Direct Energy, 2018a

8 THERMOELECTRICS 181
Seebeck Effect Peltier Effect Thomson Effect
1 2
I
1 2
I
hot
+ΔV 12 −
cold
heating/cooling
hot
cold
heating/cooling
Figure 8.2: The Seebeck eect, Peltier eect, and Thomson eect.
to be three aspects of the same phenomenon [5, p. 113].
The rst eect, now called the Seebeck eect, was discovered in1821
by Thomas Seebeck [5, p. 113]. It is observed ina junctionof two dierent
metals or semiconductors. As discussed in Section 6.5, junctions are
also used to make photovoltaic devices, LEDs, and semiconductor lasers.
When the dierent sides of the junction are held at dierent temperatures,
a voltage develops across the junction. The Seebeck coecient $, inunits
V
K
, is dened as the ratio of that voltage to the temperature dierence.
More specically, consider a junction where one side is held at a hotter
temperature thanthe other, as showninthe left part of Fig. 8.2. The difference
between the Seebeck coecient in material one $ 1 and the Seebeck
coecient in material two $ 2 is givenby the measured voltage across the
junction ΔV 12 divided by the temperature dierence between the materials
ΔT 12 [110, p. 24].
$ 1 − $ 2 = ΔV 12
(8.23)
ΔT 12
The dierence betweenthe Seebeck coecients canbe positive or negative
because both the temperature dierence and the measured voltage can be
positive or negative. However, for any given material, the Seebeck coecient
is positive. To nd the Seebeck coecient for an unknown material,
form a junction between that material and a material with known Seebeck
coecient, heat one end of the junction hotter than the other, and measure
the voltage established. For most materials, the Seebeck coecient is
less than 1 μV K
. Some of the largest values of the Seebeck coecient are
found in materials containing tellurium. For example, (Bi 0.7 Sb 0.3 ) 2
Te 3 has
$ ≈ 230 μV K and PbTe has $ ≈ 400 μV K [3].
To understand the physics behind the Seebeck eect, consider the ow
of charges across this diode-like device. In metals, valence electrons are the
charge carriers, and in semiconductors, both valence electrons and holes
are the charge carriers. These charge carriers diuse from the hot to the
cold side of the junction. Consider a junction of two metals with no net