Short-Range and Medium-Range Order in Lithium Silicate Glasses, Part II: Simulation of the Structure by the Reverse Monte Carlo Method

The results of X-ray and neutron diffraction experiments with (Li₂O) _x (SiO₂)_{100_x} glasses (x = 0, 20, 33.3, and 40) are discussed using the Reverse Monte Carlo method to produce atomic clusters, which are analyzed with respect to the O-O distribution, the angular correlations within the first and higher coordination spheres, and the bond-orientational multipole moments. Within Li₂O-containing silicate glasses, each lithium atom is surrounded by a ring of six SiO₄ tetrahedra in such a way that four oxygen atoms lie at the joints of a tetragonal cell centered by lithium. Thus, the lithium atoms increase the order of the glass.     

Structural Study on PbO–SiO2 Glasses by X-Ray and Neutron Diffraction and 29Si MAS NMR Measurements

Structure of x PbO–(100− x )SiO₂ ( x =25–89) glasses has been investigated by means of the X-ray and neutron diffraction and ²⁹Si MAS NMR measurements. In the radial distribution functions of all the glasses, the Pb–O correlation was observed at 0.23 nm, indicating that the PbO₃ trigonal pyramids units do exist in the whole glass forming composition range. Furthermore the existence of the first Pb–Pb correlation at ∼0.385 nm in the whole composition range suggests that the basic structural unit is considered to be a Pb₂O₄ unit, which consists of the edge-shared PbO₃ trigonal pyramids. These results strongly imply that the Pb₂O₄ units participate in the glass network constructed by SiO₄ tetrahedra even at low PbO content. Differing from other lead-containing glass systems, these structural characteristics of Pb ions in the PbO–SiO₂ glass system are responsible for the extremely wide glass-forming region.     

Crystal Structure of Lu4Si2O7N2 Analyzed by the Rietveld Method Using the Time-of-Flight Neutron Powder Diffraction Pattern

The crystal structure of a lutetium silicon oxynitride (Lu₄Si₂O₇N₂) was analyzed by the Rietveld method using time-of-flight (TOF) neutron powder diffraction data. The compound crystallizes in a monoclinic cell, space group P 2₁/ c (No. 14-1) with a = 7.4243(1), b = 10.2728(1), c = 10.6628(1) Å, and β= 109.773(1)° at 297 K. One nitrogen atom in Lu₄Si₂O₇N₂ occupies the bridging site between the two Si atoms, and the other one is statistically situated at the terminal sites of Si₂O₅N₂ ditetrahedra. In the local structure, Si₂O₅N₂ ditetrahedra consist of SiO₃N and SiO₂N₂ tetrahedral units sharing the N atom. Lu atoms are in sixfold, sevenfold (×2) and eightfold coordinations of O/N atoms. X-ray powder diffraction data were also analyzed with the model obtained by the neutron diffraction.     

Sol–Gel Preparation and Characterization of Lithium Disilicate Glass–Ceramic

A sol–gel process is described for preparation of crystalline lithium disilicate (Li₂Si₂O₅) from tetraethylorthosilicate and lithium ethoxide. The glass network structure and crystallinity resulting from heat treatment at temperatures from 150° to 900°C were investigated by nuclear magnetic resonance, X-ray diffraction, and differential scanning calorimetry/thermogravimetric analysis. Q³ structural units (SiO₄ tetrahedra with three bridging oxygen atoms) formed in the amorphous gel at a low temperature (≤150°C) persist to elevated temperature (≤500°C) and directly transform to crystalline Li₂Si₂O₅ at about 550°C. The heating schedule slightly affects the crystalline phase transformation.     

Pu, U, and Hf Incorporation in Gd Silicate Apatite

Trivalent Pu can be incorporated in the silicate apatite structure to form Ca₂Pu₈(SiO₄)₆O₂ by sintering under reducing conditions, while the incorporation of tetravalent Pu in the Ca/rare earth sites in oxidizing or neutral conditions is limited to only 0.6 formula units (f.u.). The d -spacings and intensities of the X-ray pattern of hexagonally structured Ca₂Pu₈(SiO₄)₆O₂ after firing at 1250°C are given, and the a and c lattice parameters are 0.9561₁ and 0.7028₁ nm, respectively. The respective solid solubility limits of U and Hf in Ca₂Gd_{8– x} (U/Hf) _x (SiO₄)₆O₂ apatite samples were 0.3 and 0.2 f.u.     

Synthesis of Blue-Emitting CaMgSi2O6:Eu2+ Phosphor Using an Electrostatic Self-Assembly Deposition Method

Eu²⁺-doped CaMgSi₂O₆ phosphor was prepared by depositing mixed hydroxides of Ca, Mg, and Eu over spherical SiO₂ particles (300 nm) pre-coated with polycations (polyethyleneimine), followed by calcination at 1200°C in a reducing atmosphere. The prepared phosphor showed intense blue emission, ascribable to the 4f⁷-4f⁶5d transition of Eu²⁺. In contrast, the luminescence intensity of the phosphor was considerably decreased when prepared without polycations. It was suggested that negatively charged hydroxides are deposited on positively charged SiO₂ surfaces pre-coated with polycations through electrostatic self-assembly interaction. On calcination, the hydroxide shells react with the SiO₂ cores to produce Eu²⁺:CaMgSi₂O₆.     

Bulk Concentration Effects on the Structure and Orientation of Adsorbed Silane on the Surface of Nanosized SiO2 Particles

In this study, the concentration effects of 3-methacryloxypropyltrimethoxysilane (MEMO) on the surface coverage extent, orientation, and structures of MEMO silane grafted onto the surface of SiO₂ particles are investigated using Fourier transform infrared (FTIR) and ²⁹Si-NMR spectroscopy. The MEMO silane concentration effects on the rheological behaviors of the SiO₂ particles–1,6-hexanedioldiacrylate (HDDA) suspensions are discussed in terms of the surface coverage extent and the MEMO silane structures grafted onto the surface of SiO₂ particles. At low MEMO concentrations, many free silanol SiOH groups were observed for the MEMO grafted onto the SiO₂ surface and the adsorbed MEMO molecules tended to orient parallel to the SiO₂ surface due to the hydrogen bonding of the MEMO-carbonyl and the hydroxyl group of the oxides. At high MEMO concentrations, the condensation reactions between neighboring grafted MEMO molecules result in the predominance of a T² and T³ silicon atom structure and complete coverage of the SiO₂ surface by the grafted MEMO. The enhanced steric hindrance and compatibility between the MEMO-modified SiO₂ particles and HDDA monomer, which originate from the complete coverage of the SiO₂ surface by MEMO silane and a higher proportion of free carbonyl groups on the grafted MEMO, may improve the dispersibility of nanosized SiO₂ in acrylate suspensions.     

Chemical Shifts of Silicon X-ray Photoelectron Spectra by Polymerization Structures of Silicates

The binding energy of Si 2 p electrons and the kinetic energy of the Si(KLL) X-ray-excited state were measured by using X-ray photoelectron spectroscopy (XPS) and X-ray Auger electron spectroscopy (XAES), respectively, for silicates with SiO₄ tetrahedra of various polymerization types. The resulting Si 2 p XPS binding energies varied from 101.3 eV in merwinite (monomeric structure) to 103.4 eV in quartz and cristobalite (three-dimensional framework structure). A clear chemical-shift relation was observed, relating the polymerization structures of SiO₄ tetrahedra to the plots of their Si 2 p XPS binding energy versus the kinetic energy of their Si(KLL) XAES spectra. Thus, the structural state of the surface silicates in various substances can be deduced from this chemical-shift relationship.     

Novel Synthesis of Silica-Coated Ferrite Nanoparticles Prepared Using Water-in-Oil Microemulsion

Ferrite nanoparticles (magnetite (Fe₃O₄) and cobalt-ferrite (Co _x Fe_{3− x} O₄)) coated with silica (SiO₂) were prepared using water-in-oil microemulsion of a polyoxyethylen(15)cetylether/cyclohexene system. Observation via transmission electron microscopy revealed that the ferrite nanoparticles were located nearly at the center of spherical SiO₂ particles. The sizes of Fe₃O₄ and Co _x Fe_{3− x} O₄ nanoparticles were in the range of 8–12 nm and 10–14 nm, respectively, and the thickness of the SiO₂ layer was ∼14.0 nm. The solid solution of cobalt ions into Fe₃O₄ to form Co _x Fe_{3− x} O₄ nanoparticles led to an increase in the coercivity of the SiO₂-coated ferrite nanoparticles. The coercivity increased with increasing amounts of added cobalt, obtaining a maximum value of 780 Oe around a Co/Fe ratio of 0.3–0.4.     

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.     

Structural Characteristics of Liquid Mixtures of Feldspar and Silica

Thermodynamic properties of liquid mixtures of feldspar and silica were considered in attempting to interpret them in terms of melt structure. Two sources of data were used: the heats of formation and free energies of formation of the crystalline and glassy compounds, and the published phase diagrams. The systems albite-silica, orthoclase-silica, and anorthite-silica were studied, and some data from the system celsian-silica were included. A model which assumes a random distribution of AlO₄ tetrahedra and SiO₄, tetrahedra is generally applicable to silica-rich liquid mixtures of albite-silica and orthoclase-silica. For mixtures of albite-silica, it is suggested that minor amounts of polymeric aluminosilicate units are also present. In liquid mixtures of anorthite and silica, however, it is necessary to assume that polymeric units like 4AlO₂.3SiO₂ or 4AlO₂.2SiO₂, are the dominating feature of the structure.     

Structural Analysis of Metadickite

The structure of metadickite, which was a dehydration phase of a dickite from Shokozan mine, Hiroshima, Japan calcined at 600°C for 24 h, was investigated by X-ray radial distribution function (RDF) analysis and the correlation method. The RDF curve showed the coordination numbers of both Si and Al atoms to be four and the structural regularity was observed up to ∼0.8 nm. Several structural models for metadickite were examined by the correlation method and the most likely model is considered to be the arrangement built up of SiO₄ tet-rahedral sheets, similar to the original dickite, and an adjacent edge-shared AlO₄ tetrahedral chain, with a 6-4-4 type oxygen atom distribution.     

Volume Relations in Na2O·B2O3 and Na2O·SiO2 B2O3, Melts at 1300·C

Density (and some viscosity) data are presented for binary sodium borate melts containing as much as 60 mole % Na₂O and for ternary sodium silicoborate melts with B/Si <2.0 between 1000°C and 1300°C. The high-temperature partial molar volume analysis of the binary sodium borate melts reveals about 50% BO₄ tetrahedra at the 40 mole % Na₂O composition, in agreement with recent NMR estimates for the binary glasses. No "boron anomaly" was found near 18 mole % Na₂O at high temperature. The synthetic partial molar volume model that agrees best with experiment for all ternary melts studied involves the presence of some BO₄ tetrahedra, the percentage of which varies with composition. This ternary model involves a high degree of internal consistency. No tendency toward extensive micro-immiscibility was observed for ternary melts near the SiO₂·B₂O₃ binary.     

Preparation of Ultrathin-Walled Carbon-Based Nanoporous Structures by Etching Pseudo-Amorphous Silicon Oxycarbide Ceramics

Etching polymer-derived silicon-oxycarbide ceramics with hydrofluoric acid creates nanoporous structures of specific surface areas as high as 600 m²/g. The change in composition upon etching shows the removal of silica, not carbon. The structure remaining after etching is postulated to consist of a scaffolding of graphene networks with their surfaces decorated with mixed bonds of tetrahedral silicon bonded to both oxygen and carbon (SiO _m C_{4− m} , where m =1, 2, or 3). The pores existing within such scaffoldings are presumed to have been filled with SiO₂ tetrahedra, which are removed by etching. The measurement of the average pore size and pore volumes permits us to estimate the width of the graphitic domain walls, δ_W, left behind by the etching process. The smallest value of δ_W, which corresponds to the specimen with the highest surface area, is approximately 1 nm, which is about equal to the total width of one graphene layer and two SiO _m C_{4– m} tetrahedra, one on either side of the graphene sheet. This highest surface area specimen is also believed to have the largest size of the silica domains in the unetched samples. In specimens with smaller domains, the etching is only partially successful in removing the silica, presumably because their small size hinders the access of the etchant to the silica tetrahedra. The above behavior is found for samples with low to moderate carbon content. In one sample with a very high carbon content, the etching process removes some carbon as well as silica.     

Microstructure in Hydrated Silicate Glasses

A microstructure ranging from 5 to 20 nm in size was detected by small-angle X-ray scattering in Na₂O·3SiO₂ and 4EaO·28Na₂O·68SiO₂ glasses hydrated at ∼100° and 80°C, respectively. This observation suggests the presence of silica gel-solvent (water) phase separation.     

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.     

Paralinear Oxidation of Silicon Nitride in a Water-Vapor/Oxygen Environment

Three Si₃N₄ materials were exposed to dry oxygen flowing at 0.44 cm/s at temperatures between 1200° and 1400°C. Weight change was measured using a continuously recording microbalance. Parabolic kinetics were observed. When the same materials were exposed to a 50% H₂O–50% O₂ gas mixture flowing at 4.4 cm/s, all three types exhibited paralinear kinetics. The material was oxidized by water vapor to form solid SiO₂. The protective SiO₂ was in turn volatilized by water vapor to form primarily gaseous Si(OH)₄. Nonlinear least-squares analysis and a paralinear kinetic model were used to determine parabolic and linear rate constants from the kinetic data. Volatilization of the protective SiO₂ scale could result in accelerated consumption of Si₃N₄. Recession rates under conditions more representative of actual combustors were compared with the furnace data.     

Hydration of Beta Dicalcium Silicate Alone and in the Presence of CaCl2 or C2H5OH

Beta C₂S was hydrated at room temperature with and without added CaCl₂ or C₂H₅OH by methods previously studied for the hydration of C₃S, i.e. paste, bottle, and ball-mill hydration. The amount of reacted β-C₂S, the Ca(OH)₂ concentration in the liquid phase, the CaO/SiO₂ molar ratio, and the specific surface area of the hydrate were investigated. A topochemical reaction occurs between water and β-C₂S, resulting in the appearance of solid Ca(OH)₂ and a hydrated silicate with a CaO/SiO₂ molar ratio of ≃1. As the liquid phase becomes richer in Ca(OH)₂, the first hydrate transforms to one with a higher CaO/SiO₂ ratio. Addition of CaCl₂ increases the reaction rate and the surface area of the hydrate but to a much lesser extent than in the hydration of C₃S, whereas C₂H₆OH strongly depresses the hydration rate of β-C₂S, as observed for C₃S hydration.     

Structure of Borosilicate and Borogermanate Melts at 1300°C; a Viscosity and Density Study

An improved counterbalanced sphere viscometer-densitometer was used to obtain information at temperatures between 1000° and 1400°C for (a) molten B₂O₃, (b) a series of borosilicate melts containing up to 55 mole % SiO₂, and (c) a series of borogermanate melts containing up to 65 mole % GeO₂. The dependence on composition of these structure-sensitive parameters was used to develop a model for the alteration of molten B₂O₈ by other network-forming species. The B/Si or B/Ge ratio is shown to be a significant factor in determining melt structure. The silicon (or germanium) atoms appear to be widely separated in the B₂O₂ solvent for compositions in the 0 to 10-20 mole % SiO₂ (or GeO₂) region. The evidence suggests that a gradual microclustering of SiO₂ (or GeO₂) accompanies moderate departures from ideality for B₂O₃ in the 10-20 to 60 mole % SiO₂ (or GeO₂) region. Extensive micro-clustering of SiO₂ (or GeO₂), approaching network formation, appears to occur in the 60 to 100 mole % SiO₂ (or GeO₂) region; some of the boron atoms remain in clusters and/or are forced to adopt a tetrahedral configuration in this region.     

Hollow Beads Composed of Nanosize Ca α-SiAlON Grains

Carbothermal reduction—nitridation (CRN) of SiO₂ is an attractive method to manufacture Si₃N₄ powders with controlled grain morphology. Moreover, β-SiAlON powders could also be synthesized from either pure powder mixture or some inexpensive raw minerals by CRN and the resulting powders favored the sintering of SiAlON product. However, there have been few works on preparing α-SiAlON powders so far. In this work, Ca α-SiAlON powder was synthesized by CRN of a SiO₂—Al₂O₃—CaCO₃ mixture. An unusual morphology of hollow beads 200 to 500 nm in diameter with a great deal of nanosize α-SiAlON particles around 10 to 30 nm in diameter was observed from the resultant Ca α-SiAlON powders, which has not been reported for SiAlON ceramics before.