Download Nanostructured Thermoelectric Materials

Nanostructured Thermoelectric Materials
Alexandre Cuenat and Laurie Winkless, NPL Nanomaterials
What are thermoelectric materials?
Thermoelectric (TE) materials can produce direct electrical power from an applied temperature difference. They are extremely reliable, ‘fuel-free’ solid-state
devices with no moving parts.
TE devices can play an important role in energy harvesting and energy efficiency. They are a long-proven technology – TE materials have been used for many years
as generators for deep space probes. However, their everyday use is limited by low efficiency. Recent developments based on nanomaterials have shown dramatic
improvements (by a factor 2 to 3) in the performance of TE devices. However, the accurate measurement of their efficiency is extremely difficult.
Heat Source
What impact can these materials have?
Recent years have seen a huge increase in worldwide energy demand. Fossil fuels are a finite resource, so alternatives are actively
sought after, along with more efficient ways of using fossil fuels. Most of our systems are also incredibly inefficient – consider for
example than more than 70% of the energy generated from a typical car engine is wasted, mostly in the form of heat. The
exhaust temperature can be up to 500°C, providing a large temperature difference – perfect for TE materials.
NPL Nanomaterials group are working on two European projects to develop accurate and traceable characterisation techniques
for Nanostructured TE Materials and devices
The basics
The Origins of Thermoelectricity
The thermoelectric effect is the conversion of temperature
difference to electric voltage (and vice versa). It refers to three
separately identified effects – the Seebeck Effect, the Peltier
effect and the Thomson Effect.
Lord Kelvin was the first to realize the connection between these
effects, and created the first non-equilibrium thermodynamic theory.
e-
Generating electrical current is based on the Seebeck effect, which is the creation of a
voltage across a material with a temperature gradient.
In materials the density of states (DOS) describes the number of states at
each energy level that are available. The Fermi level (EF) is the median
energy of of the electrons distributed along a Fermi-Dirac distribution (blue
df
line)
df
− D (E ) ×
D(E)
f
dE
dE
E
h+
Ef
This voltage can be used to drive a current through the material. The current will flow in
different direction, depending of the type of doping (n or p-type, see inset) of the
materials.
• Conductivity requires large area under differential conductivity (Green line)
• Thermopower (Seebeck coefficient) requires asymmetry in differential conductivity about
the Fermi Level, Ef (Blue area)
Thermoelectric Materials have been used for a number of years, but
have, until recently, only operated with low thermodynamic efficiency.
This efficiency is related to the Figure of merit Z of the TE
material:
• Thermal Conductivity, κ
• Electrical Conductivity, σ
ZT =
S 2σT
e2
df ⎞
⎛
τ (E)v 2 (E) ⎜ −D(E) 0 ⎟ dE
⎝
3∫
dE ⎠
σ=
κ
Thermoelectric generator (TEG)
• Seebeck Coefficient, S
Thermoelectric Materials are mostly semiconductor materials – both p and n-type (see text
bubbles). The reason for this can be seen in the table below. The goal of the engineer is
to MAXIMISE the electrical conductivity of a material, while MINIMISING the thermal
conductivity. As these transport characteristics depend on interrelated material properties,
there are great challenges surrounding the optimisation of the efficiency of the final device.
Metals
Semiconductors
Insulators
S (x10-6 VK-1)
5
200
1000
σ = ηeμ (Ω-1m-1)
108
105
10-10
λ tot = λ l + λ el
λ tot (Wm-1K-1)
λ tot ≈ λ el
10 - 1000
λ tot < λ el
1 - 100
λ tot ≈ λ l
0.1 - 1
ZT
10-3
0.1 - 1
10-14
e
df ⎞
⎛
τ (E)v 2 (E) ⎜ −D(E) 0 ⎟ (E − E f )dE
⎝
3T σ ∫
dE ⎠
It has been shown that it is possible to enhancing Z using low-dimensional materials
Electron Crystal-Phonon Glass
Standard degenerate semiconductor
Nanostructured semiconductor
>50% of the heat is carried by
phonons with mean free path over
40nm
>90% of the heat is carried by
phonons with mean free path over
5nm
P-type Materials
Dresselhaus, 2006
Impurity/Dopant
Table and plot: Comparison of thermoelectric properties of metals,
semiconductors and insulators (Fleurial, 1993, Kushch, 2004)
The critical role of metrology
N-type Materials
The difficulty in TE materials metrology is
that accurate efficiency measurements
are as complex as building an entire
device. Results can vary considerably,
particularly above room temperature
where thermal gradients in the
measurement system add to systematic
inaccuracies. An uncertainty of 50% is
quite common in TE measurements.
A doped
semiconductor where
charge is carried by
electrons (which are
negative)
e-
Phonon
Electron
• Increased Density of States near the Fermi Level
Quantum confinement S↑ lph ↓
Bulk
Six key elements of a
thermoelectric waste heat
recovery module for
vehicle applications.
NSF_DOE partnership for
thermoelectric devices
S=−
D (E )
Single Nanocrystal
D (E )
QD Superlattice
D (E )
A doped
semiconductor
where
P-type Materials
charge is carried by
A dopedare
holes (which
semiconductor where
positive)
charge is carried by
holes (which are
positive)
h+
e-
−
d f
d E
• Heavy fermion compounds: S↑ s~ le
Ideal DOS: Delta function several kT from EF will maximize power factor if mobility can be
enhanced by eliminating ionized impurity scattering
The NPL Context
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NPL is involved in two European projects on the characterisation of thermoelectric materials and devices. This fits into two of NPL’s core priorities - Energy and Sustainability. Thermoelectric
generators (TEGs) have wide-ranging applications in energy harvesting. Some of the obvious applications include: Power supply in remote locations, Wireless sensors, Wearable TEG, Body-heatpowered medical devices, Fuel reduction in cars, Industrial waste heat recovery, MEMS power, computer and IT heat management. TE devices have also been successfully deployed in space in the form of
Radioisotope thermoelectric generators (RTGs) so are already considered stable and reliable.
NPL’s aim is to develop accurate characterisation techniques for TE materials and the final devices. This work will go some way to removing the uncertainties surrounding TE efficiency measurement.
Partners on the EMRP project are: PTB (Germany), LNE (France), INRIM (Italy), MIKES (Finland), CMI (Czech Republic), SIQ (Slovenia)
Cold Source
Cold Junction
Partners on the FP7 project are: KTH (Sweden), Siemens (Germany), NCSR ”Demokritos” (Greece), Cidete (Spain), SEAT (Spain), ELECTROLUX (Sweden), CSIC (Spain), LEITAT (Spain), Cardiff
University (UK), Fraunhofer Inst (Germany), Uppsala University (Sweden)
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