THERMAL CONDUCTIVITY: A perspective from Nanotechnology Diego A Gomez-Gualdron Seminar II Nanotechnology CHEN 689-601 Texas A&M University April 13 th.

THERMAL CONDUCTIVITY: A perspective from Nanotechnology

Diego A Gomez-GualdronSeminar II

Nanotechnology CHEN 689-601Texas 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

3

RelevanceUnsuitable values of thermal conductivity might render a new material useless for an application.

POWER DISSIPATION

INSULATIONTHERMO

ELECTRICITY

HEAT EXCHANGE

FLUIDS

4

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

5

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

6

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

www.iav.com

THE FIGURE OF MERIT

Materials with a high Seebeck coefficient (S=∆V/∆T) are needed

Also a low thermal and a high electrical conductivity would be ideal

7

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

8

Preliminary Approaches:

INSULATION

bricks asbestos fiber glass

Evolution of new materials from ceramics to modern composites

www.wikipedia.com www.scrapetv.com www.coolandquiet.com

9

Preliminary Approaches

POWER DISSIPATION

BJT transistorvacuum tube CMOS technology

Changes in the electronics technology rather than in cooling methods

www.noveltyradiocom www.digitalcounterproducer.comwww.solarbotics.com

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THERMOELECTRICS

Thermoelectric Module Radioactive heating

Not much interest until the 90’s, because of conflicting characteristics of materials (figure of merit)

www.thermoelectrics.caltech.edu


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HEAT EXCHANGE Playing with the design equation Q=UA (Ti-To) and making heat

integration

www.cerematec.com

Microchannel heat exchangerHelically baffled heat exchanger

www.alltecho.co.uk

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Contextualization

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

Nanotechnology-based revolution!!!

Computational and experimental resources to determine k at the nanolevel

www.salaswildthoughts.blogspot.com

14

Current Research: Nanotechnology

Aerogels/Insulation

www.boingboing.net

Deionized water prior to(left) and after (right)dispersion of Al2O3

nanoparticles

Oil prior to (left) andafter (right) evaporationof Cu nanoparticles

Nanofluids/Heat Exchange

www.kostic.niu.edu

Reduce k Increase k

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Current Research: Nanotechnology

Thin Film/Thermoelectrics

Nature Materials (2008) Vol 7, 105

MEMS/Power Dissipation

Nature Nanotechnology(2008) Vol 3, 275

Reduce k

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Emphasis: Polymer Industry

www.wikipedia.com

www.epotek.com

www.batchglow.co.uk

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Motivation: Polymer Industry

One of the most pervasive materials in modern society

• Ease of processing and versatility

• Attractive for the development of new materials

• Integral part of high-tech applications

Bayern chemical Plant, Baytown, Texas


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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 compoundsFree Electrons

Metal Atoms

HOT REGION

Strong vibration

High Kinetic Energy Electrons

Interaction between energetic

electron and atom

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

A Diamond lattice

HOT REGION

Strong vibration

Vibrational excitation being transmitted

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Heat is transferred through lattice vibrations

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Phonons are quantized analogous to the vibrations of a guitar string

www.wikipedia.com

L

k=1/3(CV v l)

Heat capacity

Phonon velocity (sound speed)

Mean free path length

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Imperfections in the structure enhance phonon scattering and decrease k

Scattering point

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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 SimulationThe Green-Kubo expression for thermal conductivity

is widely used

k= V ∫dt <JQ(t)JQ(0)>kBT2

www.zeolites.nqs.northwetern.edu

• Force Field defining potential energy

• Instantaneous velocities related to kinetic energy

• Sometimes and external field

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Thermal Conductivity Design

Serial Resistances

www.boingboing.net

Analogy with electric circuits with R ~ 1/kAerogel structure

www.aip.org

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Analogy with electric circuits with R ~ 1/k

Parallel Resistances

www.ntu.edu.vn

31


Altering the value of the resistances…

www.chemistryland.com

Incr

ease

res

ista

nce

Adding defects


decr

ease

res

ista

nce

Improving crystallinity

32

PART III

CASE STUDY:A Polymer more conductive than metal

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Alternative work: Polymer Composites

Embedding thermally conductive nanostructures in a polymeric matrix

Nature (2007), Vol 447, p. 1066 www.physorg.news

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TEM image of a composite

Alternative work:Carbon Nanotube Conductivity

Phys. Rev. Let. (2000) Vol 84, p. 4663

Molecular simulations reveal a thermal conductivity of ~ 104 W/m.K

Nanotube (10,10)

Green-Kubo relation

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Alternative Work: Nanotube-Polymer Composites

Ideal structure model

An effort to conduct through the nanotube network instead of the polymer matrix

TEM side view

Adv. Mat. (2005) Vol 17, p. 1562

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Alternative Work:Nanotube-Polymer Composites

Even the most promising results only enhance 6.5 W/m.K

Adv. Mat. (2005) Vol 17, p. 1562

38

Preliminary Work: Conduction in Molecular Chains

Experimental work shows ultrafast thermal transport in self-assembled molecules

Self-assembly

Set-up schematics

Science (2007) Vol 317, p. 787

• Sample is heated with a pulsed laser

• Sum Frequency Generation (SFG) spectroscopy is performed

Summary

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Heat is transferred in a time frame of picoseconds

Molecular excitations Heat transfer

Science (2007) Vol 317, p. 787

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Science (2007) Vol 317, p. 787

Molecular Dynamics results provide further inside

Thermal Disorder after 10 ps

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Preliminary work:Thermal Conductivity of Polymer ChainsPolyethylene chains were shown to have k in

the order of 103 W/m.KThermal conductivity for different

domain sizes

Phys. Rev. Let. (2008) Vol 101, p. 235502

Polyethylene chain

42

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

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Featured Paper:Synthesis Procedure

Fiber Drawing Schematics a) Polyethylene gel preparation

b) Gel sample heating

c) Tungsten tip contact wit gel

d) Tungsten tip withdrawing

e) Microscope inspection

f) Secondary heating activatedNature nanotechnology (2010), Vol. 5, p. 251

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Featured Paper:Nanostructure Changes

Molecular chains are expected to align, thus approaching the ideal case of a thermal transport on a single chain

nanostructure in gel sample nanostructure in nanofiber

Nature Nanotechnology (2010), Vol. 5, p. 251

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Featured Paper:Nanostructure Changes

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

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Featured Paper:Thermal Conductivity Measurements

Measurement Setupa) 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

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Featured Paper:Thermal Conductivity Results

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 ResearchExploration of fillers that reduce thermal contact

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Design of processes exploiting 1-D heat transport

Electrical Component HEAT SINK

Q

Nanofiber

Nanofiber

Thermal Contact

Questions?

53

G4Rebuttal: 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

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 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”

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

G1Review: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 .

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.

http://images.iop.org/objects/ntw/news/7/3/21/070321-right.jpg

G2Review: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 polymernanofiber in ‘networks’ for potential applications, because it’s desired not loosing the outstanding 1-D thermal conductivity of a single nanofiber.

Alfredo D. Bobadilla

REVIEW:THERMAL

CONDUCTIVITYG4 PRESENTATION

Mary Coan, G3

Chemical Engineering

Review Defines Thermal Conductivity and it’s

applicationsNew Nanostructure Materials

○ PolymersStructural ReinforcementIncrease Electrical ConductivityIncrease of Thermal Conductivity

- Polyethylene Nanofibres

DefinedElectron Heat TransportPhonon 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 compositesEmbedded 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 NanofibersSynthesis

○ Nanostructure Changes as Nanofiber is pulledThermal Conductivity Measured

○ K = 110 W/(m*K)Higher than most pure metals

Challenges○ Understand Mechanisms, Scale Up, Uniformity issues

Future work was discussed

G5Review: 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.

G6Review: 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.

THERMAL CONDUCTIVITY: A perspective from Nanotechnology Diego A Gomez-Gualdron Seminar II Nanotechnology CHEN 689-601 Texas A&M University April 13 th.

Documents

heat integration

poor thermal conductivity

heat sink materials

microchannel heat exchanger

baffled heat exchanger

heat exchanger sizes

heat fouriers law

vast quantity of heat