Environmental Engineering Reference
In-Depth Information
6.2.2 Thermionics
The concept of thermionic energy conversion is not really a new scienti
c area. The
basic principle of operation is the
ow of an electric current through the vacuum
gap between the hot cathode and the cold anode, due to the voltage potential. When
the cathode is exposed to the heat source it starts to emit electrons, which are
absorbed by the anode on the other side of the vacuum gap. The electrons are, in
this case, heat carriers [
21
]. The problem of thermionics is in its high-temperature
operating range (above 230
fl
C[
22
]), which is at present thought to be useless for
devices operating at or near room temperature. This is a great weakness, especially
because thermionic devices show higher ef
°
ciencies than thermoelectrics (above
30 %) [
19
,
23
]. Therefore, researchers have spent the last 15 years involved in
extensive studies and designed a new principle of thermionics operating at or near
room temperature. This new technology is called Hetero-Structured Integrated
Thermionics (HIT) [
24
lm
thermionic cooling devices (especially HIT devices). These micro devices work at/
near room temperature with high cooling-power densities (600 Wm
−
2
, temperature
span 4.1 K) [
26
26
]. Furthermore, researchers have also developed thin-
-
29
] and with fast response times (40
μ
s) [
30
], which is used in
-
high-speed micro on-spot cooling.
6.2.3 Spincaloritronics
Spintronics [
31
] is an emerging technology in the
eld of microelectronics and
nano-electronics and is based on the combination of electron spin and its charge.
Mainly, the focus of the research is in the areas of computer, military and space
technologies [
32
,
33
]. At the moment, spintronics technology is not directly
applicable as thermal diodes; however, it contains some segments that could be
combined with some other mechanisms of thermal diodes, discussed in this chapter.
One of the directions, in which the research of spintronics is heading and has some
potential for synergy with thermal diodes, is the manipulation of the so-called
magnetoresistance effect. This phenomenon occurs in some metals when they are
exposed to a magnetic
cantly, thus
increasing the electrical conductivity. This phenomenon has become even more
interesting since the discovery of the
eld. The electrical resistance decreases signi
magnetoresistance effect (GMR),
which was independently discovered by Albert Fert [
34
] and Peter Gr
“
giant
”
nberg [
35
].
The 2007 Nobel Prize in physics was awarded to them for the discovery of GMR.
The GMR exhibits an even larger change in electrical resistance in the presence of a
magnetic
ü
eld. The problem of spintronics is in materials, which for GMR is useful
only at extremely low temperatures (below
C), so it has no applicable value.
Again, researchers are struggling with the development and discovery of new
materials that are useful at/near room temperature [
36
150
°
−
42
].
-
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