PRIZ riddle #04 Medium About 4 minutes

The Tantalum Capacitor Paradox

Titanium oxide has a higher dielectric permittivity than tantalum oxide. So why are porous electrolytic capacitors made from tantalum rather than titanium?

The challenge

A large surface inside a small capacitor

The anode of a tantalum capacitor is produced by pressing and sintering fine Ta powder.

The resulting pellet is highly porous. Although the capacitor is small, the interconnected particles create a very large internal surface area.

During electrochemical oxidation, also called anodic formation, part of the tantalum surface is converted into a thin dielectric film:

2\mathrm{Ta} + 5\mathrm{H_2O} \rightarrow \mathrm{Ta_2O_5} + 10\mathrm{H^+} + 10\mathrm{e^-}

Every accessible Ta particle surface becomes coated with amorphous Ta₂O₅.

The capacitance is described approximately by:

C = \frac{\varepsilon_0 \varepsilon_r A}{d}

where A is the internal surface area, d is the dielectric thickness and εᵣ is the relative dielectric permittivity.

Titanium appears better

$Ta₂O₅$ has a relative dielectric permittivity of approximately 25–30.

Anodic $TiO₂$ can have a considerably higher value. A higher dielectric permittivity should provide greater capacitance for the same area and oxide thickness.

Titanium can also be formed into a porous sintered anode and oxidized electrochemically.

Everything appears to favor titanium, but conventional porous titanium electrolytic capacitors are not used as replacements for tantalum capacitors.

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Your task

Why can’t Ti replace Ta in an electrolytic capacitor?

Do not guess and do not search the internet. It can find information, but it cannot think for you. Think first.

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Answer revealed

The key difference is electrical conductivity.

Amorphous $Ta₂O₅$ is a stable, highly resistive dielectric. It allows electrical polarization but strongly blocks the flow of electrons, so the capacitor can store charge with very low leakage.

Stoichiometric $TiO₂$ can also behave as a dielectric, but it has a strong tendency to lose oxygen and become nonstoichiometric:

\mathrm{TiO_2} \rightarrow \mathrm{TiO_{2-x}} + \frac{x}{2}\mathrm{O_2}

Oxygen loss creates vacancies, converts some $Ti⁴⁺$ into $Ti³⁺$ and may produce lower oxides such as $Ti₂O₃$ . These defects introduce electronic states inside the bandgap through which electrons can move.

The titanium-oxide film therefore becomes partially semiconductive. Across the enormous internal surface of a porous anode, these conductive paths produce unacceptable leakage current and increase the risk of dielectric breakdown.

$Ta₂O₅$ reliably stores charge. $TiO₂$ tends to lose oxygen and conduct it away.

Why this works

$Ta₂O₅$ reliably stores charge. $TiO₂$ tends to lose oxygen and conduct it away.

Thinking principle: A high dielectric constant is useful only when electrical conductivity remains extremely low.

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