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Author

Nita Dragoe

Other affiliations: Supélec, University of Bucharest, University of Paris-Sud  ...read more
Bio: Nita Dragoe is an academic researcher from Université Paris-Saclay. The author has contributed to research in topics: Thermoelectric effect & Seebeck coefficient. The author has an hindex of 30, co-authored 119 publications receiving 4366 citations. Previous affiliations of Nita Dragoe include Supélec & University of Bucharest.


Papers
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Journal ArticleDOI
TL;DR: In this paper, the authors summarized the crystal structures, microstructures, electronic structures and physical/chemical properties of BiCuSeO oxyselenides and discussed the approaches that successfully enhanced the thermoelectric performances of these materials.
Abstract: BiCuSeO oxyselenides have recently acquired ever-increasing attention and have been extensively studied as very promising thermoelectric materials. The ZT of the BiCuSeO system was significantly increased from 0.5 to 1.4 in the past three years, which indicates that BiCuSeO oxyselenides are robust candidates for energy conversion applications. In this review, we first discuss and summarize the crystal structures, microstructures, electronic structures and physical/chemical properties of BiCuSeO oxyselenides. Then, the approaches that successfully enhanced the thermoelectric performances in the BiCuSeO system are outlined, which include increasing carrier concentration, optimizing Cu vacancies, a simple and facile ball milling method, multifunctional Pb doping, band gap tuning, and increasing carrier mobility through texturing. Theoretical calculations to predict a maximum ZT in the BiCuSeO system are also described. Finally, a discussion of future possible strategies is proposed to aim at further enhancing the thermoelectric figure of merit of these materials.

498 citations

Journal ArticleDOI
TL;DR: In this paper, a new family of oxide-based materials with the general formula (MgCoNiCuZn)1−x−yGayAxO (with A = Li, Na, K).
Abstract: Impedance spectroscopy measurements evidence superionic Li+ mobility (>10−3 S cm−1) at room temperature and fast ionic mobility for Na+ (5 × 10−6 S cm−1) in high entropy oxides, a new family of oxide-based materials with the general formula (MgCoNiCuZn)1−x−yGayAxO (with A = Li, Na, K). Structural investigations indicate that the conduction path probably involves oxygen vacancies.

417 citations

Journal ArticleDOI
TL;DR: Zhang et al. as mentioned in this paper showed that the configurational disorder can be used for stabilizing simple solid solutions of oxides, which should normally not form solid solutions, and these new materials were called entropy-stabilized oxides.
Abstract: Entropic contributions to the stability of solids are very well understood and the mixing entropy has been used for forming various solids, for instance such as inverse spinels, see Nawrotsky et al., J. Inorg. Nucl. Chem. 29, 2701 (1967) [1]. A particular development was related to high entropy alloys by Yeh et al., Adv. Eng. Mater. 6, 299 (2004) [2] and Cantor et al., Mater. Sci. Eng. A 375–377, 213 (2004) [3] (for recent reviews see Zhang et al., Prog. Mater. Sci. 61, 1 (2014) [4] and Tsai et al., Mater. Res. Lett. 2, 107 (2014) [5]) in which the configurational disorder is responsible for forming simple solid solutions and which are thoroughly studied for various applications especially due to their mechanical properties, e.g. Gludovatz et al., Science 345, 1153 (2014) [6] and Lu et al., Sci. Rep. 4, 6200 (2014) [7], but also electrical properties, Kozelj et al., Phys. Rev. Lett. 113, 107001 (2014) [8], hydrogen storage, Kao et al., Int. J. Hydrogen Energy 35, 9046 (2010) [9], magnetic properties, Zhang et al., Sci. Rep. 3, 1455 (2013) [10]. Many unexplored compositions and properties still remain for this class of materials due to their large phase space. In a recent report it has been shown that the configurational disorder can be used for stabilizing simple solid solutions of oxides, which should normally not form solid solutions, see Rost et al., Nature Commun. 6, 8485 (2015) [11] these new materials were called ”entropy-stabilized oxides”. In this pioneering report, it was shown that mixing five equimolar binary oxides yielded, after heating at high temperature and quenching, an unexpected rock salt structure compound with statistical distribution of the cations in a face centered cubic lattice. Following this seminal study, we show here that these high entropy oxides (named HEOx hereafter) can be substituted by aliovalent elements with a charge compensation mechanism. This possibility largely increases the potential development of new materials by widening their (already complex) phase space. As a first example, we report here that at least one HEOx composition exhibits colossal dielectric constants, which could make it very promising for applications as large-k dielectric materials. (© 2016 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim)

383 citations

Journal ArticleDOI
TL;DR: In this article, a layered oxyselenide composed of conductive (Cu2Se2) 2− layers alternately stacked with insulating (Bi2O2)2+ layers, shows an enhancement of the electrical conductivity after substituting Bi3+ by Sr2+, from 470 S 1/m−1 (BiCuSeO) to 4.8×104 S 2/m −1 (Sr 0.15CuO) at 293 K. Maximum ZT values reach 0.76 at 873 K.
Abstract: p-type BiCuSeO, a layered oxyselenide composed of conductive (Cu2Se2)2− layers alternately stacked with insulating (Bi2O2)2+ layers, shows an enhancement of the electrical conductivity after substituting Bi3+ by Sr2+, from 470 S m−1 (BiCuSeO) to 4.8×104 S m−1 (Bi0.85Sr0.15CuSeO) at 293 K. Coupled to high Seebeck coefficients, this leads to promising values of the thermoelectric power factor that exceeds 500 μW m−1 K−2 at 873 K. Moreover, the thermal conductivity of these layered compounds is lower than 1 W m−1 K−1 at 873 K. Maximum ZT values reach 0.76 at 873 K, making this family promising for thermoelectric applications in the medium temperature range.

342 citations

Journal ArticleDOI
TL;DR: Zhao et al. as discussed by the authors reported on the high thermoelectric performance of p-type polycrystalline BiCuSeO, a layered oxyselenide composed of alternating conductive and insulating (Bi2O2)2+ layers.
Abstract: We report on the high thermoelectric performance of p-type polycrystalline BiCuSeO, a layered oxyselenide composed of alternating conductive (Cu2Se2)2− and insulating (Bi2O2)2+ layers. The electrical transport properties of BiCuSeO materials can be significantly improved by substituting Bi3+ with Ca2+. The resulting materials exhibit a large positive Seebeck coefficient of ∼+330 μV K−1 at 300 K, which may be due to the ‘natural superlattice’ layered structure and the moderate effective mass suggested by both electronic density of states and carrier concentration calculations. After doping with Ca, enhanced electrical conductivity coupled with a moderate Seebeck coefficient leads to a power factor of ∼4.74 μW cm−1 K−2 at 923 K. Moreover, BiCuSeO shows very low thermal conductivity in the temperature range of 300 (∼0.9 W m−1 K−1) to 923 K (∼0.45 W m−1 K−1). Such low thermal conductivity values are most likely a result of the weak chemical bonds (Young’s modulus, E∼76.5 GPa) and the strong anharmonicity of the bonding arrangement (Gruneisen parameter, γ∼1.5). In addition to increasing the power factor, Ca doping reduces the thermal conductivity of the lattice, as confirmed by both experimental results and Callaway model calculations. The combination of optimized power factor and intrinsically low thermal conductivity results in a high ZT of ∼0.9 at 923 K for Bi0.925Ca0.075CuSeO. Li-Dong Zhao, Jiaqing He and co-workers have gained insight into the highly thermoelectric properties of a bismuth–copper oxyselenide (BiCuSeO), a polycrystalline, layered compound. BiCuSeO's ability to produce a significant electric potential from a temperature difference, and vice versa, arises from its intrinsically low thermal conductivity, and can be further improved by boosting the material's electrical conductivity through doping with strontium or barium, or introducing copper deficiencies. The researchers have now carried out an extensive characterization of the oxyselenide and propose that its conveniently low thermal conductivity results from the weak chemical bonds that exist between two different kinds of layers, and a particular bonding arrangement, in the material's lattice. Moreover, by substituting bismuth ions (Bi3+) with calcium ions (Ca3+) the thermal conductivity of the lattice could be lowered further, leading to an improvement in the oxyselenide's thermoelectric properties. We report on the promising thermoelectric performance of p-type polycrystalline BiCuSeO, which is a layered oxyselenide composed of conductive (Cu2Se2)2− layers that alternate with insulating (Bi2O2)2+ layers. Electrical transport properties can be optimized by substituting Bi3+ with Ca2+. Moreover, BiCuSeO shows very low thermal conductivity in the temperature ranges of 300 (∼0.9 W m−1K−1) to 923 K (∼0.45 W m−1 K−1). These intrinsically low thermal conductivity values may result from the weak chemical bonds of the material as well as the strong anharmonicity of the bonding arrangement. The combination of the optimized power factor and the intrinsically low thermal conductivity results in a high ZT of ∼0.9 at 923 K for Bi0.925Ca0.075CuSeO.

327 citations


Cited by
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Journal ArticleDOI
01 Apr 1988-Nature
TL;DR: In this paper, a sedimentological core and petrographic characterisation of samples from eleven boreholes from the Lower Carboniferous of Bowland Basin (Northwest England) is presented.
Abstract: Deposits of clastic carbonate-dominated (calciclastic) sedimentary slope systems in the rock record have been identified mostly as linearly-consistent carbonate apron deposits, even though most ancient clastic carbonate slope deposits fit the submarine fan systems better. Calciclastic submarine fans are consequently rarely described and are poorly understood. Subsequently, very little is known especially in mud-dominated calciclastic submarine fan systems. Presented in this study are a sedimentological core and petrographic characterisation of samples from eleven boreholes from the Lower Carboniferous of Bowland Basin (Northwest England) that reveals a >250 m thick calciturbidite complex deposited in a calciclastic submarine fan setting. Seven facies are recognised from core and thin section characterisation and are grouped into three carbonate turbidite sequences. They include: 1) Calciturbidites, comprising mostly of highto low-density, wavy-laminated bioclast-rich facies; 2) low-density densite mudstones which are characterised by planar laminated and unlaminated muddominated facies; and 3) Calcidebrites which are muddy or hyper-concentrated debrisflow deposits occurring as poorly-sorted, chaotic, mud-supported floatstones. These

9,929 citations

Journal ArticleDOI
TL;DR: This Review discusses model high-entropy alloys with interesting properties, the physical mechanisms responsible for their behaviour and fruitful ways to probe and discover new materials in the vast compositional space that remains to be explored.
Abstract: Alloying has long been used to confer desirable properties to materials. Typically, it involves the addition of relatively small amounts of secondary elements to a primary element. For the past decade and a half, however, a new alloying strategy that involves the combination of multiple principal elements in high concentrations to create new materials called high-entropy alloys has been in vogue. The multi-dimensional compositional space that can be tackled with this approach is practically limitless, and only tiny regions have been investigated so far. Nevertheless, a few high-entropy alloys have already been shown to possess exceptional properties, exceeding those of conventional alloys, and other outstanding high-entropy alloys are likely to be discovered in the future. Here, we review recent progress in understanding the salient features of high-entropy alloys. Model alloys whose behaviour has been carefully investigated are highlighted and their fundamental properties and underlying elementary mechanisms discussed. We also address the vast compositional space that remains to be explored and outline fruitful ways to identify regions within this space where high-entropy alloys with potentially interesting properties may be lurking. High-entropy alloys have greatly expanded the compositional space for alloy design. In this Review, the authors discuss model high-entropy alloys with interesting properties, the physical mechanisms responsible for their behaviour and fruitful ways to probe and discover new materials in the vast compositional space that remains to be explored.

1,798 citations

Journal ArticleDOI
TL;DR: This review describes the recent advances in designing high-performance bulk thermoelectric materials and highlights the decoupling of the electron and phonon transport through coherent interface, matrix/precipitate electronic bands alignment, and compositionally alloyed nanostructures.
Abstract: There has been a renaissance of interest in exploring highly efficient thermoelectric materials as a possible route to address the worldwide energy generation, utilization, and management. This review describes the recent advances in designing high-performance bulk thermoelectric materials. We begin with the fundamental stratagem of achieving the greatest thermoelectric figure of merit ZT of a given material by carrier concentration engineering, including Fermi level regulation and optimum carrier density stabilization. We proceed to discuss ways of maximizing ZT at a constant doping level, such as increase of band degeneracy (crystal structure symmetry, band convergence), enhancement of band effective mass (resonant levels, band flattening), improvement of carrier mobility (modulation doping, texturing), and decrease of lattice thermal conductivity (synergistic alloying, second-phase nanostructuring, mesostructuring, and all-length-scale hierarchical architectures). We then highlight the decoupling of th...

1,469 citations

Journal ArticleDOI
29 Sep 2017-Science
TL;DR: The mechanisms and strategies for improving thermoelectric efficiency are reviewed and how to report material performance is discussed, as well as how to develop high-performance materials out of nontoxic and earth-abundant elements.
Abstract: BACKGROUND Heat and electricity are two forms of energy that are at opposite ends of a spectrum Heat is ubiquitous, but with low quality, whereas electricity is versatile, but its production is demanding Thermoelectrics offers a simple and environmentally friendly solution for direct heat-to-electricity conversion A thermoelectric (TE) device can directly convert heat emanating from the Sun, radioisotopes, automobiles, industrial sectors, or even the human body to electricity Electricity also can drive a TE device to work as a solid-state heat pump for distributed spot-size refrigeration TE devices are free of moving parts and feasible for miniaturization, run quietly, and do not emit greenhouse gasses The full potential of TE devices may be unleashed by working in tandem with other energy-conversion technologies Thermoelectrics found niche applications in the 20th century, especially where efficiency was of a lower priority than energy availability and reliability Broader (beyond niche) application of thermoelectrics in the 21st century requires developing higher-performance materials The figure of merit, ZT, is the primary measure of material performance Enhancing the ZT requires optimizing the adversely interdependent electrical resistivity, Seebeck coefficient, and thermal conductivity, as a group On the microscopic level, high material performance stems from a delicate concert among trade-offs between phase stability and instability, structural order and disorder, bond covalency and ionicity, band convergence and splitting, itinerant and localized electronic states, and carrier mobility and effective mass ADVANCES Innovative transport mechanisms are the fountain of youth of TE materials research In the past two decades, many potentially paradigm-changing mechanisms were identified, eg, resonant levels, modulation doping, band convergence, classical and quantum size effects, anharmonicity, the Rashba effect, the spin Seebeck effect, and topological states These mechanisms embody the current states of understanding and manipulating the interplay among the charge, lattice, orbital, and spin degrees of freedom in TE materials Many strategies were successfully implemented in a wide range of materials, eg, V2VI3 compounds, VVI compounds, filled skutterudites and clathrates, half-Heusler alloys, diamond-like structured compounds, Zintl phases, oxides and mixed-anion oxides, silicides, transition metal chalcogenides, and organic materials In addition, advanced material synthesis and processing techniques, for example, melt spinning, self-sustaining heating synthesis, and field-assisted sintering, helped reach a much broader phase space where traditional metallurgy and melt-growth recipes fell short Given the ubiquity of heat and the modular aspects of TE devices, these advances ensure that thermoelectrics plays an important role as part of a solutions package to address our global energy needs OUTLOOK The emerging roles of spin and orbital states, new breakthroughs in multiscale defect engineering, and controlled anharmonicity may hold the key to developing next generation TE materials To accelerate exploring the broad phase space of higher multinary compounds, we need a synergy of theory, machine learning, three-dimensional printing, and fast experimental characterizations We expect this synergy to help refine current materials selection and make TE materials research more data driven We also expect increasing efforts to develop high-performance materials out of nontoxic and earth-abundant elements The desire to move away from Freon and other refrigerant-based cooling should shift TE materials research from power generation to solid-state refrigeration International round-robin measurements to cross-check the high ZT values of emerging materials will help identify those that hold the most promise We hope the renewable energy landscape will be reshaped if the recent trend of progress continues into the foreseeable future

1,457 citations

Journal ArticleDOI
TL;DR: Novel concepts and paradigms are described here that have emerged, targeting superior TE materials and higher TE performance, including band convergence, "phonon-glass electron-crystal", multiscale phonon scattering, resonant states, anharmonicity, etc.
Abstract: The past two decades have witnessed the rapid growth of thermoelectric (TE) research. Novel concepts and paradigms are described here that have emerged, targeting superior TE materials and higher TE performance. These superior aspects include band convergence, "phonon-glass electron-crystal", multiscale phonon scattering, resonant states, anharmonicity, etc. Based on these concepts, some new TE materials with distinct features have been identified, including solids with high band degeneracy, with cages in which atoms rattle, with nanostructures at various length scales, etc. In addition, the performance of classical materials has been improved remarkably. However, the figure of merit zT of most TE materials is still lower than 2.0, generally around 1.0, due to interrelated TE properties. In order to realize an "overall zT > 2.0," it is imperative that the interrelated properties are decoupled more thoroughly, or new degrees of freedom are added to the overall optimization problem. The electrical and thermal transport must be synergistically optimized. Here, a detailed discussion about the commonly adopted strategies to optimize individual TE properties is presented. Then, four main compromises between the TE properties are elaborated from the point of view of the underlying mechanisms and decoupling strategies. Finally, some representative systems of synergistic optimization are also presented, which can serve as references for other TE materials. In conclusion, some of the newest ideas for the future are discussed.

1,014 citations