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Saturday, 27 July 2019

Carbon - Properties and Uses

Carbon is mostly available at the earth in the form of graphite. This contains about 90% carbon. The armorphous carbon is found in the form of coal coke, charcoal, petroleum etc. The manufacturing process of electrical carbon products consists of grinding of the raw carbon materials, With the binding agent, mix the powered carbon (coal tar) moulding of requisite components and baking them. Copper or bronze powder is mixed with carbon moulding compound to increase the conductivity. Carbon has very high resistivity 4600 x 10-8 ohm-m in graphite form. It has negative temperature co-efficient of resistance, pressure sensitive resistance, and low surface friction. The main application of carbon is brush for DC motors. Some important properties of carbon are listed below:




(a) Pure carbon is a semi-conductor.
(b) Carbon has negative temperature co-efficient of resistance.
(c) It has conductivity slightly less than that of metals and their alloys.
(d) Its melting point is 3900°C
(e) Its density is 1.7 to 35.
(f) Its electrical resistance of carbon contact decreases as the pressure increases.
(g) It has a low surface friction.
(h) It is not affected by moisture, acids and bases.
(i) Its resistivity is 1000 to 70000 micro ohm-cm.
(j) It temperature co-efficient is — 0.0002 to — 0.0008.
(k) It gets oxidised in air beyond 300°C.
(I) It possesses good thermal conductivity.
(m) It can withstand arcing very nicely.
(n) Powdered carbon in conjunction with binding material can be used for making moulds of suitable shapes.

The Uses of Carbon are:

(a) It is used extensively as brushes as it possesses mirror smooth surface, sufficient strength and adequate conductivity.
(b) Used as electrodes for electric arc furnaces.
(c) Used in electrolytic baths and arc welding.
(d) Can be used in arc lamp and as carbon resistors film type and solid type,
(e) Applicable in battery cells.

Table gives the properties of different forms of carbon.

Properties of different forms of Carbon

Type of Brushes
Commutator brushes at velocity of 25 m /sec current density of 20 A/sq. cm and pressure of 0.8 kg/sq. cm
Kind of Brush
Hardness kg/sq.mm
Resistivity x ohm-m
Contact voltage drop / brush pair V
Maximum wear in 20 hrs mm
Max. Friction, co-efficient
Graphite
8 - 22
800 - 2200
0.6 – 1.8
0.4 - 0.5
0.3
Electro- graphitised
3- 50
6- 50
1.1- 2.5
0.4 - 0.6
0.23 - 0.3
Carbon graphite
1 - 42
40 - 50
1.5 – 2.5
0.1
0.3
Copper graphite
4 - 25
0.0 - 15
0.1 – 2.2
0.15 - 3 8
0.2 - 0.26



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