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

The solid/liquid interface migration of niobium‐doped strontium titanate (SrTiO 3 ) 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 SrTiO 3 that is in equilibrium with Cu‐Ba‐Ca‐O melts can be determined by measuring the solute concentration in the migrated Sr(Ba,Ca)TiO 3 regions of SrTiO 3 grains. No interface migration is observed for a certain composition range of Cu‐Ba‐Ca‐O infiltrant. For SrTiO 3 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)TiO 3 solid solution. This value is in good agreement with a predicted value of zero coherency strain in a thin diffusional Sr(Ba,Ca)TiO 3 layer on bulk SrTiO 3 . For the specimens that have been sintered in 5H 2 ‐95N 2 , 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 5H 2 ‐95N 2 is attributed to the change in defect concentration via the change in atmosphere. The estimated difference in the lattice parameters of SrTiO 3 sintered in air and those of SrTiO 3 sintered in 5H 2 ‐95N 2 is ∼1.8 10 −3 Å (∼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 5H 2 ‐95N 2 .