solid state energy conversion

Solid State Energy ConversionSolid State Energy Conversion

Auburn University October 9, 1998

Cronin B. ViningZT Services, Inc.

Auburn, Alabama USAPhone: (1) (334) 887-2404FAX: (1) (334) 887-2604

[email protected]://www.zts.com

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Dilbert 10-10-1993

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OutlineOutline

Introduction Alternative energy conversion technologies Modern ideas for new thermoelectric materials Summary

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Show CassiniShow Cassini

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Show UnicoupleShow Unicouple

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Igloo-Brand CoolerIgloo-Brand Cooler

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Production Cost ReductionProduction Cost Reduction

Production costs have decreased steadily

Significant consumer markets have opened

– Picnic Baskets use >500,000 modules/year (Igloo, Coleman, etc...)

Reliability is very high Efficiency remains near 1960

levels

Price per Watt/1993 Price

0

10

20

30

40

50

60 65 70 75 80 85 90 95Year

After R.J. Buist, 1993

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Physical Origin of ZTPhysical Origin of ZT

Any efficiency calculation will involve the same factors shown here

S, and always occur together in the end

ZT (not just Z) is the preferred quantity– this is the only unitless

combination of S, and.

– ZT also occurs in thermodynamics

Efficiency

V R

Q

S T R

K TS

RKT

S T T

T

ZTT

T

Power Out

Heat In2

2 2

2

/

/

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COPT

T T

ZT T T

ZTi

c

h c

h c

1

1 1

/

For a single stage cooler: For Power Generation

N P

Cold

Heat Rejection

Current

Typical CFC System

Current TE's

Next Generation TE's

K 300

K 355

c

h

T

T

TT

ZT

ZT TT

h c

h

1 1

1

NP

Heat Source

Heat Sink

Current

0%

10%

20%

30%

40%

50%

60%

0 2 4 6 8 10 12 14

ZT

Eff

icie

ncy

Carnot

Today

K 573

K 1323

c

h

T

T

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Applications Drive R&DApplications Drive R&D

In the ‘50-60’s semiconductors were new, hopes were high From the mid-1960s until the mid-1990s, most US R&D

has been directed to support Space Nuclear Power– Systems & hardware oriented

– Materials work focused on ‘modest’ improvements in SiGe

– Evolutionary, not revolutionary

Space Power R&D has essentially ended New ideas being supported mostly by new sponsors

– Emerging R&D hopes to significantly improve efficiencies to approach mechanical engines

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Trends in the US & the FutureTrends in the US & the Future

US TrendsSpace Defence Commercial

RTGs Reactors

PropulsionWaste Heat

Small Refrigeration Large Refrigeration

Near Cryogenic

=Stable=Cancelled=Decreasing=Growing

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BiSb with B=0.2T

Bi2Te3

PbTe

SiGe

CuNi

Temperature (K)

ZT

Experimental ZT Results

Essentially unchanged since the beginning of the SPACE AGE. Why?

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OutlineOutline

Introduction Alternative energy conversion technologies

– Thermionic– Thermophotovoltaic– AMTEC - Alkali Metal Thermal to Electric Conversion– Thermoelectric

Modern ideas for new thermoelectric materials Summary

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Thermionic ConverterThermionic Converter Plates emit electrons at different

rates Work function values of several

eV– Te ~ 1500-2000 K

– Tc ~ 1000 K Recently Mahan pointed out

refrigeration is possible with sufficiently small work functions

– Borealis is pursuing Or, replace vacuum with a solid

to reduce the work function– receiving real attention

from Angrist, 1982

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OutlineOutline




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From Angrist, 1982 A solar cell is a heat engine– The heat source is 1 AU away and at 6000 K

Thermophotovoltaics use– A lower temperature heat source, 1000-1400 K– Insulation/reflectors to return unused radiation back to the source– Narrow spectrum emitters– Lower band-gap semiconductors

ThermophotovoltaicsThermophotovoltaics

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NP Thermodynamic Cycle for AMTECThermodynamic Cycle for AMTEC

Schematic of a liquid-fed AMTEC cell

a-d: liquid vaporizes, isothermald-e: gas coolse-f: gas condenses, isothermalf-g: liquid pressurizedg-a: liquid heated

Two converters in one:1)Condensable gas cycle turns heat

into mechanical (=Pressure )2)BASE turns Pressure Drop

(= concentration difference) into electrical

Liquid-Fed AMTEC

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NP AMTEC P-V and T-S PlotsAMTEC P-V and T-S Plots

SodiumAMTEC

a

b c

ef

g

T =1100 KT =1200 Ka-b-c-d:

e-f-g: c

h

dSodiumAMTEC

(a,b) c

e(g,f)T =1100 K

T =1200 K dh

c

a-d: liquid vaporizes, isothermala-b: de-pressurization of liquidb-c: vaporization - saturatied liquidc-d: isothermal expansion

d-e: gas coolse-f: gas condenses, isothermalf-g: liquid pressurized

Na is so incompresible it doesn’t matterPump efficiency is not critical

g-a: liquid heated

Liquid-Fed AMTEC- illustration, small T

Cycle: a-b-c-d-e-f-g-aHeat in: g-a-b-c-d

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Introduction Alternative energy conversion technologies Modern ideas for new thermoelectric materials Summary

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Thermoelectricity : Coupled electrical and thermal transport

–Heat Flow resulting from an Electrical Current

–Electrical Voltages from Thermal Gradients, or

• Important applications:–Peltier Cooling, Power Generation, Thermometry

• Existing technology is reliable, but inefficient–Efficiency is related to a single material property: ZT

–Best known materials have at best ZT~1

»Efficiency limited to about 1/6 of Carnot

• If ZT~1 is the best possible: Why?• If ZT>>1 is possible: How to achieve it?

TE BasicsTE Basics

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BACKGROUNDBACKGROUND

Thermoelectricity dates to 1822 (or before) and matured to present status in the 1950’s-1960’s, along with other semiconductor technologies

The coefficient of performance () for a thermoelectric power generator or cooler depends on the active thermoelectric material through the Figure of Merit:

where

Neither equilibrium thermodynamics nor non-equilibrium thermodynamics place any upper bound on ZT.

ZTS T

2

Electrical Conductivity

S = Seebeck Coeffcient

= Thermal Conductivity

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NP DefinitionsDefinitions The Definitions of Transport Coefficients of Interest in Thermoelectricity:

Thermoelectric Property Definition Under Condition Type

Electrical Conductivity Direct

Thermal Conductivity Direct

Seebeck Coefficient Cross

Peltier Coefficient Cross

i E T 0Q T E T Q i T 0

i 0i 0

i E T ( )

Q i T i E T ( )

Generalized Ohm’s Law,Good Under All Conditions:

Symmetrical Version,Currents are created by Forces

Q E T T 2 ( )

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NP Single Band ModelSingle Band Model

( )

( )

( )

( ) ( )

( )

( )

( )

1

2

3

4 2

5

6

7

2 3 2

2

2

2

1

2

2

= lnn

ne

Sk

er

n

n

rk

eT

ZTS T

r

e r

k

e

enT

o

omkT

phonon electronic

phonon

o

p

h

total

lattice electronic

SS2

ZT=S2

T

Carrier Concentration (cm )-3

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NP Today: Doping and Alloying are the Major Effects

total

lattice electronic

S

S 2

ZT= S

2

T

Carrier Concentration (cm ) -3

total

lattice electronic

S

S 2

ZT= S

2

T

Pure "A" Pure "B"

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More Detailed ModelMore Detailed Model• Today, increasingly detailed and quantitative

models are possible

• Any transport or thermodynamic property of interest can be calculated self-consistently

• Quantitative agreement with known materials is excellent

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How To Increase ZTHow To Increase ZT• All approaches fall into one of three categories:

1. Decrease the lattice thermal conductivity• Focus on phonons• Larger unit cell and higher mass to decrease sound velocity• Increase disorder to decrease phonon mean free path

2. Increase the carrier mobility• New, covalently bonded materials• Heterostructures to physically separate carriers from scattering centers

3. Increase the thermopower• Larger effective mass materials• Barriers to inhibit transport of low energy carriers• Novel band structures and/or scattering mechanisms

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EnergyEnergy

f

df

d

of states}{density

I0=

I1=S

I2= + S2T

MobilityEdge

MobilityEdge

ZT=0.1 ZT=3

Schematic ZT CalculationSchematic ZT Calculation

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There is no theoretical basis for a limit near ZT~1– Discovery of a theoretical limit could have broad implications

Estimates indicate thermoelectric performance can triple, or more– Even incremental progress is significant

A systematic approach to advanced thermoelectric materials is very promising– Utilize recent advances in experimental and theoretical methods

– Explore new materials discovered since the 1960’s

The challenge is to accurately evaluate many possibilities

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OutlineOutline

Thermoelectric Technology Today Thermoelectric Fundamentals and Physical Phenomena Thermoelectric Materials of the Future Summary

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Where do we start to look?Where do we start to look?

Today’s materials are based on: Bi2Te3, PbTe, SiGe

– might also include BiSb, TAGS and FeSi2

These will not be replaced in the near future– Mature device technologies available

– Current markets are too small to develop new technologies quickly

By establishing a deeper understanding of today’s materials we lay the foundation for new materials– Use well understood materials to test novel ideas

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Conventional SemiconductorsConventional Semiconductors Are there semiconductors which “work” according to

conventional rules, but have more favorable parameters?– Large meff, &

– Small ph (approach the minimum possible)

– Eg > 4kT

Binary Compounds– Most (but not all) binary compounds have already been studied

– Novel binary compounds studied in recent years:» B4C, La3-xS4, La3-xTe4

» Ru2Si3, Ir3Si5, IrSi3, Ru2Ge3, Re3Ge7, Mo13Ge23, Cr11Ge19, CoGe2

» RuSb2, IrSb2, IrSb3, and CoSb3

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Conventional SemiconductorsConventional Semiconductors

Slack has surveyed all the binary compounds!

– in CRC Handbook on Thermoelectricity

– Emphasis on small electronegativity difference for high mobility values

– 28 candidate binary compounds tabulated!

– Particularly promising: IrSb3, Re6Te15, and Mo6Te8

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Skutterudites - IrSb3Skutterudites - IrSb3

Large family of compounds like MX3

– M=Co, Rh, Ir; X=P, As, Sb, many other, more complex substitutions also possible

High p-type mobility values reported– 1000-1200 cm2/V-s for IrSb3, 8000 cm2/V-s for RhSb3

Low thermal conductivity values– as low as 0.008 W/cm-K for selected alloys

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IrSb3: Premature BreakthroughIrSb3: Premature Breakthrough

Reports of ZT~1.5-2 for IrSb3 have been retracted

–Annealing causes formation of a ‘skin’ of IrSb2

–High ZT measurements now considered unreliable

–interesting physics, but much more work is required

NASA Tech Briefs, December 1994, p. 54

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Skutterudite Crystal StructureSkutterudite Crystal Structure

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Groups Working on SkutteruditesGroups Working on Skutterudites

– Slack, Nolas, Rensealear Polytechnical Institute» Experimental

– Caillat, Fleurial, Borshchevsky at JPL» Everything!

– Cook, Canfield at Ames Laboratory» Experimental

– Morelli at GM– Tritt, Gillespie, Ehrlich at Naval Research Laboratory

» Experimental

– Reineke, Naval Research Laboratory» Band Structure, Theory

– Matsubara et al, Yamaguchi University (now moved).» Experimental

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HeterostructuresHeterostructures

• Apply modern fabrication techniques to thermoelectric materials

– allows materials and properties not previously possible

– extensively applied to control electronic properties

– extension to thermal and thermoelectric properties is only starting

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• Hicks and Dresselhaus: Quantum wells

– ZT increases with decreasing size of quantum well

– Factor of 14 increase in ZT predicted for Bi2Te3!

– Another factor of 2 increase predicted for 1D quantum wires

– Theoretical work also at Naval Research Lab, Oak Ridge

• Harman at MIT Lincoln Labs is pursuing this type of approach by Molecular Beam Epitaxy

- Some early experimental results consistent with theory have been reported

• Note criticisms by Sofo, Mahan and Lyon (Oak Ridge) and by Whitlow

• Other effects could also enhance ZT

– Mobility enhancement due to physical separation between carriers and ionized impurities

– Phonon scattering and/or Bragg reflection at heterostructure boundaries

HeterostructuresHeterostructures

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HeterostructuresHeterostructures• Moizhes and Nemchinsky: Barriers enhance the Seebeck

– Carriers below the chemical potential degrade the Seebeck

– Energy barriers allow “good carriers” to pass, inhibit bad carriers

Energy

x

E>S>0

E<S<0

S E E dE ( ) ( )

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Ternary and More Complex CompoundsTernary and More Complex Compounds

Vast number of ternary compounds known

–Thousands studied for superconductivity, but TE data is rare

Copper Oxides - evaluated by Mason

– only low ZT expected, due to poor mobilities

Mn4Al3Si5 - studied by Marchuk et al

– Such anomalous RH and S results are always worth careful study

HfNiSn - studied by Dashevsky et al

– 67% metal and still a promising semiconductor!

This approach is being pursued at Mich. St. by Kanatzidis

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Unconventional SemiconductorsUnconventional Semiconductors

• Not all semiconductors work the same way

– In hopping conductors, carriers interact so strongly with “phonons” that the lattice distorts around the carrier

– In some materials, charge carriers interact with each other so strongly that electrons cannot be considered as “independent”

– Conventional selection criteria may fail for such materials

Pursue the anomalies

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Strong Carrier-Lattice InteractionStrong Carrier-Lattice Interaction

• n-type FeSi2 is a hopping conductor

– for n-type FeSi2 is about 50 times smaller than for SiGe

– but ZTmax~0.4 for FeSi2, less than 3 times small than for SiGe

– low cost and “anomalous” behavior are good reasons for further studies

• BxC has ZT~0.4-0.5

– too small mobility (~ 1 cm2/V-s), too high carrier concentration (~1021 cm-3)

– Very high melting point and composed of very light elements

– All conventional rules suggest this material has no promise

– Still, it is within 2-3 of the very best

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Strong Carrier-Carrier InteractionStrong Carrier-Carrier Interaction

• U3Pt3Bi4 - suggested by Slack

– many isostructural compounds, such as Ce3Pt3Bi4

– so-called “heavy fermion semiconductor”

– carriers behave as if they have large effective mass

– Should have high Seebeck values

• Related materials suggested by Louie and Radebaugh

– (Ce1-xLax)Ni2, (Ce1-xLax)In3, CePd3, and CeInCu2

• This General Approach is being pursued at:

– Cornell (DiSalvo), Ames Lab (Cook, Vining)

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Organic ConductorsOrganic Conductors

• Many organic polymers with high electrical conductivity are now known

– Doped polyacetylene can have electrical conductivity comparable to good metals

– At low doping levels, high Seebeck values (>1000 V/K) have been observed

– Sometimes, electrical mobility values can be quite good

– Give the low cost and the great ability to modify organic materials, some closer attention seems justified

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OutlineOutline

Thermoelectric Technology Today Thermoelectric Fundamentals and Physical Phenomena Thermoelectric Materials of the Future Summary

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Status of New Materials ResearchStatus of New Materials Research

Large ZT values have not yet been confirmed There is no easy path to large ZT But there are many plausible approaches that have yet to be

tried Persistent efforts are bound to yield exciting results

The challenge is not the generation of plausible ideas, but the rapid and accurate evaluation of those ideas

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The Goal is not NewThe Goal is not New

The Westinghouse Thermoelectric Generator Program goal for efficiency was “only 35%” because

“Frankly, I wish the goal to be one that we can attain.”

From C. Zener, 1959

solid state energy conversion

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