Binary Silicate Melts Research Articles

Composition-dependent signinversion of the Soret coefficient of SiO2 in binary borosilicate melts.

Using a laser-induced local-heating experiment combined with temperature analysis, we observed the composition-dependent signinversion of the Soret coefficient of SiO2 in binary silicate melts, which was successfully explained by a modified Kempers model used for describing the Soret effect in oxide melts. In particular, the diffusion of SiO2 to the cold side under a temperature gradient, which is an anomaly in silicate melts, was observed in the SiO2-poor compositions. The theoretical model indicates that the thermodynamic mixing properties of oxides, partial molar enthalpy of mixing, and partial molar volume are the dominant factors for determining the migration direction of the SiO2 component under a temperature gradient.

Modelling of Viscosity of Melts Containing Iron Oxide in Ternary Silicate Systems

Abstract The motivation of this work is to show that the structural model, which was initially used to estimate the thermodynamic properties of binary silicate systems, can be also used to estimate the viscosity of binary and ternary silicate melts in terms of temperature and composition. The model links the viscosity to the internal structure of melts through the concentration of the oxygen bridges present in the slag. A previously proposed structural thermodynamic model was used to calculate the content of oxygen bridges. The viscosity model requires only three parameters to obtain a good agreement between experimental and calculated data for the SiO2−FeO binary system and for the SiO2−CaO−FeO, SiO2−MgO−FeO and SiO2−MnO−FeO ternary systems. The viscosity of ternary systems was calculated with the model while assuming a linear function of the parameters from binary systems; however, the content of the oxygen bridges was calculated using the thermodynamic model for ternary systems.

Direct and indirect evidence for free oxygen (O2−) in MO-silicate glasses and melts (M = Mg, Ca, Pb)

Oxygen 1s XPS spectra of a Pb-silicate glass containing 76.6 mol% PbO provide the first accurate, direct measurement of free oxide ion (O2−) in these glasses. O2− constitutes 35 (±3) mol% of total oxygen, with NBO and BO constituting, respectively, 52 (±3) and 13 (±3) mol%. All 29Si NMR and O 1s XPS results for Pb-silicate glasses indicate mol% levels of O2− containing more than ~30 mol% PbO. The O2− abundances are consistent with equilibrium thermodynamic considerations where K ~ 12 for the mass action equation involving NBO, BO, and O2−. Raman and 17O NMR spectra of two CaMg-silicate glasses indicate ~10 (±4) mol% O2− in CaMgSiO4 glass and ~18 (±4) mol% O2− in a Ca0.36Mg0.36Si0.28O1.28 glass. Oxygen species abundances are calculated using experimental results from 13 separate 29Si NMR, 17O NMR, and Raman measurements of Mg-, Ca-, and CaMg-silicate glasses. All reveal mol% levels of O2− with ~1 to 2.6 mol% in metasilicate glass and ~5 to 10 mol% in orthosilicate glass. Recent Raman experimental results also indicate O2− in CaMg-silicate glasses at levels ranging from about 1 to 10 mol%. In all there are 23 separate 29Si NMR, 17O NMR, and Raman measurements indicating mol% levels of O2− in alkaline earth silicate glasses. Eight recent MD simulations of Mg, Ca, and CaMg-silicate glasses include 21 separate simulations over a wide compositional range. All indicate mol% levels of O2− in the glasses demonstrating that the MD simulations and experimental results on these systems are in accord. There are two fundamentally important implications of these studies. First, free oxygen (O2−) is an essential constituent of Pb, Mg, Ca, and CaMg binary silicate melts and glasses. It is not an “accidental” product associated with glass or melt defects. It is instead, a thermodynamically important constituent of these binary melts (and glasses). Second, where melts are equilibrated, the mass action equation relating BO, NBO, and O2− must hold across the entire Ca, Mg, CaMg, and Pb binary systems, thereby requiring the activities and mole fractions of all three species to be defined and finite in the melts. Free oxygen, however, may be too low to be detected in highly siliceous glasses using conventional spectroscopic techniques.

Prediction of the thermodynamic functions of mixing of binary oxide melts in the PbO–SiO2, Al2O3–SiO2 and CaO–Al2O3 systems by structure-based modification of the quasi-chemical model

A structure-based modification of the Guggenheim's quasi-chemical model for the prediction of the thermodynamic functions of mixing in binary silicate and aluminate melts is presented and the extension of this approach to ternary systems is discussed. Based on the “anionic” point of view on the oxide melts structure, the novel method for determination of the interchange energy (a principal parameter of the quasi-chemical theory) was proposed along with a simple and general set of the equations. The activity calculations of the end-member components and integral and partial enthalpies and Gibbs free of mixing along with the whole range of compositions for PbO–SiO2 and Al2O3–SiO2 systems as well as the model calculations of components in CaO–Al2O3 melts were made. The reasonable agreement between results of model calculations and experimental data showed the reliability of this approach in predicting of the thermodynamic functions of mixing in binary and aluminate systems. This approach has the advantage that prior knowledge of the experimental mixing properties of liquid oxide systems is not required.

Surface tension of bismuth borosilicate melts

The surface tension of Bi2O3–B2O3–SiO2 melts was measured using the ring method over the temperature range 973–1573K. The composition and temperature dependences of the surface tension were investigated. The surface tension of Bi2O3–B2O3–SiO2 melts showed a maximum near 60–70mol% Bi2O3 and decreased with increasing B2O3/SiO2 ratio at 1373K. By assuming a linear relationship between the surface tension and the Bi2O3 content of binary silicate melts, the surface tension factor of Bi2O3 was determined to be 211mNm−1 at 1373K. The surface tension values calculated using the surface tension factor of each single component showed the largest deviation from the measured values when the Bi2O3 content was around 50–60% and were dependent on the B2O3 content. The temperature coefficient of surface tension had positive values at low Bi2O3 contents (less than 30–40mol%) and negative values at high Bi2O3 contents.

Relation Between Viscosity and Electrical Conductivity of Silicate Melts

The relation between viscosity and electrical conductivity of binary silicate melts is studied. The logarithm of viscosity varied linearly with the logarithm of conductivity. Furthermore, the lines are different for a MO-SiO2 system than for a M2O-SiO2 system. Compared with the MO-SiO2 system, the viscosity of the M2O-SiO2 system decreases more with the increase in conductivity.

Density and Surface Tension of CaO–SiO2–Al2O3–R2O (R=Li, Na, K) Melts

In the present work, we measured the density and the surface tension of CaO–SiO2–Al2O3–R2O (CaO/SiO2=0.67, Al2O3=20 mass%, R2O=10.8 mol%, R=Li, Na, K) quaternary melts at elevated temperature using the double-bob Archimedean method and the ring method, respectively. The density of the CaO–SiO2–Al2O3–R2O melts decreased with temperature due to the thermal expansion of the melts. In addition, the density of the CaO–SiO2–Al2O3 ternary melt decreased with the addition of the alkali oxides. We converted the density into the molar volume to have a consideration on the microstructure of the melts. The molar volumes of the CaO–SiO2–Al2O3–R2O melts are proportional to cube of the cationic radius for the alkali ions, which indicates the structural roles of the alkali oxides are mainly charge compensator to AlO45– anions. The surface tension of the CaO–SiO2–Al2O3–R2O melts changed only slightly with temperature. The surface tension of the CaO–SiO2–Al2O3 ternary melt increased with the addition of Li2O. In contrast, the surface tension decreased with the addition of Na2O or K2O. As for quaternary system, the surface tension increased with the ionic potential of the additive alkali ions; it is the same tendency with the binary alkali silicate melts. However, the mechanism of the surface tension change with the addition of the alkali oxides has not been clarified in the present study.

Analysis of disproportionation of Q n structons in the simulation of the structure of melts in the Na2O-SiO2 system

A new version of the STRUCTON�1.2 computer program (2009) has been presented. The pro� gram combines the algorithm for calculating real distributions of Q n structons in binary silicate melts (with allowance made for their disproportionation) and the statistical simulation of molecularmass distributions of polymerized ions at different temperatures. This model has been used to perform test calculations for two melts in the Na2 O-SiO 2 system (Na 6 Si 2 O 7 , Na 6 Si 3 O 9 ). The results of the calculations have made it possible to trace variations in the set and concentrations of chain and ring silicon-oxygen complexes with a decrease in the temperature in the order: stochastic molecularmass distribution mole cularmass distribution at T = 2000 K molecularmass distribution at the liqu idus temperature. The main result of these calcula� tions is that the dominant species of silicon-oxygen anions at the liquidus temperatures (in contrast to the stochastic distributions) exactly correspond to the stoichiometry of the initial melts: the chain anions and (SinO3n) 3n- ring complexes are dominant in the Na6Si2O7 and Na6Si3O9 melts, respectively. It has been established that, with a decrease in the temperature, the average size of polymer complexes varies weakly in the Na6 Si 2 O 7 melt but increases by a factor of approximately 1.5 in the metasilicate system.

Optimization and calculation of the MCl–ZnCl 2 (M = Li, Na, K) phase diagrams

An earlier structural model for binary silicate melts and glasses is extended to zinc chloride–alkali metal chloride systems. The evaluation of the available thermodynamic and phase diagrams data for the MCl–ZnCl 2 (M = Li, Na, K) binary systems have been carried out using the structural model for the liquid phase. This thermodynamic model is based on the assumption that each alkali chloride produces the depolymerization of ZnCl 2 network with a characteristic free-energy change. A least-squares optimization program permits all available thermodynamic and phase diagram data to be optimized simultaneously. In this manner, data for these binary systems have been analysed and represented with a small number of parameters.

Thermodynamic analysis of LiF–BeF 2 and KF–BeF 2 melts by a structural model

An earlier structural model for binary silicate melts and glasses is extended to MF–BeF 2 (M = Li, K) systems. The evaluation of the thermodynamic properties as well as the phase diagrams for the binary LiF–BeF 2 system and the integral enthalpy of mixing of the KF–BeF 2 system are carried out with this model. This thermodynamic model is based on the assumption that each alkali fluoride produces the depolymerization of BeF 2 network with a characteristic free energy change. A least squares optimization program permits all available thermodynamic and phase diagram data to be optimized simultaneously. In this manner, data for these binary systems have been analysed and represented with a small number of parameters. The model predicts the chain-length distribution of polymeric ions, even though these are not explicitly treated as structural units of the model. The calculated fluoride polyanion chain-length distribution for the LiF–BeF 2 system is in quantitative agreement with the predictions reported in the literature.

A model to calculate the viscosity of silicate melts

Abstract Our recently developed model to describe the viscosity of binary silicate melts is extended to describe and predict the viscosities of multicomponent silicate melts. The viscosity of multicomponent melts containing no AlO1.5 is modeled to vary linearly as a function of the mole fractions of the basic oxides at constant SiO2 mole fraction. Systems containing AlO1.5 show a more or less pronounced viscosity maximum close to the charge compensating composition. This maximum is caused by some of the Al3+ taking on the same structural role as Si4+, thereby participating in the formation of the silica network. The network-forming Al3+ must remain associated with either one Na+, or two Al3+ ions must remain associated with one Mg2+ or Ca2+, in order to assure charge neutrality. To take this into account we introduce the associates NaAlO2, CaAl2O4 and MgAl2O4 that correspond to charge compensated network-forming Al3+. The Gibbs energy of formation of these associates determines the amount of Al3+ that takes on the network-forming role. We assume the effect on viscosity of network-forming Al3+ to be the same as Si4+ and optimize the Gibbs energies of the associates to reproduce the experimental viscosity data. Viscosities in ternary silicate systems without AlO1.5 are quantitatively predicted with no additional ternary model parameters. Ternary systems MeO x – SiO2 – AlO1.5 are modeled with only two temperature-independent ternary parameters per system. The model not only reproduces the magnitude of the observed viscosity maximum, but also its complex shape, asymmetry and temperature dependence.

On calculating the activity of components in binary silicate melts

A strict scientific analysis of thermodynamics, kinetics, and mechanism of oxide melts, as well as metal and gas phase interaction, cannot be possible without studying the effect of wire former quantities and nature on the ionic composition and physicochemical properties of those melts. Quantitative evaluation of the ionic composition and thermodynamic properties of silicate systems can be based on the polymer theory. This paper concerns the methods of calculating components’ activity and ionic compositions in silicate melts, as based on the anion volume ratio. The design and experimental data on component activity in binary silicate melts are provided.

Quantitative Raman spectroscopy: Principles and application to potassium silicate glasses

A mathematical approach was developed to interpret Raman spectra of binary silicate glasses and melts without the necessity of external calibration, e.g., from NMR spectroscopy. The developed approach is based on Principal Component Analysis (PCA), linear combinations of partial Raman spectra and a linear optimization technique. In order to apply and to test this approach, we developed an experimental method to collect a large number of Raman spectra efficiently. We applied the quantification and the experimental approaches to investigate potassium silicate glasses with compositions from 17.4 to 38 mol% K 2O. The equilibrium constant for the reaction 2 Q 3 ⇄ Q 2 + Q 4 was found log K 3 = −2.37 ± 0.07, in excellent agreement with NMR studies for the same glasses.

Quantitative Raman spectroscopy: High-temperature speciation of potassium silicate melts

In situ, high-temperature Raman spectroscopy was used to study the Q n speciation in binary potassium silicate melts. Over 300 Raman spectra in the compositional range from 20 to 38 mol% K 2O were collected at temperatures between 800 and 1200 K. Quantitative information on the relative abundances of species in melts was obtained from the Raman spectra through a quantification procedure that does not require any a priori assumptions about the line shapes or external calibration of the Raman scattering efficiencies for the various Q n species. The Δ H 0 associated with the speciation reaction 2 Q 3 = Q 4 + Q 2 was found to be 33.1 ± 7.3 kJ/mol.

An Approach to Modeling Al2O3 Containing Slags with the Cell Model

The cell model originally developed by Kapoor and Frohberg for binary and ternary silicate melts was later extended to multi-component liquid slags by Gaye and Welfringer. CSIRO has extended the application of this model to describe the behaviour of a large number of oxide species commonly found in metallurgical slags. One limitation of the model was that when using only binary parameters to model ternary or higher order systems, the thermodynamic behaviour of the components in Al2O3 containing systems cannot be represented accurately. Based on an analysis of the structural role played by oxide components in the CaO-Al2O3-SiO2 system, two ternary parameters have been introduced to account for the influence of Al2O3 on Ca-Si interaction and CaO on Al-Si interaction, respectively. The introduction of the ternary parameters resulted in significant improvements in the model description of Al2O3 containing ternary and higher order systems. As a structurally based model, the cell model calculates parameters related to the degree of polymerization in silicate melts. These parameters have been used in a structurally based viscosity model for calculating the viscosity of silicate melts. The fit to a recent set of data has demonstrated the capability of the viscosity model to account closely for the cation effects.

Raman spectra and structure study of silicate glasses and their liquids

Stress index of tetrahedron (SIT) was defined to describe the topological connectivities among various silicon-oxygen tetrahedra (SiOT) in anionic clusters of binary silicate crystals, glasses, and melts. It was found that the value of SIT was well correlated with the wavenumber of Raman active symmetric stretching vibration of non-bridging oxygen of SiOT. The spatial fractional dimension of hyperfine structure was introduced while comparative analysis was made with the value of SIT. It can be concluded that the concepts of SIT, vibrational wavenumber, and spatial fractional dimension were inherently and holographically correlated and exhibit isomorphic representations of complex structure of binary silicates. Experimental Raman spectra of binary silicates with different alkali cations were investigated. It was demonstrated that alkali cations have little effect on the vibrational wavenumber of symmetric stretching of non-bridging oxygen (NBO) of SiOT, but remarkably affect its Raman active optical cross section, as was consensus resulted from ab initio calculation. It can also be concluded that the spatial fractional dimension of binary silicate is predominantly determined by the hyperfine structure of the anionic clusters and little affected by alkali cations, although the species of anionic clusters and their distributions were originally assigned by the content of alkali oxides. And Raman optical activity extinct effect of isolated SiOT at high basicity should be considered while being applied to quantitatively analysis.

A Semi-empirical Model for Viscosity Estimation of Molten Slags in CaO–FeO–MgO–MnO–SiO2 Systems

A semi-empirical viscosity model was proposed for silicate melts in this paper. The binary silicate melts MO–SiO2 (MO is a bivalent metal oxide) was treated as 2MO·SiO2–“SiO2” or “MO”–2MO·SiO2 system when XMO≤0.667 or XMO>0.667. Viscous activation energy of silicate melts was divided into three parts which come from the contributions of “SiO2”, “MO” and 2MO·SiO2. A relationship between pre-exponential factor A and activation energy E in Arrenhius equation was used for silicate melts. The model parameters were extracted from binary silicate melts and applied in ternary or higher order systems. The viscosities of silicate melts within CaO–MgO–MnO–FeO–SiO2 system were estimated using the present model. Good agreements have been achieved between calculated values and measured values. The mean deviation Δ of present model for slag systems investigated in our study is in the vicinity of 20%.

Isomorphic representations of hyperfine structure of binary silicates by interior stress, vibrational wavenumber and spacial fractional dimension

Stress index of tetrahedron (SIT) was defined to describe the topological connectivities among various silicon-oxygen tetrahedra (SiOT) in anionic clusters of binary silicate crystals, glasses and melts. It was found that the value of SIT was well correlated with the wavenumber of Raman active symmetric stretching vibration of non-bridging oxygen of SiOT. And also the spatial fractional dimension of hyperfine structure was introduced while comparative analysis being made with the value of SIT. It can be concluded that the concepts of SIT, vibrational wavenumber and spatial fractional dimension were inherently and holographically correlated and exhibit isomorphic representations of complex structure of binary silicates. Experimental Raman spectra of binary silicates with different alkali cation were investigated. It was demonstrated that alkali cation has little effect on the vibrational wavenumber of symmetric stretching of non-bridging oxygen (NBO) of SiOT, but remarkably affects its Raman active optical cross section, as was consensus resulted from ab initio calculation. It also can be concluded that the spatial fractional dimension of binary silicate is predominantly determined by the hyperfine structure of the anionic clusters and little affected by alkali cation, although the species of anionic clusters and their distributions were originally assigned by the content of alkali oxides.

Thermodynamic approach to physical properties of silicate melts

The thermophysical properties of silicate melts are strongly structure dependent. It is well known that the viscosity of slags increases with increasing degree of polymerisation of the silicate anion. Even the thermodynamic properties of slags are dependent on the species type and population in the melt. Thus, a link between the thermophysical and thermochemical properties of silicate melts is logically expected. The present paper elucidates the salient features of Darken's excess stability approach to the Gibbs energy of solution as applied to the viscosities of silicate melts. It is demonstrated that the second derivatives of the viscosities of binary silicate melts with respect to composition indicate maxima corresponding to the existence of stable compounds in these systems. The concept has been successfully applied to the following systems: Al2O3–SiO2, CaO–SiO2, FeO–SiO2, MgO–SiO2 and MnO–SiO2. In all cases, the second derivative plots of viscosities with respect to composition show peaks corresponding to the metasilicates. The second derivatives of the activation energies of viscous flow with respect to temperature have earlier been shown to reflect the formation of associates/embryos in homogeneous silicate melts, indicating the readiness of the melt to separate a solid phase. Thermodynamic coupling of thermal diffusivities in the case of the CaO–Al2O3–SiO2 system from laser flash measurements of these slags, as a function of temperature, has been examined as part of the present study. Densities have been estimated from integral molar enthalpies in the case of silicate systems, and the results are presented.

Lux-Flood basicity of binary silicate melts

Application of the hybrid polymeric model to depict silicate melt energetics allows to distinguish chemical interaction terms from strain energy contributions. It is thus shown that Lux-Flood basic oxides when admixed with silica in various proportions along simple binaries give rise to purely endothermic effects, while acidic Lux-Flood oxides give rise to athermal mixtures and amphoteric oxides promote both enthalpic and entropic (non-configurational) chemical interactions. The Lux-Flood acid–base character of the various oxides is shown then to be consistent with experimental determinations of nephelauxetic properties of the limiting oxide components in the mean donor ligand field approximation (optical basicity of Duffy and coworkers). The linear proportionality observed between endothermic heat of mixing and optical basicity difference allows to predict the polymerization extent in simple MO–SiO 2 molten systems in a wide T range with reasonable precision.