Surface diffusion study of single crystal and bicrystal ceramics by AFM

本研究中使用的一组样品是双晶MgO，其表面和晶界向[010]方向倾斜24^0。样品经AFM检测后，在1100-1400℃的温度下加热，再进行AFM检测。晶界宽度(w)与t^<1/4>成正比，其中t为退火时间，表明表面扩散控制了这一过程。结果表明，原子力显微镜可以在较低的退火温度和较短的退火时间下研究晶界沟槽。用这种方法测定的表面扩散系数与文献中干涉显微镜测定的数据吻合较好。利用光刻技术和后续退火技术，在MgO单晶（100）平面表面形成了正弦曲线。然后研究了这些试样的正弦曲线衰减。计算得到的扩散系数比晶界开槽法计算得到的扩散系数快几个数量级。这说明这两个过程的机制是不同的。还研究了与MgO具有相同取向的SrTiO_3双晶。与MgO相反，SrTiO_3形成凹槽后的表面横截面与Mullins预测的完全一致。在测量中还观察到SrTiO_3的晶界开槽是受表面扩散控制的。由于干涉显微镜无法测量凹槽高度h，因此对h的测量是AFM所独有的。在SrTiO_3中，由晶界开槽确定的表面扩散与氧晶格扩散几乎在同一范围内，但比Sr晶格扩散快。虽然还不知道Ti在SrTiO_3中的扩散系数，但目前的结果是慢速移动阳离子的表面扩散。

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.