Nanoscale Materials and Devices for Energy Transport and

Fisher Group OverviewNanoscale Materials and Devices for Enhanced Energy Transport and Conversion
An Overview of the Nanoscale Transport Research Group at Purdue
Nanoscale Transport Research Group T.S. Fisher, Feb08 Slide 2
• My Background – Born and raised in Aurora, IL USA
– BS (1991) & PhD (1998), Cornell University, Mechanical Eng. • 19911993, Design Engineer, Motorola Automotive Group
– 19982002, Assistant Professor, Vanderbilt University
– 2002present, Associate Professor and now Professor, Purdue
– JanMay 2008, Visiting Professor, JNCASR
• Special Thanks – Prof. C.N.R. Rao and Prof. G.U. Kulkarni for agreeing to my visit
and for so kindly making arrangements for me, my family, and my student (Kyle Smith)
– Faculty and staff of JNCASR for such a warm welcome
– Dr. Pankaj Sharma (Purdue, Discovery Park) for coordinating activities with JNCASR and other Indian institutions
Background & Acknowledgments
Subjects Heat, mass and charge transfer
at small scales
Transfer
CNTbased Thermal and Bio
Sensors
CNTbased Electron
Emission for Energy
Chemical Hydrides
Computer Power and Cooling Trends
500
1000
1500
2000
2500
3000
0 PC tower Mini tower μ−tower Slim line Small pc
Sy st em
V ol um
e ( c ub
Th er m al B ud
ge t (
V ol um
100
250
FM )
Thermal Contact Resistance in Electronics
• Resistance across a Thermal Interface Material (TIM) comprises a significant fraction of the total thermal budget in modern microprocessor packages
• Substantial technological progress has been achieved to date in improving conductivity and process control
• Polymer, hybrid and solder TIM’s have helped accomplish ~10x reduction in resistance (from 100 mm2K/W to 10 mm2K/W) in the past decade
• Accompanied by major metrology improvements
• nanoTIMs being actively explored to move toward 1 mm2K/W
TIMs
What is Contact Resistance?
• Contact resistance is resistance to heat transfer across a real interface.
• The interface poses a resistance to the heat flow, which is seen as a temperature drop across the interface.
Source: Williamson & Majumdar, 1992
Factors that Affect Thermal Contact Resistance • Contact geometry
– Surface roughness and waviness – Flatness of contact
• Thermal/physical properties – Contacting members – Gas gap (or filler material)
• Applied pressure – Elastic or plastic surface asperity
deformation • Interface temperature
contact spots
Gas gap
filler
filler
• Hence, total is the sum of the solid spot and the interstitial gap conductances.
Heat Flow Through a Real Interface T2
T1 > T2
• Thermal contact conductance (TCC) is the ratio of heat flux to the temperature drop
• Thermal contact resistance (TCR) is just the inverse of TCC, therefore
• Alternate definition of TCR
Thermal Contact Conductance / Resistance
Δ Δ = = =
= = Δ Δ
Note that contact resistance at a joint is not physical property of the interface.
• CNTs are highly conductive – Room temperature thermal conductivity ≈3000 W/mK (8X copper, Kim et al. 2001)
– Ballistic electron conductor, R =6500 Ω, independent of length up to ~0.1 μm at RT (Frank et al., 1998)
• CNTs have high aspect ratios – Length more than 1000 x diameter
• CNTs are mechanically resilient – Young’s modulus ~ 1 TPa (5X steel, Treacy et al. 1996)
• CNTs have strong van der Waals interactions – Increased nanotubesubstrate contact area (Hertel et al., 1998)
• CNTs are chemically stable in a large temperature range (in air up to ≈ 450 °C)
Carbon Nanotube (CNT) Interfaces With Faculty: X. Xu, S. Garimella Students: B. Cola, S. Aradhya
Microwave Plasma CVD
[email protected] Microwave Generator
Dual Wavelength Pyrometer
75 mm of Stage Translation
Seki Technotron Corp. AX5200 Series
Process gases: H2 – 1000cm3/min CH4 10cm3/min N2 – 10cm3/min Other – O2, Ar
Substrate Bias: 0 – 600 V; 0 – 1.7 A
Stage Temperature Control with Heating
up to 1000oC
• silicon/SiO2
• copper • quartz • sapphire • diamond • stainless steel • iron • nickel • silicon carbide • aluminum • titanium • and more…
Some Growth Substrates for CNT Arrays
• Full multiwalled CNT coverage over the sample (macroscopic)
• Uniform CNT layer thickness for each array (array heights up to 100μm)
• Strong CNTsubstrate bonding
• Dense and vertically oriented CNT ‘forest’ – Density ≈ several hundred million to
more than one billion CNTs/mm2
– MWCNT diameters can range from 5 to 90 nm
General Characteristics of CNT Arrays Broken stems of CNTs after scratching
CNT Array Thermal and Electrical Interfaces
Solid 1
Solid 2
Solid 1
Solid 2
Solid 1
Solid 2
Cu foil
CNT/foil interface
Thermal Interface Results: Summary
References:
B.A. Cola, J. Xu, C. Cheng, H. Hu, X. Xu, and T.S. Fisher, J. Appl. Phys. 101, 054313 (2007).
B.A. Cola, X. Xu, and T.S. Fisher, Appl. Phys. Lett. 90, 093513 (2007).
J. Xu and T.S. Fisher, Int. J. Heat Mass Trans. 49, 1658 (2006).
J. Xu, T.S. Fisher, IEEE Trans. CPT, 29, 261 (2006).
silicon/copper substrates at room temperature
Stateoftheart commercial materials (nonbonded interfaces)
Projected performance range
Resolving Component Resistances: OneSided
• Resolution enabled by photoacoustic measurement
B.A. Cola et al., J. Appl. Phys. 101, 054313 (2007)
Results: Twosided CNT Interfaces silicon/copper substrates at room temperature
Resolving Component Resistances: TwoSided
• Resolution enabled by PA measurement B.A. Cola et al., J. Appl. Phys. 101, 054313 (2007)
Recently measured, nonlinear bulk modulus, B, of CNF arrays
predicted well by adapted wool theory
Contact Conductance Model: CNT Array Mechanics
CNT arrays deform similarly to wool fibers C.M. Van Wyk, J. Textile Inst. 37, T285 (1946)
c – constant that depends on CNT orientation, quality, aspect ratio, and packing density
E – Young’s modulus
t – CNT array height
A – substrate area
= −
aluminum
silicon
aluminumaluminum
siliconsilicon
force
silicon
aluminum
siliconsilicon
aluminumaluminum
force
Contact Conductance Model for CNT Array Interfaces
CNT growth substrate (Si)
• For steady, continuum heat flow, the critical characteristics are thermal conductivity (k) and contact spot size (a)
• Thermal conductivity can be expressed as k = C vg λ/3, where – C is the volumetric heat capacity of the energy carriers
– vg is the group velocity of the energy carriers
– λ is the mean free path of the energy carriers
Diffusive Heat Flow
• When the carrier mean free path becomes comparable to the contact spot size, subcontinuum effects become significant
• As a result, the Knudsen number (Kn = λ/a) becomes an important parameter, in addition to k and a alone
Ballistic Heat Flow
Phonon Ballistic Transport
• Contact size a < λ, phonon ballistic transport – λ, phonon mean free path
– vg, frequency independent phonon group velocity
– Cv, Debye volumetric specific heat
– Γ, average transmissivity
4 2221211121 gvgv vTCvTC
Thermal Constriction Resistance: Total
as PBT 0R → and as expected,
Total diffusive PBT≈ +R R R
210Kn ∼if (typical of nanocontacts)
then 2
:
Preliminary Results
Points: Experimental Data
Electrical CNT Interfaces to Thermoelectrics
0
5
10
15
20
25
30
35
R es
is ta
nc e
TE film
MWCNT array
Faculty: T. Sands Student: H. Mishra
Mishra et al., MRS Fall Meeting (2007)
Boiling chamber
OxygenFree Copper Block
Faculty: Issam Mudawar Students: Sebastine Ujereh, Vikash Khanikar
Patterned CNT Arrays on Si
12.7
12.7
Pool Boiling from an Si Substrate with FC72
Δ Tsat (oC) 0 20 40 60
0
5
10
15
20
CHF
Incipience
S. Ujereh Jr., T.S. Fisher, I. Mudawar, Int. J. Heat Mass. Transfer, 50, 4023 (2007).
heated substrate
vapor embryo
heated substrate
growing bubble
ΔTsat = 9.8 K
q” = 1.3 W/cm2
ΔTsat = 26.5 K
Ionic Winds for Electronics Cooling
• Macroscale corona discharges require high voltages (>2 kV) • Field emission in microgaps require much lower voltage (~100V)
– Nanostructured carbon field enhancement shows potential of lowering voltage to ~10V→ suitable for microelectronics
emitted electrons
eeee e
ionized air
upstream of ionic wind
wind
~10μm
D.B. Go et al., J. Appl. Phys. 102 053302 (2007)
at small scales
Transfer
Sensors
CNTbased Electron
Emission for Energy
Chemical Hydrides
SWCNT Transistors ftp://download.intel.com/technology/silicon/
Javey et al., Nano Lett, 5, 2005
• Placement and addressability
• In situ ohmic contact metallization • Lithography (photo or electron beam)
• Focused ion beam
Challenges in SWCNT Device Fabrication
Kong, J., et al. Appl. Phys. A 69, 305308 (1999) Choi, W.B., et al. Adv. Func. Mat. 13, 8084 (2003)
Templated Vertical Growth of SWCNT Devices
• Start with Fe (SWCNT catalyst) embedded in Al film stack
SWCNT Catalyst (Fe)
Faculty: Tim Sands, David Janes Students: Aaron Franklin, Matt Maschmann
• Film anodized to create porous anodic alumina (PAA), exposing Fe catalyst within pores
PAA
A. D. Franklin, et al. J. Vac. Sci. Technol. B 25, 343 (2007)
Hydrogen plasma treatment penetrates alumina barrier, exposing Ti
Si
Ti
• SWCNTs grown from Fe catalyst using
microwave plasmaenhanced CVD
Maschmann, et al. Nanotechnology 17, 3925 (2006)
Si
Ti
• Pd electrodeposited into pores, contacting SWCNTs with Pd nanowires
Contacting SWCNTs in situ
Gating/Device Approach
Selective etchback of Al2O3 and deposition of gate metal to coat dielectric pillars
Gate metal etch to define channel length Passivation layer deposition (Al2O3), planarization and top contact definition
Interesting ByProduct: Pd Nanoclusters on SWCNTs
• Pd nanowires contact SWCNTs, adding them to the cathodic electrode during electrodeposition
• Defect sites have been shown to serve as nucleation points for contact metals Fan, Y., et al. Nat. Mat. (2005)
200nm 150nm
Faculty: K.S. Choi
A.D. Franklin et al., J Phys Chem C 111, 13756 (2007).
Addressable ‘Fields’ of Templated SWCNTs
Franklin et al., Appl. Phys Lett, 92 013122 (2008)
at small scales
Transfer
Sensors
CNTbased Electron
Emission for Energy
Chemical Hydrides
Ligand (Biotin-SH)
Pd crystals on CNTs
Bright Field Image Fluorescence Image
Manuscript in preparation
Biosensing with MWCNT Arrays
ti on
GlucoseGlucose Oxidase Reaction • Three electrode cell
– Ag/AgCl electrode (reference)
– Platinum electrode (counter)
– CNT/CNF substrate (working)
• Cyclic Voltammetry: Initial potential 0V, switching potential 800mV, final 0V
• Scan rate of 100mV/sec
0.0
1.0x10-8
2.0x10-8
3.0x10-8
4.0x10-8
C ur
re nt
p er
C N
T A
re a
[A /m
m 2 ]
• Reduction of sensor size and detection time
• Elimination of target labeling/signal amplification and complicated/expensive fabrication protocols
• Optimization of CNT array sensors – Correlation of growth conditions to nanostructure
– Influence of quality and morphology of CNTs/CNFs on degree of enzyme adsorption • More defect sites better adsorption of enzyme?
– Biofunctionalization of CNTs for different assays
– Effects of tethered vs. immobilized receptors
Important Issues in Biosensing with CNT Arrays
• Motivation – Measuring the temperature is
nanostructures is extremely difficult, yet critically important to many applications
– Noise thermometry offers the possibility of a selfcalibrating, primary thermometer
• Applications – Atomicscale thermal interface
resistances
– Convective boiling processes
( ), 2 coth (7) 2 θ
+ ∞
= +
Modification to include selfheating (T QRθ + T∞)
Student: Bob Sayer Manuscript in review
PAA
SWCNT Pd
at small scales
Transfer
Sensors
CNTbased Electron
Emission for Energy
Chemical Hydrides
• Motivation – More efficient atomistic simulation tools are needed to address problems at practical scales
– Include effects of bulk contacts through selfenergy matrices
– Suitable for ballistic transport • Low temperature and/or very small dimensions (~10 nm)
• Required inputs – Equilibrium atomic positions – Interatomic potentials – Contact temperatures
Atomistic Green’s Function (AGF) Modeling of Phonon Transport
Faculty: Jayathi Murthy, Natalio Mingo (UCSC) Students: Zhen Huang, Dhruv Singh
The AGF Algorithm
Calculate device G and phonon transmission (Ξ)
Integrate (Ξ) over phonon frequencies and k|| to obtain the thermal conductance
Uses decimation algorithm
interatomic potential parameters
Results for Simple Atomic Chains
Homogeneous chain density of states Homogeneous vs heterogeneous
“heavy” device
“light” device
W. Zhang, T.S. Fisher, N. Mingo, Numerical Heat Transfer: Part B, 51, 333 (2007).
Contacted Si Nanowire
W. Zhang, N. Mingo, T.S. Fisher, Phys Rev B, 76, 195429 (2007).
Effect of Diameter
Thin film results
W. Zhang, N. Mingo, T.S. Fisher, Phys Rev B, 76, 195429 (2007).
Electron Green’s Functions: Modeling of the Nottingham Effect
Electron Emission: Electrons cross over vacuum barrier
(thermionic emission) or tunnel through barrier (field emission)
Cooling Mechanism: Emitting electrons carry net energy from emitter to collector (heat sink)
EF
rc e
Cooling by Vacuum Electron Emission? Compare to Thermoelectric Refrigeration • Theory indicates possibility of large local cooling rates ( > 100 W/cm2)
• No lattice thermal conductivity
• Vacuum gap must be small
• Emission area must be large enough to produce necessary current
H ea
Heat flow
G. D. Mahan, J. Appl. Phys. 76, 4362 (1994). Y. Hishinuma, T. H. Geballe, B.Y. Moyzhes and T.W. Kenny, Appl. Phys. Lett. 78, 2572 (2001); J. Appl. Phys. 94, 4690 (2003). N. M. Miskovsky and P. H. Cutler, Appl. Phys. Lett., 75, 2147 (1999). T. S. Fisher and D. G. Walker, J. Heat Tran. 124, 954 (2002).
NonEquilibrium Green’s Function Model
• Vacuum discretized with a oneband effective mass model
• Energy levels described by a Hamiltonian matrix [H]
• Coupling at interfaces described by selfenergy matrices [Σi]
1, 2i i i ii + = Γ = Σ − Σ
[ ] 1 21
Cathode
Theory based on work of S. Datta: S. Datta. Electronic Transport in Mesoscopic Systems (Cambridge University Press, 1995); S. Datta. Superlattices and Microstructures 28, 253 (2000).
• Not practical in a cooling device
• Need a lower work function, φ
Flat Plate Cooling
• NEGF simulation yields accurate results
• WKB approximation overpredicts cooling
• Maximum cooling density ≈ 80 W/cm2
• But, COP ~ 0.001, Electronic Coefficient of Performance (cooling power to power input ratio)
Effect of Gap Spacing
COP = Electronic Coefficient of Performance (ratio of cooling power to electrical power input)
Westover and Fisher, Phys. Rev. B, accepted
at small scales
Transfer
Sensors
CNTbased Electron
Emission for Energy
Chemical Hydrides
Thermionic Power Generation
• Alluring direct power generation scheme because vacuum separation minimizes parasitic thermal losses.
• At a surface, excited electrons may escape the material if their energy exceeds a surface potential barrier, or work function φ.
• Additional potential barriers exist due to space charge and/or generated voltage, qV0 = μ2 μ1. For these cases, the sum of all such barriers and the work function is denoted by Φ.
Electron motive diagram for a thermionic power generation diode, with T1 > T2.
Students: Tyler Westover, Vance Robinson Faculty: Ron Reifenberger, Chuck Lukehart (Vanderbilt)
0
0.2
0.4
0.6
0.8
1
Energy (eV)
In te
ns ity
(a . u
600C w/ K 700C w/ K 750 C w/o K
Thermionic Electron Energy Distributions from Nanomaterials
• Experiments on arrays of graphitic carbon nanofibers with and without potassium intercalation
intercalation
Robinson et al., Appl. Phys. Lett., 87, 061501 (2005) Li et al., Nanotechnology, 18, 325606 (2007)
• Basic (highly idealized) theory predicts ultrahigh efficiency (Ross, Nozik, J Appl Phys 53 3813 (1982))
• Concentrated solar irradiation provides some photoemission, perhaps amplified by CNTinduced field concentration and low scattering rates in CNTs
• Solar energy that does not directly produce photoemission heats the CNT absorber, promoting thermionic emission
• New modeling tools are needed that incorporate quantum considerations and accurate electronphonon scattering models to explain measured performance
Concept for Solar Thermionics
Concentrated simulated solar irradiation from LS1000 and
parabolic concentrator
Water-cooled anode
Coolant water
Possible alkali metal source
at small scales
Transfer
Sensors
CNTbased Electron
Emission for Energy
Chemical Hydrides
• Specific energy: 6 wt% H2 or 2 kWh/kg Energy density: 0.045kgH2/L or 1.5 kWh/L Cost: $133/kgH2 or $4/kWh
• These are for the system: fuel + tank + BOP • Storage system for a compact car
– Hydrogen: 5.6 kg – System weight: 93.3 kg – System volume: 124.4 liters – Cost: $745 – Fueling rate: 1.5 to 2.0 kg H2/min
US DoE 2010 Targets for Hydrogen Storage
Source: U.S. Department of Energy, Office of Basic Energy Sciences, 2003, Basic Research Needs for the Hydrogen Economy, http://www.sc.doe.gov/bes/hydrogen.pdf
• Compressed gas and liquid hydrogen – 700 bar, 300K: 143.5 liters
– LH2: 78.9 liters (fuel only)
• Materialbased storage (adsorption or absorption) – Hydrogen itself can occupy much less space
– Adds extra volume and weight from additional chemical species
– Complex system: Useful hydrogen + material + tank + thermal management subsystem + BOP
• So far, no system has been demonstrated to meet the 2010 targets
Storage Approaches
• Reviewed heat transfer in onboard hydrogen storage technologies using compressed H2, LH2, chemical hydrides and metal hydrides – Zhang et al., J. Heat Transfer 127 1391 (2005)
• Sodium borohydride (SBH) systems – Heat of reaction measurements – Kinetics measurements – Subscale (1kWe) system design, construction and tests – Subscale (1kWe) system modeling
• Highpressure metal hydride systems – Subscale (1/50) system design, construction and tests – Subscale (1/50) system modeling – Vehiclescale facilities and testing
Purdue Research
Hydrogen Storage Using NaBH4
NaBH +2H O NaBO +4H→
(U.S. DOE, Office of Basic Energy Science, 2003, Basic Research Needs for the Hydrogen Economy)
Heat of Reaction for SBH Hydrolysis
Zhang et al., Int J Hyd Energy 31 2292 (2006).
– Commonly accepted: 300 kJ/mol NaBH4
– Measured: 210 kJ/mol ± 11 kJ/mol
Schematic of a 1 kWe NaBH4 System
Effects of Flow Rate
Students: Jinsong Zhang, Varsha Velagapudi, Kyle Smith, Avanthi Boopalan, Scott Flueckiger, Andrew Steiner, Yen Yu, Casey Porta, Tyler Voskiulen, Aaron Sisto
Faculty: Issam Mudawar, Timothee Pourpoint, Yuan Zheng
Funding: General Motors
Chaffee Hall
• Most important rule at the Hydrogen Systems Lab:
“Safety comes first.”
• Significant effort placed on: – Training:
• High Pressure Lab procedures and safety training followed by all students involved in project
• Linde Gas training on high pressure systems
– Procedures:
• Careful development of test procedures for safe and repeatable testing
– Safety:
• Safety measures and evacuation procedures developed with the Purdue Fire and Safety Department
• Emergency alarm system: – 3 sirens (122 dB at 10 feet) with amber flashing lights installed to notify all
personnel in ZL1 of an emergency situation
– To be activated in case of uncontrolled fire, catastrophic test article failure
• Event tree developed to assess and solve emergency situations
Safety
• Introduction – Reversible metal hydrides used for hydrogen storage generate a large amount of heat (2030 kJ/mol H2) during the hydrogen absorption process
– Practical onboard hydrogen refueling rate of 2 kg H2/min leads to extremely high heat release rates (Q ≈ 0.5 MW over 3 minutes)
• Key Technical Issues – High heat release rates require active convective cooling – Highpressure systems for gases and coolants – Low thermal conductivity of nanoparticulate hydride bed – Adequate storage capacity and reversibility
Heat Transfer Systems for Metal Hydrides
• Design and implement a heat exchanger with an integrated metal hydride containment system ensuring thermostructural integrity of the tank and its contents
• Modular design for flexibility of scaling: scale range extends from 1/50th to fullsize vehiclescale metal hydride hydrogen storage tank
• Measure fillingdependent thermal performance of a well instrumented subscale tank
• Verify thermodynamic model with operation of vehiclescale reacting system at desired hydriding and dehydriding rates
Experimental Objectives
Pressure Vessel and Tank Inlet Connections
Coolant Loop
Test Article Installed in Pressure Vessel
Preliminary Experimental Results
• Conduction is most significant contributor to heat dissipation
• Reaction progress and conduction are coupled due to temperature dependent kinetics
• Conduction limits reaction progress for kinetically good materials
40 45 50 55 60 65 70
-4
-2
0
2
4
6
8
10
12
14
16
Contribution of volumetric energy rates as a function of time
Sensible Energy Gas Compression Conduction Reaction Generation
( ) qTk t p
t TC eff +∇+
Conduction Modeling
Nanoscale constrictions at
Conclusions • Many opportunities exist for improved understanding of energy and
charge transfer processes at the nanoscale • Spatial confinement may enable the engineering of higherefficiency
energy transfer and conversion processes in carbon nanotubes • Promising results for early ‘benchtop’ CNT applications
– Weldlike heat transfer with ‘thermal velcro’ – Ideal boiling behavior – Exceptional electrical transistor behavior – Large work function reduction of carbon nanofibers for direct energy conversion,
possibly solar – Major breakthroughs needed in hydrogen storage, thermal issues paramount
• Progress generally requires coordinated efforts in a variety of disciplines and skills – Material synthesis and scaleup – Transport property characterization – Systemlevel scaling/engineering – Health, safety, and environmental impact
• Collaborators – As listed in the slides
• Funding – General Motors – Cooling Technologies Research Center – National Science Foundation – US Air Force Research Laboratory – NASA – Creare – Intel Corp. – Nanoconduction, Inc.
Acknowledgments
THANK YOU
Nanoscale Materials and Devices for Enhanced Energy Transport and Conversion An Overview of the Nanoscale Transport Research Group at Purdue
Background & Acknowledgments
What is Contact Resistance?
Thermal Contact Conductance / Resistance
Carbon Nanotube (CNT) Interfaces
General Characteristics of CNT Arrays
CNT Array Thermal and Electrical Interfaces
Thermal Interface Results: Summary
Resolving Component Resistances: One-Sided
Results: Two-sided CNT Interfaces
Resolving Component Resistances: Two-Sided
CNT Array Contact Mechanics
Diffusive Heat Flow
Ballistic Heat Flow
Phonon Ballistic Transport
Summary of Pool Boiling from CNT Arrays
Patterned CNT Arrays on Si
Pool Boiling from an Si Substrate with FC-72
Enhancement Hypothesis
Outline
Contacting SWCNTs in situ
Addressable ‘Fields’ of Templated SWCNTs
Outline
Noise Thermometry with CNTs
The AGF Algorithm
Contacted Si Nanowire
Effect of Diameter
Cooling by Vacuum Electron Emission?
Non-Equilibrium Green’s Function Model
Flat Plate Cooling
Concept for Solar Thermionics
Storage Approaches
Purdue Research
Schematic of a 1 kWe NaBH4 System
Effects of Flow Rate
High-Pressure Metal Hydride Research
Experimental Objectives
Heat Conduction and Reaction Rate
Conduction Modeling

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