Structural Study of PbO–B2O3 Glasses by X-ray Diffraction and 11B MAS NMR Techniques

The structure of PbO–B₂O₃ glasses has been investigated in the composition range of 25–80 mol% PbO by using X-ray diffraction and ¹¹B NMR techniques. The well-separated peaks due to Pb–O and Pb–Pb pairs were first observed in the radial distribution function, and peak deconvolution was performed by using a pair function method. The average coordination number of lead atoms was determined to be about 6 in the low-PbO-content region and about 3 in the high-PbO-content region. With an increase of PbO content, the peaks at 0.24 and 0.40 nm shifted to shorter distances, and especially the latter peak drastically increased in intensity in the composition range >50 mol% PbO. It was suggested that PbO is a main glass former in the high-PbO-content region. Based on the results obtained, we propose structural models of lead borate glasses.     

Reaction of CaTiO3 with PbO–B2O3 and PbO–SiO2 Glasses

Interfacial and powder reactions between CaTiO₃ and 90PbO–10B₂O₃ and 75PbO–25SiO₂ binary glasses were studied. The reaction has been analyzed as the effect of B₂O₃ and SiO₂ additions on the interaction between CaTiO₃ and PbO, and discussed from thermodynamic and kinetic points of view. For a fixed CaTiO₃/PbO ratio₂ the product perovskite phase became enriched with lead as the amount of additives increased, which is more pronounced with B₂O₃ addition. The reaction of CaTiO₃ with the lead–boron glass was controlled by a dissolution-precipitation mechanism, and that with the lead-silica glass by a diffusion mechanism.     

27Al and 29Si MAS NMR Study of Kaolinite Thermal Decomposition by Controlled Rate Thermal Analysis

The processing of kaolinite under controlled rate thermal analysis conditions (CRTA) provides homogeneously transformed samples at defined dehydroxylation rates. The kaolinite-to-metakaolinite transformation has been monitored by solid-state NMR and X-ray diffraction. The metakaolinite product is an amorphous material with no resolved XRD pattern and is based on a disordered polymerized silicon/aluminum network in which aluminum preferably occupies four- and five-fold structural positions in the ratio 60% Al_VI, 30% Al_V, and 10% Al_VI. This complex dehydroxylation process has been viewed at different scales, with the aluminum NMR tracing a site-by-site destructon of the octahedral sheet and the silicon NMR and XRD following a more rapid destruction of the longrange order.     

29Si MAS NMR Study of Dicalcium Silicate: The Structural Influence of Sulfate and Alumina Stabilizers

The effect on β-C₂S of two stabilizing agents, calcium sulfate and alumina, has been investigated using high-resolution ²⁹Si solid state NMR spectroscopy. Syntheses were achieved via the gel route, wet or dry processes. Room-temperature NMR spectra characteristics were analyzed as a function of the sintering temperature. The incorporation of Al³⁺ and S⁶⁺ ions, which finds expression in a noticeable line broadening, is shown to be effective above 1200°C. The ²⁹Si chemical shift is unchanged upon doping, suggesting a mean SiO₄ tetrahedra geometry identical to that in pure β-C₂S. General trends on the structure adopted by C₂S upon Al³⁺ and S⁶⁺ doping are also discussed.     

31P and 29Si Magic-Angle Sample-Spinning NMR Investigation of the Structural Environment of Phosphorus in Alkaline-Earth Silicate Glasses

²⁹Si and ³¹P magic-angle sample-spinning NMR spectroscopy indicates that phosphorus added as P₂O₅ to alkaline-earth metasilicate glasses is present as monomeric (PO₄)^3– structural units and that incorporation of this phosphorus increases the average polymerization of the silicate portion of the glass. These results are consistent with published interpretations of Raman spectra of similar composition.     

Acid-Base Equilibria in the System PbO–SiO2

The acid-base equilibria in the liquid silicates in the system PbO–SiO₂ are discussed, Data reported by Richardson and Webb, wherein the PbO activity is determined over a composition range of 0 to 60 mole % SiO₂, are used for comparison with activities computed from structural models with consideration of the acid-base equilibria. The results suggest that the liquid silicates in the system PbO–SiO₂, for the composition and temperature ranges studied, are constituted of a relatively low number of anionic species and that these anions are of a relatively small size (i.e., O_2–, SiO_4–, (SiO₃)₃⁶⁻. and (SiO_2.5)₆⁶⁻).     

29Si MAS-NMR Study of the Short-Range Order in Lithium Borosilicate Glasses

²⁹Si MAS-NMR measurements have been made on a series of lithium borosilicate glasses of general composition R Li₂O.B₂O₃· K SiO₂. At low alkali contents ( R < 1), the ²⁹Si resonance envelope is broadened and indicates a distribution of Si sites. As R increases above 1, the FWHM of the ²⁹Si resonance narrows considerably to that representative of a single chemical site. Simultaneously, the average chemical shift of the resonance shifts upfield in agreement with the trends found in the binary lithium silicate glass system. Using the chemical shifts for the individual Q species in the binary system it was found that very good agreement between the chemical shifts of the binary glasses and the ternary glasses examined here could be achieved if a model of proportional sharing of the added oxygen (from lithia) between silicate and borate units was used. In contrast to the ¹¹B NMR studies of these same glasses, the ²⁹Si NMR data are quantitatively best-fit if it is assumed that the proportional sharing of the oxygen from the added lithia begins at R = 0. Models of sharing developed from the ¹¹B NMR studies of these glasses, where proportional sharing above a certain fixed (independent of K ) or variable (dependent on K ) minimum R ₀, have been reexamined and were quantitatively shown through residual analysis to give consistently poorer fits to our data. At present the reasons for the discrepancy between the two sets of NMR data are unknown.     

29Si and 27Al MASNMR Study of Stratlingite

Strätlingite (2CaO·Al₂O₃·SiO₂·8H₂O) is a complex calcium aluminosilicate hydrate commonly associated with the hydration of slag-containing cements or other cements enriched in alumina. Strätlingite can coexist with the hydrogarnet solid solution [hydrogarnet (3CaO·Al₂O₃·6H₂O)-katoite (3CaO·Al₂O₃·SiO₂·4H₂O)] and calcium silicate hydrate (C-S-H). Since Strätlingite is present in many blended cements, the knowledge of strätlingite's characteristic silicate anion structure and how aluminum is accommodated by the structure is important. Phase pure Strätlingite samples have been synthesized from oxides in the presence of excess water and from metakaolinite, calcium aluminate cement, CaO, NaOH, and water. The samples were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), and thermogravimetric analysis (TGA) and then further examined using ²⁹Si, with and without cross-polarization (CP), and ²⁷Al solid-state magic angle nuclear magnetic resonance spectroscopy (MASNMR). For the most part, NMR data for these strätlingites corroborate structural information available in the literature. The aluminum atoms are both tetrahedrally and octahedrally coordinated, and the silicon atoms exist predominantly as Q₂, Q₂(1Al), and Q₂(2Al) species. The presence of alkali affects the structure of strätlingite in subtle ways, significantly reducing the Al^IV/A1^VI ratio.     

Hydration of Tricalcium Silicate Followed by 29Si NMR with Cross-Polarization

The use of cross-polarization (CP) NMR in conjunction with magic angle sample spinning (MASS) to examine the hydration reaction of tricalcium silicate (C₃S) is described. In particular the very early stages of the reaction both with and without admixtures has been studied as well as the hydration in a ball mill. The combination of CP and non-CP ²⁹Si NMR permits the distinction between silicate units associated with protons, i.e., in hydrated material, and those in anhydrous material. It has been found that in paste hydration there is steady formation of a small amount of hydrated monomeric silicate units during the induction period. In ball mill hydration the formation of the crystalline calcium silicate hydrate, afwillite, which contains only hydrated monomeric silicate species, can be monitored. These results are interpreted in terms of possible mechanisms for C₃S hydration.     

Structural Analysis of SiO2-Al2O3 Glasses

The structure of SiO₂-Al₂O₃ glasses with up to 60 wt% Al₂O₃ was investigated using the radial distribution function together with the correlation method based on X-ray scattering intensity data. Radial distribution curves are interpreted on the basis of glass-in-glass separation with the constituents of SiO₂-rich and Al₂O₃-rich glasses. The structure of the Al₂O₃-rich glass has a short-range ordering similar to the crystal structure of mullite. The calculated S- i (S) curve of this model gives good agreement with the observed one.     

29Si and 31P MAS-NMR Spectra of Li2S-SiS2-Li3PO4 Rapidly Quenched Glasses

²⁹Si and ³¹P MAS-NMR spectra were measured for the Li₂S-SiS₂-Li₃PO₄ glasses. ²⁹Si MAS-NMR spectra revealed that a large number of silicon atoms in the glasses were coordinated with both sulfur and oxygen atoms. ³¹P MAS-NMR spectra showed that some phosphorus atoms were also coordinated with both sulfur and oxygen atoms. Such structural units resulted in the improvement of the conductivity and the glass stability against crystallization by the doping of Li₃PO₄ to the Li₂S-SiS₂ glasses.     

Coordination of Zinc(II) in ZnO–K2O–SiO2 Glasses by X-ray Diffraction

The radial distribution functions of ZnO–K₂O–SiO₂ glasses with 7 and 10 wt% ZnO are compared with that of the corresponding K₂O–SiO₂ matrix leading to "difference distribution curves'representative of the zinc structural arrangement. Analysis of the curves indicates that Zn²⁺ ions are prevalent (65% to 80%) in the glasses in tetracoordinated form.     

Influence of TiO2 on Properties of Glasses in the System K2O–PbO–SiO2–TiO2 and Its Relation to Structure

By a progressive weight percent substitution of TiO₂ for SiO₂ at various rations of concentration of K₂O and PbO, the entire region of glass formation in the quaternary system K₂O–PbO–SiO₂–TiO₂ was covered with 51 glass compositions. The properties of these glasses were determined and studied with respect to the role of TiO₂ in the system. The results indicated that the dielectric constant increased progressively with increasing TiO₂ concentration whereas the dissipation factor showed an overall decrease, when measured at 1 Mc and 25°C. Density and the refractive index increased progressively with increasing TiO₂ concentration but deviated from the additive relation. Chemical durability, expansivity, and softening temperature vs. composition curves showed definite inflections. The effect of TiO₂ on oxygen packing indicated that Ti⁴⁺ strengthens the network in lower concentrations and weakens the network in higher concentrations in this system. It appears to be likely that Ti⁴⁺ changes its coordination number form 4 to 6.     

PbTiO3–PbO–SiO2 Glass-Ceramic Thin Film by a Sol–Gel Process

PbTiO₃(PT)-PbO-SiO₂ glass-ceramic thin films were pro-duced by a sol-gel process. The crystallization of PT oc-curred at ∼700°C and was higher than that in PT-PbO-B₂ O₃ sol-gel glass-ceramics. A pinhole-free thin film was obtained by a rapid thermal annealing process when the designed glass-forming phase content in the thin film was >24 vol%. The measured dielectric constants of the films fairly agreed with the predicted values, based on a parallel mixing model. The dielectric constant was 219 and the di-electric loss was 0.04 in the 0.6PT-0.4(PbO-SiO₂) film that was fired at 700°C.     

29Si NMR Spectroscopy of Silicate Anions in Hydrothermally Formed C-S-H

Hydrothermal treatment of lime–silica mixtures under saturated steam pressures below 200°C usually gives C-S-H as an initial product, which reacts further to give crystalline calcium silicate hydrates. In this paper, C-S-H was hydro–thermally prepared using CaO and silicic acid at Ca/Si ratios of 0.3 to 2.0 and 120° to 180°C for 2 h. The C-S-H was examined mainly using ²⁹Si NMR by the magic angle spinning gate proton decoupling and cross polarization magic angle spinning methods. XRD for all of the C-S-H showed bands at 0.304, 0.280, 0.183, and 0.166 nm. NMR results showed that all of the C-S-H contained single chains of silicate anion, which became progressively longer as the Ca/Si ratio decreased, i.e., as the system became richer in silica. This was independent of the preparation temperature. The 0.8 ratio preparations at 180°C contained small amounts of double-chain structure of 1.1-nm tobermorite. The reaction processing in the lime- silicic acid mixtures is also discussed.     

Solid-State27Al and 29Si Magic-Angle Spinning NMR of Aluminosilicate Gels

The environment of the Al and Si atoms of diphasic and single-phase aluminosilicate gels was determined by solid-state ²⁷Al and ²⁹Si magic-angle spinning nuclear magnetic resonance (MASNMR) spectroscopy, The tetrahedral coordination of Al increased with decreasing Al/Si ratio in single-phase gels but not in diphasic gels. The results showed that ultrahomogeneous mixing on a next-nearest-neighbor level is achieved in single-phase aluminosilicate gels, while in diphasic gels this has successfully been avoided .     

Characterization of Synthetic and Naturally Occurring Clays by 27Al and 29Si Magic-Angle Spinning NMR Spectroscopy

Magic-angle spinning nuclear magnetic resonance (MASNMR) spectroscopy (²⁷Al) detected the coordination of Al in clays containing as little as 0.26% Al₂O₃. The ²⁹Si MASNMR of fluorphlogopite showed three distinct Si chemical environments which suggested short-range ordering. Synthetic laponite and mica-montmorillonite showed broad ²⁹Si resonances indicative of short-range disorder. A saponite showed four ²⁹Si resonances. Considerable insight into the short-range ordering of clays can be gained by ²⁷Al and ²⁹Si MASNMR.     

Effects of Preparation History on TiO2· SiO2 Glasses

Differences in the Raman spectra of various heat-treated TiO₂· SiO₂ glasses could be related to their thermal and chemical histories. For instance, while rutile could be detected in batch-prepared glasses heated at 1100°C, only α-cristobalite could be detected in heat-treated devitrified flame-prepared glasses with comparative TiO₂-concentrations. Thermal expansion coefficients increased for batch-prepared glasses upon heat treatment due to exsolution of rutile from the glasses. Earlier work had noted similar behavior at lower temperatures due to exsolution of anatase.