imperfection in ceramics

1. IMPERFECTION IN CERAMICS

2. • The word "ceramics" comes from the Greek word "Keramos"
meaning "Pottery," "Potter's Clay," or "a Potter." This Greek word is
related to an old Sanskrit root meaning "to burn" but was primarily
used to mean "burnt stuff."
• Ceramics are defined as products made from inorganic materials
having non-metallic properties, usually processed at a high
temperature at some time during their manufacture.
Ceramic products have a number of outstanding properties which
determine their usefulness. One of the most unusual of these is their
great durability. This durability can be divided into three types:
chemical, mechanical and thermal.

3. Chemical Durability - The high chemical durability of the great majority of ceramic products
makes them resistant to almost all acids, alkalis, and organic solvents. - Of further
importance is the fact that ceramic materials are not affected by oxygen. The materials
generally contained in the ceramic products have already combined with all of the oxygen
for which they have an affinity, and therefore, are not affected further by the presence of
oxygen in their environment.
Mechanical Durability The mechanical durability of ceramics is evidenced by their strength
and hardness. The compressive strengths of ceramic materials are extremely high,
normally 50,000 to 100,000 lbs/sq. in. The hardness makes ceramic materials very resistant
to abrasion. It is this property which makes them useful for floors, and for the grinding of
metals and other materials.
Thermal Durability Most ceramics have the ability to withstand high temperatures. This is
why they are useful in the production of all types of heat-containing equipment such as kilns
for the ceramic industry, and such products as the inner linings of fireplaces and home
heating furnaces.

4. Ceramics can also be classified into three distinct material categories:
Oxides-based: Silicate and non-silicate oxide ceramics (alumina, zirconia,etc)
Non-oxides: Carbides, borides, nitrides, silicides.
Composites: Particulate reinforced, combinations of oxides/non-oxides.
Properties: - oxidation resistant, - chemically inert, - electrically insulating -
generally low thermal conductivity, Notes: - relatively simple manufacturing and
low cost for Al2O3 - more complex manufacturing and higher cost for ZrO2

5. Most ceramic materials are neither purely covalently or ionically
bonded materials. In most ionically bonded materials, there is a
significant level of covalency which is decreases as the
difference between the electronegativities of cations and anions
increases. While covalent bonding is prevalent among the group
IV solids such as diamond and many other compound
semiconductors, most ceramics such as NaCl, MgO, BaTiO3,
Fe3O4 etc are predominantly ionically bonded. Covalent
bonding, arises from the sharing of orbitals and as a result
materials with this type of bonding are characterized by
significant hybridization of orbitals and directionality of the bonds
which play a crucial role in determining the crystal structure. In
contrast, ionically bonded solids are predominantly based on the
size difference between the cations and the anions and the
formation of structures in them

6. Materials in general consist of defects which
can be divided into a variety of categories such
as
point defects or 0-D defects,
line defects or 1-D defects and
2-D or surface defects.
These defects play an important role in
determining the properties of ceramic materials
and in this context, the role of point defects is
extremely important.

7. • Point defect can be primarily classified into three categories, viz. Vacancy,
Interstitial, and Substitutional. Sometime Self-Interstitial defect is also
considered another one category. However, in case of ionic crystals
(ceramics), Schottky defect and Frenkel defect can occur, which are nothing
but the combination of two different types of the above mentioned four basic
types of point defects
Vacancy – A Point Defect
A vacancy is produced when an atom is missing from its original lattice site. So vacancy creates
an empty lattice site as depicted below. Like other point defects, vacancy is also a zero-
dimensional defect. Vacancy defect puts the neighboring atoms under tension. Due to the
reduction in number of atoms in the crystalline solid, vacancy defect results in reduction of the
density. However, hardness of the solid may increase. The number of vacancies present within a
crystalline solid depends exponentially with temperature, thus with increase in temperature of
solid, number of vacancies also increases

8. Interstitial – A Point Defect
An interstitial defect occurs when an atom takes the interstitial position of the lattice
structure. This interstitial atom may be of the same crystal or of a foreign material.
Accordingly, interstitial defect can be of two types:
•Self-Interstitial Defect—occurs when atom of the same crystalline solid occupies
the interstitial position leaving its original lattice site.
•Interstitial Defect—occurs when a foreign atom occupies the interstitial position.
Although extra atom occupies the empty interstitial space, the size of the atom is
usually larger than that of the empty space. Thus the surrounding atoms are
compressed and distorted. Presence of substantial number of interstitial atoms can
change the mechanical and thermal properties of the solid. However, this is
sometime beneficial, and thus interstitial defects can be applied in a controlled way to
enhance various properties of the solid. For example, in steel production, carbon is
added with iron

9. Substitutional – A Point Defect
Substitutional Defect occurs when the original atom in the lattice
site of a crystalline solid is replaced by a different type of atom.
Unlike interstitial defect, foreign atom should occupy the lattice
site only and not the interstitial position, as depicted below. The
foreign atom may be of same size or different (either larger or
smaller). Depending on the size of the substituted foreign atom,
the neighboring atoms may remain either in tension or in
compression. Substitutional defects can be found in brass,
where zinc atoms replace copper atoms

10. Schottky Defect – Point Defect in Ionic Crystal (Ceramic)
It is one type of Point Defect that occurs in ionic crystals (ceramics). Schottky defect
occurs when oppositely charged atoms (cation and anion) leave their corresponding
lattice sites and create a pair of Vacancy Defects. So, one Schottky defect leads to the
formation of two vacancies. Since both cation and anion leave the lattice sites at the
same time, so overall electrical neutrality of the crystal is maintained; however, density
reduces because of the vacancies

11. Frenkel Defect – Point Defect in Ionic Crystal (Ceramic)
Frenkel Defect is one type of Point Defect; in fact, it is a combination of
both Vacancy and Interstitial type of point defects. Usually, this type of
defect is observed in ionic solids, where size of anion is substantially
larger than the size of cation. Basically, a Frenkel Defect is occurs when
an atom (better to say ion, especially cation) leaves its original lattice site
and occupies an interstitial position on the same crystal. Although
both—Schottky and Frenkel defects occur in ionic materials, Frenkel defect
occurs if size of anion is quite large as compared to that of the cation;
whereas, Schottky defect occurs if difference in size between cation and
anion is small. In Frenkel defect, only the smaller ion (cation) leaves its
original lattice site; whereas, the anion remains in corresponding lattice
sites. However, in Schottky defect, both cation and anion leave the solid
crystal

12. Thermal energy keeps the atoms vibrating vigorously about their lattice
positions and continually bumping into each other and exchanging energy
with their neighbors and surroundings. Every now and then, an atom will gain
sufficient energy to leave its mooring and migrate. This motion is termed
diffusion, without which the sintering of ceramics, oxidation of metals,
tempering of steels, precipitation hardening of alloys, and doping of
semiconductors, just to name a few phenomena, would not be possible
There are essentially three mechanisms by which atoms will diffuse,
as shown schematically in Fig. a to c. The first, the vacancy
mechanism, involves the jump of an atom or ion from a regular site
into an adjacent vacant site (Fig.). The second, interstitial diffusion,
occurs as shown schematically in Fig. and requires the presence of
interstitial atoms or ions. The third, less common mechanism is the
interstitialcy mechanism, shown in Fig. c, where an interstitial atom
pushes an atom from a regular site into an interstitial site.

14. • The self-diffusivity D of an atom or ion is a measure of the
ease and frequency with which that atom or ion jumps around
in a crystal lattice in the absence of external forces,
• where Q is the activation energy for diffusion which is not a
function of temperature, whereas D0 was a weak function of
temperature. It also has been long appreciated that diffusivity
depends critically on the stoichiometry and purity level of a
ceramic.

15. Fick's Second Law of Diffusion :

20. NaCl containing small amounts of CdCl2 ln such scenario, for each Cd ion, Cd occupying Na
site with an extra positive charge and a sodium vacancy with one negative charge is created
according to the following defect reaction
In addition, NaCl will also have certain intrinsic sodium and chlorine vacancy
concentration (VNa′ and VCl
•) due to Schottky dissociation, depending on the temperature. In
such a scenario, the diffusivity of sodium ions is governed by vacancy diffusion and can be
worked out as