Thermoelectric Materials and Devices - Hierarchical Nanostructures for Energy Devices - page 124

such as p-type Bi 0.5 Sb 1.5 Te 3 , n-type Bi 2 Te 2.4 Se 0.6 and n-type Bi 2 Te 2.7 Se 0.3 . The
highest figure of merit for the Bi 2 Te 3 system as a pure alloy is ZT 1of

Bi 0.5 Sb 1.5 Te 3 alloys. 52 This system was developed in the 1950s and now is
widely used in commercial thermoelectric devices. Recent research work
demonstrated enhanced thermoelectric properties for the nanograined
composites of the BiSbTe system. 21 The first step in making nanograins is to
make a nanopowder. A BiSbTe ingot is made by a conventional melting
method and loaded into a jar with balls. The process is similar to that of MA,
but the alloying is performed not by mechanical action but by melting.
Secondly, high-energy ball milling is performed for several hours. This yields
a nanosized BiSbTe powder. The next step is to form the powder by hot
pressing or spark plasma sintering (SPS). Figure 6.12 shows the schematics
of hot pressing and SPS processes. 1,21,51-56 Hot pressing is a type of sintering
with applied pressure and temperature. Hot pressing has many advantages.
Firstly, it can reduce the sintering time and temperature. Therefore, grain
growth can be controlled. Further, it can eliminate the pores inside the
sample because it applies a high pressure to the sample. SPS is a special type
of hot pressing that uses a DC current supply instead of a furnace. The DC
current is applied inside the powder during the process. The current flow
produces considerable joule heating at the powder surface. The temperature
of the powders rapidly increases, so their surfaces are melted in a moment.

B

d n 3 r 4 n g | 7

.

Figure 6.12 Schematics of hot pressing and spark plasma sintering (SPS) processes.

Reprinted with permission from Ref. 101. Copyright r 2004, John
Wiley and Sons.