Design of the high-temperature ceramic materials by small amount of dopant addition

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In this research the high-temperature mechanical properties in engineering ceramic materials such as polycrystalline Al_2O_3 was examined. It was found that the doping of trivalent ions such as Y or Lu is very effective to improve the high-temperature creep resistance in Al_2O_3 ceramics. On the other hand, some of divalent or tetravalent ions reduce the creep resistance in Al_2O_3. The origin of the dopant effect was revealed by using transmission electron microscopy, especially high-resolution transmission electron microscopy. By using high-resolution transmission electron microscccpy and attached EELS (electron energy loss spectroscopy) or EDS (energy dispersive X-ray spectroscopy) analysis, it has been found in the present work that the doped cations tend to segregate at the grain boundary, and change chemical bonding state at the grain boundaries. The segregation was detected in uniformly in the oxide ceramics by STEM (Scanning TEM)-EDS method. The change in the chemical bonding state was able to be quantitatively estimated by the first-principle molecular orbital calculations. For example, from the calculation of DOS (density of states in unoccupied molecular orbital) the experimental EELS spectra can be explained, and it was found that the change in the EELS means that the ionicity at the grain boundary is increased due to the dopant cation segregation. Moreover, it is revealed that the improved creep resistance in cation-doped oxide ceramics results from the increment in the atomic bonding strength. The present results would bring an important and ultimately new method to develop high-temperature ceramic materials.