STRUCTURES OF CERAMICSREFF: 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 Introduction • CERAMICS: Greek keramikos = burn stuff solid compounds formed by heat (&/P) applications followed by cooling desirable properties are achieved through high-T process (firing) Firing causes irreversible transformation resulting a material that has lost its plasticity & no longer capable to rehydrate at least 2 elements; 1 is a non-metal, the other may be (a) metal(s) or (an)other non • The number of covalent bonds is determined by the number of valence electrons • For N valence electrons. N= 7 for chlorine.N= 1. an atom can bond with other atoms 8 – N • For example.valence • The valence of an atom is the number of electrons in an atom that participate in bonding or chemical reactions • Usually. the valence is the number of electrons in the outer s and p energy levels. • The valence of an atom is related to the ability of the atom to enter into chemical combination with other elements • Examples of the valence are: Mg: 1s2 2s2 2p6 3s2 valence = 2 . and 8 . which means that one Cl atom can bond to only one other atom. Electronegativity (e greed) the tendency of an atom to gain an electron.the greater the difference in electronegativity) in general the more ionic the bond.7 covalent . e attracted to the more electronegative element ion attract each other In general ∆x > 1. the smaller the difference in electronegativity). Conversely. the greater the degree of covalency.e. If the electronegativity difference between them (∆x)is large (indicating 1 element is greedier than other).e. However. atom with nearly empty outer levels—such as sodium—readily give up electrons and have low electronegativity.. the power of atom to attract electrons to itself Atoms with almost completely filled outer energy levels—such as chlorine—are strongly electronegative and readily accept electrons. The wider the separation (both horizontally and vertically) from the lower left to the upper-right-hand corner (i. the closer the atoms are together (i..7 ionic ∆x < 1.electronegativity • • • • • • • • • • It is possible to have interatomic bonds that are partially ionic and partially covalent the degree of either bond type depends on the relative positions of the constituent atoms in the periodic table or the difference in their electronegativities. IONIC BONDING • • • • When one atom may donate its valence electrons to a different atom. . filling the outer energy shell of the second atom Both atoms now have filled/emptied outer energy levels. The oppositely charged ions are then attracted to one another and produce the ionic bond. but both have acquired an electrical charge and behave as ions. while the atom that accepts the electrons acquires a net negative charge and is called an anion. The atom that contributes the electrons is left with a net positive charge and is called a cation. A loses e easily. and vice versa • The predominant bonding in ceramic materials is ionic. X accepts e without too much energy input • It follows that for ionic materials to be stable. . • form between very active metallic & non metallic elements • Atoms of a metallic element easily give up their valence electrons to the nonmetallic atoms • to form AX ionic bonding. all positive ions must have as nearest neighbors negatively charged ions in a three dimensional scheme.• Occured by transfer electron. • forms when atoms have the same electronegativity energies of bonding electrons of A & X are comparable • If the electron energy of the atoms is different transfer energy (ionic bonding) • Each instance of sharing represents one covalent bond • e. • Two atoms will each contribute at least one electron to the bond.g: silicon atom. Cl2. F2 etc) as well as molecules containing dissimilar atoms. obtains eight electrons in its outer energy shell by sharing its electrons with four surrounding silicon atoms each silicon atom is bonded to 4 neighboring atoms by 4 covalent bonds • Many nonmetallic elemental molecules (H2. and HF. such as CH4. HNO3. are covalently .H2O. and the shared electrons may be considered to belong to both atoms. has a valence of four.COVALENT BONDING • the sharing of covalent bonding electrons between adjacent atoms. 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.• 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. anions also desire a maximum number of cation. • Stable structures require that cations and anions are in “touch” . number of anions neighbors for a cation) related to rc/ra This is one indication of how tightly and effisiently atoms are packed together. . The coordination number of anions is the number of nearest cations.Coordination number • • • • • the number of atoms touching a particular atom. For ionic solids. the coordination number of cations is defined as the number of nearest anions. or the number of nearest neighbors for that particular atom. 12 . 6 and 8 • rc/ra>1 coordinate no.• Table: Coordination numbers and geometries for various rc/ra • red cation • whte anion • Common coordination numbers for ceramic: 4. 077: 0. charge on an ion • Removing e from atom/ion.g: 1.124: 0.The size of an ion depend several factors. 4<6<8) 2. 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. coordination number • Ionic radius increase as the number of opposite charge neighbor ions increases • ionic radii for (coord no. e.069 . Atom arrangement • 1 unit cell: the smallest group of atoms form a repetitive pattern in describing crystal structure represent crystal stucture . 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. ceramics . e. 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. glass 3.g. In monoatomic gases. such as argon (Ar) atoms or ions have no orderly arrangement.No Order These materials randomly fillup whatever space is available to them. 2. in three dimension crystalline materials. gridlike pattern.• Types of atomic or ionic arrangements: 1. e.g. . SRO-non crystalline solid • • • • • lack a systematic and regular arrangement arrangement of atoms over relatively large atomic distances. which may exist in both states. 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). . also called amorphous or supercooled liquids. rhombohedral (also called trigonal).Crystal structure • • • • • based on the unit cell geometry only. y. and triclinic The cubic system. and γ . and the three interaxial angles a. for which a = b = c and a = ß = γ = 90 . The unit cell geometry is completely defined in terms of six parameters: the three edge lengths a. Least symmetry is displayed by the triclinic system. These seven crystal systems are cubic. an x. y. each of the x. . and z axes coincides with one of the three parallelepiped edges that extend from this corner. z coordinate system is established with its origin at one of the unit cell corners. Within this framework. ß. has the greatest degree of symmetry. tetragonal. hexagonal. monoclinic. and c. orthorhombic. b. since a ≠ b ≠ c and a ≠ß≠γ. . .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. . The borders between tiny crystals. where the crystals are in misalignment and are known as grain boundaries. These crystals in a polycrystalline material are known as grains. 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.• Stages in the solidification of a polycrystalline: • Initially. small crystals or nuclei form at various positions. . ZnS • AmXp • AmBnXp .Type of crystal structure • AX: structure of NaCl. CsCl. AX-type crystal structures • • • • • • equal number of A (cation) & X (anion) Referred as AX 3 structures: rock salt. 2 e of A transferred to X. sharing elektron . CsCl and ZnS Ionic & or covalent bonding Ionic MgO. result in Mg2+ & O2Covalent ZnS. LiF and FeO .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. no Na+ neighbours (vice versa) • Coordination number for both + & . MgO. electrostatic interaction each Na+ has 6 Cl-.hold the crystal together • Max. MnS.Rock salt (NaCl) structure • The most common AX crystal structure • Electrostatic attraction between Na+ & Cl. 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 . vice versa • Most often the atomic bonding is highly covalent • ZnS. ZnTe.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. and SiC . 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 .AmXp – Type crystal structures • Charges of + & . ions is at the centre of 6 faces . O2. BaTiO3 Ba2+ ions are situated at all 8 corners of the cube. single Ti4+ is at the centre.AmBnXp – Type crystal structure • • • • 2 types of cation. A & B Chemical formula AxBnXp Ex. every oxygen becomes a bridge between 2 silicon atoms • Every corner oxygen atom is shared by adjacent tetrahedra • The material’s electrically neutral.silica • The most simple: silicon dioxide/silica • Pure silica no metal ions. all atoms have stable electronic structures • Ratio Si to O 1:2 (indicated by chemical formula) • If tetrahedras are arranged in a regular & order crystalline . into dense (< 2 % for fine and < 6 % for coarse) or porous ceramics (> 2% and > 6 %. . clay Each silicon atom bond strongly to 4 oxygen atom Basic unit in all silicates tetrahedron (oxygen are situated at the corners. rock. system MgO-Al2O3-SiO2).SILICATE CERAMIC • • • • • Silicates are composed primarily of silicon and oxygen. system K2O-Al2O3-SiO2) and 2) magnesium silicates (talc-based technical ceramics. according to water absorption. Silicate ceramics are conventionally subdivided into coarse or fine and. abundant elements in earth’s crush. soil. oxygen is at the centre) The main types of silicate ceramics: 1) alumosilicates (kaolin.or clay-based ceramic such as porcelain and bricks. respectively). a fused silica/vitreous silica • As a result. glasses are mechanically rigid like solids. yet have the disordered arrangement of molecules like liquids • noncrystalline solid or glass.k. low thermal expansion but hard to work with because high in viscosity . high randomness • Basic unit tetrahedron (same as the crystalline) • Pure silica forms glass with high softening T (1200 C) • Great strength and stability.Silica Glasses • A. CaO. each attaches to 1 oxygen of tetrahedron non bridging .g.• Commercial glasses silica glasses add with other metal oxide to reduce viscosity • E. Na2O add positive ion to the structure &break up the network network modifiers • Add 1 Na2O molecules introduces 2 Na+.