1Gh.V.Cimpoca, 1I.Bancuta, 2Gh.Brezeanu, 3Ileana Cernica, 3Maria Cimpoca, 4I.Grozescu

1Valahia University of Targoviste

2Politehnica University of Bucharest

3National Institute for R&D Microtechnology, Bucharest

4National Institute for R&D Electrochemical Materials, Timisoara

Al

Al 5-lea Seminar National de “Nano”, Academia Română, 2 februarie 2006

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 What makes a good Thermoelectric Material?Thermoelectric Material?

Figure of Merit Figure of Merit ZT = (αZT = (α22σ/K)Tσ/K)T

T = Absolute TemperatureT = Absolute Temperature αα22= (Seebeckcoefficient)= (Seebeckcoefficient)22

Tells how much average thermal Tells how much average thermal energy is transported by each energy is transported by each carriercarrier

σ= electrical conductivityσ= electrical conductivity Tells how much the carriers can Tells how much the carriers can

transport energy without Joule transport energy without Joule lossloss

K = thermal conductivityK = thermal conductivity Tells how small is the reverse Tells how small is the reverse

flow of heat from the cold-side to flow of heat from the cold-side to the hot-side, opposing the the hot-side, opposing the electron-transport of heatelectron-transport of heat

Minimize thermal conductivity and Minimize thermal conductivity and maximize electrical conductivitymaximize electrical conductivity

Has been the biggest dilemma for the last Has been the biggest dilemma for the last 40 years40 years

Can the conflicting requirements be Can the conflicting requirements be met by nano-scale material design?met by nano-scale material design?

Efficiency versus ZT and T

ZT need to improve over 1.3 at ZT need to improve over 1.3 at 300K for a major impact in 300K for a major impact in electronics cooling and around 2.5 electronics cooling and around 2.5 for a revolutionary impact in air – for a revolutionary impact in air – conditioning, and power from waste conditioning, and power from waste - heat- heat

With AdvancedWith Advanced

SemiconductorSemiconductor Materials?Materials?

Big Jump in ZT with the Phonon-Blocking, Electron-Transmitting StructuresBig Jump in ZT with the Phonon-Blocking, Electron-Transmitting Structures

Some of ApproachesSome of Approaches New Bulk MaterialsNew Bulk Materials

Skutterudites (Rensselaer, Oak Ridge, JPL, 1992)Skutterudites (Rensselaer, Oak Ridge, JPL, 1992) Cage – structures with ratting atoms to scatter phononsCage – structures with ratting atoms to scatter phonons

Novel Chalcogenides and Clathrates (Michigan State and Arizona, 1994)Novel Chalcogenides and Clathrates (Michigan State and Arizona, 1994) Complex Variations of BiComplex Variations of Bi22TeTe3 3 to reduce phonon mean-free pathsto reduce phonon mean-free paths

Nano-scale MaterialsNano-scale Materials Low-Dimensional Structures (MIT, Mit Lincon Labs, 1992)Low-Dimensional Structures (MIT, Mit Lincon Labs, 1992)

Quantum – confinement to Enhance Density of states which increase Quantum – confinement to Enhance Density of states which increase Seebeck coefficient Seebeck coefficient

Nano-scale Superlattice (RTI, 1992)Nano-scale Superlattice (RTI, 1992) Phonon blocking from acoustic mismatch between superlattice Phonon blocking from acoustic mismatch between superlattice

components but electron-transmitting due to negligible electron-energy components but electron-transmitting due to negligible electron-energy offsetsoffsets

Heterostructure Thermionics (UCSB, Oak Ridge, 1996) Heterostructure Thermionics (UCSB, Oak Ridge, 1996) Thermionic-like effects using energy barriers that can be controlled in Thermionic-like effects using energy barriers that can be controlled in

hetero-structures hetero-structures

Some Bulk Material and Nano-Material ProgressSome Bulk Material and Nano-Material Progress

o Cs BiCs Bi44TeTe66(Michigan State University)(Michigan State University) Bulk Materials with a ZT~ 0.8 at 225K but less than 0.8 at 300K (Science 287, Bulk Materials with a ZT~ 0.8 at 225K but less than 0.8 at 300K (Science 287,

1024-1027, 2000)1024-1027, 2000)

o Filled Skuterrudites (JPL)Filled Skuterrudites (JPL) Bulk materials with a ZT ~1.35 at 900K(Proc. Of 15Bulk materials with a ZT ~1.35 at 900K(Proc. Of 15th th International Conf. On International Conf. On

Thermoelectrics, 1996)Thermoelectrics, 1996)

o PbTe/PbTeSe Quantum-dots (Harman, MIT Lincoln Labs.)PbTe/PbTeSe Quantum-dots (Harman, MIT Lincoln Labs.) ZT~ 1.6 at 300K based on cooling data (Science 297, Sep. 2002)ZT~ 1.6 at 300K based on cooling data (Science 297, Sep. 2002)

o BiBi22TeTe33/Sb/Sb22TeTe33Superlattices(RTI)Superlattices(RTI)

ZT~2.4 at 300K in devices with all properties measured at the same place, 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 same time, with current flowing and verified by two independent techniques (Nature, 597-602, Oct. 2001)(Nature, 597-602, Oct. 2001)

New Bulk Materials

The skutterudite structure was originally The skutterudite structure was originally attributed to a mineral from Skutterud (Norway) with a attributed to a mineral from Skutterud (Norway) with a general formula (Fe, Co, Ni) Asgeneral formula (Fe, Co, Ni) As33. The skutterudite . The skutterudite structure (cubic space group Imstructure (cubic space group Im33, prototype CoAs, prototype CoAs33) is ) is illustrated in figure. The unit cell contains 8 ABillustrated in figure. The unit cell contains 8 AB33 groups. The unit cell is relatively large and contains 32 groups. The unit cell is relatively large and contains 32 atoms which indicates that a low lattice” thermal atoms which indicates that a low lattice” thermal conductivity might be possible. For the state of the art conductivity might be possible. For the state of the art thermoelectric materials such as PbTe and Bithermoelectric materials such as PbTe and Bi22TeTe33 alloys, alloys, the number of isostructural compounds is limited and the number of isostructural compounds is limited and the possibilities to optimize their properties for the possibilities to optimize their properties for maximum performance in different temperature ranges maximum performance in different temperature ranges of operation are also very limited.of operation are also very limited.

The skutterudite unit cell of formula TPnThe skutterudite unit cell of formula TPn33 (T- transition metal, Pn - pnicogen).(T- transition metal, Pn - pnicogen).

CrCryystal stal typetype Chevrel Chevrel

Experimental techniques and sample preparation

Modes Of WorkingModes Of WorkingTwo modes of working are anticipated for the in-plane thermoelectric micro-Two modes of working are anticipated for the in-plane thermoelectric micro-

generator. The first mode of working (i.e. mRTG) is when the heat source is on the generator. The first mode of working (i.e. mRTG) is when the heat source is on the

membranemembrane (Fig. 2a). (Fig. 2a). The silicon frame that sustains the membrane is the cold side. A The silicon frame that sustains the membrane is the cold side. A

large temperature difference along the thermoelectric legs should be created with small 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 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(1250 nm) and its thermal conductivity is low (3.9 W.m-1-1.K.K-1-1). This large temperature ). This large temperature

difference is interesting to get high efficiency.difference is interesting to get high efficiency.

The second mode of working (i.e. BHPW) takes advantage of the large surface-to-The second mode of working (i.e. BHPW) takes advantage of the large surface-to-

volume ratio of the membrane to use it as a radiator, the hot side being the silicon frame volume 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 (Fig. 2b). The heat source may be the heat generated by a living creature while the coolant

could be simply air.could be simply air.

Fig.2a Fig.2bFig.2b

The fabrication The fabrication methodmethod

Low stress-silicon nitride and silicon Low stress-silicon nitride and silicon

dioxide sandwich layers were deposited on a dioxide sandwich layers were deposited on a

<100>-oriented silicon wafer by low pressure <100>-oriented silicon wafer by low pressure

chemical vapor deposition (LPCVD). A chemical vapor deposition (LPCVD). A

window was etched in the dielectric multilayer window was etched in the dielectric multilayer

on the back of the silicon wafer by plasma on the back of the silicon wafer by plasma

etching. A polycrystalline silicon layer was etching. A polycrystalline silicon layer was

deposited by LPCVD at 600°C and patterned deposited by LPCVD at 600°C and patterned

by wet etching, to define the position of the by wet etching, to define the position of the

thermoelectric legs on the front side of the thermoelectric legs on the front side of the

wafer. Selected legs were implanted with wafer. Selected legs were implanted with

boron (p-type) while other legs were boron (p-type) while other legs were

implanted with phosphorus (n-type). implanted with phosphorus (n-type). Top view of a silicon-based thermoelectric micro-generator.

Figure 2. Section of microgenerator

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:

1 d1 = d2

where1, 2 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.

Polisilicon εKT

[W/mK]

dT

[nm]ZTm

ΔT [K]

ΔV [V]W

[W]

L =1,6 x 1,6 mm50 couplesP = 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 mm500 couplesP = 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 mm500 couplesP = 10 mW

vacuum 270 140 0,014 20 3,3 2,3

air 200 240 0,018 8,4 1,4 0,98

Advantages of Superlattice Advantages of Superlattice Thermoelectric TechnologyThermoelectric Technology

• • Enhanced efficiency Enhanced efficiency

• • Super-fast cooling and heatingSuper-fast cooling and heating

• • Enhanced power densityEnhanced power density

• • Localized cooling/ heating technologyLocalized cooling/ heating technology

• • 1/40,0001/40,000thth the actual TE material requirement of the actual TE material requirement of

bulk technology for same functionality – low recycle bulk technology for same functionality – low recycle

costs –Eco-friendly technologycosts –Eco-friendly technology

CONCLUSIONCONCLUSION A new family of promising thermoelectric materials with the skutterudite crystal A new family of promising thermoelectric materials with the skutterudite crystal structure has been presented. The possibilities of finding candidates for a particular 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 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 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 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 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 skutterudite compounds can be somewhat preserved, there are several approaches for large reductions in thermal could lead to ZT values substantially larger than 1.large reductions in thermal could lead to ZT values substantially larger than 1.

In-plane thermoelectric micro-generators are very promising for powering micro-In-plane thermoelectric micro-generators are very promising for powering micro-systems. A heating power of about systems. A heating power of about 100 mW100 mW may be enough to produce may be enough to produce 1 mW1 mW of useful of useful electrical power in vacuum, using thin film technology. electrical power in vacuum, using thin film technology.

Thermoelectric micro-generators based on thick-film technology will be able to Thermoelectric micro-generators based on thick-film technology will be able to work in airwork in air. They will take advantage of their large surface-to-volume ratios to improve the . 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 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 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 compact thermoelectric device may be able to produce as much as 60 mW60 mW with an output with an output voltage of about voltage of about 1.5 Volt1.5 Volt. Nevertheless, the electrical contact resistances have to be . Nevertheless, the electrical contact resistances have to be lowered to a satisfactory level, good thermoelectric materials have to be used and 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 thermoelectric thick-film technology needs to be improved or developed to get films with good thermoelectric properties at an acceptable economical cost. 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.