Fabrication and Characterization of MgO Bicrystals
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ABSTRACT High purity bicrystals of MgO have been grown using chemical vapor transport for the purpose of studying oxygen grain boundary diffusion. Preliminary data indicate preferential diffusion of oxygen along E 13 symmetric tilt boundaries. The measured grain boundary diffusivities were approximately 4 orders of magnitude higher than the corresponding bulk values. The activation energies for bulk and grain boundary diffusion were found to be equal to within experimental error. (• 3.9eV)
INTRODUCTION There have been many diffusion studies in MgO due to the fact that it is a stable, high melting compound with a simple structure and little deviation from stoichiometry. Moreover, large single crystals have been available for many years. The first quantitative study of oxygen diffusion in MgO was performed by Oishi and Kingery [1]. Crushed MgO single crystals that had been annealed in an 180 atmosphere were subsequently reduced to CO 2 . The relative amounts of the stable isotopes 160 and 180
were determined by gas phase mass spectrometry. This method was not precise however as it required assumptions about the size and shape of the particles to make the mathematics of analysis tractable. Systematic studies of grain boundary diffusion in MgO are both scarce and at present not well understood. Two early studies of oxygen grain boundary diffusion done by Condit [2] and McKenzie [3] used the techniques of proton activation and audioradiography to examine the penetration of the stable isotope 80 along the grain boundary in Fe doped MgO bicrystals. The crystals were grown from the melt and annealed in an atmosphere of 180. In both cases the audioradiography revealed 180 penetration along the grain boundary to be much deeper than in the bulk region. A study of Ni 2+ diffusion in MgO polycrystals and pressure sintered bicrystals by Wuensch and Vasilos [4] revealed enhanced transport at the boundaries. A subsequent study of Ni 2+ and Co 2 + grain boundary diffusion in pressure sintered bicrystals was done by the same authors [5]. As in the previous study, fast diffusion was observed along 637 Mat. Res. Soc. Symp. Proc. Vol. 318. @1994 Materials Research Society
the grain boundaries. In both cases, though, the enhanced transport was correlated with the presence of impurities segregated from the bulk, most notably Ca, Si and Fe. In two more recent studies of oxygen grain boundary diffusion performed by Dolhert [6] and Roshko [7], doped and undoped polycrystals and deformed single crystals of MgO were analyzed with secondary ion mass spectrometry after annealing in an atmosphere of 180. Both studies confirm enhanced diffusion of oxygen along grain boundaries but the results are again clouded somewhat by the mitigating presence of segregated impurities and second phases at triple junctions. High purity boundaries are clearly needed to permit elucidation of structure-property relations of grain boundaries. High purity MgO single crystals have been prepared by Gruber [8] and Yoo [9] using chemical vapor transport.' It is the g
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