Search

Higher manganese silicides have evolved as an efficient thermoelectric material because of their exceptional thermoelectric performance at intermediate temperatures. In this work, we report the ultralow-thermal conductivity with improved thermoelectric properties of Mn 1-x Cr x Si 1.8 (x = 0, 0.025, 0.05 and 0.1) prepared by induction melting followed by ball milling and uni-axial induction hot-pressing. Bragg diffraction patterns confirmed that the as-synthesized compound belongs to the Mn 15 Si 26 phase along with the solid solubility of Cr in the HMS lattice. In addition, the backscattered electron micrographs revealed the precipitation of excess Si in the HMS matrix, originating from Si-rich stoichiometry. Furthermore, an elevation in electrical conductivity with increasing Cr content has been observed because of its acceptor dopant behaviour at the Mn site. On the contrary, the Seebeck coefficient decreased upon an increase in Cr content, and bulk MnSi 1.8 has shown the highest Seebeck coefficient of 220 μV/K at 773 K. Impressively, nanostructuring along with the generated point defects as a result of Cr dopant facilitates an intensified phonon scattering at the nanostructured grain boundaries that results in a very low κ total of 0.99 W/mK at 773 K for ball-milled Mn 0.9 Cr 0.1 Si 1.8 , leading to an elevated zT of 0.46 at 773 K. [ABSTRACT FROM AUTHOR]

The effects of Ta doping on the thermal and electrical properties of SnO 2 varistors were researched. The results showed that the thermal conductivity is 2.01 WK−1m−1, the nonlinearity coefficient is 43, and the leakage current density is 0.7 μA/cm2 when the Ta doping level reaches 0.2 mol%. According to the measurement results of the EPR, SEM/EDS, and XRD, the Ta dopant can reduce the oxygen vacancy concentration, which was generated during the sintering. The decline of the oxygen vacancy concentration reduces the phonon scattering, then the thermal conductivity of the varistors is improved. On the other hand, the reduction of the oxygen vacancy concentration elevates the barrier height, so the nonlinearity coefficient is improved and the leakage current density is declined. [ABSTRACT FROM AUTHOR]

In present work, high entropy (MoTaTiVW)C X ceramics with different carbon stoichiometries (X = 3–6) were produced using spark plasma sintering. The microstructure, mechanical properties, and thermal properties of (MoTaTiVW)C X with different carbon stoichiometries were studied. At X = 3, the carbon deficiency resulted in a multiphase structure of the sample, (MoTaTiVW)C 3 with a Mo-rich carbide phase. At X = 3.5, the single-phase rock salt structure was formed despite carbon vacancies of up to 30 %. The relative density of (MoTaTiVW)C X gradually decreases as the carbon stoichiometry increases from X = 3 to X = 6. The concentration of carbon vacancies affects the sintering activation energy, which can enhance mass transfer and promote the densification process. The Vickers hardness of (MoTaTiVW)C X shows an overall decreasing trend as the carbon stoichiometry increases from X = 3.5 to X = 6, peaking at 20.4 ± 0.4 GPa at X = 3.5. The fracture toughness increases and then decreases, peaking at 4.4 ± 0.2 MPa m1/2 at X = 4. The thermal conductivity of (MoTaTiVW)C X gradually increases, then decreases and finally increases with the increase of carbon stoichiometry. At room temperature, the maximum value of thermal conductivity is 6.35 ± 0.29 W/m·K of (MoTaTiVW)C 5 and the minimum value is 5.32 ± 0.35 W/m·K of (MoTaTiVW)C 5.5. At 1000 °C, the thermal conductivity of (MoTaTiVW)C X ranges from 15.94 ± 0.37 W/m·K to 19.56 ± 0.45 W/m·K. As the concentration of carbon vacancies decreases, the decrease in point defect concentration will reduce the degree of lattice distortion, thereby reducing phonon scattering and affecting thermal conductivity. The study suggests that carbon stoichiometry can be used to adjust the microstructure, mechanical properties, and thermal properties of high entropy carbide ceramics. [ABSTRACT FROM AUTHOR]

Texturation and anisotropic performance of Ca 3 Co 4 O 9 -based ceramics fabricated by spark plasma sintering were reported, showing promising thermoelectric properties. With template seeds simultaneous additives, the high degree of texturing was obtained. Electrical resistivity decreased from 16.9 to 6.1 mΩ cm and Seebeck coefficient increased from 75.8 to 187.6 μV/K for the sample with 10 wt% templates, which were dominated by a synergistic combination of the grain orientation and grain boundary density. The obvious anisotropic power factors presented, mainly coming from the texturing characteristic. A low total thermal conductivity of 1.16 W/(m·K) were greatly suppressed by tuning broad wavelength phonon scattering, which was ascribed to the layered nanostructures at grain boundaries. Furthermore, the ZT value of 0.31 was obtained at 1073 K for the sample perpendicular to pressure direction, yielding 138 % enhancement compared with that of the other direction. The work confirmed clear evidence that the textured ceramics with template seeds addition have a great influence on thermoelectricity in oxides materials. [ABSTRACT FROM AUTHOR]

The pyrochlore oxide La 2 Hf 2 O 7 (LHO) has attracted extensive attention in thermal management applications. Accurate determination and comprehensive understanding of thermal conductivity (κ) stand as imperative prerequisites for the effective utilization and manipulation of LHO in various applications. In this study, with the assistance of machine learning interatomic potential, we investigate the microscopic mechanisms of phonon thermal transport in LHO crystals. Our findings show that the thermal transport in LHO defies a precise explication through the conventional framework of the Boltzmann transport equation (BTE) due to the emergence of diffusive phonon modes, engendered by strong anharmonicity. Using the state-of-the-art dual-phonon model incorporating both propagating and diffusive modes, we successfully reproduce the experimentally measured results in both magnitude and temperature dependence. It is also shown that the thermal conduction in LHO crystals is dominated by the propagating modes at low temperatures while by the diffusive modes at high temperatures, which synergistically lead to a low κ for LHO crystals. The novel insights into phonon thermal transport provide a stepping stone for regulating the thermal transport properties of LHO crystals. [ABSTRACT FROM AUTHOR]

BiCuSeO, an oxide thermoelectric material with a superlattice structure, presents the advantages of affordability, low toxicity, and environmental friendliness. However, its intrinsic low electrical conductivity has impeded its possible applications. To address this challenge, we put tiny PbZr 0.52 Ti 0.48 O 3 (PZT) with varying weight percentages (x = 0, 5, 7, 9, 11) into BiCuSeO (BCSO) to fabricate thermoelectric nanocomposites. Through the spark plasma sintering process, PZT particles underwent in-situ decomposition, generating nano-sized second phases such as PbO 2 , TiO 2 , and PbZrO 3 , which are homogenously distributed within BCSO matrix (validated by XRD and TEM). The introduced PbO 2 , TiO 2 , and PbZrO 3 second phases served a dual purpose: inhibiting the grain growth of BCSO and introducing additional nano-grain boundaries. This resulted in more effective phonon scattering sites, thereby reducing lattice thermal conductivity. Simultaneously, the Pb element derived from PZT decomposition was doped into the Bi site of the BCSO matrix, causing a reduction in the band gap of BCSO from 0.49 eV to 0.388 eV and a significant increase in carrier concentration. Consequently, this doping enhanced the electrical conductivity of BCSO-based composites, with the composite with 11 wt% PZT additive amount exhibiting a remarkable 318-fold increase, reaching 550 S/cm compared to pristine BCSO. Notably, the composite with 7 wt% PZT additive amount achieved an exceptionally low lattice thermal conductivity of 0.333 W m−1 K−1 at 823 K. Through the synergistic optimization of electrical and thermal transport properties, the BCSO-based composite with 7 wt% PZT additive amount obtained a ZT value of ∼0.9 at 873 K, representing a 2.45-fold increase compared to pristine BiCuSeO. This straightforward method presents a cost-effective and scalable approach to enhance the thermoelectric performance of various materials, opening new avenues for potential commercial applications. [ABSTRACT FROM AUTHOR]

Clean energy advances can drive solid heat storage technology. Because of their high-temperature, corrosion, and excellent thermal shock resistance properties, refractory oxides are widely used as solid heat storage materials. However, their applications are limited owing to their poor heat-transfer performance resulting from their multicomponent, multiphase, and complex nature and thus understanding their heat-transfer mechanism will be crucial for their performance optimization. In this study, the thermal properties and mechanisms of corundum, high-alumina, and magnesia bricks were characterized using heat-transfer theories and models. Macroscopic analysis of the bricks revealed that they are all "internal porosity" materials. By comparing the weighting parameter j , the multiphase high-alumina brick shows poor heat-transfer path patency and the measured thermal conductivity is the lowest at 3.55 W m−1 K−1. Microscopic analysis found that in contrast to the thermal conductivity of other materials, the thermal conductivity of high-alumina bricks was significantly affected by phonon intrinsic scattering. Moreover, corundum and magnesia bricks exhibited a wider range of ultimate heat-transfer performance than the high-alumina brick. Hence, the heat-transfer performance of refractory oxide can be enhanced by increasing their particle contact, reducing the negative effects of microcracks, phases, and impurities, and by reducing the number of defects on phonon heat transfer, thereby providing an optimization scheme for improving the heat-transfer performance of the materials. [ABSTRACT FROM AUTHOR]

1% vanadium doped ZnO powders have been successfully synthesized via two different preparation techniques viz., glycine assisted combustion method and mechanical attrition method to understand their impact on the structure-property relation. X-ray Diffraction study validates the Wurtzite phase formation of ZnO with hexagonal structure. X-ray diffraction peak discloses the marked role of size and strain component. Evidently, the shift of lattice strain with the preparation techniques from compressive to tensile nature show the importance of structure property relations in the ZnO compounds. SEM micrographs depict the formation of densely populated nanograins with networking like structures which is characteristics of combustion synthesized product whereas as top down approach exhibits the presence of coarse grains amidst the broken sea of fine particles. Selected area electron diffraction (SAED) images display ring and bright circular spot pattern with changes in method of preparation, which further clarifies the distinct nature of investigated ZnO:V particles. Vibrational spectral study gives clear evidence on the structural disorder as deduced from the changes associated with longitudinal optical mode phonon scattering. Interestingly, the UV spectral study divulges the increment in optical band gap as well as a strong visible light absorption (about 30 %) for milled ZnO:V over their chemically prepared counterparts. Photoluminescence spectra of milled V doped ZnO upheld the structural disorder taken place with the suppression of Near Band Edge (NBE) transition and the shifting of broad asymmetric defect emission band towards the lower wavelength regime. Impedance study enlightens the crucial role of synthesis method with 5 times enhanced grain boundary resistance along with suppression of grain contribution in the milled samples. Magnetic study highlights the remarkable distinction of preparation route which alters the ferromagnetic ordering in the combustion synthesized samples to conventional diamagnetic signature of the host ZnO in the milled samples. [ABSTRACT FROM AUTHOR]

SnSe materials have garnered significant interest due to the intrinsic low thermal conductivity. However, its limited electrical conductivity hinders their practical implementation. This study achieved a high ZT value of 1.40 at 773 K in the samples of n-type polycrystalline SnSe 0.95 + x wt% SmCl 3 + y wt% ErCl 3 (x = 0, 0.75, 1.0, 1.25, and 1.5; y = 0, 0.0675, 0.125, and 0.25) since (SmCl 3 , ErCl 3) co-doping decoupled the electrical-thermal transport properties. First, the released electron carrier concentration due to SmCl 3 doping enhanced the electrical transport properties. The SnSe 0.95 + 1.25 wt% SmCl 3 sample exhibited a high power factor of 563 μWm−1K−2 at 773 K. Then, the lattice thermal conductivity markedly decreased with further ErCl 3 doping due to the high-density dislocations and coherent boundary, resulting in effective the phonon scattering. Additionally, the special microtopography was conducive to the decreased lattice thermal conductivity. Therefore, the SnSe 0.95 + 1.25 wt% SmCl 3 + 0.125 wt% ErCl 3 sample achieved a low lattice thermal conductivity value of 0.28 Wm−1K−1 at 773 K. Simultaneously, the (SmCl 3 , ErCl 3) co-doped samples maintained the electrical transport properties than the SmCl 3 -doped samples. Consequently, the SnSe 0.95 + 1.25 wt% SmCl 3 + 0.125 wt% ErCl 3 sample obtained a peak ZT value of 1.40 at 773 K and a competitive average ZT value of 0.37 from 323 to 773 K, and a theoretically calculated conversion efficiency reached 7.2%, which can be applied in the deep space for power generation. This strategy can be utilized to optimize the thermoelectric performance of other thermoelectrical material systems. [ABSTRACT FROM AUTHOR]

High-entropy rare-earth niobates have garnered significant attention in the field of thermal barrier coatings (TBCs) due to their ultra-low thermal conductivity. However, the exceptional thermal properties of fluorite-phase high-entropy rare-earth niobates and the outstanding mechanical properties of monoclinic-phase high-entropy rare-earth niobates cannot be simultaneously harnessed. To address this limitation, this paper investigates and synthesizes five single-phase high-entropy rare-earth niobates: (RE 1/5 Ho 1/5 Er 1/5 Y 1/5 Yb 1/5) 3 NbO 7 (RE = Ce, Sm, Eu, Dy, Lu), and five composite-phase high-entropy rare-earth niobates with a molar ratio of (RE 1/5 Ho 1/5 Er 1/5 Y 1/5 Yb 1/5)NbO 7 : (RE 1/5 Ho 1/5 Er 1/5 Y 1/5 Yb 1/5)NbO 4 = 1:2 (RE = Ce, Sm, Eu, Dy, Lu). The objective is to fabricate niobate coatings that exhibit both outstanding thermal and mechanical properties. The test results reveal that the intrinsic thermal conductivity of the composite-phase samples stands at a mere 1.46 W m−1·K−1 at 1000 °C. Comparatively, the experimental thermal conductivity of the single-phase samples slightly surpasses that of the composite-phase at 1000 °C, registering a minimum of 1.57 W m−1·K−1. Importantly, both values adhere to the TBC standard. The mechanisms behind the low thermal conductivity of the composite-phase and single-phase samples are elucidated by the phonon scattering mechanism and the Cahill model, respectively. Furthermore, the thermal expansion coefficient of the composite-phase samples exceeds that of the single-phase samples overall, and they demonstrate exceptional mechanical properties, offering nearly twice the fracture toughness of the single-phase samples while maintaining high hardness. Experimental evidence shows that composite-phase niobates exhibit enhanced thermodynamic properties compared to single-phase niobates. This work serves as a valuable reference for the utilization of high-entropy rare-earth niobates in the realm of thermal barrier coatings. [ABSTRACT FROM AUTHOR]

The order-disorder transition (ODT) of RE zirconate (RE 2 Zr 2 O 7) has received much attention due to its potential applications in the design of thermal barrier coatings (TBCs). A series of high entropy RE zirconates (RE = La, Sm, Gd, Er and Lu) were designed and synthesized in order to regulate the fluorite-pyrochlore phases, and to investigate the effect of phase composition on the microstructures and performances. The results showed that the sintering temperature, as well as equivalent ion radius, played an important role in regulating the phase transformation of fluorite-pyrochlore. The coexistence of pyrochlore-fluorite phases not only can result in the decrease in grain size but also increased the lattice strain of each phase, thereby enhancing the phonon scattering and contributing to the reduction in the thermal conductivity. The CMAS corrosion resistance was mainly related to phase compositions, and the thicknesses of reaction layer decreased gradually with the increasing fluorite phase content. The results will provide theoretical guidance for the microstructural design of high entropy RE zirconates as next generation TBC materials in future. [ABSTRACT FROM AUTHOR]

Developing high-performance infrared radiation ceramic materials with desired broadband emissivity while reducing thermal conductivity to prevent heat transfer is highly desirable for emerging industrial and aerospace applications. Nevertheless, it remains a grand challenge to simultaneously meet these requirements in existing infrared radiation ceramic materials. Herein, a high-entropy strategy is employed to enhance the high-temperature infrared radiation property with a high emissivity above 0.9 at room temperature and 0.68 at 1200 °C across the entire range of wavelength (1–14 μm), and integrate a low thermal conductivity (<0.88 W m−1 K−1 at 1000 °C) and remarkable mechanical properties. High-entropy rare earth (RE) disilicates ((Y 0.4 Yb 0.4 Tm 0.1 Lu 0.05 Ho 0.05) 2 Si 2 O 7) ceramic coating with high lattice entropy has a more complex electronic structure, inducing lattice distortion and extra multi-mode vibrations, which boosts the emissivity in mid-infrared range (3–14 μm). Meanwhile, the high-entropy strategy prompts the formation of impurity energy levels as gap states, achieving optical absorption at low photon energies, and thus enhancing the emissivity in near-infrared range (1–3 μm). Simultaneously, (Y 0.4 Yb 0.4 Tm 0.1 Lu 0.05 Ho 0.05) 2 Si 2 O 7) ceramic coating owns diffusion-mediated thermal transport properties with strong phonon scattering, assisted further by the lamellar porous structure, thereby enabling a low thermal conductivity. The excellent mechanical properties ensure the reliability of the coating in extreme environments. All these merits render the high-entropy ceramic coating competitive for the development of high-temperature broadband high-emissivity thermal radiation materials. [ABSTRACT FROM AUTHOR]

Bi 1.6 Pb 0.4 Sr 2 Co 2 O 8 samples have been textured by the Laser Floating Zone (LFZ) process using Nd:YAG, and CO 2 laser radiation. Using different wavelengths resulted in significant structural and microstructural modifications. Powder XRD patterns showed that the thermoelectric phase is the major one in both cases. Microstructural studies revealed that all samples presented the same phases but with much lower content of secondary ones in those processed with the CO 2 laser. Electrical resistivity showed different behavior for the two types of samples, being in general, lower for the CO 2 grown rods. Seebeck coefficient is lower for the CO 2 grown samples up to 300 °C, and higher in the high-temperature range, reaching 240 μV/K at 650 °C, which is one of the highest values obtained so far in these compounds. Moreover, thermal conductivity at 600 °C for these samples (0.93 W/K m) is among the lowest reported in the literature. As a consequence, ZT values at 600 °C reached 0.42 in CO 2 textured materials, about two times higher than the obtained in Nd:YAG ones. This value is among the highest reported so far in the literature, and is comparable to the performance attained for the same composition containing nanoparticles addition. All these properties, combined with the fact that the processed materials can be directly integrated into thermoelectric modules, render them highly attractive for industrial production. [ABSTRACT FROM AUTHOR]

Developing highly reliable infrared radiation materials with broadband high emissivity at high temperatures, low thermal conductivity, and excellent thermal stability is highly desirable for aerospace thermal protection applications. However, it remains a huge challenge to take into account infrared radiation heat dissipation and blocking heat transfer through low thermal conduction simultaneously. In this work, we reported a broadband high emissivity Ca2+-doped YbCrO 3 ceramic with emissivity above 0.89 at room temperature across the entire range of wavelength (1–14 μm). This doping strategy leads to the introduction of impurity energy levels into the band gap of YbCrO 3 , which increases the possibility of light absorption to promote electron transition, improving the emissivity of the near-infrared band (1–3 μm). Simultaneously, un-equivalent doping induces electron exchange between chromate ceramic ions, which complicates the electronic structure (producing lattice distortion and extra multi-mode vibrations) and reduces the band gap width, thus boosting the emissivity in the mid-infrared band (3–14 μm). More importantly, (Yb 0.8 Ca 0.2)CrO 3 presents a high emissivity (0.76) at an elevated temperature of 1200 °C, together with low thermal conductivity (2.5 W m−1 K−1 at 1000 °C) due to strong phonon scattering. Moreover, the doping-dominating phase stabilization effect contributes to impressive thermal stability (stable at 1300 °C for 50 h) and a high coefficient of thermal expansion (9.0–9.5 × 10−6 K−1), which makes it suitable for long-term high-temperature thermal protection application. All these merits render the development of thermally stable high-temperature infrared radiation ceramic materials core competitive. [ABSTRACT FROM AUTHOR]

Higher manganese silicide (HMS) is a promising thermoelectric (TE) semiconductor that operates effectively at medium temperatures. It is characterized by its abundance, environmental friendliness, and desirable impact resistance properties. To enhance the TE properties of HMS, this study employs isoelectronic anion and cation codoping, combined with embedded quantum dot (QD) techniques. The introduction of element doping and nanoinclusions, such as Mn 0.96 Re 0.04 (Si 0.96 Ge 0.04) 1.79 +1.5%Ag 2 Pt QDs sample, leads to an approximately 85% increase in electrical conductivity. This boost is attributed to the adjustment of the band structure through Re, Ge, and Ag element doping and the charge transfer facilitated by MnSi, Si, and Pt precipitates. Furthermore, the enhanced PF of 1.85 × 10−3 W m−1 K−2 at 773 K is approximately 29% higher than that of pure HMS. Simultaneously, the increase in point defects hampers the improvement of lattice thermal conductivity (κ l) by intensively scattering short-wave phonons. This effect is complemented by the precipitation of Si, MnSi, Ag, and Pt nanoparticles, which increases the density of grain boundaries, enhances medium-wave phonon scattering, and substantially reduces κ l to 1.45 W m–l K−1 (at 773 K), ∼30% lower than that of pure HMS. The figure of merit of the sample containing 1.5%Ag 2 Pt QDs at 823 K attains a value of 0.69, which is ∼52% and ∼19% higher than that of pure HMS and Re-Ge-double-doped samples, respectively. The incorporation of isoelectronic codoping, along with the integration of metastable QDs, is demonstrated as an effective strategy for enhancing TE properties. [ABSTRACT FROM AUTHOR]

Bismuth telluride-based materials are well known for their excellent thermoelectric (TE) properties within the near-room temperature range. However, the poor thermoelectric performance of their n-type counterparts has posed a major hindrance to the application of bismuth telluride-based thermoelectric generator (TEG). In this study, a dual optimization approach was implemented. Specifically, 2D ceramic nano-sized hexagonal boron nitride (h-BN) was introduced into n-type Bi 2 Te 2.7 Se 0.3 (BTS) to enhance its performance, aligning it with p-type materials. It is indicated that ceramic h-BN decoration leads to the emergence of a high density of dislocations, which boosts the phonon scattering and reduces lattice thermal conductivity significantly. Meanwhile, a high carrier mobility is maintained to ensure an improved power factor. With the addition of moderate ceramic h-BN, the composite material achieved a ZT value of 1.1 at 400 K. The geometric configuration of TEG was optimized using finite element methods to strike the optimal balance between output power (P o u t ) and efficiency (η). Based on the structure optimization results, the predicted P o u t and η increased by 48 % and 36 %, respectively. The fabricated TEG demonstrated to achieve an efficiency of 6.4 %. This work allows the evolution from bismuth telluride-based prototype to a functional TEG advanced in room-temperature heat recovery. [ABSTRACT FROM AUTHOR]

The effect of average feedstock powder size on the evolution of microstructure and thermal conductivity are comprehensively analyzed. It was found that the porosity and thermal conductivity of the LaYbZr 2 O 7 coatings were positively correlated with the average powder size. The conventional notion that higher porosity results in lower thermal conductivity is challenged by the above results. The abnormal porosity and thermal conductivity relationship can be ascribed to the density of horizontal cracks which dominates the thermal conductivity reduction. Reducing the average powder size leads to a greater concentration of horizontal cracks and hence reduced thermal conductivity. Moreover, the LaYbZr 2 O 7 coatings not only show significantly lower initial thermal conductivity of under 0.70 W m−1 K−1, but also exhibit stronger sintering resistance in terms of porosity stability and thermal insulation ability compared to the conventional YSZ coatings. The present work affords a new insight in developing advanced thermal barrier coatings with ultralow thermal conductivity. [ABSTRACT FROM AUTHOR]

Cu 1.8 S-based materials are regarded as promising thermoelectric materials due to their super ionic nature. The Cu 1.8-x Zn x S (x = 0, 0.01, 0.03, and 0.09) samples were synthesized by the hydrothermal method followed by cold pressing and sintering. The powder X-ray diffraction pattern has confirmed the phase purity and rhombohedral crystal structure of Cu 1.8 S. Additionally, FE-SEM micrographs along with the EDX elemental mapping detected the existence of CuS as the secondary phase in the Cu 1.8 S matrix and uniform distribution of the constituent elements. Upon doping the Zn atom, the Seebeck coefficient subsequently increases owing to the decrement in the carrier concentration, whereas the electrical conductivity gradually reduces. Essentially, the presence of CuS secondary phase and the defects induced by the Zn atom effectively scattered low-mid wavelength phonons, where a very low thermal conductivity of 3.65 W/mK was achieved for Cu 1.71 Zn 0.09 S at room temperature. Ultimately, an overall maximum zT of 0.07 at 573 K was observed for Cu 1.77 Zn 0.03 S, which is about 3.5 times greater than that of the pristine sample. [ABSTRACT FROM AUTHOR]

SnSe compound (Nature , 2014 , 508 , 373–377) has become the star in the field of thermoelectric (TE) materials owing to the environmental friendliness, abundant reserves, and excellent performance. In the present work, the electronic structures, thermal and electronic transport, as well as TE properties of a two-dimensional (2D) wrinkled SnSe monolayer are comprehensively investigated in combination with first-principles calculations and Boltzmann transport theory. The SnSe monolayer is an indirect semiconductor with a bandgap of 2.88 eV using Heyd-Scuseria-Ernzerhof (HSE06) hybrid functional. The "multivalley" band structure of SnSe monolayer is not only beneficial for maximizing the power factor, but also leads to low lattice thermal conductivity (κ l) (1.37/1.42 W/mK @ 300 K armchair-/zigzag-direction). The further crystal orbital Hamilton population (COHP) analysis shows that the low κ l in SnSe monolayer is attributed to weak antibonding state below the Fermi level, which in turn weakens the chemical bonding and leads to the mutual exclusion of Sn and Se atoms. As a consequence, the softening of phonon modes and a significant reduction in the phonon group velocity are discovered for the SnSe monolayer, which is beneficial for strong anharmonicity and significant phonon scatterings. Additionally, the electronic transport properties of the 2D SnSe monolayer are evaluated by taking into account the electron-phonon interactions, and the optimal ZT s of 2.13 (armchair-direction) and 3.01 (zigzag-direction) are achieved for n -type and p -type SnSe monolayer at 900 K, respectively. Our present work can not only provide fundamental understanding of thermal and electronic transport properties of wrinkled SnSe monolayer, but also shed some light on the theoretical design of low dimensional SnSe-layered material in thermoelectric applications. [ABSTRACT FROM AUTHOR]

Thermal conductivity is a key property of oxide glass, especially for building applications such as thermal insulation materials and windows or glazed facades. However, this property is difficult to be predicted since it depends on several factors such as the degree of order/disorder and porosity. Here, we report on the effects of crystallization, crystalline phase, and crystal size on the thermal conductivity of a melt-quenched silicate glass. These effects were studied by heat-treating the glass at the onset crystallization temperature for different durations to vary crystallinity in the samples. The results show a general increase in thermal conductivity with crystallinity and crystal size in the nano-range (<75 nm). The growth of devitrite and combeite phases in the glass has a great impact on the thermal conductivity. Interestingly, an anomaly of thermal conductivity is found, i.e., the thermal conductivity of the sample with a relatively low crystallinity of <15 % is reduced by >20 % compared to that of the pure glass phase. This may be attributed to the grain boundaries between amorphous and crystalline phases, which scatter the phonons and thus reduce the thermal conductivity. These results imply that nano-crystallization in window glass might be a useful way to reduce the heat loss from glazed facades in the building envelope. [ABSTRACT FROM AUTHOR]

In this paper, by regulating the A-site doped rare earth elements to achieve the transformation of entropy, the composition of (La 1/3 Nd 1/3 Eu 1/3) 2 Zr 2 O 7 (LNEZ) and (La 1/4 Nd 1/4 Eu 1/4 Y 1/4) 2 Zr 2 O 7 (LNEYZ) medium entropy ceramic system and (La 1/5 Nd 1/5 Eu 1/5 Y 1/5 Gd 1/5) 2 Zr 2 O 7 (LNEYGZ) high entropy ceramic system were designed and prepared. The results show that the thermal and mechanical properties of the ceramics are enhanced as the entropy value increases, and from this it is demonstrated that the crystal structure of the high-entropy LNEYGZ ceramics is the most disordered. Severe lattice distortions have the effect of hindering and enhancing the behavior of elemental expansion, phonon scattering, etc., and have the effect of enhancing the resistance of ceramics to sintering and heat insulation. Moreover, the small grain size enables the ceramic to have an enhanced ability to deflect cracks, and the fracture toughness of the ceramic is thus enhanced. [ABSTRACT FROM AUTHOR]

The electrical transport performance of bulk SrTiO 3 thermoelectric materials can be optimized by incorporating uniformly dispersed conductive inclusions. Herein, the impact of Bi 2 O 3 addition on the thermoelectric properties of La-doped strontium titanate composite nano Ti powder (Sr 0. 875 La 0. 1 TiO 3-δ /20 wt% Ti) has been examined. During the sintering process, Bi 2 O 3 gradually transforms into a liquid phase to facilitate Redox reactions with nano Ti and mass transfer, followed by reduction and transformation into Bi–Bi 2 O 3 particles that precipitate at grain boundaries. The presence of nanoscale Bi-rich phases reduces carrier transport barriers at grain boundaries and enhances phonon relaxation time associated with grain boundary scattering. Ultimately, the sample containing 10 wt% bismuth oxide exhibits the lowest lattice thermal conductivity and the highest weighted mobility. This led to a maximum power factor of 498 μW/mK2 at 873 K and a maximum ZT value of 0.188 at 1073 K. This study presents a novel approach to decouple electroacoustic transport through the utilization of Redox reactions between multiple phases, offering valuable insights for the development of high-performance thermoelectric oxide composites. [ABSTRACT FROM AUTHOR]

To our knowledge, this is the first study to successfully synthesise high-temperature-phase copper selenide (β -Cu 2 Se) at room temperature using rapid microwave hybrid heating (MHH). Controlling the starting Cu/Se ratio is the critical parameter for adjusting the content of α - and β -phases in the as-synthesised sample. The relatively low Cu composition causes impurities to form in the Cu 3 Se 2 phase, deteriorating the thermoelectric (TE) properties of the Cu 2 Se material. The β phase formation at room temperature promotes electrical conductivity. The thermal conductivities of the Cu 2.0 Se samples were 0.5–0.8 Wm−1K−1 at 303–673 K. A strong electronic-phonon interaction may potentially couple electronic thermal conductivity (κ e) and lattice thermal conductivity (κ L), resulting in incomplete separability of κ L and κ e in the β -Cu 2.0 Se sample. The Cu 2.0 Se exhibited a ZT value of 0.65 at 523 K because of its considerably lowered thermal conductivity. [ABSTRACT FROM AUTHOR]

Thermoelectric (TE) materials have attracted considerable interest for their potential and the figure of merit ZT serves as a crucial metric for evaluating TE performance. Chalcogenide materials, such as tellurium (Te) and selenium (Se), have been favored for high-performance TE applications. However, their toxic nature raises concerns, demanding exploration of non-toxic alternatives suitable for human-contacting devices. This study focused on antimony trioxide (Sb 2 O 3) due to its high Seebeck coefficient and explored the potential of layered double hydroxide (LDH) materials as a non-toxic alternative for efficient TE materials. Composites of Sb 2 O 3 nanowires integrated with 2D CaAl-LDH were synthesized to reduce thermal conductivity and improve TE performance. The composites displayed enhanced electrical conductivity, resulting in a significantly improved power factor. The introduction of CaAl-LDH facilitated the charge carrier transport paths, leading to better electrical performance. Furthermore, the composites exhibited reduced thermal conductivity due to phonon scattering at the heterointerfaces between Sb 2 O 3 and CaAl-LDH. As a result, the Sb 2 O 3 /CaAl-LDH_70 composite achieved a maximal ZT value of 0.0485 at 473 K, demonstrating promising potential for LDH-based composites as environmentally friendly TE materials. These findings contribute to sustainable energy conversion technologies, ensuring progress towards more efficient and biocompatible TE materials. [ABSTRACT FROM AUTHOR]

Laser powder bed fusion (LPBF) additive manufacturing, as a novel technique provides broad benefits in thermoelectric materials synthesis, such as enhanced printing speed, reduced waste materials, and customized dimension design. The high energy density of laser could potentially balance the synthesis rate and thermoelectric figure-of-merit zT , but the correlations between laser energy and material performance are still vague. Herein, Bi 0.5 Sb 1.5 Te 3 bulks are directly synthesized by LPBF with a recorded synthesis rate of 254 g h−1. The maximum zT , without any post-processing, reaches 1.1 at 75 °C. Induced nanoscale pores by high laser energy printing, obtain a comparable diameter to phonon mean free path, leading to reduced lattice thermal conductivity and enhanced TE performance. We found that input laser energy critically affected thermoelectric performance and concluded the correlation between volumetric energy density E V and power factor PF , PF = 9.05 E V 2 – 30.01 E V + 48.84. It potentially predicts thermoelectric performance with inputted laser energy density. [ABSTRACT FROM AUTHOR]

SiC has a high theoretical thermal conductivity, excellent mechanical, and corrosion-resistant properties, making it a promising material for heat exchanger applications. However, the thermal conductivity of SiC can't satisfy the requirement at present. This study proposes the strategy of incorporating β-SiC nano-particles (β-SiC np) into SiC through pressureless solid phase sintering (PSS-SiC) to enhance its thermal conductivity, resulting in an increase from 153 W/(m·K) to 207.5 W/(m·K). The increase in thermal conductivity in PSS-SiC by β-SiC np is believed to be primarily caused by the longer phonon mean free path due to high density and large grain size. [ABSTRACT FROM AUTHOR]

Nanostructured phonon-glass thermoelectric materials are highly sought after for heat waste recovery applications. This work introduces a novel phonon-glass multi-phase ceramic material synthesized from sintered mixtures of two sub-stoichiometric tungsten oxides, WO 2.90 and WO 2.72. Notable measurements were recorded for our sample, denoted as 0.1WO 2.72 /0.9WO 2.90 , at 800 °C, with S 2 σ = 0.51 ± 0.03 mWm−1K−2, representing a tenfold increase compared to the base materials. The lattice thermal conductivity, already displaying phonon-glass characteristics, is further suppressed by the addition of the secondary WO 2.72 pentacolumn phase, reaching its lowest value at 0.54 ± 0.25 Wm−1K−1. As a result, the ZT value is enhanced to twice the intrinsic value of the pristine phase. This study demonstrates a novel strategy for improving the thermoelectric performance of oxide-based materials by leveraging a synergistic relationship among several key factors, including phonon-glass nanostructures, tunable electrical properties, and the reduction of lattice thermal conductivity within a homogeneous composite system. [ABSTRACT FROM AUTHOR]

The foremost objective of this work is to employ an entropy strategy to optimize the thermophysical properties of rare-earth (RE) tantalates. The stabilizing single-phase structure of seven-component rare-earth tantalates is verified by density functional theory calculations, and entropy-stabilized (7RE 1/7)TaO 4 with a defective structure is successfully synthesized via spark plasma sintering. (7RE 1/7)TaO 4 exhibits lower intrinsic thermal conductivity over the entire temperature range from 100 to 1200 °C. Furthermore, its intrinsic thermal conductivity (0.83–0.90 W m−1·K−1) is 39–51% and 68–71% lower compared to those of single-RE RETaO 4 and 8YSZ at 1200 °C, respectively, and is even close to the limited thermal conductivity from the Cahill model. This is the result of multiscale phonon scattering by Umklapp phonon-phonon, point defect, dislocation, ferroelastic domain and distorted structures. Moreover, (7RE 1/7)TaO 4 exhibits superior high-temperature phase stability and excellent mechanical properties. Therefore, entropy-stabilized (7RE 1/7)TaO 4 compounds with ultralow thermal conductivity can be used as promising thermal insulating materials. [ABSTRACT FROM AUTHOR]

The amorphous aerogel precursor with a BET-specific surface area of 203.58 m2/g was synthesized by sol-gel and CO 2 supercritical drying using metal chlorides as cation source. It was then calcined at 950 °C to obtain a monolithic defective fluorite (Zr 0.25 Ce 0.25 Hf 0.25 Y 0.25)O 2-δ ceramic aerogel with a grain size of 20 nm and an ultra-low thermal conductivity of 0.054 W m−1 K−1 at room temperature. Bulk (Zr 0.25 Ce 0.25 Hf 0.25 Y 0.25)O 2-δ with porosity of 62.24–66.11% and flexural strength of 2.359–4.487 MPa were synthesized by pressing and sintering. The sintered porous (Zr 0.25 Ce 0.25 Hf 0.25 Y 0.25)O 2-δ ceramic has a low thermal diffusivity of 0.152–0.207 mm2/s and a thermal conductivity of 0.138–0.201 W m−1 K−1 with a thermal transfer behavior distinct from that of micro-grain ceramics, which is caused by the severe lattice distortion, porous structure, and nanoscale phonon boundary scattering. [ABSTRACT FROM AUTHOR]

(La 0.2 Nd 0.2 Sm 0.2 Gd 0.2 Yb 0.2) 2 Zr 2 O 7 (Ln 2 Zr 2 O 7) ceramic powders with nanoscale were synthesized by concurrent chemical coprecipitation method. The thermal behaviors and structural evolution of Ln 2 Zr 2 O 7 precursor were investigated. The synthesis mechanism and thermal conductivity of Ln 2 Zr 2 O 7 ceramic were also discussed in detail. Results show that a kind of complex polymeric compound containing -[Zr–OH–Zr]- and -[Zr–OH-Ln]- network structure was formed in the as-synthesized Ln 2 Zr 2 O 7 precursor, which directly transformed to Ln 2 Zr 2 O 7 crystals through dehydroxylation and structural ordering during the calcination process. The bulk Ln 2 Zr 2 O 7 ceramics have a lower thermal conductivity of 1.20–1.47 W/(m·K), which can be attributed to the high-entropy effect resulting from the introduction of multiple rare earth elements. [ABSTRACT FROM AUTHOR]

In this study, a novel approach is presented which can be utilized to enhance the thermal conductivity and densification of sintered AlN pellets. AlN pellets with a CaO additive were produced by spark plasma sintering. The influence of CaO powder properties such as particle size (micron-, submicron- and nano-scale), dispersion, flowability and structural characteristics, on the mixing behavior of AlN and CaO mixed powders, and on the porosity and thermal conductivity of AlN pellets, were investigated. Additionally, the influence of mixing conditions, including additive content and rotational speed in a novel high-shear mixer (Picomix), were studied and various mixing techniques were compared. The results indicate that when the CaO content of the AlN pellets is increased to 3 wt%, their thermal conductivity rises from 48.1 W/m·K to 112.9 W/m·K. This increase can be attributed to the formation of a liquid phase, and to a reduction in pellet porosity during sintering. Furthermore, when the rotational speed of the mixer is increased to 5000 rpm, the pellets' thermal conductivity further rises to 126.2 W/m·K. This improvement is a result of the enhanced dispersion of CaO in the mixed powder. The shearing forces in the Picomix facilitate the coating of AlN particles with submicron- or nano-scale CaO particles, thereby improving the flowability of the mixed powder and narrowing the particle size distribution. This further improves the liquid phase distribution, thus enhancing the thermal conductivity of the AlN pellets (135.3 W/m·K). The novel high-shear mixer (Picomix) has demonstrated enhanced performance when compared with traditional mixing techniques (ball milling and manual mixing), since it involves a solvent-free process, is faster, has a lower energy consumption, causes less powder-oxidation and leads to a higher value of thermal conductivity, making it an excellent choice for the preparation of mixed powders for solar thermal applications. [ABSTRACT FROM AUTHOR]

Tin sulfide (SnS), as a low-cost and eco-friendly thermoelectric (TE) material, exhibits relatively poor phonon and charge transport capacities when the long-range ordered layered crystal structure is broken by the grain boundaries of polycrystalline. Nevertheless, doping and compositing are effective strategies for improving the electrical and thermal conductivity. Herein, utilizing the composite powders synthesized via Cu-doped SnS nanosheets grown in-situ on the surface of Cu-doped SnS nanofibers, we design a homogeneous nanofibers composited Cu-doped SnS polycrystalline bulk to boost its TE performance. Comparing with pure phase SnS, Cu doping can effectively increase the carrier concentration and mobility of SnS, realizing an increase of electrical conductivity from 4.11 S·cm−1 to 31.57 S·cm−1. Simultaneously, quite a number of fiber/matrix composite interfaces for phonon scattering are formed due to incorporating of composite nanofibers, which significantly reduce the lattice thermal conductivity of Cu-doped SnS polycrystalline bulk. The results show that thermal conductivity reduced as low as 0.64 W·m−1 K−1 at 324 K when the additive amount of nanofibers was fix at 2 wt%. Ultimately, the power factor and ZT value are markedly enhanced at low and medium temperature ranges (300–500 K) compared with SnS polycrystalline bulk without fiber incorporation. This provides an effective strategy for optimizing the thermoelectric performance of SnS-based polycrystalline. [Display omitted] • In-situ growth of SnS-based nanofiber composite thermoelectric materials synthesized using a simple approach. • Cu doping increased the carrier concentration and increased the conductivity from 4.11 S·cm−1 to 31.57 S·cm−1. • The composite SnS fibers reduced the thermal conductivity from 1.38 W·m−1·K−1. to 0.64 W m−1·K−1. [ABSTRACT FROM AUTHOR]

The phase instability of 6–8 wt% Y 2 O 3 partially stabilized ZrO 2 (YSZ) in high temperature and molten salt corrosion environment seriously limits its application above 1200 °C. In this study, the chemical co-precipitation method prepared Yb 2 O 3 and CeO 2 co-doping YSZ (YbCeYSZ) ceramic materials to improve the above issues. It was proposed to improve the defect of high-temperature phase instability of CeO 2 -doped YSZ materials by adding Yb 2 O 3. The phase stability, thermal insulation properties, and Na 2 SO 4 + V 2 O 5 molten salt corrosion behavior of YbCeYSZ ceramic materials were studied. The results showed that 2.5 mol% Yb 2 O 3 -4 mol% CeO 2 -YSZ material (2.5Yb4CeYSZ) had no monoclinic phase (m-ZrO 2) after holding at 1500 °C for 120 h due to forming more defect clusters in the system. Meanwhile, the 2.5Yb4CeYSZ had excellent thermal insulation performance and molten salt corrosion resistance, which was attributed to the strong phonon scattering in the 2.5Yb4CeYSZ material system and the low reactivity between the stabilizer and the corrosive salt. [ABSTRACT FROM AUTHOR]

Due to their superior thermoelectric capabilities, Bi 2 Te 3 -based materials have long been a popular area of study in the thermoelectric field. However, further advancements in the energy conversion efficiency of Bi 2 Te 3 -based thermoelectric modules are constrained by the low figure of merit of n -type Bi 2 Te 3. Here, we describe an efficient method for enhancing the electrical and thermal characteristics of n -type Bi 2 Te 2.7 Se 0.3 (BTS) material by dispersing TiO 2 ceramic nanoparticles (NPs) in the matrix of the BTS. In more detail, the energy potential barrier, created by the matrix and the addition of TiO 2 NPs, causes an energy filtering effect, which in turn raises the Seebeck coefficient. The lattice thermal conductivity is decreased as a result of dislocations at the TiO 2 /BTS interface and an increase in grain boundary density brought on by the fine grain strengthening of TiO 2 ceramic NPs. Last but not least, the ZT max for the BTS + 3 wt% TiO 2 sample increased to 1.31, representing an improvement of 52% over the pristine sample. Additionally, at a maximum temperature differential of 150K, we have built a thermoelectric module with a power output and energy conversion efficiency of 7.2 W and 5.9%, respectively. This study introduces a viable approach towards the rational design of advanced n -type Bi 2 Te 2.7 Se 0.3 materials. [ABSTRACT FROM AUTHOR]

An equimolar quaternary (Gd 0.25 Sm 0.25 Yb 0.25 Y 0.25) 3 TaO 7 high-entropy oxide was synthesised using the sol-gel technique and high-temperature calcination technology. The obtained nanopowder and compact bulk sample are all of the single fluorite phase. The relatively low thermal conductivity (1.170‒1.211W·m‒1·K‒1, 50‒1000 °C) of bulk sample is ascribed to the typical phonon scattering mechanisms and high-entropy distortion effect. The high average thermal expansion coefficient reaches 10.855 × 10–6 K−1 (1000–1400 °C) owing to the relatively weak ionic bond strength and lattice energy. The outstanding phase stability up to 1400 °C is in favour of lengthening the service life at high temperatures. These results reveal that (Gd 0.25 Sm 0.25 Yb 0.25 Y 0.25) 3 TaO 7 high-entropy oxide can be explored as a promising thermal barrier coating material. [ABSTRACT FROM AUTHOR]

Bismuth telluride (Bi 2 Te 3) is a widely used thermoelectric (TE) material in practical applications. However, it contains expensive and toxic tellurium, a promising TE material should include low-cost and non-toxic element. Bismuth selenide (Bi 2 Se 3) is similar to Bi 2 Te 3 in electronic and crystal structure. The electron effective mass of Bi 2 Se 3 is smaller than that of Bi 2 Te 3 , while its electrical transport performance is inferior to that of Bi 2 Te 3. Therefore, in this study, Cl and K co-doped Bi 2 Se 3 thermoelectric material with high density, low thermal conductivity, and excellent thermoelectric performance was prepared using KCl as the solvent. It is found that the effective electron mass and carrier concentration of the doped sample is effectively improved, which significantly increases the doped sample's electrical transport properties and ZT value. Finally, the Bi 2 Se 3 (KCl) 3.5 sample reached a maximum ZT value of 0.79 at 550 K, and the average ZT in the measured temperature range reached 0.45. [ABSTRACT FROM AUTHOR]

In this work, digital light processing (DLP) – common 3D-printed technology – was used to shape yttria-stabilized zirconia (YSZ) ceramic green bodies. Influences of sintering temperature on thermal conductivity of YSZ ceramics were studied systematically. Results showed that thermal conductivity of ceramics increased from 1.89 W/(m·K) to 2.99 W/(m·K) with increasing sintering temperature from 1200 °C to 1600 °C. Porosity of YSZ ceramics decreased from 15.00% to 1.59%, and grain size increased from 0.13±0.03 μm to 0.70±0.20 μm. At lower sintering temperature from 1200 °C to 1400 °C, excessive pores exerted an impact on thermal conductivity due to phonon scattering processes. With further increase in temperature to 1600 °C, the amount of grain boundaries controlled by grain size was main factor affecting thermal conductivity of ceramics. In particular, fewer grain boundaries reduced phonon scattering, resulting in the increase in thermal conductivity. Therefore, sintering temperature is key parameter influencing thermal conductivity of YSZ ceramics through porosity, grain size and grain boundary at different stages. [ABSTRACT FROM AUTHOR]

Using the first-principle simulations and the Boltzmann transport equation, our study investigated the properties of single-layer SnBi 2 Se 4 and PbBi 2 Se 4 , including stability, elasticity, electronic and thermoelectric transport properties. We discovered that both 2D materials have acceptable cleavage energies ranging from 0.27 to 0.28 J/m2 and that they are indirect semiconductors with narrow band gaps of 0.68 eV and 0.94 eV, respectively. Interestingly, the valence band maximum exhibits 'multi-valley' energy dispersion. Furthermore, SnBi 2 Se 4 and PbBi 2 Se 4 have comparable electron and hole mobility of about ∼102 cm2/Vs and ∼103 cm2/Vs, respectively resulting in high conductivity and a high thermoelectric power factor. Owing to low group velocities and strong phonon–phonon scattering rates, the materials exhibit low lattice thermal conductivities of 2.59 W/mK (SnBi2Se 4) and 1.73 W/mK (PbBi 2 Se 4). Thus, they demonstrate high thermoelectric figures of merit, namely 0.31 (SnBi2Se 4) and 0.37 (PbBi 2 Se 4) at 300 K, which rise further to 1.22 and 1.82, respectively, at 700 K. Our results suggest that these two single-layer materials are promising candidates for use in nanoelectronics and thermoelectric appliances. [ABSTRACT FROM AUTHOR]

Tremendous efforts have been devoted to exploring alternative materials to 8 wt% yttria-stabilized zirconia used for thermal barrier coatings. In this work, novel composite ceramic GdPO 4 /GdTaO 4 was designed by introducing GdTaO 4 as second reinforcement phase into GdPO 4 matrix. Mechanical and thermophysical properties of resulting composites were studied as a function of doping concentration of GdTaO 4. Results suggested GdPO 4 /GdTaO 4 composite ceramics consisting of GdPO 4 monazite and GdTaO 4 monoclinic phases, indicating GdPO 4 and GdTaO 4 with maintained excellent chemical compatibility at high temperatures. GdPO 4 /GdTaO 4 composite ceramics achieved superior hardness of 5.82 GPa, fracture toughness of 2.72 MPa m1/2, and thermal conductivity of 1.36 W•m−1•k−1 at GT doping concentration of 50 wt% owing to grain refinement and scattering of phonons caused by the doped GdTaO 4 phase. In sum, excellent mechanical and thermophysical properties of GdPO 4 /GdTaO 4 composite ceramics look potential for thermal barrier coatings materials. [ABSTRACT FROM AUTHOR]

Phonons play a pivotal role in the lattice dynamics and thermal transport properties of AlN epilayers. Revealing the phonon anharmonicity and epilayer-substrate interactions is the most fundamental step to develop high-performance AlN-based optoelectronic devices. We systematically report the temperature-dependent behavior of phonons in AlN epilayers grown on sapphire substrates in the temperature range of 4 – 870 K. The nonlinear frequency redshifts (softening) of the two main phonon modes, E 2 (high) and A 1 (LO), are fitted by our proposed formula with robust generalizability. Strikingly, the weakly anomalous phonon behavior of AlN epilayers and bulk AlN was observed below 100 K, which may be attributed to the negative thermal expansion of AlN crystals. More pronounced phonon softening due to substrate induced strain effect was observed in AlN epilayers than in bulk AlN. This stronger temperature dependence in AlN epilayers can be used as a non-invasive thermal probe to determine the local temperature variation of AlN-based devices. Combined with first-principles calculations, the frequency redshift and linewidth broadening of E 2 (high) and A 1 (LO) modes are further quantified by a physical model involving thermal expansion, phonon anharmonic effects, and substrate effects. Our findings shed insight into phonon-phonon interactions in AlN and provide an indicator for its extreme device applications. This process can also be fundamentally applicable to other emerging epitaxial films and 2D materials. Additionally, the biaxial stress coefficient and pure temperature coefficient of the E 2 (high) mode determined by the contribution of substrate-induced strain effects to the phonon frequency shift are −6.63 ± 0.27 cm−1/GPa and −0.0217 cm−1/K, respectively. These coefficients provide critical information for accurately assessing the impact of temperature and stress on device reliability. [ABSTRACT FROM AUTHOR]

The present work aims to develop a novel thermal barrier coating (TBC) material for potential applications above 1400 °C. Specifically, novel Yb x Sc y Zr 1- x - y O 2 (y = 0.02, 0.04, 0.06; x + y = 0.05, 0.06, 0.07) ceramics with tailored thermal conductivity consist of mixtures of tetragonal and cubic phases. The cubic phase ratio of Yb x Sc y Zr 1- x - y O 2 ceramics gradually increase with an increase of Yb 2 O 3 content. The thermal conductivity of Yb x Sc y Zr 1- x - y O 2 ceramics decreases with the increase of Yb 2 O 3 and Sc 2 O 3 doping. When the total dopant content is kept constant, the thermal conductivity of the specimens with higher Yb 2 O 3 content is lower than those of the specimens with low Yb 2 O 3 content. Quantitative results indicate that the phonon scattering associated with the elastic field around a point imperfection plays a dominant role in the reduction of thermal conductivity. These results suggest that the co-doing of Sc 2 O 3 and Yb 2 O 3 provides a feasible path to achieve low thermal conductivity. [ABSTRACT FROM AUTHOR]

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 CoSb 3 -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. [ABSTRACT FROM AUTHOR]

In this study, we fabricated mechanically deformable thermoelectric sponges comprising transition metal dichalcogenides (TMDs) and polyethyleneimine (PEI) through a layer-by-layer (LBL) self-assembly technique for a thermoelectric power supply for electronic skin. Chemically exfoliated molybdenum sulfide (MoS 2) and niobium diselenide (NbSe 2) were prepared as p- and n-type room-temperature thermoelectric materials, respectively, and deposited on a melamine sponge via electrostatic bonding with PEI to obtain stable mechanical stretchability and low thermal conductivity. Five bilayers of LBL self-assembled thermoelectric sponges exhibited an enhanced thermoelectric performance and figure of merit, which resulted from the improvement in the Seebeck coefficient compared with that of pristine chemically exfoliated TMDs owing to the energy filtering effect and the extremely low thermal conductivity owing to the phonon scattering effect at several created interfaces and the porous structure of the sponge. Additionally, the thermoelectric sponges showed mechanical stability during operation under stretching and compression and mechanical durability over 10,000 cycles under 30% tensile strain. Finally, based on the proposed thermoelectric sponge, a power patch that can be installed on the back of a hand to produce electrical energy in real time was successfully demonstrated. [ABSTRACT FROM AUTHOR]

A novel (Sm 0.2 Lu 0.2 Dy 0.2 Yb 0.2 Y 0.2) 3 TaO 7 (SLT-5RE 0.2) oxide with a single-fluorite structure was synthesized via an optimized sol-gel and sintering method, and its crystal structure, mechanical and thermophysical properties were investigated. The results indicate that the calcined nanoscale powder is of high crystallinity, and bulk sample is of a uniform elemental distribution. Compared to YSZ (6–8 wt.% Y 2 O 3 partially stabilized by ZrO 2), SLT-5RE 0.2 exhibits lower Young's modulus, less mean acoustic velocity, and higher Vickers microhardness. Owing to the strengthened anharmonic vibration and phonon scattering, SLT-5RE 0.2 exhibits low thermal conductivity (1.107 W K−1·m−1, 900 °C). The high thermal expansion coefficient (11.3 × 10−6 K−1, 1200 °C) of SLT-5RE 0.2 ceramic can be ascribed to the reduced lattice energy and ionic spacing as well as the cocktail effect of high-entropy ceramics. The excellent mechanical and thermophysical properties, and excellent phase steadiness during the whole testing temperature cope, indicate that SLT-5RE 0.2 high-entropy ceramic can be a candidate material for thermal barrier coatings. [ABSTRACT FROM AUTHOR]

The high-ordered crystal structure of a thermoelectric material does not delimit the phonon conductivity but shows improved electrical properties. Disordered crystal structures act as scattering centers for phonon, but at the expense of the electrical properties of the material. Hence, a diminished thermal (phonon) conductivity and improved electronic conductivity can be obtained from an incommensurate (IC) phase structure. This paper reports that TlBiSe 2 polycrystal material exhibits incommensurate (IC) phase (disordered structure). The 'Tl+' connecting chains and the disordered structure inherent in the IC phase enhance the TE properties in TlBiSe 2. The crystalline phase was confirmed for TlBiSe 2 by X-ray diffraction (XRD). These polycrystals have shown their characteristic Raman modes and micro flake-like morphology, as observed by Raman spectroscopy and field emission scanning electron microscopy (FESEM). X-ray photoelectron spectroscopy confirmed the IC phase formation by observing the characteristic binding energy shift of 'Tl' and 'Se' states. The parallel plane symmetry break in the IC phase has resulted in a hall carrier concentration (n) of 3.49 × 1020 cm−3 and mobility (μ) of 8.27 m2V−1s−1. Further, Seebeck coefficient showed 80 μV/K; the Wiedemann Franz law calculation reserved a lower electronic thermal conductivity of 0.003381 Wm−1K−1 and a power factor of 2.956 μW/m K2 has been attained. [ABSTRACT FROM AUTHOR]

Bi 2 Sr 2 Co 2 O y is a thermoelectric material with low thermal conductivity. The Bi 2 Sr 2 Co 2 O y /Si 80 Ge 20 composite samples were prepared by solid phase sintering at high temperature to investigate the effects of Si 80 Ge 20 alloys as the second phase on the microstructure and thermoelectric properties of the fabricated composites. An appropriate amount of the dispersed Si 80 Ge 20 in the Bi 2 Sr 2 Co 2 O y matrix can reduce the resistivity of the composite successfully. In particular, the increase in phonon scattering caused by the second phase leads to a significant decrease in thermal conductivity, which improves the thermoelectric properties of the material significantly. At 923 K, the thermal conductivity of the Bi 2 Sr 2 Co 2 O y + 0.2 wt% Si 80 Ge 20 sample achieves an ultralow value of 0.58 W/K·m. Its corresponding optimal dimensionless thermoelectric figure of merit value is 0.36, which is 56% higher than that of the pure Bi 2 Sr 2 Co 2 O y sample. [ABSTRACT FROM AUTHOR]

The electron and phonon thermal transport behavior of Ag + doped KSr 2 Nb 5 O 15 were discussed by using the first-principles calculations. The band gap was reduced after Ag+ doping, and the electrons near the Fermi level had stronger transition capability, which effectively increased the carrier concentration and electrical conductivity and reduced the thermal conductivity, thereby improving the ZT of the doped KSr 2 Nb 5 O 15 from 0.6298 to 0.7214 (1200 K) under ideal conditions. In addition, the solid-state reaction method was used to prepare Ag nanoparticle added KSr 2 Nb 5 O 15 samples, and their thermoelectric performance was tested. The experimental results and the calculated results showed a good consistent trend in which Ag improved the thermoelectric properties of KSr 2 Nb 5 O 15. When the amount of addition of nanosized Ag was 20 wt%, the power factor and ZT of the material were the highest at 1073 K, which were 0.228 mW/(K2·m) and 0.1090, respectively. This research shows how to improve the thermoelectric performance of KSr 2 Nb 5 O 15 ceramics and broaden their temperature range for application. [Display omitted] [ABSTRACT FROM AUTHOR]

Low thermal conductivity plays an essential role in application relevant to thermal energy conversion and management. In this paper, we utilize molecular dynamics to investigate the thermal transport and lattice variation modes in yttria-stabilized hafnia, which only contains binary oxides of Y 2 O 3 and HfO 2. It is found that the thermal conductivity κ of yttria-stabilized hafnia decreases significantly with the increase of doping ratio of Y 2 O 3 , and then reaches a limiting value (∼2.1 W m−1K−1), because of the strong phonon scattering of oxygen vacancies. Importantly, a glass-like thermal conductivity κ is achieved in yttria-stabilized hafnia samples when the content of Y 2 O 3 exceeds 15 mol%. By decomposing the phonon vibrational modes, we find that most of the heat is transported by diffusive modes. As a result, the κ exhibits a glass-like feature in yttria-stabilized hafnia samples with high content of Y 2 O 3. Notably, the κ of yttria-stabilized hafnia is much lower than those of classical functional ceramics materials. The insight into the κ in yttria-stabilized hafnia system is beneficial for understanding and reducing the κ of materials through defect engineering. Despite its simple composition, yttria-stabilized hafnia with different doping ratios demonstrates unexpected high scattering rate of phonon vibration density states, which is confirmed by the diffused wavevector-frequency dispersion. Eigenvector periodicity and phonon participation ratio of phonon have been visualized to capture the distribution of phonon modes in yttria-stabilized hafnia with various dopant. This work investigates into the details of phonon vibrational modes in yttria-stabilized hafnia, which would be valuable for conducting experiments to acquire low thermal conductivity materials in laboratory. [ABSTRACT FROM AUTHOR]

In this paper, thermoelectric (TE) properties of n- type InSb 1-x Bi x (x = 0.00, 0.02, 0.05 and 0.10) polycrystalline samples, synthesized by single-step vacuum melting reaction, have been studied in the temperature range of 300–623 K. P-XRD confirms the phase purity and preferred orientation of (220) plane. FE-SEM back-scattered electron (BSE) micrographs reveal surface morphology and phase homogeneity, and elemental mapping with EDS is used to identify the chemical composition. The elemental existence and oxidation state of In3+, Sb3+, and Bi3+ in InSb 1-x Bi x are rechecked through XPS analysis. With Bi substitution, the electrical conductivity of InSb increases, whereas Seebeck coefficient starts decreasing. However, the maximum Seebeck coefficient, S of −241 μV/K at 423 K, was achieved for the composition of InSb 0.98 Bi 0.02. Interestingly, a huge reduction in lattice thermal conductivity, κ lattice from ∼17.5 W/mK (InSb) to 13.1 W/mK (InSb 0.9 Bi 0.10) at 300 K is observed due to increased phonon scattering from mass-fluctuation and created point defects. Further, the maximum thermoelectric figure of merit, zT was achieved as ∼0.56 at 623 K with higher Vickers micro-hardness values of ∼140 H V for the composition of InSb 0.98 Bi 0.02 which is notably higher than the recently reported InSb based materials. [ABSTRACT FROM AUTHOR]

Bulk and nanoparticles of n -type CuFeS 2 Chalcopyrite have been synthesized using vacuum melting reaction and conventional hydrothermal method, respectively. The powder XRD pattern of both the samples has confirmed phase formation, and the crystallite size was calculated to be ∼30 nm for nano-CuFeS 2. FE-SEM micrographs illustrate the cuboid morphology of nano-CuFeS 2 with particles sizes in the range of 50–200 nm, and EDS analysis confirms the nominal composition. The elemental composition and electronic states of Cu1+, Fe3+, and S2− present in CuFeS 2 are further confirmed by XPS studies. The weak ferromagnetic nature of both bulk and nano-CuFeS 2 with magnetic saturation of ∼0.075 emu/g and ∼0.036 emu/g, respectively, has been observed from VSM measurements. The bulk n -type CuFeS 2 exhibits the Seebeck coefficient of −160 μV/K at room temperature, increasing to −187 μV/K for nano-CuFeS 2. Interestingly, a huge reduction in total thermal conductivity of ∼0.69 W/mK is observed in nano-CuFeS 2 compared to the bulk sample (∼4.18 W/mK) at room temperature, resulting in an overall enhanced thermoelectric figure of merit, zT of ∼0.28 at 623 K for nano-CuFeS 2. Mid-to-long wavelength phonon scattering from the increased grain boundaries and reduced grain sizes of nano-CuFeS 2 and their corresponding reduction in lattice thermal conductivity as a function of temperature. [Display omitted] • Bulk and nano CuFeS 2 samples have been successfully synthesized using vacuum melting and hydrothermal method, respectively. • The mean particle size of nano-CuFeS 2 was calculated to be ∼130 nm. • An increase in Seebeck co-efficient (S) of −305 μV/K at 623 K was observed for nano-CuFeS 2. • Ultra-low thermal conductivity of ∼0.69 W/mK at 623 K was observed for nano-CuFeS 2. • Overall, a maximum figure of merit (zT) of 0.28 at 623 K is achieved for nano-CuFeS 2. [ABSTRACT FROM AUTHOR]