Muscovite
Muscovite is Potassium-sodium hydroxyl alumino-silicate.
Magnesium and iron substitute for aluminum, also chromium or vanadium. Also
known as RUBY MICA is chemically inert when in contact with the chemicals
typically used in electrical insulation. Mica is attacked by HF and H2SO4,
but it is resistant to other acids, solvents, and alkalis. Unless precautions
are taken, liquids such as water or oil can work their way in between the
laminae of mica by capillary action. Water lowers the electrostatic force of
attraction between the layers of mica by an exchange mechanism of hydroxyl for
potassium ions.
Out gassing becomes intense above 600 °C, and prolonged exposure at high temperatures results in further release of water and finally in decomposition of mica. For this reason, mica spacers and insulators should be out gassed before being used in vacuum tubes or other devices where out gassing is a problem.
The electrical resistivity of muscovite decreases with increasing temperature. Because of the removal of moisture from the surface as well as from between the exfoliated layers, the electrical resistivity of the degassed mica is higher than that of the untreated material.
Muscovite has a high dielectric strength that averages between 3000 and 6000 volts/mil at 60 Hz. It has a relatively stable dielectric constant over a wide frequency range. Muscovite also has a dissipation factor (tan d) that decreases with frequency. This makes Muscovite Ruby Mica an especially useful dielectric for high frequency application.
The Muscovite mica provides excellent electrical
properties and the thermal decomposition temperature is about 600 °C.
