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1 Chapter 10 Thermoelectric Materials Only for Teaching 1

2 What is Thermoelectric (TE)? -Solid-state energy conversion between thermal energy and electricity -Three different TE phenomena - Seebeck effect - Peltier effect - Thomson effect 2

The magnitude of the deflection varies with the type of conducting material and the

3 Seebeck effect Seebeck discovered that a compass needle deflected when the two junctions were maintained at different temperatures. The magnitude of the deflection varies with the type of conducting material and the temperature difference between the junctions, and does NOT depend on the temperature distribution along the conductors. 3

4 Definition of Seebeck coefficient A V A S AB = S A -S B = (ΔV/ΔT) Je=0 T B T+ΔT Known as Thermocouple Measuring junction, 84.5% Ni, 14.2% Cr, 1.4% Si 95% Ni, 2% Mn, 2% Al, Alumel 1% Si ΔV T M ΔT K type TC Reference junction, T 0 4

5 Peltier effect Heat absorption A A Heat generation J Q B J Q Electric current direction J E A thermodynamically reversible process Peltier coefficient is defined π AB =J Q /J E 5

6 Thomson effect Heat generation or absorption (J Q ) by passing a current J E through a homogeneous conductor that is subject to a temperature gradient. Heat absorption occurs when electrical carriers flow in a direction opposite to a thermal gradient and heat liberation when carriers flow in the same direction as a thermal gradient. J Q β = J Q /(J E ΔT) J E T T+ΔT 6

7 How Thermoelectric works? 電子由右導體流入 n-type 半導體時, 僅高能 carrier( 電子 ) 克服能障 (E c -E f ) 而流入, 使右導體剩下低能量電子 ( 溫度降低 ), 左導體則累積高能量電子 ( 溫度升高 ). 熱流與電流方向相反 7

8 How Thermoelectric works? 8

9 How Thermoelectric works? TE cooler or generator 9

10 Thermoelectric energy conversion 10

11 Solid-state refrigeration T Q C Peltier πi=sti Thermal conduction K(T h -T c ) Electric current n Joule heat (I 2 R)/2 T H Q c = (Peltier) - (Joule heat)/2 - (Thermal conduction) Efficiency φ = q c /W W = S (T H -T)I+ I 2 r q c = STI - K(T H -T) - I 2 r / 2 Peltier heat extraction Heat conduction Joule s heating Z : Figure of merit( 熱電優值 ) Z = S 2 /rk ( 單位 1/T) ZT ( 無單位 ) COP opt QC = W = T [(1 + ZTM ) T) [( 1+ ZT 1 2 T 2 ( T 1 H M ) H + 1] T] ( T T) = H max ZT 2 2 M 11

12 Power generation Efficiency η = W/Q S I = S(T H -T C )/(R L +r) T H W = I 2 R L Q S = ST H I + K(T H -T C ) - I 2 r / 2 Peltier heat extraction Heat conduction Joule s heating p n η opt = ( T H T T H C ) T C ( 1 + ZT T H ' ) ( 1 + ZT ' ) 1 2 T C 2 S Z = = Kr 2 S σ λ Load R L 12

13 Requirements of good TE materials High figure of merit Z (=S 2 σ/λ) Both η and φ increase with (1+ZT) 1/2 High Seebeck coefficient S High Electrical conductivity σ Low Thermal conductivity λ 13

14 Consideration of Thermoelectric Properties 14

15 Approaches to ideal TE materials ZT=f (carrier concentration,microstructure) Ideal TE materials should be - Compound Semiconductor (highly doping with narrow bandgap) - Carrier concentration optimized - Lattice thermal conductivity minimized - Low dimensional structures preferred (Nano-sized materials) 15

16 ZT of thermoelectric materials on application 16

17 History of the Thermoelectric Figure of Merit(ZT) 17

18 Thermoelectric materials Schematic plots of S, σ, λand S 2 σas functions of carrier concentration n n(opt) ~ cm -3 for maximum S 2 σ λ= λ L + λ e ; λ e /σ=(1/3)π 2 (k B /e) 2 T (Wiedmann-Franz Law) 18

Thermoelectric materials ZT for p-type thermoelectric materials ZT for n-type thermoelectric materials Material Form N-type Bi 2 Te 3-x Se x (small doping)

19 Thermoelectric materials ZT for p-type thermoelectric materials ZT for n-type thermoelectric materials Material Form N-type Bi 2 Te 3-x Se x (small doping) Unidirectional P-type Bi y Sb 2-y Te 3 (small doping) unidirectional σ ( 10 3 (Ω-cm) -1 ) α (μv/k) κ (W/mK) ZT (T=300K) 19

20 Thermoelectric materials BiTe based Te or Se Bi Bi or Sb A B C a b Based on BiTe material : Atom size Bi : 1.70A Te : 1.60A Sb: 1.59A Se : 1.40A c Te Corresponding hexagonal cell a 0 = Å c 0 = Å 20

Sb; 2 out of 8 sub-cells are empty cages Skutterudites typical S~200 μv/k, but high λ ~ 100 W/mK;

21 Thermoelectric materials Skutterudite Skutterudite - a mining town in Norway called Skutterud known for CoAs 3 - based mineral MX3 structure w/ 32 atoms in a unit cell; M- Co, Rh or Ir; X- P, As or Sb; 2 out of 8 sub-cells are empty cages Skutterudites typical S~200 μv/k, but high λ ~ 100 W/mK; Approach : reduce λ L by inserting heavy atoms such as rare-earth elements into the cages in the lattice 21

Applications: Cooling/Temperature Control Biotechnology DNA Sequence reactor,

Microscope stage cooler, Cold/hot therapy blanket, Liquid-chiller, Electronics

Industrial Si wafer cooling plate, Chemical circulation system, General purpose

22 Applications: Cooling/Temperature Control Biotechnology DNA Sequence reactor, Minifridge for blood, Programmable thermal controller, Medical Hot/cold stimulator, Microscope stage cooler, Cold/hot therapy blanket, Liquid-chiller, Electronics Photoelectric detectors, laser module, micro-processor and high density electronics Industrial Si wafer cooling plate, Chemical circulation system, General purpose Immersion cooler, Vacuum pump baffle, Air conditioner, Consumer Picnic box, Cu/Al heat sink Al2O3 Al2O3 Chip 22

Precise λcontrol Fiber optics cooling modules Peltier TE module can serve as

23 Applications: Cooling/Temperature Control Fiber-optic telecommunications Stable output power Laser module Tight temperature control required! Precise λcontrol Fiber optics cooling modules Peltier TE module can serve as Temperature Stabilizer Heating or cooling is just a function of the direction of electric current. 23

24 Applications: Power generator Miniature power generator - Nuclear battery for cardiac pacemaker - Thermoelectric wrist watch Detectors and sensors - Infrared detector and thermal sensor Commercial Thermoelectric Generator - Cathodic protection - Data gathering system - Telecommunication system Space applications - Radioisotope Thermoelectric Generator 24

25 Radioisotope Thermoelectric Generator Radioactive decay of Pu-238 Si-Ge TE module 25

Each cell consists of rod-shaped TE elements (80x600um) sandwiched between two Si substrates.

26 Thermoelectric Wrist-watch SEIKO Thermic released in 1998 priced US$2,550 Thermic The watch is powered by 1000 serially connected (BiTe) TE cells, that yield ΔT=1 o C. Each cell consists of rod-shaped TE elements (80x600um) sandwiched between two Si substrates. The power is capable to charge a secondary Ti-Li battery with a capacity 4.5 mah, that is sufficient to drive the watch for 10 months without supplying any heat. 26

27 焚化爐廢熱回收 27

General TEG Module Uses Bismuth Telluride Based Semiconductors Physical Properties Value Tolerance Width & Length 2.95" (7.5 cm) ±0.01 (0.25) Thickness 0.2" (0.508cm) ±0.01 (0.25) Special Order ±0.

28 General TEG Module Uses Bismuth Telluride Based Semiconductors Physical Properties Value Tolerance Width & Length 2.95" (7.5 cm) ±0.01 (0.25) Thickness 0.2" (0.508cm) ±0.01 (0.25) Special Order ±0.002 (0.05) Weight 115 grams ±3 grams Compressive Yield Stress 10 ksi (70 MPa) minimum Number of active couples 71 couples Thermal Properties Design Hot Side Temperature 230 C (450 F) ±10 (20) Design Cold Side Temperature 30 C (85 F) ±5 (10) Maximum Continuous Temperature 250 C (480 F) Minimum Continuous Temperature none Maximum Intermittent Temperature 400 C (750 F) Thermal Conductivity* W/cm*K Heat Flux* 9.54 W/sqcm ±0.5 Electrical Properties (as a generator)* Power** 19 Watts minimum Load Voltage 2.38 Volts ±0.1 Internal Resistance 0.3 Ohm ±0.05 Current 8 Amps ±1 Open Circuit Voltage 5.0 Volts ±0.3 Efficiency 4.5 % minimum 28

29 汽車引擎廢熱回收 29

30 汽車引擎廢熱回收 30

31 汽車引擎廢熱回收 31

32 汽車引擎廢熱回收 32

33 汽車引擎廢熱回收 33

34 Combine with Solar Cell <500μm <2mm 34

35 Combine with Solar Cell 35

36 Low dimension thermoelectric materials Hicks and Dresselhaus, Phys. Rev. B, 47, 16631, 1993 Mean free path of phonon in Bi 2 Te 3 : 620 nm ~ 56 nm (Temp 310K ~ 450 K) (Mater. Chem. Phys., vol.57, p.57) 36

37 Low dimension thermoelectric materials 美國 RESEARCH TRIANGLE ISTITUTE (RTI) 研究所 Rama Venkatasubramanian NATURE VOL OCTOBER

38 Low dimension thermoelectric materials 美國 RESEARCH TRIANGLE ISTITUTE (RTI) 研究所 Superlattice Thin-film Thermoelectric Material and Device Technologies (ICT 2004) 38

39 Low dimension thermoelectric device 2005 年 RTI spin off Nextreme Thermal Solutions 39

40 Low dimension thermoelectric device 2005 年 RTI spin off Nextreme Thermal Solutions 40

41 Low dimension thermoelectric device Micropelt/Infineon (Micropelt Miniaturized Thermoelectric Devices: Small Size, High Cooling Power Densities, Short Response Time ICT2005) 41

42 Low dimension thermoelectric device Micropelt/Infineon (Micropelt Miniaturized Thermoelectric Devices: Small Size, High Cooling Power Densities, Short Response Time ICT2005) 42

43 TE thin film device 43

44 Micro TE Generator 44

45 High Efficiency TE Generator 增進系統效率提升材料 ZT 值改進集熱板與模組設計 High κ 複材 Al/graphite Al/diamond Partial melting 接合?? Heat Exchanger 1 Qh in P1 N1 P1: CeFeSb P2: BiTe N1: CoSb N2: BiTe P2 N2 Heat Exchanger 2 Load Qc out 45

46 CPU 熱電散熱片資料來源 : 工研院 46

47 CPU 熱電散熱片資料來源 : 工研院 47

48 CPU 熱電散熱片資料來源 : 工研院 48

49 Summary Thermoelectric is a direct energy conversion between thermal and electrical energy. Efficiency of TE modules depends on figure of merit ZT. Good TE materials require large S and σ,, while low lattice thermal conductivity λ L. High potential of low-dimensional TE materials. A wide variety of TE Applications power generation and refrigeration. Design and integration of TE devices are critical for device performance and reliability. 49

中溫矽基熱電材料介紹及其應用

中溫矽基熱電材料介紹及其應用探討不同成分比例合金熔煉與 Te 元素摻雜對中溫熱電合金 Zn 4 之熱電性能影響 Probing thermoelectric effect in Zn 4 compounds of medium-temperature thermoelectric materials by different alloy elements and Te doping