Electrical Properties of Materials - HILLS：材料的电学性质-山.doc

Electrical Properties of Materials For Structural Materials we looked at the atomic bonding to determine the properties. For Electronic Materials we will look at the electronic structure: Energy levels Energy bands Valance bands Conduction bands Conductors Insulators Semiconductors The material property of conductivity has the widest range of values of any other property. It is determined by: ( = n q ( where ( is a material property (reciprocal of resistivity, () n is q is ( is On a macroscopic level Ohm’s Law gives: V = IR With some manipulation we can get Ohm’s Law on a microscopic level: J = E( Let’s Consider the Conductivity of Metals: Consider many Na atoms together in the solid: 3s 2p 2s 1s The 3s electrons are considered “delocalized”, i.e. part of the whole solid, and not associated with any one atom. The inner electrons are essentially unchanged when part of the solid. Question: If the 3s electrons are considered part of the whole, is the Pauli Exclusion Principle violated? Answer: NO! Because the 3s level will split into a Pseudo-continuous Energy Band, i.e. a narrow range of energy levels with millions (as many as there are Na atoms in the solid) of discrete energy levels. Note: Hunds Rule says that each level will be half filled. This is the Valance Band because it contains the valance electrons. There is high mobility in this band because the energy levels are so close. Hence n is large and ( is high ( high value of conductivity, ( The electron energy level picture for metals looks like: Valance Band (partially filled) This is also the Conduction Band Forbidden Energies Inner Energy levels (completely filled) Now let’s Consider the Conductivity of Insulators: Consider Carbon: diamond cubic structure valance electrons are all covalently bonded in sp3 orbitals the next available energy level is 6eV above the highest occupied level Conduction Band is empty (if electrons could get here, they would conduct)