Surface diffusion study of single crystal and bicrystal ceramics by AFM

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One group of the samples used in this study was bicrystal MgO which was made with (100) surface and grain boundary tilted 24^0 in [010] direction. After examining by AFM,the samples were heated at 1100-1400ﾟC and reexamined by AFM.The grain boundary width (w) is proportional to t^<1/4>, where t is the annealing time, implying that surface diffusion controls the process. It was found that AFM made it possible to study grain boundary grooving at lower temperatures and short annealing time. The surface diffusion coefficient determined in this way was in good agreement with the data in the literature determined by interference microscope.Using photoetching technique and subsequent annealing, sinusoidal profile was formed on (100) plane surface of MgO single crystals. The sinusoidal profile decay was then studied on these specimens. The diffusion coefficients calculated from the results were several orders of magnitude faster than the results obtained by grain boundary grooving. This suggests that mechanism is different in these two processes.Bicrystal SrTiO_3 with the same orientation as bicrystal MgO was also studied. Contrary to MgO the cross section of the surface of SrTiO_3 after the formation of groove was exactly the same as that predicted by Mullins. It was also observed in the measurement that grain boundary grooving in SrTiO_3 was controlled surface diffusion. This was supported by the measurement of groove height, h. Since h can not be measured by interference microscope, the measurement of his unique to AFM.The surface diffusion in SrTiO_3 determined by grain boundary grooving is almost in the same range as the oxygen lattice diffusion in SrTiO_3 but is faster than Sr lattice diffusion. Although the diffusion coeffcient of Ti in SrTiO_3 is not known, the present result is the surface diffusion of slower moving cation.