Analytical Expressions for the Heat Capacities of Alkali Silicate Liquids

The available thermochemical data for silicate liquids and glasses indicate that the customary assumption of heat-capacity additivity is not valid, particularly for liquid silicates near 1000 K. Estimates are obtained for the heat capacities of KO_0.3-SiO₂, NaO_0.3-SiO₂, and CaO-SiO₂ liquids. The excess heat capacity of mixing of alkali silicates at 1000 K is:
in units of cal mol⁻¹ K⁻¹. Analytical expressions are given for C ^e_p for KO_0.5-NaO_0.5-SiO₂ and NaO_0.3-CaO-SiO₂ liquids at 900 to 1800 K.     

Contamination Effects on Interfacial Porosity during Cyclic Oxidation of Mullite-Coated Silicon Carbide

Plasma-sprayed mullite (3Al₂O₃2SiO₂) and mullite/yttria-stabilized zirconia (mullite/YSZ) dual-layer coatings have been developed to protect silicon-based ceramics from environmental attack. The mullite/SiC system develops interfacial pores during cyclic oxidation. The development of pores at the mullite/sintered SiC interface in air has been investigated as a function of the purity of mullite at 1230°–1350°C in an atmospheric-pressure furnace. The silica scale is readily contaminated by impurities of alkali or alkaline-earth metal oxides from the mullite coating. The contamination enhances oxidation and reduces the scale viscosity by forming alkali or alkaline-earth metal silicates. The porosity increases as the temperature and contamination increase and decreases as the purity of the mullite increases.     

Accelerated Curing of Compacted Calcium Silicate Mortars on Exposure to CO2

The parameters affecting strength development in compacted cylinders of 3CaO- SiO₂ and β-2CaO- SiO₂ mortars exposed to CO₂ were investigated. Strength increased with time up to 81 min, the duration of the longest detailed study. The β-2CaO- SiO₂ develops strength more slowly initially, but both silicates achieved compressive strengths of 7,000 to 10,000 psi. The rate of increase in strength depends on both the amount of water used in molding the compact and the amount of water present in the CO₂. Increasing CO₂ pressures from 1 to 2 atm increased the rate of reaction, but a further increase to 4 atm had little additional effect. Carbonation occurs mainly in the outer portions of the cylindrical compacts. The initial reaction on exposure to CO₂ appears to be accelerated hydration of the silicates to a CaO-SiO₂-H₂O-like gel and calcite. The gel has a stoichiometry similar to that found in conventional hydration. Further reaction results in progressive carbonation of the gel, which decreases its lime content. The reaction products appear to be intimately dispersed in the microstructure.     

Effects of Glass Frit Oxides on Crystallization and Zircon Pigment Dissolution in Whiteware Coatings

In experimental whiteware coatings comprised of zircon pigment and multi-oxide silicate glass frits that incorporated ZrO₂, the quantity, size, and morphology of zircon that precipitated during "fast-firing" were mainly dependent upon the ZnO, SrO, and Al₂O₃:alkali levels. In these frits, little or no zircon pigment dissolution occurred, and zircon crystallization was nearly complete by 1000°C. Microstructures of the coatings were consistent over a range of peak firing temperatures from 1000° to 1100°C. Fritted ZnO in these coatings stimulated zircon crystallization and produced high opacity. Replacing ZnO with SrO in the same frits prevented zircon from precipitating and resulted in transparent coatings. In frits without ZrO₂, significant zircon pigment dissolution and crystallization of calcium-based silicates occurred during firing. In these unstable coatings, crystallization and dissolution increased from 1000° to 1100°C and produced a range of microstructures. Raising the Al₂O₃:alkali ratio caused frits with ZrO₂ to precipitate more zircon, and frits without ZrO₂ to form less of the calcium-based silicates and dissolve less pigment.     

Space-Charge Development in Glass

Data are given that graphically demonstrate space-charge build-up in a glass containing alkali and in a glass relatively free of alkali. The observed potential distributions show a decided asymmetry and have large potential drops near the electrodes with a linear potential change near the center of the sample. The observations qualitatively fit the expected theoretical distributions, according to Proctor and Sutton, for material having cations mobile and anions essentially immobile. Crude estimates arising from comparison with the theory yield carrier concentration near 2 × 10¹⁶ per cm³. mobility of the cation near 3 × 10^-6 cm.² per volt second, and diffusion constant near 2 × 10^-6 cm.² per second. These mobility and diffusion values are two orders of magnitude larger than values computed from conductivity data or computed from measurements of the diffusion coefficient for sodium cations, both in simple alkali silicates. According to the theory, these higher values are perhaps reasonable, since they are associated with only the more mobile portion of the cation population.     

Products of Reaction Between PbO and Nb2O5 in Molten KCl or NaCl

Lead oxide and niobium oxide were heated in molten KCl or NaCl to examine the participation of chlorides in the reaction between the oxides. Alkali ions partially replace Pb ions in PbNb₂O₆, and Cl ions form PbCl₂. The substitution of alkali ions for Pb stabilizes the tetragonal form ofPbNb₂O₆. The reactivity of NaCl with Nb₂O₅ was much greater than that of KCl; the extended substitution resulted in the formation of an NaNbO₃ phase with incorporated Pb .     

Atomic Force Microscopy and X-ray Photoelectron Spectroscopy Investigation of the Onset of Reactions on Alkali Silicate Glass Surfaces

Atomic force microscopy was used to measure forces acting on a sharp tungsten tip as it was brought into contact with silica and 30 mol% binary alkali silicate glasses. Experiments were performed in controlled atmospheres and under vacuum. Attractive forces and liquid-layer thicknesses were found to vary markedly between the glasses, and heterogeneity was observed on the binary alkali silicates analyzed in vacuo . Air or wet carbon dioxide exposure resulted in the penetration of the tip into a soft surface layer on the alkali silicates. In addition, liquid layer formation on the alkali silicates was found to be promoted by exposure to water vapor in the order lithium < sodium < potassium. X-ray photoelectron spectroscopy indicated that reaction between the potassium silicate surface and water vapor occurred on exposure to only 10⁻⁴ torr (1 torr = 1.33 × 10² Pa) water. Surface segregation and leaching of potassium occurred under the same conditions.     

Surface and Bulk -OH Infrared Absorption in ZrF4- and HfF4-Based Glasses

Infrared absorption at the 3400 cm⁻¹ -OH peak has been measured as a function of thickness for several ZrF₄-BaF₂-LaF₃ and HfF₄-BaF₂LaF₃ glasses to separate contributions from bulk and surface -OH. For glasses melted under CCl₄ reactive atmosphere, the peak is due almost entirely to surface -OH. In ambient atmosphere the -OH peak exhibited no time dependence over a 30 d period, indicating a very small rate of surface attack by atmospheric H₂O.     

Acid Durability of Sodium and Potassium Galliosilicate Glasses

The relative chemical durability in nitric acid of three series of glasses in the (Na,K)₂O-Ga₂O₃-SiO₂ system have been studied. In these compositions, gallium is proposed to behave as either a network modifier or former depending on the gallium to alkali ion ratio of the glass. The mechanisms of acid attack as found in the present work are postulated to be a combination of ion exchange and network dissolution dependent upon the glass composition/structure. This results in complex trends in the acid dissolution of these glasses as functions of the gallium to alkali ion ratio, and silica content.     

Oxygen Diffusion Coefficients in Alkali Silicates

Oxygen self-diffusion coefficients in molten alkali silicates were measured by the technique of heterogeneous isotopic exchange with a gaseous phase enriched in ¹⁸O. For the composition 64 wt% SiO₂-36 wt% K₂O, the diffusion coefficients from 700° to 1000°C under 100 torr O₂ pressure are described by The effect of pressure on D _O*, studied for the same composition at 900°C under O₂ pressures of 20 to 400 torr, is described by D _O*= kP _O2^{−(0.44±0.09)}. For the composition 75 wt%SiO₂-25 wt% K₂O, the diffusion coefficients from 750° to 1000°C under 100 torr O₂ pressure are described by The effect of pressure on the self-diffusion coefficients can be explained by a diffusion mechanism involving O vacancies.     

Pentacoordinate Silicon Complexes as Precursors to Silicate Glasses and Ceramics

The recent discovery that pentacoordinate alkali glycolato silicates such as monomeric MSi(OCH₂CH₂O)₂OCH₂-CH₂OH or dimeric M₂Si₂(OCH₂CH₂O)s (where M = Li, Na, K, or Cs) can be synthesized directly from SiO₂, ethylene glycol, and MOH suggests that these compounds may serve as useful, inexpensive precursors to alkali silicate glasses or ceramics, either by sol–gel processing or by simple pyrolysis. We report here studies on the chemical changes and phase transformations that occur during the pyrolytic transformation of these compounds to ceramic and glassy materials. The evolutionary processes encountered as the materials are heated to selected temperatures were followed by X-ray powder diffraction, diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and thermal analysis techniques. In general, the crystalline compounds oxidatively decompose at temperatures above 300°C to form amorphous materials. At higher temperatures, the line compounds M₂Si₂O₅ begin to crystallize. During the transformation process, ligand oxidation forms CO₂ and H₂O, which react with the alkali metals to form small amounts of carbonates as seen by DRIFTS. At higher temperatures, the carbonates decompose with coincident formation of the primary crystalline phase, except in the case of the potassium compound, which exhibits some phase segregation.     

Chemical Interactions Between Cement and E-Glass Fibers with a CaO–BaO–SiO2–TiO2 Coating

E-glass fibers were coated with a 15CaO–15BaO–20SiO₂–50TiO₂ thin film by the sol–gel method. Mechanical and chemical tests were performed on coated and uncoated fibers in cement and cement extract solutions to investigate the interactions between cement and gel-glass film. The results show that the resistance of E-glass fibers to the alkali cement medium is enhanced by the 15CaO–15BaO–20SiO₂–50TiO₂ coating. The significant roles of TiO₂, CaO, and BaO in the protection fibers from the alkaline attack of cement are described. Some evidence is presented that the alkali corrosion of the coated fibers results in the formation of a thick and compact Ti film that suppresses further corrosion reaction.     

Water Vapor Corrosion Behavior of Scandium Silicates at 1400°C

The corrosion behavior of scandium silicates with different scandia contents were investigated in an atmosphere of 50%H₂O–50%O₂ water vapor with a total pressure of 1 atm at 1400°C. The weight changes of scandium silicates as a function of annealing time were recorded to illustrate the corrosion behavior of these materials. The phases of as-prepared and corroded materials were compared by using X-ray diffraction and Fourier transform infrared spectroscopy. The results indicate that the water vapor does not corrode the scandium disilicate. The weight gain during corrosion is attributed to the hydroxylation of free scandia.     

Corrosion Kinetics of Silicon Nitride in Dry Air Containing Sodium Nitrate Vapors

The gaseous alkali corrosion kinetics of Si₃N₄ were systematically investigated from 950° to 1100°C in dry air containing 0.98 vol% sodium nitrate vapors. The linear reaction rates observed at all temperatures indicate that the alkali corrosion of Si₃N₄ is interface-controlled with an activation energy of 199 kJ/mol. The overall reaction involves a complex absorption–dissolution–oxidation process and the rate-controlling step appears to be the interfacial oxidation of Si₃N₄ to SiO₂. A comparison of the reaction kinetics and mechanism between the oxidation of Si₃N₄ in pure oxygen and in the alkali-containing atmosphere is presented.     

Hydrothermal Corrosion of Pure, Hot Isostatically Pressed Silicon Nitride

The aqueous corrosion behavior of hot isostatically pressed Si₃N₄ (HIPed-Si₃N₄) without additives was studied under hydrothermal conditions at 300°C and 8.6 MPa (86 atm). The accelerated weight loss in the HIPed-Si₃N₄ was attributed to uniform thinning of the specimen accompanied by dislodgement of Si₃N₄ grains from the substrate due to preferential attack at grain boundaries. Enhanced attack at grain boundaries was due to the presence of amorphous SiO₂ from impurities in the starting powder.     

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⁻).     

Substituted Hydrated Calcium Silicates Obtained in Autoclave Hydration

Hydrated calcium silicates containing Al^3+] or Fe^3+] were prepared by autoclaving C₃S and β-C₂S in the presence of C₃A or C₂F at 190°C. Al^3+] and Fe^3+] diffuse into the crystal lattice of α-C₂SH and C₃SH_1.5. Solid solutions containing Al^3+] and Fe^3+] were placed in contact at 25°C with sources of sulfates, either in aqueous stirred suspensions or as pastes. Al^3+] and Fe^3+] remain stable in the solid solution, inhibiting the formation of ettringite. This absence of ettringite can explain the resistance of autoclaved cement pastes and concretes to sulfate attack.     

Preparation of Dicalcium Siliciate at 950°C

β-Ca₂Si0₄ can be obtained from a mixture ofCaC₂O₄-H₂O and amorphous silica by firing at 950°C as opposed to a normal sintering temperature around 1450°C. If CaCO₃ is used instead of CaC₂O₄·H₂O, four repeated firings under CO₂ atmosphere are needed to obtain β-Ca₂SiO₄. The role of CO₂ atmosphere during firing and the influence of specific surface of reactanm on the rate of reaction are discussed.     

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.     

Translucent Polycrystalline Alumina with Improved Resistance to Sodium Attack

Reaction of sodium with polycrystalline alumina (PCA) arc tubes in high-pressure sodium lamps can limit lamp performance. The rate of degradation depends on the grain-boundary diffusion of a reaction product, aluminum, through the PCA wall. The effects of sintering aids and other dopants on the sodium resistance of PCA are investigated via accelerated lamp tests and microstructural analyses. Key material parameters for sodium resistance are the spinel (MgAl₂O₄) second phase and the grain-boundary MgO level. Eliminating spinel in the sintered body, doping with tetravalent cations to charge-compensate the Mg²⁺ solutes, and creating a second phase to absorb MgO in situ , significantly improve the resistance of PCA to sodium attack.