2.3.4 Summary to: Conductors - Special Applications

Thermionic emission provides electron beams.
The electron beam current (density) is given by the Richardson equation:
j  =  A · T 2 · exp – EA
Atheo = 120 A · cm–2 · K–2 for free electron gas model
Aexp » (20 - 160) A · cm–2 · K–2
EA = work function » (2 - >6) eV  
Materials of choice: W, LaB6 single crystal  
High field effects (tunneling, barrier lowering) allow large currents at low T from small (nm) size emitter  
Needs UHV!
There are several thermoelectric effects for metal junctions; always encountered in non-equilibrium.    
Seebeck effect:
Thermovoltage develops if a metal A-metal B junction is at a temperature different form the "rest", i.e. if there is a temperature gradeient
Essential for measuring (high) temperatures with a "thermoelement"
Future use for efficient conversion of heat to electricity ???
Peltier effect:
Electrical current I through a metal - metal (or metal - semiconductor) junction induces a temperature gradient µ I, i.e. one of the junction may "cool down".
Used for electrical cooling of (relatively small) devices. Only big effect if electrical heating (µ I2) is small.
All Multiple Choice questions to 2.3

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© H. Föll (Electronic Materials - Script)