Thermoelectric Materials and Devices - Hierarchical Nanostructures for Energy Devices - page 129

then used for a second-step reaction at 973 K to promote solid solution
formation and increase the homogeneity. Finally, the products were ground
to fine powders in the glove box, and SPS processing was performed at
823-1073 K. The two-step SSR method can effectively improve the degree of
reaction and solve the problem of the raw elements having entirely different
melting temperatures.

d n 3 r 4 n g | 7

6.4.1.2 Mechanochemical Synthesis, Incremental Milling
Technique and Hot Pressing
Mechanochemical synthesis was initially a synthetic method for preparing
toxic materials. 64 Many researchers have introduced this technique to be
applied to thermoelectric materials. It has proven to be an effective synthetic
method for some materials. In particular, it is useful when the stoichio-
metric proportion is dicult to control owing to evaporation or other
problems. In this process, the elemental powders are loaded into a milling
vial with ball bearings; the vial is then sealed, and ball milling is performed.
Some of the mechanical work among the powders and balls during ball
milling is converted into chemical work, as mentioned above. Chemical
reactions among the elemental powders occur during ball milling, and their
reaction energy comes from the mechanical work. This method can be used
to prepare materials at room temperature and avoid the vaporisation of Mg
at high temperature. However, the elemental Mg is a ductile material with a
low bulk modulus, so aggregation of Mg may occur in the vial. To solve this
problem, the amount of Mg is often controlled in the mechanochemical
synthesis process, so an incremental milling technique, i.e. adding more raw
materials during milling, is chosen. Finally, the pure phase of a Mg 2 Si-based
thermoelectric material can be formed by mechanochemical synthesis and
incremental milling. 63 We will consider Bi-doped Mg 2 Si as an example. Mg
metals are added incrementally to a stoichiometric amount of silicon and
bismuth and ball-milled for intervals of less than 1 h. Additional increments
of Mg are then added and ball-milled until the correct stoichiometry is
achieved. This technique is called incremental milling. These processes can
prevent the oxidation and evaporation of Mg and effectively prepare phase-
pure thermoelectric materials. Figure 6.15 shows the highest figure of merit
of Mg 2 Si-based materials produced by this method and the two methods
discussed above.

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6.4.2 Oxide Materials
Since 1997, oxides have received attention as a new environmentally friendly
thermoelectric material. Ca 3 Co 4 O 9 -based and SrTiO 3 -based ceramics are
chosen as examples of typical p- and n-type oxide thermoelectric materials,
respectively, in order to discuss the synthetic methods in this section.
Conventional SSR technology and sol-gel methods are two typical synthesis
methods.