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Thermionic emission provides electron beams.
The electron beam current (density)
is given by the Richardson equation: |
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| 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 |
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EA = work function » (2 - >6)
eV | |
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Materials of choice: W, LaB6 single crystal |
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High field effects (tunneling, barrier lowering) allow large currents at low T
from small (nm) size emitter | |
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There are several thermoelectric effects for metal junctions; always encountered
in non-equilibrium. | |
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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 ??? |
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© H. Föll (Electronic Materials - Script)
