The thermo electric effect

The thermoelectric effect is the direct conversion temperature difference into electricity

1. Introduction to thermoelectrics Anandhu Thampi M.Sc Physics Cochin university of science and technology Cochin-22

2. Thermo electric effect The thermoelectric effect is the direct conversion of temperature difference into electric voltage and vise versa In a thermoelectric material there are free carriers which carry both charge and heat.

3. History In the 1820’s Thomas Seebeck (Germany) discovered that if two metals at different temperatures were touching could create an electric current. In 1834, Jean Peltier (France) discovered that an electrical current would produce heating or cooling at the junction of two dissimilar metals. In 1851, Lord Kelvin discovered that when the current flows through the unequally heated conductor, heat is evolved or absorbed through the conductor

4. Thomas Johann Seebeck Jean Charles Athanase Peltier William Thomson( Lord Kelvin) Abram F. Ioffe H. Julian Goldsmid 1821 1834 1854 1928 1954 1851  Gustav Magnus 

5. The first thermoelectrics 1930 1959 1959 1970 1998 1999 Radio Home refrigerator Radioactive Thermoelectric Generator (RTG) Cardiac pacemaker Seiko introduces the Thermic watch Seat coolers in the Lincoln Navigator and Toyota's Lexus Voyager 1 1977

6. Seebeck effect Thermoelectricity - known in physics as the " Seebeck Effect“ Thomas Johann Seebeck found that a circuit made from two dissimilar metals, with junctions at different temperatures would deflect a compass magnet . Discovered a small current flow and so demonstrated that heat could be converted to electricity.

7. When the junctions of two different metals are maintained at different temperature, the emf is produced in the circuit This is known as Seebeck effect. The conductor 1 is maintained at T+∆ T temperature The conductor 2 is maintained at temperature T Since the junctions are maintained at different temperature, the emf ‘U’ flows across the circuit

8. Peltier Effect The heating or cooling at an electrified junction of two different conductors The Peltier heat generated at the junction per unit time The peltire coefficient represents how much heat is carried per unit charge

9. THOMSON EFFECT The current flows through the unequally heated conductor, heat is evolved or absorbed through the conductor Heat production rate unit per unit volume q´=-KJ• ∇T K-   Thomson coefficient ∇T- T emperature gradient   J- Current density

10. Positive Thomson effect Current flows from lower T to high T, heat is absorbed throughout the conductor Eg :- Sn , Au, Ag, Zn, Cd , Sb Negative Thomson effect Current flows from lower T to high T heat is liberated throughout the conductor Eg :- Bi, Ni, Pt, Co, Hg Nill Thomson effect Current flow from high T to Low T or Low T to high T heat is neither liberated nor absorbed Eg :- Pb

11. Thermoelectric figure of merit ( zt ) The good thermoelectric materials should possess Large Seebeck coefficients High electrical conductivity Low thermal conductivity Ѕ – Seebeck coefficient σ – Electrical conductivity Κ – Thermal conductivity Т - Temperature

12. Thermoelectric efficiency maximum efficiency of a thermoelectric material depends on two terms Carnot efficiency, for all heat engines can not exceed Carnot efficiency Depends on the thermoelectric properties, Seebeck coefficient, electrical resistivity and thermal conductivity

13. Band structure

14. Direct band gap semiconductor Indirect band gap semiconductor

15. The first Brillouin zone of bismuth telluride

16. The fermi level should be a little below (n type) or above (p type) the band edge. Maximize the no. of channels in the fermi window (large effective mass). Maximize the velocity (small effective mass). Minimize scattering (small DOS – small effective mass)

17. The band structure without spin orbit coupling The conduction band minimum and the valance band maximum are both at the Γ point making it a direct band gap, with a size of 0.33 eV

18. The band structure including spin orbit coupling The conduction band minimum is now between the Γ and the Z points and the valance band maximum is now between the Z and F points, because they are not at the same k point the band structure now has an indirect band gap, with a size of 0.11 eV

19. Darbble et.al introduced six valley model Highest valence band and lowest conduction band have six valleys Bands are described in terms of effective mass tensor All valleys with the extrema inside first brillouin zone Multi valley band structure shows good thermoelectric properties

20.  thermoelectric modules A thermoelectric module is an array of thermocouples connected electrically in series but thermally in parallel Many couples are used (in both power generation and cooling) becuause the voltage drop across one couple is only on the order of millivolts

21. The Seebeck voltage of the couple, S is derived from the Seebeck coefficient of the n-type and p-type elements and the number of couples, n The electrical resistance of the device depends on electrical resistance of the thermoelectric, electrical resistnace of the metal interconnects & contact resistance between the interconnects and the thermoelectric materials

22. The total thermal conductance K l  is the parallel thermal loss per couples associated with gas conduction, radiation, or other losses

23. advantages Environmental friendly Recycles wasted heat energy Reliable source of energy scalability Lower production cost Silent in operation Simple, compact & safe

24. disadvantages Low energy conversion efficiency Requires relatively constant heat source Slow technology progression

25. applications Thermoelectric generator Cooling Computers Drink Coolers Recharging Devices Space Probes Solar Power Low power remote controller system Nuclear power stations Automotive TEGs

26. reference H. Julian Goldsmid :, Introduction to Thermoelectricity, Springer international publisher,2010 J.appl.phys.111,113707(2012) M G Kanatzidis etal :, Chemistry,physics,and material science of thermoelectric materialsn beyond bismuth telluride, First edition, Springer international publisher, 2003 http://thermoelectrics.matsci.northwestern.edu/thermoelectrics.html Liouise Henderson:, Calculating crystal properties of bismuth telluride using wien22, senior thesis,2014

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