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Design of thermoelectric Materials and Characterization

Outline:

Introduction

Various Sources of Energy

Why Thermoelectric Materials?

Materials Designed

a) Sucessesful completion of isoelectronic doping of Bi at Sb site in

Mg3Sb2: Mg3Sb2-xBix (0 < x < 0.4)

b) n-type in-situ composite HH/FH in new generic composition

Ti9Ni7Sn8 and Zr9Ni7Sn8 thermoelectric materials.

Prospective of present study and Future plan

• Thrust for Renewable Energy sources

• Variable outputs

• Energy Buffering

• Importance in the present context

• Why new technologies and devices?

Renewable Energy Resources

Wind Energy

Solar Energy

Geothermal Energy

Nuclear Energy

Hydroelectrical Energy

Wind Energy- Technology

PLF of the Wind Farm is normally in the range of 20% to 30% depending upon the site conditions and WTG rating.

Pros Cons

Free and in abundance Reliability of wind is not always good

Can generate enough energy for large number of people by using larger turbines

Can only be stored with a battery

Wind power can be used when electricity is down during a blackout.

Not a lot of places are suitable for turbines.

Solar Energy - Technology

PLF of Concentrating Solar Thermal Plant (CSP) – In the Range of 20% to 30 %

PLF of Photovoltaic Plants (PV) – In the range of 15% to 20% Pros Cons

Greatly reduced contribution to global warming

Some countries receive less sunlight and thus solar energy is not very effective there

Infinite Energy Resource Solar energy cannot be directly available during the night or when there are clouds blocking sun’s rays

Geothermal Energy - Technology

Pros Cons

Geothermal power is cost effective, reliable, sustainable and environmental friendly.

Hot water from geothermal sources may hold in solution trace amount of toxic chemicals such as mercury, arsenic, boron and antimony.

Adversely affect land stability

Geothermal plants use a lot of fossil fuel or coal or nuclear, and freshwater.

Nuclear Energy - Technology

Pros Cons

Little pollution High-known risks in an accident.

Quite reliable High construction costs due to complex radiation

Large power-generating capacity able to meet industrial and city needs

Waste lasts 200 – 500 thousand years

Hydroelectricity Energy - Technology

Pros Cons

Power generation is almost instantaneous

It requires a lot of time and money, and a lot of land.

May effect the environment around the area.

Thermoelectrical Energy

Pros Cons

Lack of moving parts or circulating fluid

They cannot simultaneously have low cost and high power efficiency

small size and flexible shape

Ideal when precise temperature control is required

Introduction to Thermoelectrics Materials

• Thermoelectric materials include:

– Alternating thin film layers of

Sb2Te3 and Bi2Te3.:

– Lead telluride and its alloys

– SiGe

– Materials based on

nanotechnology

– Skutterrudites

– Oxide thermoelectric materials

– Half heusler alloys

– Zintl phase compounds.

Optimal Thermoelectric Materials

The most commonly used thermoelectric material is Bi2Te3 because of its relatively high figure of merit. However, the performance of this material is still relatively low and alternate materials are being investigated with possibly better performance.

Influence of the Bi2Te3 constituents Elements on total alloy Price .

The most commonly used thermoelectric material is Bi2Te3 because of its relatively high figure of merit. However, the performance of this material is still relatively low and alternate materials are being investigated with possibly better performance.

The price of tellurium is mostly impacting bismuth telluride alloy’s price.

Phenomenology of thermoelectric materials

Metals have high σ, but low S

Insulators have high S, but low σ

Heavily doped semiconductors (n=1018-1020) makes the best thermoelectrics

J. Snyder (2003) http://www.its.caltech.edu/~jsnyder/thermoelectrics/science_page.htm

Good TE have Eg= 10 KBTC where TC is operating temperature For 600 K ; Eg = 0.5 eV For 800 K ; Eg = 0.7 eV

Small Eg leads to high mobility

However, If too small, then thermal excitation of minority carriers adversely effect FOM. (Bi-polar conduction)

The 10 KBTC rule:


Multi-valley semiconductors

Z = µ × (m*)3/2 ; µ = carrier mobility and m* =DoS effective mass

Maximize both m and m*Possible only if the semiconductors has many equivalent (degenerate) bands

Crystals with high symmetry produce several equivalent bands


Large electronegativity difference indicate ionic bonding, large charge transfer, and strong scattering of electrons by optical phonons.

This leads to low carrier mobilities

(one reason why oxides are poor thermoelectrics)

Exceptions : (i) Ruddleson-Popper phases : (SrTiO3)n (SrO)m

(ii) NaxCoO2

Oxide thermoelectrics with ZT = 2.7 @ 900 K (wikipedia)

Bonding properties


High average atomic mass Heavy atoms lead to small sound velocities and correspondingly low thermal conductivity, KL=(1/3) × CV × VS × d

Mass fluctuationsIsovalent substitutions scatter heat carrying phonons strongly because the λ of these phonons is about the as the distance between the scattering centers. Electrons, on other hand have longer λ and will be scattered less.

Thermal conductivity-I


Many atoms per cell There are 3n phonon modes. As “n” increases modes overlap. Thermal transport, then, resembles like that of glass.

High coordination numberStrictly phenomenological. Seems to facilitate rattling(Skutterudites, Clathrates, ...)

Thermal conductivity-II (contd...)


Hexagonal (P-3m1 ; # 164)

a = 4.559, c= 7.228 A

Mg(I) : (0.0, 0.0, 0.000)Mg(II) : (1/3, 2/3, 0.869)Sb(I) : (1/3, 2/3, 0.276)

Mg3Sb2 : Crystal Structure

Thermoelectric properties of

1.New n-type Ti9Ni7Sn8 compound in the form of composite

HH/FH (About to be communicated)

2.n-type Zr9Ni7Sn8 compound in the form of composite HH/Ni3Sn4

(Publishable)

3.p- and n-type TiCoSb half-Heusler Compound

•n- Zr0.5Hf0.5Ni0.8Pd0.2Sn0.99Sb0.01 ZT = 0.7 at 800 (Shen, Q. et al. Appl. Phys. Lett. 2001, 79, 4166.)•n- Zr0.4Hf0.6NiSn0.98Sb0.02 ZT = ~1.0 at 1000 K (C. Yu et al. Acta Materialia 2009, 57, 2757–2764.)•n-Zr0.25Hf0.75NiSn0.975Sb0.025 ZT = ~0.8 at 1000 K (Culp, R. S. et al. Appl. Phys. Lett. 2001, 88, 042106.)

• (Zr0.5Hf0.5)0.5Ti0.5NiSn1−ySby ZT= 1.4 at 1.4 at @700K (Sakurada, Set al.. Appl. Phys. Lett., 86, No. 8 (2005), pp.2105-2107

VacancyZX

M

Why half-Heusler Alloys as TE Materials? Motivation

High seebeck cofficient (S -500 V/K) and relatively high electrical conductivity @ 300K

Low ZT because of high lattice thermal conductivity (~7 to 17 W/mK @ 300K)

Promising for high temperature (>1000K) power generation

Solid solution alloying and doping

•High thermal stability•Involve cheap, abundant, lightweight and environmentally friendly elements;•Easy to prepare in large scale

MgAgAs-type; F4̅C3m (By W. Jeischko, Metall. Trans. A (1970)

Narrow-gap indirect-transition type semiconductor (∂ρ/∂T < 0, semimetal ∂ρ/∂T=0 or metal ∂ρ/∂T > 0)

High thermal stability (melting above 1470K with almost no sublimation at temperatures near 1270K)

Effect of nanostructuring on figure-of-merit of TE materials

Micro-composite(μm) Nano-composite(nm)

Nanostructuring substantially increases the figure of merit (ZT)

X-ray diffraction patterns Zr0.25 Hf0.75NiSn samples.

20 nm

10 nm

5.0 nm

TEM of nanostructured sample

Development of undoped half-Heusler alloy Zr0.25Hf0.75NiSn &

Nanostructuring Effect (high ZT ~ 1.09)

TEM images of nanostructured Zr0.25Hf0.75NiSn samples.

Thermoelectric Properties of Zr0.25Hf0.75NiSn

Strategy to HH/FH composites: Microstructures

ZrNiSn + x Ni (1-x) ZrNiSn (matrix) + x ZrNi2Sn (inclusion) (0≤ x ≤ 1)

MNiSn (M = Ti, Zr, Hf)

FH

4Ni

TiNi2Sn (FH) MnCu2Al-type; Fm-3m; a = 6.20 Å

VacancySnNi

Zr

TiNiSn (HH) MgAgAs-type; F-43m; a = 6.08 Å

+Ni

{MNiSn + MNi2Sn}

FH

“Twin” boundary

HH

J. Am. Chem. Soc. 2011, 133, 18843–18852

TiNi1+xSn

Stiocheometries of compounds of Zr9Ni7Sn8 and Ti9Ni7Sn8have been synthesized in order to make a super cell structure; (F m O 3m with a= 2ahhÅ) 2ahh×2ahh×2ahh ) of normal half-Heusler similar to Ru9Zn7Sb8 (super cell structure of half-Heusler;

2ahh×2ahh×2ahh ) with the hope of achieving high ZT. (Motivation)

Ru9Zn7Sb8 (Fm3O m with a= 2ahhÅ)Supercell structure 2ahh×2ahh×2ahh of normal half-Heusler phase

Metallic having 15 valence electron concentration per unit cell

So, question is whether Zr9Ni7Sn8, and Ti9Ni7Sn8 can be formed as super cell structure or not.If it forms, we can add three electron by chemical doping to achieve 18 VE equivalent to that of half-Heusler, being a semiconducting which could be a potential thermoelectric materials with significantly low thermal conductivity

Synthesis of novel compound of Ti9Ni7Sn8, Zr9Ni7Sn8 as n-type: M1+xN1-xP ( M= Ti, Zr, Hf; N= Ni, Co etc; P= Sn, Sb etc)

Synthesis Details of Ti9Ni7Sn8, Zr9Ni7Sn

SPS

TEM TE

Annealed in quartz tube using a tube furnace 800 oC for 12 hrs at residual pressure of 10-5 torr

Pieces of Zr, Ti Ni, Sn

Mechanical Milling

Arc melted

Normal Bulk Sample

Nanostructured Sample

Ti1+xNi1-xSn with x=0.125

2 2 0

2 0

2 0 2

2

0 0 0

[1 1 1 ]200 nmA

A

A

B

20 nm

B

B

5 nm5 nm 2 nm

200 HH

TEM Investigation of Bulk half-Heusler TiNiSn alloy

2 nm HH

FH

100 nm

HH

ATi6Sn5

FH

[1 1C 2]]

000 1 -1 1

3 112 2 0 HH

1 nm

[1C 1 2][1C 1 2]220

1� 1 1�

200 HH

220 HH

311 HH222 HH

513 Ti6Sn5

TEM Investigation of Bulk Ti9Ni7Sn8

FH 2.0 nm

111 FH

HH

[1 � 12]

HH

2.0 nm

FHHH

200 FH

200 HH

TEM Investigation of Nano Ti9Ni7Sn8

100 200 300 400 500

500

1000

1500

Ele

ctr

ical c

onductivity(S

/cm

)

Temperature(oC)

TiNiSn

Ti9Ni7Sn8-bulk

Ti9Ni7Sn8-nano

100 200 300 400 500

-140

-120

-100

-80

-60

-40

-20

0

Temperature(oC)

See

bec

k C

oeffi

cien

t(V

/K) TiNiSn

Ti9Ni7Sn8-bulk

Ti9Ni7Sn8-nano

Electronic Transport Behavior of Ti9Ni7Sn8 composite:

100 200 300 400 500

0.0000

0.0002

0.0004

0.0006

0.0008

0.0010

0.0012

0.0014

Pow

er

Facto

r(W

/m K

2 )

Temperture(oC)

TiNiSn

Ti9Ni7Sn8-bulk

Ti9Ni7Sn8-nano

Thermal Transport and Figure of Merit (ZT)

100 200 300 400 5000

1

2

3

4

5

Therm

al C

onductivity(W

/mK

)

Temperature(oC)

TiNiSn

Ti9Ni7Sn8-bulk

Ti9Ni7Sn8-nano

ZTBulk = 0.18 @ 773 KZTNano = 0.64 @ 773 K

Expected ZTNano > 1.0 @ elevated temperature of 973 K

0 100 200 300 400 500

0.0

0.2

0.4

0.6

0.8

Fig

ure

Of M

erit(ZT)

Temperature(oC)

TiNiSn Ti9Ni7Sn8-bulk

Ti9Ni7Sn8-nano

Thermoelectric Properties of Zr9Ni7Sn8:

HH/Ni3Sn4 Composite

XRD Investigation:

20 30 40 50 60 70 80

300 400 500 600 700 800 900 1000

1000

1200

1400

1600

Ele

ctrica

l Conduct

ivity(

S/c

m)

Temperature (K)

Bulk

Nano (500 oC)

Nano(700 oC)

Zr9Ni7Sn8 : HH/Ni3Sn4

Thermoelectric Properties of Zr9Ni7Sn8:

HH/Ni3Sn4 Composite

300 400 500 600 700 800 900 1000

300

600

900

1200

1500

1800

Bulk

Nano (500 oC)

Nano(700 oC)

Zr9Ni7Sn8 : HH/Ni3Sn4 composite

Temperature(K)

Pow

er F

acto

r(W

/K2m

)

300 400 500 600 700 800 900 1000-120

-100

-80

-60

-40

Bulk

Nano (500 oC)

Nano(700 oC)


Temperature(K)

See

bec

k C

oeffi

cien

t(V/K

)

Bulk sample measurment upto 700 C is awaitedZrNiSn, HH wiil be synthesized and will be added for comparison

1st run2nd run


300 400 500 600 700 8000.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Bulk

Nano (500 oC)

Nano(500 oC)

Temperature(K)


Ther

moel

ectr

ic F

igure

Of m

erit (ZT)

ZTBulk = 0.31 @ 773 KZTNano = 0.81 @ 773 K

Expected ZTNano > 1.0 @ elevated temperature of 973 K

300 400 500 600 700 8001.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

Bulk

Nano (500 oC)

Nano(500 oC)


Ther

mal

Conduci

tvity(

W/k

m)

Temperature(K)

Two times meaurement

1. In-situ composites of HH/Ti6Sn5 and HH/Ni3Sn4 are synthesized in Ti9Ni7Sn8 and Zr9Ni7Sn8 respectively.

2. High ZT is observed in these materials by nanostructuring approach.

3. Increased ZT is due to the drastic reduction in the thermal conductivity which might be due to large level of defects and many interfacial phonon scattering

Summary

XRD pattern of TiFexCo1-xSb1-xSnx

20 30 40 50 60 70 800

8000

16000

Temperature(oC)

Inte

nsi

ty (ar

b u

nit)

TiFe0.15Co0.85Sb0.85Sn0.15-SPSTiFe0.15Co0.85Sb0.85Sn0.15 TiFe0.15Co0.85Sb-SPS

TiFe0.15Co0.85Sb

TiCoSb-SPS

TiCoSb

Processing: mechanical milling

TiCoSb (HH)

TiFe0.15Co0.85Sb (HH)

TiFe0.15Co0.85Sb0.85Sn0.15 (HH)

0 100 200 300 400 500 600 7000

500

1000

1500

2000

2500

Ele

ctrica

l conduct

ivity(

S/c

m)

Temperature(oC)

TiCoSb TiFe

0.15Co

0.85Sb

0.85Sn

0.15

0 100 200 300 400 500 600 700-30

-25

-20

-15

-10

-5

0

5

10

15

See

bec

k C

oeffi

cien

t(V

/K)

Temperature(oC)

TiCoSb TiFe0.15Co0.85Sb0.85Sn0.15

Electrical Transport behavior of TiFexCo1-xSb1-xSnx :

n and p-type TE materials:

Thermal conductivity measurements on these samples is yet to be done ??

ZT is not calculated. Incomplete

Thermoelectric Properties of Mg3Sb2-xPbx

20 30 40 50 60 70 80

(115

)

(006

)

(115

)

(032

)

(123

)(3

00)

(105

)(1

22)

(114

)

(211

)

(210

)(0

23)

(104

)

(202

)

(004

)

(201

)(1

12)

(200

)(103

)

(110

)

(012

)(011

)

(100

)

x=0.30

x=0.20

x=0.10

Mg3Sb2-xPbx

x=0

Inte

nsi

ty (a.

u.)

Angle 2()

(002

)

Pb

XRD Investigation of Mg3Sb2-xPbx

SEM Investigation

Mg3Sb2

Mg3Sb1.8Pb0.2

Mg3Sb2 & Mg3Sb1.8Pb0.2 : EDAX analysis

Element`````` Weight% Atomic%

Mg K 23.4̅5 60.54̅

Sb L 76.55 39.4̅6

Totals 100.00

EDAX: Mg3.027Sb1.973

Element`````` Weight% Atomic%

Mg K 23.08 60.24̅

Sb L 71.4̅2 35.30

Pb M 5.50 4̅.4̅6

Totals 100.00

EDAX: Mg3.012Sb1.765Pb0.223

Mg3Sb2

Mg3Sb1.8Pb0.2

Electronic Transport Behavior of Mg3Sb2-xPbx

300 400 500 600 700 8000

750

1500

2250

3000

Temperature (K)

Ele

ctr

ical c

onductivity(S

/m)

Mg3Sb2-xPbx

X=0.0 X=0.1 X=0.2 X=0.3

300 400 500 600 700 800150

200

250

300

350

Seebeck C

oeffi

cie

nt(V

/K)

Temperature (K)

Mg3Sb2-xPbx

x=0.0 x=0.1 x=0.2 x=0.3

300 400 500 600 700 8000.00000

0.00005

0.00010

0.00015

0.00020

0.00025

0.00030

0.00035

Mg3Sb2-xPbx

Temperture (K)

x=0.0 x=0.1 x=0.2 x=0.3

Pow

er Facto

r(W

/m K

2 )


300 400 500 600 700 800

0.3

0.6

0.9

1.2

1.5

Mg3Sb2-xPbx x=0.0 x=0.1 x=0.2 x=0.3

Therm

al C

onductivity(W

/mK

)

Temperature(K)300 400 500 600 700 800

0.0

0.2

0.4

0.6

0.8

1.0

Fig

ure

Of M

erit (Z

T)

Temperature (K)

x=0.0 x=0.1 x=0.2 x=0.3

Mg3Sb2-xPbx

ZT Mg3Sb2 = 0.26 @ 773 KZT Mg3Sb1.8Pb0.2 = 0.83 @ 773 K

XRD Investigation of Mg2PbSb2 And Mg3Sb2

Thermoelectric Properties of New Zintl Compound- Mg3PbSb2 And Mg3Sb2

20 30 40 50 60 70 8012

3

11421

1

300

11520

2

20111

2

006

032

105

122

023

104

004

210

200

103

110

102

101

002

100

Inte

nsi

ty (ab

s)

2-theta (deg.)

Mg2PbSb

2 SPS 800 oC

Mg3Sb

2 SPS 800 oC

300 400 500 600 700 800

0

500

1000

1500

2000

2500

Ele

ctr

ical conductivity(S

/m)

Temperature (K)

Mg3Sb2 Mg2PbSb2

Electronic Transport Behavior of Mg3Sb2 And Mg2PbSb2

300 400 500 600 700 800

200

300

400

500

600

Seebeck C

oeffi

cie

nt(V/K

)

Temperature(K)

Mg3Sb2 Mg2PbSb2

300 400 500 600 700 800

0.00000

0.00005

0.00010

0.00015

0.00020

Mg3Sb2 Mg2PbSb2

Pow

er

Facto

r(W/m

K2 )

Temperture (K)

300 400 500 600 700 8000.3

0.6

0.9

1.2

1.5

Mg3Sb2 Mg2PbSb2

Temperature(K)

Therm

al C

onductivity(W

/mK

)

300 400 500 600 700 800

0.00

0.05

0.10

0.15

0.20

0.25

0.30

Therm

oele

ctr

ic F

igure

of m

eri

t (Z

T)

Temperature (K)

Mg3Sb2 Mg2PbSb2

Thermal Transport and Figure of Merit (ZT) of Mg3Sb2 And Mg2PbSb2

ZT Mg3Sb2 = 0.26 @ 773 KZT Mg2PbSb2 = 0.16 @ 773 K

Thank you for your attention!!

ZrNi1+xSn

ZrNiSn:Half-Heusler semiconductor (Eg=0.51 eV) a= 6.11 A

ZrNi2Sn:Full-Heusler Metal a= 6.27 A

ZrNi1+xSn: Electronic Structure

final pptw

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