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The thermal process parameters are crucial in metal-sulfides ceramics as they affect significantly the resulting physico-chemical properties. In the present work, we investigated the sintering effect in the kesterite Cu 2.125 Zn 0.875 SnS 4 on its structural, microstructural, and thermoelectric (TE) properties to highlight the non-negligible contribution of the thermal process often ignored in metal-sulfide ceramics. For this purpose, we developed an approach combining data science with the conventional material experiment/theory approach which can be used as a tool to shortcut the time-consuming steps of TE material optimization. We confirmed that the optimization and control of the densification process is critical in unravelling the highest potential on metal sulfide TE ceramics with a non-negligible increase of its z T up to 60 %. We propose a scientific tool, the synergic combination of active machine learning with conventional chemistry/theory approaches, to either identify the most proficient sintering process as well as the process to avoid the degradation of the metal-sulfide ceramic properties and thus in a shorten number of experiments. This approach can be extended not only to other metal-sulfide ceramics for thermoelectricity but also to other research fields. [Display omitted] [ABSTRACT FROM AUTHOR]

Alongiside the growing demand for wearable and implantable electronics, the development of flexible thermoelectric (FTE) materials holds great promise and has recently become a highly necessitated and efficient method for converting heat to electricity. Conductive polymers were widely used in previous research; however, n-type polymers suffer from instability compared to the p-type polymers, which results in a deficiency in the n-type TE leg for FTE devices. The development of the n-type FTE is still at a relatively early stage with limited applicable materials, insufficient conversion efficiency, and issues such as an undesirably high cost or toxic element consumption. In this work, as a prototype, a flexible n-type rare-earth free skutterudite (CoSb3)/poly(3,4-ethylenedioxythiophene)-polystyrene sulfonate (PEDOT:PSS) binary thermoelectric film was fabricated based on ball-milled skutterudite via a facile top-down method, which is promising to be widely applicable to the hybridization of conventional bulk TE materials. The polymers bridge the separated thermoelectric particles and provide a conducting pathway for carriers, leading to an enhancement in electrical conductivity and a competitive Seebeck coefficient. The current work proposes a rational design towards FTE devices and provides a perspective for the exploration of conventional thermoelectric materials for wearable electronics. [ABSTRACT FROM AUTHOR]

Abstract A complete thiospinel solid solution CuCo x Ti 2- x S 4 (0 ≤ x < 1) has been synthesized by conventional sealed tube synthesis followed by Spark Plasma Sintering, whereas, for x = 1, the presence of secondary phases is observed. The compounds exhibit a semi-metallic behavior in the whole composition range in spite of the existence of two subdomains characterized by different major redox species: (I) involving Cu+, Ti3+, Ti4+, Co2+ for 0 ≤ x ≤ 0.5 and (II) involving Cu+, Cu2+, Ti4+, Co2+for 0.5 < x ≤ 1. The thermoelectric properties show that, in the compositional range x = 0.3–0.8, the figure of merit does not depend significantly on x , ZT ranging from 0.13 to 0.18 at 675 K, whereas for the limit x = 0, it drops down to 0.04. This behavior is interpreted in terms of the predominant influence of the redox couple Ti3+/Ti4+ on the transport properties, as long as Cu+ is present, according to the equilibrium Cu+ + Ti4+⇌ Cu2+ + Ti3+. The thiospinel CuCo 0.6 Ti 1.4 S 4 , which exhibits the higher ZT of 0.18 at 675 K, opens the route to the research of new "Ti4+-Cu+" based thermoelectric sulfides with different structures. Highlights • Thiospinel CuCo x Ti 2- x S 4 (0 ≤ x ≤ 1) synthesized by sealed tube synthesis and SPS. • Metallic behavior from x = 0 to x = 0.8 • The couple Ti3+/Ti4+ dominates the n -type transport properties. • Small amounts of cobalt in the titanium sites decrease the thermal conductivity. [ABSTRACT FROM AUTHOR]

Ternary copper-containing sulfides Cu 12 Sb 4 S 13 and Cu 4 Sn 7 S 16 have attracted considerable interest since few years due to their high-efficiency conversion as absorbers for solar energy and promising thermoelectric materials. We report therein on the decomposition study of Cu 12 Sb 4 S 13 and Cu 4 Sn 7 S 16 phases using high temperature in situ neutron powder diffraction. Our results obtained at a heating rate of 2.5 K/min indicate that: (i) Cu 12 Sb 4 S 13 decomposes above ≈792 K into Cu 3 SbS 3 , and (ii) Cu 4 Sn 7 S 16 decomposes above ≈891 K into Sn 2 S 3 and a copper-rich sulfide phase of sphalerite ZnS-type structure with an assumed Cu 3 SnS 4 stoichiometry. Both phase decompositions are associated to a sulfur volatilization. While the results on Cu 12 Sb 4 S 13 are in fair agreement with recent published data, the decomposition behavior of Cu 4 Sn 7 S 16 differs from other studies in terms of decomposition temperature, thermal stability and products of reaction. Finally, the crystal structure refinements from neutron powder diffraction data are reported and discussed for the Cu 4 Sn 7 S 16 and tetrahedrite Cu 12 Sb 4 S 13 phases at 300 K, and for the high temperature form of skinnerite Cu 3 SbS 3 at 843 K. [ABSTRACT FROM AUTHOR]

Bulk polycrystalline samples in the series Ti 1− x Nb x S 2 (0 ≤ x ≤ 0.075) were prepared using mechanical alloying synthesis and spark plasma sintering. X-ray diffraction analysis coupled with high resolution transmission electron microscopy indicates the formation of trigonal TiS 2 by high energy ball-milling. The as-synthesized particles consist of pseudo-ordered TiS 2 domains of around 20–50 nm, joined by bent atomic planes. This bottom-up approach leads, after spark plasma sintering, to homogeneous solid solutions, with a niobium solubility limit of x = 0.075. Microstructural observations evidence the formation of small crystallites in the bulk compounds with a high density of stacking faults. The large grain boundary concentration coupled with the presence of planar defects, leads to a substantial decrease in the thermal conductivity to 1.8 W/mK at 700 K. This enables the figure of merit to reach ZT = 0.3 at 700 K for x = 0.05, despite the lower electron mobility in mechanically alloyed samples due to small crystallite/grain size and structural defects. [ABSTRACT FROM AUTHOR]

This paper describes Cu 4 Sn 7 S 16 material as a potential thermoelectric comprising low cost and non-toxic elements, whose complex crystal structure permits intrinsic low thermal conductivity. We begin by identifying two different synthesis approaches to obtain pure and fully dense samples, using conventional sealed tube and mechanical alloying methods combined with Spark Plasma Sintering. We then confirm the complex rhombohedral structure (R-3m, a ∼ 7.37 Å, c ∼ 36.02 Å) using both Rietveld refinement of powder X-ray diffraction patterns and High Resolution Transmission Electron Microscopy (HRTEM) analyses. Finally, we characterize the electrical and thermal properties at high temperatures on highly dense samples. The electrical behavior is also shown for the first time to change from semiconducting to metallic behavior according to sample preparation, suggesting that the material is highly sensitive to cationic site occupancy and Cu:Sn:S ratio. The complex crystallographic structure of Cu 4 Sn 7 S 16 causes low thermal conductivity, which is found below 1.1 W/mK above room temperature. The focus of this paper is to drawn attention to the way in which complex structures specifically designed to induce intrinsic low thermal conductivity can be potentially attractive thermoelectrics, even in materials that are intrinsically poor conductors. [ABSTRACT FROM AUTHOR]

As the alternative for the high-performance thermoelectric semiconductors such as GeTe, PbTe, and other compounds with rare and expensive elements, the cost-effective and abundant sulfides have been considered as one of the promising materials and garnered intensive interest. In this study, a rarely reported metastable wurtzite-type phase of CuFeS 2 has been successfully obtained as thin film grown on quartz substrates via magnetron sputtering. Subsequent temperature-dependent depositions were performed to establish a phase transition between the metastable wurtzite and the chalcopyrite phase in the temperature range between room temperature and 473 K while maintaining the nominal composition ratio of CuFeS 2. The wurtzite metastable analogue can be recovered and stabilized when the temperature reached 573 K and above. Thermoelectric properties were evaluated and constituted the first experimental result of the inorganic CuFeS 2 thin film for thermoelectrics. The phase change between the wurtzite and chalcopyrite structure induced a significant effect on the thermoelectric properties associated with the lattice disorder and carrier scattering. The reduced thermal conductivity compared with bulk was demonstrated by the picosecond time-domain thermoreflectance method, which was found to be well correlated with the polycrystal sizes. The work provides new insight into synthesizing and manipulating the metastable wurtzite phase of the ternary I-III-VI for future applications in thermoelectrics, solar cells, and other electronic applications. [Display omitted] [ABSTRACT FROM AUTHOR]

In this work, a series of Bi2Te3/X mol% MoS2 (X = 0, 25, 50, 75) bulk nanocomposites were prepared by hydrothermal reaction followed by reactive spark plasma sintering (SPS). X-ray diffraction analysis (XRD) indicates that the native nanopowders, comprising of Bi2Te3/MoS2 heterostructure, are highly reactive during the electric field-assisted sintering by SPS. The nano-sized MoS2 particles react with the Bi2Te3 plates matrix forming a mixed-anion compound, Bi2Te2S, at the interface between the nanoplates. The transport properties characterizations revealed a significant influence of the nanocomposite structure formation on the native electrical conductivity, Seebeck coefficient, and thermal conductivity of the initial Bi2Te3 matrix. As a result, enhanced ZT values have been obtained in Bi2Te3/25 mol% MoS2 over the temperature range of 300–475 K induced mainly by a significant increase in the electrical conductivity. [ABSTRACT FROM AUTHOR]

An in-depth analysis of Cu 2+ x Co 1- x GeS 4 (CCGS) has been performed to illustrate its potential as a new p -type compound for thermoelectric (TE) applications with suitable thermal stability. An intrinsic semiconductor behavior is reported with attractive TE properties, characterized by a large Seebeck coefficient and intrinsic low thermal conductivity. Band structures and elastic properties calculations sustained the experimental observations and revealed the dominant role of Cu/Co ratio in electrical transport properties. A promising TE performance is achieved through a carrier concentration tuning by Cu/Co ratio control, with a zT = 0.37 at 725 K resulting from combined large PF enhancement and a reduced κ. • We performed the band structure and elastic properties calculations of Cu2CoGeS4 compound. • We synthesized the Cu2+ x Co1-xGeS4 (0 = x ≤ 0.225) bulk compounds series by sealed tube reaction and Spark Plasma Sintering. • We studied the structural and thermal stability of the compounds above 775 K. • The thermoelectric properties were reported and optimized up to zT = 0.37 at 725 K. • The effect of the Cu/Co ratio in the transport properties was highlighted. [ABSTRACT FROM AUTHOR]