Journal of the American Ceramic Society

COLLEPARDI, MARIO; MASSIDDA, LUIGI

doi: 10.1111/j.1151-2916.1973.tb12450.xpmid: N/A

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.

PRABRIPUTALOONG, K.; PIGGOTT, M. R.

doi: 10.1111/j.1151-2916.1973.tb12451.xpmid: N/A

The bulk reaction of Al and SiO2 was reexamined in light of results of thin‐film work. When the bulk reaction was conducted under vacuum in such a way that the oxide film on the Al was almost completely absent, the reaction started immediately and proceeded at a rate more rapid than that observed by previous workers, who conducted the experiment in air at atmospheric pressure. Results obtained at 700° to 820°C indicate that the activation energy for the reaction is 42±5 kcal/mol when oxide films are absent from the surface of the molten Al.

EYSEL, W.; WOLFE, R. W.; NEWNHAM, R. E.

doi: 10.1111/j.1151-2916.1973.tb12452.xpmid: N/A

The compound Pb5Ge3O11 is an optically active ferroelectric with a transition temperature near 177°C. Solid solutions containing up to 62% Si replacing Ge were prepared from the melt. Increasing the Si content lowers the melting point, Curie point, and coercive field. Permittivity measurements on single crystals indicate a simple second‐order phase change.

YEE, J.; KRÖGER, F. A.

doi: 10.1111/j.1151-2916.1973.tb12453.xpmid: N/A

When guards are used to eliminate surface and gas conduction in emf measurements, thermocouple effects of the electrodes and guards must be taken into account, and unwanted gradients in gas composition must be eliminated. Results for single‐crystal and polycrystalline Al2O3 are given. Single crystals have an ionic transference number, t̄l of ≅1, with a possible small decrease as a result of electronic (hole) conduction at T>1450°C. Polycrystalline samples exhibit ti values of <I, with tel(=1‐tl) increasing with decreasing temperature and increasing O2 pressure. The electronic conduction results from movement of holes along grain boundaries.

WIEDERHORN, S. M.; JOHNSON, H.

doi: 10.1111/j.1151-2916.1973.tb12454.xpmid: N/A

Crack velocities in glass in various acids, bases, and neutral solutions were studied using the double‐cantilever‐beam technique. Results are explained in terms of crack‐tip pH, which is controlled by the electrolyte at low crack velocities and by the glass composition at high velocities. The crack‐velocity data are consistent with the known dependence of strength on pH for soda‐lime silicate glass. Results also suggest that the slope of the universal fatigue curve should depend on surface pH.

NAVROTSKY, A.; KLEPPA, O. J.

doi: 10.1111/j.1151-2916.1973.tb12455.xpmid: N/A

The enthalpies of solution in molten 2PbO·B2O3 at 690°±2°C of cordierite, Mg2Al4Si5O18, and of a glass of cordierite composition were determined. From these data, the enthalpy of formation of cordierite from the component oxides is ‐16.1±1.7 kcal/mol and its heat of fusion +54.0±2.0 kcal/mol.

LEIPOLD, M. H.; KAPADIA, C. M.

doi: 10.1111/j.1151-2916.1973.tb12456.xpmid: N/A

Anion impurities (S2−, Cl−, F− and OH−) are shown to be deleterious to the densification of MgO by hot‐pressing. The magnitude of the effect decreases in the order in which the anions are listed and is reduced as temperature increases. The effect is suggested to result from gas pressures within the compact and die cavity, and the magnitude of interference with densification is related to both the vapor pressure of the impurity and its rejection from the lattice (insolubility).

SCHEIDECKER, RALPH; BERARD, M. F.

doi: 10.1111/j.1151-2916.1973.tb12457.xpmid: N/A

Interdiffusion coefficients were determined in single‐crystal (Ca,Sr)F2 solid solutions in the temperature range 1100° to 1320°C. The composition dependence of the coefficients passes through a maximum that shifts toward higher CaF2 concentrations at higher temperatures. Extrapolation of the interdiffusion coefficient to extremes of composition provides impurity diffusion coefficients that agree well with tracer results. The changes in activation energy with composition can be explained readily on the basis of cationic radii and lattice parameters.