Ceramics - To print - Chapter 13 Structure and Properties of Ceramic Chapter Outline Ceramics Structure and Properties of Ceramics 1 Crystal Structures

Ceramics - To print - Chapter 13 Structure and Properties...

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Chapter 13, Structure and Properties of Ceramic 1 Chapter Outline: Ceramics Structure and Properties of Ceramics 1. Crystal Structures 2. Silicate Ceramics 3. Imperfections in Ceramic 4. Mechanical Properties a. Brittle Fracture of Ceramics b. Stress-Strain Behaviour c. Mechanism of Plastic Deformation Chapter 13, Structure and Properties of Ceramics 2 ¾ keramikos - burnt stuff in Greek - desirable properties of ceramics are normally achieved through a high- temperature heat treatment process (firing). ¾ Usually a compound between metallic and non- metallic elements ¾ Always composed of more than one element (e.g., Al 2 O 3 , NaCl, SiC, SiO 2 ) ¾ Bonds are partially or totally ionic, and can have combination of ionic and covalent bonding ¾ Generally hard and brittle ¾ Generally electrical and thermal insulators ¾ Can be optically opaque, semi-transparent, or transparent ¾ Traditional ceramics – based on clay (china, bricks, tiles, porcelain), glasses. ¾ “New ceramics” for electronic, computer, aerospace industries. Ceramics
Chapter 13, Structure and Properties of Ceramics 3 Electronegativity - a measure of how willing atoms are to accept electrons (subshells with one electron - low electronegativity; subshells with one missing electron - high electronegativity). Electronegativity increases from left to right. Bonding in Ceramics The 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 Ceramics University of Tennessee, Dept. of Materials Science and Engineering 4 Crystal structure is defined by ¾ Magnitude of the electrical charge on each ion. Charge balance dictates chemical formula (Ca 2+ and F - form CaF 2 ). ¾ Relative 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 bonding Stable 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 r C /r A for which this contact can be maintained.
Chapter 13, Structure and Properties of Ceramics 5 Coordination Number The number of adjacent atoms (ions) surrounding a reference atom (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).

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