7.structures and properties of ceramics
TRANSCRIPT
-
7/28/2019 7.Structures and Properties of Ceramics
1/16
Structures and Properties ofCeramics
-
7/28/2019 7.Structures and Properties of Ceramics
2/16
Introduction
Ceramics are groups of non-metallicinorganic solids formed between metallic(e.g. Mg, Al, Zr etc.) or semi-metallic (e.g.
Si ) and non-metallic elements (e.g. O, C,N etc.).
Ceramics may be fully crystalline (e.g.magnesia), fully amorphous (glasses) or
partially crystalline where crystallinegrains are cemented together by anamorphous matrix.
-
7/28/2019 7.Structures and Properties of Ceramics
3/16
Familiar ceramics such as tableware areclay based, but for advanced engineeringapplications ceramics are often chemically
synthesized (or use highly reformednatural materials).
The group of engineering ceramics of mostinterest for SOFCs is electroceramics, e.g.
alumina (Al2O3), and yttria-stabilised-zirconia (YSZ).
-
7/28/2019 7.Structures and Properties of Ceramics
4/16
Bonding in Ceramics
Ionic Bonding is very common inceramics containing mixtures of metallicand nonmetallic elements such asmagnesia (MgO). Ionic bonding is non-directional.
Covalent Bonding is common in specialceramics such as silicon carbide (SiC).Covalent bonding is directional.
Mixed Bonding consisting of ionic,covalent (and often molecular) bonding isthe norm for ceramics.
-
7/28/2019 7.Structures and Properties of Ceramics
5/16
Ceramic material are inorganic compounds consisting of metallic
and non-metallic elements which are held together with ionic
and/or covalent bonds
-
7/28/2019 7.Structures and Properties of Ceramics
6/16
THE DEGREE OF IONIC CHARACTER
The degree of ionic character of a ceramic compound can beestimated approximately
using the following formula:
% Ionic Character = { 1 - exp[-0.25(XA-XB )2]} x 100XA = Electronegativity of element AXB = Electronegativity of element B
-
7/28/2019 7.Structures and Properties of Ceramics
7/16
Structure Of Ceramics
Number of Atoms per Unit Cell
When counting the number of lattice points belonging toeach unit cell, we must recognize that lattice points may beshared more than one unit cell.
-
7/28/2019 7.Structures and Properties of Ceramics
8/16
-
7/28/2019 7.Structures and Properties of Ceramics
9/16
-
7/28/2019 7.Structures and Properties of Ceramics
10/16
Simple Ionic Ceramic Structures
Factors which influence the crystal structures:
(i) Electrical charges on the ions needed to bebalanced.
(ii) Relative sizes of ions: Radius Ratio = rc /ra
COORDINATION NUMBERS (CN):the number ofanion nearest neighbours for a cation is related
to cation/anion radius ratio.
(The most common CNs are 4, 6, and 8).
-
7/28/2019 7.Structures and Properties of Ceramics
11/16
iii.Ions tend to be packed together as densely aspossibly to lower the overall energy of the solid.
-
7/28/2019 7.Structures and Properties of Ceramics
12/16
-
7/28/2019 7.Structures and Properties of Ceramics
13/16
Properties of Ceramics
Most of the properties outlined below follow fromthe ionic and/or covalent bonding in ceramics.
1. Very high elastic modulii which are retained to
elevated temperatures.2. Very high melting points.
3. Chemically stable and resistant to decompositionor oxidation.
4. Good compressive strength.
-
7/28/2019 7.Structures and Properties of Ceramics
14/16
5. Generally brittle:
(a) dislocations in ceramics have narrow cores and hencerequire strong thermal activation to move;
(b) only few slip systems are operative, either because of highresistance to motion of dislocations on a particular set of
slip systems and/or because of the low symmetry of thematerial;
(c) modes of slip which would cause ions of like charges tocome closer are not favourable and
(d) grain boundaries, pores, interface boundaries all act as
good crack paths in ceramics, making ceramics very proneto brittleness. Ceramics and inorganic glasses are oftensensitive to thermal shock, i.e., differential thermalexpansion or contraction induced cracking.
-
7/28/2019 7.Structures and Properties of Ceramics
15/16
6. Usually ceramics are electrical insulators asall electrons are tied up in ionic/covalentbonds and thus are not available forconduction. However free electrons can be
generated thermally and/or extrinsically andseveral ceramics acquire semi-conducting oreven metallic properties. Defects in the ioniccrystals can also result in ionic conduction
which can be enhanced or combined withelectronic conduction by doping.
-
7/28/2019 7.Structures and Properties of Ceramics
16/16
7. Generally poor thermal conductors as heat isconducted by phonons alone, without help fromfree electrons. Insulating materials can beexcellent thermal conductors when phonon-
phonon interaction becomes favourable in thecrystal structure, as for example in diamond andSiC.
8. Generally have low densities, since they tend to
be comprised of atoms of low atomic numbers,but there are exceptions such as zirconia.