Oxygen diffusion along the short-circuit paths in bicrystal SrTiO 3
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Oxygen diffusion in bicrystal SrTiO3 was investigated with the aid of resistive anode encoder of secondary-ion mass spectrometry. The diffusion profiles of 18O on volume, grain boundary (joining interface), and dislocation were separately determined from the three-dimensional distribution of 18O in the bicrystal SrTiO3. The volume diffusion of oxygen is found to be dependent on (i) Nb doping concentration, (ii) crystallographic orientation, and (iii) nonequilibrium oxygen defects that are annihilated by thermal annealing. The feature of 18O diffusion image along the dislocation in (100) and (110) bicrystals can be explained by the dislocation array introduced by the mechanical polishing.
Strontium titanate (SrTiO3) is widely known as an oxygen sensor material at high temperature. The response time depends on oxygen bulk diffusion and surface exchange processes. The defect chemistry of SrTiO3 has been studied intensively.1,2 The undoped SrTiO3 normally shows acceptorlike behavior because the most common impurities (iron, aluminum, and nickel) act as acceptors. The oxygen vacancy concentration is determined by the concentration of the acceptors. In contrast to this, donor dopants (such as lanthanum and niobium) are charge compensated by electrons. It follows that donor dopants reduce the oxygen vacancy concentration. Due to these reasons, the oxygen diffusion dependence on the oxygen vacancy was measured in polycrystalline SrTiO3 with the various dopants3,4 and in the single crystal as well.5–7 Dislocations can act as a short-circuit path for oxygen diffusion in single-crystal SrTiO3, and have been shown to play a significant role in the redox reaction.8 However, the grain-boundary and dislocation diffusion in SrTiO3 has not been well understood yet. Previously, the grainboundary diffusion was studied in oxygen materials such as MgO,9 MgAl2O4,10 and NiO.11 The grain-boundary diffusion was strongly dependent on the grain-boundary orientation and its mechanism was indicated to be dislocation pipe mechanism. The polycrystalline form is not suitable as a substrate for investigating orientation effect on grain-boundary diffusion due to the difficulty in defining grain-boundary orientation. Recently, our coworkers developed a new technique for producing bicrystal,12 and grain boundary orientation became clear. On the determination of grain-boundary diffusion profile, secondary ion-mass spectrometry (SIMS) is a technique well suited for the analysis of diffusion.13 Recently, the 2598
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J. Mater. Res., Vol. 15, No. 12, Dec 2000 Downloaded: 11 Mar 2015
study of diffusion using the secondary ion image of SIMS was reported.14 It is indicated that this technique is useful for the anisotropy of grain-boundary diffusion and impurity segregation. In this study, the oxygen diffusion profiles of grain boundary joined in the same planes in SrTiO3 and of mechanical damages were determined with the aid of a resistive anode encoder system of SIMS. The oxygen diffusion mechanism in SrTiO3 is discussed from
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