diffusion, are captured well by this 6×6 diffusion matrix. The slowest diffusing eigenvector is largely due to the exchange between Si and Al, and the fastest diffusing eigenvector is the exchange of Na with all other components. An anorthite dissolution experiment was also conducted to test whether the diffusion matrix can be applied to mineral dissolution experiments. The calculated diffusion profiles in the melt during anorthite dissolution roughly match the measured profiles, demonstrating the validity and utility of the diffusion matrix in this FeO-free aluminosilicate melt system.

Molecular dynamics simulations of the structural properties and electrical conductivities of CaO–MgO–Al2O3–SiO2 melts

The structural properties and electrical conductivities of nine CaO−MgO−Al2O3−SiO2 slags with compositions in the peralkaline region have been calculated using molecular dynamics simulations. The proportion of nonbridging oxygen (NBO) and bridging oxygen (BO) atoms were determined from simulation and shown to be in reasonable agreement with theoretical prediction. Bridging oxygen atoms were classified into Si−O−Si, Si−O−Al or Al−O−Al linkages and the results used to establish whether the Al avoidance principle is applicable. The number of bridging oxygens coordinated to tetrahedral [SiO4]4− and [AlO4]5− units, namely the Qn distribution, was determined. Although a good comparison to the theoretical average Q was found at low basicity, at higher basicity greater deviation was seen. Electrical conductivities, taking cross-correlations into account, were in excellent agreement with experimentally measured values, although Nernst–Einstein conductivities, estimated from self-diffusion coefficients alone, showed large deviations. Molecular dynamics simulations are therefore able to reliably predict conductivity, but values calculated indirectly, i.e. using the Nernst–Einstein relationship, should be used with care. At low basicity theoretical predictions of structural disorder, based on Zachariasen's Random Network Model, are in reasonable agreement with simulation, but this agreement worsens as the proportion of network modifying cations increases.

Influence of Ti(C0.3N0.7) on viscosity of blast furnace slags

During the blast furnace smelting process of titanium-bearing ores, TiC0.3N0.7 was proved to be formed on the protective layer of the hearth, which greatly affects the fluidity of blast furnace slag and the successful separation of slag and metal. The present study reported an experimental investigation into the effect of solid particles on the viscosity of molten slags by the rotating-cylinder method. The viscosities of CaO–MgO–Al2O3–SiO2 melts containing different volume fractions of TiC0.3N0.7 particles were measured at different temperatures. It was found that viscosity of TiC0.3N0.7-containing melt increased with volume fraction of solid particles, but decreased as increasing the rotation speed. Furthermore, the Arrhenius law was obeyed for both the melts with and without TiC0.3N0.7 particles. It was also found that the temperature has little influence on the relative viscosity of TiC0.3N0.7 bearing slag. By comparison, TiC0.3N0.7 can increase the viscosity of suspension system more than TiN, but less than TiC.