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Thermoelectrics, 1999. Eighteenth International Conference on

Date Aug. 29 1999-Sept. 2 1999

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Displaying Results 1 - 25 of 172
  • Eighteenth International Conference on Thermoelectrics. Proceedings, ICT'99 (Cat. No.99TH8407)

    Publication Year: 1999
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    Freely Available from IEEE
  • An introduction to the DARPA program in advanced thermoelectric materials and devices

    Publication Year: 1999 , Page(s): 1 - 4
    Cited by:  Papers (4)
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    The Defense Advanced Research Projects Agency (DARPA) initiated its program in advanced thermoelectric materials and devices in 1995 in response to a belief that these materials could play a significant role in the development of new and enhanced military capabilities. The current and future use of these all-solid-state devices for both cooling and power generation is driven by their inherent advantages: high reliability with no maintenance, silent operation with no moving parts or vibrations, and environmental compatibility (no compressed gases or chemicals). View full abstract»

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  • Synthesis of skutterudite superlattices by controlled crystallization of elementary modulated reactants

    Publication Year: 1999 , Page(s): 5 - 8
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    Modulated elemental reactants were used to prepare a number of superlattices containing ternary skutterudites. For the superlattice [LaCo/sub 4/Sb/sub 12/]/sub n/[YCo/sub 4/Sb/sub 12/]/sub m/, low angle diffraction maxima from the superlattice structure were observed. Diffraction data collected as a function of annealing time and temperature are presented which show the evolution of the initially amorphous modulated reactant into the desired superlattice. For the [LaCo/sub 4/Sb/sub 12/]/sub n/[LaFe/sub 4/Sb/sub 12/]/sub m/ superlattice, rocking curve data were collected on both the low and high angle diffraction maxima. These data demonstrate that the skutterudite grains are randomly oriented with respect to the original superlattice but the chemical modulation of the superlattice is aligned with the substrate. Attempts to prepare skutterudite superlattices containing unfilled skutterudites were unsuccessful at short modulation distances, implying the facile inter diffusion of both the ternary cation and the transition metal through the empty cage sites in the structure. View full abstract»

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  • Lattice dynamics and LDA calculations for LaFe/sub 4/Sb/sub 12/

    Publication Year: 1999 , Page(s): 9 - 12
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    We perform first principles calculations of the total energy and Hellmann Feynman forces generated by displacements of the La atom as well as by other atoms of the filled skutterudite LaFe/sub 4/Sb/sub 12/ within the LAPW method. Assuming short ranged interatomic forces we thereby evaluate the force constant matrix between La and Fe and between La and Sb. The strongest interactions are central bond stretching and that force constant is positive for the La-Sb interaction and negative for the La-Fe interaction. The potential seen by a La atom in the fixed environment of Sb and Fe atoms is found to be quite harmonic for energies even corresponding to T somewhat greater than room temperature, and the quartic term is positive and hence constraining. By incorporating the LDA information into a lattice dynamical model for CoSb/sub 3/ the two peak structure discovered in an analysis of inelastic neutron scattering data is explained. The specific heat is also calculated and found to disagree with previous experimental results, but those results have recently been superseded by other experimental results that agree with our calculations. View full abstract»

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  • Skutterudite based new thermoelectric compounds. Cerium filled Ni substituted skutterudites Ce/sub y/Fe/sub 4-x/Ni/sub x/Sb/sub 12/ type XANES and neutron diffraction studies

    Publication Year: 1999 , Page(s): 13 - 15
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    Low thermal conductivity of filled and partially filled skutterudites is due to specific properties of filling atom. In order to achieve a better fundamental understanding of this behavior, we have performed XANES and neutron diffraction experiments for the Ce/sub y/Fe/sub 4-x/Ni/sub x/Sb/sub 12/ class compounds. View full abstract»

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  • Prediction of room temperature high thermopower in n-type filled skutterudites La-Rh-Ru-Sb

    Publication Year: 1999 , Page(s): 16 - 18
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    First principles density functional calculations for the La(Ru,Rh)/sub 4/Sb/sub 12/ system reveal a multi-valley conduction band structure with highly anisotropic, heavy mass band minima. Analysis of these results using kinetic transport theory yields high n-type thermopower above 150 /spl mu/V/K in the technologically important range 150-300 K with doping in the mid 10/sup 19/ cm/sup -3/ range. The calculations also show evidence for weak Ru-Rh alloy scattering implying that reasonable mobilities are possible if La-filling can be controlled. View full abstract»

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  • The effect of various filler atoms on the thermoelectric properties of ternary skutterudites

    Publication Year: 1999 , Page(s): 19 - 22
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    The conventional synthesis method restricts the search for the best thermoelectric material to thermodynamically stable skutterudites. Modulated reactants allows the nucleation of skutterudite antimonides that are metastable with respect to binary compounds. However, many materials with very interesting properties are only kinetically stable. A technique that employs controlled crystallization of elementally modulated reactants allows the nucleation of skutterudite antimonides that are metastable with respect to binary compounds. Using this method, a series of samples which are partially filled with Gd, Lu, La, Hf and Pb has successfully been synthesized. For the first time, these metastable materials were prepared in bulk amounts. View full abstract»

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  • Design of integrated thin film coolers

    Publication Year: 1999 , Page(s): 23 - 26
    Cited by:  Papers (2)  |  Patents (2)
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    Thin film coolers can be monolithically integrated with optoelectronic and high-speed electronic components. Important parameters in the design of such coolers are investigated theoretically and experimentally. A three-dimensional finite element simulator (ANSYS) is used to model self consistently thermal and electrical properties of a complete device structure. Heat conduction, Joule heating, thermoelectric and thermionic cooling are included as well as non ideal effects such as contact resistance, finite thermal resistance of the substrate and the heat sink, and heat generation in the wire bonds. Various substrate materials (InP, Si, Cu, diamond) and thicknesses are studied as well as the effect of current spreading in different sample geometries. Comparisons to experimental results with InGaAsP-based thermionic coolers are made. Simulations demonstrate that single stage thin film coolers can provide up to 20-30 degrees centigrade cooling with cooling power densities of several 1000's W/cm/sup 2/. View full abstract»

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  • Thermo-electro-mechanical refrigeration based on transient thermoelectric effects

    Publication Year: 1999 , Page(s): 27 - 30
    Cited by:  Papers (1)
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    This paper introduces the concept of a thermo-electromechanical cooler (TEMC), which modifies a traditional thermoelectric cooler (TEC) by using intermittent contact of a mechanical element synchronized with an applied pulsed current. Using Bi/sub 2/Te/sub 3/ as the thermoelectric material, it is predicted that the maximum temperature drop across a TEMC may be as much as 35 percent higher than that of a TEC in low heat flux applications. This effectively increases the figure of merit by a factor of 1.8. View full abstract»

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  • Performance assessment of thermoelectric coolers for use in high temperature electronics applications

    Publication Year: 1999 , Page(s): 31 - 34
    Cited by:  Papers (2)
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    Thermoelectric cooling (TEC) modules which are rated for operation up to 200/spl deg/C are now widely available commercially, making them potential candidates for use in the thermal management of high temperature electronics applications. Through the use of these TECs, traditional electronic devices could be employed at much higher temperatures than they might otherwise tolerate, by creating a low temperature "micro-environment" around the specific components of interest, To gauge the feasibility of using TECs for high temperature thermal management, a single stage TEC was operated at constant current, near its maximum temperature limit for up to 1500 hours to assess the long term effect of elevated temperature on the module's performance. Results of the test showed a gradual decline in the overall temperature differential generated by the TEC during powered operation. Analysis of the exposed module by E-SEM suggests diffusion of Te from the thermoelements into the Bi solder region and a degradation of the Ni diffusion barrier between the copper tabs and the bismuth solder layer. View full abstract»

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  • Material optimization for heterostructure integrated thermionic coolers

    Publication Year: 1999 , Page(s): 35 - 39
    Cited by:  Papers (4)
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    The material figure-of-merit for conventional thermoelectrics is /spl mu/m/sub eff/ /sup 1.5///spl beta/ where /spl mu/ is the electron or hole mobility, m/sub eff/ its effective mass, and /spl beta/ the material thermal conductivity. From the electronic point of view, in order to optimize the cooler performance, there is a trade off between electron effective mass and its mobility. While high mobility is inherently important to facilitate electron transport in the material and reduce the Joule heating, a large effective mass is only required due to the symmetry of electronic density-of-states with respect to the Fermi energy in an energy range on the order of thermal energy (k/sub B/*T) near the Fermi level. It is possible to increase this asymmetry by using doping densities so that the Fermi level is close to the bandedge. In this case there is a small number of elections participating in the conduction and the net transport of heat is small. We clarify how this trade off is alleviated in high barrier thermionic coolers. Prospects for different material systems to realize bulk and superlattice thermionic coolers are also discussed. View full abstract»

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  • Thermoelectric cooling of high power extremely localized heat sources: system aspects

    Publication Year: 1999 , Page(s): 40 - 44
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    A generalized approach to the cooling of localized high power heat sources is given. A system is considered, which consists of a heat source and a cooling unit including a cascade thermoelectric cooler (TEC) and a finned heat exchanger with a fan. The method of optimal integration of a TEC into the cooling system, which provides minimal input power is described. The cooling unit destined to maintain a concentrated (0.8/spl times/2.5 mm) 8 W heat source at 25/spl deg/C, the ambient temperature being of 70/spl deg/C, is fabricated and tested. View full abstract»

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  • Bandgap features and thermoelectric properties of Ti-based half-Heusler alloys

    Publication Year: 1999 , Page(s): 45 - 51
    Cited by:  Patents (4)
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    Electronic transport properties of narrow-gap TiNiSn and presumed wider-gap TiCoSb half-Heusler alloys are investigated by systematically doping the three sublattice sites. The two alloys are found to exhibit different doping trends. While all three sites in TiCoSb can be doped to enhance semimetallic behavior, only the Ti and Ni sites in TiNiSn can be efficiently doped. Meanwhile, several 3d dopants are found to lead to more localized electronic properties. These findings, together with results on Hall effect and thermopower measurements, have shed light on the bandgap structure of these metal-based semiconductors. Power factor and dimensionless figure of merit ZT reaching /spl sim/5.7/spl times/10/sup -3/ W/m-K/sup 2/ and /spl sim/0.5 at 680 K, respectively, are obtained in the Sb-doped (TiHf)NiSn system. The quite favorable thermoelectric parameters obtained in these low-mobility alloys are attributed to the existence of a moderately heavy electron band mass. View full abstract»

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  • Physics of the gap formation in half-Heusler compounds and bismuth chalcogenide systems

    Publication Year: 1999 , Page(s): 52 - 55
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    Using first principles electronic structure calculations based on the density functional theory, we discuss the reasons behind the formation of energy gaps in different classes of narrow-gap semiconductors which are either good or promising thermoelectrics. We find that in half-Heusler compounds such as ZrNiSn and YNiSb, the Ni atoms take active role in the gap formation, both through local symmetry breaking and hybridization. In Bi/sub 2/Te/sub 3/, the best known room temperature thermoelectric, the subtle gap structure is determined by both spin-orbit interaction and hybridization of Bi p and Te p bands. In other Bi chalcogenides and complex ternary systems containing Bi and Te, it appears that spin-orbit interaction does not play as important a role. We discuss possible reasons for this difference. View full abstract»

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  • Thermoelectric properties of Bi-doped half-Heusler alloys

    Publication Year: 1999 , Page(s): 56 - 59
    Cited by:  Patents (2)
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    Based on our preliminary promising results for Sb-doped ZrNiSn-type half-Heusler intermetallics, we made a new series of Bi-doped samples Zr/sub 0.5/Hf/sub 0.5/NiSn/sub 1-x/Bi/sub x/ with concentrations x in the range 0/spl les/x/spl les/0.02. While the isoelectronic alloying of Zr and Hf reduces the lattice thermal conductivity, doping on the Sn site with Hf controls the carrier density and thus the nature of transport. This confirms the trend we reported previously for Sb, namely, that a small amount of the group V semimetal substituted on the Sn-site has a spectacular effect on all transport properties. Specifically, the electrical resistivity is much reduced due to an order of magnitude higher carrier density, and the thermopower is exceptionally large (/spl sim/-250 /spl mu/V/K for x=0.01), comparable to the thermopower of the parent (undoped) compound. Doping on the Sn site with Sb or Hi is thus an effective way to achieve high power factors in these half-Heusler intermetallics. The optimal operational range of the intermetallics is above room temperature. View full abstract»

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  • Electrical properties of half-metallic PtMnSb-based Heusler alloys

    Publication Year: 1999 , Page(s): 60 - 63
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    We have studied the electrical properties of the Heusler the alloys, PtMnSb, PdMnSb and PtGdBi, in relationship to the electronic band structures. Measurements of the electrical conductivity, Hall mobility and thermoelectric power for the alloys were performed in the temperature range from 77 to 850 K. From the results, it was found that PtMnSb and PdMnSb exhibit half-metallic conductivities, i.e., metallic for majority spin while semiconducting for minority spin bands, while PtGdBi is a narrow-gap semiconductor of p-type. The activation energy E/sub a/ of PtGdBi was found to be about 14 meV, which. Was obtained from the slope of the hole concentration p versus 1/T curve at low temperatures region up to 300 K. The energy gap E/sub G/ in PtGdBi could not be determined for the present. The values of electrical conductivity, Seebeck coefficient, mobility and hole concentration were also found to be of about 2/spl times/10/sup 3/ S/cm, 60 /spl mu/V/K, 120 cm/sup 2//Vs, and 7/spl times/10/sup 19/ cm/sup -3/ at room temperature, respectively. View full abstract»

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  • High temperature thermoelectric properties of MNiSn (M=Zr, Hf)

    Publication Year: 1999 , Page(s): 64 - 67
    Cited by:  Papers (1)
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    The high temperature transport properties in a series of intermetallic half-Heusler alloys of the form MNiSn, where M=Zr, Hf, have been examined. The semiconducting nature of these materials due to the formation of a pseudo-gap in the density of states make them promising candidates for intermediate temperature thermoelectric applications. Samples of pure and Sb-doped ZrNiSn, HfNiSn, and (Zr-Hf)NiSn were prepared by arc melting and homogenized by heat treatment. Phase purity was determined by X-ray diffraction and the microstructures were examined by scanning electron microscopy. The temperature dependence of the electrical resistivity and Seebeck coefficient of these samples was characterized between 300 K and 1050 K. At room temperature, the data match closely with the results recently reported by us. The thermopower initially increases with temperature, exhibits a broad maximum between 400 K and 600 K, and decreases to a common value, characteristic of the magnitude of the forbidden gap. The electrical resistivity decreases with temperature following a T/sup -1/ dependence. A correlation between the magnitude of the thermopower and the Hf/Zr ratio was observed. An estimate of the magnitude of the gap was made from a plot of 1n(/spl sigma/) versus reciprocal temperature, giving a value of 0.21 eV which is in good agreement with previous estimates. The effects of antimony and bismuth doping on the electrical properties are discussed. View full abstract»

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  • Heat transfer and thermoelectric design considerations for a ground-source thermo generator

    Publication Year: 1999 , Page(s): 68 - 71
    Cited by:  Papers (2)
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    An energy harvesting device designed to produce small (sensor-scale) amounts of electric power is being developed at Mississippi State University, USA. The device will use the daily temperature difference between the air and the ground as input to a thermoelectric generator. The advantages of such a passive device include high reliability, long life-span, low visibility, night-time power production and ruggedness. In this paper, previous results which demonstrated optimum thermoelectric design parameters for this small /spl Delta/T case are extended to include effects of linearly and sinusoidally varying thermal resistance in the heat exchangers. Such variation would occur in an operating device from changes in the air-side convective heat transfer coefficient, for example. It is shown that for the linear case an optimum is reached at the geometric mean of the two extremes in thermal resistance. A closed form function of the mean value and fluctuation magnitude provides the optimum for the sinusoidal case. View full abstract»

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  • Research and development on a thermoelectric power generating system using low-calorie exhaust gas. II

    Publication Year: 1999 , Page(s): 72 - 75
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    The authors have developed U-type Fe-Si porous thermoelectric devices (ex./spl phi/ 30 mm/spl times/20 mm) and succeeded to generate electricity by the reciprocating combustion of low-calorie gas. The obtained electric power and efficiency are not presently unsatisfied, but they assure the first step of design criteria of a 10 kW generating system for the near future. In this paper, the authors present a concept of this system and key technologies such as materials, porous processing, reciprocating combustion, etc. View full abstract»

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  • Enhancement of thermoelectric power factor in composite thermoelectrics

    Publication Year: 1999 , Page(s): 76 - 79
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    An approach is described for obtaining improved values for the thermoelectric power factor by making a composite where one constituent is a "high quality thermoelectric" while the other constituent is a "benign metal", with large electrical and thermal conductivities but poor thermoelectric properties. As an example, the authors show calculated results for a composite mixture of Cu and of the high quality thermoelectric alloy (Bi/sub 2/Te/sub 3/)/sub 0.2/(Sb/sub 2/Te/sub 3/)/sub 0.8/. View full abstract»

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  • Further development of "self-powered boilers"

    Publication Year: 1999 , Page(s): 80 - 83
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    The concept of "self-powered" appliances is one where a normally flame-heated appliance generates enough electricity also to operate its electrical components, and therefore the unit operates free from the electric grid. This would be in contrast to cogeneration, in which surplus electric power is generated to be used for other functions beyond the principal purpose of the appliance. Thermoelectric generating technology is very well suited for "self-powering". A series of "self-powered" residential-scale hydronic central heating units, often referred to as "boilers" although the circulating water does not boil, have been built with thermoelectric generating modules incorporated. These are modifications of compact, wall-mounted, highly efficient units that are a standard in the Netherlands and also popular in Great Britain and northern Germany. As well as adding the thermoelectrics, these modifications have involved conversion of the existing AC power consumers to DC wherever possible within the unit and in other ways reducing electric power demand. Each of the units built in the development phase has been more prototypic and more spartan in its electricity need. The present objective is a 22 kW/sub th/ (75,000 BTU/hr) heating unit that runs on 55 to 60 W/sub el/. Twenty of these units are being built and will be field tested in 1999-2000. It appears that the case can be made that the "self-powered boiler" is marketable now in economies such as the Netherlands, where the price gap between natural gas and electricity is wide, the prices of both are high and where comfort heating is utilized a large fraction of the year. View full abstract»

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  • New proposal of high temperature thermoelectric conversion in power plant

    Publication Year: 1999 , Page(s): 84 - 87
    Cited by:  Papers (1)  |  Patents (1)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (352 KB)  

    Efficiency of thermoelectric conversion is not high, and therefore this is not used in an usual conventional energy system in spite of the advantages. Even if the product of figure of merit and temperature is improved up to 2.0 and the heat cycle temperature difference is 1000 K, the total efficiency of the conversion is around 23%. This result requests us to find a different way to use this technique in the conventional conversion system. Here, we propose a new system of thermoelectric conversion, which is applied to a steam power plant, in order to improve the total efficiency of the system. This is called the topping system. Since temperature difference between the flame and the pressured steam is large, the loss of exergy is large in the boiler and the thermoelectric conversion is available in this heat cycle. This structure does not lose the energy because the low temperature side of the element is that of the steam. If we insert the element between the inner wall and steam tube of the boiler, the resistance of the heat circuit is increased, but this is not a serious problem if GeSi semiconductor is used. A scale of the thermoelectric generation is 10000 times higher than that of the present generator, and therefore the mass production research is needed. Rough cost estimation has been done and the cost of the thermoelectric conversion is 2.1 US$/Watt, and this is almost one-third of the present price of the element based on bismuth telluride. View full abstract»

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  • New proposal of medium temperature thermoelectric conversion in power plant

    Publication Year: 1999 , Page(s): 88 - 91
    Cited by:  Papers (1)
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    There exists a large amount of unused energy in power generation plants, such as condenser waste heat which accounts for about 50% of plant thermal output. However, it consists of relatively low temperature energy, so it is difficult to generate electricity by the conventional water-steam cycle. In order to raise the efficiency, we have to install an energy recovering system to heat exchangers which have high entropy production as the boiler and the feedwater heaters instead of the condenser. A new concept of electricity generating system for the feedwater heater has been proposed. In this concept, the total generation efficiency is improved by applying the medium temperature thermoelectric generation module into heat exchanging walls or tubes in the feedwater heaters focusing on the reduction of the entropy production. The efficiency improvement of the proposed system is 1% and the allowed cost of the proposed system is proportional to the net increase of the electric power. View full abstract»

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  • Advances in 1D and 2D thermoelectric materials

    Publication Year: 1999 , Page(s): 92 - 99
    Cited by:  Papers (1)  |  Patents (2)
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    Recent advances in our understanding of 1D and 2D thermoelectric materials in the form of quantum wires (1D) and quantum wells (2D) are reviewed, with emphasis given to the physical mechanisms responsible for the enhanced thermoelectric figure of merit (ZT) in these low dimensional systems. Starting with 2D superlattices, progress in demonstrating proof-of-principle in the PbTe/Pb/sub 1-x/Eu/sub x/Te and Si/Si/sub 1-x/Ge/sub x/ systems is presented. The concept of carrier pocket engineering regarding improved thermoelectric performance for the whole superlattice Z/sub 3D/T, including both the quantum well and the barrier region, is reviewed. Particular attention is given to recent results obtained for 1D bismuth nanowire arrays and for individual Bi nanowires. View full abstract»

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  • Phonon-blocking electron-transmitting structures

    Publication Year: 1999 , Page(s): 100 - 103
    Cited by:  Papers (1)  |  Patents (19)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (419 KB)  

    We present the concept of phonon-blocking electron-transmitting structures, desirable for high-performance thermoelectric devices, with the measured properties of p-type Bi/sub 2/Te/sub 3//Sb/sub 2/Te/sub 3/ superlattice materials. Transmission electron microscopy data indicate that these superlattices are structurally nearly ideal; this is also supported by the in-plane carrier transport measurements which suggest that superlattices offer higher mobilities compared to alloy films. The cross-plane carrier transport has been evaluated by a transmission line model technique, adapted to measure the electrical conductivities normal to the plane of the thin films. From a phonon-transport point of view, the superlattices offer significantly reduced lattice thermal conductivities compared to that of alloys. Prototypal thermoelements have been prepared with these superlattice films and other reference materials. Peltier-effect differential temperatures in the range of 2 to 4K are inferred across the thermoelements during quasi-steady state current flow. Figure of merit (ZT), from isothermal cross-plane electrical resistivities and adiabatic Peltier voltages, in bulk, thin-film alloys, and superlattice thermoelements were /spl sim/1, /spl sim/0.4, and 1.6 to 3, respectively. View full abstract»

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