ceramic structures
DESCRIPTION
material engineering theory : ceramic structureTRANSCRIPT
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
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
Atom arrangement• 1 unit cell: the smallest group of atoms form a repetitive pattern in describing crystal
structure represent crystal stucture
• 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”
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.
• 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
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
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
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.
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.
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.
• 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.
Type of crystal structure
• AX: structure of NaCl, CsCl, ZnS• AmXp• AmBnXp
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
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
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
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
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
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