1. Strain Engineering of Thermal
Transport in Nanocrystalline
Materials
Brandon N. Davis
PhD Student
Department of Mechanical Engineering
Oral Preliminary Exam
May 14, 2014
Advisor: Prof. Sandeep Kumar
Committee: Prof. Javier Garay, Prof. Masaru Rao, Prof. Lorenzo Mangolni

2. Nanomechanics and Multiphysics Lab
Presentation Outline
• Background
– Part I: Thermoelectric Materials
– Part II: Strain Engineering
– Part III: Lead Telluride
• Proposed Research Plan
• Future Work
2

3. Nanomechanics and Multiphysics Lab
Presentation Outline
• Objective
• Background
– Part I: Thermoelectric Materials
– Part II: Strain Engineering
– Part III: Lead Telluride
• Proposed Research Plan
• Future Work
3

4. Nanomechanics and Multiphysics Lab
Background: Applications
• We can take advantage of the “Seebeck Effect”
and use the heat generated to create electrical
current
4
Example: Satellite Example: Car Exhaust
(1) http://www.spacetoday.org/
(2) http://www.gizmag.com/

5. Nanomechanics and Multiphysics Lab
Background: Applications
• Thermoelectric Generators is an example of a
thermoelectric material exhibiting the “Seebeck
Effect”
5
• Using p and n type semiconductors
• Connected electrically in series thermally in
parallel
• Quiet, Reliable, Cheap, Durable
• Potential for heat reclamation in car
exhaust systems
• VERY INEFFICIENT

6. Nanomechanics and Multiphysics Lab
Background: Thermoelectric Materials
6
Temperature
Gradient
Electrical
Potential
Materials that exhibit a
change in temperature
can create an electrical
potential
Materials that exhibit a
change in electrical
potential can generate a
temperature difference
Known as Seebeck
Effect
Known as Peltier
Effect
http://www.thermoelectrics.caltech.edu/

7. Nanomechanics and Multiphysics Lab
Background: Seebeck Effect
7
http://www.thermoelectrics.caltech.edu/
Hot
Cold
NP
• Discovered by Thomas Seebeck in 1821
• Hot and Cold side
• Electron build up causes electric potential
• Voltage drop is the result
Holes Electrons

8. Nanomechanics and Multiphysics Lab
• Thermal Efficiency equation describes the
maximum efficiency of thermoelectric materials
Background: Thermal Efficiency
8
𝑧𝑇 =
𝑆2
𝜎𝑇
𝑘
S – Seebeck coefficient (add units)
σ– Electric conductivity (add units)
T – Absolute temperature
k – Thermal conductivity
zT – Figure of merit
• Part of my goal is to increase
the zT of a material
• Typical zT <1
G. Jeffrey Snyder et. Al. :complex thermoelectric materials. Nature publishing group February
2008

9. Nanomechanics and Multiphysics Lab
Background: Thermal Transport
• Our goal is to optimize the properties of
thermoelectric materials by specifically
improving the thermal transport of the material
9
PbTe
Strategies to improve the
Figure of Merit (zT)
New Material Design
Nanostructuring/ Interface
Engineering
Alloying Nanoinclusions
Nanocrystalline grain structure
Heterostructures

10. Nanomechanics and Multiphysics Lab
Presentation Outline
• Objective
• Background
– Part I: Thermoelectric Materials
– Part II: Strain Engineering
– Part III: Lead Telluride
• Proposed Research Plan
• Future Work
10

11. Nanomechanics and Multiphysics Lab
Background: Strain Engineering
• Strain engineering is a technique used to improve the
performance of materials
• Using strain engineering to improve the performance of
the thermoelectric material, PbTe
11
Strain Engineering can be used for and applied to:
• Influence the properties of a
material
• Tune to specific parameters
• Effect the carrier mobility and
band gap of materials
• Nanocrystalline &
Nanostructured Materials
• Semiconductors
• Thermoelectrics

12. Nanomechanics and Multiphysics Lab
Background: Current Methods
• Current method of strain engineering
12
Tension
Compression
Compression
Tension
Lattice match
Dislocation +
Defect Trap
Relaxation
Lattice Mismatch
EpilayerSubstrate

13. Nanomechanics and Multiphysics Lab
Background: Four Key of Strain Engineering
• The implementation of strain engineering can be
classified by four processes
13
This process will be further outlined and applied to our proposed process
Ju Li et. al. “Elastic strain engineering for unprecedented materials properties”
Materials research Socciety February 2014 vol 39
Synthesizing
Load Bearing
Nanostructures
Applying Force
to the Material
Measuring
Strain
Prediction of
Strain Effect

14. Nanomechanics and Multiphysics Lab
Background: Characterizing Strain Engineering
• Relating strain engineering to the figure of merit (zT)
14
Small Grain
𝒛𝑻 =
𝑺 𝟐 𝝈𝑻
𝒌
Electric Conductivity
Thermal Conductivity
Phonon
Large Grain
Electron

15. Nanomechanics and Multiphysics Lab
Presentation Outline
• Objective
• Background
– Part I: Thermoelectric Materials
– Part II: Strain Engineering
– Part III: Lead Telluride
• Proposed Research Plan
• Future Work
15

16. Nanomechanics and Multiphysics Lab
Background: Lead Telluride
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• Narrow gap material
• Rock Salt Structure (NaCl)
• Is optimum for mid-temperature application
• Operates in the temperature range of 500k-
900 K
• Has shown to have a maximum zT of 2
1. http://www.webelements.com/
2. Y. Q Cao et. al. “Low thermal conductivity and improved figure of merit in fine-grain binary PbTe
thermoeletric alloys

17. Nanomechanics and Multiphysics Lab
Presentation Outline
• Objective
• Background
– Part I: Thermoelectric Materials
– Part II: Strain Engineering
– Part III: Lead Telluride
• Proposed Research Plan
• Future Work
17

18. Nanomechanics and Multiphysics Lab
Nanofab : Photolithography and Sputtering
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Synthesizing
Load Bearing
Nanostructures
Applying Force
to the Material
Measuring
Strain
Prediction of
Strain Effect

19. Nanomechanics and Multiphysics Lab
Proposed Research: Nanofab Process
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Step 1:
Create Mask
Design
Step 2:
Use photolithography
to transfer pattern
(frontside and
backside)
Step 3:
DRIE Etch
Step 4:
Hydro Fluoric (HF)
Vapor Etch
Specimen and MEMS
Device Ready for
Experimentation and
Analysis

20. Nanomechanics and Multiphysics Lab
Nanofab: Mask
• L-edit Mask Design
20
Backside Alignment
MEMS Device
Mask with MEMS Device

21. Nanomechanics and Multiphysics Lab
Nanofab: Process Flow
21
Photo Resist Substrate PbTeSilicon Oxide
MASK
MASK
Deep Reactive
Ion Etching

22. Nanomechanics and Multiphysics Lab
Nanofab: MEMS Device and Experiment
22
Synthesizing
Load Bearing
Nanostructures
Applying Force
to the Material
Measuring
Strain
Prediction of
Strain Effect

23. Nanomechanics and Multiphysics Lab
Experimental Setup
23
Tensile Strain
Applied Current

24. Nanomechanics and Multiphysics Lab
MEMS Device
24

25. Nanomechanics and Multiphysics Lab
Experiment and Analysis
25
Synthesizing
Load Bearing
Nanostructures
Applying Force
to the Material
Measuring
Strain
Prediction of
Strain Effect

26. Nanomechanics and Multiphysics Lab
Raman Spectroscopy
26
• A laser is focused on to the sample
• This excites and scatters the
phonons across the material
• Raman light reflected and collected
• Measure the total phonon
scattering to understand thermal
conductivity and strain being
applied
http://chemie.uni-paderborn.de/

27. Nanomechanics and Multiphysics Lab
Prediction of Strain
27
Synthesizing
Load Bearing
Nanostructures
Applying Force
to the Material
Measuring
Strain
Prediction of
Strain Effect

28. Nanomechanics and Multiphysics Lab
Presentation Outline
• Objective
• Background
– Part I: Thermoelectric Materials
– Part II: Strain Engineering
– Part III: Lead Telluride
• Proposed Research Plan
• Future Work
28

29. Nanomechanics and Multiphysics Lab
Future Work
• 3 omega method to measure the eletrical
conductivity
• Use 4 probe method to measure the thermal
conductivity
29

30. Nanomechanics and Multiphysics Lab
Proposed Research Timeline
30
2013-14 2014-15 2015-2016 2016-2017
Su Fa W Spr Su fa w Spr Su Fa W Spr Su Fa W Spr
Phase
1
Phase2
phase3

31. Nanomechanics and Multiphysics Lab
Acknowledgements
• Nanomechanics and Multiphysics Lab
– Principal Investigator Prof. Sandeep Kumar
– Mr. Devil Garcia
• Nanofabrication Facility @ UCR & UCSD
– Mr. Mark Heiden
– Mr. Dexter Humphrey
– Other names from UCSD
• Oral Prelim Committee
– Principal Investigaor Prof. Sandeep Kumar
– Prof. Lorenzo Mangolini
– Prof. Javier E. Garay (double check middle initial)
– Prof. Masaru P. Rao
31
GEM Fellowship
Award Year 2014

32. Nanomechanics and Multiphysics Lab
Questions?
32

33. Nanomechanics and Multiphysics Lab
Background: PbTE response to Temp
• At Temperature range 400 C – 600 C Dramatic
Increase in zT
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34. Nanomechanics and Multiphysics Lab
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35. Nanomechanics and Multiphysics Lab
Sputtering and Liftoff
35