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Thermoelectricity

Thermoelectric Effect & applications

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Thermoelectricity

  1. 1. 1 Thermoelectric materials: Promising candidates for waste heat conversion BY Dr. LEENA JOSHI
  2. 2. 2 www.gapminder.org C:UsersleenaDocumentsCamt asia Studiotrial CO2 Emission vs. Energy use
  3. 3. 3 Thermoelectric materials: Promising candidates for waste heat conversion
  4. 4. 4 Outline of the talk  Basics of thermoelectricity  Advantages and applications  Factors affecting ZT  Strategies for improving ZT  Major classes of TE materials  Thermoelectric efficiency
  5. 5. 5 Thermoelectric Effect Thermoelectric Generator
  6. 6. 6 Thermoelectricity Applications Electrical power generation Refrigeration  Release no pollution into environment  Have no moving parts so maintenance free  Compact and less weight  High reliability  No noise  Can be used in zero gravity environment  Have life span of more than 14 years Advantages
  7. 7. 7 Thermoelectricity Applications Food storage Biological specimen Radioisotope thermal generators CPUs Power plant Watch driven by body heat Automobiles
  8. 8. 8 S- Seebeck Coefficient σ- Electrical Resistivity κ- Thermal Conductivity κe – Electronic contribution κl – Lattice contribution T – Average temperature between cold and hot side The good thermoelectric materials should possess : S ↑ σ ↑ κ↓ Figure of merit (zT) Power factor
  9. 9. 9 Most suitable TE materials are heavily doped semiconductors which typically possess carrier concentrations of 1019-1021 carriers/cm3. Low ~10-2-10-4 W/m-K TE Parameters Materials Metals Insulators Semiconductors Electrical Conductivity (σ) Seebeck Coefficient (S) Thermal Conductivity (κ) High ~102 W/m-K High Moderate 10-3S/m High ~120 μV/K Very High ~107 S/m Low ~ 10μV/K Low ~10 W/m-K Extremely low (~10-10S/m) Material of choice for thermoelectricity
  10. 10. 10 Factors affecting But m* is inversely proportional to electrical conductivity These contradictory requirements hampered the progress towards a higher ZT for many years, where it was stagnant at a nominal value of 1. So σ requires:-  High carrier concentrations  High mobility Higher value of S requires:-  large effective mass  Smaller carrier concentrations
  11. 11. 11 A phonon is a quantum of vibrational energy, and each has a different wavelength. When heat flows through a material, a spectrum of phonons needs to be scattered at different wavelengths (short, intermediate and long). Heat Conduction in Solids ω(k) Speed of propagation of phonon = dω(k)/dk Saco > Sopt
  12. 12. 12 Thermal conductivity Chem. Mater. 2010, 22, 624.  can only be controlled by changing l, as e depends on n and if we decrease n, clearly σ will also decrease Wiedemann-Franz Law Lattice thermal conductivity can be reduced by :-  Introducing heavy elements as they are difficult to vibrate.  Forming complex crystal structures.  Increasing percentage of interfaces and surfaces to enhance phonon scattering.  By increasing no. of atoms /unit cell which results in increase in no. of optical modes. Where L is Lorentz number
  13. 13. 13 Vineis et al., Adv. Mater. 22 3970, 2012 Scattering Mechanism d- size of particle λ - wavelength Rayleigh scattering
  14. 14. Phonon-glass electron-crystal 14 Strategies for improving zT Rattlers Cage for e- conduction Trapped atom for scattering phonons Recently, Liu et al. (2012) have extended this idea using superionic conductors (Cu2Se) to eliminate shear vibrations. They have successfully managed to achieve Cv values below 3NkB (the DulongPetit limit for solids, where N is number of particles) Phonon-liquid electron-crystal a semiconducting host framework with good electrical conductivity like in crystals, while the extreme thermal motion - “rattling” of the loosely bound guest atoms scatters phonons and reduces the thermal conductivity as in glasses. This approach is used for skutterudites and clathrates.
  15. 15. Dresselhaus et al. proposed that: (i) S is directly related to the slope of the DOS at the Fermi surface (ii) Boundary scattering at the interface reduces the κ much more than the σ can lead to superior TE properties in nanostructured materials. Nano-dimensional Approach With the decrease in dimension of the material, the motion of the electrons (or holes) is confined in one direction, leading to a change in the shape of the electronic density of states (DOS). G. Dresselhaus et al., in International Conference on Thermoelectrics, 1998.
  16. 16. 16 Band Engineering •Steep bands result in smaller effective mass which increases conductivity •Shallow bands result in larger effective mass which increases Seebeck Coefficient It is ideal to have both steep and shallow band simultaneously to achieve high power factor Effective mass of electron in a particular band is related the curvature of band in E-k diagram and is given by -
  17. 17. 17 Band valley degeneracy Higher value of S requires large m* Where, , is greater for shallow bands and is valley degeneracy which can be increased by multiple parabolic bands Figure of merit is proportional to band degeneracy Many valley Fermi surface Snyder, Materials Today 14 526 (2011) For same number of carriers multiple valleys produce larger Seebeck coefficient Single Valley Multiple Valley Band broadening occurs in a crystal system with increase in anisotropy which leads to band degeneracy PbTe
  18. 18. From Bulk to …. Nanostructures 18 Phonon Confinement Balandin et al., JAP 84 (1998) 6149
  19. 19. (Ca2CoO3)0.61CoO2 ZT ~ 1, at 1000 K Thermoelectric oxides 19 Bi2Te3 Alloys (ZT ~ 1 at 300 K) Skutterdites (ZT ~ 1.4 at 900K) Clathrates Ba8Ga16Ge 30 ZT~ 0.7 at 700 K Half-Heuslers (ZT ~ 0.8 at 900 K) Major classes of TE materials
  20. 20. 20 A narrow gap semiconductor with an indirect band gap of 160 meV at 300 K. large thermopower (S ≈ 200 μV/K), large electrical conductivity ( σ ±1000 1/cm), low thermal conductivity (Κ ≈ 1.5W/mK), and high thermoelectric figure of merit (ZT ≈ 1) at room temperature. It was firstly reported by Goldsmid for its use in thermoelectric refrigeration Properties:- Till date best TE material for room temperature applications Bi2Te3 & Alloys Disadvantages:- Toxicity Low thermal stability
  21. 21. 21 The first Heusler compound Cu2MnAl was made in 1903 by Heusler. Heusler Alloys Typically X and Y are transition metals (Cations) and Z is anion from main group (although X and Y can also be alkali metals, alkaline earth elements, or lanthanides).
  22. 22. 22 Removal of one X fcc sublattice from X2YZ, gives HH compounds Structure and properties  These are semiconductor in nature with 18 or 24 valance electrons count (VEC).  It is possible to control the VEC of HH alloys by the partial substitution for the X, Y or Z site by other elements.  For example, the calculative VEC for TiNiSn1−xSbx, where the Sn site is partially substituted by Sb atom, is 18 + x ( ) and similarly TiNiSb1−xSnx is 18 - x ( ).  Have potential for high temperature power generation applications especially as n-type material.  High value of the thermo power that arises as a consequence of the narrow band with heavy carrier mass.  In addition, show very rich physical properties +
  23. 23. 23 XYZ Ti/Zr/Hf Doping with same group element is used for introducing defects Sn/Sb Ni/Co For introducing magnetic phase For valance electron count = 18 For 18 + x :- % doping of group 5 for n-type e. g. Sb/Bi For 18 - x :- % doping of group 3 for p-type e. g. Sn in excess Compositions of interest in HH alloys
  24. 24. 24 Liquid-like thermoelectric T> 400 K (β-phase) cubic anti-fluorite structure. Cu2Se Liu et al., Nature Materials 11 (2012) 422 liquid-like material in which selenium atoms make a crystal lattice and copper atoms flow through the crystal structure like a liquid.
  25. 25. 25 Energy conversion efficiency for thermoelectric device Where TH and TC and temperatures and hot and cold junction respectively and is the modified figure of merit taking into consideration both n and p-type thermoelectric materials and is given by & Effective for device is the average of p-type and n-type segments, so it turns out to be quite lower than materials zT . e. g. state of art TE material Bi2Te3 having peak zT 1.1, actual device efficiency is only 0.7.
  26. 26. 26 Figure of Merit & Carnot efficiency ‘T’ is the average temperature between hot and cold end so it is very important to have larger value of ZT over large ∆T. Carnot efficiency
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