Thermoelectric and Transport Properties of Permingeatite Cu3SbSe4 Prepared Using Mechanical Alloying and Hot Pressing

Permingeatite (Cu₃SbSe₄) is a promising thermoelectric material because it has a narrow band gap, large carrier effective mass, and abundant and nontoxic components. Mechanical alloying (MA), which is a high-energy ball mill process, has various advantages, e.g., segregation/evaporation is not required and homogeneous powders can be prepared in a short time. In this study, the effects of MA and hot-pressing (HP) conditions on the synthesis of the Cu₃SbSe₄ phase and its thermoelectric properties were evaluated. The electrical conductivity decreased with increasing HP temperature, while the Seebeck coefficient increased. The power factor (PF) was 0.38–0.50 mW m⁻¹ K⁻² and the thermal conductivity was 0.76–0.78 W m⁻¹ K⁻¹ at 623 K. The dimensionless figure of merit, ZT, increased with increasing temperature, and a reliable and maximum ZT value of 0.39 was obtained at 623 K for Cu₃SbSe₄ prepared using MA at 350 rpm for 12 h and HP at 573 K for 2 h.     

Enhancing Thermoelectric Properties of (Cu2Te)1−x-(BiCuTeO)x Composites by Optimizing Carrier Concentration

Because of the high carrier concentration, copper telluride (Cu₂Te) has a relatively low Seebeck coefficient and high thermal conductivity, which are not good for its thermoelectric performance. To simultaneously optimize carrier concentration, lower thermal conductivity and improve the stability, BiCuTeO, an oxygen containing compound with lower carrier concentration, is in situ formed in Cu₂Te by a method of combining self-propagating high-temperature synthesis (SHS) with spark plasma sintering (SPS). With the incorporation of BiCuTeO, the carrier concentration decreased from 8.1 × 10²⁰ to 3.8 × 10²⁰ cm⁻³, bringing the increase of power factor from ~1.91 to ~2.97 μW cm⁻¹ K⁻² at normal temperature. At the same time, thermal conductivity reduced from 2.61 to 1.48 W m⁻¹ K⁻¹ at 623 K. Consequently, (Cu₂Te)_0.95-(BiCuTeO)_0.05 composite sample reached a relatively high ZT value of 0.13 at 723 K, which is 41% higher than that of Cu₂Te.     

Phase Analysis and Thermoelectric Properties of Cu-Rich Tetrahedrite Prepared by Solvothermal Synthesis

Because of the large Seebeck coefficient, low thermal conductivity, and earth-abundant nature of components, tetrahedrites are promising thermoelectric materials. DFT calculations reveal that the additional copper atoms in Cu-rich Cu₁₄Sb₄S₁₃ tetrahedrite can effectively engineer the chemical potential towards high thermoelectric performance. Here, the Cu-rich tetrahedrite phase was prepared using a novel approach, which is based on the solvothermal method and piperazine serving both as solvent and reagent. As only pure elements were used for the synthesis, the offered method allows us to avoid the typically observed inorganic salt contaminations in products. Prepared in such a way, Cu₁₄Sb₄S₁₃ tetrahedrite materials possess a very high Seebeck coefficient (above 400 μVK⁻¹) and low thermal conductivity (below 0.3 Wm⁻¹K⁻¹), yielding to an excellent dimensionless thermoelectric figure of merit ZT ≈ 0.65 at 723 K. The further enhancement of the thermoelectric performance is expected after attuning the carrier concentration to the optimal value for achieving the highest possible power factor in this system.     

Thermoelectric Properties of Cu2Se Synthesized by Hydrothermal Method and Densified by SPS Technique

The aim of the work was to obtain copper (I) selenide Cu₂Se material with excellent thermoelectric properties, synthesized using the hydrothermal method and densified by the spark plasma sintering (SPS) method. Chemical and phase composition studies were carried out by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) methods. Measurements of thermoelectric transport properties, i.e., electrical conductivity, the Seebeck coefficient, and thermal conductivity in the temperature range from 300 to 965 K were carried out. Based on these results, the temperature dependence of the thermoelectric figure of merit ZT as a function of temperature was determined. The obtained, very high ZT parameter (ZT~1.75, T = 965 K) is one of the highest obtained so far for undoped Cu₂Se.     

Ce Filling Limit and Its Influence on Thermoelectric Performance of Fe3CoSb12-Based Skutterudite Grown by a Temperature Gradient Zone Melting Method

CoSb₃-based skutterudite is a promising mid-temperature thermoelectric material. However, the high lattice thermal conductivity limits its further application. Filling is one of the most effective methods to reduce the lattice thermal conductivity. In this study, we investigate the Ce filling limit and its influence on thermoelectric properties of p-type Fe₃CoSb₁₂-based skutterudites grown by a temperature gradient zone melting (TGZM) method. Crystal structure and composition characterization suggests that a maximum filling fraction of Ce reaches 0.73 in a composition of Ce_0.73Fe_2.73Co_1.18Sb₁₂ prepared by the TGZM method. The Ce filling reduces the carrier concentration to 1.03 × 10²⁰ cm⁻³ in the Ce_1.25Fe₃CoSb₁₂, leading to an increased Seebeck coefficient. Density functional theory (DFT) calculation indicates that the Ce-filling introduces an impurity level near the Fermi level. Moreover, the rattling effect of the Ce fillers strengthens the short-wavelength phonon scattering and reduces the lattice thermal conductivity to 0.91 W m⁻¹ K⁻¹. These effects induce a maximum Seebeck coefficient of 168 μV K⁻¹ and a lowest κ of 1.52 W m⁻¹ K⁻¹ at 693 K in the Ce_1.25Fe₃CoSb₁₂, leading to a peak zT value of 0.65, which is 9 times higher than that of the unfilled Fe₃CoSb₁₂.     

Chemical Potential Tuning and Enhancement of Thermoelectric Properties in Indium Selenides

Researchers have long been searching for the materials to enhance thermoelectric performance in terms of nano scale approach in order to realize phonon-glass-electron-crystal and quantum confinement effects. Peierls distortion can be a pathway to enhance thermoelectric figure-of-merit ZT by employing natural nano-wire-like electronic and thermal transport. The phonon-softening known as Kohn anomaly, and Peierls lattice distortion decrease phonon energy and increase phonon scattering, respectively, and, as a result, they lower thermal conductivity. The quasi-one-dimensional electrical transport from anisotropic band structure ensures high Seebeck coefficient in Indium Selenide. The routes for high ZT materials development of In₄Se_3−δ are discussed from quasi-one-dimensional property and electronic band structure calculation to materials synthesis, crystal growth, and their thermoelectric properties investigations. The thermoelectric properties of In₄Se_3−δ can be enhanced by electron doping, as suggested from the Boltzmann transport calculation. Regarding the enhancement of chemical potential, the chlorine doped In₄Se_3−δCl_0.03 compound exhibits high ZT over a wide temperature range and shows state-of-the-art thermoelectric performance of ZT = 1.53 at 450 °C as an n-type material. It was proven that multiple elements doping can enhance chemical potential further. Here, we discuss the recent progress on the enhancement of thermoelectric properties in Indium Selenides by increasing chemical potential.     

Extremely Fast and Cheap Densification of Cu2S by Induction Melting Method

The aim of this work was to obtain dense Cu₂S superionic thermoelectric materials, homogeneous in terms of phase and chemical composition, using a very fast and cheap induction-melting technique. The chemical composition was investigated via scanning electron microscopy (SEM) combined with an energy-dispersive spectroscopy (EDS) method, and the phase composition was established by X-ray diffraction (XRD). The thermoelectric figure of merit ZT was determined on the basis of thermoelectric transport properties, i.e., Seebeck coefficient, electrical and thermal conductivity in the temperature range of 300–923 K. The obtained values of the ZT parameter are comparable with those obtained using the induction hot pressing (IHP) technique and about 30–45% higher in the temperature range of 773–923 K in comparison with Cu₂S samples densified with the spark plasma sintering (SPS) technique.     

Microstructural,Thermoelectric and Mechanical Properties of Cu Substituted NaCo2O4

Polycrystalline samples of NaCo_2−xCu_xO₄ (x = 0, 0.01, 0.03, 0.05) were obtained from powder precursors synthesized by a mechanochemically assisted solid-state reaction method (MASSR) and a citric acid complex method (CAC). Ceramic samples were prepared by pressing into disc-shaped pellets and subsequently sintering at 880 °C in an argon atmosphere. Effects of low concentrations of Cu doping and the above-mentioned synthesis procedures on the thermoelectric and mechanical properties were observed. The electrical resistivity (ρ), the thermal conductivity (κ) and the Seebeck coefficient (S) were measured simultaneously in the temperature gradient (ΔT) between the hot and cold side of the sample, and the figure of merit (ZT) was subsequently calculated. The ZT of the CAC samples was higher compared with the MASSR samples. The highest ZT value of 0.061 at ΔT = 473 K was obtained for the sample with 5 mol% of Cu prepared by the CAC method. The CAC samples showed better mechanical properties compared to the MASSR samples due to the higher hardness of the CAC samples which is a consequence of homogeneous microstructure and higher density obtained during sintering of these samples. The results confirmed that, besides the concentration of Cu, the synthesis procedure considerably affected the thermoelectric and mechanical properties of NaCo₂O₄ (NCO) ceramics.     

Development of High-Performance Thermoelectric Materials by Microstructure Control of P-Type BiSbTe Based Alloys Fabricated by Water Atomization

Developing inexpensive and rapid fabrication methods for high efficiency thermoelectric alloys is a crucial challenge for the thermoelectric industry, especially for energy conversion applications. Here, we fabricated large amounts of p-type Cu_0.07Bi_0.5Sb_1.5Te₃ alloys, using water atomization to control its microstructure and improve thermoelectric performance by optimizing its initial powder size. All the water atomized powders were sieved with different aperture sizes, of 32–75 μm, 75–125 μm, 125–200 μm, and <200 μm, and subsequently consolidated using hot pressing at 490 °C. The grain sizes were found to increase with increasing powder particle size, which also increased carrier mobility due to improved carrier transport. The maximum electrical conductivity of 1457.33 Ω⁻¹ cm⁻¹ was obtained for the 125–200 μm samples due to their large grain sizes and subsequent high mobility. The Seebeck coefficient slightly increased with decreasing particle size due to scattering of carriers at fine grain boundaries. The higher power factor values of 4.20, 4.22 × 10⁻³ W/mk² were, respectively, obtained for large powder specimens, such as 125–200 μm and 75–125 μm, due to their higher electrical conductivity. In addition, thermal conductivity increased with increasing particle size due to the improvement in carriers and phonons transport. The 75–125 μm powder specimen exhibited a relatively high thermoelectric figure of merit, ZT of 1.257 due to this higher electric conductivity.     

Cement-Based Thermoelectric Device for Protection of Carbon Steel in Alkaline Chloride Solution

The thermoelectric cement-based materials can convert heat into electricity; this makes them promising candidates for impressed current cathodic protection of carbon steel. However, attempts to use the thermoelectric cement-based materials for energy conversion usually results in low conversion efficiency, because of the low electrical conductivity and Seebeck coefficient. Herein, we deposited polyaniline on the surface of MnO₂ and fabricated a cement-based thermoelectric device with added PANI/MnO₂ composite for the protection of carbon steel in alkaline chloride solution. The nanorod structure (70~80 nm in diameter) and evenly dispersed conductive PANI provide the PANI/MnO₂ composite with good electrical conductivity (1.9 ± 0.03 S/cm) and Seebeck coefficient (−7.71 × 10³ ± 50 μV/K) and, thereby, increase the Seebeck coefficient of cement-based materials to −2.02 × 10³ ± 40 μV/K and the electrical conductivity of cement-based materials to 0.015 ± 0.0003 S/cm. Based on this, the corrosion of the carbon steel was delayed after cathodic protection, which was demonstrated by the electrochemical experiment results, such as the increased resistance of the carbon steel surface from 5.16 × 10² Ω·cm² to 5.14 × 10⁴ Ω·cm², increased charge transfer resistance from 11.4 kΩ·cm² to 1.98 × 10⁶ kΩ·cm², and the decreased corrosion current density from 1.67 μA/cm² to 0.32 μA/cm², underlining the role of anti-corrosion of the PANI/MnO₂ composite in the cathodic protection system.     

Doping Effect on Cu2Se Thermoelectric Performance: A Review

Cu₂Se, owing to its intrinsic excellent thermoelectric (TE) performance emerging from the peculiar nature of “liquid-like” Cu⁺ ions, has been regarded as one of the most promising thermoelectric materials recently. However, the commercial use is still something far from reach unless effective approaches can be applied to further increase the figure of merit (ZT) of Cu₂Se, and doping has shown wide development prospect. Until now, the highest ZT value of 2.62 has been achieved in Al doped samples, which is twice as much as the original pure Cu₂Se. Herein, various doping elements from all main groups and some transitional groups that have been used as dopants in enhancing the TE performance of Cu₂Se are summarized, and the mechanisms of TE performance enhancement are analyzed. In addition, points of great concern for further enhancing the TE performance of doped Cu₂Se are proposed.     

Solid-State-Activated Sintering of ZnAl2O4 Ceramics Containing Cu3Nb2O8 with Superior Dielectric and Thermal Properties

Low-temperature co-fired ceramics (LTCCs) are dielectric materials that can be co-fired with Ag or Cu; however, conventional LTCC materials are mostly poorly thermally conductive, which is problematic and requires improvement. We focused on ZnAl₂O₄ (gahnite) as a base material. With its high thermal conductivity (~59 W·m⁻¹·K⁻¹ reported for 0.83ZnAl₂O₄–0.17TiO₂), ZnAl₂O₄ is potentially more thermally conductive than Al₂O₃ (alumina); however, it sinters densely at a moderate temperature (~1500 °C). The addition of only 4 wt.% of Cu₃Nb₂O₈ significantly lowered the sintering temperature of ZnAl₂O₄ to 910 °C, which is lower than the melting point of silver (961 °C). The sample fired at 960 °C for 384 h exhibited a relative permittivity (ε_r) of 9.2, a quality factor by resonant frequency (Q × f) value of 105,000 GHz, and a temperature coefficient of the resonant frequency (τ_f) of −56 ppm·K⁻¹. The sample exhibited a thermal conductivity of 10.1 W·m⁻¹·K⁻¹, which exceeds that of conventional LTCCs (~2–7 W·m⁻¹·K⁻¹); hence, it is a superior LTCC candidate. In addition, a mixed powder of the Cu₃Nb₂O₈ additive and ZnAl₂O₄ has a melting temperature that is not significantly different from that (~970 °C) of the pristine Cu₃Nb₂O₈ additive. The sample appears to densify in the solid state through a solid-state-activated sintering mechanism.     

Thermoelectric Properties of Cu2Te Nanoparticle Incorporated N-Type Bi2Te2.7Se0.3

To develop highly efficient thermoelectric materials, the generation of homogeneous heterostructures in a matrix is considered to mitigate the interdependency of the thermoelectric compartments. In this study, Cu₂Te nanoparticles were introduced onto Bi₂Te_2.7Se_0.3 n-type materials and their thermoelectric properties were investigated in terms of the amount of Cu₂Te nanoparticles. A homogeneous dispersion of Cu₂Te nanoparticles was obtained up to 0.4 wt.% Cu₂Te, whereas the Cu₂Te nanoparticles tended to agglomerate with each other at greater than 0.6 wt.% Cu₂Te. The highest power factor was obtained under the optimal dispersion conditions (0.4 wt.% Cu₂Te incorporation), which was considered to originate from the potential barrier on the interface between Cu₂Te and Bi₂Te_2.7Se_0.3. The Cu₂Te incorporation also reduced the lattice thermal conductivity, and the dimensionless figure of merit ZT was increased to 0.75 at 374 K for 0.4 wt.% Cu₂Te incorporation compared with that of 0.65 at 425 K for pristine Bi₂Te_2.7Se_0.3. This approach could also be an effective means of controlling the temperature dependence of ZT, which could be modulated against target applications.     

Normally-off Hydrogen-Terminated Diamond Field-Effect Transistor with SnOx Dielectric Layer Formed by Thermal Oxidation of Sn

SnO_x films were deposited on a hydrogen-terminated diamond by thermal oxidation of Sn. The X-ray photoelectron spectroscopy result implies partial oxidation of Sn film on the diamond surface. The leakage current and capacitance–voltage properties of Al/SnO_x/H-diamond metal-oxide-semiconductor diodes were investigated. The maximum leakage current density value at −8.0 V is 1.6 × 10⁻⁴ A/cm², and the maximum capacitance value is measured to be 0.207 μF/cm². According to the C–V results, trapped charge density and fixed charge density are determined to be 2.39 × 10¹² and 4.5 × 10¹¹ cm⁻², respectively. Finally, an enhancement-mode H-diamond field effect transistor was obtained with a V_TH of −0.5 V. Its I_DMAX is −21.9 mA/mm when V_GS is −5, V_DS is −10 V. The effective mobility and transconductance are 92.5 cm²V⁻¹ s⁻¹ and 5.6 mS/mm, respectively. We suspect that the normally-off characteristic is caused by unoxidized Sn, whose outermost electron could deplete the hole in the channel.     

Reduction of Thermal Conductivity for Icosahedral Al-Cu-Fe Quasicrystal through Heavy Element Substitution

Icosahedral Al-Cu-Fe quasicrystal (QC) shows moderate electrical conductivity and low thermal conductivity, and both p- and n-type conduction can be controlled by tuning the sample composition, making it potentially suited for thermoelectric materials. In this work, we investigated the effect of introducing chemical disorder through heavy element substitution on the thermal conductivity of Al-Cu-Fe QC. We substituted Au and Pt elements for Cu up to 3 at% in a composition of Al₆₃Cu₂₅Fe₁₂, i.e., Al₆₃Cu_25−x(Au,Pt)_xFe₁₂ (x = 0, 1, 2, 3). The substitutions of Au and Pt for Cu reduced the phonon thermal conductivity at 300 K (κ_ph,300K) by up to 17%. The reduction of κ_ph,300K is attributed to a decrease in the specific heat and phonon relaxation time through heavy element substitution. We found that increasing the Pt content reduced the specific heat at high temperatures, which may be caused by the locked state of phasons. The observed glass-like low values of κ_ph,300K (0.9–1.1 W m⁻¹ K⁻¹ at 300 K) for Al₆₃Cu_25−x(Au,Pt)_xFe₁₂ are close to the lower limit calculated using the Cahill model.     

Copper Chalcogenide–Copper Tetrahedrite Composites—A New Concept for Stable Thermoelectric Materials Based on the Chalcogenide System

For the first time, an alternative way of improving the stability of Cu-based thermoelectric materials is proposed, with the investigation of two different copper chalcogenide–copper tetrahedrite composites, rich in sulfur and selenium anions, respectively. Based on the preliminary DFT results, which indicate the instability of Sb-doped copper chalcogenide, the Cu_1.97S–Cu₁₂Sb₄S₁₃ and Cu_2−xSe–Cu₃SbSe₃ composites are obtained using melt-solidification techniques, with the tetrahedrite phase concentration varying from 1 to 10 wt.%. Room temperature structural analysis (XRD, SEM) indicates the two-phase structure of the materials, with ternary phase precipitates embed within the copper chalcogenide matrix. The proposed solution allows for successful blocking of excessive Cu migration, with stable electrical conductivity and Seebeck coefficient values over subsequent thermal cycles. The materials exhibit a p-type, semimetallic character with high stability, represented by a near-constant power factor (PF)—temperature dependences between individual cycles. Finally, the thermoelectric figure-of-merit ZT parameter reaches about 0.26 (623 K) for the Cu_1.97S–Cu₁₂Sb₄S₁₃ system, in which case increasing content of tetrahedrite is a beneficial effect, and about 0.44 (623 K) for the Cu_2−xSe–Cu₃SbSe₃ system, where increasing the content of Cu₃SbSe₃ negatively influences the thermoelectric performance.     

Preparation and Characterization of Thermoelectric PEDOT/Te Nanorod Array Composite Films

In this study, we prepared Te nanorod arrays via a galvanic displacement reaction (GDR) on a Si wafer, and their composite with poly(3,4-ethylenedioxythiophene) (PEDOT) were successfully synthesized by electrochemical polymerization with lithium perchlorate (LiClO₄) as a counter ion. The thermoelectric performance of the composite film was optimized by adjusting the polymerization time. As a result, a maximum power factor (PF) of 235 µW/mK² was obtained from a PEDOT/Te composite film electrochemically polymerized for 15 s at room temperature, which was 11.7 times higher than that of the PEDOT film, corresponding to a Seebeck coefficient (S) of 290 µV/K and electrical conductivity (σ) of 28 S/cm. This outstanding PF was due to the enhanced interface interaction and carrier energy filtering effect at the interfacial potential barrier between the PEDOT and Te nanorods. This study demonstrates that the combination of an inorganic Te nanorod array with electrodeposited PEDOT is a promising strategy for developing high-performance thermoelectric materials.     

Electrical and Structural Properties of Li1.3Al0.3Ti1.7(PO4)3—Based Ceramics Prepared with the Addition of Li4SiO4

The currently studied materials considered as potential candidates to be solid electrolytes for Li-ion batteries usually suffer from low total ionic conductivity. One of them, the NASICON-type ceramic of the chemical formula Li_1.3Al_0.3Ti_1.7(PO₄)₃, seems to be an appropriate material for the modification of its electrical properties due to its high bulk ionic conductivity of the order of 10⁻³ S∙cm⁻¹. For this purpose, we propose an approach concerning modifying the grain boundary composition towards the higher conducting one. To achieve this goal, Li₄SiO₄ was selected and added to the LATP base matrix to support Li⁺ diffusion between the grains. The properties of the Li_1.3Al_0.3Ti_1.7(PO₄)₃−xLi₄SiO₄ (0.02 ≤ x ≤ 0.1) system were studied by means of high-temperature X-ray diffractometry (HTXRD); ⁶Li, ²⁷Al, ²⁹Si, and ³¹P magic angle spinning nuclear magnetic resonance spectroscopy (MAS NMR); thermogravimetry (TG); scanning electron microscopy (SEM); and impedance spectroscopy (IS) techniques. Referring to the experimental results, the Li₄SiO₄ additive material leads to the improvement of the electrical properties and the value of the total ionic conductivity exceeds 10⁻⁴ S∙cm⁻¹ in most studied cases. The factors affecting the enhancement of the total ionic conductivity are discussed. The highest value of σ_tot = 1.4 × 10⁻⁴ S∙cm⁻¹ has been obtained for LATP–0.1LSO material sintered at 1000 °C for 6 h.     

Synthesis and Characterization of Al- and SnO2-Doped ZnO Thermoelectric Thin Films

The effect of SnO₂ addition (0, 1, 2, 4 wt.%) on thermoelectric properties of c-axis oriented Al-doped ZnO thin films (AZO) fabricated by pulsed laser deposition on silica and Al₂O₃ substrates was investigated. The best thermoelectric performance was obtained on the AZO + 2% SnO₂ thin film grown on silica, with a power factor (PF) of 211.8 μW/m·K² at 573 K and a room-temperature (300 K) thermal conductivity of 8.56 W/m·K. PF was of the same order of magnitude as the value reported for typical AZO bulk material at the same measurement conditions (340 μW/m·K²) while thermal conductivity κ was reduced about four times.     

Investigation on the Power Factor of Skutterudite Smy(FexNi1−x)4Sb12 Thin Films: Effects of Deposition and Annealing Temperature

Filled skutterudites are currently studied as promising thermoelectric materials due to their high power factor and low thermal conductivity. The latter property, in particular, can be enhanced by adding scattering centers, such as the ones deriving from low dimensionality and the presence of interfaces. This work reports on the synthesis and characterization of thin films belonging to the Sm_y(Fe_xNi_1−x)₄Sb₁₂-filled skutterudite system. Films were deposited under vacuum conditions by the pulsed laser deposition (PLD) method on fused silica substrates, and the deposition temperature was varied. The effect of the annealing process was studied by subjecting a set of films to a thermal treatment for 1 h at 423 K. Electrical conductivity σ and Seebeck coefficient S were acquired by the four-probe method using a ZEM-3 apparatus performing cycles in the 348–523 K temperature range, recording both heating and cooling processes. Films deposited at room temperature required three cycles up to 523 K before being stabilized, thus revealing the importance of a proper annealing process in order to obtain reliable physical data. XRD analyses confirm the previous result, as only annealed films present a highly crystalline skutterudite not accompanied by extra phases. The power factor of annealed films is shown to be lower than in the corresponding bulk samples due to the lower Seebeck coefficients occurring in films. Room temperature thermal conductivity, on the contrary, shows values comparable to the ones of doubly doped bulk samples, thus highlighting the positive effect of interfaces on the introduction of scattering centers, and therefore on the reduction of thermal conductivity.