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Miscellaneous Dielectrics Glass Glass
capacitors can be roughly divided into two general classes, the low-K glasses with properties similar to film capacitors, and the medium-K glasses with properties similar to the medium-K ceramics. The low-K types
are available to about 0.01 uF. They have a fairly low and predictable temperature coefficient, negligible voltage coefficient, and reasonably low dissipation factor. The high-K types are available to about
0.1 uF. They have a much higher temperature coefficient, a significant voltage coefficient (varies by type), and a higher dissipation factor (varies by type). I have not seen them in surface mount.
Recently, AVX has introduced the CY31 series of glass capacitors with much improved performance. AVX claims a C0G temperature drift, with a dielectric
absorption and dissipation factor as good or better than polystyrene. They are much cheaper than old styles, around $6 in 1000s. I have tried to find other companies who make fixed glass capacitors. I
have seen a number of lists (EEM is one) and search engines that show a number of glass capacitor vendors. After tracking them down however, I found that they really made of porcelain, glass encapsulated ceramics,
glass trimmers, or nothing related to glass at all. Companies that advertise glass capacitors include:
Mica
The best ruby muscovite mica capacitors have a low dissipation factor at high frequencies (but poor dielectric absorption) and are usable up to 1 GHz. Their
stability over time is one of the best of all capacitors and they have a fairly low temperature drift. This varies by type and size, however. Many mica capacitors are guaranteed to less than +90 ppm/C, but
others can be +200 ppm/C or even more. Mica capacitors are best known for general-purpose RF applications including filters and oscillators, although C0G ceramic, with smaller size and lower temperature drift, has
largely replaced them. Micas are also used in high-power and fast-pulse applications such as transmitting capacitors, high-energy storage, and snubbers. Mica is very popular for high-voltage applications
because its voltage breakdown and corona resistance are better than most other dielectrics. Mica capacitors have very good temperature resistance, limited mostly by packaging; even common ones are usually
rated from -55 to +125C. A very few are available for use up to 300C, with customs to 400C. They are available from <1 pF to at least 20 uF and to at least 15,000 volts. Mica capacitors are
available in SMD. Like anything else dug out of the ground, mica varies significantly in its properties, and may contain a variety of metallic impurities (iron
etc.). If your application is critical, make sure you know exactly what you are getting. Temperature drift can be quite variable part-to-part, unless guaranteed. Hams tell me to beware of micas made in
third-world countries where quality control may be poor. Ruby mica RF-quality capacitors generally have dissipation factors well under 0.1% below 1 MHz. Capacitors like these are usually only available in
small sizes, to about 0.01 uF. This because the best quality mica is no longer available in large size plates. If you need a large high-performance mica capacitor, you may have to look to old surplus
parts. The small RF micas are commonly referred to as "silver mica" or "silvered mica" capacitors, both because of the silver metallization and mica´s silvery appearance. Mica is found around the world,
including the US, but the best ruby mica is only found in India to my knowledge. ASTM-D-351 is a complex system for grading mica based on visual defects
such as color, air inclusions, and other flaws that degrade electrical properties. A rating of V-1 is best, and V-12 is worst. V-1 through V-4 is generally considered to be "capacitor-grade". Also see
ASTM-D-748: Mica For Fixed Capacitors. Large-value high-voltage capacitors are made from "reconstituted mica", which is made from pieces of lesser quality
capacitor-grade mica. These are made into sheets bonded with another dielectric, such as polyester, silicone, or epoxy, depending on the temperature resistance required. Reconstituted mica capacitors generally
have dissipation factors between 0.1% and 1% to 1 MHz over -50C to 100C. Muscovite mica is usable to about 500C. Another form of mica called
phlogopite mica (potassium magnesium aluminum silicate) is usable to about 1000C, but has poorer electrical properties. A synthetic fluorine-modified form of phlogopite, fluorophlogopite, has electrical
properties similar to muscovite, and thermal properties even better than phlogopite. Fluorophlogopite is used in many refractory products, but is not presently used in any electronic applications that I know
of. It may not be available in large enough plates.
Companies that advertise mica capacitors include. http://www.cornell-dubilier.com/
Paper and Oil
Oil is necessary for reliability in high-voltage film capacitors where it displaces air and moisture, and
helps prevent corona and voltage breakdown. Traditionally, the oil was held in a paper separator, but a new
method called "hazy-film" does away with the paper. With hazy-film, the film is roughened to allow it to hold
oil on its surface. In recent years, improved dry-film capacitors have seen increased usage at the low end of the high-voltage range (<2000 volts).
Small-value (a few uF and below) paper-oil capacitors have gained favor for high-end audio applications for some reason.
Porcelain Companies that advertise porcelain capacitors include: Companies that advertise vacuum/gas capacitors include:
Air Variable Capacitors
Single-Layer Variations on the single-layer RF capacitor theme include laser-trimmable chips, and multiple capacitors
on the same substrate. Companies that advertise single-layer capacitors include: http://www.avx.com/
For additional information on unusual capacitors see: http://www.geocities.com/the_ark_solved/ Too bad...dead link.
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