Technology · Established
Storing charge in an oxide a few atoms thick
A tantalum capacitor's dielectric is an oxide film grown electrochemically on a porous metal pellet, which is how a large capacitance fits in a small volume.
Explore the supporting references at the foot of this article.
THE MATERIAL THAT DOES THE WORK
Chemical form
The is a pressed and sintered pellet of tantalum powder with a very large internal surface area. The dielectric is tantalum pentoxide, grown to a controlled thickness by anodising. The is manganese dioxide formed in place, or a conducting in newer parts.
- 01
Press and sinter
Fine tantalum powder is pressed around a lead wire and sintered into a porous pellet with a large internal surface.
- 02
Anodise the dielectric
A voltage-controlled anodisation grows tantalum pentoxide uniformly through the pore structure, setting the thickness.
- 03
Form the cathode and derate the part
Manganese dioxide or a conducting polymer is formed in place to contact the throughout the pellet. Working voltage is then set well below the formation voltage, because the failure mode of the oxide, not its capacitance, governs reliability.
Why it works
Capacitance rises with surface area and falls with dielectric thickness, and anodising delivers both: the oxide grows uniformly over a convoluted internal surface at a thickness set by the applied voltage, so a pellet a few millimeters across can present a surface area of hundreds of square centimeters. Tantalum pentoxide also has a high permittivity and the oxide is self-limiting as it grows.
How it is made & used
Tantalum powder is pressed around a wire, sintered under vacuum, and anodised in an at a voltage that fixes the oxide thickness. Manganese nitrate is then impregnated and pyrolysed to manganese dioxide, or a polymer is deposited, followed by carbon and silver layers and encapsulation.
Inside the application
Tantalum is one of the subject to conflict-mineral reporting, so a capacitor's supply chain is documented back to the smelter. That traceability requirement is unusual for a passive component and has changed how the industry buys raw material.
Limits & alternatives
Manganese dioxide parts can fail by ignition if a defect in the oxide allows a local current to run away, so circuit designers derate the working voltage substantially; polymer cathodes fail more benignly but tolerate less heat and have higher leakage. Ceramic and aluminum capacitors cover overlapping ranges, and the choice depends on volume, ripple current, temperature and failure mode rather than on capacitance alone.
Part of a larger system
This connects an element chosen for the behavior of its oxide, rather than of the metal, to a component whose performance is decided almost entirely by geometry created during manufacture.
Sources & further reading
- USGS — Tantalum ↗United States Geological SurveyReference explanation · Source checked
- LANL Periodic Table — Tantalum ↗Los Alamos National LaboratoryReference explanation · Source checked
- RSC Periodic Table — Tantalum ↗Royal Society of ChemistryReference explanation · Source checked
- Manganese: history, properties and uses ↗Royal Society of ChemistryReference explanation · Source checked
- Manganese: history and uses ↗Los Alamos National LaboratoryReference explanation · Source checked
Sources and further reading. What the review labels mean