Introduction to Thermoelectric Materials and Devices

Space probe to the Jupiter From JPL, NASA Radioisotope

Thermoelectric Generator (PbTe)

Introduction to Thermoelectric Materials and Devices

4th Semester of 2012 2012.03.29, 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 and Film

6 Thermoelectric System : Current and Future of Modules

7 Applications : Power Generation and Heat Cooling

8 Mid-term Exam

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

Thermoelectric Energy Conversion Efficiency

ellat

: Power Factor /2

lat : Lattice Thermal Conductivity

el : Electronic Thermal Conductivity

TZT

2

or S : Seebeck Coefficient

(Thermoelectric Power)

: Electrical Resistivity

: Thermal Conductivity

Dimensionless Figure of Merit, ZT

Measurement : Quiz

Prof. M at a conference talked a bulk material with ZT over 2.5 was obtained at 1000 K.

Mainly due to the high Seebeck coefficient

Specimen

Tc = 300K

Th = 1000K

Furnace

V

Why not at 350K ???

If a material with 100V/K : Acceptable range of Temperature difference ???

I

Quasi 4-point probe

Measurement : Seebeck Coefficient

1. Integral method 2. Differential method

1. Integral method : Measurement of V generated by two thermocouples consisting of a sample wire and a reference material wire. Known Seebeck coefficient of a third wire.

But, Who makes wire ??? Nobody use now.

Measurement : Seebeck Coefficient

2. Differential method : When the net current in the sample is ZERO, T and V are measured. Due to the electrical field in the sample is due to the E=T.

The sample should be homogeneous !!!

Measurement : Electrical Conductivity

Contact resistance should be small

To Minimize the Joule heating on the resistivity measurement, the current density should be lower.

The electromagnetic noise gives a random contribution and can be reduced -By appropriate shielding of the circuit, -By using low-noise measuring equipment -By averaging the measured values

Spurious voltage originating from thermoelectric effect can be eliminated -By additional measurement with the current set to zero -With different current value -By reversing the current flow direction

V = I R

DC : Direct Current AC : Alternating Current

Measurement : Electrical Conductivity

2-point probe 4-point probe

Applying Voltage, Measuring Current

Simple Not correct, unreliable

Thickness, t << distance of electrode, s

Bulk

Measurement : Seebeck Coefficient and Electrical conductivity

Measurement : Seebeck Coefficient and Electrical conductivity

Apparatus for Seebeck coefficient and electrical conductivity at 100K to 1300K

Measurement : Seebeck Coefficient in Magnetic Field

1: sample, 2:sample pressing and sample-supporting plates, 3: AlN plates, 4: Cu plate, 5: heater, 6: heatsink (Cu-Be alloy), 7: temperature sensor, 8: alumina tube holding thermocouple, 9: spring (Cu-Be alloy)

Measurement : Seebeck Coefficient and Electrical conductivity

Apparatus for Seebeck coefficient and electrical conductivity at 300K to 2000K

1: heatsink, 2: Mo tubing, 3: sample, 4, 5: sample supports, 6: Mo stopper, 7: alumina insulating ring, 8: Mo pressing rod, 9 stainless pressing rod, 10: alumina tube

Measurement : Mistakes or Ignorance

Prof. M at a conference talked a bulk material with ZT over 2.5 was obtained at 1000 K.

Mainly due to the high Seebeck coefficient

Specimen

Tc = 300K

Th = 1000K

Furnace

V

Why not at 350K ???

If a material with 100V/K : Acceptable range of Temperature difference ???

I

Quasi 4-point probe

Thermal Conductivity : Steady-State Technique

TA

LQT

0 QT : heating power through sample

L0 : length between thermocouple

(Heater, I2R)

Substantial Errors : Radiation loss or gain, Convection, Conduction through lead wires

(0.004 in)

(0.001 in)

(Conducting Epoxy)

At low Temp, Cernox (resistance temperature sensors)

Adiabatic Condition !!!

Thermal Conductivity : Steady-State Technique at low Temp.

PPMS (Quantum Design Inc.)

Thermal Conductivity : Laser Flash method, = Cp

One surface of a disc sample is irradiated by a short pulse of heat from a laser times being 1 msec.

The resultant temperature rise of the opposite surface is recorded, from which the thermal diffusivity is computed from temperature rise vs. time data.

CL

QTm

is the density of specimen with dimension of g/cm3. C is the specific heat with dimension of J/gK.

Maximum temperature rise of rear surface

Thermal Conductivity : Laser Flash

2/1

2

1388.0t

La

Only Bulk Sample available Impossible for Low temperature measurement Steady state

Thermal Conductivity : in-plain of thin film

Anisotropy Problem : Out-plain of thin film (Direction to thickness) ???

Thermal Conductivity : 2 (3) method for thin film

Thin metal strip evaporated on the sample acts as heat source and a thermometer

The heater is driven with AC current at frequency ω, which causes heat source to oscillate at frequency 2ω

Thermal conductivity is evaluated along the thickness direction Clk

PT

ln

2

Penetration depth

d2/1)2

(

: Thermal diffusivity

Films are neglected

d2/1)2

(

Confined to the films

Thermal Conductivity : 2 (3) method for thin film

We will have in 3 months.

Z Meter, Haman Technique

Under steady-state or adiabatic condition, the heat pumped by the Peltier effect will be equal to heat carried by the thermal conduction;

Direct method for measuring ZT of a material and device

)()(L

TAQIT P

)1(

IR

TEIR

V

VVZT

Valid for Ideal case : Contact, Radiation, loss

Reference material with ZT of 0.1 is necessary

We will have in 3 months too.

Z Meter, Haman Technique

Thermocouples

Electrode Electrode

Wire

+Q Q

l

Probes

Electrode Electrode

Wire

+Q

l

Time

V b

etw

een

Pro

bes

Q

IR

Seebeck Coefficient

Presentation by Group 1

• Variable-Range Hopping conduction • Electron-Electron Interaction • Electron Localization • Sign Change of Seebeck coefficient

Crystal Structure of 12CaO7Al2O3 (C12A7)

1 Molecule = 12CaO · 7Al2O3

1 Unit Cell = 2 molecules = [Ca24Al28O64]4+ · 2O2–

Lattice: Positive framework (12 cages per UC)

Free O2– ions (2 per UC)

Charge per framework cage: + 1/3

Three dimensionally connected

subnanometer-sized cages

Cage: 5 Å wide

Crystal Structure of 12CaO7Al2O3 (C12A7)

1 Molecule = 12CaO · 7Al2O3

1 Unit Cell = 2 molecules = [Ca24Al28O64]4+ · 2O2–

1 Unit Cell = 2 molecules = [Ca24Al28O64]4+ · 4X–

Lattice: Positive framework (12 cages per UC)

X– ions (4 per UC)

X– = O–, H–, e–, etc

Charge per framework cage: + 1/3

Three dimensionally connected

subnanometer-sized cages

monovalent anion (4X–)

Metal – Insulator Transition

Hopping Conduction

Band Conduction

Nc = ~1 1021 cm–3

Metal – Insulator Transition

Metal composed of typical insulators, lime and alumina !

Polaron : electron localized by

lattice distortion

Thermal Annealing with metal Ti

Concentration

of electrons

Thermoelectric Properties of C12A7:e

C12A7:O2 + Ti C12A7:e + TiOX

Treatment Temperature, 700 – 1100oC

Time, 12 – 24 hr

|S| decreases with Ne

Sign change with Ne

: Impurity-band-conduction like Si

T1/2 dependence

: Variable range hopping

Thermoelectric Properties of C12A7:e

Electronic Structure of C12A7:e

DOS of FVB is mainly formed by the O atoms of lattice framework cages

DOS of FCB and CCB is mainly formed by the component of Ca atoms

The DOS at EF decreases with Ne

m* of C12A7 electride

Semiconducting : 1.1 – 1.5m0

Metallic : 0.8m0

m* of SrTiO3

Nb-doped STO : 7.3 – 7.7m0

[Ca24Al28O64]4+ · 4e–

DOS of FVB is mainly formed by the O atoms of lattice framework cages

DOS of FCB and CCB is mainly formed by the component of Ca atoms

Density of State (DOS)

Thermoelectric Properties of C12A7:e

CaO : ~ 15 mW/K, Al2O3 : ~ 30 mW/K

Thermal Conductivity

Amorphous-like thermal conduction

Acoustic Properties

Varshni’s equation

Phonon Mean Free Path : 0.7 nm

Seebeck Coefficient

Presentation by Group 1

Presentation Articles by Group 1

Presentation by Group 1

Presentation Articles by Group 1

Presentation by Group 1