BDavisprelimPPT

Strain Engineering of Thermal Transport in Nanocrystalline

Materials

Brandon N. DavisPhD Student

Department of Mechanical Engineering

Oral Preliminary ExamMay 14, 2014

Advisor: Prof. Sandeep KumarCommittee: 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

3



• Objective• Background– Part I: Thermoelectric Materials– Part II: Strain Engineering– Part III: Lead Telluride


4


Background: Applications

• We can take advantage of the “Seebeck Effect” and use the heat generated to create electrical current

Example: Satellite Example: Car Exhaust

(1) http://www.spacetoday.org/(2) http://www.gizmag.com/

5


Background: Applications

• Thermoelectric Generators is an example of a thermoelectric material exhibiting the “Seebeck Effect”

• 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


Background: Thermoelectric Materials

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


Background: Seebeck Effect

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


• Thermal Efficiency equation describes the maximum efficiency of thermoelectric materials

Background: Thermal Efficiency

𝑧𝑇= 𝑆2𝜎𝑇𝑘

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


Background: Thermal Transport

• Our goal is to optimize the properties of thermoelectric materials by specifically improving the thermal transport of the material

PbTeStrategies to improve the Figure of Merit (zT)

New Material DesignNanostructuring/ Interface

Engineering

Alloying NanoinclusionsNanocrystalline grain structure

Heterostructures

10





11


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

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


Background: Current Methods

• Current method of strain engineeringTension

Compression

Compression

Tension

Lattice match

Dislocation + Defect Trap

Relaxation

Lattice Mismatch

EpilayerSubstrate

13


Background: Four Key of Strain Engineering

• The implementation of strain engineering can be classified by four processes

This process will be further outlined and applied to our proposed processJu 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


Background: Characterizing Strain Engineering

• Relating strain engineering to the figure of merit (zT)

Small Grain

𝒛𝑻=𝑺𝟐𝝈𝑻𝒌

Electric Conductivity

Thermal ConductivityPhonon

Large Grain

Electron

15





16


Background: Lead Telluride

• 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





18


Nanofab : Photolithography and Sputtering


Nanostructures


Measuring Strain


19


Proposed Research: Nanofab Process

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


Nanofab: Mask

• L-edit Mask Design

Backside Alignment

MEMS DeviceMask with MEMS Device

21


Nanofab: Process Flow

Photo Resist Substrate PbTeSilicon Oxide

MASK

MASK

Deep Reactive Ion Etching

22


Nanofab: MEMS Device and Experiment


Nanostructures


Measuring Strain


23


Experimental Setup

Tensile Strain

Applied Current

24


MEMS Device

25


Experiment and Analysis


Nanostructures


Measuring Strain


26


Raman Spectroscopy

• 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


Prediction of Strain


Nanostructures


Measuring Strain


28





29


Future Work

• 3 omega method to measure the eletrical conductivity

• Use 4 probe method to measure the thermal conductivity

30


Proposed Research Timeline

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


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

GEM Fellowship Award Year 2014

32


Questions?

33


Background: PbTE response to Temp

• At Temperature range 400 C – 600 C Dramatic Increase in zT

34


35


Sputtering and Liftoff

Documents

BDavisprelimPPT