260 Chapter 10
copper. A few metals form dense, stable, tightly
adhering, electrically insulating oxides. These are the
so-called valve metals and include titanium, zirco-
nium, niobium, tantalum, hafnium, and aluminum.
Only a few of these permit the accurate control of
oxide thickness by electrochemical means. Of these,
the most valuable for the electronics industry are
aluminum and tantalum.
The dielectric used in all tantalum electrolytic
capacitors is tantalum pentoxide. Although wet foil
capacitors use a porous paper separator between their
foil plates, its function is merely to hold the electro-
lyte solution and to keep the foils from touching.
The tantalum pentoxide compound possesses high
dielectric strength and a high dielectric constant. As
capacitors are being manufactured, a film of tantalum
pentoxide is applied to their electrodes by an electro-
lytic process. The film is applied in various thick-
nesses and at various voltages. Although transparent
at first, it takes on different colors as light refracts
through it. This coloring occurs on the tantalum elec-
trodes of all three types of tantalum capacitors.
Rating for rating, tantalum capacitors tend to have
as much as three times better capacitance/volume
efficiency than aluminum electrolytic capacitors,
because tantalum pentoxide has a dielectric constant
of 26, some three times greater than that of aluminum
oxide. This, in addition to the fact that extremely thin
films can be deposited during manufacturing, makes
the tantalum capacitor extremely efficient with
respect to the number of microfarads available per
unit volume.
The capacitance of any capacitor is determined by
the surface area of the two conducting plates, the
distance between the plates, and the dielectric
constant of the insulating material between the plates.
The distance between the plates in tantalum elec-
trolytic capacitors is very small since it is only the
thickness of the tantalum pentoxide film. The dielec-
tric constant of the tantalum pentoxide is high, there-
fore, the capacitance of a tantalum capacitor is high.
Tantalum capacitors contain either liquid or solid
electrolytes. The liquid electrolyte in wet-slug and
foil capacitors, usually sulfuric acid, forms the
cathode or negative plate. In solid-electrolyte capaci-
tors a dry material, manganese dioxide, forms the
cathode plate.
The anode lead wire from the tantalum pellet
consists of two pieces. A tantalum lead is embedded
in, or welded to, the pellet, which is welded, in turn,
to a nickel lead. In hermetically sealed types, the
nickel lead is terminated to a tubular eyelet. Anexternal lead of nickel or solder-coated nickel is
soldered or welded to the eyelet. In encapsulated or
plastic-encased designs, the nickel lead, which is
welded to the basic tantalum lead, extends through
the external epoxy resin coating or the epoxy end fill
in the plastic outer shell.Foil Tantalum Capacitors. Foil tantalum capacitors
are made by rolling two strips of thin foil, separated by
a paper saturated with electrolyte, into a convolute roll.
The tantalum foil, which is to be the anode, is chemi-
cally etched to increase its effective surface area,
providing more capacitance in a given volume. This is
followed by anodizing in a chemical solution under
direct voltage. This produces the dielectric tantalum
pentoxide film on the foil surface.
Foil tantalum capacitors can be manufactured in dc
working voltage values up to 300 V. However, of the
three types of tantalum electrolytic capacitors, the foil
design has the lowest capacitance per unit volume. It is
also the least often encountered since it is best suited for
the higher voltages primarily found in older designs of
equipment and requires more manufacturing operations
than do the two other types. Consequently, it is more
expensive and is used only where neither a solid electro-
lyte nor a wet-slug tantalum capacitor can be employed.
Foil tantalum capacitors are generally designed for
operation over the temperature range of 55°C to
+125°C (67°F to +257°F) and are found primarily in
industrial and military electronics equipment.Wet-Electrolyte Sintered Anode Tantalum Capaci-
tors. Wet-electrolyte sintered anode tantalum capaci-
tors often called wet-slug tantalum capacitors, use a
pellet of sintered tantalum powder to which a lead has
been attached. This anode has an enormous surface area
for its size because of its construction. Tantalum powder
of suitable fineness, sometimes mixed with binding
agents, is machine-pressed into pellets. The second step
is a sintering operation in which binders, impurities, and
contaminants are vaporized and the tantalum particles
are sintered into a porous mass with a very large internal
surface area. A tantalum lead wire is attached by
welding the wire to the pellet. (In some cases, the lead is
embedded during pressing of the pellet before sintering.)
A film of tantalum pentoxide is electrochemically
formed on the surface areas of the fused tantalum parti-
cles. The oxide is then grown to a thickness determined
by the applied voltage.
Finally the pellet is inserted into a tantalum or silver
container that contains an electrolyte solution. Most
liquid electrolytes are gelled to prevent the free move-
ment of the solution inside the container and to keep the