Towards the Sustainable Use of

Thermoelectrics

R Freer

School of Materials

University of Manchester,

Manchester M13 9PL UK

Thermoelectrics - Applications

• Mmm

Exploitation of Thermoelectrics

• Seebeck 1821

• Thermocouples

- Becquerel 1826;

- Johnson Matthey 1890s

• Arthur C Clarke 1945

• – Letter to Wireless World: ……the operating period of rockets might be indefinitely prolonged by the use of thermocouples

Peltier 1834

Principles of Thermoelectrics

Seebeck Effect Peltier Effect

Material Properties

• Figure of Merit ZT

• Conversion Efficiency

Power generation from ∆T• From 1950s - TEGs

• 1960s RTEGs

• – NASA

• Pioneer 10 (1972) 4 x RTEGs – Pu-238; 140W

• New Horizon (2006) – RTG (10 Kg Pu) – 250W

Pluto 2015

Power Generation from WASTE Heat

• Autos

Preferred Materials

Want High SHigh σLow K (or λ)

Traditional Thermoelectric Materials

.Bi2Te3 Industry Standard

Future Prospects for Thermoelectric technology

Applications Sectors

• Automotive/Internal Combustion

• Wireless Sensing

• Aerospace/Marine

• Wearable/Implantable

• Building Scale Integration

• General Industry and Power Generation

• Nuclear Industry

• Geothermal Applications

High Temperatures in Industry

• Exhaust manifolds of combustion engines

• Small segments of hot pipes but millions of cars.

• Manufacturing plants

• Chemical plants -- exothermic reactions. Kilometers of pipes,

• Metallurgical industries - smelting, casting

• Gas and steam turbines -Tens of thousands of turbines

• Nuclear reactors

(Clark Review 2014)

Automotive

• Light Vehicles

Experience from VIPER1-2 – Bi2Te3, metal silicides, skutterudites.. >500 W

IPM Fraunhoffer – Half Hauslers

Impact of 2040 legislation? From Viper1 and 2, and IPM Fraunhoffer

Auto 2• Materials and modules – cost issues

(After IPM Fraunhoffer)

Aero and Marine

• Marine sector – 5% global CO2 emissions (2x Aero) – target 0.5-1.5 kW output

• Aero Now: cooling - Black Box; Drinking Water, Space Telescopes, Cameras; Space Vehicle Refrigerator, Freezers;

• Aero Future – Waste heat recovery – e.g. power for in-flight entertainment etc; military air crew clothing

Wearables• Use of the body as a power source: indoors -

heat flow between 1 to 10 mW cm-2

• Examples include ECG system

• Future - Sporting and military would be the most promising markets

(After NCSU ASSIST program)

Buildings – Heating systems

• IPM Fraunhoffer Institute.

Others….

• Buildings – sensor networks - 200 µW of electrical power from a temperature difference of 3.5C

• IoT expected to consist of about 50 billion objects by 2020; sensors, control systems…

• Geothermal - 80-100C higher than at the surface – estimate up to 10 MW may be generated

Market predictions

• Current global market for TE devices is approximately $300M and is predicted to grow to over $1Bn by 2024

• Main sectors - Automotive, Aerospace, Industrial heat, low power, sub-watt TEG

Materials Issues and Future TE Technology

Materials and Temperature Ranges• Using materials at its best operating temperature

• Development of advanced modules

300 400 500 600 700 800 900[K]

TheTce

Bi-TeBi-Te

Bi-Te

Co-Sb

Zn-Sb

Silicide

Bi-Te

Cascade Bi-Te

Cascade Bi-TeCascade

Materials and Interfaces

Cascade modules

Materials Needs• High conversion efficiency High ZT

SnSe max ZT~2.7

(Zhao et al 2015)

Which Thermoelectric Properties?

• Max ZT or average ZT?

Average ZT and η for SKD and SnSe

SnSe (Zhao et al 2015)

Strategies for Improving TE Performance

• To increase ZT

• Increase (S) Seebeck coefficient – new or changed compositions/doping

• Increase σ – doping/Band structure (DOS) engineering?

• Reduce λ – increase phonon scattering sites; nanostructuring

Microstructural Engineering and ZT

Data miningS and ρ

Significant increase in ZT (Mori et al 2017)

(Kanatzidis 2015)

Automated Manufacturing to Reduce Module Costs

IMP Fraunhoffer – Half Heusler (Zr-Hf-Co-Sb-Sn) modules (0.25$/W)

Routes to Manufacture Films and Flexible TE

.

Cambridge Display Technology 2017

Needs and Directions to New Materials Development

.

Beyond Bi2Te3 Thermoelectric Materials

.

Needs - Wish list

• Materials made from earth abundant starting materials

• Understanding factors controlling TE properties

• Design rules for achieving desired TE properties

• TE materials with high ‘average’ ZT

• TE materials suitable for wide ranges of applications, which are cost effective

He and Tritt (2017)

Earth Abundant Starting Materials

• Sulphides - Bi2S3, Cu2xS,

• Mg (Mg3Sb2) and

• Si (Mg2Si) based materials

Ge et al (2015)

.

.

SrTiO3

Composites

• Inorganic – metal nanoparticles

• Inorganic-organic hybrids

• Designed nanstructures/superstructures

SrTiO3 – Adding metal particles

• Sr0.8La0.06Ti0.8Nb0.2O3 2.5 wt% of Fe or Cu.

• Sinter at 1700 K for 24 h under Ar-5% H2.

Srivastava et al (2017)

Inorganic-Organic Hybrids

• TiS2 - [tetrabutylammonium]x[hexylammonium]• TiS2 - (HA)0.08(H2O)0.22(DMSO)0.03

Wan et al Nature 2017

SrTiO3 – Graphene Nanocomposites

• Limitation of SrTiO3 – high resistivity, high λ

• Preparation

• La(0.067)Sr(0.9)TiO3 powder, exfoliated nanographite platelets >> graphene sheets; G solution + LSTO soln. Pellets pressed at sint at 1427C (95% Ar/5% H2)

Graphene – to increase electrical conductivity and decrease thermal conductivity

Headline results

Lin, Freer, Kinloch et al, ACS Appl Mat Inter 2015Freer, Kinloch et al, WO2014125292 A1

Add up to 1 wt % graphene to lanthanum strontium titanium oxide (LSTO)ZT increaseThermal window went from 500 °C to room temperature.

SrTiO3 – Graphene Nanocomposites

• Benefits – very wide temperature ‘window’ of operation

• No need for cascade structure modules

• Current limitation – modest ZT

Designed Structures –Two-Dimensional Electron Gas (2DEG)

• 2D superlattice films grown using PLD several-unit-cell-thick 20% Nb-doped STO QW layers, sandwiched in undoped STO barrier layers.

Resulting S ~4x bulk material; ZT estimated ~2.5Ohta et al 2007

Designed Structures –3D Superlattice Ceramics

• Koumoto et al 2010 predicted room temperature ZT up to 1.2 –comparable with Bi2Te3.

• Zhang et al Nature 2018 – PLD Superstructures: PF 2x bulk

ALD Production of Superlattice Films

• .

Giri PRB 2016

Greater Understanding

• Modelling combined with

• Atom Level Characterisation

Layered CCO – charge transfer

1nm

J.D. Baran, M. Molinari, N. Kulwongwit et al., J. Phys. Chem. C 119, 21818-21827 (2015)G. Yang, Q.M. Ramasse and R. Klie, Physical Review B (2009)

G. Yang, Q.M. Ramasse and R. Klie, Applied Physics Letters (2010)

• Ca3Co4O9: incommensurately layered thermoelectric with high Seebeck coefficient. CoO2 acts as p-type crystal, Ca2CoO3 rock-salt layer acts as the charge reservoir.

• Spin-state transition observed at ~500K from EELS fine structure.

• Strain (through chemical doping) can modulate the Seebeck coefficient.

Thermoelectric Roadmap

Kajikawa 2012

Roadmap - 2

Themoelectric Network – Roadmap 2018

Future – Novel Hybrid Structures: For example: Combined PV-TE……..