Christophe Candolfi

Assessment of the thermoelectric performance of polycrystalline p-type SnSe

We report the evaluation of the thermoelectric performance of polycrystalline p-type SnSe, a material in which unprecedented values of the thermoelectric figure of merit ZT have been recently discovered in single crystals. Besides anisotropic transport properties, our results confirm that this compound exhibits intrinsically very low thermal conductivity values. The electrical properties show trends typical of lightly doped, intrinsic semiconductors with thermopower values reaching 500 μV K−1 in a broad temperature range. An orthorhombic-to-orthorhombic transition sets in at 823 K, a temperature at which the power factor reaches its maximum value. A maximum ZT of 0.5 was obtained at 823 K, suggesting that proper optimization of the transport properties of SnSe might lead to higher ZT values. These findings indicate that this system represents an interesting experimental platform for the search of highly efficient thermoelectric materials.

Atomic Interactions in the p-Type Clathrate I Ba8Au5.3Ge40.7

Single crystals of Ba(8)Au(5.3)Ge(40.7) [space group Pm(3)n (No. 223), a = 10.79891(8) Å] were prepared by a Bridgman technique. The crystal structure refinement based on single-crystal X-ray diffraction data does not reveal any vacancies in the Au/Ge framework or in the cages. In addition to the ionic bonding between Ba and the anionic framework, a direct interaction between Ba and Au atoms was identified in Ba(8)Au(5.3)Ge(40.7) by applying the electron localizability indicator. As expected by the chemical-bonding picture, Ba(8)Au(5.3)Ge(40.7) is a diamagnet and shows p-type electrical conductivity with a hole carrier concentration of 7.14 × 10(19) cm(-3) at 300 K and very low lattice thermal conductivity of ≈0.6 W m(-1) K(-1) at 500 K. The thermoelectric figure of merit ZT of single crystals of Ba(8)Au(5.3)Ge(40.7) attains 0.3 at 511 K and reaches 0.9 at 680 K in a polycrystalline sample of closely similar composition. This opens up an opportunity for tuning of the thermoelectric properties of materials in the Ba-Au-Ge clathrate system by changing the chemical composition.

Facile General Route toward Tunable Magneli Nanostructures and Their Use As Thermoelectric Metal Oxide/Carbon Nanocomposites

Engineering nanoscale interfaces is a requisite for harnessing electrical and thermal transports within nanostructured materials, especially those destined for thermoelectric applications requiring an unusual combination of low thermal conductivity and electrical resistivity. Nanocomposites open up possibilities in this area, but are still bound to a very narrow range of materials. Here, we report a new approach combining the sol-gel process toward hybrid materials with spark plasma sintering (SPS) to yield functional nanocomposites based on substoichiometric titanium oxides Ti(n)O(2n-1), so-called Magnéli phases. The potential of this new approach is demonstrated by three results. First, multiple Ti(n)O(2n-1) compounds (n = 3, 4, 5, 6, 8) are obtained for the first time as sole nano-Magnéli crystalline phases with controlled specific surface areas from 55 to 300 m(2)·g(-1), classified as potential thermoelectric n-type metal oxides and paving the way toward advanced systems for energy-harvesting devices and optoelectronics. Second, this work combines the use of sol-gel and SPS processes to yield percolated nanocomposites based on metal oxide nanoparticles embedded in a carbon matrix with low electrical resistivity (2 × 10(-4) Ω·m for a Ti(4)O(7) compound) and reduced thermal conductivity (1 W·m(-1)·K(-1)) with respect to bulk phases. Finally, the discovered materials are reliable with thermoelectric figures of merit (ZT = 0.08) relatively high for n-type Ti-O-based systems and metal oxides. Thereby this study represents a proof of concept for the development of promising, cheaper, and more efficient thermoelectric conversion devices.

Crystal structure and transport properties of Ba8Ge43□3

The single phase clathrate-I Ba(8)Ge(43)square(3) (space group Ia3d (no. 230), a = 21.307(1) A) was synthesized by quenching the melt between cold steel plates. Specimens for physical property measurements were characterized by microstructure analysis and X-ray diffraction on polycrystalline samples as well as single crystals. Transport properties including thermopower, electrical resistivity, thermal conductivity and specific heat were investigated in a temperature range of 2-673 K. The electrical resistivity exhibits a metal-like temperature dependence below 300 K turning into a semiconductor-like behaviour above 300 K. The analysis of the specific heat at low temperature indicates a finite density of states at the Fermi level, thus corroborating the metallic character below 300 K. The temperature dependence of the specific heat was modelled assuming Einstein-like localized vibrations of Ba atoms inside the cages of the Ge framework. A conventional crystal-like behaviour of the thermal conductivity with a low lattice contribution (kappa(l)(300 K) = 2.7 W m(-1) K(-1)) has been evidenced.

High temperature thermoelectric properties of the type-I clathrate Ba8NixGe46-x-ysquarey

Polycrystalline samples of the type-I clathrate Ba(8)Ni(x)Ge(46-x-y)□(y) were synthesized for 0.2 ⩽ x ⩽ 3.5 by melt quenching and for 3.5<x ⩽ 6.0 by melting with subsequent annealing at 700 °C. The maximum Ni content in the clathrate framework at this temperature was found to be x ≈ 4.2 atoms per unit cell. Thermoelectric and thermodynamic properties of the type-I clathrate were investigated from 300 to 700 K by means of electrical resistivity, thermopower, thermal conductivity and specific heat measurements. As the Ni content increases, the electronic properties gradually evolve from a metallic character (x < 3.5) towards a highly doped semiconducting state (x ⩾ 3.5). Below x ≈ 4.0 transport is dominated by electrons, while further addition of Ni (x ≈ 4.2) switches the electrical conduction to p-type. Maximum value of the dimensionless thermoelectric figure of merit ZT ≈ 0.2 was achieved at 500 K and 650 K for x ≈ 2.0 and x ≈ 3.8, respectively.

Crystal structure, electronic band structure and high-temperature thermoelectric properties of Te-substituted tetrahedrites Cu12Sb4-xTexS13 (0.5 <= x <= 2.0)

Polycrystalline samples of the tetrahedrite phase Cu12Sb4−xTexS13 with nominal compositions 0.5 ≤ x ≤ 2.0 were synthesized by two different synthesis routes: from precursors and from direct melting of elements. The crystal structure was verified by single-crystal and powder X-ray diffraction (PXRD), both confirming the successful substitution of Te for Sb in both series. Our chemical analyses evidenced differences between the chemical compositions of the two series of samples likely tied to the synthesis method employed and suggesting off-stoichiometry on the Sb site. High-temperature PXRD and differential scanning calorimetry measurements indicate that these materials are stable up to 623 K. Above this temperature, the decomposition process starts and ends up near 748 K where a Cu2−yS-type phase is solely observed. In agreement with the simple electron counting rule and electronic band structure calculations, the electrical resistivity and thermopower increase with increasing x reflecting the gradual shift from a p-type metallic state (x = 0.0) to a p-type semiconducting behavior (x = 2.0). Combined with extremely low lattice thermal conductivity values (κ ≈ 0.5 W m−1 K−1 at 623 K), this substitution enables us to optimize the power factor leading to a maximum thermoelectric figure of merit ZT of about 0.8 at 623 K. These results parallel those obtained in prior studies dealing with partial substitutions on the Cu site and enlarge the possibilities to tune the electrical properties of tetrahedrites by extrinsic dopants.

BaGe5: A New Type of Intermetallic Clathrate

BaGe(5) constitutes a new type of intermetallic clathrate obtained by decomposition of clathrate-I Ba(8)Ge(43)(3) at low temperatures. The crystal structure consists of characteristic layers interconnected by covalent bonds. BaGe(5) is a semiconducting Zintl phase.

Invited Article: A round robin test of the uncertainty on the measurement of the thermoelectric dimensionless figure of merit of Co0.97Ni0.03Sb3

A round robin test aiming at measuring the high-temperature thermoelectric properties was carried out by a group of European (mainly French) laboratories (labs). Polycrystalline skutterudite Co0.97Ni0.03Sb3 was characterized by Seebeck coefficient (8 labs), electrical resistivity (9 labs), thermal diffusivity (6 labs), mass volume density (6 labs), and specific heat (6 labs) measurements. These data were statistically processed to determine the uncertainty on all these measured quantities as a function of temperature and combined to obtain an overall uncertainty on the thermal conductivity (product of thermal diffusivity by density and by specific heat) and on the thermoelectric figure of merit ZT. An increase with temperature of all these uncertainties is observed, in agreement with growing difficulties to measure these quantities when temperature increases. The uncertainties on the electrical resistivity and thermal diffusivity are most likely dominated by the uncertainty on the sample dimensions. The temperature-averaged (300-700 K) relative standard uncertainties at the confidence level of 68% amount to 6%, 8%, 11%, and 19% for the Seebeck coefficient, electrical resistivity, thermal conductivity, and figure of merit ZT, respectively. Thermal conductivity measurements appear as the least accurate. The moderate value of the temperature-averaged relative expanded (confidence level of 95%) uncertainty of 17% on the mean of ZT is essential in establishing Co0.97Ni0.03Sb3 as a high temperature standard n-type thermoelectric material.

X-ray Characterization, Electronic Band Structure, and Thermoelectric Properties of the Cluster Compound Ag2Tl2Mo9Se11

We report on a detailed investigation of the crystal and electronic band structures and of the transport and thermodynamic properties of the Mo-based cluster compound Ag2Tl2Mo9Se11. This novel structure type crystallizes in the trigonal space group R3̅c and is built of a three-dimensional network of interconnected Mo9Se11 units. Single-crystal X-ray diffraction indicates that the Ag and Tl atoms are distributed in the voids of the cluster framework, both of which show unusually large anisotropic thermal ellipsoids indicative of strong local disorder. First-principles calculations show a weakly dispersive band structure around the Fermi level as well as a semiconducting ground state. The former feature naturally explains the presence of both hole-like and electron-like signals observed in Hall effect. Of particular interest is the very low thermal conductivity that remains quasi-constant between 150 and 800 K at a value of approximately 0.6 W·m(-1)·K(-1). The lattice thermal conductivity is close to its minimum possible value, that is, in a regime where the phonon mean free path nears the mean interatomic distance. Such extremely low values likely originate from the disorder induced by the Ag and Tl atoms giving rise to strong anharmonicity of the lattice vibrations. The strongly limited ability of this compound to transport heat is the key feature that leads to a dimensionless thermoelectric figure of merit ZT of 0.6 at 800 K.

Synthesis, Crystal Structure, and Physical Properties of the Type-I Clathrate Ba8−δNix□ySi46–x–y

Type-I clathrate phase Ba(8)Ni(x)□(y)Si(46-x-y) (□ = vacancy) was obtained from the elements at 1000 °C with the homogeneity range 2.4 ≤ x ≤ 3.8 and 0 ≤ y ≤ 0.9. In addition, samples with low Ni content (x = 1.4 and 1.6; y = 0) and small Ba deficiency were prepared from the melt by steel-quenching. Compositions were established by microprobe analysis and crystal structure determination. Ba(8-δ)Ni(x)□(y)Si(46-x-y) crystallizes in the space group Pm ̅3n (No. 223) with lattice parameter ranging from a = 10.3088(1) Å for Ba(7.9(1))Ni(1.4(1))Si(44.6(1)) to a = 10.2896(1) Å for Ba(8.00(3))Ni(3.82(4))Si(41.33(6)). Single-crystal X-ray diffraction data together with microprobe analysis indicate an increasing number of framework vacancies toward compositions with higher Ni content. For all compositions investigated, Ni K-edge X-ray absorption spectroscopy measurements showed an electronic state close to that of elemental Ni. All samples exhibit metallic-like behavior with moderate thermopower and low thermal conductivity in the temperature range 300-773 K. Samples with compositions Ba(7.9(1))Ni(1.4(1))Si(44.6(1)) and Ba(7.9(1))Ni(1.6(1))Si(44.4(1)) are superconducting with T(c) values of 6.0 and 5.5 K, respectively.