Tetrahedrite Cu12Sb4S13 Research Articles

Improved Thermoelectric Performance in Ga‐ and Te‐Co‐introduced Tetrahedrite Cu12Sb4S13

As an earth‐abundant, environment‐friendly, and cost‐effective material, tetrahedrite Cu12Sb4S13 has attracted much attention in thermoelectrics (TEs). However, the TE performance of the pristine Cu12Sb4S13 (CSS) is mediocre, thereby much improvement is required. In this work, both the electronic and phonon transports of the CSS are engineered using Ga2Te3 as a dopant. In the results, it is indicated that dual substitutions of Ga for Cu and Te for Sb not only cause the enhancement of power factor via electronic band structure tuning, but also leads to a drop in both the electronic and lattice thermal conductivities. As a result, the total thermal conductivity (κ) decreases to 0.98 W m−1 K−1 at 770 K, which is about 58% that of the pristine CSS, thus improving the TE performance with the highest dimensionless figure of merit value increasing to 0.95 for (Cu12Sb4S13)0.9(Ga2Te3)0.1 at 773 K, 1.9 times that of the pristine CSS (≈0.5 at 770 K).

Modification of tetrahedrite Cu12Sb4S13 thermoelectric performance via the combined treatment of mechanochemistry and composite formation

Tetrahedrite Cu12Sb4S13 with its low thermal conductivity represents a flagship in sulphide thermoelectrics. However, to achieve a reasonable figure-of-merit ZT (measure of thermoelectric efficiency), adequate doping or special sample processing is needed. In this work, a different approach (without doping) is illustrated for the two tetrahedrite-containing systems. In the first approach binary composite tetrahedrite Cu12Sb4S13/chalcopyrite CuFeS2 was prepared by mechanochemical leaching with the aim to obtain partly decomposed tetrahedrite. In this approach, the alkaline leaching medium (Na2S + NaOH) was applied to extract Sb from tetrahedrite thus changing its composition. The obtained composite (formed from its own phases in an intrinsic mode) shows low values of ZT = 0.0022@673 K in comparison with the non-treated tetrahedrite where ZT was 0.0090@673 K. In this case the extremely high electric resistivity (6–20 mΩ cm−1) was documented. In the second approach binary composite tetrahedrite Cu12Sb4S13/muscovite KAl2(AlSi3O10)(OH)2 (formed from its own and foreign phases in an extrinsic mode) was prepared by two-step mechanical activation in which combined treatment of industrial vibratory milling and subsequent laboratory planetary milling was applied. The addition of a foreign phase, muscovite, did not give extraordinary thermoelectric performance results. However, the two-step milling process (without the addition of foreign phase) gives the value of ZT = 0.752@673 K which belongs to the highest in the tetrahedrite thermoelectric community. In this case, the two-times increase in specific surface area and the increased amount of tetrahedrite in comparison to famatinite are suspectable for this effect. Both applied non-traditional approaches to synthesize tetrahedrite composites form a platform for potential modification of its thermoelectric performance.

Tetrahedrite copper antimony sulfide (Cu12Sb4S13) thin films for photovoltaic and self-powered photodetector applications grown by single step ultrasonic spray pyrolysis

This work reports in situ growth of tetrahedrite Cu12Sb4S13 thin films via ultrasonic spray pyrolysis using a non-aqueous solution containing CuCl2, SbCl3 and C2H5NS. The thin films deposited by varying S/(Cu + Sb) ratios in the precursor solution were studied using X-ray diffraction and Raman spectroscopy to evaluate the phase formation. Optical characterization showed a direct band gap of nearly 1.5 eV, consistent with the theoretical simulation of electronic band structure and optical spectra based on First principles calculations. Cu12Sb4S13 thin films formed as a function of S/(Cu + Sb) precursor ratio spans semiconducting (p) to semi-metallic (p+) behavior, confirmed by Hall effect. Photodetection properties of the p type Cu12Sb4S13 films showed a spectral response ranging from UV to visible and NIR wavelengths. Both p and p+ thin films were incorporated into photovoltaic structures using CdS as window layer, resulting: Voc = 0.273–0.398 V, Jsc = 2.5–16.3 mAcm−2, FF = 32–35% and efficiency = 0.33–1.4%. These heterostructures also performed as self-powered photodiodes for UV–Vis-NIR wavelength detection. This is the first report of a single-step synthesis of phase pure Cu12Sb4S13 thin films and their performance as self-biased photodiode and photovoltaic devices.

Thermoelectric Performance of Tetrahedrite (Cu12Sb4S13) Thin Films: The Influence of the Substrate and Interlayer.

In the present work, tetrahedrite Cu12Sb4S13 thin films were deposited on various substrates via aerosol-assisted chemical vapor deposition (AACVD) using diethyldithiocarbamate complexes as precursors. A buffer layer of Sb2O3 with a small lattice mismatch to Cu12Sb4S13 was applied to one of the glass substrates to improve the quality of the deposited thin film. The buffer layer increased the coverage of the Cu12Sb4S13 thin film, resulting in improved electrical transport properties. The growth of the Cu12Sb4S13 thin films on the other substrates, including ITO-coated glass, a SiO2-coated Si wafer, and mica, was also investigated. Compared to the films grown on the other substrates, the Cu12Sb4S13 thin film deposited on the SiO2-coated Si wafer showed a dense and compact microstructure and a larger grain size (qualities that are beneficial for carrier transport), yielding a champion power factor (PF) of ∼362 μW cm-1 K-2 at 625 K. The choice of substrate strongly influenced the composition, microstructure, and electrical transport properties of the deposited Cu12Sb4S13 thin film. At 460 K, the highest zT value that was obtained for the thin films was ∼0.18. This is comparable to values reported for Cu-Sb-S bulk materials at the same temperature. Cu12Sb4S13 thin films deposited using AACVD are promising for thermoelectric applications. To the best of our knowledge, the first full thermoelectric characterization of the Cu12Sb4S13 thin film is performed in this work.

Molecular precursor-mediated facile synthesis of photo-responsive stibnite Sb2S3 nanorods and tetrahedrite Cu12Sb4S13 nanocrystals.

Stibnite Sb2S3 and tetrahedrite Cu12Sb4S13 nanostructures being economical, environmentally benign and having a high absorption coefficient are highly promising materials for energy conversion applications. However, producing these materials especially tetrahedrite in the phase pure form is a challenging task. In this report we present a structurally characterized single source molecular precursor [Sb(4,6-Me2pymS)3] for the facile synthesis of binary Sb2S3 as well as ternary Cu12Sb4S13 in oleylamine (OAm) at a relatively lower temperature. The as-prepared Sb2S3 and Cu12Sb4S13 nanostructures were thoroughly checked for their phase purity, elemental composition and morphology by powder X-ray diffraction (pXRD), electron dispersive spectroscopy (EDS) and electron microscopy techniques. pXRD and EDS studies confirm the formation of phase pure, crystalline orthorhombic Sb2S3 and cubic Cu12Sb4S13. The SEM, TEM and HRTEM images depict the formation of well-defined nanorods and nearly spherical nanocrystals for Sb2S3 and Cu12Sb4S13, respectively. The Sb2S3 nanorods and Cu12Sb4S13 nanocrystals exhibit an optical bandgap of ∼1.88 and 2.07 eV, respectively, which are slightly blue-shifted relative to their bulk bandgap, indicating the quantum confinement effect. Finally, efficient photoresponsivity and good photo-stability were achieved in the as-prepared Sb2S3 and Cu12Sb4S13 nanostructure-based prototype photo-electrochemical cell, which make them promising candidates for alternative low-cost photon absorber materials.

Achieving high average power factor in tetrahedrite Cu12Sb4S13 via regulating electron-phonon coupling strength

Lattice anharmonicity usually leads to low lattice thermal conductivity and thus is favored by thermoelectrics. However, if the anharmonicity is driven by strong lattice distortion, it is possible that the strong coupling between electrons and phonons would result in low carrier mobility. Yet, the fundamental physics of the competition between anharmonicity and mobility is not fully understood. In this study, taking thermoelectric tetrahedrite Cu12Sb4S13 as example, we try to elucidate such mechanism experimentally by analyzing electrical transport and local chemical bonding environment. It is found that substituting Sb with Sn can effectively suppress the local vibrational amplitude of Cu atoms, which is the origin of anharmonicity. As a result, electron-phonon coupling strength is regulated as well as the electrical conductivity and Seebeck coefficient are spontaneously promoted, leading to a substantially enhanced average power factor by more than 60%. Combined with the suppressed of electron-phonon coupling strength and exsolution process, a maximum zT of 1.1 at 723 K is achieved. These findings shed some light on the relationship between anharmonicity and carrier mobility, further emphasizing the importance of considering electron-phonon coupling for designing novel thermoelectric materials.

Substitution Site Selection and Thermoelectric Performance‐Enhancing Mechanism of Cu12Sb4S13 Doped with Pb/Ge/Sn

Tetrahedrite Cu12Sb4S13 has attracted much attention as an earth‐abundant and ecofriendly thermoelectric material with intrinsic low thermal conductivity. Herein, the effects of carbon group elements Pb, Ge, and Sn on the electronic structure and thermoelectric properties of Cu12Sb4S13 are extensively investigated based on first‐principles electronic structure calculations and Boltzmann transport theories. It is found that the most energetically favorable sites for the elements Pb and Ge are the Cu(1) sites, while for Sn atoms they are the Cu(2) sites. And the engineered electronic density of states by Pb/Ge/Sn doping results in a sharp increase of the Seebeck coefficient and a great decrease in carrier thermal conductivity of the host. In addition, the Cu12Sb4S13 compounds substituted with Pb/Ge at Cu(2) sites and Sn at Cu(1) sites have a preferable optimizing thermoelectric performance in a whole. Especially, the optimizing ZT of the host could get an about 15‐time improvement at T = 470 K upon Sn substituting at Cu(1) sites.

Thermoelectric Cu12 Sb4 S13 -Based Synthetic Minerals with a Sublimation-Derived Porous Network.

Pores in a solid can effectively reduce thermal conduction, but they are not favored in thermoelectric materials due to simultaneous deterioration of electrical conductivity. Conceivably, creating a porous structure may endow thermoelectric performance enhancement provided that overwhelming reduction of electrical conductivity can be suppressed. This work demonstrates such an example, in which a porous structure is formed leading to a significant enhancement in the thermoelectric figure of merit (zT). By a unique BiI3 sublimation technique, pore networks can be introduced into tetrahedrite Cu12 Sb4 S13 -based materials, accompanied by changes in their hierarchical structures. The addition of a small quantity of BiI3 (0.7 vol%) results in a ≈72% reduction in the lattice thermal conductivity, whereas the electrical conductivity is improved due to unexpected enhanced carrier mobility. As a result, an enhanced zT of 1.15 at 723 K in porous tetrahedrite and a high conversion efficiency of 6% at ΔT= 419 K in a fabricated segmented single-leg based on this porous material are achieved. This work offers an effective way to concurrently modulate the electrical and thermal properties during the synthesis of high-performance porous thermoelectric materials.

Bismuth Doping in Nanostructured Tetrahedrite: Scalable Synthesis and Thermoelectric Performance.

In this study, we demonstrate the feasibility of Bi-doped tetrahedrite Cu12Sb4−xBixS13 (x = 0.02–0.20) synthesis in an industrial eccentric vibratory mill using Cu, Sb, Bi and S elemental precursors. High-energy milling was followed by spark plasma sintering. In all the samples, the prevailing content of tetrahedrite Cu12Sb4S13 (71–87%) and famatinite Cu3SbS4 (13–21%), together with small amounts of skinnerite Cu3SbS3, have been detected. The occurrence of the individual Cu-Sb-S phases and oxidation states of bismuth identified as Bi0 and Bi3+ are correlated. The most prominent effect of the simultaneous milling and doping on the thermoelectric properties is a decrease in the total thermal conductivity (κ) with increasing Bi content, in relation with the increasing amount of famatinite and skinnerite contents. The lowest value of κ was achieved for x = 0.2 (1.1 W m−1 K−1 at 675 K). However, this sample also manifests the lowest electrical conductivity σ, combined with relatively unchanged values for the Seebeck coefficient (S) compared with the un-doped sample. Overall, the lowered electrical performances outweigh the benefits from the decrease in thermal conductivity and the resulting figure-of-merit values illustrate a degradation effect of Bi doping on the thermoelectric properties of tetrahedrite in these synthesis conditions.

Effects of Se Doping on Thermoelectric Properties of Tetrahedrite Cu12Sb4S13−zSez

Tetrahedrite Cu12Sb4S13 is composed of abundant non-toxic components and has attracted attention as a promising thermoelectric material with low thermal conductivity at intermediate temperatures. The carrier concentration can be optimized by doping (substituting), thereby maximizing its power factor and reducing its thermal conductivity. In this study, Cu12Sb4S13−zSez (z = 0.1–0.4) compounds were synthesized using mechanical alloying and hot pressing. Our objective was to maintain a high power factor through Se doping and to reduce the lattice thermal conductivity through additional phonon scattering. X-ray diffraction analysis revealed that the lattice constant increased with an increase in Se substitution for the S sites, and all the specimens appeared as a single tetrahedrite phase. As the Se doping level increased, the carrier (hole) concentration decreased while the mobility increased. The Hall and Seebeck coefficients were both positive, indicating that Se-doped tetrahedrites exhibit p-type conduction. As the Se substitution increased, the electrical conductivity decreased, but the Seebeck coefficient increased. In addition, Se doping lowered both, the electronic and lattice thermal conductivities, which resulted in decreased thermal conductivity. A maximum dimensionless figure of merit (ZT) of 0.87 was obtained at 723 K for Cu12Sb4S12.8Se0.2 with a high power factor of 0.96 mW m−1 K−2 and a low thermal conductivity of 0.77 W m−1 K−1.

Thermoelectric Cu–S-Based Materials Synthesized via a Scalable Mechanochemical Process

In this work, Cu-based sulfides (chalcopyrite CuFeS₂, mohite Cu₂SnS₃, tetrahedrite Cu₁₂Sb₄S₁₃, mawsonite Cu₆Fe₂SnS₈, and kesterite Cu₂ZnSnS₄) were synthesized by industrial milling in an eccentric vibratory mill to demonstrate the scalability of their synthesis. For a comparison, laboratory-scale milling in a planetary mill was performed. The properties of the obtained samples were characterized by X-ray diffraction and, in some cases, also by Mossbauer spectroscopy. For the densification of powders, the method of spark plasma sintering was applied to prepare suitable samples for thermoelectric (TE) characterization which created the core of this paper. A comparison of the figure-of-merit ZT, representative of the efficiency of thermoelectric performance, shows that the scaling process of mechanochemical synthesis leads to similar values as compared to using laboratory methods. This makes the cost-effective production of Cu-based sulfides as prospective energy materials for converting heat to electricity feasible. Several new concepts that have been developed involving combinations of natural and synthetic species (tetrahedrite) and nanocomposite formation (tetrahedrite/digenite, mawsonite/stannite) offer sustainable approaches in solid-state chemistry. Mechanochemical synthesis is selected as a simple, one-pot, and facile solid-state synthesis of thermoelectric materials with the capability to reduce, or even eliminate, solvents, toxic gases, and high temperatures with controllable enhanced yields. The synthesis is environmentally friendly and essentially waste-free. The obtained results illustrate the possibility of large-scale deployment of energy-related materials.

The influence of self-doping of stibnite ore with impurities on the preparation, heat capacity, magnetic and transport properties of tetrahedrite Cu12Sb4S13

Abstract Stibnite mineral (mainly Sb2S3) has been employed for the synthesis of tetrahedrite Cu12Sb4S13 bulk material by spark plasma sintering. High purity Cu12Sb4S13 can be quickly obtained by two sintering procedures at temperatures from the range of 420 °C to 440 °C for 1 h. Appropriate reduction of Cu content (Cu12+xSb4S13, x ⩽ –0.05) or CuS content (Cu12−ySb4S13−y, y = 0.1 or 0.3) was beneficial to fabricate Cu12Sb4S13. The secondary resintering improved the purity of Cu12Sb4S13 material. The first-order magnetic phase transformation with magnetic hysteresis effect was confirmed by the behavior of susceptibility, heat capacity and resistivity. The magnetization showed a linear increase with increasing field (up to 7 T) and non-saturation behavior was observed. The impurities in stibnite mineral Sb2S3 had a weak influence on the transformation temperature but affected the low-temperature magnetization value (~0.15, close to natural tetrahedrite). Similar transformation was observed by the analysis of heat capacity. The properties such as electrical resistivity, Seebeck coefficient and thermal conductivity were also measured for Cu11:9Sb4S13 and Cu11:9Sb4S12:9. The maximum figure of merit ZT of Cu11:9Sb4S12:9 was 0.22 at 367 K.

Effects of Sb Deviation from Its Stoichiometric Ratio on the Micro- and Electronic Structures and Thermoelectric Properties of Cu12Sb4S13.

Thermoelectric material tetrahedrite Cu12Sb4S13 has attracted much attention because of its intrinsic low lattice thermal conductivity, excellent electrical transport property, and environment-friendly constituents. However, its thermoelectric figure merit, ZT, is limited because of the low Seebeck coefficient (S) and power factor (PF). Hence, it is indispensable to enhance its S and PF to increase its ZT. Here, we show that when Sb deviation from its stoichiometric ratio in the Cu12Sb4S13 band structure is modulated, it gives rise to increased density of states and enhancement of the Seebeck coefficient. Moreover, carrier concentration is tuned by changing sulfur and copper vacancies through controlling the Cu3SbS4 phase with an atomic ratio of Sb, leading to increased electrical conductivity. In addition, as large as ∼60% reduction of lattice thermal conductivity is obtained by intensified phonon scattering using an impurity phase/element and vacancy-like defects induced by different Sb contents. As a result, a high ZT = 0.86 is achieved at 723 K for the Cu12Sb4+δS13 sample with δ = 0.2, which is ∼50% larger than that of stoichiometric Cu12Sb4S13 studied here, indicating that ZT of Cu12Sb4S13 can be improved through simple modulation of the Sb stoichiometric ratio.

Tetrahedrites synthesized via scalable mechanochemical process and spark plasma sintering

In this study, we demonstrate the use of elemental precursors (Cu,Sb,S) to synthesize tetrahedrite Cu12Sb4S13 using an industrial eccentric vibratory mill. Mechanochemical synthesis of tetrahedrite leads to the formation of covellite (CuS), skinnerite (Cu3SbS3) or famatinite (Cu3SbS4) in dependence on milling time. However, the composite product can be modified in favour of prevailing tetrahedrite when Spark Plasma Sintering (SPS) treatment is applied after milling. The as-synthesized and sintered products are composed of polydisperse nanosized particles with dimensions up to 250 nm. The thermoelectric measurements reveal a maximum value of figure-of-merit ZT = 0.67 @ 700 K, as a consequence of a relatively high power factor (1.07 mW m−1 K−2) and a low thermal conductivity (1.12 W m−1 K−1). The obtained zT values of products prepared in an industrial mill are comparable to the ones reported for compounds synthesized using laboratory mills. The synthesis of ternary and quaternary sulphides by a scalable and industrializable milling process represents a prospective route for mass production of thermoelectric materials.

Achieving high power factor and thermoelectric performance through dual substitution of Zn and Se in tetrahedrites Cu12Sb4S13

As an environmentally friendly thermoelectric material with its constituents being free of Pb/Te, tetrahedrite Cu12Sb4S13 absorbs much research interest. However, its low thermoelectric performance inhibits its applications. Here, we show that through dual substitution of Se for S and Zn for Cu in the compound, both the electrical conductivity and the thermopower are enhanced, leading to the elevation of the power factor as high as ∼33% (at 723 K). Analyses indicate that the substitution of Se for S gives rise to changes in stoichiometry of Cu12Sb4S13 through precipitation of impurity phase Cu3SbS4, which causes variations of S vacancies and hole concentrations, while Zn2+ substitution for Cu1+ introduces donors, both of which tune and optimize the carrier concentration. Besides, the lattice thermal conductivity of dual substituted samples is reduced by as low as ∼30% (at 723 K) due to intensified phonon scattering of the impurities (Se and Zn). As a result, a large figure of merit ZT = 0.9 (at 723 K) is achieved in Cu12−yZnySb4S12.8Se0.2 samples with y = 0.025 and 0.05, which is ∼41% higher than that of pristine tetrahedrite Cu12Sb4S13, indicating that dual substitution is an effective approach to improving its thermoelectric performance.

Thermal Stability, Mechanical Properties and Thermoelectric Performance of Cu11TrSb4S13 (Tr = Mn, Fe, Co, Ni, Cu, and Zn)

Tetrahedrites substituted with transition elements, Cu11TrSb4S13 (Tr = Mn, Fe, Co, Ni, Cu, and Zn), were synthesized by mechanically alloying and hot pressing, and their thermal stability, mechanical properties and thermoelectric performance, including phase transition (decomposition), elemental redistributions, microstructures, thermoelectric parameters, hardness, and bending strength, were examined. Hot-pressed compacts showed relative densities of 97.4–99.8%. As the atomic number of the transition element substituted for 29Cu decreased (28Ni, 27Co, 26Fe, and 25Mn), the lattice constant increased; however, the lattice constant also increased when 29Cu was substituted with 30Zn (higher atomic number). The electrical conductivity of tetrahedrites doped with transition elements decreased compared with that of intrinsic tetrahedrite Cu12Sb4S13. This was because transition elements were successfully substituted at Cu+ sites, and the carrier (hole) concentration decreased owing to electron donation. The Seebeck coefficient of Cu11TrSb4S13 was greater than that of Cu12Sb4S13, except for Cu11FeSb4S13. However, the thermal conductivity of the tetrahedrite decreased upon the substitution of transition elements owing to enhanced impurity phonon scattering. Endothermic reactions were observed at temperatures between 882 K and 984 K, which corresponded to each melting point, and the tetrahedrite melting point increased upon doping with transition elements. The Vickers hardness and three-point bending strength of Cu12Sb4S13 were 2.2 GPa and 23 MPa, respectively. However, the hardness (2.5–2.7 GPa) and bending strength (23–44 MPa) increased for Cu11TrSb4S13 as the result of the solid-solution hardening effect.

First-Order Metal–Semiconductor Transition Triggered by Rattling Transition in Tetrahedrite Cu12Sb4S13: Cu-Nuclear Magnetic Resonance Studies

Tetrahedrite Cu12Sb4S13 shows a metal–semiconductor transition (MST) at TMST = 85 K. We have studied the mechanism of the MST by measurements of Cu-NMR. Above TMST, the Cu-NMR spectrum consists of signals from a tetrahedral Cu(1) 12d site and a trigonally coordinated Cu(2) 12e site. Analyses of the spectra at 95 and 20 K yield NMR and nuclear quadrupole resonance (NQR) parameters above and below TMST. The Cu(1) signal does not show clear quadrupole splitting, which remains unchanged on cooling across TMST. On the other hand, the Cu(2) signal at T > TMST clearly shows the quadrupole splitting characterized by a quadrupole frequency νQ(2) ≈ 18.6 MHz and an asymmetry parameter η(2) ≈ 0.03. Below TMST, the values of νQ(2) and η(2) change markedly. Such marked changes in NQR parameters provide evidence of a remarkable change in the local electronic structure around the Cu(2) site. Together with νQ from NMR spectra and that from first-principles calculation, we conclude that the Cu(2) atoms are displaced from the sulfur triangles below TMST. We also found that the NMR Knight shift and the spin–lattice relaxation rate divided by temperature, 1/T1T, for the Cu(1) signal markedly change below TMST, which is ascribable to the reduction in the electronic density of states at the Fermi level. We further discuss the relationship among the MST, structural transformation, and crystal structure below TMST.

Effects of Aging on Thermoelectric Properties of Tetrahedrite Cu12Sb4S13

Tetrahedrite Cu12Sb4S13 was prepared by mechanical alloying and hot pressing. The phase transition, microstructure, and thermoelectric properties (electronic conductivity, Seebeck coefficient, power factor, thermal conductivity, and dimensionless figure of merit) were examined under various aging conditions (atmosphere, temperature, and time). When aged at temperatures above 723 K in air, various oxides (SbO2, Sb2O3, and Sb6O7(SO4)2) and sulfides (Cu9S8, Cu2S, Cu1.96S, and CuSbS2) were formed through oxidation, volatilization, and decomposition of the tetrahedrite phase. However, the tetrahedrite phase was stable up to 723 K in vacuum. The pristine specimen exhibited a dimensionless figure of merit of 0.86 at 723 K, resulting from a power factor of 0.95 mW·m−1K−2 and a thermal conductivity of 0.78 W·m−1K−1 at 723 K. For the specimens aged at 523–723 K for 10 h in air, the dimensionless figure of merit varied from 0.81 to 0.92 as the power factor ranged from 0.97 to 1.04 mW·m−1K−2 and the thermal conductivity ranged from 0.80 to 0.85 W·m−1K−1. However, the thermoelectric performance was slightly degraded when the specimen was aged at 723 K for 100 h because of a decreased power factor and increased thermal conductivity.

Cu 2p-1s x-ray emission spectroscopy of mineral tetrahedrite Cu12Sb4S13

The electronic structure of mineral tetrahedrite Cu12Sb4S13 showing a metal-semiconductor transition (MST) at TMST = 85 K has been investigated by means of Cu 1s x-ray absorption spectroscopy with the partial fluorescence yield mode (PFY-XAS) and Cu 2p-1s x-ray emission spectroscopy (XES). Remarkable changes are detected in both the PFY-XAS and XES spectra on cooling across TMST. The changes in the spectra indicate that the unoccupied Cu 3d and 4p densities of states (DOS) shift away from the Fermi energy (EF). We conclude that the MST of Cu12Sb4S13 is associated with the decrease of the Cu 3d DOS at EF.

Thermal Stability and Mechanical Properties of Thermoelectric Tetrahedrite Cu12Sb4S13

Tetrahedrite Cu12Sb4S13 was prepared by mechanical alloying and hot pressing, and its thermal stability and mechanical properties were examined. The phase transformation (decomposition), chemical composition, elemental redistribution, microstructure, hardness, and three-point bending strength were studied under various aging conditions (atmospheric, temperature, and time). Endothermic peaks were observed at temperatures from 845 K to 892 K and were found to be related to tetrahedrite decompositions. The Vickers hardness of the pristine specimen was 2.2 GPa on average, and did not significantly change with the aging conditions. The bending strength of the pristine specimen was 26.7 MPa on average, and it remarkably decreased to 6.2 MPa after aging at 723 K for 100 h in air. However, it was 21.4 MPa after aging at 723 K for 100 h in vacuum.