Lec 21 - ENGR ENGR 4250 Advanced Materials Engineering...

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ENGR 4250 ENGR 4250 – Advanced Materials Engineering Lecture 21 Electrical Properties
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Electrical Conduction Ohm's Law: Δ V = I R voltage drop (volts = J/C) resistance (Ohms) C = Coulomb current (amps = C/s) I e - A (cross sect. area) Δ V L • Geometry-independent forms of Ohm's Law J = σ ε <= another way to state Ohm’s law J current density ε electric field potential = V / l or ( Δ V / Δ l )
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Conductivity: Comparison Room T values (Ohm-m) -1 METALS conductors = ( Ω -m) -1 CERAMICS 0 1 Silver 6.8 x 10 7 Copper 6.0 x 10 7 Soda-lime glass 10 Concrete 10 -9 3 -10 -10 -11 Iron 1.0 x 10 7 Aluminum oxide <10 -13 Silicon 4 x 10 -4 SEMICONDUCTORS Polystyrene <10 -14 5 POLYMERS Germanium 2 x 10 0 GaAs 10 -6 Polyethylene 10 -15 -10 -17 sulators Selected values from Tables 18.1, 18.3, and 18.4, Callister 7e . semiconductors insulators
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Energy Bands
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Electronic Band Structures Adapted from Fig. 18.2, Callister 7e .
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Band Structure
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Band Structure Valence band – filled – highest occupied energy levels Conduction band – empty – lowest unoccupied energy levels alence band Conduction band valence band Adapted from Fig. 18.3, Callister 7e .
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Energy States
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Metals: Fermi Energy – 0ºK
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Charge Carriers Two charge carrying mechanisms Adapted from Fig. 18.6 (b), Callister 7e . Electron – negative charge ole qual & opposite Hole equal & opposite positive charge
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Electron Mobility • Electrons move due to electric field opposite direction – in opposite direction – Continuous acceleration? – Frictional forces? Move at different speeds - drift velocity ; mobility Higher temp. promotes more electrons into the conduction band ∴σ Ç as T Ç Electrons scattered by impurities, grain boundaries, etc.
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Electron Mobility Drift Velocity ξ μ υ e d = Constant of proportionality electron mobility is an indication of scattering events e Conductivity e e n σ= C x e 19 10 6 . 1 =
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Electrical Resistivity of Metals d i t total ρ + + =
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This note was uploaded on 10/18/2010 for the course ENGR 3360 taught by Professor Ahmadbarari during the Fall '10 term at UOIT.

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Lec 21 - ENGR ENGR 4250 Advanced Materials Engineering...

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