Michaël Pollet

Rapid Hydrothermal Synthesis of VO2 (B) and Its Conversion to Thermochromic VO2 (M1)

The present study provides a rapid way to obtain VO2 (B) under economical and environmentally friendly conditions. VO2 (B) is one of the well-known polymorphs of vanadium dioxide and is a promising cathode material for aqueous lithium ion batteries. VO2 (B) was successfully synthesized by rapid single-step hydrothermal process using V2O5 and citric acid as precursors. The present study shows that phase-pure VO2 (B) polytype can be easily obtained at 180 °C for 2 h and 220 °C for 1 h, that is, the lowest combination of temperature and duration reported so far. The obtained VO2 (B) is characterized by X-ray powder diffraction, high-resolution scanning electron microscopy, and Fourier transform infrared spectroscopy. In addition, we present an indirect way to obtain VO2 (M1) by annealing VO2 (B) under vacuum for 1 h.

Large thermoelectric power factors and impact of texturing on the thermal conductivity in polycrystalline SnSe

Single crystals of SnSe have been reported to have very high thermoelectric efficiencies with a maximum figure merit zT = 2.5. This outstanding performance is due to ultralow thermal conductivities. We report on the synthesis of highly textured polycrystalline SnSe ingots with large single-crystal magnitude power factors, S2/ρ = 0.2–0.4 mW m−1 K−2 between 300–600 K, increasing to 0.9 mW m−1 K−2 at 800 K, and bulk thermal conductivity values κ300K = 1.5 W m−1 K−1. However, small SnSe ingots, which were measured in their entirety, were found to have a substantially reduced κ300K = 0.6 W m−1 K−1. Microscopy and diffraction revealed two distinct types of texturing within the hot-pressed ingots. In the interior, large coherent domains of SnSe platelets with a ∼45° orientation with respect to the pressing direction are found, while the platelets are preferentially oriented at 90° to the pressing direction at the top and bottom of the ingots. Fitting the κ(T) data suggests an increase in defect scattering for the smaller ingots, which is in keeping with the presence of regions of structural disorder due to the change in texturing. Combining the measured S2/ρ with the bulk ingot κ values yields zT = 1.1 at 873 K.

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.

Reinvestigation of the magnetic behavior of O3–LiCoO2

Stoichiometric high temperature LiCoO2 obtained by long annealing in oxygen was characterized by electron spin resonance and magnetization measurements. Both methods allow identifying unambiguously not only the presence of traces of cobalt oxides in the material but also paramagnetic defects in lithium cobaltite itself. We report on the presence of surface Li+–O− centers in pure LiCoO2, which has not been observed before in this material, and on the presence of Co2+ related centers in argon- and subsequent oxygen-annealed samples.

Evidence for hard and soft substructures in thermoelectric SnSe

SnSe is a topical thermoelectric material with a low thermal conductivity which is linked to its unique crystal structure. We use low-temperature heat capacity measurements to demonstrate the presence of two characteristic vibrational energy scales in SnSe with Debye temperatures θD1 = 345(9) K and θD2 = 154(2) K. These hard and soft substructures are quantitatively linked to the strong and weak Sn-Se bonds in the crystal structure. The heat capacity model predicts the temperature evolution of the unit cell volume, confirming that this two-substructure model captures the basic thermal properties. Comparison with phonon calculations reveals that the soft substructure is associated with the low energy phonon modes that are responsible for the thermal transport. This suggests that searching for materials containing highly divergent bond distances should be a fruitful route for discovering low thermal conductivity materials.

Thermopower of 2D-alkali cobaltites A x CoO 2 with A = Li, Na and K

This paper is devoted to a comparative study of the AxCoO2 (A = Li, Na and K) systems. For x ~ 0.6, all the investigated alkali cobaltites exhibit a metallic-like behavior. Potassium cobaltites exhibit a T2 dependence of the resistivity - suggesting a Fermi liquid character - and an enhanced Pauli type paramagnetism ascribed to electronic correlations. On the other hand lithium and sodium compounds show linear temperature dependence for the resistivity down to 4 K, a large thermopower above room temperature and a Curie-Weiss paramagnetism denoting larger electronic correlations. Physical properties of these cobaltites are discussed in details especially in the case of K-ordered and disordered K0.6CoO2 phases.

Electronic Properties of 2D Alkali Cobaltites and Related Oxides

Strong thermoelectric power (TEP) can be achieved in semiconducting oxides when a low carrier density is created by appropriate atomic substitutions or intercalations, but the electrical resistivity remains too large. Actually, oxides such as layered cobaltites for which the best balance between high TEP and small resistivity is reached contain a large concentration of strongly correlated carriers. However, the origin of the large TEP values still remains an open question. In mixed valence oxides two main transport mechanisms are generally involved: either a metallic type transport with a mean free path larger than that predicted by the Ioffe-Regel limit or a hopping type transport. In the first case, according to Motts equation, large TEP values could result from peculiar energy dependence of the density of states and relaxation time at the Fermi level; in the second case TEP can be calculated using Heikes formula. In both cases, TEP can be enhanced by spin entropy effects mainly expected for spin polarized metallic oxides (or half metals) rather than Pauli metals. For hopping transport an additional term in Heikes formula arising from the spin as well as orbital degeneracy must be taken into account. The specificity of the Co3+ ions (3d6) that can exhibit three different electronic configurations (S = 0, S = 1 and S = 2) in oxides, depending on the interplay of exchange and crystal field energies, as well as the dimensionality of the crystal structure, the site symmetry and correlation effects is also discussed. The behavior of recently investigated potassium-intercalated 2D-cobaltites is compared to that of the corresponding sodium oxides

Structural features and transport properties of iodine intercalated misfit layer [BiCaO/sub 2/]/sub 2/[CoO/sub 2/]/sub 1.69/ single crystals

The thermopower and the electrical resistivity of [BiCaO/sub 2/]/sub 2/[CoO/sub 2/]/sub 1.69/ and corresponding iodine intercalated single crystals have been measured. Upon intercalation, the thermopower is drastically decreased, indicating that there is a hole doping by charge transfer from the intercalated iodine layer to the hexagonal CoO/sub 2/ layer. The resistivity is increased due to stacking faults and disordered structures. Structural analyses confirmed the stacking scheme along the c direction, with the localisation of iodine between the [BiO] double layers. The effect of intercalation on the thermoelectric properties suggested discussions from the view point of hole doping and nano-block layer coupling effect.