1997 MRS Spring Meeting & Exhibit

Chairs

Mercouri Kanatzidis, Michigan State Univ
Hylan Lyon, Marlow Industries Inc
Gerald Mahan, Univ of Tennessee
Terry Tritt, Clemson Univ

Symposium Support

• Naval Research Laboratory in conjunction with DARPA
• Office of Naval Research
• Army Research Office

Proceedings published as Volume 478
of the Materials Research Society
Symposium Proceedings Series.

In sessions below "*" indicates an invited paper.

SESSION Q1: INTRODUCTION TO THERMOELECTRIC MATERIALS 
Monday Afternoon, March 31, 1997
Salon 14

1:30 PM Q1.1 
BASIC THERMOELECTRICITY: FROM ANCIENT TIMES TO PRESENT, Cronin B. Vining, ZT Services Inc, Auburn, AL.

Thermoelectricity today is receiving renewed attention spurred by recent theoretical and experimental developments, as well as by growth in niche markets for current technology. Modern applications range from coolers for infrared detectors and DNA testing, to power supplies for remote locations and space probes. And of course consumer items such as beverage coolers. As an aid to place current activities in context, it seems appropriate, therefore, to review the history, basic principles, and present state of the art of thermoelectric technology. The origin of thermoelectric effects in solids, basic temperature measurement, cooling and power generation principles and applications will be discussed. Selected aspects of the history of thermoelectricity, particularly the influence of the progress in semiconductors on thermoelectricity in the 1950s and 1960s, the role of Ioffe, and space power applications, will be discussed.

2:00 PM Q1.2 
SELECTION AND EVALUATION OF MATERIALS FOR THERMOELECTRIC APPLICATIONS I, Jean-Pierre Fleurial, California Inst of Technology, Jet Propulsion Laboratory, Pasadena, CA.

This lecture aims at presenting an overview of the selection, preparation, characterization and optimization of semiconducting materials for thermoelectric applications. After shortly reviewing the various processes and techniques for preparing and characterizing such materials, approaches for optimization of their thermoelectric properties will be described. Following a brief description of the properties of state-of-the art materials for various thermoelectric application, the search for novel high performance thermoelectric materials and structures will be discussed. 
1. General requirements for semiconducting thermoelectric materials Introduction; Characteristics of materials resulting in good thermoelectric properties; general requirements for electrical and thermal transport properties; special considerations for high temperature materials (power generation) and low temperature materials (cooling). 
2. Preparation and evaluation of thermoelectric materials Experimental preparation of thermoelectric materials; crystal growth techniques; powder metallurgy processes; thermodynamics and phase diagrams; metallographic evaluations and transport properties measurements. 
3. Achieving optimum thermoelectric properties Optimization of thermoelectric properties; materials quality; thermal conductivity reductions, optimum doping; maximum figure of merit. 
4. Performance of state-of-the-art materials State-of-the-art materials; properties and temperature ranges of application; optimizing n-type and p-type materials; limits on current state-of-the-art materials. 
5. Novel materials for high performance thermoelectrics Improvement in performance of thermoelectrics; search for new thermoelectric materials; new classes of materials; thin film structures; special considerations for cooling and power generation applications.

2:30 PM Q1.3 
SELECTION AND EVALUATION OF MATERIALS FOR THERMOELECTRIC APPLICATIONS II, Jeff W. Sharp, Marlow Industries Inc, Dallas, TX.

In good thermoelectrics, phonons have short mean free paths, and charge carriers have long ones. The other requirements are a multivalley band structure and a bandgap greater than 0.1 eV for the 200 to 300 K temperature range. We discuss the use of solid state physics and chemistry concepts, along with atomic and crystal structure data, to select the new materials most likely to meet these criteria.

3:30 PM Q1.4 
MEASUREMENT AND CHARACTERIZATION TECHNIQUES FOR THERMOELECTRIC MATERIALS, Terry M. Tritt, Clemson Univ, Dept of Physics & Astronomy, Clemson, SC.

Many new researchers are coming into the research field of thermoelectric materials. Characterizing a material for its potential for use in thermoelectric applications involves a broad spectrum of measurements, including parameters related to both electrical and thermal transport. Basic measurement techniques will be discussed including conductivity, thermopower, thermal conductivity and Hall effect. The ''Harman technique'' for measuring the figure of merit will also be discussed and addressed. Specifically, errors and pitfalls encountered in the various measurement methods will be discussed. Contact effects will be discussed and ways to reduce these effects as well as other errors will be suggested. These errors or pitfalls will then be discussed in relation to the effect they may have on calculating the figure of merit for a thermoelectric material. Many new measurement techniques exist today that did not exist 30 years ago but the problems that did exist still have to be overcome. Hopefully a more complete appreciation of the difficulties and care that must be taken in characterizing a thermoelectric material will be conveyed to these new people in the thermoelectrics field.

4:00 PM Q1.5 
EXPERIMENTAL TRANSPORT PHENOMENA AND OPTIMIZATION STRATEGIES FOR THERMOELECTRICS, A. C. Ehrlich, Naval Research Laboratory, Washington, DC.

When a new and promising thermoelectric material is discovered, an effort is undertaken to improve its ''figure of merit.'' If the effort is to be more efficient than one of trial and error with perhaps some ''rule of thumb guidance'' then it is important to be able to make the connection between experimental data and the underlying material characteristics, electronic and phononic, that influence the figure of merit. Transport and fermiology experimental data can be used to evaluate these material characteristics and thus establish trends as a function of some controllable parameter, such as composition. In this tutorial some of the generic materials characteristics generally believed to be required for a high figure of merit will be discussed in terms of the experimental approach to their evaluation and optimization. Transport and fermiology experiments will be emphasized and both what they can reveal and what can be obscured by the simplifying assumptions generally used in their interpretation will be outlined.

4:30 PM *Q1.6 
DESIGN CONCEPTS FOR IMPROVED THERMOELECTIC MATERIALS, Glen A. Slack, Rensselaer Polytechnic Inst, Dept of Physics, Troy, NY.

The design concepts for thermoelectric materials used by A. Ioffe in the l950s were to, first, employ semiconductors chosen from heavy-element compounds; second, choose those whose lattice thermal conductivity could be reduced by making mixed crystals. These concepts can now be supplemented by looking for covalent compounds with small electronegativity differences between their constituent elements, highly polarizable elements, and some loosely bound atoms within the structure that produce very low lattice thermal conductivities approaching the minimum values. The skutterudites and other crystal structures possess some of these attributes. Clathrate structures and crystals with atoms of 3 or 4 coordination are potentially interesting. Some examples will be given. Some speculations on crystals with peaks in the electron density of states will be given as another possible approach to new materials.

SESSION Q2: ARTIFICIAL STRUCTURES I (QUANTUM CONFINEMENT ETC.) 
Chair: A. C. Ehrlich
Tuesday Morning, April 1, 1997
Salon 14

8:30 AM *Q2.1 
PROSPECTS FOR HIGH THERMOELECTRIC FIGURES OF MERIT IN 2D SYSTEMS, Mildred S. Dresselhaus, MIT, Dept of EECS & Physics, Cambridge, MA; Theodore C. Harman, MIT Lincoln Laboratory, Lexington, MA; Xiangzhong Sun, S. B. Cronin, MIT, Dept of Physics, Cambridge, MA; K. L. Wang, Univ of California-Los Angeles, Dept of EE, Los Angeles, CA.

Theoretical models indicate that the quantum confinement of carriers can enhance the thermodynamic figure of merit (), so that 2D quantum wells are expected to be favorable for enhancing values. Experimental confirmation of this prediction has been achieved in both -type and -type PbTe/PbEuTe quantum wells. Having accomplished this milestone, the field is now moving in several directions. One of these directions involves efforts to demonstrate enhanced in a variety of other quantum well materials to gain a better understanding of the confinement phenomenon in actual systems. A second thrust involves strategies to reduce the barrier width and increase the barrier height. Progress in both of these directions will be reviewed.

9:00 AM Q2.2 
THERMOELECTRIC POWER OF Bi AND BiSb ALLOY THIN FILMS AND SUPERLATTICES GROWN BY MBE, Sunglae Cho, John B. Ketterson, Antonio DiVenere, Northwestern Univ, Dept of Physics & Astronomy, Evanston, IL; Jerry R. Meyer, Naval Research Laboratory, Washington, DC; George K. Wong, Northwestern Univ, Dept of Physics & Astronomy, Evanston, IL; Craig A. Hoffman, Naval Research Laboratory, Washington, DC.

Predictions of the enhancement of the thermoelectric figure of merit, ZT, for quantum well structures have created tremendous excitement and opportunity for the application of new material systems for enhanced device performance. Two particularly promising materials are Bi and BiSb alloys. The thermoelectric power of Bi and BiSb alloy thin films as well as Bi/CdTe and BiSb/CdTe superlattices were measured as a function of temperature. The films were grown by molecular beam epitaxy on CdTe(111)B substrates prepared by standard procedures. Initial results show an enhancement of the thermopower over bulk values. Conductivity measurements are in progress and will be reported. We will also discuss the advantages of the BiSb alloy system over the pure Bi system.

9:15 AM *Q2.3 
ENHANCEMENT IN FIGURE-OF-MERIT WITH SUPERLATTICE STRUCTURES AND PROGRESS TOWARDS THIN-FILM THERMOELECTRIC DEVICES, R. Venkatasubramanian, Research Triangle Inst, Ctr for Semiconductor Research, Research Triangle Pk, NC.

Superlattice (SL) structures in thermoelectric materials appear to be an exciting route to obtaining enhanced figure-of-merit (ZT) over bulk alloyed materials. We will describe our experimental results with the SL approach in two material systems, Bi and Si/Ge, relevant to thermoelectric cooling and power conversion, respectively. it First, the SL structures offer higher carrier mobilities compared to alloyed materials, through reduced alloy scattering of carriers as well as reduced impurity scattering of carriers. , the effects of improved carrier mobility and the average higher energy of carriers (with respect to the Fermi-energy) in an SL, compared to an alloy, lead to enhanced Seebeck coefficients and larger power factors. Our results indicate that the SL structures, with little or negligible quantum-confinement, can offer improved power factors. , and most importantly, the short-period SL structures can significantly reduce the thermal conductivity due to the various phonon scattering processes unique to SL structures. We first reported experimental measurements of thermal conductivities in SL structures in thermoelectric materials and indicated them to be substantially lower than their respective alloys. The short-period SL structures, as opposed to quantum-well structures where the thick barrier layers can increase the thermal conductivity and deteriorate the effective ZT, offer factorial improvements in the three-dimensional ZT compared to current state-of the-art bulk alloys. We will also briefly describe a device strategy called Bipolarity Assembled, Series-Inter-Connected, Thin-Film Thermoelectric Device (BASIC-TFTD) to utilize these thin-film SL structures, only ten to twenty microns thick, for a variety of applications. We will present results on low resistivity (2 to 3E-8 Ohm-cm) electrical contacts to Bi-based SL materials to indicate the practical feasibility of such thin-film thermoelectric devices.

9:45 AM Q2.4 
THERMAL CONDUCTIVITY AND HEAT TRANSFER IN SUPERLATTICES, Gang Chen, Maria Neagu, Theodorian Borca-Tasciuc, Duke Univ, Dept of ME&MS, Durham, NC.

This paper presents both theoretical and experimental studies on the thermal conductivity of superlattice structures. The theoretical approach is based on the solution of the phonon Boltzmann transport equation (BTE). Different interface conditions including specular, diffuse, and partially specular and partially diffuse scattering of phonons are included in the formulation. Solutions of the BTE for heat conduction in the directions both parallel and perpendicular to the film plane are obtained. The results show that the thermal conductivity of superlattice structures is very sensitive to the interface conditions. Perfectly specular interfaces have little influence on the thermal conductivity of superlattices, while a small increase in the diffuse scattering of phonons may cause significant reduction in the thermal conductivity. A comparison of the theoretical results with experimentally measured thermal conductivity of GaAs/AlAs superlattice structures demonstrates that the observed reduction in the thermal conductivity of these structures can only be explained by assuming that the phonon scattering are partially specular and partially diffuse. The models established are then applied to other superlattice structures of current interests in thermoelectric applications including Si/Ge and Be2Te3/Sb2Te3.

SESSION Q3: NEW MATERIALS I 
Chair: Terry M. Tritt
Tuesday Morning, April 1, 1997
Salon 14

10:30 AM *Q3.1 
STUDIES OF BULK MATERIALS FOR THERMOELECTRIC COOLING, Jeff W. Sharp, George S. Nolas, Edward H. Volckmann, Hylan B. Lyon, Marlow Industries Inc, Dallas, TX.

We briefly discuss the selection of new materials for thermoelectrics, then report crystallographic and thermoelectric data for materials that we are investigating. These materials belong to one of three categories of semiconductors: 1) cage-like structures that allow the introduction of guest atoms, 2) compounds featuring anion networks, or 3) rare earth compounds. In the first case, the approach is to lower the lattice thermal conductivity to near the theoretical minimum. In the second case, good electronic properties are expected due to the absence of polarity. In the third case, we are beginning an effort to learn if f bands are superior conductivity channels for thermoelectrics. 400 C. Electromigration activation energy was obtained for films deposited at the different accelerating voltages.

11:00 AM Q3.2 
INITIAL ASSESSMENT OF THE THERMOELECTRIC PROPERTIES OF ZnCdSb SOLID SOLUTIONS, Thierry Caillat, A. Borshchevsky, Jean-Pierre Fleurial, California Inst of Technology, Jet Propulsion Laboratory, Pasadena, CA.

-ZnSb was recently identified as a new high performance p-type thermoelectric material [1]. A maximum dimensionless thermoelectric figure of merit, ZT, of about 1.3 was obtained on p-type -ZnSb samples at a temperature of about 400C. This is the highest figure of merit ever obtained at this temperature for a p-type thermoelectric material. The possibility of improving ZT values further by forming solid solutions between the isostuctural compounds ZnSb and CdSb was studied. ZnCdSb crystals with 0 x <1 were grown using a Bridgman gradient freeze technique and characterized by x-ray diffractometry and microprobe analysis. Initial results show that ZnCdSb crystals showed significantly lower room temperature thermal conductivity values than those obtained for -ZnSb. Room temperature lattice thermal conductivity values as low as 4 mW/cmK ware calculated for some compositions. The temperature dependence is also strongly reduced for the alloys. In addition, ZnCdSb crystals are characterized by relatively large effective masses, resulting in good Seebeck coefficient values. A maximum ZT value of 1.4 at a temperature of 250C was obtained for a sample with a composition ZnCdSb. The very low thermal conductivity of these materials suggest that high ZT values might be achievable at low temperatures (200-300 K). The optimization required to achieve such performance in this temperature range is discussed. v 11:15 AM *Q3.3 
RARE EARTH INTERMETALLICS: WILL THEY MAKE THE GRADE?, Francis J. DiSalvo, Cornell Univ, Dept of Chemistry, Ithaca, NY.

A small number of rare earth compounds have unusually high thermopowers for metallic conductors (S 100uV/K). In a few cases, this high thermopower is maintained over a broad temperature interval (say 100-300 K). All the materials which show such high thermopower display intermediate valence effects of the rare earth atoms (typically Ce or Yb). 
In these metallic systems, it is expected that the product pK has a minimum value determined by the Wiedemann Franz (WF) law. Even in the heavily doped Bi and PbTe materials used in TE cooling, the measured product is given by WF. In order to increase the figure of merit (Z) above that in Bi, the thermopower (S) of any candidate bulk material must be considerably larger than the 200 V found in Bi. Since Z = S, thermopowers in excess of 300 V/K are necessary for a better than factor of two improvement in Z. In this presentation we report on our efforts to increase S by chemical doping and our search for other intermediate valence materials with even higher thermopowers.

11:45 AM Q3.4 
A NEW CLASS OF MATERIALS WITH PROMISING THERMOELECTRIC PROPERTIES: MNiSn (M = Ti, Zr, Hf), Heinrich Hohl, A. P. Ramirez, Bell Labs, Lucent Technologies, Murray Hill, NJ; W. Kaefer, K. Fess, Ch. Thurner, Ch. Kloc, Univ of Konstanz, Faculty of Physics, Konstanz, GERMANY; E. Bucher, Bell Labs, Lucent Technologies, Murray Hill, NJ.

TiNiSn, ZrNiSn, and HfNiSn are members of a large group of intermetallic compounds which crystallize in the cubic MgAgAs structure. Both single crystals and polycrystalline samples of these compounds have been prepared and investigated for their thermoelectric properties. With thermopowers of up to -300 V/K and resistivities of a few mcm, power factors S as high as 35 W/Kcm were obtained at 400 K. These remarkably high power factors are, however, accompanied by a thermal conductivity which is too high for applications. In order to reduce the parasitic lattice thermal conductivity, solid solutions (Ti,Zr)NiSn, (Ti,Hf)NiSn and (Zr,Hf)NiSn were formed. Thermoelectric properties of these solid solutions and the resulting figures of merit will be presented.

SESSION Q4: RECENT ADVANCES IN BULK BiTe, BiSb AND PbTe RESEARCH 
Chair: Thierry Caillat
Tuesday Afternoon, April 1, 1997
Salon 14

1:30 PM Q4.1 
SYNTHESIS OF NANOSTRUCTURED BiTe, Liu Yang, E. J. Benstock, S. Pirzada, T Yadav, Nanomaterials Research Corporation, Tucson, AZ.

It has been suggested that engineering traditional thermoelectric materials into ultrafine nanostructure may lead to a significant improvement in their thermoelectric performance because of the enhancement of Seebeck coefficient and reduction of thermal conductivity. This work focuses on the synthesis of nanostructured BiTe. The microstructure features of nanostructured BiTe will be discussed. Preliminary results of the measurement of their thermoelectric properties were also presented.

1:45 PM Q4.2 
TRANSPORT PROPERTIES OF BiTe:Sn, Sergei A. Nemov, Yuri I. Ravich, Marina K. Zhitinskaya, St Petersburg Technical Univ, Dept of Semiconductors Physics, St. Petersburg, RUSSIA.

Resonant states of impurities and defects, finded in AB compounds, permit to receive improved thermoelectrical properties of these materials through Fermi level's pinning and selective character of carrier's scattering into resonant states. Therefore it is interesting to search similar resonant states in AB. From our and literature data it follows that there is a good chance to find these states in BiTe:Sn. Our investigations of galvano- and thermomagnetic effects on the perfect monocrystals BiTe:Sn are presented in this paper. Our experimental data confirm the existence of impurity's states (Sn) against the background of the allowed valence band. This fact permits to hope on the improvement of figure of merit in compounds traditionally used for middletemperature thermoelectric performances. It should be noted that semiconductors with resonant states constitute a new class of materials. There are considerable opportunities for control of physical properties and technical parameters of these materials.

2:00 PM Q4.3 
DEVELOPMENT OF BI-SB-TE TERNARY ALLOY WITH COMPOSITIONALLY GRADED STRUCTURE, Atsushi Yamamoto, Toshitaka Ohta, Electrotechnical Laboratory, Energy Fundamentals Div, Ibaraki, JAPAN.

An attempt was made to develop compositionally graded Bi-Sb-Te thermoelectric materials by PIES (Pulverized and Intermixed Elements Sintering) method, which is featured with a mechanical grinding of powder elements by using ball-milling and a sintering by using hot-press. The materials were developed by following two steps. In the first step, the thermoelectric properties of the samples with different compositions were measured in order to collect data for designing the optimum graded structure. The composition of Bi-Sb-Te alloy was varied in wide spans as a parameter, and Seebeck coefficients, resistivities and thermal conductivities were measured as a function of temperature. As a result, it was clarified that p-n transition took place at mole fraction of antimony telluride y=0.7 and suitable compositions for designing p-type samples were ranged between y=0.75 and y=0.9. Figure of merit Z increased in higher temperature region with increased antimony telluride fraction. In the second step, segmented p-type samples were designed and prepared by using hot-pressing. The samples were designed to maximize the electrical output at the temperature difference of 200K and consisted of three segments of different compositions, y=0.8, 0.825 and 0.9. The electrical output of the samples showed an asymmetry according to the direction of heat flux produced by external heat source and heat sink. This asymmetry of electrical output seemed to stem from compositionally graded structure.

2:15 PM Q4.4 
THERMOELECTRIC PROPERTIES OF P-TYPE (BiSb)Te FABRICATED BY MECHANICAL ALLOYING PROCESS, Boo Yang Jung, Jae Sik Choi, Tae-Sung Oh, Hong-Ik Univ, Dept of Metallurgy & Matl Sci, Seoul, SOUTH KOREA; Dow-Bin Hyun, Korea Inst of Science and Technology, Metals Division, Seoul, SOUTH KOREA.

Thermoelectric properties of polycrystalline (Bi (0.75 0.85), fabricated by mechanical alloying and hot pressing methods, have been investigated. Formation of (Bi alloy powders was completed by mechanical alloying for 5 hours at ball-to-material ratio of : 5:1 and processing time for (Bi formation increased with Sb content x. When (Bi was hot pressed at temperatures ranging from 300C to 550C for 30 minutes, figure of merit increased with hot pressing temperature and maximum value of 2.8x10/K could be obtained by hot pressing at 550C. When hot pressed at 550C, (Bi exhibited figure of merit of 2.92x10/K, which could be improved to 2.97x10/K with addition of 1 wt Sb as acceptor dopant.

2:45 PM Q4.5 
MICROSTRUCTURAL AND THERMOELECTRIC PROPERTIES OF P-TYPE TeDOPED BiSbTe AND N-TYPE SbI-DOPED BiTeSe COMPOUNDS, Jun-Ho Seo, Chi-Hwan Lee, Inha Univ, Dept of Metallurgical Engr, Inchon, SOUTH KOREA; Kyeongsoon Park, Chung-ju National Univ, Dept of Matls Engr, Chungbuk, SOUTH KOREA; Jong-Hoon Kim, Korea Academy of Industrial Technology, Dept of Adv Mfg Processing, Siheung, SOUTH KOREA.

The p-type, Te-doped BiSbTe and n-type SbI doped BiTeSe thermoelectric compounds were fabricated by hot pressing at the temperature range of 380 to 440C under 200 MPa in Ar. The microstructure and thermoelectric properties of the compounds were investigated. The microstructure was examined by optical microscopy, scanning electron microscopy, transmission electron microscopy, and x-ray diffraction. The microstructure of the compounds was relatively dense and equixed. The grains of the compounds were preferentially oriented through the hot pressing and also the degree of preferred orientation increased with the hot pressing temperature. The fractured surface showed transgranular cleavage features. The fracture path followed transgranular cleavage planes, which are perpendicular to the c-axis. In addition, with increasing the hot pressing temperature, the figure of merit was increased due to the decrease in porosity and the increase in preferred orientation. The highest values of figure of merit for the p-type and n-type compounds were obtained at 420C and their values were 2.69 x 10/K and 2.35 x 10/K, respectively.

SESSION Q5: ALTERNATIVE DIRECTIONS IN THERMOELECTRICS 
Chair: Armin Heinrich
Tuesday Afternoon, April 1, 1997
Salon 14

3:30 PM *Q5.1 
CONTROLLING CARRIER CHARGE DENSITIES, CONDUCTIVITY, AND THERMOPOWER WITH HOST/GUEST CAGE VARIABLES, Galen D. Stucky, Univ of California-S Barbara, Dept of Chemistry, Santa Barbara, CA; Nick Blake, Univ of California-S Barbara, Dept of Chemistry/Materials, Santa Barbara, CA; Vojislav I. Srdanov, Univ of California-S Barbara, Dept of Chemistry, Santa Barbara, CA; Susan Latturner, Beatrice Chinh, Horia Metiu, Univ of California-S Barbara, Dept of Chemistry/Materials, Santa Barbara, CA.

We are exploring in this research the optimization of thermoelectric figure-of-merit variables through the synthesis of cage structures that have a) complex unit cells (50 to 600 atoms/unit cell) to reduce lattice thermal conductivity, b) host cage structures with insulating (aluminosilicate) or conducting (Si,Ge) frameworks and c) alkali metal guests. Two examples will be presented. Wide bandgap (6 eV) aluminosilicate 60 atom cage ( diameter) structures have been variably doped with alkali metal atoms to give in the limit a periodic lattice of F centers containing up to 3x10 spins/cm. These electrons are associated with paramagnetic clusters, akin to F centers in ionic solids and are coupled between cages to give a semimetal that can be described by Maxwell-Boltzman statistics [1,2]. The second class of cage structures being studied are of the form M(Si,Ge) (x 11) and M(Si,Ge) (x 8). In this case, semiconducting properties arise from transport through the silicon or germanium cage walls which are one atomic layer thick. The concentration of the host alkali metals can be varied to give varying conduction electron densities and thermopower values ranging up to -270 V/K. The synthesis and characterization of these cage structures by solid state nmr, epr, optical spectroscopy, electron transport and thermopower measurements will be described. The results of theoretical modeling of the electronic properties will also be presented in the context of further optimization of thermoelectric response.

4:00 PM Q5.2 
TO THE PROBLEM OF MULTICASCADE THERMOELEMENTS, Nickolay Marchuk, Inst for Nuclear Research, Energy Converter Physics Lab, Kiev, UKRAINE; Yuri Goryachev, E. I. Shwarzman, Inst for Materials Research, Kiev, UKRAINE; N. N. Kolychev, Inst for Nuclear Research, Energy Converter Physics Lab, Kiev, UKRAINE; L. I. Fiyalka, Inst for Materials Research, Kiev, UKRAINE.

Literature data and results of authors' investigations indicate that the problem of increase of thermoelectric converters' efficiency and specific power is closely connected with rising of operating temperatures and temperature gradients in thermoelements. But the other problem of keeping high values of thermoelectric figure of merit of thermoelement branches in the wide temperature intervals arises in parallel with the first one. In this paper, results of investigation of kinetic properties of some thermoelectric materials used in multicascade thermoelements (YbB, CrSi, HMS, NiZrSn, PbTe-Bi) are presented. By means of application new refractory borides upper operating temperature is rising up to 1700 K. New data of investigation of composite commutative materials based on NiB as conductive phase, compose borate glasses with different melting temperatures and materials of thermoelement branches [1, 2] are given.

4:15 PM Q5.3 
THE ENHANCEMENT OF THERMOELECTRIC POWER AND SCATTERING OF CARRIERS IN BiSnTe SINGLE CRYSTALS, Vladimir Kulbachinskii, Moscow State Inst, Dept of Low Temperature Physics, Moscow, RUSSIA; Hiroshi Negishi, Minoru Sasaki, Yioshiaki Giman, Masasi Inoue, Hirsohima Univ, Dept of Materials Science, Hiroshima, JAPAN.

Thermoelectric power, electrical resistivity, and Hall effect of p-type BiSnTe (0 4:30 PM Q5.4 
EFFECT OF LARGE TEMPERATURE GRADIENTS ON THERMOELECTRIC FIGURE OF MERIT, Lev P. Bulat, St Petersburg State Academy of Refrigeration, Dept of Electrical Engr, St Petersburg, RUSSIA.

There are a number of patents in which are offered several thermoelectric converters with giant temperature gradients. According to the patents, a thermoelectric figure of merit of some of these converters reaches to Z = 1000/T. In this paper, results of a theoretical study of the thermoelectric figure of merit under large temperature gradients are reported. A solving of two problems are presented: 
1. It was investigated an effect of large temperature gradients on the thermoelectric figure of merit of a homogeneous semiconductor. We have used following methods in accordance with conditions of the problem: 1) solution of a system of kinetic equations for electrons and phonons; 2) solution of a system of equations of an energy balance for electrons and phonons. So, the nonlinear and nonlocal theory of a phenomena of transport in semiconductors with large temperature gradients has been developed. It appeared that under the large temperature gradients a new parameter plays the part of the thermoelectric figure of merit. 
2. It was studied an effect of large temperature gradients on a figure of merit of macroscopically heterogeneous media. A plane-laminated two-phase structure was considered to investigate nonlinear and nonlocal responses in such media. Tensors of the effective kinetic coefficients in this media have been calculated with regard to the electrons and phonons temperature mismatch on layer boundaries. In the case when one of the phases is a metal and the other is a semiconductor, a sharp increase of the effective thermoelectric figure of merit can be observed. 
The main result of this paper consists in the fact that the use of large temperature gradients really can lead to the increasing of the figure of merit in homogeneous and heterogeneous media.

4:45 PM Q5.5 
ANISOTROPIC CRYSTALS FOR TE DEVICES, Eduard Osipov, Algirdas P. Smilga, Vilnius Univ, Dept of Solid State Electronics, Vilnius, LITHUANIA.

The main requirement for the selfconversion of the thermal energy to the electrical one and the other way around on the free surface of the anisoptropic thermoelectrical converter (ATE) is presence an ansotropic thermoelectrical power in the TE material. The growing technique of the mixed crystals of ZnCdSb and BlSb is presented. Among the known TE materials the first one has the largest value of the anisotropy of the thermoelectric power coefficient at liquid nitrogen as well as at room temperatures and higher. Some compositions of the BlSb with the Sn doping shows the largest values of the transversal thermoelectric performance and they are successfully used for the manufacturing of the ATE converters. Optimal crystal growth conditions for the above mentioned alloys are discussed.

SESSION Q6: POSTER SESSION 
Chair: Michael L. Wilson
Tuesday Evening, April 1, 1997
8:00 P.M. 
Salon 7

Q6.1 
SOLID STATE CHEMISTRY AND THERMOELECTRIC PROPERTIES OF NEW QUATERNARY A-Ln-Bi-S (A=K, Rb, Ba, Sr; Ln=La, Ce) COMPOUNDS, Lykourgos Iordanidis, Mercouri G. Kanatzidis, Michigan State Univ, Dept of Chemistry, East Lansing, MI; Jon L. Schindler, Paul W. Brazis, Carl R. Kannewurf, Northwestern Univ, Dept of E&CE, Evanston, IL; Baoxing Chen, Ctirad Uher, Univ of Michigan, Dept of Physics, Ann Arbor, MI.

We are interested in new semiconducting bismuth chalcogenide phases with structural and compositional complexity because these characteristics are likely to lead to materials with low thermal conductivities and high thermopowers. We have synthesized four new quaternary bismuth sulfides, KLa1.24Bi3.76S8, KCeBi4S8, and their Rb derivatives. We will also report on BaLaBi2S6 and SrLa2Bi6S14. These compounds have unusual structures with tunnels in them which are hybrid between the Bi2Se3 and La2S3 structure types. In all compounds substantial site occupancy disorder between the alkali (or alkaline earth), the bismuth and lanthanide elements is observed, leading to stoichiometric deviations from the ideal formula. This paper describes the synthesis, crystal structures, optical, and charge transport properties of the new compounds. Preliminary results in the ternary Rb-Bi-S and Rb-Bi-Se systems will also be reported.

Q6.2 
SYNTHESIS AND THERMOELECTRIC CHARACTERIZATION OF NEW PHASES IN THE A/Pb/Bi/Se SYSTEMS., Duck-Young Chung, Mercouri G. Kanatzidis, Michigan State Univ, Dept of Chemistry, East Lansing, MI; Jon L. Schindler, Paul W. Brazis, Carl R. Kannewurf, Northwestern Univ, Dept of E&CE, Evanston, IL; Baoxing Chen, Ctirad Uher, Univ of Michigan, Dept of Physics, Ann Arbor, MI.

Bismuth is very attractive for study because of its inert 6s2 lone pair of electrons which may or may not be manifested structurally in a given compound. Whether the lone pair is stereochemically active or not affects both the lattice structure and the electronic structure and thus the electronic properties of the resulting compounds. Given that Bi2Te3 is an excellent thermoelectric material, it is surprising that little exploratory synthesis of new ternary bismuth chalcogenide phases has been reported. We have synthesized several new ternary bismuth selenides and examined their thermoelectric properties. These compounds include, b-K2Bi8Se13, K2.5Bi8.5Se14 and CsPbBi3Se6. These compounds possess three-dimensional tunnel frameworks based on Bi2Te3- and CdI2-type fragments with alkali ions found in the channels. Single crystal b-K2Bi8Se13 has conductivity of 225 S/cm and a thermopower of -180 µV/K at 23 0C. The thermoelectric properties of K2.5Bi8.5Se14 and CsPbBi3Se6 are comparable and will be discussed.

Q6.3 
MICROSTRUCTURE, MECHANICAL PROPERTIES, AND THERMOELECTRIC PROPERTIES OF HOT-EXTRUDED P-TYPE Te-DOPED BiSbTe COMPOUNDS, Kyeongsoon Park, Chung-ju National Univ, Dept of Matls Engr, Chungbuk, SOUTH KOREA; Jun-Ho Seo, Chi-Hwan Lee, Inha Univ, Dept of Metallurgical Engr, Inchon, SOUTH KOREA.

The p-type BiSbTe compounds with Te dopant (2 6wt) and without dopant were fabricated by the hot extrusion at the extrusion temperature of 300 to 510C at an extrusion ratio of 20:1 and a ram speed of 5 cm/min. The microstructure, mechanical properties, and thermoelectric properties of the compounds were investigated. The optical microscopy, scanning electron microscopy, transmission electron microscopy, and X-ray diffraction were used to examine the microstructure. The microstructure of the undoped and Te doped compounds was highly dense and equiaxed fine-grained (m). The hot extrusion gave rise to a preferred orientation of grains. The bending strength of the undoped and Te-doped compounds was increased with increasing the extrusion temperature owing to the porosity decrease. The fractographs showed transgranular cleavage features. The small grains and high density of the compounds led to an improvement in thermoelectric properties. The highest figure of merit (2.11 x 10/K) for the undoped compounds was obtained at 420C. It was also found that the Te dopant significantly increased the figure of merit. The 4 wt Te-doped compounds hot extruded at 420C showed a very high figure of merit (2.94 x 10/K). We believe that the hot extrusion reduced the grain size and increased the density, resulting in enhanced bending strength and figure of merit.

Q6.4 
DETERMINATION OF THERMAL PROPERTIES BY WAFER BENDING UNDER PERIODICAL HEATING, Margarita L. Shendeleva, Inst of Physics, Laboratory of Photoacoustics & Optics, Kiev, UKRAINE.

Theoretical background of noncontact, nondestructive method for thermal properties determination, based on photoacoustic effect in thin plate, have been developed. When a light source (laser or light emitting diode) of periodically modulated intensity illuminates the wafer under investigation and is absorbed in it, this causes a periodical heating of the wafer. Strongly inhomogeneous temperature distribution inside the wafer leads to its thermal expansion and bending. The radius of wafer curvature can be measured. This model was studied theoretically under some approximations. We consider, that light radiation is strongly absorbed in thin surface layer of the plate. Temperature distribution in the plate is found from heat conduction equation with periodical heat source located on the surface of wafer. Solution of this equation is superposition of thermal waves, which properties depend on modulation frequency and thermal diffusivity of the wafer material. Then a periodical part of thermoelastic strains and curvature of wafer are calculated by means of elastic theory of thin plates. Analytical expressions for wafer curvature in the quasistatic approximation have been obtained for homogeneous and layered wafers. It is shown, that in the case of homogeneous wafer the dependences of wafer curvature versus modulation frequency allow for determination the value of thermal diffusivity. For layered wafer this dependences have more complicated character and represent mismatch of thermal diffusivities and thermal expansion coefficients of layers.

Q6.5 
THERMOELECTRIC PROPERTIES AND FIGURE-OF-MERIT OF THE BaPbO HOMOLOGOUS COMPOUNDS, Masahiro Yasukawa, Norimitsu Murayama, Nat Ind Research Inst Nagoya, Nagoya, JAPAN.

Electrically conducting oxides are considered to take advantages in long term use in air at high temperatures for thermoelectric energy conversion. However, few oxide materials with figure-of-merit higher than 10K have been found. We have found that thermoelectric figure-of-merit (Z) and ZT of a BaSrPbO sintered ceramics are 1.8 x 10K and 0.12 at 673 K, respectively, which are 4.5 times higher than those of nondoped BaPbO3. The (BaSr)PbO(n = ) compound is one of the BaPbO homologous series. It is known that the crystal structure of BaPbO consists of n perovskite layers separated by a BaO layer, and an increase in n from 1 to brings about the change in conduction behavior from semiconducting to semimetallic, which is caused by energy lowering of the conduction band edge. In addition, these perovskite-related oxides have high capability for cation doping, which is necessary to decrease thermal conductivity and to modify the electronic structure. Higher thermoelectric figure-of-merit will be reported by optimization of the value of n and the doping in the BaPbO homologous compounds.

Q6.6 
THERMOELECTRIC PROPERTIES OF ZnSb FILMS GROWN BY MOCVD, R. Venkatasubramanian, E. Watko, T. Colpitts, Research Triangle Inst, Ctr for Semiconductor Research, Research Triangle Pk, NC.

Recent work indicates that stoichiometric ZnSb (Zn:Sb = 1:1) thin-films can offer high Seebeck coefficients near 480 K. The thermoelectric properties of metallorganic chemical vapor deposited (MOCVD) ZnSb films will be reported. The growth conditions necessary to obtain the desired stoichiometry in the ZnSb films and the effects of various growth parameters on the electrical conductivity and Seebeck coefficients of the films would be described. The residual doping level in these films were rather high, although monotonically decreasing from 8.5E20 cm for a layer thickness of 0.1 m to 2.4E20 cm for a layer thickness of 1.3 m. The thicker ZnSb films also exhibit improved carrier mobilities. The variation of the Seebeck coefficient with temperature has been characterized in the ZnSb films. The Seebeck coefficients of the ZnSb films were found to rise rapidly near 420 K, with peak Seebeck coefficients of 470 V/K at 490 K. The growth conditions including the use of intentional dopants to improve the Seebeck coefficients have been studied. Thermal conductivity of the ZnSb thin films measured by the 3- method, between 300 K to 423 K, are essentially similar to those reported for bulk ZnSb crystals. Initial results suggest that the ZnSb films have a ZT 0.5 at 300 K, perhaps limited by the high residual p-type doping levels. Significantly higher ZT values are estimated for the ZnSb films at higher temperatures, essentially due to higher Seebeck coefficients. This work suggests a considerable potential for the use of ZnSb thin-film materials in ultralow grade heat power conversion devices.

Q6.7 
THERMOELECTRIC PROPERTIES ON N-TYPE Bi(TeSe) FABRICATED BY MECHANICAL ALLOYING AND HOT PRESSING, Hee Jung Kim, Hong-Ik Univ, Dept of Metallurgy & Matl Sci, Seoul, SOUTH KOREA; Jae Sik Choi, Tae-Sung Oh, Hong-Ik Univ, Dept of Metallurgy & Matl Sci , Seoul, SOUTH KOREA; Dow-Bin Hyun, Korea Inst of Science and Technology, Metals Division, Seoul, SOUTH KOREA.

Thermoelectric properties of polycrystalline Bi (0.05 x 0.25), fabricated by mechanical alloying and hot pressing, have been investigated. Formation of n-type BiSe alloy powders was completed by mechanical alloying for 2 hours at ball-to-material ratio of 5:1, and processing time for Bi formation increased with Bi content x. Figure of merit of BiSe) was markedly increased by hot pressing at temperatures above 450C, and maximum value of 1.9x10/K was obtained by hot pressing at 550C. With addition of 0.015 wt Bi as acceptor dopant, figure of merit of Bi2(Te0.9Se0.1)3, hot pressed at 550C, could be improved to 2.1x10/K. When Bi was hot pressed at 550C, figure of merit increased from 1.14x10/K to 1.92x10/K with increasing Bi content x from 0.05 to 0.15, and then decreased to 1.30x10/K for x = 0.25 composition.

Q6.8 
THERMOELECTRIC PROPERTIES OF M2MO6X6(M-Tl,In,Rb,Cs AND X - Se, Te, Darren Verebeyi, Clemson Univ, Dept of Physical Science, Orangesburg, SC; Guebre X. Tessema, Clemson Univ, Dept of Physics & Matls Sci, Clemson, SC.

The family of M2Mo6X6(M=Tl,In,Rb, Cs and X = Se, Te)of quasi-one-dimensional compounds exhibit a metal insulator transition possibly driven by a Charge Density Wave transition. Above the metal insulator transition,temperature (Tp = 200K) the materials are semi metallic with a rather high thermopower, 40 to 100 microvolt/K for Tl2Mo6Se6. We will report results of a systematic investigation of the thermoelectric properties: conductivity, thermopower, and thermal conductivity of these materials.

Q6.9 
FABRICATION OF BISMUTH TELLURIDE BASED THIN FILM BY DUAL-BEAM PULSED LASER DEPOSITION, Yusuke Mori, M. Matsubara, H. Sonobe, Y. Takaba, I. Yamasaki, Takatomo Sasaki, Osaka Univ, Dept of Electrical Engr, Osaka, JAPAN; H. Morisaki, H. Nishino, Osaka Gas Co Ltd, R & D, Kyoto, JAPAN; Y. Yamada, Kansai Research Inst, Sensor Contract Research, Kyoto, JAPAN.

The Bi-Te based thin films have been grown on glass substrate at temperatures of 200-350C by dual-beam pulsed laser deposition (PLD). The deposition was carried out inside a vacuum chamber evacuated by a turbomolecular pump to a base pressure of 10 Torr. The fourth harmonic of Nd:YAG laser radiation was employed with the repetition rate of 10 Hz and a nominal pulse width of 3-5 ns. The Bi and Te metals were used as targets. A single laser beam was split up into two beams for simultaneous ablation of these two targets which were standing side by side. The deposited films were characterized by means of x-ray diffraction, secondary electron microscopy, Rutherford backscattering spectrometry and energy dispersion spectroscopy. The temperature dependence of the electrical resistivity and Seebeck coefficient of films were also measured. The Bi:Te composition ratio varied from 1:1.1 to 1:1.5 gradually along the horizontal direction in 10 mm length on the substrate when the target substrate distance was 30 mm. This is because the overlapping ratio between the two plumes from the respective target changed in this direction. This indicates that the dual-beam PLD is useful to fabricate functionally graded thermoelectric material. The typical Seebeck coefficient and electrical resistivity obtained were 170 V/K and 2 x 10 Ohm.cm, respectively.

SESSION Q7: ARTIFICIAL STRUCTURES II (QUANTUM CONFINEMENT ETC) 
Chair: Gerald D. Mahan
Wednesday Morning, April 2, 1997
Salon 14

8:30 AM *Q7.1 
THERMOELECTRIC PROPERTIES OF SUPERLATTICES, Thomas L. Reinecke, Naval Research Laboratory, Washington, DC; David A. Broido, Boston College, Dept of Physics, Chestnut Hill, MA.

The thermoelectric properties of superlattices have been of considerable interest recently because of their potential as improved thermoelectric materials. The quantity that provides a measure of the quality of a material for thermoelectric applications is the "figure of merit," ZT = S/, where S is the thermopower, is the electrical conductivity, and is the total thermal conductivity. It has been suggested that ZT in superlattices formed from quantum wells and quantum wires may he dramatically increased over those for bulk because of the increase of the electronic density of states that occurs in ideal two- and one-dimensional systems. We have developed quantitative theoretical treatments of thermoelectric transport in realistic superlattice systems. Their electrical properties have been studied within the envelope function approach in order to include tunneling between the layers. We find that it is essential to include the thermal transport through the barriers and the electronic tunneling between the layers in order to obtain a proper understanding of these systems. We have also included the changes in the elastic and inelastic carrier-phonon scattering rates and the lifting of carrier valley degeneracy due to quantum confinement of carriers. Our formulation provides an exact solution to the Boltzmann equation for in-plane thermoelectric transport, from which we obtain the electrical conductivity, the thermopower, the electrical component of the thermal conductivity, and ZT. We find significant differences between ZT obtained when the well width and energy dependences of the phonon scattering are included as compared to those obtained in the constant relaxation time approximation. In this way we have obtained an accurate and realistic description of the figure of merit of superlattice systems, and we are able to describe its dependences on the parameters of the system. Results for ZT are given for BiTe, PbTe, GaAs, and Bi superlattices. We find that ZT reaches a maximum for decreasing superlattice period and then decreases for smaller periods. The maximum values of ZT can be larger than those for bulk. ZT has a significant dependence on the potential offset; it is larger for superlattices formed from wires than for wells, and it depends an the electronic density of states of the materials used.

9:00 AM *Q7.2 
HIGH THERMOELECTRIC FIGURES OF MERIT IN PbTe QUANTUM WELLS, Theodore C. Harman, MIT Lincoln Laboratory, Lexington, MA; David L. Spears, MIT Lincoln Laboratory, Electro-Optic Matls & Devices Group, Lexington, MA; M. P. Walsh, MIT, Lincoln Lab, Lexington, MA; Xiangzhong Sun, S. B. Cronin, MIT, Dept of Physics, Cambridge, MA; Mildred S. Dresselhaus, MIT, Dept of EECS & Physics, Cambridge, MA.

High-quality PbEuTe/PbTe multiple quantum wells (MQWs) have been grown by molecular beam epitaxy and designed for high thermoelectric power factors P and figures of merit Z T. The measured 300 and 400 K thermoelectric properties have been compared with that of the best bulk PbTe. P values up to 130 and over 150 WcmK have been measured at 300 K for n-type and p-type MQW samples, respectively. Values of Z T >1.2 has been achieved for these n type PbTe quantum wells and Z T >1.5 has been achieved for the p-type quantum wells at 300 K. Z T up to 2.3 has been achieved at 400 K. Background information on the band structure and bulk properties of PbTe will be presented along with a detailed description of the experimental MQW results. Hall coefficient, electrical resistivity, and Seebeck coefficient data from 80 to 400 K will be shown. This work was sponsored by the Department of the Navy, the Army Research Office, and the Defense Advanced Research Projects Agency (DARPA) under AF Contract No. F19628-95-0002. The opinions, interpretations, conclusions and recommendations are those of the authors and are not necessarily endorsed by the Department of Defense.

9:30 AM Q7.3 
QUANTUM CONFINEMENT EFFECTS ON THE THERMOELECTRIC FIGURE OF MERIT IN Si/SiGe SYSTEM, Xiangzhong Sun, MIT, Dept of Physics, Cambridge, MA; Mildred S. Dresselhaus, MIT, Dept of EECS & Physics, Cambridge, MA; K. L. Wang, Univ of California-Los Angeles, Dept of EE, Los Angeles, CA.

In bulk form, Si is a promising thermoelectric material for high temperature application. It has been shown recently that is may be possible to increase the thermoelectric figure of merit () of certain materials by preparing them in the form of two-dimensional quantum-well structures. It has also been shown that an increase in over bulk values may be possible in the Si/Si system through quantum confinement effects using quantum-well structure. In this paper, we report results from theoretical modeling as well as experimental investigations of quantum confinement effects in the presence of -doping within the barrier layers. The doping layers are introduced by growing very thin layers of wide bandgap materials within the barrier layers in order to increase the effective barrier height within the barriers and thereby reduce the barrier width necessary for the quantum confinement of carriers within the quantum well. The overall figure of merit is enhanced due to the reduced barrier width. The -doping would also serve to reduce the thermal conductivity in the barriers, by introducing phonon scattering centers within the barrier region. The optimum placement of the -doping layers within the barrier region will be discussed.

SESSION Q8: SKUTTERUDITES 
Chair: George S. Nolas
Wednesday Morning, April 2, 1997
Salon 14

10:15 AM *Q8.1 
LOW THERMAL CONDUCTIVITY SKUTTERUDITES, Jean-Pierre Fleurial, Thierry Caillat, A. Borshchevsky, California Inst of Technology, Jet Propulsion Laboratory, Pasadena, CA.

Recent experimental results on semiconductors with the skutterudite crystal structure show that these materials possess attractive transport properties and have a good potential for achieving ZT values substantially larger than for state-of-the-art thermoelectric materials. Both n-type and p-type conductivity samples have been obtained, using several preparation techniques. Associated with a low hole effective mass, very high carrier mobilities, low electrical resistivities, and moderate Seebeck coefficients are obtained in p-type skutterudites. For a comparable doping level, the carrier mobilities of n-type samples are about an order of magnitude lower than the values achieved on p-type samples. However, the much larger electron effective masses and Seebeck coefficients make n-type skutterudites even more promising candidates. Unfortunately, the thermal conductivities of the binary skutterudite compounds are too large, particularly at low temperatures, to be useful for thermoelectric applications. Several approaches to the reduction of the lattice thermal conductivity in skutterudites are being pursued: heavy doping by ionized impurities, formation of solid solutions and alloys, study of novel ternary and filled skutterudite compounds. All those approaches have already resulted in skutterudite compositions with substantially lower thermal conductivity values in these materials. Recently, superior thermoelectric properties in the moderate to high temperature range were achieved for compositions combining alloying and ''filling'' of the skutterudite structure [1,2]. The experimental results on low thermal conductivity skutterudites are reviewed and their potential for high ZT values near room temperature is discussed.

10:45 AM *Q8.2 
COMPUTATIONAL STUDIES OF NOVEL THERMOELECTRIC MATERIALS, David J. Singh, Naval Research Laboratory, Complex Systems Theory Branch, Washington, DC.

Thermoelectric materials are characterized by a figure of merit, Z. This consists of the ratio of a power factor and the thermal conductivity. This ratio requires very low lattice thermal conductivity for good thermoelectric performance. When combined with the required favorable electronic properties, this is most likely in crystalline materials with complex structures. The power factor, which depends on electronic transport properties, is often highly dependent on the doping level and temperature. The difficulty in understanding and optimizing such materials provides a potentially important role for materials specific first principles theory. In this talk, some of the uses of density functional based electronic structure calculations in the search for high Z materials are illustrated by a series of examples, including novel Skutterudite, filled Skutterudite and Zn-Sb based thermoelectrics

11:15 AM *Q8.3 
THERMODYNAMICS AND TRANSPORT IN FILLED SKUTTERUDITE ANTIMONIDES, David G. Mandrus, Brian C. Sales, Oak Ridge National Laboratory, Solid State Div, Oak Ridge, TN; Ronert Williams, J. R. Thompson, B. Chakoumakos, Veerle M. Keppens, Lynn A. Boatner, Oak Ridge National Laboratory, Oak Ridge, TN; Tim Darling, Los Alamos National Laboratory, Los Alamos, NM; Albert Migliori, Los Alamos National Laboratory, Dept of Conds Matter & Thermal Physics, Los Alamos, NM; M. B. Maple, Don Gajewski, Univ of California-San Diego, Dept of Physics, La Jolla, CA; E. J. Freeman, Univ of California-San Diego, Dept of Physics 0319, La Jolla, CA.

One of the most promising new ideas in the field of thermoelectrics is the ''electron crystal, phonon glass'' (ECPG) concept originally proposed by Slack [1]. In this picture, a loosely bound atom with a large thermal parameter scatters phonons much mare strongly than electrons, thus permitting a glasslike thermal conductivity to coexist with the high electron mobilities found in crystals. A physical realization of an ECPG may occur in the filled skutterudite antimonides, in which a rare earth atom fills the two voids per unit cell that exist in the skutterudite structure. Very recently, Sales et al. [2] reported a high thermoelectric figure of merit in LaFeCoSb, which was primarily due to a drastic reduction in the lattice contribution to the thermal conductivity of this material compared to its unfilled analogue. Here we review the crystal chemistry and transport properties of the filled skutterudite antimonides, and present new results on the temperature dependence of the thermal parameters, specific heat, and elastic response of these materials in order to evaluate to what extent filled skutterudite antimonides represent ideal ECPG materials.

11:45 AM Q8.4 
THE SYNTHESIS OF TERNARY IRON ANTIMONIDE SKUTTERUDITES USING SUPERLATTICE REACTANTS, Mark Hornbostel, David C. Johnson, Univ of Oregon, Dept of Chemistry & Matls Science, Eugene, OR; Ed Hyer, Univ of Oregon, Dept of Chemistry, Eugene, OR.

A series of kinetically stable, crystalline skutterudites (MFeSb where M = vacancy, RE, Hf, Al and Y) have been synthesized by controlled crystallization of elementally modulated reactants. Low angle diffraction data demonstrates that the elemental layers interdiffuse at temperatures below 150C. Nucleation of the skutterudite structure occurs with a large exotherm on annealing at temperatures above 200C regardless of the ternary metal. The metastable binary compound, FeSb was also formed via nucleation of an amorphous binary intermediate at 200C. All of the ternary compounds and the new binary compound were found to decompose exothermically on higher temperature annealing. The decomposition temperature ranged from 300C for the binary compound to approximately 550C for the europium compound. The occupation of the ternary cation was found to depend on the composition of the initial reactant.

12:00 NOON Q8.5 
PROPERTIES OF CoSb FILMS GROWN BY PULSED LASER DEPOSITION, Hans-Martin Christen, David G. Mandrus, David P. Norton, Oak Ridge National Laboratory, Solid State Div, Oak Ridge, TN; Lynn A. Boatner, Oak Ridge National Laboratory, Oak Ridge, TN; Brian C. Sales, Oak Ridge National Laboratory, Solid State Div, Oak Ridge, TN.

Polycrystalline CoSb films were grown on a variety of electrically insulating substrates by pulsed laser ablation from a stoichiometric hot-pressed target. These films are fully crystallized in the skutterudite structure, and the grains exhibit a strongly preferred alignment of the cubic [310]-axis perpendicular to the substrate surface. The film quality and morphology are studied for different single-crystal substrates (including SrTiO, AlO, and TiO) and as a function of growth temperature and background gas pressure. Hall measurements show that the films are p-type semiconducting with a room-temperature carrier density of 310 holes/cm. The Hall mobility is found to be 65 cm/(Vs), which is high for such a heavily-doped material. The Seebeck coefficient and the resistivity are measured as a function of temperature and are compared to bulk measurements. This research was sponsored by the Division of Materials Sciences, U.S. Department of Energy under Contract No. DE-AC05-96OR22464 with Lockheed Martin Energy Research Corp.TROPY.ints of the BZ.esults indicate incorporation of aluminum into the organic layer upon failure of the devices.ITO/TPD/Alq/Al devices.affect of pyrolysis and sample temperature on polymerization initiation, film growth rates, and film composition. The FT-IRRAS compositional studies are complimented with ex-situ XPS. AFM and SEM characterization will be presented to illustrate the affect of processing conditions on film morphology. Implications of the CVD PPV film characteristics on device performance will also be discussed.

SESSION Q9: NEW MATERIALS II 
Chair: Mercouri G. Kanatzidis
Wednesday Afternoon, April 2, 1997
Salon 14

1:30 PM *Q9.1 
RARE EARTH THERMOELECTRICS, Gerald D. Mahan, Univ of Tennessee, Dept of Physics, Knoxville, TN.

We discuss the possibility of obtaining a high figure of merit, for thermoelectrics, by using compounds containing rare-earth compounds. We note that our recent theory for the ''Best thermoelectric'' [1] suggested that high values of ZT were obtained from solids with singular features in their density-of-states. This is best achieved using the -levels of rare-earth compounds. Most compounds to date contain either Ce or Yb, and we show that they give < 0.5.

2:00 PM *Q9.2 
CRYOGENIC THERMOELECTRICS AND THE ETTINGSHAUSEN EFFECT, Albert Migliori, Los Alamos National Laboratory, Dept of Conds Matter & Thermal Physics, Los Alamos, NM.

In 1822, Seebeck discovered that charge carriers in electrical conductors could be moved using temperature gradients as well as by application of electric fields. As is typical of thermodynamically-driven processes, this effect works in reverse -heat can be moved when electric fields move the carriers. And as is typical of transport processes, Nernst and Ettingshausen in the 19th century discovered that magnetic fields cause charge and heat flows perpendicular to the driving forces. These latter effects turn out to be the most powerful of the lot, and the most amenable to study using modern solid-state theory. We will describe how such theory guides the experimentalist in creating new materials for solid-state refrigeration, and some of those materials we have developed.

2:30 PM Q9.3 
THERMOELECTRIC ORGANICS - NEW DIRECTIONS AND APPROACHES, Brian G. Dixon, M. Aldissi, M. Bhamadapati, E. Lazaro, E. Miller, Cape Cod Research, Inc, East Falmouth, MA.

A new direction in thermoelectric compositions will be described. Research is discussed which has investigated the feasibility of developing hybrid organic inorganic compositions that possess superior thermoelectric properties. More specifically, the reported results identify dimensional composite materials in which very high Seebeck coefficients of over 1000 V/K are coupled with very good electrical conductivities, resulting in an increase in ZT of five orders of magnitude, from 10 to 0.03. The ability to process the innovative hybrids was also demonstrated. A distinct advantage of the new materials, compared to state-of-the art thermoelectrics, is the ease with which they can be prepared. In addition, computational studies have indicated the ability to correlate electrical properties with molecular structure. Large, multi-atom organic-inorganic hybrids have been effectively modeled, leading to the design of compositions with superior performance characteristics. A series of dimensional and superlattice organic inorganic hybrid compositions have been prepared and characterized both structurally and thermoelectrically.

2:45 PM Q9.4 
ELECTRICAL TRANSPORT PROPERTIES OF THE POLYCRYSTALLINE PENTATELLURIDE MATERIALS HfTe and ZrTe, Terry M. Tritt, M. Fakhruddin, Clemson Univ, Dept of Physics & Astronomy, Clemson, SC; C. Feger, Clemson Univ, Dept of Chemistry, Clemson, SC; S. J. Hwu, Clemson Univ, Dept of Physics, Clemson, SC; Joseph W. Kolis, Clemson Univ, Dept of Chemistry, Clemson, SC; A. Johnson, R. L. Littleton, Clemson Univ, Dept of Physics & Astronomy, Clemson, SC; Darren T. Verebelyi, Clemson Univ, Dept of Physics, Clemson, SC; Michael L. Wilson, Clemson Univ, Dept of Physics & Astronomy, Clemson, SC.

We have synthesized pressed powders of polycrystalline pentatelluride materials, HfTe and ZrTe. We have measured the resistivity and thermopower of these materials as a function of temperature between 77K and 300K. The room temperature thermopower for each of these materials is relatively high, +95 V/K and +65 V/K for HfTe and ZrTe, respectively. These values compare closely to thermopower values for single crystals of these materials. The thermopower decreases monotonically with temperature to 77K, where the thermopower is +55 V/K for HfTe and +35 V/K for ZrTe. This indicates that these materials exhibit p-type behavior over this entire range of temperature. As expected, the resistivity for these materials is higher than the single crystal material, with values of 430 m-cm and 24 m-cm for HfTe and ZrTe, respectively. The resistivity monotonically increases with decreasing temperature from 300K to 77K. We have found that the peak in the resistivity evident in both single crystal materials is absent in these polycrystalline materials. Single crystal growth of these materials is in progress as well as Se doping of the polycrystalline and the single crystal materials. Hall coefficient and thermal conductivity measurements are in progress on these materials as well as for the tetratelluride materials and will be reported. We have found values for the thermal conductivity of a very similar material, TaTe, to be on the order of 5 W/m-K. Results on the Se doping and single crystal properties of these materials will be reported as well as a discussion of the potential of these low dimensional telluride materials for thermoelectric applications.

SESSION Q10: SILICIDES 
Chair: Cronin B. Vining
Wednesday Afternoon, April 2, 1997
Salon 14

3:30 PM *Q10.1 
THERMOPOWER AND ELECTRICAL CONDUCTIVITY OF UNDOPED AND DOPED IRON DISILICIDE SINGLE CRYSTALS, Armin Heinrich, Inst of Solid State & Materials Research, Dept of Thin Films, Dresden, GERMANY; Gunther Behr, Inst of Solid State & Materials Research, Dept of Solid State Chem, Dresden, GERMANY; Horst Griessmann, Inst of Solid State & Materials Research, Dept of Thin Films, Dresden, GERMANY; Horst Lange, Hahn-Meitner-Inst, Dept of Photovoltaics, Berlin, GERMANY.

Thermopower and electrical conductivity of undoped and doped iron disiliced single crystals have been measured in the temperature range from 30K to 1200K. The single crystals were grown in closed ampoules by chemical vapour transport in a temperature gradient between 1100K and 800K(750K) using iodine as transport agent. As starting material a powder was used of Si and Fe of high purity (5N). Needle like crystals were obtained with a length of about 10-15mm and a diameter of 1-2mm. Choosing Fe/Si ratios larger and smaller than the stoichiometric ratio single crystals have been obtained at the lower and upper phase boundaries of the homogeinity region, respectively. All undoped crystals are of n-type. Both thermopower and electrical conductivt show a distinct dependence on the deviation from strict stoichiometry. Intrinsic behaviour was found above 500K. The activation energies of the shallow donors determined from the conductivity at room temperature depend also on the deviation from stoichiometry and reflect probably intrinsic defects. Between 100K and 200K a strong phonon drag effect was found which gives a maximum thermopower of about 2000µV/K. The results will be discussed in comparison with single crystals grown with iron of less purity (4N). In contrast to the former crystals these samples are always of p-type. Doping experiments have been carried out with Co and Cr. First results will be discussed of the dependence of doping effects on the phase boundaries of the homogeneity region.

4:00 PM Q10.2 
FABRICATION OF p-FeMnSi / n-Si HETEROSTRUCTURE DIODE AND THEIR ELECTRICAL AND OPTICAL PROPERTIES, Takeaki Takada, Electrotechnical Laboratory, Photonprocess Section, Ibaraki, JAPAN; Hiroshi Katsumata, Y. Makita, Electrotechnical Laboratory, Tsukuba, JAPAN; Naoto Kobayashi, Electrotechnical Laboratory, Quantum Radiation Div, Tsukuba, JAPAN; Hajime Shibata, Electrotechnical Laboratory, Photonprocess Section, Ibaraki, JAPAN; Masataka Hasegawa, Electrotechnical Laboratory, Quantum Radiation Div, Ibaraki, JAPAN; S. Uekusa, Electrotechnical Laboratory, Photonprocess Section, Tsukuba, JAPAN.

Semiconducting -FeSi has attracted increasing attention as a promising material for thermoelectronic devices and optoelectronic devices due to its chemical stability at higher temperatures, low cost, and a strong optical absorption coefficient (about 10 cm). We have so far prepared unintentionally doped -FeSi layers on n type Si(100) substrate by Electron-Beam-Deposition (EBD) of ferrosilicon (FeSi) at room temperature (RT) and subsequent furnace annealing (FA) at 900C for 2 hours. They have shown typically n-type conductivity. The purpose of this work is, therefore, to make p-type -FeSi layers on n-type Si(100) and to investigate their p-n diode characteristic. The p-type layers were formed by introduction of Mn into EBD -FeSi using two kinds of doping methods: one is EBD of Fe (x = 0.1) at RT and subsequent FA at 900C for 1-120 min, where FeSi lumps added with Mn (10) were used as starting material. The other is ion implantation of Mn into EBD -FeSi and subsequent FA. By these processes, p-type, -Fe layers with resistivity of 0.0036 0.031 [-cm] and Hall mobility of 11.9 89.0 [cm/Vsec] were obtained. The electrical I-V and C-V characteristics of these p--FeSi/n-Si heterostructure diodes were examined. These results were compared with those obtained from the optical and structural characterizations by Raman scattering, transmittance, reflectance, XRD and SEM measurements.

4:15 PM Q10.3 
STRUCTURAL AND OPTICAL PROPERTIES OF -FeSi/Si(100) PREPARED BY LASER ABLATION METHOD, Hirofumi Kakemoto, Electrotechnical Laboratory, Photonprocess Section, Ibaraki, JAPAN; A. Obara, Electrotechnical Laboratory, Ibaraki, JAPAN; Hajime Shibata, Electrotechnical Laboratory, Photonprocess Section, Ibaraki, JAPAN; Y. Tsai, S. Sakuragi, Union Materials Inc, Ibaraki, JAPAN; Y. Makita, Electrotechnical Laboratory, Tsukuba, JAPAN; Naoto Kobayashi, Masataka Hasegawa, Electrotechnical Laboratory, Quantum Radiation Div, Tsukuba, JAPAN; T. Tsukamoto, Science Univ of Tokyo, Tokyo, JAPAN.

Optimized and repeatable ultrashallow junctions not only require stringent temperature control and repeatability, but also tight control and reproducibility of the annealing ambient. In the annealing of boron and BF2 doped wafers, boron can diffuse from the surface as volatile compounds of boron, or be consumed into an oxide layer grown intentionally or unintentionally during the annealing stage. The control of this complicated surface chemistry is critical to optimize the ' 'retained'' boron dose in the Si substrate. It is this retained boron dose which directly affects the sheet resistance value of the junctions. In addition, the junction depth and its reproducibility can also be significantly affected by the ambient, e.g., the partial pressure of the residual oxygen. This paper addresses these issues by systematically investigating the effects of various ambients, such as N, N + O, N + NH, Ar, and others, on the sheet resistance, retained boron dose in Si and the junction depth of 2.2 keV, BF and 2.0 keV ion implanted wafers and 1.0 kV BF plasma doped (PLAD) wafers. RGA data was collected during the anneal stage to assist in identifying the complex surface chemistry responsible for the boron out diffusion. Subsequent to the anneals, ellipsometric, XPS, four-point probe resistance and SIMS measurements were performed to further elucidate the effects of the different ambients on the retained boron dose, the sheet resistance value, the RTP grown oxide layer and the junction depth. This study not only yields detailed knowledge of the surface chemistry involved in the boron dose loss, but also identifies optimum anneal conditions required to minimize the sheet resistance value for a desired junction depth.

SESSION Q11: INDUSTRY AND APPLICATION INTEREST IN NEW THERMOELECTRIC MATERIALS 
Chair: Jean-Pierre Fleurial
Thursday Morning, April 3, 1997
Salon 14

8:30 AM Q11.1 
THERMOELECTRIC COOLING CONTAINER FOR MEDICAL APPLICATIONS, Arcady Aivazov, Yu. L. Shtern, B. G. Budaguan, K. B. Makhrachev, Moscow Inst of Electronic Technology, Dept of Microtechnology, Zelenograd, RUSSIA; Miquel Pastor, NTE, Barcelona, SPAIN.

In the work the thermoelectric cooling container for storing and transportation of the medicine, particularly for insulin, was developed In the working volume the temperature is supported on the level of +4C The container can work in two operating conditions with the power supply and without power supply. Two removable blocks are used for this purpose. One block (thermoelectric) is used for the work with the power supply and another (passive) for the work without power supply. Thermoelectric block have +12 V power supply, which is used in the automobiles, yachts, and other kinds of transport. The temperature in the working volume is supported by the use of the Peltier effect. Electronic device used in this block stabilizes temperature on the level of +4C and indicates information about working conditions. Thermoelectric container has a power supply block for work at 220(110) V. The working temperature in the container can be maintained in the absence of the power supply. In this case the necessary temperature conditions are supported by melting of the crystallized salt. For this purpose container has hermetic volume containing this salt and contacting with the working volume.

8:45 AM *Q11.2 
OVERVIEW OF INDUSTRY INTEREST IN NEW THERMOELECTRIC MATERIALS, Hylan Benton Lyon, Marlow Industries Inc, Dept of Thermogenic, Dallas, TX.

The technology base for air conditioning, refrigeration, component cooling below ambient temperatures and power generation will be required to meet several new challenges. The main lines of these challenges will be presented in a way which relates them to the several new materials and materials engineering options being pursued by the research community. Since the potential benefits of thermoelectric devices are only partially met by enhancing the figure of merit ZT, the nature of the design challenge and the econometric constraints which contain the rest of the roadmap to success will be presented. The presentation is designed to enable researchers to understand the challenges that each of their specific approaches creates for the device engineer, and the original equipment manufacturer.

9:15 AM *Q11.3 
ADVANCED THERMOELECTRIC MATERIALS AND SYSTEMS FOR AUTOMOTIVE APPLICATIONS IN THE NEXT MILLENNIUM, Donald T. Morelli, General Motors R & D Center, Dept Physics & Phys. Chem MS 480-106-224, Warren, MI.

I will highlight some of the key potential applications of thermoelectrics in the car of the future, how these systems might work, and why we are interested in developing them. Generator applications as well as both small- and large-scale heating/cooling systems will be considered. The widespread use of thermoelectrics in these applications will become a reality when new, inexpensive materials of increased figure of merit are discovered and developed. Within the past two years, the first small steps in this direction have been taken. High figure of merit has been demonstrated in the laboratory in rare-earth-filled skutterudite compounds in the 400-600C temperature range. I will describe several candidate high figure of merit materials systems under investigation in our laboratory, including both filled skutterudite compounds and 1-1 1 intermetallic semiconductor systems. I will discuss our experimental results on the fabrication. characterization, and transport properties of these and related systems. It is stressed that a detailed experimental and theoretical understanding of the fundamental physical properties of these and related materials is required in order to assess their ultimate potential as advanced thermoelectrics.

SESSION Q12: GOVERNMENT AND MILITARY INTEREST IN NEW THERMOELECTRIC MATERIALS 
Chair: Hylan B. Lyon
Thursday Morning, April 3, 1997
Salon 14

10:30 AM *Q12.1 
THERMOELECTRIC SCIENCE AND TECHNOLOGY IN THE NAVY, John C. Pazik, Office of Naval Research, Dept of Physical Sciences S&T, Arlington, VA.

The Navy is currently supporting science and technology research in the field of thermoelectric (TE) materials and devices. The Navy program is a multi-disciplinary effort which brings together a diverse group of individuals with expertise in theory, solid state synthesis, thin film deposition, characterization, thermal and electronic property measurement, and device prototyping. A major objective of this program is to design, synthesize and characterize new TE materials with significant enhancement of the materials figure of merit which can ultimately lead to more efficient cooling and power generation devices. Areas of interest include disordered solids, thin film superlattices, and nanostructured composites. Recent developments in these promising areas will be described as well the significance of thermoelectric science and technology to the Navy.

11:00 AM *Q12.2 
THERMOELECTRIC APPLICATIONS AS RELATED TO BIOMEDICAL ENGINEERING FOR THE NASA JOHNSON SPACE CENTER, Catherine Kramer, NASA Johnson Space Center, Houston, TX.

This paper presents current NASA Biomedical developments and applications using thermoelectrics. Discussion will include future technology enhancements that would be most beneficial to the application of thermoelectric technology. 
A great deal of thermoelectric applications have focused on electronic cooling. As with all technological developments within NASA, if the application cannot be related to the average consumer, the technology will not be mass-produced and widely available to public (a key to research and development expenditures and thermoelectric companies). Included are discussions of thermoelectric applications to cool astronauts during launch and reentry. The earth-based applications, or spin-offs, include such innovations as tank and race car driver cooling, to cooling infants with high temperatures, as well as the prevention of hair loss during chemotherapy. In order to preserve the scientific value of metabolic samples during long-term space missions, cooling is required to enable scientific studies. Results of one such study should provide a better understanding of osteoporosis and may lead to a possible cure for this disease. In the space environment, noise has to be kept to a minimum. In long-term space applications such as the International Space Station, thermoelectric technology provides the acoustic relief and the reliability for food, as well as scientific refrigeration/freezers. Applications and future needs are discussed as NASA moves closer to a continued space presence in MIR, International Space Station, and Lunar-Mars Exploration.

11:30 AM *Q12.3 
THE DARPA ADVANCED THERMOELECTRIC MATERIALS PROGRAM, Stuart Wolf, DARPA, Arlington, VA.

DARPA has recognized a unique opportunity to develop advanced thermoelectric materials and devices for both cooling and power generation applications. Recent advances in the theoretical understanding of complex materials and the ability to control the microstructure and chemistry of tonary and quaternary compounds as well as multilayer films, superlattices and quantum well structures were instrumental for the development of this program. In this talk, I will present in some detail, the goals we hope to achieve and some of the potential paths toward these goals.

SESSION Q13: NEW MATERIALS III 
Chair: David G. Mandrus
Thursday Afternoon, April 3, 1997
Salon 14

1:30 PM Q13.1 
LOW TEMPERATURE TRANSPORT PROPERTIES OF CERIUM-FILLED ANTIMONIDE SKUTTERUDITES, Ctirad Uher, Baoxing Chen, Univ of Michigan, Dept of Physics, Ann Arbor, MI.

Experimental results on the low temperature electronic and thermal transport properties of Ce will be described. These studies reveal important information on the subtle interplay between rare earth filling factor and cobalt concentration x in this compound. The substitution of Co for Fe has a dramatic effect on all transport properties. While CeFeSb exhibits a metallic resistivity, Co substitution leads to a progressively stronger activated behavior and a decrease in hole concentration. The thermopower increases with increasing Co concentration and the thermal conductivity is depressed, notably at low temperatures. Because the amount of Ce filling the voids, , depends strongly on Co concentration, careful control over both of these compositional variables is required in order to elucidate their individual influences on transport in this system.

1:45 PM Q13.2 
MEASUREMENT OF THE THERMOELECTRIC PROPERTIES OF QUASICRYSTALLINE AlPdRe AND AlCuFe ALLOYS, Michael L. Wilson, Clemson Univ, Dept of Physics & Astronomy, Clemson, SC; Stephane LeGault, McGill Univ, Dept of Physics & Astronomy, Montreal, PQ; Terry M. Tritt, Clemson Univ, Dept of Physics & Astronomy, Clemson, SC.

We have performed some of the first measurements to determine the potential for the use of quasicrystalline materials as the basis for a new generation of thermoelectric materials. Quasicrystals are promising for thermoelectric applications due to their naturally low thermal conductivities ( typically 1 W/mK at 300 K), relatively low electrical resistivities (1 m-cm are not uncommon) and in the cases where the thermopower has been measured, Seebeck coefficients as large as 60 V/K. Thermopower measurements on quasicrystalline materials, however, are few and far between, and reported studies were not performed as part of a search for thermoelectric materials. We have examined the thermopower, electrical resistivity, and thermal conductivity of the quasicrystalline alloys Al, and Al. Their thermal conductivities were nearly identical, changing linearly from 0.5 W/mK at 50 K to 0.7 W/mK at 150 K. Their electrical resistivity and thermopower, however, were dramatically different. AlCuFe was found to have a resistivity of 4 m-cm and thermopower of -3 V/K at 300 K. AlPdRe, however, was found to have a much larger thermopower, with opposite sign (S = +31 V/K) and an increased resistivity (p = 31 m-cm) at 300 K. The promise that these materials show is that the electrical resistivity and magnitude as well as sign of the thermopower can all be varied without significantly altering the thermal conductivity. This bodes well for the examination of doping series and chemically tuning the materials to improve their thermoelectric properties.

2:00 PM Q13.3 
ELECTRICAL PROPERTIES AND FIGURES OF MERIT FOR NEW CHALCOGENIDE-BASED THERMOELECTRIC MATERIALS, Jon L. Schindler, Tim P. Hogan, Paul W. Brazis, Carl R. Kannewurf, Northwestern Univ, Dept of E&CE, Evanston, IL; Duck-Young Chung, Mercouri G. Kanatzidis, Michigan State Univ, Dept of Chemistry, East Lansing, MI.

Renewed Interest In thermoelectric cooling and power generation applications has prompted investigations to search for new materials with the desired properties of a large Seebeck voltage, good electrical conductivity, and an acceptably low thermal conductivity. These properties are the key factors which determine the thermoelectric figure of merit, Z, for a given material. Such exploratory materials studies require transport characterization techniques with the flexibility to handle various sample types, such as pressed pellets, single crystals, and films. In previous work we have demonstrated computer automation methods that permit rapid and simultaneous multisample characterization with good accuracy and reproducibility. This report discusses refinements in variable temperature electrical conductivity and Seebeck measurements, as well as the development of an automated thermal conductivity characterization method. Experimental results from new chalcogenide-based materials, prepared using the reactive polychalcogenide flux method, will be presented. These materials show encouraging properties, with some compounds approaching a ZT product of unity near room temperature.

2:15 PM *Q13.4 
NEW CHEMICALLY AND STRUCTURALLY COMPLEX BISMUTH TELLURIDES AS ADVANCED THERMOELECTRICS, Mercouri G. Kanatzidis, Michigan State Univ, Dept of Chemistry, East Lansing, MI; Tim L. Hogan, Jon L. Schindler, Carl R. Kannewurf, Northwestern Univ, Dept of E&CE, Evanston, IL.

Bi2Te3 and its solid solutions are currently the leading thermoelectric materials for near room temperature cooling applications. This reflects the fact that this material exhibits the unusual combination of simultaneously high electrical conductivity, high thermoelectric power and low thermal conductivity. We have embarked in a chemically based approach to prepare and study the properties of structural and chemical derivatives of Bi2Te3. The variation in properties of such materials could reveal interesting trends which could point the way to a better theoretical framework for understanding and designing advanced thermoelectric materials. We have examined the Ba/Bi/Te, Cs/Bi/Te and Rb/Bi/Te systems in detail and discovered several new promising materials. One of the challenges in this chemistry is the high stability of Bi2Te3 in the temperature range from 200 to 800 oC. We will report on the synthesis, structure, infrared absorption properties, electrical conductivity, and thermoelectric power of these materials. The synthesis, structures and structure/property relationships for the new materials will be discussed and compared to those of Bi2Te3. We also report thermal conductivity measurements which show that these materials have very low kL (lattice thermal conductivity) values and in some cases much lower than that of Bi2Te3.

2:45 PM Q13.5 
THERMOELECTRIC PROPERTIES OF DOPED ZnO, Bruce A. Cook, Iowa State Univ, Ames Laboratory, Ames, IA; J. A. Harringa, Iowa State Univ, Ames, IA; S. L. Purdum, A. M. Russell, Iowa State Univ, Dept of MS&E, Ames, IA.

Oxides have traditionally been regarded as poor candidates for thermoelectric applications due to their low electrical conductivity. However, recent consideration of the possibility of introducing donor states within the bandgap to increase the conductivity in ZnO has yielded polycrystalline samples in which electrical conductivities as high as 4.4 x 10 (-m) and electron mobilities of 6x10/V-s at 300 K have been observed. The electrical conductivity exhibits metal-like behavior between room temperature and 1300 K. Effects of incorporating other n-type substitutional dopants such as In, Al, and Ge are reported. Measurements of thermal diffusivity to 1300 K indicate that the dominant scattering mechanism is phonon-phonon throughout the entire temperature range. The Seebeck coefficient is a function of doping concentration and values as large as -50 V/K have been observed. Unoptimized ZnO samples have a ZT which increases with temperature and reaches a maximum of 0.4 at 1300 K. Effects of isostructural alloying with ZnS are shown to reduce the lattice component of the thermal conductivity.

SESSION Q14: THERMOELECTRIC PANEL DISCUSSION: NEW DIRECTIONS AND APPROACHES 
Chairs: Mercouri G. Kanatzidis and Terry M. Tritt 
Thursday Afternoon, April 3, 1997
Salon 14