Raman Study of Isothermal Devitrificatn Kinetics of NaPO3 Glass

A laser Raman spectroscopic method was developed by which the increase in crystallinity of a devitrifying glass can be monitored continuously and isothermally at the devitrification temperature, without sample quenching, in a molecularly specific manner that yields high-quality kinetic data for the glass-crystal transformation. Applied to the study of vitreous (NaPO₃)_n( gl ), the method yielded continuous data which were fitted, by a nonlinear least-squares method, to an equation of the form α( t ) = 1- exp (- k_Nt^N ) with sufficient precision to permit optimization with respect to both N and k_N. The values of N and k_N vary with glass thermal history, i.e. with variations in time of heating at T_g for otherwise identical samples, although the material does not phase separate. The activation energy from any plot of In ( k_N )^{1/ N} vs (1/ T ) does not vary, but the offset of the curves indicates different preexponential terms in the Arrhenius expression. The results are analyzed in terms of current theories.     

Defect Structure of NiO-CoO Solid Solutions

Measurements at high temperature of integrated intensities of Bragg peaks were used to examine the defect structure in two NiO-CoO solid solutions. The results indicate that the concentrations of vacant cation sites at 1100° to 1500°C are 4 to 10% and the tetrahedral occupations 2 to 5%. The concentration of these defects increases with temperature and partial pressure of O₂. The mean-square displacements of oxygen ions and cations increase with temperature and CoO concentration.     

Systems of Na-Fe-SiO2 Glasses and Steel: I, Wetting and Adherence

Wetting behavior of molten Na₂O-Fe₂O₃-SiO₂ and Na₂O-FeO-SiO₂ glasses on very-low-carbon steel at 900°C in purified Ar was studied. The steel surface was dry-polished and cleaned by ion bombardment. Glass frit was then placed on the metal, and the system was introduced into the furnace without exposure to air. Dynamic advancing and receding contact angles were measured. Shear (push-off) tests were performed on the glass-steel composite at room temperature to determine adherence. The equilibrium contact angles were obtained from the arithmetic mean of the final advancing and receding angles. As the Fe oxide content of the glasses increased, the degree of wetting improved; Fe³⁺ was more effective than a combination of Fe³⁺ and Fe²⁺ in improving the wettability. Glasses very nearly saturated with Fe oxide exhibited high adherence values and chemically adhered to steel.     

Swelling Control in Uranium Carbide

Fine particles were dispersed in a fuel structure to trap fission gases as very small bubbles and thereby reduce fuel swelling. The dispersions were made by adding 1.5 to 3.0 wt% W to the UC; the specimens were irradiated to a maximum of ∼2 at.% burnup. Density changes were used as the measure of swelling; there was much scatter in the results. Tungsten reduced the swelling when it was uniformly dispersed. For some samples having excess U and W or a segregated structure, W increased the swelling. The results tentatively confirm that fine particles are of value in reducing fission-gas swelling of carbide fuels.     

Strain Effects and Spalling in Alpha-Bombarded ThO2

Samples of bulk polycrystalline ThO₂ were bombarded with 5-MeV α-particles to doses between 9.4×10¹⁶ and 6.0×10¹⁷ ions/cm². The sample which received the highest dose spalled during bombardment; those receiving lower doses either did not spall or did so only after postirradiation annealing. The spalling was investigated by X-ray analysis and replica and transmission electron microscopy. It is concluded that spalling resulted from severe lattice strains at the interface between damaged and undamaged material and that sintering pores played a part in the fracture process. The role of lattice defects in initiating fracture is discussed.     

Effect of Aspect Ratio and Viscosity Gradients on Flow Through Open Channels

The effect of aspect (height to half width) ratio on flow through open channels is considered by evaluation of St. Venant's solution. Near the center line, flow through moderately wide channels is unatrected by the side walls. Therefore, analytical solutions for the single-dimension case of flow above a flat plate are determined. They reveal that until viscosity differentials are extreme, the velocity distribution is relatively insensitive to viscosity gradients. With several simplifying assumptions the temperature distribution and the corresponding viscosity field in a cross section of the melting end of a glass tank can be calculated. This permits a numerical solution of open-channel flow for conditions appropriate to a glass tank. Calculated temperature and velocity distributions are presented for three values of the ratio of effective thermal conductivity of glass to thermal conductivity of refractory. Applications of this type of information to glass-tank technology are briefly discussed.     

Lanthanum Dependence of Elastic and Piezoelectric Properties of PLZT Ceramics with a Zr/Ti Ratio of 65/35

The elastic and piezoelectric properties of PLZT ceramics with a 65/35 Zr/Ti ratio undergo pronounced changes as La is added to the structure. At the rhombohedral-tetragonal phase boundary, material constants plotted as functions of La concentration show strong discontinuities in slope; these discontinuities provide a precise means of locating the boundary. The radial-mode coupling factor, k_p , compliance coefficient, S₁₁ ^E , and piezoelectric coefficient, d₃₁ , attain peak values at the boundary, whereas Poisson's ratio, σ^E, and the resonance-frequency constant exhibit pronounced minima. This behavior is evidence of the increasing ferroelastic sensitivity of the material as the distortion angle of the rhombohedral phase approaches a minimum.     

A STUDY ON THE PERFORMANCE OF THE CATALYTIC POROUS-WALL THREE-PHASE REACTOR

A theoretical investigation of a catalytic porous-wall reactor in which gaseous and liquid reactants approach each other from opposite sides of the catalyst is undertaken. Equations for the annular liquid-channel are coupled with those for the catalytic wall and solved numerically and analytically using a simplified model. For the model reaction under study, the main design and operation parameters which affect reactor performance are the Thiele modulus, Peclet number, width of the liquid channel and the inlet concentration of the reactant in the liquid phase.

The effect of reactor configuration is peculiar to the cylindrical geometry because the thickness and relative location of the catalytic wall as well as the selection of the liquid and gas channels can influence the reactor performance. Thin-walled catalyst tubes have larger effectiveness factors and as the tube radius approaches that of the reactor, conversion in the reactor increases especially when the liquid is saturated with the gaseous reactant. Concentration of the liquid reactant in the feed has a significant effect if the reactant is depleted at some point inside the catalyst wall. Since the reaction zone width can be adjusted by changing the feed composition, this might have important implications with respect to selectivity.     

Kinetics of Opacification of a TiO2-Opacified Cover Coat Enamel

Quantitative X-ray diffraction measurements were used to study the firing behavior of a typical TiO₂-opacified cover coat enamel. A model was developed for predicting the anatase and rutile concentrations as a function of time and temperature of firing. The model consists of three first order reactions: crystallization of anatase, crystallization of rutile, and inversion of anatase to rutile. It predicts successfully over a wide range of time-temperature conditions. Discrepancies are found, however, for short-time, low-temperature firings. The reflectance data do not correlate well with the crystalline concentration data. The correlation with color data is better, but is not perfect.     

Dissolution in Ceramic Systems: III, Boundary Layer Concentration Gradients

By use of electron microbeam probe analysis on quenched samples, the concentration distribution of CaO, A1₂O₃, and SiO₂ was determined across the boundary layer between molten calcium aluminum silicates and dissolving or growing sapphire and fused silica. A definite shift in the concentration ratios of the solvent components was found near the interface. Analysis of diffusion flux equations for a ternary system successfully related the shift in concentration ratio to the intrinsic diffusion coefficient for each component. For alumina dissolution in a melt rich in CaO, evidence of incongruent dissolution was observed with the formation of new phases, CaO· 6Al₂O₃ and CaO· 2A1₂O₃.