ME 381R Lecture 20: Nanostructured Thermoelectric Materials

1. ME 381R Lecture 20: Nanostructured Thermoelectric Materials Dr. Li Shi Department of Mechanical Engineering The University of Texas at Austin Austin, TX 78712 www.me.utexas.edu/~lishi lishi@mail.utexas.edu

2. Thermoelectric (Peltier) cooler: T1 T2 • Seebeck effect: Bi, Cr, Si… Pt Pt V Q Metal Cold p n I I • Thermoelectric refrigeration: • no toxic CFC, no moving parts Hot • Automobile • Electronics • Optoelectronics 250C Thermoelectrics • Thermocouple: 250C

3. Thermoelectric Cooling Performance Q Metal p n I I Venkatasubramanian et al. Nature 413, 597 Cold Nanostructured thermoelectric materials 2.5-25nm Bi2Te3/Sb2Te3 Superlattices Harman et al., Science 297, 2229 Hot Quantum dot superlattices • Coefficient of Performance (COPQ/IV) 2 CFC unit 1 COP Bi2Te3 0 0 1 2 3 4 5 ZT • ZT: Figure of Merit Seebeck coefficient Electrical conductivity Thermal conductivity

4. Phonon (lattice vibration wave) transmission at an interface Incident phonons Reflection Interface Transmission Thin Film Superlattice Thermoelectric Materials • Thin film superlattice • Approaches to improve Z  S2s/k : --Frequent phonon-boundary scattering: low k --High density of states near EF: high S2sin QWs Quantum well (smaller Eg) Barrier (larger Eg)

5. Electronic Density of States in 3D 2D projection of 3D k space • Each state can hold 2 electrons • of opposite spin(Pauli’s principle) • Number of states with wavevectore<k: ky dk k kx 2p/L • Number of states with energy<E: Density of States Number of k-states available between energy E and E+dE

6. Electronic Density of States in 2D 2D k space (kz = 0) • Each state can hold 2 electrons • of opposite spin(Pauli’s principle) • Number of states with wavevectore<k: ky dk k kx 2p/L • Number of states with energy<E: Density of States Number of k-states available between energy E and E+dE

7. k = 2np/L; n = ±1, ± 2, ± 3, ± 4, ….. (x+L) = (x) 0 -6p/L -4p/L 4p/L 2p/L -2p/L Electronic Density of States in 1 D 1D k space (ky = kz =0) k • Each state can hold 2 electrons • of opposite spin(Pauli’s principle) • Number of states with wavevectore<k: • Number of states with energy < E: Density of States Number of k-states available between energy E and E+dE

8. Electronic Density of States Ref: Chen and Shakouri, J. Heat Transfer124, p. 242 (2002)

9. Nanowires of Bi, BiSb,Bi2Te3,SiGe Al2O3 template Top View Nanowire • Thin Film Superlattices of Bi2Te3,Si/Ge, GaAs/AlAs Low-Dimensional Thermoelectric Materials Barrier Quantum well Ec E Ev x

10. Potential Z Enhancement in Low-Dimensional Materials • Increased Density of States near the Fermi Level: • high S2s (power factor) • Increased phonon-boundary scattering: low k  high Z = S2s/k:

11. Thin Film Superlattices for TE CoolingVenkatasubramanian et al, Nature413, P. 597 (2001)

12. Experiment Theory Z Enhancement in Nanowires Nanowire Prof. Dresselhaus, MIT Phys. Rev. B. 62, 4610 Heremans et at, Phys. Rev. Lett. 88, 216801 Challenge: Epitaxial growth of TE nanowires with a precise doping and size control

13. Imbedded Nanostructures in Bulk Materials 5x1018 Si-doped InGaAs Si-Doped ErAs/InGaAs SL (0.4ML) Undoped ErAs/InGaAs SL (0.4ML) Hsu et al., Science303, 818 (2004) AgPb18SbTe20 ZT = 2 @ 800K AgSb rich • Nanodot Superlattice Data from A. Majumdar et al. • Bulk materials with embedded nanodots Images from Elisabeth Müller Paul Scherrer Institut Wueren-lingen und Villigen, Switzerland

14. Phonon Scattering with Imbedded Nanostructures Phonon Scattering v eb Nanostructures Atoms/Alloys wmax Frequency, w Spectral distribution of phonon energy (eb) & group velocity (v) @ 300 K Long-wavelength or low-frequency phonons are scattered by imbedded nanostructures!

15. Challenges and Opportunities • Designing interfaces for low thermal conductance at high temperatures • Fabrication of thermoelectric coolers using low-thermal conductivity, high-ZT nanowire materials • Large-scale manufacturing of bulk materials with imbedded nanostructures to suppress the thermal conductivity