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LINDEMER, T. B.; ALLEN, M. D.; LEITNAKER, J. M.
doi: 10.1111/j.1151-2916.1969.tb09173.xpmid: N/A
The reaction of microspheres of UOz with graphite was studied from 1400° to 1756°C. When a spherically symmetrical layer of carbide was produced around the UO2 core, only UC2 was formed, and the diffusion of oxygen through this layer was rate‐controlling. The Arrhenius relation for this system is kD= 21.0 exp(−90, 000/RT) cm2/s The reaction of a geometrically nonsymmetrical configuration of UO2 and UC2 was also studied. Comparison of the reactions in the symmetrical and nonsymmetrical systems demonstrated that the kinetic behavior of the two systems is quite different and that the conversion in the nonsymmetrical system was 2 to 5 times faster. The importance of these observations to kinetic results reported in the literature for analogous systems is discussed.
BURGGRAAF, A. J.; VELZEN, H. C.
doi: 10.1111/j.1151-2916.1969.tb09174.xpmid: N/A
Glasses resistant to attack by sodium vapor at temperatures up to 700°C were developed from the CaO‐Al2O3‐MgO‐BaO‐B2O3 system. The resistance of these glasses decreases sharply above the transformation temperature. The glasses have a high water content, have good wetting properties, and adhere well to materials such as aluminum oxide and niobium; thus they can be used as solder glasses and metalizing components. The surface layer formed on the glass during attack by sodium vapor has a specific electrical resistance which is lower by 2 to 3 orders of magnitude than that of glass not exposed to sodium vapor.
doi: 10.1111/j.1151-2916.1969.tb09175.xpmid: N/A
The kinetics of secondary grain growth in a Ti02‐nucleated β‐spodumene solid‐solution glass‐ceramic was studied. The thermal stability of the grains was excellent. Grain growth followed the cube‐root‐of‐time law. The activation energy of the grain boundary migration was 55 ± 10 kcal/mol. Grain growth inhibition due to Ti02 precipitates and the residual glassy phase was closely examined. The excellent thermal stability of the grains is due to grain growth inhibition by the residual glassy phase, not by rutile precipitates. It is suggested that the diffusion of A2+, and probably the simultaneous diffusion of Li+, through the residual glass is the rate‐limiting process for the grain boundary migration.
doi: 10.1111/j.1151-2916.1969.tb09176.xpmid: N/A
The sound velocities of polycrystalline BeO and their pressure and temperature derivatives were determined by a pulse superposition method. The derivatives found from the measurements at pressures up to 2 kbars and near 25° are: (∂ul/∂P)T= 6.48×10−3 km/s kbars, ∂S/∂P)T= 0.33×10−3 km/s kbars, ∂L/∂T)P=−2.8×10−4 km/s deg, and ∂S/∂P)T=−2.0×10−4 km/s deg. The very low value of ∂S/∂P)T is anomalous for crystalline solids but may be expected from the crystal structure in view of similar anomalous behavior found in other solids with the wurtzite structure. The Grüneisen parameter calculated from the pressure derivatives of sound velocities was small compared with the Grüneisen parameter obtained from thermal properties. This result seems to indicate that the thermal expansivity of BeO does not change with temperature in proportion to the specific heat at low temperatures.
doi: 10.1111/j.1151-2916.1969.tb09177.xpmid: N/A
New data for monticellite crystalline solutions confirm that monticellite (CaMgSiO4 is unstable at atmospheric pressure. On the join forsterite‐monticellite, the estimated range of stable monticellite crystalline solutions is 89 to 93 mol% monticellite at 1200°C and 75 to 92 mol% at 1490°C. The range of forsterite crystalline solution is 93 to 100 mol% forsterite at 1200°C and 81 t0 100 mol% at 1490°C. These results can be applied to the proportions of phases present in impure periclase refractories at service temperatures.
WESTON, T. B.; WEBSTER, A. H.; McNAMARA, V. M.
doi: 10.1111/j.1151-2916.1969.tb09178.xpmid: N/A
An investigation of the effects of small additions of Fe2O3 to lead zirconate‐lead titanate ceramics with compositions between 45 and 60 mol% lead zirconate has shown that, with 2 moles of PbO to balance each mole of Fe2O3, the solubility of Fe2O3 was about 0.8 wt% in compositions near the tetragonal‐rhombohedra1 boundary. The dielectric constant and dissipation factor for both rhombohedra1 and tetragonal materials were decreased by addition of iron oxide, whereas the mechanical quality factor and frequency constant were increased. The dependence of the electromechanical properties on grain size was qualitatively similar for both undoped and iron oxide‐doped materials; the presence of iron oxide inhibited grain growth and lowered the limiting grain size below which the electromechanical properties change rapidly with grain size.
SPEAR, KARL E.; LEITNAKER, JAMES M.
doi: 10.1111/j.1151-2916.1969.tb09179.xpmid: N/A
Twin‐crucible experiments with vanadium carbonitride solutions in equilibrium with carbon and nitrogen gas gave evidence for the formation of carbon in a state more energetic than graphite. These studies show that the V(C, N) lattice parameter depends on whether the starting material was vanadium carbide or a mixture of vanadium nitride and graphite. The postulate that a high‐energy form of carbon may result from any decomposition or reaction leading to free carbon is discussed relative to metal carbide evaporation and to the decomposition of uranium dicarbide to the sesquicarbide and carbon.
EPPLER, R. A.; SPENCER‐STRONG, G. H.
doi: 10.1111/j.1151-2916.1969.tb09180.xpmid: N/A
Phosphorus pentoxide is known to affect the reflectance and color of TiO2‐opacified porcelain enamels. It has been postulated that P2O5, impedes the anatase‐to‐rutile phase transformation. This hypothesis was tested by applying recently developed analytical procedures based on X‐ray diffraction analysis to a typical porcelain enamel cover coat system. The inversion of anatase to rutile was not significantly affected by P2O5. Instead, in the range 0.5 to 4%, as P2O5 concentration was increased in the frit, the rate of crystallization of both anatase and rutile also increased, with the rate of increase of anatase being about twice that of rutile.
doi: 10.1111/j.1151-2916.1969.tb09181.xpmid: N/A
The factors that influence the measurement of stress by the “two‐exposure” X‐ray diffractometer technique in the WC phase of WC‐Co cermets were studied. Chromium Karadiation was used on the (1012) line of WC (2θ= 135.8°). The X‐ray values of Young's modulus and Poisson's ratio are 105×106 psi and 0.19, respectively, and the X‐ray stress factor is 1.51 × 108 psi/Å. The measured stresses are biaxial. Because penetration is limited to less than 5 μm for CrKa radiation, extreme care in surface preparation and heat treatment is required. A suitable preparation is metallographic polishing with 6 μm diamond followed by annealing in hydrogen at 750°C. This treatment allows measurement of about 40, 000 psi compressive temperature stress inherent in the WC particles resulting from differences in coefficients of thermal expansion between WC and Co. Mechanical stresses can also be measured; they arise from external forces such as grinding, polishing, and sand blasting. Mechanical and temperature stresses cannot always be clearly distinguished, however.
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