### 2.4.4 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 kT
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|  | 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.
Questionaire
All Multiple Choice questions to 2.4

© H. Föll (Advanced Materials B, part 1 - script)