7.Structures and Properties of Ceramics

7/28/2019 7.Structures and Properties of Ceramics

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Structures and Properties ofCeramics


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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.


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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).


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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.


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Ceramic material are inorganic compounds consisting of metallic

and non-metallic elements which are held together with ionic

and/or covalent bonds


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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


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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.


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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).


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iii.Ions tend to be packed together as densely aspossibly to lower the overall energy of the solid.


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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.


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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.


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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.


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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.

7.Structures and Properties of Ceramics

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