Thermoelectric Materials and Devices - Hierarchical Nanostructures for Energy Devices - page 123

6.3.1.1 Melting Quenching
Melting quenching is a structure formation and crystallisation process that
begins with mixing of the elements constituting the raw materials. For
melting, the mixture of raw materials is heated beyond the melting tem-
peratures. When the temperature exceeds the melting temperature of each
element, the elements start to change phase to a homogeneous liquid. Then
the temperature is held above the highest melting point of the elements for
several hours until full liquefaction occurs. Quenching means 'rapid cooling'
and is generally performed in water, oil or air. The melted liquid mixture is
placed in the cooling medium; it is cooled rapidly and will crystallize
quickly. The mixture does not have sucient time to grow large crystals, so
one can obtain alloys with a very small grain size and homogeneous struc-
ture. Although this melting quenching method is very simple, it is dicult to
control the secondary phase of the quenching process. Thus, it is easy to
obtain non-crystalline phases.

d n 3 r 4 n g | 7

6.3.1.2 Mechanical Alloying
Mechanical alloying (MA) uses collisional energy for alloying. It is usually
performed using a ball mill. First, the powdered elements are prepared. Each
elemental powder is placed in a jar with an alumina or steel ball, and ball
milling is performed. During the ball milling, the powders collide with each
other, and fracturing and cold welding among them occur. As these pro-
cesses are repeated during ball milling, the mixture powder becomes an
alloy powder. The milled powder is then consolidated into a bulk shape and
heat-treated to obtain the desired microstructure and properties. The results
of this MA process depend on the milling speed, type of mill, type of pro-
tective gas and so on. MA can avoid the high temperatures used in the
melting process, and it can reduce the synthesis time.

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6.3.2 Nanocomposites Based on Conventional
Thermoelectric Materials
The two synthesis methods, i.e. melting quenching and MA, have yielded
good results in bulk alloy thermoelectric materials. Many studies have re-
ported further improvement in the thermoelectric performance by using
special microstructures and nanostructures. In this section, we will intro-
duce several special synthesis methods used for obtaining nanostructures
and compare them to the conventional synthesis methods.

6.3.2.1 Bismuth Telluride/Antimony Telluride
Bi 2 Te 3 and related compounds are the most popular thermoelectric ma-
terials for refrigeration applications because they have a maximum ZT range
at around room temperature. There are many compounds based on Bi 2 Te 3 ,