3.12 CERAMIC INSULATORS
More typically we think of ceramics as being good electrical insulators and there are many ceramics that have ?>1014?•cm. Examples of common ceramic insulators include
- Aluminum oxide (Al2O3)
- Mullite (3Al2O3 • 2SiO2)
- Forsterite (2MgO • SiO2)
- Beryllium oxide (BeO)
- Aluminum nitride (AlN)
In an electrical insulator, there is a wide energy gap between the bottom of the conduction band and the top of the valence band. Figure 3.16 shows the energy band diagram for MgO. The valence band is formed by the 2p energy levels of oxygen (O2-ions) and the conduction band is formed from the empty 3s orbitals of the Mg2+ ions. The energy band gap is ~8 eV and the concentration of thermally excited electrons in the conduction band of MgO is low right up to its melting point, 2800°C. MgO is therefore an excellent high temperature insulator.
Very wide energy band gaps (>6eV) are associated with compounds that have high fractions of ionic character in their bonding. Ions have a stable noble gas electron configuration.
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