Na2O-SiO2 System Research Articles

Phase equilibria and thermodynamic analysis of liquid–liquid immiscibility in the system SiO2–Na2O–MoO3

AbstractPhase equilibrium experiments were carried out in the SiO2–Na2O–MoO3 system at 1200ºC and 1000ºC. We found that immiscible phase region in this system is narrower than that reported previously. By combining phase relationship and thermodynamic data and assuming associate species model, interaction parameters to describe enthalpy of mixing of SiO2–Na2O–MoO3 liquid were optimized by CALPHAD methodology. The thermodynamic database can be applied to thermodynamic calculations of silicate systems containing molybdenum, such as the vitrification process of high-level radioactive waste.

Viscosity, Boson Peak and Elastic Moduli in the Na2O-SiO2 System

The temperature and chemical dependence of the melt viscosity are ubiquitous in the model development of the volcanic dynamics, as well as in the glass production and design. We focussed on the yet-explored relationship between the bulk and shear moduli ratio and boson peak with the melt fragility of their parental glasses. Here, we explored the extension of the observed trend by testing the conventional binary system Na2O-SiO2, thus providing new evidence supporting the link between the flow of melts and supercooled liquids and the vibrational dynamics of their parental glasses. This was accomplished by integrating new low-frequency Raman measurements and integrating data from the literature on Brillouin light scattering and viscometry. This approach allows us to feed the MYEGA equation with reliable input parameters to quantitatively predict the viscosity of the Na2O-SiO2 system from the liquid up to the glass transition.

Effect of Na Ions on Melt Structure and Viscosity of CaO–SiO<sub>2</sub>–Na<sub>2</sub>O by Molecular Dynamics Simulations

Molecular dynamics (MD) simulations have been used to study the effect of Na ions on the structure properties of CaO–SiO2–Na2O melts. The short-range structure and medium-range structure of CaO–SiO2–Na2O in this study are consistent with existing data. The replacement of Ca2+ with Na+ in CaO–SiO2–Na2O melts has almost no effect on the degree of polymerization and distribution of bond angles of Si–O tetrahedron. From micro perspective, Na ions enhance the mobility of CaO–SiO2–Na2O melts by multiple ways. Firstly, the modification effect of Na+ on the melt network structure is weaker than that of Ca2+, the Si–O tetrahedron around Na+ is sparser than Ca2+, which is more conducive to ions movement. Secondly, the diffusion capacity of Na+ is much greater than other ions in CaO–SiO2–Na2O system, which the overall diffusion capacity of the system can be improved by adding more Na+. Thirdly, since Na+ has only one charge, there is no electrostatic restraint on the depolymerized tetrahedron which happened in multivalent charges such as Ca2+, so that the mobility of CaO–SiO2–Na2O is stronger than that of CaO–SiO2. The micro changes provide an explanation for the improvement of macro liquidity.

Determination of thermodynamic and microscopic origins of the Soret effect in sodium silicate melts: Prediction of sign change of the Soret coefficient.

The Soret effect in silicate melts has attracted attention in earth and material sciences, particularly in glass science and engineering, because a compositional change caused by the Soret effect modifies the material properties of silicate melts. We investigated the Soret effect in an Na2O-SiO2 system, which is the most common representative of silicate melts. Our theoretical approach based on the modified Kempers model and non-equilibrium molecular dynamics simulation was validated for 30Na2O-70SiO2(mol. %). The sign and order of the absolute values of the calculated Soret coefficients were consistent with the experimental values. The positive Soret coefficient of SiO2 in the SiO2-poor composition range was accurately predicted. Previous experimental studies have focused on SiO2-rich compositions, and only the negative sign, indicating SiO2 migration to the hot side, has been observed. In the SiO2-poor composition range, the Q0 structure was dominant and had four Si-O-Na bonds around an SiO4 unit. The Si-O-Na bond had high enthalpic stability and contributed to the large negative enthalpy of SiO2 mixing. According to our model, components with a large negative partial molar enthalpy of mixing will concentrate in the cold region. The microscopic and thermodynamic origins of the sign change in the Soret effect were determined.

Phase-field simulation of spinodal phase separation in the Na2O-SiO2 glasses

The phase-field method was used for the kinetic simulation of the spinodal phase separation of glasses in the Na2O-SiO2 system. The miscibility gap and spinodal curve were calculated based on the previous interaction parameters of the liquid phase. The kinetic simulations showed that the resultant morphology depended on the initial concentration and the temperature. In the early stages of spinodal decomposition, the interconnected structure developed continuously and in the later stages began to coarsen. During the coarsening stage, the evolution of morphology of the microstructures could result in the discrete spherical particles as well as the interconnected structure, depending on the heat treatment condition. To analyze the phase separation rate, the amplification factors for the initial concentrations were calculated as a function of temperature and could explain the morphology developed from the present simulation. Also, the present work could explain the evolution of experimental morphologies available in the literature reasonably.

FEATURES OF MELTING IN THE THERMOCHEMICAL PLUME CONDUIT AND HEAT AND MASS TRANSFER DURING CRYSTALLIZATION DIFFERENTIATION OF BASALTIC MELT IN A MUSHROOM-SHAPED PLUME HEAD

The number Ka=N/N1is used to evaluate the thermal power of a plume;Nis the thermal power transferred from the plume base to its conduit, andN1is the thermal power transferred from the plume conduit into the surrounding mantle. At the relative thermal power 1.9<Ka<10, after eruption of the melt from the plume conduit to the surface, melting occurs in the crustal block above the plume roof, resulting in the formation of a mushroom-shaped head of the plume. A thermochemical plume originates at the core-mantle boundary and ascends (melts up) to the surface. Based on laboratory and theoretical modeling data, we present the flow structure of melt in the conduit and the head of the thermochemical plume. The features of melting in the plume conduit are elucidated on the basis of the phase diagram of the CaO-MgO-Al2O3-SiO2model system. The two upper convection cells of the plume conduit relate to the region of basic and ultrabasic compositions. Our study shows that melting in these cells proceeds according to monovariant equilibria of eutectic type L=Cpx+Opx+An+Sp and L=Fo+An+Cpx+Opx. In case of the CaO–MgO–Al2O3–SiO2–Na2O system, crystallization differentiation proceeds as separation of plagioclase crystals. Separation of plagioclase crystals enriched in anorthite component leads to enrichment of the residual melt in silica and alkaline components. Assuming the initial basaltic melt, we calculated the compositional changes in the melt, which are powered by the heat and mass transfer processes in the mushroom-shaped plume head. The calculations were performed in two stages: (1) after settling of refractory minerals; (2) after settling of plagioclase in the melt resulting from the first stage. In the second stage, the melt contains 88.5 % of plagioclase component. The calculations were performed for melt temperatureTmelt=1410 °C and pressureP=2.6 kbar and 6.3 kbar. The calculated weight contents of oxides, the normative compositions for solid phase, and the oxide content and normative composition for the residual melt were tabulated. The SiO2content in the residual melt amounts to 59.6–62.3 % and corresponds to the crustal SiO2content.

Structural evolution of CaF2 nanoparticles during the photoinduced crystallization of a Na2O–K2O–CaO–CaF2–Al2O3–ZnO–SiO2 glass

A glass in the system Na2O–SiO2–Al2O3–K2O–CaO–CaF2–ZnO was doped with Ce, Ag, Sn, Sb and Br. Homogeneous and transparent glass ceramics are obtained from this glass by the precipitation of CaF2 nanoparticles. An interface-controlled crystallization mechanism hinders crystal growth after some time. By adding photosensitive agents, a tailored photoinduced CaF2 crystallization was achieved. Structural evolution during irradiation and heat treatment was investigated by different electron microscopic techniques, e.g., scanning transmission electron microscopy (STEM) including energy-dispersive X-ray analysis (EDXS) and scanning electron microscopy (SEM) including electron backscatter diffraction (EBSD). Agglomerated structures with a size of ca. 35 nm were observed after the first thermal treatment at 530 °C. The Ag nucleation is accompanied by clustering of several components within the glass. CaF2 particles with diameters of ca. 300 nm with a spherulitic structure are observed in the glass ceramic after the second heat treatment step at 560 °C. Calcium and fluoride are depleted from the glass matrix during crystallization, while SiO2 is enriched. From (S)TEM micrographs, supplemented by selected area diffraction (SAD), outward growth of CaF2 from a central point is observed. STEM–EDXS analyses show residual glass within the spherulitic structure.

MAS-NMR support for Hench model in the case of bioactive glass microspheres

The structural changes occurred in bioactive glass microspheres belonging to the system SiO2–Na2O–P2O5–CaO–K2O–MgO incorporating yttrium were investigated before and after soaking in simulated body fluid (SBF) by X-ray diffraction (XRD) and 31P and 29Si magic angle sample spinning nuclear magnetic resonance (MAS-NMR). The addition of yttrium to the bioactive glass composition induces changes in the behavior of the glass microspheres in SBF. The XRD analysis proves that after the immersion in SBF a crystalline hydroxyapatite-like phase is developed on the microspheres surface. The 29Si and 31P MAS-NMR results show that silicate species with two and three bridging oxygens per SiO4 tetrahedra and PO4 monomeric units are present in the glass structure. After immersion in SBF, new silicate species with four bridging oxygens appear as result of silica-gel layer formed on microspheres surface. The formation of crystalline hydroxyapatite-type layer is reflected by the occurrence of narrow components in 31P MAS-NMR spectra. The NMR results support the Hench model for bioactive glasses behavior in biological environments.

Angiogenic potential of boron-containing bioactive glasses: in vitro study

Boron-containing bioactive glasses (BGs) are being extensively researched for the treatment and regeneration of bone defects because of their osteostimulatory and neovascularization potential. In this study, we report the effects of the ionic dissolution products (IDPs) of different boron-doped, borosilicate, and borate BG scaffolds on mouse bone marrow stromal cells in vitro, using an angiogenesis assay. Five different BG scaffolds of the system SiO2–Na2O–K2O–MgO–CaO–P2O5–B2O3 (with varying amounts of SiO2 and B2O3) were fabricated by the foam replication technique. Bone marrow stromal cells were cultivated in contact with the IDPs of the boron-containing BG scaffolds at different concentrations for 48 h. The expression and secretion of vascular endothelial growth factor (VEGF) from the cultured cells was measured quantitatively using the VEGF ELISA Kit. Cell viability and cell morphology were determined using WST-8 assay and H&E staining, respectively. The cellular response was found to be dependent on boron content and the B release profile from the glasses corresponded to the positive or negative biological activity of the BGs.

Mechanical Properties andIn VitroPhysico‐chemical Reactivity of Gel‐derived SiO2–Na2O–CaO–P2O5Glass from Sand

AbstractIn the present report, a bioactive glass was synthesized from silica sand as economic substitute to alkoxy silane reagents. Sodium metasilicate (Na2SiO3) obtained from the sand was hydrolyzed and gelled using appropriate reagents before sintering at 950 °C for 3 h to produce glass in the system SiO2Na2OCaOP2O5. Compression test was conducted to investigate the mechanical strength of the glass, while immersion studies in simulated body fluid (SBF) was used to evaluate reactivity, bioactivity and degradability. Furthermore, the glass samples were characterized by scanning electron microscopy (SEM), X‐ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and energy dispersive X‐ray spectroscopy (EDX) to evaluate the microstructure and confirm apatite formation on the glass surface. The glass, dominated by bioactive sodium calcium silicate, Na2Ca2Si3O9(combeite) crystals, had mechanical strength of 0.37 MPa and showed potentials for application as scaffold in bone repair.

Phase equilibria of the Na2O–SiO2 system between 1173 and 1873 K

Knowledge of phase equilibria in the Na2O–SiO2 system at elevated temperatures is important for mastering the metallurgical process and for the glass and ceramic industries. The phase diagram at metasilicate and silica saturations in the Na2O–SiO2 system between 1173 and 1873 K has been investigated by quenching and phase analyses. The experimental technique involved equilibration of high purity oxide mixtures at sub-liquidus temperatures, rapid quenching in cold water and followed by compositional measurement of the phases using electron probe X-ray microanalyser. The phases stable in the equilibrium assemblages have been characterised over a wide range of compositions and temperatures. The results were compared with previous experimental data and an optimised phase diagram.

Synthesis and physical characterization of amorphous silicates in the system SiO2-Na2O– RO (R = Zn, Pb or Cd)

Glasses of the composition (mol%) 65 SiO2, 20 RO, 15 Na2O, where R is Zn, Pb or Cd, were prepared by the melting quenching technique. It is aimed to investigate the structural, optical and, for the first time, their electrical and dielectric properties of these glasses. FTIR revealed that modifier oxide tetrahedral structural units and chemical resistivity increase, while non-bridging oxygens (NBOs) decrease as ZnO is replaced by CdO or PbO. The absorption edge shows a red shift in the same direction of replacement, which was ascribed to the decrease of the strength of oxygen bonding and increase of polarizable NBOs. Refractive index, density, molar volume and optical band gap energy (Eopt) were determined. The dielectric constant attained higher values for glass containing ZnO than those containing CdO or PbO. The ac conductivity behavior is interpreted in terms of overlapping-large polaron tunneling model and the correlated barrier hopping model. The electronic-type conductivity dominates in the studied glasses containing CdO or PbO. The ac conductivity values of the studied glasses increase with increasing frequency, indicating their semiconducting nature. The obtained values of electrical conductivity and capacitance may allow considering the investigated glasses for use in electronic devices.

Thermal Conductivity of Molten B2O3, B2O3–SiO2, Na2O–B2O3, and Na2O–SiO2 Systems

With the aid of the transient hot‐wire method, the thermal conductivity of molten B2O3, B2O3–SiO2, Na2O–SiO2, and Na2O–B2O3 systems was measured along with their temperature and composition. It was observed that the thermal conductivity of pure B2O3 increased with temperature, until about 1400 K, and then decreased subsequently. Using the MAS‐NMR, 3Q‐MAS, and Raman spectroscopy, the structure of B2O3 and SiO2 in the B2O3–SiO2 system was confirmed. Findings show that an addition of B2O3 into the pure SiO2 system causes a significant decrease in thermal conductivity, due to the formation of boroxol rings. The thermal conductivity of the Na2O–SiO2 system was measured and its phonon mean free path was calculated. In addition, a positive linear relation between viscosity and thermal conductivity was observed. In the Na2O–B2O3 system, it was found that a change in the relative fraction of 4‐coordinated boron has an influence on the thermal conductivity when the concentration of Na2O is between 10 and 30 mol%, in which case the tetraborate unit is dominant.

Evolution of the heterogeneous structure of model sodium silicate glasses during binodal decomposition: X-ray small-angle scattering investigation

The kinetics of binodal decomposition in model glasses of the Na2O-SiO2 system from the initial stage to the stage of the Ostwald ripening inclusive has been investigated in situ. It has been found that the spatial-temporal evolution of the heterogeneous structure during binodal decomposition has a multistage character. The characteristic size of the phase regions at each stage of the binodal decomposition varies with time according to the power law L ∼ tβ. The values of exponents β for different stages of the binodal decomposition are as follows: 1/20 (fluctuation stage), 1/2 (growth stage), 0 (transient stage), and 1/3 (Ostwald ripening stage). The particle size distributions for all stages of the binodal decomposition have been presented.

Composite bone cements loaded with a bioactive and ferrimagnetic glass-ceramic. Part I: Morphological, mechanical and calorimetric characterization

Hyperthermia is a technique for destroying cancer cells which involves the exposition of body's tissue to a controlled heat, normally between 41℃ and 46℃. It has been reported that ferro- or ferrimagnetic materials can heat locally, if they are placed (after being implanted) under an alternating magnetic field, damaging only tumoral cells and not the healthy ones. The power loss produced by the magnetic materials can be dissipated in the form of heat. This phenomenon has to be regulated in order to obtain a controlled temperature inside the tissues. The material that was produced and characterized in this work is composed of two phases: a polymethylmethacrylate (PMMA) matrix in which a ferrimagnetic biocompatible/bioactive glass ceramic is dispersed. This composite material is intended to be applied as bone filler for the hyperthermic treatment of bone tumors. The ferrimagnetic bioactive glass-ceramic belongs to the system SiO2-Na2O-CaO-P2O5-FeO-Fe2O3 and contains magnetite (FeO*Fe2O3) inside an amorphous bioactive residual phase. The composite material possesses structural, magnetic and bioactivity properties. The structural ones are conferred by PMMA which acts as filler for the bone defect or its damaged area. Bioactivity is conferred by the composition of the residual amorphous phase of the glass-ceramic and magnetic properties are conferred by magnetite crystals embedded in the bioactive glass-ceramic. The characterization involved the following tests: morphological and chemical characterization (scanning electron microscopy-energy dispersion spectrometry-micro computed tomography analysis), calorimetric tests and mechanical test (compression and flexural four point test). In vitro assessment of biological behavior will be the object of the part II of this work.

MD-SLAG-MELT software package for simulating the nanostructure and properties of multicomponent melts

The possibilities of computer experiments performed with the MD-SLAG-MELT v. 10.0 information-research system (IRS) with remote access are described. The main classes of mathematical models and methods and the sets of properties obtained in computer experiments are considered. An information model is developed for an oxide melt to study multidimensional composition-temperature-property-structure relations. A nanostructural model based on generalized graph network descriptors is considered in detail. The results of simulating the structure-sensitive properties of the SiO2-Na2O system using IRS are presented.

Effect of nitrogen and fluorine on mechanical properties and bioactivity in two series of bioactive glasses

Bioactive glasses are able to bond to bone through formation of carbonated hydroxyapatite in body fluids, and fluoride-releasing bioactive glasses are of interest for both orthopaedic and, in particular, dental applications for caries inhibition. However, because of their poor strength their use is restricted to non-load-bearing applications. In order to increase their mechanical properties, doping with nitrogen has been performed on two series of bioactive glasses: series (I) was a “bioglass” composition (without P2O5) within the quaternary system SiO2Na2OCaOSi3N4 and series (II) was a simple substitution of CaF2 for CaO in series (I) glasses keeping the Na:Ca ratio constant. The objective of this work was to evaluate the effect of the variation in nitrogen and fluorine content on the properties of these glasses. The density, glass transition temperature, hardness and elastic modulus all increased linearly with nitrogen content which indicates that the incorporation of nitrogen stiffens the glass network because N is mainly in 3-fold coordination with Si atoms. Fluorine addition significantly decreases the thermal property values but the mechanical properties of these glasses remain unchanged with fluorine. The combination of both nitrogen and fluorine in oxyfluoronitride glasses gives better mechanical properties at much lower melting temperatures since fluorine reduces the melting point, allows higher solubility of nitrogen and does not affect the higher mechanical properties arising from incorporation of nitrogen. The characterization of these N and F substituted bioactive glasses using 29Si MAS NMR has shown that the increase in rigidity of the glass network can be explained by the formation of SiO3N, SiO2N2 tetrahedra and Q4 units with extra bridging anions at the expense of Q3 units. Bioactivity of the glasses was investigated in vitro by examining apatite formation on the surface of glasses treated in acellular simulated body fluid (SBF) with ion concentrations similar to those in human blood plasma. Formation of a bioactive apatite layer on the samples treated in SBF was confirmed by grazing incidence X-ray diffraction and scanning electron microscopy (SEM) combined with energy dispersive X-ray spectroscopy (EDS). The crystallinity of this layer decreases with increasing N content suggesting that N may decrease bioactivity slightly.

AN ANALYSIS OF THE GEM‐BLUE GLAZE OF YE WANG'S KOJI POTTERY*

Koji pottery is a glazed ceramic art used widely for figurines. In early Taiwan, it was employed in temple construction for decorative purposes. Ye Wang (1826–87) is the first historically documented Koji artist of Taiwan and also the most prominent Koji pottery artist, noted for his modelling and glazing skills. Unfortunately, his unique technique was lost following his death in 1887. In order to provide vital information for ongoing conservation work on Koji pottery, this study analysed the physical and chemical characteristics of Ye Wang's gem‐blue glaze, to discover the glaze formula. DSC combined with the two‐thirds rule revealed that the firing temperature of Ye Wang's works of art was most probably around 878–923°C. EPMA revealed that the gem‐blue glaze has high alkali levels, and belongs to the PbO–K2O–B2O3–Na2O–SiO2 system, deriving its unique colour from copper, iron, manganese and cobalt. This study found high potassium levels in the gem‐blue glaze, which are generally a characteristic of traditional Chinese glazes. In addition, a unique discovery of boron, commonly used in famille rose, was also identified in the glaze. By comparing spectra of historical and reconstructed glazes and adjusting the proportion of chromophoric elements, this study found a glazing formula with colours close to those of Ye Wang's gem‐blue glaze.

Experimental small-angle X-ray scattering investigation of initial stages of the spinodal decomposition in model sodium silicate glasses

The kinetics of the initial stages of the spinodal decomposition in model glasses of the Na2O-SiO2 system has been investigated in situ. It has been demonstrated that there is a quantitative agreement of the experimental results obtained in the framework of the Stephenson theory with the basic principles of modern theories and the data on direct determination of the viscosity, mobility, and diffusion. It has been found that the spatial-temporal evolution of the heterogeneous structure has a multistage character during spinodal decomposition. The characteristic size of the phase regions at each stage varies with time according to the power law. The sequence of stages and the values of exponents for the spinodal decomposition are as follows: 1/20, 1/4, 1/2, and 1/3.

Simulation of the molecular mass distributions of Si anions on the liquidus of the Na2O-SiO2 system with regard for the disproportionation of Q structons

A new version of the STRUCTON (2009) computer model is proposed for the simulation of the molecular mass distributions (MMD) characterizing the diversity of anions in silicate melts depending on their polymerization and temperature. In contrast to earlier versions, the new version of the model accounts for disproportionation reactions of Qn species and makes use of their proportions in the statistical simulations of the origin of real Si-O complexes. The new potentialities of the STRUCTON program package are illustrated by its application to studying the structural-chemical characteristics of melts in the Na2O-SiO2 system along its liquidus line, including the points of eutectics and phase transitions at 0.333 ≤ \( N_{SiO_2 } \) 0.40, this ion dominates over the SiO44− monomer. More silicic melts with \( N_{SiO_2 } \) ≥ 0.45, are dominated by (SinO3n)3n− ring species, and the concentrations of these species are related as (Si3O9)6− > (Si4O12)8− > (Si5O15)10−. The maximum concentration of these flat rings also occurs near the composition of stoichiometric metasilicate with Si/O = 0.333. The comparison of the dependence of the average size of anions iav and the average number of their species on depolymerization indicates that a change in the proportion of Qn species in melt at decreasing temperature results in structural restyling and an increase in the average size of Si-O complexes. The average number of anion species thereby decreases compared to that in a stochastic MMD. The results presented in this publication direct the progress in the thermodynamic theory of silicate melts to a new avenue that makes use of the capabilities and advantages of the ion-polymer model, the theory of associated solutions, spectroscopic data, and the experimental study of variations in oxide activities depending on composition and temperature.