ceramic structures

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STRUCTURES OF CERAMICS REFF: Materials Science & Engineering; An Introduction Callister, W. D, Jr, 2007, John Wiley & Sons Fundamental of Ceramics, Barsoum, M. W., 2003, McGraw-Hill Engineering Materials 2; An Introduction to Microstructures, Processing and Design, Ashby, M. F and Jones, D. R. H, 1986, Pergamon Press

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material engineering theory : ceramic structure

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Page 1: Ceramic Structures

STRUCTURES OF CERAMICS

REFF: Materials Science & Engineering; An IntroductionCallister, W. D, Jr, 2007, John Wiley & Sons Fundamental of Ceramics, Barsoum, M. W., 2003, McGraw-HillEngineering Materials 2; An Introduction to Microstructures, Processing and Design, Ashby, M. F and Jones, D. R. H, 1986, Pergamon Press

Page 2: Ceramic Structures
Page 3: Ceramic Structures

Introduction• CERAMICS: Greek keramikos = burn stuffsolid compounds formed by heat (&/P) applications followed by

coolingdesirable properties are achieved through high-T process (firing)Firing causes irreversible transformation resulting a material that

has lost its plasticity & no longer capable to rehydrateat least 2 elements; 1 is a non-metal, the other may be (a) metal(s)

or (an)other non

Page 4: Ceramic Structures

Atom arrangement• 1 unit cell: the smallest group of atoms form a repetitive pattern in describing crystal

structure represent crystal stucture

Page 5: Ceramic Structures

• Characteristics of ions which affect crystal structure:1. magnitude of electrical charged of each ions

• Crystal electrically neutral• (+) charges must be balanced by an equal number of (–) • chemical formula indicates ratio of + to –• Ex CaF2 calcium ions (+2) & fluoride (-)

2. relative size of + and – ion• Involve size/ionic radii (rc & ra)• Metalic elements give up electrons when ionized cations are smaller than

anions rc/ra <1• Each cation prefers as many neighbour anions, anions also desire a maximum

number of cation.

• Stable structures require that cations and anions are in “touch”

Page 6: Ceramic Structures

Coordination number• the number of atoms touching a particular atom, or the number of nearest

neighbors for that particular atom.• number of anions neighbors for a cation) related to rc/ra• This is one indication of how tightly and effisiently atoms are packed together. • For ionic solids, the coordination number of cations is defined as the number of

nearest anions. • The coordination number of anions is the number of nearest cations.

Page 7: Ceramic Structures

• Table: Coordination numbers and geometries for various rc/ra

• blue cation• red anion• Common coordination

numbers for ceramic: 4, 6 and 8

• rc/ra>1 coordinate no. 12

Page 8: Ceramic Structures

The size of an ion depend several factors, e.g:1. coordination number

• Ionic radius increase as the number of opposite charge neighbor ions increases

• ionic radii for (coord no. 4<6<8)

2. charge on an ion • Removing e from atom/ion, the remaining valence electrons become more

tightly bound to the nucleus decrease ionic radius• Ionic size increases when electrons are added to an atom or ion• Radii for Fe: Fe2+: Fe3+ = 0.124: 0.077: 0.069

Page 9: Ceramic Structures

Crystal structure• Solid materials may be classified according to the regularity with which atoms or

ions are arranged with respect to one another. :1.No Order=amorphous

These materials randomly fillup whatever space is available to them.In monoatomic gases, such as argon (Ar) atoms or ions have no orderly arrangement.

2. Short-Range Order (SRO) A material displays short-range order (SRO) if the special arrangement of the atoms extends only to the atom’s nearest neighbors Amorphous/glassy/non crystalline material; e.g. glass

3. Long-Range Order (LRO) the special atomic arrangement extends repeat periodicity >>bond length over much larger ~>100 nm up to few cm The atoms or ions in these materials form a regular repetitive, gridlike pattern, in three dimension crystalline materials; e.g. ceramics

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SRO-non crystalline solid• lack a systematic and regular arrangement• arrangement of atoms over relatively large atomic distances. • also called amorphous or supercooled liquids, inasmuch as their atomic structure

resembles that of a liquid.• Whether a crystalline or amorphous solid forms depends on the ease with which a

random atomic structure in the liquid can transform to an ordered state during solidification

• An amorphous condition may be illustrated by comparison of the crystalline and noncrystalline structures of the ceramic compound silicon dioxide (SiO2), which may exist in both states.

Page 12: Ceramic Structures

Single crystal• when the periodic and repeated arrangement of atoms is perfect or extends

throughout the entirety of the specimen without interruption, the result is a single crystal.

• All unit cells interlock in the same way and have the same single crystal orientation.

Page 13: Ceramic Structures

Polycrystalline material• A polycrystalline material is comprised of many crystals with varying orientations in

space. These crystals in a polycrystalline material are known as grains. • The borders between tiny crystals, where the crystals are in misalignment and are

known as grain boundaries.

Page 14: Ceramic Structures

• Stages in the solidification of a polycrystalline:• Initially, small crystals or nuclei form at various positions. These have

random crystallographic orientations. The small grains grow by the successive addition from the surrounding liquid of atoms to the structure of each. The extremities of adjacent grains impinge on one another as the

solidification process approaches completion.

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Type of crystal structure

• AX: structure of NaCl, CsCl, ZnS• AmXp• AmBnXp

Page 16: Ceramic Structures

AX-type crystal structures• equal number of A (cation) & X (anion)• Referred as AX• 3 structures: rock salt, CsCl and ZnS • Ionic & or covalent bonding• Ionic MgO; 2 e of A transferred to X, result in Mg2+ & O2- • Covalent ZnS; sharing elektron

Page 17: Ceramic Structures

Rock salt (NaCl) structure• The most common AX crystal

structure• Electrostatic attraction between

Na+ & Cl- hold the crystal together• Coordination number for both + &

- is 6 (octahedral)• 1 unit cell generated from FCC of

anion with 1 cation in cubic center & 1 at centered of each of 12 cube edge

• NaCl, MgO, MnS, LiF and FeO

Page 18: Ceramic Structures

Cesium cloride (CsCl) stucture

• Coordination number for both ions is 8 (cubic)

• The anions are at each of the corners of a cube

• Single cation is at the cube center

• This structure is possible when the anion and the cation have the same valence

Page 19: Ceramic Structures

Zinc Blende (ZnS) structure• Coordinate number for both

ions is 4 (tetrahedral)• all corner and face positions

of the cubic cell are occupied by S atoms

• the Zn atoms fill interior tetrahedral positions

• Each Zn atom bonded to 4 S atoms, vice versa

• Most often the atomic bonding is highly covalent

• ZnS and SiC

Page 20: Ceramic Structures

AmXp – Type crystal structures

• Charges of + & - are not the same, m ≠ p;

• Example: AX2 CaF2• Ca ion at the centers of cube,

F ion in the corner• 1 unit cell consists of 8 cubes

Page 21: Ceramic Structures

AmBnXp – Type crystal structure

• 2 types of cation, A & B• Chemical formula AxBnXp• Ex. BaTiO3• Ba2+ ions are situated at all 8

corners of the cube, single Ti4+ is at the centre, O2- ions is at the centre of 6 faces