THERMAL CONDUCTIVITY:
A perspective from Nanotechnology
Diego A Gomez-Gualdron
Seminar II
Nanotechnology CHEN 689-601
Texas A&M University
April 13th 2010
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PART I
INTRODUCTION & CONTEXTUALIZATION
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Definition
The thermal conductivity relates to the ability
of a material to transfer heat
Fourier’s law
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Relevance
Unsuitable values of thermal conductivity might
render a new material useless for an application.
POWER
DISSIPATION
THERMO
ELECTRICITY
INSULATION
HEAT
EXCHANGE
FLUIDS
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Overview: Power Dissipation
 Decrease in size of electronic devices requires ingenuous ways
to dissipate heat and protect the device components structure and
performance
THINGS TO LOOK FOR:
 Good thermal contact between
components and heat sink
 Materials with high thermal
conductivity and low coefficient of
thermal expansion
www.epotek.com
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Overview: Insulation
 The basic principle is the protection of a system from the harsh
(hot or cold) conditions in a neighboring region, while fulfilling
additional requirements
A MATTER OF COMPROMISE
 Space suits require insulating
materials, while being light enough to
be handled by the astronaut
 Skylights
require
insulating
characteristics, while allowing light to
pass through
www.wikipedia.com
www.mygreenhomeblog.com
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Overview: Thermoelectricity
 In many technologies a vast quantity of heat is eliminated as
waste. Nonetheless, the efficiency of the process would be much
higher if some of the heat were transformed into electricity
THE FIGURE OF MERIT
 Materials with a high Seebeck
coefficient (S=∆V/∆T) are needed
www.iav.com
Also a low thermal and a high
electrical conductivity would be ideal
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Overview: Heat-Exchange Fluids
 Conventional heat-transfer fluids have inherently poor thermal
conductivity compared to solids. Several industries would benefit from
increasing their thermal conductivity to reduce heat exchanger sizes
and pumping needs
TO HAVE IN MIND
 High thermal conductivity
 Low friction coefficient
 Clogging of microchannels is undesired
 Lubricating behavior is a plus
www.engadget.com
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Preliminary Approaches:
INSULATION
 Evolution of new materials from ceramics to modern composites
bricks
www.wikipedia.com
asbestos
www.scrapetv.com
fiber glass
www.coolandquiet.com
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Preliminary Approaches
POWER DISSIPATION
 Changes in the electronics technology rather than in cooling methods
vacuum tube
www.noveltyradiocom
BJT transistor
www.solarbotics.com
CMOS technology
www.digitalcounterproducer.com
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Preliminary Approaches
THERMOELECTRICS
 Not much interest until the 90’s, because of conflicting characteristics
of materials (figure of merit)
Thermoelectric Module
Radioactive heating
www.thermoelectrics.caltech.edu
www.thermoelectrics.caltech.edu
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www.thermoelectrics.caltech.edu
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Preliminary Approaches
HEAT EXCHANGE
 Playing with the design equation Q=UA (Ti-To) and making heat
integration
Helically baffled heat exchanger
www.alltecho.co.uk
Microchannel heat exchanger
www.cerematec.com
Contextualization
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Nanotechnology-based revolution!!!
 The intelligent design of the nanostructure of a material can provide
all the desired properties, including the thermal conductivity
REQUIREMENTS
 Understanding the heat transfer phenomena
at the molecular level
 Modification of the structure of the material
accordingly
 Computational and experimental resources
to determine k at the nanolevel
www.salaswildthoughts.blogspot.com
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Current Research: Nanotechnology
Aerogels/Insulation
Reduce k
www.boingboing.net
Nanofluids/Heat Exchange
Increase k
www.kostic.niu.edu
Deionized water prior to Oil prior to (left) and
after (right) evaporation
(left) and after (right)
of Cu nanoparticles
dispersion of Al2O3
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Current Research: Nanotechnology
Thin Film/Thermoelectrics
MEMS/Power Dissipation
Reduce k
Nature Materials (2008) Vol 7, 105
Nature Nanotechnology(2008) Vol 3, 275
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Emphasis: Polymer Industry
www.epotek.com
www.wikipedia.com
www.batchglow.co.uk
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Motivation: Polymer Industry
 One of the most pervasive materials in modern society
Bayern chemical Plant, Baytown, Texas
• Ease of processing and versatility
• Attractive for the development
of new materials
• Integral part
applications
of
high-tech
Nature Materials (2008) Vol 7, 261
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Research Status: Polymer Industry
 Structural Reinforcement
 Increase of Electrical
Conductivity
 Increase of Thermal
Conductivity
www.silmore.cn
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PART II
THEORETICAL BACKGROUND
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Mechanism: Electron Heat Transport
 Characteristic of metallic compounds
High Kinetic
Energy Electrons
Strong vibration
Free Electrons
Metal Atoms
Interaction
between
energetic
electron and
atom
HOT REGION
Increased vibration
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Mechanism: Electron Heat Transport
 Very effective heat transport mechanism
 Characterized by electron mean free path
Not so sensitive to lattice defects
Typically 20-400 W/m.K
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Mechanism: Phonon Heat Transport
 Characteristic of most compounds
Vibrational excitation being
transmitted
Strong vibration
HOT REGION
A Diamond lattice
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Mechanism: Phonon Heat Transport
 Heat is transferred through lattice vibrations
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Mechanism: Phonon Heat Transport
 Phonons are quantized analogous to the vibrations
of a guitar string
L
Phonon velocity (sound speed)
k=1/3(CV v l)
Heat capacity
www.wikipedia.com
Mean free path
length
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Mechanism: Phonon Heat Transport
Imperfections in the structure enhance phonon
scattering and decrease k
Scattering point
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Mechanism: Phonon Heat Transport
Not as efficient as electron heat transport
Characterized by phonon free path and velocity
Very sensitive to defects (e.g. amorphous
structure of polymers)
Typical values range from 0.01-50 W/m.K
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Molecular Simulation
The Green-Kubo expression for thermal conductivity
is widely used
k= V ∫dt <JQ(t)JQ(0)>
kBT2
• Force Field defining potential energy
• Instantaneous velocities related to kinetic energy
• Sometimes and external field
www.zeolites.nqs.northwetern.edu
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Thermal Conductivity Design
Analogy with electric circuits with R ~ 1/k
Serial Resistances
www.boingboing.net
Aerogel structure
www.aip.org
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Thermal Conductivity Design
Analogy with electric circuits with R ~ 1/k
Parallel Resistances
www.ntu.edu.vn
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Thermal Conductivity Design
 Altering the value of the resistances…
Improving crystallinity
Increase resistance
decrease resistance
Adding defects
Nature Materials (2008) Vol 7, 105
www.chemistryland.com
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PART III
CASE STUDY:
A Polymer more conductive than metal
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Alternative work:
Polymer Composites
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 Embedding thermally conductive nanostructures
in a polymeric matrix
TEM image of a composite
www.physorg.news
Nature (2007), Vol 447, p. 1066
Alternative work:
Carbon Nanotube Conductivity
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 Molecular simulations reveal a thermal conductivity
of ~ 104 W/m.K
Nanotube (10,10)
Green-Kubo relation
Phys. Rev. Let. (2000) Vol 84, p. 4663
Alternative Work:
Nanotube-Polymer Composites
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 An effort to conduct through the nanotube
network instead of the polymer matrix
Ideal structure model
TEM side view
Adv. Mat. (2005) Vol 17, p. 1562
Alternative Work:
Nanotube-Polymer Composites
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 Even the most promising results only enhance 6.5 W/m.K
Adv. Mat. (2005) Vol 17, p. 1562
Preliminary Work:
Conduction in Molecular Chains
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 Experimental work shows ultrafast thermal transport
in self-assembled molecules
Set-up schematics
Self-assembly
Summary
• Sample is heated with a
pulsed laser
• Sum Frequency Generation
(SFG)
spectroscopy
is
performed
Science (2007) Vol 317, p. 787
Preliminary Work:
Conduction in Molecular Chains
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 Heat is transferred in a time frame of picoseconds
Molecular excitations
Heat transfer
Science (2007) Vol 317, p. 787
Preliminary Work:
Conduction in Molecular Chains
 Molecular Dynamics results provide further inside
Thermal Disorder after 10 ps
Science (2007) Vol 317, p. 787
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Preliminary work:
Thermal Conductivity of Polymer Chains
Polyethylene chains were shown to have k in
the order of 103 W/m.K
Polyethylene chain
Thermal conductivity for different
domain sizes
Phys. Rev. Let. (2008) Vol 101, p. 235502
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Motivation
Modification of thermal properties in polymers
composites not as good
 Molecular simulations and experiments suggest
high thermal conduction in hydrocarbon chains
 Thermal conductivity enhancement done on
microfibers
Featured Paper:
Synthesis Procedure
Fiber Drawing Schematics
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a) Polyethylene gel preparation
b) Gel sample heating
c) Tungsten tip contact wit gel
d) Tungsten tip withdrawing
e) Microscope inspection
f) Secondary heating activated
Nature nanotechnology (2010), Vol. 5, p. 251
Featured Paper:
Nanostructure Changes
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 Molecular chains are expected to align, thus approaching
the ideal case of a thermal transport on a single chain
nanostructure in gel sample
Nature Nanotechnology (2010), Vol. 5, p. 251
nanostructure in nanofiber
Featured Paper:
Nanostructure Changes
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 The structure achieves crystallinity as confirmed by
diffraction measurements
TEM image of the fiber
Diffraction pattern of the fiber
Nature Nanotechnology (2010), Vol. 5, p. 251
Orthorhombic Structure
Featured Paper:
Thermal Conductivity Measurements
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Measurement Setup
a) Cantilever holds the fiber
b) Fiber cut at 300µm from the tip
c) Loose end joined to thermocouple
d) Thermocouple heated up
e) Cantilever is stimulated
f) Laser picks up the signal
Featured Paper:
Thermal Conductivity Results
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 A thermal conductivity around 110 W/m.K was
achieved. This is higher than for most pure metals!!!
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General Challenges
 Improve uncertainty in measurements
 Understand mechanism in nanostructures
 Trade-off in design of material properties
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Particular Challenges
Structure uniformity along the nanofiber
 Adapt process for future scaling up
Vanish thermal resistance among fibers
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Follow-up Research
 Dependence of fiber structure from process
parameters:
1) Heating rate and strategy
2) Nature of gel preparation
3) Drawing rate
4) Composition
 Is it possible to make ‘Doped’ nanofibers?
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Follow-up Research
 Exploration of fillers that reduce thermal contact
Nanofiber
Thermal Contact
Nanofiber
 Design of processes exploiting 1-D heat transport
Q
Electrical Component
HEAT SINK
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Questions?
G4
Rebuttal: Thermal Conductivity
Diego A. Gómez-Gualdrón
Reviewer G1: “The presenter gave an overview and contextualization of the topic.
However, this part of the talk lasted too long and was a little disorganized and there was
not direct relation with the papers he talk”
A:/ 1) The thermal conductivity is an important parameter in the design of an
overwhelming number of applications and worth of a careful review. The reviewer is
assessing part I as it were and introduction to part III. The three sections of the
presentations are meant to be independent, and were timed accordingly.
2) I invite the reviewer to check the slides again and he will clearly see the following
structure for part I:
a) Definition of thermal conductivity
b) Relevance and fields of application
c) Overview: Power Dissipation → Insulators →
thermoelectricity → Heat Exchange
d) Preliminary Approach: Power Dissipation →
Insulators → thermoelectricity → Heat Exchange
e) Nanotechnology Approach: : Power Dissipation →
Insulators → thermoelectricity → Heat Exchange
Then, It is stated the interest and motivation of manipulating TC in polymers in
particular, and the featured paper is announced
Reviewer G1: “The presenter gave an overview and contextualization of the topic.
However, this part of the talk lasted too long and was a little disorganized and there was
not direct relation with the papers he talk”
A:/
3) There was not direct relation, because the three sections are independent. Part I
reviews the role of thermal conductivity in several fields, and the role that
nanotechnology has started playing in them. Part II visits the theoretical background
needed to be able to understand and manipulate thermal transport at the nanoscale.
Parts III explores the latest progress in manipulating the thermal conductivity of a
material (a polymer in this case) using nanotechnology.
Reviewer G1: “The overall presentation was good, however I think he had the opportunity
of exploiting a little more the topic since there are some recent applications of thermal
transport to create logical circuits using the rectification capability of designed graphene
sheets. This phenomenon opens the possibility to a large variety of applications”
A:/ There has been more than enough examples of nanotechnology applications in
electronics during the class. I can understand that due to the academic background of
the reviewer he prefers focusing on circuits and whatnot. However, I think that for a
class of chemical engineers, an application involving polymers is much more attractive.
Besides, the featured application is a beautiful example how nanotechnology can alter
commonplace conceptions such as polymers being poor thermal conductors.
Reviewer G2: “It would have been good mentioning the reason for the difference on the
nature of main thermal carriers when comparing metals and polymers”
A:/ During the oral presentation, from slides 22 through 24, this was explained. In slide 22 the
graph shows the existence of free electrons in metallic compounds and described a
mechanism based on them. In slide 24, the mechanism in all other compounds (this
includes polymers) is explained. Diamond was used as a example of a material with no
free electrons, hence featuring a phonon-controlled thermal transport
Reviewer G2: “The typical or approximate values of electron and phonon mean free path for
metal and polymers were not mentioned”
A:/ I agree. Here are the values : mean free path of electrons varies between 5-50 Å; mean
free path of phonons varies from 500 to 700 nm
Reviewer G2: “The Green-Kubo expression for thermal transport was mentioned but not well
depicted, neither its relation with Fourier’s law”
A:/ The impact this would have had on the overall presentation is not worth the additional
time needed to go into the mathematical details of the equation. The term autocorrelation
function was briefly explained, as well what the terms of the equation were, and what you
needed to run the simulation. The gist of that slide is that there exists an equation to
calculate the thermal conductivity using molecular simulations
G1
Review:Thermal Conductivity
Edson Bellido
The presenter gave an overview and contextualization of the topic. However, this part of
the talk lasted too long and was a little disorganized and there was not direct relation
with the papers he talk. He talk in the second part about the difference between
electron and phonon heat transport and theoretical background that help to understand
the topic.
He showed some attempts to improve the
thermal conductivity of polymer using
carbon nanotubes. He also showed some
Molecular Dynamics simulations and how
Polyethylene chains were shown to have k
in the order of 103 W/mK. In the actual
paper he described the synthesis of the
nanofibers. He explained how they were
able to measure the thermal conductivity
on the nanofibers that was in the range of
110W/mK .
http://images.iop.org/objects/ntw/news/7/3/21/070321-right.jpg
The overall presentation was good, however I think he had the opportunity of exploiting a
little more the topic since there are some recent applications of thermal transport to
create logical circuits using the rectification capability of designed graphene sheets. This
phenomenon opens the possibility to a large variety of applications.
G2
Review:Thermal Conductivity
Alfredo Bobadilla
Thermal conductivity lecture review
It would have been good mentioning the reason for the difference on the
nature of main thermal carriers when comparing metals and polymers.
The typical or approximate values of electron and phonon mean free path
for metal and polymers were not mentioned.
The Green-Kubo expression for thermal transport was mentioned but not well
depicted, neither its relation with Fourier’s law.
It’s noticeable the effort of the presenter on trying to explain the concepts as
far as possible using graphic illustrations.
It was well emphasized the challenges when trying to integrate the polymer
nanofiber in ‘networks’ for potential applications, because it’s desired not
loosing the outstanding 1-D thermal conductivity of a single nanofiber.
Alfredo D. Bobadilla
Mary Coan, G3
Chemical Engineering
Review

Defines Thermal Conductivity and it’s
applications
 New Nanostructure Materials
○ Polymers
 Structural Reinforcement
 Increase Electrical Conductivity
 Increase of Thermal Conductivity
- Polyethylene Nanofibres

Defined
 Electron Heat Transport
 Phonon Heat Transport
Review

Thermal Conductivity design
 Can be viewed as an electrical series of resistors or
Parallel Resistances
○ Increase defects or Decrease defects to increase or
decrease resistance

Polymer composites
 Embedded thermally conductive nanostructure into
polymer matrix

Nanotube-Polymer Composites
 Uses a Nanotube matrix instead of a Polymer matrix
Review

Conduction through Molecular Chains
 Polyethylene Chains, k = 103 W/(m*K)
○ Addition of nanofibers might help

Polyethylene Nanofibers
 Synthesis
○ Nanostructure Changes as Nanofiber is pulled
 Thermal Conductivity Measured
○ K = 110 W/(m*K)
 Higher than most pure metals
 Challenges
○ Understand Mechanisms, Scale Up, Uniformity issues
 Future work was discussed
G5
Review: Thermal Conductivity
Norma L. Rangel
Thermal Conductivity,
by Diego Gomez-Gualdron
• Diego did an excellent job in his presentation, he
has very good skills that he implements well in his
oral presentations. Very fluent, well prepared,
organized and able to deliver concepts and ideas
to the audience.
• The information presented was highly oriented
for undergraduates and Chemical Engineers, I
understand his motivation to do that but I believe
he underestimated the audience capability to
digest more state of the art and deep
information.
G6
Review: Thermal Conductivity
Jung Hwan Woo
• The preparation was very well organized.
• The oral presentation was also very good. It
flowed very well and was sequenced nicely to
let the audiences to understand the
presentation.
• There were very interesting ideas such as
aerogels.
• The introduction was quite well organized as
the topic was a very broad and hard to gather
and present ideas.