Tantalum and Niobium

Tantalum electrolytic capacitors are a step up from aluminum capacitors.  They come in a number of types with different
advantages, but in general, they have smaller size, lower leakage, lower dissipation factor, lower ESR, more stable
capacitance over temperature, and good service life.  Tantalum capacitors aren't made in the monster sizes that aluminum
capacitors are, but are available to several hundred µF in common voltage ratings (to about 100 volts) and to several
thousand  uF at low voltage (6-10 volts).

The first tantalum capacitors were made from tantalum foil with a sulfuric acid electrolyte.  Foil tantalums are made for
use to at least 300 volts, and to 125C.  They are made in both industrial and military styles but their usage is probably
mostly military.  Foil tantalums are now rare however, and seem be out of production.

The foil anode is a poor use of what is an expensive metal, and not in good supply.  Most modern tantalum capacitors
are what are called "slug" capacitors.  Tantalum powder is sintered into a porous yet strong slug, with a tantalum wire,
which forms the anode of the capacitor.  The many small particles produce a very high surface area.  A variety of particle
shapes are used, depending on the operating voltage, and making them is a science in itself.  A layer of tantalum
pentoxide (Ta2O5 with a K of about 25) is grown over the particles for the dielectric.  In one version of the capacitor,
the electrolyte is gelled sulfuric acid.  This is called a "wet-slug" and, as you can imagine, it must be well sealed.  Wet-slug
tantalums are the capacitor of choice for some applications, especially at very high temperatures (to 200C for some,
often with derating).  They are available for up to 900 volts operation.  Construction details vary.   

The vast majority of solid tantalums are the "dry-slug" or "solid tantalum" capacitors.  A layer of manganese dioxide
(MnO2) is layered over the pentoxide followed by a layer of colloidal graphite and a layer of silver paint. There are minor
variations on this.  With no liquid involved, they can be sealed with just an epoxy dip, although the better ones may have
a molded body.  The pentoxide layer is prone to defects, and the key to the solid tantalum capacitor's reliability is that the
MnO2 provides self healing.  If a flaw in the pentoxide layer develops, the leakage will cause localized heating in the
MnO2 and convert it to Mn2O3 , a much less conductive oxide, sealing off the flaw.  This mechanism is not perfect,
however, and failures due to dielectric flaws have been a traditional problem for solid tantalums.   If the temperature of
the pentoxide gets high enough, about 500C, the pentoxide converts from its nonconducting amorphous form to its
conducting crystalline form and the capacitor goes up in flame.  Solid tantalums aren't usually made with high working
voltages because the particle size limits the dielectric thickness that can be grown; 50 volts is usually the upper limit. 
There are however, a very few solid tantalums with working voltages as high as 125 volts.  Most dry tantalums are rated
to no more than 85C to 125C, but a few are rated for use to 175C with voltage derating.  Solid tantalum capacitors are
commonly available in surface-mount packages, in molded bodies. 

A weakness of slug tantalums is high-frequency performance.  Depending on construction details, the capacitance can fall
to 50% in the 100-200 kHz region, compared to 1 kHz.  Lower ESR parts can be made using larger particles (although
at the expense of somewhat lower capacitance) and by various manufacturing refinements. 

A rarely mentioned characteristic of solid tantalums is their rapid decline in leakage current.  When both aluminum and
tantalum electrolytics are powered up, their leakage current starts high, but declines over time.  For aluminums, leakage
take minutes to decline to a stable value.  For tantalums, this occurs in seconds. 

Even though "dry-slug" tantalums are not wet-chemistry devices, they are still polarized.  Although many sources claim
that dry tantalums are very sensitive to reverse bias, they can actually to be very slow to fail when reverse biased.  In fact,
failure of tantalums installed backwards may occur from seconds to more than a year after manufacture. 

Because of the growing popularity of tantalum in miniature electronic equipment, tantalums (mainly SMD) have diverged
into a number special types. They include:

•High temperature, to at least 200C with voltage derating, MnO2.  
•Low ESR using specially sized powders and multiple anodes.
•Miniature package sizes.
•Very low height.  Standard SMD tantalums are 1.9 to >4 mm in height.  Low profile are 1.2 or 1.5 mm. 
•Fused, for power supply bypassing on digital boards.
•Tantalum capacitors with the semiconducting polymer "electrolyte" TCNQ have appeared.  Sanyo, for one,
makes them under the POSCAP name.  See Sanyo OS-CON capacitors above.  They tend to have the same
advantages as their aluminum cousins, including lower ESR and flatter temperature characteristics. 
Manufacturers also claim that they can fail without bursting into flame.  A new conductive polymer, PEDOT has
also appeared, see above. 


          Companies that advertise wet-electrolyte tantalum capacitors include:

Mallory  Gone.

Niobium has been under investigation for many years as a lower-cost replacement for tantalum in electrolytic capacitors.  The Russians were making very poor quality devices as early as the 1960s.  This research is paying off however as several companies are close to (or actually are) shipping niobium electrolytics in volume. 

A much more common metal, niobium should not have the supply and cost problems seen by tantalum.   However, it has proven to be much more difficult to work with than tantalum, a major problem being to get leakage under control.  Niobium, unlike tantalum, tends to want to form other oxides than the pentoxide, such as NbO and NbO2 which are conductive to some degree.  The fewer the oxygen atoms, the higher the conductance.  Oxygen also will dissolve in niobium to some extent.  At elevated temperature the niobium will start forming lower oxides, increasing leakage.  At first this kept the rated operating temperature to 85C.  More recently AVX as introduced 105C parts that are suitable for ROHS soldering temperatures (260C peak).  Niobium´s dielectric (Nb2O5 ) has a higher K than tantalum, about 42, but a lower breakdown voltage and a greater tendency for leakage requires that the film be thicker.  The volumetric efficiency may turn out to be similar.  Niobium caps, like tantalum, require a voltage derating for applications that involve current surges.  The fate of niobium will probably depend most on user acceptance, which will rise and fall with the price of tantalum. 

The next step is the use niobium oxide, NbO, for the anode.  NbO has advantages over Nb, including lower weight, a much lower tendency to ignite (something niobium has in common with tantalum), and an improved ability to handle current surges.  There seems to be no true niobium capacitors out there, but AVX is selling the NbO type. 

Niobium capacitors have promise for when tantalum becomes too expensive, but availability seems poor.  Also, going to ceramic capacitors or polymer-aluminums may be a better option for many applications.

Companies that at least are talking about niobium electrolytic capacitors, all in SMD, include:

Double-Layer Capacitors: 

Sometimes called super caps, electrochemical caps, or Gold caps (an early Panasonic trade name), the double-layer (DL) capacitor is a relative newcomer.  I tend to think of DL capacitors as an odd form of electrolytic, but others regard it as a new class of capacitor.  The classic DL capacitor does not use a traditional dielectric, but rather uses air-gel carbon electrodes and a sulfuric acid electrolyte which forms a vary thin "dielectric" layer.  Other systems are now appearing.  Some DL capacitors are polarized, but others are not.  Recent DL capacitors all seem to use organic electrolytes.  DL capacitors have limitations.  They are very low voltage, about 2.7 volts, and their temperature range is only about -20 to +60C.

DL caps live halfway between a capacitor and a battery.  They have a much higher energy density than a capacitor but a higher discharge rate than most batteries.  By combining a DL cap with a battery that has good energy density but a hight output you can use the battery in an application that has a high transient current requirement. 

Lithium ion DL capacitors

The lithium ion double-layer capacitor is fairly new.  It uses lithium doping of cathode to get least twice the energy density, near that of lead-acid batteries.  They have a voltage of 3.8 volts (a little lower at high temperature), a much lower self-discharge rate, and a little better temperature range, -20 to _80C.  Cells are available in the 100s to 1000s of farads range.  Manufacturers mention a minimum voltage but don't explain the consequences of that. 

          Manufacturers of lithium double-layer capacitors include: 


Polyacene capacitor

A new variation on the double-layer. 

Double click to edit
http://www.vishay.com/  Parts to 200C. 
http://www.cde.com/capacitors/tantalum/wet-tantalum-electrolytic  obsolete
http://www.transitor.com   I think all Transitor does is tantalum.  But they are now a Vishay company.
http://www.tantalum-pellet.com/  Parts to 85C. 
http://www.kemet.com  No longer seen on their web site.
http://www.ncc-matsuo.co.jp/product/tantalum/indexe.html  maybe
http://www.dearbornelectronics.com/tantalum-capacitors/  Made by Exxelia? 
https://ec.kemet.com/  Excellent. 
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http://www.elecond.ru/main3e.html  A Russian Company, they seem to have little or no presence in the west.
http://www.epcos.com  Had announced volume production, nothing on their website at present.  Now TDK-Epcos. 
http://www.vishay.com  Vishay was shipping samples at one time.  
http://www.nec-tokin.com/english/product/cap/topics/0107_qa.html  The only ones using a polymer electrolyte so far, but production seems limited. 
http://www.avxcorp.com/   They have been shipping niobium caps for some time.  They also have good technical information.
http://www.kemet.com/   Kemet has talked about niobium but there is nothing on their website. 
http://www.holystonecaps.com/index_us.php   Their website shows a cross to an AVX part. 
http://www.niccomp.com/index.php  They once advertised niobium capacitors but I can't find them on their website now.
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http://www.idtechex.com/supercapacitors-usa/conference.asp  Supercaps are rapidly mutating and improving.