A tantalum electrolytic capacitor is an electrolytic capacitor, a passive component of electronic circuits. It consists of a pellet of porous tantalum metal as an anode, covered by an insulating oxide layer that forms the dielectric, surrounded by liquid or solid electrolyte as a cathode. The tantalum capacitor, because of its very thin and relatively high permittivity dielectric layer, distinguishes itself from other conventional and electrolytic capacitors in having high capacitance per volume (high volumetric efficiency) and lower weight. Tantalum is a conflict resource. Tantalum electrolytic capacitors are considerably more expensive than comparable aluminum electrolytic capacitors. Tantalum capacitors are inherently polarized components. Applying a reverse voltage can destroy the capacitor. Non-polar or bipolar tantalum capacitors are made by effectively connecting two polarized capacitors in series, with the anodes oriented in opposite directions. Tantalum electrolytic capacitors are extensively used in electronic devices that require stable capacitance, low leakage current, and where reliability is crucial. Due to its reliability, durability and performance under extreme conditions, it is used in medical equipment, aerospace and military applications. Other applications include power supply units, measuring instruments, telecommunications equipment, and computer peripherals.
Basic information
Basic principle
Electrolytic capacitors use a chemical feature of some special metals, historically called valve metals, which can form an insulating oxide layer. Applying a positive voltage to the tantalum anode material in an electrolytic bath forms an oxide barrier layer with a thickness proportional to the applied voltage. This oxide layer serves as the dielectric in an electrolytic capacitor. The properties of this oxide layer are compared with those of a niobium electrolytic capacitor oxide layer in the following table:
After forming a dielectric oxide on the rough anode structures, a cathode is needed. An electrolyte acts as the cathode of electrolytic capacitors. There are many different electrolytes in use. Generally, the electrolytes will be distinguished into two species, non-solid and solid electrolytes. Non-solid electrolytes are a liquid medium whose conductivity is ionic. The oxide layer may be destroyed if the polarity of the applied voltage is reversed.
Every electrolytic capacitor in principle forms a plate capacitor whose capacitance is greater the larger the electrode area, A, and the permittivity, ε, are and the thinner the thickness, d, of the dielectric is.
C = ε ⋅ A d {\displaystyle C=\varepsilon \cdot {\frac {A}{d}}}
The dielectric thickness of electrolytic capacitors is very thin, in the range of nanometers per volt. Despite this, the dielectric strengths of these oxide layers are quite high. Thus, tantalum capacitors can achieve a high volumetric capacitance compared to other capacitor types. All etched or sintered anodes have a much larger total surface area compared to a smooth surface of the same overall dimensions. This surface area increase boosts the capacitance value by a factor of up to 200 (depending on the rated voltage) for solid tantalum electrolytic capacitors. The volume of an electrolytic capacitor is defined by the product of capacitance and voltage, the so-called CV-volume. However, in comparing the permittivities of different oxide materials, it is seen that tantalum pentoxide has an approximately 3 times higher permittivity than aluminum oxide. Tantalum electrolytic capacitors of a given CV value can therefore be smaller than aluminum electrolytic capacitors.
Basic construction of solid tantalum electrolytic capacitors
A typical tantalum capacitor is a chip capacitor and consists of tantalum powder pressed and sintered into a pellet as the anode of the capacitor, with the oxide layer of tantalum pentoxide as a dielectric, and a solid manganese dioxide electrolyte as the cathode.
Materials, production and styles
Anode
Tantalum capacitors are manufactured from a powder of relatively pure elemental tantalum metal. A common figure of merit for comparing volumetric efficiency of powders is expressed in capacitance (C, usually in μF) times volts (V) per gram (g). Since the mid-1980s, manufactured tantalum powders have exhibited around a ten-fold improvement in CV/g values (from approximately 20k to 200k). The typical particle size is between 2 and 10 μm. Figure 1 shows powders of successively finer grain, resulting in greater surface area per unit volume. Note the very great difference in particle size between the powders.
The powder is compressed around a tantalum wire (known as the riser wire) to form a "pellet". The riser wire ultimately becomes the anode connection to the capacitor. This pellet/wire combination is subsequently vacuum sintered at high temperature (typically 1200 to 1800 °C) which produces a mechanically strong pellet and drives off many impurities within the powder. During sintering, the powder takes on a sponge-like structure, with all the particles interconnected into a monolithic spatial lattice. This structure is of predictable mechanical strength and density, but is also highly porous, producing a large internal surface area (see Figure 2). Larger surface areas produce higher capacitance; thus high CV/g powders, which have lower average particle sizes, are used for low voltage, high capacitance parts. By choosing the correct powder type and sintering temperature, a specific capacitance or voltage rating can be achieved. For example, a 220 μF 6 V capacitor will have a surface area close to 346 cm2, or 80% of the size of a sheet of paper (US Letter, 8.5×11 inch paper has area ~413 cm2), although the total volume of the pellet is only about 0.0016 cm3.
Dielectric
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