using simulations to discover new materials with energy ... - rthowe...• motivation: thermionic...

Using Simulations to Discover New Materials with (Thermionic) Energy Conversion Applications

Roger T. Howe

William E. Ayer Professor of Engineering Faculty Director, Stanford Nanofabrication Facility (SNF)

October 14-15, 2014

Outline

• Motivation: thermionic energy conversion – Reinventing a century old technology – Critical role of the anode (collector) work function

• Candidate anode materials – Alkali earth oxides – Emission current from first principles

• Alternative anodes and future prospects for thermionic energy

Thermionic Energy Converters (TECs)

4

TEC Energy Diagram

kTE

CEout

EeATJ

V/2 ϕ

ϕϕ−=

−≈

RL

Em

itter

Col

lect

or

φE

φC

EF,E

EF,C

(Cathode) (Anode)

Anode work function: minimize!

Vout

(Richardson-Dushman law)

Radiative heat transfer Conduction through leads

EVacuum

• Basic Concept: W. Schlichter, Germany, 1915

Thermionic Converters Through the 20th Century

• Lab demonstration (cesium vapor diode): M. Y. Gurtovoy and G. I. Kovalenko, USSR, 1941

• Ignited cesium plasma TEC with efficiencies of 5-10% at power densities of 3-10 W/cm2: K. G. Herqvist (RCA), US, 1957

• Cold War: US and USSR pursue TECs for direct

conversion of heat from nuclear pile to electricity for space missions; 20% efficiency achieved in 1968 (GE)

• 6 kW TOPAZ II, flown in 1987 on a Soviet RORSAT naval reconnaissance satellite Photo of TOPAZ II arriving at Kirkland AFB, New Mexico, early 1990s, purchased by SDIO for $6.5M

•

Ivanpah (2014), Mojave Desert, CA (ivanpah.nrgenergy.com/)

A 21st Century Application for TEC: Concentrated Solar Thermal Power

Gemasolar (2011), Spain (http://www.torresolenergy.com/TORRESOL/gemasolar-plant/en)

Thermionics: Post Cold-War Decline • Technology:

– Tungsten electrodes with adsorbed cesium to lower work functions

– Electrode gaps of around 0.1 mm space charge limits current

– Work-around: “ignited mode” operation … cesium plasma, with loss of available voltage due to potential drop

• Assessment: – Thermionics: Quo Vadis?

An Assessment of the DTRA's Advanced Thermionics Research and Development Program, National Academies Press, 2001.

Optimal Electrode Gaps We can easily fabricate gaps from 100 nm to 100 μm and control thermal expansion using MEMS design approaches Large gaps space charge limits current Small gaps excessive heat transfer due to near-field coupling from emitter to collector First-order model: parallel plate, metal electrodes with Langmuir-Child space charge limit + all heat transfer sources; use Drude model for evanescent coupling

J. H. Lee, I. Bargatin, N. A. Melosh, and R. T. Howe, Appl. Phys. Lett., 100, 173904 (2012).

Model Results

Emitter – collector gap of 1 μm – 10 μm maximizes efficiency!

J. H. Lee, I. Bargatin, N. A. Melosh, and R. T. Howe, Appl. Phys. Lett., 100, 173904 (2012).

Thermal Isolation: A Show-Stopper?

“… the MTC (micro thermionic converter) configuration has

the two electrode surfaces, differing in temperature by

approximately 500 K, separated by approximately 1 μm.

This situation creates a temperature gradient of 5 x 105

K/mm in the connecting structure.… In the opinion of the

committee, sustaining such an enormous gradient with

tolerable thermal conduction losses is not credible.”

Thermionics, Quo Vadis? An Assessment of the DTRA's Advanced Thermionics Research and Development Program, National Academies Press, 2001.

Not Really! Thermal Isolation Micro-Platforms are Well-Known …

• Micro-bolometer arrays for uncooled IR imaging (Honeywell Research, 1980s)

P. W. Kruse, et al., SPIE Proc., 3436, 572-577 (1998).

> Temperatures on the order of milli-K can be detected > Renewed interest for heads-up night vision for automobiles

Poly-SiC Micro-TEC Pixel

J. H. Lee, et al, Hilton Head 2012

U-trough suspension: 1.5 μm thick, 30 μm high walls Substrate (silicon) is the common anode In-plane interconnects

Optical Heating of Poly-SiC Microcathode

J.-H. Lee, et al, IEEE MEMS 2012, Paris.

Conversion Efficiency: A Strong Function of Low Anode (Collector) Work Function φc

I. Bargatin based on Hatsopoulos and Gyftopoulos, Thermionic Energy Conversion, vol. 1 (MIT Press, 1973)

Collector (heat sink) assumed to be at 300K for all cases

How to find these materials?

Outline




Mixed Alkali Earth Oxide Coatings: Dramatically Lower the Workfunction of Tungsten

~ 5μm

grain boundary

W base metal

electron flow

{Ba, Sr, Ca}O coated films

{Ba, Sr, Ca} carbonates to oxides

{Ba, Sr, Ca}O through grains

S. H. Chou from Y. Ohuchi, et al. IECE Technical Report ED82-89, 1992

W(100) substrate

AOx film

Ratio of A to O in AOx films? A= {Ca, Sr, Ba}

Most favorable configurations

S. H. Chou, J. Voss, A. Vojvodic, R. T. Howe, F. Abild-Pedersen, J. Phys. Chem. C (2014)

Most Favorable Configurations of AO4 on W(100) A = {Ca, Sr, Ba} with dopants D = {various metals}

(a) Top view of AO4 (b) Top view of D-AO4 (c) Side view of D-AO4


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Selecting dopants with AO4 on W(100), A = {Ca, Sr, Ba}; D = {Sc, Li}


(no D) CaO4 SrO4

BaO4

More stable

Virtual Crystal Approximation (VCA) Study of A’O4 , Sc-A’O4 , Li-A’O4 on W(100)

A’ = Ca+Sr+Ba


St. dev. in WF due to VCA ≈ 0.06 eV, less than experimental variations ~0.1 eV

Type of atoms

Cell size Run time

(each)

Regular 4x4, 2x6 ~7±2 DAYS

VCA 2x2 ~8±3 hours

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http://en.wikipedia.org/wiki/Pseudopotential

1. Generate atomic orbitals with DFT

• Atoms are represented by their pseudopotentials • Pseudopotentials only include outermost electrons • Plane wave DFT needs pseudopotentials to be computationally feasible

2. Generate smooth "pseudo- orbitals" from atomic all- electron orbitals

3. Check accuracy, reformulate and repeat 1-3 as required

DFT Simulation: New Pseudopotentials are Needed for VCA Blending


A’O4 only Sc-A’O4 Li-A’O4

Work functions

Stabilities

570 total compositions

(a) Work function and (b) Stability of Mixed Oxide Films on W(100), A’ = Ca+Sr+Ba

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A’O4 films on W(100), A’ = Ca+Sr+Ba (a) work function (b) stability

• Low WF & high stability – want optimal regions to overlap

• Ca amount most affects WF


• Small amounts of calcium improves surface stability

Experimental ratio

D. B. King, The Microminiature Thermionic Converter, AIP Conf. Proc. 552, 1152 (2001)

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Li-A’O4 films on W(100), A’ = Ca+Sr+Ba (a) low WF ~1.2 eV (b) as stable as A’O4

• Li lowers WF and keeps stability of A’O4 films


• Good for long-lifetime electron emissive applications

Alkali (Cs), alkali-earth (Ba, Sr, Ca)

• Strong surface dipoles → lower WF • Loosely bound outer electrons • Large ionic radii → larger dipoles

WF Depends on Surface Dipoles: vertical offset between A’ and O for A’O4 , Sc-A’O4 , Li-A’O4

100% Ca

100% Ba

~20% Ca ~15% Ca

• A’O4 :

max O-A’ distance → lowest WF

• D-A’O4 , D = {Sc, Li} :

• Adsorbate interactions ↓ vertical dipole


A’ = Ca+Sr+Ba

• Adjacent D and A’ atoms interact

100% Ca

Ideal Converter Efficiency at φc = 1.15 eV

Collector (heat sink) assumed at 300 K in all cases

Emitter:

I. Bargatin, based on Hatsopoulos and Gyftopoulos, Thermionic Energy Conversion, vol. 1 (MIT Press, 1973)

Computation of Thermionic Emission Current from First Principles using DFT

Johannes Voss, S. H. Chou, R. T. Howe, I. Bargatin, and F. Abild-Pedersen J. Chemical Physics, 138, 204701 (2013).

Supercell scheme in DFT calculation of thermionic emission current

Enhanced Thermionic Emission from LaB6/BaB6 Heterostructures

J. Voss, A. Vojvodic, S. H. Chou, R. T. Howe, and F. Abild-Pedersen, Phys. Rev. Appl. 2, 024004 (2014)

Work function lowering (0.4 eV) without inducing dipole barriers

Working temperature could be lowered by 400K!

Enhanced emission without sacrificing stability

Outline




Another Way to Lower the Work Function

1. Lower the vacuum level using surface coatings

2. Raise the Fermi level using built-in or applied electric fields (through another electrode) … this approach doesn’t work in 3D materials due to pinning by surface states … but it does work in 2D materials*!

Y.-J. Yu, et al (P. Kim group, Columbia), Nano Lett. 9, 3430– 3434 (2009).

Electrostatic Doping of Graphene

Insulator (SiO2)

conductor

graphene

Work Function Shift vs. Substrate Voltage

~0.65eV Kelvin Force Microscope (Agilent, SNSF*)

SNSF = Stanford Nano Shared Facilities – new name for merged SNC and SNL

Photoelectron measurements at SLAC’s SSRL (beam line 8-1) indicate a 0.4 eV shift averaged over a much larger area.

Graphene is not coated with a surface dipole

Testing a Micro-TEC

External laser heats micro-cathode with ALD-deposited mixed oxide coating, assembled adjacent to low-workfunction anode.

Prospects for Wafer-Scale Thermionic Energy Converters

• Thermionic converters are fundamentally simple devices that present challenges in high-temperature materials and fabrication

• MEMS technology can be used to make thermally isolated micro-cathode arrays and for wafer-to-wafer vacuum sealing

• Using DFT, both the workfunction and the emission current density can be found from first principles, which will accelerate the discovery of new materials for high-efficiency TECs …

Photon-Enhanced Thermionic Emission (PETE) Energy Converters

• Stanford invention by N. Melosh and J. Schwede uses both the optical energy of solar photons and the broad-spectrum thermal energy

• PETE converters benefit from low work function anodes, too!

J. Schwede, et al, Nature Materials (2010)

Acknowledgments

• Prof. Jens Nørskov • Dr. Frank Abild-Pedersen • Dr. Aleksandra Vojvodic • Dr. Johannes Voss

• Sharon Chou (now FirstFuel, Lexington, Mass.) • Dr. Igor Bargatin (now U. Penn, MEAMS Dept.) • Hongyuan Yuan • Blair Huffman • Jae Lee (now CEO, Stratio Technologies) • Dr. J Provine • Dr. Justin Snapp

PETE Research Group

• Prof. Nick Melosh, MSE Dept. • Prof. Z-X Shen,

Physics/AP/Photon Sciences/SLAC

• Dr. Karl Littau (now IMI, San Jose) • Jared Schwede • Dan Riley • Vijay Narasimhan • Sam Rosenthal • Kunal Sahasrabuddhe

R. T. Howe, J. Nørskov, and P. Pianetta, co-PIs

• Prof. Piero Pianetta, SSRL and EE Dept.

using simulations to discover new materials with energy ... - rthowe...• motivation: thermionic...

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