Aluminum Electrolytics

The primary strength of aluminum capacitors is their ability to provide a large capacitance value in a small package, and do so for a relatively low cost. Additionally, they tend to have good self-healing characteristics; when a localized weak spot in the aluminum oxide dielectric layer develops, the increased leakage current flow through the weak point in the dielectric causes a chemical reaction similar to that used during the initial formation of the dielectric layer, resulting in a thickening of the dielectric at the weak point, and a consequent reduction in leakage current.

The shortcomings of aluminum capacitors are mostly related to (a) the chemically-reactive nature of the materials used in their construction, (b) the conductive properties of the electrolyte solutions, and (c) the volatility of liquid electrolytes.

The chemically reactive nature of the materials used in aluminum capacitors is problematic on two points; the stability of the dielectric layer and the long-term mechanical integrity of the device. Since the aluminum oxide dielectric layer in these devices is formed through an electrochemical process, it can also be eroded by an electrochemical process simply by reversing the applied voltage. This is why most aluminum capacitors are polarized; application of voltage with the wrong polarity causes rapid erosion & thinning of the dielectric, resulting in high leakage current and excessive internal heating.

From a mechanical integrity standpoint, mixing a highly reactive metal (aluminum) with a corrosive electrolyte solution is a delicate proposition; errors in electrolyte composition can result in premature failure, as evidenced by the “capacitor plague” of the early 2000’s.

Another shortcoming of aluminum electrolytic capacitors is the fact that the electrolytes used aren’t particularly efficient conductors, because conduction in electrolyte solutions is achieved through ionic, rather than electronic conduction; instead of loose electrons moving between atoms serving as the charge carriers, ions (atoms or small groups thereof that have a charge due to a surplus or deficit of electrons) are moving about through the solution. Since ions are more bulky than electrons, they don’t move as easily and hence ionic conduction generally tends to be a higher-resistance proposition than electronic conduction. The extent to which this is the case is influenced significantly by temperature; the lower the temperature, the more difficult it is for ions in an electrolyte solution to move about through the solution, which translates into a higher resistance. Thus, electrolytic capacitors tend to have a relatively high ESR that exhibits a strong inverse correlation with temperature.

The third major downside to aluminum capacitors (with the exception of the solid polymer types) is that the liquid electrolyte solutions tend to evaporate over time, eventually being lost to the atmosphere by diffusion through the rubber sealing plug, leaks in safety vent structures, or similar phenomena.

Acknowledgement

This lesson was contributed by Dr. Arne Albertsen, Jianghai Europe Electronic Components GmbH