Chapter 13, Structure and Properties of Ceramic1Chapter Outline: CeramicsStructure and Properties of Ceramics1. Crystal Structures2. Silicate Ceramics3. Imperfections in Ceramic4. Mechanical Propertiesa. Brittle Fracture of Ceramicsb. Stress-Strain Behaviourc. Mechanism of Plastic Deformation Chapter 13, Structure and Properties of Ceramics2¾keramikos- burnt stuff in Greek - desirable properties of ceramics are normally achieved through a high-temperature heat treatment process (firing).¾Usually a compound between metallic and non-metallic elements¾Always composed of more than one element (e.g., Al2O3, NaCl, SiC, SiO2)¾Bonds are partially or totally ionic, and can have combination of ionic and covalent bonding¾Generally hard and brittle¾Generally electrical and thermal insulators¾Can be optically opaque, semi-transparent, or transparent¾Traditional ceramics – based on clay (china, bricks, tiles, porcelain), glasses.¾“New ceramics” for electronic, computer, aerospace industries.Ceramics
Chapter 13, Structure and Properties of Ceramics3Electronegativity - a measure of how willing atoms are to accept electrons (subshellswith one electron -low electronegativity;subshells with one missing electron -high electronegativity). Electronegativity increases from left to right.Bonding in CeramicsThe atomic bonding in ceramics is mixed, ionic and covalent, the degree of ionic character depends on the difference of electronegativity between the cations (+) and anions (-).Chapter 13, Structure and Properties of CeramicsUniversity of Tennessee, Dept. of Materials Science and Engineering4Crystal structure is defined by¾Magnitude of the electrical charge on each ion. Charge balance dictates chemical formula (Ca2+and F-form CaF2).¾Relative sizes of the cations and anions. Cations wants maximum possible number of anion nearest neighbors and vice-versa.Crystal Structures in Ceramics with predominantly ionic bondingStable ceramic crystal structures: anions surrounding a cation are all in contact with that cation. For a specific coordination number there is a critical or minimum cation-anion radius ratio rC/rAfor which this contact can be maintained.
Chapter 13, Structure and Properties of Ceramics5Coordination NumberThe number of adjacent atoms (ions) surrounding a referenceatom (ion) without overlap of electron orbitals.• Also called ligancy• Depends on ion size (close packed)• Ideal: Like-sized atoms = 12• Calculated by considering the greatest number of larger ions(radius R) that can be in contact with the smaller one (radius r).