Modeling the Behavior of the Dielectric Properties in Low Formation Voltage Electrolytic Tantalum Capacitors

Open Access
Author:
Kramer, Angela J
Graduate Program:
Materials Science and Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
December 16, 2010
Committee Members:
  • Sarah Elizabeth Dickey, Thesis Advisor
  • Elizabeth C Dickey, Thesis Advisor
Keywords:
  • electric field
  • Poole-Frenkel effect
  • specific charge
  • dielectric
  • tantalum pentoxide
  • tantalum
  • electrolytic capacitors
  • Schottky emission
Abstract:
With the miniaturization of electronics, there is a continual demand for capacitive components to have greater capacitance per volume with low leakage currents. This research was motivated by the need to understand the impact of low formation voltage dielectrics and high surface area powders on the electrical properties of electrolytic tantalum capacitors. To help address the specific charge (CV/g) loss with decreasing formation voltage (below approximately 12 V), a physical, mathematical model based on the cylindrical geometry of the tantalum anode was developed to qualitatively explain this phenomenon as well as investigating the influence of curvature on the localized electric field and D.C. leakage current. The model developed qualitatively explains the CV/g behavior and low voltage % CV/g loss, which agrees well with the experimental data observed. The model was useful to qualitatively predict the capacitance per volume increase with decreasing particle size. It was found that the CV/g decreases with decreasing formation voltage due to the presence of the native oxide thickness (which corresponds to the zero formation voltage) becoming a more significant fraction of the total dielectric thickness. Also, since the native oxide is present regardless of tantalum particle size, the percentage of % CV/g loss is almost independent of the initial tantalum particle size, although the absolute losses are greater for smaller particle sizes. The model provided insight into the effects of curvature on electric field, where it was observed that there is electric field enhancement at the anode for features with positive curvature and at the cathode for features of negative curvature. Likewise, there is electric field depletion at the cathode for features with positive curvature and at the anode for features of negative curvature. Since the D.C. leakage current is directly related to the electric field, using the same physical model allows for the effects of particle curvature on the D.C. leakage to be predicted. It was found that for Schottky emission, there is leakage current enhancement at the cathode for negative curvature features. The leakage due to Schottky emission is predicted to be inhomogeneous at the cathodes depending on the features curvature. For Poole-Frenkel conduction, curvature does not affect the leakage current density, but the total leakage current per volume scales with the surface area per volume and is thus greater for smaller particle sizes.