Effect of Trace A12O3 on Transformation of Quartz to Cristobalite

The effect of additions of 0.22, 0.44, 0.88, and 1.76% A1₂O₃ (Si⁴⁺/A1³⁺ ratio of 200:1, 100:1, 50:1, and 25:1) on the transformation of Brazilian quartz to cristobalite was studied at 1500°, 1530°, and 1570°C. The smaller percentages of A1₂O₃ (0.22 and 0.44%) catalyzed the transformation of quartz and the formation of cristobalite considerably. The rates of transformation of quartz with 0.88 and 1.76% A1₂O₃ were slower than with 0.22 or 0.44%, indicating a critical A1³⁺/Si⁴⁺ ratio where the catalytic effect was found to be maximum. This appeared to occur at about 0.5% A1₂O₃. The transformation rate of quartz indicated that the reaction was first order. Cristobalite, however, showed two different rates; the initial rapid growth was followed by a slower rate. The point of changeover was found to be at about 30 ± 5% cristobalite. The critical nature of the A1³⁻/Si⁴⁺ ratio at about 0.01 (or A1₂O₃/SiO₂± 0.005) may have some bearing on the properties of silica refractories with more or less than 0.5% A1₂O₃.     

Ca Segregation to Basal Surfaces in α-Alumina

High-resolution transmission electron microscopy (HRTEM) and analytical electron microscopy (AEM) were used to investigate the structure and chemistry of (0001) α-A1₂O₃/ A1 interfaces in melt-infiltrated polycrystalline alumina composites. HRTEM revealed an interfacial region different from both Al and α-A1₂O₃, with a structural width of 0.8 ± 0.2 nm. AEM of the same interfaces revealed a Ca excess of Γ = 2.5 ± 0.5 Ca atoms per nm² (Ca/nm²). AEM of a basal twin boundary in the α-A1₂O₃ also revealed a Ca excess (Γ= 1.0 ± 0.5 Ca/nm²). Since the metal-ceramic interfaces were the free surfaces of pores before melt infiltration, it can be concluded that Ca segregates to the basal surface of alumina, as well as to basal twin boundaries. Furthermore, the Ca at the free surfaces does not reside on only one cation plane, but is spread over 4 ± 1 basal cation layers and forms an interfacial phase with a nominal composition of CaO-6A1₂O₃.     

Improvement in Wear Resistance of High-Strength and High-Toughness Silicon Nitride Modified by Ion Implantation

Surface modification by ion implantation has been conducted to improve the tribological properties of a high-strength and high-fracture-toughness unidirectionally aligned silicon nitride (UA-SN). B⁺, N⁺, Si⁺, and Ti⁺ ions were implanted into the planes parallel and normal to the grain alignment of the UA-SN with a fluence of 2 × 10¹⁷ ions/cm² at an energy of 200 keV. The ion implanted UA-SN showed a dramatic improvement in wear resistance. For example, the specific wear rate of the Si⁺-implanted specimen in the direction parallel to the grain alignment was reduced to a value of 3 × 10⁻¹⁰ mm²/N, equal to 1/20 of the unimplanted one. Cross-sectional transmission electron microscopy indicates the high wear resistance was attributed to the amorphous surface caused by the ion implantation.     

Selective Cation Exchange in Substituted Tobermorites

Selective cation exchange in tobermorites with three levels of Al³⁺ and Na⁺ substitution for Si⁴⁺ has been investigated. The cation exchange selectivity for a tobermorite with 2 mol% Al³⁺ and 0 mol% Na⁺ substitution increased as follows: Mg²⁺ > Ba²⁺ > Sr²⁺. Cation exchange in the above tobermorite is postulated to take place mainly from edge and planar surface sites and apparently from interlayer Ca²⁺ sites. Tobermorites with 10 and 15 mol% Al³⁺ (and Na⁺) substitution have additional Na⁺ exchange sites in the interlayers and show the following selectivity: Ba²⁺ > Sr²⁺ > Mg²⁺. The cation exchange selectivity in the substituted tobermorites can be explained by the hydrated radii of cations and the steric limitations of the ion exchange cavity in tobermorite; the latter was determined by ²⁷Al and ²⁹Si magic angle spinning nuclear magnetic resonance spectroscopy. These basic selective cation exchange studies of substituted tobermorites are of relevance in nuclear waste treatment and disposal.     

Reactions and Bonding of Sodium Disilicate Glass with Chromium

Reactions and reaction mechanisms occurring at sessile drop interfacial regions of sodium disilicate glass on chromium were identified at 1000°C and an ambient atmosphere of 2.7 × 10⁻⁴Pa with a p(O₂) of 1 × 10⁻¹⁵ and 1 × 10⁻⁵ Pa. Depending on the experimental conditions, the principal reactions were redox reactions of Cr⁰ with Na⁺ to form Cr²+ and Na⁰, Cr²+ with Na⁺ to form Cr³+ and Na⁺, Cr⁰ with Si⁴+ to form CrSi_x, alloy dendrites and Cr²⁺, and Cr²⁺ with Si⁴⁺ to form Cr³⁺ and SiO(gas). Adherence was developed when the interfacial region was saturated with Cr³⁺, i.e., Cr₂O₃.     

Relation of Composition of Hydrogarnet to Resistance to Sulfate Attack

Crystallographic changes resulting from the entry of Si into the cubic lattice of tricalcium aluminate hexahydrate were examined in an effort to understand the sulfate resistance of hydrogarnets. Low-SiO₂ hydrogarnets were prepared by hydrothermal treatment of specially prepared glasses. The lattice constants of the unit cell of C₃AH₆ decreased when Si⁴⁺ ions were introduced. In investigating the low-Si members of the C₃AH₆-grossularite series, only those with well-defined Si contents of 1, 3, and 6 Si⁴⁺ ions/unit cell were found; the lattice constants were 12.52, 12.47, and 12.41 Å, respectively. Intermediate lattice constants represent mixtures of at least two members. A content of 0.125 SiO₂/formula unit, corresponding to 1 Si⁴⁺ ion in the unit cell of C₃AH₆, is not sufficient to produce resistance to sulfate attack; the lowest SiO₂ content which results in such resistance appears to be 0.375 SiO₂/formula unit or 3 Si atoms/unit cell.     

Si NMR Study of the Hydration of Ca3SiO5 and ß-Ca2SiO4 in the Presence of Silica Fume

²⁹Si magic-angle spinning nuclear magnetic resonance (MASNMR) was used to study the room-temperature hydration of C₃S, ß-C₂S, and reactive ß-C₂S mixed with different amounts of silica fume (SF) that had been hydrated up to nine months and longer. The overall CaO:SiO₂ molar ratios of the mixes were 0.12, 0.20, 0.35, 0.50, and 0.80. NMR spectroscopy was used to quantify the remaining starting materials and the resulting hydration products of different species. A broad peak assigned to Q³, appearing in both the fourier transform (FT) and the cross-polarization (CP) modes, increased in intensity with increased SF content and with age. This Q³ species was attributed to two sources: (1) the surface hydroxylation of SF and (2) the cross-linking of dreierketten (chains of silicate tetrahedra arranged in a repeating three-unit conformation) in the calcium silicate hydrate (C-S-H) structure. A Q⁴ species also appeared in the CP spectra of samples with large SF additions after extended hydration and was attributed to cross-polarization by adjacent hydroxylated Q³ species at the surface of amorphous SiO₂.     

Structure of Liquid Al6Si2O13 (3:2 Mullite)

We report the first measurements of the structure factor, S ( Q ), and the pair distribution function, G ( r ), of Al₆Si₂O₁₃ (3:2 mullite) in the normal and supercooled liquid states in the temperature range 1776–2203 K. Measurements are obtained by synchrotron X-ray scattering on levitated, laser-heated liquid specimens. The S ( Q ) shows a prepeak at 2.0 Å⁻¹ followed by a main peak at 4.5 Å⁻¹ and a weak feature at 8 Å⁻¹. The G ( r ) shows a strong (Si,Al)–O correlation at 1.80 Å at high temperature that moves to 1.72 Å as the liquid is supercooled. The second and third nearest neighbor peaks at 3.0 and 4.25 Å sharpen with supercooling. The short-range structure of the high-temperature liquid is similar to the corresponding glasses produced by rapid quenching. Supercooling causes an increase in the concentration of tetrahedral Si⁴⁺ ions, which is manifested by the large shift in the first peak to lower ionic distance, r , values in G ( r ). The increase in tetrahedrally coordinated Si⁴⁺ ions is offset by an increase in octahedral Al³⁺ ions. The clustering of the SiO₄⁴⁻ tetrahedral units results in increased viscosity of the liquid at temperatures below the melting point, which is consistent with Al₆Si₂O₁₃ being a fragile liquid.     

Chromium-Doped Forsterite Nanoparticle Synthesis by Flame Spray Pyrolysis

Synthesis of chromium-doped forsterite (Mg₂SiO₄:Cr) nanoparticles by flame spray pyrolysis (FSP) was investigated. The morphology, crystalline phase, and photoluminescence of the products were evaluated. Crystalline Mg₂SiO₄:Cr nanoparticles of several 10 nm in diameter were obtained, although a small amount of the submicrometer-sized particles and the unreacted MgO phase existed. The product powder showed electron-spin resonance signals from Cr⁴⁺ and photoluminescence typical for Cr⁴⁺ in Mg₂SiO₄, suggesting that a part of the Cr⁴⁺ ions were incorporated into Si⁴⁺ sites by FSP. On the other hand, the effects of excess SiO₂ addition on the structural and optical characteristics of Mg₂SiO₄:Cr were examined. Addition of excess SiO₂ up to 20 mol% did not influence these characteristics of the products. Further addition of excess SiO₂ (60–100 mol%) enhanced the formation of the amorphous phase and resulted in the emission from Cr³⁺ in the amorphous phase in addition to an emission from Cr⁴⁺ in Mg₂SiO₄.     

Attack of Glass by Chelating Agents

The chemical attack of a soda-lime glass by the combined action of ethylene diamine tetraacetic acid and ortho-dihydroxy benzene (catechol) in dilute aqueous solution is reported u a function of pH. A pronounced maximum occurs in the rate of attack at a pH value determined by the anionic species present. The effects are ascribed to the chelation of the protective cations A1³⁺, Ca²⁺, and Mg²⁺ by ethylene diamine tetraacetic acid and to the chelation of Si⁴⁺ by catechol.     

Studies in Lithium Oxide Systems: XIII, Li,O.Al2O3.2SiO2-Li2O.Al2O3.2GeO2

Complete solid solubility was demonstrated to occur between LiAlGeO₄ and the low temperature form of Li AlSiO₂ (a-eucryptite). Hydrother-mal preparation was necessary for the silicate-rich compositions. Under atmospheric pressure, about 65 mole % LiAlGeO₄ entered the β-eucryβ-tite phase at 1150°C, but solid solutions containing more than 25 mole % LiAlGeO4 exsolved if held at lower temperatures. Directional thermal expansion data were obtained by X-ray diffraction methods on both α- and β-eucryptite and their solid solutions. Substitution of Ge⁴⁺ for Si⁴⁺ produced no significant difference in the thermal expansion coefficients in the α and β phases. An increase in the lattice parameters in the a and c directions took place as expected when Ge⁴⁺ (0.53 A) was substituted for Si⁴⁺ (0.39 A).     

High-Temperature Oxidation of Chemically Vapor-Deposited Silicon Nitride in a Carbon Monoxide-Carbon Dioxide Atmosphere

Oxidation behavior of chemically vapor-deposited silicon nitride (CVD-Si₃N₄) in CO─CO₂ atmospheres between 1823 and 1923 K was investigated using a thermogravimetric technique. Mass loss of Si₃N₄ (active oxidation) was observed in a region of P _CO2/P_CO < 1, while mass gain (passive oxidation) was observed at around P _CO2 P _CO= 10. In the active oxidation region below P _CO2P_CO= 10 ^–4, carbon particles were formed on the Si³N⁴ surface as an oxidation product, and the mass-loss rates were independent of P _CO2/ P _CO In the active oxidation region above P _CO2/ P _CO= 10^–4 the mass-loss rates decreased with increasing P _CO2/ P co. The critical P _CO2/ P _CO value from the active to passive oxidation was 2 orders of magnitude larger than the calculated value predicted from the Wagner model.     

Crystallographic Data of a New Phase of Dicalcium Silicate

Unit-cell parameters and the space group of a new phase of dicalcium silicate (Ca₂SiO₄) were determined by using powder X-ray diffractometry and selected-area electron diffraction techniques. This phase could be synthesized via the dissociation of hydrothermally synthesized alpha-Ca₂(SiO₄H)OH at temperatures of ∼500°-920°C. Crystallographic data for the sample synthesized at 600°C were as follows: Ca₂SiO₄, monoclinic; P 2₁/ c space group; lattice parameters of a = 0.82147(9) nm, b = 0.9808(1) nm, c = 0.9741(1) nm, and β= 94.642(7)°; cell volume ( V ) of 0.7857(1) nm³; Z = 8 (where Z is the number of chemical formula units in a unit cell); and a density of 2.91 g/cm³. The crystallographic data for samples synthesized at 800°C had slightly different unit-cell parameters of a = 0.82124(6) nm, b = 0.97348(7) nm, c = 0.97935(7) nm, β= 94.831(5)°, and V = 0.7849(1) nm³. Structural relationships of the new phase with the other dicalcium silicates are discussed.     

Effects of Zirconium Doping on Grain-Boundary Bonding in Alumina-Silicon Carbide Composites

In this work, 800 ppm of Zr⁴⁺ dopants were added to Al₂O₃-5 vol% SiC particle composite. Zr⁴⁺ doping led to a weak Al₂O₃ grain-boundary bonding so that the fracture mode changed from transgranular in undoped composite to intergranular in Zr⁴⁺-doped composite. The fracture mode change increased the fracture toughness of the composite. Transmission electron microscopy and energy-dispersive spectroscopy examinations revealed that the weak grain-boundary bonding in the doped composite was caused by the segregation of Zr⁴⁺ and Si⁴⁺ ions at the Al₂O₃ grain boundary.     

Fast Li+ Ion Conduction in Li2O-Al2O3-TiO2-SiO2-P2O5 Glass-Ceramics

Fast lithium ion conducting glass-ceramics have been successfully prepared from the pseudobinary system 2[Li_{1+ x} Ti₂Si _x P_{3− x} O₁₂]-AlPO₄. The major phase present in the glass-ceramics was LiTi₂P₃O₁₂ in which Ti⁴⁺ ions and P⁵⁺ ions were partially replaced by Al³⁺ ions and Si⁴⁺ ions, respectively. Increasing x resulted in a considerable enhancement in conductivity, and in a wide composition range extremely high conductivity over 10⁻³ S/cm was obtained at room temperature.     

Ionic Conductivity of the System Si-Al-O-N from 850° to 1400°C

Electrical conductivity was measured from 850° to 1400°C for β-sialon and pure X phase as well as for the sintered system Si₃N₄-Al₂O₃, containing β-sialon, X phase, β-Si₃N₄, and glassy phase. Ionic conductivity was measured at >1000°C. The charge carriers were identified by electrolysis. The results showed that pure β-sialon is ionically conducting because of Si⁴⁺ migration for the temperature range studied. Pure X phase shows ionic conduction by Si⁴⁺ above 1000°; below 1000°C, it shows electronic conduction because of impurities. The conductivity of the sintered system Si₃N₄-Al₂O₃ containing β-sialon, β-Si₃N₄ X phase, and glassy phase changes as the relative quantities of β -sialon and X phase change. The apparent activation energies for the ionic and electronic conductivities are 45 and 20 kcal/mol, respectively.     

Fracture-Mode Change in Alumina-Silicon Carbide Composites Doped with Rare-Earth Impurities

Al₂O₃-5 vol% SiC particle composites doped with 800 ppm rare-earth impurities (Y³⁺, Nd³⁺, and La³⁺) were fabricated by hot-pressing at a temperature of 1550°C. Doping of rare-earth impurities in Al₂O₃-SiC composites led to a fracture- mode change from transgranular in dopant-free composites to intergranular in rare-earth-doped composites. The fracture mode change obviously increased the crack deflection so that the fracture toughness of rare-earth-doped composites was higher than that of the composites without dopants, especially for the Nd³⁺- and La³⁺-doped composites. It was found that the fracture-mode change originated from a weak grain-boundary bonding caused by co-segregation of the rare-earth dopants and Si⁴⁺ ions dissolved from the SiC particle surfaces.     

Rutherford Backscattering Spectroscopy Study of the Kinetics of Oxidation of (Mg, Fe)2SiO4

Single crystals of oilivine, (Mg_0.9Fe_0.1)₂SiO₄, have been oxidized in air at temperatures between 700° and 1100°C for times from 0.5 to 100 h. Both an internal and an external oxidation layer developed. Transmission and analytical electron microscopy observations reveal that the internal oxidation layer is composed of precipitates of magnetite plus amorphous silica, which nucleated heterogeneously on dislocations and grew in an Fedepleted matrix of olivine. Rutherford backscattering spec-trometry (RBS) demonstrates that the thin external oxidation layer is free of Si; that is, it is made up of Mg-Fe oxide phases. Thus, the oxidation process is primarily controlled by diffusion of Fe²⁺ and Mg²⁺ ions toward the surface with Si⁴⁺ and O^2- remaining largely immobile. The kinetics of oxidation, as determined from RBS analyses of the external oxidation layer, are parabolic with an activation energy of 140 kJ/mol. Although this activation energy is lower than that reported for self-diffusion of Mg in Mg₂SiO₄, the diffusivity calculated from the reaction rate constant is in good agreement with published values for lattice diffusion of Mg in the limited temperature range in which data overlap. However, the rate of accumulation of Fe in the external layer is more rapid than expected for lattice diffusion, indicating that the transport of Fe is dominated by short-circuit diffusion along the precipitate complexes which decorate dislocations.     

X-ray Powder Study of 2BaO·CuO

A compound of composition 2BaO·CuO was synthesized during the phase equilibrium study of the BaO-Y₂O₃-CuO_x system. Phase characterization has been carried out by using X-ray powder diffraction. The crystal symmetry was found to be the same as that of Ca₂CuO₃ and Sr₂CuO₃. It is orthorhombic with space group Immm and lattice parameters a=12.9655(14) Å (1.29655 nm), b=4.1007(3) Å (0.41007 nm), c=3.9069(5) Å (0.39069 nm), and V=207.72(3) ų (0.20772 nm³). The experimental pattern shows good agreement, in general, with intensity values calculated by assuming Ba₂CuO₃ to have a structure similar to that of Sr₂CuO₃ and Ca₂CuO₃. Intensity discrepancy for the h00 reflections might be due to preferred orientation.     

Relationship Between Structure and Glass Transition Temperature in Low-silica Calcium Aluminosilicate Glasses: the Origin of the Anomaly at Low Silica Content

The anomalous behavior of the glass transition temperature ( T _g) in low silica calcium aluminosilicate glasses has been related to the structural modifications observed by neutron and X-ray diffraction. The diffraction data indicate that Al and Si are in tetrahedral sites and that Ca atoms are in distorted octahedral sites. By subtracting the correlation functions for glasses at constant SiO₂ or constant Al₂O₃ content, we have shown that Si and Al atoms are introduced in a different way within the glass structure. Si is present in various Q ⁿ sites, while Al resides in Q³ and Q⁴ sites for glasses with high CaO content and enters fully polymerized Q⁴ sites with increasing SiO₂ or Al₂O₃ content. The higher proportion of Al in Q³ positions at high CaO content yields a depolymerization of the network. The lower connectivity will contribute to a decrease of the viscosity, which may be at the origin of the decrease of T _g for glasses at low silica content.