David Berthebaud

Thermoelectric properties and spark plasma sintering of doped YB22C2N

YB 22 C 2 N is one of a series of rare earth borocarbonitrides and is potentially the long awaited n-type counterpart to boron carbide. We conducted studies on YB 22 C 2 N spark plasma sintered with additions of YB 4 and YB 25 C, including the investigations of the densification process and the thermoelectric properties of the material. We discovered that a small amount of dopants can lower the starting temperature of densification during spark plasma sintering (SPS). Variations of pressure and temperature during the sintering process are also found to have an effect. Electrical conductivity of the dense samples has increased due to insertion of metal borides and also because of the improvement of the relative density. At the same time, only a slight reduction was observed for the Seebeck coefficient leading to an important improvement of power factor. The highest density of more than 95% was achieved with 5 wt% of YB 25 (C) dopant.

Thermoelectric properties of the chalcopyrite Cu1−xMxFeS2−y series (M = Mn, Co, Ni)

The natural chalcopyrite mineral CuFeS2 is a semiconductor material with potential for thermoelectric applications. This study presents the thermoelectric properties – electrical resistivity ρ, Seebeck coefficient S and thermal conductivity κ – of the substituted on the Cu site and/or sulfur deficient CuFeS2 chalcopyrite based series Cu1−xMxFeS2−y (M = Mn, Co, Ni, x ≤ 0.05 and y ≤ 0.02). All samples have been densified by spark plasma sintering, allowing proper measurements of S, ρ and κ at high temperature. All compounds show n-type semiconducting properties with large absolute values of S, from −220 to −340 μV K−1. Maximum ZT values up to 0.20 at 623 K were obtained for Cu0.97Mn0.03FeS2 and Cu0.98Co0.02FeS1.98. The veracity of Mn for Cu substitution into the structure has been confirmed by EDS analyses, coupled to electron diffraction within a transmission electron microscope. The latter study demonstrates the existence of twinned domains. The thermal conductivity reaches values as low as κ ∼ 1.2 W m−1 K−1 at 623 K. The magnetic properties of a Mn substituted sample did not show any significant modification in the magnetic behavior compared to the pristine CuFeS2 compound. The small negative magnetoresistance observed in CuFeS2 of about −2% at 5 K in 9 T is degraded in the Mn substituted sample.

Magnetodielectric Effect in Crystals of the Noncentrosymmetric CaOFeS at Low Temperature

Single crystals of the stoichiometric iron calcium oxysulfide CaOFeS have been grown by a solid-state reaction. Structural analysis of CaOFeS at room temperature by combining single-crystal X-ray diffraction data and transmission electron microscopy leads to a stoichiometric hexagonal noncentrosymmetric P63mc layered structure isostructural to CaOZnS. It is built from alternating layers made of FeOS3 tetrahedra sharing sulfur apexes and stacked with Ca(2+) planes. All Fe-O bonds are parallel to the stacking axis; this breaks the centrosymmetry, leading to a polar structure. The dielectric measurements reveal the existence of a magnetodielectric effect near 33 K in good agreement with the Neel temperature, as evidenced near 35 K by specific heat measurements reported by a different group.

Transport and magnetic properties of highly densified CoS2 ceramic

Dense ceramics of the itinerant ferromagnet CoS2 have been prepared by the spark plasma sintering technique. The structural study confirms a cubic unit-cell for CoS2 with a = 5.539(6)A (SG=Pa3¯). Scanning and transmission electron microscopy reveal the existence of sulfur deficient regions of composition “Co2S3” with a structure compatible to that of CoS2 where 0.5 S atom are missing in one out of two successive layers along the b direction. The volume of these regions estimated to be less than 1% is explained by possible S losses from the surface during the sintering. The physical properties measurements show a high TC (122 K), large saturated magnetization (0.88 μB/f.u.), with a ∼100% magnetoresistance at 5 K in 9 T. Also, the value of the residual resistivity ratio, ρ300 K/ρ5 K = 45, between those of crystal and conventional ceramics, attest for both good compacity and grain connectivity. Interestingly, the Seebeck coefficient exhibits negative values reaching S = −47.5 μV K−1 at 675 K and below TC, S ...

Synthesis and Thermoelectric Properties in the 2D Ti1 – xNbxS3 Trichalcogenides

A solid solution of Ti1 − xNbxS3 composition (x = 0, 0.05, 0.07, 0.10) was synthesized by solid-liquid-vapor reaction followed by spark plasma sintering. The obtained compounds crystallize in the monoclinic ZrSe3 structure type. For the x = 0.07 sample, a mixture of both A and B variants of the MX3 structure is evidenced by transmission electron microscopy. This result contrasts with those of pristine TiS3, prepared within the same conditions, which crystallizes as a large majority of A variant. Thermoelectric properties were investigated in the temperature range 323 to 523 K. A decrease in the electrical resistivity and absolute value of the Seebeck coefficient is observed when increasing x due to electron doping. The lattice component of the thermal conductivity is effectively reduced by the Nb for Ti substitution through a mass fluctuation effect and/or a disorder effect created by the mixture of both A and B variants. Due to the low carrier concentration and the semiconductor character of the doped compounds, the too low power factor values leads to ZT values that remain smaller by a factor of 50 than those of the TiS2 layered compound.

The solid solution series Tl(V1−xCrx)5Se8: crystal structure, magnetic and thermoelectric properties

The crystal structure of single crystalline members of the solid solution series Tl(V1−xCrx)5Se8 (x = 0–1 and Δx = 0.2) was determined and the magnetic and thermoelectric properties of bulk TlV5Se8 were investigated. All compounds crystallize in the pseudo-hollandite-type structure (C2/m) with a nonlinear increase in the unit cell volume due to a simultaneous increase/decrease in the transition metal distances across the edge/face-sharing octahedra. A slight Tl deficiency was found for single crystalline TlV5Se8 and Tl(V0.8Cr0.2)5Se8 as well as for bulk TlV5Se8 according to Rietveld and single crystal structure refinements. The metallic character of bulk TlV5Se8 and its low electrical resistivity compared to poly- and single-crystalline TlCr5Se8 can probably be associated with this probable Tl deficiency. In TlV5Se8, anomalies in the low temperature Seebeck coefficient, i.e. a maximum at T ∼ 14 K and two sign changes at T ∼ 47 K and T ∼ 167 K, were found. Bulk TlV5Se8 is a highly frustrated (f = |θ|/TN ≈ 42 K) Curie–Weiss paramagnet ordering antiferromagnetically (θ = −1287 K) below TN ≈ 31 K with an effective magnetic moment of μ = 2.68 μB corresponding to V3+. Low-field M(H) measurements revealed the existence of a small ferromagnetic component below TN in bulk TlV5Se8 and, despite the observed spin frustration, no spin glass phase was found in this compound.

Structural and thermoelectric properties of n-type isocubanite CuFe2S3

An effective strategy to enhance thermoelectric performance consists of scattering phonons by point defects, such as the ones intrinsic to the n-type isocubanite CuFe2S3. In this structure, the TEM evidences a random distribution of vacancies, and copper and iron ions of the 4c and 4d sites naturally reduce the phonon lifetime. Furthermore, orthorhombic and cubic cubanite are usually found in their natural states intimately intergrown with other sulfides such as chalcopyrite (CuFeS2) and pyrrhotite (Fe7S8). We performed comprehensive studies of Rietveld analysis of X-ray diffraction, transmission electron microscopy, and thermoelectric transport measurements of the n-type isocubanite CuFe2S3, and we obtained a maximum ZT value of 0.14 at 700 K in this environmentally friendly compound. Our electronic structure calculations support the fact that chalcopyrite CuFeS2 is an antiferromagnetic insulator, together with a higher degree of covalency of sulfide compared to oxides. The weak temperature dependence of the resistivity points towards strong disorder in the 4c and 4d sites of the isocubanite.

Coupled dielectric permittivity and magnetic susceptibility in the insulating antiferromagnet Ba2FeSbSe5

We report on coupled changes in the dielectric permittivity and the magnetic susceptibility in the insulating antiferromagnet Ba2FeSbSe5. The real part of the dielectric permittivity (e′) and the thermal conductivity (κ) shows pronounced anomalies at the Neel temperature (TN). Our findings show that there is a weak coupling between electric dipoles and magnetic spins, which is mediated by spin-lattice coupling possibly through exchange striction effects.

Suppression of superconductivity and resistivity anomaly in Rh17S15 by cobalt substitution

The chalcogenide superconductor Rh17S15 is known for having an upper critical field of nearly twice the Pauli limit and an unusual temperature dependence of the resistivity. When doped with small amounts of cobalt, superconductivity in Rh17-x Co x S15 (0  <  x  <  3) is systematically suppressed. We explore the evolution of the electrical transport properties from 2-300 K as a function of x. We identify three temperature regimes which are differently affected by doping. The disappearance of an electron-like contribution to the transport at low temperature is correlated with the suppression of superconductivity.

Substitution of indium for chromium in TlIn5−xCrxSe8: crystal structure of TlIn4.811(5)Cr0.189(5)Se8

A substitution of indium for chromium in TlIn5Se8 led to the isostructural solid solution TlIn4.8Cr0.2Se8 with only one (Wyckoff position 2a) of three In positions occupied by Cr atoms.