CaO Al2O3 SiO2 Research Articles

Investigation of Self-Diffusion and Structure in Calcium Aluminosilicate Slags by Molecular Dynamics Simulation

Molecular dynamics simulation is applied to investigate the mechanism and variation of self-diffusion in calcium aluminosilicate slags. The self-diffusion coefficients are calculated for eleven slag compositions with varying Al2O3/SiO2 ratios at a fixed CaO content. In practice, the results of the study are relevant to the significant changes in transport phenomenon caused by the changes in chemical composition during continuous casting of steels containing high amounts of dissolved aluminum. The cooperative movement between O atoms and network formers is discussed since [AlO4] and [SiO4] tetrahedra are the elementary structural units in the CaO-Al2O3-SiO2 (CAS) slag system. The diffusivities for four atomic types are affected by the degree of polymerization (DOP) of slag network characterized by the proportions of non-bridging oxygen (NBO) and Qn species in the system. On the other hand, a sudden increase in 5-coordinated Al as network modifiers in high alumina regions slightly increases the self-diffusion coefficient for Al. As another structural defect, oxygen tricluster plays an important role in the behavior of self-diffusion for O atoms, while the diffusivity for Ca is deeply influenced by its bonding and coordinating conditions.

Effect of Potassium Oxide Addition on Viscosity of Calcium Aluminosilicate Melts at 1673–1873 K

We investigated the changes induced in the viscosities of CaO–SiO2–Al2O3–K2O melts (CaO/SiO2 molar ratio = 0.68 ± 0.04, Al2O3 content = 13.4 ± 0.6 mol%) with the addition of K2O in amounts of 0–17.4 mol% for temperatures of 1673–1873 K using the rotating crucible method. The viscosity increased with an increase in the K2O content when K2O/Al2O3 molar ratio < 0.7. On the other hand, the viscosity decreased with the addition of K2O for K2O/Al2O3 molar ratio > 0.9. The maximum values of the viscosities were noticed in the range of 0.7 < K2O/Al2O3 molar ratio < 0.9. This behavior could not be explained on the basis of the polymerization degree of the aluminosilicate network structure. The 17O magic angle spinning nuclear magnetic resonance (MAS NMR) spectrum of the 31.6CaO-44.2SiO2-13.4Al2O3-10.8K2O (mol%) glass suggested that K+ ions preferentially compensate the negative charge of AlO4, while the Ca2+ ions primarily create non-bridging oxygens (NBOs). The viscosity data and the 17O MAS NMR spectrum also suggested that the increase in viscosity for K2O/Al2O3 molar ratio < 0.7 was because of the increase in the average bond strength of the aluminosilicate framework, which was attributable to the substitution of Ca2+ ions by K+ ions at the charge compensator sites. The activation energies for viscous flow indicated that the NBOs bonded with K+ ions, forming NBO-K species, for K2O contents greater than those corresponding to the viscosity maxima. Thus, the viscosities of aluminosilicate melts are indicative of the average bond strengths of the melts.

Structure Analysis of CaO–SiO2–Al2O3–TiO2 Slag by Molecular Dynamics Simulation and FT-IR Spectroscopy

The structure information in the CaO-SiO2-14 mass% Al2O3–TiO2 slag was investigated by the molecular dynamics simulation and the FT-IR spectroscopy at 1 773 K. The influence of different additions of TiO2 and varying CaO/TiO2 ratios on the structure was studied to clarify the role of TiO2 in the slag. The results show that there exist three stable units, [SiO4] tetrahedron and [AlO4] tetrahedron as well as [TiO6] octahedron in the CaO-SiO2-14 mass% Al2O3–TiO2 slag. The average coordination numbers, CNSi–Al and CNAl–Al, are all approximately equal to 1 and are barely influenced by additions of TiO2 and varying CaO/TiO2 ratios, which indicates that both the [SiO4] and [AlO4] tetrahedrons are surrounded by only one [AlO4] tetrahedron and some other units. Nevertheless, [AlO4] can be linked by more than one [SiO4] tetrahedron but [SiO4] can only be surrounded by one [AlO4] tetrahedron. The bridging oxygen, classified into Si–O–Si, Al–O–Al and Si–O–Al, is preferentially localized in Si–O–Al. However, it is found a little violation of the so-called Al avoidance principle which states that the Al–O–Al linkage is absent have been obtained with about (less than) 5% Al–O–Al, and the bond of Al–O–Al is hardly affected by TiO2 additions. Replacement of CaO by TiO2 can only result in a slight change of the degree of polymerization, indicating TiO2 has the similar role as CaO to be a basic oxide.

Thermochemical Calculation of the Quaternary CaO–SiO2–Al2O3–MgO System at 1600°C for Predicting the MgO‐Saturation and MgO–MgO · Al2O3 Equilibrium of Secondary Metallurgical Slag

Different quaternary CaO–SiO2–Al2O3–MgO systems were calculated with thermochemical‐calculations at various temperatures by using FactSage6.2. The calculated systems provide enhanced knowledge of secondary metallurgical ladle slag conditions at process temperatures. These systems can also be used for predicting the MgO‐saturation and the MgO–MgO · Al2O3 equilibrium at various temperatures for commonly used ladle slag compositions.

Research of boron removal from polysilicon using CaO–Al2O3–SiO2–CaF2 slags

Purification process of MG-Si was limited by the difficulties involved in reducing boron content. The possibility of removing impurity boron (B) in MG-Si using CaO–Al2O3–SiO2–CaF2 slags was investigated in the present study. Different from traditional research methods, the whole melting process was carried out under atmosphere condition, which was closer to the actual production conditions. The thermodynamic and kinetic mechanisms of B removal were analyzed. Based on the ionic structure theory, the relation between ionic structure of slags at high temperature and optical basicity can be explained firstly. The parameters, including the slag optical basicity, melting time and CaF2 content were discussed. The boron content was reduced from original 25 ppmw to 4.4 ppmw through 120 min melting with the optical basicity of 0.551 and 5 wt% CaF2 additions.

In Situ Observation of the Dissolution of SiO2 Particles in CaO–Al2O3–SiO2 Slags and Mathematical Analysis of its Dissolution Pattern

The dissolution of amorphous SiO2 particles in CaO–Al2O3–SiO2 slags was investigated at 1450°C by high‐temperature confocal scanning laser microscopy (HT‐CSLM) and thermodynamic/kinetic analyses. The SiO2 particles used in this experimental study had a spherical form so that any rotation of the particle did not cause errors in the determination of the particle size during the dissolution. Moreover, a wide composition range of the slag could be chosen without forming any solid reaction layer which could distort the evaluation of the dissolution mechanism. The evolution of the diameter of the spherical SiO2 particle was measured by image analysis of pictures obtained from the HT‐CSLM. It was found that the dissolution curve of the SiO2 particle (size as a function of time) exhibited either a parabolic‐like curve or an S‐shaped curve depending on the slag composition. The patterns were compared with a well‐known shrinking core model (SCM), and it was shown that the SCM could not represent the dissolution behavior of the SiO2 particle observed in this study. It was experimentally found that the shape of the dissolution curves varies as a function of the slag composition. The curve exhibited a parabolic‐like shape for low SiO2‐containing slags and changed to an S‐type shape with increasing SiO2 concentration in the slag. To elucidate the dissolution mechanism, a model based on approximations for the diffusion near the particle was proposed by modifying the previously available model [M. J. Whelan, Met. Sci. J., 3, 95–97 (1969)]. From the experimental data and the model calculations, the viscosity of the slag was shown to be the major factor affecting both dissolution rate and mechanism. Effective binary diffusion coefficients were estimated using the model and experimental data. Those were shown to be in the range of literature data.

Numerical simulation of slag foaming on bath smelting slag (CaO–SiO2–Al2O3–FeO) with population balance modeling

A computational fluid dynamic (CFD) model has been developed for the simulation of slag foaming on bath smelting slag (CaO–SiO2–Al2O3–FeO) by considering foam as a separate phase. The CFD model has been used to predict the foam height, bubble number density and the multiphase flow phenomena in the system. The height of foam is dynamically balanced by the formation of foam due to transformation of both gas and liquid into foam and its destruction due to liquid drainage and bursting of bubbles, transforming foam back to liquid and gas. The bubble break-up and coalescence were considered in gas–liquid dispersion whereas in the foam layer, the bubble coalescence due to film rupture was incorporated. A population balance modeling was used to track the number density of different bubble classes and fixed pivot method was used to discretize the population balance equation. The model predicted the foam height of the slag system (CaO–SiO2–Al2O3–FeO). The content of FeO was changed and its effect on the foam height predicted. The foaming index was calculated and the results from the model predict that the foaming index decreases with increase of FeO content in slag. The CFD model also predicts that the foaming index of a slag with Al2O3 is higher than that of slag without Al2O3. Dimensionless analysis was performed based on the model available in the literature to correlate the foaming index with the physical properties of the slag. The predicted results from the present study are in reasonable agreement with available experimental data.

Microstructural characterization of the interaction between 8YPSZ (EB-PVD) thermal barrier coatings and a synthetic CAS

Thermal barrier coatings (TBC) in service are prone to the attack by molten calcium–magnesium aluminosilicates (CMAS) resulting from the ingestion of siliceous minerals (dust, sand, volcanic ash) by the aeroengines. As CMAS deposits observed on ex-service parts present complex and highly variable compositions, progress in the understanding of the interaction mechanism between CMAS and TBC has been mostly obtained during laboratory experiments using simplified CMAS compositions.This paper consists in a microstructural characterization of the isothermal chemical interaction between a synthetic calcium aluminosilicate (CAS) and a standard 8YPSZ EB-PVD coating deposited on alumina substrates. The chosen CAS is the lowest melting temperature eutectic (1170°C) in the ternary (CaO–Al2O3–SiO2) system and it is prepared as a glass. The evolution of the reaction zone morphology and chemistry is studied after 1200°C treatments from 15min to 100h. The CAS/TBC interaction is governed by the classical mechanism of dissolution/reprecipitation with formation of Y-lean zirconia and Ca2Zr(Y)Si4O12. Ca2Zr(Y)Si4O12 formation induces local variation in the CAS composition leading to CaAl2Si2O8 (anorthite) precipitation. Y2O3 depletion in the YSZ regions infiltrated by CAS is attested using X-ray compositional mapping and the association of EDS and EBSD clearly illustrates that the morphological transformation from porous initial YSZ to dense globular Y-lean YSZ is associated with the structural transformation from initial tetragonal t′ to tetragonal Y-lean zirconia transforming into monoclinic during cooling.The degradation of the TBC columns increases with the duration of the 1200°C heat-treatment. The interfacial Ca2Zr(Y)Si4O12 phase forms on an increasing amount of columns and is finally observed on all the columns. Interfacial phase formation is discussed and compared to other phase assemblages reported in the literature.

Model for diffusion coefficient estimation of calcium ions in CaO–Al2O3–SiO2 slags

As a common component in metallurgical slag, CaO plays an important role in desulphurisation, dephosphorisation and absorption of non-metallic inclusions. In order to better understand the mechanism of the slag/metal reactions, the diffusion dynamics of calcium ions in CaO–Al2O3–SiO2 slags were studied. It was found that there was almost a linear relation between the logarithms of pre-exponential factor and diffusion activation energy. By combining the relation between the diffusion activation energy and the optical basicity corrected in the CaO–Al2O3–SiO2 slags, a simple model used to estimate the diffusion coefficient of calcium ion is proposed. The estimated values of the CaO–SiO2 and CaO–Al2O3–SiO2 systems agree well with the experiment data, with a mean deviation of 35·3 and 22·5% respectively.

Bioactivity of Glass and Glass-Seramic in the System SiO2–CaO–Al2O3–P2O5–Na2O–MgO–CaF2

Bioactivity of Glass and Glass-Seramic in the System SiO₂–CaO–Al₂O₃–P₂O₅–Na₂_O–MgO–CaF₂

Structure Characterization of Sintered CaO-Al2O3-SiO2 Glass-Ceramic Reinforced with Spodumene

A glass-ceramic belonging to the CaO-Al2O3-SiO2(CAS) system with different composition of spodumene and doping the Li2O with amount between 0~2.5 % (mass fraction) were prepared by onestage heat treatment, under sintering and crystallization temperature at 1120 °C for two hours. In this paper, differential thermal analysis, X-ray diffraction, scanning electron microscopy, energy dispersive spectrometry and bending strength test were employed to investigate the microstructure and properties of all samples. β-wollastonite crystals were identified as the major crystalline phases, and increasing Li2O was found to be benefit for the crystallization and tiny crystalline phases remelting, resulting in the content of major crystalline phases increased first and then decreased with increasing the expense of spodumene. Meanwhile, the crystal size can be positively related with the content of Li2O. The preferable admixed dosage of spodumene can be obtained, besides the strength of glass-ceramics can be more than 90 MPa.

The mechanism of boron removal in the CaO–SiO2–Al2O3 slag system for SoG-Si

The dissolution behavior of boron in a molten slag was evaluated at 1773 K to establish the boron removal mechanism from silicon refining for preparing higher purity solar-grade silicon. From the experimental results, a mechanism of boron removal from silicon into the CaO–SiO2–Al2O3 slag system was confirmed to proceed via BO33− and B62− following a linear relationship with oxygen partial pressure in the slag system. In addition, the distribution ratio of boron was seen to increase with increasing slag basicity.6B+1/3O2−=B62−+1/6O2(g)The boride capacity of the CaO–SiO2–Al2O3 slag can be expressed as a linear function of the activity of CaO and the optical basicity. It is suggested that the proposed dissolution mechanism of boron could be appropriate to slag systems, depending on oxygen partial pressure and basicity.

The effect of chlorine, fluorine and water on the viscosity of aluminosilicate melts

The influence of 1.6 to 14.5mol% water, 1.1 to 18.3mol% fluorine (F) and 0.5 to 1.4mol% chlorine (Cl) on the viscosity of peralkaline and peraluminous Na2O–CaO–Al2O3–SiO2 melts as a model system for phonolites was studied with the micropenetration method within the range of 109 to 1013Pas. Water and F alone and combined decrease viscosity non-linearly. With increasing F and water concentration the further addition of F and water becomes less effective in decreasing viscosity. Water decreases the viscosity of peraluminous melts more strongly than that of peralkaline melts. In the halogen-free peraluminous sample 5mol% water decreases Tg12 (the temperature where the viscosity=1012Pas) by 259±5K, while in the halogen-free peralkaline sample 5mol% water decreases Tg12 by 189±5K. Fluorine (on a mol% basis) decreases the viscosity of Na2O–CaO–Al2O3–SiO2 melts less than water. In the peraluminous melts the addition of 5mol% F decreases Tg12 by ~95K. In the peralkaline melt 5mol% F decreases Tg12 by ~30K. However, a significant decrease in Tg12 was only observed up to a F concentration of 1.9mol% F, another sample with 6.2mol% F showed no further decrease of Tg12. In all hydrous peraluminous and peralkaline samples that contain F alone or combined with Cl, the decrease of viscosity due to the addition of water is smaller than in the halogen-free samples. In a peraluminous melt with 18.3mol% F the addition of 5mol% water decreases Tg12 by about 152±5K compared to the dry melt. In a peralkaline melt with 1.9mol% F the addition of 5mol% water decreases Tg12 by about 146±5K compared to the dry melt. In both cases this is significantly less than for the halogen-free samples. In peralkaline melts Cl has no influence on the decrease in viscosity upon the addition of water. The effects of water and Cl on viscosity are independent of each other and add together. The effects of water and F on viscosity are not independent, which could be, in the case of the peralkaline samples related to changes in F speciation due to the presence of water.

Corrosion study of SiO2–CaO–Al2O3–Na2O–TiO2 glass coating on steel

A protective coating of 60SiO2–15CaO–10Al2O3–10Na2O–5TiO2 glass ceramic was performed on the ferritic stainless steel substrate by slurry technique. X-ray diffraction and scanning electron microscopy were used for detailed analyses of the coating. Open circuit potential, electrochemical impedance spectroscopy and potentiodynamic polarisation studies were carried out in artificial seawater to monitor the corrosion resistance behaviour of the coating at room temperature. The presence of coating reduced the corrosion current density Icorr and shifted corrosion potential Ecorr towards more noble values. It has been established through this study that glass ceramic coating on the steel substrate essentially improves the corrosion resistance property of the alloy and can be used for marine applications.

Effects of Al2O3 composition on the near-infrared emission in Bi-doped and Yb–Bi-codoped silicate glasses for broadband optical amplification

Bi-doped and Yb–Bi-codoped SiO2–Al2O3–CaO glasses were prepared, and the effect of Al2O3 composition on the near-infrared (NIR) emission and energy transfer between Yb3+ and Bin+ (Bi NIR emission centers) were investigated. The introduction of Al2O3 into the glass hosts can vary the NIR luminescence centered at 1300nm and obtain the strongest emission when the concentration of Al2O3 is 15mol%. Efficient energy transfer from Yb3+ to Bin+ is achieved by co-doping of Al2O3. With the increased Al2O3 concentration, the energy transfer efficiency between Yb3+ and Bin+ increased at first and then decreased, maximizing (~79%) at 8mol%. It is suggested that the introduction of appropriate Al2O3 concentration in Yb–Bi-codoped silicate glasses may be an efficient way to realize effective optical amplification with commercially available 976nm LD pumping.

Joining of Porous Alumina with a CaO–Al2O3–SiO2 Glass‐Ceramic

A CaO–Al2O3–SiO2 (CAS)‐based glass interlayer was developed for joining of porous alumina membrane tubes with dense alumina in this work. The results indicated that the interfacial microstructure of the joint was highly sensitive to the quench rate from the joining temperature, which rendered crystallization of CaTiSiO5 at a fast quench rate but CaAl2Si2O8 at a slow quench rate due to the interfacial reaction between the CAS glass interlayer and the substrate. An extra crystallization treatment during quench, i.e., dwelling at 800°C–900°C for 2 h, produced a multiphase interlayer consisting of LiAlSi2O6, CaTiSiO5, and CaAl2Si2O8. All joints were evaluated by the thermal shock test. The results showed that the LiAlSi2O6‐containing joint interlayer had much lower thermal shock resistance than those without LiAlSi2O6.

Synthesis of bioactive and machinable miserite glass-ceramics for dental implant applications

ObjectivesTo synthesize and characterize machinable, bioactive glass-ceramics (GCs) suitable for dental implant applications. MethodsA glass in the SiO2–Al2O3–CaO–CaF2–K2O–B2O3–La2O3 system was synthesized by wet chemical methods, followed by calcination, melting and quenching. Crystallization kinetics were determined by differential thermal analysis (DTA). GC discs were produced by cold pressing of the glass powder and sintered using schedules determined by DTA. The crystalline phases and microstructure of GC samples were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. Dynamic Young's modulus (E), true hardness (Ho), fracture toughness (KIC) and brittleness index (BI) were evaluated. Bioactivity was studied by examining the formation of hydroxyapatite (HA) on the GC surfaces after soaking in simulated body fluid (SBF). Attachment and proliferation of MC3T3-E1 osteoblastic cells were assessed in vitro. ResultsMiserite [KCa5(Si2O7)(Si6O15)(OH)F] was the main crystalline phase of the GC with additional secondary phases. Microstructural studies revealed interlocking lath-like crystalline morphology. E, Ho, and KIC values for the GCs were 96±3GPa, 5.27±0.26GPa and 4.77±0.27MPam0.5, respectively. The BI was found to be 1.11±0.05μm−0.5, indicating outstanding machinability. An HA surface layer was formed on the GC surfaces when soaked in SBF, indicating potential bioactivity. MC3T3-E1 cells exhibited attachment, spreading and proliferation on GC surfaces, demonstrating excellent biocompatibility. SignificanceWe present a novel approach for the synthesis of miserite GC with the physical and biological properties required for non-metallic dental implant applications.

Effects of nano silica on synthesis and properties of glass ceramics in SiO2–Al2O3–CaO–CaF2 glass system: A comparison

Glass ceramics of composition 34SiO2–29Al2O3–25CaO–12CaF2 (wt.%) was made by conventional melting and quenching process using either normal or nano-SiO2 respectively. The glasses were characterized by differential thermal analysis (TG/DTA), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). The crystallization, microstructure, mechanical and chemical properties were compared for the two systems. With nano-SiO2 addition, the crystallization peak temperature (Tp) decreases, activation energy (E) and Avrami parameter (n) have very little change, and the mechanism of crystallization of the glass ceramics changed from surface crystallization to two-dimensional crystallization. The crystallite size of nano-SiO2 containing glass is lower than the normal SiO2 containing glass. Introduction of nano-SiO2 particles in glass ceramics gives higher Vickers hardness, shrinkage, lower water absorption, and higher acid resistance than the normal silica containing glass ceramics, thus making it more useful for industrial building, internal and external wall facing and tiles applications.

Crystallization behavior and properties of K2O–CaO–Al2O3–SiO2 glass-ceramics

In order to obtain high-strength anorthite glass-ceramics, K2O–CaO–Al2O3–SiO2 quaternary glass and relevant glass-ceramics were prepared and investigated. The results show that anorthite along with kalsilite or leucite was precipitated from the parent glass. Kalsilite crystals were formed firstly and then converted into leucite through reacting with SiO2 in the glass phase. The morphology of the crystals was dependent on the heat-treatment temperature. Column crystals were transformed into fine granular grains when the sintering temperature changed from 900 °C to 1100 °C. The activation energy (Eα) and avrami constant (n) were also calculated as 463.81 KJ/mol and 3.74 respectively, indicating that bulk nucleation and three-dimensional crystal growth were the dominating mechanisms in the temperature range 1000–1100 °C. The maximum value of the flexural strength for the glass-ceramics containing leucite was 248 MPa and the coefficient of thermal expansion (CTE) was in the range 5.69~11.94×10−6 K−1. The leucite is the main reason for the high CTEs and high flexural strength of glass-ceramics.

Phase evolution and nature of oxide dissolution in metallurgical slags

The dissolution of solid lime particles into liquid slags at high temperatures was evaluated by means of confocal scanning laser microscopy. An additional solid layer around the lime particle was observed at the intermediate stage of the dissolution into CaOAl2O3SiO2 slags. The dissolution rate was decelerated due to the existence of the additional layer and the dissolution profile could be clearly distinguished into three stages, that is, an early, intermediate, and late stage. By adding 10 wt % MgO, this layer could be effectively eliminated and the slope of the whole dissolution profile kept relatively constant. The dissolution path and mechanisms were subsequently evaluated by incorporating thermodynamic calculations. Both direct and indirect dissolutions could be distinguished. It was realized that the decrease in composition range for solid precipitating after adding MgO could significantly reduce the interfacial reaction (IR) layer formation. Post‐mortem analyses on quenched samples were further carried out to confirm the theoretical calculations. It was found that the solid layer in slags without MgO was (CaO)2·SiO2 and (CaO)3·SiO2 which is in line with the thermodynamic calculations. However, only (CaO)2·SiO2 was noticed in slags with MgO which both (CaO)2·SiO2 and MgO phases should be present according to the calculations. The nonequilibrium during dissolution may play an important role on phase transformation and MgO particles in much smaller quantity may have dissolved into (CaO)2·SiO2 phase. The diffusion of CaO in both slags with and without MgO was additionally investigated. The local CaO concentration distributions from the direct dissolution phase to the slag bulk could be well fitted with the theoretical model proposed via Fick's second law. As a result, the local diffusion coefficient in the dissolution region was obtained and the effect of MgO addition on diffusion could be assessed. © 2013 American Institute of Chemical Engineers AIChE J, 59: 2907–2916, 2013