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Thermoelectric Generator (PbTe)

Introduction to Thermoelectric Materials and Devices

10th Semester of 2012 2012.05.31, Thursday Department of Energy Science Sungkyunkwan University

1 Thermoelectric Phenomena and Conversion Efficiency

2 Thermoelectric Transport Theory I : Electrical Properties

3 Thermoelectric Transport Theory II : Thermal Properties

4 Measurement of Thermoelectric Properties

5 Materials Preparation : Bulk

6 Materials Preparation : Thin Film

7 Thermoelectric System : Current and Future of Module

8 Applications : Power Generation and Heat Cooling

9 Thermoelectric Materials : State-of-the-art

10 Thermoelectric Materials : Intermetallics

11 Thermoelectric Materials : Oxides

12 Thermoelectric Materials : Phonon Glass and Electron Crystal (PGEC) Materials

13 Theory and Modeling in Nanostructured Thermoelectrics

14 High efficiency in Low Dimensional Materials

15 Hybrid Energy Conversion Systems of Thermoelectrics

16 Final Exam

Plan

Snyder, Toberer, Nat. Mater. 2008

Thermoelectric Research : Current and Future of Module

Target : High Conversion Efficiency of Thermoelectric Devices

Module : Accumulation of Elements, Contact Resistance (Welding), Shape Diversity for Micro-Scale (Thin Film)

System : Circumstance Suitability (Generation), Diversity of Mass Conversion System, Hybridization with other renewable energy system

Materials : High Efficiency (High ZT), New Materials and Various Categories (Limitation to Heavy Metal Compounds), Properties for Hybridization (Magnetic Semiconductor for Magnetocaloric) (Low Workfunction for Thermionic)

Thin Film

Cascade Conventional

Solar absorber + TE Custom Cooling Device D. Kraemer et al., Nat. Mater. 20011

Intermetallic Compounds : Definition

Definition Intermetallic compounds as solid phases containing two or more metallic elements, with optionally one or more non-metallic elements, whose crystal structure differs from that of the other constituents Intermetallic compounds are often simply called “Alloys” Both are metallic phases containing more than one element, but… In alloys, the various elements substitute randomly for one another in the crystal structure, forming a solid solution with a range of possible compositions, while… In intermetallic compounds, different elements are ordered into different sites in the structure, with distinct local environments and often a well-defined, fixed stoichiometry

Properties Intermetallic compounds are generally brittle and high melting. They often offer a compromise between ceramic and metallic properties when hardness and/or resistance to high temperatures is important enough to sacrifice some toughness and ease of processing.

Examples Some examples include alnico (Al-Ni-Co) and the hydrogen storage materials in nickel metal hydride batteries. Ni3Al, which is the hardening phase in the familiar nickel-base superalloys, and the various titanium aluminides have also attracted interest for turbine blade applications, while the latter is also used in very small quantities for grain refinement of titanium alloys.

Silicides, intermetallics involving silicon, involving many of the elements have been studied for, and some utilized as, barrier and contact layers in microelectronics.

Intermetallic Compounds

Laves phases Zintl phases

That have composition AB2. The phases are classified on the basis of geometry alone. There are three different classification classes: cubic MgCu2 (C15), hexagonal MgZn2 (C14), and hexagonal MgNi2 (C36). In general, the A atoms are ordered as in diamond, hexagonal diamond, or a related structure, and the B atoms form tetrahedra (AB4)around the A atoms for the AB2 structure. Laves phases are of particular interest in modern metallurgy research because of characteristic features that are the almost perfect electrical conductivity, but they are not plastically deformable at room temperature.

The product of a reaction between group 1 (alkali metals) or group 2 (alkaline earths) and post transition metals from group 13, 14, 15 or 16.

Zintl phases are a subgroup of brittle, high melting point intermetallic compounds which are diamagnetic or exhibit temperature-independent paramagnetism, are poor conductors or semiconductors.

NaTl, where it is now known that the structure consists of a polymeric anion (-Tl−-)n with a covalent diamond structure with Na+ ions fitted into the anionic lattice. NaSi where the polyanion is tetrahedral (Si4)4− similar to phosphorus molecule P4. Na2Tl which the polyanion is tetrahedral (Tl4)8− similar to phosphorus molecule P4.

FeSi2

Phase Diagram

FeSi2

According to the phase diagram of the binary system Fe–Si, the stoichiometric FeSi2 solidifies at 1493 K as a eutectic structure composed of -Fe2Si5 and -FeSi which are metallic phases with poor thermoelectric properties. The -FeSi2 phase formation is either through the peritectoid reaction between -Fe2Si5 and -FeSi or the eutectoid decomposition of -Fe2Si5 to -FeSi2 and Si. It is well known that -FeSi phase generally remains stable after normal fabrication procedure and that the transformation from -Fe2Si5 to -FeSi2 phase by heat treatment is very sluggish.

FeSi2

FeSi2

FeSi2

0.1 nm 1nm 10 nm 100 nm

Phonon distribution

Phonon wavelength

Deteriorate electrical conductivity

Point defect; doping, intercalation

Incoherent interface; grain boundary, nano-dispersion

Semi-coherent

(strained) interface; misfit dislocation

Coherent interface

FeSi2

CoSi compound

Zn4Sb3

Zn4Sb3

Half-Heusler : TiNiSn

Half-Heusler : TiNiSn

Thermal Conductivity

300 400 500 600 700 800 900 1000

3

4

5

6

7

8

9

10

11

TiNi0.9

Pt0.1

Sn

TiNi0.95

Pt0.05

Sn

Ti0.9

Zr0.1

NiSn

Ti0.9

Hf0.1

NiSn

TiNiSn

TiNiSn0.9

Si0.1

TiNiSn0.95

Si0.05

Therm

al C

onductivity (

W/m

K)

Temperature (K)

The Most Effective Element is Hf

300 400 500 600 700 800 900 1000

3

4

5

6

7

8

9

TiNi0.9

Pt0.1

Sn

TiNi0.95

Pt0.05

Sn

Ti0.9

Zr0.1

NiSn

Ti0.9

Hf0.1

NiSn

La

ttic

e T

he

rma

l C

on

du

ctivity (

W/m

K)

Temperature (K)

TiNiSn

TiNiSn0.9

Si0.1

TiNiSn0.95

Si0.05

Lattice Thermal Conductivity

Ti (MTi=48) : Hf (MHf=179) and Zr (MZr=91)

Hf

CsBi4Te6

CsBi4Te6

Yb14MnSb11

Yb14MnSb11

Yb14MnSb11

Enhancement of DOS(E)

well

barrier

Confinement of conduction

electrons leads to enhancement

of DOS(E)

m

knnn

LmE zyx

2)()

2(

2

22

22222

)(2

3)1

(znynxn

Li zyx

eL

dkkm

dE2

2 dE

k

mdk

12

dkkL

L

dkkdkkN

2

33

3

2

22

4)(

Density of State

32 ,)/1(/)(2 LVkdkkVdkkN ][)2(

2

1)( 132/1

3

2/3 eVcmdEE

mdEEN

3-dimensional system

222

22

222

2222

)(22

)(2

)()2

(2

z

z

z

z

yx nLm

km

nLm

nnLm

E

znL

e z

z

ynxnL

i yx

sin)(

2

dkkm

dE2

2 dE

k

mdk

12

kdkL

L

kdkdkkN

22

2)(

3

3

Density of State

2,//)(2 LSkdkSdkkN ][1

)( 12

2

eVcmdEm

dEEN

2-dimensional system

)()(2

)2

(2

222

,

222

2

zy

zy

x nnLm

nLm

E

zn

Lyn

Le z

z

y

y

xnL

i x

sinsin2

dkkm

dE2

2 dE

k

mdk

12

kdkL

L

dkdkkN

22)(

Density of State

//)(2 dkLdkkN ][1)2(

2

1)( 11

2/12

2/1 eVcmdE

E

mdEEN

1-dimensional system

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