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This work discloses a newly-discovered Cu8−xGe(S, Te)6 argyrodite stabilizing in a high-symmetry cubic phase in the entire low-to-mid temperature region (250–700 K).

We revisit the well-established Bi2Te3 via phase diagram engineering. Along with a phase diagram in hand, it is realized that the solubility of Ga in Bi2Te3 is coincident with the p–n transition zo...

To evaluate the effects of anodization on the friction behavior of beta-titanium (β-Ti) orthodontic archwires in conventional or self-ligating brackets in vitro. β‑Ti archwires (0.018 × 0.025 inch) pre- and postanodization were tested in combination with 0.022-inch stainless steel conventional and self-ligating brackets. The surface composition and oxide thickness of the β‑Ti archwires pre- and postanodization were measured using Auger electron spectroscopy (AES) and transmission electron microscopy (TEM). Detailed surface topography and roughness were assessed using atomic force microscopy (AFM). Surface topographies of the β‑Ti archwires pre- and postanodization were examined using scanning electron microscopy (SEM). Friction was measured using a universal testing machine; the data were statistically analyzed. Postanodization, the identified titanium oxide layer on the surface of the β‑Ti archwires increased in thickness from 10 to 100 nm; at the same time, the values for surface roughness were significantly reduced by half (p

We prepared amorphous indium–gallium–zinc oxide (a-IGZO) thin films with various Ga content ratios and investigated their feasibility as the active channel layers of top-gate thin-film transistors (TFT). First, the 2-inch IGZO sputtering targets with stoichiometric ratios of InGaZn2O5, InGaZnO4, and InGa2ZnO5.5 were fabricated using In2O3, Ga2O3, and ZnO oxide powders as raw materials via sintering treatments at temperatures ranging from 900 °C to 1300 °C for 6 h or 8 h. X-ray diffraction analysis indicated that the InGaZn2O5 and InGaZnO4 targets are single-phase structures whereas the InGa2ZnO5.5 target is a two-phase structure. Hall effect measurement indicated that the a-InGaZn2O5 and a-InGaZnO4 layers possess a carrier concentration (N) of about 1019 cm−3 and a resistivity (ρ) of about 10−2 Ω·cm; however, the N of the a-InGa2ZnO5.5 layer is only 1017 cm−3, and the ρ is about 1 to 4 Ω·cm. Moreover, the a-InGaZn2O5 layer exhibited the highest Hall-effect mobility (μHall) of 21.17 cm2·V−1·sec−1. This indicated that the impedance of Ga3+ ions to carrier migration is the main factor affecting the electrical properties of a-IGZO layers. Ga content in the a-IGZO channel similarly affects the performance of the TFT devices prepared in this study. The annealing at 300 °C for 1 h in an ambient atmosphere was found to significantly improve the electrical properties of the TFT devices. The best performance was observed in the a-InGaZnO4 TFT sample subjected to post-annealing at 300 °C with Vth = −0.85 V, μFE = 8.46 cm2, V−1·sec−1, SS = 2.31, V·decade−1, and Ion/Ioff = 2.9 × 104.

Most high-entropy alloys and medium-entropy alloys (MEAs) possess outstanding mechanical properties. In this study, a series of lightweight nonequiatomic Al50–Ti–Cr–Mn–V MEAs with a dual phase were produced through arc melting and drop casting. These cast alloys were composed of body-centered cubic and face-centered cubic phases. The density of all investigated MEAs was less than 5 g/cm3 in order to meet energy and transportation industry requirements. The effect of each element on the microstructure evolution and mechanical properties of these MEAs was investigated. All the MEAs demonstrated outstanding compressive strength, with no fractures observed after a compressive strain of 20%. Following the fine-tuning of the alloy composition, the Al50Ti20Cr10Mn15V5 MEA exhibited the most compressive strength (~1800 MPa) and ductility (~34%). A significant improvement in the mechanical compressive properties was achieved (strength of ~2000 MPa, strain of ~40%) after annealing (at 1000 °C for 0.5 h) and oil-quenching. With its extremely high specific compressive strength (452 MPa·g/cm3) and ductility, the lightweight Al50Ti20Cr10Mn15V5 MEA demonstrates good potential for energy or transportation applications in the future.

Zn4Sb3-based compounds constitute a lead-free material family with a best thermoelectric figure of merit (zT) in the midtemperature range. Unlike being a stoichiometric compound, the mutual solubil...

Hydrogen evolution reaction (HER) from water electrolysis is a promising route for hydrogen fuel production. However, an effective conversion technology from distributed energy resources, such as solar energy and thermal energy, to hydrogen energy is still a challenging research. Here, earth-abundant and multifunctional Ni nanosheets array as electrocatalysts and light absorption layer integrated thermoelectric (TE) device for overall water splitting was proposed. Instead of the traditional power source, the output voltage from thermoelectric (TE) device was directly used for water splitting. More importantly, the structural, photothermal transformation and electrocatalytic characterizations confirmed that the earth-abundant and multifunctional Ni nanosheets array grown on the hot side of TE device was not only as an efficient photothermal conversion layer for providing temperature difference (ΔT) of TE, but also as an active electrocatalysts for HER. The Electrolyzer-TE hybrid devices were designed for overall water splitting in a two-electrode configuration, which possessed high hydrogen production rate of 1.818 mmol/h and oxygen production rate of 0.912 mmol/h. The proposed integrative TE device herein offered great advantages in terms of designing the overall water splitting system, which are helpful for its practical applications for utilization of solar thermal and waste heat.

To evaluate the effects of anodization on the friction behavior of beta-titanium (β-Ti) orthodontic archwires in conventional or self-ligating brackets in vitro.β‑Ti archwires (0.018 × 0.025 inch) pre- and postanodization were tested in combination with 0.022-inch stainless steel conventional and self-ligating brackets. The surface composition and oxide thickness of the β‑Ti archwires pre- and postanodization were measured using Auger electron spectroscopy (AES) and transmission electron microscopy (TEM). Detailed surface topography and roughness were assessed using atomic force microscopy (AFM). Surface topographies of the β‑Ti archwires pre- and postanodization were examined using scanning electron microscopy (SEM). Friction was measured using a universal testing machine; the data were statistically analyzed.Postanodization, the identified titanium oxide layer on the surface of the β‑Ti archwires increased in thickness from 10 to 100 nm; at the same time, the values for surface roughness were significantly reduced by half (p 0.001). The archwire surfaces post anodization were harder and had fewer scratches after the friction test. Anodization significantly reduced 23.77% of the static (p 0.01) and 25.61% of the kinetic (p 0.001) friction of the β‑Ti archwires in conventional brackets, while it significantly reduced 85.71% of the static and 84.38% of the kinetic friction (p 0.01) in self-ligating brackets.Anodization reduced the β‑Ti archwire friction, which was particularly more effective in combination with self-ligating brackets. The friction reduction via anodization could be attributed to the increased thickness, surface hardness, and decreased surface roughness of the titanium oxide layer.ZIEL: Untersucht werden sollten die Auswirkungen einer Eloxierung auf das Reibungsverhalten von kieferorthopädischen Bogendrähten aus β‑Titan (β-Ti) in konventionellen bzw. selbstligierenden Brackets in vitro.β‑Ti-Bögen (0,018 × 0,02 Inch) vor und nach der Eloxierung wurden in Kombination mit konventionellen und selbstligierenden Brackets aus Edelstahl (0,022 Inch) getestet. Die Oberflächenzusammensetzung und die Oxiddicke der β‑Ti-Bögen vor und nach der Eloxierung wurden mit Auger-Elektronenspektroskopie (AES) und Transmissionselektronenmikroskopie (TEM) bestimmt. Die detaillierte Oberflächentopografie und -rauheit wurde mittels Rasterkraftmikroskopie (AFM) bewertet. Die Oberflächentopografien der β‑Ti-Bögen vor und nach der Eloxierung wurden mit dem Rasterelektronenmikroskop (REM) untersucht. Die Friktion wurde mit einer Universalprüfmaschine ermittelt; die Daten wurden statistisch ausgewertet.Nach der Eloxierung nahm die Dicke der identifizierten Titanoxidschicht auf der Oberfläche der β‑Ti-Bögen von 10 auf 100 nm zu; gleichzeitig wurden die Werte für die Oberflächenrauheit signifikant um die Hälfte reduziert (p 0,001). Nach der Eloxierung waren die Oberflächen der Bögen härter und hatten weniger Kratzer nach dem Friktionstest. Die Eloxierung reduzierte signifikant 23,77% der statischen (p 0,01) und 25,61 % der kinetischen (p 0,001) Reibung der β‑Ti-Bögen in konventionellen Brackets, während sie signifikant 85,71 % der statischen und 84,38 % der kinetischen Reibung (p 0,01) in selbstligierenden Brackets reduzierte.Die Eloxierung kann die Friktion des β‑Ti-Bogens reduzieren, besonders effektiv in Kombination mit selbstligierenden Brackets. Die Verringerung der Friktion durch die Eloxierung könnte auf die Zunahme der Dicke und der Oberflächenhärte sowie die Verringerung der Oberflächenrauhigkeit der Titanoxidschicht zurückzuführen sein.

A novel lightweight Al-Ti-Cr-Mn-V medium-entropy alloy (MEA) system was developed using a nonequiatiomic approach and alloys were produced through arc melting and drop casting. These alloys comprised a body-centered cubic (BCC) and face-centered cubic (FCC) dual phase with a density of approximately 4.5 g/cm3. However, the fraction of the BCC phase and morphology of the FCC phase can be controlled by incorporating other elements. The results of compression tests indicated that these Al-Ti-Cr-Mn-V alloys exhibited a prominent compression strength (~1940 MPa) and ductility (~30%). Moreover, homogenized samples maintained a high compression strength of 1900 MPa and similar ductility (30%). Due to the high specific compressive strength (0.433 GPa·g/cm3) and excellent combination of strength and ductility, the cast lightweight Al-Ti-Cr-Mn-V MEAs are a promising alloy system for application in transportation and energy industries.

In recent years the emerge of high-entropy alloys (HEAs) imposes an evolution in metallic materials, which breaks the boundaries set by the traditional alloys. Alongside the development of HEAs, the medium-entropy alloys (MEAs), which comprise two to four majority elements, also reveal the outperforming properties with less compositional complexity. Among them, the medium-entropy AlNbV alloys attract great attention owing to the existence of a body-centered cubic (BCC) solid solution that contains soluble Al, Nb, and V elements. Herein, we construct the phase diagrams for the Al-Nb-V system and define the equilibrium homogeneity by thermally-equilibrated ternary alloys underwent a post-annealing at 1073 K or 1273 K. Meanwhile, a superposition of phase diagram and thermal conductivity κ mapping suggests that the coexistence of BCC solid solution and nano-grained AlNb2 brings down the κ. With the incorporation of Ti and Cr, the HEA AlNbVCrTi, which is composed of Laves C14 phase and BCC solid solution, achieves an ultralow κ of 6 – 10 Wm-1K-1 within 323 – 723 K.

Microstructure engineering fulfilled by phase transformation elicits the reduced thermal conductivity κ in thermoelectric materials. We demonstrate that the multiscale hierarchical twinning structure could exist in the GeTe alloys as the phase transformation from cubic β-GeTe to rhombohedral α-GeTe is manipulated by two-step cooling. The coexistence of nanoscale and microscale herringbone structures yields the low-lying κ, attributing to the reorientation of martensitic variants and pile-up of martensitic twinning. Dilute dopants of Cu and Bi/Sb further suppresses hole carrier concentration nH to the order of 1020 cm−3, leading to the enhanced power factor PF = S2ρ−1 of α-GeTe. Meanwhile, the phase diagram engineering probes the solubility limit of Cu in the single-phase GeTe, diminishing the formation of undesired impurity phases. These strategies boost the peak zT value to 1.5 at 698 K in the light-doped Bi0.01Cu0.01Ge0.98Te alloy. The multiscale twin hierarchy and carrier optimization synergistically enhance the thermoelectric performance of GeTe-based alloys that provide a new chapter in search of green energy resources out from phase change materials.

For decades, the Bismuth-telluride (Bi2Te3) has been intensively studied as the thermoelectric (TE) cooler. Still, new ideas and emerging results are put forward for raising the conversion efficiency. Herein, we re-visit the Cu-doped Bi2Te3, and report the high zT values nearing the room temperature, for both the p-type (Cu2Te)0.01(Bi2Te3)0.99/Cu0.01Bi1.99Te3 (zT∼1.2 at 300 K) as well as the n-type (Cu2Te)0.09(Bi2Te3)0.91 (zT∼1.09 at 363 K), respectively. Given that the phase boundary mapping is essential for the optimization of high-efficiency TE materials, the isothermal section of ternary Bi-Cu-Te at 523 K is constructed, by collecting the phase equilibria information of various thermally-equilibrated alloys; it further guides the alloying directions for (Cu2Te)x (Bi2Te3)1-x and CuyBi2-yTe3, respectively. Small modulation in the stoichiometry leads to the carrier type transition. As a consequence, the lamellae composed of Bi2Te3 and Cu7Te5 precipitate along the grain boundary of the n-type (Cu2Te)0.09(Bi2Te3)0.91, resulting in the reduced κ, due to the stronger interfacial phonon scattering, and the higher PF, owing to the higher amounts of Te vacancy (VTe). On the contrary, the promising p-type (Cu2Te)0.01(Bi2Te3)0.99/Cu0.01Bi1.99Te3 features the single-phase Bi2Te3, whereas the soluble Cu introduces extra holes and might therefore promote the p-type conduction.

Engineering transport properties of thermoelectric (TE) materials leads to incessantly breakthroughs in the zT values. Nevertheless, modular design holds a key factor to advance the TE technology. Herein, we discuss the structures of TE module and illustrate the inter-diffusions across the interface of constituent layers. For Bi2Te3-based module, soldering is the primary bonding method, giving rise to the investigations on the selections of solder, diffusion barrier layer and electrode. For mid-temperature PbTe-based TE module, hot-pressing or spark plasma sintering are alternative bonding approaches; the inter-diffusions between the diffusion barrier layer, electrode and TE substrate are addressed as well.

The n-type I-V-VI2 AgBiSe2 features intrinsically low κ due to the anharmonicity of chemical bonds. Experimentally-determined isothermal section guides the starting compositions for the following AgBiSe2-based alloys. Among the undoped alloys, the Ag25Bi25Se50 exhibits a highest peak of zT∼0.75, and yet the neighboring Ag20Bi27.5Se52.5, which involves a Se-rich liquid phase, has a much lower zT∼0.3 at 748 K, respectively. With the incorporation of Ge, the (GeSe)0.03(AgBiSe2)0.97 exhibits an ultralow κ∼0.3 (W/mK), owing to the formation of Bi2Se3 nano-precipitate in the size of 20–40 nm. Additionally, the moire fringes with a periodicity of 0.25 nm are observed in the Bi2Se3 nano-precipitate, implying the presence of local mass fluctuation and superlattice, which could further lead to enhancing phonon scattering and reduced κ. As a result, the ultra-low κ∼0.3 (W/mK) boosts the peak of zT up to zT∼1.05 in n-type (GeSe)0.03(AgBiSe2)0.97, which shows a 140% enhancement compared with that of the undoped AgBiSe2.

The bismuth-tellurides have never ceased as the best candidates for thermoelectric (TE) coolers since the beginning of manufactures in the 1960s. As the years passed, the effort in boosting their performance continues abreast of breakthroughs in the conversion efficiencies with the occasional setbacks from the n-type alloys. Herein, a synergistic approach combining the phase diagram engineering and the carrier optimization overcomes the long-existing obstacle, eliciting an outperforming n-type Ag–Bi2Te3. The Ag0.03Bi2Te2.97 alloy attains a high figure-of-merit zT = 1.4 at 363 K and a superior averaged zTave = 1.1 within 300 K–500 K. The phase diagram pinpoints a high-zT zone whose boundary coincides with the maximal Ag solubility. A close-up upon the best-performing Ag0.03Bi2Te2.97 reveals the presence of Ag2Te nano-precipitates, explaining its low-lying κL alongside the remarkable enhancement in its power factor PF = S2ρ−1. The avenue paved by the thermodynamic route essentially revives the TE research field, in which more potential candidates are awaiting further discovery.

The capability of converting waste heat into electricity characterizes the technology of thermoelectricity as a feasible solution in dealing with the energy issue. The Bi2Te3-based alloys have long been in the spotlight as they show promising zTs and high thermal stability near the room-temperature region. Herein, the experimentally determined 523 K isothermal section of ternary Bi-Ge-Te system concludes that the four ternary compounds, the Ge3Bi2Te6, the Ge1.5Bi1.5Te5, the Ge1Bi2Te4 and the Ge1Bi4Te7, are thermally stabilized and the solubility of Ge in Bi2Te3 could reach ∼5.0 at.%Ge. The thermoelectric performance of various Ge-incorporated Bi2Te3 alloys with compositions of Bi40-yGexTe60-x+y (x = 0–12.5, y = 0–10.0) are evaluated within 300 K-650 K. The conduction type of those alloys transits from p-type, for the un-doped Bi2Te3, to n-type semiconducting as one of the following conditions are satisfied: (1) the concentration of Te drops to less than 60.0 at.%, or (2) the concentration of Te exceeds 60.0 at.% and the ratios of Ge/Bi is larger than 0.2. The zT peak values reach zT∼0.9 at 325 K for p-type alloy P2 (Bi39.0Ge1.0Te60.0), which features a Bi2Te3 single-phase microstructure, and zT∼0.45 at 525 K for n-type alloy N5 (Bi32.5Ge10Te57.5), which falls in a Bi2Te3+Ge1Bi2Te4+Te three-phase region, respectively.

Thermoelectric (TE) research is not only a course of materials by discovery but also a seedbed of novel concepts and methodologies. Herein, the focus is on recent advances in three emerging paradigms: entropy engineering, phase-boundary mapping, and liquid-like TE materials in the context of thermodynamic routes. Specifically, entropy engineering is underpinned by the core effects of high-entropy alloys; the extended solubility limit, the tendency to form a high-symmetry crystal structure, severe lattice distortions, and sluggish diffusion processes afford large phase space for performance optimization, high electronic-band degeneracy, rich multiscale microstructures, and low lattice thermal conductivity toward higher-performance TE materials. Entropy engineering is successfully implemented in half-Huesler and IV-VI compounds. In Zintl phases and skutterudites, the efficacy of phase-boundary mapping is demonstrated through unraveling the profound relations among chemical compositions, mutual solubilities of constituent elements, phase instability, microstructures, and resulting TE properties at the operation temperatures. Attention is also given to liquid-like TE materials that exhibit lattice thermal conductivity at lower than the amorphous limit due to intensive mobile ion disorder and reduced vibrational entropy. To conclude, an outlook on the development of next-generation TE materials in line with these thermodynamic routes is given.

In this work, a high thermoelectric figure of merit, zT of 1.9 at 740 K is achieved in Ge1−xBixTe crystals through the concurrent of Seebeck coefficient enhancement and thermal conductivity reduction with Bi dopants. The substitution of Bi for Ge not only compensates the superfluous hole carriers in pristine GeTe but also shifts the Fermi level (EF) to an eligible region. Experimentally, with moderate 6–10% Bi dopants, the carrier concentration is drastically decreased from 8.7 × 1020 cm−3 to 3–5 × 1020 cm−3 and the Seebeck coefficient is boosted three times to 75 μVK−1. In the meantime, based on the density functional theory (DFT) calculation, the Fermi level EF starts to intersect with the pudding mold band at L point, where the band effective mass is enhanced. The enhanced Seebeck coefficient effectively compensates the decrease of electrical conductivity and thus successfully maintain the power factor as large as or even superior than that of the pristine GeTe. In addition, the Bi doping significantly reduces both thermal conductivities of carriers and lattices to an extremely low limit of 1.57 W m−1K−1 at 740 K with 10% Bi dopants, which is an about 63% reduction as compared with that of pristine GeTe. The elevated figure of merit observed in Ge1−xBixTe specimens is therefore realized by synergistically optimizing the power factor and downgrading the thermal conductivity of alloying effect and lattice anharmonicity caused by Bi doping.

In this work, a high thermoelectric figure of merit, zT of 1.9 at 740 K is achieved in Ge1−xBixTe crystals through the concurrent of Seebeck coefficient enhancement and thermal conductivity reduction with Bi dopants. The substitution of Bi for Ge not only compensates the superfluous hole carriers in pristine GeTe but also shifts the Fermi level (EF) to an eligible region. Experimentally, with moderate 6–10% Bi dopants, the carrier concentration is drastically decreased from 8.7 × 1020 cm−3 to 3–5 × 1020 cm−3 and the Seebeck coefficient is boosted three times to 75 μVK−1. In the meantime, based on the density functional theory (DFT) calculation, the Fermi level EF starts to intersect with the pudding mold band at L point, where the band effective mass is enhanced. The enhanced Seebeck coefficient effectively compensates the decrease of electrical conductivity and thus successfully maintain the power factor as large as or even superior than that of the pristine GeTe. In addition, the Bi doping significantly reduces both thermal conductivities of carriers and lattices to an extremely low limit of 1.57 W m−1K−1 at 740 K with 10% Bi dopants, which is an about 63% reduction as compared with that of pristine GeTe. The elevated figure of merit observed in Ge1−xBixTe specimens is therefore realized by synergistically optimizing the power factor and downgrading the thermal conductivity of alloying effect and lattice anharmonicity caused by Bi doping.

Phase transition in thermoelectric (TE) material is a double-edged sword-it is undesired for device operation in applications, but the fluctuations near an electronic instability are favorable. Here, Sb doping is used to elicit a spontaneous composition fluctuation showing uphill diffusion in GeTe that is otherwise suspended by diffusionless athermal cubic-to-rhombohedral phase transition at around 700 K. The interplay between these two phase transitions yields exquisite composition fluctuations and a coexistence of cubic and rhombohedral phases in favor of exceptional figures-of-merit zT. Specifically, alloying GeTe by Sb

Among all types of thermoelectric materials, chalcopyrite CuGaTe 2 has been viewed as a promising candidate for use of thermoelectric generator due to its high figure-of-merit ( zT ) at the high temperatures. Herein the 923 K isothermal section of ternary Cu-Ga-Te system is determined, using various thermally-equilibrated Cu-Ga-Te alloys, and ternary CuGaTe 2 phase is stabilized within the compositional region of 48.0–53.0 at%Te and 25.0 at%-30.0 at%Cu. Moreover, the solubility of Cu in binary Ga 2 Te 3 and Ga 3 Te 4 compounds at 923 K is negligible, while that in GaTe phase reaches 7.9 at%Cu. The as-determined isothermal section, depicting the phase stability regime of CuGaTe 2 , provides options for precisely locating the compositions of CuGaTe 2 -based materials that lead to promising and reproducible thermoelectric properties. A zT peak of 0.6 has been achieved in the Bridgman-grown Cu 25 Ga 26 Te 49 alloy at 750 K, which is nearly eight times higher than the neighboring Cu 28 Ga 25 Te 47 alloy, presumably due to the fact that the Cu 25 Ga 26 Te 49 alloy, which exhibits high phase purity of CuGaTe 2 , has lower lattice thermal conductivity (κ L ∼1.3 (W/mK) and higher power factor ( PF ∼11.2 (μW/mK 2 )), comparing with that of Cu 28 Ga 25 Te 47 alloy (κ L ∼1.8 (W/mK) and PF ∼1.9 (μW/mK 2 )), which locates in a three-phased CuGaTe 2 +Cu 2 Te + Cu 9 Ga 4 region, with only a slight deviation in the starting composition.

Due to the demand for reduced energy consumption by transportation vehicles, lighter weight high-entropy alloys (with density comparable to that of commercial Ti alloys around 4.5–5 g/cm3) are attracting more attention in terms of alloy design and application as structural materials. In this study, a nonequiatomic quaternary alloy system, Tix(AlCrNb)100-x, was designed through the calculation of phase diagrams. Simulation results reveal that a single body-centered cubic (BCC) phase can be formed and can be stable at temperatures above 950 °C. Accordingly, a series of Tix(AlCrNb)100-x (x = 45–80) alloys containing a BCC structure were prepared through vacuum arc melting and rapid cooling. The designed alloys can exhibit desirable mechanical properties with high compression yield strength about 1500 MPa, high compression fracture strength about 1800 MPa and high compression plasticity more than 30% at room temperature. Moreover, the Ti65 alloy can demonstrate a tensile strength of 1200 MPa with a tensile elongation of 32%, after a homogenization treatment for 24 h. The specific compression and tensile strength can reach 0.36 and 0.24 GPa cm3/g.

Turning waste heat into electricity using thermoelectric materials supports the crucial goals of the sustainable development. Two ternary chalcogenide compounds, the AgSbTe_2 and AgSbSe_2, are viewed as prospective candidates for mid-temperature waste heat recovery (600–800 K). Alloying between the cubic AgSbTe_2 and AgSbSe_2 yields to the very high zT peak value of 1.4 in Ag_(25)Sb_(25)Se_(10)Te_(40) and Ag_(25)Sb_(25)Se_5Te_(45) at 680 K, which are nearly 40% and 250% higher than those of undoped AgSbTe_2 (zT ∼ 1) and AgSbSe_2 (zT ∼ 0.4). The relations between the phase stability, the carrier concentration and the electronic/thermal transport property are understood by conducting comprehensive study along the pseudo-binary AgSbSe_2–AgSbTe_2 system with varying values of x and y in Ag_(50-x)Sb_xSe_(50-y)Te_y (25 ⩽ x ⩽ 30, y ⩽ 50).

The stability of intrinsic point defects in PbTe, one of the most widely studied and efficient thermoelectric material, is explored by means of Density Functional Theory (DFT). The origin of n- and p-type conductivity in PbTe is attributed to particular intrinsic charged defects by calculating their formation energies. These DFT calculated defect formation energies are then used in the Gibbs free energy description of this phase as part of the Pb-Te thermodynamic model built using the CALPHAD method, and in the resulting phase diagram it is found that its solubility lines and non-stoichiometric range agree very well with experimental data. Such an approach of using DFT in conjunction with CALPHAD for compound semiconductor phases that exhibit very small ranges of non-stoichiometry does not only make the process of calculating phase diagrams for such systems more physical, but is necessary and critical for the assessment of unknown phase diagrams.

The layered-chalcogenide Sb2Se3 has potential for use in photovoltaic devices and thermoelectric coolers, but its practical applications are restricted as bulk Sb2Se3 exhibits high electrical resistivity (ρ). Herein, we synthesize n-type Te-doped Sb2Se3 whiskers with the average feature size of 1 cm length and 200 μm width via a self-assisted vapor–solid (VS) method. In particular, based on the crystallographic identification using techniques of powder X-ray diffraction and transmission electron microscopy, the Te-doped Sb2Se3 whisker grows along a preferred orientation of [010], which is rarely reported for an orthorhombic structure due to the fact that the bonding energy along the c-axis is much stronger. The [010]-oriented Sb2Se3 whisker shows enhanced electrical conductivity, especially compared with its bulk form, and exhibits a high Seebeck value (S) within 300–400 K, resulting in a peak power factor (S2/ρ) of ∼7.6 (μW m K−2) at 350 K. The peak PF value is 104 higher than that of Sb2Se3 bulk and is comparable to that of Sb2Se3 nanotubes, respectively, giving rise to the possibility of utilizing Sb2Se3 in the application of thermoelectric coolers.

The thermal stability of joints in thermoelectric (TE) modules, which are degraded during interdiffusion between the TE material and the contacting metal, needs to be addressed in order to utilize TE technology for competitive, sustainable energy applications. Herein, we deposit a 200 nm-thick Zr-based thin-film metallic glass (TFMG), which acts as an effective diffusion barrier layer with low electrical contact resistivity, on a high-zT Se-doped AgSbTe2 substrate. The reaction couples structured with TFMG/TE are annealed at 673 K for 8–360 hours and analyzed by electron microscopy. No observable IMCs (intermetallic compounds) are formed at the TFMG/TE interface, suggesting the effective inhibition of atomic diffusion that may be attributed to the grain-boundary-free structure of TFMG. The minor amount of Se acts as a tracer species, and a homogeneous Se-rich region is found nearing the TFMG/TE interface, which guarantees satisfactory bonding at the joint. The diffusion of Se, which has the smallest atomic volume of all the elements from the TE substrate, is found to follow Fick’s second law. The calculated diffusivity (D) of Se in TFMG falls in the range of D~10−20–10−23(m2/s), which is 106~107 and 1012~1013 times smaller than those of Ni [10−14–10−17(m2/s)] and Cu [10−8–10−11(m2/s)] in Bi2Te3, respectively.

Bi 2 (Te 1− x Se x ) 3 is an important kind of n-type thermoelectric material. In this study, interfacial reactions at 250 °C in the Sn/Bi 2 Te 3 , Sn/Bi 2 Se 3 and Sn/Bi 2 (Te 1− x Se x ) 3 couples are examined. In the Sn/Bi 2 Te 3 couple, the reaction zone is found to be non-planar and the reaction path is liquid/(liquid + SnTe)/Bi 2 Te 3 . But with only 2.0 at.%Se addition to the Bi 2 Te 3 substrate, very different reaction results are observed. In the Sn/Bi 2 (Te 1− x Se x ) 3 couple, the reaction layers are planar, and the reaction path is liquid/SnTe/BiTe x /Bi 2 (Te 1− x Se x ) 3 . In the Sn/Bi 2 Se 3 couple, the reaction layers are also planar, the reaction path is liquid/SnSe/BiSe/Bi 2 Se 3 , and the Bi content in the SnSe phase varies. The growth rates in the Sn/Bi 2 Te 3 couples are very fast, and the reaction layer is as thick as 1174 μm in just 15 min. Very dramatic reduction of growth rates has been observed with Se addition to the substrates, and the reaction layer is only 9 μm in the Sn/Bi 2 (Te 2.9 Se 0.1 ) couples after reaction for 30 min.

The Ag-Sn-Te ternary system is of interest to thermoelectric applications and its liquidus projection is determined in this study. Forty Ag-Sn-Te ternary alloys are prepared and their primary solidification phases are determined. These different primary solidification phase regions include three terminal solid solutions: Ag, Sn, and Te; six binary intermediate phases: SnTe, β-Ag5Te3, Ag1.9Te, Ag2Te (assuming no phase transformation), ζ-Ag4Sn, and e-Ag3Sn; and one ternary compound, AgSnTe2. These data, together with the phase diagrams of the three constituent binary systems, are employed to construct the univariant lines of the liquidus projection. The temperature-descending directions of these univariant lines are determined using thermal analysis results and mass balance concept. The types of invariant reactions and the reaction temperatures are determined from the temperature-descending directions of the univariant lines and by thermal analysis. There are two Class I reactions, five Class II reactions, and one Class III reaction. The invariant reaction with the highest reaction temperature is L + Ag = Ag2Te + e-Ag3Sn, at 992.7 ± 4 K (719.5 ± 4 °C), and that with the lowest reaction temperature is L = Sn + e-Ag3Sn + SnTe, at 494.2 ± 4 K (221 ± 2 °C).

The thermal stability of joints in thermoelectric (TE) modules, which are degraded during interdiffusion between the TE material and the contacting metal, needs to be addressed in order to utilize TE technology for competitive, sustainable energy applications. Herein, we deposit a 200 nm-thick Zr-based thin-film metallic glass (TFMG), which acts as an effective diffusion barrier layer with low electrical contact resistivity, on a high-zT Se-doped AgSbTe2 substrate. The reaction couples structured with TFMG/TE are annealed at 673 K for 8–360 hours and analyzed by electron microscopy. No observable IMCs (intermetallic compounds) are formed at the TFMG/TE interface, suggesting the effective inhibition of atomic diffusion that may be attributed to the grain-boundary-free structure of TFMG. The minor amount of Se acts as a tracer species, and a homogeneous Se-rich region is found nearing the TFMG/TE interface, which guarantees satisfactory bonding at the joint. The diffusion of Se, which has the smallest atomic volume of all the elements from the TE substrate, is found to follow Fick’s second law. The calculated diffusivity (D) of Se in TFMG falls in the range of D~10−20–10−23(m2/s), which is 106~107 and 1012~1013 times smaller than those of Ni [10−14–10−17(m2/s)] and Cu [10−8–10−11(m2/s)] in Bi2Te3, respectively.

The thin film metallic glass (TFMG) is an effective diffusion barrier layer for PbTe-based thermoelectric (TE) modules. Reaction couples structured with Cu/TFMG/PbTe are prepared via sputter-deposition and are annealed at 673 K for 8-96 h. The transmission line method is adopted for the assessment of electrical contact resistivity upon the PbTe/TFMG, and the value remains in the range of 3.3-2.5 × 10−9 (Ω m2). The titanium-based TFMG remains amorphous upon annealing at 673 K for 48 h and effectively blocks the inter-diffusion by not having grain-boundaries, which only allows the bulk diffusion between the metal electrode and the TE substrate.

Ag–Pb–Te is an important system in thermoelectric applications. Various Ag–Pb–Te alloys are prepared and their microstructures are examined. The liquidus projection of ternary Ag–Pb–Te system is determined both by experimental investigations and Calphad calculations, and the results are in good agreement. There are eight primary solidification phases, Ag, γ-Ag2Te, β-Ag2Te, Ag1.9Te, Ag5Te3, Te, PbTe and Pb, including a miscibility gap extended from the Ag–Te side. A ternary-eutectic reaction, L = PbTe + Te + Ag5Te3, was determined with a liquid composition of Ag-4.3at%Pb-62.6at%Te at 337 °C. A nanoscale microstructure containing a dotted PbTe and a lamellar matrix of Ag5Te3 and Te phases results from the ternary eutectic reaction. The solidification paths of selective ternary Ag–Pb–Te alloys are calculated using the Scheil model with the optimized interaction parameters obtained in this study, and the results are consistent with the experimental observations.

The ternary Bi-Sb-Te system is important for thermoelectric applications. Its liquidus projection is determined, except for the Bi-rich corner with various unconfirmed binary Bi-Te compounds. Fifty Bi-Sb-Te ternary alloys are prepared. Primary solidification phases are identified using metallographic observations and X-ray diffraction techniques. Phase transformation temperatures are determined using differential thermal analysis. No ternary compound is found. (Bi,Sb)2Te3 and (Bi,Sb) are continuous solid solutions. The Bi2Te3 and Sb2Te3 phases form a pseudo-binary section. Alloys of the primary solidification phases of Te, (Bi,Sb)2Te3, γ, δ, and (Bi,Sb) are determined and the boundaries of various primary solidification phases are delineated. The Bi-Sb-Te liquidus projection is also calculated with the CALPHAD method using preliminary thermodynamic models, and the calculated results are qualitatively in agreement with the experimental determinations.