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Tuesday, 8 October 2019

Mica and Mica Products

Mica and Mica Products :

Mica is a mineral inorganic material having natural cleavage and splits. These are obtained from mines and then refined and machined in the required forms. It is one of the best natural insulating materials available. It is found in India in huge quantity. Mica is a mineral compound of silicate of aluminium with silicates of soda potash and magnesia. It occurs in the form of crystals which can easily be split into laminate. It is least affected by heat but dehydrates at high temperatures. It is not affected by moisture, acids, alkalies and chemicals. It has a high dielectric strength 40 to 150 kV / mm and low power loss. It is mouldable at 150° C temperature. It is rigid tough and strong, but affected by oils. Its electrical properties are deteriorated with the presence of quartz and feldspar.
Mica sheets are used as insulation between commutator segments. Thick mica tape is largely used for taping the armature and field coils of traction motors and for taping stator coils of high voltage alternators. It is used in domestic appliances like electric iron, heater, toaster etc. When mica flakes are held together with adhesive, it is called manufactured mica and is used in motor slot lining, in transformers as insulation.


Muscovite Mica :

Its basic chemical composition is KH2A13 (SiO4)3. It is strong, tough and less flexible. White and ruby coloured varieties are superior to green type. It is not hygroscopic and it is not affected by alkalies, acids and chemicals. Its dielectric strength is 40 to 150 kV / mm and dielectric constant 6 to 7. It stands a maximum temperature of 500°C and looses its water at high temperature. It has high abrasion resistance. It is used in commutators due to high abrasion resistance. It is also used in capacitors due to its high dielectric constant. Table shows the general properties of two main types of mica.

Properties of Mica:

Property

Muscovite Mica
Phlogopite Mica
Max. Operating temperature (°C)
500 - 600
800 - 900
Moisture absorption
Low
Low
Specific gravity
2.7 - 3.2
2.6 - 2.8
Tensile strength (kg / cm2 x 10-3)
0.35 - 0.7
--
Resistivity (ohm - cm at 25°C)
1015 - 1016
1012- 1014
Dielectric strength (kV / mm at 25°C)
80
60
Power factor at 25°C
1 x 10-4 - 3 x 10-4
10 x 10-4 - 50 x 10-4

Phlogopite Mica :

Its basic chemical composition is KH (MgF)3 MgAI (SiO4)3. It is also known as Magnesium mica. This type of mica is dark in colour and is inferior to Muscovite Mica in electrical qualities. However, it withstands double the temperature i.e., about 1000°C. It has high thermal stability. It is lighter and more resistant to alkalies than acids. It is used in domestic appliances like electric irons, toasters, hot plates. It is more suitable for commutator segments insulation as the wear rate of this is equal to that of copper, hence lesser number of grooving is necessary.

Glass Bonded Mica:

It is made by moulding ground mica flakes and powdered glass in the ratio of 40/60 or 60/40. It can be moulded and machined very accurately. It possesses high dielectric strength and low dielectric loss. It is impervious to water and is chemically stable. This material finds its use in high humidity and high ambient temperature atmosphere.


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