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Monday, 7 October 2019

Classification of Ceramic Materials

Ceramic Materials :


Ceramics can also be called porcelain, potter's earth or clay. It is the product which is produced by the action of fire on an earthly material. These clay materials are sintered and fused at high temperature of about 1500°C. Structural ceramics are crystals bonded together. A few of known ceramics are composed as follows :

Insulator porcelain - Clay, quartz, feldspar
High frequency porcelain - Clay, quartz, barium carbonate
Steatite - Clay, talc, magnesite

Ceramics can be produced by a simple manufacturing process which involves mixing finely ground clay and metal oxide with water just sufficient to make a paste which is shaped according to the requirements of job. Then this mould is finally dried and fired at temperature ranging between 1200°C and 1700°C. The main features of the ceramics are :

(a) It is hard, strong and dense.
(b) They are completely stable at high temperatures.
(c) They possess excellent dielectric properties. Its electric strength is 24 kV / mm.
(d) They are stronger in compression than in tensions and can easily be moulded in any shape.
(e) They are weak in impact.
(f) They are not affected by chemical action by strong acids and alkalies.
(g) They are not affected by moisture.

Porcelain Insulators :

Porcelain materials are used to make different types of pole insulators, insulators for transformer bushing, pins, suspension type insulators for transmission and distribution lines, porcelain parts for switches, plugs, sockets etc. Porcelain is also used in heating appliances, discs for electric stoves, isolating beads for kettles and heaters and fuse holders. The composition consists of Kaoline (china clay), quartz as filler agents, Fledspare pegmatite as fluxing agents, chalk and dolomite as glaze producing agent. Clay gives plasticity and make it suitable for moulding. Quartz serves to reduce shrinkage, deformation and prevents cracking during firing. Fluxing agent reduce the firing temperature.



Steatite :

It is made of hydrated magnesium silicate mixed with small quantities of clay and fledspar. It has better electrical properties, excellent compressive strength, and absorbs less moisture. In order to reduce the penetration of moisture, ceramic insulators are glazed. Glazing also prevents dust from accumulating on the surface of the insulator. It is used in making equipments for high frequency systems and where thermal shock resistance is desired. It has much high tensile and bending stress than porcelain and is advantageous to be used in those transmission lines which take a sharp turn.

Alumina :

It is primarily made of aluminium oxide. Its manufacturing is difficult due to extremely high firing temperature which is of the order of 1750°C. It has high compressive strength, absorbs less moisture and is excellent in heat conduction. It is used in high temperature appliances like furnaces.

Titanate Ceramics :

This is made from metal oxides and titanium oxide. Barium titanite has very high dielectric constant and positive temperature coefficient of resistance. Its dielectric constant is 10.4. It is used in phonograph, pick up, accelerometers etc.

Some of the important properties of Ceramic insulating materials are given in below Table.

Properties of Important High Ceramics

Property
Alumina
High voltage porcelain
Low voltage porcelain
Steatite
Ziroon
Dielectric constant (60 Hz, 25°C)
8 - 9.5
5.7
5.5 - 7.0
5.7 - 6.5
7.1 - 9.1
Power factor (x 103, 25°C, 60 Hz)
0.5 - 1.5
8-25 
7-20  
1.2 - 2.0
1.5 - 36.0
Tensile strength (kg/cm2 x 103)
0.56 - 3.5
0.2 - 0.56
--
0.52 - 1.05
0.49 - 1.04
Compressive strength (kg/cm2 x 102)
5.6 - 30
1.75 - 5.6
1.75 - 4.2
4.2 - 30.5
5.6 - 7.0
Volume resistivity (ohm-cm 25°C)
1014 - 1016
1012- 1014
1011- 1014
1012- 1015
1013- 1015
Specific gravity
3.1 - 3.9
2.35 - 5.1
2.2 - 2.4
2.5 - 2.9
3.1 - 3.8
Softening temperature °C
1450-2000
1300-1335
1300-1335
1300-1450
1390-1550
Structure limiting temperature °C
1400-1802
1000-1200
1000-1100
1000-1180
1000-1300
Heat conductivity ( x 103)
7.50
2 - 8
3.4 - 4
5 - 8
1-15
Water absorption
0.0 - 0.2
0.0 - 0.5
0.5 - 2.7
0.0 - 0.03
0.0 - 0.02

Effects of Temperature and Moisture on Ceramics :

Electrical resistance of ceramics decreases very rapidly with the rise of temperature. The effect of temperature on dielectric constant for different types of ceramics is shown in Figure. Ceramics are not much affected by moisture. In order to reduce the penetration of moisture, ceramic insulators are glazed. Glazing also reduces the dust accumulation on the surface of the insulator.



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