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 ???
All Multiple Choice questions to 2.3

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© H. Föll (Electronic Materials - Script)