Behavior of Atoms at the Surface of a K2O · 3siO2 Glass—A Molecular Dynamics Simulation

Recent ion scattering spectroscopy (ISS) studies indicate that an excess of K ions occurs at the surface of a K₂O · 3SiO₂ glass. Molecular dynamics (MD) computer simulations were used to evaluate the short-time dynamic behavior of atoms at the surface of such a glass in order to determine a mechanism for the K ion enrichment. In the simulations, a bulk glass of several hundred atoms was melted using three-dimensional periodic boundary conditions, and subsequently quenched to lower temperatures. Periodic boundary conditions were removed in one dimension near room temperature so as to create free surfaces. The distribution of species perpendicular to the free surface was determined. The MD simulations show that K ions can build up at the outermost surface of the glass within several picoseconds after formation of the surface.     

Reaction Between K2O and Al2O3-SiO2 Refractories as Related to Blast-Furnace Linings

An examination was conducted to determine the mechanism of peeling of fire-clay brick in the low-temperature region of a blast furnace where 3 to 10% K₂O is the principal contaminant. In laboratory tests, as-received high-duty and superduty fire-clay brick and 70% alumina brick treated with KCl-K₂CO₃ mixtures showed no peeling at a temperature of 1600°F. Cracks were found in high-duty brick that were treated with KCN at 1500°F. under partially reducing conditions. X-ray diffraction studies of mixtures of crushed brick and K₂CO₃ indicated the formation of leucite (K₂O.Al₂O₃.4SiO₂) and kaliophilite (K₂O.-Al₂O₃.2SiO₂) at temperatures below 1700°F. These latter data, confirmed by specimens from used blast-furnace linings, showed that silica is the first constituent attacked by alkali. Since the formation of leucite and kaliophilite in fire-clay brick is the probable cause of peeling, the increased reaction of silica, in a dense Al₂O₃.SiO₂ refractory of higher silica content than fire-clay brick, should confine the alkali attack to the surface of the brick in low-temperature applications.     

Nucleation and Crystallization of Na2O · 2CaO · 3SiO2 Glass by Differential Thermal Analysis

The nucleation and crystallization of the Na₂O · 2CaO · 3SiO₂ (NC₂S₃) glass were studied by differential thermal analysis (DTA), and a (nucleation rate—temperature)-like curve was determined by plotting either the reciprocal of the temperature corresponding to the crystallization peak maximum, 1/ T_p , or the height of the crystallization peak, (δ T ) _p , as a function of nucleation temperature, T_n. The temperature where nucleation can occur for this glass ranges from 550° to 650°C and the temperature for maximum nucleation is 600°± 5°C. Both temperatures are in excellent agreement with those determined by the classical technique of nucleation followed by isothermal crystallization. The activation energy for crystallization, E_c , for this glass is the same for surface and/or bulk crystallization, and is 370 ± 15 kJ/mol. The analysis of the crystallization data with the Kissinger equation yelds the correct value for E_c only when crystal growth occurs on a fixed number of nuclei. When a majority of the nucleation occurs during the DTA measurements, a modified Kissinger equation must be used to calculate E_c . E_c is also independent of the heating rate when determined using a single-crystallization-peak analysis technique. The single-peak analysis technique is useful for a rapid determination of E_c or when only a small amount of sample is available.     

Direct Measurement of Relative Partial Molar Enthalpy of SiO2 in SiO2–M2O (M=Li, Na, K, Cs) Binary and SiO2–CaO–Al2O3 Ternary Melts

The relative partial molar enthalpies, Δ _SiO2, of SiO₂ in SiO₂–M₂O (M = Li, Na, K and Cs) binary and SiO₂–CaO–Al₂O₃ternary melts were directly measured by drop-solution calorimetry at 1465 K and 1663 K. Δ _SiO2 changes from exothermic to endothermic as silica content increases, confirming the tendency toward immisciblity seen from activity measurements. It is concluded that Δ _SiO2 is negative due to acid-base reactions and charge-coupled substitutions when the melt is composed of fewer Q ⁴ and more Q ³ and Q ² species, but positive due to structural strain when the melt is composed of mostly Q ⁴ species. The Δ _SiO2 obtained by calorimetry is a useful measure of basicity, when comparing different alkali and alkaline earth oxides.     

Thermodynamic Activities in B2O3-SiO2 Melts at 1475 K

Eight glass samples in the B₂O₃-SiO₂ system with compositions from 20 to 90 mol% B₂O₃ were prepared. The equilibrium vaporization was studied by Knudsen effusion mass spectrometry at temperatures between 1450 and 1500 K. B₂O₃ ( g ) was the most abundant boron-containing species in the vapor; no silicon-containing gaseous species were detected. Thermodynamic activities of B₂O₃ in the liquid were determined at 1475 K. Thermodynamic activities of SiO₂ and integral excess Gibbs energies were estimated from the thermodynamic activities of B₂O₃. The thermodynamic data support the results obtained by other methods indicating the existance of a miscibility gap in the metastable liquid.     

Internal Friction of Ion-Exchanged Li2O.Al2O3.2SiO2 Glass

Fibers of Li₂O.Al₂O₃.2SiO₂ glass were ion-exchanged for 1 to 300 min in an NaNO₃ bath at 366°C. The internal friction and the Li and Na concentration profiles were measured. As Na progressively replaced Li, the alkali internal friction peak became smaller while a new peak (mixed-alkali peak) appeared and increased in magnitude. These changes in internal friction are similar to those that occur when a second alkali is added to glasses prepared by conventional melting. The magnitudes of both internal friction peaks in the ion-exchanged glass depended on the overall composition of the glass; that of the alkali peak depended on the composition of the unexchanged glass core, whereas that of the mixed-alkali peak depended on the composition of the exchanged layer on the glass surface. When the exchanged surface layer was dissolved, the original alkali peak was restored, and the mixed-alkali peak disappeared. Changing the alkali distribution did not affect the mixed-alkali peak much; however, it caused the alkali peak to shift to higher temperatures and become smaller. The height of the alkali peak can be used to determine the maximum depth of penetration of the second alkali.     

Liquid Immiscibility in the Systems X2O-MO-B2O3-SiO2 (X=Na, K; M=Mg, Ca, Ba) and Na2O-MgO-BaO-B2O3-SiO2

The limits of miscibility at 650°C were determined for compositions with the mole ratio SiO₂/B₂O₃=1.07 in the systems X₂O-MO-B₂O₃-SiO₂ (X = Na,K; M = Mg,Ca,Ba) and Na₂O-MgO-BaO-B₂O₃-SiO₂. The form of the miscibility gaps in the quaternary systems is similar to that previously described for the system Na₂O-ZnO-B₂O₃-SiO₂. The topography of miscibility gaps in systems of this type is discussed in detail. The extent of the miscibility gap is correlated with the polarizing power of each cation, X and M (Na > K and Zn ≅ Mg > Ca > Ba) both among the seven quaternary systems and within the single five-component system examined. The possibility of using empirical correlations observed among the quaternary systems to predict the behavior of other compositions, or of more complex systems, is explored.     

Revised Phase Diagram for the System Al2O3—SiO2

On the basis of 190 runs made up to 1860°C in sealed noble-metal containers the following revisions have been made in the equilibrium diagram for the system A1₂O₃–SiO₂. Mullite melts congruently at 1850°C. The extent of equilibrium solid solution in mullite at solidus temperature is from approximately 60 mole % Al₂O₃ (3/2 ratio) to 63 mole % A1₂O₃. Metastable solid solutions can be prepared up to about 67 mole % Al₂O₃. There is no evidence for stable solubility of excess SiO₂ beyond the 3/2 composition at pressures below 3 kbars. Refractive indices are presented for glasses containing up to 60 mole % Al₂O₃ and from them the composition of the eutectic is confirmed at 5 mole % SiO₂. The variation in lattice constants of the mullite solid solution is not an unequivocal guide to composition since mullites at one composition produced at different temperatures show differences in spacing, no doubt reflecting Al-Si ordering phenomena. The possibility of quartz and corundum being the stable assemblage at some low temperatures and pressures cannot be ruled out. A new anhydrous phase in the system is described, which was previously thought to be synthetic andalusite; it is probably a new polymorph of the Al₂SiO₅ composition with ortho-rhombic unit-cell dimensions a =7.55 A, b =8.27 A, and c = 5.66 A.     

Phase Equilibrium Relationships at Liquidus Temperatures in the System FeO–Fe2O3–Al2O3–SiO2

Phase equilibrium data at liquidus temperatures are presented for mixtures in the system FeO–Fe₂O₃–Al₂O₃–SiO₂. The volume located between the 1 and 0.2 atm. O₂ isobaric surfaces of the tetrahedron representing this system was studied in detail. Scattered data were obtained at lower O₂ pressures. Results obtained in the present investigation were combined with data in the literature to construct a phase equilibrium diagram, at liquidus temperatures, for the entire system FeO–Fe₂O₃–Al 2 O₃–SiO₂. Methods for interpretation of the diagram are explained.     

Sintering and Crystallization of CaO–Al2O3–ZrO2–SiO2 Glasses Containing Different Amount of Al2O3

In this work several complementary techniques have been employed to carefully characterize the sintering and crystallization behavior of CaO–Al₂O₃–ZrO₂–SiO₂ glass powder compacts after different heat treatments. The research started from a new base glass 33.69 CaO–1.00 Al₂O₃–7.68 ZrO₂–55.43SiO₂ (mol%) to which 5 and 10 mol% Al₂O₃ were added. The glasses with higher amounts of alumina sintered at higher temperatures (953°C [lower amount] vs. 987°C [higher amount]). A combination of the linear shrinkage and viscosity data allowed to easily find the viscosity values corresponding to the beginning and the end of the sintering process. Anorthite and wollastonite crystals formed in the sintered samples, especially at lower temperatures. At higher temperatures, a new crystalline phase containing ZrO₂ (2CaO·4SiO₂·ZrO₂) appeared in all studied specimens.     

Electrical Properties of Sr0.7Ba0.3TiO3 Ceramics Doped with Nb2O5, 3Li2O · 2SiO2, and Bi2O3

The electrical properties of Sr_0.5Ba_0.3TiO₃ in the presence of Nb₂O₅ as a donor, 3Li₂O · 2SiO₂ as a sintering agent, and Bi₂O₃ as a dopant have been studied. When the compositions of the ceramics were 1 mol Sr_0.7Ba_0.3TiO₃+ 0.5 mol% Nb₂O₅+ 2 mol% 3Li₂O · 2SiO₂+ 0.2 mol% Bi₂O₃, the ceramics were sintered at 1100°C and exhibited the following characteristics: apparent dielectric constant ɛ, 25000; loss factor tan δ, 2%; insulating resistivity ρ_j, 10¹⁰Ω· cm; variation of dielectric constant with temperature Δɛ/ɛ (−25° to +85°C), +10%, −14%. ɛ and tan δ show only small changes with frequency. The study shows this ceramic can be used in multilayer technology.     

Cassiterite Solubility in High-Silica K2O-Al2O3-SiO2 Liquids

The saturation surface of cassiterite, SnO₂, was determined for liquids in the system K₂O–Al₂O₃–SiO₂ as a function of bulk composition and temperature. At fixed K₂O/Al₂O₃ cassiterite solubility varies weakly with SiO₂ concentration (76 to 84 mol%), temperature (1350° to 1550°C), and log ( f _O2) (−0.7 to −5.3). Cassiterite solubility is also approximately independent of composition in liquids with molar ratios of K₂O/Al₂O₃ lessthan equal to 1 (peraluminous liquids). As K₂O/Al₂O₃ increases from 1 (peralkaline liquids), however, cassiterite solubility increases steeply and approximately linearly with K₂O in excess of Al₂O₃. It is proposed that potassium in excess of aluminum combines with Sn⁴⁺ to form quasi-molecular complexes with an effective stoichiometry of K₄SnO₄.     

Solid Solution and Chromium Oxide Loss in Part of the System MgO-Al2O3-Cr2O3-SiO2

Thermal and X-ray studies show that there is complete solid solution between MgO.Cr₂O₃ and MgO.Al₂O₃ and that the spinel solid solutions are stable with no exsolution down to temperatures as low as 510°C. There is no solid solution of excess Cr₂O₃ in MgO.Cr₂O₃ nor of MgO.Cr₂O₃ in Cr₂O₃. The join MgO.Cr₂O₃–Al₂O₃ is found to be nonbinary; compositions along that join yield mixtures of a chromium oxide-alumina solid solution and a spinel solid solution on firing to temperatures high enough to promote solid-state reaction. Chromium oxide loss by volatilization increases at higher temperature. At a given temperature, chromium oxide loss is found to vary directly with the partial pressure of oxygen in the furnace atmosphere and with the ratio of MgO to SiO₂ in the charges heated.     

Phase Equilibria in the System Na2SiO3-Li2SiO3

Phase equilibria were studied for the system Na₂SiO_:rLi₂SiO₃. The 2 end-member metasilicates show limited mutual solid solubility with the (Na_2-χLi_χ)SiO₃ solid solutions being particularly extensive at solidus temperatures (0 x 1.06). Ordering of the latter solid solution occurs at the NaLiSiO_;) composition with a_supercellx= 6a_subcell During cooling of the solid solutions, metastable phase transformations occur; a twinned monoclinic metastable phase, low (Na, Li)₂SiO₃, has been characterized.     

Preparation of NiAl2O4/SiO2 and Co2+-Doped NiAl2O4/SiO2 Nanocomposites by the Sol–Gel Route

NiAl₂O₄/SiO₂ and Co²⁺-doped NiAl₂O₄/SiO₂ nanocomposite materials of compositions 5% NiO – 6% Al₂O₃– 89% SiO₂ and 0.2% CoO – 4.8% NiO – 6% Al₂O₃– 89% SiO₂, respectively, were prepared by a sol–gel process. NiAl₂O₄ and cobalt-doped NiAl₂O₄ nanocrystals were grown in a SiO₂ amorphous matrix at around 1073 K by heating the dried gels from 333 to 1173 K at the rate of 1 K/min. The formations of NiAl₂O₄ and cobalt-doped NiAl₂O₄ nanocrystals in SiO₂ amorphous matrix were confirmed through X-ray powder diffraction, Fourier transform infrared spectroscopy, differential scanning calorimeter, transmission electron microscopy (TEM), and optical absorption spectroscopy techniques. The TEM images revealed the uniform distribution of NiAl₂O₄ and cobalt-doped NiAl₂O₄ nanocrystals in the amorphous SiO₂ matrix and the size was found to be ∼5–8 nm.     

Immiscibility Area in the System TiO2–ZrO2–SiO2

A furnace for use in conjunction with the X-ray spectrometer was developed which was capable of heating small powdered specimens in air to temperatures as high as 1850°C. This furnace was also used for the heating and quenching of specimens in air from temperatures as high as 1850°C. An area of two liquids coexisting between 20 and 93 weight % TiO₂ above 1765°± 10°C. was found to exist in the system TiO₂–SiO₂, which is in substantial agreement with the previous work of other investigators. The area of immiscibility in the system TiO₂–SiO₂ was found to extend well into the system TiO₂–ZrO₂–SiO₂. The two liquids were found to coexist over a major portion of the TiO₂ (rutile) primary-phase area with TiO₂ (rutile) being the primary crystal beneath both liquids. The temperature of two-liquid formation in the ternary was found to fall about 80°C. with the first additions of ZrO₂ up to 3%. With larger amounts of ZrO₂ the change in the temperature of the boundary of the two-liquid area was so slight as to be within the limits of error of the temperature measurement. Primary-phase fields for TiO₂ (rutile), tetragonal ZrO₂, and ZrTiO₄ were found to exist in the system TiO₂–ZrO₂–SiO₂. SiO₂ as high cristobalite is known to exist in the system TiO₂–ZrO₂–SiO₂.     

Phase Diagram for the SiF4 Join in the System SiO2-Al2O3-SiF4

Alumina reacts with 1 atm of SiF₄ below 660°± 7°C to form A1F₃ and SiO₂. At higher temperatures the product is a mixture of fluorotopaz and AIF₃. Mixtures of fluorotopaz and AIF3 decompose in 1 atm of SiF₄ at 973°± 8°C and form tabular α-alumina. The equilibrium vapor pressure of SiF₄ above mixtures of fluorotopaz and AlF₃ is log p (atm) = 9.198 – 11460/ T (K). Fluorotopaz itself decomposes at 1056°± 5°C in 1 atm of SiF₄ to give acicular mullite, 2Al₂O₃^.1.07SiO₂. Alumina and mullite are stable in the presence of 1 atm of SiF₄ above 973° and 1056°C, respectively. The phase diagram of the system SiO₂-Al₂O₃-SiF₄ shows only gas-solid equilibria.     

Effects of Amorphous and Crystalline SiO2 Additives on γ-Al2O3-to-alpha-Al2O3 Phase Transitions

This paper focused on the effects of various phases of SiO₂ additives on the γ-Al₂O₃-to-α-Al₂O₃ phase transition. In the differential thermal analysis, the exothermic peak temperature that corresponded to the theta-to-α phase transition was elevated by adding amorphous SiO₂, such as fumed silica and silica gel obtained from the hydrolysis of tetraethyl orthosilicate. In contrast, the peak temperature was reduced by adding crystalline SiO₂, such as quartz and cristobalite. Amorphous SiO₂ was considered to retard the γ-to-α phase transition by preventing γ-Al₂O₃ particles from coming into contact and suppressing heterogeneous nucleation on the γ-Al₂O₃ surface. On the other hand, crystalline SiO₂ accelerated the α-Al₂O₃ transition; thus, this SiO₂ may be considered to act as heterogeneous nucleation sites. The structural difference among the various SiO₂ additives, especially amorphous and crystalline phases, largely influenced the temperature of γ-Al₂O₃-to-α-Al₂O₃ phase transition.     

Optical Properties of Er-Doped Al2O3–SiO2 Films Prepared by a Modified Sol–Gel Process

Er-doped Al₂O₃–SiO₂ (1/9 in mol ratio of Al₂O₃/SiO₂) thin films were prepared by using a modified sol–gel process. The modified process entails the precipitation and digestion of Er(OH)₃, obtained from the reaction between Er ions and NH₄OH in solution. Thin films were deposited on Si wafers by using a spin coating technique (3000 rpm) and the coated films were heat treated at different temperatures for 1 h in an oxygen-purged furnace. All the films were structurally characterized by the X-ray diffraction technique using Cu K α radiation. Refractive indices and the morphologies of the films were studied using a spectroscopic phase modulated ellipsometer and atomic force microscopy, respectively. It was observed that the films were crack free and of about 0.4 μm thickness in a single spin coating and both the lifetime and the photoluminescence intensity of Er ions increased with increasing the annealing temperature. The luminescence properties of the Er-doped Al₂O₃–SiO₂ made by a conventional and our modified doping process were compared and discussed from the stand point of peak intensities and lifetimes as a function of annealing temperatures. It is to be noted here that our modified process was found to be more effective in reducing the clustering of Er ions in Al₂O₃–SiO₂ materials as compared to that of the conventional method.     

Optical and NMR Studies on Interdiffusion of Silver in SiO2-B2O3-AI2O3-R2O Glasses

Interdiffusion of silver ions in SiO₂-B₂O₃-AI₂O₃-R₂O glasses where R=Na or K was investigated, using optical transmission, ESR, and wet chemical methods to determine concentration and the chemical state of silver, and NMR spectra as a probe of the glass structure. The concentration of silver introduced by ion exchange increased monotonically, as the line widths of²⁷AI NMR spectra decreased. The sharp and narrow features of ²⁷Al line shapes were broadened and the amount of colloidal silver produced by ion exchange decreased, as R₂O/B₂O₃ approached unity with fixed AI₂O₃. The BO₄ to BO₃ ratio approached unity and the quadrupole coupling constant of BO₃ units varied from 2.70 to 2.96 MH_Z, as R₂O/AI₂O₃, approached unity for fixed B₂O₃. These diverse data suggest a relation between silver diffusion and glass structure, although the phenomena of phase separation and the mixed-alkali effect could also influence silver-colloid formation in the glasses studied.