Direct Energy, 2018a

8 THERMOELECTRICS 187
The gure of merit does not incorporate all of the temperature related
factors to consider in selecting materials for thermoelectric devices. Melting
temperature is also important. A thermoelectric device converts more
energy when a larger temperature dierence is placed across the device.
The Seebeck coecient is inversely proportional to the temperature dierential,
$= ΔV . However,too large of a temperature dierential will melt
ΔT
the hot end of the device,and dierent materials can have very dierent
melting temperatures. For example,lead telluride PbTe melts at 924 ◦ C,
and Bi 2 Te 3 melts at 580 ◦ C [114,p. 4-52,4-71].
The gure of merit also depends on doping level because the electrical
conductivity is directly proportional to the charge concentrations n and p
[110]. Thus,a thermoelectric device made from a doped semiconductor has
a higher electrical conductivity and thermoelectric eciency than a device
made from an undoped semiconductor of the same material. The Seebeck
coecient is also dependent on doping level but not as strongly [110]. Thermal
conductivity is not a strong function of charge concentrations n and p
[110]. Thus,thermoelectric materials are often made from heavily doped
semiconductors or from conductors.
The gure of merit also depends on degree of crystallinity. Typically,
both the electrical conductivity and thermal conductivity are much higher
in crystals than glasses because charge and heat carriers are less likely to
get scattered as they travel through crystals than glasses [113]. Since both
electrical and thermal conductivity are inuenced,the eect of degree of
crystallinity on the gure of merit can be complicated.
Thermoelectric devices are typically made from junctions of two different
metals or semiconductors. Essentially,a thermoelectric device is a
diode. Common materials used include bismuth telluride,lead telluride,
and antimony telluride,all of which are semimetals. Bi,Sb,and Pb are all
located near each other on the periodic table. Other materials studied for
use in thermoelectric devices include [110],BiSeTe,LiMnO,LiFeO,LiCoO,
LiNiO,PbS,and ZnSb. These materials are either small gap semiconductors
or semimetals. In semiconductor materials with small energy gaps,the
ratio of electrical conductivity to thermal conductivity is large. However,
this fact must be balanced against the fact that smaller gap semiconductors
tend to have lower melting temperatures than larger gap semiconductors
[110,ch. 1].
Recently,layered materials and superlattices have been considered as
materials for thermoelectric devices [115] [116]. The layers can be tailored
to aect the thermal and electrical properties dierently and can act like
a lter to select out dierent conduction mechanisms. Understanding of
the conductivity mechanisms is a prerequisite to understanding such more