1,324 research outputs found
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Recent developments in nanostructured materials for high-performance thermoelectrics
This highlight discusses recent trends in the search for new high-efficiency thermoelectric materials. Thermoelectric materials offer considerable attractions in the pursuit of a more efficient use of existing energy resources, as they may be used to construct power-generation devices that allow useful electrical power to be extracted from otherwise waste heat. Here, we focus on the significant enhancements in thermoelectric performance that have been achieved through nanostructuring. The principal factor behind the improved performance appears to be increased phonon scattering at interfaces. This results in a substantial reduction in the lattice contribution to thermal conductivity, a low value of which is a key requirement for improved thermoelectric performance
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Ball milling as an effective route for the preparation of doped bornite: synthesis, stability and thermoelectric properties
Bornite, Cu5FeS4, is a naturally-occuring mineral with an ultralow thermal conductivity and potential for thermoelectric power generation. We describe here a new, easy and scalable route to synthesise bornite, together with the thermoelectric behaviour of manganese-substituted derivatives, Cu5Fe1-xMnxS4 (0 ≤ x ≤ 0.10). The electrical and thermal transport properties of Cu5Fe1-xMnxS4 (0 ≤ x ≤ 0.10), which are p-type semiconductors, were measured from room temperature to 573 K. The stability of bornite was investigated by thermogravimetric analysis under inert and oxidising atmospheres. Repeated measurements of the electrical transport properties confirm that bornite is stable up to 580 K under an inert atmosphere, while heating to 890 K results in rapid degradation. Ball milling leads to a substantial improvement in the thermoelectric figure of merit of unsusbtituted bornite (ZT = 0.55 at 543 K), when compared to bornite prepared by conventional high-temperature synthesis (ZT < 0.3 at 543 K). Manganese-substituted samples have a ZT comparable to that of unsubstituted bornite
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Improved thermoelectric performance through double substitution in shandite-type mixed-metal sulphides
Substitution of tin by indium in shandite-type phases, A3Sn2S2 with mixed Co/Fe occupancy of the A-sites is used to tune the Fermi level within a region of the density of states in which there are sharp, narrow bands of predominantly metal d-character. Materials of general formula Co2.5+xFe0.5-xSn2-yInyS2 (x = 0, 0.167; 0.0 x 0.7) have been prepared by solid-state reaction and the products characterised by powder X-ray diffraction. Electrical transport property data reveal that the progressive depopulation of the upper conduction band as tin is replaced by indium, increases the electrical resistivity and the weakly temperature-dependent (T) becomes more semiconducting in character. Concomitant changes in the negative Seebeck coefficient, the temperature dependence of which becomes increasingly linear, suggests the more highly substituted materials are n-type degenerate semiconductors. The power factors of the substituted phases, while increased, exhibit a weak temperature dependence. The observed reductions in thermal conductivity are principally due to reductions in the charge-carrier contribution on hole doping. A maximum figure-of-merit of (ZT)max = 0.29 is obtained for the composition Co2.667Fe0.333Sn1.6In0.4S2 at 573 K: among the highest values for an n-type sulphide at this temperature
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Crystal design approaches for the synthesis of paracetamol co-crystals
Crystal engineering principles were used to design three new co-crystals of paracetamol. A variety of potential cocrystal formers were initially identified from a search of the Cambridge Structural Database for molecules with complementary hydrogen-bond forming functionalities. Subsequent screening by powder X-ray diffraction of the products of the reaction of this library of molecules with paracetamol led to the discovery of new binary crystalline phases of paracetamol with trans-1,4- diaminocyclohexane (1); trans-1,4-di(4-pyridyl)ethylene (2); and 1,2-bis(4-pyridyl)ethane (3). The co-crystals were characterized by IR spectroscopy, differential scanning calorimetry, and 1H NMR spectroscopy. Single crystal X-ray structure analysis reveals that in all three co-crystals the co-crystal formers (CCF) are hydrogen bonded to the paracetamol molecules through O−H···N interactions. In co-crystals (1) and (2) the CCFs are interleaved between the chains of paracetamol molecules, while in co-crystal (3) there is an additional N−H···N hydrogen bond between the two components. A hierarchy of hydrogen bond formation is observed in which the best donor in the system, the phenolic O−H group of paracetamol, is preferentially
hydrogen bonded to the best acceptor, the basic nitrogen atom of the co-crystal former. The geometric aspects of the hydrogen bonds in co-crystals 1−3 are discussed in terms of their electrostatic and charge-transfer components
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Recent developments in Earth-abundant copper-sulfide thermoelectric materials
The ability of thermoelectric devices to convert waste heat into useful electrical power has stimulated a remarkable growth in research into thermoelectric materials. There is however, growing recognition that limited reserves of tellurium together with the reduction in performance that occurs at elevated temperatures, places constraints on the widespread implementation of thermoelectric technology based on the current generation of telluride-based devices. Metal sulfides have attracted considerable attention as potential tellurium-free alternatives. This perspective provides an overview of the key characteristics of sulfide thermoelectrics and the advantages they offer in the development of devices for energy recovery in the temperature range 373 T/K 773. The structures and properties of a group of synthetic materials, related to the minerals chalcocite (Cu2S), stannite (Cu₂FeSnS₄) / kesterite (Cu2SnS4), chalcopyrite (CuFeS2), bornite (Cu5FeS4), colusite (Cu26V2(As,Sn,Sb)6S32) and tetrahedrite ((Cu,Fe)12Sb4S13), are discussed. In addition to all being composed of Earth-abundant elements, these sulfides share a common tetrahedral CuS4 structural building block. The use of chemical substitution to manipulate electrical and thermal transport properties is described and common features are identified. This includes the presence of low-energy vibrational modes, the onset of copper-ion mobility and the emergence of a liquid-like sub-lattice, which serve to reduce thermal conductivity. Issues associated with materials’ stability during synthesis, consolidation and device operation, due to sulfur volatilization and migration of mobile copper ions are also highlighted. Future prospects for sulfide thermoelectrics are discussed in the light of the performance of materials investigated to date
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Structural, magnetic, and electronic properties of VxCr2−xS3(0 <x< 2)
A new family of vanadium-substituted chromium sulfides (VxCr2-xS3, 0 < x < 2) has been prepared
and characterized by powder X-ray and neutron diffraction, SQUID magnetometry, electrical resistivity,
and Seebeck coefficient measurements. Vanadium substitution leads to a single-phase region with a
rhombohedral Cr2S3 structure over the composition range 0.0 < x e 0.75, while at higher vanadium
contents (1.6 e x < 2.0) a second single-phase region, in which materials adopt a cation-deficient Cr3S4
structure, is observed. Materials with the Cr2S3 structure all exhibit semiconducting behavior. However,
both transport and magnetic properties indicate an increasing degree of electron delocalization with
increasing vanadium content in this compositional region. Materials that adopt a Cr3S4-type structure
exhibit metallic behavior. Magnetic susceptibility data reveal that all materials undergo a magnetic ordering
transition at temperatures in the range 90–118 K. Low-temperature magnetization data suggest that this
involves a transition to a ferrimagnetic state
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Ordered-defect sulfides as thermoelectric materials
The thermoelectric behaviour of the transition-metal disulphides n-type NiCr2S4 and p-type CuCrS2 is investigated. Materials prepared by high-temperature reaction were consolidated using cold-pressing and sintering, hot-pressing (HP) in graphite dies or spark-plasma sintering (SPS) in tungsten carbide dies. The consolidation conditions have a marked influence on the electrical transport properties. In addition to the effect on sample density, altering the consolidation conditions results in changes to the sample composition, including the formation of impurity phases. Maximum room-temperature power factors are 0.18 mW m-1 K-2 and 0.09 mW m-1 K-2 for NiCr2S4 and CuCrS2, respectively. Thermal conductivities of ca. 1.4 and 1.2 W m-1 K-1 lead to figures of merit of 0.024 and 0.023 for NiCr2S4 and CuCrS2, respectively
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High thermoelectric performance of bornite through control of the Cu(II) content and vacancy concentration
The thermoelectric performance of the p-type semiconductor bornite, Cu5FeS4, is greatly enhanced through chemical substitution. Non-stoichiometric materials in which the Cu:Fe ratio and overall cation-vacancy con-tent were adjusted are reported and a figure of merit, ZT = 0.79, is achieved at temperatures as low as 550 K in Cu4.972Fe0.968S4. All materials were synthesised mechanochemically and characterised by powder X-ray diffrac-tion, DSC and thermal and electrical transport property measurements. Single-phase behaviour is retained in copper deficient phases, Cu5-xFeS4, for vacancy levels up to x = 0.1, while in materials Cu5+yFe1-yS4, in which the Cu:Fe ratio is varied whilst maintaining full occupancy of cation sites, single-phase behaviour persists for y≤0.08. Adjusting the Cu:Fe ratio at a constant cation-vacancy level of 0.06 in Cu4.94+zFe1-zS4, leads to single-phases for z ≤0.04. DSC measurements indicate the temperature of the intermediate- (2a) to high-temperature (a) phase transition shows a more marked dependence on the Cu:Fe ratio than the lower temperature 4a to 2a transition. The thermoelectric power factor increases almost linearly with increasing Cu(II) content. The maximum figures of merit are obtained for materials with Cu(II) contents in the range 0.10 to 0.15 (corresponding to 2.0 – 2.8 % Cu(II)) which simultaneously contain ca .1 % of cation vacancies
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Realising the potential of thermoelectric technology: a roadmap
All machines from jet engines to microprocessors generate heat, as do manufacturing processes ranging from steel to food production. Thermoelectric generators (TEGs) are solid-state devices able to convert the resulting heat flux directly into electrical power. TEGs therefore have the potential to offer a simple, compact route to power generation in almost every industrial sector. Here, in a Roadmap developed with wide-ranging contributions from the UK Thermoelectric Network and international partners, we present the science and technology that underpins TEGs. We outline how thermoelectric (TE) technology capable of generating power outputs from microwatts to tens/hundreds kW, and potentially to MW, can have an impact across a wide range of applications in powering devices, ranging from medical to building monitoring, the Internet of things, transportation and industrial sectors. The complementary application of TE technology in cooling affords additional opportunities in refrigeration and thermal management. Improved waste heat harvesting and recovery and more efficient cooling offer significant opportunities to reduce energy usage and CO2 emissions. We provide an overview of the key challenges associated with the development of new materials and devices that offer higher power output, while matching TE solutions to the wide range of applications that would benefit from energy harvesting. There is an existing supply chain to develop, manufacture and integrate thermoelectric devices into a broad range of end-user sectors all with global market potential: the full realisation of which will require new state-of-the-art manufacturing techniques to be embraced in order to drive down costs through high-volume manufacturing to widen the application base
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Skutterudite thermoelectric modules with high volume-power-density: scalability and reproducibility
The construction and evaluation of wholly-skutterudite thermoelectric modules with a high volume-power-density is described. Such modules afford the maximum power output for the minimum use of material. Synthesis of the component n-type unfilled skutterudite CoSb2.75Sn0.05Te0.20 and p-type filled skutterudite Ce0.5Yb0.5Fe3.25Co0.75Sb12, was achieved using a scalable ball-milling route that provides sufficient material for the construction and assessment of performance of 12 modules. Impedance spectroscopy at room temperature is shown to provide a rapid means of evaluating the quality of module fabrication. The results show a high degree of reproducibility in module performance across the 12 modules, with an average internal resistance of 102(4) m. Electrical measurements on the component n- and p-type materials reveal power factors (S2σ) of 1.92 and 1.33 mW m-1 K-2, respectively, at room temperature and maximum figures of merit of ZT = 1.13 (n-type) and ZT = 0.91 (p-type) at 673 K and 823 K, respectively. The figure of merit of the module at room temperature (ZT = 0.12) is reduced by ca. 39% from the average of the n- and p-type component materials at the same temperature, as a result of thermal- and electrical-contact resistance losses associated with the architecture of the module. I-V curves for the module determined for T in the range 50 – 450 K show an almost linear dependence of the open-circuit voltage on T and allow calculation of the power output, which reaches a maximum value of 1.8 W (0.9 W cm-2) at T = 448 K
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