Capacitors You Probably Won't See Very Often.

          Parylene
          Parylene is a group of versatile, very high-performance polymers most commonly used for board coating.  Parylene capacitors were manufactured by at least two companies but have disappeared.  Parylene is said to have properties similar to polystyrene but a much higher temperature rating.

          Polyethylene
          Polyethylene, in various forms, is hugely popular as a high-voltage cable insulation material, but I have never seen it used in commercial capacitors, probably because of poor heat resistance.   It has some popularity with hobbyists doing high-voltage experiments however; the film is cheap and readily available.  Breakdown voltage is relatively high compared to other polymers.  Polyethylene has negligible moisture absorption.  Said to have a very low dissipation factor, but I haven´t seen actual numbers.

          Polyimide
          Polyimide covers a number of related high-performance polymers that are used in a wide range of applications.  Some have been investigated as high-temperature capacitor dielectrics, but I don´t know of much usage beyond that.  Operating temperature for polyimides range from about 200- 400C, depending on type.  High breakdown voltage and moderately low dissipation factor (as low as .3% for some grades).  Polyimides tend to be be fairly hydroscopic.  In any case, high cost and average DF might relegate polyimide to niche high-temperature applications.  Trade names include Kapton and Ultem. 

          Companies that advertise polyimide capacitors include:
http://www.eci-capacitors.com/  polyetherimide

          Polyvinylidene fluoride:
          Polyvinylidene fluoride (PVDF, trade name Kynar) is rare as a capacitor material, I have never seen it in a stock capacitor.  Its high dielectric constant (around 8) has apparently made it useful in certain high-voltage pulse applications.  High-voltage storage capacitor in a defibrillation machine is one application I have seen.  Its temperature drift is good above 0C but very high below, and its dissipation factor is the highest of any plastic film.  More information would be welcome.

          Companies that advertise PVDF capacitors include:
http://www.dei2000.com/

          Polyethersulfone:
          Polyethersulfone is roughly similar to polysulfone.  I have seen hints that it has been used in capacitors.

          Cellulose acetate
          High dielectric constant (6 to 8) but poor dissipation factor and limited temperature range, to 85C.  Only Siemens used this material to my knowledge.

          Wax paper
          Wax-impregnated paper was once the standard for low-cost, small-value capacitors (100 pF-1 uF).  The original ones were built in wax-sealed cardboard tubes but later parts (1940s-'60s), were often made with plastic cases.  Performance and reliability was poor by modern standards and operating life limited.  Humidity was a problem, making the parts prone to leakage.  In damp climates, repairmen sometimes retrofitted TV sets with light bulbs to keep the insides warm and dry when the set was turned off.
          Hobbyist rehabilitating old radios and amplifiers will normally replace wax paper caps with modern polyester or polypropylene parts. 

          Class 4 Ceramics
          These are the barrier-layer and reduced-titanate ceramics.  These ceramics have low breakdown voltage, high leakage and all-around poor electrical properties.  High Ks made them useful at one time but modern multilayer ceramics have made them all but obsolete.

          Possible next-generation dielectrics
          Some dielectrics that have attracted attention at the laboratory level include cellulose triacetate (cellulose acetate´s tougher cousin) and beryllium oxide (very high temperature).  Also presently under investigation are "diamond-like carbon" and a polymer called flourene polyester.
          DuPont has promoted the use of various Teflons for capacitor use, PFA and FEP for example, by publishing detailed  information on their electrical performance.  No one seems to have taken them up on this.  PFA and FEP have better mechanical properties than PTFE which is difficult to work with. 
          Any number of promising polymer films have been investigated in the lab, often aimed at high-temperature operation, but manufacturers have rarely shown great interest in developing them into capacitors.  This is both because of lack of faith that a new dielectric will find a profitable market, and uncertainty that new materials will be available in production quantity.  To make a capacitor, usually three people are involved, the material maker, the film maker, and the capacitor maker.  Only one company I know of does even two of these.  All must be convinced of a profit to commercialize a new dielectric and keep an existing one on the market. 

         Various fluorocarbon polymers:
          Teflon AF (Dupont).  A little known material with remarkable optical and mechanical properties.  Its index of refraction is actually lower than water.  It is highly permeable to gases, making it useful for degassing applications.  It is soluble in certain perflourocarbon solvents.  Its dissipation factor and dielectric constant, about 1.92, are both low and relatively constant over a wide frequency range.  A high material cost would probably prevent its use at present.
          Tedlar PVF (Dupont).   High dielectric constant and high dissipation factor.  Both highly variable with temperature and frequency .  These have a limited availability. 
          Tefzel (Dupont).  Dielectric constant around 2.6 and relatively stable over temperature and frequency.  Dissipation factor fairly low over a relatively wide temperature and frequency range (through at least 1 MHz). 
          Teflon FEP (Dupont).  Dielectric constant is about 2, and is fairly stable over a wide temperature range and very stable into the GHz range. 
         

          The Final Frontier
          An ambitious goal of electronics system manufactures is to integrate formed-in-place capacitors (and other things) between the layers of conventional circuit boards (glass-epoxy, etc.).  This would allow for miniaturization of portable equipment beyond what is already being done with chip-scale ICs and size 0201 SMD parts.  Dielectrics under investigation for this application include the conventional ones such as barium titanate and other titanates, silicon dioxide, titanium dioxide, tantalum pentoxide, and several high-performance polymers.  Less common dielectrics include silicon carbide, silicon nitride, and diamond film.