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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