Thermoelectric Materials and Devices - Hierarchical Nanostructures for Energy Devices - page 125

d n 3 r 4 n g | 7

Figure 6.13 Thermoelectric figures of merit of BiSbTe, PbTe and SiGe systems in
various crystal structures. 1,21,51-56

Finally, the melted surfaces adhere to each other. SPS has the great advan-
tage of providing extreme confinement of grain growth. Grain growth occurs
by diffusion, and the diffusion rate is determined by the diffusivity of the
material and time during the process. In SPS, the process is completed in
about 5-10 minutes. Because the time is short, the degree of diffusion is
limited.
Figure 6.13 shows the thermoelectric figures of merit of the BiSbTe system
in various crystal structures: a single crystal made by the zone melting
method, 52 a polycrystal made by melting, 21 and nanograins made by ball
milling and hot pressing. ZT increases as the grain size decreases. One of the
reasons could be the thermal conductivity reduction owing to a decrease in
the grain size.

.

6.3.2.2 Silicon Germanium
Silicon germanium is ideal for high temperature applications. It was first
used in radioisotope thermoelectric generators for spacecraft. 1 SiGe devices
can operate at up to 1300 K without performance degradation. The optimal
composition is generally considered to be Si 0.8 Ge 0.2 . 1 It can be made by
conventional methods such as zone melting, ball milling and hot pressing. It
can also produce nanograined composites although a high operating tem-
perature is required. A nanograined SiGe composite 54 yields a ZT of

B 1.5 at

0.9 at 1173 K). 1
Figure 6.13 shows the thermoelectric figures of merit of the Si 0.8 Ge 0.2 system
in various crystal structures: a single crystal made by the zone levelling
method, 53 a polycrystal made by a conventional sintering method 53
and a nanocomposite with nanosized grains produced by SPS. 54

1173 K, which is almost twice that of a SiGe bulk alloy (ZT B

The