characterizinghalf heuslerthermoelectric...
TRANSCRIPT
Results
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Tensile Stress [MPa
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Applied Brazing Pressure [MPa]
Incusil-‐ABA
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Num
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f Cycles [#]
Sample Number [#]
Cusil-‐ABA Cusil
Materials and Methods
Background
FabricaDon ObjecDves • Strong bond between half-‐Heusler and conducDng material • Consistent braze joints • Desired electrical and thermal properDes of braze • High efficiency unicouples and devices
¡ High electrical conducDvity across braze ¡ Low thermal conducDvity
Half-Heusler Material
Braze Foil
Direct Bonded Copper
Acknowledgements
Emily Johnson 1, Nicholas Kempf 2, Yanliang Zhang 2 1 Department of Physics, St. Olaf College, Northfield, MN; 2 Department of Mechanical & Biomedical Engineering, Boise State University, Boise, ID
References 1. Adapted from Snyder et al., 2008 Nature Materials, 105-‐114 2. GMZ Energy, Inc., Voltage Measurement
Characterizing Half-‐Heusler Thermoelectric Unicouple ProperDes
• Increase material uniformity to create more consistent devices ¡ Decrease variability in part sizes during fabricaDon ¡ Increase braze consistency to overall leg performance
• Improve braze fixture to streamline braze process ¡ Apply pressure uniformly throughout braze cycle ¡ Higher quanDty fabricaDon yield
• Develop beZer tensile tesDng apparatus to aZach TE legs for tesDng • Improve resistance measurement technique across joint • Test material limits with current fixtures and fabricaDon techniques • Implement efficiency measurements, thermal cycling tests, and thermoelectric generator (TEG) tesDng to determine power output and lifeDme of TE devices for various applicaDons
Future DirecDons
Seebeck Effect • Applied temperature difference creates a current • Temperature gradient across the TE material causes higher energy charge carriers to diffuse toward colder side • ConDnuously applied heat keeps charge carriers moving
Thermoelectric Figure of Merit (ZT) • Provides a measure of the thermoelectric performance of the TE material
Efficiency • Percent energy output from TE device
α = −ΔVΔΤ
ZT=α2σκT
ηmax =THot −TCold
THot
1+ ZTMean −11+ ZTMean +TCold /THot
"
#$$
%
&''
PelDer Effect • Applied current causes a temperature difference • Charge carriers have different heat carrying capaciDes in different materials • Charge carries moving between materials cause heat to be absorbed or rejected
η =PowerOutputHeat Input
α = Seebeck coefficient σ = electrical conducDvity κ = thermal conducDvity ΔV = Seebeck voltage ΔT = temperature difference
Heat Sink
Heat Source
n-‐type p-‐type
Power Output Cooling
n-‐type p-‐type
Heat Rejected
Current Source
Assembly and Joint Brazing Procedure • Sand parts to appropriate size (approximately 2×2×3 mm) • Clean components in sonicaDon process • Assemble half-‐Heusler legs, braze foil, and direct bonded copper (DBC); or copper, braze foil, and copper in brazing fixture • Braze components in vacuum furnace (approximately 0.01-‐0.06 mbar at highest vacuum)
IntroducDon • Increases in global energy usage have created a growing need for new, efficient energy technologies • Thermal energy losses pose many problems in current technologies • Thermoelectric (TE) materials provide a method to convert waste heat directly into energy, making them a strong contender for energy producDon
Diagram of TE module internal structure1
• Most current TE generators and materials are costly and inefficient • In order to improve feasibility of thermoelectric energy generaDon we need to improve modules • OpDmizing fabricaDon methods will help improve TE module efficiency
Braze Alloy IncusilTM-‐ABA CusilTM CusilTM-‐ABA ComposiDon (%wt.) Silver 59 72 63 Copper 27.25 28 35.25 Titanium 1.25 -‐-‐-‐ 1.75 Indium 12.5 -‐-‐-‐ -‐-‐-‐ Solidus(°C) 605 779 779 Liquidus (°C) 715 779 816 CTE (×10-‐6K-‐1) 18.2 19.6 18.5
a) Tensile stress test results from Copper – IncusilTM-‐ABA – Copper joints brazed at 825°C for 5 minutes with various pressures applied to legs; and b) completely reversed bending test results from Copper – CusilTM – Copper and Copper – CusilTM-‐ABA – Copper joints brazed at 825°C for 5 minutes (samples from data points marked red can be seen below)
This research was supported by the Boise State University REU in Materials for Energy and Sustainability and sponsored by NaDonal Science FoundaDon grant 1359344.
~ 6 mm
Braze Fixture
TesDng Process • Tensile tests performed on brazed legs using homemade tesDng fixture • Completely reversed bending cycle test with a full cycle across a maximum angle of approximately ± 24°
~ 2 mm
DBC
Half-‐Heusler
Brazed half-‐Heusler, DBC leg
~ 3 mm
Brazed Copper leg
Copper
b) a)
c)
Copper leg braze interface aqer c) tensile; and d) bending tests for various numbered samples shown in red on graphs above (leg dimensions ~2×2 mm)
d)
# 1
# 6
# 9
# 12
# 3
# 5
# 7
# 9
Incusil-‐ABA Cusil-‐ABA Cusil
f) e)
250 μm
e) SEM images of Copper – CusilTM – Copper braze joint interface; and f) GMZ Energy example voltage measurement across braze joint2 to measure contact resistance
PosiDon (mm)
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15
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Measured Contact Resistance: 0.827 μΩcm2
0.3 0.4 0.5 0.6 0.7 0.2 5
Volta
ge (μ
V)
Heat Flow
Heat Flow
Braze Process ConsideraDons • Material preparaDon and assembly • Atmosphere, applied pressure, surface roughness, and cleanliness • Braze temperature and Dme for different materials • Diffusion bonding, joint coverage, and braze foil dimensions
Materials SelecDon • Half-‐Heusler compound
¡ Nanostructured alloys increase ZT • Braze alloy
¡ Diffusion bonding influences joint strength ¡ Similar thermal expansion coefficients are ideal ¡ Braze foil thickness changes joint properDes
• ConducDve material ¡ Copper provides high conducDvity electrical connecDon ¡ Direct Bonded Copper (DBC) provides insulaDon
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Tempe
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Time [min]
Typical Furnace Braze Cycle