M. Falmbigl

Thermal expansion of skutterudites

The current paper gives an overview of the newly obtained thermal expansion coefficients of skutterudites as well as those so far available in literature. Thermal expansion was determined for CoSb3, Pt4Sn4.4Sb7.6, for As- and Ge-based skutterudites as well as for various high-ZT skutterudites (micro- and nanostructured) with didymium (DD) and mischmetal (Mm) as filler atoms in frameworks of (Fe1−xCox)4Sb12 and (Fe1−xNix)4Sb12, and for double and triple-filled skutterudites such as Ca0.07Ba0.23Co3.95Ni0.05Sb12 and Sr0.025Ba0.075Yb0.1Co4Sb12. For low temperatures, a capacitance dilatometer was used (4–300 K), whereas for temperatures 300<T<750 K, a dynamic mechanical analyzer was employed. For a set of Ge-, P-, and Sb-based skutterudites, lattice parameters of single crystals, measured at three different temperatures, were used to derive the thermal expansion coefficient. The semiclassical model of Mukherjee [Phys. Rev. Lett. 76, 1876 (1996)] has been successfully used to quantitatively describe the thermal...

Thermal expansion of thermoelectric type-I-clathrates

Thermal expansion was determined for two series of ternary compounds, Ba8MxGe46−x and Ba8MxSi46−x, with M=Cu, Zn, Pd, Ag, Cd, Pt, and Au and for several quaternary compounds for which we investigated the influence of substitution by Zn/Ni in Ba8ZnxGe46−x as well as the dependence of thermal expansion on the Si/Ge ratio in Ba8Cu5SixGe41−x. In the temperature range from 4.2 to 300 K the thermal expansion of all ternary compounds was measured with a capacitance dilatometer, whereas from 300 to 700 K for several selected samples a dynamic mechanical analyzer was employed. The low temperature data compare well with the lattice parameters of single crystals, gained from measurements at three different temperatures (100, 200, and 300 K). For a quantitative description of thermal expansion the semiclassical model of Mukherjee et al. [Phys. Rev. Lett. 76, 1876 (1996)] was used, which also provided reliable accurate values of the Debye and Einstein temperatures. Results in this respect show good agreement with the ...

Type-I clathrate Ba8NixSi46−x: Phase relations, crystal chemistry and thermoelectric properties

The phase relations, crystal structure and thermoelectric properties of the type-I solid solution Ba(8)Ni(x)Si(46-x) were investigated. Based on X-ray diffraction, differential thermal analysis and electron probe microanalysis data, a partial phase diagram was constructed for the Si-rich part of ternary system Ba-Ni-Si at 800 °C. The solubility range of Ni in the clathrate-I phase at 800 °C was determined (2.9 ≤x≤ 3.8) and thermoelectric properties, namely electrical resistivity, Seebeck-coefficient and thermal conductivity, were measured in the temperature range from 300 to 850 K. A shift of the thermoelectric properties from a predominantly metallic to a more semiconducting behavior was observed for an increasing Ni-content. Density functional calculations revealed a significant decrease of the gap width in the density of states induced by the incorporation of Ni. Electrical resistivity and Seebeck coefficients for Ba(8)Ni(x)Si(46-x) with 3.3 ≤x≤ 3.8 have been modeled within the rigid band approximation.

Thermoelectric and magnetic properties of nanocrystalline La0.7Sr0.3CoO3

This work reports on the effect of grain size on the electrical, thermal, and magnetic properties of La0.7Sr0.3CoO3 samples obtained by solid-state reaction combined with ball milling. Electrical characterization made for samples with grain sizes ranging from 26 nm to 0.57 μm shows a size-induced metal-to-insulator transition for the sample with smallest grain size. As a consequence of the grain size reduction, there is a change of the thermopower sign and the thermal conductivity displays extremely low values. Interestingly, the thermoelectric figure of merit displays an enhancement as a consequence of the grain size reduction. The Curie temperature was found to be constant whereas the saturation magnetization decreases as grain size decreases. Thereby electrical and magnetic properties of nanocrystalline samples are interpreted in terms of a core-shell structure in which grain boundaries display an insulating behavior while the core region of each grain exhibits the bulk properties.

High-Pressure Torsion to Improve Thermoelectric Efficiency of Clathrates?

High-pressure torsion (HPT), as a technique to produce severe plastic deformation, has been proven effective to improve the thermoelectric performance of skutterudites. In this report, we present microstructural and thermoelectric properties of the clathrate Ba8Cu3.5Ge41In1.5 processed by HPT. The sample was synthesized from high-purity elements, subsequently annealed, ball milled, and hot pressed, and finally subject to HPT. Compared with the ball-milled and hot-pressed sample, the HPT-processed sample has higher electrical resistivity and Seebeck coefficient, and lower thermal conductivity, electron concentration, and mobility, which is attributed to the reduced grain size and increased density of dislocations, point defects, and cracks. No essential improvement of the dimensionless thermoelectric figure of merit is observed in the investigated temperature range, questioning the universal versatility of this technique for improvement of thermoelectric materials.

Cage-forming compounds in the Ba-Rh-Ge system: from thermoelectrics to superconductivity.

Phase relations and solidification behavior in the Ge-rich part of the phase diagram have been determined in two isothermal sections at 700 and 750 °C and in a liquidus projection. A reaction scheme has been derived in the form of a Schulz–Scheil diagram. Phase equilibria are characterized by three ternary compounds: τ1-BaRhGe3 (BaNiSn3-type) and two novel phases, τ2-Ba3Rh4Ge16 and τ3-Ba5Rh15Ge36-x, both forming in peritectic reactions. The crystal structures of τ2 and τ3 have been elucidated from single-crystal X-ray intensity data and were found to crystallize in unique structure types: Ba3Rh4Ge16 is tetragonal (I4/mmm, a = 0.65643(2) nm, c = 2.20367(8) nm, and RF = 0.0273), whereas atoms in Ba5Rh15Ge36–x (x = 0.25) arrange in a large orthorhombic unit cell (Fddd, a = 0.84570(2) nm, b = 1.4725(2) nm, c = 6.644(3) nm, and RF = 0.034). The body-centered-cubic superstructure of binary Ba8Ge43□3 was observed to extend at 800 °C to Ba8Rh0.6Ge43□2.4, while the clathrate type I phase, κI-Ba8RhxGe46–x–y□y, reveals a maximum solubility of x = 1.2 Rh atoms in the structure at a vacancy level of y = 2.0. The cubic lattice parameter increases with increasing Rh content. Clathrate I decomposes eutectoidally at 740 °C: κI ⇔ (Ge) + κIX + τ2. A very small solubility range is observed at 750 °C for the clathrate IX, κIX-Ba6RhxGe25–x (x ∼ 0.16). Density functional theory calculations have been performed to derive the enthalpies of formation and densities of states for various compositions Ba8RhxGe46–x (x = 0–6). The physical properties have been investigated for the phases κI, τ1, τ2, and τ3, documenting a change from thermoelectric (κI) to superconducting behavior (τ2). The electrical resistivity of κI-Ba8Rh1.2Ge42.8□2.0 increases almost linearly with the temperature from room temperature to 730 K, and the Seebeck coefficient is negative throughout the same temperature range. τ1-BaRhGe3 has a typical metallic electrical resistivity. A superconducting transition at TC = 6.5 K was observed for τ2-Ba3Rh4Ge16, whereas τ3-Ba5Rh15Ge35.75 showed metallic-like behavior down to 4 K.

Evaluation of the thermoelectric potential of the type-I clathrate Ba8NiyZnxGe46−x−y

A detailed investigation of the high-temperature thermoelectric properties of the ternary clathrate of type-I Ba8ZnxGe46−x for the range 7.2 ≤ x ≤ 7.8 is presented. Electrical resistivity, Seebeck coefficient, thermal conductivity and Hall-effect measurements clearly demonstrate that the maximum ZT-value can be expected for the composition Ba8Zn7.7Ge38.3, which reaches ZT ~ 0.46 at a temperature of 730 K. Furthermore, the influence on the thermoelectric properties was studied for partial substitution of Zn by one Ni-atom (Ba8NiyZnxGe46−x−y with ynom = 1, and 6.5 ≤ x ≤ 7.2) or by three Ni-atoms per unit cell (Ba8NiyZnxGe46−x−y with ynom = 3 and x = 1 to 3). Whereas a small amount of Ni increases the figure of merit ZT to 0.65 at 800 K, a higher Ni-content reduces the thermoelectric performance. Hall-effect measurements reveal that this improvement is essentially due to an increased charge carrier mobility.

Unusual behaviour of (Np,Pu)B2C

Two transuranium metal boron carbides, NpB2C and PuB2C have been synthesized by argon arc melting. The crystal structures of the {Np,Pu}B2C compounds were determined from single-crystal X-ray data to be isotypic with the ThB2C-type (space group , a = 0.6532(2) nm; c = 1.0769(3) nm for NpB2C and a = 0.6509(2) nm; c = 1.0818(3) nm for PuB2C; Z = 9). Physical properties have been derived from polycrystalline bulk material in the temperature range from 2 to 300 K and in magnetic fields up to 9 T. Magnetic susceptibility and heat capacity data indicate the occurrence of antiferromagnetic ordering for NpB2C with a Neel temperature TN = 68 K. PuB2C is a Pauli paramagnet most likely due to a strong hybridization of s(p,d) electrons with the Pu-5f states. A pseudo-gap, as concluded from the Sommerfeld value and the electronic transport, is thought to be a consequence of the hybridization. The magnetic behaviour of {Np,Pu}B2C is consistent with the criterion of Hill.

Ferro- and antiferro-magnetism in (Np, Pu)BC

Two new transuranium metal boron carbides, NpBC and PuBC, have been synthesized. Rietveld refinements of powder XRD patterns of {Np,Pu}BC confirmed in both cases isotypism with the structure type of UBC. Temperature dependent magnetic susceptibility data reveal antiferromagnetic ordering for PuBC below TN = 44 K, whereas ferromagnetic ordering was found for NpBC below TC = 61 K. Heat capacity measurements prove the bulk character of the observed magnetic transition for both compounds. The total energy electronic band structure calculations support formation of the ferromagnetic ground state for NpBC and the antiferromagnetic ground state for PuBC.

Mechanical Properties of Intermetallic Clathrates

The present work provides a comprehensive compilation and discussion covering hardness, experimentally determined elastic properties, thermal expansion, and Debye and Einstein temperatures for intermetallic clathrates. Comparing hardness values and elastic properties a major influence of the framework atoms is observed. Hardness and elastic moduli change linearly with the Si/Ge-ratio. Also a linear temperature dependence of the elastic properties is observed. In the case that vacancies are present in the clathrate framework, the hardness of different compounds decreases almost linearly with an increasing vacancy-content. As Debye and Einstein temperatures are of significance for the vibrational spectra and performance of thermoelectric materials, corresponding values extracted from various measurement techniques such as thermal expansion, sound velocity, specific heat, electrical resistivity or X-ray absorption and diffraction measurements are evaluated. The Debye temperatures correlate with the melting temperatures of the intermetallic clathrates, and the Einstein temperatures for similar guest atoms show a linear dependence on the cage size. Wherever available, experimental data are compared with those from DFT model simulations. In general the experimentally derived values match those theoretically calculated.