Control of Interface Migration in Melt‐Infiltrated Niobium‐Doped Strontium Titanate by Solute Species and Atmosphere

Abstract

The solid/liquid interface migration of niobium‐doped strontium titanate (SrTiO3) has been investigated by using barium and calcium solute sources in the form of a Cu‐Ba‐Ca‐O liquid. The solubilities of barium and calcium in SrTiO3 that is in equilibrium with Cu‐Ba‐Ca‐O melts can be determined by measuring the solute concentration in the migrated Sr(Ba,Ca)TiO3 regions of SrTiO3 grains. No interface migration is observed for a certain composition range of Cu‐Ba‐Ca‐O infiltrant. For SrTiO3 specimens that have been sintered in air, the zero driving force for the interface migration is found at a Ba:Ca ratio of ∼0.75 in Sr(Ba,Ca)TiO3 solid solution. This value is in good agreement with a predicted value of zero coherency strain in a thin diffusional Sr(Ba,Ca)TiO3 layer on bulk SrTiO3. For the specimens that have been sintered in 5H2‐95N2, however, a zero driving force results when the ratio is ∼0.84. The difference between the value for the specimens sintered in air and that for the specimens sintered in 5H2‐95N2 is attributed to the change in defect concentration via the change in atmosphere. The estimated difference in the lattice parameters of SrTiO3 sintered in air and those of SrTiO3 sintered in 5H2‐95N2 is ∼1.8 10−3Å (∼1.8 10−2 nm). The difference in lattice parameters has been further confirmed by using synchrotron X‐ray scattering experiment, which has revealed the lattice parameter to be 3.9053 Å (0.39053 nm) in air and 3.9070 Å (0.39070 nm) in 5H2‐95N2.