Didier Ravot

Nickel-substituted skutterudites: synthesis, structural and electrical properties

Abstract Based on AB 3 (A=Co, Ir, Rh and B=P, As and Sb) compounds, filled skutterudites (MA 4 B 12 ; M=rare earth) are promising new thermoelectric elements. Increasing their figure of merit requires realizing substitutions to give these compounds semiconducting character with a carrier concentration around 10 19 cm −3 and decreasing their thermal conductivity. In this framework, many published works have been devoted to CeFe 4 Sb 12 which, although metallic due to the trivalent character of the cerium ion, is the starting compound for numerous substitutions. For Ce y Fe 4− x Ni x Sb 12 , the substitution of nickel for iron gives semiconducting compounds. In this paper, we present our results on these compounds synthesized by the thermal treatment usually applied to obtain CeFe 4 Sb 12 .

Improved thermoelectric properties in double-filled Cey∕2Yby∕2Fe4−x(Co∕Ni)xSb12 skutterudites

We have investigated the thermal and electrical properties of filled skutterudites belonging to the series Ce1−zYbzFe4Sb12 and Cey∕2Yby∕2Fe4−x(Co∕Ni)xSb12, as well as their potential for thermoelectric power generation. In the first series, increasing the Yb fraction decreases the resistivity and thermopower. These effects are related to the variations of valence of Yb with the Yb fraction (from 2.2 in Yb0.92Fe4Sb12 to 2.7 in Ce0.85Yb0.05Fe4Sb12). The power factor is increased in Ce0.40Yb0.53Fe4Sb12 while the thermal conductivity is reduced and this leads to an improved figure of merit in this compound when compared to Ce0.85Fe4Sb12. Co or Ni substitution was used to tune the carrier concentration and improve the power factor: we measure a ZT=0.5 at 500K in Ce0.44Yb0.32Fe3.02Co0.98Sb12 and an extrapolation leads to ZT=0.95 at 800K, which is close to state-of-the-art Ce0.28Fe1.52Co2.48Sb12 although the composition is far from being optimized. Double filling happens to be a promising path to provide good th...

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.

Anomalous physical properties of cerium–lanthanum filled skutterudites

Abstract Thermoelectric materials generate an electromotive force when submitted to a thermal gradient with an efficiency scaled by the so-called figure of merit: Z = α 2 σ / κ , where α is the Seebeck coefficient, σ the electronic conductivity and κ the thermal conductivity. Theoretical considerations have shown that a large value of Z requires not only a small value of κ but also a large effective mass of charge carriers. The first condition is met in filled skutterudites, which have been studied extensively for this reason. The second condition suggests we focus our attention on skutterudites filled with an anomalous rare-earth element such as cerium, presenting heavy fermion characteristics. In this context, we report in this paper a study of the magnetic properties of CeFe 4 Sb 12 and their evolution upon cerium dilution.

Zinc oxide nanostructures confined in porous silicas.

We report on molecular simulations of zinc oxide nanostructures obtained within silica nanopores of diameter D = 1.6 nm and D = 3.2 nm. Both the effects of confinement (by varying the pore size) and degree of pore filling on the structure of the nanomaterial are addressed. Two complementary approaches are adopted: 1) the stability of the three crystalline phases of ZnO (wurtzite, rocksalt, and blende) in the silica nanopores is studied, and 2) ZnO nanostructures are obtained by slowly cooling down a homogeneous liquid phase confined in the silica pores. None of the ideal nanostructures (wurtzite, rocksalt, blende) retains the ideal structure of the initial crystal when confined within the silica pores. Only the structure starting from the ideal wurtzite nanocrystal remains significantly crystalline after relaxation, as revealed by the marked peaks in the pair correlation functions for this system. The morphology and degree of cristallinity of the structures are found to depend on the parameters involved in the synthesis (pore size, filling density). Nanograin boundaries are observed between domains of different crystal structures. Reminiscent features of the bulk behavior, such as faceting of the nanostructures, are also observed when the system size becomes large. We show that the use of nanopores as a template imposes that the confined particles exhibit neutral (basal) surfaces. These predictions provide a guide to experiments on semiconductor nanoparticles.

Influence of the nickel concentration on the magnetic properties of the cerium filled nickel substituted skutterudites

Abstract According to numerous authors, filled skutterudites which are promising thermoelectric compounds are characterized by very anomalous magnetic properties. Although containing cerium, iron and nickel or cobalt, these compounds, whatever the composition, do not order at low temperature. The magnetic susceptibility of nickel-poor compounds show a large deviation from the classical Curie–Weiss law. The effective magnetic moment of the semiconducting samples indicates that neither nickel nor iron are magnetic. These properties could indicate a temperature depending valency of the cerium ion, a crystal field effect or a heavy fermion behavior. In order to check some of these hypotheses we have performed magnetic and X-ray absorption experiments on nickel substituted cerium filled skutterudites. Our results presented in this paper confirm the valency 3+ of the cerium ion and corroborate the heavy fermion hypothesis.

Experimental determination of the phonon density of states in filled skutterudites: evidence for a localized mode of the filling atom

The generalized density of states of LaFe4Sb12 and CeFe4Sb12 has been determined by inelastic neutron scattering and its main features are found to be in agreement with recently published calculations (J. L. Feldman, D. L. Singh, C. Kendziora, D. Mandrus and B. C. Sales, Phys. Rev. B, 2003, 68, 094301). In both compounds a localized vibrational contribution appears superposed on the low-energy Debye response. The distinct inelastic response of La in LaFe4Sb12 is obtained by subtraction of the data for the Ce filled compound and it shows even more clearly the resolution limited peak at 7 meV, attributed to the localized mode of La-atoms.

Revisited phonon assignment and electro-mechanical properties of chromium disilicide

We report a complete study of the lattice dynamics, dielectric, elastic and piezoelectric properties of hexagonal semiconducting chromium disilicide (CrSi2). From a combined experimental and theoretical study, we have revisited the phonon mode assignments at the zone-center, so that the contradictions met in previous experimental studies between 250 and 300 cm−1 are now explained and understood. We found that the temperature dependence of the Raman frequencies is mainly due to an implicit volume contribution and related to the large Gruneisen parameter. This explains why CrSi2 has a moderate thermal conductivity although its Debye temperature is quite large. Optic and static dielectric constants have also been analyzed and discussed. The elastic constants of CrSi2 are large, but this compound is quite brittle. In addition, the relatively low Poisson coefficient associated to the large negative Cauchy pressure of CrSi2 indicate the angular nature of its bonding. The calculation of its piezoelectric coefficient shows a sizable value with a magnitude similar to that reported for α-quartz. This prediction requires, however, experimental confirmation.

Low-temperature anomalies of photoinduced second harmonic generation in skutterudites

Photoinduced second harmonic generation (PISHG) was found in skutterudite compounds of CeFe4Sb12 and Ce0.7Fe3.5Ni0.5 Sb12. Measurements versus temperature, pump–probe delaying time and external magnetic field were performed. The studied compounds belong to moderate heavy fermion compounds (HFC) in the ground state. The PISHG signals appear at 6.8 and 4.9 K for CeFe4Sb12 and Ce0.7Fe3.5Ni0.5Sb12, respectively. We suspect that these signals are due to anharmonic electron–phonon interactions creating a charge density non-centrosymmetry. The observed effects are caused either by a possible phase transition or by drastic changes in the electron structure of the HFC with decreasing temperature.

EXAFS and 125Te spectroscopy study of the Yb1−xEuxTe, Pb1−xEuxTe and Yb1−xGdxTe solid solutions

Abstract We report EXAFS (Extended X-Ray Absorption Fine Structure) measurements on three solid solutions of rare-earth monotelluride in order to measure the x-dependence of the cation-anion distance and Debye-Waller factor. We find Yb1−xEuxTe is an ideal solid solution with a bimodal distribution of distances, accommodated by the Te sublattice, at contrast with the two other solid solutions Pb1−xEuxTe and Yb1−xGdxTe. The anomalous behaviour of the EXAFS parameters in the last system is related to the shallow nature of the Gd donor.