Direct Energy Conversion: Chemistry, Physics, Materials Science and Thermoelectrics

Direct Energy Conversion: Chemistry, Physics, Materials Science and Thermoelectrics Mercouri G Kanatzidis MURI KANATZIDIS KANATZIDIS MSU, MSU, CHEMISTRY CHEMISTRY Solid Solid State State Chemistry Chemistry Synthesis, Synthesis, Discovery Discovery TIM HOGAN MSU, E. ENGINEERING Measurements, Devices HAROLD SCHOCK MSU, Mechanical Engineering Applications S. D. MAHANTI MSU, PHYSICS Theory CTIRAD UHER Univ of of Michigan Physics Measurements ELDON CASE MSU, Mechanical Engineering Metallurgy American Physical Society Meeting, Baltimore March 2006

Heat to Electrical Energy Directly Up to 20% conversion efficiency with right materials http://www.dts-generator.com/ Schock group

Thermoelectric applications Waste heat recovery Automobiles Over the road trucks Utilities Chemical plants Space power Remote Power Generation Solar energy Geothermal power generation Direct nuclear to electrical

U.S. Energy Flow, 1999 web site: www.eia.doe.gov Given that ~60% of energy becomes waste heat, even a 10% capture and conversion to useful forms can have huge impact on overall energy utilization

How does it work? http://www.designinsite.dk TE devices have no moving parts, no noise, reliable

Figure of Merit 0.25 0.2 δ = 0.0 δ = 0.1 electrical conductivity thermopower η 0.15 0.1 δ = 1.1 ZT = σ S2 T κ total 0.05 0 0 0.5 1 1.5 2 2.5 3 Total thermal conductivity Power factor σ S 2 ZT avg δ=r c /R For T h = 800K T c = 300K

Today s situation The most efficient materials today for power generation: PbTe and TAGS (TeSbGeAg alloy) The most efficient material for cooling Bi 2 Te 3 PbTe: ZT~0.8 at 800 K (n-type) TAGS: ZT~1.2 700 K (p-type) Bi 2 Te 3-x Se x : ZT~1 at 300 K Further improvements are needed. New materials needed Quantum Dot Layers in thin MBE-grown PbSe/PbTe superlattices (Harman et al, ZT~3) PbTe PbSe 20 nm dot Hicks LD, Dresselhaus MS Phys. Rev. B 47 1993 (24): 16631

Some promising systems under investigation half-heusler alloys (ZrNiSn) Zn 4 Sb 3 Clathrates Skutterudites (CoSb 3 ) Bulk nanocomposites based on PbTe Bulk nanocomposites based on Si-Ge AgSbTe 2 /PbTe, NaSbTe 2 /PbTe See March 2006 issue of MRS Bulletin

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

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

Chemistry as a source of materials Investigating the System: A 2 Q + PbQ + M 2 Q 3 > (A 2 Q) n (PbQ) m (M 2 Q 3 ) p Map generates target compounds Cubic materials AB m MQ 2+m Phases shown are promising new TE Materials A 2 Q Rb 0.5 Bi 1.83 Te 3 A=Ag, K, Rb, Cs M=Sb, Bi Q=Se, Te PbQ APb 2 Bi 3 Te 7 A 1+x Pb 4-2x Bi 7+x Se 15 Pb 6 Bi 2 Se 9 Pb 5 Bi 6 Se 14 Pb 5 Bi 12 Se 23 β-k 2 Bi 8 Se 13 CsBi 4 Te 6 Bi 2 Q 3

Our first contact with cubic AgPb m SbTe 2+m AgBi 3 S 5, KPbBi 9 Se 13, KPb 4 Sb 7 Se 15 CsPbBi 3 Te 6, CsPb 2 Bi 3 Te 7, CsPb 3 Bi 3 Te 8, RbPbBi 3 Te 6, RbPb 2 Bi 3 Te 7, RbPb 3 Bi 3 Te 8, KPbBiSe 3, K 2 PbBi 2 Se 5 K 2 Pb 3 Bi 2 Te 7, KPb 4 SbTe 6

AgPb m SbTe 2+m (LAST-m) AgPb m (Sb,Bi)Te 2+m (BLAST-m) Pb Te Sb Ag 268 Unit cell volume (Å) 267 266 265 264 Vegard's law 263 6 8 10 12 14 16 18 m value (1) (a) Rodot, H. Compt. Rend. 1959, 249, 1872-4. (2) (a) Rosi, F. D.; Hockings, E. S.; Lindenblad, N. E. Adv. Energy Convers. 1961, 1, 151.

(LAST-18) Ag 1-x Pb 18 SbTe 20 : Tunable properties Changing x Thermopower (µv/k) 50 0-50 -100-150 -200 Comparison of Thermopower Ag0.73 (KF2207R2b) Ag0.76 (KF2228R3A2) Ag0.78 (KF2213R1B) Ag0.80 (KF2213R2C) Ag0.82 (KF2229R1A) Ag0.82 (KF2229R1B) Ag0.82 (KF2229R1C) Ag0.82 (KF2229R1D) -250 50 100 150 200 250 300 350 400 450 Temperature (K) Comparison of Power Factors ZT 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 Ag0.73 (KF2207R2b) Ag0.76 (KF2228R3A2) Ag0.78 (KF2213R1B) Ag0.80 (KF2213R2C) Ag0.82 (KF2229R1A) Ag0.82 (KF2229R1B) Ag0.82 (KF2229R1C) Ag0.82 (KF2229R1D) Comparison of ZT 0 80 120 160 200 240 280 320 360 400 Temperature, K Power Factor S 2 σ (µw/cm K 2 ) 40 30 20 10 Ag0.73 (KF2207R2b) Ag0.76 (KF2228R3A2) Ag0.78 (KF2213R1B) Ag0.80 (KF2213R2C) Ag0.82 (KF2229R1A) Ag0.82 (KF2229R1B) Ag0.82 (KF2229R1C) Ag0.82 (KF2229R1D) 0 50 100 150 200 250 300 350 400 450 Temperature (K)

Synthesis: Heating cooling profiles rock T, K 950-1000 ºC Cool to 50 ºC in 72 h 13 deg/h Ingot properties very sensitive to cooling profile Gravity induced inhomogeneity LAST-18 time, h T, K 1000 ºC rock Cool to 50 ºC in 24 h time, h Wernick, J. H.. Metallurg. Soc. Conf. Proc. (1960), 5 69-87. R. G. Maier Z. Metallkunde 1963, 311

Samples cooled slowly from liquid to solid Strongly varying composition from top to m= 8 bottom. m= 14 Strongly varying properties from top to bottom. m= 18 Sweet spot exists with very high ZT. m= 25 Mechanical properties weak. m= 40 σ (S/cm) S (µv/k) Strong composition grading along ingot

Properties of Ag 1-x Pb 18 SbTe 20 2000-100 2.5 Lattice thermal conductivity σ (S/cm) -150 1500-200 1000-250 -300 500-350 0-400 300 350 400 450 500 550 600 650 700 Temperature, K Lattice Thermal Conductivity (W/mK) S (µv/k) 2 1.5 1 0.5 0 PbTe LAST-18 0.35 W/mK (Harman PbTe/PbSe superlattice) 300 400 500 600 700 800 Temperature (K)

Ag 1-x Pb 18 SbTe 20 2.00 Ag 0.86 Pb 18 SbTe 20 1.50 ZT 1.00 0.50 0.00 300 350 400 450 500 550 600 650 700 Temperature, K Hsu KF, Loo S, Guo F, Chen W, Dyck JS, Uher C, Hogan T, Polychroniadis EK, Kanatzidis MG Science, 2004, 303, 818

What is the origin of the TE properties of AgPb m SbTe m+2 systems?

HRTEM Coherently embedded nanocrystals Pb- rich region 8-10 nm Ag-Sb rich region Polychroniadis, Frangis, 2004 LAST-18 κ latt =1.2 W/m-K at 300 K PbTe κ latt =2.2 W/m-K at 300 K

Coherent compositional fluctuations in AgPb m SbTe m+2 FCC lattice 10 nm Ag, Sb, Pb ordering

Driving force for segregation Ag + /Sb 3+ pair: stable Ag Te Pb Te Pb Te Pb Te Pb Te Pb Te Sb Te Pb Te Pb Te Pb Te Pb Te Ag Te Pb Te Pb Te Pb Te Pb Te Pb Te Pb Te Pb Te Pb Te Pb Te Pb Te Sb Te Pb Te Pb Te Ag Te Ag Te Pb Te Pb Te Pb Te Sb Te Pb Pb Te Sb Te Pb Te Pb Te Pb Te Pb Te Pb Te Te Pb Te Pb Te Pb Te Dissociated state..unstable Associated state..stable Any +1/+3 pair

LAST-m Solid solutions in (AgSbTe 2 ) 1-x (PbTe) x Lattice Lattice Thermal Thermal Conductivity (mw/cm K) (W/cmK) 20 18 16 14 12 10 8 6 4 0 20 40 60 80 100 Klemmens-Drabble theory m=65 m=62 m~62 Random alloy m~30 m=18 m=10 m=18 Experimental Points below theoretical 0 20 40 60 80 100 AgSbTe2 PbTe Mol % PbTe T. Irie Jap. J. Appl. Phys. 1966, 5, 854

P-type materials, LASTT1000 (LASTT-m) Ag(Pb 1-x Sn x ) m SbTe 2+m Sn atoms act as acceptors Ag atoms act as acceptors Sb atoms act as donors e.g AgPb 10 Sn 8 SbTe 20, Ag x Pb 7 Sn 3 Sb y Te 12, Very low lattice thermal conductivity Good homogeneity Temperature ( o C) 800 600 400 200 0 0 10 20 30 40 50 60 t (hours) 2 hours rocking 12000 10000 Ag 0.3 Pb 7 Sn 3 Sb 0.1 Te 12 Intensity (arb. units) 8000 6000 4000 2000 0 20 30 40 50 60 70 80 90 100 2Theta

LASTT-16 Very low lattice thermal conductivity κ latt =0.5 W/m K at 650 K Androulakis, Hsu, Hogan, Uher, Kanatzidis Advanced Mater. 2006, in press

LASTT-16: AgPb 12 Sn 4 Sb 0.4 Te 20 320 280 240 Seebeck (uv/k) 200 160 120 80 40 0 320 400 480 560 640 720 Temperature, K Androulakis, Hsu, Hogan, Uher, Kanatzidis Advanced Mater. 2006, in press

Figure of Merit LASTT (p-type) ZT 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 Ag 0.5 Pb 6 Sn 2 Sb 0.2 Te 10 Ag 0.9 Pb 5 Sn 3 Sb 0.7 Te 10 Ag 0.6 Pb 7 Sn 3 Sb 0.2 Te 12 (b) LASTT TAGS PbTe 300 400 500 600 700 800 Temperature (K) Androulakis, Hsu, Hogan, Uher, Kanatzidis Advanced Mater. 2006, in press

NaPb 20 SbTe 22 (SALT-20) (A) Lattice Thermal conductivity (W/mK) 2 (B) 1.5 Pb Sn Te 0.9 0.1 not-nano 1 PbTe Se 0.9 0.1 0.5 Na Pb SbTe 1-x 20 22 nano 0 300 350 400 450 500 550 600 650 700 Temperature, K 50 nm Poudeu, Hogan, Kanatzidis Angew. Chemie, 2006,

SALT-20 Poudeu, Hogan, Kanatzidis Angew. Chemie, 2006,

State of the art - bulk 2.5 2 LAST ZT 1.5 Tl 9 BiTe 6 NaPb SbTe 20 22 LASTT TAGS 1 CsBi 4 Te 6 Bi 2 Te 3 Ce 0.9 Fe 3 CoSb 12 SiGe PbTe 0.5 0 0 200 400 600 800 1000 1200 1400 Temperature, K

Conclusions New approaches are succeeding in raising ZT The (A 2 Q) n (PbQ) m (Bi 2 Q 3 ) p (Q=Se, Te) system is a rich source of new materials Several new promising compounds identified strongly anisotropic cubic nanostructured LAST, LASTT and SALT family of materials nanostructured superior ZT Strong thermal conductivity reduction achieved through nanostructuring Doping studies and processing conditions are important in ZT optimization

Outlook Further progress is expected on the TE figure of merit Fundamental challenge: Translate current theoretical physical predictions on how to enhance Power Factor (σ S 2 ) into actual chemistry in the laboratory Achieve minimum thermal conductivity (~0.3 W/mK) in a bulk (nano)crystalline TE material Research in new materials should focus on Understanding and controlling carrier scattering Controlling nanostructuring to manipulate phonon propagation Discovering new compounds Long term: Waste heat recovery and conversion could be impacted on a massive scale with low cost materials if ZT>2-3. Thermoelectrics could help utilize existing depletable energy resources more effectively Thermoelectrics could also play role in renewable energy (e.g. solar, etc)

Collaborators Tim Hogan, Dept of Electrical Engineering, MSU S. D. (Bhanu) Mahanti, Dept. of Physics, MSU Ctirad Uher, Dept. of Physics, U of Michigan Simon Billinge, Physics, MSU Eldon Case, MSU Harold Schock, MSU Bruce Cook, Iowa State Terry Tritt, Clemson U Art Schultz, Argonne NL

TE Research group Dr Duck young Chung Joseph Sootsman Dr Kuei fang Hsu Dr Eric Quarez Aurelie Guegen Ferdinand Poudeu Jun-Ho Kim Dr John Androulakis