Papers by Author: Yuichi Ikuhara

Abstract: SrTiO3 bicrystals with various types of grain boundaries were prepared by joining two single crystals at high temperature. By using the bicrystals, we examined their current-voltage characteristics across single grain boundaries from a viewpoint of point defect segregation in the vicinity of the grain boundaries. Current-voltage property in SrTiO3 bicrystals was confirmed to show a cooling rate dependency from annealing temperature, indicating that cation vacancies accumulate due to grain boundary oxidation. The theoretical results obtained by ab-initio calculation clearly showed that the formation energy of Sr vacancies is the lowest comparing with Ti and O vacancies in oxidized atomosphere. The formation of a double Schottky barrier (DSB) in n-type SrTiO3 is considered to be closely related to the accumulation of the charged Sr vacancies. Meanwhile, by using three types of low angle boundaries, the excess charges related to one grain boundary dislocation par unit length was estimated. In this study, we summarized our results obtained in our group.

Abstract: Structure and configuration of boundary dislocations on the low angle tilt grain boundaries in alumina were considered based on the ideas that the boundary is composed of regularly arrayed edge dislocations and that the dislocations could dissociate into partial dislocations as well as glide dislocations in bulk. Moreover, the structure of the dissociated boundary dislocations were evaluated by the calculations based on an elastic theory. The calculations indicated that the largeness of the stacking fault region between partial dislocations formed by the dissociation will decrease with increasing tilt angles. It can be said that the idea and calculations used here will be powerful in considering the dislocation structure of low angle tilt grain boundaries that are not or are difficult to be identified.

Abstract: Dislocation structure of {1120}/<1100> 2º tilt grain boundary in alumina was observed by transmission electron microscopy (TEM). The grain boundary consisted of periodical array of basal dislocations, which were dissociated into pairs of 1/3<1010> and 1/3<0110> partials with {1120} stacking-fault in between. The relationship between the separation distance of partials and the stacking-fault was modeled by considering the force balances of periodical dislocations. The estimated stacking-fault energy for 2o tilt grain boundary was consistent with the previous reports.

Abstract: High temperature creep and superplastic flow in high-purity oxide ceramics such as alumina and tetragonal zirconia polycrystals is very sensitive to a small amount of doping by various oxides. High-resolution transmission electron microscopy and an energy-dispersive X-ray spectroscopy analysis revealed that grain boundaries in high-purity oxide ceramics are free from amorphous phase, and that the doped cations tend to segregate along the grain boundaries. Since the high temperature plastic flow in oxide ceramics takes place mainly by grain boundary matter transport by atomic diffusion, the grain boundary nano-structure control must be a useful way to develop new high-performance functional ceramics in the near future.

Abstract: Grain boundary diffusion coefficients of oxygen (δDgb) at 1793K in high purity α-alumina bicrystals with Σ7{2 _ ,310}/[0001] and Σ31{7 _ ,1140}/[0001] symmetric tilt grain boundaries were measured by means of the isotopic exchange and diffusion depth profiling using SIMS. δDgb of both grain boundaries were determined to be 7.1x10-24 [m3/sec] for Σ7 grain boundary and 5.3 x10-24 [m3/sec] for Σ31 grain boundary, respectively. These results indicate that Σ values do not directly relate to grain boundary diffusion properties.

Abstract: Superplastic behaviors were investigated for fine-grained yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) from a viewpoint of GeO2 or TiO2 doping. It was found that both dopants enhance the ductility in TZP. In particular, elongation to failure of 988% could be obtained in 3Y-TZP co-doped with 2mol%GeO2 and 2mol%TiO2. In addition, it was revealed that lower flow stress did not always give a larger elongation in this system. On the other hand, the strong segregation of dopants along grain boundaries was confirmed by high-resolution electron transmission microscopy study (HRTEM) with X-ray energy dispersive spectrometer (EDS). The unique phenomena observed in the relation between flow stress and ductility is closely related to the strong segregation of dopants along grain boundaries in this system. The grain size at fracture is determined by covalency because of the dopant segregation. It could be concluded that elongation to failure is closely related to a balance between grain size at fracture and grain growth rate during deformation.

Abstract: Our recent studies on current-voltage characteristic and grain boundary structure in ZnO bicrystals are reviewed in this paper. All types of undoped ZnO bicrystals showed Ohmic characteristics. This indicates that nonlinear current-voltage characteristic cannot be generated solely by the atomic disarrangement in undoped ZnO. On the other hand, co-doped ZnO bicrystals showed nonlinear current-voltage characteristics, which depended on the types of grain boundary. It is considered that the nonlinearity depends on the Pr concentrations at the respective grain boundaries.

Abstract: Interfacial atomic and electronic structures of Cu/Al2O3(0001) and Cu/Al2O3(11 ＿ ,20) prepared by a pulsed-laser deposition technique were characterized by high-resolution transmission electron microscopy (HRTEM) and electron energy-loss spectroscopy (EELS). It was found that both systems have O-terminated interfaces, irrespective of different substrate orientations. This indicates that Cu-O interactions across the interface play an important role for the Cu/Al2O3 systems.