Chemistry Reference
In-Depth Information
sample properties may change during operation. For example, stable
K-doped PbTe
1-x
Se
x
have been prepared in this way,
60
and a higher figure
of merit (ZT
1.7) was obtained. In addition, they show high stability at
high temperatures above the hot pressing temperature. Therefore, the pro-
cess of melting, annealing, hot pressing and post-annealing has the ad-
vantage of stability and extends the operating temperature range.
B
d
n
3
r
4
n
g
|
7
6.4 Nanodot Nanocomposites
Nanostructured materials are currently a focus of research to improve
thermoelectric properties. As noted, it is dicult to obtain nanosized PbTe
grains because of rapid grain growth during the annealing process. However,
some PbTe systems have excellent intrinsic nanostructures that can enhance
the thermoelectric properties. The melting annealing method is suitable for
making nanostructures consisting of nanosized precipitations. Generally,
precipitations appear at a very specific temperature range, so it is very im-
portant to control the temperature of the solid solution properly.
20
To con-
trol the temperature precisely, the solid solution should be cooled slowly
from the melting temperature and be held at the temperature at which
precipitation occurs. The PbTe-SrTe system was also made by the melting
annealing method. These systems exhibited ZT values of 2.2 and 1.7,
51,55
respectively, at 800 K. Later, the Kanatzidis group achieved a ZT of
2.2 at
950 K in the same material by using all-scale hierarchical structures.
51
They
adopted the quenching and SPS processes in the PbTe-SrTe system. Despite
the fast grain growth of the PbTe system, relatively small grains could be
formed. In addition, quenching produces many atomic-scale defects. Thus,
this system has intrinsic atomic-scale defects, nanodots and small grains
(Figure 6.11(a)). Quenching and SPS are both rapid synthesis processes that
may produce special microstructures and nanostructures. Figure 6.13
shows
1,21,51-56
the thermoelectric figures of merit of PbTe and related
compounds in various crystal structures. The figure shows the highest figure
of merit for each of the following three methods: the Bridgman method of
producing single crystals,
56
the melting annealing method of producing
polycrystals
55
and melting, quenching and SPS for producing a nano-
composite.
51
The PbTe system and related compounds could possess in-
trinsic nanostructures depending on synthesis schemes, so the polycrystals
in particular could have high ZTs. In addition to this, SPS reduces the grain
size, yielding the highest ZT. In summary, melting, quenching, annealing,
hot pressing, post-annealing and SPS can be combined to prepare PbTe-
based thermoelectric materials. However, the specific synthesis method for a
given sample depends on the specific requirements, such as high density,
stability and special microstructure. Wang et al.
99
reported that 2% Na
doped PbTe can have an enhanced zT of 2.0 at 773 K by modifying synthesis
conditions. A rapid quenching process makes nano- and microstructures
inside the PbTe matrix. These structures scatter phonons so that the thermal
conductivity decreases (Figure 6.14).
B
.
Search WWH ::
Custom Search