Application of TEM to the Characterisation of Commercial BaTiO3 Capacitors

Abstract

Ferroelectric BaTiO3 with its high dielectric constant is a material widely used for miniature high capacitance capacitors. Pure BaTiO3 has the perovskite crystal structure (cubic) above 120°C and undergoes a phase transformation to a ferroelectric state at 120°C: the electric dipole is created as the Ti+4 ion shifts slightly along a <100> direction, resulting in a slightly tetragonal structure (c/a 1.01). Ferroelectric domains form to minimize strain energy. Commercial polycrystalline BaTiO3 contains various impurities and dopants and the purpose of this TEM study is to assess whether similar behaviour occurs compared to the pure material.BaTiO3 capacitors from several industrial sources were examined, containing up to several atomic per cent of various oxides (Bi, Zr, Sn, Ca) which stabilize the high dielectric constant over a wide range of temperatures and “tie up” oxygen vacancies. Previous TEM work in the present study using EDS has shown that the presence of impurities prevents the ferroelectric phase transformation and the formation of ferroelectric domains at room temperature. No ferroelectric domains exist in region A of fig. 1(d) where Bi, Sn, and Ca impurities were detected, while conversely in the ferroelectric region B no impurities could be found. (Delta fringes from overlapping ferroelectric domains delineate the ferroelectric portion of the grain.) It is thought that impurity ions substituting on the BaTiO3 lattice distort the lattice sufficiently to prevent the phase transformation from occuring.