Transcript PHYSICS AND MODELING OF THERMOELECTRIC …

Al 5-lea Seminar National de “Nano”, Academia Română, 2 februarie 2006
1Gh.V.Cimpoca, 1I.Bancuta, 2Gh.Brezeanu, 3Ileana
3Maria
1Valahia
4National
Cimpoca, 4I.Grozescu
University of Targoviste
2Politehnica
3National
Cernica,
University of Bucharest
Institute for R&D Microtechnology, Bucharest
Institute for R&D Electrochemical Materials, Timisoara
Al
Proiect finanţat prin programul MATNANTECH, contract 250/2004
A thermoelectric in-plane micro-generator with nanometric films has been
fabricated using compatible standard semiconductor technologies (MEMS). The active
material is a nanolayer polycrystalline silicon material laid on a dielectric membrane
sustained by a silicon frame. Hicks and Dresselhouse predicted a huge increase of
figure of merit ZT if the dimensionality of the electron system in thermoelectric
materials is redused from 3D behaviour in bulk materials to 2D behaviour via
nanoscaled layers. Reduced dimensionality offers one strategy for increasing ZT
relative to bulk values [1-2].
The use of low-dimensional systems for thermoelectric applications is of
interest because low dimensionality provides:
(1) - a method for enhancing the density of states near EF, leading to an enhancement
of the Seebeck coefficient;
(2) - opportunities to take advantage of the anisotropic Fermi surfaces in multi-valley
cubic semiconductors;
(3) - opportunities to increase the boundary scattering of phonons at the barrier-well
interfaces, without as large an increase in electron scattering at the interface,
(4) - opportunities for increased carrier mobilities at a given carrier concentration when
quantum confinement conditions are satisfied [3].
What makes a good Thermoelectric Material?

Figure of Merit ZT = (α2σ/K)T
T = Absolute Temperature
 α2= (Seebeckcoefficient)2
 Tells how much average thermal
energy is transported by each
carrier
 σ= electrical conductivity
 Tells how much the carriers can
transport energy without Joule
loss
 K = thermal conductivity
 Tells how small is the reverse
flow of heat from the cold-side to
the hot-side, opposing the
electron-transport of heat
Minimize thermal conductivity and
maximize electrical conductivity
 Has been the biggest dilemma for the last
40 years
 Can the conflicting requirements be
met by nano-scale material design?


Efficiency versus ZT and DT
Big Jump in ZT with the Phonon-Blocking, Electron-Transmitting Structures
With Advanced
Semiconductor Materials?

ZT need to improve over 1.3 at
300K for a major impact in
electronics cooling and around 2.5
for a revolutionary impact in air –
conditioning, and power from waste
- heat
Some of Approaches




New Bulk Materials
 Skutterudites (Rensselaer, Oak Ridge, JPL, 1992)
 Cage – structures with ratting atoms to scatter phonons
 Novel Chalcogenides and Clathrates (Michigan State and Arizona, 1994)
 Complex Variations of Bi2Te3 to reduce phonon mean-free paths
Nano-scale Materials
 Low-Dimensional Structures (MIT, Mit Lincon Labs, 1992)
 Quantum – confinement to Enhance Density of states which increase
Seebeck coefficient
Nano-scale Superlattice (RTI, 1992)
 Phonon blocking from acoustic mismatch between superlattice
components but electron-transmitting due to negligible electron-energy
offsets
Heterostructure Thermionics (UCSB, Oak Ridge, 1996)
 Thermionic-like effects using energy barriers that can be controlled in
hetero-structures
Some Bulk Material and Nano-Material Progress
o
Cs Bi4Te6(Michigan State University)
 Bulk Materials with a ZT~ 0.8 at 225K but less than 0.8 at 300K (Science 287,
1024-1027, 2000)
o
Filled Skuterrudites (JPL)
 Bulk materials with a ZT ~1.35 at 900K(Proc. Of 15th International Conf. On
Thermoelectrics, 1996)
o
PbTe/PbTeSe Quantum-dots (Harman, MIT Lincoln Labs.)
 ZT~ 1.6 at 300K based on cooling data (Science 297, Sep. 2002)
Bi2Te3/Sb2Te3Superlattices(RTI)
o

ZT~2.4 at 300K in devices with all properties measured at the same place,
same time, with current flowing and verified by two independent techniques
(Nature, 597-602, Oct. 2001)
New Bulk Materials
The skutterudite structure was originally
attributed to a mineral from Skutterud (Norway) with a
general formula (Fe, Co, Ni) As3. The skutterudite
structure (cubic space group Im3, prototype CoAs3) is
illustrated in figure. The unit cell contains 8 AB3
groups. The unit cell is relatively large and contains 32
atoms which indicates that a low lattice” thermal
conductivity might be possible. For the state of the art
thermoelectric materials such as PbTe and Bi2Te3 alloys,
the number of isostructural compounds is limited and
the possibilities to optimize their properties for
maximum performance in different temperature ranges
of operation are also very limited.
The skutterudite unit cell of formula TPn3
(T- transition metal, Pn - pnicogen).
Crystal type Chevrel
Experimental techniques and sample preparation
Modes Of Working
Two modes of working are anticipated for the in-plane thermoelectric microgenerator. The first mode of working (i.e. mRTG) is when the heat source is on the
membrane (Fig. 2a). The silicon frame that sustains the membrane is the cold side. A
large temperature difference along the thermoelectric legs should be created with small
heat sources because the thickness of the area covered by the thermoelectric leg is thin
(1250 nm) and its thermal conductivity is low (3.9 W.m-1.K-1). This large temperature
difference is interesting to get high efficiency.
Fig.2a
Fig.2b
Fig.2b
The second mode of working (i.e. BHPW) takes advantage of the large surface-tovolume ratio of the membrane to use it as a radiator, the hot side being the silicon frame
(Fig. 2b). The heat source may be the heat generated by a living creature while the coolant
could be simply air.
The fabrication method
Low stress-silicon nitride and silicon
dioxide sandwich layers were deposited on a
<100>-oriented silicon wafer by low pressure
chemical vapor deposition (LPCVD). A
window was etched in the dielectric multilayer
on the back of the silicon wafer by plasma
etching. A polycrystalline silicon layer was
deposited by LPCVD at 600°C and patterned
by wet etching, to define the position of the
thermoelectric legs on the front side of the
wafer. Selected legs were implanted with
boron (p-type) while other legs were
implanted with phosphorus (n-type).
Figure 2. Section of microgenerator
Top view of a silicon-based
thermoelectric micro-generator.
Using the model proposed by Koslov, assuming a onedimensional heat transfer along the
thermoelectric legs, an active material with a low figure-of-merit and neglecting the heat
losses by radiation and convection, it can be easily demonstrated that the maximum
electrical power produced by a mRTG, at a given heating power is obtained for a
thermoelectric leg thickness calculated from:
K1 d1 = K2 d2
where K1, K2 and d1, d2 are the thermal conductivities and thicknesses of the dielectric
membrane and of the thermoelectric material, respectively
The optimum leg length is calculated from:
l2/l1=1/3
where l1 is half of the self-standing membrane width and l2 is the thermoelectric leg length.
ε
KT
[mW/m
K]
dT
[nm]
ZTm
ΔT
[K]
ΔV [V]
W
[mW]
L =1,6 x 1,6 mm
50 couples
P = 1 mW
vacuum
180
150
0,014
8,0
0,13
0,090
air
150
190
0,016
5,1
0,084
0,058
L =1,6 x 1,6 mm
500 couples
P = 5 mW
vacuum
270
140
0,014
9,9
1,6
0,58
air
200
240
0,018
4,2
0,70
0,24
L =1,6 x 1,6 mm
500 couples
P = 10 mW
vacuum
270
140
0,014
20
3,3
2,3
air
200
240
0,018
8,4
1,4
0,98
Polisilicon
Advantages of Superlattice
Thermoelectric Technology
• Enhanced efficiency
• Super-fast cooling and heating
• Enhanced power density
• Localized cooling/ heating technology
• 1/40,000th the actual TE material requirement of
bulk technology for same functionality – low recycle
costs –Eco-friendly technology
CONCLUSION
A new family of promising thermoelectric materials with the skutterudite
crystal structure has been presented. The possibilities of finding candidates for a
particular operating temperature are great in such a large family of materials. Initial
results obtained on some of their representatives demonstrate the great potential of
skutterudites for high ZT values as very high mobility and very low lattice thermal
conductivity can be obtained with materials of the same crystal structure. In particular,
that if the high mobility of the binary skutterudite compounds can be somewhat
preserved, there are several approaches for large reductions in thermal could lead to ZT
values substantially larger than 1.

In-plane thermoelectric micro-generators are very promising for powering
micro-systems. A heating power of about 100 mW may be enough to produce 1 mW of
useful electrical power in vacuum, using thin film technology.

Thermoelectric micro-generators based on thick-film technology will be able
to work in air. They will take advantage of their large surface-to-volume ratios to
improve the coupling between the heat reservoirs and the thermo elements. This makes
it a very promising device to efficiently convert heat wasted by our body to electrical
power. A compact thermoelectric device may be able to produce as much as 60 mW
with an output voltage of about 1.5 Volt. Nevertheless, the electrical contact resistances
have to be lowered to a satisfactory level, good thermoelectric materials have to be
used and thermoelectric thick-film technology needs to be improved or developed to get
films with good thermoelectric properties at an acceptable economical cost.

REFERENCES
[1] Hicks, L.D et al., “Effect of quantum-well structures on thermoelectronic figure of merit”
Phys.Rev.B.Vol 47, No 19 (1993), pp.12727-12731
[2] Dresselhouse, M.S. et al., „Low Dimensional Thermoelectrics“, Proc.16 th International
Conference on termoelectrics, Dresden, Germany, August 1997, pp 92-99
[3] D.-J. YAO, C.-J. KIM, and G. CHEN – „Design of thin-film thermoelectric microcoolers”,
in HTD-Vol. 366-2, Proceedings of the ASME Heat Transfer Division – 2000, Volume 2,
ASME 2000.
[4] Gh.V.Cimpoca et all, „Physics and Modeling of Thermoelectric Microgenerators”, 6 th
International Balkan Worckshop on Applied Physics, Constanta, Romania, 5-7 July,
2005.