Effect of the pyrochlore (Y 2 Ti 2 O 7 ) phase on the resistance degradation in yttrium-doped BaTiO 3 ceramic capacitors
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Degradation of insulation resistance due to the formation of the Y2Ti2O7 (pyrochlore) phases in yttrium(Y)-doped BaTiO3 ceramics was investigated. The addition of Y2O3 and sintering aid SiO2 to the BaTiO3 ceramics caused the formation of barium-related BaSiO3 and titanium-related Y2Ti2O7 second phases. The appearance of the abnormally grown large pyrochlore grains with the increase of sintering temperature critically degraded insulation resistance at high temperatures. When the volume fraction of the pyrochlore phase increased, the resistance degradation was also observed although the grain size of the pyrochlore phase became smaller. The highly oxygen ionic conductive nature of the Y2Ti2O7 pyrochlore phase is supposed to accelerate electromigration of oxygen vacancies resulting in the resistance degradation. The formation of pyrochlore phase should be prevented to guarantee the high-temperature reliability of BaTiO3 bases ceramic capacitors.
I. INTRODUCTION
In a barium titanate-based multilayer ceramic capacitors, there are a number of electrical properties that should be considered, such as dielectric constant, temperature coefficient of capacitance (TCC), dissipation factor (DF), breakdown voltage (BDV), insulation resistance (IR), reliability at high temperature, etc. Of these properties the degradation of insulation resistance at high temperature under high electric field is of great concern in the base metal electrode (BME) multilayer ceramic capacitors (MLCC) that are sintered in reducing atmospheres. The principles of degradation mechanism of insulation resistance in perovskite materials have been well established by Waser et al.1,2 The phenomenon has usually been observed in undoped or acceptor-doped BaTiO3, where the majority defect is the oxygen vacancy, but not in donor-doped BaTiO3. Under simultaneous temperature and direct current (dc) field stress, the electromigration of highly mobile oxygen vacancies occurs, resulting in depletion and pileup of oxygen vacancies and consequent generation of holes and electrons in the positive-biased and negative-biased regions, respectively. This situation is similar to the forward-biased p-n junction diode, and thus leakage current is enhanced, resulting in resistance degradation. Recently, the direct experimental evidences of the polarization of oxygen vaa)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2007.0326 J. Mater. Res., Vol. 22, No. 9, Sep 2007
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cancies in the resistance-degraded specimen have also been provided.3 It is reported that several dopants are effective in retarding such resistance degradation in acceptor-doped BaTiO3. The transition metal elements such as Mn, Cr, Fe, etc., due to their multivalent nature, can partly compensate electrons and holes generated by the polarization of oxygen vacancies, which improves resistance degradation.2 Besides, some of the most important and widely used elements in the BME BaTiO3-based MLCC materials are
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