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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]

Transition metal dichalcogenides (TMDs) are uniquely multifunctional materials with optical, electronic, and catalytic properties. Despite the advantages of low cost, low toxicity, and high abundance, the thermoelectric transport properties of MoSe2 were not extensively investigated. Meanwhile, MoSe2 bulk material with 3R phase was rarely reported compared to 2H phase. In this work, controllable phase transition from 2H to 3 R for MoSe2 bulk polycrystalline material was achieved with various Nb contents by a simple and feasible high-pressure method. The preferred orientation resulted in anisotropy of both electrical and thermal transport. The samples converted from n type for pristine sample to p type conduction after Nb doping. Meanwhile, the conduction type gradually changed from semiconductor to degenerated semiconductor. The electrical properties were distinctly improved by Nb doping systematically from the reduced bandgap and the enhanced carrier concentration and mobility. The lattice thermal conductivity was reduced by point defects and grain/phase boundaries generating from Nb doping. Maximum zT of 0.17 at 873 K was obtained for Nb0.04Mo0.96Se2, which is among the highest values for Te-free Mo dichalcogenides. The strategy of chemical doping and high-pressure synthesis provides an alternative route to achieve MoSe2 bulk materials with a controllable 2H/3R phase ratio for potential applications, which can be extended to other TMDs. [ABSTRACT FROM AUTHOR]

Motivated by prototypes that the heavy-element doping can effectively tune carrier and phonon transport behavior, we studied the influence of Bi doping on the thermoelectric properties of Cu2Se synthesized by high-pressure and high-temperature technique. With the increased Bi contents, the carrier mobility of BixCu2Se samples distinctly decreased, while the Seebeck coefficient and electrical resistivity increased. The BixCu2Se samples exhibited multiple microstructures including abundant grain boundaries, micropores and lattice defects. Various phonon scattering mechanisms generated relatively low lattice thermal conductivity below 0.55 Wm−1 K−1 for all Cu2Se-based samples. The lattice thermal conductivity of Cu2Se-based samples increased after Bi doping due to reduced degree of disorder of Cu+ ions, which weakened phonon scattering. Due to the significantly reduced thermal conductivity, a peak zT value of 1.57 at 873 K was obtained for Bi0.005Cu2Se sample, which was 25% higher than that of pristine Cu2Se (zT ~ 1.25). This work indicates the potential of heavy-element doping in boosting performance for liquid-like thermoelectrics. [ABSTRACT FROM AUTHOR]

High-pressure effects can be used to effectively optimize the thermoelectric performance of wide-bandgap oxides. We build upon this finding to show that the synergistic effects of doping with aluminum and graphene composite give excellent thermoelectric performance under high pressure. By employing the parabolic band model, the Pisarenko curves are derived at different pressures, coupled with the band structure of Al-doped ZnO. An analysis indicates that pressure-induced narrowing of the bandgap in Al-doped ZnO reduces the electron excitation energy, thereby optimizing the electrical properties of the samples. The Debye Callaway model is also used to fit the lattice thermal conductivity of the samples, thus enabling an in-depth analysis of the phonon scattering mechanisms. Our analysis suggests that graphene composites significantly enhance point defect scattering and Umklapp scattering, thus increasing the phonon relaxation time and effectively reducing the lattice thermal conductivity of the samples. The improvement in the figure of merit (zT) of ZnO is attributed to the simultaneous enhancement of its electrical properties under high pressure and the synergistic optimization of reducing the lattice thermal conductivity through doping and composites. In this work, we consider a fixed doping ratio of Al and a constant proportion of graphene composite, and analyze the microstructure and thermoelectric properties of 0.1C–ZnAl 0.04 O (C–ZnAlO) samples synthesized under pressures ranging from 1 to 5 GPa, with the aim of elucidating the influence of pressure and graphene composite on the electro-acoustic transport properties of ZnO. We find that the zT value for a sample synthesized at 5 GPa is increased to 0.27 at 973 K. [ABSTRACT FROM AUTHOR]

The chemical composition can directly tune the transport properties of Cu2Se liquid-like materials, including the carrier concentration, carrier mobility and superionic feature. In this work, the off-stoichiometry effects of both the deficiency and excess of Cu on the phase transition and thermoelectric properties of Cu2+δSe (−0.1 ≤ δ ≤ 0.05) compounds are investigated. The Cu2+δSe samples were synthesized by a high-pressure and high-temperature (HPHT) technique in 30 minutes. Cu2+δSe (−0.1 ≤ δ < 0) existed as a mixture of the monoclinic α-Cu2Se phase and cubic β-Cu2Se phase at room temperature, while only the monoclinic α-Cu2Se phase was detected in the Cu2+δSe (0 ≤ δ ≤ 0.05) compounds. Increasing the Cu content can effectively regulate the carrier concentration and reduce the thermal conductivity from 1.45 W m−1 K−1 (δ = −0.1) to 0.47 W m−1 K−1 (δ = 0.05) at 843 K. The ultralow lattice thermal conductivity of 0.3 W m−1 K−1 was achieved for Cu1.97Se and Cu2.0Se by stoichiometry tuning. A maximum zT value of 1.46 at 843 K was obtained for Cu2.0Se, which is 100% higher than that of Cu1.9Se. This work demonstrates the potential of off-stoichiometry for tuning the TE properties of Ag+ and Cu+ based superionic materials. [ABSTRACT FROM AUTHOR]

High-pressure technique is an effective route to synthesize thermoelectric materials and tune transport properties simultaneously. In this work, S-doped copper–selenium compounds ( Cu 2 Se 1 − x S x , 0 ≤ x ≤ 0. 0 5) were successfully synthesized by high-pressure and high-temperature (HPHT) technology in just 30 min. Cu 2 Se 1 − x S x samples show layered morphology composed of abundant pores and lattice defects. The appropriate S introduction (x = 0. 0 1 and 0.03) can effectively enhance Seebeck coefficient and reduce the thermal conductivity of Cu 2 Se. Compared with the pure Cu 2 Se sample, Cu 2 Se 0. 9 7 S 0. 0 3 exhibits a 30% lower thermal conductivity, but the decline of power factor by the distinctly increased electrical resistivity at high temperature results in a smaller zT at temperature > 6 5 0  K. The variations of thermoelectric properties are resulted from the competitive effects between S-doping and actual composition change (Cu:S). It indicates that S-doping is not so effective in improving the zT value of Cu 2 Se materials by high-pressure synthesis. [ABSTRACT FROM AUTHOR]

Thermoelectric materials are widely used in space, solar energy, refrigeration, flexible materials and so on, because they can directly convert electric energy and heat energy into each other. In this paper, the polycrystalline ceramic materials Bi 2 Sr 2 Co 2 O y (abbreviated as BSCO) were prepared by the conventional solid-phase reaction sintering method, and their thermoelectric properties were optimized under the dual action of cationic Pb2+ substitution and nano-SiC dispersion. The change of Seebeck coefficients after Pb doping is different from that in theory, which does not accord with the coupling trend of the decrease of resistivity with the increase of Seebeck coefficients. This is because Pb doping opens a pseudogap in BSCO system, which leads to the increase of the Seebeck coefficient. Thus the electrical transportation properties of the materials are improved. Nano-SiC dispersion can increase phonon scattering, thus improve the thermal transport properties of the materials. Under the condition of 973 K and q = 0.025, the power factor of Bi 1.6 Pb 0.4 Sr 2 Co 2 O y + q wt% SiC (q = 0, 0.025, 0.05, 0.1 and 0.15) system reaches 307 × 10−6 Wm−1K−2, and the dimensionless thermoelectric figure of merit (ZT) value reaches 0.48, which is the maximum in the previously reported BSCO polycrystalline ceramics. • High electrical conductivity and low thermal conductivity are obtained simultaneously by Pb-doping and nano-SiC dispersion. • A high ZT value of 0.48 is achieved in Bi 1.6 Pb 0.4 Sr 2 Co 2 O y + 0.25 wt% SiC at 973 K. • Seebeck coefficients increase because Pb doping opens a pseudogap in BSCO system. [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]

Cu2Se has been considered as a promising thermoelectric material due to its unique structure and excellent performance. Herein, a series of Cu2-xNaxSe (x = 0, 0.01, 0.02, 0.03, and 0.04) polycrystalline samples were synthesized by combining hydrothermal synthesis and hot pressing to investigate the effects of Na-doping on the microstructure and thermoelectric properties of Cu2Se. Compared with the pristine Cu2Se, the Na-doping introduces numerous micro-pores that can optimize the thermal transport performance by strong phonon scattering effects of interfaces between the micro-pores and grains. The doped samples have excellent electrical properties and low thermal conductivity. The maximum value of ZT = 2.1 is obtained at 973 K for the Cu1.96Na0.04Se sample with nominal composition. The results confirm that introducing Na into Cu2Se is an effective and convenient strategy to improve the thermoelectric performance of the Cu2Se alloy by decreasing the lattice thermal conductivity. [ABSTRACT FROM AUTHOR]

In this work, Bi 2 Ba 2 Co 2 O y + v wt% SiC (v = 0.00, 0.025, 0.05, 0.1, 0.2) and Bi 1.975 Na 0.025 Ba 2 Co 2 O y + w wt% SiC (w = 0.00, 0.025, 0.05, 0.1, 0.2) ceramic samples were synthesised. The increase in the carrier concentration caused by a small amount of Na element doping leads to a decrease in the resistivity and Seebeck coefficients. The lattice distortion caused by Na element doping decrease the sample thermal conductivity. After adding SiC nanoparticles, the resistivity can be reduced further. Especially more phonons are scattered by more grain boundaries, which leads to reduced thermal conductivity. The Seebeck coefficients increase slightly due to the low energy carrier scattering effect. Considering the influence of these two factors, dispersing SiC particles on the basis of Na element doping can effectively improve the thermoelectric performance of Bi 2 Ba 2 Co 2 O y -based samples. The ZT value of the Bi 1.975 Na 0.025 Ba 2 Co 2 O y + 0.1 wt% SiC sample reaches a maximum value of 0.21 at 923 K, 26.6% higher than the Bi 1.975 Na 0.025 Ba 2 Co 2 O y sample and 93% higher than the pure Bi 2 Ba 2 Co 2 O y sample. [ABSTRACT FROM AUTHOR]

In this letter, Ni substitution for Co and Ba filling into the voids were simultaneously applied to the CoSb 3 bulks by the high pressure and high temperature (HPHT) method. As the results, the polycrystalline Ba 0.3 Ni 0.15 Co 3.85 Sb 12 skutterudite bulks were successfully synthesized and the systematic thermoelectric properties were investigated. In our experiments, the synthesized time was sharply shortened to 30 min, which is markedly an advantage for mass production. The multiple textures and microstructures were revealed, including vortices-like structures, defects, and lattice dislocations. Remarkably, pressure tuning led to the increase of the Seebeck coefficient and electrical resistivity, yielding an optimized power factor of 11.7 × 10 −4 Wm −1 K −2 in the Ba 0.3 Ni 0.15 Co 3.85 Sb 12 sample prepared at 3 GPa at room temperature, which yielded a 10-fold improvement in power factor and showed distinct reduction of thermal conductivity compared with pure CoSb 3 sample. The maximum zT value 0.91 was obtained at 700 K for Ba 0.3 Ni 0.15 Co 3.85 Sb 12 sample. The experimental results suggest that the effective elements doping combining with HPHT synthesis could optimize electrical and thermal transport properties in skutterudites. [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]

Chalcogenide perovskites are expected to be promising thermoelectric materials, since they not only possess efficient carrier transport and defect tolerance, but also demonstrate unique advantages of high thermodynamic stability, eco-friendly and earth-abundant constituents. Especially, theoretical reports have predicted their "glass-like" thermal conductivities. However, experimental investigation on thermoelectric performances of chalcogenide perovskite BaZrS 3 is extremely scarce due to the difficulty in preparing high-quality bulk samples, which originates from the brittle nature, high melting point, and the large difference in melting points between Ba/S and Zr. In this work, pure phase BaZrS 3 bulks with high relative density reaching 100 % are realized by optimized sulfurization from low-cost BaZrO 3 powders combined with fast spark plasma sintering. A maximum zT value of 0.37 at 623 K in BaZrS 3 bulks is achieved, which is the record-high value among the reported sulfide, halide, and hybrid perovskite materials. A room-temperature electron mobility up to 385 cm2V−1s−1 is among the highest values for perovskites due to the high phase purity, dense morphology and corner-sharing ZrS 6 octahedral three-dimensional network as effective carrier channels. Meanwhile, a measured low lattice thermal conductivity of 1.11 Wm−1K−1 at 623 K is attributed to the intense phonon scattering from the intrinsic distorted-perovskite structure and the lattice defects by sulfur deficiency. Moreover, the BaZrS 3 bulks in this work are stable against moisture/air and high temperature test. This work provides new insights into the fundamental electrical and thermal properties of chalcogenide perovskites, and highlights their great potential in the practical thermoelectric applications. [Display omitted] [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]

• Carbon nanodots dispersant. • Simultaneous optimization of power factors (S2σ) and thermal conductivity (κ). • A high ZT value of ~0.91 at 423 K and averaged ZT of ~0.81 at 320–520 K. With excellent performance at room temperature as well as being the most mature n -type thermoelectric material in commercialization, Bi 2 Te 2.7 Se 0.3 has been attracting increased attention. In this study, its carrier concentration and mobility were optimized by introducing excessive Te. When the excessive content of Te was 0.4, its power factor was the largest, and its thermal conductivity declined slightly. Based on Te excess, carbon nanodots were introduced as the dispersed nanosecond phase. The power factors were improved further due to the energy filter effect. In particular, the lattice thermal conductivity was depressed remarkably due to enhanced phonon scattering. Finally, at 423 K, the maximum ZT value of the sample reached 0.91, which was approximately 42% higher than that of the pure Bi 2 Te 2.7 Se 0.3. The experimental results demonstrate that carbon nanodots dispersion acting as a nanosecond phase is an effective method to improve the thermoelectric performance of n -type Bi 2 Te 2.7 Se 0.3 -based materials. [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]

By embedding organic compound carbon nanodots (CDs) into the inorganic compound Cu 2 Se, a series of Cu 2 Se + x wt% CDs (x = 0, 0.2, 0.4, 0.6, and 0.8) nanopowders are obtained through hydrothermal synthesis and then carried out by hot-pressing. The effects of CDs dispersant on resistivity, Seebeck coefficients, and thermal conductivity are investigated. It is found that the resistivity deteriorates, but the Seebeck coefficient increases greatly due to the energy filtering effect of carbon nanodots in the matrix. Higher Seebeck coefficients also lead to higher power factors compared with the matrix. Importantly, carbon nanodots are well distributed in the matrix without obvious agglomeration, and suppress the re-crystallization of Cu 2 Se. With increasing of the content of CDs, phonon scattering increases due to the newly formed interfaces between CDs dispersant and Cu 2 Se matrix, which reduces thermal conductivity effectively. Correspondingly, the lowest total thermal conductivity reaches 0.45 W/Km at 973 K when x = 0.8. Because of the relatively high power factor and exceptional low thermal conductivity, the Cu 2 Se + 0.8 wt% CDs sample presents an excellent ZT value of 1.98. The present work indicates that CDs are novel and effective dispersants, and could be furtherly extended to use in other thermoelectric materials. • Cu 2 Se + x wt% CDs hybrid materials were synthesized successfully by combining hydrothermal synthesis and hot-pressing. • Exceptional low thermal conductivity is obtained by CDs scattering. • A high ZT value of 1.98 is achieved in Cu 2 Se + 0.8 wt% CDs at 973 K. [ABSTRACT FROM AUTHOR]

Abstract In this study, we optimized the high-temperature parameter of high-pressure and high-temperature (HPHT) for enhancing the thermoelectric properties of TiO 1.76. The phase compositions, morphologies and microstructures were significantly modulated during the optimization process of HPHT. Correspondingly, the electrical properties were improved yet the thermal conductivity reduced compared with other means. As a result, a zT of 0.35 at 700 °C was achieved, outperforming all reported results of nonstoichiometric titanium oxide obtained using ambient pressure. Besides, the mechanical performance was also significantly better than most alloy-based thermoelectric materials. For all these both high-pressure and high-temperature are indispensable and the coordination between them is the key to transforming potential edges into performance advantages. [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]

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]

Clathrate compounds of Ba 8 Cu 6 Ge 8x Si 40−8x (x = 0, 1, 2, 3) were synthesized using a high-temperature, high-pressure (HPHT) method. The study explored chemical doping with Ge at the Si site to optimize the thermoelectric figure of merit in a series of Ba 8 Cu 6 Ge 8x Si 40−8x alloys. The synthesis time was significantly reduced from a few days to 30 min, and the electrical resistivities, Seebeck coefficients and thermal conductivities of the samples were measured in the temperature range of 303–773 K. Among all of the samples, the sample with x = 3 showed the lowest thermal conductivity of 0.91 W m −1 K −1 at 573 K and the maximum the dimensionless figure of merit (ZT) value of 0.42 at 673 K. Combining elemental substitution and HPHT effectively improved the thermoelectric properties of clathrates. [ABSTRACT FROM AUTHOR]

Ba 8 Cu 6 Si 16 Ge 24 clathrate alloys were synthesized by high temperature and high pressure (HPHT) method, and the effect of the different pressures on the structure and thermoelectric properties was investigated. The synthesis time was sharply reduced from a few days to 30 min. The lowest possible thermal conductivity 0.76 Wm −1 K −1 was obtained due to the lattice disorders and dislocations. A relatively good ZT value 0.55 was achieved, which was attributed to both the enhanced power factor and the relatively low thermal conductivity. [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]

In 0.1 Co 4 sb 11 Te 0.8 Ge 0.2 skutterudite alloys were synthesized by high temperature and high pressure (HTHP) method, and the effect of the In filling and Te, Ge co-doping atoms on thermoelectric properties was investigated under different pressures. The synthetic time was sharply reduced from a few days to 30 min. A fairly good ZT value of 1.12 at 773 K was obtained due to both the remarkably enhanced power factor and the low thermal conductivity. [ABSTRACT FROM AUTHOR]

Highlights: [•] We research the synthesized Bi2Te3 at different synthesis pressures. [•] The preferred orientation and texture have been changed by synthesis pressure. [•] The electrical and thermal conductivity have been improved by synthesis pressure. [•] The maximum ZT value of synthesized Bi2Te3 is 0.97 at 365K. [Copyright &y& Elsevier]

• 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]

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]

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]

The Cu 2 Se samples were synthesized by high pressure directly at room temperature in several minutes. The composition evolution under high pressure demonstrates that the critical conditions to synthesize Cu 2 Se are the pressure of 1 GPa and the reaction time of 5 min. The synthetic pressure can effectively tune the morphology, carrier concentration and the electrical transport properties. The low lattice thermal conductivity less than 0.5 Wm−1 K−1 is obtained because of the intrinsic superionic character and the microstructures by high pressure including abundant micropores and lattice defects. A maximum zT of 0.92 at 783 K is achieved for Cu 2 Se synthesized at 1 GPa. This work indicates the potentiality of high pressure technique to further enhance the thermoelectric properties of Cu 2 Se materials. • Synthesizing Cu 2 Se by high-pressure method is time-saving and energy-saving. • The critical condition for synthesizing Cu 2 Se is to keep it for 5 min under 1 GPa. • High pressure can tune crystallinity, microstructure, and reaction process of Cu 2 Se. • High pressure generates a low lattice thermal conductivity of 0.35 Wm−1 K−1. • A maximum zT of 0.92 at 783 K is achieved for Cu 2 Se synthesized at 1 GPa. [ABSTRACT FROM AUTHOR]

The title clathrate is synthesized from a stoichiometric mixture of the elements and BaSi2 (3-5 GPa, 1125 K, 30 min). [ABSTRACT FROM AUTHOR]