Browsing by Author "Chen, J"

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    Copper diffusion rates and hopping pathways in superionic Cu 2Se: implications for thermoelectricity
    (SSRN, 2020-10-21) Nazrul Islam, SMK; Mayank, P; Ouyang, Y; Chen, J; Sagotra, AK; Li, M; Cortie, MB; Mole, RA; Cazorla, C; Yu, DH; Wang, XL; Robinson, RA; Cortie, DL
    The ultra-low thermal conductivity of Cu2Se is well established, but there is so far no consensus on the underlying mechanism. One proposal is that the fast-ionic diffusion of copper suppresses the acoustic phonons. The diffusion coefficients reported previously, however, differ by two orders of magnitude between the various studies and it remains unclear whether the diffusion is fast enough to impact the heat-bearing phonons. Here, a two-fold approach is used to accurately re-determine the diffusion rates. Ab-initio molecular dynamics simulations, incorporating landmark analysis techniques, were closely compared with experimental quasielastic/inelastic neutron spectroscopy. Reasonable agreement was found between these approaches, consistent with the experimental coefficient of 3.1 ± 1.3 10-5 cm2.s-1 and an activation barrier of 140 ± 60 meV. The hopping mechanism includes short 2 Å hops between tetragonal and interstitial octahedral sites. This process forms dynamic Frenkel defects, however, there is no indication of additional broadening in the density-of-states indicating the intrinsic anharmonic interactions dictate the phonon lifetimes. © Preprint article - 2023 Elsevier Inc.
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    Copper diffusion rates and hopping pathways in superionic Cu2Se
    (Elsevier, 2021-08-15) Nazrul Islam, SMK; Mayank, P; Ouyang, Y; Chen, J; Sagotra, AK; Li, M; Cortie, MB; Mole, RA; Cazorla, C; Yu, DH; Wang, XL; Robinson, RA; Cortie, DL
    The ultra-low thermal conductivity of Cu2Se is well established, but so far there is no consensus on the underlying mechanism. One proposal is that the fast-ionic diffusion of copper suppresses the acoustic phonons. The diffusion coefficients reported previously, however, differ by two orders of magnitude between the various studies and it remains unclear whether the diffusion is fast enough to impact the heat-bearing phonons. Here, a two-fold approach is used to accurately re-determine the diffusion rates. Ab-initio molecular dynamics simulations, incorporating landmark analysis techniques, were closely compared with experimental quasielastic/inelastic neutron scattering. Reasonable agreement was found between these approaches, consistent with a diffusion coefficient of 3.1 ± 1.3 x 10−5 cm2.s−1 at 675 K and an activation barrier of 140 ± 60 meV. The hopping mechanism includes short 2 Å hops between tetrahedral and interstitial octahedral sites. This process forms dynamic Frenkel defects. Despite the latter processes, there is no major loss of the phonon mode intensity in the superionic state, and there is no strong correlation between the phonon spectra and the increased diffusion rates. Instead, intrinsic anharmonic phonon interactions appear to dictate the thermal conductivity above and below the superionic transition, and there is only subtle mode broadening associated with the monoclinic-cubic structural transition point, with the phonon density-of-states remaining almost constant at higher temperatures. © 2021 Acta Materialia Inc.
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    Defect structure and property consequence when small Li+ ions meet BaTiO3
    (American Physical Society, 2020-08-31) Narayanan, N; Lou, Q; Rawal, A; Lu, T; Liu, Z; Chen, J; Langley, J; Chen, H; Hester, JR; Cox, N; Fuess, H; McIntyre, GJ; Li, G; Yu, DH; Liu, Y
    In the present work the longstanding issue of the structure and dynamics of smaller ions in oxides and its impact on the properties was investigated on 7% Li-doped BaTiO3. The investigation combined several techniques, notably neutron powder diffraction (NPD), nuclear magnetic resonance (7Li-NMR), electron paramagnetic resonance (EPR), electron microprobe, electric polarization (EP) measurement, and electronic structure calculations based on density-functional theory (DFT). Electron microprobe confirmed multiple phases, one containing incorporated Li in the BaTiO3 host lattice and another glassy phase which breaks the host lattice due to excessive Li accumulation. While the average structure of Li in BaTiO3 could not be determined by NPD, 7Li-NMR revealed one broad “disordered” and multiple “ordered” peaks. Local structure models with different defect types involving Li+ were modeled and the corresponding chemical shifts (δ) were compared with experimental values. It is found that the closest defect model describing the ordered peaks, is with Ti4+ being replaced by four Li+ ions. The biexponential behavior of the spin-lattice relaxation of the ordered peaks each with a short and a long relaxation discloses the existence of paramagnetic ions. Finally, EPR revealed the existence of the paramagnetic ion Ti3+ as a charge-transfer defect. DFT calculations disclosed local antipolar displacements of Ti ions around both types of defect sites upon insertion of Li+. This is in accordance with the experimental observation of pinching effects of the EP in Li-doped BaTiO3. These studies demonstrate the huge impact of the local structure of the doped smaller/lighter ions on the functional properties of oxides. ©2020 American Physical Society
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    Defect structure-property correlations in Li doped BaTiO3
    (Australian Institute of Nuclear Science and Engineering (AINSE), 2020-11-11) Narayanan, N; Lou, Q; Rawal, A; Lu, T; Liu, Z; Chen, J; Langley, J; Chen, H; Hester, JR; Cox, N; Fuess, H; McIntyre, GJ; Li, G; Yu, DH; Liu, Y; Li, G
    In the present work we investigate the important issue of the structure and dynamics of smaller ions in oxides and the resulting impact on its functional properties. For this purpose, we selected a 7% Li-doped BaTiO3. Li is a vital ingredient in novel energy storage technologies such as Li-ion batteries. The smaller Li-ion can influence the structural stability, homogeneity, local environment, and dynamic behavior of the host lattice, affecting and optimizing the dielectric and multiferroic properties of novel polar functional materials [1-2]. However, the Li-ion positions and dynamics in functional materials are not completely understood, controversially discussed and are the subject of extensive ongoing research [3]. Furthermore, sample inhomogeneity due to Li migration to the grain boundary and/or development of multiple phases complicates the elucidation of the structure-property correlations that may lead to incorrect interpretations [4]. The selection of BaTiO3 as the host lattice is due to materials based on this being considered as the alternative to the piezoelectric lead zirconate titanate, citing environmental issues [5]. BaTiO3 also crystallizes in a simple perovskite structure and Li ions can be effectively doped into it at lower doping levels. Very recently, field-dependent electric polarization measurements on BaTiO3 exhibited a polarization–electric field double hysteresis loop upon Li doping [4]. These drastic changes to the electric polarization, related to the doping poses a good test case for the investigation of the Li induced defect structure model and its influence on the functional properties. To elucidate the above structure-property correlations, we combined several techniques, such as neutron powder diffraction electron microprobe associated with the wavelength-dispersive spectroscopy, 7Li nuclear magnetic resonance spectroscopy (NMR), electron paramagnetic resonance (EPR), electric polarization measurement, and theoretical calculations based on density functional theory [6].
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    Domain wall and interphase boundary motion in a two-phase morphotropic phase boundary ferroelectric: Frequency dispersion and contribution to piezoelectric and dielectric properties
    (American Physical Society, 2012-07-12) Jones, JL; Aksel, E; Tutuncu, G; Usher, TM; Chen, J; Xing, XR; Studer, AJ
    In ferroelectric materials, enhanced dielectric and piezoelectric property coefficients are found in compositions near morphotropic phase boundaries (MPBs). The material response in these compositions may be contributed by enhanced intrinsic piezoelectric distortions or increased interface motion, e.g., contributions from domain wall and interphase boundary motion, though the relative effect of these mechanisms in different materials is not yet well understood. One of the major challenges to developing this understanding is the availability and sensitivity of in situ characterization techniques, particularly during the application of cyclic electric fields of subcoercive or weak amplitude, conditions at which the property coefficients are measured. Here, we use time-resolved neutron diffraction to resolve the subtle electric-field-induced crystallographic strain mechanisms in a prototypical MPB composition, 36%BiScO(3)-64%PbTiO(3), that contains coexisting monoclinic and tetragonal phases. We observe multiple cooperative electromechanical effects including domain wall motion in both the monoclinic and tetragonal phases, interphase boundary motion between the two phases, and electric-field-induced lattice strains. The measured effects span four orders of magnitude in frequency, facilitating the discrimination of intrinsic and extrinsic contributions to properties. Domain wall motion in the monoclinic phase dominates the response, leading to shifts of diffraction peaks as high as 2300 pm/V; these shifts reflect the field-induced changes in average pseudocubic (00h) lattice spacing of the monoclinic phase parallel to the electric field. Domain wall motion in the tetragonal phase is also readily apparent and exhibits a degree of frequency dispersion similar to that measured in both the relative permittivity and piezoelectric coefficients at similar conditions. © 2012, American Physical Society.
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    Hierarchically porous carbon–zirconium carbide spheres as potentially reusable transmutation targets
    (Elsevier, 2015-08-01) Scales, N; Chen, J; Hanley, TL; Riley, DP; Lumpkin, GR; Luca, V
    The preparation of hierarchically porous phase-pure carbon–zirconium carbide spheres with surface areas upwards of 70 m2/g and diameters in the 1–2 mm range has been achieved. The zirconium carbide beads were prepared through carbothermal reduction of polyacrylonitrile-zirconium composites prepared via three different routes including infiltration of a zirconium precursor into preformed polyacrylonitrile (PAN) beads and two one-pot co-precipitation methods. Depending on the route used to prepare the composites, relatively high surface area phase-pure zirconium carbides with the radial macroporous internal structure of the PAN template could be prepared. The adsorption of the elements U, Mo, Cs and Sr by the zirconium carbide beads was studied as a function of pH in the range 1–13 and target element concentrations of 0.025 mmol/L. The as-prepared hierarchical zirconium carbide beads demonstrated almost quantitative uptake of Mo below pH 4 with uptake decreasing to close to zero and above pH 10.5. On the other hand, U uptake was negligible below pH 4 and became quantitative in the range 4–12. Cesium was not extracted to any significant extent in the pH range studied and Sr only to a limited extent above pH 10. The engineered hierarchical porosities, neutron transparency, thermal stability and anticipated chemical stability of the present zirconium carbide spheres makes them promising candidates as reusable transmutation targets for Minor Actinide incineration. © 2015 Elsevier Inc.
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    Neutron diffraction study of unusual magnetic behaviors in the Ho2Fe11Al6 intermetallic compound
    (American Chemical Society, 2019-09-30) Cao, Y; Lin, K; Liu, ZN; Hu, JY; Wang, CW; Avdeev, M; Li, Q; Deng, JX; Chen, J; Zhang, HJ; Xing, XR
    Knowledge of structure–property relationships is fundamental but significant in the exploitation of magnetic materials. Here we report that the high Al substitution for Fe transformed the crystal structure from a hexagonal Ho2Fe17 compound to a rhombohedral Ho2Fe11Al6 compound. Intriguingly, the latter shows unusual evolution of magnetization around 86 and 220 K compared with the former. Integrated investigations of the detailed structure analysis and magnetic performance on the Ho2Fe11Al6 compound demonstrate that the Ho2Fe11Al6 compound possesses a stable rhombohedral structure (R3̅m) from 5 to 430 K with preferred occupation of Al atoms and ferrimagnetic structure in which the magnetic moments of Ho and Fe lie antiparallel in the basal plane below the Curie temperature. The results of the temperature dependence of moments reveal that the disparate rates of change of the moments for Ho and Fe sublattices give rise to unusual evolution of magnetization around 86 and 220 K and then turn to paramagnetic above 280 K. This work provides clear structure and magnetization information on the Ho2Fe11Al6 compound, which may be beneficial to guiding the future development of magnetic materials. © 2019 American Chemical Society
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    Optimizing the structure of layered cathode material for higher electrochemical performance by elucidating structural evolution during heat processing
    (Elsevier, 2020-12-01) Huang, ZY; Chu, MH; Wang, R; Zhu, WM; Zhao, WG; Wang, CQ; Zhang, YJ; He, LH; Chen, J; Deng, SH; Mei, LW; Kan, WH; Avdeev, M; Pan, F; Xiao, YG
    Improving electrochemical performance of cathode materials for lithium-ion batteries requires comprehensive understanding of their structural properties which could facilitate or impede the diffusion of lithium during charge-discharge. In order to optimize the structure and improve the electrochemical performance of layered cathode material, the detailed structural evolution as a function of heat treatment temperature in LiNi0.8Co0.1Mn0.1O2 was investigated by in-situ and ex-situ neutron powder diffraction methods. We show that both cycling stability and rate performance of LiNi0.8Co0.1Mn0.1O2 can be improved by performing heat treatment at 400 °C, which is attributed to the optimization of surface structure and the enlargement of c/a ratio. Heat treatment of LiNi0.8Co0.1Mn0.1O2 at higher temperature induces a layered-to-rock-salt structure phase transition accompanied with the precipitation of lithium oxide. A 3D phase diagram, which correlates the high temperature phases and room temperature phases, is constructed. The presentation of comprehensive phase diagrams up to 1000 °C could provide the basis for further research on not only synthesis strategy but also thermal stability in Ni-rich layered cathode materials. © 2020 Elsevier Ltd.
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    Thermal batteries based on inverse barocaloric effects
    (Science Advances, 2023-02) Zhang, Z; Li, K; Lin, SC; Song, R; Yu, DH; Wang, Y; Wang, JF; Kawaguchi, S; Zhang, Z; Yu, CY; Li, XD; Chen, J; He, LH; Mole, RA; Yuan, B; Ren, QY; Qian, K; Cai, ZL; Yu, JG; Wang, MC; Zhao, CY; Tong, X; Zhang, ZD; Li, B
    To harvest and reuse low-temperature waste heat, we propose and realize an emergent concept-barocaloric thermal batteries based on the large inverse barocaloric effect of ammonium thiocyanate (NH4SCN). Thermal charging is initialized upon pressurization through an order-to-disorder phase transition, and the discharging of 43 J g-1 takes place at depressurization, which is 11 times more than the input mechanical energy. The thermodynamic equilibrium nature of the pressure-restrained heat-carrying phase guarantees stable long-duration storage. The barocaloric thermal batteries reinforced by their solid microscopic mechanism are expected to substantially advance the ability to take advantage of waste heat. Copyright © 2023 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works. Distributed under a Creative Commons Attribution NonCommercial License 4.0 (CC BY-NC).
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    Ultralow thermal conductivity from transverse acoustic phonon suppression in distorted crystalline α-MgAgSb
    (Springer Nature, 2020-02-18) Li, XY; Liu, PF; Zhao, EY; Zhang, ZG; Guidi, T; Le, MD; Avdeev, M; Ikeda, K; Otomo, T; Kofu, M; Nakajima, K; Chen, J; He, LH; Ren, Y; Wang, XL; Wang, BT; Ren, ZF; Zhao, HZ; Wang, FW
    Low thermal conductivity is favorable for preserving the temperature gradient between the two ends of a thermoelectric material, in order to ensure continuous electron current generation. In high-performance thermoelectric materials, there are two main low thermal conductivity mechanisms: the phonon anharmonic in PbTe and SnSe, and phonon scattering resulting from the dynamic disorder in AgCrSe2 and CuCrSe2, which have been successfully revealed by inelastic neutron scattering. Using neutron scattering and ab initio calculations, we report here a mechanism of static local structure distortion combined with phonon-anharmonic-induced ultralow lattice thermal conductivity in α-MgAgSb. Since the transverse acoustic phonons are almost fully scattered by the compound’s intrinsic distorted rocksalt sublattice, the heat is mainly transported by the longitudinal acoustic phonons. The ultralow thermal conductivity in α-MgAgSb is attributed to its atomic dynamics being altered by the structure distortion, which presents a possible microscopic route to enhance the performance of similar thermoelectric materials. © The Author(s) 2020.
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    Ultrawide temperature range super-invar behavior of R2(Fe, Co)17 materials (R = rare earth)
    (American Physical Society, 2021-07-30) Cao, YL; Lin, KM; Khmelevskyi, S; Avdeev, M; Taddei, KM; Zhang, Q; Huang, QZ; Li, Q; Kato, K; Tang, CC; Gibbs, A; Wang, CW; Deng, JX; Chen, J; Zhang, HJ; Xing, XR
    Super Invar (SIV), i.e., zero thermal expansion of metallic materials underpinned by magnetic ordering, is of great practical merit for a wide range of high precision engineering. However, the relatively narrow temperature window of SIV in most materials restricts its potential applications in many critical fields. Here, we demonstrate the controlled design of thermal expansion in a family of R2(Fe,Co)17 materials (R=rare Earth). We find that adjusting the Fe-Co content tunes the thermal expansion behavior and its optimization leads to a record-wide SIV with good cyclic stability from 3–461 K, almost twice the range of currently known SIV. In situ neutron diffraction, Mössbauer spectra and first-principles calculations reveal the 3d bonding state transition of the Fe-sublattice favors extra lattice stress upon magnetic ordering. On the other hand, Co content induces a dramatic enhancement of the internal molecular field, which can be manipulated to achieve “ultrawide” SIV over broad temperature, composition and magnetic field windows. These findings pave the way for exploiting thermal-expansion-control engineering and related functional materials. © 2021 American Physical Society