Ceramic1 - Structure - Structure Properties of Ceramics Lecture 3 EBB113 Why study ceramic materials Very traditional(crude civil engineering material

Ceramic1 - Structure - Structure Properties of Ceramics...

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Lecture 3 EBB113 Structure & Properties of Ceramics
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3 Introduction keramikos - 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.,Al 2 O 3 , NaCl, SiC, SiO 2 ) Interaction bond either totally ionic or having some covalent character Properties : • Generally hard and brittle Generally electrical and thermal insulators (exceptions: graphite, diamond, AlN… and others) • Can be optically opaque, semi-transparent, or transparent High chemical stability and high melting temperature.
4 Class of ceramic Traditional Ceramics : primary raw materials is clay Example: porcelain, bricks, tiles, glasses.
5 Class of ceramic Now new generation of this materials have evolved Engineering Ceramics ¾ High-temperature ceramic ¾ Advance ceramic ¾ Electroceramic Oxygen sensor Aerospace product-Ni Ti, Stainless steel
6 Ceramics Structure
7 Ceramic Crystal Structures Ceramic 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: small CaF 2 : large Ionic 89% Ionic 12%
8 Crystal Structure Two characteristic which influence the crystal structure 1. Magnitude of electrical charge of each component ions 2. The relative sizes of the cation and anions
9 1. Magnitude of electrical charge of each component ions CaF 2 : Ca 2+ cation F - F - anions + A m X p m, p determined by charge neutrality Electrically must be neutral-Net charge in structure should be zero
2. The relative sizes of the cation (rC) and anions (rA)
11 Coordination number Coordination 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 r C /r A ratio for which this cation-anion contact is established and this ratio can be determined from geometrical consideration (see following example) r cation r anion r cation r anion