Intragranular Voids and dc Degradation in (CaO+MgO) Codoped BaTiO_3 Multilayer Ceramic Capacitors

摘要

The microstructure of multilayer ceramic capacitors (MLCC) based on BaTiO_3 and nickel electrode, have been analyzed using the scanning and the transmission electron microscopy. In order to investigate how MgO improves MLCC against dc degradation, both CaO doped and (CaO+MgO) codoped chips, pristine as well as highly accelerated life-tested, are studied. BaTiO_3 grains are characterized by both the types I and II core shell structure, which is typical of MLCC exhibiting the X7R dielectric behavior. Intragranular voids are found in BaTiO_3 grains in the vicinity of the electrode-dielectric interface. Void-containing grains are more frequently observed and voids are more abundant in (CaO+MgO) codoped chips than in CaO doped ones. Higher concentration of oxygen vacancies (V_O~(··)) is induced extrinsically from both MgO codoping and Ni diffusion into BaTiO_3 grains along the electrode-dielectric interface during sintering. Such oxygen vacancies have reacted with both cation vacancies (V_(Ba)'' and V_(Ti)'''' by an inverse Schottky defect reaction and condensed to form voids. This reaction requiring cation and oxygen vacancies in the stoichiometric BaTiO_3 composition of 1:1:3 has significantly decreased the randomly distributed mobile oxygen vacancies, and contributes to improve against dc degradation.