Phonon-Polariton Physics: Thermal Conductivity, Phonon ... · NanoTech Institute University of Texas at Dallas Phonon-Polariton Physics: Thermal Conductivity, Phonon-Polariton Lasers

1

11Anvar A. Zakhidov, University of Texas at Dallas

NanoTech InstituteUniversity of Texas at Dallas

Phonon-Polariton Physics:

Thermal Conductivity, Phonon-Polariton Lasers and

Phonon Transistors in Nanostructures

Anvar Zakhidov on behalf of UTD PEOM Team

“DSRC Nanoscopic Phonon Engineering Workshop”

Booz Allen Hamilton, Arlington, May 9, 2005


Outline :

Motivation for Tuning Phonon K(T) in Our 2 Main Systems.

System 1.

Phonon-Polaritons: New Mechanism of Thermal Conductivity PhononPhonon--PolaritonPolariton LasersLasers

Overview of PhononOverview of Phonon--PolaritonicsPolaritonics

System 2.

Carbon Nanotube Yarns and Sheets for Enhanced Thermal ConductivityPhonon Transistor with Charge Injection GatePhonon Transistor with Charge Injection Gate

Report Documentation Page Form ApprovedOMB No. 0704-0188

Public reporting burden for the collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering andmaintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information,including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway, Suite 1204, ArlingtonVA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to a penalty for failing to comply with a collection of information if itdoes not display a currently valid OMB control number.

1. REPORT DATE 09 MAY 2005

2. REPORT TYPE N/A

3. DATES COVERED -

4. TITLE AND SUBTITLE Phonon-Polariton Physics: Thermal Conductivity, Phonon-PolaritonLasers and Phonon Transistors in Nanostructures

5a. CONTRACT NUMBER

5b. GRANT NUMBER

5c. PROGRAM ELEMENT NUMBER

6. AUTHOR(S) 5d. PROJECT NUMBER

5e. TASK NUMBER

5f. WORK UNIT NUMBER

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) NanoTech Institute University of Texas at Dallas

8. PERFORMING ORGANIZATIONREPORT NUMBER

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES) 10. SPONSOR/MONITOR’S ACRONYM(S)

11. SPONSOR/MONITOR’S REPORT NUMBER(S)

12. DISTRIBUTION/AVAILABILITY STATEMENT Approved for public release, distribution unlimited

13. SUPPLEMENTARY NOTES See also ADM001801., The original document contains color images.

14. ABSTRACT

15. SUBJECT TERMS

16. SECURITY CLASSIFICATION OF: 17. LIMITATION OF ABSTRACT

UU

18. NUMBEROF PAGES

33

19a. NAME OFRESPONSIBLE PERSON

a. REPORT unclassified

b. ABSTRACT unclassified

c. THIS PAGE unclassified

Standard Form 298 (Rev. 8-98) Prescribed by ANSI Std Z39-18

2


Our Main Systems and Materials:

1. New Mechanism of Phonon-Polariton Thermal Conductivity

2. Carbon Nanotubes with Enhanced K(T) CNT in CNT Yarns and

Oriented CNT bucky-aerogels


Our Main Concepts: “Solid State Heat Pipes”And “Phonon Transistor or Valve”

Two types of heat pipes:1. We predict an analog of optical fiber for heat transfer by lightmixed with optical phonon

Wave-guide in which Phonon-Polaritoncan propagate like light and contribute to K(T) of low-K organic matter

2. CNT are nanoscale analog of optical fibers in which phonons flow is ballistic and 1-d. Can it be a Heat-pipeHow to preserve this ballistic transport and high K in fibers of CNTs and CNT dispersed in polymers?

Polariton Heat transfer

Giant K(T) = 3000 – 6000 W/mK,Predicted in SWCNT and measured in MWCNTs

a) Conjugated Polymers-CNTfor distributed heat removal

in Organic Electronics

3-+

++

3-+

++

3-+

++3-+

++

3-+

++

3-+

++3-+

++

3-+

++

3-+

++

3-+

++3-+

++

3-+

++

3-+

++3-+

++

Low K Expect High Kupon unbundling

Charging

3


Physical Ideas for Engineering Phonons at Nanoscale for Enhanced thermal conductivity K(T)

1. Optical Phonons dispersion is flat, group velocity small: no contribution to K(T)

2. It can be really engineered: changed to Phonon-Polariton with large Vg and we found sizable contribute to thermal conductivity K(T).

3. In Organic matter a Phonon-Laser is proposed, which can be pumped by heat and produce monochromatic and coherent IR

4. In single Carbon nanotubes phonons are ballistic and Lm ~1 µm, providing high K(T) for one CNT. Our Approach is to achieve and control high K(T) in real systems:

- unbundle tubes: decrease Phonon-Phonon scattering both inter-tube and UP- coat tubes with polymers and mix into a matrix of low-K conjugated

polymer at concentration lower or close to percolation for nano-scale distributed heat removal.

5. Phonon-Transistor concept appeared as a result of tunable K(T)


DoD Needs Better Cooling Systems for Electronic Warfare Chips

4


DARPA has a strong interest in Thermal Management: Previous HERETIC Program and Phonon EngineeringProgram are examples


State-of-Art Heat Removal Circuitry Targets Fluids:Solid State Systems for cooling would be much more attractive for DoD Applications

5


Impact 1: for Heat Management Circuitry for DoD Applications

Goal:Develop micro- and nano-scale solid state heat removal circuitry

with control devices (valves, couplers, switches,etc.) which can be Ultimately integrated with electronic and photonic circuitry.This technology will be unique, since it will enable controllable, adaptable,distributed and programmable thermal management

Impact: Efficient, compact and controllable (with active feedback)

heat removal circuitry will enable design of low power, small form-factorDoD systems, such as radars, high performance computers, and otherelectronic and photonic warfare subsystems


Our Concept: Solid State Cooling Circuitry Integrated on Chip,Heat Pipes (CNTs or “Polaritonic Fibers”

Active Cold Spots (by TEC)

PassiveCNT fibers and sheetsheat pipe in laminate

Novel light weightPolymer-CNT compositeHeat exchanger

Active Cold spots

Phonon-polaritonT-selective heat pipe

e.g. Kmax at 250K

CNT

Chips operating at low T

Phonon valvePolariton coupler

6


Photon-phonon mixed states:Polaritons will be usedfor Radiative heat transferinside thin films and “opticalfibers”

Carbon nanotube 1-d nano-heat-pipes arranged into fibers will be explored

Physical Concepts for Solid State Heat Pipes:1. Heat Transfer by Polaritons in Organic Thin Films 2. Achieve high K(T) of Carbon Nanotubes

with engineered Phonon-phonon interaction


PhononPhonon--PolaritonsPolaritons Contribution to Thermal Contribution to Thermal ConductivityConductivity

Prospects for Organic Phonon-PolaritonHeat Pipes

And Phonon-Polariton Laser

Overview of Polaritonics (MIT, France,)

7


• Known: by phonons, photons, electrons• New: by Phonon-Polaritons

• Bulk and Thin Films (Microcavity)• Ballistic regime: Λ(ω) > d

• Statistical, diffusive regime Λ(ω) << d

• Phonon-Polariton Microcavity Laser

Thermal conductivity

in solis:


Motivation

•Contribution of phonon-polaritons to thermal conductivity has never been estimated

(Klemens P.G. et al. Term. Conduct. (1988), 19, 453 )

•Phonon-polaritons can have very long mean free paths Λ(ω) of mm and even cm – ballistic propagation

•Enhancement of K(T) may be expected in nanostructureswith sizes, d smaller than Λ(ω)

In thick samples K(T) still can have contribution from polaritons

8


What is phonon-polariton?

Phonon + Photon = Phonon Polariton

Polariton is a mixed excitation, has phonon and electromagnetic components, and can propagate with high velocity

+=

ωLO

ωTO


POLARITONS vs PHOTONS

Photons in the Bulk , ε = ε∞ - constant

For Transverse Waves:

q2 e2/ ω2 = ε∞

ω = q e/√ε∞

Density of states Vs ω

Density of States:

DOS ~ q2 dq/(2π)3 ~ ω2 dω

Polaritons: Coherent mixture

of Photon with Phonon

DOS becomes singular at ωT

9


Phonon-Polaritons in Bulk: Dispersion and DOS

photons

Phonon-polarions


PHOTONS: Radiative Contribution to Thermal Conductivity in ballistic regime (Landauer)

φθθωωπ ω ddk

dkdk

ekJ TBkkz

cossin)))(()(1

1())2(

(2 /)(3

2h

h −= ∫

Total current J = JZ (T+∆T) - JZ (T)

3

2)2(26)(

⎟⎟⎠

⎞⎜⎜⎝

⎛≅

∆∞

∞ cTkck

TTJ BB

h

εεπ

10


Ballistic Propagation

[ ]coldhotdqddqqTJ ηηωωθ

πθ

εε

−=

=

∫∞

∞

cos)2(

sin2)(0

3

2

h

dxe

exc

TkckTTJ

x

xBB ∫

∞∞

∞ −⎟⎟⎠

⎞⎜⎜⎝

⎛=

∆ 02

43

2 )1()2()(

h

εεπ

But we have, 26)1(0

2

4

≅−∫

∞

dxe

exx

x

3

2)2(26)(

⎟⎟⎠

⎞⎜⎜⎝

⎛≅

∆∞

∞ cTkck

TTJ BB

h

εεπ


Phonon-Polaritons:Contribution to Thermal Conductivity:

Resonance with Optical Phonon at ω=ω⊥

⎟⎟⎠

⎞⎜⎜⎝

⎛

−

−=⎟

⎟⎠

⎞⎜⎜⎝

⎛

−

−=

⊥∞

⊥

∞22

22||

22

22||

2

22 )( ,

ωωωω

εωεωωωωωε

ck

⎪⎭

⎪⎬⎫

−⎟⎟⎠

⎞⎜⎜⎝

⎛

+−

−−+

+⎪⎩

⎪⎨⎧

−⎟⎟⎠

⎞⎜⎜⎝

⎛

+−

−−⎟⎟⎠

⎞⎜⎜⎝

⎛=

∆

∫

∫∞

⊥⊥

⊥

⊥⊥

⊥∞

∞

⊥

dxe

exxxxxxxxx

dxe

exxxxxxxxx

cTkck

TTJ

x

x

x

x

xxBB

BULK

2

4

222222

2222||

2

4

0222222

2222||

3

2

)1(4)())((

)1(4)())((

)2()(

||δ

δε

επ h

Tkx

Tkx

BB

|||| ,

ωω hh== ⊥

⊥

11


sphotons

Phonon-polarions

GROUPvDOS ×∝Γ )(ω

Major Difference: Density of States


0.3=∞ε

s

⊥

⊥−=

xxx||η

Resulting Enhancement of Heat Conductance

0 50 100 150 200 250 300

0

5

10

15

20

25

30

width=10-2

x⊥=566.4/T

BULK

Without resonance With Resonance

η=0.1 0.5 1.0

G(W

/m2 K

)

T(K)0 50 100 150 200 250 300

1

2

3

4

5

6

7

8

9

10

width=10-2

BULK x=566.4/T

η 0.1 0.2 0.3 0.4 0.5 0.8 1.0

GR

ES/G

NO

NR

ES

T(K)

for parameters of MgO

12


0 100 200 300 400 5001

2

3

4

5

6

7

8

9

10

ω=796cm-1

ε=6.52

η 0.1 0.2 0.4 0.6 0.8 1.0

GR

ES/G

NO

NR

ES

T(K)

Position of the enhancement peak is tunable

For parameters of phonon in SiC


0.0 0.2 0.4 0.6 0.8 1.0165

170

175

180

185

190

Pic

o P

ositi

on (K

)

η

0.0 0.2 0.4 0.6 0.8 1.0

128

132

136

140

144

Pic

o w

idth

(K)

η

The Dependence of T-Peak Position on the Tuning Parameter

13


Comparison of Polaritonic K(T) inSiC and MgO

0 200 400 600 800 10000

1

2

3

4

5

6

7

8

T= 176 K

T= 92 K

BULK

MgO SiC

GR

ES/G

NO

NR

ES

T(K)


Depends on:•Frequency ωTO• Line width δω

•Polaritonic gap ∆0 200 400 600 800 1000

1.0

1.2

1.4

1.6

1.8

2.0

2.2

ε=4.0

η=0.1

wres=10-2

w (cm-1) 100 394 796 1000 1500

GR

ES/

GN

ON

RE

S

0 200 400 600 800 1000

1.0

1.2

1.4

1.6

1.8

2.0

2.2

wres=10-5

η=0.1

0 200 400 600 800 10000

4

8

12

16

20

T(K)

η=1.0

0 200 400 600 800 10000

4

8

12

16

20η=1.0

To get a peak at RT, we need optical phonons with ωTO =1500 cm-1

Position and intensity of the enhancement peak is tunable

14


Thin Films: Radiative contribution to thermoconductivity

L

∞

==ε

πωL

cq )0(

Film thickness

L = λTO /2


Dispersion of Cavity polariton

15


GROUPMC vDOS ×∝Γ )(ω


Thin Films: Phonon-polariton contribution

to thermal conductivity

Thin films = Planar waveguides = Planar microcavities

16


Thin films: resulting enhancement of thermal conductivity

(Again from difference in DOS)

0 100 200 300 400

0

5

10

15

20

Resonance Non-Resonance

G(W

/m2 K

)

T(K)

MgO


0 100 200 300 400 500 600 700 800

0

20

40

60

80

100

120

140

ε=3.0x=566.7/T

wres=10-2

MC

GR

ES /G

NO

NR

ES

T(K)

Relative Enhancement of Heat Conductanceby Polaritonic Effect in

MICROCAVITY: The ratio of G res to G nonres

0.3=∞εMgO

17


Thick samples: Λ(ω) < dDiffusive heat transfer


Specific Heat C(ω)

18


Mean free path Λ(ω) of Phonon-Polaritons:


Check of Approximation for Λ(ω)

in MgO

19


Polaritonic K(T) in kinetic limit


Comparison of Polaritonic contribution with experimental K(T)

20


Promise of Fullerene M6C60 for Polaritonic K(T)

F4u optical IR mode is a great candidate forOP-Polariton with tunable K(T).

The intensity of optical absorption increase88 times (!) upon doping x=6 electrons in C60

Oscillator strength S ~ x2, increases dramatically,

Polariton Gap ( TO-LO splitting): ∆ ~ S1/2 , So that the parameter η ~ x

Polaritonc Thermal conductivity K(T) ~ η ~ x,

K becomes tunable by doping level x.K can be increased x times ( 6 in measured case)


1.1711.9660304LiF

0.7241.72424246NaF

1.0832.75425204LiCl

0.7191.85330192KF

1.0463.16354173LiBr

0.5982.31262164NaCl

0.8311.93293160RbF

0.5002.16216144KCl

0.5562.63210135NaBr

0.4962.14178119RbCl

0.5473.03181117NaI

0.4482.34168116Kbr

0.3982.59144103K-I

0.6604.04171103AgCl

0.6794.6213681AgBr

0.3732.6210375RbI

1.5165.116164TlCl

1.3755.4111448TlBr

ηε∞ω|| (cm-1)ω⊥ (cm-1)Material

⊥

⊥−=

ωωω

η ||

Map of materialsto chooseω and η

21


0.2196.52970796SiC

0.8252.956719394MgO

0.0989.1402366GaP

0.2745.2349274ZnS

0.303--305234CdS

0.8052.05464257CaF2

0.03614.4233225GaSb

0.7832.07387217SrF2

0.15712.3243210InAs

0.2213.71210172CuCl

0.8812.4380202CdFr

0.2415.9252203ZnSl

0.8262.16336184BaF2

0.11715.7200179InSb

0.1757.3208177ZnTl

0.251--214171CdSl

0.2217.2171140CdTl

ηε∞ω|| (cm-1)ω⊥ (cm-1)Material

Map of materialsto chooseω and η


Idea of IR Phonon-Lasers: Condensation of phonon-polaritons in planar

microcavities at Room-T•Exciton-polariton condensation has been recently demonstrated

in semiconductor MC.

•If condensation of lowest energy cavity phonon-polaritonsis achievable, IR lasing would

become possible with zero pumping threshold.

•The life-time of phonon-polaritons is a crucial parameter:

tlifetime > trelaxation

is needed

Lasing at KII =0

22


New Concepts:“Phonon-Polariton Laser”

1. We propose: Phonon-Polariton laser in the microcavity, pumped by heating

Polariton Heat transfer

Heater

T ~ 300 C

coolerHeat transfer by Polaritonic K

Mirror semi-transparent

Microcavity

Bose condensate ofPhonon-Polaritons

IR lasing beamwith ω = Ω op - ∆


Phonon-Polariton Laser• Polariton Laser - The polariton laser is a relatively new lasing

mechanism postulated by several research groups and recently observed in GaAs at cryogenic temperatures by a Japanese researcher visiting in the US. The laser operates on the principle of Bose-Einstein condensation of excitonic polaritons within microcavities to align phase, with radiation from the condensate. Theoretical calculations by Professor KavokinЃfs group indicate that the same opto-electronic effects will be possible in wide-bandgap semiconductors at room temperature. Their calculations show that the kinetic blocking of polaritonrelaxation preventing formation of the B-E condensation of polariton phase at low temperatures should disappear at higher temperatures. These lasers have very low threshhold currents, are very efficient, produce very little heat, and should have applications in very low power optical communications and optical computing.

23


European Activities

• New Laser Principle for Low Power and Fast Optoelectronic Devices:Dr. Harvey, of ARL-ERO, visited Blaise Pascal University, FR, to discuss new room temperature lasing mechanism based on Bose-Einstein condensation in wide-bandgap semiconductor microcavities. This opens the door to very low power laser communications, THz optical signal processing, quantum computing, spintronic devices, THz modulation in photonic bandgap structures, and THz electronic signal processing. Such devices will enable the communication and processing of the massive amounts of data necessary to support FCS concepts. The Physics Dept at, Blaise Pascal University, has a very strong theoretical program and a good experimental program which is focusing on electron-light interactions in semiconductors, in particular excitonic polariton effects in semiconductor microcavities. Professor Kavokin leads a European collaboration funded by the European Community and focused on phenomena in semiconductor microcavities under the EC Framework program on “High Technology for Communications and Information Processing”.


Polariton Transistor

• THz Excitonic Polariton Transistor and THz Optoelectronic Devices - The Blaise Pascal group has shown theoretically and experimentally that the excitonic polaritons can be accelerated in the plane of microcavities, with the gradient of the microcavitythickness acting as the forcing function. Observed velocities are one to two orders of magnitude faster than the electronic ballistic transport in bulk semiconductors, with the potential for using the polaritons as carriers in a very fast transistor type device and for ultrafast optical processing.

•

24


Polaritonics: bridging the gap between electronics and photonics

• Between electronics and photonics there exists a frequency gap of approximately 2 octaves, i.e. the frequency range between 100~GHz and 10~THz. Here we demonstrate that phonon-polaritons in ferroelectric crystals like LiNbO$_3$ or LiTaO$_3$ may be able to bridge this gap. The ability to fabricate structures within the crystal by femtosecond laser machining facilitates all integrated signal guiding and processing. Spatiotemporal imaging is employed for direct visualization of the electromagnetic field within the crystal. Polaritonic resonators, waveguides, photonic crystals and focusing, dispersive, and diffractive elements will be demonstrated.

• Authors: David Ward, Thomas Feurer, Eric Stats, JoushuaVaughan, Keith Nelson, Massachusetts Institute of Technology)


Conclusions on K(T) by Phonon-Polaritons1. Ph-Polaritons are found to contribute to K(T) of thin films, with T-peak.

Position of T-peak depends on Ω op, the line width of OP and the TO-LO splitting,

2. K(T) can be 10-20 times stronger than the conventional radiative contribution to K by free photons.

3. T-peak shifts to lowest T in microcavities (L ~ 1-10 µm), which can be used in cryogenic heat transfer.

4. To create a material with high enough polaritonic K(T) at RT, compared to the usual, phonon Kphone should create an organic material with OP at 1500-2000 cm-1, which has large oscillator strengthIn organic materials Kph is usually low (< 0.1-1 W/mK), the Kpol can become a main contribution.

5. One candidate for polaritonic heat pipe, can be a doped fullerene film MxC60 in which a giant oscillator strength S enhancement is found, which is quadratic in doping level x: S ~ x2.

6.The strong dependence of Kpol(T) on S(x) leads to tunability of K(T) by charge transferand thus may be used in “polariton-transistors”, in which K can be amplified by charging gate G.

7. Phonon-Polaritons can be used for “Polariton-lasers”, which will emit monochromaticand coherent IR radiation, due to Bose-Einstein condensation in microcavity.

25


Our Main Systems and Materials:

1. New Mechanism of Phonon-Polariton Thermal Conductivity

2. Carbon Nanotube Systems with Enhanced K(T) CNT:

- CNT Fibers and Yarns and - Oriented CNT-ribbon aerogels


PHONON TRANSISTOR inNANOTUBE FIBERS with Electrochemical Gate

Capacitance of 134 F/gm, versus usual 15-30 F/gm.Electrolyte: 1.0 M tetrathylammonium hexafluorophosphate in acetonitrile.

Increased capacitance results from debundling. (Raman measurements of radial breathing mode shifts.)

3-+

++

3-+

++

3-+

++3-+

++

3-+

++

3-+

++3-+

++

3-+

++

3-+

++

3-+

++3-+

++

3-+

++

3-+

++3-+

++Low K Expect High K upon unbundlingCharging the bundles

Debundling should decrease tube-tube phonon scattering, and increase K(T) dramatically:

Phonon Transistor

50 100 150 200 250 300T0

200

400

600

800

1000

ka WmK

∆K

26


SEM images of PurifiedUnannealed Oleum -spun

HiPCo fibers

Before cycling in EMIImFiber diameter Å 50 µm

After cycling in EMIImFiber diameter Å 100 µm

51

Prove of Unbundling: Twice Increased Diameter of Fibers

Transistor in ‘ON” statePhonon Transistor in ‘OFF” state


Phonon Transistor withElectrochemical Charge Injection Gate

C= charge/voltageC= Area × dielectric constant /dArea/weight is above 300 m2/gm;d is in nanometers

Gate function:

Charge injection will cause changein tube-tube interaction, which changesΩ of intertube phonon and modulate The Tube-Tube scattering.

Modulation of Thermal Conductivity K(T)

Increased K(T) upon G voltage

27


Charging Setup:

• SWNT paper was used to charge and study the effect of charge injection in field emission characteristics. SWNT paper was was used as both the positive and negative electrode.

• Chronopotentiometry was used for performing the double layer charging. Princeton model 273A instrument contains both a potentiostat and a galvanostat, and hence can perform both controlled potential (potentiostatic) and controlled current (galvanostatic) experiments.

• Cyclic Voltametry was also performed before the charging by cycling from –0.2 to 0.9V. The direction of the potential is reversed at the end of the first scan. This has the advantage that the product of the electron transfer reaction that occurred in the forward scan can be probed again in the reverse scan. It is a powerful tool for the determination of formal redox potentials, detection of chemical reactions that precede or follow the electrochemical reaction and evaluation of electron transfer kinetics.


Charging I-t Curves

Capacitance of SWCNT paper is very large: C ~ 20-30 F/g

28


Tuning K by Charge Injection

0.0 0.5 1.0 1.5 2.0 2.5 3.0

50

100

150

200

250

300

αcharged Na, 10Hz=12.6 mm2/sαcharged Cl, 10Hz=11.5 mm2/s

αpristine, 5 Hz = 10.8 mm2/sαcharged Na, 5 Hz =12.9 mm2/sαcharged Cl, 5 Hz =11.5 mm2/sPh

ase

shift

, deg

ree

Distance, mm

HipCO (15x0.5 mm), prisitine Wetted in 2M NaCl, washed DI Charged, Na+, 1h, 0.8V Charged overnight, Na+, 0.8V Charged overnight, repeated Charged overnight, repeated, 10 Hz Charged Cl, 10 Hz Charged Cl, 5 Hz


Tuning K by Charge Injection:A Step towards Phonon Transistor

0.0 0.5 1.0 1.5 2.0

50

100

150

200

250

300

350

400

450Two MagAlign SWNT(II) paper electrodes charged at 0.5V for 1h

Phas

e sh

ift, d

egre

e

Distance, mm

Cl- α = 14.2 mm2/s Pristine α = 15.2 mm2/s Na+ α = 21.6 mm2/s

Thermal diffusivity and thus thermalConductivity of CNT is provedto be tuned by Charge injection into

Oriented CNT paper at V = 0.5 V

Effect is 25 %:

From D = 15.2 mm2/s to 21.6 mm2/s

29


Draw and Twist of yarns from MWNT forests

Fig 1a

Fig 1b

Fig 2a

Fig 2b


0.0 0.1 0.2 0.3 0.4 0.5 0.6-5

0

5

10

15

20

25

30

35

40

45Thermal diffusivity of carbon nanotube yarn, d=11.3 µm

D=220 mm2/s

216

220

219

Phas

e sh

ift, d

egre

e

Distance, mm

80 Hz 40 Hz 20 Hz

High Thermal Diffusivity of Single CNT Yarn: Better than Cu

30


Thermal conductivity of Single CNT Yarn: Better than Cu

• The thermal conductivity λ of single-strand MWNT yarn was obtained at room temperature from the relationship λ= ρCpD , by measuring the thermal diffusivity D, density ρ, and specific heat capacity Cp. The measurements of D were carried out using the laser flash technique.

• One end of a specimen of length L is uniformly irradiated by a laser beam Q=Qosin ωt.

•• λ= ρСpD = 0.8 g/cm3 0.715 J/gK 2.20 cm2/s = • 1.25 W/cmK = 125 W/mK,• where the specific heat capacity of graphite with density 2.26 g/cm2:

Cp(300K) = 8.58 J/(mol K) = 0.715 J/gK [1], (For comparison the specific heat capacity value for 10 μm Amoco P-55 carbon fibers, with density of 2 g/cm2 at 25oC is 0.717 J/gK [2]), ρ=0.8 g/cm3 is the density of fiber, andD=2.20 cm2/s is the thermal diffusivity of the fiber.

• For comparison, the thermal diffusivity of thin wire (100µm) of copper and gold are much lower: Dcopper = 117 mm2/s, Dgold = 130 mm2/s.


Multifunctional Carbon Nanotube Yarns by Downsizing an Ancient TechnologyM. Zhang, Ken Atkinson, Ray Baughman, Science 306 (2004) 1358

‘Draw-Twist’ process to convert MWNT

in a forest to ‘Twisted Yarns’

31


SEM images of ‘Twisted Yarns’


Oriented Multiwall CNT Thin Sheets:Aerogels with strongly anisotropic K(T)

32


NanoTech Institute MWNTSheet Fabrication Process

Sheets (presently 5 cm X 1 m) are fabricated At 3 m/minute. These width and length are not fundamentally limited and the rate is limited by our present draw apparatus.

We have promising initial results for diverse applications: * transparent elastomeric electrodes; * light-emitting diodes; * incandescent sources of polarized light; * two-dimensionally reinforced composites; & microwave absorbing appliqués


Spun nanotube sheets as an incandescent light source.

The light output is polarized, with a degree of polarization that increases withwave length from 0.6 at 500 nm to 0.66 at 780 nm.

33


400 500 600 700 800

0.0

1.0x10-2

2.0x10-2

3.0x10-2

4.0x10-2

5.0x10-2

a)

II

⊥

Rad

ianc

e ( W

/sr/m

2 )

Wavelength ( nm )

U = 50V ⊥ 1st measurement ⊥ 2nd measurement II 1st measurement II 2nd measurement

1 layer ribbon

400 500 600 700 80010-6

10-5

10-4

10-3

10-2

10-1

b)

II⊥

Rad

ianc

e ( W

/sr/m

2 )

Wavelength ( nm )

U = 50V ⊥ 1st measurement ⊥ 2nd measurement II 1st measurement II 2nd measurement

1 layer ribbon

Polarized Emission of Nanotube Sheet Incandescent Light Source

Spectral radiance of incandescent light from single sheet of parallel carbon nanotubes: a) linear scale, b) semi logarithmic scale. The solid line in a) is a fit by black body radiation with T=1350 K.


0

20

40

60

80

100

200 600 1000 1400 1800 2200Wavelength (nm)

Tran

smit

(%)

Polarizer Alignment:

Perpendicular

Parallel

Optical Transparency of MWNT Sheet

Resistance: ~600 Ω/ (in aligned direction)~15 KΩ/ (in cross direction)

Documents

Phonon-Polariton Physics: Thermal Conductivity, Phonon ... · NanoTech Institute University of Texas at Dallas Phonon-Polariton Physics: Thermal Conductivity, Phonon-Polariton Lasers