Thermoelectric energy Oleksandr Shpak. Outline 1. Overview 1.1 Why Thermoelectricity? 1.2...

Thermoelectric energy

Oleksandr Shpak

Outline 1. Overview

1.1 Why Thermoelectricity? 1.2 Possible Applications 1.3 How Does Thermoelectricity Work? 1.4 Efficiency 1.5 Historical Overview

2. Nano-scale Materials Technology 2.1 Quantum-Dot Superlattice (QDSL) 2.2 Silicon Nanowires 2.3 Nanocomposites

3. Summary

"Energy & Nano" - Top Master in Nanoscience Symposium 17 June 2009

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1.1 Why Thermoelectricity?

Solid-state reliability, predictability and stability

Does not use any moving parts

Vibration/noise free Chlorofluorocarbon-free, no

environmentally harmful fluids.


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A. J. Minnich, M. S. Dresselhaus, Z. F. Ren and G. Chen, Energy Environ. Sci., 2009, 2, 466–479

With thermoelectric energy conversion heat is converted directly into electricity using a class of materials known as thermoelectric materials.

1.2 Applications


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Vehicle waste heat recovery to improve fuel economy.

Industrial waste heat recovery (incinerators, cement, steel mills, and so on).

Site-specific and on-demand cooling in electronicTE Climate Control Technologies.

Radioisotope heat-powered thermoelectric generators in space probes sent beyond Mars

1.3 How thermoelectricity works?


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F.J. DiSalvo, Science 1999, 285, 703

Conversion of temperature differences directly into electricity (Seebeck effect )

Diffusion of mobile carriers from the hot side to the cold side

Electrochemical potential will form in response to a temperature gradient (Seebeck voltage)

Thermoelectric characteristic of the material is the amount of voltage generated per unit temperature gradient (Seebeck coefficient )

Can also be used as solid-state refrigerators or heat pumps by driving a current in a circuit with two dissimilar materials (Peltier effect)

1.4 Efficiency


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G.J Snyder and E.S. Toberer, Nature Materials 2008, 7, 105

Modern devices consist of many legs of alternating n-type and p-type materials, allowing a current to flow through each leg sequentially while heat flows through each leg in parallel

Today thermoelectric devices are not in common use because of low efficiency and engineering considerations

We desire materials with high electrical conductivity, high Seebeck coefficient, and low thermal conductivity

1.4 Efficiency

Power conversion efficiency is

critically dependent on the

material Figure of Merit (ZT)


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www.eere.energy.gov/vehiclesandfuels/pdfs/deer_2002/session2/2002_deer_ venkatasubramanian1.pdf

S - Seebeck coefficient σ - electrical conductivityk - thermal conductivityT - absolute temperature at which the properties are measured

Optimizing all the parameters together turns out to be very difficult because the properties are interdependent.

1.5 Historical Overview


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http://epa.gov/ncer/publications/workshop/9-15-2003/pdf/Rama.pdf

During the period 1960–1990 the (Bi1–xSbx)2(Se1–yTey)3 alloy family remained the best commercial material with ZT≈ 1.

An alternative approach is to create nanostructured materials. By using the same materials but in a nanostructured form, it is possible to modify thermoelectric properties in ways that are not possible with bulk materials, which can lead to an enhancement in ZT.

2.1 Quantum-Dot Superlattice (QDSL)


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Schematic drawing of a QDSL

T. C. Harman, P. J. Taylor, M. P. Walsh, B. E. LaForge, Science 2002, 297, 2229.

Two strategies: the use of quantum-confinement phenomena to enhance S (to control S and σ somewhat independently), and the use of numerous interfaces to scatter phonons more effectively than electrons

With a quantum well width below 4 nm S could be increased relative to bulk.

2.1 Quantum-Dot Superlattice (QDSL)


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T. C. Harman, M. P. Walsh, B. E. LaForge, G. W. Turner, J. Electron.Mater. 2005, 34, L19.

Now it is possible to grow superlattices of such sandwich structures over thousands of periods to produce a QDSL of composition PbTe/PbSe0.98Te0.02. Using Bi as an n-type dopant for this QDSL, a value of ZT~1.6 was achieved at 300K.

These materials are not practical for large-scale commercial use because they are fabricated by atomic layer deposition processes (molecular beam epitaxy etc.), making them slow and expensive to fabricate.

2.2 Silicon Nanowires


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l.I. Hochbaum, R. Chen, R.D. Delgado, W. Liang, E.C. Garnett, M. Najarian, A. Majumdar, and P. Yang, Nature 2008, 451, 163

Si is the most abundant and widely used semiconductor, with a large industrial infrastructure for low-cost and high-yield processing.

Arrays of Si nanowires were synthesized by an aqueous electroless etching (EE) method.

The nanowires varied from 20 to 300 nm in diameter with an average diameter of approximately 100 nm.

2.2 Silicon Nanowires


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A.I. Boukai, Y. Bunimovich, J. Tahir-Kheli, J.-K. Yu, W.A. Goddard III and J.R. Heath,,Nature 2008, 451, 168

Large difference in mean free path lengths between electrons and phonons at room temperature: 110 nm for electrons in highly doped samples and 300 nm for phonons. Reduction in thermal conductivity without significantly affecting S.

It is possible to achieve ZT = 0.6 at room temperature in rough Si nanowires of 50 nm diameter. With optimized doping, diameter reduction and roughness control, the ZT is likely to rise even higher.

2.3 Nanocomposites


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A.J. Minnich, M.S. Dresselhaus, Z.F. Ren and G. Chen, Energy Environ. Sci. 2009, 2, 466–479

Reduced lattice thermal conductivity does not require an atomically perfect interface or an exact geometry.

Ball milling and hot pressing can be used to create nanograined materials.

Nanocomposites retain the high density of interfaces but do not have a special geometry or structure, significantly simplifying the fabrication process and allowing the material to be produced in large quantities.

3. Summary


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Even with the current efficiencies of thermoelectric devices billions of dollars could be saved each year if thermoelectric generators were used on the exhaust of vehicles.

Modeling and experimental efforts help to understand the fundamental physics of phonon transport which is the key to further reducing the thermal conductivity. Further increases in ZT should be possible.

Thank you for you attention


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Thermoelectric energy Oleksandr Shpak. Outline 1. Overview 1.1 Why Thermoelectricity? 1.2...

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