Role of Interfaces in Nanocomposite Materials. Experimentally obtained improvements in
ZT in two superlattice structures benefited mainly from reductions on the phonon thermal
conductivity. A further increase in ZT in a wide range of structures and materials is possible by
engineering phonon transport through interfaces. For example, it has been demonstrated
experimentally that the phonon thermal conductivity of superlattices can be significantly smaller
than the theoretical minima of their constituent bulk materials^ (Costescu et al. 2004). Modeling
suggests that it is the incoherent superposition of interface reflection of phonons that is the major
cause of phonon thermal conductivity reduction^ (Chen 2001; Chen et al. 2003). However,
phonon reflection and transmissivity at single interfaces cannot be predicted at this stage, except
at very low temperatures.
Electron Transport in Nanoscale Materials. While there has been significant attention paid to
phonon transport in nanoscale systems, only a limited study of electronic transport in nanoscale
thermoelectric materials and structures has been conducted. Significant opportunities exist for
fine-tuning in two-dimensional superlattices to optimize the mini-band conduction as well as
obtain a delta function in DOS. A major development in itself could be the general theory of
electronic transport in solid state materials, with ZT >1, where isothermal conditions cannot be
assumed during current flow. A new theoretical framework needs to be developed in the study of
solid-state thermoelectrics, where quantum effects, multi-valley effects, strain-induced band-gap
engineering effects, sharp DOS, and nonisothermal electronic transport are all brought into play.
Theoretical and experimental methodologies to determine these quantities should be developed.
Thermophotovoltaics
We need to gain a basic understanding of novel materials for spectral control^ (Fleming et al.
2002; Greffet et al. 2002). Photonic crystals, plasmonics, phonon-polaritons, coherent thermal
emission, left-handed materials, and doping with lanthanides are concepts from the optics
community that can be exploited for the spectral control components required in TPV systems.
Insight has to be gained into fundamental processes, such as emission.
Nanostructured Metallic and Dielectric Materials with Low Diffusion and Evaporation
Rates. The major challenge of spectral control for TPV systems is given by the high operating
temperatures of ∼1,200°C in fuel-powered TPV and ∼2,000°C in solar TPV. Diffusion processes
and evaporation of material may limit the durability of the components significantly. Suitable
concepts of material engineering to reduce theses effects have to be developed and fully
understood by using multiscale models.
Scalable Manufacturing Processes Applicable to Various Geometries. The optical
approaches mentioned are based on materials properties and on precise nanostructuring of the
materials. The techniques for producing nanostructures are top-down approaches that are limited
to small, homogeneously structured areas and to flat surfaces nowadays. They are also not cost-
efficient. Thus, novel techniques have to be developed, which very likely incorporate self-
organization processes.