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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]

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]

The high stability and low cost are considered comparative advantages of the SrTiO 3 chalcogenide oxide thermoelectric material. However, the pure SrTiO 3 material is characterized by low electrical conductivity and high thermal conductivity, which could eventually lead to poor overall thermoelectric performance, and limit its application in the field of high temperature thermoelectricity. The development of composite-based metal materials with SrTiO 3 by using suitable methods is arising as the most promising approach to improve their electrical properties. Along these lines, in this work, xNb: (1-x) SrTiO 3 composite ceramics were successfully synthesized by using the high temperature and high-pressure (HPHT) method. The Nb powder was oxidized into various niobium oxides exhibiting their own thermoelectric properties under HPHT conditions to optimize the thermoelectric properties of the Nb/SrTiO 3 composites. From our analysis, it was demonstrated that with the increase of the Nb powder content, the electrical properties were significantly improved. In addition, the fabricated Nb/SrTiO 3 composite ceramics possessed a special "pore" microstructure, which significantly reduced the thermal conductivity of the material. In this work, the physical phase, microscopic morphology, and thermoelectric properties of the synthesized samples were also systematically tested and analyzed, and the maximum zT value of 0.28@973 K was obtained for the final sample with x = 0.4. The implementation of the HPHT method for the preparation of chalcogenide oxide composite ceramics provides valuable insights for the improvement of their thermoelectric properties. [ABSTRACT FROM AUTHOR]

SrTiO 3 (STO) is considered a promising high-temperature N-type thermoelectric material. However, its intrinsic insulating properties and high thermal conductivity limit the dimensionless quality factor (zT) due to its simple crystal structure and the existence of light elements. Along these lines, in this work, the optimization of the thermoelectric properties of STO-based materials was achieved by using the high temperature and high pressure (HPHT) method and adjusting the doping ratios of the La and Nb heavy elements. To enhance the doping effect, oxygen vacancies need to be formed within the sample. For this reason, titanium powder was added to the sample. Due to the confined environment induced by the high pressure, which can be attributed to the application of the HPHT method and the strong oxygen trapping ability of Ti at high temperatures, a reducing environment was attained for the samples, the latter, promoted the formation of oxygen vacancies in the synthesized samples, and semiconducting properties were achieved. The thermoelectric properties and microscopic morphology of the synthesized samples with different doping ratios of La and Nb were also thoroughly investigated. Finally, the maximum zT value obtained for the samples was 0.28@973 K. A fast, simple, and effective method for the preparation of oxygen-deficient packet-crystalline oxide materials was developed in this work, which paves the way for significantly improving their thermoelectric properties. [ABSTRACT FROM AUTHOR]

SrTiO 3 (STO)-based perovskite oxide is regarded as a promising high-temperature n-type thermoelectric material. However, its intrinsic high thermal conductivity leads to poor thermoelectric properties. Using entropy engineering, lower thermal conductivity can be obtained. However, the high configuration entropy can also lead to poor carrier mobility, which inhibits electron transport and consequently reduces the electrical conductivity. Along these lines, in this work, an ultra-low thermal conductivity was obtained, which is significantly lower than the majority of the values reported in the literature, and the concept of phononic glass electronic crystal was attained at the same time. The enhanced effective scattering of phonons through the multi-scale defects gives rise to a low thermal conductivity of 1.8 W m−1 K−1 at 973K. Optimization of electrical properties due to in situ reduction at high temperature and pressure，the high-entropy ceramics possess the maximum power factor of 7.03 μW cm−1 K−2 at 973K. The application of high pressure is considered an important approach for the development of new materials with special properties. A new strategy for the composition design and in-situ reduction of oxide thermoelectric materials was provided in this work, which paves the way for the optimization and application of both the electrical and thermal properties of perovskite-based materials. [ABSTRACT FROM AUTHOR]

Because of its high stability and low cost, titanium oxide is a very promising thermoelectric material. Improvement of its thermoelectric performance is limited by the strong correlations among conductivity, Seebeck coefficient, and thermal conductivity. It is therefore important to optimize at least two of these three parameters. This paper provides a new idea for the synergistic optimization of the conductivity and thermal conductivity of titanium oxide. Under high temperature and high pressure, the elementary substance Ta can reduce TiO 2 to Magnèli phase titanium oxide (mainly Ti 8 O 15), which greatly increases its conductivity. At the same time, the submicron tantalum oxide (Ta 2 O 5) generated during the reaction process can effectively reduce the lattice thermal conductivity and total thermal conductivity, and realize the synergistic optimization of the conductivity and thermal conductivity of the material. The titanium-tantalum-oxygen composite thermoelectric ceramics prepared by the high-pressure and high-temperature (HPHT) method have good thermoelectric and mechanical properties. The maximum zT value of the material at 973 K is 0.176, and its Vickers hardness is 11.14. • The addition of elementary substance Ta achieves the synergistic optimization of the conductivity and thermal conductivity of the sample. • The synthesis time is short, and the process is convenient and rapid. • A titanium-tantalum-oxygen composite thermoelectric material is obtained, which has good thermoelectric and mechanical properties. [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]

Ceramic materials with low thermal conductivity are critically important in applications such as thermoelectric conversion and thermal barrier coatings. Introducing the concept of entropy and its singular physical properties permits attainment of low thermal conductivity in ceramics, and the high-pressure synthesis conditions can also reduce the thermal conductivity of the synthesized samples by adjusting the micro-morphology. In this work, high-entropy (Sr 0.2 La 0.2 Nd 0.2 Sm 0.2 Eu 0.2)TiO 3 ceramics were synthesized by solid-state reaction at high temperature and high pressure, which significantly shortened the synthesis time. X-ray diffraction (XRD) results affirmed that high-entropy cubic perovskite ceramics can be synthesized using high temperature and high pressure. High-resolution transmission electron microscopy (HRTEM) results show that the multi-component cations are uniformly distributed at the A-site. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) results show that the pressure provided by the synthesis method can effectively adjust the microstructure of the synthesized samples. Ultra-low thermal conductivity was obtained by synthesizing high-entropy samples under high pressure; the lowest thermal conductivity of resulting high-entropy (Sr 0.2 La 0.2 Nd 0.2 Sm 0.2 Eu 0.2)TiO 3 perovskite ceramics was 0.82 Wm−1 K−1 at the test temperature of 973 K, which is the lower value of perovskite strontium-titanate-based ceramics reported in the literature at present. The high-entropy perovskite ceramics prepared by this new high-temperature and high-pressure method also have good mechanical properties, with Vickers hardness of 10.05 GPa. This study provides a new synthesis strategy for the rapid and effective synthesis of perovskite ceramics with low thermal conductivity, which is a new method for the synthesis of high-entropy materials. • The high-entropy perovskite material synthesized by the high-pressure method has ultra-low lattice thermal conductivity. • High pressure-induced micromorphological changes reduce thermal conductivity. • The whole synthesis procedure last only for 60 min. • Introducing high-pressure synthesis conditions and incorporating high entropy enhances the scattering of all phonon spectra. [ABSTRACT FROM AUTHOR]

The electrical properties of the perovskite oxide thermoelectric material SrTiO 3 (STO) should be improved in order to realize its thermoelectric application due to its intrinsic insulation characteristics. The bulk STO materials usually optimize their electrical properties by annealing in reducing environment after synthesis. However, this method inevitably leads to the gradient distribution of oxygen defect concentration in materials. In this study, the combination of High Temperature and High Pressure (HPHT) method is used, and Ti powder is added to optimize the electrical properties of STO materials. HPHT provides a closed synthesis environment. In addition, with the strong oxygen-capturing ability of Ti at high temperature, the semi-conductivity of materials is performed. The thermoelectric properties of the synthesized materials under HPHT are studied using XRD, XPS, SEM, and TEM. The properties and micro-morphology of the synthesized samples with different Ti contents are investigated. The results show that, with the increase of the Ti content, the oxygen defect concentration increases and the electrical properties are significantly improved. Simultaneously, the sample synthesized using the HPHT method changes its micro-morphology and reduces the thermal conductivity of the material. The increase of oxygen vacancy concentration can also effectively reduce the thermal conductivity. Under the collaborative optimization of HPHT while adding Ti, the maximum obtained zT value at 20 wt% addition is 0.19 @ 973 K. The density functional theory calculation also shows that, compared with pure STO, the band gap of STO with oxygen vacancy decreases, which increases the conductivity. Therefore, a high pressure and the addition of Ti powder provide a fast, simple and efficient method for preparing oxygen-deficient perovskite oxide materials, which is of great significance for improving its thermoelectric properties. • The introduction of oxygen vacancies effectively improves the electrical properties. • High pressure-induced micromorphological changes and generated oxygen vacancies reduce thermal conductivity. • The whole synthesis procedure last only for 30 min • Theoretical calculations show that oxygen vacancies can significantly improve the electrothermal transport properties of perovskite-structured materials. [ABSTRACT FROM AUTHOR]