Thermoelectric Materials and Devices - Hierarchical Nanostructures for Energy Devices - page 120

In 3D materials, the thermal conductivity can be reduced by increasing
grain boundary scattering. To achieve this, the grain size should be kept
small. In Figure 6.9(a), the thermal conductivity of a BiSbTe nanograin
composite was reduced over the entire temperature range compared to that
of a normal crystalline state-of-the-art (SOA) BiSbTe alloy. Black lines below
the symbols in Figure 6.9(a) represent the calculated lattice thermal con-
ductivities of nano and bulk BiSbTe. As a result of this, Poudel et al. 21

d n 3 r 4 n g | 7

re-

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ported the BiSbTe nanograin composite that yielded a ZT of 1.4 at 373 K

(Figure 6.9(b)). Nanostructured BiSbTe has a lower thermal conductivity
than the bulk material due to the fact that nanograins increased the phonon
scattering (Figure 6.9(c)). Small grains reduce the phonon mean free path by
increasing grain-boundary scattering, which results in an enhancement of
the thermoelectric figure of merit. 21

6.2.4.2 Nanosized Defect Scattering
Figure 6.10(a) shows the mechanism by which nanoparticles reduce the
thermal conductivity. The aforementioned atomic substitutions such as
alloy atoms in the matrix can scatter short-wavelength phonons effectively,
but they cannot scatter mid- to long-wavelength phonons due to their small
sizes. 45,50 However, if we embed nanosize defects inside the matrix, they can
scatter mid- to long-wavelength phonons effectively. Thus, the thermal
conductivity is reduced over the wide phonon wavelength range by embed-
ding nanoparticles inside the matrix. Hsu et al. 20 reported a AgPb m SbTe 21m
nanocomposite that exhibits epitaxial nanoprecipitation inside the matrix.
The nanoprecipitations scatter phonons effectively, so that the material has
a low thermal conductivity. In addition, it also has a large power factor,
which resulted in a maximum ZT of

.

2.2 at 800 K. Kim et al. 46 demonstrated
the reduced thermal conductivity for InGaAs semiconductors by embedding
ErAs nanodots epitaxially inside the matrix (Figure 6.10(b) inset). The ther-
mal conductivity was reduced by almost a factor of two. In this temperature
range, defect scattering dominates the thermal conductivity (Figure 6.10 (b)). 43
As a result, ZT increases by a factor of two owing to the reduced thermal
conductivity (Figure 6.10(c)).
Biswas et al. 51 reported Na-doped PbTe-SrTe with all-scale hierarchical
structure, where they reported a ZT of

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2.2 at 950 K. PbTe-SrTe has intrinsic
nanoprecipitations inside the matrix, which scatter mid-wavelength pho-
nons effectively (Figure 6.11(a)). Moreover, they produced grains of a few
micrometres in size by spark plasma sintering (SPS). These small grains can
scatter the long-wavelength phonons, and intrinsic atomic substitutions in
the alloy scatter short-wavelength phonons. By using this all-scale hier-
archical structure, phonons in a wide range of wavelengths can be effectively
scattered. Interestingly, Biswas et al. 51 observed a rich concentration of Na at
the interfaces between the matrix and the precipitation (Figure 6.11(b)). As
temperature increases, sodium atoms that were located at the interface
diffuse into the bulk region and become electrically active, donating charge
carriers to the matrix.

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