Thermoelectric Materials and Devices - Hierarchical Nanostructures for Energy Devices - page 119

shorter as the diameter decreases. In 2008, Hochbaum et al. 19 observed ex-
tremely low thermal conductivities in silicon nanowires with rough surfaces,
where they used the electroless etching (EE) method for the synthesis of the
rough nanowires. The thermal conductivity was reduced almost fourfold in
the wires of the same diameter at room temperature, compared to the vapor-
liquid-solid nanowires 19 (Figure 6.8, red squares).
Two-dimensional superlattices can also have low thermal conductivities
due to the increased interface scattering. Venkatasubramanian et al. 49

d n 3 r 4 n g | 7

re-

ported a ZT of 2.4 at room temperature with Bi 2 Te 3 /Sb 2 Te 3 superlattices.

They obtained a cross-plane lattice thermal conductivity of 0.22 W/m-K,
which is only 50% of that of a Bi 2-x Sb x Te 3 bulk alloy. To minimize reduction
in power factor while achieving more reduction in thermal conductivity, the
superlattice period was carefully chosen to be larger than the electron mean
free path, but smaller than the phonon mean free path.

B

.

Figure 6.9 (a) Thermal conductivities of the state-of-the-art (SOA) BiSbTe alloy and

nanograined BiSbTe. Solid lines are calculated phonon contributions of
bulk and nanograin composite. (b) Figures of merit of SOA (white
squares) and nanograined BiSbTe alloy (black squares). Inset shows a
scanning electron microscope image of nanograin BiSbTe. Reprinted
with permission from Ref. 21. Copyright r 2008 American Association
for the Advancement of Science. (c) Schematic of phonon grain boundary
scattering in normal polycrystal and nanograins. 21