Direct Energy, 2018a

182 8.6 Thermoelectric Eects
charge on either side initially. If an electron moves from the hot side to the
cold side, the hot side then will have a net positive charge, and the cold
side will have a net negative charge. This movement of charges sets up an
electric eld and hence a voltage.
If we let the sample reach an equilibrium temperature, no voltage will
be measured. A voltage is measured only during the time when charge
carriers have diused from one material to the other but when the material
has not reached a uniform temperature. Thus, for a material to have a
large thermoelectric eect, it must have a large electrical conductivity and
small thermal conductivity. Thermoelectric devices are typically made from
metals or semimetals because these materials satisfy this condition.
The second eect was discovered by Jean Peltier in 1834 [5, p. 113].
The Peltier eect is also observed in a junction of two dierent metals,
semimetals, or semiconductors. It is illustrated in the middle part of Fig.
8.2. When a current, I in amperes, is supplied across a junction, heat is
transferred. This eect occurs because charges from the supplied current
ow through dierent materials with dierent thermal conductivities on
the dierent sides of the junction. The eect is quantied by the Peltier
coecient for the junction, Π 12 , or Peltier coecients for the materials
forming the junction, Π 1 and Π 2 . More specically, the Peltier coecient
is dened as
Π 12 =Π 1 − Π 2 =
( dQ
dt
I
)
(8.24)
in the units of volts [110, p. 24]. The term dQ represents the rate heat is
dt
transferred in J s , and it may be positive or negative because the thermal
conductivity in the rst material may be higher or lower than in the second
material. The Seebeck coecient and the Peltier coecient are related by
Π 1 − Π 2 =($ 1 − $ 2 ) T. (8.25)
PbTe is a material with a relatively high Seebeck coecient.
temperature, it has coecients $ = 400 μV K and
At room
Π = 400 μV K
· 300K =0.12 V. (8.26)
The third eect was rst discovered by William Thomson in the 1860s
[3]. Thomson also derived the relationship between these three eects. It
is illustrated on the right part of Fig. 8.2. When a current passes through
a uniform piece of material which has a temperature gradient, heating or
cooling will occur, and this result is known as the Thomson eect [3, p.
148] [110, p. 24] [5, p. 115]. To observe this eect, apply a temperature