Melt‐spun Eu8Ga16–xGe30+x Clathrates

Intermetallic clathrates are promising materials for thermoelectric applications. This is not only due to their low thermal and high electrical conductivities (“phonon glass – electron crystal”) but also due to the expectation that they are semiconductors and thus have large thermopower values. Band structure calculations of the stoichiometric compound Eu₈Ga₁₆Ge₃₀ indeed yield a semiconducting ground state. However, with conventional synthesis methods the exact 8:16:30 stoichiometry could not be reached. Herein we use the melt‐spinning technique to obtain Eu₈Ga_16–xGe_30+x samples with smaller x than previously realized. The quenching procedure and the results of the characterization of the quenched phases by X‐ray powder diffraction and electron microscopy are presented. The electrical resistivity shows that, in contrast to conventional synthesis procedures, semiconducting compounds can be produced.     

Bulk and surface structure and high-temperature thermoelectric properties of inverse clathrate-III in the Si-P-Te system

The creation of thermoelectric materials for waste heat recovery and direct solar energy conversion is a challenge that forces the development of compounds that combine appreciable thermoelectric figure‐of‐merit with high thermal and chemical stability. Here we propose a new candidate for high‐temperature thermoelectric materials, the type‐III Si_172?xP_xTe_y cationic clathrate, in which the framework is composed of partially ordered silicon and phosphorus atoms, whereas tellurium atoms occupy guest positions. We show that the utmost stability of this clathrate (up to 1500 K) in air is ensured by the formation of a nanosized layer of phosphorus‐doped silica on the surface, which prevents further oxidation and degradation. As‐cast (non‐optimized) Si‐P‐Te clathrates display rather high values of the thermoelectric figure‐of‐merit (ZT=0.24–0.36) in the temperature range of 700–1100 K. These ZT values are comparable to the best values achieved for the properly doped transition‐metal‐oxide materials. The methods of the thermoelectric efficiency optimization are discussed.     

Air‐tolerant Fabrication and Enhanced Thermoelectric Performance of n‐Type Single‐walled Carbon Nanotubes Encapsulating 1,1′‐Bis(diphenylphosphino)ferrocene

The thermally‐triggered n‐type doping of single‐walled carbon nanotubes is demonstrated using 1,1′‐bis(diphenylphosphino)ferrocene, a novel n‐type dopant. Through a simple thermal vacuum process, the phosphine compounds are moderately encapsulated inside single‐walled carbon nanotubes. The encapsulation into SWNTs is carefully characterized using Raman/X‐ray spectroscopy and transmission electron microscopy. This easy‐to‐handle doping with air‐stable precursors for n‐type SWNTs enables the large‐scale fabrication of thermoelectric materials showing an excellent power factor exceeding approximately 240 μW mK^?2.     

Highly Porous Thermoelectric Nanocomposites with Low Thermal Conductivity and High Figure of Merit from Large‐Scale Solution‐Synthesized Bi2Te2.5Se0.5 Hollow Nanostructures

To enhance the performance of thermoelectric materials and enable access to their widespread applications, it is beneficial yet challenging to synthesize hollow nanostructures in large quantities, with high porosity, low thermal conductivity (κ ) and excellent figure of merit (z T ). Herein we report a scalable (ca. 11.0 g per batch) and low‐temperature colloidal processing route for Bi₂Te_2.5Se_0.5 hollow nanostructures. They are sintered into porous, bulk nanocomposites (phi 10 mm×h 10 mm) with low κ (0.48 W m⁻¹ K⁻¹) and the highest z T (1.18) among state‐of‐the‐art Bi₂Te_3−x Se_x materilas. Additional benefits of the unprecedented low relative density (68–77 %) are the large demand reduction of raw materials and the improved portability. This method can be adopted to fabricate other porous phase‐transition and thermoelectric chalcogenide materials and will pave the way for the implementation of hollow nanostructures in other fields.     

Rapid One‐Step Synthesis and Compaction of High‐Performance n‐Type Mg3Sb2 Thermoelectrics

n‐type Mg₃Sb₂‐based compounds have emerged as a promising class of low‐cost thermoelectric materials due to their extraordinary performance at low and intermediate temperatures. However, so far, high thermoelectric performance has merely been reported in n‐type Mg₃Sb₂‐Mg₃Bi₂ alloys with a large amount of Bi. Moreover, current synthesis methods of n‐type Mg₃Sb₂ bulk thermoelectrics involve multi‐step processes that are time‐ and energy‐consuming. Herein, we report a fast and straightforward approach to fabricate n‐type Mg₃Sb₂ thermoelectrics using spark plasma sintering, which combines the synthesis and compaction in one step. Using this method, we achieve a high thermoelectric figure of merit zT of about 0.4–1.5 at 300–725 K in n‐type (Sc, Te)‐co‐doped Mg₃Sb₂ without alloying with Mg₃Bi₂. In comparison with the currently reported synthesis methods, the complexity, process time, and cost of our method are significantly reduced. This work demonstrates a simple, low‐cost route for the potential large‐scale production of n‐type Mg₃Sb₂ thermoelectrics.     

Bismuth Interfacial Doping of Organic Small Molecules for High Performance n‐type Thermoelectric Materials

Development of chemically doped high performance n‐type organic thermoelectric (TE) materials is of vital importance for flexible power generating applications. For the first time, bismuth (Bi) n‐type chemical doping of organic semiconductors is described, enabling high performance TE materials. The Bi interfacial doping of thiophene‐diketopyrrolopyrrole‐based quinoidal (TDPPQ) molecules endows the film with a balanced electrical conductivity of 3.3 S cm^?1 and a Seebeck coefficient of 585 μV K^?1. The newly developed TE material possesses a maximum power factor of 113 μW m^?1 K^?2, which is at the forefront for organic small molecule‐based n‐type TE materials. These studies reveal that fine‐tuning of the heavy metal doping of organic semiconductors opens up a new strategy for exploring high performance organic TE materials.     

Selenium‐Substituted Diketopyrrolopyrrole Polymer for High‐Performance p‐Type Organic Thermoelectric Materials

Development of high‐performance organic thermoelectric (TE) materials is of vital importance for flexible power generation and solid‐cooling applications. Demonstrated here is the significant enhancement in TE performance of selenium‐substituted diketopyrrolopyrrole (DPP) derivatives. Along with strong intermolecular interactions and high Hall mobilities of 1.0–2.3 cm² V^?1 s^?1 in doping‐states for polymers, PDPPSe‐12 exhibits a maximum power factor and ZT of up to 364 μW m^?1 K^?2 and 0.25, respectively. The performance is more than twice that of the sulfur‐based DPP derivative and represents the highest value for p‐type organic thermoelectric materials based on high‐mobility polymers. These results reveal that selenium substitution can serve as a powerful strategy towards rationally designed thermoelectric polymers with state‐of‐the‐art performances.     

Mechanochemical Synthesis and Temperature‐Dependent Optical Properties of Thermochromic (Ag1−xCux)2HgI4

Thermochromic materials are generally synthesized via high‐temperature melting reaction or solution‐based synthesis. Herein, all‐inorganic thermochromic compounds of (Ag_1?xCu_x)₂HgI₄ were synthesized by solvent‐free simple and scalable mechanochemical grinding at room temperature. Temperature‐dependent electronic absorption spectroscopy along with DSC analysis confirmed the thermochromic events within these materials, and the phase transition temperature varied with solid solution compositions. The photoluminescence (PL) spectra is red‐shifted with the increase in the Cu content in (Ag_1?xCu_x)₂HgI₄ (x=0–1).     

Coated/Sandwiched rGO/CoSx Composites Derived from Metal–Organic Frameworks/GO as Advanced Anode Materials for Lithium‐Ion Batteries

Rational composite materials made from transition metal sulfides and reduced graphene oxide (rGO) are highly desirable for designing high‐performance lithium‐ion batteries (LIBs). Here, rGO‐coated or sandwiched CoS_x composites are fabricated through facile thermal sulfurization of metal–organic framework/GO precursors. By scrupulously changing the proportion of Co²⁺ and organic ligands and the solvent of the reaction system, we can tune the forms of GO as either a coating or a supporting layer. Upon testing as anode materials for LIBs, the as‐prepared CoS_x‐rGO‐CoS_x and rGO@CoS_x composites demonstrate brilliant electrochemical performances such as high initial specific capacities of 1248 and 1320 mA h g^?1, respectively, at a current density of 100 mA g^?1, and stable cycling abilities of 670 and 613 mA h g^?1, respectively, after 100 charge/discharge cycles, as well as superior rate capabilities. The excellent electrical conductivity and porous structure of the CoS_x/rGO composites can promote Li⁺ transfer and mitigate internal stress during the charge/discharge process, thus significantly improving the electrochemical performance of electrode materials.     

Water‐Mediated Proton Conduction in a Robust Triazolyl Phosphonate Metal–Organic Framework with Hydrophilic Nanochannels

The development of water‐mediated proton‐conducting materials operating above 100 °C remains challenging because the extended structures of existing materials usually deteriorate at high temperatures. A new triazolyl phosphonate metal–organic framework (MOF) [La₃ L ₄(H₂O)₆]Cl ? x H₂O ( 1 , L ^2?=4‐(4H‐1,2,4‐triazol‐4‐yl)phenyl phosphonate) with highly hydrophilic 1D channels was synthesized hydrothermally. Compound 1 is an example of a phosphonate MOF with large regular pores with 1.9 nm in diameter. It forms a water‐stable, porous structure that can be reversibly hydrated and dehydrated. The proton‐conducting properties of 1 were investigated by impedance spectroscopy. Magic‐angle spinning (MAS) and pulse field gradient (PFG) NMR spectroscopies confirm the dynamic nature of the incorporated water molecules. The diffusivities, determined by PFG NMR and IR microscopy, were found to be close to that of liquid water. This porous framework accomplishes the challenges of water stability and proton conduction even at 110 °C. The conductivity in 1 is proposed to occur by the vehicle mechanism.     

Plasmapolymerisierte Organosilane – Photolumineszenz,Infrarotabsorption und Struktur

Plasma Polymerized Organosilanes – Photoluminescence, Infrared Absorption and Structure The structure of amorphous hydrogen‐containing silicon‐carbon thin films (a‐Si_1–xC_x : H) is difficult to access as known for disordered materials. In this paper we attempt to develop structure models for a‐Si_1–xC_x : H from photoluminescence and infrared transmission, and to support these models through molecular dynamic and LCAO calculations. The modelling is further based on the knowledge of the starting materials in the plasma.     

Potential of Chevrel phases for thermoelectric applications

A low lattice thermal conductivity is one of the requirements to achieve high thermoelectric figures of merit. Several low thermal conductivity materials were identified and developed over the past few years at the Jet Propulsion Laboratory (JPL), including filled skutterudites and Zn₄Sb₃-based materials. A study of the mechanisms responsible for the high phonon scattering rates in these compounds has demonstrated that materials with structures that can accommodate additional atoms in their lattice are likely to possess low lattice thermal conductivity values. Chevrel phases (Mo₆Se₈-type) are just such materials and are currently being investigated at JPL for thermoelectric applications. The crystal structures of the Chevrel phases present cavities which can greatly vary in size and can contain a large variety of atoms ranging from large ones such as Pb to small ones such as Cu. In these materials, small inserted atoms usually show large thermal parameters which indicate that they move around and can significantly scatter the phonons. The electronic and thermal properties of these materials can potentially be controlled by a careful selection of the filling element(s). We have synthesized (Cu, Cu/Fe, Ti)_xMo₆Se₈ samples and report in this paper on their thermoelectric properties. Approaches to optimize the properties of these materials for thermoelectric applications are discussed. Solid State Sciences, 1293-2558/99/7-8/© 1999 Éditions scientifiques et médicales Elsevier SAS. All rights reserved.     

All‐Inorganic Nanocrystals as a Glue for BiSbTe Grains: Design of Interfaces in Mesostructured Thermoelectric Materials

Nano‐ and mesostructuring is widely used in thermoelectric (TE) materials. It introduces numerous interfaces and grain boundaries that scatter phonons and decrease thermal conductivity. A new approach has been developed for the rational design of the interfaces in TE materials by using all‐inorganic nanocrystals (NCs) that serve as a “glue” for mesoscopic grains. For example, circa 10 nm Bi NCs capped with (N₂H₅)₄Sb₂Te₇ chalcogenidometallate ligands can be used as an additive to BiSbTe particles. During heat treatment, NCs fill up the voids between particles and act as a “glue”, joining grains in hot‐pressed pellets or solution‐processed films. The chemical design of NC glue allowed the selective enhancement or decrease of the majority‐carrier concentration near the grain boundaries, and thus resulted in doped or de‐doped interfaces in granular TE material. Chemically engineered interfaces can be used as to optimize power factor and thermal conductivity.     

Superior Na‐Storage Performance of Low‐Temperature‐Synthesized Na3(VO1−xPO4)2F1+2x (0≤x≤1) Nanoparticles for Na‐Ion Batteries

Na‐ion batteries are becoming comparable to Li‐ion batteries because of their similar chemical characteristics and abundant sources of sodium. However, the materials production should be cost‐effective in order to meet the demand for large‐scale application. Here, a series of nanosized high‐performance cathode materials, Na₃(VO_1?xPO₄)₂F_1+2x (0≤x≤1), has been synthesized by a solvothermal low‐temperature (60–120 °C) strategy without the use of organic ligands or surfactants. The as‐synthesized Na₃(VOPO₄)₂F nanoparticles show the best Na‐storage performance reported so far in terms of both high rate capability (up to 10 C rate) and long cycle stability over 1200 cycles. To the best of our knowledge, the current developed synthetic strategy for Na₃(VO_1?xPO₄)₂F_1+2x is by far one of the least expensive and energy‐consuming methods, much superior to the conventional high‐temperature solid‐state method.     

Microemulsion‐Assisted Preparation of a Mesoporous Ferrihydrite/SiO2 Composite for the Efficient Removal of Formaldehyde from Air

Mesoporous ferrihydrite/SiO₂ composites were synthesized according to a water‐in‐oil microemulsion method and characterized by X‐ray diffraction, scanning electron microscopy, transmission electron microscopy, Fourier transform infrared spectroscopy, thermogravimetry, nitrogen‐adsorption/desorption, and by X‐ray photoelectron spectroscopy. The as‐prepared porous ferrihydrite/SiO₂ composites showed an excellent adsorption performance for formaldehyde (HCHO) removal from indoor air at ambient temperature. It was found that the aging time during the synthesis had a significant impact on the pore structure, surface area, and HCHO adsorption of these materials. The ferrihydrite/SiO₂ composite that was aged for 3 h in the presence of tetraethyl orthosilicate (TEOS) exhibited a relatively high HCHO adsorption capacity, as well as good recyclability, which was attributed to a relatively large BET surface area, optimal pore size, a suitable Si/Fe atomic ratio, and a synergistic effect between ferrihydrite and SiO₂. This work not only demonstrates that porous ferrihydrite/SiO₂ composites can act as an efficient adsorbent toward HCHO, but suggests a new route for the rational design of cost‐effective and environmentally benign adsorbents with high performance for indoor air purification.     

Engineered Molecular Chain Ordering in Single‐Walled Carbon Nanotubes/Polyaniline Composite Films for High‐Performance Organic Thermoelectric Materials

Single‐walled carbon nanotubes (SWNTs)/polyaniline (PANI) composite films with enhanced thermoelectric properties were prepared by combining in situ polymerization and solution processing. Conductive atomic force microscopy and X‐ray diffraction measurements confirmed that solution processing and strong π–π interactions between the PANI and SWNTs induced the PANI molecules to form a highly ordered structure. The improved degree of order of the PANI molecular arrangement increased the carrier mobility and thereby enhanced the electrical transport properties of PANI. The maximum in‐plane electrical conductivity and power factor of the SWNTs/PANI composite films reached 1.44×10³ S cm^?1 and 217 μW m^?1 K^?2, respectively, at room temperature. Furthermore, a thermoelectric generator fabricated with the SWNTs/PANI composite films showed good electric generation ability and stability. A high power density of 10.4 μW cm^?2 K^?1 was obtained, which is superior to most reported results obtained in organic thermoelectric modules.     

Creating Zipper‐Like van der Waals Gap Discontinuity in Low‐Temperature‐Processed Nanostructured PbBi2nTe1+3n: Enhanced Phonon Scattering and Improved Thermoelectric Performance

Nanoengineered materials can embody distinct atomic structures which deviate from that of the bulk‐grain counterpart and induce significantly modified electronic structures and physical/chemical properties. The phonon structure and thermal properties, which can also be potentially modulated by the modified atomic structure in nanostructured materials, however, are seldom investigated. Employed here is a mild approach to fabricate nanostructured PbBi_2nTe_1+3n using a solution‐synthesized PbTe‐Bi₂Te₃ nano‐heterostructure as a precursor. The as‐obtained monoliths have unprecedented atomic structure, differing from that of the bulk counterpart, especially the zipper‐like van der Waals gap discontinuity and the random arrangement of septuple‐quintuple layers. These structural motifs break the lattice periodicity and coherence of phonon transport, leading to ultralow thermal conductivity and excellent thermoelectric z T.     

Structure Design and Performance of LiNixCoyMn1‐x‐yO2 Cathode Materials for Lithium‐ion Batteries: A Review

Lithium‐ion batteries are now considered to be the technology of choice for future hybrid electric and full electric vehicles to address global warming. One of the challenges for improving the performance of lithium ion batteries to meet increasingly demanding requirements for energy storage is the development of suitable cathode materials. The recent advancement of lithium nickel cobalt manganese oxides are investigated as advanced positive cathode materials for lithium‐ion batteries. This review aims at providing the reader with an understanding of the critical scientific challenges facing the development of LiNi_xCo_yMn_1‐x‐yO₂ materials, the latest developments in crystal structure, synthesis methods, and structure designs to unravel the mechanisms of charge and mass transport processes associated with battery performance, and the outlook for future‐generation batteries that exploit gradient structures materials for significantly improved performance to meet the ever‐increasing demands of emerging technologies.     

Cobalt‐Doped FeS2 Nanospheres with Complete Solid Solubility as a High‐Performance Anode Material for Sodium‐Ion Batteries

Considering that the high capacity, long‐term cycle life, and high‐rate capability of anode materials for sodium‐ion batteries (SIBs) is a bottleneck currently, a series of Co‐doped FeS₂ solid solutions with different Co contents were prepared by a facile solvothermal method, and for the first time their Na‐storage properties were investigated. The optimized Co_0.5Fe_0.5S₂ (Fe0.5) has discharge capacities of 0.220 Ah g^?1 after 5000 cycles at 2 A g^?1 and 0.172 Ah g^?1 even at 20 A g^?1 with compatible ether‐based electrolyte in a voltage window of 0.8–2.9 V. The Fe0.5 sample transforms to layered Na_xCo_0.5Fe_0.5S₂ by initial activation, and the layered structure is maintained during following cycles. The redox reactions of Na_xCo_0.5Fe_0.5S₂ are dominated by pseudocapacitive behavior, leading to fast Na⁺ insertion/extraction and durable cycle life. A Na₃V₂(PO₄)₃/Fe0.5 full cell was assembled, delivering an initial capacity of 0.340 Ah g^?1.     

Properties and Structure of Spinel Li‐Mn‐O‐F Compounds for Cathode Materials of Secondary Lithium‐ion Battery

The spinel Li‐Mn‐O‐F compound cathode materials were synthesized by solid‐state reaction from calculated amounts LiOH‐H₂O, MnO₂(EMD) and LiF. The results of the electrochemical test demonstrated that these materials exhibited excellent electrochemical properties. It's initial capacity is ‐ 115 mAh.g¹ and reversible efficiency is about 100%. After 60 cycles, its capacity is still around 110 mAh.g¹ with nearly 100% reversible efficiency. The spinel Li‐Mn‐O‐F compound possibly has two structure models: interstitial model [Li]‐[Mn³⁺_xMn⁴⁺_2‐x]O₄F_δ, in which the fluorine is located on the interstice of crystal lattice, and substituted model [Li]‐[Mn³⁺_xMn⁴⁺_2‐x]O_4‐δF_δ, which the fluorine atom substituted the oxygen atom. The electrochemical result supports the interstitial model [Li][Mn³⁺_xMn⁴⁺_2‐x]O₄F_δ.