Detection of Degraded Formulated BaTiO₃ Induced by Oxygen Vacancy Migration Using Ultrasonic Attenuation

Insulation resistance degradation caused by electromigration of oxygen vacancies under DC bias is a common mechanism that limits lifetime in multilayer ceramic capacitors (MLCCs). This work explores the use of high frequency ultrasound (100 MHz) as a non-electrical, nondestructive characterization tool to spatially identify regions of degradation caused by oxygen vacancy migration through ultrasonic attenuation, which results from acoustic impedance changes leading to scattering contrast. To complement this technique, thermally stimulated depolarization current (TSDC) measurements were utilized to both degrade parts and evaluate the presence of electromigrated oxygen vacancy space charge. Commercial 1812 BME X7R MLCCs (100 V rated, 560 nF) were characterized using a custom ultrasonic immersion setup in both a pristine state and after TSDC (degraded at 130°C, 140°C, or 150°C for 8 h and measured using a 10°C min^-1 heating rate). Each TSDC condition induced oxygen vacancy migration across grain boundaries, which was confirmed using passive voltage contrast SEM. The average ultrasonic attenuation increased (1-16.4%) after TSDC stress. Additionally, more extreme poling conditions resulted in larger variations in the spatial attenuation response. Thus, ultrasonic attenuation is sensitive to MLCC degradation caused by electromigration of oxygen vacancies and may be useful as a non-electrical evaluation of this mechanism.