Thermoelectric Materials and Devices - Hierarchical Nanostructures for Energy Devices - page 131

0.4

0.3

d n 3 r 4 n g | 7

0.2

SrTiO 3 bulk system

0.1

HT SSR CS+SPS SG+SPS

Synthesis Methods

Figure 6.16 Figure of merit for the SrTiO 3 system produced by different methods.
HT: hydrothermal; SSR: conventional solid-state reaction technology;
CS þ SPS: combustion synthesis and SPS; SG þ SPS: sol-gel and
SPS. 65-68

of precursors as starting materials. It can offer homogeneous particle growth
(at the nanoscale), a uniform size distribution and monodispersive particles.
Therefore, this is one of the most favourable techniques for making single-
phase ceramics. Na/La-doped CaCo 3.8 Cu 0.2 O 9 ceramics have reportedly been
synthesized by sol-gel and SPS. 66 The sol-gel method started with precursors
in stoichiometric proportions. The precursors were dissolved in aqueous
solutions; heat was applied, and the pH was adjusted. The mixture solutions
were stirred continuously. This yielded a xerogel, which was calcinated at
high temperature in air. The obtained powders were then consolidated into
bulk ceramics by SPS. From the above description, we can see that the sol-
gel method is very cost effective and easy to handle and set up, and can yield
predefined stoichiometric compounds. Conventional SSR and the sol-gel
method are simple techniques in many types of synthesis methods for oxide
thermoelectric materials. Many other methods, such as hydrothermal, 67
combustion, 68 sol-gel, 66 etc, are available, but we do not discuss them in
detail. For the SrTiO 3 -based materials, we list only the highest figure of merit
obtained using each synthesis method in Figure 6.16.

.

6.5 Thermoelectric Devices

6.5.1 Power Generation
A typical structure of a thermoelectric (TE) device comprised of both n-type
and p-type elements is shown in Figure 6.17. For simplicity, only a pair of
n-type and p-type elements are presented in the figure, 1 but a real device for