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Ceramics

Ceramic materials are the compounds which are inorganic and nonmetallic. Examples are oxides, nitrides and carbides. Traditional ceramics are porcelain, bricks, tiles, etc.

Crystal Structure of Ceramics

Structure of ceramic materials depends on following criterions:

1. Magnitude of electrical charge on each component ions.

2. Relative sizes (radii) of the cations (rC) and anions (rA). Cation are normally smaller than anions (rC/rA < 1.)

Stable ceramic crystal structures form when all anions surrounding the cations are in contact with it. Coordination number is related with cation to anion radius ratio.

UnstableStable

Coordination number Cation-Anion radius ratio

2 <0.155

3 0.115-0.225

4 0.225-0.414

6 0.414-0.732

8 0.732-1

12 >1

Most common coordination numbers are 4, 6 and 8.

AX-type crystal structures: Rock salt structures-NaCl, CsCl. CN = 6 for both cations and anions.

Coordination number for both cations and anions = 6

Cation/ anion radius ratio = 0.414-0.732

Some common ceramic materials with this type of structure = NaCl, MgO, MnS, LiF, FeO.

Structure: FCC

Coordination number for both cations and anions = 8

Cation/ anion radius ratio = 0.732-1

Structure: Simple Cubic

Coordination number for both cations and anions = 4

Structure is known as zinc blend or sphalerite structure

Cation/ anion radius ratio = 0.225-0.414

Some common ceramic materials with this type of structure = ZnS, ZnTe, SiC

Structure: FCC

Fig. A unit cell of ZnS

Zn

S

AmXp-type crystal structures: If the charges on cation and anion are

not same then a compound can exist with AmXp chemical formula. Example: CaF2, Fluorite structure.

rC/rA = 0.8

Coordination number

(for cation) = 8

(for anion) = 4

Examples: UO2, PuO2, ThO2. Structure: SC

F-

Ca2+

AmBnXp type crystal structures: Ceramic materials having more

than one type of cations represented by A and B in the general formula type. Example: BaTiO3 (Barium Titanate) having B2+ and Ti4+ cations.

Coordination number

(for A cation, Ba2+ in this case) = 12

(for B cation, Ti4+ in this case) = 6

(for anion, O2- in this case) = 6

Structure: Perovskite - FCC

4Ti

2Ba2O

Another AmBnXp type crystal structures is spinel structure exhibited by MgAl2O4. In FCC lattice of O2-, Mg2+ fills tetrahedral sites and Al3+ fills octahedral sites. Magnetic ceramics (or ferrites) have similar structure with slight variation.

Ceramic Density Calculation:

The theoretical determination of density of ceramics can be done with the help of unit cell data by using the equation,

AC

AC

NV

AAn

)('

Where,

cellunit e within thunits formula ofNumber ' n

cellunit in the cations all ofAW of Sum CA

cellunit in the anions all ofAW of Sum AA

volumecell Unit CV

number sAvogadro' AN

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Example of Ceramic Density Calculation: Calculation of theoretical density of NaCl.

a

Na Cl

4' n

In the case of NaCl

g/mol 45.35A Cl AA

g/mol 99.22A Na

CA

33 )22(a ClNaC rrV

AClNa

ClNa

Nrr

AAn3

'

)22(

)(

Thus,

23377 1002.6)]10181.010102.0(2[

)45.3599.22(4

3g/cm 14.2

Silicate Ceramics: Silicates are the materials which are composed are silicon and oxygen. There are various forms of silicate materials depending upon different ways in which SiO4

4-

tetrahedron units can be combined in one-, two-, three-dimensional arrangements. In this tetrahedron each silicon atom is bonded with four oxygen atoms, which are positioned at four corners of the tetrahedron.

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There are three polymorphic crystalline forms of silica depending upon the arrangement of SiO4

4- tetrahedron – Quartz, crystobalite and tridymite.

Quartz Cristobalite Tridymite

Structure of various forms of silica are complicated an open i.e. atoms are not closely packed together. Thus these crystalline forms have low density. However, strength of Si-O bonding is very high resulting into a very high melting temperature, 1710 0C.

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Silica also exists in non-crystalline solid or glass which is known as vitreous silica or fused silica. In the glass also the basic unit is SiO4

4-

beyond this structure disorder exists.

Other oxides which may form glassy structure are B2O3, GeO2.

Crystalline sheet Amorphous silica

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Mechanical properties of ceramics:

Mechanically ceramics are inferior than metals. Both crystalline and amorphous ceramics fracture before any plastic deformation in response to the tensile stress at room temperature.

Despite strong interatomic bonding forces, the inferior mechanical properties of ceramic materials can be explained by formation and propagation of cracks in it. The measure of ceramic material's ability to resist fracture when a crack is present is specified in terms of fracture toughness. The plain strain fracture toughness (KIc) is given as

aY IcK

Where Y is dimensionless quantity which depends on specimen and crack geometry. is the applied stress and “a” is the length of surface crack.

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Crack propagation leading to fracture.

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Other mechanical considerations:

Influence of porosity:

Despite heat treatment of ceramic materials during synthesis, some pores are left into the material which have deleterious influence on both the elastic and strength of the materials. The effect of porosity on modulus of elasticity E is given by the equation,

)9.09.11( 20 PPEE

E

P

Where P is volume fraction porosity. There are two reasons for the reduced flexural strength

(1)Pores reduce cross sectional area across which a load is applied and

(2)Pores act as stress concentrators as for an isolated pore, an applied stress is amplified 2 times.

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Hardness of ceramic materials: This is one beneficial mechanical property of a ceramic material. This very property is very often utilized when used as an abrasive.