George S. Nolas

Semiconducting Ge clathrates: Promising candidates for thermoelectric applications

Transport properties of polycrystalline Ge clathrates with general composition Sr8Ga16Ge30 are reported in the temperature range 5 K⩽T⩽300 K. These compounds exhibit N-type semiconducting behavior with relatively high Seebeck coefficients and electrical conductivity, and room temperature carrier concentrations in the range of 1017–1018 cm−3. The thermal conductivity is more than an order of magnitude smaller than that of crystalline germanium and has a glasslike temperature dependence. The resulting thermoelectric figure of merit, ZT, at room temperature for the present samples is 14 that of Bi2Te3 alloys currently used in devices for thermoelectric cooling. Extrapolating our measurements to above room temperature, we estimate that ZT>1 at T>700 K, thus exceeding that of most known materials.

High figure of merit in partially filled ytterbium skutterudite materials

We present evidence of a relatively high dimensionless figure of merit (ZT) in a polycrystalline skutterudite partially filled with ytterbium ions. The small-diameter yet heavy-mass Yb atoms partially filling the voids of the host CoSb3 system exhibit low values of thermal conductivity while the quite favorable electronic properties are not substantially perturbed by the addition of Yb. This combination is ideal for thermoelectric applications exemplifying the “phonon-glass electron-crystal” concept of a thermoelectric material, resulting in ZT=0.3 at room temperature and ZT∼1 at 600 K for Yb0.19Co4Sb12.

Glasslike Heat Conduction in High-Mobility Crystalline Semiconductors

The thermal conductivity of polycrystalline semiconductors with type-I clathrate hydrate crystal structure is reported. Ge clathrates (doped with Sr and /or Eu) exhibit lattice thermal conductivities typical of amorphous materials. Remarkably, this behavior occurs in spite of the well-defined crystalline structure and relatively high electron mobility ( ,100 cm 2 y Vs ). The dynamics of dopant ions and their interaction with the polyhedral cages of the structure are a likely source of the strong phonon scattering. [S0031-9007(98)08334-3]

High figure of merit in Eu-filled CoSb3-based skutterudites

We report measurements of electrical resistivity, thermopower, thermal conductivity, and Hall coefficient of polycrystalline Eu-doped CoSb3-based skutterudites with compositions Eu0.20Co4Sb12, Eu0.43Co4Sb11.59Ge0.31, and Eu0.42Co4Sb11.37Ge0.50. The relatively high mobility of these compounds, as compared to that of La- and Ce-filled skutterudites, may play a role in the large thermoelectric figure of merit (ZT>1 at 700 K) of Eu0.42Co4Sb11.37Ge0.50. We discuss the significant potential of these compounds for thermoelectric applications.

Structural disorder and thermal conductivity of the semiconducting clathrate Sr8Ga16Ge30

Abstract The temperature dependence of the atomic displacement parameters for Sr8Ga16Ge30 determined from refinements of neutron powder and single-crystal diffraction data shows that the anomalously large values for one of the two unique Sr atoms persist from 295 to 11 K. Its position is better described by a fractionally occupied four-fold split site, but the rms displacement remains the largest of all of the atoms in the structure. Difference Fourier maps of this Sr site show a residual nuclear density with lobes in the directions of the split-atom positions. The Ga and Ge atoms appear to be fully disordered on the three distinct framework sites. The measured atomic displacement parameters are used to derive estimates of the following thermodynamic related quantities: Debye temperature, 271 K; mean velocity of sound, 2600 m/s; temperature of the Einstein “rattler”, 85 K; mean free path of heat-carrying phonons, 5.36 A; and lattice thermal conductivity, 0.008 W/cm-K.

Thermoelectric properties of Sn-filled skutterudites

Thermal conductivity, resistivity, Seebeck coefficient, and structure measurements of CoSb3 with tin interstitially placed in the voids are reported. These tin-filled skutterudites were synthesized under high pressure and temperature conditions; they cannot be synthesized under “normal” synthesis approaches. The tin atoms exhibit very large atomic displacement parameters indicating a large “rattling” motion inside their atomic “cages.” The disorder induced by the Sn atoms is a very good phonon scattering mechanism. The thermal conductivity of these compounds is very low with a temperature dependence that is atypical of simple solids. The tin-filled compounds exhibit n-type semiconducting behavior with relatively high Seebeck coefficients for compounds whose electronic properties have not been optimized. The potential for thermoelectric applications is discussed.

Thermal and Electrical Conductivity of Size‐Tuned Bismuth Telluride Nanoparticles

Quantum-confined semiconductors composed of heavy elements hold great promise as thermoelectric materials. An increase in the density of states near the Fermi level due to quantum confinement effects and an increased scattering of boundary phonons due to nanostructuring can lead to an increase in the dimensionless figure of merit, ZT, which is defined as s 2 STk � 1 . [1,2] Here, s is the electrical conductivity, S is the Seebeck coefficient, T is the absolute temperature, and k is the thermal conductivity. To realize the highest ZTquantum-confined material possible from a conventional thermoelectric semiconductor material such as bismuth telluride, meeting several criteria are important. First, it is reasonable to expect that uniform, optimal size quantum domains will lead to the highest ZT at a given doping level. The electronic structure of the semiconductor, which determines the thermoelectric power, depends on the degree of quantum confinement and thus the domain size. Second, the interfacial electrical transport must be optimized. The presence of impurities between the domains/particles typically leads to barriers to transport that reduce the electrical conductivity. Third, while optimizing electrical transport between domains, it is important that the quantum architecture not be destroyed, or the phonon scattering will decrease and the thermal conductivity will increase. Fourth, it is well knownthatproperdopingiscriticaltotheperformanceofbulk

Inorganic clathrate-II materials of group 14: synthetic routes and physical properties

Crystalline open-framework and nanoporous materials have long attracted the attention of chemists, physicists, and materials scientists. The intriguing structures such materials exhibit are often intimately related to the unique physical properties they possess. This article reviews recent developments in the preparation and characterization of one such class of inorganic materials, those comprised of group 14 elements crystallizing with the clathrate-II crystal structure. These materials correspond to expanded forms and, in some cases, metastable allotropes of Si, Ge, and Sn. Just as interesting as the structure and properties that group 14 clathrate-II materials display is the diverse range of synthetic techniques used to prepare them and these techniques are discussed. We review the work to date characterizing the physical properties of group 14 clathrate-II materials with emphasis placed on structure–property relationships, in particular with regard to the electronic and transport properties.

PbTe nanocomposites synthesized from PbTe nanocrystals

Dense lead telluride (PbTe) nanocomposites were prepared from PbTe nanocrystals synthesized employing an aqueous solution-phase reaction. This approach reproducibly synthesizes 100–150nm nanocrystals with a high yield of over 2g per batch. Densification using spark plasma sintering dimensionally integrated nanoscale grains within a bulk matrix, resulting in a uniform dispersion of nonconglomerated nanocrystals. Transport properties of PbTe nanocomposites were evaluated through temperature dependent resistivity, Hall, Seebeck coefficient, and thermal conductivity measurements. These nanocomposites show an enhancement in the thermoelectric properties compared to bulk polycrystalline PbTe with similar carrier concentrations. Our results also indicate a strong sensitivity to stoichiometry, surface oxygen adsorption, and porosity.

Table-like magnetocaloric effect and enhanced refrigerant capacity in Eu8Ga16Ge30-EuO composite materials

A large reversible magnetocaloric effect (MCE) and enhanced refrigerant capacity (RC) were observed in multiphase composite materials composed of type-I clathrate Eu8Ga16Ge30 and EuO. Eu8Ga16Ge30 undergoes two successive ferromagnetic transitions at 10 K and 35 K, and EuO exhibits a ferromagnetic transition at 75 K. A large RC of 794 J/kg for a field change of 5 T over a temperature interval of 70 K was achieved in the Eu8Ga16Ge30–EuO composite with a 40%-60% weight ratio. This is the largest value ever achieved among existing magnetocaloric materials for magnetic refrigeration in the temperature range 10 K-100 K. Adjusting the Eu8Ga16Ge30 to EuO ratio is shown to produce composites with table-like MCE, desirable for ideal Ericsson-cycle magnetic refrigeration. The excellent magnetocaloric properties of these Eu8Ga16Ge30–EuO composites make them attractive for active magnetic refrigeration in the liquid nitrogen temperature range.