Grain boundaries, interfaces and surfaces are very attractive areas from the viewpoint of lattice discontinuity. Doped elements and point defects often segregate there, which largely changes bulk properties such as microstructure, mechanical strength, electrical conductivity. To understand the details of the dopant effects, it is necessary to use nano-scale analysis techniques based on transmission electron microscopy with energy dispersive X-ray spectroscopy and electron energy-loss spectroscopy with nano/sub-nano electron probe. This review contains some nano-scale analysis studies performed in our research group for the grain size control of BaTiO₃ polycrystals, electrical properties across single grain boundaries of n-type BaTiO₃, SrTiO₃ and ZnO, interface structural change of WC–Co based cemented carbides and the preparation of novel dislocation nano-wires.
Grain boundaries in BaTiO₃ sinters were revealed to exhibit grain boundary faceting by doping a very small amount of excess TiO₂. The formation of the facets due to extra Ti–O₂ bonding is closely related to abnormal grain growth, which is often observed in TiO₂-excess BaTiO₃ sinters. A similar faceting feature is also observed in VC-doped WC–Co cemented carbides. Doped VC strongly segregates to WC/Co interfaces to form micro facets. The formation of the micro facets resulted in WC grain size reduction. In ZnO boundaries, doped Pr was revealed to segregate at the grain boundary. The segregated Pr ions are situated in specific atomic columns. Further, electron energy-loss spectroscopy has revealed that the valence of the segregated Pr ions is 3+, which means that they are not acceptors but donors to ZnO. The improvement of varistic effect observed in Pr-doped ZnO is closely related to Zn vacancies enhanced by Pr doping. Meanwhile, dislocations are a kind of one-dimensional lattice defect including extra half planes. By pipe-diffusion of dopants at dislocation cores, we can successfully develop conducting nano-wires in insulating sapphire crystals.