1. Introduction to Materials Science, Chapter 13, Structure and Properties of
Ceramics. University of Tennessee, Dept. of Materials Science and Engineering.
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Introduction to Materials Science, Chapter 13, Structure and Properties of Ceramics
Chapter Outline: Ceramics
Chapter 13: Structure and Properties of Ceramics ¾ Crystal Structures ¾ Silicate Ceramics ¾ Carbon ¾ Imperfections in Ceramics Optional reading: 13.6 – 13.10
Chapter 14: Applications and Processing of Ceramics ¾ Short review of glass/ceramics applications and processing (14.1 - 14.7) Optional reading: 14.8 – 14.18
University of Tennessee, Dept. of Materials Science and Engineering
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Introduction to Materials Science, Chapter 13, Structure and Properties of Ceramics
Ceramics ¾ keramikos - burnt stuff in Greek - desirable properties of ceramics are normally achieved through a hightemperature heat treatment process (firing). ¾ Usually a compound between metallic and nonmetallic elements ¾ Always composed of more than one element (e.g., Al2O3, NaCl, SiC, SiO2) 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 transparent
opaque,
semi-transparent,
or
¾ Traditional ceramics – based on clay (china, bricks, tiles, porcelain), glasses. ¾ “New ceramics” for electronic, computer, aerospace industries. University of Tennessee, Dept. of Materials Science and Engineering
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Introduction to Materials Science, Chapter 13, Structure and Properties of Ceramics
Bonding in Ceramics (Review of Chapter 2)
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. 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 (-). University of Tennessee, Dept. of Materials Science and Engineering
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Introduction to Materials Science, Chapter 13, Structure and Properties of Ceramics
Crystal Structures in Ceramics with predominantly ionic bonding Crystal structure is defined by ¾ Magnitude of the electrical charge on each ion. Charge balance dictates chemical formula (Ca2+ and F- form CaF2). ¾ Relative sizes of the cations and anions. Cations wants maximum possible number of anion nearest neighbors and vice-versa. 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 cationanion radius ratio rC/rA for which this contact can be maintained.
University of Tennessee, Dept. of Materials Science and Engineering
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Introduction to Materials Science, Chapter 13, Structure and Properties of Ceramics
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). R=1.0 r =0.2
CN = 1 possible
CN = 2 possible
30°
Cos 30°=0.866=R/(r+R) → r/R = 0.155 CN = 4 possible University of Tennessee, Dept. of Materials Science and Engineering
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Introduction to Materials Science, Chapter 13, Structure and Properties of Ceramics
Coordination Number CN numbers for ionic bonding
CN r/R 2 0
