Enhanced thermoelectric performance based on special micro-configuration of graphene-ZnO induced by high-pressure and high-temperature

Zinc oxide (ZnO) is a promising high-temperature thermoelectric material. Graphene is typically a two-dimensional material, and its development and application have attracted wide attention due to its excellent thermal stability and mechanical properties. To the best of our knowledge, the graphene-ZnO (C–ZnO) composite has never been studied in the field of thermoelectric conversion. The high-pressure and high-temperature (HPHT) technique has unique advantages in improving the thermoelectric properties of ZnO. In this study, for the first time, C–ZnO bulk energy materials with novel micro-configuration were prepared by rapid sintering using the HPHT method. Observation under a microscope revealed that as the doping amount of graphene increased, a large number of graphene nanowires formed connected between the ZnO grains, and with the excess amount of graphene introduced the morphology of the ZnO grains changed and their size became smaller. This novel micro-configuration of the 0.1C–ZnO sample showed an ultrahigh electrical conductivity of 2.8 × 10⁴ S/m with a significantly lower lattice thermal conductivity of 4.3 Wm^?1K^?1 at 973 K. Ultimately, at 973 K, the zT value of the 0.1C–ZnO sample was 129 times higher than that of pure ZnO. Therefore, the high-temperature thermoelectric material C–ZnO prepared by the HPHT method can be used in automobile exhaust systems and industrial boilers to effectively recover and reuse the waste heat.     

Synthesis,magnetic and transport properties of oxygen-free CrN ceramics

In the literature, CrN is usually synthetized as thin films and often contains impurities such as oxygen or Cr₂N. We successfully prepared pure CrN powder by ammonolysis of anhydrous chromium (III) chloride, which was confirmed by X-ray diffraction analysis (XRD) and SEM-EDS. CrN ceramics were then prepared by classical ceramic route (CCR) and by spark plasma sintering (SPS) at different sintering conditions. Transport properties (Seebeck coefficient, electrical resistivity and thermal conductivity) were measured in the range 2–600 K and the magnetic behavior was examined by SQUID. Above the room temperature thermal conductivity was obtained by laser flash analysis (LFA). Thermal stability and thermodynamic characteristics were probed by simultaneous thermal analysis (STA). Transport properties are strongly affected by the fabrication process and the resulting figure of merit (ZT) is surprisingly high. CrN has big potential as a new high temperature thermoelectric material.     

Rapid fabrication of Se-modified skutterudites obtained via self-propagating high-temperature synthesis and pulse plasma sintering route

Selenium is an effective dopant in skutterudite-based thermoelectric materials. It strongly influences thermal transport properties due to effective phonon scattering. This study proposes a short-term fabrication route to Se-modified CoSb₃-based materials. Alloy synthesis was conducted via self-propagating high-temperature synthesis. Subsequently, pulse plasma sintering consolidated all materials. As a result, thermoelectric materials with high electrical properties homogeneity were obtained. Seebeck potential mapping showed the measured deviation of the Seebeck coefficient for all fabricated samples was between 5 and 7%. A very low thermal conductivity (1.59 W m^?1 K^?1, at 573 K) was achieved for the highest doped sample, and one of the lowest reported results obtained for bulk skutterudite-based thermoelectric materials ever. This resulted in a low lattice thermal conductivity (1.51 W m^?1 K^?1, at 573 K). This led to the highest ZT (0.27 at 623 K) for the highest doped sample.     

Enhancing thermoelectric properties of p-type (Bi,Sb)2Te3 via porous structures

Bismuth telluride is a widely used commercial thermoelectric material with excellent thermoelectric performances near room temperature. Reducing thermal conductivity is one of the most effective ways to improve performances of thermoelectric materials. In this study, the thermal conductivity of the material was reduced by fabricating porous structures. Highly dense NaCl-(Bi,Sb)₂Te₃ composites were fabricated by a high-pressure technology. The NaCl phase was then removed from the composites by ultrasonic washing to produce porous structures. The produced (Bi,Sb)₂Te₃ porous materials possessed excellent thermoelectric properties. The porosity and pore size of the (Bi,Sb)₂Te₃ porous materials increased with the increasing NaCl content, decreasing the thermal conductivity significantly. An ultra-low lattice thermal conductivity of 0.21 Wm^?1K^?1 at 493 K was achieved when the porosity was 39%, almost the lowest lattice thermal conductivity reported for (Bi,Sb)₂Te₃ bulk materials. The figure of merit ZT value was enhanced to 1.05 at 493 K when the porosity was 25%. Compared with the most compacted samples (ZT = 0.79 and porosity of 10%) prepared under the same conditions, the ZT value of the porous samples increased by 33%. This study indicated that porous thermoelectric materials can be prepared simply, quickly and efficiently by high-pressure/ultrasonication washing to improve thermoelectric performances, which has evident reference values for preparing other thermoelectric pore materials with enhancing behaviors.     

Synergistically optimizing thermoelectric performance of ZnO ceramics by interfacial band alignment and self-doping defects

ZnO is a promising thermoelectric ceramic material due to non-toxicity and abundance in resources. However, its thermoelectric performance is limited by the intrinsic low carrier concentration and high thermal conductivity. In this work, we synthesized the (1 ? x)ZnO/xZnS (x = 0–0.05) powders by a two-step solution method followed by microwave sintering in an oxygen-deficient environment at 1000 ℃, and then produced the self-doped ZnO ceramics with ZnO/ZnS interfaces. The electrical and thermal properties was investigated from room temperature to 900 K. The ZnO/ZnS interface and self-doping significantly increased the electrical properties of ZnO ceramics, the electrical conductivity (σ) and Seebeck coefficient (α) increased simultaneously with temperature for (1 ? x)ZnO/xZnS (x > 0), and the highest power factor (PF, 3675 µW·m^?1·K^?2) was obtained from 0.98ZnO/0.02ZnS at 900 K. At the same time, the ZnO/ZnS interfaces and self-doped defects greatly reduced the lattice thermal conductivity. Finally, the highest ZT value of 0.94 has been reached in 0.95ZnO/0.05ZnS at 900 K.     

Synthesis and characterization of G/TiO1.80 bulk composite thermoelectric material under high temperature and high pressure

The primary purpose of this work is to introduce the second phase of graphene (G) into non-stoichiometric TiO_1.80 successfully and optimize the thermoelectric properties of this composite material through high pressure and high temperature (HPHT) technology. The purpose of doping Ti powder under high pressure is to create a closed reducing atmosphere to change the ratio of titanium to oxygen in the titanium oxide base. The addition of graphene can considerably improve the electrical properties of the material and reduce its resistivity. An X-ray diffractometer, X-ray photoelectron spectrometer, scanning electron microscope, and transmission electron microscope were used to analyze and characterize the phase structure, chemical bond, micro morphology and crystal morphology of the samples. An abundance of grain boundaries and lattice dislocation defects can inhibit the lattice thermal conductivity. We also tested and analyzed the thermoelectric performance of the high-temperature and high-pressure synthetic samples through a variable temperature system. The variation of the absorption intensity of the ultraviolet UV spectrum with wavelength shows that high pressure can reduce the band gap, which is beneficial to the carrier transition and improves the conductivity of semiconductors. HPHT optimizes both the electrical and the thermal parameters of the sample. At a final sintering pressure of 5.0 GPa, the dimensionless figure of merit (zT) of the bulk composite material G/TiO_1.80 was found to be 0.23 at 700 °C.     

High temperature thermoelectric performance of p-type TaRhSn half Heusler compound: A computational assessment

Half Heusler compounds are gaining greater attractions as high temperature thermoelectric materials owing of their giant thermal power and promising thermoelectric performance. In the light of current study, we present a larger library having an overview of structural and physical details including electronic, mechanical, phonon and thermoelectric properties of TaRhSn material within the framework of the density functional theory and semi classical Boltzmann transport approach. The band structure calculations reveal the fact that TaRhSn is an indirect band gap semiconductor with energy gap of 1.25 eV. An in-depth insight on the thermoelectric properties such as the Seebeck coefficient, electrical conductivity, total thermal conductivity and figure of merit has been strictly put along with variation in temperature. The compound has the figure of merit with the maximum numeric value of 0.55 at 1100 K. The value of lattice thermal conductivity is found to be 19.4 W/mK at room temperature and shows a significant reduction in its value towards higher temperatures. These theoretical calculations have the capabilities to stimulate experimental research towards designing and improving the thermoelectric performance of TaRhSn based half Heusler compounds, thus permitting this compound as an efficient thermoelectric material at high temperatures.     

Effect of texturing on thermal,electric and elastic properties of MoAlB,Fe2AlB2, and Mn2AlB2

Herein, MoAlB, Fe₂AlB₂, and Mn₂AlB₂ with textured microstructures were sinter-forged from pre-reacted preforms in a hot press at 1400, 1200, and 1050 °C, respectively. X-ray diffraction and energy dispersive X-ray spectroscopy confirmed the textured nature and high phase purities. The effect of texturing on the thermoelectric properties was evaluated by measuring thermal diffusivities, electrical resistivities, and Seebeck coefficients on the sinter-forged samples and those of less textured, reactively hot-pressed samples. While the thermoelectric properties were essentially isotropic for sinter-forged Fe₂AlB₂ and Mn₂AlB₂, differences included electron-dominated thermal transport in Fe₂AlB₂ and phonon-dominated transport in Mn₂AlB₂ over most of the 323–873 K range. Magnetic phase transitions at 281 K in Fe₂AlB₂ and 308 K in Mn₂AlB₂ were identified in electrical resistivity measurements. MoAlB was found to have anisotropic thermoelectric properties, with notably small relative Seebeck coefficients ranging from ? 2 μV/K at 323 K and increasing to 5 μV/K at 873 K.     

Synthesis,characterization, and thermoelectric properties of nanostructured bulk p-type MnSi1.73, MnSi1.75, and MnSi1.77

P-type higher manganese silicide (HMS) has attracted considerable interest due to its remarkable thermoelectric (TE) properties and potential applications at intermediate and high temperature TE devices. In this study, a series of nanostructured bulk p-type HMS materials with different compositions of MnSi_x (where x=1.73, 1.75 and 1.77) were synthesized via mechanical ball milling and hot-press sintering. The X-ray diffraction analysis of the synthesized materials showed that increasing the Si contents yields to a slight shift to higher diffraction angles. The increase in Si content further resulted in a decrease in electrical conductivity and increase in Seebeck coefficient. The power factor of all three compositions are approximately identical. However, the lowest thermal conductivity was achieved in MnSi_1.75 and resulted in the highest figure-of-merit among all the compositions.     

High‐temperature electrical and thermal transport behaviors in layered structure WSe2

Two‐dimensional transition‐metal dichalcogenides semiconductors (TMDCs) with layered structures have been concerned as promising thermoelectric (TE) materials for several years. However, WSe₂ as one of TMDCs is barely investigated. Herein, we systematically investigated the high‐temperature electrical and thermal transport behaviors in layered structure WSe₂, demonstrating that WSe₂ possesses high Seebeck coefficient and low thermal conductivity. The study of sintering process shows that the best electrical properties can be obtained in the sample sintered at 1123 K. Besides, anisotropic thermoelectric properties were also revealed. It is found that the dimensionless figure of merit (ZT) in the direction parallel to the pressing direction is higher than that in the direction perpendicular to the pressing direction. The highest ZT value of 0.03 is obtained at 923 K, which is an appreciable value for the pristine alloy material.     

Glass-ceramic oxidation protection of higher manganese silicide thermoelectrics

A higher manganese silicide (HMS) thermoelectric, with composition MnSi1.74, densified by spark plasma sintering, was successfully coated with a glass-ceramic, in order to be used at temperatures higher than 500 °C. Compositional changes in both the HMS substrate and the glass-ceramic coating are reviewed and discussed with respect to the electrical properties of the uncoated and coated HMS before and after thermal cycles from RT to 600 °C in air. The formation of a Si-deficient layer (MnSi) on the uncoated HMS surface is due to the reaction between the HMS and oxygen at 600 °C, thus contributing to a lower power factor in comparison with the as-sintered HMS. Coated HMS samples (after thermal cycles RT-600 °C) show a lower electrical resistivity and a significantly higher power factor in comparison with the uncoated ones. The glass-ceramic coating is self-reparable at 600 °C, as demonstrated by the complete sealing of an induced scratch on its surface.     

Isoelectronic Re-Ge-codoped higher manganese silicides with enhanced thermoelectric properties via band optimization,charge transfer,and phonon scattering

Higher manganese silicide (HMS) is a naturally abundant, environmentally friendly, and mechanically robust P-type thermoelectric (TE) semiconductor. Isoelectronic doping elements Re and Ge are used to partially occupy cation (Mn) and anion (Si) sites, respectively. Compared with the single Re-doped strategy, TE performance is further improved in the isoelectronic double-doped Mn_0.96Re_0.04(Si_0.96Ge_0.04)_1.79 composition. The electrical conductivity (∼54.2 × 10³ S m^–1 at 823 K) is double that of pure HMS (∼26.1 × 10³ S m^–1), whereas the power factor increases from ∼1.42 × 10^–3 W m^–1 K^–2 to ∼1.80 × 10^–3 W m^–1 K^–2 due to the increase in carrier concentration caused by the decreased band gap (from 0.82 eV to 0.74 eV), as well as the embedding of the Si, Ge nanodispersions with low work functions. Furthermore, the substitution of Re and Ge increases the amounts of lattice defects, and the presence of nanoscale Ge precipitates cause the broadband phonon-scattering effect in the HMS matrix. Finally, the lattice thermal conductivity at 673 K decreases from ∼2.18 W m^–1 K^–1 to ∼1.60 W m^–1 K^–1. The zT value at 823 K increases by ∼36% from 0.45 to 0.61. The strategy of cation and anion double doping can effectively improve the TE properties of materials, which has significance for the optimization of other materials.     

Enhanced thermoelectric performance of p-type sintered BiSbTe-based composites with AgSbTe2 addition

Bismuth telluride-based materials have been widely used in the field of thermoelectric cooling near room temperature. However, the material utilization and device conversion efficiency were limited by the low thermoelectric performance and poor mechanical properties of commercial zone-melting materials. With an aim to optimize the comprehensive properties, we prepared the composite samples of Bi_0.48Sb_1.52Te₃ (BST)-x wt% AgSbTe₂ (x = 0, 0.05, 0.1, 0.2) via the hot pressing method. It was found that the AgSbTe₂ addition can effectively increase the carrier concentration and improve the power factor to 46 μW cm^?1 K^?2 at 300 K. Due to the introduction of dislocations, stress and Te inhomogeneities, the lattice thermal conductivity of the composite was significantly reduced to 0.69 W m^?1 K^?1 at 325 K. As a result, a maximum ZT of 1.15 at 325 K is obtained for the x = 0.1 sample. Interestingly, BST-0.1 wt% AgSbTe₂ exhibits roughly isotropic thermoelectric performance perpendicular to and parallel to the pressing direction. Our study suggests that the BST-AgSbTe₂ composite is very promising for the application of thermoelectric refrigeration near room temperature.     

Influence of SiC dispersion and Ba(Sr) substitution on the thermoelectric properties of Ca3Co4O9+δ

Ca₃Co₄O_9+δ is a typical p-type thermoelectric oxide material with a low thermal conductivity. In this study, double-layered oxide samples Ca(Ba,Sr)₃Co₄O_9+δ dispersed with different SiC contents were obtained via the traditional solid phase reaction method. The effects of different elemental substitutions and SiC dispersion contents on the microstructure and thermoelectric properties of the samples were studied. The double optimisation of partial substitution of Ca-site atoms and SiC dispersion considerably improved the thermoelectric properties of Ca₃Co₄O_9+δ. Through the elemental substitution, the resistivity of the Ca₃Co₄O_9+δ material was reduced. Conversely, introducing an appropriate amount of SiC nanoparticles enhanced phonon scattering and was crucial in reducing its thermal conductivity. After double optimisations, the dimensionless thermoelectric figure of merit (ZT) values of both Ca_2.93Sr_0.07Co₄O_9+δ + 0.1 wt% SiC and Ca_2.9Ba_0.1Co₄O_9+δ + 0.1 wt% SiC achieved an optimum value of 0.25 at 923 K.     

Achieving weak anisotropy in N-type I-doped SnSe polycrystalline thermoelectric materials

SnSe is a very strong anisotropic material; sometimes, strong anisotropy is unenviable for producing parts of thermoelectric (TE) devices. In order to study the efficient preparation of high-performance n-type polycrystalline SnSe with weak anisotropy, in this work, we combine mechanical alloying at 450 RPM for 10 h and spark plasma sintering at 773 K under 50 MPa pressure for the preparation of polycrystalline SnSe _0.95-xI_x (x = 0,0.01,0.02,0.03) samples, and investigate the TE properties. The prepared samples show very weak anisotropy. With iodine doping, increased carrier concentration is observed, in agreement with DFT calculations. A peak ZT ≈ 1.02 at 723 K is observed with I-doping of x = 0.02, which is about 225% higher than that of undoped sample with ZT ≈ 0.31 at 723 K in parallel direction, mainly attributed to the enhanced power factor and about 56% reduced thermal conductivity from 0.68 Wm^?1K^?1 to 0.30 Wm^?1K^?1. TE properties in both directions are not much different, and the ratios of electrical and thermal conductivities in both directions are very close to unity.     

Effect of annealing temperature in carbon powder on thermoelectric properties of the SrTiO3−δ single crystal in the [111] crystal orientation

SrTiO₃ is a promising thermoelectric material for applications in harsh environments, owing to its excellent thermoelectric (TE) properties and high-temperature stability. The effect of annealing in carbon powders on the TE properties of SrTiO_3?δ in the [111] direction was experimentally investigated and analysed using first-principles calculation. The electrical conductivity of the SrTiO_3?δ single crystals in the [111] direction increases with an increase in annealing temperature in the range of 1573–1673 K, which is opposite to that in the [510] direction as shown in a previous experiment. This is attributed to the different variation trend of carrier concentration. First-principle calculation results show that the carrier concentration in the [111] direction increases with increasing oxygen vacancy concentration, but it is the opposite in the [510] direction. Strong anisotropy of carrier concentration should be caused by the difference in atomic arrangement, making difference in the amount of free electrons that maybe constrained at high oxygen vacancy in different directions. Although the SrTiO_3?δ single crystal annealed at 1573 K shows a lower electrical conductivity, it obtains higher power factor with the maximum value of 2.24 mW m^?1 K^?2 at 323 K, which is predominantly due to its higher effective mass. It also yields the highest ZT value of 0.18 at 873 K owing to the lower thermal conductivity. The results of this study are imperative for the design and development of perovskite TE materials.     

Enhanced thermoelectric efficiency of Cu2−xSe–Cu2S composite by incorporating Cu2S nanoparticles

The study of thermoelectric transport properties in Cu_2−xSe and Cu₂S has gained an importance in the thermoelectric research during last few years due to their superionic behavior and low cost. Here, we reported a facile method to enhance the thermoelectric efficiency of Cu_2−xSe by introducing Cu₂S nanoparticles (NPs) in the matrix of Cu_2−xSe. The observed efficiency is a direct result of simultaneous improvement of Seebeck coefficient (S) because of the external strain induced by Cu₂S nanoinclusions in Cu_2−xSe and decline in the total thermal conductivity by suppressing both electronic and lattice thermal contributions. Such higher S and lower thermal conductivity is realized for a composite structure with 10 wt% nanoinclusions of Cu₂S which effectively contributed to higher ZT value of 0.90 at moderate temperature of 773 K. Thus, it is believed that the proposed hybrid structure is a promising p-type thermoelectric material for mid-temperature range energy harvesting applications.     

Thermoelectric properties of Sb-doped tin oxide by a one-step solid-state reaction

Recently, oxide-based materials have proven to be potential thermoelectric materials at high temperatures. In this work, the thermoelectric properties of one-step solid-state sintered Sn_1-xSb_xO₂ (x = 0, 0.005, 0.01, 0.02, 0.03, 0.04) ceramic pellets were investigated in detail. It was confirmed that the addition of Sb significantly alters the thermoelectric properties of SnO₂ due to the increase in the carrier concentration, which increases the electrical conductivity. The Seebeck coefficient values of all the solid solutions were negative, which indicates that these samples have n-type conduction. The thermoelectric performance of the material was evaluated by determining the zT value and the best composition was Sn_0·98Sb_0·02O₂ with zT ～0.06 at 1073 K.     

Fabrication and high-temperature thermoelectric properties of Ti-doped Sr0.9La0.1TiO3 ceramics

Ti-doped Sr_0.9La_0.1TiO₃ ceramics with high density were successfully prepared in argon atmosphere by conventional solid state reaction. The influences of titanium doping content on the microstructure and thermoelectric properties were investigated. The results showed that titanium was oxidized during the calcination procedure. TiO₂ phase survived and coexisted with Sr_0.9La_0.1TiO₃ phase in the sintered ceramics. The Seebeck coefficients were increased from −163 to −259 μV/K as the temperature increased from 350 K to 1073 K. The thermal conductivity can be significantly reduced by doping Ti. Thermoelectric figure of merit (ZT) first decreased and then increased with increasing Ti doping content. Ceramics showed the best thermoelectric properties when Ti doping amount was 5 wt%, the maximum PF was 7.13 μW/K²/cm, and ZT value was 0.144 at 1073 K.     

Studies of thermoelectric transport properties of atomic layer deposited gallium-doped ZnO

The thermoelectric transport properties of atomic layer deposited (ALD) gallium doped zinc oxide (GZO) thin films were investigated to identify their potential as a thermoelectric material. The overall thermoelectric properties, such as the Seebeck coefficient and electrical conductivity, were probed as a function of Ga concentration in ZnO. The doping concentration was tuned by varying the ALD cycle ratio of zinc oxide and gallium oxide. The GZO was deposited at 250 °C and the doping concentration was modified from 1% to 10%. Sufficient thermoelectric properties appeared at a doping concentration of 1%. The crystallinity and electronic state, such as the effective mass, were investigated to determine the enhancement of the thermoelectric properties. The efficient Ga doping of GZO showed a Seebeck coefficient of 60 μV/K and an electrical conductivity of 1808.32 S/cm, with a maximum power factor of 0.66 mW/mK².