Nanostructured Thermoelectric Materials 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

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

Coefficient of Performance (COPQ/IV) Thermoelectric Cooling Performance Q p n I Metal 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 COP 1 Bi2Te3 1 2 3 4 5 ZT ZT: Figure of Merit Seebeck coefficient Electrical conductivity Thermal conductivity

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

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

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

Electronic Density of States in 1 D (x+L) = (x) k = 2np/L; n = ±1, ± 2, ± 3, ± 4, ….. 2p/L 4p/L -6p/L -2p/L -4p/L 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

Electronic Density of States Ref: Chen and Shakouri, J. Heat Transfer 124, p. 242 (2002)

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

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:

Thin Film Superlattices for TE Cooling Venkatasubramanian et al, Nature 413, P. 597 (2001)

Z Enhancement in Nanowires Experiment Theory 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

Imbedded Nanostructures in Bulk Materials 5x1018 Si-doped InGaAs Si-Doped ErAs/InGaAs SL (0.4ML) Undoped ErAs/InGaAs SL (0.4ML) Nanodot Superlattice Data from A. Majumdar et al. AgPb18SbTe20 ZT = 2 @ 800K AgSb rich Hsu et al., Science 303, 818 (2004) Bulk materials with embedded nanodots Images from Elisabeth Müller Paul Scherrer Institut Wueren-lingen und Villigen, Switzerland

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

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