These are a kind of mica capacitor which has an additional advantage of reduced moisture infiltration. These capacitors are expensive and are used often in HF and low VHF radio frequency circuits as low value accurate capacitors particularly in the oscillators and filters. The reasons that these capacitors are still in use regardless of high cost, large size and availability of other low cost capacitors are due to its remarkable features such as:
·low tolerance of +/- 1%
·positive temperature coefficient of 35 to 75 ppm/C
·greater range from few pF to two or three pF
·little voltage dependence,
·Good Q factor.
However, these capacitors are not widely used these days.
These capacitors are fabricated of glass dielectrics and are very expensive which are used for highly accurate, stable and reliable operation in harsh environmental conditions. These are resistant to nuclear radiations and available in range of 10pF to 1000pF.