Birck Nanotechnology Center

Birck Nanotechnology Center

TDS


2009 NSF NSE Grantees Conference, Dec 7-9, 2009, Arlington, VA

Tim SandsDirector, Birck Nanotechnology CenterPurdue University

Tim SandsDirector, Birck Nanotechnology CenterPurdue University

Nanoscale design for scalable sustainable energy technologies

Nanoscale design for scalable sustainable energy technologies


TDS

ThemesThemes The magnitude of the challenge The nanoscale opportunity Focus on solid state approaches to energy efficiency

Solid state lighting Waste heat conversion Solid state refrigeration

Constraining the problem – filling the gap between basic and applied research What is the technological discontinuity to be bridged? What fundamental problem is to be solved? Are the source materials abundant and of low toxicity? Is there potential for scalable manufacturing? Are there near-term niche application for early solutions?


TDS

The Terawatt ChallengeThe Terawatt Challenge

Adapted from Richard Smalley’s presentation on “Our Energy Challenge” at the 2004 International Electron Devices Meeting (IEDM), San Francisco, CA 12/14/04

Primary source for 2003 data:International Energy Agency

5

10

15

Oil Coal Gas Nuclear Biomass Hydro Other

2003 14 TW total

2050 30 TW total scenario

Global Rate of Primary EnergyConsumption

SolarWindGeothermal…

{

0.5%

0


TDS

Spectrum of opportunities for nanoSpectrum of opportunities for nano

Portfolio of solar/thermal/electrochemical energy conversion, storage, and

conservation technologies, and their interactions

Workshop on Nanotechnologies for Thermal and Solar Energy Conversion and Storage, August 10,11, 2008, Jacksonville, FL

“Nanoscale design to enable the revolution in renewable energy” J. Baxter, Z. Bian, G. Chen, D. Danielson, M.S. Dresselhaus, A.G. Fedorov, T.S. Fisher, C.W. Jones, E. Maginn, U. Kortshagen, A. Manthiram, A. Nozik, D.R. Rolison, T. Sands, L. Shi, D. Sholl and Y. Wuo, Energy & Environmental Science 2 (2009) 559-588.


TDS

The solid-state part of the solution…The solid-state part of the solution…

More efficient devices for… LED-based lighting Thermoelectric refrigeration Thermoelectric and thermophotovoltaic conversion of waste heat Photovoltaic conversion of solar energy and production of hydrogen

Added benefits Compact Robust Low environmental impact

Challenges Efficiency breakthroughs needed! Availability and price of raw materials Manufacturing costs

Nanostructured semiconductors?Thin films instead of bulk?

“Bottom-up” nanofabrication?


TDS

New degrees of freedom in nanocompositesNew degrees of freedom in nanocomposites

Length scales on the order of electron, phonon and photon wavelengths, and scattering lengths of electrons, phonons and electron spin

Dimensions below critical values for relaxation of lattice misfit strain and epitaxial stabilization of high pressure phases

High surface (interface)-to-volume ratios for sensing, catalysis, and chemical storage

Fundamentally new materials with new properties

Nanoparticles suspended in solution (Frankel, MIT)

5 nm diameter CdSe nanocrystal (Manna et al., J. Cluster Sci. 2002)

Strain relaxation in nanowire

heterostructures (Ertekin et al.,

JAP,2005)


TDS

Solid State LightingSolid State Lighting


TDS

Solid state lighting – the opportunitySolid state lighting – the opportunity

Electricity generation accounts for about 37% of primary energy consumption in the U.S.*

Lighting accounts for 22% of the nation’s electric power usage.

The DoE SSL Goal: a solid-state lamp that is more efficient, longer lasting and cost competitive compared to conventional technologies, targeting a system efficiency of 50% and the color quality of sunlight.

Implications of Success: 33% reduction in energy consumed for lighting by 2025, eliminating need for 41 1000MW power plants, and saving consumers $128 B+.

+Navigant Consulting (11/03)*Annual Energy Outlook (02)


TDS

LEDs – the technological discontinuityLEDs – the technological discontinuity

III-V LEDs cover the visible spectrum, but not with one materials system Low cost solution:

Blue (In,Ga)N LED with partially absorbing yellow phosphorLimitations: poor color rendering, low efficiency due to Stokes shift

Warm light solution: Board-level integration of (In,Ga)N/yellow phosphor and (Al,Ga,In)P red LEDsLimitations: “green gap”, high cost of assembly

Compound Semiconductor, June 2008, pg. 17


TDS

The nano aspect of a solution?The nano aspect of a solution? Elastic relaxation of lattice misfit strain

Reduced electric field in recombination volume Higher local injection current densities without

Auger recombination Greater InN incorporation, longer emission

wavelengths Filtering of threading dislocations

Lower-cost, larger diameter, higher thermal conductivity substrates (metallized silicon)

Improved geometry for light extraction

EL

EDS

TEM


TDS

Thermoelectric Waste Heat Generators

Thermoelectric Waste Heat Generators


TDS

www.greencarcongress.com/thermoelectrics

Waste heat factoids…Waste heat factoids… >60% of primary energy consumed is dissipated as waste heat Energy costs account for approximately 8~12% of the total cost of glass

production, and about 15% for steel* The world’s data centers are projected to surpass the airline industry as a

greenhouse gas polluter by 2020** About 40 percent of the energy content of gasoline burned in automobile IC

engines is lost as exhaust heat and another 30 percent is lost through engine cooling***

*http://www1.eere.energy.gov/industry/program_areas/industries.html **Study by McKinsey & Co., Green Enterprise Computing Symposium in Orlando, Fla., May 1, 2008***http://www.washingtontimes.com/news/2008/aug/12/researchers-eye-exhaust-for-fuel/?page=2

www.eere.doe.gov


TDS

Overview: thermoelectricsOverview: thermoelectrics

Thermoelectric primer Recent progress in ZT enhancement by

nanostructuring (MBE and bulk) A scalable approach with nanoscale control:

electrodeposition into porous templates Mitigating parasitics – removing/replacing the

template, and scaling to thicknesses > 100 m TE element fabrication from nanoscale materials


TDS

Physical basis of thermoelectricityPhysical basis of thermoelectricity

Thermoelectricity arises from the difference between the average energy of the electrons (or holes) responsible for conduction and the Fermi energy

Densityof

States(DOS)

EnergyFermi-DiracOccupation

Function

0 1

Densityof

Occupied States

* =EF

Average energy of conduction electrons

In an n-type semiconductor…


TDS

The Seebeck effect (1821)The Seebeck effect (1821) Generation of voltage along a conductor subjected to a temperature

difference Initially, carriers (electrons or holes) move from hot to cold Resulting potential difference opposes further current flow Open circuit voltage is proportional to T

where S is the Seebeck coefficient [V/K] Completing the circuit through a load generator

lim {Voc}= STT0

Initial

qVoc]

Steady State

Hot Side Cold Side

eElectron

energy


TDS

Thermoelectric generatorsThermoelectric generators

heatsource

n-type semiconductor

p-type semiconductor

hot side

heatsink

current

cold sideh+

e-

•Large Seebeck coefficient (S) large open circuit voltage for generators; large Peltier coefficient for refrigerators

•Low thermal conductivity () easier to maintain T for generators; reduced conduction of heat back to cold side for refrigerators

•High electrical conductivity () reduced Joule heating

Dimensionless figure-of-merit

Altenkirch (1909,1911)

zT = S2T/


TDS

Applications of TE today (zT ~ 1)Applications of TE today (zT ~ 1)

Radioisotope TE generators powering deep space probes

Automotive seat coolers/heaters (500,000/yr) Picnic coolers DNA PCR precision temperature cyclers Temperature stabilizers for laser diodes used in

fiber optic communications systems Electronics cooling at the cabinet level Remote power generators in harsh environments

Materials zT~1; System ZT~0.7

Heat source (PuO2)

Thermoelectric modules

http://www.its.caltech.edu/~jsnyder/thermoelectrics/

Bell, L 2008, “Cooling, heating, generating power, and recovering waste heat with thermoelectric systems,” Science 321, p.1457

Can we expect more from thermoelectrics?


TDS

Applications of TE tomorrow (zT =2-3)Applications of TE tomorrow (zT =2-3)

Exhaust waste heat recovery for gasoline and diesel engines to improve mileage by 10%

Cogenerators for 5-20 kW diesel generators, improving system efficiency by 5-10%

Split-spectrum solar generators (PV + TE) Industrial waste heat recovery in metal, glass and cement processing Flex fuel powered small engines Microprocessor cooling Greenhouse-gas-free HVAC for vehicles and residences

Materials zT=2-3; System ZT~1.5-2

Bell, L 2008, “Cooling, heating, generating power, and recovering waste heat with thermoelectric systems,” Science 321, p.1457


TDS

Materials design challengesMaterials design challenges

• Increasing increases e (Weideman-Franz-Lorenz Law)

• Decreasing ph by adding defects decreases mobility (and )

• Reducing m* increases mobility (and ), but decreases S• Increasing carrier concentration decreases S

S2zT = T

For most bulk materials…

= e + ph = (LoT) + ph

zT =S2

Lo + (ph/T)


TDS

Device design challengesDevice design challenges

CTE and CTE mismatch Assembly of 10s to 100s of p-n couples – electrically in

series to match load resistance Thermal and electrical parasitic resistances

Contact resistance – leg length should be greater than ~200 m (total contact resistance <10% of total resistance for c = 1 x 10-6 -cm2 and TE = 1 m-cm)

Maintaining T – leg length should be between ~100 m and 1 mm to maximize power density


TDS

20% efficient TE generators?20% efficient TE generators?

Target: >20% efficiency; >104 W/m2 power density

0 50 100 150 200 250 300 350 400 450 500 550 600

0.4

0.3

0.2

0.1

0

Thot [°C]

ηZT= 1

ZT= 2

ZT= 3ZT= 4ZT= 5

Carnot

Tcold =50°C

ZT > 2; T ~ 500K; 10% areal coverage; forced convection


TDS

Optimum leg lengthOptimum leg lengthMayer and Ram, Nano-Microscale Thermophys Eng, 10 (2006)

10 100 1000L (µm)

100

10-1

10-2

101

102

103

Pow

er d

ensi

ty (W

/cm

2 )

Leg length for maximum power density decreases with increasing heat transfer coefficient, minimizing use of less abundant materials

Heat flux limited

Interface limited


TDS

Design of bulk thermoelectricsDesign of bulk thermoelectrics

S2ZT = T

• Power factor, S2, optimized for degenerate semiconductors

• Heavy masses, low “spring constants”, and large unit cells to reduce ph

• Solid-solution alloying to reduce ph

Bi2Te3 and its alloys for cooling near room temperature

“Phonon glass, electron crystal”


TDS

Materials zTMaterials zT

State-of-the-art commercial and NASA TE materials (from G.J. Snyder and E.S. Toberer, Nat. Mater. 7 (2008) 105.)

Commercial bulk materials (not intentionally nanostructured) are limited to materials zT of ~ 1

Sb1.5Bi0.5Te3


TDS

Quantum confinement…the solution?Quantum confinement…the solution?

S ph

Density of States

Ele

ctro

n E

nerg

y

bulk

well

wire

After Esaki, 1990

Ideal DOS: delta function several kT from EF will maximize power factor if mobility can be enhanced by eliminating ionized impurity scattering

Does not include deleterious interface or barrier effects!

Hicks and Dresselhaus, PRB 47 (1993) 12727

15

10

5

0

1D - quantum wire

3D - bulk

Fig

ure-

of-

Mer

it Z

T

Well or Wire Width [nm]

0 1 2 3 4 5 6

Bi2Te3

2D - quantum well


TDS

zT enhancement: p-type leg, 300-400KzT enhancement: p-type leg, 300-400K 1950’s to 2000: Bulk (Bi,Sb)2Te3 with zT~1

2001: RTI group reports zT = 2.4 for Bi2Te3/Sb2Te3 superlattice grown by MBE1

[1] Venkatasubramanian, Nature 413(2001)597[2] Poudel, Science 320(2008)634

• 2008: Boston College/GMZ/Nanjing/MIT group reports ball milled (Bi,Sb)2Te3 bulk alloy with nanoscale grain size yielding zT ~ 1.4 at 373K2

[3] Xie et al. APL 94 (2009)102111

• 2009: Wuhan/Clemson group reports zT ~ 1.56 at 300K for melt spun and spark plasma sintered bulk (Bi,Sb)2Te3 with nanocrystalline grains embedded in amorphous matrix3


TDS

Is zT > 2 possible for a scalable material?Is zT > 2 possible for a scalable material?

Common thread: zT > 1 achieved by enhancing scattering of longer wavelength (> 1 nm) phonons, suppressing lattice thermal conductivity without reducing power factor

zT > 2 only reached in MBE material; difficult to scale to optimal leg lengths of 200 m to ~1 mm

Is there a synthesis approach that combines control of vapor deposition with scaling to practical leg lengths to achieve zT > 2?

Can we enhance the power factor (S2) as well?


TDS

Electrodeposited nanowire arrays?Electrodeposited nanowire arrays? Nanoscale control of grain size and composition modulation

(lateral and radial) with a scalable synthesis technique First electrodeposited Bi2Te3 nanowire (~280 nm dia.) arrays into

porous anodic alumina (Sapp et al., 1999)

Sapp, Lakshmi, and Martin, Advan. Mater. 11 (1999) 402; Prieto et al., JACS 123 (2001) 7160.

40 nm diameter Bi2Te3 nanowires electrodeposited into porous anodic alumina (Prieto et al., 2001)

Texture control


TDS

(Potential) advantages of nanowires(Potential) advantages of nanowires

Phonon scattering - Phonon scattering from free surfaces and grain boundaries decreases the lattice contribution to the thermal conductivity, thereby increasing zT.

Lateral elastic relaxation - Lateral elastic relaxation of strain in nanowires enables coherent interfaces in large lattice misfit heterostructures.

Elastic compliance - Improved elastic compliance resulting from free surfaces and/or interfaces with compliant matrix materials.

Chemical modification of free surfaces - Access to free surfaces permits chemical modification that may enhance charge mobility while suppressing phonon transport; short diffusion lengths less grain growth

Crystallographic texture control – interface energy driven Quantum confinement – Potential for power factor enhancement, but

only for nanowires with diameters < 5 nm (for Bi2Te3)


TDS

Challenges of nanowire topologyChallenges of nanowire topology

Minimizing parasitics Leg lengths > 100 m – thicker than typical PAA Minimizing matrix thermal shunt – matrix <<TE and/or matrix

volume fraction, fmatrix, should be as small as possible

Enhancing mechanical strength and toughness Protecting internal (nanowire) surfaces Making ohmic contacts …


TDS

Bi2Te3 nanowires – property measurementsBi2Te3 nanowires – property measurements

Wang et al. (JAP 96 2004) – arrays, PAA removed 50 nm diameter S = 270 mV/K at 300K; Semiconducting

Zhou et al. (APL 87 2005) – single nanowires 43.5-120 nm diameter Highly variable S and lower than bulk by 28-57%

Li et al. (Nanotech. 17 2006) – arrays, PAA removed 40-60 nm diameter Semiconducting

Mavrokefalos et al. (JAP 105 2009) – single nanowires 52 and 55 nm diameter zT estimated to be 0.1 at 400K; Doping level too high;

minimal reduction (~20%) in due to surfaces and grain boundaries; better control of stoichiometry needed

No power factor or ZT measurements of alloy nanowires yetMavrokefalos et al.(JAP 2009)


TDS

Minimizing matrix effectsMinimizing matrix effects

Approach 1 – replace PAA matrix with low polymer Approach 2 – eliminate matrix by synthesizing self-

supporting nanowire arrays

5 m5 m

11

1nwnw

m

nwcomp

f


TDS

PAA replacement with SU-8PAA replacement with SU-8

PAA selectively etched in 3 wt% KOH for 24h; rinsed with water and then IPA

SU-8 2005 spin coated at 2000 rpm to thickness of 40 m; uv exposed, then hard baked on hot plate at 150°C

SU-8 backfilledNW array

NW arrayin PAA

Free-standingNW array

Etch PAA SU-8 infiltration

K.G. Biswas, T.D. Sands, B.A. Cola and X. Xu, APL 94 (2009) 223116


TDSBi2Te3/SU-8 compositeBi2Te3/SU-8 composite

FESEM images of fractured composite in cross-section

K.G. Biswas, T.D. Sands, B.A. Cola and X. Xu, APL 94 (2009) 223116

cleavage plane

Bi2Te3 NWs

500 nm


TDS

Photoacoustic measurement of Photoacoustic measurement of

Composite Bi2Te3/PAA Bi2Te3/SU-8

comp (W/m-K) 1.40 ± 0.07 1.10 ± 0.06

Bi2Te3(W/m-K) 1.44 ± 0.10 1.45 ± 0.09

matrix (W/m-K) 1.31 ± 0.10 0.2*

Replacement of PAA with SU-8 reduces the matrix penalty in ZT from 27% to ~5%. The effect will be larger with lower TE

K.G. Biswas, T.D. Sands, B.A. Cola and X. Xu, APL 94 (2009) 223116*data from product literature


TDS

Composition modulation in Bi2(Te,Se)3Composition modulation in Bi2(Te,Se)3

0.2SU-8

Bi2Te3

(pure)

Bi2(Te,Se)3

(composition-modulated)

(W/m-K) 1.48 ~0.23

Thermal conductivity measurements obtained by the photoacoustic technique (Baratunda Cola, Xianfan Xu; Purdue University).

Thermal Conductivity Measurements

Cross-section FESEM image of composition-modulated Bi2(Te,Se)3

nanowires.

By varying the electrodeposition potential from +40 mV to -60 mV, the composition was varied from 0.18 to 0.07 mole fraction Bi2Se3.

• Substantial suppression of apparent thermal conductivity

• Compare to minimum of 0.31 for Bi2Te3 and ~0.20 for (Bi,Sb)2Te3[Chiritescu et al., JAP 106 (2009)]


TDS

Next step…Next step…

Can we eliminate the matrix altogether? Suggests interconnected, self-supporting nanowire array

• Template must be branched as well• Solution: Branched Porous Anodic Alumina (B-PAA)


TDS

Interconnecting pores in B-PAAInterconnecting pores in B-PAA

K.G. Biswas, et al., Appl. Phys. Lett. 95 (2009) 073108


TDS

B-PAA synthesisB-PAA synthesis

Time variation of voltage, current, and temperature during B-PAA synthesis

K.G. Biswas, et al., Appl. Phys. Lett. 95 (2009) 073108

165-190°C in 0.4 M phosphoric acid with a current density limit of 1.1 A/cm2 (275X current density for mild anodization)

Thermal runaway branched pores and a rate of anodization > 300 m/hr, 60X faster than mild anodization


TDS

Self-supporting nanowire arraySelf-supporting nanowire array

Cross-section FESEM images of self-supporting interconnected Bi2Te3 nanowires after B-PAA template removal by selective etching.

Pt electrode, electrolyte: 0.035M Bi(NO3)3 · 5H2O and 0.05M HTeO2+ in

1M nitric acid at pH = 1; 3 s pulses of current density 5 mA / cm2 followed by a 3 s standby.


TDS

Fabrication of elementsFabrication of elements

340 m

B-PAA template before electrodeposition of (Bi,Sb)2Te3

Top view of Au/Ni metallized 370 m x 370 m (Bi,Sb)2Te3 self-supporting nanowire array

element

Side view of Au/Ni metallized 370 m x 370 m (Bi,Sb)2Te3 self-supporting nanowire array element of 100 m thickness


TDS

Where are we?Where are we?

Thermal conductivity measurements (photoacoustic method) validate PAA replacement approach

B-PAA process understood and scaled to ~300 m in thickness (~1h)

Element fabrication (electrodeposition, annealing planarization, metallization, template removal, and dicing) demonstrated

Present: Optimization of element fabrication process (uniformity and reproducibility)

Next: zT measurement and optimization through composition modulation and post-growth modification (e.g., annealing in Te/Se vapor)….a platform for systematic analysis of the nanowire approach


TDS

Another example: nano to bulkAnother example: nano to bulk

HAADF/STEM image of (Zr,W)N/ScN superlattice Courtesy: Joel Cagnon and Susanne Stemmer, UCSB

Sputtered metal/semiconductor superlattices for “thermoelectric metamaterials”


TDS

ConclusionsConclusions

Parasitic thermal and electrical effects must be overcome before “artificially nanostructured” materials can be accurately assessed

Leg (segment) lengths greater than 100 m are required

Electrodeposition into porous templates is a viable approach to scalable artificially nanostructured TE materials

Not there yet!


TDS

Closing thoughts…Closing thoughts…

A portfolio solution to the global energy challenge will include solid-state devices that conserve energy

Nanoengineered materials and devices can offer more efficient use of materials with limited availability

Size-dependent properties may allow substitution for abundant non-toxic materials

Considering scaling and parasitics at the outset alters materials and device design choices

Birck Nanotechnology Center

Documents

Transcript of Birck Nanotechnology Center