Glass is seldom seen in industrial electronics but is more popular in big-dollar military and aerospace applications. Glass capacitors are in the Mars Rovers
for example. AVX is the only manufacturer I know of. Glass capacitors are seriously expensive, I have seen list prices in the 10s to 100s of dollars. The main attractions include negligible aging rate,
very high radiation tolerance, a very wide temperature range (-75 to +200C for some types), and an overall ruggedness and reliability.
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:http://www.avx.com/
Mica is any one of 28 minerals, all of which have aluminum silicate as their main building
block. Most are very rare, and only a few have commercial value. For those that do however, applications abound. Micas in general, tend to be tough, heat and chemical resistant materials. The one
preferred for capacitors is a selected form of muscovite mica (potassium aluminum silicate, K2Al4Al2Si6O20(OH)4) variously called "India ruby mica"
or "ruby muscovite mica". Like other micas, muscovite mica splits easily into thin, tough sheets. To make capacitors, the plates are usually plated with silver. Mica was the first
mass-produced capacitor, dating back to before 1920. Over the years, mica has been replaced by newer capacitors in many applications, but it still has many important niches.
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.susco.com/ SUSCO imports SAHA brand micas from India
http://www.reynoldsindustries.com/product/7micacapacitor/ Look for graphs on dissipation factor and other
http://www.iihr.uiowa.edu/~hml/people/kruger/Publications/ChipCenter/micacap.pdf General information on mica.
Paper and Oil
You're probably as likely to find a paper capacitor in your TV set as you are to find a vacuum tube.
However, oil-impregnated paper capacitors live on as high-capacity (hundreds of uF, at least), high-voltage
(>100,000 volts, at least) power capacitors. Some capacitors have paper-oil with polyester or polypropylene
film. High dissipation factor means that oil-paper capacitors are best suited for DC applications or for AC
applications with limited duty cycles, like motor starting. Continuous duty AC applications (motor run, power-factor correction) are best left to large polypropylene capacitors.
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 capacitors are rather similar to C0G ceramics. They are used in RF and microwave
applications for their very high Qs. Temperature drift is usually specified as +90 ppm/C (P90), but can be found with TCs of +-30 ppm/C, and even +-15 ppm/C. In fact, some are referred to as "NP0 (or C0G)
porcelains". They are commonly available in SMD.
Companies that advertise porcelain capacitors include:
http://www.dilabs.com +-15 ppm/C ceramics
Vacuum and gas
Vacuum and gas capacitors are used in RF transmitter applications that require characteristics such as
working voltages in the 10s of thousands of volts, RF currents in the hundreds (or even thousands) of amperes, and good ruggedness and overload capacity. Capacitance is limited to a few thousand pF. They
come in both glass and ceramic envelopes, some are water-cooled. Both fixed and variable types are available.
Companies that advertise vacuum/gas capacitors include:http://www.jenningstech.com/ Jennings also makes vacuum relays and other
devices. Now a Danaher company.
www.omnicor.com/vacuum-capacitors.htm Also vacuum interrupters.
http://www.comet.ch/index.php?id=22 Swiss company.
Distributor for vacuum and mica capacitors.
Air Variable Capacitors
Air variable capacitors (AVC) were once used as the tuning element in almost every radio, but
electronic tuning schemes have largely eliminated them in modern radio equipment. Although AVCs are not made in the numbers they once were, they have found a number of important niche applications that call for
their unique properties. These include RF test equipment, ham radio gear (including antenna tuning) , nuclear magnetic resonance imaging machines, and controlled plasma etch equipment used in semiconductor
Construction details vary. The insulating materials may be silicone-treated steatite, or a number of
plastics. Some have bronze bushings, others have ball bearings. Most have aluminum plates, but other metals
can be used for better current carrying capacity, or better temperature stability. Other options include silver
plated plates, high-current rotor contacts, various capacitance-vs-rotation curves, multiple sections, built-in
trimmers, and gear reduction drives. AVCs are available to at least 1500 pF, and with operating voltage ratings to over 10 kV.
Companies that advertise air variable capacitors include:
Oren Elliot Products
Single-layer SMD capacitors are used in RF and microwave applications, to at least 50 GHz, for their
low inductance, and low ESR. C0G ceramic and porcelain are typically used for tuning and filtering, while higher K (and lower quality) dielectrics are often used for less demanding applications, like dc blocking.
AVX (and others) makes single-layer SMD microwave capacitors by the vapor deposition of advanced
dielectrics on glass substrates. These include silicon dioxide, silicon nitride, and tantalum pentoxide. They are
claimed to have characteristics better than porcelain and C0G capacitors, including lower ESR, superior power handling capability, and low and consistent inductance. Capacitors made of silicon dioxide grown on
silicon chips are known as MOS capacitors.
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://onchip.com SiC and SiN capacitors
For additional information on unusual capacitors see:
http://www.geocities.com/the_ark_solved/ Too bad...dead link.
http://www.fractalspin.com/x/product.php?productid=125 Not sure what market this is aimed at.