Franck Gascoin

Macroscopic thermoelectric inhomogeneities in (AgSbTe 2) x (PbTe) 1-x

Exceptionally high thermoelectric figure of merit (zT>2), has been reported for (Ag1−ySbTe2)0.05(PbTe)0.95, which may involve the nanoscale microstructure. However, conflicting reports on the same materials claim only zT of 1 or less. Here we show that (Ag1−ySbTe2)0.05(PbTe)0.95 materials are multiphase on the scale of millimeters despite appearing homogeneous by x-ray diffraction and routine electron microscopy. Using a scanning Seebeck microprobe, we find significant variation of Seebeck coefficient (including both n-type and p-type behavior in the same sample) that can explain the discrepancy in reported zT. More homogeneous samples can be prepared with faster cooling rates.

Measuring thermoelectric transport properties of materials

In this review we discuss considerations regarding the common techniques used for measuring thermoelectric transport properties necessary for calculating the thermoelectric figure of merit, zT. Advice for improving the data quality in Seebeck coefficient, electrical resistivity, and thermal conductivity (from flash diffusivity and heat capacity) measurements are given together with methods for identifying possible erroneous data. Measurement of the Hall coefficient and calculation of the charge carrier concentration and mobility is also included due to its importance for understanding materials. It is not intended to be a complete record or comparison of all the different techniques employed in thermoelectrics. Rather, by providing an overview of common techniques and their inherent difficulties it is an aid to new researchers or students in the field. The focus is mainly on high temperature measurements but low temperature techniques are also briefly discussed.

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.

Macroscopic thermoelectric inhomogeneities in (AgSbTe2)x(PbTe)1−x

Exceptionally high thermoelectric figure of merit (zT>2), has been reported for (Ag1−ySbTe2)0.05(PbTe)0.95, which may involve the nanoscale microstructure. However, conflicting reports on the same materials claim only zT of 1 or less. Here we show that (Ag1−ySbTe2)0.05(PbTe)0.95 materials are multiphase on the scale of millimeters despite appearing homogeneous by x-ray diffraction and routine electron microscopy. Using a scanning Seebeck microprobe, we find significant variation of Seebeck coefficient (including both n-type and p-type behavior in the same sample) that can explain the discrepancy in reported zT. More homogeneous samples can be prepared with faster cooling rates.

Fast synthesis of nanocrystalline Mg2Si by microwave heating: a new route to nano-structured thermoelectric materials

The ultra fast synthesis of nanocrystalline Mg(2)Si was carried out using microwave radiation. The elemental precursors were first milled together under dry conditions to get fine particles. The resulting mixture of powders of Mg and Si was cold pressed before being heated by microwave irradiation. Precursors and products were analyzed by X-ray diffraction and scanning electron microscopy. The high energy ball milling parameters utilized to prepare the reactive powders have quite an influence on the behavior of the mixture under irradiation. Moreover, SEM imaging demonstrates that the power and time of irradiation are crucial for the grain growth of the Mg(2)Si and must be adequately controlled in order to avoid the decomposition of the phase. Our results show that we successfully managed to easily and quickly synthesize homogeneous nanocrystalline Mg(2)Si with particle size smaller than 100 nm using a microwave power of only 175 W for two minutes on powders ball milled for two hours.

Thermoelectric bulk glasses based on the Cu–As–Te–Se system

Stable bulk glasses from the quaternary system Cu–As–Te–Se are investigated for thermoelectric applications. These materials exhibit a low thermal conductivity κ ∼ 0.3 W K−1 m−1 which is appealing for raising the thermoelectric figure of merit ZT. The addition of small amounts of selenium within the telluride amorphous matrix plays two fundamental roles. First, the increased disorder associated with the size mismatch improves glass-formation and widens the glass-formation domain, and second, it increases phonon scattering and slightly decreases the thermal conductivity. Furthermore, the addition of copper up to 32% dramatically increases the electrical conductivity without notably affecting the thermal conductivity. This permits us to obtain bulk glass samples with promising thermoelectric properties, which could be manufactured through conventional low-cost glass casting methods. While addition of copper permits the increase of electrical conductivity by more than six orders of magnitude, another three orders of magnitude are required to obtain thermoelectric materials with competitive ZT. Nevertheless, predicted values of ZT > 1.2 are estimated which would constitute some of the highest reported figure of merit for a bulk solid at room temperature. The effect of glass annealing on thermoelectric properties is also discussed.

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.

Thermoelectric Properties of Highly-Crystallized Ge-Te-Se Glasses Doped with Cu/Bi

Chalcogenide semiconducting systems are of growing interest for mid-temperature range (~500 K) thermoelectric applications. In this work, Ge20Te77Se3 glasses were intentionally crystallized by doping with Cu and Bi. These effectively-crystallized materials of composition (Ge20Te77Se3)100−xMx (M = Cu or Bi; x = 5, 10, 15), obtained by vacuum-melting and quenching techniques, were found to have multiple crystalline phases and exhibit increased electrical conductivity due to excess hole concentration. These materials also have ultra-low thermal conductivity, especially the heavily-doped (Ge20Te77Se3)100−xBix (x = 10, 15) samples, which possess lattice thermal conductivity of ~0.7 Wm−1 K−1 at 525 K due to the assumable formation of nano-precipitates rich in Bi, which are effective phonon scatterers. Owing to their high metallic behavior, Cu-doped samples did not manifest as low thermal conductivity as Bi-doped samples. The exceptionally low thermal conductivity of the Bi-doped materials did not, alone, significantly enhance the thermoelectric figure of merit, zT. The attempt to improve the thermoelectric properties by crystallizing the chalcogenide glass compositions by excess doping did not yield power factors comparable with the state of the art thermoelectric materials, as these highly electrically conductive crystallized materials could not retain the characteristic high Seebeck coefficient values of semiconducting telluride glasses.

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.

Layered tellurides: stacking faults induce low thermal conductivity in the new In2Ge2Te6 and thermoelectric properties of related compounds

A new ternary layered compound In2Ge2Te6, belonging to the hexatellurogermanate family has been synthesized from the reaction of appropriate amounts of the pure elements at high temperature in sealed silica tubes. In2Ge2Te6 crystallizes in the rhombohedral space-group R:H with lattice parameters a = 7.0863(3) A and c = 21.206(2) A and its structure is resolved using single crystal X-ray diffraction. The transport properties (Seebeck coefficient, resistivity and thermal conductivity) of compounds belonging to the family AMTe3 (A = In and Cr; M = Ge and Si) are reported. All compounds are p-type semiconductors. InSiTe3 and Cr2Si2Te6 are too resistive to be good thermoelectric materials, with maximal power factors of 10−6 and 10−5 W m−2 K−2 at 473 K, while In2Ge2Te6 and Cr2Ge2Te6 exhibit maximal values of about 10−4 and 10−3 W m−2 K−2 at 673 K, respectively. All compounds exhibit thermal conductivity below 2 W m−1 K−1, with values dropping to 0.35 W m−1 K−1 at 673 K for In2Ge2Te6. Transmission electron microscopy evidences stacking faults explaining such low thermal conductivities. The best ZT values are observed for Cr2Ge2Te6 with 0.45 at 773 K and In2Ge2Te6 with 0.18 at 673 K. Among these layered structures, a spark plasma sintered Cr2Ge2Te6 sample exhibits some thermal conductivity anisotropy but only weakly due to crystallite orientations.