Mica is a mineral that can be readily sliced into thin chips, makes an excellent dielectric and is used in capacitance incorporated in circuits requiring stable and accurate capacitors with low losses and high leakage resistance. Mica sheets are brittle and cannot be rolled up like a paper dielectric. The capacitor consists either of alternate layers of metallic foils, with mica forming a stack which is then hermetically sealed in some hard plastic or alternatively the mica can be silvered stacked and enclosed in a plastic moulding. The main characteristics of this type of capacitor are low power factor, high voltage operation and long term stability at room temperature. Precision mica capacitors, used as sub-standards Can be adjusted to better than 0.01 percent stability for values over 1μf. They are sealed in cases to protect against moisture. The silvered mica plate capacitor has a better temperature coefficient when compared to the stacked plate type. Both types exhibit slight non-cyclic capacitance shifts during temperature variation yielding a nonlinear temperature capacitance characteristics.
Specifications
Capacitance range 1pf to few micro farad
Ratings up to 2000V
Temperature 550C to 1250C
Dielectric constant 8
Power factor approx 0.0003 at 1 MHz
Applications: They are used in filter circuits and tuning circuits.