Intrinsic degradation mechanisms of barium titanate based multilayer ceramic capacitors

Abstract

A study was conducted into the intrinsic degradation behavior of X7R and Z5U capacitors. The major goals of this research were:

1) to determine current-voltage, activation energy and leakage current characteristics for such capacitors.

2) to investigate how these characteristics change with degradation;

3) to investigate possible correlations between observed behavior and capacitor microstructure or composition.

Examination of capacitor microstructures revealed large differences in grain morphology and electrode spacings. The development of a color gradient between positive and negative electrodes with. degradation was observed in one type of capacitor.

Compositional studies using EDAX, AUGER, and microprobe analyses failed to detect gradients in dielectric composition in degraded capacitors indicating that gradients are less than 0.1 atomic percent. Current-voltage studies showed a 3/2 power dependance, at voltages above one volt for X7R capacitors indicating space charge limited current. This dependance was attributed to point emission from electrode protuberances. The 3/2 dependance was observed to shift to a square law behavior with degradation. This change was attributed to a increase in conductivity of the dielectric near the cathode, blunting the effect of electrode protuberances. Z5U capacitors were found to have ohmic behavior. No Schottky or Poole Frenkel currents were observed.

Degradation studies were carried out on capacitors at 2 to 8 times the rated voltage, and at temperatures from 100 to 1500℃. Leakage currents in actively degrading capacitors were observed to rise exponentially with time. This rise was accompanied by a gradual decrease in activation energy. A model is proposed to explain the observed current vs degradation behavior.