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Gao Min School of Engineering, Cardiff University, UK
Key Issues in Design and Fabrication of
Thermoelectric Modules
Gao Min
School of Engineering, Cardiff University
N P
UK Thermoelectric Network Meeting
Herriot Watt University, Edinburgh
13th February 2018
Gao Min School of Engineering, Cardiff University, UK
Design and Fabrication Processes
Design
Fabrication
Evaluation
SPECIFICATIONS
Materials Selection Geometry Design
Contact Resistance Barrier & Brazing
Power Output Conversion Efficiency
Gao Min School of Engineering, Cardiff University, UK
Selection of Thermoelectric Materials
• Large average ZT
• Matching N and P
• Thermal stability
• Non-toxicity
• Abundances
• Manufacturability
TEMPERATURE (K)
0 200 400 600 800 1000 1200 1400
TH
ER
MO
ELE
CT
RIC
FIG
UR
E-O
F-M
ER
IT, Z
T
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
2.2
2.4
2.6
2.8
3.0
SnSe
SKD
300K - 850K
h ~ 9%
ZT~ 0.5
h ~ 12%
ZT~ 0.8
(e.g. SKD vs Silicide)
(e.g. Half-heusler vs SKD)
(e.g. Half-heusler vs SKD)
(e.g. Silicide vs PbTe)
(e.g. Silicide vs Bi2Te3)
Gao Min School of Engineering, Cardiff University, UK
Design of Thermoelement Geometry
Nature of Heat SourceConstant DT Constant heat flux
l
A
A/l = ?
Pmax =(a ×DT )2
4r
A ×2N
(n + l)(1+ 2rlc / l)2
f =
DT
TH
æ
èç
ö
ø÷
1+ 2rlc
l
æ
èç
ö
ø÷
2
2 -1
2
DT
TH
é
ëê
ù
ûú+
4
ZTH
é
ëê
ù
ûú
l + n
l + 2rlc
é
ëê
ù
ûú
æ
èç
ö
ø÷
G Min, CRC Handbook of Thermoelectrics, 1996, 11-1
Shorter legs (down to ~500mm)
G Min and Rowe, 2002, Energy Conversion & Management , (43) 221-228
P =s
(1+ s)2×
Z
(1+ ZTM )2×
˙ Q 2
l×
l
A×
l
(n + l)(1+ 2rlc / l)2
f =P
˙ Q =
s
(1+ s)2×
Z
(1+ ZTM )2×
˙ Q
l×
l
A×
l
(n + l)(1+ 2rlc / l)2
H Hashim, JJ Bomphrey, G Min, Renewable Energy, 87(2016), 458-463
Longer legs (typically 3-8 mm)
G Min, “TE Design for a Given Thermal Input” J.Electr. Mater. (2013), 42(7), 2239-2242.
Temperature difference (K)
0 20 40 60 80 100
Po
we
r o
utp
ut
(W)
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4Theory
Best commercially available module
optimised
cooling water
Solar radiation
Solar absorber
TE generator
cooling water
Solar radiation
Solar radiation
cooling water
Gao Min School of Engineering, Cardiff University, UK
Thermoelectric
Materials
Thermal contacts
N P Solder N P Cu
Electric contacts
Thermoelectric
n-p junctionThermoelectric
Module
Materials to Module – Interfaces Challenge
Requirements:
To join and hold the N- and P-type thermoelements together.
• Mechanical strength.
• Temperature stability and durability.
• No reduction in thermoelectric properties.
Role:
Gao Min School of Engineering, Cardiff University, UK
oTD
Q
l
oTDTD
2cTD
2cTD
ReR
cR
cR
Q
Influence of Contacts – Theoretical Consideration
Bulk properties:
Contact properties:
ZT will be reduced due to the interfaces. The degree of reduction
depends on contact properties, materials properties, and leg length
ZT =a 2T
RK
ZTe =ae
2T
ReKe
= ?
G Min and M Phillips, 2016, “Preparation and Characterisation of TE Interface” Chapter 6 in Thermoelectric Energy Conversion (Eds. Pineda & Rezania, Wiley-Vch), 111-125
Gao Min School of Engineering, Cardiff University, UK
l (mm)
0.1
0.5
1.0
1.5
2.0
2.5
3
5
Manufacturing Requirements
N P
Case study based on Bi2Te3: aims to limiting ZT reduction within 10%.
Assume:
It requires: andL = 2mm
Gao Min School of Engineering, Cardiff University, UK
Sample Preparation for Interface Studies
Skutterudite with barrier layer
Copper block
1.3- 1.5 mm
Solder/braze
120-200 mm
Brazing holder
Brazing (or soldering) in argon gas or vacuum
Gao Min School of Engineering, Cardiff University, UK
High-resolution Scanning Voltage Probe for Measuring Electrical Contact Resistance
R(x) =VS (x)
Vref
× Rrefr = dR(x) / dx
rc = DR × A
DR
DR
Dx
x
R(x)
perfect contact
- contact DR
- no alloy
- contact DR
- alloy region Dx High Resolution Scanning Voltage Probe (1 mm)
The key is appropriate spatial resolution (~1mm for for bulk modules)
G Min and M Phillips, 2016, “Preparation and Characterisation of TE Interface” Chapter 6 in Thermoelectric Energy Conversion (Eds. Pineda & Rezania, Wiley-Vch), 111-125
Gao Min School of Engineering, Cardiff University, UK
Characterisation of Skutterudite Interfaces
1 2 3 4
0.028
0.030
0.032
0.034
0.036
0.038
0.040
0.042
Resi
stance
/
Distance along sample/mm
Copper region
Unaffected region
Inter-diffusion region
SVP Measurement
High resolution scanning voltage probe
measurement indicates the formation of
alloying region.
Spectrum Co Cu Zn Ag Sb Total
Spectrum 1 12.29 2.39 0.00 1.93 83.39 100.00
Spectrum 2 3.64 34.17 7.50 14.78 39.90 100.00
Spectrum 3 0.00 97.59 0.00 0.00 2.41 100.00
SEM and EDX Analysis
EDX analysis confirms the formation of
the alloying region, which needs to be
minimised in good TE modules.
Pd/Cu/Ag50Cu20Zn28Ni2
Gao Min School of Engineering, Cardiff University, UK
Influence of Brazing Temperature on Interface Quality
-0.10 -0.08 -0.06 -0.04 -0.02 0.00 0.02 0.04 0.06 0.08 0.10
0
50
100
150
200
250
300
Conta
ct
resis
tivity/m
cm
2
Sample position/cm
654 oC
663 oC
668 oC
2 x 10-5 /cm2
7 x 10-5 /cm2
1.5 x 10-4 /cm2
• Brazing temperature
• Thermoelectric materials
• Brazing atmosphere
• Brazing materials
• Barrier layers
INFLUENTIAL FACTORS
Gao Min School of Engineering, Cardiff University, UK
Thermal Contact – Principle of Measurement
x
)(xT
Perfect contact
TE TECu
CTD2
HT
CT
xD
TD
Ensure constant heat flow
through the sample:
G Min and M Phillips, 2016, “Preparation and Characterisation of TE Interface” Chapter 6 in Thermoelectric Energy Conversion (Eds. Pineda & Rezania, Wiley-Vch), 111-125
Gao Min School of Engineering, Cardiff University, UK
Evaluation of Thermal Contact using IR Microscopy
Hot contact Cold contact
Sample
(a)
Bi2Te3
Pressure contact
Bi2Te3
(b)
Bi2Te3
Sn/Pb solder
Bi2Te3
Similar results obtained using indium or heat sink compound
Gao Min School of Engineering, Cardiff University, UK
Module Assembly
DESIGN
Thermoelectric Module Assembly Process
Assembly rig
Bracing materials
Ceramic substrate
Metallisation
High Temperature Skutterudite Module
by CU-RU-LU
Cutting TE legs
Coating barrier layers
ASSEMBLY
J Garcia-Canadas, A Powell, A Kaltzoglou, P Voqueiro and G Min, Journal of Electronic Materials, (2013), 42(7), 1369-137
Gao Min School of Engineering, Cardiff University, UK
Rapid Screening by Thermoelectric Impedance Spectroscope
J. Garcia-Cañadas and G. Min, “Impedance Spectroscopy for Thermoelectric Characterisation” J. Appl. Phys. 116 (17), 174510 (2014).
ZT =RTE
Ri
TE
module
ReferencemoduleZT=0.170
~620oCbrazebothsidesZT=0.084
~650oCbrazebothsidesZT=0.117
~620oCbrazeonesideZT=0.103
Gao Min School of Engineering, Cardiff University, UK
Determination of Power Output by I-V Curves
0 1 2 3
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
Voltage/V
Current/A
50K
100K
150K
200K
250K
300K
350K
400K
440K
0 1 2 3 4 5
0
200
400
600
800
1000 50K
100K
150K
200K
250K
300K
350K
400K
440K
Pow
er/
mW
Current/A
Be aware of errors when used for
determining conversion efficiency
Gao Min School of Engineering, Cardiff University, UK
Determine Conversion Efficiency by Triple I-V Curves
hmax =1
4×DTo
T× 1-
VOCO
VOCS
æ
èç
ö
ø÷
Fast Scan
Slow Scan
VOCO
VOCS
Fast Scan – constant temperature difference
Slow Scan – constant heat flux
VOCO - corresponding to the first fast scan
VOCS - corresponding to the second fast scan
s
Open-circuitShort-circuit
Gao Min School of Engineering, Cardiff University, UK
Ring-structured Thermoelectric Module
Hot
FluidCold
Fluid
Hot-pressing
Copper
Insulator
N-type
thermoelement
Copper
Insulator
P-type
thermoelement
Insulator
Gao Min School of Engineering, Cardiff University, UK
Ring-Structured Thermoelectric Tube
Gao Min and Rowe, 2007, Semicond. Sci. Technol., (27) 21-28
Gao Min School of Engineering, Cardiff University, UK
Micro/Nano Converters for Integrated Circuits
Insulating layer/silicon substrate
Membrane
Cold junction
Hot junction
IR focal plane
Metalization
P-type
N-type
Bonding pads
IR focal planeInsulating layer
(membrane)Substrate
Horizontal structure
Ref: G Min and D Rowe, Electronics Letter, 1998, 34(2), 222-223
L.M. Goncalves. P. Alpuim, G. Min, D. M. Rowe, C. Couto, and J. H. Correia, “Optimization of Bi2Te3 and Sb2Te3 thin
films deposited by co-evaporation on polyimide for thermoelectric applications”, Vacuum, 82 (12), (2008), 1499-1502
Gao Min School of Engineering, Cardiff University, UK
Thank You
Acknowledgements
Dr. Matthew Philips, Dr. Jorge Garcia-Canadas and Dr. Tanuji Singh
The work is funded by the United Kingdom Engineering and
Physical Science Research Council (EPSRC) under the projects of
EP/K029142 and EP/K022156.