Solar Thermoelectrics. Mercouri Kanatzidis, Materials Science Division

Transcription

1 Solar Thermoelectrics Mercouri Kanatzidis, Materials Science Division December 15, 2009

2 Heat to Electrical Energy Directly Up to 20% conversion efficiency with right materials cold hot Thermopower S = ΔV/ΔT TE devices have no moving parts, no noise, reliable 2

3 Figure of Merit 0.25 δ = 0.0 δ = electrical conductivity 0.15 σ S ZT = T κ to ta l δ = 1.1 η Total thermal conductivity thermopower 3 ZT Power factor σ S 2 avg δ=rc/r For Th = 800K Tc = 300K 3

4 ZT and Electronic Structure Isotropic structure Anisotropic structure mx m y T τ mz (+ ) γ e r 1/ 2 κ latt 3/ 2 Z max m= effective mass τ=scattering time r= scattering parameter κlatt= lattice thermal conductivity T = temperature For acoustic phonon scattering r=-1/2 E E γ= band degeneracy Large γ comes with (a) high symmetry e.g. rhombohedral, cubic (b) off-center band extrema Ef k k k Complex electronic structure 4

5 Selection criteria for candidate materials Narrow band-gap semiconductors Heavy elements High μ, low κ Large unit cell, complex structure low κ Highly anisotropic or highly symmetric Complex compositions low κ, complex electronic structure 5

6 Investigating the A/Bi/Q system A2Q + Q + M2Q3 > (A2Q)n(Q)m(M2Q3)p Map generates target compounds Cubic materials AmBnMmQ2m+n A=Ag, K, Rb, Cs M=Sb, Bi Q=Se, A2Q Phases shown are promising new TE materials Rb0.5Bi1.833 A2Bi37 A1+x4-2xBi7+xSe15 Q 6Bi2Se9 5Bi6Se14 5Bi12Se23 β-k2bi8se13 CsBi46 Bi2Q3 6

7 AgmSb2+m (LAST-m) NamSb2+m (SALT-m) Sb Ag Unit cell volume (Å) Single crystal data 266 Powder data 265 Vegard's law m value (1) (a) Rodot, H. Compt. Rend. 1959, 249, (2) (a) Rosi, F. D.; Hockings, E. S.; Lindenblad, N. E. Adv. Energy Convers. 1961, 1, 151. No phase transitions to melting point 7

8 Synthesis Ingot properties very sensitive to cooling profile ºC T, K rock Cool to 50 ºC in 72 h Gravity induced inhomogeneity LAST deg/h time, h T, K 1000 ºC rock Cool to 50 ºC in 24 h Wernick, J. H.. Metallurg. Soc. Conf. Proc. (1960), time, h R. G. Maier Z. Metallkunde 1963, 311 8

9 LAST-18: Synthesis with Slow Cooling ~2deg/hr Ag Sb amount g 1000 oc 4h 7d 700 oc fast cooled sample 12 h 12 h 50 oc ETN125 σ (S/cm) S (µv/k) PF (µw/cm K2) A B C D slow cooled sample 9

10 Lattice thermal conductivity σ (S/cm) S ( µ V/K) Lattice Thermal Conductivity (W/mK) Properties of Ag1-x18Sb20 LAST mperature, K mperature (K)

11 LAST-18 ZT~ Ag 0.86 Sb ZT mperature, K

12 HRTEM of LAST-18 12

13 What is the dot made of? Cook, Kramer 13

14 Nanostructures reduce the lattice thermal conductivity 2.5 Lattice Thermal Conductivity (W/mK) Lattice thermal conductivity LAST W/mK (Harman /Se superlattice) 300 Clemens-Drabble theory mperature (K) 14

15 Why do the LAST materials nanostructure? Ag Ag Driving force for segregation Ag+/Sb3+ pair: thermodynamics Sb Sb Ag Ag Sb Sb Dissociated state..unstable Associated state..stable Any +1/+3 pair 15

16 1.4 Ag0.67Sn3Sb0.212 Ag0.56Sn2Sb ZT Figure of Merit LASTT (p-type) Ag0.95Sn3Sb (b) Ag0.5 6Sn2Sb Ag0.9 5Sn3Sb Ag0.6 7Sn3Sb LASTT ZT 1.2 TAGS mperature (K) J. Androulakis, K. F. Hsu, R. Pcionek, H. Kong, C. Uher, J. J. D'Angelo, A. Downey, T. Hogan, M. G. Kanatzidis, Advanced Materials 2006, 18,

17 Na-based materials (SALT-m) New high ZT p-type material Na1-xmSb2+m m~ nm 2 nm 17

18 What is nanostructuring worth? Sb Na Na Sb P. F. P. Poudeu, J. D'Angelo, A. D. Downey, J. L. Short, T. P. Hogan, M. G. Kanatzidis, Angew. Chem. Int. Ed. 2006, 45, 1 18

19 Matrix Encapsulation as a Route to Nanostructured X = Sb + X X = Bi X= InS b 2 nm 20 nm 100 nm 100 nm 2 nm 2 nm 19

20 Nanocrystals of Sb in A 2.5 B Sb(16%) - Sb(4%) lat nm κ -1 κ lat, Wm K -1-1, Wm K % Sb 4% Bi % InSb - Sb(2%) mperature, K mperature, K An optimum concentration of nanoscale second phase is necessary Mass fluctuations play a role in thermal conductivity reduction Lattice thermal conductivity reduced, however ZT low due to small Seebeck 20

21 Phonons 21

22 Electrons 22

23 23

24 Completed and Processed Ingot Schock Composition: Ag0.4318Sb1.220 Weight: 200 grams Increasing T 2 - Decreasing T 3 - Increasing T 4 - Decreasing T 1200 Ag Sb Increasing T 2 - Decreasing T 3 - Increasing T 4 - Decreasing T TEP (µv/k) Electrical Conductivity (S/cm) mperature (K) mperature cyclability 24

25 Module Fabrication Hot side diffusion contacts, and cold side solder contacts with <10 µw cm2 have been achieved Th=495K Th=545K Th=585K 1.78mΩ total 16.0µΩ cm2 Power Generated (W) Load resistance (ohms)

26 Best ZT Materials 2 LAST Na Sb LASTT Tl Bi 9 β -K Bi Se 1 2 CsBi 4 8 TAG S 6 13 Ce Fe CoSb SiGe Bi mperature (K) 26

27 Conclusions LAST, LASTT and SALT: promising thermoelectric materials for next generation power generation modules. (expected device efficiency ~14%) Nanostructures strongly reduce thermal conductivity. Nanostructures are closely linked to high ZT. Scaleup successful in producing large quantities but material is brittle and contains microcracks. Hot pressing and powder processing yield 3x improvement in strength. Higher average ZT (>2) needed to reach 20% efficiency. 27