Molybdenum Phosphate Glasses Containing Ag2O or K2O

Glasses in the systems MoO₃-P₂O₅-Ag₂O and MoO₃-P₂O₅-K₂O were investigated and general areas of composition for good glass formation were determined by X-ray diffraction methods. Temperature coefficients of electrical resistivity and linear expansion were determined for the glasses along with density, durability, and other thermal characteristics. Resistivity of the glasses was effectively reduced by the addition of Ag₂O.     

Ag2O-Doped Bi2Sr2Ca1Cu2Ox Superconductors Prepared via Melt-Quenching

Ag₂O-doped superconducting Bi₂Sr₂Ca₁Cu₂O _x ceramics were prepared by a melt-quenching–reheating method. It is found that the Ag₂O-doped, as-cast specimens exhibit superconductivity ( T _c= around 80 K) by heat treatment at temperatures around 800°C even in an evacuated and sealed silica glass tube, while the undoped specimens do not and vaporize by the corresponding heat treatment. Conversion of the Ag₂O-doped, as-cast specimens into superconducting ceramics when heated in an evacuated vessel is explained in terms of the oxygen donor of Ag₂O in the specimen. This finding enables us to fabricate a desired shape of superconducting Bi₂Sr₂Ca₁Cu₂O _x ceramics sealed in metals or glasses. The addition of Ag₂O to Bi₂Sr₂Ca₁Cu₂O _x melt, however, had deleterious influences on the superconducting properties ( T _c and J _c) of the resultant ceramics when obtained by heat treatment in air.     

The System Na2O-CaO-SiO2-H2O

A portion of the quaternary phase diagram for Na₂O-CaO-SiO₂-H₂O has been constructed. Plotting concentrations as their 10th roots allows compounds having solubilities which differ by several orders of magnitude to be represented on a single diagram. The compositional relationships among sodium-substituted calcium silicate hydrate, calcium-substituted sodium silicate hydrate, calcium bydroxide, a quaternary compound of approximate composition 0.25Na₂O · CaO · SiO₂· 3H₂O, sodium hydroxide monohydrate, and miscellaneous sodium silicate hydrates are presented. The quaternary diagram constructed shows the quaternary compound to exist in equilibrium with sodium-substituted calcium silicate hydrate and calcium hydroxide. Conditions in concrete pore solutions which favor the formation of this quaternary compound may also favor the occurrence of the alkali-silica reaction.     

Heavy-Water (D2O) Take-Up-Induced Lattice Expansion in the High-Temperature Proton Conductor Ba3Ca1.18Nb1.82O9−δ

Dimensional changes of the high-temperature proton conductor Ba₃Ca_1.18Nb_1.82O_9−δ, when exposed to D₂O vapor, were investigated using length-change and precision density measurements. Such information is essential for possible applications of proton conductors in solid oxide fuel cells and humidity and hydrogen sensors. A linear increase of the sample lengths with increases in the deuterium content was observed. Comparison of the present D₂O data with those that were previously obtained for H₂O showed that there was a small isotope effect in the lattice expansion. The fact that the length-change-versus-hydrogen-isotope-concentration curves were almost isotope independent supported the validity of the take-up reaction H₂O( g ) + V_Oö+ Ox_O is an ⇄ 2OH_O. In regard to the latter equation, it was found that small islands of silver, palladium, nickel, and platinum act as catalytic promoters of the reaction and lead, for given heavy-water-steam exposure conditions, to an increase of water absorption of up to 100%, when compared with samples without the catalytic surface layers.     

The Formation of Magnesium Oxychloride Phases in the Systems MgO-MgCl2-H2O and NaOH-MgCl2-H2O

The reactions leading to the formation of crystalline Mg₃(OH)₅Cl·4H₂O (phase 5), Mg₂(OH)₃,Cl·4H₂O (phase 3), and Mg(OH)₂ are compared for the systems MgO-MgCl₂-H₂O and NaOH-MgCl₂-H₂O. The crystalline phases were determined by X-ray diffraction analysis. The concentration of the total magnesium and chloride in the solution and the pH of the solution determine the reaction product(s) in both systems. The influence of MgO reactivity and the molar ratio of reactants on the formation and stability of reaction products is discussed and the mechanism of the formation of phases 3 and 5 is explained. In the system MgO-MgCl₂-H₂O, MgO serves only to increase the concentration of total magnesium and the pH of the MgCl₂ solution.     

The System CaO-Al2O3-H2O

Phase equilibria have been determined in the system CaO-Al₂O₃-H₂O in the temperature range 100° to 1000°C. under water pressures of up to 3000 atmospheres. Only three hydrated phases are formed stably in the system: Ca(OH)₂, 3CaO·Al₂O₃·6H₂O, and 4CaO·3Al₂O₃-3H₂O. Pressure-temperature curves delineating the equilibrium decomposition of each of these phases have been determined, and some ther-mochemical data have been deduced therefrom. It has been established that both the compounds CaO·Al₂O₃ and 3CaO·Al₂O₃ have a minimum temperature of stability which is above 1000°C. The relevance of the new data to some aspects of cement chemistry is discussed.     

Some Properties of the System Na2O-NaF-B2O2-H2O

It was first shown that the enamel slips which have the best suspnding characteristics contain equal amounts of Na₂O and B₂O₃ and at least a moderate amount of NaF. The solubilities of mixtures of Na₂O, NaF, and B₂O₃ were then investigated. The pH of these solutions and the primary crystalline phases separating on evaporation also were determined. The solubility data obtained at room temperature were summarized. When the solutions were evaporated, NaF was the first crystalline phase to separate from a large proportion of the mixtures investigated. It was concluded that the desirable handling characteristics of enamels whose mill liquors contain the proper proportion of Na₂O, NaF, and B₂O₃ are not due to the formation of complex salts but to the following combination of properties: (1) the presence of salts with a moderate solubility which changes very slightly with temperature, (2) a moderate pH of about 10 in a probably well-buffered solution, (3) a relatively stable crystalline material, NaF, as a primary phase, and (4) a secondary phase which crystallizes slowly with relatively little shrinkage.     

Interdiffusion Coefficients in SiO2-K2O Melts

Chemical diffusion coefficients for Si were measured in potassium silicate compositions containing from 64 to 85 wt% SiO₂; unidirectional diffusion couples were heated for 0.1 to 720 h at 600° to 1400°C. The diffusion coefficients, which were calculated by the Boltzmann-Matano method, are described for a silicate containing 70 wt% SiO₂ by:
The diffusion coefficients decreased with increasing O₂ pressure. Below 800°C, the coefficients decreased with increasing SiO₂ concentration; above 800°C, their dependence on SiO₂ concentration was too weak to detect by the methods used. On the basis of a simple structural model for the glass, it was possible, from a phenomenological analysis of the diffusion, to establish a relation between the oxygen self-diffusion coefficient and the chemical diffusion coefficient for a given composition.     

Subsolidus Relations in the System SiO2— Al2O3— P2O5

Phase relations in that part of the system SiO₂-Al₂O₅-P₂O6 below 50 mole % P₂O₃ were investigated at subsolidus temperatures. The only ternary compositions which crystallized readily were those lying close to binary compounds and no evidence for ternary compound formation was found. Solid solution between silica and aluminum phosphate, expected on the basis of closely similar lattice dimensions and physical properties, was not indicated by X-ray diffraction, refractive index, or electron-beam microprobe results. Above 80% silica and at a temperature of 1400°C, the only crystalline species was cristobalite whereas below 60% silica, only the cristobalite form of AlPO₄ was present. Coexistence of these two constituents seemed to be inhibited, possibly because of the strong glass-forming properties of the component oxides.     

Effect of Na2O/K2O Ratio on Chemical Durability of Alkali-Lime-Silica Glasses

The chemical resistance of mixed-alkali glasses to water was measured by determining the sodium, potassium, calcium, and silicon ions extracted instead of evaluating it from the alkalinity of the mixed-alkali extracts. In glasses of the composition K₂O and Na₂O 18, CaO 10, and SiO₂ 72% the minimum extraction of alkali (maximum durability) occurred for a K₂O/-Na₂O ratio of 2.6/1.0 (by weight). Based on an ionic-composition analysis of the results, an empirical index is suggested for determining the relative degree of ion-exchange and network-breakdown modes of reactions that occur when such mixed-alkali glasses are attacked by water.     

Synthesis and Characterization of Amorphous Si2N2O

Amorphous silicon oxynitride powder was synthesized by nitridation of high-purity silica in ammonia at 1120°C. The resulting material was X-ray amorphous, and its chemical characteristics were determined by X-ray photoelectron spectroscopy (XPS) and ²⁹Si nuclear magnetic resonance (NMR). The XPS analysis showed a shift to lower binding energies for the Si₂ p peak with increasing nitrogen content. Upon initial nitridation, the full width at half maximum (FWHM) of the Si₂ p peak increased, but decreased again at higher nitrogen contents, thus showing the formation of a silicon oxynitride phase with a single or small range of composition. The ²⁹Si NMR analysis showed the formation of (amorphous) Si₃N₄ (Si–N₄) and possibly two oxynitride phases (Si–N₃O, Si–N₂O₂). It is concluded that while XPS, FT-IR, and nitrogen analysis may show the formation of an homogeneous, amorphous silicon oxynitride (Si₂N₂O) phase, the formation of phase–pure, amorphous Si₂N₂O is extremely difficult via this route.     

The Structure of Some B2O3-H2O Glasses

Boron oxide glasses with compositions corresponding to B₂O₃, B₂O₃.0.42H₂O, B₂O₃.0.50H₂O, and B₂O₃.0.63H₂) have been prepared and their structures have been studied. Diffractometric X-ray scattering measurements have been made, and from these, radial distribution functions have been computed. Comparison of the radial distribution functions indicates that the fundamental triangular coordination characteristic of vitreous B₂O₃ is maintained in the water-containing glasses. Consideration of the results of complementary infrared and nuclear magnetic resonance studies along with preliminary results of physical property measurements indicates that the glasses of high water content are heterogeneous. It is probable that disordered regions of high hydrogen bond density, approaching HBO₂ in composition, are embedded in a matrix approximating vitreous B₂O₃.     

Glass Formation in the System GeO2-Bi2O3-Tl2O

The occurrence of glass in the three binary and one ternary oxide systems containing Ge, Bi, and Tl was studied by air- and water-quenching melted samples. Bulk glass occurred widely in these systems and was obtained with as little as 10 mol% GeO₂, combined with 68 mol% BiO_1.5 and 22 mol% TIO_0.5. Some glass occurred in all compositions except those with the highest Bi₂O₃ or TI₂O contents. Glass specimens were studied by differential thermal analysis to yield glass transition and crystallization exotherm temperatures; the thermal expansion coefficients and densities were also measured. Diagrams are presented to show the variation of these properties as well as λ₀, the wavelength for zero material dispersion, in the binary and ternary systems. Based on these data, it is demonstrated that ultralow-loss optical waveguides can be constructed in the 3-3.5-μm regionb.     

Infrared Absorption Due to H2O and D2O in a Fluorozirconate Glass

A comparative study of infrared absorption due to H₂O and D₂O impurities in a fluorozirconate glass (53ZrF₄·20BaF₂·4LaF₃·3AlF₃·20NaF) was carried out. The H₂O and D₂O were introduced into the glass by reaction of the surface at 260°C with H₂O and D₂O vapor entrained in a stream of N₂. Reaction with H₂O produced IR absorption bands at 2.9 μm (O–H stretch) and 6.1 μm (H₂O bend). Reaction with D₂O produced bands at 3.9 μm (O–D stretch) and 8.3 μm (D₂O bend). The ratios of the corresponding D₂O/H₂O peak frequencies are 0.74 for both the stretching and bending vibrations, in good agreement with the value of 0.727 predicted from the difference in the OH and OD reduced masses.     

Mechanical Properties of Si2N2O/SiC-Whisker Composites

The mechanical and thermal properties of Si₂N₂O/SiC-whisker composites were studied with emphasis on the effect of matrix composition and of whisker content. The fracture toughness of Si₂N₂O was remarkably improved by 90% with a concomitant 70% strength improvement by addition of SiC whiskers of only 10 vol%. Optimum mechanical and thermal properties of Si₂N₂O/SiC-whisker composites were obtained at an equimolar ratio of Si₃N₄/SiO₂, which is the stoichiometric composition for Si₂N₂O. Additional investigation concerning the Si₂N₂O-matrix/SiC-whisker interface by controlling sintering additives is necessary for further improvement of mechanical and thermal properties of Si₂N₂O/SiC-whisker composites.     

Conversion of CaO·Al2O3·10H2O to 3CaO·Al2O3·6H2O

The morphological changes accompanying the conversion of the hexagonal CaO·Al₂O₃·10H₂O phase to the cubic 3CaO·Al₂O₃·6H₂O phase were studied by scanning electron microscopy. The hydration and conversion reactions were monitored by X-ray diffraction analysis. From the micrographs, it was inferred that changes in the pore structure and the presence of large cubic crystals of questionable adhesive value were probably the principal factors responsible for the loss of strength in converted calcium aluminate cement pastes.     

Growth of α-Al2O3 Crystals by Na2O Evaporation

A technique for growing α-Al₂O₃ crystals is described in which Na₂O·11Al₂O₃ is dissolved in a liquid of composition Na₂O·4TiO₂·3Al₂O₃. Alpha Al₂O₃ is precipitated as Na₂O evaporates from the system; Na₂O·11Al₂O₃ serves as a source of Al₂O₃, and Na₂O in the liquid. The content of solids in the mixture is always such that it does not melt completely. The size of the α-Al₂O₃ crystals grown is related to the Na₂O content of the composition. Crystals as large as 4000 by 3000 μm in the α-axis direction and 500 μm in the c -axis direction have been grown.