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Electronegative S-filled skutterudite compounds have attracted considerable attention due to scattering of low-frequency phonons (35–50 cm − 1 ), but there are few reports on S-filled double-substituted skutterudite. In this work, a series of S-filled Te-Sn double-substituted skutterudite compounds were synthesized using high pressure and high temperature (HPHT) method. The phase composition, microstructure, and thermoelectric properties of SxCo4Sb 1 1 Te 0. 7 Sn 0. 3 (x = 0 , 0.05, 0.10, 0.20) samples were systematically determined. The results show that the introduction of S increases the substitution limit of Te in the Sb site. Under the combined effect of the sample synthesis pressure and S filling, the nucleation density of the sample grains increases and the grain size decreases. At the same time, there are numerous special microstructures in the samples synthesized in a high-pressure environment. Thermoelectric performance studies show that a small amount of S filling optimizes the Seebeck coefficient of the sample and increases the power factor (PF). With an increase of the amount of S introduced, the thermal conductivity of the samples decreases significantly. Therefore, the maximum zT value of an S 0. 0 5 Co4Sb 1 1 Te 0. 7 Sn 0. 3 sample synthesized at 2.5 GPa pressure is 0.17 at room temperature. The maximum zT value of the sample is 1.26 at 773 K. [ABSTRACT FROM AUTHOR]

A series of graphene-S 0.05 Co 4 Sb 11.6 Te 0.4 (C x -S 0.05 Co 4 Sb 11.6 Te 0.4) composite polycrystalline skutterudite materials with graphene stoichiometric ratio of x = 0, 0.05, 0.10, 0.20 have been successfully prepared by high pressure and high temperature (HPHT) technology. Graphene is a two-dimensional material with large carrier mobility and large specific surface area. Through microscopic observation, it is found that graphene is attached to the grains in the sample. At the same time, with the increase of graphene content, grain growth is inhibited. Graphene addition reduces the thermal conductivity of skutterudite by increasing grain boundaries and achieves the purpose of optimizing its thermoelectric properties. At the same time, there are numerous lattice defects and distortions introduced in skutterudite synthesized by HPHT technology. Finally, the samples were synthesized under the conditions of 1.5 GPa and 900 K. The lattice thermal conductivity of the graphene composite sample C x -S 0.05 Co 4 Sb 11.6 Te 0.4 with x = 0.10 reaches a minimum of 0.99 at 773 K, and the zT value of this sample is 1.25 at 773 K, which is greater than pure S 0. 05 Co 4 Sb 11.6 Te 0.4 , its zT value is 1.00 at 773 K. Compared with the method of synthesizing skutterudite under normal pressure, the HPHT technology can dramatically reduce the reaction time from several days to less than 30 min, while forming a high-pressure airtight reaction environment, which can effectively prevent the volatilization and oxidation of samples during the reaction process, thus providing a convenient method for synthesizing thermoelectric materials quickly and efficiently. [ABSTRACT FROM AUTHOR]

The reaction between sulfur and iron under high pressure and high temperature (HPHT) was studied. Sulfur–iron reaction models under different pressure levels were constructed. The morphology and formation mechanism of the reactants were comprehensively analyzed by scanning electron microscopy, energy-dispersive spectroscopy–line scanning, metallographic microscopy, and Raman spectroscopy. The results indicated that the pressure of the reaction could significantly affect the diffusion behavior of sulfur and iron during the reaction. With an increase in pressure, the diffusion of iron in the system was inhibited, whereas that of sulfur was enhanced. The pressure distribution gradient at the reaction interface was simulated by finite element calculation. The effect of pressure gradient as the driving force of the reaction on the diffusion behavior of elements was evaluated by thermodynamics combined with experimental results. Based on the experimental results, finite element simulation, and formula derivation, a new standpoint was proposed: the diffusion of substances in the HPHT system was affected by the pressure gradient at the interface. [ABSTRACT FROM AUTHOR]

Compared to Te, Ni is found in abundance in the earth's crust. At the same time, the mechanical properties of Ni atom-substituted skutterudite are significantly improved. In this study, a series of S-filled Ni-substituted skutterudite compounds were synthesized by a high-pressure and high-temperature (HPHT) method in the synthesis pressure range of 1.0–3.0 GPa. The phase composition, microscopic morphology, and electrothermal transmission properties of the S x Co 3.6 Ni 0.4 Sb 12 (x = 0, 0.05, 0.10, 0.20) samples were systematically characterized. Phase composition analysis showed that the introduction of S atoms into the intrinsic pores of skutterudite can improve the solid solution limit of Ni atoms in the Co sites of skutterudite. The filling limit of S increased with the synthetic pressure. Moreover, microscopic morphology analysis revealed that the filling of S atoms inhibited the growth of grains. A large number of lattice fringes in different directions were found in the sample, containing abundant microstructures such as lattice distortions and dislocation defects. Compared with the synthesized samples, S 0.05 Co 3.6 Ni 0.4 Sb 12 synthesized at 1.0 GPa had a maximum room temperature power factor of 7.98 × 10−4 Wm−1K−2. The electrical properties of S 0.05 Co 3.6 Ni 0.4 Sb 12 samples stored for 6 months without any protective measures were tested at room temperature. No obvious changes in performance were observed, which proved that the HPHT method can synthesize stable samples. After several thermal cycles, the electrical properties of the S 0.05 Co 3.6 Ni 0.4 Sb 12 sample was tested for variable temperature, and it was found that the sample still had good stability. How the substitution of S atoms with Ni atoms reduces the lattice thermal conductivity can be explained by fitting the Callaway model. The S 0.05 Co 3.6 Ni 0.4 Sb 12 sample synthesized at 1.0 GPa had a maximum zT value of 0.46 at the test temperature of 773 K, which decreased to 0.43 after multiple thermal cycles at the same temperature. [ABSTRACT FROM AUTHOR]

Bi 2 S 3 is considered a promising n-type thermoelectric (TE) material due to its inherent extremely low thermal conductivity and earth-abundant elements. Herein, we have successfully synthesized a series of Nb-doped Bi 2 S 3 (Bi 2-x Nb x S 3 , x = 0, 0.01, 0.02, 0.03) bulk materials by combining mechanical alloying (MA) method and high pressure and high temperature (HPHT) technology. The enhanced electrical transport properties of the Nb-doped Bi 2 S 3 samples is derived from the collaborative optimization of carrier concentration and effective mass, which indicated that the Nb dopants acted as donors. In addition, differences in the direction of the lattice fringes were observed in the samples, and a large number of lattice distortions and dislocation defects occurred, which significantly reduced the lattice thermal conductivity of the Bi 2 S 3 material. Consequently, a peak zT of 0.22 was achieved for the Bi 1.98 Nb 0.02 S 3 sample at 543 K, which is 120% higher than that of pure Bi 2 S 3. • Nb-doped Bi 2 S 3 samples were fabricated via mechanical alloying method followed by high-pressure and high-temperature. • The carrier concentration, carrier mobility, and effective mass of Bi 2 S 3 can be synergistically optimized via Nb doping. • The high synthetic pressure and Nb-doping introduce defects to enhance phonon scattering. • The maximum zT value for Bi 1.98 Nb 0.02 S 3 reached 0.22 at 543 K. [ABSTRACT FROM AUTHOR]