Emmanuel Guilmeau

Transport and thermoelectric properties in Copper intercalated TiS2 chalcogenide

We report on the thermoelectric properties of CuxTiS2 bulk compounds. Copper cations have been intercalated into the layered chalcogenide TiS2 by spark plasma sintering. X-ray diffraction analysis coupled to transmission electron microscopy shows that the lattice constant c expands linearly as the Cu content x increases. The Cu-intercalation into TiS2 leads to substantial decrease in both electrical resistivity and lattice thermal conductivity as compared to those of pristine TiS2. The figure of merit, ZT, is increased up to 0.45 at 800 K for x = 0.02. The power factor, PF, reaches 1.7 mW/mK2 in TiS2 at 325 K.

Tuning the transport and thermoelectric properties of In2O3 bulk ceramics through doping at In-site

Bulk ceramics In2−xMxO3, with a metal-like behavior, have been synthesized in air for M=Ti4+, Zr4+, Sn4+, Ta5+, and Nb5+, with rather low solubility limits xl, ranging from 0.01 to 0.1. An abrupt increase in the electrical conductivity and of the carrier concentration with x is observed in the monophasic region (x xl) these values do not vary significantly. These results show that the valence of the doping element plays a crucial role in such properties, similar to degenerated semiconductors. Similarly, the thermopower |S| value is correlated with this evolution decreasing as x increases for x<xl. For the Sn doped samples, the maximum carrier concentration n=10.8×1020 cm−3 and electrical conductivity σ=5×103 S cm−1 are both achieved at x=0.06–0.1 (∼3–5 at. %) for a thermopower S of −20 μV/K at room temperature. In comparison, the minimum |S| and maxima σ and n of In2O3 compounds doped with the other cations occur at lower doping levels, e.g., xl∼0.02 and xl∼0.015 for...

Revisiting some chalcogenides for thermoelectricity

Abstract Thermoelectric materials that are efficient well above ambient temperature are needed to convert waste-heat into electricity. Many thermoelectric oxides were investigated for this purpose, but their power factor (PF) values were too small (∼10−4 W m−1 K−2) to yield a satisfactory figure of merit zT. Changing the anions from O2− to S2− and then to Se2− is a way to increase the covalency. In this review, some examples of sulfides (binary Cr–S or derived from layered TiS2) and an example of selenides, AgCrSe2, have been selected to illustrate the characteristic features of their physical properties. The comparison of the only two semiconducting binary chromium sulfides and of a layered AgCrSe2 selenide shows that the PF values are also in the same order of magnitude as those of transition metal oxides. In contrast, the PF values of the layered sulfides TiS2 and Cu0.1TiS2 are higher, reaching ∼10−3 W m−1 K−2. Apparently the magnetism related to the Cr–S network is detrimental for the PF when compared to the d0 character of the Ti4+ based sulfides. Finally, the very low PF in AgCrSe2 (PF = 2.25 × 10−4 W m1 K−2 at 700 K) is compensated by a very low thermal conductivity (κ = 0.2 W m−1 K−1 from the measured Cp) leading to the highest zT value among the reviewed compounds (zT700K = 0.8). The existence of a glassy-like state for the Ag+ cations above 475 K is believed to be responsible for this result. This result demonstrates that the phonon engineering in open frameworks is a very interesting way to generate efficient thermoelectric materials.

Thermoelectric properties–texture relationship in highly oriented Ca3Co4O9 composites

The correlation between thermoelectric properties and texture strength is discussed within the framework of Ca3Co4O9 textured ceramics. Based on an innovative method of x-ray diffraction analysis, the distribution density (i.e., the degree of orientation) of composite material composed of Ca3Co4O9 powder and single crystals was determined. Electrical resistivity of the prepared composites was shown to be reduced with increasing single crystals weight ratios and, in parallel, was directly correlated to an improvement of grain alignment. The incorporated single crystals help the texture development of the powder via an enhanced stacking of grains and fulfill a role as bypasses of the grain boundaries. This letter highlights the value of quantitative texture analysis to explain the evolution of anisotropic physical properties, as demonstrated here concerning textured thermoelectric materials.

The effect of MgO addition on the formation and the superconducting properties of the Bi2223 phase

Abstract MgO particles were introduced into (Bi,Pb)2Sr2Ca2Cu3O10+δ powder in various weight fractions. Bulk samples were made from the Bi2223/MgO powder and heat-treated at various temperatures. The phase formation and the microstructure were studied for different amounts of MgO and for different sintering conditions. It was found that, below 845 °C, the ratio of Bi2223 phase decreased when the amount of MgO increased. The addition of MgO caused the Bi2201 phase content to decrease via an increase of the liquid viscosity. The presence of MgO influenced the microstructure of the samples and decreased the mean size of Bi2223 grains. The effects of the MgO addition on the intergranular critical current density, Jc as a function of magnetic field was investigated by magnetisation hysteresis measurements. MgO presence significantly expands the magnetisation hysteresis loop and thus increases Jc for amount of MgO, which does not exceed 15 wt.%.

On the strong impact of doping in the triangular antiferromagnet CuCrO2

Abstract Electronic band structure calculations using the augmented spherical wave method have been performed for CuCrO2. For this antiferromagnetic ( T N = 24 K ) semiconductor crystallizing in the delafossite structure, it is found that the valence band maximum is mainly due to the t2g orbitals of Cr3+ and that spin polarization is predicted with 3 μ B per Cr3+. The structural characterizations of CuCr1−xMgxO2 reveal a very limited range of Mg2+ substitution for Cr3+ in this series. As soon as x = 0.02 , a maximum of 1% Cr ions are substituted by Mg site is measured in the sample. This result is also consistent with the detection of Mg spinel impurities from X-ray diffraction for x = 0.01 . This explains the saturation of the Mg2+ effect upon the electrical resistivity and thermoelectric power observed for x > 0.01 . Such a very weak solubility limit could also be responsible for the discrepancies found in the literature. Furthermore, the measurements made under magnetic field (magnetic susceptibility, electrical resistivity and Seebeck coefficient) support that the Cr4+ “holes”, created by the Mg2+substitution, in the matrix of high spin Cr3+ ( S = 3 / 2 ) are responsible for the transport properties of these compounds.

Magnetic properties of bulk Fe-doped indium oxide

This paper presents the influence of the iron fraction and of the sintering atmosphere on the magnetic properties of bulk iron-doped indium oxide In2−xFexO3. The formation of a solid solution between β-Fe2O3 and In2O3 up to x~0.3 under argon atmosphere and x~0.55 under air has been evidenced. All single-phase samples are paramagnetic, with dominant antiferromagnetic interactions and a paramagnetic effective moment originating from Fe3+ ions. For the higher x values, a cluster glass or superparamagnetic behaviour can be closely linked to the presence of Fe2O3 or Fe3O4 inclusions. No trace of ferromagnetism has been detected, even with samples prepared under argon.

Synthesis and thermoelectric properties of Bi 2.5 Ca 2.5 Co 2 O x layered cobaltites

Bi–Ca–Co–O polycrystalline materials with a layered structure were prepared. The synthesis of the sintered specimens from two starting compositions, Bi1.8Ca2Co2Ox and Bi2.5Ca2.5Co2Ox, revealed the latter is preferentially formed at high temperature (850 °C). The increase in sintering time was shown by growth of large platelike grains (up to 50 m in diameter and several micrometers in thickness). The reaction mechanisms during the heat treatment and the preferential formation of the Bi2.5Ca2.5Co2Ox phase were observed by x-ray diffraction, thermogravimetry– differential thermal analysis, and scanning electron microscopy. These techniques supposed the presence of a liquid phase at high temperature, origin of a highly kinetic phase formation, and the growth of large grains. Interestingly the liquid phase reaction promotes an efficient stacking and sliding of grains during hot-forging treatment, and highly (00l) oriented materials were prepared. A relationship between thermoelectric performance, texture strength, and microstructure is clarified.

Promising thermoelectric properties in AgxMo9Se11 compounds (3.4 ≤ x ≤ 3.9)

Polycrystalline Mo9 cluster chalcogenides AgxMo9Se11 (3.4≤x≤3.9) have been prepared by powder metallurgy techniques, sintered by spark plasma sintering and characterized by x-ray diffraction. Their thermoelectric properties (electrical resistivity, thermopower, thermal conductivity) have been determined in the 300–800 K temperature range. The AgxMo9Se11 compounds show p-type conduction characteristics. The outstanding low lattice thermal conductivities give rise to a rather high value of the dimensionless thermoelectric figure of merit ZT of ∼0.65 at 800 K for x=3.8–3.9, making this family of materials particularly promising for thermoelectric power generation applications.

A copper-containing oxytelluride as a promising thermoelectric material for waste heat recovery

The new thermoelectric material BiOCuTe exhibits an electrical conductivity of 224 S cm−1 and a Seebeck coefficient of +186 μV K−1 at 373 K, together with an extremely low lattice thermal conductivity of ∼0.5 W m−1 K−1. This results in a ZT of 0.42 at 373 K, which increases to 0.66 at the maximum temperature investigated, 673 K.