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Lecture 3EBB113Structure &Properties of Ceramics
3Introductionkeramikos- burnt stuff in Greek →desirable properties of ceramics are normally achieved through a high-temperature heat treatment process (firing).Usually a compound between metallic and non-metallic elements.Always composed of more than one element (e.g.,Al2O3, NaCl, SiC, SiO2)Interaction bond either totally ionic or having some covalent characterProperties : • Generally hard and brittle•Generally electrical and thermal insulators (exceptions: graphite,diamond, AlN… and others)• Can be optically opaque, semi-transparent, or transparentHigh chemical stability and high melting temperature.
4Class of ceramicTraditional Ceramics: primary raw materials is clayExample: porcelain, bricks, tiles, glasses.
5Class of ceramicNow new generation of this materials have evolvedEngineering Ceramics¾High-temperature ceramic¾Advance ceramic¾ElectroceramicOxygen sensorAerospace product-Ni Ti, Stainless steel
7Ceramic Crystal StructuresCeramic which predominately ionic-composed of cation (+ve charge) and anion (-ve charge)Atomic Bonding: Mostly ionic, some covalent.% ionic character increases with difference in electronegativity.SiC: smallCaF2: largeIonic 89%Ionic 12%
8Crystal StructureTwo characteristic which influence the crystal structure1.Magnitude of electrical charge of each component ions2.The relative sizes of the cation and anions
91. Magnitude of electrical charge of each component ionsCaF2:Ca2+cationF-F-anions+AmXpm, p determined by charge neutralityElectrically must be neutral-Net charge in structure should be zero
2. The relative sizes of the cation (rC) and anions (rA)
11Coordination numberCoordination no (i.e no of anions nearest-neighbors for a cation) is related to the Coordination no increases with There is a critical or minimum rC/rAratio for which this cation-anion contact is established and this ratio can be determined from geometrical consideration (see following example)rcationranionrcationranion