Diffusion coefficients for transition metal ions in single-crystal MgO have been determined in the temperature range 1000°-1850°C. from concentration distributions obtained with the aid of x-ray absorption analysis and electron microbeam probe spectroscopy. The variation of the diffusion coefficients with temperature yields D0 = 8.83 (10)-5, 5.78 (10)-5, and 1.80 (10)-5 cm2/sec and activation energies for diffusion of 1.81, 2.06 and 2.10 electron volts for Fe2+, Co2+, and Ni2+, respectively. It is found that the activation energy for diffusion increases exponentially with the ratio ionic radius/ionic electronic polarizability for the diffusing cation and that D0 increases exponentially with the cation radius cubed. This correlation suggests that the diffusion mechanism for all three ions is the same and that self-diffusion in MgO takes place via the same process.

An excess absorption has been observed in castor oil by pulse propagation measurements. It is most pronounced at the higher temperatures and the lower frequencies of the range investigated.

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Preferential diffusion of Ni2+ and Co2+ along grain boundaries was observed in certain bicrystals of MgO. This enhancement is attributed to impurity segregation at the boundary. The identified impurities responsible for the effect are the principal impurities in the single-crystal MgO: Ca, Si, and Fe. No enhancement was observed in any bicrystal prepared above 130O0C, a temperature similar to that at which studies of the mechanical properties of MgO have implied a reabsorption of impurity precipitates into solid solution. It is concluded that enhanced grainboundary diffusion of cations in MgO is an extrinsic, rather than an intrinsic, property of the boundary.* Received from the Norton Company, Worcester, Mass.soon discovered, however, that the effect was extrinsic. The origin of the effect could successfully be interpreted through examination of the distribution of impurities in the specimens. 13-89 (1961).Ways of describing and defining progress of fining or the standard to be achieved are discussed. Methods of measurement are also discussed. Published quantitative data which can be used to test proposed hypotheses or theories are reviewed. Finally, processes known to be important in eliminating gas bubbles from liquids, namely rise and bursting at the surface, coalescence, and growth or dissolving are discussed.

Grain-boundary diffusion of Ni2+ was investigated in polycrystalline MgO and also in isolated grain boundaries in natural and prepared bicrystals of MgO. Concentration distributions were determined with the aid of both electron microbeam probe spectroscopy and X-ray absorption analysis. Results from diffusion couples in which a surface was maintained at constant concentration during the diffusion annealing indicate that grain-boundary diffusion is the predominant transport mechanism in polycrystalline M g O at temperatures below 1700°C. Lattice diffusion becomes increasingly important at higher temperatures and eventually becomes the dominant mechanism. Concentration distributions for diffusion couples in which a fixed amount of NiO was supplied to the surface of the couple as a thin initial plating resemble those for lattice diffusion but yield anomalously high values for apparent lattice diffusion coefficients. Grain-boundary diffusion in MgO is not confined to a layer of atomic dimensions but extends over a zone of the order of microns. The activation energy for grain-boundary diffusion is less than that for lattice diffusion and is between 1 and 2 ev.Grain-boundary diffusion was observed even in tilt boundaries with a mismatch as low a s 6".