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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]

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]

Introducing the nanosecond phase into thermoelectric materials is an effective strategy to improve their thermoelectric properties. In this study, the influence of nano Y2O3 dispersion on the resistivity, Seebeck coefficient and thermal conductivity of Cu2Se + x mol% Y2O3 (x = 0, 0.5, 1.0, 2.0, 3.0) was investigated. Their resistivity and Seebeck coefficient have no obvious change under the combined effects of the microstructure and the energy filtering. Especially, Y2O3 dispersion can inhibit the recrystallization of Cu2Se and make the lamellar grains thinner, thus scattering phonon and reducing the thermal conductivity effectively. When x = 1.0, the thermal conductivity reaches a minimum of 0.65 W/K·m at 923 K. The dimensionless thermoelectric figure of merit (ZT) value of the Cu2Se + 1.0 mol% Y2O3 sample reaches 1.81 at 923 K. The study indicates that nano rare earth oxides are relatively effective second-phase that can reduce thermal conductivity, which may be further promoted for other thermoelectric materials. [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]

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]

• High pressure increases the solid solubility of Al 2 O 3 in ZnO. • The thermoelectric properties of the sample are explained by the given band gap model under high pressure. • The whole synthesis procedure last only for 30 min. • High pressure can induce rich defects. ZnO has excellent electrical properties such as high conductivity (σ) and Seebeck coefficient (S) that are important for its thermoelectric properties and have provided the basis for obtaining improved ZnO-based thermoelectric materials by doping. In particular, the most representative Al-doped ZnO (AZO) has excellent thermoelectric performance. Although the Al doping is beneficial for reducing the lattice thermal conductivity of ZnO, the solubility of Al 2 O 3 in ZnO is limited and excessive doping of Al gives rise to the generation of a large amount of the ZnAl 2 O 4 secondary phase that limits the further improvement of the electrical properties. Thus, it is crucial to develop approaches to reduce thermal conductivity while still maintaining high electrical properties. In this work, high pressure and high temperature (HPHT) synthesis was used to adjust the ZnO optical band gap by modifying the lattice parameters of ZnO, and thereby improving the electrical properties of the sample. It was found that the optimal doping content for the samples synthesized at high pressure is different from that of the samples synthesized at normal pressure, providing a new approach for the further improvement of the thermoelectric properties of ZnO. Additionally, high pressure can promote grain refinement that is conducive to the generation of the multi-scale hierarchical structure of the sample that introduces a variety of phonon scattering mechanisms to reduce the thermal conductivity while maintaining a high power factor. The Zn 0.96 Al 0.04 O material obtained by sintering at a pressure of 3 GPa and a temperature of 1053 K for 30 min reached the highest power factor (7.8 µW cm−1 K−2) and the highest zT (0.16) at 973 K. [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]

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]

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]

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]

Here, the thermal and electrical properties of Al doped ZnO were synergistically improved by using high temperature and high pressure (HPHT) technology. HPHT treatment to modulate its crystal structure and microstructure. We simulated this process using first-principles calculations and finite element analysis. Increasing the synthesis pressure improved the electrical and thermal properties. As the synthesis pressure increased, the band gap decreased, and the electrical properties were improved. HPHT also changed the microstructure, and full-spectrum phonon scattering provided the sample with a relatively low thermal conductivity. The sample synthesized at 4.5 GPa achieved a dimensionless figure of merit (zT) of 0.165 at 973 K. Compared to related literature, the synthesized Al doped ZnO showed better thermoelectric properties than previous achievements at the same temperature, showing that HPHT provides a promising method for improving its thermoelectric performance. • The grain size and band gaps of ZnO were effective optimized. • Partial high pressure features of ZnO have been captured to the ordinary pressure. • The whole synthesis procedure last only for 30 min. • High pressure can induce rich defects which are beneficial for thermal properties. [ABSTRACT FROM AUTHOR]

Bismuth sulfide (Bi 2 S 3) is a promising non-tellurium candidate for thermoelectric application due to its low thermal conductivity and abundance of low-toxicity elements. In this work, Bi 2 S 3 -based materials are fabricated by combining mechanical alloying, high-pressure and high-temperature treatment, and spark plasma sintering (MA ​+ ​HPHT ​+ ​SPS). Cation and anion doping (copper and chlorine) are then employed to enhance the thermoelectric properties. While chlorine doping can effectively improve the electrical transport properties of Bi 2 S 3 , it results in a corresponding increase in thermal conductivity. Upon copper doping, the electrical properties show a moderate and steady improvement, which also plays a role in reducing thermal conductivity. Thus, the collaborative optimization of electrical and thermal properties is achieved, whereby a maximum zT value of 0.45 is finally obtained for the Bi 2 S 3 -0.25 ​mol% Cu sample at 573 ​K. [Display omitted] • The Bi 2 S 3 materials were fabricated by a MA + HPHT + SPS route. • Chlorine doping can optimize carrier concentration, it results in a corresponding increase in thermal conductivity. • Copper doping significantly suppress lattice thermal conductivity. • A high zT value of 0.45 is achieved in Bi 2 S 3 -0.25 ​mol% Cu at 573 ​K. [ABSTRACT FROM AUTHOR]

The thermoelectric properties of Bi 2 Ba 2 Co 2 O y and Bi 1.975 Na 0.025 Ba 2 Co 2 O y +x wt% carbon nanotubes (CNT; x=0.00, 0.05, 0.10, 0.15, 0.5, and 1.0) ceramic samples synthesised by the solid-state reaction method were investigated from 300K to 950K. Na doping with a small amount played an important role in reducing resistivity and slightly reduced the Seebeck coefficients and the thermal conductivity. The CNT dispersant increased resistivity, but the thermal conductivity was reduced remarkably. In particular, the Bi 1.975 Na 0.025 Ba 2 Co 2 O y +1.0wt% CNT sample exhibited an ultralow thermal conductivity of 0.39 W K −1 m −1 at 923K. This was attributed to the point defects caused by Na doping and the interface scattering caused by the CNT dispersant. The combination of Na doping and CNT dispersion had better effects on thermoelectric properties. The Bi 1.975 Na 0.025 Ba 2 Co 2 O y +0.5wt% CNT sample exhibited a better dimensionless figure of merit ( ZT ) value of 0.2 at 923K, which was improved by 78.2%, compared with the undoped Bi 2 Ba 2 Co 2 O y sample. [ABSTRACT FROM AUTHOR]

Preparation of TiAl x O 1.80-x (x = 0.06, 0.04, 0.02) under high temperature and high pressure (HPHT) can adjust the energy band gap and modify the resistivity of the material, and gives rise to special microstructure and morphology that reduce the phonon thermal conductivity. In this work, we sought to obtain improved the thermoelectric performance of oxide materials by adjusting the doping ratio of Al and Ti in TiAl x O 1.80-x. The phase structures of different samples were studied by X-ray diffraction. The morphology characteristics generated by HPHT synthesis were studied by electron microscopy （FSEM, HRTEM）and the thermoelectric properties of TiAl x O 1.80-x (x = 0.06, 0.04, 0.02) were evaluated. It was found that an appropriate Al and Ti doping ratio combined with the use of HPHT synthesis can reduce the resistivity of the sample and inhibit the rapid increase of thermal conductivity with increasing temperature. The best zT value (0.22) was obtained at the temperature of 973 K for the TiAl 0 · 04 O 1.76 sample. • Prepare samples with different Al and Ti doping ratios under high temperature and high pressure. • The typical non-stoichiometric titanium oxide structure obtained at high temperature improves the thermoelectric performance of the sample. • Adjust the doping ratio of different Al and Ti to obtain the best thermoelectric performance of the sample, which is 10% higher than that of the sample prepared under the same conditions. • The effect of the ratio of doping with Al and Ti on the performance of the samples was studied. [ABSTRACT FROM AUTHOR]