Thermoelectric Materials and Devices - Hierarchical Nanostructures for Energy Devices - page 118

The terms on the right-hand side of eqn (6.11) represent, respectively from
the left, the boundary scattering, Umklapp scattering, normal scattering,
alloy scattering, nanoparticle scattering and electron-phonon scattering
mean free path. This equation shows that the shortest mean free path
mechanism dominates over other mean free paths. Nanostructured ther-
moelectric materials usually have reduced mean free paths by boundary
scattering and/or nanoparticle scattering.

d n 3 r 4 n g | 7

6.2.4.1 Interface and/or Boundary Scattering
Several types of nanostructures are used to reduce the lattice thermal con-
ductivity. They can be classified as either 1-dimensional (1D), 2D or 3D
materials. A nanowire is a 1D material with a nanometre-scale diameter, so
phonons are scattered at the boundaries of the nanowires. 17 A representative
2D material is a superlattice or multi-layers. When material A is wetted and
adheres to material B epitaxially, a two-dimensional (2D) film growth results
and a traditional superlattice (SL) may be created if films A and B are al-
ternated in a periodic manner. 47 Each layer is a few nanometres thick, and
the layers are stacked periodically. Phonons are scattered at the interfaces
between the two layers, which results in a dramatic decrease of the thermal
conductivity. 16,18,39,48
Li et al. 40 reported reduced thermal conductivities in individual Si nano-
wires grown by the vapor-liquid-solid method. The thermal conductivities
of Si nanowires decrease as the diameter is reduced (Figure 6.8, black
squares). The reduction in thermal conductivity is caused by the decreasing
boundary size. The phonon mean free path for boundary scattering becomes

.

Figure 6.8 Thermal conductivities of rough and smooth Si nanowires.
Reprinted with permission from Ref. 19. Copyright r 2008 Nature
Publishing Group.