Chemistry Reference
In-Depth Information
thermoelectric figure of merit increases as the average grain size decreases
owing to the increased phonon scattering at the grain boundaries.
6.3.2.3 Lead Telluride and Related Compounds
Lead telluride is widely used for a temperature range of 600-900 K, i.e. mid-
temperature thermoelectric material, which corresponds to a use in waste
heat recovery of exhaust gas in vehicles. Therefore, the lead telluride system
could be used to harvest waste heat from vehicles. There are many reports of
lead telluride synthesis. It is known that it is dicult to make small PbTe
grains because the grains grow very rapidly.
57
However, the PbTe system has
its own advantages. Lead antimony silver telluride (LAST) has intrinsic
nanodots inside the matrix.
20
These nanodots scatter phonons, so LAST has
low thermal conductivity. Similarly, the PbTe-SrTe system also has intrinsic
nanostructures.
55
In the following subsections, we will discuss this synthesis
method.
d
n
3
r
4
n
g
|
7
6.3.2.3.1 Melting, Quenching, Annealing and Hot Pressing. Annealing is
a heat motional dispersion process. It operates at a relatively high tem-
perature below the melting point and includes a recrystallization process.
The actual annealing is conducted in a furnace for over a long period of
time. After annealing, a homogeneous composition can be obtained in the
ingot. Hot pressing is a typical method to consolidate into a bulk shape,
and heat treatment is a method of obtaining a desired microstructure and
improving the density of materials. Therefore, annealing and hot pressing
have often been added to the synthesis process for PbTe-based thermo-
electric materials made by the conventional melting quenching method.
For example, Na-doped PbTe
58
and Na-doped PbTe
1-x
Se
x
alloys
36
have been
synthesized in this way. For these two examples, homogeneous samples
with high relative density have been synthesized successfully. Higher ther-
moelectric figures of merit
.
1.8) were obtained for
Na-doped PbTe and Na-doped PbTe
1-x
Se
x
, respectively. Therefore, the syn-
thesis method of melting, quenching, annealing and hot pressing is a
good choice for successfully synthesizing PbTe-based materials with a
high thermoelectric performance.
(ZT
B
1.4 and
B
6.3.2.3.2 Melting, Annealing, Hot Pressing and Post Annealing. Quench-
ing is a process of solidification and crystallisation. The desired com-
position should appear after quenching. However, some defects, such as
non-crystalline phases, nanoparticles and nanopores also occur during
this process because the temperature drops in a very short time.
59
There-
fore, to obtain a pure phase without defects, some researchers omit the
quenching process. Instead, they set the temperature to decrease slowly
from the melting point to the annealing temperature, and post-annealing
is added after hot pressing. The main aim of post-annealing is to ensure
stable properties at a high temperature. Because the operating tempera-
ture of thermoelectric materials, especially PbTe, is relatively high, the
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