Advanced ceramics:
1. Glass ceramics:
Glass-ceramics are polycrystalline materials of fine micro structure that are produced through controlled crystallisation (devitrification) of base glass. Glass ceramic materials share many properties with both glass and ceramics. These are special glass composition that are thermally treated prior to forming operations to devirtify or precipitate a crystalline phase from the material; this phase gives special properties such as zero thermal expansion for applications involving high thermal-shock applications.
Numerous silicate based wastes, such as coal combustion ash, slag from steel production, fly ash and filter dusts from waste incinerators, mud from metal hydrometallurgy, different types of sludge as well as glass cullet or mixtures of them have been considered for the production of glass-ceramics.
The composition in which nucleation and crystallisation have been commercially produced are : Li2O-Al2O3-SiO2, MgO-CaO-SiO2, MgO-CaO-SiO2-P2O5
Charectirestics:
u Very low coefficient of thermal expansion.
u High thermal conductivity.
u High mechanical strength.
u Easily fabricated.
Properties and Uses of Some Glasses and Glass-Ceramics
Composition
|
Property
|
Use
| ||
Glasses
| ||||
Al2O3, MgO, CaO, SiO2
|
Translucent, chemically
resistant |
Window glass, bottles
| ||
PbO, SiO2
|
High refractive index
|
Lead crystal
| ||
B2O3, SiO2, Na2O
|
Acid resistant, low
expansion on heating |
Pyrex
| ||
Glass Ceramics
| ||||
MgO, Al2O3, SiO2
|
Insulator with high
mechanical strength at high temperatures |
Spark plug insulators
| ||
CaSiO3, CaMgSi2O6,
CaAl2Si2O8 |
Wear resistant
|
Building materials
| ||
Li2Si2O5
|
Resistant to thermal
shock |
Nose cones on
rockets, cookware |
2. Dielectric ceramics:
These ceramic material are used as both electrical insulators and as functional parts of an electric circuit such as capacitors, memories, sensors and actuators.
An important property of a dielectric is its ability to support an electrostatic field while dissipating minimal energy in the form of heat. The lower the dielectric loss (The operation of energy lost as heat), the more effective is a dielectric material.
The electric insulators can be breakdown under high electrical voltages, the insulators are designed with lengthened surface paths to decrease the possibility of surface shorting. Since internal pores and cracks provide opportunity for additional surface failure, the insulator are glazed to make them non-absorbent.
Non-linear dielectric ceramics are suitable in the miniaturising the electronic parts which have led to the development of increasingly sophisticated electrical circuitry.
Examples : Ferroelectrics such as BaTiO3 and PbTiO3 are used as sensors and actuators.
Lead zirconate titanate (Chemical formula Pb[ZrₓTi₁₋ₓ]O₃ ).
3. Cermets:
Cermets are composites in which ceramic material and metals join together (Ceramic plus metal = Ceramet), typically to give something with the high temperature performance or wear resistance of a ceramic and the toughness, flexibility or electrical conductivity of metal.
- Electrical components are one obvious application, Because they can get extremely hot, they need to behave like ceramics but, since they also need to conduct electricity, it helps if they work like metals. Cermets also make them useful for reducing friction in machine parts.
- These cermets are used in break shoes lining, oxidation resistant parts and inject engines.
- Machine tools are another increasingly common use for cermets, which offer greater toughness and wear resistance than more traditional materials. Cemented carbide such like composites are extensively used as cutting tools for hardened steels.
Cermets meaning --https://youtu.be/KF6RvzXlXAE
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