Thermoelectric Materials and Devices - Hierarchical Nanostructures for Energy Devices - page 128

d n 3 r 4 n g | 7

Figure 6.14 Thermoelectric figure of merit for 2% Na doped PbTe made using
various synthesis processes and their structures.

6.4.1 Mg 2 Si as a Thermoelectric Material
Mg 2 Si-based materials have a high effective mass, high carrier mobility and
low lattice thermal conductivity. 61 Thus, they are good for power generation
in the 500-800 K temperature range. In addition, their low cost and non-
toxicity are great advantages over other conventional thermoelectric ma-
terials based on telluride. Therefore, they are considered to be promising
Te-free thermoelectric materials. It is hard to make Mg 2 Si using con-
ventional processes because Mg evaporates and oxidizes easily in air.
Therefore, special synthesis processes should be used to prevent these
problems. We will introduce two types of synthesis methods for Mg 2 Si. One
is a two-step solid-state reaction (SSR) method based on the processes of
melting and quenching. 62 The other is a mechanochemical method that
combines a chemical reaction and MA. 63 These two methods will be dis-
cussed in the following examples.

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6.4.1.1 Two-step Solid-state Reaction Method and SPS
The two-step SSR method consists of two processes in a melting reaction.
This method can supply sucient reaction opportunities for particles of
each element. It is easy to obtain a homogeneous sample. Because it is easy
to lose Mg in the reaction, Mg is generally added during synthesis to com-
pensate for its loss. The Sb-doped Mg 2 Si 1-x Sn x alloy is chosen as an example
to explain each step in the synthesis process in detail. 62 Sb-doped
Mg 2 Si 1-x Sn x compounds were prepared by the two-step SSR method and SPS.
High-purity powders of the elements were weighed in stoichiometric quan-
tities and Mg powder was added with an excess of 5-10 mol% over the
stoichiometric amount. The raw materials were mixed in a glove box, shaped
by cold pressing and sealed in quartz tubes under vacuum for the first-step
SSR. The reaction temperature was 873-973 K. The reaction products were