Mica, muscovite dielectric

Mica for use as a dielectric is obtained from the mineral muscovite (KAl2(Si3Al)O10(OH)2) which can be cleaved into single-crystal plates between 0.25 and 50 fim thick. 

India is the largest producer of muscovite sheet mica. Madagascar is the principal supplier of phlogopite sheet mica. The prices for sheet mica range from less than l/kg for low-quality material to more than 2,000/kg for the highest quality. High-quality muscovite mica is used as a dielectric in capacitors. 

Mineral fillers used are calcium carbonate (CaCOj), China Clay (Al/0H [Si.,0 J), miea especially muscovite (KAyOHjFlJAlSijOj J), silica flour (SiO ), talc (Mg, (OH)j[Si Oj J), and wollastonite (CaSiOj). Parameters that need to be considered during inorganic filler selection are basicity, filler impurities that may influence the resin euring proeess, and filler hardness that eauses abrasion of the mold [1]. Mica filled phenolics are used in electrical parts because mica adds outstanding dielectric properties, high thermal and chemical resistance and low water absorption [1]. 

Muscovite has a high dielectric strength that averages between 3,000 and 6,000 volts/mil at 60 Hz for specimens 1 to 3 mils thick. It has a relatively stable dielectric constant over a wide frequency range, as is shown in Fig. 2.43. Muscovite also has a dissipation factor (tan S) that decreases with frequency. This makes ruby mica an especially useful dielectric for high-frequency apphcations. 

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