Thermoelectric Materials and Devices - Hierarchical Nanostructures for Energy Devices - page 108

radioisotope thermoelectric generators (RTG). 1 NASA devoted its efforts to
developing highly reliable and robust RTGs, and successfully used a series of
RTGs in their spacecraft without any failures for more than 10 years.
A thermoelectric generator is highly robust because it is a solid-state device
that has no moving parts. The pioneering efforts by NASA and others in the
1950s led to the discoveries of many important thermoelectric materials
such as Bi 2 Te 3 , PbTe and SiGe, all of which are still widely used. 9 These bulk
thermoelectric materials are semiconductor alloys that have high electrical
conductivities by sucient doping, and possess inherently low lattice ther-
mal conductivities. Typically, semiconductors are found to be more ecient
thermoelectric materials than insulators or metals. 6 We will discuss the
reason for this later in this chapter.
Thermoelectric refrigeration drew renewed attention as an alternative
method of refrigeration in the 1990s, when the eco-toxicity of refrigerant
fluids used in conventional refrigerators was revealed. 9 Thermoelectric de-
vices have many advantages as alternative refrigerators; they have high
power density and do not have moving parts, so their operation is quiet and
robust. A critical advantage is that they do not require a toxic or environ-
mentally harmful refrigerant. 10
In the past few decades, both thermoelectric refrigeration and power
generation have been explored extensively for various potential applications
both in academia and industry. The most promising application is waste
heat recovery. Thermoelectric devices can generate electrical power even
from a small temperature gradient, whereas conventional energy conversion
devices such as turbines or combustion engines are not suitable to produce
electricity under a low temperature gradient. Therefore, thermoelectric
generators can be used to harvest energy from low-quality heat sources such
as waste heat. 11 Thermoelectric refrigerators are widely used for cooling
small solid-state devices as well these days. Controlling the hot spots in a
CPU and cooling optoelectronic laser modules are representative appli-
cations for thermoelectric refrigerators. 12
Although thermoelectric devices have many advantages over conventional
energy conversion systems, their main weakness comes from their low en-
ergy conversion eciency. 10 Significant efforts have been made throughout
the 20 th century to enhance the thermoelectric eciency. 13 Figure 6.1(a)
shows several important studies conducted on thermoelectric materials
with high figures of merit in each year for various temperature ranges. The
dimensionless thermoelectric figure of merit ZT, which represents the e-
ciency of a thermoelectric material, is defined as
ZT ¼ S 2 sT
k

d n 3 r 4 n g | 7

.

(6 : 1)

where S, s, and k are the Seebeck coecient, electrical conductivity, thermal
conductivity of the material used, respectively, and T is the absolute tem-
perature. The Seebeck coecient is defined as the ratio of the induced
electric, E, field to the temperature gradient within the solid, which is equal