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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.

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 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.

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.

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.

Pb-alloyed AgSbTe_2 (Pb_xAg_(20)Sb_(30−x)Te_(50) (x = 3, 4, 5 and 6)) composites were synthesized using a modified Bridgman method with a graphite mold to form plate-like samples. The Bridgman-grown specimens were dense, with few solidification cavities, and were sufficiently mechanically robust for a variety of electronic/thermal transport measurements. Inhomogeneity was found on the grain boundary, and was embedded with the nanoprecipitates of δ-Sb_2Te with a feature size of 100 nm of the 5 at.% Pb and 6 at.% Pb specimens. A combined effect of alloying, inhomogeneity and nanoprecipitates leads to a low thermal conductivity of 0.3–0.4 W m^(−1) K^(−1), which approaches the theoretical minimum thermal conductivity of the amorphous material (κ_(min) ∼ 0.36 W m^(−1) K^(−1)). A peak of the zT value, ranging from 0.7 to 0.8, is achieved at 425 K. Further annealing at 673 K increases the grain size and causes a reduction in the value of the zT peak to 0.4.

Regular arrays of nano-wire microstructures containing Ag_2Te rods with diameter

The ternary Ag–Sb–Te system is important for thermoelectric applications because the AgSbTe2 ternary compound has a high figure-of-merit and because Ag–Sb–Te is a subsystem of the very promising quaternary Pb–Ag–Sb–Te thermoelectric alloys. In this study we prepared equilibrated Ag–Sb–Te ternary alloys at 250 and 400 °C. These alloys were metallographically examined, and their microstructures and equilibrium phases were determined. Based on the phase equilibria information available in the literature and the experimental results determined in this study, the isothermal sections of the ternary Ag–Sb–Te system at 250 and 400 °C were constructed. The AgSbTe2 compound is stable at 400 °C but not at 250 °C. Moreover, high-resolution transmission electronic microscopy images of the ternary AgSbTe2 compound and its crystal structure were determined. The crystal structure of AgSbTe2 is rock salt (NaCl type), and its homogeneous compositional regime is determined to be 49.0–53.0 at.% Te and 28.0–30.0 at.% Sb. The solubilities of Ag in the Sb–Te binary compounds, γ and δ, at 400 °C are as high as 8.6 and 5.7 at.%, respectively.