Thermoelectrics Mitglied der HelmholtzGemeinschaft Benedikt Klobes JCNS2 PGI4

Thermoelectrics Mitglied der Helmholtz-Gemeinschaft Benedikt Klobes JCNS-2 & PGI-4, Forschungszentrum Jülich, Germany 19 th September 2014 | Hercules Specialized Course 17

What are thermoelectrics? Why using synchroton X-rays and neutrons? R. Simon wondering at ID 18. 1. Basics of thermoelectricity (TE) Mitglied der Helmholtz-Gemeinschaft 2. Dynamical aspects of thermoelectric materials 3. Synchrotron X-rays for TE research 4. Neutrons for TE research examples

Basics of Thermoelectrics G. Snyder et al. , Nat. Mater. 7 (2008) 105. Mitglied der Helmholtz-Gemeinschaft based on the Seebeck effect hot cold reversed: Peltier effect can be used for thermoelectric cooling Kelvin relation

Basics of Thermoelectrics www. aist. go. jp Mitglied der Helmholtz-Gemeinschaft wikipedia actual usage of thermoelectric generators (TEG): 1. space missions/probes 2. remote locations 3. gadgets (e. g. charging …) there is no actual large-scale application of TEG

Basics of Thermoelectrics – the figure-of-merit QH hot junction @ TH Mitglied der Helmholtz-Gemeinschaft I n-type p-type cold junctions @ TC QC RL I thermoelectric efficiency: R some assumptions and redefinitions !!!!!

Basics of Thermoelectrics – the figure-of-merit Mitglied der Helmholtz-Gemeinschaft ZT www. nistep. go. jp h > 0. 1 ZT > 1 , but also depending on the temperature range Carnot efficiency for a 800 K to 300 K „machine“ ~ 0. 63 real-world applications require h > 0. 1 (economically …) temperature (K)

Basics of Thermoelectrics – routes for research the electronic transport part maximizing the power factor S 2 s Mitglied der Helmholtz-Gemeinschaft but carrier concentration and thermal transport, i. e. k, are coupled via the Wiedemann-Franz-law A. Shakouri, Ann. Rev. Mat. Sci. 41 (2011) 399.

Basics of Thermoelectrics – routes for research thermal transport part reducing the lattice part of thermal conductivity without impeding electronic transport properties Mitglied der Helmholtz-Gemeinschaft electron crystal phonon glass reduce phonon mean free path l reduce phonon group velocity v. G reduce phonon heat capacity Cs

Mitglied der Helmholtz-Gemeinschaft Basics of Thermoelectrics – routes for research possible strategies: • introduce scattering centers: defects, rattling atoms and many more … • use low density materials • push the acoustic phonon branches down using complex cells … • make use of highly anisotropic properties (layered materials …) • … in any case, X-rays and neutrons are mandatory for a microscopic understanding of the vibrational properties of thermoelectrics

Lattice dynamics in simple Pb. Te is the gold standard of TE: • applied since mid of the 1950 s • high ZT values • great flexibility concerning doping • possibility to tune microstructure Mitglied der Helmholtz-Gemeinschaft special Ti alloys glass K. Biswas et al. , Nature 489 (2012) 414.

Lattice dynamics in simple Pb. Te and in related Sn. Te and Ge. Te … v. S <u 2> EA Eint P. Bauer Pereira et al. , Phys. Status Solidi B 250 (2013) 1300. Mitglied der Helmholtz-Gemeinschaft using nuclear inelastic scattering in order to obtain the element specific densities of phonon states, here via the 119 Sn & 125 Te resonances f. LM <F> Svib ID 18 @ ESRF

Fourier-Log decomposition P. Bauer Pereira et al. , Phys. Status Solidi B 250 (2013) 1300. Mitglied der Helmholtz-Gemeinschaft Lattice dynamics in simple Pb. Te

Mitglied der Helmholtz-Gemeinschaft Lattice dynamics in simple Pb. Te Sn. Te Ge. Te v. S = 1850(80) m/s v. S = 1800(80) m/s v. S = 1900(70) m/s Pb. Te Sn. Te Ge. Te QD = 170(2) m/s QD = 160(5) m/s QD = 180(5) m/s Ge. Te slightly „harder“ than Pb. Te and Sn. Te …

Lattice dynamics in simple Pb. Te in some sense just numbers, but test for acoustic mismatch hypothesis from Pb. Te to Ag. Pb 18 Sb. Te 20 : Pb. Te matrix plus coherent precipitates rich in Ag & Sb Mitglied der Helmholtz-Gemeinschaft reduction of lattice thermal conductivity due to „impedance“ mismatch between matrix and precipitates? Pb. Te Sn. Te Ge. Te v. S = 1850(80) m/s v. S = 1800(80) m/s v. S = 1900(70) m/s Pb. Te Sn. Te Ge. Te QD = 170(2) m/s QD = 160(5) m/s QD = 180(5) m/s 125 Te & 121 Sb NIS on Ag. Pb 18 Sb. Te 20 speak with Atefeh during coffee break

Nanocrystallinity and lattice dynamics – Si high-energy ball milling spark plasma sintering Mitglied der Helmholtz-Gemeinschaft IN 6 @ ILL T. Claudio et al. , J. Mater. Sci. 48 (2013) 2863. www 2. cpfs. mpg. de How to achieve nanocrystalline, but bulk compounds?

Mitglied der Helmholtz-Gemeinschaft T. Claudio et al. , J. Mater. Sci. 48 (2013) 2863. Nanocrystallinity and lattice dynamics – Si • nanocrystallinity has a strong impact on thermal conductivity • peak at around 6 me. V indicative for amorphous Si. O 2 confirmed by PGAA, Raman • broad feature 80 – 160 me. V : H impurities in Si • Deybe level changes drastically different acoustic group velocities from 6000 m/s to ~ 3400 m/s

Limits of k – Bi 2 Te 3 based thermoelectrics Besides Pb. Te, Bi 2 Te 3 is the other gold standard of thermoelectrics Mitglied der Helmholtz-Gemeinschaft grainsizes: 5 mm (as-cast) vs. 25 nm (nano) FOCUS @ SINQ

Mitglied der Helmholtz-Gemeinschaft Limits of k – Bi 2 Te 3 based thermoelectrics • boundary scattering not sufficient • other mechanisms due to synthesis? point defects, dislocations … strain and mass fluctuations

Rattling atoms in thermoelectrics Mitglied der Helmholtz-Gemeinschaft Introducing additional atoms in voids of some structures significant decrease of thermal conductivity skutterudite, e. g. In 0. 2 Co 4 Sb 12 clathrate, e. g. Sr 8 Ga 16 Ge 30 possible reasons: mass density fluctuations, lower speed of sound, rattling behavior …

Rattling of In in In 0. 2 Co 4 Sb 12 Mitglied der Helmholtz-Gemeinschaft In 0. 2 Co 4 Sb 12

Rattling of In in In 0. 2 Co 4 Sb 12 Mitglied der Helmholtz-Gemeinschaft Atomic dynamics also present in „simple“ neutron diffraction, i. e. in the atomic displacement parameters (ADP) POWGEN @ SNS • huge difference between guest- and host ADP • excessive specific heat at low-T Einstein

Lattice dynamics of rattling atoms besides Einstein-like ADP and specific heat, „confined“ and low energetic phonons are found e. g. using NIS support for „rattling“ notion Mitglied der Helmholtz-Gemeinschaft ID 18 @ ESRF Xe clathrate hydrate G. J. Long et al. , Phys. Rev. B 71 (2005) 140302. B. Klobes et al. , EPL 103 (2013) 36001.

Do rattling atoms really rattle (independently)? large ADP and Einstein-mode like behavior independent rattlers? resonant scattering mechanism? Maybe: avoided crossing between acoustical and optical branches? Mitglied der Helmholtz-Gemeinschaft RITA-II @ SINQ Ba 8 Ga 16 Ge 30 M. Christensen et al. , Nat. Mater. 7 (2008) 811.

Do rattling atoms really rattle (independently)? large ADP and Einstein-mode like behavior independent rattlers? resonant scattering mechanism? Maybe: avoided crossing between acoustical and optical branches? M. Christensen et al. , Nat. Mater. 7 (2008) 811. H. Euchner et al. , Phys. Rev. B 86 (2012) 224303. Ba 8 Ni 3. 5 Ge 42. 1□ 0. 4 T 2 @ LLB Mitglied der Helmholtz-Gemeinschaft Ba 8 Ga 16 Ge 30

Do rattling atoms really rattle (independently)? well, not that independently … and probably not solely rattling … comparative study using La and Ce filled Fe 4 Sb 12 IN 4, IN 6 @ LLB G. Nolas et al. , Phys. Rev. B 58 (1998) 164. calculation assuming coupling M. Koza et al. , Nat. Mater. 7 (2008) 805. Mitglied der Helmholtz-Gemeinschaft measurement Lax. Co 4 Sn 2 Sb 10 first decrease, then increase in x

Mitglied der Helmholtz-Gemeinschaft nanoengineering: • quantum dots • nanowires • superlattices • … specific design of thermal and electronic transport properties M. Beekman et al. , Semicond. Sci. Technol. 29 (2004) to be published Artificial structures – misfit layered compounds

NIS by [(Sn. Se)1. 04]n[Mo. Se 2]m k 3 -ID-B @ APS Mitglied der Helmholtz-Gemeinschaft NIS probes phonons with polarization along k

Thermoelectrics – other challenges Zhao et al. , Nature 508 (2014) 373. G. Snyder et al. , Nat. Mater. 7 (2008) 105. Mitglied der Helmholtz-Gemeinschaft in the present discussion, electronic transport properties were completely omitted • band engineering: narrow gaps, sharp slope of electronic DOS • exploit crystal anisotropy • control synthesis of complex alloys • scalability !!! • module related issues contacts • replacement of toxic elements

Thermoelectrics, neutrons and X-rays … Improvement of Thermoelectrics Dynamical Properties Lattice dynamics X-ray and Neutron Scattering Pb. Te based Systems Nanocrystalline Compounds Notion of Rattling in Skutterudites Mitglied der Helmholtz-Gemeinschaft Artifically Engineered Systems Other Challenges for Thermoelectric Research

The people behind the science … Mitglied der Helmholtz-Gemeinschaft R. Hermann Group leader B. Klobes Post. Doc V. Potapkin Post. Doc A. Mahmoud Post. Doc M. Herlitschke Ph. D student R. Simon Ph. D student A. Jafari Ph. D student 10 former members: 4 Ph. D, 1 Diploma, 3 B. Sc. thesis P. Alexeev Ph. D student M. Mebonia Ph. D student F. Deng M. Sc. student