Jean-Pierre Fleurial

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...

Measurement of the electrical resistivity and Hall coefficient at high temperatures

The implementation of the van der Pauw (VDP) technique for combined high temperature measurement of the electrical resistivity and Hall coefficient is described. The VDP method is convenient for use since it accepts sample geometries compatible with other measurements. The technique is simple to use and can be used with samples showing a broad range of shapes and physical properties, from near insulators to metals. Three instruments utilizing the VDP method for measurement of heavily doped semiconductors, such as thermoelectrics, are discussed.

Mechanochemical synthesis and thermoelectric properties of high quality magnesium silicide

Magnesium silicide and related alloys are attractive for thermoelectric applications due to their low toxicity, thermal stability, low density, relative abundance and low cost of production. Earlier work on the synthesis of Mg2Siviahigh energy ball milling resulted in incomplete product formation, oxide impurities, and contamination from milling media. Here we present an improved solid-state synthesis of n-type magnesium silicide using the mechanochemical technique of high energy ball milling of the elements followed by high pressure sintering using hot uniaxial compaction. This robust synthetic method permits a detailed investigation of thermoelectric properties as a function of Bi doping. The thermoelectric properties of Mg2Si1−xBix (0 ≤ x ≤ 0.021) samples are characterized from 300 K to 775 K. These results are analyzed within a single parabolic band (SPB) model to determine the effective conduction band parameters and identify regimes of non-SPB behavior.

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...

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...

Thermal properties of electrodeposited bismuth telluride nanowires embedded in amorphous alumina

Bismuth telluride nanowires are of interest for thermoelectric applications because of the predicted enhancement in the thermoelectric figure-of-merit in nanowire structures. In this letter, we carried out temperature-dependent thermal diffusivity characterization of a 40nm diameter Bi2Te3 nanowires∕alumina nanocomposite. Measured thermal diffusivity of the composite decreases from 9.2×10−7m2s−1 at 150Kto6.9×10−7m2s−1 at 300K and is lower than thermal diffusivity of unfilled alumina templates. Effective medium calculations indicate that the thermal conductivity along nanowires axis is at least an order of magnitude lower than thermal conductivity of the bulk bismuth telluride.

Thermal conductivity reduction in SiGe alloys by the addition of nanophase particles

Abstract Transport models have predicted that the thermal conductivity of SiGe alloys could be appreciably reduced by incorporating discrete 40 A particles within the SiGe grains. These particles would scatter the thermal phonons which transport most of the heat in these alloys. Such a thermal conductivity reduction would lead to substantial improvements in the figure-of-merit and efficiency of thermoelectric materials used in power generation applications. This paper reports on the results of adding 40 A particles to SiGe by using a spark erosion process. Thermal conductivity reductions consistent with the transport models have been achieved; however, the improvement in figure-of-merit has not been as large as predicted.