transport properties. For nanostructured materials, a crucial issue is the role of interfaces on
electron and phonon transport. Although the ultimate goal should be set at predictive tools,
modeling should help in pointing directions for materials synthesis and structural engineering.
Insights gained through combined theoretical and experimental studies on fundamental
thermoelectric transport processes are invaluable
in the search of materials with high values of ZT,
the thermoelectric figure of merit.
New, High-performance Bulk Materials.
Several new bulk materials that exceeded ZT of
1 have been identified over the last 10 years.
Diverse classes of potential materials need to be
developed so they may serve as sources for novel
high ZT compounds. Mechanisms for decoupling
electron transport from phonon transport in such
materials through modification need to be
identified. Research opportunities along these
directions need to be systematically pursued.
Nanoengineered Materials. Nanoscale
engineering may be a revolutionary approach to
achieving high-performance bulk thermoelectric
materials. Recent results in bulk materials (based
on AgPbSbTe called LAST) have shown ZT > 2
in a bulk thermoelectric material (Hsu et al.
2004). An intriguing finding is that this material
exhibited a nanoscale substructure. Given the
former successes for high ZT in nanomaterials
(quantum dots and superlattice materials), the
nanostructure observed in the LAST material
may be essential for achieving a ZT > 2.
Therefore, one approach to nanoscale
engineering is to synthesize hybrid or composite
materials that have nanoscale thermoelectric
materials inserted into the matrix of the parent
thermoelectric material (see Figure 56).
Developing synthetic processes to fabricate
controlled nanoscale substructures is an
important undertaking.
Nanoscale thermoelectric materials that can
independently reduce phonon transport without
deteriorating electronic transport have been
implemented in Bi 2 Te 3 /Sb 2 Te 3 superlattices^
(Venkatasubramanian et al. 2001) offering a ZT
CFigure 56 The difficulty of searching
experimentally for the optimal high-ZT
material is illustrated in this figure.
AgPbmMTe2+m (where M is either Sb or Bi
and m varies from 10 to 18) is a candidate for
a high ZT material. What is the optimal
composition? Fig. 56A shows the average
ideal crystal structure; repeated x-ray
diffraction experiments indicate that the lattice
constant varies with m for M=Sb, as shown in
Fig. 56B. But TEM reveals that the x-ray
diffraction has not detected the presence of
nanodots of differing composition in Fig. 56C.
(Courtesy of M. Kanatzidis)