Thermoelectricity
•Transferir como PPTX, PDF•
7 gostaram•6,690 visualizações
This document summarizes thermoelectric materials and their potential for waste heat recovery. It discusses the basics of thermoelectricity, factors that influence performance like the figure of merit ZT, and strategies for improving ZT such as nanostructuring, band engineering, and using materials with low lattice thermal conductivity. Examples of promising thermoelectric materials classes are provided, like Bi2Te3 alloys, skutterudites, clathrates, and half-Heusler compounds. The talk outlines advantages of thermoelectric generators and their applications in areas like automotive waste heat recovery and concludes with equations for calculating thermoelectric efficiency.
Recomendados
Mais conteúdo relacionado
Mais procurados
Mais procurados (20)
Destaque
Destaque (16)
Semelhante a Thermoelectricity
Semelhante a Thermoelectricity (20)
Último
Último (20)
Thermoelectricity
- 1. 1 Thermoelectric materials: Promising candidates for waste heat conversion BY Dr. LEENA JOSHI
- 2. 2 www.gapminder.org C:UsersleenaDocumentsCamt asia Studiotrial CO2 Emission vs. Energy use
- 3. 3 Thermoelectric materials: Promising candidates for waste heat conversion
- 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
- 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 Thermoelectricity Applications Food storage Biological specimen Radioisotope thermal generators CPUs Power plant Watch driven by body heat Automobiles
- 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 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 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 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 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 Vineis et al., Adv. Mater. 22 3970, 2012 Scattering Mechanism d- size of particle λ - wavelength Rayleigh scattering
- 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. 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 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 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. From Bulk to …. Nanostructures 18 Phonon Confinement Balandin et al., JAP 84 (1998) 6149
- 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 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 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 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 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 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 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 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
- 27. 27
Notas do Editor
- I would like to start my talk by showing CO2 emission data over the period of time. Data which I will be showing is taken from a www.gapminder.org, which is a free web-service displaying time series of development statistics for all countries. In this graph every bubble is a country and size of the bubble correspond to the population of that respective country. So, this in this video I have taken 4 countries for comparison, developed countries US in yellow, Germany in orange, and two fastest growing economies in the world, India in blue and China in red. Y- axis is CO2 emission in tonnes/person which is in log scale and X- axis is time in years from 1899 to 2010. So from this video it is visible that there is almost linear increase in amount of CO2 emission for developing countries whereas it has come to a saturation for developed countries. So, according to these statistics if we don’t take any preventive measures to cease CO2 emission in coming years we will reach and even cross developed countries. Increase in accumulation of CO2 in the atmosphere is the indicator of increase in pollution. Now if we look at energy demand/per person for these countries on time scale energy demand is also increasing with the industrial development in these countries and again slope of these graphs for developing countries is much higher than that of developed countries, if we compare India with Germany gap in our energy demand is much higher that gap in CO2 emission. That means we are polluting the environment much more as compared to our energy demand. This is the reflection that our country is still relying mainly on conventional carbon based energy sources. Hence, such situation can not go forever, The solution to this problem is finding alternatives that can reduce the use of carbon-based fuels and their emission of greenhouse gases to the atmosphere. One of the probable solution is Thermoelectricity
- About which I am going to talk are Thermoelectric materials which are Promising candidates for waste heat conversion. Using thermoelectric device, one can directly convert heat into electricity. Researchers all over the globe are trying to maximise the efficiency of thermoelectric devices so that waste heat from automobile exhaust, power plant etc can be efficiently converted in to useful work. So…plan of the talk is as follows
- These material directly convert temperature differences to electric voltage and vice-versa. The 3 basic effects associated with TE are namely, Seebeck, Peltier and Thomson effect, but here we will not go into the details of that. The thermoelectric effect is based on a property called thermo power, It was discovered by Thomas Seebeck the way back in 1821 . He found that there is voltage induced when two dissimilar metals are joined and kept at different temperature and known as Seebeck effect. The beauty of this phenomenon is that it is a pure solid state effect so it requires no intermediaries. This is a typical TE generator which can capture heat to produce electricity, In this n-type thermo-element and a p-type thermo-element are connected in electrical series and thermally parallel between a heat source and a heat sink. The open circuit voltage that is generated in a given thermo-element is proportional to the temperature difference, ΔT, across that thermo-element. A device generally has many such pairs
- ~65% of energy become waste heat Even if a change about 10% is made it will greatly impact the total energy scenario in the world
- It is defined for particular material or device in order to determine their relative utility for an application
- “It is hard to increase one without compromising the other” For 50 years, researchers have struggled to push that number past 1.
- optical modes interact strongly with electromagnetic radiation in polar crystals. As the packets of vibrations (called phonons) move through the lattice they may hit atoms, impurities, defects or each other. When they do this they bounce off each other or scatter. In most cases they do this elasctically - like two billiard balls hitting each other. Phonon momentum and energy are conserved. The biggest difference between electron transport and phonon transport is that phonons obey Bose-Einstein statistics and electrons obey Fermi-Dirac statistics. Without the Pauli exclusion principle, one has to include all phonon modes in the calculation of heat transport. Only those electrons near the Fermi surface contribute to transport. The Pauli exclusion principle and the narrow energy range of electrons contributing to electrical transport are the main factors responsible for the large difference of more than 20 orders of magnitude in electrical conductivity of well-conducting materials (metals) versus the electrical conductivity of insulators (e.g., plastics). In contrast, the thermal conductivity of the best-conducting materials for heat (i.e., diamond) is only 4–5 orders of magnitude larger than that of thermal insulators (e.g., glass). In crystalline semiconductors, heat is carried predominantly by a wide spectrum of wave-like vibration modes called phonons. At high temperatures, most phonon modes are excited, but heat is carried mainly by a small fraction of these excited modes. Acoustic phonons near the edge and the center of Brillouin zone and optical phonons are inefficient in carrying heat due to their low group velocity. In crystals, phonons are scattered by imperfections including defects, anharmonicity and grain boundaries.
- a semiconducting host framework has high Seebeck coefficient and electrical conductivity (like in crystals), while the extreme thermal motion - “rattling” of the looselybound guest atoms scatters efficiently the heat-carrying phonons and reduces the thermal conductivity (like in glasses).
- The parabolic shape of the DOS for bulk materials implies that the electron density surrounding the Fermi level is small. The spike-like shape of the DOS for quantum wires implies that the electronic states are enhanced near the Fermi level, thus resulting in an increased thermal power factor S2σ.
- The high-temperature specific heat capacity of Cu2Se. The theoretical value (Dulong–Petit) for the high-temperature specific heat at constant volume Cv (Cph) is 3NkB in a solid crystal. The lowest Cv theoretical value in a liquid is 2NkB. Cu2Se shows a reduced Cv, approaching 2NkB at high temperatures. The dashed blue line shows the expected value of the specific heat at constant pressure Cp in a solid crystal Cu2Se without liquid-like properties, which is usually beyond the Dulong–Petit limit at high temperatures owing to the extra contributions by carriers (Ce) and lattice thermal expansion B is the bulk modulus, V is the volume per atom and is the thermal expansion coefficient.