Transcript ceramics - 123SeminarsOnly.com

Properties of Ceramic
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They are distiguished by its bonding which
is ionic or covalent bonding
High elastic modulus and hardness
High melting point
Low thermal expansion
Good chemical resistance
Brittle
Thermal insulators
Electrical insulators
Nonmagnetic
Ceramic Structures : Crystal
Structure
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Factors that affect crystal structure of
ceramics :
- the magnitude of the electrical charge
on each of the component ions and the
relative sizes of the cations and anions
- the sizes or ionic radii of the cations
and anions rC/rA
Ceramic Structures
Crystal structures
Atomic bonding: ionic bonding
Positive charges (metal) cation
Negative charges (non metal) anion
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anion
kation
AX-Type Crystal Structures
A: cation X: anion
1. Rock Salt Structure
~e.g: NaCl, MgO, MnS, LiF
~The coordination
number for both
cations&anions: 6
~Structure: FCC
2. Cesium Chloride Structure
~The coordination number: 8
~e.g: CsCl
3. Zinc Blende Structure
~The coordination
number: 4
~atomic bonding:
covalent bonding
~e.g: ZnS, ZnTe, SiC
AmXp-Type Crrystal Structure
When charges of cations and anions are
not the same; where m and/or p ≠ 1
 AX2 e.g: CaF2, UO2, PuO2
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AmBnXp-Type Crystal Structures
It is possible for ceramic compounds to
have more than one type of cation
 A & B: cation
X: anion
 Structure: perovskite
 e.g: BaTiO3
(Barium titanat)
cation: Ba2+, Ti4+
anion: O2
Classification of Ceramics
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Glasses : 1. Glasses
2. Glass-ceramics
Clay Products : 1. Structural clay products
2. Whitewares
Refractories
: 1. Fireclay
2. Silica
3. Basic
4. Special
Abrasives
Cements
Advanced ceramics
Glasses (1)
1.
Glasses
They are noncrystalline silicates containing other
oxides
Glass
Type
Composition
SiO2 Na2O
Other
Fused
Silica
>99.5
96% Silica
(VycorTM)
96
Borosilicate 81
(PyrexTM)
CaO
Al2O3
B2O3
Characteristics and
Application
High melting temperature,
very low coefficient of
expansion (thermally
schok resistant)
3.5
2.5
4
Thermally shock and
chemically resistant –
laboratory ware
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Thermally shock and
cehimcally resistant –
ovenware
Glasses (2)
2. Glass-ceramics
 Formed glass ceramics : We use a process that
called crystallization which is the most
inorganic glasses can be made to transform
from a noncrystalline state to one that is
crystalline by the proper high temperature heat
treatment
 Properties : relatively high mechanical strength,
low coefficients of thermal expansion, relatively
high temperature capabilities, good dielectric
properties, good biological compability
 Applications : ovenware, tableware, oven
windows, and rangetops, and etc
Clay Products
1. Structural clay products
 Applications : building bricks, tiles, and
sewer pipes
2. Whitewares
Whitewares become white after the high
temperature firing
 Applications : porcelain, pottery,
tableware, china and plumbing fixtures
(sanitary ware)
Refractories (1)
Fireclay refractories
 The primary ingredients : high purity fireclays,
alumina, and silica mixtures usually containing
between 25 and 45 wt% alumina.
 Applications : in furnace construction, to cofine hot
atmospheres and to thermally insulates structural
members from excessive temperatures.
2. Silica refractories
 Contains over 95% of SiO 2 and not more than 3%
CaO both by weight
 Well known for their high temperature load bearing
capacitiy
 Applications : arched roof of steel and glass making
furnaces
1.
Refractories (2)
3. Basic refractories
 Have a high propotions of basic content
like CaO and MgO. Also contain chromium
and iron compounds
 Applications : use in some steel making
open hearth furnaces
4. Special refractories
 Some of these are relatively high purity
oxide materials
 Applications : For example SiC are used
for electrical resistance heating elements
Abrasives
The most common examples of
abrasives are diamond, silicon carbide,
tungsten carbide, corundum, and silica
sand
 The prime requisite of this material is
hardness
 Applications : bonded in grinding
wheels, as coated abrasives, as loose
grains, etc
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Cements
Cement used in construction is
characterized as hydraulic and nonhydraulic
 The characteristic feature in this material
is when mixed with water, form a paste
 Cementitious bond develops at room
temperature
 Applications : mortar and concrete
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Advanced Ceramics (1)
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MEMS (Microelectromechanical
Systems)
Miniature “smart” system consisting of a
multitude of mechanical devices that
are integrared with a large numbers of
electrical elements on a substrate of
silicon
Applications : accelerometer,
microsystem for DNA, chemical
detectors, etc
Advanved Ceramics (2)
2. Optical Fibers
 Is made of extremely high purity silica
3. Ceramic Ball Bearings
 Consists of balls and races that are in
contact with and rub against one
another when in use
Fabrication and Processing Of
Ceramics (1)
Fabrication and Processing Of Glasses
and Glass-ceramics
 Glasses are formed at elevated
temperature, hence, we have to
consider the temperature viscosity
 Glass forming techniques : pressing,
blowing, drawing and fiber forming
 After fabrication, glasses may be
annealed and/or tempered to improve
mechanical characteristics
1.
Fabrication and Processing Of
Ceramics (2)
2. Fabrication and Processing of Clay Products
 Commonly, there are two forming techniques, hydroplastic
forming and slip casting
 After forming, the body must be dried and then fired at
elevated temperature
3. Powder Pressing
 Used to fabricate both clay and non clay compositions
 Three basic powder pressing : uniaxial, isostatic, and hydro
pressing
4. Tape Casting
 Pouring the slip onto a flat surface
 A doctor blade spreads the slip into a thin tape of uniform
thickness
 Tape thickness normally range between 0.1 and 2 mm
 Usually used in the production of ceramic substrates