Flexible thermoelectric energy harvesters using bulk ... · Flexible thermoelectric energy...
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Flexible thermoelectric energy harvesters using bulk thermoelectric materials and low-resistivity liquid metal interconnectsMehmet C. Ozturk and M. DickeyYasaman Sargolzaeiaval, Viswanath Padmanabhan Ramesh & Taylor Neumann
North Carolina State University
Advanced Self-Powered Systems of Integrated Sensors and Technologies
(ASSIST) Nanosystems Engineering Research Center
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ASSIST vision
ASSIST’s vision of health and wellness is enabled by its
disrup ve system features
Self-Powered/Ultra Low Power
Wireless
Hassle-Free/Comfortable
Medically Validated
Non-invasive/minimally invasive
Mul -modal health and environmental sensors
Long-term monitoring of personal health & environment enabled by always-on pla orms
Sophis cated picture of health via correla on of mul ple sensors
Enable a pathway towards personalized medicine
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Heat - ThermoelectricMotion - Piezoelectric
ASSISTRoadmap
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ASSIST Low-Power Sensor Technologies
Long term monitoring of environmental exposures & health
DetectionWheeze / Cough
Critical Data CorrelationOzone / Respiratory &
Cardiac Health
HET 1Asthma Management
ECG PPG VOC Ozone
Critical Data CorrelationFood Intake - Glucose / Lactate
Levels & Uric Acid - Wound Health
HET 2Diet Management &
Wound Healing
Wound Fluid ISF Sweat
Non-Invasive Glucose/Lactate & Wound Health Monitoring
Glucose Lactate Uric AcidpH Breathable Materials
ASSIST Zero-Power Extraction and Transportation Technologies
ASSIST Low-Power Detection Technologies
Critical Data CorrelationMedication Intake & Physiological
Parameters from HET 1.0 & 2.0
Sweat
Non-Invasive Medication Detection
Monitoring of Medication Intake
ASSIST Zero-Power Extraction and Transportation Technologies
ASSIST Low-Power Detection Technologies
ISF
HET 3Medication Adherence
Vigilant ECG (R-R) and Motion
Critical Data CorrelationA-Fib and Activity Identification
Optimized for Low-Power
SAP 1Cardiac Health
ECG AFE
ASSIST Energy Harvesting and Low-Power Circuit Technologies
SoC & RadioSupercapacitorsEnergy Harvesting
SAP 1.0 + HET 1.0
Critical Data Correlation HET 1.0 and Pulsed Transit Time
Blood Pressure
SAP 2Cardiac Health, Blood
Pressure & Asthma
Ozone
ASSIST Energy Harvesting, Sensor & Low-Power CircuitTechnologies
PPGEnergy Harvesting
ECG VOC
SAP 2.0 + HET 2.0
Critical Data CorrelationMetabolic Changes &
Cardiac Health
SAP 3Cardio-Metabolic Health
Ozone
ASSIST Energy Harvesting, Sensor & Low-Power CircuitTechnologies
PPGEnergy Harvesting VOC
ECGBiomarkers < Sweat/ISFBP
Health and Environmental Tracker
Self-Powered Adaptive Platform
Harvesting Heat from the Body
The application imposes large thermal resistances
Flexible thermoelectric generators (TEGs) are desirable:● Conformal to the body
● Better contact with the skin
● Large area harvesting● Simple Integration
● Electrical resistance
● Aesthetics
SkinResistance
ContactResistance
Flexible Heatsink &Small Form Factor
Heat
LateralHeat Spread
Heat Flowthrough Filler
Vertical Heat FlowThrough Polymeric Substrate
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Harvesting Heat from the Body
Key Challenge: Small ΔT across the harvester
SkinResistance
ContactResistance
Flexible Heatsink &Small Form Factor
Heat
LateralHeat Spread
Heat Flow through Filler
Vertical Heat FlowThrough Polymeric Substrate
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Our approach to Flexible Thermoelectrics
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● Bulk thermoelectric materials● Best materials used in rigid TEGs
● No new material development
● Pick-and-Place Tooling● Standard technique
● Flexible packaging● Material Innovations
Ag/AgCl
Ag Nanowires
EGaIn
A flexible approach with a low cost-of-ownership that can rival the performance of rigid TEGs
Eutectic Gallium Indium (EGaIn)Gallium Indium
+ =30 °C 157 °C 16 °C
Liquid• Low viscosity• Low toxicity• Near-zero vapor pressure
Adv. Fun. Mat. 2009
Can be encapsulated by an elastomer
Adv. Mater. 2016
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TEG Fabrication with EGaIn Interconnects
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TE Leg PDMS
1.31mm
1.45mm
EGaIn
TEG with liquid interconnects
PDMS Encapsulation
Suarez at. Al, Flexible thermoelectric generator using bulk legs and liquid metal interconnects for wearable electronics, Applied Energy, 2017
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101520253035404550
0 0.5 1M
easu
red
Pow
er (µ
W)
Air Velocity (m/s)
5°C
22°C
32°C
Testing on the Human Body
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Promising performance despite• No heat spreaders• No heatsink• Thick PDMS encapsulation
Mechanical Testing
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Mechanical Testing
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10mm 35mm
• Low Resistance - negligible contribution from interconnects• Spikes recovered due to “self-healing” nature of EGaIn
Further Improvements
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Thin Metal Spreaders
Reduction of Filler Thermal Conductivity
Thinner Top and Bottom
Elastomers &Higher Thermal Conductivity
Taller Legs
Printing and Spray Coating of EGaIn
● Printing provides faster and more reliable interconnects on TEGs with large leg count
● Spray coating of encapsulation provides a much thinner encapsulation 13
Flexible TEG with 256 legs & Spray Coated Encapsulation
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0 20 40Ther
mal
Con
duct
ivity
(W/m
K)
Volume percentage
Alumina doped PDMS
Al2O3, AlN, and BN Doped PDMSIncreasing the thermal conductivity of the encapsulating layer
153X enhancement can be achieved in the thermal conductivity of PDMS
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mal
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duct
ivity
(W/m
K)
Volume percentage
AlN doped PDMS
Material Thermal Conductivity Powder size
Al2O3 Powder ~35 W/mK ≤10 µm
AlN Powder ~150 W/mK ~10 µm
BN Powder ~30 W/mK ~1 µm
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duct
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BN doped PDMS
Output Power – Device characterization on the wrist – Impact of time on the wrist
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utpu
t pow
er (µ
W)
Air velocity (m/s)
Room Temperature (22 °C)
Immediate
1 minute
5 minutes0
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Out
put P
ower
(µW
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Air Velocity (m/s)
Outdoor (~32 °C)
Immediate
1 minute
5 minutes0
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Out
put P
ower
(uW
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Air Velocity (m/s)
Refrigerator (1 °C)
Immediate
1 minute
5 minutes
3X improvement in performance with• Thin PDMS encapsulation• Al2O3 doped PDMS top/bottom layers
Still… No heatsink or heat spreaders
Time on body: Time on body: Time on body:
Device characterization on the wristVoltage, Temperature Differential and Power
32°C
22°C
1°C
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pera
ture
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Air Velocity (m/s)
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Out
put P
ower
(µW
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Air Velocity (m/s)
Benchmarking (w/o Air Flow)
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0.05 0.178
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33.2
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Outp
ut P
ower
(µW
/cm
2 )
Author/Temperature (°C)
PDM
S +
Dis
pens
ing
prin
ting
of T
E
legs
, Yon
seiU
nive
rsity
(Kor
ea)/2
012
Flex
ible
fabr
ic+
Dis
pens
ing
prin
ting
of
TE l
egs
Yons
eiU
nive
rsity
(Kor
ea)/2
014
CN
T TE
legs
/ No
enca
psul
atio
nTe
xas
A&M
Uni
vers
ity/2
014
PDM
S +
Dis
pens
ing
prin
ting
of T
E le
gsYo
nsei
Uni
vers
ity (K
orea
)/201
5
CN
T TE
legs
/ No
enca
psul
atio
nTe
xas
A&M
Uni
vers
ity/2
014
PDM
S +
Dis
pens
ing
prin
ting
of T
E le
gsYo
nsei
Uni
vers
ity (K
orea
)/201
5
Mul
ti st
age
TEG
/ PD
MS+
El
ectro
chem
ical
dep
ositi
on
of T
E le
gs/T
ohok
oU
nive
rsity
(Jap
an)/2
017 ASSIST
Tamb = 15oC
Tamb = 22oC
Tamb = 1oC
Further Improvements – COMSOL simulation
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Low Thermal Conductivity Elastomer
Air bubbles
Aerogel Particles
Aerogel Islands
A low thermal conductivity elastomer between the legs can increase the power by ~ 2X
Graphene/EGaIn Doped Elastomer● Our approach was elastomer
doping with high thermal conductivity particles
● We have tried● Al2O3, AlN, BN, Graphene and
EGaIn
● Best results were obtained with EGaIn + Graphene Doping
● 0.85 W/mK (5.6X improvement)
● We can increase the encapsulating thickness to 200 um without paying significant penalty0
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4
6
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10
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0 0.5 1
Outp
ut p
ower
(uW
/cm
2 )
Fill Factor
50um PDMS50um 1W/mK elastomer100um 1W/mK elastomer200 um 1 W/mK elastomer
3D COMSOL Modeling(Natural Convection)
Graphene percentage
PDMS + Graphene
PDMS + EGaIn+ Graphene
2.2% 0.602 W/mK 0.838 W/mK
4.3% 0.652 W/mK 0.853 W/mK
6.3% Cannot measure 0.871 W/mK
Improved TEG design
P N P N P N P
Low Thermal ConductivityElastomer
NanocompositeBiTe Legs
Low-ResistivityEGaIn Interconnects
High Thermal ConductivityElastomer
TitaniumGold
Copper Heat Spreader
Spray CoatedThin Elastomer
Conclusions
● EGaIn provides ● Printable, low-resistivity, stretchability
● Self-healing, reliability
● Room-temperature bonding to TE legs
● There is still much room for improvement● Thermal management - Device packaging and materials
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There is potential for producing flexible TEGs that rival the performance of rigid TEGs
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