Nanostructured bulk thermoelectric materials

Nanostructured Bulk Thermoelectric Materials

We need to develop materials which shows a high figure of merit to enhance the thermoelectric conversion efficiency. A promissing way to high thermoelectric figure of merit is lowering lattice thermal conductivity. We study to lower lattice thermal conductivity through contolling microstructures of thermoelectric materials.

Lattice thermal conductivity lowered by enhanced phonon scattering

Lattice thermal conducitivity is given by k_L = 1/3Cvl, where C, v, and l are specific heat, speed of sound, and phonon mean free path, respectively. l is the average distance between consecutive scattering events of phonons. There are various modes of phonon scattering. Efficient ways to lower k_L include introduing impuring atoms or increasing interfaces such as grain boundaries or heterophase boundaries to shorten l.

Nanostructuring thermoelectric materials utilizing phase transformations

Nanostructure can be introduced to thermoelectric materials utilizing solid state phase transformations resulting in lowered thermal conductivity. The left figure shows an example of nanostructuring using a solid state reaction (a eutectoid reaction), Pb₂Sb₆Te₁₁ → PbTe (bright phase) + Sb₂Te₃ (dark phase).

Examining the phase diagram in details, nanostructures from phase transformations can be precisely controlled based on phase transformation theories (thermodynamics and kinetics).

Another example of nanostructuring utilizng a solid state precipitation (PbTe → PbTe (bright phase) + Sb₂Te₃(dark phase)) also shows the reduction of lattice thermal conductivity.

Nanostructuring via a high energy nonequilibrium intermediate state

Interface density has a significant impact on the thermoelectric properties. We have proposed a process to fabricate nanostructured thermoelectric materials with less restriction from phase diagrams — a process via a high energy nonequilibrium state. In cases the energy we need to give to a material for achieving the nonequilibrium state is high (like cases for line compounds), the chemical driving force for the back transfor¬mation to the equilib¬rium state is expected to be large resulting in nanoscale microstructure formation and the great reduction of κ_L.

References

1. N.A. Heinz, T. Ikeda, Y. Pei, G.J. Snyder, Quantitative microstructure control in bulk thermoelectric composites, Adv. Funct. Mater., 24 (2013), 2135-2153.

2. T. Ikeda, L. Haviez, Y. Li, G.J. Snyder, Nanostructuring of thermoelectric Mg2Si via a nonequilibrium intermediate state, Small, 8 (2012), 2350-2355.
3. T. Ikeda, N.J. Marolf, K. Bergum, M.B. Toussaint, N.A. Heinz, V.A. Ravi, G.J. Snyder, Size control of Sb2Te3 Widmanstätten precipitates in thermoelectric PbTe, Acta Mater., 59 (2011), 2679-2692
4. T. Ikeda, L.A. Collins, V.A. Ravi, F.S. Gascoin, S.M. Haile, G.J. Snyder, Self-assembled nanometer lamellae of thermoelectric PbTe and Sb2Te3 with epitaxy-like interfaces, Chem. Mater., 19 (2007), 763-767.