Thermoelectric materials Presentation in MENA5010 by Simen Nut Hansen Eliassen
Outline Motivation Background Efficiency Thermoelectrics goes nano Summary
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Waste heat is everywhere! e.g. industrial parks, solar heating, combustion engine Thermal energy Electricity Example of application areas:
Pros: Environmentally friendly High reliability Very little maintenance Cons: Expensive Lack of efficiency Challenges in producing at a large scale Need to improve the efficiency!!
Seebeck effect Thermal energy Electricity Thomas J. Seebeck in 1821 Deflection of compass needle e- N-type semiconductor Inhomogeneous charge distribution Electric field reduces diffusion Electrochemical potential Seebeck voltage Seebeck coefficient, α + <--- ΔU, ΔT ---> -
Thermoelectic module Several «legs» Alternating n-type and p-type Closed circuit Current flow
Peltier effect Jean-Charles Peltier in 1834 Two dissimilar conductors in contact Electricity Thermal energy Cooling/heating Depending on applied voltage Peltier coefficient Material dependent Thermal energy per charge carrier
Theory of thermoelectric conversion Abram Fedorovich Ioffe et al. (1949) The ability to generate electrical potential by a temperature gradient The ability of electrons to diffuse in the material Figure of merit: ZT = α2 σ κ e + κ l T We want: High electrical conductivity High voltage per unit temperature gradient Low thermal conductivity Heat transport by electrons Heat transport by lattice vibrations
Breakdown of the figure of merit: Mott relation: α = 8π2 k B 2 3eh 2 m T( π 3n )2 3 Electrical conductivity: σ = 1 ρ = neμ α m 1 μ ZT = α2 σ κ e + κ l T Wiedemann-Franz law: κ e = neμ k 2 B π 2 e 3 T Lattice vibrations Scattering of phonons
Problem: Interdependence of the parameters Optimization of the carrier concentration for maximization of ZT Engineer material to favor a high ZT Other methods to optimize ZT: Going into the nano regime
Approach: Reduce dimension for improved σ and reduced κ l 2D thin-films 1D nano-wire Atomic layer deposition: Expensive, slow 0D quantum-dots Nanostructured materials Nanosized grains Nanocomposites Nano coated grain boundaries Nano inclusions Multilayer structure D.L. Medlin, G.J. Snyder / Current Opinion in Colloid & Interface Science 14 (2009) 226 235
Example: Si 0.8 Ge 0.2 ZT: Nanocrystalline > Bulk (n- and p-type) Grains ~10-50 nm Reduced κ l due boundary scattering Boundaries and interfaces A. J. Minnich, M. S. Dresselhaus, Z. F. Ren and G. Chen, Energy Environ. Sci., 2009, 2, 466 479 Electronic properties and phonon scattering But why?
Host material, e.g. bulk PbTe Pb nanoparticles embedded in the structure matrix of the host Metal-Semiconductor interface Match Ψ M and Ψ SC Gap states decay into the semiconductor Fermi levels must match charge transfer n-type M-SC interface
Metal p-type Semiconductor D.L. Medlin, G.J. Snyder / Current Opinion in Colloid & Interface Science 14 (2009) 226 235 Wenjie Xie, Anke Weidenkaff, Xinfeng Tang, Qingjie Zhang, Joseph Poon and Terry M. Tritt Nanomaterials (2012) 379-412 Scattering potential Filter out low energetic electrons Enhancement of the Seebeck coefficient
Spectrum of phonon Different phonons contribute to heat transport Example: Phonon dispersion for bulk Si Accumulated κ l at 300 K
Increase of scattering centers (e.g. reduce grains, alloying, nanoparticles) Decrease in κ l for enhanced ZT H B Radousky and H Liang 2012 Nanotechnology 23 502001
Thermoelectric materials are «clean» and reliable Problem: Expensive and inefficient Solution: Tune material of interest for higher efficiency Approach: Go into the nano regime (e.g. reduce grain size, dimensions, nanoparticles in bulk material, etc.) Interfaces enhances α and reduces κ l Further research on new materials and enhancing ZT in existing materials are needed
Thank you for your attention! Reference list: H. B. Radousky and H. Liang (2012) Nanotechnology 23 D.L. Medlin, G.J. Snyder, Current Opinion in Colloid & Interface Science 14 (2009) 226 235 A. J. Minnich, M. S. Dresselhaus, Z. F. Ren and G. Chen, Energy Environ. Sci., (2009) 466-479 Wenjie Xie, Anke Weidenkaff, Xinfeng Tang, Qingjie Zhang, Joseph Poon and Terry M. Tritt Nanomaterials (2012) 379-412 Jesús Carrete, Natalio Mingo, Shidong Wang and Stefano Curtarolo, Adv. Funct. Mater. (2014), 24, 7427 7432 Atsushi Togo, Fumiyasu Oba, and Isao Tanaka, Phys. Rev. B, 78 (2008) Atsushi Togo, Laurent Chaput, and Isao Tanaka, Phys. Rev. B, 91 (2015)