This document discusses the properties and types of ceramics. Ceramics are inorganic materials consisting of metallic and nonmetallic elements bonded by ionic and/or covalent bonds. They are generally hard, brittle, dense, corrosion resistant and stable at high temperatures. Common ceramics include oxides like alumina and silica. Ceramics are classified based on their composition and dielectric constant, and used in applications like insulation or capacitors. The document also describes different ceramic crystal structures defined by the ratio of cation to anion radii and coordination numbers.

1. By Rohit Tripathi

2. Ceramic
 Generally inorganic materials that consist of metallic and
nonmetallic (or two nonmetallic) elements
 Bonded by ionic and / or covalent bonds
 Has nonmetallic properties (good electrical and thermal
insulators)
 Hard , brittle and dense
 Corrosion resistant
 High temperature stability
 Generally crystalline (exception is amorphous glass)
 Ex: ZnS, Alumina (Al2O3), Silica (SiO2), Rock salt (NaCl),
CsCl, BaTiO3, Rutile (TiO2, ), Fluorite(CaF2) etc.

3. Types of ceramic
1. Porcelain
 Chemical composition of SiO2 and Al2O3.
 Used in low and high voltage applications.
 Used as insulator in transmission and distribution
system, plugs and sockets.
2. Steatite
 Composed of SiO2, MgO , Al2O3, Na2O and BaO.
 Used in high frequency, low loss and high voltage
application.

4. Types of ceramic
3. Alumina
 Used in high temperature application.
 Used in circuit breakers and resistance cores etc.
 Used as an abrasive. Many types of sandpaper use
aluminium oxide crystals.
4. Titanate
 High dielectric constant.
 Used in capacitors.
 Ex: BaTiO3, FeTiO3, PbTiO3, Al2O3·TiO2 and SrTiO3 etc.

5. Ceramic types based on ϵr
(relative permittivity)
1. ϵr˂12
 Used as insulators.
 Ex: Porcelain, steatite, alumina.
2. ϵr˃12
 Used in capacitor applications due to high dielectric
constant
 Ex: Titanate.

6. Ceramic Crystal Structure
 Still based on 14 Bravais lattices .
 The atomic bonding in ceramics is mixed( ionic and
covalent).
 The degree of ionic character depends on the difference of
electronegativity between the cations (+) and anions (-).
 Cation: Metal usually alkalis or alkaline-earths (first two
columns of the periodic table), positively charged, usually
smaller in size.
 Anion: Non metals usually O, C, or N, negative charge,
usually larger in size .

7. Ceramic Crystal Structure
 Note: Atomic or Ionic Radii
o The size of an atom or ion depends on the size of the
nucleus and the number of electrons.
o Atoms with higher numbers of electrons have larger radii
than those with smaller numbers of electrons.
o Ions will have radii different from the atoms because ions
will have either gained or lost electrons.
o As the charge on the ion becomes more positive, there will
be less electrons and the ion will have a smaller radius.
o As the charge on the ion becomes more negative, there will
be more electrons and the ion will have a larger radius.

8. Ceramic Crystal Structure
 The ratio of ionic radii (R cation or Rc/ R anion or Ra )
dictates the coordination number of anions around each
cation.
 As the ratio gets larger (i.e. Rc/Ra 1 ) the coordination
number gets larger and larger.
 Note : Coordination number (CN)
The number of atoms or ions that are in physical contact
of atom or ion of intrest.

9. Ceramic Crystal Structure
 The table 1.1 here summarizes the cation to anion radius
ratios, Rc/Ra, for various coordination numbers.
Rc/Ra C.N. Type
1.0 12
Hexagonal or Cubic
Close Packing
1.0 - 0.732 8 Cubic
0.732 - 0.414 6 Octahedral
0.414 - 0.225 4 Tetrahedral
0.225 - 0.155 3 Triangular
<0.155 2 Linear

10. Ceramic Crystal Structure
 Note
1. Tetrahedral: Geometric structure where one atom or
sphere is surrounded by 4 nearest neighbors.

11. Ceramic Crystal Structure
2. Octahedral: Geometric structure where one atom or
sphere is surrounded by 6 nearest neighbors.. It looks like
double pyramid.

12. Ceramic Crystal Structure
3. Trianlgular: Geometric structure where one atom or
sphere is surrounded by 3 nearest neighbors.
4. Linear: Geometric structure where one atom or sphere is
surrounded by 2 nearest neighbors.

13. Ceramic Crystal Structure
 Example
1. Rock Salt Structure
 NaCl structure: Rc = RNa = 0.116 nm, Ra = RCl = 0.167 nm
 ⇒ Rc/Ra = 0.69
 From the table 1.1 we see that C.N. = 6
 NaCl, MgO, CaO, BaO, CdO, MnO,
FeO, NiO, CrO, LiF have this crystal
structure.
 Structure is FCC for Na or Cl ions.
 Connection between ions is
octahedral.

14. Ceramic Crystal Structure
2. CsCl Structure
 Rc = RCs= 0.181 nm,
Ra = RCl = 0.167 nm
 Rc/Ra = 1.08 .
 Structure is SC for Cs
or Cl ions(not BCC
since there are different ions at corner
and body center positions).
 C.N. = 8 (Violates radius ratio rule).

15. Ceramic Crystal Structure
 A mono atomic simple cubic will have
coordination number of six, but this is an
ionic compound. Thus, each cation or anion
that sits in the center of the cube is bonded
to the eight anions or cations (respectively)
that sit in the vertices of the cube.
 CsCl, CsBr, CsI have this crystal structure.

16. Ceramic Crystal Structure
3. Fluorite (CaF2) Structure
 Rc = RCa = 0.100 nm, Ra = RF = 0.133 nm
 ⇒ Rc/Ra = 0.75
 From the table 1.1, C.N. = 8 for Ca
and CN = 4 for F.
 Structure is FCC for Ca ions.
 Connection between ions is
tetrahedral.
 CaF2,SrF2, BaF2, SrCl2, CdF2, HgF2, and PbF2 have this
crystal structure.

17. Ceramic Crystal Structure
4. Zinc Blende Structure(Cubic ZnS):
 Rc = RZn = 0.074 nm, Ra = Rs = 0.184 nm
 Rc/Ra = 0.402
 From table 1.1,C.N. =4
 Structure is FCC for Zn or S ions.
 Connection between ions is
tetrahedral.
 ZnS, ZnTe, SiC, AlP, CuCl, CuBr, CuI have this crystal
structure.
 Group 3-5 compounds also have this structure.Ex-GaAs,
GaP, AlP etc.

18. Ceramic Crystal Structure
5. Wurtzite(Hexagonal ZnS)
 Rc/Ra = 0.402
 From table 1.1,C.N. =4
 Structure is HCP .
 Connection between ions is
tetrahedral.
 ZnO, CdS, CdTe and BeO have
this crystal structure.

19. Ceramic Crystal Structure
6. Silica (SiO2)
 Silica can be crystalline or amorphous (Glass).
 There are three crystalline forms of silica; quartz,
tridymite, cristobalite.
 C.N. of Si =4
C.N. of O =2
 Connection between ions is tetrahedral.
 SiO2 and GeO2 have this structure.

20. Ceramic Crystal Structure
7. Alumina(Al2O3)
 Coordination Numbers/Geometry
Al CN= 6 Octahedral coordination
O CN= 4 Tetrahedral coordination
 Structure is HCP .
 Al2O3, Cr2O3, Fe2O3, Ti2O3,
V2O3 and Ga2O3 have
this crystal structure.