Thierry Caillat

Recent developments in thermoelectric materials

Abstract Efficient solid state energy conversion based on the Peltier effect for cooling and the Seebeck effect for power generation calls for materials with high electrical conductivity σ, high Seebeck coefficient S, and low thermal conductivity k. Identifying materials with a high thermoelectric figure of merit Z(= S2σ/k) has proven to be an extremely challenging task. After 30 years of slow progress, thermoelectric materials research experienced a resurgence, inspired by the developments of new concepts and theories to engineer electron and phonon transport in both nanostructures and bulk materials. This review provides a critical summary of some recent developments of new concepts and new materials. In nanostructures, quantum and classical size effects provide opportunities to tailor the electron and phonon transport through structural engineering. Quantum wells, superlattices, quantum wires, and quantum dots have been employed to change the band structure, energy levels, and density of states of electrons, and have led to improved energy conversion capability of charged carriers compared to those of their bulk counterparts. Interface reflection and the scattering of phonons in these nanostructures have been utilised to reduce the heat conduction loss. Increases in the thermoelectric figure of merit based on size effects for either electrons or phonons have been demonstrated. In bulk materials, new synthetic routes have led to engineered complex crystal structures with the desired phonon-glass electron-crystal behaviour. Recent studies on new materials have shown that dimensionless figure of merit (Z ×temperature) values close to 1·5 could be obtained at elevated temperatures. These results have led to intensified scientific efforts to identify, design, engineer and characterise novel materials with a high potential for achieving ZT much greater than 1 near room temperature.

Preparation and thermoelectric properties of semiconducting Zn4Sb3

Hot-pressed samples of the semiconducting compound β-Zn4Sb3 were prepared and characterized by X-ray and microprobe analysis. Some physical properties of β-Zn4Sb3 were determined and its thermoelectric properties measured between room temperature and 650 K. Exceptionally low thermal conductivity values were obtained in the 300–650 K temperature range and the room temperature lattice thermal conductivity was estimated at 6.5 W cm−1 K−1. High thermoelectric figures of merit (ZTs) were obtained between 450 and 670 K and a maximum of about 1.3 was obtained at a temperature of 670 K, the highest known at this temperature. The stability of the compound was investigated by several techniques, including thermogravimetric studies. The results showed that the samples were stable under argon atmosphere and static vacuum up to about 670 K and up to 520 K in dynamic vacuum. The high thermoelectric performance of β-Zn4Sb3 in the 300–670 K temperature range fills the existing gap in the ZT spectrum of p-type state-of-the-art thermoelectric materials between Bi2Te3-based alloys and PbTe-based alloys. This material, relatively inexpensive, could be used in more efficient thermoelectric generators for waste heat recovery and automobile industry applications, for example.

Properties of Single Crystalline Semiconducting CoSb3

A study of the thermoelectric properties of the skutterudite compound CoSb3 was carried out on single crystals grown by the Bridgman gradient freeze technique. p‐ and n‐type samples were obtained over a wide range of carrier concentration. Undoped As‐grown crystals show p‐type conductivity while n‐type samples were obtained by addition of Te or Pd. Samples were characterized by x‐ray diffractometry, electron microprobe analysis, and density measurements. The physical properties of CoSb3 such as linear thermal expansion coefficient, sound velocity, and Debye temperature were also determined and are presented. Seebeck coefficient, electrical resistivity, thermal conductivity, and Hall effect measurements were performed between room temperature and about 900 K. Exceptionally high Hall mobilities were obtained on p‐type samples with a maximum room‐temperature Hall mobility of 3300 cm2 V−1 s−1 at a carrier concentration of 1×1017 cm−3. The results of the transport property measurements are discussed and are in...

Supercooling of Peltier cooler using a current pulse

The operation of a Peltier cooler can be temporarily enhanced by utilizing the transient response of a current pulse. The performance of such a device, using (Bi,Sb)2Te3-based thermoelectric elements, was examined from −70 to 55 °C. We establish both theoretically and experimentally the essential parameters that describe the pulse cooling effect, such as the minimum temperature achieved, maximum temperature overshoot, time to reach minimum temperature, time while cooled, and time between pulses. Using simple theoretical and semiempirical relationships the dependence of these parameters on the current pulse amplitude, temperature, thermoelectric element length, thermoelectric figure of merit and thermal diffusivity is established. At large pulse amplitudes the amount of pulse supercooling is proportional to the maximum steady-state difference in temperature. This proportionality factor is about half that expected theoretically. This suggests that the thermoelectric figure of merit is the key materials para...

High figure of merit in Ce-filled skutterudites

New thermoelectric materials with superior transport properties at high temperatures have been discovered. These materials are part of the large family of skutterudites, a class of compounds which have shown a good potential for thermoelectric applications. The composition of these novel materials, called filled skutterudites, is derived from the skutterudite crystal structure and can be represented by the formula LnT/sub 4/Pn/sub 12/ (Ln=rare earth, Th; T=Fe, Rn, Os, Co, Rh, Ir; Pn=P, As, Sb). In these compounds, the empty octants of the skutterudite structure which are formed in the TPn/sub 3/ (/spl sim/T/sub 4/Pn/sub 12/) framework are filled with a rare earth element. Some of these compositions, based on CeFe/sub 4/Sb/sub 12/, have been prepared by a combination of melting and powder metallurgy techniques and have shown exceptional thermoelectric properties in the 350-700/spl deg/C temperature range. At room temperature, CeFe/sub 4/Sb/sub 12/ behaves as a p-type semimetal, but with a low thermal conductivity and surprisingly large Seebeck coefficient. These results are consistent with some recent band structure calculations on these compounds. Replacing Fe with Co in CeFe/sub 4/Sb/sub 12/ and increasing the Co:Fe atomic ratio resulted in an increase in the Seebeck coefficient values. The possibility of obtaining n-type conductivity filled skutterudites for Co:Fe values higher than 1:3 is currently being investigated. Measurements on bulk samples with a CeFe/sub 3.5/Co/sub 0.5/Sb/sub 12/ atomic composition and p-type conductivity resulted in dimensionless figure of merit ZT values of 1.4 at 600/spl deg/C.

Bridgman-solution crystal growth and characterization of the skutterudite compounds CoSb3 and RhSb3

Abstract Compounds with the skutterudite structure have recently been identified as advanced thermoelectric materials. We report on the crystal growth and characterization of the skutterudite compounds CoSb 3 and RhSb 3 which form peritectically at 873 and 900°C, respectively. Large single crystals were obtained by the vertical gradient freeze technique from solutions rich in antimony. The samples were characterized by high-temperature Hall-effect and electrical resistivity measurements. Bandgaps of 0.56 and 0.80 eV were estimated from these measurements for CoSb 3 and RhSb 3 , respectively. N-type CoSb 3 samples were obtained by doping with Te. Exceptionally high p-type Hall-mobility values have been measured and a room-temperature value of 3445 cm 2 V −1 s −1 was obtained for CoSb 3 at a carrier concentration of 4 × 10 17 cm −3 and 8000 cm 2 V −1 s −1 was obtained for RhSb 3 at a carrier concentration of 3.5 × 10 18 cm −3 .

Preparation and thermoelectric properties of the skutterudite‐related phase Ru0.5Pd0.5Sb3

A new skutterudite phase Ru0.5Pd0.5Sb3 was prepared. This new phase adds to a large number of already known materials with the skutterudite structure which have shown good potential for thermoelectric applications. Single phase, polycrystalline samples were prepared and characterized by x‐ray analysis, electron probe microanalysis, density, sound velocity, thermal‐expansion coefficient, and differential thermal analysis measurements. Ru0.5Pd0.5Sb3 has a cubic lattice, space group Im3 (T5h), with a=9.298 A and decomposes at about 920 K. The Seebeck coefficient, the electrical resistivity, the Hall effect, and the thermal conductivity were measured on hot‐pressed samples over a wide range of temperatures. Preliminary results show that Ru0.5Pd0.5Sb3 behaves as a heavily doped semiconductor with an estimated band gap of about 0.6 eV. The lattice thermal conductivity of Ru0.5Pd0.5Sb3 is substantially lower than that of the binary isostructural compounds CoSb3 and IrSb3. The unusually low thermal conductivity m...

RAMAN SCATTERING STUDY OF ANTIMONY-BASED SKUTTERUDITES

Raman spectra of single‐crystal CoSb3 and RhSb3 and of polycrystalline IrSb3 have been studied. We have also studied four different polycrystalline‐filled skutterudite samples, Ir4LaGe3Sb9, Ir4NdGe3Sb9, Ir4SmGe3Sb9, and Fe4CeSb12, where rare‐earth ions occupy the voids in the skutterudite structure. These void‐filling ions interact with some of the lattice vibrations in this structure and produce a shift and broadening of the observed lines. The most prominent lines in all of the samples are the Sb4 ring‐breathing modes.Raman spectra of single‐crystal CoSb3 and RhSb3 and of polycrystalline IrSb3 have been studied. We have also studied four different polycrystalline‐filled skutterudite samples, Ir4LaGe3Sb9, Ir4NdGe3Sb9, Ir4SmGe3Sb9, and Fe4CeSb12, where rare‐earth ions occupy the voids in the skutterudite structure. These void‐filling ions interact with some of the lattice vibrations in this structure and produce a shift and broadening of the observed lines. The most prominent lines in all of the samples are the Sb4 ring‐breathing modes.

Chemical Stability of (Ag,Cu)2Se: a Historical Overview

Recent work on Cu2−xSe has caused strong interest in this material due to its high reported peak zT (1.5) and the reduction of thermal conductivity through the mechanism of liquid-like suppression of heat capacity. In the 1960s, 3M patented Cu1.97Ag0.03Se as “TPM-217.” Over the following decade it was tested and developed by the 3M Corporation, at the National Aeronautics and Space Administration (NASA) Jet Propulsion Laboratory, Teledyne Energy Systems, and the General Atomics Corporation for use as a next-generation thermoelectric material. During these tests, extreme problems with material loss through Se vaporization and chemical reactions between the material and the device contacts were found. These problems were especially severe while operating under conditions of high