Thursday, 24 January 2013

What are Ceramics | Their Classification | Structure | Properties


The word "ceramics" is derived from an ancient Greek word "keramos",
which refers to potter's clay and the objects made from it.

What are ceramics ???? 

Defination :-
1) Ceramics are Inorganic, Non-Metallic compounds for which Interatomic Bonds may be either of the following two types :-
a) Ionic, but with some Covalent character or
b) Covalent in some cases.

2) Ceramics are prepared by heating inorganic , nonmetallic solids and
then cooling them to retain the given shape.

Classification of Ceramics :-
Ceramics can be classified on the basis of their Industrial applications and structure as follows :-

i) Functional Classification :- Abrasives, Fireclay products, Cementing products, Glass, Rocks, Minerals, Clay, Refractories etc.

ii) Structural Classification :- Crystalline ceramics & Non-Crystalline ceramics.

Structure :-
1) Ceramics have crystal structure which is similar to a metal structure.
2) Atoms of ceramics are ionically bonded or covalently bonded.
3) Ionically bonded ceramics are hard ,rigid and posses high chemical stability. Example:- Nacl.
4) Covalently bonded ceramics are also hard,rigid and posses high electrical resistance. Example:- SiO
5) Combinedly bonded ceramics are similar in properties and show better characteristics than both. Example :- MgO.

Properties :- The following properties of ceramic materials make them useful for Engineering Applications :-

1) Mechanical Properties :-

i) Ceramic materials have low tensile strength.
ii) They generally fail due to stress concentration on cracks, pores etc.
iii) Ceramics posses great hardness and resistance to wear and can be used for grinding.
iv) Ceramics posses high compressive strength.
v) Most ceramics posses low fracture strength and fail in a brittle manner.
vi) Values of Modulus of Elasticity for ceramics ranges from 7 x 1010
N/m2 to 40 x 1010 N/m2 .

2) Electrical Properties :-

i) Ceramics are often used for electric insulation.
ii) Some ceramics conduct electrically well and are used as Semiconductors i.e N.T.C & P.T.C resistors.
iii) Many ceramics have a dielectric constant value upto 100 and very low dielectric losses.
iv) Some ceramics also exhibit piezoelectric properties and can transfer mechanical deformations in to voltage changes.

3) Chemical Properties :-

i) Majority of ceramics are highly resistant to all chemicals and
organic solvents.
ii) Ceramics are completely resistant to oxidation even at high temperature.
iii) Glazed porcelain ceramics is used for chemical vessels.

4) Optical Properties :-

i) Many types of glasses are used for windows and optical lenses.
ii) They also find use in selective transmission or absorption of certain wavelengths.

5) Thermal Properties :-

i) Ceramic materials do not have enough electrons for bringing about
electronic thermal conductivity.


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